Stable isotope and trace element composition of foraminiferal calcite - from incorporation to dissolution

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(1)Stable isotope and trace element composition of foraminiferal calcite from incorporation to dissolution. Dissertation zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften - Dr. rer. nat. im Fachbereich 2 (Biologie/Chemie) der Universität Bremen. vorgelegt von. Bärbel Hönisch Bremen, 2002.

(2) Tag des öffentlichen Kolloquiums: 14. November 2002. Gutachter der Dissertation:. Prof. Dieter Wolf-Gladrow PD Dr. Ralph Schneider.

(3) Abstract Long-term reconstruction of climate and environmental parameters from marine sediments relies heavily on the reliability of proxy indicators from planktonic and benthic foraminifera. Therefore, the aim of this dissertation is to improve our understanding and confidence in planktonic foraminiferal proxies as indicators of seawater carbonate chemistry and their stability in response to dissolution. Towards this goal laboratory experiments with living specimens and empty shells collected from sediments were carried out. In culture experiments with the living planktonic foraminifer Orbulina universa the control of symbiont photosynthetic activity on the boron isotopic composition of shell calcite was investigated (Publication I). Under low light (reduced photosynthetic rates) the boron isotopic composition of the tests is 1.5‰ lower compared to shells grown under high light (elevated photosynthetic rates). As boron isotope values trace seawater-pH, the lower δ11B translates to a reduction in recorded pH of approximately 0.2 units. Data obtained for fieldgrown, symbiont-barren Globigerina bulloides record significantly lighter δ11B than the symbiont-bearing O. universa and therefore support the hypothesis that respiration and photosynthesis are the key physiological parameters responsible for species-specific vital effects. Although this experiment may indicate that symbiont-barren foraminifera reflect ambient seawater chemistry more accurately than symbiont-bearing foraminifera, model results suggest that photosynthesis- and respiration-driven offsets are constant over a wide pH-range and do not compromise the reliability of δ11B in symbiont-bearing foraminifera as a paleo-pH indicator. The Ba/Ca ratio in foraminiferal shells has been shown to reflect seawater Ba concentrations, which in turn are correlated to alkalinity. To investigate whether alkalinity may directly influence the Ba incorporation into foraminiferal calcite and thereby compromise the reliability of this alkalinity proxy, symbiont-bearing O. universa and symbiont-barren G. bulloides were grown in seawater of constant Ba concentration at five different alkalinities (Publication II). A weak negative correlation between the Ba uptake ratio in O. universa shells and alkalinity was found under high light conditions. For an increase in alkalinity of 100 µmol kg-1 the distribution coefficient DBa (= Ba/Ca. shell. / Ba/Caseawater) decreased by. 0.004. This change is well within the error of DBa determined to date and the weak influence of alkalinity on Ba incorporation into foraminiferal shells is insignificant for paleoreconstructions. Globigerina bulloides has not been calibrated for Ba before and the experiments revealed that DBa in this species is the same as DBa in O. universa. In line with.

(4) the similar Ba/Ca uptake ratio of symbiont-bearing and symbiont-barren species, varying light levels do not affect the Ba incorporation of O. universa. To investigate the effect of undersaturated seawater on foraminiferal shell chemistry, well preserved shells of the planktonic foraminifera species Globigerinoides sacculifer and Neogloboquadrina pachyderma (sinistral coiling) were partially dissolved under controlled conditions in the laboratory (Publication III). In addition to known dissolution effects on Mg/Ca, δ18O and δ13C, significant effects on Sr/Ca and δ11B could be determined which are in the same order of magnitude as observed for glacial/interglacial changes. Using previous hypotheses to explain and discuss dissolution patterns, it becomes evident that the overall process is not yet fully understood. While δ18O, δ13C, Mg/Ca and maybe Sr/Ca can be explained by preferential dissolution of ontogenetic calcite and a shift of the bulk shell chemistry to calcite secreted at greater depth (gametogenic and/or crust calcite), δ11B and δ44Ca seem to be inconsistent with such an explanation and the dissolution patterns of these elements need to be dominated by other processes. Reduced calcite stability due to higher Mg/Ca was found to be insignificant to control the overall dissolution behavior in foraminiferal shells and increasing Sr/Ca ratios demonstrate that crystal impurities are not necessarily more prone to dissolution. The microstructural breakdown of shell surfaces, i.e. the formation of fissures and crevices, indicates an increase in porosity upon shell corrosion rather than the removal of outer calcite layers. The resulting increase in surface area leads to the exposition of otherwise protected lattice areas and possibly allows certain elements to be leached out. In summary, dissolution effects appear to be species-specific and depend on the physico-chemical gradients encountered by vertically migrating foraminifera at different locations. The dissolution-driven weight loss of planktonic foraminifera shells in a defined narrow size range has been proposed to reflect bottom water carbonate ion concentration ([CO32-]). A number of recent studies used this relationship but disregarded a number of complications that may limit the reconstructions. Publication IV combines experimental results on cultured foraminifera and theoretical arguments to evaluate these complications: (1) The value chosen for the pressure impact on the [CO32-] at calcite saturation is overestimated and should be 16 µmol kg-1 km-1 instead of 20 µmol kg-1 km-1. (2) The offset in [CO32-] between bottom and pore water depends on the amount of organic matter that is being degraded within the sediment and consequently the assumed constancy of the offset over time and space is highly unlikely. (3) The initial weight of undissolved shells also changes between sites and over geological timescales. Growth conditions such as [CO32-], light and temperature affect.

(5) respiration, symbiont photosynthesis and calcification processes and cause a significant variability in initial shell weight. (4) As the dissolution susceptibility of foraminifera shells varies between species, a single weight loss slope cannot be used for different species. Correcting the published estimate of glacial bottom water [CO32-] for the various effects and uncertainties discussed in Publication IV considerably reduces the published estimate of the Atlantic glacial upper deep water [CO32-]-increase from +14 mol kg-1 to only +4 µmol kg-1..

(6) Zusammenfassung. Die Rekonstruktion vergangener Klima- und Umweltbedingungen anhand von marinen Sedimenten wird maßgeblich durch die Verläßlichkeit der Proxyindikatoren bestimmt, die man aus Schalen planktischer und benthischer Foraminiferen gewinnen kann. Das Ziel dieser Arbeit ist deshalb die Verbesserung des Verständnisses und Vertrauens in Proxies aus planktischen Foraminiferen als Indikatoren mariner Karbonatchemie und der Stabilität von Proxies in angelösten Schalen. Um mögliche Einflüsse zu untersuchen, wurden Laborexperimente mit lebenden Foraminiferen und leeren Schalen aus Sedimenten durchgeführt. In Kulturexperimenten mit der lebenden planktischen Foraminifere Orbulina universa wurde der Einfluß der Symbiontenaktivität auf die Borisotopenzusammensetzung der Kalkschale untersucht (Publikation I). Im Vergleich zu Individuen, die unter hohem Lichtangebot. gewachsen. sind. (hohe. Photosyntheseraten),. ist. die. Borisotopenzusammensetzung von Schalen, die unter geringem Lichtangebot (geringe Photosyntheseraten) gebildet wurden, etwa 1.5‰ leichter. Da die Borisotopie ein Anzeiger für den marinen pH-Wert ist, entspricht der geringere δ11B-Wert einer Verminderung des pHWerts um etwa 0,2 Einheiten. Die Borisotopenzusammensetzung der Symbionten-freien Foraminifere Globigerina bulloides ist deutlich leichter als die der Symbionten-tragenden O. universa und unterstützt damit die Hypothese, daß Respiration und Photosynthese die physiologischen Schlüsselparameter für artspezifische Vitaleffekte darstellen. Obwohl dieses Experiment nahelegen könnte, daß Symbionten-freie Foraminiferen die Meerwasserchemie genauer aufzeichnen als Symbionten-tragende Arten, deuten Modellergebnisse daraufhin, daß die durch Photosynthese und Respiration bedingten Unterschiede über weite pH-Bereiche konstant sind und die Zuverlässigkeit von δ11B aus Schalen Symbionten-tragender Foraminiferen als paläo-pH Indikator nicht beeinträchtigen. Das Ba/Ca Verhältnis in Foraminiferenschalen spiegelt die Ba-Konzentration des Meerwassers wider. Da die Ba-Konzentration des Meerwassers mit der Alkalität korreliert, findet das Ba/Ca Verhältnis in Foraminiferenschalen Anwendung als Proxy für die Alkalität. Um zu untersuchen, ob die Alkalität auch einen direkten Einfluß auf den Einbau von Ba2+ in Foraminiferenkalk haben und damit die Zuverlässigkeit dieses Proxies beeinträchtigen könnte, wurden die Symbionten-tragende Foraminifere O. universa und die Symbionten-freie G. bulloides bei konstanter Ba Konzentration unter fünf unterschiedlichen Alkalitäten gehältert. (Publikation II). Unter hohem Lichtangebot konnte dabei eine schwach negative.

(7) Korrelation zwischen Ba-Aufnahme in Schalen von O. universa und der Alkalität festgestellt werden. Eine Zunahme der Alkalität in Höhe von 100 µmol kg-1 führt zu einer Abnahme von 0,004 im Verteilungskoeffizienten DBa (=Ba/CaForaminifere / Ba/CaMeerwasser). Dieser Unterschied liegt innerhalb der natürlichen Varianz mit der planktische Foraminiferen Ba2+ einbauen und ist damit unbedeutend für Paläorekonstruktionen. Über mögliche Vitaleffekte beim Einbau von Ba/Ca in Schalen von Globigerina bulloides gab es vor dieser Studie keine Daten und die Experimente konnten nun zeigen, daß diese Art Ba2+ im gleichen Verhältnis zur Meerwasserkonzentration einbaut wie O. universa. In Übereinstimmung mit dem gleichen Ba/Ca Aufnahmeverhältnis von Symbionten-tragenden und Symbionten-freien Arten zeigt variierendes Lichtangebot keinen Einfluß auf den Ba2+-Einbau in O. universa. Um den Einfluß von untersättigtem Meerwasser auf die Schalenchemie von Foraminiferen zu untersuchen, wurden sehr gut erhaltene Schalen der planktischen Foraminiferen Globigerinoides sacculifer und Neogloboquadrina pachyderma (linksdrehender Morphotyp) unter kontrollierten Bedingungen im Labor angelöst (Publikation III). Zusätzlich zu den bereits bekannten Lösungseffekten auf Mg/Ca, δ18O und δ13C konnten signifikante Effekte auf Sr/Ca und δ11B beobachtet werden, die in der Größenordnung von Änderungen zwischen Warm- und Kaltzeiten liegen. Die Diskussion der gefundenen Lösungsmuster anhand von früheren Erklärungshypothesen zeigt deutlich, daß man den zugrundeliegenden Prozeß noch nicht vollständig versteht. δ18O, δ13C, Mg/Ca und eventuell auch Sr/Ca können durch die bevorzugte Lösung von ontogenetischem Kalzit hinreichend gut erklärt werden. Dabei wird das Gesamtsignal zur Chemie der äußeren Schale verschoben, die in größeren Wassertiefen gebildet wird (gametogenetischer Kalzit und/oder Kruste). δ11B und δ44Ca hingegen können nicht durch denselben Prozeß erklärt werden und die Lösungsmuster dieser Elemente müssen deshalb durch andere Faktoren dominiert werden. Verstärkte Lösungsanfälligkeit aufgrund von höheren Mg/Ca-Verhältnissen reicht nicht aus, um das gesamte Lösungsverhalten von Foraminiferenschalen zu kontrollieren und zunehmende Sr/Ca Verhältnisse zeigen, daß Verunreinigungen des Kristallgitters nicht notwendigerweise stärker lösungsanfällig sind. Die Veränderung der Mikrostruktur einer Schalenoberfläche, d.h. die Entstehung von Fissuren und kleinen Spalten, weist daraufhin, daß Lösung die Porosität erhöht und die Schale nicht einfach schichtweise von außen nach innen abträgt. Das Resultat ist eine Zunahme der Schalenoberfläche und damit die Exposition von Kristallgitterbereichen, die in ungelösten Schalen vom aggressiven Meerwasser abgeschirmt sind. Damit besteht die Möglichkeit,. daß. bestimmte. Elemente. bevorzugt. herausgelöst. werden. können.. Zusammenfassend kann man sagen, daß Lösungseffekte artspezifisch sind und von den.

(8) physikochemischen Gradienten abhängen, die vertikal migrierende Foraminiferen an unterschiedlichen Orten erfahren haben. Der lösungsbedingte Gewichtsverlust planktischer Foraminiferenschalen einer definierten Größenklasse korreliert mit der Karbonationenkonzentration ([CO32-]) des Wassers am Meeresboden. Einige Studien haben diese Beziehung kürzlich angewendet, dabei aber eine Reihe von Komplikationen nicht berücksichtigt, die die Aussagekraft der Rekonstruktionen begrenzen. Publikation IV nutzt experimentelle Ergebnisse und theoretische Argumente, um diese Komplikationen zu bestimmen und zu quantifizieren: (1) Der Wert für die Druckabhängigkeit der [CO32-] bei Kalzitsättigung wurde zu groß gewählt und sollte 16 µmol kg-1km-1 anstatt 20 µmol kg-1km-1 betragen. (2) Der Abbau organischen Materials im Sediment verändert die Karbonationenkonzentration und bestimmt daher den [CO32-]Unterschied zwischen Boden- und Porenwasser. Räumlich und zeitlich gesehen ist es daher höchst unwahrscheinlich, daß dieser Unterschied konstant ist. (3) Das Gewicht ungelöster Schalen variiert zwischen verschiedenen Orten und über geologische Zeitskalen. Wachstumsbedingungen. wie. [CO32-],. Lichtangebot. und. Temperatur. beeinflussen. physiologische Prozesse wie Respiration, Photosynthese der Symbionten und Kalzifizierung und bedingen maßgebliche Unterschiede im Schalengewicht. (4) Da die Lösungsanfälligkeit von Schalen unterschiedlicher Foraminiferenarten variiert, kann eine einzelne Beziehung nicht universell für alle Arten angewendet werden. Korrigiert man Abschätzungen der [CO32]-Zunahme des glazialen Bodenwassers für die in Publikation IV diskutierten verschiedenen Effekte, so reduziert sich die Zunahme für das obere Tiefenwasser des glazialen Atlantiks von +14 mol kg-1 auf nur +4 µmol kg-1..

(9) Danksagung Das Gelingen dieser Arbeit verdanke ich der wissenschaftlichen Betreuung durch und Freundschaft mit Jelle Bijma, seinen Ideen, seinem Optimismus, Enthusiasmus und Überlebenswillen in Bor-schweren Zeiten. Dieter Wolf-Gladrow danke ich für die Begutachtung und Betreuung der Arbeit, sowie für die Schaffung einer unvergleichlichen Arbeitsatmosphäre. Ebenso möchte ich mich bei Ralph Schneider für die Begutachtung bedanken. I am especially grateful to Howie Spero, Ann Russell, David Lea, Dirk Nürnberg, GeertJan Brummer, Neven Loncaric, Abhijit Sanyal, Gary Hemming, Douglas Adams, Uli Groß, Michel Stoll and Frank Peeters. They have all contributed their time and considerable expertise to my work. Nikolaus Gussone, Toni Eisenhauer, Silke Vetter, Folkmar Hauff und Anette Deyhle haben endlose Geduld mit immer wiederkehrenden Problemen am TIMS bewiesen und massgeblich zu den erfolgreichen Messungen beigetragen. Similarly, this work would not have been possible without the advice and/or laboratory help and work of Pam Martin, Georges Paradis, Dotti Pak, Dave Winter and Sylvia Duncan. I would like to thank Laurie Juranek, Megan Thomas and Heidi Iverson for their field help and a great summer on Catalina Island. André Wischmeyer möchte ich für gute Nachbarschaft, Versorgung mit Kuchen, Schokolade und Musik danken. Ohne seine aufopfernde Unterstützung bei mathematischen und Computerfragen würde ich jetzt noch an den Problemen verzweifeln und hätte das Rennen nie gewonnen. Richard Zeebe, Gert-Jan Reichart, Heiko Jansen und Christoph Völker danke ich für hilfreiche Diskussionen über Isotope und Spurenelemente, Lösungskinetik und Statistik. Anja Terbrüggen gebührt besonderer Dank für die Organisation des C-Labors und seelische Aufbauarbeit bei der schlimmsten aller Laborkrankheiten: Coulometerfrust. Ebenso danke ich Frau Schwarz und Friedel Hinz für gute Zusammenarbeit. Jan Helmke und Jürgen Pätzold danke ich für die Bereitstellung von Foraminiferen und Sedimentmaterial. Das Überleben der Doktorarbeit bedeutet nicht nur wissenschaftliche Zusammenarbeit und Diskussion, sondern auch freundschaftliches Miteinander in der Arbeitsgruppe. Neben vorher genannten Gruppenmitgliedern möchte ich mich hier insbesondere bei Björn Rost, Albert Benthien, Ingrid Zondervan, Uta Schneider, Gerald Langer, Peter Köhler, Kai Schulz, Frank Gervais, Irini Mataliotaki, Ulf Riebesell, Uta Passow, Claudia Sprengel, Markus Geisen, Anja Engel, Markus Schartau, Ignacio Tebas, Silke Thoms und Christel Heemann bedanken. Zuletzt, aber in mancherlei Hinsicht mehr als allen anderen, danke ich Hubertus Fischer, Uli Holzwarth und all denjenigen, die mich mit ihrer Freundschaft und ihrem Interesse in den vergangenen Jahren unterstützt haben..

(10) Structure This thesis is subdivided into 5 parts. Part 1 refers to the main context of this study. Part 2 presents 4 manuscripts dealing with the main topic submitted or in preparation to be submitted to reviewed scientific journals. To reduce repetitions, the references were excluded from the manuscripts and combined in a separate chapter. Part 3 contains the conclusions of this thesis and provides implications for future research. The appendix (part 4) presents the report of working group 3 of the ESF Explanatory Workshop on "The ocean carbon cycle and climate change", Delmenhorst, September 1-4, 2001, which deals with currently available carbonate proxies and their major limitations..

(11) Table of contents 1. INTRODUCTION AND MOTIVATION. 1. 1.1 The oceanic carbon cycle 1.1.1 The marine carbonate system 1.1.2 Carbonate chemistry in the light of biological activity 1.1.3 Glacial to interglacial changes in CO2 and future scenarios 1.2 The use of proxies in paleoceanography 1.3 Proxies and their limitations. 1 1 3 5 6 11. 2. PUBLICATIONS. 15. 2.1 Focus and outline of this study 15 2.2 Publication I: The influence of symbiont photosynthesis on the boron isotopic composition of foraminiferal shells 18 2.3 Publication II: Assessing the reliability of Ba/Ca as a tracer for alkalinity 31 2.4 Publication III: Post-depositional effects on trace metals and stable isotopes in foraminiferal calcite – Evidence from dissolution experiments 42 2.5 Publication IV: Comment to Broecker and Clark "Carbonate ion concentration in glacial-age deep waters of the Caribbean Sea" 71 80. 3. SUMMARY AND OUTLOOK. 3.1 Effects of symbiont photosynthesis and respiration on the stable boron isotopic composition of foraminiferal shells 3.2 The effect of alkalinity on planktonic foraminiferal Ba/Ca 3.3 Changes in planktonic foraminiferal shell chemistry after incubation in undersaturated seawater 3.4 Foraminifera collected from sediment cores - identifying their preservation state 3.5 Perspectives for future research. 82 83 84. 4. APPENDIX. 87. 80 81. Reconstructing and modeling past ocean carbonate chemistry – Working Group 3 report of the ESF Explanatory Workshop on "The ocean carbon cycle and climate change", Delmenhorst, September 1-4, 2001 5. REFERENCES. 106.

(12) List of figures page 1 Typical vertical seawater profiles of carbonate parameters....................................................4 2 Changes in surface ocean carbonate chemistry in response to increasing atmospheric CO2.........................................................................................................................................6 3 Four species of planktonic foraminifera................................................................................8 4 The oxygen isotopic composition of marine calcites as a function of temperature and seawater δ18O..........................................................................................................................9 5 Boron speciation and isotope partitioning between B(OH)4- and B(OH)3 as a function of seawater pH..........................................................................................................................10 6 Reconstructing past ocean alkalinity from foraminiferal Ba/Ca..........................................10 7 Comparison of the boron isotopic composition in shells of O. universa cultured under HL and LL............................................................................................................................24 8 Comparison of the boron isotopic composition of the symbiont-bearing foraminifera O. universa and the symbiont-barren G. bulloides taken from plankton tows and inorganic carbonates.............................................................................................................................26 9 DBa in the subtropical, spinose, symbiont-bearing foraminifera Orbulina universa vs. alkalinity...............................................................................................................................37 10 DBa in the subpolar, spinose planktonic foraminifera Globigerina bulloides compared to alkalinity......................................... ................................................................................37 11 Comparison of Sr/Ca and Ba/Ca versus seawater pH in O. universa................................40 12 Microstructural breakdown of G. sacculifer shell surfaces monitored by scanning electron microscopy............................................................................................................50 13 The effect of partial dissolution on Mg/Ca in G. sacculifer and N. pachyderma (sin.).......51 14 Averages of Mg/Ca in inner and outer calcite of 10 G. sacculifer shells as determined by microprobe analysis of wall profiles...............................................................................53 15 The effect of partial dissolution on Sr/Ca in G. sacculifer and N. pachyderma (sin.).........54 16 The effect of partial dissolution on δ18O in G. sacculifer and N. pachyderma (sin.)...........55 17 The effect of partial dissolution on δ13C in G. sacculifer and N. pachyderma (sin.)...........55 18 The effect of partial dissolution on δ44Ca in G. sacculifer and N. pachyderma (sin.).........56.

(13) 19 The effect of partial dissolution on δ11B in G. sacculifer.....................................................56 20 Schematic presentation of the life cycle of G. sacculifer: vertical migration and varying calcification depths.................................................................................................59. 21 Comparison of different dissolution rates of ontogenetic and gametogenic calcite and the respective effect on changes of a heterogeneously distributed element in foraminiferal calcite.............................................................................................................63 22 The effect of [CO3=] on planktonic foraminiferal shell weight............................................73 23 Foraminiferal shell weights versus pressure corrected [CO3=]...........................................77 A1 Present state of the δ11B proxy calibration..........................................................................91 A2 Increased foraminiferal shell weight under higher [CO32-] during shell growth .................96 A3 Calcium carbonate content and calcite saturation in a modern sediment profile ...............99. List of tables page 1 Dissolution effects on foraminiferal shell chemistry as observed in sediment studies and laboratory dissolution experiments....................................................................13 2 Boron isotopic composition of cultured O. universa and modified seawater.......................22 3 Boron isotopic composition of plankton tow O. universa and G. bulloides........................ 22 4 Experimental Ba/Ca data for cultured shells.........................................................................36 5 Average weights of undissolved and dissolved foraminifera shells, dissolution estimates and calcite saturation of experimental seawater....................................................................48 6 Dissolution experimental data: Minor and trace elements....................................................52 7 Numerical experiment on the dissolution susceptibility of foraminiferal Mg-calcite..........62 A1 Reconstructing past ocean carbonate chemistry: proxies, limitations and estimates..........90.

(14) A2 In situ investigation of sedimentary carbonate dissolution................................................95.

(15) General introduction. 1. 1. Introduction and motivation Knowledge of the origin and amplitude of natural fluctuations in past climate systems can be used to assess the stability of modern terrestrial and marine subsystems and their potential range of variations in the future. Changes in the cycling of organic and inorganic carbon in the ocean have been proposed (see for an overview Falkowski et al., 2000; Raven and Falkowski, 1999; Sigman and Boyle, 2000) as mechanisms leading to the glacialinterglacial changes in atmospheric carbon dioxide measured in ice-cores (Fischer et al., 1999; Petit et al., 1999). In spite of the ocean's acknowledged importance in controlling atmospheric carbon dioxide concentrations on glacial-interglacial timescales, the roles of chemical and physical processes governing carbon transfers between the ocean and atmosphere are still poorly understood. The chemical reactions determining the exchange of CO2 between atmosphere and ocean are very complex and before we can go into the theory of paleoceanographic reconstructions, the exchange reactions between ocean, atmosphere and marine biosphere shall be introduced briefly.. 1.1 The oceanic carbon cycle 1.1.1 The marine carbonate system The marine carbonate system encompasses the different dissolved inorganic carbon species (CO2, H2CO3, HCO3- and CO32-), H+- and OH-- ions. These species are interrelated by chemical reactions which determine their relative abundances in seawater. Following Henry's law, gaseous CO2 dissolves into surface water directly proportional to the atmospheric partial CO2 pressure (pCO2): [CO2] aquatic = K0 (T, S) * pCO2,. (1). where K0 is the solubility coefficient of CO2 in seawater at a given temperature (T) and salinity (S). The dissolved CO2 hydrates immediately with water to carbonic acid (H2CO3), which itself dissociates to bicarbonate (HCO3-), carbonate (CO32-) and H+-ions: CO2 (aq.) + H2O ⇔ H2CO3 ⇔ HCO3- + H+ ⇔ CO32- + 2 H+. (2). CO2 is therefore not only dissolved physically but dissociates to ionic species which do not contribute to the aquatic partial pressure of CO2 (PCO2). This is the reason why.

(16) General introduction. 2. significantly more CO2 dissolves in seawater than any other inert gas such as nitrogen or oxygen. Because the concentration of H2CO3 is very small, it is usually combined with CO2 (aq.) to [CO2]. For the description of the carbonate system in seawater, stoichiometric equilibrium constants, K1 and K2, are used which are related to the ion concentrations and depend on temperature, pressure (P) and salinity:. [H ][HCO ] K (T,S, P) = +. 1. [CO2 ]. 3. −. [H ][CO ] K (T,S, P) = [HCO ] +. 2. (3). 2−. 3. 3 −. (4). Decreasing T and S and increasing P result in a shift of the relative ion concentrations to the left-hand side of equation (2), i.e. especially [CO32-] will decrease and [CO2] (aq.) increase in colder, deeper and less saline waters. The sum of the dissolved inorganic carbon species is abbreviated as ΣCO2 or DIC and defined as follows: DIC = [CO2] + [HCO3-] + [CO32-]. (5). In seawater, about 90% of the DIC is present as bicarbonate, approximately 9% as carbonate and about 1% as dissolved CO2. Another essential quantity for the description of the carbonate system is alkalinity, which is closely related to the electrical charge balance in the ocean. The concept of alkalinity is anything but trivial and has been regarded and defined in many different ways (Dickson, 1981). In general, alkalinity depends on a small charge excess of conservative cations ([Na+] + 2[Mg2+] + 2[Ca2+] + [K+]) over anions ([Cl-] + 2[SO42-]) which is mainly compensated for by the anions of carbonic and boric acid ([HCO3-] + 2[CO32-] + [B(OH)4-]). As a very good practical approximation, total alkalinity (TA) can also be described as the sum of the charges of the major weak acids in seawater plus the charge of OH- and minus the charge of H+. TA ≈ [HCO3-] + 2 [CO32-] + [B(OH)4-] + [OH-] - [H+] ± minor constituents. (6). Analytically, total alkalinity is regarded in terms of buffer capacity, i.e. the ability to neutralize strong acids. This property is used to quantitatively determine alkalinity by titration with HCl..

(17) General introduction. 3. Next to alkalinity, only DIC, pH and PCO2 can be determined analytically (for details see DOE, 1994). As none of the carbonate system parameters varies independent from the others, the interrelated dependency enables the oceanographer to calculate the entire carbonate system (i.e. alkalinity, DIC, pH, PCO2, [HCO3-] and [CO32-]) with the knowledge of no more than two of the constituents. This was a very brief summary of the carbonate equilibria in the ocean. A detailed description can be found in Zeebe and Wolf-Gladrow (2001). In the next section, we will see how biological activity interacts with the thermodynamic equilibria just described.. 1.1.2 Carbonate chemistry in the light of biological activity. One of the critical processes controlling the ocean-atmosphere CO2 exchange is primary production in the surface ocean, and regeneration and cycling of biogenic materials in the sea (e.g. Longhurst, 1991). Oceanic primary production takes place in the euphotic zone, i.e. the upper layer of the ocean where sufficient light is available for photosynthesis. The export of biogenic material from the surface to the deep ocean is called the biological carbon pump, as it transfers inorganic carbon assimilated in the surface waters against the gradient to the deep sea. Two biological carbon pumps can be distinguished, the organic carbon and the inorganic calcium carbonate pump. The two pumps have opposite effects on the CO2 partitioning between ocean and atmosphere. While photosynthetically active organisms sequester CO2 for the purpose of biomass production, the secretion of calcitic and aragonitic skeletons by foraminifera, corals, pteropods and coccolithophores primarily increases surface PCO2 (e.g. Frankignoulle and Canon, 1994; Wollast, 1994): photosynthetic carbon fixation: 6 CO2 + 12 H2O → C6H12O6 + 6 O2 + 6 H2O carbonate precipitation:. Ca2+ + 2 HCO3- → CaCO3 + CO2 + H2O. (7) (8). Respiration processes in the deep ocean invert reaction (7) and release CO2 which lowers the pH in the deep ocean and leads, in addition to the effects of higher pressure and lower temperature, to the dissolution of calcium carbonates (reverse of reaction 8). An example of the effects of photosynthesis, calcification, respiration and CaCO3 dissolution on the distribution of the main dissolved constituents in seawater is displayed in Figure 1..

(18) General introduction. 4. Figure 1: Typical vertical seawater profiles of carbonate parameters. The hydrographic station was sampled on RV PELAGIA cruise 157P in April 2000 and is located at 12°30' S and 53°68' N in the Southern Indian Ocean (Hönisch et al., unpublished data). Measured variables are DIC and alkalinity; other parameters were calculated using the CO2SYS program provided by Lewis and Wallace (1998). K1 and K2 were used as determined by Roy et al. (1993) and KSO4 as determined by Dickson (1990). In the surface layer, PCO2 (a), DIC and, to a minor extent, alkalinity (b) are reduced due to the activities of the biological community (carbon fixation and calcification processes). Carbon dioxide sequestration by photosynthesizing organisms thus leads to higher pH and [CO32-]. Organic matter degradation predominates below the euphotic zone, where it increases PCO2 and DIC. At 1500 m depth the aragonite saturation falls to undersaturated values (shaded area in c), i.e. aragonite starts dissolving at this depth. As a consequence, pH increases and PCO2 is lowered. The correlation between lower temperature, higher pressure and reduced carbonate saturation is also indicated in c. At this locality calcite saturation does not fall below values where calcite would dissolve within the water column.. Since calcifying organisms are often associated with symbiotic algae (foraminifera, corals) or are themselves autotrophic (coccolithophores, coralline algae), the net effects of photosynthesis and calcification may balance each other to a certain degree (e.g. Crawford and Purdie, 1997; Gattuso et al., 1995; McConnaughey and Whelan, 1997; Spero and Parker, 1985). These symbiotic associations do not only affect the estimates of CO2 sinks and sources.

(19) General introduction. 5. for the ocean-atmosphere-interaction, but, as we will see later on, may also affect the reliability of chemical recorders of past ocean conditions. 1.1.3 Glacial to interglacial changes in atmospheric CO2 and future scenarios. Observations from glacier icecores have shown that cyclic changes in atmospheric CO2 levels occurred over the last 420,000 years with glacial periods displaying about 80 ppmv lower values compared to interglacials (~ 280 ppmv) (Fischer et al., 1999; Petit et al., 1999). Isotope paleothermometry on the Vostok ice core revealed significant covariation between air temperature and pCO2 of the past glacial cycles (Cuffey and Vimeux, 2001), suggesting that CO2 may be an important forcing factor for climate. In contrast, Fischer et al. (1999) observed that the pCO2 increase lags the warming of the last three deglaciations by 600 ± 400 years, rather arguing for an important feedback mechanism than a real climate forcing function. However, the cyclicity between glacial and interglacial pCO2 cannot be simply explained by higher oceanic CO2 solubility due to lower temperatures because the concomitant sealevel decrease and salinity increase (e.g. Fairbanks, 1989) largely compensate the pCO2 decrease due to cooling. Although many approaches have been made to determine the major processes that control the state of the glacial ocean (e.g. Archer and Maier-Reimer, 1994; Boyle, 1988b; Broecker, 1997; Broecker and Clark, 2001b; Martin, 1990), contradictions between theories and observations could not yet be excluded so that the interactions between glacial-interglacial shifts in atmospheric CO2 and oceanic carbon sequestration remain elusive (e.g. Anderson and Archer, 2002; Elderfield, 2002; Maher and Dennis, 2001). Understanding the origin of natural fluctuations in the past is crucial for predictions of future variations (e.g. Stott and Kettleborough, 2002). Crowley (2000) estimated that only about 25% of the 20th-century temperature increase can be attributed to natural variability. Instead, most of the 20th-century warming is consistent with that predicted from green house gas increases. Greenhouse gases absorb longwave (infra-red) radiation emitted from the earth surface and thereby prevent the loss of solar energy to space. Concomitantly the global heat budget increases. Atmospheric CO2 is one of those greenhouse gases. Since the industrial revolution in the 19th-century, the atmospheric CO2 concentration has increased by >30% from the average interglacial value of ~280 ppmv to 368 ppmv in 2000. The predictions for the future exceed 900 ppmv by the year 2100 (Cox et al., 2000) if we do not manage to reduce the current magnitude of CO2 emissions. The rise in atmospheric CO2 leads to changes in the ocean carbonate chemistry (Figure 2) which could have strong impacts on the marine biota.

(20) General introduction. 6. (e.g. Gattuso et al., 1998; Riebesell et al., 2000; Wolf-Gladrow et al., 1999b), and may change oceanic carbon uptake and cycling. The corresponding global temperature increase is estimated to be as high as 1.4 to 5.8 K (Cox et al., 2000; IPCC, 2001). Thermal expansion and loss of mass from glaciers and ice caps will lead to a global mean sealevel rise of 9 to 88 cm. 8.25. 8.15. 35 30. pH1800 = 8.2. pH. [CO 32-] 8.05. 25 20 15. 7.95 10. [CO 2] 7.85 1800. pH2100 = 7.9 1850. 1900. 1950. 2000. 2050. 5 2100. 2-1 [CO 2 ] and [CO 3 ] /10 (µmol kg ). by the year 2100 (IPCC, 2001).. year Figure 2: Changes in surface ocean carbonate chemistry in response to an atmospheric CO2 increase since the beginning of the industrial revolution and future estimates according to the business as usual scenario IS92a (T = 25°C, S = 35‰). Note the different scales for [CO2] and [CO32-]. Figure modified after Zeebe and Wolf-Gladrow (2001).. From these numbers it is obvious that we need to quantify and understand the past to develop a sound understanding for potential future variability. While the composition of the paleoatmosphere can be determined quite well from ancient air bubbles trapped in polar ice caps, no such tool exists for the ocean. To determine the physico-chemical state of the ancient ocean, paleoceanographers rely on other measurable quantities, which will be introduced in the following chapter. 1.2 The use of proxies in paleoceanography. In order to deduce past ocean and climate variability, the reconstruction of physicochemical seawater conditions is the primary objective of paleoceanography. As direct measurements of past physico-chemical seawater conditions are no longer possible, secondary indicators which have a close relationship to any one environmental parameter are employed.

(21) General introduction. 7. for this task. These measurable descriptors for desired (but unobservable) variables are called "proxies" (Wefer et al., 1999). According to Lea (1999a), proxies can be divided into three classes: biotic components (i.e. morphologic or taxonomic climate responses such as stomata density, floral and faunal assemblage compositions etc.), chemical tracers and physical and mineralogical sediment properties. This study is focussed on chemical tracers which comprise the largest proxy group. They can be organized in three sub-categories: proxies of physical seawater properties, such as temperature; proxies of seawater composition, such as nutrient concentration and carbonate chemistry; and proxies of sediment particle flux, such as productivity (see also Wefer et al., 1999). Empirical relationships between proxies and their respective environmental parameters have been established in either laboratory studies or field calibrations. Many of these chemical proxy relationships are based on foraminifera, a group of unicellular organisms which secrete multi-chambered calcareous shells1. Foraminifera occur in all ocean basins and may dwell in surface waters (planktonic species) as well as on the seafloor (benthic species). Widespread as they occur, they have the potential to record oceanwide seawater properties. The morphologic and geometric features of their skeleton, i.e. the arrangement of their successive chambers, enable the micropaleontologist to identify the different species (e.g. Kemle-von Mücke and Hemleben, 1999). Each of these species favors different environmental conditions (e.g. Bijma et al., 1990; Darling et al., 1999; Rutherford et al., 1999) and the knowledge of these habitat preferences allows to focus paleoreconstructions on specific locations and timescales. This study is focussed on planktonic foraminifera (Figure 3), whose individual life spans are on the order of 2-4 weeks (Bijma et al., 1990; Spindler et al., 1979). High abundances in the world ocean in addition to a short reproductive cycle make foraminifera an important contributor of biogenic calcite to open ocean marine sediments (Bé et al., 1977) and a valuable tool for the reconstruction of past ocean conditions. Planktonic foraminifera shells are composed of extremely pure calcite, typically about 99% by weight. The remaining 1% is comprised of minor and trace elements such as Mg, Sr, Ba and U. Since trace elements and different isotopes of major and minor elements are incorporated directly from seawater during shell precipitation, shell composition reflects both seawater composition and the physical and biological conditions encountered during precipitation. We will now see how these shell constituents can help to elucidate past ocean carbonate chemistry. 1. Some benthic species do not actively secrete shells but collect sediment material to construct exoskeletons. For obvious reasons these species are not used as chemical recorders in paleoceanography..

(22) General introduction. 8. living Orbulina universa with symbiont halo. living Globigerina bulloides. Globigerinoides ruber. Globigerinoides sacculifer. Figure 3: Four species of planktonic foraminifera; a and b are living specimens, c and d are sediment derived shells photographed by scanning electron microscopy. With the exception of G. bulloides, all shown species bear symbionts when alive. Photographies: H.J. Spero, UC Davis (a, b) and H. Hüttemann, University of Tübingen (c, d).. To define the ocean carbonate system, two elements of the system have to be known in addition to temperature, salinity and pressure (depth). This can be any combination of pH, alkalinity, DIC or related ion concentrations such as [CO32-]. Using stable oxygen isotopes, the foundation of seawater temperature reconstructions was established as early as 1947 by Urey. Following Urey's suggestion, Epstein et al. (1953) used mollusks grown in known environments to establish a relationship linking the. 18. O/16O ratio of shell carbonate to the. temperature and isotopic composition of the water in which the carbonate was secreted. Since that time numerous studies have used δ18O to deduce the oxygen isotopic composition of seawater (e.g. Fairbanks, 1989) and calcification temperature (e.g. Emiliani, 1955). In addition to the development of other paleotemperature proxies, recent work has also revived interest in the use of Mg/Ca in foraminiferal calcite as a paleothermometer (Dekens et al., 2002; Nürnberg, 1995; Nürnberg et al., 1996). Foraminiferal Mg/Ca proved especially useful in separating the effects of temperature and salinity on δ18O (Elderfield and Ganssen, 2000; Lea et al., 2002; Rosenthal et al., 2000). To give an example of proxy relationships, Figure 4 presents the temperature dependence of δ18O and Mg/Ca as recorded in shells of Globigerina bulloides and in inorganic calcite..

(23) General introduction. 9 0.5. 0 7 -0.5. inorganic calcite. 6 -1. 13-chambered shell. 5. 12-chambered shell 11-chambered shell. -1.5. 4. -2. Mg/Ca (mmol/mol). δ 18 O calcite - δ 18 O water (‰ V-PDB). 8. 3. -2.5. 2 14. 16. 18. 20. 22. 24. 26. temperature (°C). Figure 4: The oxygen isotopic composition (δ18O) of marine calcites (blue regressions, here e.g. for the foraminifera G. bulloides: Bemis et al., 1998) is determined by a temperature dependent fractionation and the isotopic composition of seawater. Isotope values are given in δ-notation relative to a specific standard (here: Vienna Pee Dee Belemnite): δ18O = [(18O/16Osample)/18O/16Ostandard-1)*1000]. In addition to temperature and salinity effects, physiological processes may cause species-specific vital effects so that the δ18O of biogenic calcites may deviate from inorganically precipitated calcium carbonate (Kim and O'Neil, 1997). For instance, amputation of successive laboratory grown chambers of the subpolar foraminifera G. bulloides revealed an ontogenetic effect with larger shells being progressively enriched in 18 O relative to smaller shells and less depleted in 18O relative to seawater (Bemis et al., 1998). Red symbols indicate Mg/Ca uptake in G. bulloides (Lea et al., 1999b). Mg/Ca is predominantly controlled by temperature, which can be used to subtract the temperature effect from foraminiferal δ18O and derive the salinity signal. Error bars are standard deviations of individual analyses of multiple amputated chambers.. With regard to specific carbonate chemistry parameters, foraminiferal Ba/Ca was used to infer ocean alkalinity (Lea, 1993; Lea and Boyle, 1989), the stable boron isotopic composition in foraminifera shells has become a powerful tool in reconstructing marine pH (Figure 5, Pearson and Palmer, 1999; Sanyal et al., 1995; Spivack et al., 1993), differences between the influence of [CO32-] on the stable carbon isotopic composition of Globigerinoides sacculifer and G. ruber were found to be useful for past carbonate ion concentration estimates (Bijma et al., 1999; Spero et al., 1999), and U/Ca and S/Ca have recently been suggested to record [CO32-] as well (Erez et al., 2001; Russell, 2001; Russell et al., in prep.). Zn concentrations in benthic foraminifera appear to correlate with bottom water carbonate saturation (Marchitto Jr. et al., 2000). Because this study focuses especially on δ11B and Ba/Ca, the functioning of these two proxies is briefly explained in Figures 5 and 6. General.

(24) General introduction. 10. descriptions of carbonate chemistry proxies and a discussion of their specific limitations can also be found in the Appendix (Working Group 3 report of the ESF Explanatory Workshop on "The ocean carbon cycle and climate change", Delmenhorst, September 1-4, 2001). 70. b. a 400. B(OH) 3. ) -1. concentration (µmol kg. 60. B(OH) 4-. B(OH) 3 50. δ 11 B (‰). 300. 200. 40. seawater B(OH) 4-. 30 100. modern marine carbonates. 20. 0. 7. 7.5. 8. 8.5. 9. 9.5. 10. 10. 7. 7.5. 8. 8.5. 9. 9.5. 10. pH. pH. Figure 5. a) Typical seawater concentrations of dissolved boron species as a function of pH (T=25°C, S=35‰). At low pH, essentially all aqueous boron is in the B(OH)3 species while at high pH, essentially all aqueous boron is in the B(OH)4- species. b) Because there is an isotopic fractionation between B(OH)3 and B(OH)4-, the boron isotopic composition (δ11B) of each species is also pH dependent. The charged B(OH)4- is supposedly the only species being incorporated in marine carbonates (Hemming and Hanson, 1992), resulting in a narrow range of δ11B in various biogenic and inorganic carbonates grown at modern seawater pH (grey box). Past changes in seawater pH are reflected in foraminiferal δ11B (Pearson and Palmer, 1999; Pearson and Palmer, 2000; Sanyal et al., 1995). Figures modified after Hemming and Hanson (1992) and Zeebe and Wolf-Gladrow (2001). 5. 4. 3. 2. 1. 5. 10. 15. 20. Ba/Ca in seawater (µmol mol. 25 -1. ). Indo-Pacific deep water. 130. 110. 90. circumpolar surface water. circumpolar deep water. 70. North Atlantic deep water. 50. b. warm surface water. a 0 0. R2 = 0.96. Ba = 0.67 * Alk -1515. 150 seawater. Ba (nmol kg -1 ) normalized to S = 34.7‰. Ba/Ca in foraminifera shells (µmol mol. -1. ). Ba/Ca shell = 0.16 * Ba/Ca. 30. 30 2280. 2320. 2360. alkalinity (µmol kg. -1. 2400. 2440. 2480. ) normalized to S = 34.7‰. Figure 6. a) Planktonic foraminifera incorporate Ba/Ca proportional to the Ba/Ca ratio of seawater (empirical relationship here for O. universa, Lea and Spero, 1992). b) Barium cycling and ocean alkalinity are similarly affected by biological uptake in surface waters and regeneration in deep waters. Although the apparent oceanwide correlation is mechanistically not well understood, changes in thermohaline circulation are supposed to redistribute Ba and alkalinity similarly, thereby allowing reconstruction of past alkalinity distributions from foraminiferal Ba/Ca (e.g. Lea, 1993)..

(25) General introduction. 11. In addition to chemical proxies of the seawater carbonate system, the preservation state of carbonates can be used to estimate bottom water undersaturation for calcite and aragonite. Relating the preservation state of carbonates in the sediment to the saturation state of bottom water yields information on [CO32-]in situ. For example, the dissolution driven shellthinning of planktonic foraminifera (Broecker and Clark, 2001a; Lohmann, 1995) and the CaCO3 size fraction index (Broecker and Clark, 1999) were found to approximate bottom water [CO32-]. 1.3 Proxies and their limitations. The above mentioned proxies are but a small selection of the already available ones. A remarkable range of available proxies (see, for an overview, Wefer et al., 1999) suggests that the tools needed to do a comprehensive survey of past ocean-climate variability have already been identified. However, many proxies bear uncertainties that complicate their interpretation. There is abundant evidence that the incorporation of trace elements in foraminiferal calcite does not take place according to thermodynamic equilibrium. Foraminifera, as living organisms, actively precipitate their shells, thereby affecting both the structure and chemistry of shell calcite. Active precipitation argues for significant biological and kinetic controls of trace element substitution and isotope incorporation. As paleoceanographic reconstructions can be no better than the proxies themselves, the principle task remaining is to refine and validate these tools and ascertain which ones yield consistently reliable information. Approaches to develop, calibrate and validate proxies are based on either field observations (i.e. coretop sediments, sediment traps and plankton tows) or laboratory culture experiments. Because environmental conditions often change in unison, using field data to quantify the influence of variations in any single parameter is the more difficult approach. On the other hand, laboratory culture experiments are limited by the lack of physico-chemical gradients usually encountered by the vertically migrating foraminifera. However, the focus on selected conditions is a major advantage of laboratory cultures. Culture data therefore provide an important means by which sediment observations can be interpreted. Beyond the uncertainties involved in specific element incorporation, one of the basic assumptions in the use of proxies is that the primary signal remains unaltered after burial in the geological record. However, sediment observations revealed significant variability in the shell chemistry of planktonic foraminifera that could not be explained by oceanographic or climatologic changes in the former habitat (Table 1). For instance, Savin and Douglas (1973).

(26) General introduction. 12. and Bender et al. (1975) first demonstrated that planktonic foraminiferal Mg/Ca decreases with water depth and attributed this change to partial dissolution. Subsequently, Brown and Elderfield (1996), Rosenthal et al. (2000) and Dekens et al. (2002) have attempted to explore how this dissolution effect varies between species and within different ocean basins. Similarly, stable oxygen isotope compositions were demonstrated to increase in deeper sediment cores (Erez, 1979b; Rosenthal et al., 2000; Savin and Douglas, 1973). Ignoring postdepositional alterations, for instance with regard to Mg/Ca and δ18O, would lead to underestimates of the real habitat temperatures. Although it seems a reasonable assumption that enhanced porewater acidity causes the observed variability, sediment observations and the use of dilute acid in laboratory experiments bear uncertainties which limit data interpretation. Furthermore, many chemical proxies have not yet been regarded in the light of selective dissolution, and quantification of the amount of foraminiferal shell corrosion - using indices of size fraction (Berger et al., 1982; Broecker and Clark, 1999), weight loss estimates (Lohmann, 1995; Lohmann et al., 1999), reflectance (Helmke and Bauch, 2002) and microstructural breakdown (Bé et al., 1974) of foraminiferal shells - underlies a number of assumptions and restrictions (e.g. Publication IV of this study). All these uncertainties limit the quality of the proxy data base. Much effort has already been spent on refining available proxies. However, although remarkable progress has been made on this field, the expansion of our knowledge also raises new questions. This study aims to contribute to the understanding of the functioning and reliability of planktonic foraminiferal proxies..

(27) Species. G. sacculifer G. sacculifer G. sacculifer G. sacculifer G. sacculifer G. sacculifer G. sacculifer G. sacculifer G. sacculifer G. sacculifer G. sacculifer G. sacculifer G. ruber G. ruber G. ruber G. ruber G. ruber G. ruber N. dutertrei N. dutertrei N. dutertrei N. dutertrei G. tumida G. tumida G. tumida G. tumida G. truncatulinoides G. inflata G. hirsuta G. conglobatus G. conglobatus G. siphonifera O. universa P. obliquiloculata. C. wuellerstorffi C. wuellerstorffi. Planktonic foraminifera Russell et al. (1994) Russell et al. (1994) Brown & Elderfield (1996) Erez (1979) Hastings et al. (1998) Lorens et al. (1977) Dekens et al. (2002) Dekens et al. (2002) Dekens et al. (2002) Rosenthal et al. (2000) Rosenthal et al. (2000) Rosenthal et al. (2000) Lea et al. (2000) Erez (1979) Lorens et al. (1977) Dekens et al. (2002) Dekens et al. (2002) Dekens et al. (2002) Lorens et al. (1977) Dekens et al. (2002) Dekens et al. (2002) Dekens et al. (2002) Russell et al. (1994) Russell et al. (1994) Brown & Elderfield (1996) Lorens et al. (1977) Erez (1979) Erez (1979) Erez (1979) Erez (1979) Lorens et al. (1977) Erez (1979) Erez (1979) Erez (1979). Benthic foraminifera Russell et al. (1994) McCorkle et al. (1995). SEDIMENT CORETOP OBSERVATIONS:. Reference. Ceara Rise Ontong Java Plateau. Ceara Rise Ontong Java Plateau Ontong Java Plateau North Atlantic Equatorial Atlantic/Caribbean East Pacific Rise, central Pacific Ontong Java Plateau Ceara Rise Sierra Leone Rise Ontong Java Plateau Ceara Rise Sierra Leone Rise Ontong Java Plateau North Atlantic East Pacific Rise, central Pacific Ontong Java Plateau Ceara Rise Sierra Leone Rise East Pacific Rise, central Pacific Ontong Java Plateau Ceara Rise Sierra Leone Rise Ceara Rise Ontong Java Plateau Ontong Java Plateau East Pacific Rise, central Pacific North Atlantic North Atlantic North Atlantic North Atlantic East Pacific Rise, central Pacific North Atlantic North Atlantic North Atlantic. Core location. 0.5 0.1 -0.4. 0.9 0.6 0.7 0.7. 0.3 0.2 -0.1. 0.2 0.3 0?. 0.6. 0.2 0.3 0? 0.2 0.2 0. 0.5. δ18O ‰/km. -0.26. -0.1. δ13C ‰/km. -29. 0. 0. -8. 0. -5.3 0. 0. -4. 0. Sr/Ca %/km. -8 -14 -5 -7 -21 -20 -21 -16 -36 -16 -10 -16. 0 -14 -5 -7 -14 -12 -6 0 -12. -20 0 0. Mg/Ca %/km. -13. -44 -20. -15 0?. U/Ca %/km. -12. Ba/Ca %/km. -25. Cd/Ca %/km. 3000-4700m 1500-4500m. 3000-4700m 1600-4500m 1600-4400m 4500-4950m 2540-3645m 580-4000m 1600-4500m 2800-4600m 3100-5100m 1600-3400m 2800-4200m 2900-5100m 1600-2500m 4500-4950m 580-3800m 1600-4500m 2800-4600m 3100-5100m 1900-4700m 1600-4500m 2800-4600m 3100-5100m 3000-4700m 1600-4500m 1600-4400m 1900-4700m 4500-4950m 4500-4950m 4500-4950m 4500-4950m 580-3800m 4500-4950m 4500-4950m 4500-4950m. Water depth. Table 1: Dissolution effects on foraminiferal shell chemistry as observed in sediment studies and laboratory dissolution experiments.. 39 to -3 20 to -15. 39 to -3 20 to -15 20 to -15 -11 to -22 27 to 2 47 to -19 20 to -15 44 to -3 32 to -7 20 to -3 44 to 21 32 to -7 20 to 9 -11 to -22 47 to -21 20 to -15 44 to -3 32 to -7 20 to -45 20 to -15 44 to -3 32 to -7 39 to -3 20 to -15 20 to 15 20 to -45 -11 to -22 -11 to -22 -11 to -22 -11 to -22 20 to -45 -11 to -22 -11 to -22 -11 to -22. ∆CO32µmol kg-1.

(28) Species. C. wuellerstorffi Uvigerina spp. Uvigerina spp. Oridorsalis spp.. Benthic foraminifera Lea & Boyle (1993) Lea & Boyle (1993) Boyle (1988) Lea & Boyle (1993) Norwegian Sea Northwestern Atlantic Northwestern Atlantic Eastern equatorial Pacific. 0.2. 0.2. Bermuda Rise Caribbean Norwegian Sea -54 -25. 0. 0. Mg/Ca %. Gulf of Aqaba. 0. δ13C ‰. -7. δ18O ‰. Caribbean. Core location. -55 3.9. 11 4.4 1.6. Sr/Ca %. -42 ?. 0. U/Ca %. 0?. -13 ? -12. 0. 0 0 0. Ba/Ca %. 27. 43 ?. 0. Cd/Ca %. 3430m 3430m 3210m. 4500m 2540m 1800m. 300m. 2540m. Water depth. Changes in element/Ca ratios of coretop sediment samples are calculated as percentage change per km water depth, relative to the dissolution onset in a water depth profile. Similarly, isotope values are given in absolute change per km. Values for ∆CO32- (= [CO32-]in situ - [CO32]saturation) indicate the ∆CO32--range over the respective depth profile and were calculated using hydrographic data from nearby WOCE and GEOSECS stations. Marked changes in shell chemistry occur where ∆CO32- falls below 20 µmol kg-1 (e.g. Dekens et al., 2002). Observed dissolution trends vary intra- and interspecifically. Laboratory dissolution experiments have been carried out under atmospheric pressure using modified seawater (Hönisch et al., this study) or dilute acid (all other studies). Water depth denotes the depth from which the study material was collected. Indicated values are maximum changes between undissolved and most dissolved shells. Note that differences in dissolution state exist between laboratory studies. Ambiguous dissolution trends are denoted by question marks.. G. sacculifer G. sacculifer G. sacculifer G. truncatulinoides G. conglobatus O. universa N. pachyderma (sin.). Planktonic foraminifera Haley & Klinkhammer (2002) Bender et al. (1975) Hönisch et al., this study Lea & Boyle (1991) Lea & Boyle (1991) Haley & Klinkhammer (2002) Hönisch et al., this study. LABORATORY DISSOLUTION EXPERIMENTS:. Reference. Table 1 continued: Dissolution effects on foraminiferal shell chemistry as observed in sediment studies and laboratory dissolution experiments..

(29) Publications. 15. 2. Publications 2.1 Focus and outline of this study. This dissertation reports of research on the use of planktonic foraminiferal proxies as indicators of changes in seawater carbonate chemistry. In a first experimental series, living planktonic foraminifera were investigated with regard to the incorporation of Ba/Ca and δ11B as a function of physiological processes and seawater carbonate chemistry. A second set of experiments concentrates on the preservation of foraminiferal shells and their chemical composition after burial in the sediment. I. The influence of symbiont photosynthesis on the boron isotopic composition of foraminiferal shells. Hönisch, B., J. Bijma, A.D. Russell, H.J. Spero, M.R. Palmer, A. Eisenhauer: The influence of symbiont photosynthesis on the boron isotopic composition of foraminiferal shells, Marine Micropaleontology, submitted 2002. This part of the thesis investigates the reliability of δ11B as a proxy for paleo-pH and the influence of symbiont photosynthetic activity. As microsensor studies have shown that pH within the spine environment of planktonic foraminifera shows large variations due to respiration and photosynthesis (Rink et al., 1998), it was investigated whether the known boron isotopic fractionation between seawater and foraminiferal shells may be altered by these physiological processes. The manuscript is based on laboratory experiments with living Orbulina universa. Results of culture experiments are compared with field-grown O. universa and Globigerina bulloides collected in plankton tows. II. Assessing the reliability of Ba/Ca as a tracer for alkalinity. Hönisch, B., A.D. Russell, J. Bijma, D.W. Lea, H.J. Spero: Assessing the reliability of Ba/Ca as a tracer for alkalinity; in preparation. Culture experiments with the planktonic foraminifera Orbulina universa and Globigerina bulloides have been carried out in order to investigate whether Ba- incorporation during shell secretion is affected by seawater alkalinity. Inorganic precipitation experiments predict such a linkage via increased in precipitation rates at higher alkalinities. As Ba2+ and alkalinity vary proportionately in the ocean and the Ba/Ca ratio in foraminiferal shells is assumed to reflect the seawater Ba2+ concentration, an influence of alkalinity on the Ba incorporation could compromise the use of this proxy..

(30) Publications. 16. III. Post-depositional effects on trace metals and stable isotopes in foraminiferal calcite – Evidence from dissolution experiments. Hönisch, B., J. Bijma, N. Gussone, H.J. Spero, D. Nürnberg, D.W. Lea, A. Eisenhauer: Postdepositional effects on trace metals and stable isotopes in foraminiferal calcite – Evidence from dissolution experiments; in preparation. One of the basic assumptions in the use of proxies for paleoceanographic reconstructions is that the primary signal remains unaltered after burial in the geological archive. However, observations on sediment cores revealed significant variability in the shell chemistry of planktonic foraminifera that could not be explained by oceanographic or climatologic changes in the former habitat. Although it seems a reasonable assumption that partial shell dissolution causes the observed variability, a number of uncertainties still remain. Using extraordinarily well preserved shells of the tropical Globigerinoides sacculifer and the polar Neogloboquadrina pachyderma (sin.), dissolution experiments under simulated natural conditions have been carried out in the laboratory. Partially dissolved shells have been analyzed with respect to minor and trace element to calcium ratios (Mg/Ca, Sr/Ca, Ba/Ca, U/Ca, Cd/Ca) and stable isotopic compositions (δ18O, δ13C, δ44Ca, δ11B). The combination of controlled laboratory conditions and the investigation of numerous proxies allows a detailed discussion of the results with regard to the underlying dissolution mechanisms. IV. The impact of the ocean carbonate chemistry on living foraminiferal shell weight: A comment to Broecker and Clark’s „Carbonate ion concentration in glacial-age deep waters of the Caribbean Sea“. Bijma, J., B. Hönisch, R.E. Zeebe: The impact of the ocean carbonate chemistry on living foraminiferal shell weight: A comment to Broecker and Clark’s „Carbonate ion concentration in glacial-age deep waters of the Caribbean Sea“. Geochemistry Geophysics Geosystems; in press. Using the size normalized weight of planktonic foraminifera to determine their preservation state and estimate the carbonate ion content of oceanic deep waters may be compromised by a number of physico-chemical parameters. The assumptions made by Broecker and Clark (2002) disregard existing evidence from culture experiments which predict differences in shell wall thickness of planktonic foraminifera upon different growth conditions. In addition to estimating the impact of uncertainties in the corrosivity of bottom and pore waters, we quantify the effect of carbonate chemistry on shell growth of planktonic foraminifera using combined data of various culture experiments..

(31) Publications. 17. Erklärung über den von mir geleisteten Anteil an den Publikationen. Publikation I Die Laborexperimente wurden von J. Bijma und mir geplant und in Zusammenarbeit mit A. Russell und H. Spero durchgeführt. Ich habe die Proben gemessen, die Daten ausgewertet und das Manuskript verfaßt.. Publikation II Die Laborexperimente wurden von J. Bijma und mir geplant und in Zusammenarbeit mit D. Lea, A. Russell und H. Spero durchgeführt. Ich habe die Daten ausgewertet und das Manuskript verfaßt.. Publikation III Die Laborexperimente habe ich in Zusammenarbeit mit Jelle Bijma geplant. Ich habe die Experimente durchgeführt, ausgewertet und die Borisotope gemessen. Ich habe das Manuskript verfaßt.. Publikation IV Das Manuskript wurde in Zusammenarbeit mit J. Bijma und R. Zeebe geplant und verfaßt. Ich habe die Kulturdaten zusammengestellt und ausgewertet..

(32) Publication I. 18. Publication I. The influence of symbiont photosynthesis on the boron isotopic composition of foraminiferal shells. Bärbel Hönisch, Jelle Bijma, Ann D. Russell, Howard J. Spero, Martin R. Palmer and Anton Eisenhauer Marine Micropaleontology (submitted 2002) ........................................................................................................................................................ Abstract. Culture experiments were carried out with the planktonic foraminifer Orbulina universa under high and low light levels in order to determine the influence of symbiont photosynthetic activity on the boron isotopic composition of shell calcite. Under low light (reduced photosynthetic rates) the boron isotopic composition of the tests is 1.5‰ lower compared to shells grown under high light (elevated photosynthetic rates). In terms of inferred pH, the lower boron isotope values correspond to a reduction in pH of approximately 0.2 units. The boron isotopic composition of Orbulina universa from plankton tows is similar to shells grown under low light conditions in the laboratory. These data are consistent with reduced symbiont concentrations in recently secreted shells. In addition to laboratory and field grown O. universa, we present the first data for a symbiont-barren foraminifer, Globigerina bulloides. Data obtained for G. bulloides fall ~1.4‰ below the field grown O. universa. Although the plankton tow results are preliminary, they support the hypothesis that respiration and photosynthesis are the key physiological parameters responsible for speciesspecific vital effects. Model results have predicted that photosynthesis- and respiration-driven offsets as presented here are constant over a wide pH range and thus do not reduce the reliability of δ11B as a paleo-pH indicator..

(33) Publication I. 19. Introduction. Data from experiments with living foraminifera have confirmed the hypothesis that seawater pH is the dominant environmental control on the. 11. B/10B content (δ11B) of. planktonic foraminifera shells (Hemming and Hanson, 1992; Sanyal et al., 2001; Sanyal et al., 1996; Sanyal et al., 2000; Spivack et al., 1993). Although measurements of foraminiferal δ11B are not yet a routine tool in paleoceanography, several studies have published paleo-pH reconstructions across different geological timescales with encouraging results (Palmer et al., 1998; Pearson and Palmer, 2000; Sanyal and Bijma, 1999; Sanyal et al., 1997; Sanyal et al., 1995; Spivack et al., 1993). Whereas pH is the primary environmental control on shell δ11B, several physiological processes can modify the pH of the calcifying microenvironment, potentially complicating straightforward interpretation of δ11B data. For instance, microelectrode studies have revealed that pH in the calcifying microenvironment of symbiont-bearing foraminifera varies with light levels (Jørgensen et al., 1985; Rink et al., 1998). Although symbionts remove CO2 during photosynthesis, thereby increasing pH in the foraminiferal microenvironment, respiration releases CO2 and decreases pH. Results from diffusion-reaction model simulations support these microsensor studies (Wolf-Gladrow et al., 1999a), showing that respiration and symbiont-photosynthesis, along with diffusion and chemical reactions, control the availability of CO32- and HCO3- for the calcification process. The carbonate ion effect on shell δ13C and δ18O of planktonic foraminifera (Bijma et al., 1998; Spero et al., 1997) can also be partly explained by the influence of these physiological processes (Zeebe, 1999; Zeebe et al., 1999). Comparison of empirical δ11B versus pH-relationships has revealed significant offsets between inorganic and biogenic calcification as well as between foraminifera species (Sanyal et al., 2001). It was speculated that species-specificity could be due to differences in microenvironment pH and/or due to differences in the relative proportion of calcite precipitated during day and night (Sanyal et al., 2001). Similarly, Hemming et al. (1998) attributed heavier boron isotopic compositions recorded in a coral during periods of high productivity to enhanced symbiont photosynthetic activity and a therefore higher pH. This study investigates the influence of symbiont photosynthetic activity on the boron isotopic composition of O. universa grown in the laboratory. In order to estimate the effects on naturally grown foraminifera, we compare experimental data with plankton tow samples of O. universa and the symbiont-barren G. bulloides..

(34) Publication I. 20. 2. Methods. 2.1 Foraminifera collection and culturing Foraminifera were cultured using previously established methods (Lea and Spero, 1992; Mashiotta et al., 1997; Spero et al., 1997). Juvenile (presphere) O. universa were hand collected by scuba divers in July and August 2000 from surface waters of the San Pedro Basin, approximately 2 km NNE of the Wrigley Institute for Environmental Studies, Santa Catalina Island, California. Surface seawater for culturing was collected at the dive site, filtered through a 0.8 µm membrane filter and its carbonate chemistry was subsequently modified using the method of Sanyal et al. (2001). To reduce the number of shells required for isotope analysis, the boron concentration in the culture solution was increased tenfold by adding 0.27 g of boric acid (H3BO3) per L seawater. The pH was readjusted to ambient pH of 8.16 by titration with NaOH. Samples of the culture solution were taken at the beginning and end of the experiment, acidified with ultrapure HCl and archived for later determination of the isotopic value. After collection, individual foraminifera were examined under an inverted light microscope for identification of species and general condition and then transferred to 115-ml glass jars containing the experimental filtered seawater. All culture jars were maintained at a constant temperature in a 22 ± 0.3°C water bath, the approximate summer SST at the collection site. For each experiment, seventy individuals were grown in the laboratory. Foraminifera were grown under the following conditions: 1) a 12-hr high light:12-hr dark cycle where light levels were adjusted to above Pmax (315-326 µmol photons m-2 s-1), and 2) a 12-hr low light (18-20 µmol photons m-2 s-1):12-hr dark cycle. Both experiments utilized high output, cool white, fluorescent bulbs. The former light levels exceed the saturating irradiances for symbionts in O. universa, whereas the latter are lower than the light compensation point (Rink et al., 1998). During the 6-to 15-day culture period, O. universa secretes and calcifies a spherical chamber. The foraminifera were fed a 1-day old Artemia sp. nauplius (brine shrimp) every third day. Upon termination of the experiment following foraminiferal gametogenesis, the empty shells were rinsed in ultrapure water and archived for later analysis. Alkalinity was determined by Gran-titration at the start and termination of the experiment. At the same time, dissolved inorganic carbon (DIC) samples were collected, poisoned with saturated HgCl2 solution and measured coulometrically at the Alfred Wegener Institute in Bremerhaven, Germany. Seawater pH values were determined potentiometrically and are given on the NBS scale. Carbonate chemistry analyses were calibrated against.

(35) Publication I. 21. certified reference material supplied by Dr. A.G. Dickson, University of California, San Diego. Plankton tow samples were collected at the dive site in order to determine the boron isotopic composition of naturally grown O. universa and the symbiont-barren Globigerina bulloides. Nets with a mesh size of 153 µm were towed at 0-20 m depth. Selected foraminifera shells were rinsed in distilled water, dried and archived. The samples were treated in a low temperature asher to remove organic matter and to better distinguish between juvenile O. universa and G. bulloides. Approximately 300 shells of each species were collected. Most O. universa had built their spherical chambers shortly before collection. Shells were very thin and none of the collected specimens from the two species showed signs of gametogenic calcification. Total sample weight before cleaning was no more than 1 mg for O. universa and 0.6 mg for G. bulloides. 2.2 Analytical techniques With the exception of the plankton tow samples, only gametogenic individuals from the culture experiments were used for analysis. All specimens were rinsed in distilled water to remove sea salts, dried and weighed. The shells of each experiment were pooled, crushed and bleached with 4-6% sodium hypochlorite to remove organic matter and then rinsed, ultrasonicated and centrifuged repeatedly with distilled water to remove soluble salt and eventually adsorbed B. In a laminar flow bench, the cleaned carbonate was dissolved in 2N quartz distilled HCl. The dissolved sample, containing approximately 5 ng of B, was loaded on a rhenium zone refined filament, and 1 µl of boron-free seawater was added to enhance ionization and suppress fractionation (Hemming and Hanson, 1994). Samples were dried at an initial ion current of 0.8 A, followed by a 1 minute period at 1.2 A. Loaded filaments were kept under an infrared lamp until mounted into the mass spectrometer. Isotope data were collected on a Finnigan MAT 262 RPQ+ Thermal Ionization Mass Spectrometer (TIMS) at GEOMAR in Kiel, Germany. The BO2- ion method was used following previously published procedures (1997; Sanyal et al., 1996). For the culture experiments each sample was run at least 4 times. Cultured foraminifera samples were measured at a filament temperature of 915 ± 10°C. While we seldom observed time-dependent fractionation in these boron enriched samples, the small plankton tow samples started fractionating after 20-30 minutes of acquisition. We could therefore only complete two acceptable runs for O. universa and a single acquisition for G. bulloides. However, initial values of the fractionating runs were consistent with the results of acceptable analyses..

(36) Publication I. 22. Table 2. Boron isotopic composition of cultured O. universa and modified seawater. light. pH. Seawater. n. (µmol photons m-2 s-1) (culture water) δ11BMS (‰). O. universa. δ11BNC. n. δ11BC (‰). (‰). 321 ± 8. 8.16 ± 0.02. -8.9 ± 0.1. 5. -25.6 ± 0.6. 4. 22.0 ± 0.6. 19 ± 2. 8.15 ± 0.03. -9.1 ± 0.4. 6. -27.2 ± 0.3. 4. 20.5 ± 0.3. Results are based on 70 shells per sample. Errors are expressed as 2σmean for multiple sample runs. δ11B (‰) = (Rs/Rstd-1)*1000, Rs = 11B/10B of sample, Rstd = 11B/10B of NBS 951 boric acid standard. Seawater standard = 39.5 ± 0.34‰. n = number of replicate analyses. δ11BNC is the δ11BC after conversion to the natural seawater scale (δ11BNS = 39.5‰), see text and Eq. 9 for details.. Table 3. Boron isotopic composition of plankton tow O. universa and G. bulloides. species. ambient pH. δ11B. n. (‰) O. universa. 8.12 ± 0.02. 20.5 ± 0.5. 2. G. bulloides. 8.12 ± 0.02. 19.0 ± 0.9. 2*. Results are based on approximately 300 shells per sample. Errors are expressed as 2σmean for multiple sample runs. δ11B (‰) = (Rs/Rstd-1)*1000, Rs = 11B/10B of sample, Rstd = 11B/10B of NBS 951 boric acid standard. Seawater standard = 39.5±0.34‰. n = number of replicate analyses. * = runs incomplete according to criteria for acceptable runs, see text for details.. To rule out isobaric interferences on mass 42 with organic contamination (12C14N16Oions), mass 26 (12C14N-ions) was monitored during each measurement. No interferences were detected. The. 11. B/10B ratio was corrected for isotopic interferences on mass 43 (10B16O17O-. ions) by subtraction of 0.00078 from the 43/42 ratio (Spivack and Edmond, 1986). The fractionation ε between natural seawater (NS) and calcite (C) is usually calculated as: ε(NS-C) = δ11BNS-δ11BC. This equation gives a good approximation when the isotopic composition of NS and modified seawater (MS) are the same. Because the modified seawater used in the culture experiments had a significantly different isotopic composition from natural seawater, all analyses were corrected for this difference in order to allow comparisons to.