Seawater Calcium Concentrations Research Articles

Electrochemical ocean alkalinity enhancement using a calcium ion battery

The oceans play a major role in moderating atmospheric CO2 levels. Enhanced CO2 uptake into ocean waters can be achieved by the provision of appropriate cations to the surface ocean, an approach known as ocean alkalinity enhancement (OAE). Here, we present a calcium ion battery approach that enhances alkalinity via electrochemical manipulation of seawater calcium concentrations. We demonstrate the efficacy of this approach using a potassium barium iron cyanide [K2BaFe(CN)6] (PBFC) electrode, a Prussian blue analogue, to move calcium ions from one reservoir of seawater to another. Using material and electrochemical characterization of the Ca2+ ion insertion and expulsion properties of PBFC in synthetic seawater, we determine the repeatability of Ca2+ ion insertion and expulsion from the PBFC electrode. Our analyses prove a 2.75 % increase in seawater alkalinity via the PBFC electrode, which yields 2.64 mg CO2 (0.72 mg C) uptake per liter of seawater. This proof-of-concept method offers a unique, low-cost, energy efficient electrochemical approach for atmospheric carbon dioxide removal that can combine with marine-based renewable energy to enable a new family of effective, scalable climate change solutions.

Estimating Coccolithophore PIC:POC Based on Coccosphere and Coccolith Geometry

AbstractThe marine biological calcification and photosynthesis, which can produce particulate inorganic and organic carbon (PIC and POC), have the opposite effects on seawater pCO2. Coccolithophores are a kind of marine unicellular algae with both of the two biological processes, and PIC and POC productions of them can shape the water column rain ratio as a dominant driver for Earth's carbon cycle. Thus, the changes in ancient coccolithophore PIC:POC can be important for the paleoceanographic and paleoclimatic studies of carbon cycle modeling. However, ancient coccolithophore PIC:POC is poorly constrained because of the occasional occurrences of intact coccospheres in deep ocean sediments, as detached coccoliths are commonly the remnants of fossilized coccolithophores. Here, we carry out the biometric analysis of coccosphere and coccolith from the living cells of Emiliania huxleyi and Gephyrocapsa oceanica in the South China Sea, and confirm a significant correlationship between their PIC:POC and lateral coccolith aspect ratio (ARL). ARL here is defined by the ratio of mean coccolith thickness with respect to coccolith length. A linear regression is given, (R2 = 0.59, n = 121), for the reconstruction of ancient Noelaerhabdaceae coccolithophore PIC:POC based on individual coccoliths in marine sediments. Based on this equation, we reconstruct ancient Noelaerhabdaceae coccolithophore PIC:POC since 14 million years ago (Ma) using published coccolith data, which reveal a long‐term decrease in PIC:POC from 7 to 4 Ma. We suggest that such a change in coccolithophore physiology may be induced by a simultaneous long‐term decline in seawater calcium concentration.

Salinity Effect on Trace Element Incorporation in Cultured Shells of the Large Benthic Foraminifer Operculinaammonoides

AbstractThe ratio of sodium to calcium in the shells of foraminifera (Na/Cashell) has been experimentally calibrated as a proxy for past ocean Ca concentrations (Hauzer et al., 2018, https://doi.org/10.1016/j.epsl.2018.06.004). In parallel, it has been suggested that Na/Cashell could be used as a proxy for paleo‐salinity. In this study, we determined the extent to which foraminiferal Na/Ca (and other elements) change with salinity for the shallow‐dwelling large benthic foraminifer Operculina ammonoides, an extant relative of the abundant Eocene Nummulites. The culture experiment was conducted under four salinities between 33 and 43 psu. Shell chemistry was measured by LA‐ICPMS with the newly precipitated CaCO3 identified by a 135Ba‐spike added to the experimental seawater. Na/Cashell, Mg/Cashell and Li/Cashell in O. ammonoides increased slightly with salinity, while Sr/Cashell showed no resolvable change. The change in Na/Cashell due to salinity was small (∼1.4%/psu) compared to the changes in this ratio caused by varying seawater calcium concentrations (Casw) with a sensitivity of ∼5%/(mmol kg−1) Casw. Moreover, the change in salinity in most regions of the past open oceans is minor compared to the large secular variations in Casw during the Phanerozoic (10–40 mmol kg−1). Thus, if at all, paleo‐salinity may be reconstructed based on Na/Cashell only for samples younger than Casw residence time (∼1 Myr). Furthermore, both regional and global changes in ocean salinity over geological time do not pose a significant complication for the use of Na/Cashell as a proxy for past changes in seawater calcium concentrations.

Planktic foraminiferal Na/Ca: A potential proxy for seawater calcium concentration

The reconstruction of seawater calcium concentration ([Ca2+]SW) in the geologic past is crucial to our understanding of biogeochemical processes and elemental cycling as linked to long-term climate change. Published [Ca2+]SW estimates for the Cenozoic differ from each other in both the direction and magnitude of the change, and are associated with large uncertainties. Here we demonstrate the potential of Na/Ca in planktic foraminifera as a new proxy for reconstructing Cenozoic variations in seawater Na/Ca. Because of the long oceanic residence time of Na+ (≫50 Myr), variations in foraminiferal Na/Ca should in principle reflect changes in [Ca2+]SW. Our culture experiments on live planktic species validate the approach, showing that foraminiferal Na/Ca responds to changes in [Ca2+]SW when [Na+]SW is kept constant, consistent with previous experiments on a shallow water benthic foraminifer and on inorganic calcite. The culture study suggests that planktic Na/Ca is affected, to a small extent, by calcite saturation state (Ωcalcite) but not resolvably affected by temperature or salinity. A core tops transect of the planktic foraminifer Trilobatus sacculifer shows similar decreasing trends in Na/Ca and Sr/Ca with water depth that can be associated with dissolution of the tests. The strong covariance with Sr supports the hypothesis that a dominant fraction of the Na resides in lattice positions within the calcite test.Down core planktic foraminiferal records from the Atlantic and Pacific Oceans consistently show 13–28% lower Na/Ca during the mid-Miocene than during the late Pleistocene. After considering the effects of temperature, salinity and diagenesis, we conclude that the observed down core decrease in Na/Ca primarily reflects changes in seawater calcium concentration. Using the calibrations generated from our culture experiments and core top data we reconstructed [Ca2+]SW, suggesting [Ca2+]SW in the mid-Miocene was 46 ± 22% higher than at present, and decreased toward present with a pattern resembling the Neogene climate evolution represented by the benthic δ18O record. The new reconstruction of [Ca2+]SW for the past 16 Myr narrows the range suggested by other [Ca2+]SW proxies, and potentially offers a new way to generate continuous records of seawater calcium concentration at sub-million years resolution.

On calcium-to-alkalinity anomalies in the North Pacific, Red Sea, Indian Ocean and Southern Ocean

An important factor for predicting the effect of increased CO2 on future acidification of the ocean is a proper understanding of the interactions controlling production and dissolution of calcium carbonate minerals (CaCO3). The production and dissolution of CaCO3 in the ocean can be assessed over large spatial scales by measuring seawater calcium concentrations and total alkalinity (AT), yet past studies suggest that there could be large discrepancies between calcium and AT-based balances of the CaCO3 cycle in the North Pacific and Indian Oceans. Here, we analyse water column samples collected along transects in the North Pacific, Southern Ocean, tropical Indian Ocean and Red Sea for their concentrations of calcium, nutrients, and AT. We find that there is an excess calcium over AT anomaly in the top 1000 m of the tropical Indian Ocean water-column. The source of this anomaly is the dissolution of subsurface gypsum deposits in the Red Sea. We find no evidence for calcium-over-AT anomalies in the North Pacific, in contrast to previous studies. Our results show that, in most cases, calcium and AT data agree well and can be used to reconstruct the marine CaCO3 cycle.

Testing the Influence of Changing Seawater Ca Concentration on Elements/Ca Ratios in Planktic Foraminifera: A Culture Experiment

AbstractElement/Ca ratios in foraminifera shells are used to assess oceanic physical‐chemical properties (e.g., paleo‐temperature, paleo‐pH…). On geological time scales, these proxies may also depend on changes in the chemical composition of seawater. Here we investigate the effect of changing seawater calcium concentration on the Li/Ca, Mg/Ca, Sr/Ca, B/Ca, and Na/Ca ratios in low‐Mg calcite shells of the planktic foraminifer Globigerinella siphonifera. Specifically, we evaluate the possibility that Na/Ca in planktic foraminifera shells may serve as a proxy for Ca concentrations in the past ocean. Since Ca concentration in today's open ocean is largely conservative with relation to salinity, field calibrations are not possible. Therefore, we cultured the planktic foraminifer (G. siphonifera) collected from the Gulf of Eilat (Red Sea) in variable Casw (10.5–29 mmol/kg), at constant temperature (25°C) and under two salinities (37 and 40.6). When changing the Casw of the growth medium, the relationships between Mg/Ca and Na/Ca in the shells and their corresponding ratios in the seawater are best described by power functions implying a variable distribution coefficient function. For Sr/Ca and Li/Ca, the relations between the shells and the seawater are described by linear functions intersecting the origin implying their distribution coefficients are insensitive to changes in Casw. The results also confirm that foraminiferal B/Ca is not influenced by seawater Casw. We conclude that Na/Ca ratios in planktic foraminifera may serve as reliable proxy for Cenozoic ocean Ca, and that other El/Ca ratios may be used to determine their past concentrations relative to Ca.

Effect of Eh and pH on the flotation of enargite using seawater

The presence of enargite in porphyry-copper deposits is an important matter as this sulfosalt displays good floatability in the presence of the collectors which are usually used in the flotation of copper sulfides, thus it readily floats and contaminate the copper concentrates. Scarcity of water resources is a relevant problem in the Chilean mining industry and the use of seawater in the mineral processing plants is today a feasible solution. The flotation behaviors of copper sulfides and molybdenite in seawater was studied to some extent, but the enargite behavior in this aqueous medium has seldom been investigated. The objective of this work was to study the flotation behavior of enargite in seawater under different pulp electrochemical potentials (Eh) and pH. The independent effects of calcium and magnesium were also evaluated. The experimental results show that enargite flotation in seawater is depressed at both reducing and oxidizing conditions at pH = 9. At pH = 10.5 enargite is strongly depressed to values even below 20% at the whole range of Eh studied which may be explained by the adhesion of magnesium hydroxo-complexes to the enargite surfaces rendering it hydrophilic. The depressing effect of calcium on enargite flotation is stronger than the depressing effect of magnesium, although calcium concentration in seawater is lower. This conclusion may be related to the presence of highly stable compounds such as hydrophilic calcium arsenates on the enargite surface.

The role of calcium in regulating marine phosphorus burial and atmospheric oxygenation

The marine phosphorus cycle plays a critical role in controlling the extent of global primary productivity and thus atmospheric pO2 on geologic time scales. However, previous attempts to model carbon–phosphorus-oxygen feedbacks have neglected key parameters that could shape the global P cycle. Here we present new diagenetic models to fully parameterize marine P burial. We have also coupled this diagenetic framework to a global carbon cycle model. We find that seawater calcium concentration, by strongly influencing carbonate fluorapatite (CFA) formation, is a key factor controlling global phosphorus cycling, and therefore plays a critical role in shaping the global oxygen cycle. A compilation of Cenozoic deep-sea sedimentary phosphorus speciation data provides empirical support for the idea that CFA formation is strongly influenced by marine Ca concentrations. Therefore, we propose a previously overlooked coupling between Phanerozoic tectonic cycles, the major-element composition of seawater, the marine phosphorus cycle, and atmospheric pO2.

The influence of seawater calcium ions on coral calcification mechanisms: Constraints from boron and carbon isotopes and B/Ca ratios in Pocillopora damicornis

In order to better understand the response of coral calcification to changes in seawater chemistry, fragments of the coral Pocillopora damicornis were grown in seawater with varying [Ca2+]. Using a combined trace element (B/Ca) and stable isotope (δ11B, δ13C) approach, this paper explores the effect of seawater calcium concentrations ([Ca2+]SW) on coral calcification mechanisms and describes the manipulation of the extracellular calcifying fluid (ECF) pH (pHECF), saturation state (ΩECF) and dissolved inorganic carbon (DIC) speciation of the ECF. In these experiments, increases in [Ca2+]SW did not significantly influence calcification rates but caused the skeletal B/Ca ratio to significantly increase. To explain these results we propose a mechanism by which [CO32−]ECF is influenced by [Ca2+]SW. This mechanism suggests that the pHECF will be lowest in our highest [Ca2+] treatment and using this relationship we can place constraints on the pHECF. The difference in pHECF in our treatments is supported in part by a slight, but insignificant decrease in skeletal δ11B and δ13C values. We propose a novel dual-proxy approach to estimate pHECF, where calcification occurs at a pHECF > 9. This work emphasizes the importance of multi-element proxy approaches in understanding pH up-regulation and CO32− concentrations during calcification of corals.

Calibration of Na partitioning in the calcitic foraminifer Operculina ammonoides under variable Ca concentration: Toward reconstructing past seawater composition

Reconstructions of past changes in the seawater calcium concentration (Casw) are critical for understanding the long-term changes in ocean chemistry, the carbon cycle and for accurate application of elemental proxies (El/CaCaCO3) in foraminifera (e.g., Mg/Ca as proxy of temperature). Here we show that Na/Ca ratios in foraminiferal shells could be used for reconstructing Casw and Mg/Ca in the past oceans. Ca has a short residence time in the ocean (∼1 My), whereas Na in seawater has a residence time of ∼100 My. Hence it may be reasonably assumed that Nasw is invariant over the Cenozoic, enabling variations in oceanic Ca to be deduced from foraminiferal Na/Ca (Na/Cashell) if Na incorporation into foraminiferal shells depends on Na/Ca in seawater. Furthermore, the paleo-concentrations of other major and minor elements may then be calculated relative to the Ca in the shells, provided that other environmental or biological factors do not present a further complication. To evaluate this hypothesis, we cultured the benthic foraminifer Operculina ammonoides, an extant relative of the Eocene Nummulites, under varying Casw and temperature. The foraminifera grew well under the experimental conditions and increased their weight by 40–90%. The newly grown calcite (identified using a 135Ba labeling in the experimental seawater) was analyzed by Laser-Ablation ICP-MS for Li, Na, Mg and Sr to Ca ratios. The relationship between Na/Ca and Mg/Ca in the shell and their ratio in the solution are best described as a power function, where the instantaneous distribution coefficient is the derivative of the power fit to the El/Cashell versus El/Casw. In contrast, DSr and DLi are invariant with El/Casw. The influence of temperature on Li, Na and Sr incorporation was smaller than the uncertainty of our measurements. We conclude that Na/Ca in foraminiferal shells can be used to calculate paleo-calcium concentrations in the oceans and also other elements that may change relative to calcium (e.g., Mg, Sr, Li and others).

Phytoplankton calcification as an effective mechanism to alleviate cellular calcium poisoning

Abstract. Marine phytoplankton have developed the remarkable ability to tightly regulate the concentration of free calcium ions in the intracellular cytosol at a level of ~ 0.1 μmol L−1 in the presence of seawater Ca2+ concentrations of 10 mmol L−1. The low cytosolic calcium ion concentration is of utmost importance for proper cell signalling function. While the regulatory mechanisms responsible for the tight control of intracellular Ca2+ concentration are not completely understood, phytoplankton taxonomic groups appear to have evolved different strategies, which may affect their ability to cope with changes in seawater Ca2+ concentrations in their environment on geological timescales. For example, the Cretaceous (145 to 66 Ma), an era known for the high abundance of coccolithophores and the production of enormous calcium carbonate deposits, exhibited seawater calcium concentrations up to 4 times present-day levels. We show that calcifying coccolithophore species (Emiliania huxleyi, Gephyrocapsa oceanica and Coccolithus braarudii) are able to maintain their relative fitness (in terms of growth rate and photosynthesis) at simulated Cretaceous seawater calcium concentrations, whereas these rates are severely reduced under these conditions in some non-calcareous phytoplankton species (Chaetoceros sp., Ceratoneis closterium and Heterosigma akashiwo). Most notably, this also applies to a non-calcifying strain of E. huxleyi which displays a calcium sensitivity similar to the non-calcareous species. We hypothesize that the process of calcification in coccolithophores provides an efficient mechanism to alleviate cellular calcium poisoning and thereby offered a potential key evolutionary advantage, responsible for the proliferation of coccolithophores during times of high seawater calcium concentrations. The exact function of calcification and the reason behind the highly ornate physical structures of coccoliths remain elusive.

Field and In-Lab Determination of Ca2+ in Seawater

Portions of classic undergraduate quantitative analysis experiments in complexiometric titration and potentiometry are combined with a field-sampling experience to create a two period (2 × 3 h) comparison-based experiment for second-year students. A multifunctional chemical analysis device is used with calcium ion-selective electrode for field measurement of calcium concentration in seawater. The results are compared with later simultaneous potentiometric and colorimetric titration with ethylenediaminetetraacetic acid (EDTA). Students compare precision, difficulty, and sources of error of the different methods and present a written argument supporting their choice of best method including advantages and limitations. The experiment provides an introduction to chemistry in-the-field, sample collection and handling, complex matrices, imperfect methods, fallibility of instrumentation, real-world performance, and decision-making.

Oceanic calcium changes from enhanced weathering during the Paleocene‐Eocene thermal maximum: No effect on calcium‐based proxies

During the Paleocene‐Eocene thermal maximum (PETM ∼55 Myr ago), prominent climatic and biogeochemical changes took place in the atmosphere, ocean, and on land. For example, deep‐sea temperatures rose by 5°C to 6°C, while sea surface temperatures at high latitudes increased by up to 9°C. In the sedimentary record, the onset of the PETM is marked by widespread dissolution of calcium carbonate on the seafloor. In addition, there is evidence for globally higher humidity, precipitation and increased weathering during the PETM. Both calcium carbonate dissolution and enhanced weathering probably affected the seawater calcium concentration. Here we investigate implications that possible changes in the ocean's calcium inventory may have had on boron/calcium (B/Ca) and magnesium/calcium (Mg/Ca) ratios, which are used as proxies for deep water carbonate chemistry and temperature, respectively. We also examine effects on δ44Ca of seawater, which is used as an indicator for variations in the marine calcium cycle. We focus on the magnitude of change in the ocean's calcium ion concentration as a result of the carbon perturbation, which resulted in increased weathering fluxes and the dissolution of calcite on the ocean floor during the PETM. Different ranges of carbon input scenarios and their effect on ocean chemistry were examined using the Long‐term Ocean‐atmosphere‐Sediment CArbon cycle Reservoir (LOSCAR) model. We found that under the most plausible scenario, the calcium ion concentration change (Δ[Ca2+]) was less than 0.7% and around 2% in the most extreme scenario. Our results show that B/Ca and Mg/Ca proxies were not affected within analytical precision by changes in oceanic calcium due to weathering and carbonate dissolution during the PETM. The most extreme scenario (Δ[Ca2+] = 2%) would result in ∼4 μmol kg−1 uncertainty in reconstruction of Δ[CO32−]. The same scenario affects the temperature reconstruction by ∼0.2°C. The effect on the ocean's calcium isotope budget was insignificant as well, resulting in Δδ44Casw of less than 0.05‰.

Response of the coccolithophores Emiliania huxleyi and Coccolithus braarudii to changing seawater Mg 2+ and Ca 2+ concentrations: Mg/Ca, Sr/Ca ratios and δ 44/40Ca, δ 26/24Mg of coccolith calcite

Calcium and magnesium concentrations in seawater have varied over geological time scales. On short time scales, variations in the major ion composition of seawater influences coccolithophorid physiology and the chemistry of biogenically produced coccoliths. Validation of those changes via controlled laboratory experiments is a crucial step in applying coccolithophorid based paleoproxies for the reconstruction of past environmental conditions. Therefore, we examined the response of two species of coccolithophores, Emiliania huxleyi and Coccolithus braarudii, to changes in the seawater Mg/Ca ratio (≈0.5 to 10 mol/mol) by either manipulating the magnesium or calcium concentration under controlled laboratory conditions. Concurrently, seawater Sr/Ca ratios were also modified (≈2 to 40 mmol/mol), while keeping salinity constant at 35. The physiological response was monitored by measurements of the cell growth rate as well as the production rates of particulate inorganic and organic carbon, and chlorophyll a. Additionally, coccolithophorid calcite was analyzed for its elemental composition (Sr/Ca and Mg/Ca) as well as isotope fractionation of calcium and magnesium (Δ 44/40Ca and Δ 26/24Mg). Our results reveal that physiological rates were substantially influenced by changes in seawater calcium rather than magnesium concentration within the range estimated to have occurred over the past 250 million years when coccolithophores appear in the fossil record. All physiological rates of E. huxleyi decreased at a calcium concentration above 25 mmol L −1, whereas C. braarudii displayed a higher tolerance to increased seawater calcium concentrations. Partition coefficient of Sr was calculated as 0.36 ± 0.04 (±2 σ) independent of species. Partition coefficient of Mg 2+ increased with increasing seawater Ca 2+ concentrations in both coccolithophore species. Calcium isotope fractionation was constant at 1.1 ± 0.1‰ (±2 σ) and not altered by changes in seawater Mg/Ca ratio. There is a well-defined inverse linear relationship between calcium isotope fractionation and partition coefficient of Sr 2+ in all experiments, suggesting similar controls on both proxies in the investigated species. Magnesium isotope ratios were relatively stable for seawater Mg/Ca ratios ranging from 1 to 5, with a higher degree of fractionation in Emiliania huxleyi (by ≈0.2‰ in Δ 26/24Mg). Although Mg/Ca ratios in the calcite of coccolithophores and foraminifera are similar, the former have considerably higher Δ 26/24Mg (by >+3‰), presumably due to differences in calcification mechanisms between the two taxa. These observations suggest, a physiological control over magnesium elemental and isotopic fractionation during the process of calcification in coccolithophores.

Incorporation of Mg and Sr in calcite of cultured benthic foraminifera: impact of calcium concentration and associated calcite saturation state

Abstract. We investigated the effect of the calcium concentration in seawater and thereby the calcite saturation state (Ω) on the magnesium and strontium incorporation into benthic foraminiferal calcite under laboratory conditions. For this purpose individuals of the shallow-water species Heterostegina depressa (precipitating high-Mg calcite, symbiont-bearing) and Ammonia tepida (low-Mg calcite, symbiont-barren) were cultured in media under a range of [Ca2+], but similar Mg/Ca ratios. Trace element/Ca ratios of newly formed calcite were analysed with Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and normalized to the seawater elemental composition using the equation DTE=(TE/Cacalcite)/(TE/Caseawater). The culturing study shows that DMg of A. tepida significantly decreases with increasing Ω at a gradient of −4.3×10−5 per Ω unit. The DSr value of A. tepida does not change with Ω, suggesting that fossil Sr/Ca in this species may be a potential tool to reconstruct past variations in seawater Sr/Ca. Conversely, DMg of H. depressa shows only a minor decrease with increasing Ω, while DSr increases considerably with Ω at a gradient of 0.009 per Ω unit. The different responses to seawater chemistry of the two species may be explained by a difference in the calcification pathway that is, at the same time, responsible for the variation in the total Mg incorporation between the two species. Since the Mg/Ca ratio in H. depressa is 50–100 times higher than that of A. tepida, it is suggested that the latter exhibits a mechanism that decreases the Mg/Ca ratio of the calcification fluid, while the high-Mg calcite forming species may not have this physiological tool. If the dependency of Mg incorporation on seawater [Ca2+] is also valid for deep-sea benthic foraminifera typically used for paleostudies, the higher Ca concentrations in the past may potentially bias temperature reconstructions to a considerable degree. For instance, 25 Myr ago Mg/Ca ratios in A. tepida would have been 0.2 mmol/mol lower than today, due to the 1.5 times higher [Ca2+] of seawater, which in turn would lead to a temperature underestimation of more than 2 °C.

A Dynamic Marine Calcium Cycle During the Past 28 Million Years

Multiple lines of evidence have shown that the isotopic composition and concentration of calcium in seawater have changed over the past 28 million years. A high-resolution, continuous seawater calcium isotope ratio curve from marine (pelagic) barite reveals distinct features in the evolution of the seawater calcium isotopic ratio suggesting changes in seawater calcium concentrations. The most pronounced increase in the delta44/40Ca value of seawater (of 0.3 per mil) occurred over roughly 4 million years following a period of low values around 13 million years ago. The major change in marine calcium corresponds to a climatic transition and global change in the carbon cycle and suggests a reorganization of the global biogeochemical system.

Temporal record of lithium in seawater: A tracer for silicate weathering?

This paper presents multi-species records of the Li/Ca ratio and Li isotopic composition (δ 7Li) of planktonic foraminifera from the Pacific and Atlantic oceans for the past 18 Ma. The Li/Ca record is corrected for interspecific offsets determined from recent (Holocene) foraminifera; interspecific offsets in δ 7Li are not significant. Despite different diagenetic histories, the records produced from both oceans are remarkably consistent. Corrected planktonic foraminiferal Li/Ca ratios range from 6.3 to 10.9 μmol/mol, while planktonic foraminiferal δ 7Li ranges from 25 to 31‰. Both records are interpreted in terms of long-term changes in seawater Li/Ca and δ 7Li, enabling issues related to higher-resolution variability in Li/Ca and δ 7Li to be ignored. By assuming that the hydrothermal flux of Li into the oceans, and the flux of Li removed from the oceans during low-temperature uptake by marine basalts and sediments, have not changed significantly since 18 Ma, and using published records for the seawater calcium concentration, the seawater Li/Ca and δ 7Li records can be used to estimate global average river δ 7Li and Li fluxes. Our records indicate that the river flux of dissolved Li decreased between 16 and ∼8 Ma while the δ 7Li value of the river input increased. These data imply that both silicate weathering rates and weathering intensity decreased over this interval which may have been responsible for putative increases in levels of atmospheric CO 2. In contrast, the riverine flux of Li has increased since ∼8 Ma while its δ 7Li value has increased. This implies that the silicate weathering rate has increased, while weathering intensity has decreased, since that time.

Variations in the marine Ca cycle over the past 20 million years

A 20 million year marine Ca isotope record is constructed based on measurements of 36 nannofossil ooze samples from DSDP Site 590. The Ca isotopic composition (δ44Ca, relative to our “bulk Earth” average) of carbonate ooze samples varies between −0.2 and −0.9, and the inferred δ44Ca of seawater calcium varies between +1.1 and +0.4 (modern seawater value is +0.95 on our scale). Fluctuations in the seawater Ca isotope ratio occur in 2 to 4 million year intervals and indicate that the supply and removal of Ca from the oceans are seldom in balance. The Ca isotope record is used to reconstruct past weathering fluxes and marine calcium concentrations. The inferred marine concentration of Ca fluctuates by about ±20% on 2–4 million year time scales and by about ±50% over the 20 million year record. These variations are presumably superimposed on the previously inferred 100 million year variations. The seawater Ca concentration was about 0.5 times present at 20 Ma and generally increased until 6 Ma (1.7 times present). Local maxima in seawater calcium concentrations occur at 6.4 and 4 Ma, preceding periods of enhanced marine productivity and high mass accumulation rates at Site 590. While the calculated calcium concentrations are sensitive to the assumed δ44Ca value for the weathering flux, the 2–4 million year features of the derived curves are not.

A Cenozoic seawater Sr/Ca record from benthic foraminiferal calcite and its application in determining global weathering fluxes

A Cenozoic multi-species record of benthic foraminiferal calcite Sr/Ca has been produced and is corrected for inter-specific offsets (typically less than 0.3 mmol/mol) and for the linear relationship between decreasing benthic foraminiferal Sr/Ca and increasing water depth. The water depth correction, determined from Holocene, Late Glacial Maximum and Eocene paleowater-depth transects, is ∼0.1 mmol/mol/km. The corrected Cenozoic benthic foraminiferal Sr/Ca record ranges from 1.2 to 2.0 mmol/mol, and has been interpreted in terms of long-term changes in seawater Sr/Ca, enabling issues related to higher-resolution variability in Sr/Ca to be ignored. We estimate that seawater Sr/Ca was ∼1.5 times modern values in the late Cretaceous, but declined rapidly into the Paleogene. Following a minimum in the Eocene, seawater Sr/Ca increased gradually through to the present day with a minimum superimposed on this trend centered in the late Miocene. By assuming scenarios for changing seawater calcium concentration, and using published carbonate accumulation rate data combined with suitable values for Sr partition coefficients into carbonates, the seawater Sr/Ca record is used to estimate global average river Sr fluxes. These fluxes are used in conjunction with the seawater strontium isotope curve and estimates of hydrothermal activity/tectonic outgassing to calculate changes in global average river 87Sr/ 86Sr through the Cenozoic. The absolute magnitude of Sr fluxes and isotopic compositions calculated in this way are subject to relatively large uncertainties. Nevertheless, our results suggest that river Sr flux increased from 35 Ma to the present day (roughly two-fold) accompanied by an overall increase in 87Sr/ 86Sr (by ∼0 to 0.001). Between 75 and 35 Ma, river 87Sr/ 86Sr also increased (by ∼0.001 to 0.002) but was accompanied by a decrease (two- to three-fold) in river Sr flux.

Effect of increased calcium concentration in sea water on calcification and photosynthesis in the scleractinian coral Galaxea fascicularis.

The relationship between calcification and photosynthesis in coral was investigated using standard sea water with enhanced calcium concentration. In standard sea water at 23 degrees C with the calcium concentration increased by 2.5 mmol l(-1), incorporation of calcium into the skeleton increased by 30-61 %, depending on the method of data normalisation, and photosynthesis, measured as (14)C incorporation into the tissues, also increased by 87 %. At 29 degrees C, calcium incorporation into the skeleton increased by 54-84 % and (14)C incorporation increased by 32 % when sea water calcium concentration was increased by 5 mmol l(-1). However, photosynthesis measured as net photosynthetic oxygen production did not increase. Similarly there was no change in respiration rate when coral polyps were incubated in high-calcium sea water. It is conjectured that an increase in photorespiration may be responsible for the latter observations. Bisphosphonate has been considered to inhibit calcification but not photosynthesis in corals. We show that bisphosphonate may not inhibit formation of amorphous calcium carbonate and that the inhibition of calcification is possibly illusory. The data are consistent with the trans-calcification model, which suggests that calcification is a source of CO(2) for photosynthesis in corals.