Seawater Ca Research Articles

Comment on “Empirical partition coefficients for Sr and Ca in marine barite: Implications for reconstructing seawater Sr and Ca concentrations” by Kristen B. Averyt and Adina Paytan

Comment on “Empirical partition coefficients for Sr and Ca in marine barite: Implications for reconstructing seawater Sr and Ca concentrations” by Kristen B. Averyt and Adina Paytan

Evolution of the oceanic calcium cycle during the late Mesozoic: Evidence from δ44/40Ca of marine skeletal carbonates

The Ca isotope compositions of 37 late Mesozoic skeletal carbonates, belemnites and brachiopods, from the Tethyan realm were analyzed by thermal (TIMS) and plasma (MC-ICP-MS) ionization mass spectrometry. A poor correlation between δ 44/40Ca and δ 18O values of belemnites suggests only a weak temperature dependency for the Ca isotope composition of belemnites, likely less than 0.02‰/°C. The δ 44/40Ca record of belemnites was therefore used to reconstruct the Ca isotope composition of paleo-seawater ( δ 44/40Ca SW), based on an experimentally determined fractionation factor between seawater Ca and belemnite calcite ( α CC–SW) of ∼ 0.9986. The inferred δ 44/40Ca SW record, with an average stratigraphic resolution of 1 Ma, shows systematic temporal variation of ∼ 0.5‰ with the Middle/Late Jurassic (∼ 154 Ma) minimum of ∼ 1.4‰ and a subsequent general increase to the Early Cretaceous (∼ 124 Ma) maximum of ∼ 1.9‰. The global nature of the δ 44/40Ca SW record is supported by identical Ca isotope compositions of coeval (Kimmeridgian) belemnites collected from two distinct paleogeographic regions, the southern (New Zealand) and northern (Germany) margin of the Tethys Ocean. The observed late Mesozoic δ 44/40Ca SW record was simulated using a simple Ca isotope mass balance model, and the results indicate that the variation in δ 44/40Ca SW record can be explained by changes in oceanic input fluxes of Ca that were independent of the carbonate ion fluxes, such as the hydrothermal Ca flux or the release of Ca to the oceans via dolomitization of marine carbonates.

Chemical Speciation of Trace Metals in Seawater: a Review

The recent development of the chemical speciation of trace metals in seawater is described. The speciation studies reveal that metal ion complexation is one of the most important processes in seawater; especially, most bioactive trace metals, such as Fe(III) and Cu, exist as complexes with ligands in dissolved organic matter. The organic ligands in seawater are characterized with metal ions selected by the HSAB concept. A strong organic ligand, which originates from marine microorganisms, is classified as a hard base including carboxylates. The free organic ligand concentrations in seawater are buffered by complexation with excess amounts of Ca and Mg in seawater. The chemical equilibrium model suggested that the concentrations of bioactive free metal ions are at an optimal level to activities of marine microorganisms. For chemical speciation, it is important to have a better understanding of the ecological roles of trace metals in seawater.

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.

Calcium isotope (δ44/40Ca) variations of Neogene planktonic foraminifera

Measurements of the calcium isotopic composition (δ44/40Ca) of planktonic foraminifera from the western equatorial Pacific and the Indian sector of the Southern Ocean show variations of about 0.6‰ over the past 24 Myr. The stacked δ44/40Ca record of Globigerinoides trilobus and Globigerina bulloides indicates a minimum in δ44/40Casw (seawater calcium) at 15 to 16 Ma and a subsequent general increase toward the present, interrupted by a second minimum at 3 to 5 Ma. Applying a coupled calcium/carbon cycle model, we find two scenarios that can explain a large portion of the observed δ44/40Casw variations. In both cases, variations in the Ca input flux to the ocean without proportional changes in the carbonate flux are invoked. The first scenario increases the riverine calcium input to the ocean without a proportional increase of the carbonate flux. The second scenario generates an additional calcium flux from the exchange of Ca by Mg during dolomitization. In both cases the calcium flux variations lead to drastic changes in the seawater Ca concentrations on million year timescales. Our δ44/40Casw record therefore indicates that the global calcium cycle may be much more dynamic than previously assumed.

Negligible temperature dependence of calcium isotope fractionation in 12 species of planktonic foraminifera

The influence of temperature on calcium (Ca) isotope fractionation during biomineralization was investigated through the paired analyses of δ 44/42Ca (via MC-ICP-MS) and δ 18O on the calcite tests of 12 species of planktonic foraminifera from core-top sediments. Samples were collected from a suite of box-cores from sites between 60°N and 30°S in the North Atlantic and West Indian oceans, spanning a range in sea surface temperature of 9–27 °C. The δ 44/42Ca SRM915a of the samples ranged from +0.21‰ to +0.81‰ over a range in the δ 18O VPDB of the foraminiferal calcite of between −2.3‰ and +2.1‰. The bulk of the species investigated produced calcite with an average calcite–seawater Ca enrichment factor ( ε) of −0.65±0.08‰ ( n=45). The globorotaliid species and a single Globigerina bulloides sample were confined to the upper 0.4‰ of the observed range in δ 44/42Ca values, with the globorotaliids displaying a lesser degree of fractionation of −0.56±0.23‰ ( n=15), possibly as a result of physiological or metabolic processes such as the production of calcite crusts at depth, leading to the depletion of internal stores of Ca. No significant correlation between temperature and Ca enrichment factor was observed in any of the 12 species of foraminifera investigated. The influence of temperature on ε was close to zero for the combined non-globorotaliid species, −0.07±0.10 K ( ε versus 1000/ K; 0.0008±0.0012‰ °C −1). The results suggest that the theoretically expected relationship between the Ca enrichment factor and temperature can be obscured by, as yet, unquantified metabolic and physiological processes in nature. These processes are particularly relevant to the globorotaliid species and G. bulloides in this core-top study. Consequently, their effects must be better understood before laboratory-determined temperature calibrations can be applied to downcore material with confidence. Provided that the effects of metabolic and physiological processes on ε remain constant through time, Orbulina universa, Globigerinoides sacculifer, Neogloboquadrina pachyderma and Globigerinella aequilateralis are suitable for the investigation of temporal changes in the calcium isotopic composition of seawater.

Influence of seawater Sr content on coral Sr/Ca and Sr thermometry

The Ca content of a Porites coral from Xisha, South China Sea is quite uniform along its 18-year growth axis. A comparison with previously published data shows that the Ca content of corals from different sites varies by only 0.4%. This is much smaller than the variation of Ca in seawater (2.2%), indicating that Ca variations in seawater do not significantly affect the Ca compositions of coral skeletons. The variation in skeletal Ca contents results in only +/- 0.6 degrees C of uncertainty in SST calculations, which is much smaller than the large disparities observed for previously established coral Sr/Ca thermometers. In contrast, Sr in tropical seawater varies spatially by as much as 2.4%, corresponding to similar to 4 degrees C offset for coral Sr/Ca calibrations. The effect of seawater Sr variations on coral Sr/Ca thermometers is evaluated and we demonstrate that the content of seawater Sr is the major factor responsible for disparities in these coral Sr/Ca thermometers. The disparities can be significantly reduced when seawater Sr contents are included in the Sr/Ca thermometers.

Seawater chemistry and the advent of biocalcification

Major ion compositions of primary fluid inclusions from terminal Proterozoic (ca. 544 Ma) and Early Cambrian (ca. 515 Ma) marine halites indicate that seawater Ca{sup 2+} concentrations increased approximately threefold during the Early Cambrian. The timing of this shift in seawater chemistry broadly coincides with the 'Cambrian explosion,' a brief drop in marine {sup 87}Sr/{sup 86}Sr values, and an increase in tectonic activity, suggesting a link between the advent of biocalcification, hydrothermal mid-ocean-ridge brine production, and the composition of seawater. The Early Cambrian surge in oceanic [Ca{sup 2+}] was likely the first such increase following the rise of metazoans and may have spurred evolutionary changes in marine biota.

Controls on the cretaceous and cenozoic evolution of seawater composition, atmospheric CO2 and climate

A new box model for the global carbon-calcium-strontium cycle is developed to simulate the evolution of Cretaceous and Cenozoic seawater and atmosphere. The model accounts for carbon masses in ocean and atmosphere, in carbonate, in particulate organic carbon (POC), and in the mantle. Major processes considered in the model are mantle degassing and hydrothermal fluxes, alteration of oceanic crust, chemical weathering, metamorphism of carbonates, carbonate accumulation, carbonate turnover in subduction zones, and the turnover of POC. Model outputs are partial pressure of CO2 (pCO2), seawater pH, concentrations of Ca, Sr, and HCO3 in seawater as well as the C and Sr isotopic composition of seawater and marine carbonates. A comprehensive review of current fluxes is given to define the flux equations and parameters. Secular trends recorded in marine carbonates are used to constrain the remaining open model parameters. The model includes a new parameterization of silicate weathering considering the contribution of young volcanic deposits. The weathering of these deposits consumes a large fraction of volcanically-released CO2 and maintains moderate pCO2 levels during periods of intense mantle degassing and volcanic/tectonic activity. Further negative feed-back is provided by POC burial which is coupled to pCO2-dependent weathering rates. The model produces high Ca concentrations during the Cretaceous and a strong increase in both pH and carbonate alkalinity during the late Cenozoic. Moreover, it predicts high atmospheric CO2 and surface temperatures for the mid-Cretaceous and early Cenozoic and low values for the late Cenozoic icehouse world thus suggesting a close coupling between climate and pCO2. Finally, it demonstrates that the mid-Cretaceous greenhouse was caused by enhanced volcanic/tectonic activity and confirms that the late Cenozoic cooling has been induced by an increase in silicate weatherability caused by enhanced mountain building and erosion.

Correlation of decreased calcium contents with proline accumulation in the marine green macroalga Ulva fasciata exposed to elevated NaCl contents in seawater

The involvement of Na + , K + , Cl - or Ca 2 + in the regulation of salinity stress-induced proline accumulation via the inhibition of the activity of proline dehydrogenase (PDH; EC 1.4.3.1), a catabolic enzyme of proline, was investigated in the marine green macroalga Ulva fasciata Delile. After 6 h of exposure to elevated artificial seawater (ASW) salinity, adjusted either by increasing the NaCI content in 30‰ ASW (a change in ion ratio) or by concentrating ASW (a constant ion ratio), the contents of Na + , K + and Cl - linearly accumulated with increasing salinity from 30-90‰ (parts per thousand); the accumulation pattern of each ion was similar between the two treatments. An increase in NaCI content in ASW induced proline accumulation, but decreased both the PDH activity and the total and water-soluble Ca 2+ contents, while concentrated ASW had no effect. As compared to a constant value at 30‰, both the contents of total and water-soluble Ca 2 + and the activity of PDH decreased 1 h after exposure to 60‰ (adjusted by increasing NaCI content in 30‰ ASW) and concomitantly the content of seawater Ca 2+ increased, while proline accumulated after 3 h. The addition of 15 mM ethylene glycol-bis-(2-aminoethyl ether) N,N,N'-tetraacetic acid (EGTA) in 60‰ ASW (adjusted by increasing the NaCI content in 30‰ ASW) enhanced both the proline accumulation and the decrease in the content of total and water-soluble cellular Ca 2+ and the activity of PDH; the effects of EGTA were reversed by 10 mM CaCl 2 or 10 mM CaSO 4 , These results indicate that a loss of cellular Ca 2+ is associated with the NaCI induction of proline accumulation via an inhibition of PDH activity in U. fasciata.

Correlation of decreased calcium contents with proline accumulation in the marine green macroalga Ulva fasciata exposed to elevated NaCl contents in seawater

The involvement of Na+, K+, Cl− or Ca2+ in the regulation of salinity stress-induced proline accumulation via the inhibition of the activity of proline dehydrogenase (PDH; EC 1.4.3.1), a catabolic enzyme of proline, was investigated in the marine green macroalga Ulva fasciata Delile. After 6 h of exposure to elevated artificial seawater (ASW) salinity, adjusted either by increasing the NaCl content in 30‰ ASW (a change in ion ratio) or by concentrating ASW (a constant ion ratio), the contents of Na+, K+ and Cl− linearly accumulated with increasing salinity from 30–90‰ (parts per thousand); the accumulation pattern of each ion was similar between the two treatments. An increase in NaCl content in ASW induced proline accumulation, but decreased both the PDH activity and the total and water-soluble Ca2+ contents, while concentrated ASW had no effect. As compared to a constant value at 30‰, both the contents of total and water-soluble Ca2+ and the activity of PDH decreased 1 h after exposure to 60‰ (adjusted by increasing NaCl content in 30‰ ASW) and concomitantly the content of seawater Ca2+ increased, while proline accumulated after 3 h. The addition of 15 mM ethylene glycol-bis(2-aminoethyl ether) N,N,N'-tetraacetic acid (EGTA) in 60‰ ASW (adjusted by increasing the NaCl content in 30‰ ASW) enhanced both the proline accumulation and the decrease in the content of total and watersoluble cellular Ca2+ and the activity of PDH; the effects of EGTA were reversed by 10 mM CaCl2 or 10 mM CaSO4. These results indicate that a loss of cellular Ca2+ is associated with the NaCl induction of proline accumulation via an inhibition of PDH activity in U. fasciata.

Independent Initiation of Calcium Dependent Glycosidase Release and Cortical Contractions during the Activation of Ascidian Eggs: (Ascidian, Ryanodine, Thimerosal, Thapsigargin, N-acetylglucosaminidase).

During fertilization or ionophore induced activation, ascidian eggs rapidly release cell surface N-acetylglucosaminidase activity used in the block against polyspermy and undergo cortical contractions before they re-initiate meiosis. To better understand the activation process, we probed the relationship between these two processes in Ascidia ceratodes eggs by activating with different agents that increase intracellular Ca levels and under different ionic conditions. Glycosidase activity release was followed by the use of a fluorogenic substrate, and cortical contractions were followed by examining changes in cell shape with light microscopy. Ionomycin (2.7 μM) and thimerosal (1 mM) initiate glycosidase release and cortical contractions when administered in complete sea water (SW) but only the contractions in low Ca SW. Ryanodine (0.67 mM), known to raise free intracellular Ca in a number of cell types by release from the endoplasmic reticulum, causes glycosidase release but fails to initiate cortical contractions in complete SW. Thapsigargin (10 μM), which inhibits Ca dependent ATPase in the ER, causes glycosidase release but induces the contractions only about 50% of the time. These experiments show that, although glycosidase release normally precedes the ooplasmic shape changes that accompany the resumption of meiosis in ascidian eggs, they are not obligately coupled. That both processes can be induced by treatments known to raise intracellular Ca in other systems but under different conditions indicates that there may be a multiplicity of Ca requiring but functionally independent events during egg activation.

Toxicity of lipid-soluble copper(II) complexes to the marine diatom Nitzschia closterium: Amelioration by humic substances

Algal assays, using the marine diatom Nitzschia closterium, have established that humic acid (5 mg kg −1) can ameliorate the toxicity of the lipophilic complex Cu(oxine) 2 (3 × 10 −8 mol l −1 in unsupplemented seawater). The toxicity of Cu(PAN) 2 is not ameliorated [PAN = 1-(2-pyridylazo)-2-napthol]. In conjunction with previous visible absorption spectrophotometry and polarographic measurements it was established that humic acid sequesters copper(II) from the hydrophobic complexes, releasing a ligand molecule. The copper(II) toxicity may be ameliorated provided the ligand itself is not toxic. Fulvic acid was significantly less effective in ameliorating toxicity. Because of the significant competition from Ca(II) and Mg(II) in seawater, it is inferred that humic substances may be more effective in ameliorating toxicity of hydrophobic copper complexes in fresh water.

Subduction-induced pore fluid venting and the formation of authigenic carbonates along the cascadia continental margin: Implications for the global Ca-cycle

Pore fluid venting associated with subduction-induced sediment deformation causes precipitation of calcium carbonate as prominent carbonate chimneys or cement in the accreted sediments across the active continental margin off Oregon and Washington. A depletion of interstitial Ca 2+ with a maximum decrease of 50% relative to seawater Ca 2+ over only 1.5m depth and reduction in porosity in the deformed sediments suggest that interstitial Ca 2+ is removed to form calcium carbonate cement. In contrast, the pore waters of the undeformed abyssal plain sediments show no depletion in dissolved Ca 2+. They are either enriched to a maximum of 5% or show no change in dissolved Ca 2+. Here the background level of CaCO 3 content in the sediment is only 0.1 to 1%. Calcium carbonate precipitation in the deformed sediments probably occurs as the result of upward migration and oxidation of biogenic methane and of the increase in carbonate saturation due to release of excess pore pressure during fluid venting. Upward advection of fluids at rates of 1–28 cm y −1 is predicted from diffusion-advection-reaction models applied to the downcore concentration profiles of dissolved Ca 2+ and NH 4 + in the tectonically-deformed sediments. The range of predicted flow rates is related to the type of calcium carbonate lithification; i.e. slow rates generate cement and fast rates generate chimneys. Carbonate mineral precipitation associated with pore fluid venting requires direct transfer of Ca 2+ from the oceanic basement to the accretionary complex. Such a mechanism leads us to propose that the accretionary complexes of the global plate subduction zones are a major sink for crustal Ca 2+. A global flux of crustal Ca 2+ that is removed by carbonate mineral precipitation may be as muc3 as the hydrothermal Ca-input. This significant Ca-flux, not previously considered in the global geochemical budget, implies that pore fluid venting in subduction zones may also act as a global sink or source for other elements.

Sr, Mg, and Ca chemistry of the skeleton of Nautilus

Concentrations of Sr, Mg, and Ca were quantified for the shell and septa formed during the final 3 to 4 yr of growth for four species of the extant cephalopod genus Nautilus [ N. belauensis Saunders 1981, N. macromphalus Sowerby 1849, N. scrobiculatus (Lightfoot 1786), and N. pompilius Linnaeus 1758]. These data are used to describe several aspects of the physiochemistry of Nautilus in relation to biomineralization of shell and septal aragonite. Species of Nautilus discriminate against the concentrations of Sr and Mg relative to Ca in seawater by 79% and 99.9%, respectively. The power of this discrimination against Sr and Mg varies for shell and septal aragonite within and among species. Although the among-species variation in the Sr and Mg content of shell and septal aragonite is greater than the within-species variation, differences in these concentrations are not unique among all species. However, when the covariance of Sr and Mg is computed as a transformed variable (discriminant function), species can be distinguished with significance ( P <0.001) by the combined Sr and Mg chemistry of septal aragonite. Similarly, all species with the exception of N. macromphalus and N. pompilius can be distinguished by the combined Sr and Mg chemistry of shell aragonite.

Determination of trace elements in particulate and soluble fractions of seawater by neutron activation analysis

Determination of trace element concentrations in marine samples is of considerable interest in studying diverse types of oceanic pollution sources, mode of transport and residence times of pollutants in seawater, and in revealing interactions that may exist among elements in suspended particulate and soluble fractions of seawater. Neutron activation analysis (NAA) methods are being investigated in this laboratory for measuring multi-element concentrations of oceanic suspended particulate matter (SPM) and seawater. An instrumental NAA (INAA) method has been developed for determining Al, Au, Ba, Br, Ca, Ce, Cl, Co, Cr, Eu, Fe, Hf, Hg, I, K, La, Lu, Mg, Mn, Na, Nd, Rb, Sb, Sc, Se, Sm, Sr, Th, Ti, V and Zn in microgram quantities of oceanic SPM. The method consists of three sequential irradiations at two different neutron fluxes and subsequent counting after five decay periods using high-resolution detectors. The method for determining trace elements in seawater involves preirradiation separations of Br and Cl using nitric acid and hydrogen peroxide and of Na with hydrated antimony pentoxide. Concentrations of Ag, Ca, Co, Cr, Fe, La, Lu, Mn, Mo, Sm, Sr, Ti, V and Zn in seawater have been measured. Details of analytical methodologies for determining trace elements in both suspended particulate and soluble fractions of seawater are presented.

A Historical Note Concerning Salt in Vertebrate Blood and in the Sea

A Historical Note Concerning Salt in Vertebrate Blood and in the Sea

The impact of solution chemistry on Mytilus edulis calcite and aragonite

Magnesium/calcium, Sr/Ca, and Na/Ca atom ratios were determined in the calcite and aragonite regions of Mytilus edulis shells which were grown in semi-artificial ‘seawater’ solutions having varying Mg/Ca, Sr/Ca, and Na/Ca ratios. These ratios were measured by instrumental neutron activation, atomic absorption, and electron microprobe analytical techniques. Strontium/calcium ratios in both calcite and aragonite were linearly proportional to solution Sr/Ca ratios. Magnesium/calcium ratios in calcite increased exponentially when solution Mg/Ca ratios were raised above the normal seawater ratio; whereas in aragonite, Mg/Ca ratios increased linearly with increases in solution Mg/Ca ratios. Sodium/calcium and sulfur/calcium ratios in calcite covaried with Mg/Ga solution ratios. Conversely, in aragonite, Na/Ca ratios varied linearly with solution Na/Ca ratios. Magnesium is known to inhibit calcite precipitation at its normal seawater concentration. We infer from the results of the work reported here that Mytilus edulis controls the Mg activity of the outer extrapallial fluid, thus facilitating the precipitation of calcitic shell. Increases in sulfur content suggest that changes in shell organic matrix content occur as a result of environmental stress. Certain increases in Mg content may also be correlated to stress. Sodium/calcium variations, and their absolute amounts in calcite and aragonite, are best explained by assuming that a substantial amount of Na is adsorbed on the calcium carbonate crystal surface. Strontium/calcium ratios show more promise than either Mg/Ca or Na/Ca ratios as seawater paleochemistry indicators, because the Sr/Ca distribution coefficients for both aragonite and calcite are independent of seawater Ca and Sr concentrations.

Calcium measurement in the periphery of an axon.

Aequorin was microinjected into squid giant axons, the axons were stimulated, and the change in light emission was followed. This response was compared with that found when the axon, in addition to being microinjected with aequorin, is also injected with the dye phenol red. Large concentrations of phenol red injected into axons result in a high probability that photons emitted by aequorin, when it reacts with Ca in the core of the axoplasm, will be absorbed before they escape from the axon; photons produced by the aequorin reaction at the periphery of the axoplasm are much less likely to be absorbed. This technique thus favors observing changes in Cai taking place in the periphery of the axon. Stimulation in 50 mM Ca seawater of an aequorin-phenol red-injected axon at 180 s-1 for 1 min produces a scarcely detectable change in Cai; the addition of 2 mM cyanide (CN) to the seawater produces an easily measureable increase in Cai, suggesting that mitochondrial buffering in the periphery is substantial. Making the pH of the axoplasm of a normal axon alkaline with 30 mM NH4+ -50 mM Ca seawater, reduces the resting glow of the axon but results in an even more rapid increase in Cai with stimulation. In a phenol red-injected axon, this treatment results in a measureable response to stimulation in the absence of CN.

Calcium content and net fluxes in squid giant axons.

Axons freshly dissected from living specimens of the tropical squid Dorytheutis plei have a calcium content of 68 mumol/kg of axoplasm. Fibers stimulated at 100 impulses/s in 100 mM Ca seawater increase their Ca content by 150 mumol/kg.min; axons placed in 3 Ca (choline) seawater increase their Ca content by 12 mumol/kg.min. Axons loaded with 0.2--1.5 mmol Ca/kg of axoplasm extruded Ca with a half time of 15--30 min when allowed to recover in 3 Ca (Na) seawater. The half time for recovery of loaded axons poisoned with carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and iodoacetic acid (IAA) is about the same as control axons. Axons placed in 40 mM Na choline seawater (to reduce chemical gradient for Na) or in 40 mM Na, 410 mM K seawater to reduce the electrochemical gradient for Na to near zero either fail to lose previously loaded Ca or gain further Ca.