Present-day Seawater Research Articles

Preservation of primary REE patterns without Ce anomaly during dolomitization of Mid-Paleoproterozoic limestone and the potential re-establishment of marine anoxia immediately after the "Great Oxidation Event"

Comparison of shallow-water limestone and silicified dolomite from the Mid-Paleoproterozoic Mooidraai Formation, Transvaal Supergroup, South Africa, shows that the primary rare earth element (REE) distribution of these pure marine sedimentary carbonates has been preserved during dolomitization and silicification. Both lithologies display REE (and Y) patterns closely resembling those of present-day seawater, i.e. they show enrichment of the heavy relative to the light REE, positive anomalies of La, Gd and Lu, and super-chondritic Y/Ho ratios. However, these shallow-water carbonates lack any Ce anomalies, indicating that in the Mid-Paleoproterozoic the redox-level of surface water in the Griqualand-West sub-basin of the Kaapvaal Craton did not allow for oxidation of Ce(III). The absence of Ce anomalies from shallow water Mooidraai carbonates indicates a return to marine anoxia immediately after large amounts of marine sedimentary Mn oxides had been deposited in a highly oxygenated marine environment in the underlying Hotazel Formation. This suggests that the “Great Oxidation Event” in the Paleoproterozoic was a transition period characterized by strong fluctuations of the redox level of the Earth’s surface ocean.

Germanium incorporation into sponge spicules: Development of a proxy for reconstructing inorganic germanium and silicon concentrations in seawater

Measurements of germanium (Ge) in deep-sea sponge skeletons are presented for sponges collected by dredge and for spicules isolated from a range of deep-sea sediment cores. Germanium to silicon (Si) ratios (Ge / Si sp) for sponge silica ranged between 0.075 and 0.380 μmol/mol, which are much lower than the present-day seawater Ge / Si ratio of 0.7 μmol/mol. A plot of Ge / Si sp versus estimated seawater Ge (and Si) concentration produced a linear relationship with the Ge content of spicules increasing with seawater Ge concentration. Plots of Ge / Si sp versus depth for both dredged sponges and sediment-bound spicules produced oceanic profiles similar to those of dissolved Ge (and Si) concentration. To explain the fractionation seen in the sponge Ge / Si sp data two models are presented. The first model used to interpret the data assumes that sponges only respond to the Ge content of the ambient seawater, implying that, Ge incorporation into sponge silica is independent of the seawater Ge / Si ratio up to a Si concentration of about 100 μmol/L. This model is consistent with the 68Ge uptake results of Davie et al. [E.I. Davie, T.L. Simpson, R. Garonne, Experimental germanium incorporation into siliceous sponge spicules, Biol. Cell 48 (1983) 191–202] for cultured spicules. Their results showed that the incorporation of Ge in sponge silica is only dependent on the Ge concentration of the water in which a sponge is growing. The second model used to explain the data assumes that Ge / Si sp fractionation results from subtle differences in the uptake kinetics of Ge and Si. While the assumptions used by each model to describe the data are different, it is possible to use sponge Ge / Si sp data to reconstruct palaeo-Ge concentrations using model I, and to reconstruct palaeo-Si concentrations using both models. Palaeo-Si concentrations estimated using both models are in good agreement.

REE abundances in ancient seawater inferred from marine limestone and experimental REE partition coefficients between calcite and aqueous solution

In our earlier work, we determined relative REE partition coefficients between calcite and aqueous (CaCl2 + NaCl) solution, but absolute values could not be obtained because of small overgrowths of calcite. In this study, we have made similar experiments in order to determine the absolute REE partition coefficients between calcite and aqueous solution, and have evaluated partition coefficients between calcite and seawater. Calcite-supersaturated solution doped with REEs was constantly pumped into a reactor to form calcite overgrowths on the seeds under CO2 + N2 gas. Absolute values of REE partition coefficients, Kd(REE)=(XREE/XCa)calcite/([REE,tot.]/[Ca])solution, have been obtained. The patterns of Kd(REE) show convex tetrad effects and Y/Ho fractionation, which is consistent with our earlier work. We have calculated REE abundances in Permian seawater using the partition coefficients corrected to seawater compositions and REE analyses of Permian seamount-type limestones. All the estimated REE abundances in Permian seawater indicate concave tetrad effects, large Y/Ho fractionation and negative Ce and Eu anomalies in their chondrite-normalized patterns, which are quite similar to those for present-day seawater. The estimated REE abundances in Permian seawater, however, are lower than those in present-day seawater by approximately one order of magnitude. This suggests that significant parts of marine limestones could not incorporate seawater REEs after the permeability and porosity of the original biogenic carbonate sediments with low-REE contents had been reduced to certain critical levels.

Mineralogical and biological influences on groundwater chemistry of the Boso Peninsula, Chiba, central Japan: Implications for the origin of groundwater in sedimentary basins

In this study we investigated the geological and biogeochemical processes that influence groundwater chemistry in the Boso Peninsula, Chiba, central Japan. We also discuss the factors that control groundwater chemistry within a sedimentary basin. Saline groundwater is found throughout the study area, while fresh groundwater is distributed unevenly in the central and southern parts of the study area. The distribution of Cl ‐ concentration indicates that groundwater in the study area is influenced by not only present-day seawater penetration but also presence of fossil brine water, although their extents remain uncertain. Since thermodynamic calculation of mineral saturation indices indicates that the groundwater is not in equilibrium with the host rock, it is considered that seawater has intruded to inland areas with and/or after progress of geochemical reactions. Consequently, changes in the concentration of various chemical species by geochemical processes (seawater-freshwater mixing is excluded) were calculated and used as a basis for the classification of samples into six reaction types according to the changing trends of the concentration values calculated by assuming that seasalt components of rainwater affect the groundwater chemistry. Chemical composition is not always correlated with a certain reaction type. The groundwater geochemistry of some reaction types is governed mainly by the dissolution of silicates (plagioclase, illite, chlorite and montmorillonite) and carbonates (calcite, dolomite, and siderite) and precipitation of carbonate minerals, as well as ion exchange of cations. The chemical properties of saline water are influenced by bacterial sulfate reduction, bromine, and the production of methane.

The major-ion composition of Permian seawater

The major-ion (Mg 2+, Ca 2+, Na +, K +, SO 4 2−, and Cl −) composition of Permian seawater was determined from chemical analyses of fluid inclusions in marine halites. New data from the Upper Permian San Andres Formation of Texas (274–272 Ma) and Salado Formation of New Mexico (251 Ma), analyzed by the environmental scanning electron microscopy (ESEM) X-ray energy-dispersive spectrometry (EDS) method, along with published chemical compositions of fluid inclusions in Permian marine halites from North America (two formations of different ages) and the Central and Eastern European basins (eight formations of four different ages) show that Permian seawater shares chemical characteristics with modern seawater, including SO 4 2− > Ca 2+ at the point of gypsum precipitation, evolution into Mg 2+-Na +-K +-SO 4 2−-Cl − brines, and Mg 2+/K + ratios ∼5. Permian seawater, however, is slightly depleted in SO 4 2− and enriched in Ca 2+, although modeling results do not rule out Ca 2+ concentrations close to those in present-day seawater. Na + and Mg 2+ in Permian seawater are close to (slightly below) their concentrations in modern seawater. Permian and modern seawater are both classified as aragonite seas, with Mg 2+/Ca 2+ ratios >2, conditions favorable for precipitation of aragonite and magnesian calcite as ooids and cements. The chemistry of Permian seawater was modeled using the chemical composition of brine inclusions for three periods: Lower Permian Asselian-Sakmarian (296–283 Ma), Lower Permian Artinskian-Kungurian (283–274 Ma), and Upper Permian Tatarian (258–251 Ma). Parallel changes in the chemistry of brine inclusions from equivalent age evaporites in North America, Central Europe, and Eastern Europe show that seawater underwent secular variations in chemistry over the 50 million years of the Permian. Modeled SO 4 2− concentrations are 20 mmol per kg H 2O (mmolal) and 19 mmolal in the Asselian-Sakmarian and Artinskian-Kungurian, with higher concentrations in the Upper Permian Tatarian (23 mmolal). Modeled Ca 2+ is at or above its concentration in modern seawater throughout the Permian. Mg 2+ is close to (slightly below) its concentration in modern seawater (55 mmolal) in the Asselian-Sakmarian (52 mmolal), and Tatarian (52 mmolal), but slightly higher than modern seawater in the Artinskian-Kungurian (60 mmolal). Mg 2+/Ca 2+ ratios are 3.5 (total range = 2.7 to 5.5) in the Lower Permian and rose slightly to 3.7 (total range = 3.1 to 5.8) in the Upper Permian, primarily due to decreases in Ca 2+. These results are consistent with models that predict oscillations in the major-ion composition of Phanerozoic seawater on the basis of changes in the midocean ridge/river water flux ratio driven by changes in the rate of midocean ridge crust production. The Permian was characterized by low sea levels, icehouse conditions, and southern hemisphere glaciation. Such conditions, analogous to the present ice age, and the similarities between Permian seawater and modern seawater, all suggest that general Phanerozoic supercycles, driven by mantle convection and global volcanicity, also control the major-ion chemistry of seawater.

ANDRA underground research laboratory: interpretation of the mineralogical and geochemical data acquired in the Callovian–Oxfordian formation by investigative drilling

Under the ANDRA Meuse/Haute Marne underground research laboratory scientific programme, two boreholes (EST204 and EST205) were drilled to a depth of 510 m for the purposes of scientific characterisation. Twenty-nine core samples were taken in borehole EST205 every 3 m between 422 and 504 m depth. Physical property measurements (water content, porosity, density, specific surface), geochemical analyses (major and trace elements, cation exchange capacity [CEC] and surface cation occupancy, leaching anions, redox state, organic matter concentration), and a semi-quantitative mineralogical study were conducted on the samples. As the rocks are in a reduced state, the core samples were stored under nitrogen immediately after drilling. All the steps of the sampling and of the characterisation are done with a research of limitation of the oxidation to obtain representative samples of the in situ conditions. The top of the formation is more carbonate-rich, with interbedded clayey layers and carbonate rock. The formation is more homogeneous in its central section with a clay mineral concentration of 45–50%, which corresponds to a maximum of flooding within the area. In the upper part of this section, micas and mixed-layer illite/smectite R0 dominate, whereas in the lower part of the section we find an abrupt transition to mixed-layer illite/smectite R1 associated with kaolinite. A statistical analysis, including the data of major and trace elements with the semi-quantitative mineralogy, enabled the identification of some mineralogical traps for trace elements. The values of cation occupancy at the surface of the clay minerals provided a good image of the pore water chemistry; pore water is in equilibrium with the clay surfaces. Leaching experiments revealed the pore water salinity and provided profiles of Cl and Br concentrations. Cl/Br values in the centre of the formation are close to the present-day seawater ratio, which could indicate a seawater origin of the pore water. Some measurements of total reduced capacity, provide quantified results of the reduced state of the rock. Processing the data on water content, helium and petroleum pycnometry enabled calculation of total rock porosity and gave an uncertainty range for this value. Finally, the high BET(N 2) specific surfaces are consistent with the clayey nature of the rock.

Re-Os isotope systematics of sediments of the Brahmaputra River system

Re-Os analyses were performed on suspended loads and coarser grained bank sediments of the Brahmaputra River system. Re and Os concentrations of these sediments vary from 7 to 1154 ppt and from 3 to 173 ppt, respectively. 187Os/ 188Os ratios range from 0.178 to 6.8, and thus vary from nearly mantle to very radiogenic crustal values. Nevertheless, most of the sediments have 187Os/ 188Os ratios less than 1.5, and nearly all of the samples of the Brahmaputra main channel have ratios less than 1.2. Thus, as previously suggested, the Brahmaputra is much less radiogenic than the Ganga. The Siang River, the northern extension of the Brahmaputra, is quite radiogenic in Os despite receiving sediments from the Tsangpo River, which flows along a suture zone with ultramafic outcrops. The Brahmaputra main channel has a fairly constant 187Os/ 188Os ratio even though its tributaries contribute sediments with very heterogeneous Os isotopic compositions. These data, along with the corresponding Nd isotopic compositions, suggest that about 60–90% of the sediment in the Brahmaputra system is derived from Himalayan formations (Higher Himalaya and Lesser Himalaya) whereas 10–40% comes from ophiolite-bearing sequences, perhaps eastern equivalents of those of the Transhimalayan Plutonic Belt. Os data also confirm previously published Sr and Nd results, indicating that about half of the sediments delivered to the Brahmaputra are supplied by the Siang River, while the Himalayan and the eastern tributaries account for 40 and 10%, respectively. The lower 187Os/ 188Os of the Brahmaputra River compared to that of the Ganga is due to two factors. One is the more limited presence of the Lesser Himalaya and hence the lower black shale content of the eastern Himalaya. The other is the non-radiogenic Os supplied by the eastern and southern tributaries, reflecting the presence of mantle-derived lithologies in this region. Despite the lower sediment supply from these tributaries, they contribute greatly to the Os budget of the Brahmaputra River. This study indicates that the Brahmaputra River has little effect on the present-day seawater Os budget. However, reconsideration of this budget suggests that the Ganga, which provides the most radiogenic Os of major rivers studied to date, may have significant impact on the marine Os isotopic composition. The Indo-Asian collision cannot be excluded as an important cause of the increase in the marine 187Os/ 188Os over the past 16 million years until the contributions of all of the rivers draining the Himalayan Tibetan Plateau are known.

Ancient sub-seafloor alteration of basaltic andesites of the Ongeluk Formation, South Africa: implications for the chemistry of Paleoproterozoic seawater

Basaltic andesites of the Ongeluk Formation, Transvaal Supergroup in Griqualand West constitute part of a large flood volcanic province that extruded at ca. 2.22 Ga onto the partly submerged Kaapvaal Craton. Pods and short veins of megaquartz, with albite, epidote and traces of calcite and Cu–Ni–Co sulphides are closely associated with beds and pods of jasper and chert that are products of low-temperature seafloor alteration. Characterization of the megaquartz pods and veins yields evidence for an origin in a “passive” seafloor alteration regime. Fluid inclusion studies suggest the involvement of two aqueous fluid end members, one NaCl-dominated with salinity similar to modern seawater, the other Ca-dominated and with distinctly elevated salinity. The chemical composition of both fluids is akin to that of seawater, modified in its composition by interaction with the volcanic host rock. The low salinity fluid appears only little affected by fluid–rock interaction processes; the composition of the more saline Ca-rich fluid is more distinctly modified. The chemical composition of the two fluids has important implications for our understanding of the composition of ocean water during the Paleoproterozoic Era. Cl/Br ratios, widely regarded as being conservative in hydrothermal solutions, are significantly below those of present-day seawater, but remarkably similar to that predicted for Archean seawater. This observation suggests that Paleoproterozoic seawater was still buffered by vent fluids, and lacked sufficient organic matter to fractionate Cl from Br.

The calcium riverine and hydrothermal isotopic fluxes and the oceanic calcium mass balance

Calcium isotope ratios relative to seawater were studied in natural waters in order to constrain the Ca isotopic flux to the oceans and to discuss the oceanic Ca budget. This study includes the analysis of (1) Ca isotopes in continental waters at the regional scale of the Upper Rhine valley and (2) the determination of the δ44Ca values of some major rivers and hydrothermal vents. The data indicate that the global δ44Ca variability of the studied waters, with a maximal range of 1‰, is limited and must be linked to mass fractionation processes. At the scale of a small watershed (Aubure, Vosges, France), the δ44Ca river flux changes due to the variation of the proportion of waters resulting from chemical alteration of rocks and biologically fractionated soil solutions. Ca isotopes could therefore be an important tool to quantify the impact of the plants and the biosphere on the river water chemistry. At the regional scale of the Rhine valley as well as at the global scale of world rivers, the δ44Ca variability of river samples ranges from 0.5 to 1‰ without any relationship with the lithology of the watershed or the climate. Similarly, the δ44Ca value of hydrothermal vents is also uniform and close to the mean value of river waters. Consequently, the δ44Ca flux to the ocean should remain rather constant through time with a mean of −1.1±0.2‰. This value confirms that the Ca budget of the present-day seawater is in steady-state. This was not necessarily the case in the past. Non-steady-state periods should have been due to intensity variations of incoming and outcoming Ca fluxes rather than to the variations in their isotopic signatures.

Variations of the 44Ca/ 40Ca ratio in seawater during the past 24 million years: evidence from δ 44Ca and δ 18O values of Miocene phosphates

The Ca isotope variation of 11 Miocene and Pleistocene, authigenic, marine phosphates is rather small compared to the corresponding variation in δ 18O values. The δ 44Ca values are not correlated with the δ 18O values and, therefore, they are not temperature controlled. It is likely that the δ 44Ca values of the phosphorites reflect the variation in the δ 44Ca values of paleo-seawater but, in contrast to Sr, not the isotopic composition of seawater itself. Furthermore, Ca and Sr isotopic compositions are decoupled with decreasing stratigraphic age of the phosphate peloids with Sr isotopic compositions changing to more radiogenic values while Ca isotopic compositions remain rather stable. All samples have δ 44Ca values below present-day seawater values, suggesting that phosphate formation discriminates against heavy Ca isotopes. Phosphorites and carbonaceous sediments have a similar Ca isotopic variation during the Miocene. A systematic and more or less constant shift between marine carbonates and phosphates is observed: the phosphate samples are slightly less enriched in 40Ca compared to carbonates. This shift has been related to a mineral-dependent kinetic mass fractionation during precipitation from seawater. The rather stable δ 44Ca value for the 19 to 9 Ma old phosphorites points to a constant δ 44Ca fractionation of about 1.1 between seawater and phosphorites during the past and suggests steady-state conditions for the Mid-Miocene seawater (sedimentation flux equals erosion flux).

Re-Os Dating of Polymetallic Ni-Mo-PGE-Au Mineralization in Lower Cambrian Black Shales of South China and Its Geologic Significance

Black shales of the basal Lower Cambrian Niutitang Formation, southeast China, host a regionally distributed concordant, several centimeter-thick, sulfide layer which displays extreme metal enrichment, i.e., Mo-NiSe-Re-Os-As-Hg-Sb >1,000 times enriched and Ag-Au-Pt-Pd >100 times enriched over bulk continental crust. Mineable portions have about 5.5 wt percent Mo, 3.5 wt percent Ni, and 1 g/t PGE + Au. A six-point 187 Os/ 188 Os versus 187 Re/ 188 Os isochron on molybdenum-nickel ore samples defines an age of 541 ± 16 Ma (2σ) with an initial 187 Os/ 188 Os ratio of 0.78 ± 0.19. This age is in agreement with the depositional age of the black shale host; the initial ratio is close to present-day seawater. The sulfide layer/average seawater metal ratio is on the order of 10 6 to 10 8 , about 10 to 100 times higher than that for the black shale host and for average metalliferous black shale. Synsedimentary metal enrichment from seawater under anoxic (sulfate-reducing) conditions appears likely but requires an unusually low sedimentation rate and/or high replenishment rate of fresh seawater to the marine basin. The paleogeographic setting of the Lower Cambrian continental margin of the Yangtze craton indicates local basins controlled by synsedimentary rifting. Stagnant water episodically replenished by upwelling oxidized seawater is thought to be responsible for the formation of the polymetallic sulfide layer and of phosphorite, barite, and sapropelic “stone coal” (combustible black shale) beds.

The major-ion composition of silurian seawater

One-hundred fluid inclusions in Silurian marine halite were analyzed in order to determine the major-ion composition of Silurian seawater. The samples analyzed were from three formations in the Late Silurian Michigan Basin, the A-1, A-2, and B Evaporites of the Salina Group, and one formation in the Early Silurian Canning Basin (Australia), the Mallowa Salt of the Carribuddy Group. The results indicate that the major-ion composition of Silurian seawater was not the same as present-day seawater. The Silurian ocean had lower concentrations of Mg 2+, Na +, and SO 4 2−, and much higher concentrations of Ca 2+ relative to the ocean’s present-day composition. Furthermore, Silurian seawater had Ca 2+ in excess of SO 4 2−. Evaporation of Silurian seawater of the composition determined in this study produces KCl-type potash minerals that lack the MgSO 4-type late stage salts formed during the evaporation of present-day seawater. The relatively low Na + concentrations in Silurian seawater support the hypothesis that oscillations in the major-ion composition of the oceans are primarily controlled by changes in the flux of mid-ocean ridge brine and riverine inputs and not global or basin-scale, seawater-driven dolomitization. The Mg 2+/Ca 2+ ratio of Silurian seawater was ∼1.4, and the K +/Ca 2+ ratio was ∼0.3, both of which differ from the present-day counterparts of 5 and 1, respectively. Seawaters with Mg 2+/Ca 2+ <2 facilitate the precipitation of low-magnesian calcite (mol % Mg < 4) marine ooids and submarine carbonate cements whereas seawaters with Mg 2+/Ca 2+ >2 (e.g., modern seawater) facilitate the precipitation of aragonite and high-magnesian calcite. Therefore, the early Paleozoic calcite seas were likely due to the low Mg 2+/Ca 2+ ratio of seawater, not the pCO 2 of the Silurian atmosphere.

Quaternary phosphorites off the southeast coast of India

Detailed petrological, mineralogical, geochemical and radiogenic (U, Sr, Nd) and stable isotope (C, O, S) studies have been carried out on the Quaternary phosphorites of the continental margin off Chennai, southeast coast of India. These phosphorites are formed as a result of trapping and binding of sediments by microbial mats and are similar to phosphate stratiform stromatolites. Detrital and biogenic constituents enclosed in the phosphorites controlled the major and minor element composition. Except for Sr and U, the concentrations of most of the trace metals are lower than those in average shale and phosphorite. Middle rare earth element (MREE)-enriched patterns are the characteristic feature. The U–Th dating method indicates that the ages of the phosphorites are beyond 300,000 years. 87Sr/ 86Sr ratios of the phosphorites are higher than that of present-day seawater and εNd values are more negative than those of seawater of the Atlantic Ocean. Carbon isotope ratios are within the range expected for the oxic/suboxic zone but sulfur isotope ratios indicate suboxic conditions during phosphorite formation. These results imply that the benthic microbial mats thrived on the shallow shelf during the Quaternary low sea level conditions. Periodic or episodic sedimentation onto the mats led to their death. The bacteria associated with decaying microbial mats utilised phosphorus supplied by continental sources and rapidly precipitated phosphate. The availability of a high percentage of phosphorus in seawater seems to be an important controlling factor for the formation of phosphate stromatolites. The composition of these phosphorites differs from the modern phosphorites in upwelling regions, but are similar to Cambrian apatite stromatolites. These phosphorites provide evidence that the replicates of ancient phosphate stromatolites do exist in the Quaternary.

Halite dissolution derived brines in the vicinity of a Permian salt dome (N German Basin). Evidence from boron, strontium, oxygen, and hydrogen isotopes

The paper presents the isotopic (B, Sr, O, H) and chemical composition of solid salts, brines, groundwater, and surface water in the vicinity of a Permian salt dome in Northern Germany (diapir of Gorleben-Rambow) being considered for disposal of high level radioactive waste. The site is situated in the NW European Basin that is dominated by intensive diapirism of Permian salt and overprinted by glacial erosion/sedimentation. The local hydrogeology comprises an upper aquifer system of heterogeneous Pleistocene sediments and a lower aquifer system of Tertiary clastics tectonically disturbed by diapirism. There is a clear chemical and isotopic evolution with TDS, that generally increases with depth up to 328 g · l−1. The chemistry and isotope ratios of the saline groundwater can be explained by a two end-member mixing system. The Na-Cl brine end-member is derived from evaporite dissolution as indicated by major ion chemistry, Cl/Br ratios, Sr and B isotopes. It is mixed with fresh-brackish groundwater dominated by water-silicate interaction. This low salinity end-member (TDS from 0.3–10 g · l−1) shows radiogenic 87Sr/86Sr ratios ranging from 0.70910 to 0.71030 and low δ11B values between 7.4 and 16.5‰ vs. NBS951. Two halite samples and a borate sample from the unaltered parts of the diapir show 87Sr/86Sr ratios from 0.70696 to 0.70868 and δ11B values of 20.8 and 27.7‰. Brine (TDS from 100–328 g/l) Sr isotope ratios, ranging from 0.70758 to 0.70788, lie in the uppermost range of late Permian ocean waters and are within the range of halite. The δ11B values of the brines vary between 25.1‰ to 33.5‰, which is close to the range of solid salt but well below those of evaporating present-day sea water. B isotope composition is thus compatible with salt dissolution rather than with residual evaporite brines. The chemistry and isotope characteristics of the brine end-member derived from halite dissolution are not significantly modified by subsequent interaction with the siliciclastic aquifer material. Most highly saline groundwaters show cold climate O and H isotopic signatures with δ18O values ranging from −10.5 to −9‰. A limited group of four rather deep-seated brines (>200 m) plots in the range of Holocene signatures with δ18O > −9‰ close to those of the shallow groundwater which is interpreted as an indication of rapid groundwater circulation from the surface and ongoing salt dissolution.

Brine-mineral reactions in evaporite basins: Implications for the composition of ancient oceans

The chemical evolution of several European Mesozoic and Tertiary evaporite basins was reconstructed by using mineral associations, primary fluid-inclusion analyses, and numerical simulations of evaporation scenarios. The solute proportion recorded in the fluid inclusions can be explained by the evaporation of present-day seawater as a major recharge. The sulfate depletion in the brines is responsible for the type of potash deposit formed, potassium-magnessium sulfates or sylvite. This sulfate depletion can be due either to dolomitization or to the addition of a CaCl 2 -rich solution to the basin. The sulfate depletion occurred in varying intensity in basins of the same age, as well as throughout the evolution of the same basin. Therefore, changes in potash mineralogy and sulfate depletion in fluid inclusions are not conclusive arguments in favor of secular variations in the composition of the ocean, as recently proposed by several authors.

Pb and Sr isotopic evidence for sources of atmospheric heavy metals and their deposition budgets in northeastern North America

Concentrations of Al, Ba, Cd, Cu, Mg, Mn, Pb, Rb, Sr, and Zn, as well as Pb and Sr isotopic compositions were determined in samples of snowpack obtained during the 1998 winter season from northeastern North America. Median enrichment factors, relative to upper crustal abundances, for Cd, Cu, Mn, Pb, and Zn for all samples range from ca. 50 to 36,000 and are indicative of an anthropogenic origin. The Pb isotope ratios correlate with geographic location because snowpack from eastern Ontario and the northeastern USA are characterized by the most radiogenic 206Pb/ 207Pb ratios (ca. 1.18–1.19), and these decrease systematically in an easterly direction towards the Atlantic coast (ca. 1.16). The Pb isotope data for the 1998 samples of snowpack indicate that atmospheric pollution in this region of North America is dominated solely by a mixture of anthropogenic emissions from US ( 206Pb/ 207Pb ca. 1.20) and Canadian ( 206Pb/ 207Pb ca. 1.15) sources. This result contrasts with that obtained for 1997 snowpack from similar geographic regions, because the Pb isotope data for the latter suggest the involvement of an additional anthropogenic component, possibly that of Eurasian pollution transported over the high Arctic. This difference in the distribution pattern of atmospheric pollution over northeastern North America between the 1997 and 1998 winter seasons may be related to the El Niño phenomenon. Sr isotope data for all the 1998 snowpack samples define a large range in 87Sr/ 86Sr values but most are between 0.709 to 0.710, similar to the value of present-day seawater (ca. 0.7092). However, the majority of snowpack samples from eastern Ontario are characterized by lower 87Sr/ 86Sr ratios (<0.709), and these correlate negatively with their enrichment factors of Pb. Such trends suggest the presence of a relatively ‘unradiogenic’ source of anthropogenic Sr, possibly related to emissions from coal-fired power plants located in the western and midwestern regions of the USA. Annual depositional budgets estimated from 1998 snowpack indicate that those for Pb, Cd, and Cu are highest in eastern Ontario, and significantly lower in the remaining regions. The former reflect a “meteorological corridor” in northeastern North America, which coincides with heavily industrialized and densely populated areas. Depositional budgets of Zn are extremely high in all areas investigated and their exact source(s) remains enigmatic. These may reflect either an increase in atmospheric industrial emissions (since the 1980s), a lower residence time in the atmosphere, or particular chemical speciation that affects the deposition of Zn from the lower troposphere. Depositional budgets for Mn are also high and may be related to the combustion of Mn-bearing fossil fuels by automotive vehicles.

Evidence for deep fluid circulation and gas hydrate dissociation using boron and boron isotopes of pore fluids in forearc sediments from Costa Rica (ODP Leg 170)

Abstract Drilling a transect of holes across the Costa Rica forearc during ODP Leg 170 demonstrated the margin wedge to be of continental, non-accretionary origin, which is intersected by permeable thrust faults. Pore waters from four drillholes, two of which penetrated the decollement zone and reached the underthrust lower plate sedimentary sequence of the Cocos Plate, were examined for boron contents and boron isotopic signatures. The combined results show dilution of the uppermost sedimentary cover of the forearc, with boron contents lower than half of the present-day seawater values. Pore fluid “refreshening” suggests that gas hydrate water has been mixed with the sediment interstitial water, without profoundly affecting the δ 11 B values. Fault-related flux of a deeply generated fluid is inferred from high B concentration in the interval beneath the decollement, being released from the underthrust sequence with incipient burial. First-order fluid budget calculations over a cross-section across the Costa Rica forearc indicate that no significant fluid transfer from the lower to the upper plate is inferred from boron fluid profiles, at least within the frontal 40 km studied. Expulsed lower plate pore water, which is estimated to be 0.26–0.44 km 3 per km trench, is conducted efficiently along and just beneath the decollement zone, indicating effective shear-enhanced compaction. In the upper plate forearc wedge, dewatering occurs as diffuse transport as well as channelled flow. A volume of approximately 2 km 3 per km trench is expulsed due to compaction and, to a lesser extent, lateral shortening. Pore water chemistry is influenced by gas hydrate instability, so that it remains unknown whether deep processes like mineral dehydration or hydrocarbon formation may play a considerable role towards the hinterland.

Comparison of oxygen isotope profile of a fossil bivale with the modern hydrographic condition: case study of the Seoguipo Formation (Korea)

The early to middle Pleistocene Seoguipo Formation, cropping out along the southern coast of Cheju Island (Korea), consists of shallow marine deposits containing numerous macro-and micro-fossils with prolific trace fossils. Oxygen isotopic compositions of a well-preserved marine bivalve (Mizuhopecten tokyoensis hokrikuensis) from the formation were measured through a high-resolution sampling along the maximum shell growth to track the paleoceanographic conditions during the lifespan of the bivalve. The oxygen isotope profile shows two distinct cyclic patterns with seasonal contrast. The minimum (i.e., summer) and maximum (i.e., winter) δ18O values denote about 2.9 to 3.4‰ (PDB) of the seasonal variation, which can be converted to approximately 13 to 15°C in seasonal temperature variation. Using the paleotemperature equation for calcite, the bimonthly measurements of temperature and salinity at water depths of 10 m and 30 m surveyed by the National Fisheries Research and Development Agency (Korea) between 1991 and 1997 enable us to construct the theoretical oxygen isotope profile that represents the present-day environmental condition. The constant δ18O values of seawater are assumed in the equation, although the salinity varies seasonally to some extent. Direct comparison between the fossil mollusk profile (ancient condition) and the predicted isotope profile (present-day condition) explains that the summer and winter paleotemperatures during fossil shell growth were quite lower than the present. It is also possible that the bivalve might grow in the18O-enriched condition different from the present-day seawater.

Palaeoclimatic and chronostratigraphic interpretations from strontium, carbon and oxygen isotopic ratios in molluscan fossils of Quaternary Seoguipo and Shinyangri Formations, Cheju Island, Korea

Palaeotemperatures and chronostratigraphies of the Quaternary Seoguipo and Shinyangri Formations, Cheju Island, South Korea were investigated in terms of the strontium, oxygen and carbon isotopic compositions of carbonate mollusc shells. The 87Sr/ 86Sr ratios of mollusc shells from the Seoguipo Formation range from 0.70911 to 0.70914, which yield carbonate ages of 0.5–1.2 Ma (Early to middle Pleistocene) using the linear equation between ages and 87Sr/ 86Sr ratios of Quaternary seawater. The 87Sr/ 86Sr ratio (0.70917) of aragonitic shells from the Shinyangri Formation provide late Pleistocene ages (0.04 Ma). Based on AMS 14C dates of mollusc fossil shells from the Shinyangri Formation, the depositional age of the formation has been extended to 1570 yr BP. Palaeotemperatures calculated on the basis of the oxygen isotopic composition of the calcitic mollusc shells from the Seoguipo Formation give a range of values from 12.6 to 19.1°C, which is lower than the present-day shallow seawater temperature (14.9–25.5°C) in the vicinity of the Seoguipo coast. Aragonitic mollusc shells from the Shinyangri Formation provide oceanic temperatures ranging from 22.5 to 29.8°C, which is higher than present-day seawater temperatures (14.1–23.2°C). The calculated palaeotemperatures of the Seoguipo and Shinyangri Formations are inferred to reflect cold and warm currents flowing over Cheju Island during the Pleistocene to Holocene.

The accretion rate of extraterrestrial 3He based on oceanic 230Th flux and the relation to Os isotope variation over the past 200,000 years in an Indian Ocean core

In the eastern equatorial Indian Ocean, the flux of extraterrestrial 3He, a proxy of interplanetary dust particles (IDPs), has been relatively constant over the past 200 ka. The flux is equal to (1.1±0.4)×10 −12 cm 3 STP cm −2 ka −1, a value obtained using the xs 230Th profiling method. Variations in mass accumulation rates (MARs) derived assuming a constant extraterrestrial 3He flux have a 40-ka periodicity similar to that observed in the δ 18O-derived MARs. This frequency is similar to that of the Earth's obliquity. Measured 187Os/ 188Os ratios are less radiogenic than present-day seawater (0.49–0.98), reflecting the mixing of Os derived from extraterrestrial, terrigenous and hydrogenous sources. When coupled with He data measured on the same samples, Os isotope data yield important information about the terrigenous component supplied to the eastern equatorial Indian Ocean. The amount of Os in the sample derived from the extraterrestrial component can be deduced with the help of the helium systematics. Once corrected for the extraterrestrial component of Os, Os isotope signatures, in conjunction with the 4He concentrations, suggest a supply of terrigenous material from Indonesian ultramafic and Himalayan crustal sources that clearly varies through time.