Conclusions

large to make any of these non-carbonates a reasonable standard. There is a clear need to define an international carbonate standard for boron isotopic analyses.

Despite the possibility of specific laboratory offsets, relative differences between samples of the same species seem to be constant. Repeated analyses of our cultured samples at SOC revealed a difference of ~2.2‰ between shells grown under HL (δ¹¹B=23.9‰, n=2) and LL (δ¹¹B=21.7‰, n=1). Although the δ¹¹B O. universa was measured ~2‰ heavier at SOC compared to GEOMAR, the relative difference between the two cultured shell samples obtained in both laboratories is the same within error. Relative comparisons with samples of known δ¹¹B-pH relationship are therefore feasible. However, comparison of absolute values raised in different laboratories seems to be inappropriate until identification of the underlying problem.

Acknowledgements

We thank the staff of the Catalina Marine Science Center for providing facilities to make this work possible. We gratefully acknowledge field help by Laurie Juranek, M.

Thomas, H. Iverson and the Catalina dive crew. Invaluable was A. Sanyal’s introduction into the secrets of δ¹¹B analyses and boron enriched culture experiments. Of great benefit were suggestions and comments by R. Zeebe, D. Lea, D. Wolf-Gladrow and H. Fischer and technical discussions with A. Deyhle and N. Gussone. A review by N.G. Hemming was very encouraging and the helpful suggestions are gratefully acknowledged. This research was supported by DAAD grant D/00 20292 (BH), NEBROC (BH, JB), NSF grants OCE-9907044 (ADR), and OCE-9903632 (HJS). Analyses at SOC were funded by a European Commission geochemical analytical facilities grant (BH, MRP).

Publication II

Assessing the reliability of Ba/Ca as a tracer for alkalinity

Bärbel Hönisch, Ann D. Russell, Jelle Bijma, David W. Lea and Howard J. Spero in preparation

...

Abstract

Here we report results of laboratory culture studies showing that seawater alkalinity has a minor effect on the incorporation of Ba into the calcite shells of planktonic foraminifera (Orbulina universa and Globigerina bulloides). The Ba/Ca ratio in foraminiferal shells has been proposed as a proxy for alkalinity. If alkalinity itself has a significant influence on the Ba incorporation into foraminiferal calcite, it would be impossible to use foraminiferal Ba/Ca to differentiate between a change in seawater Ba²⁺ and a coincidental change in alkalinity.

Specimens of the symbiont-bearing species Orbulina universa and the symbiont-barren Globigerina bulloides were grown in seawater of constant Ba²⁺ concentration at five different alkalinities. The experimental alkalinity range comprised levels below, within and above the range presently found in the ocean. We found a weak negative correlation between DBa and alkalinity in O. universa shells under high light conditions: DBa = 0.27 (±0.04) – 4.39 (±1.58)*

10^-5 * AT. For an increase in alkalinity of 100 µmol kg^-1 DBa therefore decreases by 0.004.

This change is well within the error of DBa determined to date. The weak influence of alkalinity on Ba incorporation into foraminiferal shells should not bias paleoreconstructions based on foraminiferal Ba/Ca. 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 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.

Introduction

One of the most intriguing scientific challenges today is to understand the interaction between the atmospheric CO2 budget and the oceanic carbon cycle. Since both reservoirs are tightly linked, information about one of them provides insight about the other. The values of at least two carbonate system parameters are required to calculate the entire oceanic carbonate chemistry. This can be any combination of dissolved inorganic carbon (DIC), alkalinity, pH or related ion concentrations. Lea (1993) suggested that changes in the thermohaline circulation should redistribute Ba²⁺ and alkalinity similarly, thereby allowing reconstruction of past alkalinity distributions from benthic foraminiferal Ba/Ca. Using the modern oceanic relationship between Ba²⁺ and alkalinity, Lea (1993) proposed an increase in alkalinity of approximately 20-25 µmol kg^-1 during the last glacial maximum when compared to the Holocene. This could explain a significant amount of the glacial drop in pCO2.

Although this result is promising, recent research has shown that a number of geochemical proxies are affected by the oceanic carbonate system. For instance, Lea et al.

(1999b) found that seawater pH influences shell Mg/Ca and Sr/Ca in the planktonic foraminifera O. universa and G. bulloides. They attributed increased Sr/Ca under higher pH to higher CO32- concentrations and thereby enhanced calcification rates. Similarly, shell δ¹³C and δ¹⁸O of planktonic foraminifera vary inversely with the carbonate ion concentration of seawater under conditions of constant δ¹³CDIC and δ¹⁸O of seawater (Bijma et al., 1998; Spero et al., 1997). Vital as well as kinetic effects were found to explain the underlying mechanism (Wolf-Gladrow et al., 1999a; Zeebe, 1999; Zeebe et al., 1999). More recently, Russell et al.

(2001, Russell, in prep. #548) observed a correlation between carbonate ion concentration and U/Ca in foraminifera shells. If Ba incorporation into shell calcite is similarly affected, then it would negate the use of this proxy for estimating alkalinity in paleoceans.

The specific motivation for this investigation is based on similarities in Ba²⁺ and Sr²⁺

incorporation into inorganic and biogenic calciumcarbonates. Ba and Sr are metals with ionic radii greater than Ca and Mg (Ba²⁺: 1.47 Å, Sr²⁺: 1.31 Å, Ca²⁺: 1.00 Å, Mg²⁺: 0.72 Å, according to Shannon (1976). Like Mg²⁺, Ba²⁺ and Sr²⁺ substitute for Ca²⁺ in biogenic calcite (Lea and Boyle, 1991; Mackenzie et al., 1983; Speer, 1983). A general tendency has been observed according to which partition coefficients of metals with ionic radii smaller than Ca decrease with increasing precipitation rate, whereas those with an ionic radius larger than Ca²⁺ increase with precipitation rate (Lorens, 1981). Inorganic precipitation experiments have shown that the partition coefficients of Sr²⁺ and also Ba²⁺ depend strongly on precipitation rate (Lorens, 1981; Morse and Bender, 1990; Tesoriero and Pankow, 1996) and observations

on Sr²⁺ in planktonic foraminifera (Lea et al., 1999b), coccolithophorids (Stoll et al., 2001) and apparently also in benthic foraminifera (Elderfield et al., 1996) are in agreement with these inorganic experiments. Especially in the case of planktonic foraminifera Lea et al.

(1999b) showed that Sr/Ca increases in shells grown under higher pH. Under inorganic conditions the adsorption of Ba and Sr (when both equally concentrated in the aqueous solution) onto solid calcite is low but comparable (Zachara et al., 1991). Despite large differences in seawater ion concentration, the relative uptake of Sr and Ba into planktonic foraminiferal calcite (as reflected in the values of DSr and DBa) is similar (Lea, 1999b).

These similarities in the geochemical behavior of Ba and Sr led us to question whether the incorporation of Ba²⁺ into biogenic calcite is affected by seawater carbonate chemistry.

Using laboratory experiments, we explore the influence of alkalinity on Ba/Ca in living planktonic foraminifera.

Experimental Methods

Collection and culturing of foraminifera

Foraminifera were cultured using previously established methods (Lea and Spero, 1992; Mashiotta et al., 1997; Spero et al., 1997). Juvenile (presphere) Orbulina universa and small Globigerina bulloides 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 foraminifera collection site and filtered in the laboratory using acid-cleaned 0.4 µm polycarbonate membrane filters and an acid-leached polysulfone filter holder. After collection, individual foraminifera were examined under an inverted light microscope for identification and inspection of general condition and then transferred to 120 ml glass jars containing the filtered culturing solutions. To avoid contamination of culture water during transfer and feeding of specimens, sample handling was done wearing powder free gloves and using acid-leached glass pipettes for feeding and transferring foraminifera. All culture jars were maintained at a constant temperature in a 22 ± 0.1°C water bath, the approximate summer SST at the collection site.

Foraminifera were grown under the following conditions: 1) a 12-hr high light (HL):12-hr dark cycle where light levels were adjusted to above Pmax (299-365 µmol photons m^-2 s^-1), and 2) a 12-hr low light (LL):12hr-dark cycle (23 µmol photons m^-2 s^-1). These light

levels either exceed the saturating irradiances for symbionts in O. universa or fall below the compensation point for photosynthetic activity (Rink et al., 1998).

During a 7- to 10-day culture period O. universa secretes and calcifies a spherical chamber, whereas G. bulloides forms between two and four new chambers. Globigerina bulloides and O. universa were fed a 1-day old Artemia sp. nauplius (brine shrimp) every other or every third day respectively. After the foraminifera underwent gametogenesis, empty shells were rinsed in ultrapure water and archived for later analysis. In addition, a sample of the culture solution was acidified and archived to verify that the Ba²⁺ concentration remained constant over the course of the experiment. Because the amount of calcite precipitated by foraminifera does not require more than 0.1% of the initial Ca²⁺ present in the culture seawater, Ca²⁺ concentrations were constant over the duration of the experiments.

Total alkalinity (AT) was modified by the addition of ultrapure HCl to lower AT or the addition of ultrapure NaOH to increase AT. Initial and final alkalinity was determined by Gran-titration with a Metrohm 785 titrino auto-titrator. Samples for DIC analysis were taken at the beginning and the end of the experiment, poisoned with a few drops from a saturated HgCl2 solution and measured coulometrically at the Alfred Wegener Institute.

Sample preparation

Only gametogenic individuals were used for analysis. All specimens were rinsed in distilled water to remove sea salts, dried and weighed. Chambers secreted in the laboratory, i.e. under controlled conditions, were separated from the field grown shell. Separating and cleaning procedures followed methods established by Mashiotta et al. (1997). Briefly, shells of O. universa were cracked open with a disposable scalpel and the juvenile test (if present) was removed with a small brush. The fragments were then transferred to 0.5 ml polypropylene centrifuge vials. Individual shells of O. universa shells were analysed where possible, but for the smallest individuals, two or three shells were pooled to obtain at least 40 µg of uncleaned calcite. For G. bulloides, chambers grown in the laboratory were identified by comparing the size of the specimen at collection with the size of the postgametogenic shell. The laboratory-grown chambers were removed with a scalpel, pooled (25-35 chambers per sample) and loaded into 0.5-ml polypropylene centrifuge vials. Samples were then subject to a series of physical and chemical treatments including: oxidation in hot H2O2 – NaOH solution to remove organic matter, 2-3 weak acid leaches (0.001N HNO3) and repeated rinses in ultrapure water.

Analytical Methodology

Sample analysis followed the multi-element inductively coupled plasma mass spectrometry (ICP-MS) method previously described by Lea and Martin (1996). After cleaning, 20-30 µg of purified foraminifera shells were dissolved in 0.5 ml of a 0.1 N HNO3

solution containing calibrated concentrations of ¹³⁵Ba and ⁴⁵Sc. The solutions were aspirated into a Finnigan Element 2 high resolution magnetic sector ICP-MS. The ¹³⁵Ba/¹³⁸Ba and

45Sc/⁴⁸Ca ratios are determined by pulse and analog acquisition modes, respectively. The concentrations of Ba²⁺ and Ca²⁺ are then determined by isotope dilution and internal standard calculation, respectively. Na/Ca was determined to be certain that the hydrogen peroxide – sodium hydroxide solution used in the sample preparation was completely rinsed out. Several analyses of a consistency standard with Ba²⁺ and Ca²⁺ concentrations similar to the foraminiferal samples had a standard deviation of 0.4% for Ba²⁺, 1% for Ca²⁺, and 1.25% for the Ba/Ca ratio. Analyses were performed at the University of California, Santa Barbara.

An average of 2-4 replicates was determined on O. universa. Due to the small sample yield in G. bulloides merely one Ba/Ca analysis per alkalinity could be realised for this species.

Results

Fifteen water samples were randomly selected from 184 total culture experiments to verify the constancy of the trace and minor element concentrations over the course of the experiment. These samples yielded ambient seawater concentrations of 37.8 ± 0.35 nmol kg^-1 and 10 ± 0.09 mmol kg^-1, for Ba²⁺ and Ca²⁺ respectively. The amount of Ba²⁺ incorporated into the foraminiferal shell is negligible compared to the total Ba²⁺ present in the culturing solutions (Lea and Spero, 1992). However, problems could arise from barium contamination (e.g. during feeding) or adsorption onto the container walls. Although one of the water samples showed a substantially higher Ba²⁺ concentration by 11%, the average change in the seawater Ba²⁺ concentration was only 0.9% indicating that barium adsorption was negligible and contamination was unlikely. Nevertheless, we cannot rule out the possibility that some of the foraminifera experienced Ba²⁺ concentrations that may have differed slightly from the average of 37.8 nmol kg^-1.

Table 4. Experimental data for cultured shells

experiment alkalinity µmol kg^-1

light level µmol photons m^-2s^-1

Ba/Ca µmol mol^-1

sample weight µg

n DBa

Orbulina universa

BH-3 2047 332-349 0.76 52 3 0.189 ± 0.017

0.67 50 2

0.60 65 1

BH-6 2122 299 0.57 46 2 0.165 ± 0.018

0.70 59 2

0.61 39 1

BH-1 2253 23 0.70 58 3 0.164 ± 0.015

0.57 37 2

0.59 40 2

0.63 48 1

BH-2 2268 365 0.71 51 2 0.177 ± 0.01

0.67 54 2

1.06 * 45 1

0.65 114 1

BH-5 2436 332-365 0.64 56 1 0.163 ± 0.014

0.56 40 1

1.01 * 40 1

0.56 68 1

BH-4 2632 299 0.58 69 2 0.154 ± 0.004

0.60 39 3

0.57 57 1

Globigerina bulloides

BH-3 2047 332-349 0.69 6.2 25 0.183

BH-1 2253 23 0.65 34.1 35 0.173

BH-2 2268 365 0.55 14.7 25 0.146

BH-4 2632 299 0.70 26.6 25 0.184

n = number of pooled shells per analysis; * = rejected

Ba/Ca-variability within an experimental group is larger than the analytical error in measuring the ratio. Replicate samples were excluded from means and figures (but not from tables) if the Ba/Ca differed by more than 2σ from the experimental mean Ba/Ca. Using the experimental seawater concentrations of Ba²⁺ and Ca²⁺ to calculate the partition coefficient for O. universa yields a linear decrease of DBa from 0.189 ± 0.017 at AT=2047 µmol kg^-1 to 0.154 ± 0.004 at AT=2632 µmol kg^-1 (Fig. 9, Table 4). Statistical analysis using an ANOVA variance ratio test indicates the DBa decrease with increasing alkalinity is significant at the 90% confidence level. Linear regression yields:

DBa = 0.27 (±0.037) - 4.39 (±1.58)* 10^-5 * A^T R² = 0.37

0.12 0.14 0.16 0.18 0.20 0.22

2000 2100 2200 2300 2400 2500 2600 2700

O. universa

D Ba

alkalinity (µmol kg ^-1)

D_Ba = 0.27 (±0.037) - 4.39 *10^-5 (±1.58 *10^-5)* A_T R² = 0.37

Figure 9. DBa in the subtropical, spinose, symbiont-bearing foraminifera Orbulina universa vs. alkalinity. Closed symbol indicates low light conditions. Each point is the average of two to four analyses of either individual or 2-3 pooled spheres (Table 4). Error bars are standard deviations of replicate analyses. The regression is a linear fit through the individual O. universa points at saturating light levels.

0.12 0.14 0.16 0.18 0.20 0.22

2000 2100 2200 2300 2400 2500 2600 2700

G. bulloides

D Ba

alkalinity (µmol kg ^-1)

Figure 10. DBa in the subpolar, spinose planktonic foraminifera Globigerina bulloides compared to alkalinity. Each point is an individual analysis of multiple amputated chambers (Table 4).

DBa in G. bulloides ranges between 0.146 and 0.184 (Fig. 10). These partition coefficients are comparable to the range observed in O. universa shells over the same alkalinity range. However, given the small number of data points and non-linear distribution of these data, it is not possible to determine whether or not a relationship between alkalinity and DBa exists in G. bulloides.

In a final experimental series, we investigated the influence of symbiont photosynthetic rate (as it varied with light levels) on O. universa DBa in ambient seawater (AT

= 2257 µmol kg^-1). Under LL conditions DBa= 0.164 ± 0.015 was slightly lower than under HL where DBa= 0.177 ± 0.01 (Fig. 9, Table 4). However, these partition coefficients are not significantly different from each other.

Discussion

Recordability of Ba/Ca in planktonic foraminiferal shells

The average partition coefficients for Ba/Ca in O. universa and G. bulloides found in this study are DBa= 0.166 ± 0.01 and DBa= 0.171 ± 0.02, respectively. Lea and Spero (1994) calculated a value of DBa= 0.16 ± 0.01 (regression forced through zero) for cultured O.

universa. The combined partition coefficients of core-top fossil planktonic foraminifera yield DBa= 0.19 ± 0.05 (Lea and Boyle, 1991). Hence, our values are in good agreement with the literature.

Globigerina bulloides has not been calibrated for Ba before. Although this species is symbiont-barren, the DBa is similar to that of the symbiont-bearing O. universa and G.

sacculifer. This finding is in contrast to other elements which are incorporated to different degrees by symbiont-bearing and symbiont-barren species. For instance, Mashiotta et al.

(1997) observed strong contrasts between O. universa and G. bulloides in the uptake of Cd²⁺. They suggested that symbiotic dinoflagellates either influence foraminiferal incorporation of Cd²⁺ by sequestering Cd²⁺ from the calcifying microenvironment of O. universa, or by photosynthetically enhancing calcification rate leading to Cd²⁺ exclusion. In contrast, Ba has no reported biochemical function and in line with the similarity of HL and LL results for O.

universa, the uniformity of our results suggests that symbionts have no influence on the Ba uptake.

In order to examine DBa in relation to calcification rate we refer to Mashiotta et al.

(1997) who estimated higher calcification rates for O. universa (~3000 µmol m^-2 h^-1, Lea et

al., 1995) than for G. bulloides under identical culture conditions (~1700-2400 µmol m^-2 h^-1).

This difference is not reflected in the uptake of Ba during shell formation and the similarity in DBa between O. universa and G. bulloides supports the generally held assumption that species-specific differences are low for planktonic foraminifera and vital effects are not important for the Ba uptake.

The observed negative correlation between Ba/Ca and alkalinity was not expected from theoretical arguments which would have predicted a kinetic effect towards higher Ba/Ca with increasing precipitation rate. To better compare the magnitude of the observed effect with results on Sr/Ca, we also plotted Ba/Ca versus pH. Total CO2 was constant in our experiments so that pH and [CO32-] varied almost linearly with the experimental alkalinity range. While the kinetic effect documented for the pH-dependency of Sr/Ca is ~+1% per 0.1 pH unit (Fig. 4, Lea et al., 1999b), Ba/Ca decreases by ~2% per 0.1 pH unit (Fig. 11b). The observed effect is thus similar in magnitude but opposite in direction.

The geochemical similarities between Sr and Ba discussed earlier do not extend to the influence of alkalinity (or pH). This may be due to dissimilarities in sorptive behavior because of differences in aqueous complexation (e.g. (van Cappellen et al., 1993). Alternatively, since the mineral growth in foraminifera appears to follow a pattern which is predetermined by the molecular organisation of the organic template (Langer, 1992), it may be that the organic matrix controls the selection of certain divalent cations and discriminates against others.

Unfortunately the process of chamber formation and calcification in foraminifera (e.g. Bé et al., 1979; Hemleben et al., 1977; Spero, 1988) is not very well understood at this level so that this consideration remains rather speculative.

Paleoceanographical significance

The negative slope determined for O. universa under HL conditions implies that the error yields a change in DBa of 0.004 (~ 0.017 µmol mol^-1 in Ba/Ca) if alkalinity were to change by 100 µmol kg^-1. With such a large alkalinity change, ∆DBa would still be well within the analytical error of the partition coefficient. For comparison, Lea (1993) found an increase of 1.8 µmol mol^-1 (~50 %) in the Ba/Ca of Circumpolar Deep Water during the last glacial period. The alkalinity-dependency of Ba/Ca (<1 % per 10 µmol kg^-1) would be negligible for the proposed changes in alkalinity on glacial/interglacial time scales. Even under glacial

1.10 1.15 1.20 1.25 1.30 1.35 1.40 1.45 1.50

7.5 8 8.5 9

Sr uptake vs. seawater pH

Sr/Ca (mmol/mol)

pH (NBS)

Sr/Ca = 0.18 + 0.138 * pH R² = 0.46

a

0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85

7.5 8 8.5 9

Ba uptake vs. seawater pH

Ba/Ca (µmol/mol)

pH (NBS)

Ba/Ca = 1.56 - 0.113 * pH R² = 0.38

b

Figure 11. a) Sr/Ca in O. universa. Data were taken from Lea et al. (1999b). Shell Sr/Ca increase approximately 1% per 0.1 pH units. b) Ba/Ca in O. universa versus seawater pH.

Shell Ba/Ca decrease approximately 2% per 0.1 pH units. Theoretical arguments would have predicted an increase similar to the one observed in Sr/Ca.

conditions, when pH in surface waters may have increased by 0.2 pH units (Sanyal et al., 1995), alkalinity would not be expected to exceed the range investigated in this study. In contrast, the deep water value of +0.3 pH units as determined by Sanyal et al. (1995) for the LGM would require an increase in alkalinity by more than 300 µmol kg^-1 in order to bring estimated pH and observed atmospheric pCO2 values into line. Alkalinity changes on this

order could lead to a potentially large Ba/Ca change. However, the deep water pH estimate is based on a sample of mixed benthic foraminifera species and as such not very reliable. In fact, recent data by Anderson and Archer (2002) argue against such a strong increase in pH.

Conclusions

Our results demonstrate that species variability and symbiont effects are not important for planktonic foraminifera. Orbulina universa and G. bulloides incorporate Ba in a similar ratio to seawater concentration. Water column alkalinity and Ba/Ca are at most weakly linked for these species, and the effect is too small to be significant for paleoceanographic reconstructions. Alkalinity, the closest related component to the oceanic barium cycle, can therefore be extensively ignored as a potential factor directly affecting the incorporation of Ba²⁺ into planktonic foraminiferal calcite.

Acknowledgements

We thank the staff of the Catalina Marine Science Center for providing facilities to make this work possible. We gratefully acknowledge field help by Laurie Juranek, Megan Thomas, Heidi Iverson and the Catalina dive crew. The experiments could not have been successfully completed without the laboratory work of Georges Paradis and Dotti Pak.

Support and comments from Christoph Völker and Hubertus Fischer are greatly appreciated.

This research was supported by DAAD grant D/00 20292 (BH), NEBROC (BH), NSF grant OCE-9907044 (ADR), OCE-9906821 (DWL), OCE-9729203 and OCE-9903632 (HJS).

Publication III

Post-depositional effects on trace metals and stable isotopes in foraminiferal calcite – Evidence from dissolution experiments

Bärbel Hönisch, Jelle Bijma, Nikolaus Gussone, Howard J. Spero, Dirk Nürnberg, David W. Lea and A. Eisenhauer

in preparation

...

Abstract

Sediment observations and laboratory dissolution experiments have increased paleoceanographers' interest in post-depositional changes of chemical proxies recorded in foraminifera shells. While previous studies are limited by a number of uncertainties, young proxies like δ¹¹B and δ⁴⁴Ca have not yet been regarded in the light of dissolution. We used well preserved planktonic foraminifera species Globigerinoides sacculifer and Neogloboquadrina pachyderma (sinistral coiling) to partially dissolve them under controlled conditions in the laboratory, simulating the natural processes at the seafloor. In addition to known dissolution effects on Mg/Ca, δ¹⁸O and δ¹³C, we observe significant effects on Sr/Ca and δ¹¹B on the order of glacial/interglacial changes. Discussing previously hypothesized explanations for partial dissolution, it will be shown 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, δ¹¹B and δ⁴⁴Ca seem to be inconsistent with such a process and other explanations have to be investigated instead. Mg impurities were found to be insufficient to reduce the overall stability of certain foraminiferal shell components and increasing Sr/Ca ratios demonstrate that crystal impurities are not necessarily more prone to dissolution. The microstructural breakdown of shell surfaces is systematic and may be useful to estimate the preservation state of shells deposited in sediments.

Dissolution drives deep fissures into the shell wall, indicating that corrosion does not only remove outer layers of a shell but also increases shell porosity. The resulting exposition of

otherwise protected lattice areas possibly allows certain elements to be leached out. Dissolution effects appear to be species-specific and depend on the physico-chemical gradients encountered by vertically migrating foraminifera at different locations.

Introduction

The chemical composition of biogenic carbonates derived from marine sediments is routinely used for paleoceanographic and paleoclimatic reconstructions. One of the basic assumptions is that the primary signal remains unaltered after burial in the geological archive.

However, observations on sediment cores reveal significant variability in the shell chemistry of planktonic foraminifera that can not be linked to oceanographic or climatologic changes in the organism’s habitat. Part of the observed variability is assumed to be caused by dissolution. In order to test this assumption, dissolution studies have focussed primarily on two experimental approaches: (1) empirical observations on carbonate samples deposited in sediments at various depths (e.g. Berger, 1967; Brown and Elderfield, 1996; Honjo and Erez, 1978; McCorkle et al., 1995; Zachos et al., 1994); and (2) the determination of CaCO3 dissolution kinetics and changing chemical composition in the laboratory (e.g. Bé et al., 1974; Bender et al., 1975; Boyle, 1988a;

Caron et al., 1990; Lea and Boyle, 1991; Lorens et al., 1977). Based on these and other experiments, several hypotheses have been proposed:

1. Shell Mg increases the susceptibility to dissolution (Brown and Elderfield, 1996; Lorens et al., 1977; Savin and Douglas, 1973).

2. Dissolution selectively removes the more soluble individuals and thus reduces their contribution to the chemical bulk signal (Erez, 1979b).

3. Elements that are distributed homogeneously throughout a shell are not affected by dissolution (Bender et al., 1975; Lea and Boyle, 1991; McCorkle et al., 1995).

4. Selective dissolution of ontogenetic calcite shifts bulk Mg/Ca and δ¹⁸O toward the chemistry of the secondary crust (Brown and Elderfield, 1996; Dekens et al., 2002; Lohmann, 1995;

Rosenthal et al., 2000; Russell et al., 1994).

5. Although dissolution alters Mg/Ca and δ¹⁸O, the initial relationship remains constant such that paired analyses of δ¹⁸O and Mg/Ca can be used to reconstruct the δ¹⁸O of seawater (δ¹⁸Ow) (Rosenthal et al., 2000).

Im Dokument Stable isotope and trace element composition of foraminiferal calcite - from incorporation to dissolution (Seite 43-65)