Abhijit Sanyal

OCEANIC PH CONTROL ON THE BORON ISOTOPIC COMPOSITION OF FORAMINIFERA : EVIDENCE FROM CULTURE EXPERIMENTS

Culture experiments were carried out with the species Orbulina universa at four different pH values (7.70±0.05, 8.15±0.05, 8.60±0.05, and 9.00±0.10) in order to establish the pH-dependence of boron isotope fractionation between seawater and foraminifera. A clear relationship between the boron isotopic composition of the foraminifera and the pH of the seawater culture solutions was found, showing heavier boron isotopic composition at higher pH. This finding supports the viability of boron isotopes as a paleo-pH tool. It is important to note that Orbulina universa cultured in natural seawater, as well as those obtained from coretop samples, have a significantly lighter boron isotopic ratio than Globigerinoides sacculifer from coretop samples, suggesting that at least for this species, a vital effect is active.

Seawater pH control on the boron isotopic composition of calcite: evidence from inorganic calcite precipitation experiments

Experiments involving boron co-precipitation with calcite have been carried out inorganically under controlled pH conditions (7.9 ± 0.05, 8.3 ± 0.05 and 8.6 ± 0.05) to determine the dependence of the boron isotopic composition (δ11B) of calcite on the pH of seawater. Another purpose of these experiments was to estimate the magnitude of the biogenic influence on the δ11B value of foraminifera by comparing their boron isotopic composition with that of the inorganic calcite over a common pH range. The results show a clear relationship between δ11B of inorganic calcite and the pH of artificial seawater. The variation of boron isotopic fractionation between seawater and calcite with pH, estimated from these experiments, is similar to that estimated for cultured O. universa and the theoretically predicted trend. The results also support the hypothesis that B(OH)4− is the dominant species incorporated into the calcite structure. However, the boron isotopic fractionation between seawater and inorganic calcite is lower than that estimated for O. universa indicating the presence of a biogenic effect on the boron isotopic composition at least of this species of foraminifera. Most importantly, the results imply that in spite of a small biogenic influence on the boron isotopic composition of foraminifera, the variation in δ11B of foraminiferal shells with pH (at least for O. universa) is comparable to that for inorganic calcite, supporting the potential of this isotopic signature in foraminifera as a reliable paleo pH proxy.

Changes in pH in the eastern equatorial Pacific across stage 5–6 boundary based on boron isotopes in foraminifera

Estimates of paleo-pH for the eastern equatorial Pacific Ocean across the oxygen isotopic stage 5–6 boundary have been made based on the boron isotopic composition of planktonic (Orbulina universa) and benthic (mixed species) foraminifera from core V19-28. The estimated deep ocean pH during the penultimate glacial period was about 0.3 ± 0.1 pH units higher compared to the modern deep ocean. This is consistent with previously estimated deep ocean pH changes across the stage l–2 boundary in the western equatorial Pacific and tropical Atlantic, thus arguing against the possibility that the benthic foraminifera analyzed to estimate deep ocean pH changes have been significantly affected by anomalous local environment and/or diagenesis. The estimated changes in the deep ocean carbonate chemistry require a decoupling (of several kilometers) between the saturation horizon and the lysocline during the glacial periods. Though such a decoupling could be achieved by enhanced respiration CO2 driven calcite dissolution in sediments during glacial periods, it lacks support from the calcite sedimentary records. The boron isotopic compositions of planktonic foraminifera, on the other hand, indicate no significant pH change in the eastern equatorial Pacific surface ocean during the glacial-interglacial transition. This is inconsistent with an expected higher surface ocean pH during the glacial period due to lower atmospheric pCO2 and is also in contrast with the previously estimated boron isotope based glacial-interglacial pH change of 0.2 ± 0.1 pH units in the western equatorial Pacific and tropical Atlantic. The lack of change in eastern equatorial Pacific surface ocean pH between glacial-interglacial periods could be attributed to less nutrient utilization efficiency and/or enhanced calcite production during glacial periods. Such a decrease in nutrient utilization efficiency and/or increase in calcite production would lead to a greater disequilibrium between the pCO2 of eastern equatorial Pacific surface ocean and that of the atmosphere, making this part of the ocean a greater source of CO2 to the atmosphere during glacial periods compared to today.

Comparison of two potential strategies of planktonic foraminifera for house building: Mg2+ or H+ removal?

Marine organisms must possess strategies enabling them to initiate calcite precipitation despite the unfavorable conditions for inorganic precipitation in surface seawater. These strategies are poorly understood. Here we compare two potential strategies of marine calcifyers to manipulate seawater chemistry in order to initiate calcite precipitation: Removal of Mg2+ and H+ ions from seawater solutions. An experimental setup was used to monitor the onset of inorganic precipitation on seed crystals as a function of the Mg2+ concentration and pH in artificial seawater. We focused on precipitation rates typical for biogenic calcification in planktonic foraminifera (∼10−3 mol m−2 h−1) and time scales typical for the initiation of calcification in these organisms (minutes to hours). We find that the carbonate ion concentration has to increase by a factor of ∼13 when [Mg2+] increases from 0 to 53 mmol kg−1 in order to maintain a typical biogenic precipitation rate. Model calculations for the energy requirement for various scenarios of Mg2+ and H+ removal including Ca2+ exchange and CO2 diffusion are presented. We conclude that the more cost-effective strategy to initiate calcite precipitation in foraminifera is H+ removal, rather than Mg2+ removal.

A theoretical study of the kinetics of the boric acid-borate equilibrium in seawater

y . Dissolved boron in seawater occurs mainly in the form of boric acid B OH and borate B OH . While the 34 equilibrium properties of the dissociation of boric acid have been studied in detail, very little work has focused on the kinetics of the boric acid-borate equilibrium in seawater. Here, we present a theoretical study of the relaxation of the w

The origin of Bahamian Whitings revisited

Two schools of thought exist regarding the origin of the aragonite needles which make up the milky patches of water prominent on the Bahama Banks. One school views these so-called whitings as spontaneous precipitates [Cloud, 1962; Shinn et al.,1989; Robbins et al.,1997] and the other views them as stirred up bottom sediment [Broecker and Takahashi, 1966; Morse et al.,1984]. In the paragraphs which follow we summarize what we consider to be iron-clad radiocarbon and chemical evidence that whitings are dominated by re-suspended sediment. We offer a new and highly speculative mechanism for this re-suspension. Black-tipped sharks which inhabit whitings purposefully stir up the sediment in order to create a trap for fish, much as spiders construct webs as traps for insect prey.

Does atmospheric CO2 police the rate of chemical weathering

A case is made that in the absence of an effective feedback control on the rate of delivery of CaO to the oceans, the CO 2 content of the Earths atmosphere would have wandered over a large range threatening life either by overheating or by carbon dioxide starvation. In this paper, we defend the suggestion by Walker et al. [1981] that control is exerted by the interaction between the CO 2 content of the atmosphere and the continental weathering rates. We contend that in spite of the arguments raised against it [Raymo and Ruddiman, 1992; Edmond and Huh, 1997] the CO 2 -chemical weathering feedback is the dominant mechanism that stabilizes the atmospheric carbon dioxide content.

A comparative study of the northwest Africa and eastern equatorial Pacific upwelling zones as sources of CO2 during glacial periods based on boron isotope paleo-pH estimation

Paleo-pH reconstructions based on boron isotopic composition of foraminifera have been used to estimate glacial-interglacial changes in surface ocean pCO2 of the northwest African upwelling zone. On comparison with a similar study for the eastern equatorial Pacific upwelling zone, it can be concluded that the two major upwelling zones acted quite differently during the glacial periods as compared to today. While the pH of the surface ocean off northwest Africa was 0.2±0.07 units higher during the glacial period compared to that during Holocene, there was no significant glacial-interglacial change in the surface ocean pH in the eastern equatorial Pacific. Carbonate chemistry reconstructions based on the estimated pH changes suggest that the ocean-atmosphere pCO2 gradient off northwest Africa was lower by at least 70±40 µatm during glacial periods compared to during the Holocene. In contrast, the ocean-atmosphere pCO2 gradient in the eastern equatorial Pacific was higher by at least 80±40 µatm during glacial periods as compared to during the Holocene. Hence the eastern equatorial Pacific upwelling system was a significantly larger source of CO2 to the atmosphere, while the one off northwest Africa was a significantly smaller source of CO2 during the last glacial period. The pCO2 reconstructions further indicate that in spite of higher glacial productivity compared to during the Holocene, neither of the two areas became a sink of CO2.

Vital effects and beyond: a modelling perspective on developing palaeoceanographical proxy relationships in foraminifera

Abstract This paper mainly reviews our recent work on the biology and geochemistry of foraminifera with respect to their use as palaeoceanographic proxies. Our approach to proxy validation and development is described, primarily from a modelers point of view. The approach is based on complementary steps in understanding the inorganic chemistry, inorganic isotope fractionation, and biological controls that determine palaeo-tracer signals in organisms used in climate reconstructions. Integration of laboratory experiments, field and culture studies, theoretical considerations and numerical modelling holds the key to the methods success. We describe effects of life-processes in foraminifera on stable carbon, oxygen, and boron isotopes as well as Mg incorporation into foraminiferal calcite shells. Stable boron isotopes will be used to illustrate our approach. We show that a mechanism-based understanding is often required before primary climate signals can be extracted from the geologic record because the signals can be heavily overprinted by secondary, non-climate related phenomena. Moreover, for some of the proxies, fundamental knowledge on the thermodynamic, inorganic basis is still lacking. One example is stable boron isotopes, a palaeo-pH proxy, for which the boron isotope fractionation between the dissolved boron compounds in seawater was not precisely known until recently. Attempts to overcome such hurdles are described and implications of our work for palaeoceanographic reconstructions are discussed.

Magnitude of the CaCO3 dissolution events marking the onset of times of glaciation

The availability of CaCO3 and ³He content results for core TT 13-72 from 4.3 km depth in the equatorial Pacific [Marcantonio et al., 1996] allows the magnitude of the excess (i.e., over ambient) CaCO3 dissolution at the onset of marine glacial stages 10, 8, and 6 to be estimated. These three events are remarkably similar; during each an integrated loss of about 28 g CaCO3 per cm² occurred. While the magnitude of this loss is consistent with that expected from the interglacial to glacial pH shifts reconstructed based on boron isotope measurements on benthic foraminifera [Sanyal et al., 1995], measurements at a number of other locations and water depths will be required before this approach can be used to evaluate the global toll of these dissolution events.