periplatform sediments
(Great Bahama Bank, Florida Straits)
Dissertation
zur Erlangung des Doktorgrades
der Naturwissenschaften
am Fachbereich Geowissenschaften
der Universität Bremen
vorgelegt von
Johanna Schwarz
16. Mai 2007
Gutachter:
Rebecca Rendle-Bühring
Hildegard Westphal
Prüfer:
Gerhard Bohrmann
John Reijmer
Abstract
The oceanic carbon cycle mainly comprises the production and dissolution/ preservation of carbonate particles in the water column or within the sediment. Carbon dioxide is one of the major controlling factors for the production and dissolution of carbonate. There is a steady exchange between the ocean and atmosphere in order to achieve an equilibrium of CO2; an anthropogenic rise of CO2 in the atmosphere would therefore also increase the amount of CO2 in the ocean. The increased amount of CO2 in the ocean, due to increasing CO2-emissions into the atmosphere since the industrial revolution, has been interpreted as “ocean acidification” (Caldeira and Wickett, 2003). Its alarming effects, such as dissolution and reduced CaCO3 formation, on reefs and other carbonate shell producing organisms form the topic of current discussions (Kolbert, 2006).
Decreasing temperatures and increasing pressure and CO2 enhance the dissolution of carbonate particles at the sediment-water interface in the deep sea. Moreover, dissolution processes are dependent of the saturation state of the surrounding water with respect to calcite or aragonite. Significantly increased dissolution has been observed below the aragonite or calcite chemical lysocline; below the aragonite compensation depth (ACD), or calcite compensation depth (CCD), all aragonite or calcite particles, respectively, are dissolved. Aragonite, which is more prone to dissolution than calcite, features a shallower lysocline and compensation depth than calcite. In the 1980´s it was suggested that significant dissolution also occurs in the water column or at the sediment-water interface above the lysocline. Unknown quantities of carbonate produced at the sea surface, would be dissolved due to this process. This would affect the calculation of the carbonate production and the entire carbonate budget of the world´s ocean. Following this assumption, a number of studies have been carried out to monitor supralysoclinal dissolution at various locations: at Ceara Rise in the western equatorial Atlantic (Martin and Sayles, 1996), in the Arabian Sea (Milliman et al., 1999), in the equatorial Indian Ocean (Peterson and Prell, 1985; Schulte and Bard, 2003), and in the equatorial Pacific (Kimoto et al., 2003). Despite the evidence for supralysoclinal dissolution in some areas of the world´s ocean, the question still exists whether dissolution occurs above the lysocline in the entire ocean. The first part of this thesis seeks answers to this question, based on the global budget model of Milliman et al. (1999). As study area the Bahamas and Florida Straits are most suitable because of the high production of carbonate, and because there the depth of the lysocline is the deepest worldwide. To monitor the occurrence of supralysoclinal dissolution, the preservation of aragonitic pteropod shells was determined, using the Limacina inflata Dissolution Index (LDX; Gerhardt and Henrich,
2001). Analyses of the grain-size distribution, the mineralogy, and the foraminifera assemblage revealed further aspects concerning the preservation state of the sediment. All samples located at the Bahamian platform are well preserved. In contrast, the samples from the Florida Straits show dissolution in 800 to 1000 m and below 1500 m water depth. Degradation of organic material and the subsequent release of CO2 probably causes supralysoclinal dissolution. A northward extension of the corrosive Antarctic Intermediate Water (AAIW) flows through the Caribbean Sea into the Gulf of Mexico and might enhance dissolution processes at around 1000 m water depth.
The second part of this study deals with the preservation of Pliocene to Holocene carbonate sediments from both the windward and leeward basins adjacent to Great Bahama Bank (Ocean Drilling Program Sites 632, 633, and 1006). Detailed census counts of the sand fraction (250-500 µm) show the general composition of the coarse grained sediment. Further methods used to examine the preservation state of carbonates include the amount of organic carbon and various dissolution indices, such as the LDX and the Fragmentation Index. Carbonate concretions (nodules) have been observed in the sand fraction. They are similar to the concretions or aggregates previously mentioned by Mullins et al. (1980a) and Droxler et al. (1988a), respectively. Nonetheless, a detailed study of such grains has not been made to date, although they form an important part of periplatform sediments. Stable isotope-measurements of the nodules´ matrix confirm previous suggestions that the nodules have formed in situ as a result of early diagenetic processes (Mullins et al., 1980a). The two cores, which are located in Exuma Sound (Sites 632 and 633), at the eastern margin of Great Bahama Bank (GBB), show an increasing amount of nodules with increasing core depth. In Pliocene sediments, the amount of nodules might rise up to 100%. In contrast, nodules only occur within glacial stages in the deeper part of the studied core interval (between 30 and 70 mbsf) at Site 1006 on the western margin of GBB. Above this level the sediment is constantly being flushed by bottom water, that might also contain corrosive AAIW, which would hinder cementation. Fine carbonate particles (<63 µm) form the matrix of the nodules and do therefore not contribute to the fine fraction. At the same time, the amount of the coarse fraction (>63 µm) increases due to the nodule formation. The formation of nodules might therefore significantly alter the grain-size distribution of the sediment. A direct comparison of the amount of nodules with the grain-size distribution shows that core intervals with high amounts of nodules are indeed coarser than the intervals with low amounts of nodules. On the other hand, an initially coarser sediment might facilitate the formation of nodules, as a high porosity and permeability enhances early diagenetic processes (Westphal et al., 1999). This
suggestion was also confirmed: the glacial intervals at Site 1006 are interpreted to have already been rather coarse prior to the formation of nodules. This assumption is based on the grain-size distribution in the upper part of the core, which is not yet affected by diagenesis, but also shows coarser sediment during the glacial stages. As expected, the coarser, glacial deposits in the lower part of the core show the highest amounts of nodules. The same effect was observed at Site 632, where turbidites cause distinct coarse layers and reveal higher amounts of nodules than non-turbiditic sequences. Site 633 shows a different pattern: both the amount of nodules and the coarseness of the sediment steadily increase with increasing core depth.
Based on these sedimentological findings, the following model has been developed: a grain-size pattern characterised by prominent coarse peaks (as observed at Sites 632 and 1006) is barely altered. The greatest coarsening effect due to the nodule formation will occur in those layers, which have initially been coarser than the adjacent sediment intervals. In this case, the overall trend of the grain-size pattern before and after formation of the nodules is similar to each other. Although the sediment is altered due to diagenetic processes, grain size could be used as a proxy for e.g. changes in the bottom-water current. The other case described in the model is based on a consistent initial grain-size distribution, as observed at Site 633. In this case, the nodule reflects the increasing diagenetic alteration with increasing core depth rather than the initial grain-size pattern. In the latter scenario, the overall grain-size trend is significantly changed which makes grain size unreliable as a proxy for any palaeoenvironmental changes.
The results of this study contribute to the understanding of general sedimentation processes in the periplatform realm: the preservation state of surface samples shows the influence of supralysoclinal dissolution due to the degradation of organic matter and due to the presence of corrosive water masses; the composition of the sand fraction shows the alteration of the carbonate sediment due to early diagenetic processes. However, open questions are how and when the alteration processes occur and how geochemical parameters, such as the rise in alkalinity or the amount of strontium, are linked to them. These geochemical parameters might reveal more information about the depth in the sediment column, where dissolution and cementation processes occur.
Kurzfassung
Der Karbonatkreislauf im Ozean besteht im Wesentlichen aus der Produktion und der Lösung bzw. Erhaltung karbonatischer Partikel in der Wassersäule und im Sediment. Ein wichtiger Steuerfaktor für die Produktion und Lösung von Karbonat ist der CO2-Gehalt im umgebenden Wasser. Zwischen Ozean und Atmosphäre findet ein steter Austausch von CO2 statt, so dass durch den anthropogenen Anstieg des Kohlendioxids in der Atmosphäre auch dessen Gehalt im Ozean gestiegen ist. Die steigenden CO2-Emissionen in die Atmosphäre seit der Industrialisierung werden aktuell in Bezug auf deren drastische Auswirkungen auf marine Organismen, z.B. Riffe, aber auch alle anderen karbonatbildenden Organismen, diskutiert (Kolbert, 2006). In dem Zusammenhang wurde der Begriff „Ozean-Versauerung“ eingeführt (Caldeira und Wickett, 2003).
Je tiefer die Temperatur, je höher der Druck und je höher der CO2-Gehalt des umgebenden Wassers, desto leichter werden Karbonatpartikel an der Wasser-Sediment-Grenze in der Tiefsee wieder gelöst. Ein weiterer wichtiger Punkt ist die Sättigung des Wassers in Bezug auf Kalzit und Aragonit. Verstärkte Lösung von Aragonit und Kalzit findet unterhalb der jeweiligen chemischen Lysokline und vollständige Lösung unterhalb der jeweiligen Kompensationstiefe statt. Die Lysokline und Kompensationstiefe von Aragonit liegen dabei in geringeren Wassertiefen als die von Kalzit. Im Nordatlantik, wo die Korrosivität niedrig ist, liegen Lysokline und Kompensationstiefe in großen Tiefen, im korrosiveren Pazifik dagegen relativ flach. In den 80er Jahren wurde die These aufgestellt, dass eine signifikante Lösung von Karbonat bereits oberhalb der Lysokline stattfindet, entweder noch in der Wassersäule oder an der Sediment-Wasser-Grenze. Unbekannte Mengen an der Meeresoberfläche produzierten Karbonats würden dadurch gelöst. Diese unbekannte Menge würde sich wiederum auf die Berechnung der Karbonatproduktion und damit des gesamten Karbonathaushaltes im Ozean auswirken. Supralysoklinale Lösung wurde daraufhin tatsächlich an vielen Stellen weltweit nachgewiesen, z.B. am Ceara Rise im äquatorialen Westatlantik (Martin und Sayles, 1996), in der Arabischen See (Milliman et al., 1999), im Indik (Peterson und Prell, 1985; Schulte und Bard, 2003) und im Äquatorial-Pazifik (Kimoto et al., 2003). Trotzdem bleibt die Frage bestehen, ob supralysoklinale Lösung ein generelles Phänomen oder auf einige wenige Gebiete begrenzt ist. Der erste Teil dieser Studie versucht darauf eine Antwort zu finden, basierend auf den Berechnungen des globalen Karbonathaushalts von Milliman et al. (1999). Die Bahamas und die Floridastraße eignen sich dafür sehr gut, da die Karbonatlysokline dort aufgrund der hohen Karbonatproduktion weltweit am tiefsten liegt. Da Aragonit leichter löslich ist als Kalzit, wurde ein
Lösungsanzeiger verwendet, der den Erhaltungsgrad von aragonitischen Pteropodenschalen bewertet: der Limacina inflata-Lösungsindex (Limacina inflata Dissolution Index; LDX), entwickelt von Gerhardt und Henrich (2001). Zusätzliche Korngrößenanalysen des Sediments erlaubten Rückschlüsse auf Fragmentationsprozesse; des Weiteren wurden die mineralogische Zusammensetzung und die Faunenvergesellschaftung der planktischen Foraminiferen untersucht. Während alle Proben von der Bahama-Plattform eine gute Karbonaterhaltung aufweisen, wurden in der Florida-Straße Lösungs-Erscheinungen in 800-1000 m und unterhalb 1500 m Wassertiefe festgestellt. Als Ursache für diese supralysoklinale Lösung kann der Abbau organischen Materials und die dadurch verursachte Freisetzung von CO2 angesehen werden. Der nördliche Ausläufer des korrosiven Antarktischen Zwischenwassers fließt durch die Karibik bis in den Golf von Mexiko und weiter in die Floridastraße und könnte somit zu den korrosiveren Bedingungen im Bodenwasser bei 1000 m Wassertiefe beitragen.
Im zweiten Teil der Arbeit wurde die Erhaltung pliozäner bis holozäner Karbonatsedimente anhand dreier Kerne des Ocean Drilling Programs (ODP) aus westlich und östlich der Großen Bahama-Bank (GBB) liegenden Becken untersucht (Sites 1006 bzw. 632 und 633). Die generelle Zusammensetzung der Sandfraktion (größer als 63 µm) wurde anhand detaillierter Zählungen der repräsentativen Fraktion 250-500 µm bestimmt. Zusätzlich wurden der Anteil an organischem Kohlenstoff gemessen und verschiedene Lösungsindizes berechnet, u.a. der LDX und die Fragmentationsrate. In der Sandfraktion wurden Karbonatkonkretionen („nodules“) gefunden, die in dieser oder ähnlicher Form bereits in mehreren Arbeiten über die Sedimente rund um die Bahama-Plattform beschrieben wurden (u.a. Mullins et al., 1980a; Droxler et al., 1988a). Obwohl die „nodules“ einen wichtigen Bestandteil von Periplattform-Sedimenten bilden, wurden sie jedoch bislang nicht detailliert untersucht. Stabile Isotopen-Messungen der „nodule“-Matrix bestätigten die bisherige Vermutung, dass solche Konkretionen durch frühdiagenetische Prozesse in situ gebildet wurden. Die beiden Kerne im Exuma Sound (Sites 632 und 633), auf der Ostseite der GBB gelegen, weisen einen mit der Kerntiefe zunehmenden Anteil an „nodules“ auf, der in den pliozänen Sedimenten bis auf 100% ansteigt. Site 1006, an der Westseite der GBB gelegen, ist dagegen nur im unteren Teil des untersuchten Kernabschnittes (in 30-70 m Kerntiefe) von der „nodule“-Bildung betroffen und auch dort nur in den Sedimenten der Glazialstadien. Oberhalb 30 m Kerntiefe wird das Sediment ständig von Bodenwasser durchspült. Dieses Bodenwasser beinhaltet möglicherweise korrosives antarktisches Zwischenwasser, was wiederum eine Zementation in dieser Zone verhindern würde. Feine Karbonatpartikel (kleiner
als 63 µm) bilden die Matrix der grobkörnigen „nodules“ und sind somit nicht mehr Bestandteil der Feinfraktion. Gleichzeitig wird der Grobfraktionsanteil (>63 µm) durch die gebildeten Karbonat-konkretionen erhöht. Der gesamte Prozess kann somit die Korngrößenverteilung eines Sediments beträchtlich verändern. Ein Vergleich der „nodule“-Häufigkeit mit der Korngrößenverteilung im Sediment ergab, dass tatsächlich die Kernintervalle mit großen Anteilen an „nodules“ deutlich gröber sind als die Intervalle mit geringen Anteilen an „nodules“. Andererseits mag ein ursprünglich gröberes Sediment die Bildung von „nodules“ fördern, da eine höhere Porosität und Permeabilität einen höheren Porenfluss zur Folge hat und damit frühdiagenetische Prozesse fördert. Auch diese Vermutung wurde bestätigt: Die glazialen Bereiche in Kern 1006 waren schon vor der „nodule“-Bildung relativ grob. Diese Annahme basiert auf dem Verteilungsmuster des oberen Kernabschnitts, der noch nicht durch Diagenese überprägt wurde, aber ebenso grobe Sedimente während der Glazialstadien aufweist. Erwartungsgemäß weisen die groben, glazialen Bereiche im unteren Kernabschnitt die höchsten Anteile an „nodules“ auf. Der gleiche Effekt wurde im Kern 632 beobachtet; allerdings sind in diesem Fall Turbidite für die ursprünglich gröberen Lagen verantwortlich. Kern 633 zeigt ein anderes Muster: Das Sediment wird mit zunehmender Tiefe stetig reicher an „nodules“ und gleichzeitig gröber.
Basierend auf den sedimentologischen Befunden wurde folgendes Modell entwickelt: Die Korngrößenverteilung eines Sediments mit einzelnen, gröberen Lagen, wie in den Kernen 632 und 1006, wird durch die Bildung von „nodules“ kaum verändert, da eine verstärkte „nodule“-Bildung gerade die Lagen vergröbert, die vorher auch schon gröber waren als das umliegende Sediment. Da sich der allgemeine Trend der Korngrößenverteilung in diesen Fällen nicht ändert, könnte man die Korngröße auch nach der frühdiagenetischen Überprägung noch als Näherungswert (Proxy) für z.B. Änderungen der Bodenwasser-strömung verwenden. Im zweiten Fall ist die ursprüngliche Korngrößenverteilung sehr gleichmäßig, wie z.B. in Kern 633, sodass das „nodule“-Verteilungsmuster weniger die Korngröße widerspiegelt als vielmehr die mit der Kerntiefe zunehmende diagenetische Überprägung. Dadurch wird der Trend der Korngrößenverteilung signifikant geändert, und Korngröße kann nicht mehr zuverlässig als Proxy für Veränderungen der Paläo-Umwelt verwendet werden.
Die Ergebnisse der Arbeit tragen wesentlich zum besseren Verständnis des Ablagerungsgeschehens im Periplattform-Bereich bei: Die Erhaltung der Oberflächenproben zeigt den Einfluss supralysoklinaler Lösung aufgrund der Degradierung organischen Materials und aufgrund korrosiver Wassermassen; die genaue Zusammensetzung der Sandfraktion zeigt
die Veränderung des Sediments durch frühdiagenetische Prozesse nach der Ablagerung. Unbekannt ist allerdings immer noch, wie und wann diese Prozesse genau ablaufen und in welcher Weise z.B. ein Anstieg in der Alkalinität oder der Strontiumgehalt damit in Zusammenhang stehen. Diese geochemischen Parameter könnten Hinweise darauf geben, in welcher Sedimenttiefe „nodules“ gebildet werden.
Danksagung
In erster Linie möchte ich mich bei Rebecca Rendle-Bühring für die Vergabe und Betreuung der Doktorarbeit bedanken. Ihr großes Vertrauen in meine Arbeit hat mich immer gestärkt. Und dank ihrer Hilfe und kompetenten Ratschläge wurden aus meinen Anhäufungen von Argumenten und Ideen in manchmal wohl recht fragwürdigem Englisch am Ende doch noch vernünftige Publikationen. An Hildegard Westphal als Zweitgutachterin und Rüdiger Henrich als „Co-Betreuer“ ergeht ebenso großer Dank. In zahlreichen Doktorandenseminaren oder privaten Gesprächen haben sie mir wichtige Denkanstöße und fachlichen Rat gegeben bzw. rechtzeitig die Haken meiner Arbeit erkannt.
Ein ganz dickes Dankeschön geht an meinen lieben Kollegen Stephan Steinke für die Einweisung in planktische Foraminiferen und Schalke 04, für´s kritische Lesen aller möglichen Abstracts, Manuskripte, Posterentwürfe etc., für alle wissenschaftlichen Ratschläge und Unterstützung in jeglicher Hinsicht und natürlich für die vergnügliche Zeit miteinander, vor allem im gemeinsamen Büro im TAB-Gebäude.
Für die verschiedenen Messungen und die dazugehörenden Probenvorbereitungen danke ich ganz herzlich Michael Wendschuh und Christoph Voigt (XRD, Datenauswertung), Michael Frenz (Sedigraph), Renate Henning und Brit Kockisch (Leco), Helga Heilmann, Kalle Baumann und Rüdiger Henrich (REM), Monika Segl (Stabile Isotopen) und Barbara Donner (Bereitstellung von Labormaterial). Christoph Voigt und Kalle Baumann sei noch mal extra gedankt für zahlreiche Anregungen und Kritik, die mir im Laufe meiner Arbeit sehr geholfen haben.
Der gesamten AG Henrich danke ich für die Integration der AG Rendle in vielen Bereichen, vor allem bei der Laborbenutzung (Dank an Till Hanebuth für Schlüssel und uneingeschränktes Vertrauen), dem Doktorandenseminar, den Weihnachtsfeiern zwischen Sedigraph und Abzug und den Sommerfeiern im Henrichschen Garten.
Nicole Meyer, Inka Meyer und Florian Schroth danke ich für ihren unermüdlichen Einsatz, hunderte von Proben zu schlämmen, zu sieben, zu mörsern oder zu mikroskopieren. Eure Unterstützung hat mir wertvolle Zeit gespart, vor allem seit Linus auf der Welt war!
Für wertvolle Diskussionen bei der Erstellung meiner Manuskripte möchte ich ganz herzlich John Reijmer, Hildegard Westphal und Lars Reuning danken. John Reijmer hat mich außerdem großzügig mit Probenmaterial versorgt und mit Daten aus älteren Arbeiten, die mir eine große Hilfe bei meiner Arbeit waren. Als Editor hat John außerdem entscheidende „Geburtshilfe“ bei meinem ersten Paper geleistet. Vielen Dank dafür!
Ganz besonders möchte ich mich bei der „alten“ Kaffeerunde und Mensacrew aus dem TAB-Gebäude für eine wunderschöne Zeit bedanken, allen voran Jens Holtvoeth für seine unschätzbare Hilfestellung bei meinem ersten Paper, für seinen unerschöpflichen Vorrat an Muscheldosen, eingelegter Roter Bete und
ebenso danke ich Britta Beckmann für ihre Hilfsbereitschaft in allen Dingen, Regina Krammer für ihre unterhaltsame Art und viele sehr lustige Abende abseits von Foraminiferen und Coccolithen, Sadat Kolonic für Kaffeekochen und seine Beharrlichkeit, täglich zweimal die gesamte Kaffeerunde zusammenzutrommeln, Michael „Gustl“ Seyferth für äußerst kreative und frustmindernde Unterhaltungen per email oder bei ner Zigarette im japanischen Innenhof, Katrin Huhn für ihren Zuspruch in Krisenzeiten und einfach für ihre Freundschaft, und schließlich meinem Kollegen und Mitbewohner Ingo Kock für´s häufige Hüten von Linus und die gegenseitige Motivation bei der Beendigung unserer Doktorarbeiten.
Der Umzug ins neue marum-Gebäude ergab nicht nur den Luxus eines Einzelzimmers, sondern wieder eine neu zusammengewürfelte Mensarunde, der ich hiermit danken möchte für die nette gemeinsame Zeit: Melanie Reichelt, Petra Günnewig, Xin Li, Frank Strozyk, Julia Schneider, und besonders unser neues Arbeitsgruppenmitglied Carola Ott, mit der ich leider nur noch ein paar Monate Tür an Tür arbeiten durfte.
Basti, wir haben dank Fernbeziehung, Doktorarbeit und Nachwuchs aufregende und manchmal schwierige Jahre hinter uns, und ich möchte dir hier ganz offiziell noch mal für deine Liebe und Freundschaft danken und dafür, dass du mich immer motiviert und unterstützt hast, auch über die große Distanz zwischen München und Bremen hinweg. Ganz besonders möchte ich dir dafür danken, dass du im letzten Jahr den Großteil deiner Arbeitszeit nach Bremen verlegt hast, um deiner Familie näher zu sein.
Ein dickes Danke inkl. einem knallroten Feuerwehrauto geht an unseren kleinen Linus, dafür dass er im Alter von drei Wochen schon ohne Murren auf seinen ersten Kongress gegangen ist bzw. getragen wurde, und vor allem dafür, dass er sich von klein auf bereitwillig seinen Brei von anderen Leuten in den Mund stopfen ließ, wenn seine Eltern stattdessen mal wieder irgendwas Wichtiges in den Computer tippen mussten. Der abgerissene Doppelpunkt-Knopf an meiner Laptop-Tastatur wird allerdings vom Taschengeld abgezogen...
Schließlich möchte ich mich ganz herzlich bei meinen Eltern Otto und Marlene Schwarz und meinen Brüdern Uli und Florian Schwarz für ihre stete Unterstützung und ganz besonders für´s Korrekturlesen am Ende der Arbeit bedanken.
Table of contents
Abstract... V Kurzfassung ... VIII Danksagung ... XII
Part I - Introduction ... 1
1. Calcium carbonate production on platforms ...2
2. Carbonate preservation and carbonate dissolution ...2
2.1. Carbonate preservation at the sediment-water interface...4
2.1.1. Lysocline and compensation depth...5
2.1.2. Supralysoclinal dissolution...6
2.2. Carbonate preservation in the shallow sediment column ...7
3. Main Questions...8
4. Study Area ...10
4.1. The Bahamas – a modern example of an isolated carbonate platform...10
4.2. Modern sedimentation ...10
4.3. Water masses and currents ...12
4.4. Sample locations...13
4.4.1. Florida Straits ...13
4.4.2. Northwest and Northeast Providence Channel...14
4.4.3. Exuma Sound...15
5. Methods ...16
5.1. Grain-size analyses ...17
5.2. Mineralogy ...18
5.3. Carbonate and total organic carbon contents...18
5.4. Coarse fraction analyses ...19
5.5. Dissolution Indices ...21
5.6. Scanning Electron Microscopy...21
5.7. Stable isotopes ... 22
6. Organisation of the thesis ...22
Part II - Results... 25
Chapter 1: Controls on modern carbonate preservation in the southern Florida Straits...26
Chapter 2: Compositional variations and early diagenetic processes in Quaternary periplatform sands: an example from the Great Bahama Bank...41
Chapter 3: Diagenetic alteration of periplatform sediments: implications for palaeoenvironmental interpretations based on grain size ...65
Part III - Summary... 77
1. Conclusions ...78
1.1. Supralysoclinal dissolution of aragonite ...78
1.2. Spatial and temporal distribution of nodules (diagenetic products) ...79
1.3. Formation conditions of nodules ...79
1.4. Interplay of the grain-size distribution and the formation of nodules ...80
2. Outlook ...81
Part V - Appendix... 93 1. Samples 2. Data 2.1. Grain Size 2.2. Leco 2.3. XRD
2.4. Census Counts (main component groups)
2.5. Detailed census counts (foraminifera assemblage) 2.6. LDX
1. Calcium carbonate production on platforms
Carbonate sediments are, with approximately three billion tons annually, the second most abundant sediment in the global ocean after fluvially derived sediment (Milliman and Syvitski, 1992). Modern carbonate sediments are, apart from oolites and lime muds, mainly the result of biogenic production (Tucker and Wright, 1990). Calcium carbonate is produced in the shallow seawater through the secretion of shells by planktonic organisms; these shells then fall to the sea floor, and, in the deep ocean below several thousand meters water depth, the calcium carbonate is mostly returned to the oceans by dissolution. These processes help to maintain the calcium and carbon balance of the ocean and the carbon balance of the atmosphere (Gieskes, 1974; Broecker, 1971; Pytkowicz, 1968; Broecker and Peng, 1982).
On carbonate platforms the production of carbonates and their preservation state is high. There are five major types of carbonate platforms: shelf, ramp, epeiric platform, isolated platform, and drowned platform (Tucker and Wright, 1990). The Bahama platform is a modern example of an isolated carbonate platform (Vecsei, 2003). The growth and distribution of isolated platforms is related to current- and climate-influenced properties of the sea-water (temperature or carbonate ion saturation: Kleypas et al., 1999; nutrient concentration: Hallock and Schlager, 1986; salinity: Wilson and Roberts, 1992).
Carbonate sediments have, according to Tucker and Wright (1990) four major constituents: carbonate skeletons, silicious skeletons, terrigenous silt and clay, and authigenic components. In the shallow-water realm, benthic production dominates and carbonate sediments mainly consist of aragonite and high-Mg calcite (HMC) with more than 4 mol% of MgCO3 (Milliman, 1974; Bathurst, 1971). Pelagic production is much lower than neritic production; the mineralogy of pelagic produced carbonates is dominated by low-Mg calcite (LMC) with less than 4 mol% of MgCO3 (Garrison, 1981; Milliman and Droxler, 1996). Around the margins of carbonate platforms, the neritic and pelagic sources of carbonate sediment create a third, mixed type of carbonate sediment, referred to as periplatform ooze (Schlager and James, 1978).
2. Carbonate preservation and carbonate dissolution
After the deposition of carbonate sediments on the sea floor, they are generally affected by diagenesis (Milliman, 1974). Cementation, microbial micritization, neophormism, dissolution, compaction, and dolomitisation are the most important diagenetic processes (Tucker and Wright, 1990). These diagenetic processes are mainly controlled by the composition and mineralogy of the sediment, the pore-fluid chemistry and flow rates, changes
in burial, uplift, and sea-level, the influx of different pore-fluids and the climate (Tucker and Wright, 1990).
Fig. 1: The carbonate diagenetic environments. In sediments below
sea-level, they are divided into meteoric, mixing zone, and marine phreatic diagenesis. Meteoric or marine vadose diagenesis occurs above sea-level. The samples used in this study are affected by marine phreatic diagenesis in the shallow burial environment. Figure taken from Tucker and Wright (1990).
Schlanger and Douglas (1974) introduced the concept of “diagenetic potential” to pelagic carbonates to account for the local variations in diagenetic grade observed in sedimentary records. They interpreted the composition and nature of the original sediment to be controlled by the conditions of sedimentation; namely water depth, deposition rate, temperature, productivity, sediment compaction and grain size. These conditions determine the progress of diagenesis, such as the rates of mechanical compaction, grain breakage, grain dissolution and CaCO3 precipitation. Previous studies documented extensive surficial submarine diagenesis in high-energy, current-swept, periplatform environments (Mullins et al., 1980a) as well as in relatively low-energy, deeper-water settings, where erosion and/or low sedimentation rates resulted in long-term exposure of surficial sediment to seawater (Milliman, 1966; Fisher and Garrison, 1967; Milliman and Muller, 1977; Schlager and James, 1978). Periplatform oozes possess a high diagenetic potential in their depositional environment because of the metastability of aragonite and HMC in deep, cold seawater (James and Choquette, 1983). Diagenetic processes in periplatform carbonates generally include the dissolution of aragonite and HMC and the recrystallization of LMC. Mineralogical and geochemical studies of sediments (Dix and Mullins, 1988a,b; Malone et al., 1990), combined with studies focused on the chemistry of the associated interstitial water (Swart and Guzikowski, 1988; Swart et al., 1993), also showed that periplatform sediments have a higher diagenetic potential than monomineralic, deep-sea pelagic carbonates (Malone et al., 2001).
Carbonate diagenesis operates in three principal environments (Fig. 1): the marine, meteoric, and burial environments. Marine diagenesis takes place on the seafloor and within
the sediment, and on tidal flats and beaches. Meteoric diagenesis affects a sediment soon after deposition on a supratidal flat or if rainwater falls on the carbonates. The burial environment occurs in tens to hundreds of meters depth within the sediment column. The term “early diagenesis” refers to near-surface processes, while “late diagenesis” refers to processes during deep burial. Studies of the Bahama Transect revealed three diagenetic zones (Melim et al., 2002): meteoric, mixing-zone, and phreatic-marine diagenesis. The latter is divided into seafloor diagenesis, shallow marine-burial diagenesis, and deep burial diagenesis. The sites which have been studied here, were preferentially influenced by seafloor and shallow marine-burial diagenesis: meteoric and mixing-zone diagenesis can be omitted in water depths of more than 600 m water depth, and deep-burial diagenesis occurs in deeper sediment than has been studied.
2.1. Carbonate preservation at the sediment-water interface
At the sediment-water interface, the dissolution of carbonate components is a common feature. The dissolution of calcium carbonate in deep-sea sediments is controlled by two major factors: 1) the degree of saturation of the oceanic bottom waters overlying the sediment (e.g. Berger, 1968; Li et al., 1969; Broecker, 1971; Morse and Berner, 1972; Volat et al., 1980; Broecker and Peng, 1982) and 2) the interstitial reaction with metabolically released carbon dioxide into sediment pore waters during organic matter remineralization (Emerson and Bender, 1981; Archer et al., 1989b; Berelson et al., 1990; Jahnke et al., 1994, 1997; Hales and Emerson, 1996, 1997; Martin and Sayles, 1996; Adler et al., 2001; Wenzhöfer et al., 2001). Carbonate dissolution is enhanced by an increasing rain ration, i.e. the ratio of organic carbon to calcium carbonate, reaching the sediment (Emerson and Bender, 1981).
Calcite has the unusual property of becoming more soluble with increasing pressure and decreasing temperature, i.e. with depth in the ocean. The carbonate dissolution within the sea therefore is a result of decreased temperature, increased pressure, and increased CO2 content. Carbon dioxide and temperature are more critical in regulating carbonate dissolution than pressure (Revelle, 1934). The distinct dissolution rate of carbonate sediments depends on the particle size (Chave and Schmalz, 1966), the type, shape and amount of carbonate grains (Keir, 1980, 1982), and the reaction kinetics describing carbonate dissolution as a function of carbonate ion concentration in bottom waters (Morse and Berner, 1972; Broecker and Peng, 1982). A high sedimentation rate or a very rapid deposition by turbidites can result in the protective burial of calcite before it is dissolved in the overlying bottom water (Berner et al., 1976).
Fig. 2: Carbonate dissolution and saturation in the Pacific and Atlantic Ocean. Figure after Jenkyns (1986) and Scholle et al. (1983).
2.1.1. Lysocline and compensation depth
The calcite compensation depth (CCD) is the depth, below which carbonate is not deposited since the rate of carbonate dissolution equals the rate of carbonate deposition (Pytkowicz, 1970). At the shallower lysocline (Berger, 1968), there is a pronounced increase in the rate of carbonate dissolution (Fig. 2). Aragonite and HMC are both metastable with respect to LMC (Chave et al., 1962; Chave and Schmalz, 1966) and therefore both tend to dissolve at shallower oceanic depths than LMC. HMC is slightly less soluble than aragonite in the deep sea (Milliman, 1974), but more soluble than aragonite in the Bahama region (Swart and Guzikowski, 1988). Due to the lower solubility, the aragonite compensation depth (ACD) lies in shallower water depths than the CCD. The same accounts for the aragonite lysocline relative to the LMC lysocline.
The primary cause for the depth variations of the lysoclines has been widely debated. According to Morse and Berner (1972), the lysocline is due to chemical changes in the water column. Morse (1974) stated that the chemical lysocline is a kinetic feature that can be defined in the purely thermodynamic terms of pressure, temperature, and composition. Honjo and Erez (1978) confirmed that the lysocline has a kinetic origin. Broecker and Takahashi (1978), in contrast, suggest that the lysocline is thermodynamically controlled (transition from saturation to undersaturation).
The positions of ACD and CCD are variable in space and time. The carbonate lysocline lies deepest in the North Atlantic (Berger, 1968; Biscaye et al., 1976), and shallowest in the more corrosive northern Pacific (Berger, 1970; Parker and Berger, 1971). The depth of the chemical calcite lysocline is approx. 4000-4500 m in the Atlantic Ocean, 3000 m in the Pacific, and 3800 m in the Indian Ocean (Peterson and Prell, 1985; Milliman et al., 1999). The ACD is found at significantly shallower depths, e.g. the Pacific is saturated
with respect to aragonite down to 200 m; the northwestern Atlantic is supersaturated from 0 to 1000 m, saturated from 1000 to 2300 m, and undersaturated below 2300 m water depth (Li et al., 1969). In the Bahama region the carbonate lysoclines and compensations depths are depressed relative to the open Atlantic because of the large input of bank-derived carbonate sediments (Droxler et al., 1988b): saturation for HMC occurs between 900 and 1500 m, between 3800 and 4500 m for aragonite and at about 5500 m water depth for LMC. Relative to LMC, the supersaturation is 200-400% on the western edge of the GBB (Cloud, 1962; Broecker and Takahashi, 1966).
Fig. 3: Model of carbonate budget in the open ocean (after
Milliman et al., 1999; numbers in 1012 Mol C/a). The monitoring of
supralysoclinal dissolution in surface samples from the Bahamas and the Florida Straits was based on this model.
2.1.2. Supralysoclinal dissolution
Berger (1975) and Ku and Oba (1978) found evidence that dissolution can occur in carbonate-rich sediments above the lysocline by field-studies on carbonate preservation in the central Pacific and by laboratory studies, respectively. These findings were confirmed by several authors (Emerson and Bender, 1981; Sayles, 1981; Archer et al., 1989a; Hales et al., 1994). In various regions of the world´s ocean, dissolution above the calcite lysocline was observed; at the Ceara Rise in the western equatorial Atlantic (Martin and Sayles, 1996); in the Arabian Sea (Milliman et al., 1999); in the equatorial Indian Ocean (Peterson and Prell, 1985; Schulte and Bard, 2003) and in the western equatorial Pacific (Kimoto et al., 2003). The evaluation of the rate of carbonate dissolution above the lysocline and between the lysocline and calcite compensation depth is important for the quantification of the global carbonate budget (Martin and Sayles, 1996).
The global production of pelagic calcium carbonate (58 x 1012 mol C/yr) was calculated by Milliman et al. (1999) after the following assumptions (Fig. 3):
Alkalinity flux: 60 x 1012 mol C/yr
Carbonate accumulation in the sediment: 11 x 1012 mol C/yr
Carbonate added to the alkalinity pool from the continent: 10 x 1012 mol C/yr Hydrothermal input: 3 x 1012 mol C/yr
According to this global budget, only 20% of the planktonic carbonate production accumulates on the deep-sea floor. The rest dissolves either in the water column or at or near the sediment-water interface. Martin et al. (1993) showed a 50-60% loss of calcium carbonate in the upper 1000 m of the water column, which is in close agreement with the global model of Milliman et al. (1999). Milliman et al. (1999) conclude from their studies, that 60-80% of the surface-produced carbonate is lost at epi-pelagic depths, i.e. at depths shallower than about 800 - 1000 m (Fig. 4). A major proportion of the carbonate is lost above the lysocline by dissolution of the metastable carbonate phases aragonite and HMC (Milliman, 1993). There are two possible mechanisms suggested by Milliman et al. (1999) which could result in globally significant dissolution above the lysocline: 1) dissolution within the guts and feces of grazers, and 2) microbial oxidation of organic matter.
Fig. 4: The theory of Milliman et al. (1999):
30-60% of the yearly produced carbonate (21-24 g/m2) is dissolved. As the mean global average carbonate flux at 1000 m water depth amounts to 8-12 g/m2, they suggest that 60-80% of the dissolution might occur above the lysocline and only 20-40% below the lysocline.
2.2. Carbonate preservation in the shallow sediment column
Several authors, including Saller (1984, 1986), Mullins et al. (1985a, b), Freeman-Lynde et al. (1986), and Dix and Mullins (1988a), indicated that substantial alteration, lithification, dissolution, calcite cementation and dolomitisation can occur at shallow-burial depths in the deep-water realm. Malone et al. (2001) state that diagenetic alteration in shallow sediments (in this case dissolution of metastable components) is driven by the degradation of
organic matter; this process can occur, even when the overlying bottom waters are saturated or supersaturated with respect to CaCO3.
Early diagenetic processes in periplatform carbonates principally involve the selective dissolution of aragonite and HMC and reprecipitation of LMC and dolomite (e.g. Mullins et al., 1985b). According to Dix and Mullins (1988a), shallow-burial diagenesis of periplatform carbonates proceeds in two discrete stages. Stage 1 occurs in the top 10 m of the sediment column, where rapid, extensive diagenetic changes occur, including enrichment of oxygen isotopes, depletion of carbon isotopes, dissolution of aragonite, exsolution of Mg from HMC, and incipient lithification. Stage 2 diagenesis (in a core depth of 10-200 m) is a slower, longer-term process of calcite precipitation and lithification, gradual loss of Mg and Sr, and isotopic equilibrium with still relatively cold, marine-derived pore waters. In water depths less than 700 m, diagenetic alteration is manifested primarily through cementation by HMC in well-lithified, surficial hardgrounds or near-surface nodular oozes (Milliman, 1974; Mullins et al., 1980a; Wilber and Neumann, 1993). Cements from water depths greater than 700 m are composed of LMC (Schlager and James, 1978).
Mullins et al. (1980a) found so-called nodules (grain-supported intra-micrites, cemented by HMC) in Bahamian cores north of Little Bahama Bank (LBB) and in the Northwestern Providence Channel, which have formed during early diagenesis: stable isotope analyses showed that the nodules are in closer equilibrium with ambient bottom waters than the surrounding sediment. This strongly suggests an in situ submarine cementation process. Lantzsch et al. (in press) observed nodules similar to those of Mullins et al. (1980a), with major amounts during the transitions from glacial to interglacial stages and vice versa. Due to their temporal distribution these nodules are, in contrast, interpreted to be the result of redeposition events during sea-level change. The principal occurrence of carbonate concretions and aggregates in Bahamian periplatform sediments is well known and has been identified by several authors (e.g. Mullins et al., 1980a; Droxler et al., 1988a; Lantzsch et al., in press). However, little is known about the distribution of nodules within the sediment column. More information is necessary about the exact spatial and temporal distribution to determine the origin of the concretions, and the processes and conditions of their formation.
3. Main Questions
The main goal of this thesis is to determine the preservation state of carbonate sediments in the periplatform sediments of the Great Bahama Bank (GBB) and the adjacent Florida Straits. Despite the evidence for supralysoclinal dissolution in some areas of the
world´s ocean, the question still exists whether dissolution does occur above the lysocline in the entire ocean. The first part of this thesis seeks answers to this question, based on the global budget models of Milliman et al. (1999). As study area, the Bahamas and Florida Straits are most suitable because of the extreme conditions of the carbonate factory at this location: the production of carbonate is very high and the depth of the lysocline is the deepest worldwide. The preservation state of the surface sediment was determined using a proxy for aragonite dissolution, the Limacina inflata Dissolution Proxy (LDX; Gerhardt and Henrich, 2001), as aragonite is more prone to dissolution than calcite. This study of surface sediments was expanded by a study on downcore samples (Pliocene to Holocene) of three Ocean Drilling Program (ODP) cores from the western margin of GBB and from the more easterly located Exuma Sound. Quantitative census counts of these sedimentary sequences gave detailed insights into the composition of sand-sized material in this area and for this time interval. This revealed the opportunity to determine the preservation state of the sediments via the amount of metastable aragonite particles and the occurrence of early diagenetic products, namely carbonate concretions (nodules), which have been observed in all three cores. In the process of this study, it could be shown that the nodules have been formed in situ during early diagenesis. This finding raises the question whether the formation of nodules alters the initial grain-size distribution of the surrounding sediment.
The main questions of this thesis are: Surface samples
1. Are the surface samples of the Florida Straits and Great Bahama Bank affected by supralysoclinal dissolution? What are the reasons for the observed preservation patterns?
2. Is the previously investigated LDX also suitable as a proxy for the reconstruction of the palaeo-corrosiveness of water masses in highly saturated areas?
Downcore samples
3. How are nodules spatially and temporally distributed in the sediments of Great Bahama Bank?
4. Are the nodules present in the sediment due to resedimentation processes or due to early diagenesis?
5. What are the formation conditions? Do nodules influence the grain-size distribution of the sediment, and/or vice versa?
4. Study Area
4.1. The Bahamas – a modern example of an isolated carbonate platform
The Bahama carbonate platform is located in the western equatorial Atlantic. The southern Blake Plateau (located north of the Bahamas) correlates with the top of a drowned, shallow-water carbonate platform complex of mid-Cretaceous age. Such a platform underlies all of the northwestern Bahamas. Platform drowning occurred in steps during the mid-Cretaceous. During the Late Cretaceous and Tertiary, the north- and west- facing platform flanks prograded tens of kilometres. During the Pliocene, the GBB evolved from a ramp-system to a flat-topped, rimmed platform (Beach and Ginsburg, 1980; Schlager and Ginsburg, 1981; Beach, 1982; McNeill et al., 1988; Reijmer et al., 1992).
The average angles of the slopes along the margins of the platform are highly variable, ranging from less than 1° to 40° or more (Mullins and Neumann, 1979b). Three different types of slopes have been observed. As platform slopes become steeper, they change from “depositional slopes” to “by-pass slopes” and finally to “erosional slopes” (Schlager and Ginsburg, 1981; Rendle and Reijmer, 2002). Depositional slopes are found along the western flanks of LBB and GBB (Mullins and Neumann, 1979b). Typical by-pass slopes are the gullied flanks of Tongue of the Ocean. Erosional slopes are for example, the flanks of the Northeast Providence Channel and the ocean-facing Blake-Bahama Escarpment in the east (Freeman-Lynde et al., 1979).
4.2. Modern sedimentation
The Bahamas are a tectonically stable carbonate platform consisting of thick shallow-water banks separated by deep-water channels. This isolated platform is separated from the Florida peninsula to the west by the Florida Straits and from Cuba to the south by the Old Bahama Channel (Fig. 5). These deep channels prevent the accumulation of siliciclastic material on the platform, permitting the deposition of very pure carbonate sediments (Tucker and Wright, 1990). Sedimentation rates on the platform have been 2 cm/ky during the last glacial period, and nearly 10 cm/ky during the recent sea-level highstand (Boardman and Neumann, 1984): when the bank tops are flooded during interglacial highstands, large amounts of fine-grained aragonite are produced and exported to the bank margins (Neumann and Land, 1975). As a result, thick sections of highstand sediments accumulate along the slopes of the Bahama banks (e.g. Droxler et al., 1983; Boardman et al., 1986; Slowey and Curry, 1995; Westphal et al., 1999; Rendle and Reijmer, 2002). This phenomenon has been termed “highstand shedding” by Droxler and Schlager (1985).
Fig. 5: The Bahama carbonate platform with its major basins and surface currents. GBB = Great Bahama
Bank, LBB = Little Bahama Bank.
The slopes and basins of the Bahama platform are covered with periplatform ooze. Around carbonate platforms, the processes of resedimentation are commonly due to turbidity currents, slumps and slides. Previous studies of the Bahama platform showed that when the platform is flooded, more sediment is produced than can be accumulated on the platform top. This excess sediment is exported into the periplatform realm during storms and by tidal action (Boardman, 1978), along mid-water pycnoclines, and as low-density turbidity currents. The wind regime significantly influences the sediment distribution on the Bahama platform. During summer, the wind blows largely from easterly directions. During winter, the winds have an increasing northerly component (Smith, 1940). The mainly easterly winds push the sand to the western sides of the great islands, inducing a progradation of the platforms from east to west (Eberli and Ginsburg, 1989). On the flat platform tops, sands migrate to the leeward margins where storm pulses push them into the deep water (Hine and Neumann, 1977).
Sediments in the basins between the Bahama Banks consist of carbonate ooze (nearly 100% carbonate) separated by graded beds of lime sand and mud from sediment gravity flows (Mullins and Neumann, 1979b; Schlager and Chermak, 1979). All fine sediment is a mixture of aragonite, HMC, and LMC. The mineralogy of basin sediments shows cyclic variations of aragonite and LMC, coupled with more irregular variations of HMC (Supko, 1963; Pilkey and Rucker, 1966; Kier and Pilkey, 1971; Lynts et al., 1973; Boardman, 1978; Droxler et al., 1988a; Reijmer et al., 1988; Eberli, 2000; Kroon et al., 2000b). The distinct particle sources are for LMC: planktonic foraminifera and coccoliths; for HMC: fragments of benthic forams, red algae, calcareous sponges, echinoderms, in situ cement; and for aragonite: aragonite needles, inorganic precipitates, pteropods (Droxler et al., 1988b).
4.3. Water masses and currents
The water masses of the Bahamas essentially originate in the western Atlantic, as shown from their temperature-salinity and oxygen-density distribution (Wennekens, 1959). The mixed surface layer and the upper part of the permanent thermocline (0-180 m water depth) in the intra-platform channels of the Bahamas contain Western North Atlantic Central Water (WNACW). Below 1200 m water depth the water masses derive from the North Atlantic Deep Water (NADW). The general ocean current system around the Bahama Islands is bound by the currents forming the sources of the Gulf Stream: One part of the Northern Equatorial Current flows into the Caribbean, and the rest flows northwestward along the Atlantic Side of the Antilles/Bahama archipelago as the Antillean Current, eventually contributing to the transport of the Gulf Stream (Gunn and Watts, 1982). The Florida Channel to the west of the Great Bahama Bank contains the fast-flowing Gulf Stream, whereas the intra-platform currents are weak, e.g. little water movement to the west in Northwest Providence Channel, except after northerly winds, or the weak current through the Old Bahama Channel (Smith, 1940). However, the Old Bahama Channel is important because it provides a direct connection between the Straits of Florida and the part of the subtropical gyre that flows northwestward past the Lesser Antilles. From the subtropical North Atlantic, water flows northwestward through the channel (Atkinson et al., 1995). For more detailed information about the depth distribution of the waters east of the Bahamas and north of the Antilles see Gunn and Watts (1982).
The hydrography in the Florida Straits is dominated by the geostrophic Florida Current, which is interpreted as the major source of the Gulf Stream (Wennekens, 1959). It flows between Florida and the Bahamas into the North Atlantic, where it converges with the
smaller Antilles Current to form the Gulf Stream. Short-term sea-level falls intensify the currents in the seaways because of restriction of the channel area (Richardson et al., 1969). The water masses of the Florida Current are fed by the western North Atlantic Intermediate Water (NAIW) and by the more corrosive, low-salinity, oxygen-rich Antarctic Intermediate Water (AAIW). This source water for the Gulf of Mexico and the Straits of Florida comes from the Caribbean Sea through the Yucatan Strait: the Yucatan Current enters the Gulf of Mexico through the Yucatan Channel, becoming the Loop Current (Molinari and Morrison, 1988). The Loop Current then becomes the Florida Current, exiting the Gulf of Mexico at the Straits of Florida (Molinari and Morrison, 1988). Subsidiary channels with the most important water mass contributions to the Florida Current are the Northwest Providence Channel, the Santaren Channel, and the Old Bahama Channel (Leaman et al., 1995).
4.4. Sample locations
A large number of surface and downcore samples have been used for this study, divided into different sample sets from various regions around GBB; Florida Straits, Providence Channel, Tongue of the Ocean, Exuma Sound, north of LBB and east of GBB. The most important sampling regions are described in detail below (chapter 4.4.1. to 4.4.3.). Tables 1 and 2 give an overview of all sample sets used in this study. For more detailed information on the samples see Appendix 1.
4.4.1. Florida Straits
The southern Straits of Florida forms an eastward-trending channel bound to the south by Cuba and on to the north by the Florida shelf (Fig. 5, 6a). The axis of the Straits is tilted, rising from a depth of 2100 m near the Yucatan Channel to 1000 m at the entrance to the northern Straits of Florida. The channel is filled with late Mesozoic and Cenozoic chalk and foraminiferal ooze (Schlager W., Buffler R.T., et al., 1984). Surface sediments consist of pteropod-foraminiferal sands in the Yucatan Channel and northern Straits of Florida, and of muddy, pteropod-foraminiferal oozes in the southern Straits of Florida (Brunner, 1986). Terrigenous and pelagic sediments are transported by turbidity currents to the axis of the Straits. Neritic material is swept from carbonate banks at the south Florida shelf margin and may cascade to the axis of the Florida Straits (Mullins et al., 1980b).
Table 1: Surface samples Sample
Set
Number of
samples Location Water depth Core type Cruise
1 15 bulk
samples Florida Straits 845-2325 m Piston
Gillies-7603 (1976), Trident-149 (1974)
2 44 bulk
samples Providence Channel 434-1183 m
Piston, Gravity, Box
Oceanus-205 (1988)
3 11 bulk samples Western margin of GBB 351-658 m Drill (ODP) Leg 166 (1996)
4 13 bulk samples Around GBB and Little Bahama Bank (LBB) 553-3470 m Drill (ODP) Leg 101 (1985)
5 38 bulk
samples
Exuma Sound, Tongue of the Ocean, southeastern margin of GBB 1275-4796 m Gravity, Piston Pilsbury P6401 (1964), P6408 (1964), P6804 (1968), P6807 (1968), P7008 (1970), P7102 (1971)
Table 2: Downcore samples
Sample Set Number of samples,
age Location Water depth Core type Cruise
6 20 bulk samples, MIS 1-45, mostly peak glacial/interglacial western margin
of GBB (basin) 658 m Drill (ODP) ODP Leg 166, 1006A
7a 227 samples >63 µm,
Holocene/Pleistocene Exuma Sound 1996 m Drill (ODP)
ODP Leg 101, 632A
8a 326 samples >63 µm,
Pliocene to Holocene Exuma Sound 1681 m Drill (ODP)
ODP Leg 101, 633A
9 54 bulk samples, MIS 3-41 western margin of GBB (basin) 658 m Drill (ODP) ODP Leg 166, 1006A
a
These sample sets were kindly provided by John Reijmer and his working group at the Leibniz-Institute, Kiel, Germany, for further measurements. The samples had already been divided from the fine fraction and sieved into five sub-fractions (63-125, 125-25, 250-500, 500-1000, >1000 µm).
4.4.2. Northwest and Northeast Providence Channel
The Providence Channel is a canyon flanked by gullied slopes in the east, which passes westward into a shallower, U-shaped basin (Fig. 5, 6b). It is shoaling from 4000 m at the eastern end to 800 m at its entrance into the Florida Straits. The slopes rimming the basin are rather gentle and gradually pass into the basin floor. The Providence Channel is an open seaway, where winnowing by contour currents and sea-floor lithification is important (Mullins and Neumann, 1979b; Mullins et al., 1980a). In Northwest Providence Channel more than 75% of the Holocene fine fraction is bank-derived (Boardman, 1978) and is mostly mineralogically metastable (aragonite and HMC). The remaining part of the sediment consists of skeletons of planktonic or pelagic organisms and is mostly LMC, with minor amounts of aragonite (Boardman, 1978).
4.4.3. Exuma Sound
The Exuma Sound is a closed seaway, located at the southeastern edge of GBB (Fig. 5, 6b). It represents one of three intra-platform basins of GBB. It is a relatively narrow trough, deepening from 1200 m in the north to 2000 m water depth at the connection of the Sound with the open Atlantic; the basin is physiographically characterised by gullied slopes, basin-margin rises, and the basin floor (Crevello and Schlager, 1980). The sediments in the basins consist of interbedded coarse clastic carbonates from the platform margins and slopes, and carbonate muds derived from the perennial rain of pelagic material and platform-winnowed fines. There are three main types of gravity flow deposits: graded sand and rubble, poorly sorted sand and rubble, pebbly mud (for a more detailed description see Crevello and
Fig. 6: The study area with the location of a) surface samples and b) downcore samples, and c) a profile through Andros Island, the Tongue of the Ocean (TOTO), and Eleuthera Island, with major water masses influencing the Florida Straits to the west, TOTO, and the steep escarpment at the eastern margin of Great Bahama Bank (GBB). Providence Channel and
Exuma Sound are, similar to TOTO, influenced by the water-mass distribution east of GBB. LBB = Little Bahama Bank. For detailed information about all samples see Appendix 1.
Schlager, 1980). Turbidites in the Exuma Sound consist of largely unlithified platform sediments. Aragonite content, oxygen isotopes, nannoplankton assemblages, and palaeomagnetic signatures document cyclic variations in export and preservation of platform derived mud in the Pliocene-Pleistocene (Droxler et al., 1988b; Reijmer et al., 1988).
5. Methods
A number of methods were used to determine the sediment characteristics; grain size (GS), mineralogy (X-ray diffraction; XRD), carbonate and total organic carbon content (measurements by the Leco), and census counts of the sand fraction. The preservation state of the sediments was examined with various dissolution indices. Detailed information on the composition and formation conditions of the nodules were gained with the Scanning Electron Microscope (SEM) and stable isotope-measurements. Table 3 gives an overview of the different methods used for each sample set.
Table 3: Overview of all methods used for the different sample sets.
Sample Set GS XRD bulk sed. XRD <63 µm Leco bulk sed. Leco <63 µm Census counts LDX SEM Stable isotopes (G18 O,G13 C) 1 – Gillies/ Trident X 1) X X1) X1) X1) X1) X1) 2 – Oceanus X X1) X1) X1) X1) 3 – Leg 166 surface samples X X X X 4 – Leg 101 surface samples X X X X 5 – Pilsbury X X1) X1) X1) X1) 6 – Site 1006 7 – Site 632 X3) 8 – Site 633 X2) X2,3) X2) X2) X2) (nodules) 9 – Site 1006 X2,3) 1)
results see Part II, Chapter 1: Schwarz J., Rendle-Bühring, R., 2005. Controls on modern carbonate preservation in the southern Florida Straits. Sedimentary Geology 175, 153-167.
2)
results see Part II, Chapter 2: Schwarz J., Steinke, S., Rendle-Bühring, R., Reijmer, J.J.G., Compositional variations and early diagenetic processes in Quaternary periplatform sands: an example from Great Bahama Bank. Submitted to Marine Geology.
3)
results see Part II, Chapter 3: Schwarz J., Steinke, S., Rendle-Bühring, R., Reijmer, J.J.G., Diagenetic alteration of periplatform sediments: implications for palaeoenvironmental interpretations based on grain size. Submitted to Journal of Sedimentary Research.
5.1. Grain-size analyses
For the grain-size analyses at least 5 cc of bulk sediment were dried, weighed, and wet sieved (63 µm sieve). The sand fraction was oven dried at 60° C and weighed to obtain the dry weight percentage of mud and sand fractions. The sand fraction was then dry sieved into five subdivisions (63-125, 125-250, 250-500, 500-1000, >1000 µm) and their respective weights measured. The fine fraction was retained in glass-containers (volume: 5 l), decanted after sedimentation, and stored wet. The average material loss during dry sieving was 0.02% per sample; small particles tended to stick to the sieve. It is therefore assumed that the smaller the fraction, the more material is lost, i.e. least material is lost in the >1000 µm fraction, and most material in the 63-125 µm fraction.
Fig. 7: Comparison of XRD-measurements on fine fraction (<63 µm) and bulk samples. The cross plots
with the amount of HMC, LMC, and aragonite of both data sets shows the reliability of the fine fraction data, which have been used in this study (r2= 0.97 for aragonite; r2 = 0.90 for HMC; r2= 0.94 for LMC).
5.2. Mineralogy
The interpretation of the XRD-measurements focused on the carbonate minerals; aragonite, HMC, LMC, and dolomite. Dolomite however, was not found in any of the samples. The insoluble residue (non-carbonates) mainly comprises quartz and clay minerals (e.g. Smectite, Montmorillonite, Muscovite, Kaolinite).
Following studies of Rendle et al. (2000) the fine fraction signal was used for interpretation. These results are more reliable because the periplatform sediment in the work area is generally very fine, and singular big particles in the sand fraction such as coral fragments or big pteropods, are interpreted to significantly alter the overall grain-size pattern. However, measurements were done on both bulk sediment and fine fraction and then compared to test whether fine-fraction values are representative. Figure 7 shows cross plots of aragonite, LMC, and HMC (fine fraction measurements plotted against bulk sediment measurements), which shows a very good correlation (r2=0.97 for aragonite; r2=0.90 for HMC; r2=0.94 for LMC).
Three grams (dry weight) of bulk sediment and three grams (dry weight) of the fine fraction (<63 µm) of each sample were dried and ground with an achate mortar for three minutes to get a homogenous powder. The powder was then split for XRD and for Leco (see below) measurements. Small samples with low initial weights (some of the Oceanus-samples) could not be split and were therefore measured for XRD first, and then re-used for Leco measurements. XRD-measurements were carried out using a Philips X´Pert Pro MD diffractometer. The radiation was Cu kDand measurements were carried out within a range of 3-65° with 70 seconds per step at a calculated step size of 0.0167°. The peak area of each carbonate mineral was analysed with the MacDiff 4.2.5. software (Petschick, 2001). The results have then been calibrated using the respective calibration curves of Andresen (2000).
5.3. Carbonate and total organic carbon contents
For the calculation of the mineral amounts, the fine fraction values were necessary to calculate the carbonate content. In contrast, for the interpretation of the total organic carbon (TOC) the bulk values were used, as the fine fraction values of TOC are much smaller than its bulk sediment signal (Fig. 8a). Measurements were therefore carried out on both the fine fraction and bulk sediment. Figure 8b shows cross plots of the carbonate amount (fine fraction measurements plotted against bulk sediment measurements), which show that both signals are very close together (r2 = 0.97); these results show the reliability of fine fraction values for the calculation of the carbonate content.
Total carbon (TC) and TOC contents were measured using a Leco CS-200 elemental analyzer (error 1%). The calcium carbonate contents were calculated from the difference of TC and TOC weight percentages using a standard equation based on molecular weights (CaCO3 = [TC – TOC] * 8.33).
Fig. 8: Comparison of total carbon (TC) and total organic carbon (TOC) measurements on bulk sediment and fine fraction (<63 µm). It could be shown that bulk sediments contain much more TOC than the fine
fraction only. In contrast, the calculation of CaCO3 from the TC content reveals similar results for bulk sediment
as for the fine fraction (r2 = 0.97). Values greater than 100% are due to the error of TC-measurements (1%), which is multiplied by the calculation of CaCO3.
5.4. Coarse fraction analyses
Of the Gillies/Trident samples (sample set 1; Table 1), the dry sand fraction was sieved into the size fractions 63-125, 125-150, 150-250, 250-315, 315-400, and >400 µm. For census counts, only fractions >150 µm were used. Each fraction was reduced by a microsplitter to an equivalent containing at least 250 planktonic foraminifera. The error of census-count results depends on the relative percentage of each particle type (e.g. the amount of G. ruber) of the total fraction (van der Plas and Tobi, 1965). For details see Part II, Chapter one, methods section. For identification of the different planktonic foraminifera species the taxonomy of Hemleben et al. (1989) was used. The species were ordered in relation to their resistance to dissolution after Berger et al. (1982): thin-walled, fragile shell types of planktonic foraminifera are more prone to dissolution than thick, robust shells. In addition, pteropods, benthic foraminifera, ostracods, gastropods, bivalves, heteropods, grains, peloids, fragments of pteropods and heteropods, and fragments of planktonic foraminifera were counted.
The dry sand fraction of the ODP Sites 632, 633 and 1006 samples (sample sets 7, 8, 9; Table 1) was sieved into the size fractions 63-125, 125-250, 250-500, 500-1000, and >1000 µm. The fraction 250-500 µm, as the representative one for the sand fraction (Wolf and Thiede, 1991), was used for census counts. It was split by a microsplitter to an equivalent subsample containing at least 300 specimens. The following groups were counted: planktonic foraminifera, fragments of planktonic foraminifera, pteropods and heteropods, fragments of pteropods and heteropods, coral fragments, nodules, and others (which included e.g. benthic foraminifera, sponge spicules, ostracods, gastropods, bivalves, and otoliths). In Hole 1006A, planktonic foraminifera with a micritic overgrowth were counted additionally so that a comparison of the census-count results with previous data from the same core could be made.
To test the representativity of the 250-500 µm fraction, all fractions >125 µm of 17 samples of Hole 633A were counted. Fig. 9 a) and b) show the census-count results for the
Fig. 9: Comparison of census counts of a) >125 µm with b) 250-500 µm fraction.
Circles = planktonic foraminifera, squares = fragments of planktonic foraminifera, diamonds = pteropods, x = fragments of pteropods, crosses = coral fragments, full triangles = nodules, full circles = others. c) cross plot of all samples and counted particles to show the direct correlation of both fractions (r2=0.93).
particles in both size fractions. Planktonic foraminifera and particle types with small percentages (pteropods, fragments of foraminifera, coral fragments, and others) show each similar absolute amounts in both size fractions. For example, fragments of planktonic foraminifera range between 0% and 15% in both the 250-500 µm fraction and the whole fraction >125 µm. Fragments of pteropods are overestimated in the 250-500 µm fraction in shallower samples (above 10 m; Fig 9), but, as they are mostly bound within the matrix of nodules, underestimated in deeper samples where nodule amounts are increased. Nodules are in general slightly overestimated in the 250-500 µm fraction. A good correlation of the results of the fraction >125 µm with the fraction 250-500 µm (Fig. 9c: all particle types of all samples; r2=0.93) suggests that the 250-500 µm fraction may be counted as the representative one for the whole sand fraction.
5.5. Dissolution Indices
A recently developed proxy for aragonite dissolution, the LDX (Gerhardt and Henrich, 2001), was used to determine the aragonite dissolution rate around the highly calcite saturated platform of GBB and the southern Florida shelf. At least ten adult tests of the pteropod species Limacina inflata were picked out of the >500 µm fraction of each sample and classified after six preservation stages (see Part II, Chapter 1, Fig. 2) developed by Gerhardt and Henrich (2001), using a binocular microscope. The preservation stages range from transparent (very well preserved) to opaque-white/totally lustreless/perforated (strongly dissolved). Stage three is established as the threshold to significant dissolution.
The ratio of fragments to whole tests of planktonic foraminifera (Fragmentation Index), of fragments to whole tests of pteropods/heteropods (Aragonite Fragmentation Index, AFX), and several ratios of resistant versus non-resistant species (Resistance Indices, Benthic Foraminifera Index) in terms of dissolution, were calculated for the fraction >150 µm. For an overview of all indices, their exact equations and parameters, and the relevant references, see Part II, Chapter 1, Table 2.
5.6. Scanning Electron Microscopy
Nodules taken from nodule-rich samples of ODP Site 633 (from three different intervals, containing 63-98% nodules) were examined with the Scanning Electron Miscroscope. This revealed the internal structure of the nodules and the composition of the matrix. The nodules were glued on an aluminium stub, sputtered with gold palladium, and
examined using a Zeiss DMS 940A. Part of the nodules were cut to halves to gain better sight on the internal structure.
5.7. Stable isotopes
Oxygen and carbon isotopes of the nodules were measured in order to understand the conditions and location of their formation. The results might indicate if the nodules have formed in an environment which is or was in equilibrium with the bottom water. For measurements of stable isotopes a few nodules were picked out of 20 samples from distinct glacial and interglacial levels between MIS 1 and 23. The nodules were carefully crashed under the binocular to remove whole foraminifera tests and bigger fragments of all kinds that would falsify the signal. The remaining matrix material was then put into an autosampler and measured with a Finnigan MAT 251 mass spectrometer with a Kiel carbonate device (error <0.05‰ for G13C and <0.07‰ for G18O).
6. Organisation of the thesis
The main results have been published in or submitted to international journals. Therefore, part II of this thesis has been divided into three chapters, equivalent to three manuscripts. The first chapter, entitled Controls on modern carbonate preservation in the
southern Florida Straits and published in Sedimentary Geology, focuses on the aragonite
preservation of surface samples from the Florida Straits and from Bahamian intra-platform channels. A profile from 400 to 5000 m water depth revealed the influence of different water masses on the supralysoclinal dissolution of aragonite particles in the sediment.
Chapter two, entitled Compositional variations and early diagenetic processes in
Quaternary periplatform sands: an example from Great Bahama Bank and submitted to
Marine Geology, deals with the preservation of periplatform carbonate sediments around GBB (ODP Sites 632, 633, and 1006). Carbonate concretions (nodules) have been found in the sediments, which have formed during shallow burial diagenesis. The temporal distribution of these nodules shows how early diagenetic processes at these locations depend on the margin type, the bottom water velocity and the pore water chemistry.
Chapter three then deals with the interplay of these nodules and the grain-size distribution of the surrounding sediment in a paper entitled Diagenetic alteration of
periplatform sediments: implications for palaeoenvironmental interpretations based on grain size, submitted to the Journal of Sedimentary Research. Here is shown, that coarser
formation of nodules coarsens the sediment. The observations in three cores around GBB (see above) lead to a model, showing the alteration of different types of initial grain-size patterns due to the formation of nodules.
