Ostracod Shell Chemistry Research Articles

Late Quaternary changes in moisture availability and weathering intensity on the central Tibetan Plateau indicated by chemical signatures of ostracod shells

High-resolution multi-proxy records from two lakes on the southern Tibetan Plateau, Nam Co and Tangra Yumco, are used to infer long-term variations in the Asian monsoon system with a novel set of ostracod shell chemistry proxies. We track the moisture evolution since the Last Glacial Maximum using the trace element, rare earth element (REE) and stable isotope composition of ostracod shells. The sediment records covering the past 18.8 cal. ka BP and 17.4 cal. ka BP, respectively, demonstrate the suitability of REEs as indicators of weathering intensity and thus hydrological changes and moisture sources in the catchment. In Nam Co, high concentrations of light REEs between 14 and 13 cal. ka BP suggest an increased drainage from the glaciated Nyainqêngtanglha Mountains in the south, pointing to meltwater input. REEs in ostracod shells therefore provide additional information on water sources critical for the interpretation of stable isotope records. Mg/Ca, Sr/Ca, and Ba/Ca ratios reflect salinity and thus changes in effective moisture. Asynchronous behavior of Mg/Ca, Sr/Ca, and Ba/Ca ratios are controlled by changes in dominance of precipitating carbonate minerals in the lake. Synchronous behavior reflects calcite precipitation, indicating low-Mg/Ca warm-wet conditions. Constantly low Sr/Ca ratios reflect aragonite precipitation, indicating high-Mg dry conditions. Increased Sr/Ca and Ba/Ca relative to Mg/Ca ratios show monohydrocalcite precipitation, indicating high-Mg/Ca cold-dry conditions. Furthermore, Fe/Ca, Mn/Ca and U/Ca ratios in ostracods reflect changes in oxygen saturation in lake bottom waters controlled by lake level and microbial activity. The paleoclimate histories reconstructed from Nam Co and Tangra Yumco show high similarity throughout the late Quaternary. We identified two major dry periods, corresponding to Heinrich 1 and the Younger Dryas, followed by strengthening in Indian summer monsoon precipitation. The early Holocene is characterized by a moisture maximum, reflecting abundant water supply by a strong ISM. A time-delayed shift to dry conditions occurred at 2.6 cal. ka BP at Tangra Yumco, and at 2 cal. ka BP at Nam Co, resulting in decreasing lake levels, caused by weakened monsoon intensity due to a southeastward migration of the ISM-Westerly boundary with an estimated velocity of approximately 600 m per year.

Reconstruction of short-term storm surge-driven increases in shallow coastal lake salinity using ostracod shell chemistry

Reconstruction of short-term storm surge-driven increases in shallow coastal lake salinity using ostracod shell chemistry

Tracking the seasonal calcification of Cyprideis torosa (Crustacea, Ostracoda) using Mg/Ca-inferred temperatures, and its implications for palaeotemperature reconstruction

Abstract Ostracod shell chemistry data are widely used for palaeoenvironmental reconstruction. Despite this, there has been little systematic research into the implications of the timing of calcification or the duration of each moult stage. Consequently, it is unclear whether palaeoenvironmental reconstructions are recording restricted (inter-seasonal) time periods or reflect the mean annual conditions. The seasonality of shell formation can therefore have implications for palaeoenvironmental reconstructions based on geochemical signatures, especially palaeotemperature, particularly in environments that show large inter-annual variations in water conditions. Cyprideis torosa is a geographically widespread and eurytopic species that has great potential for a range of palaeoenvironmental reconstructions, but inhabits environments with large seasonal and inter-annual variation. Using hourly water and air temperature data, ostracod shell and surface water chemistry from a shallow coastal pond in SE UK, we improve knowledge of the timing of Cyprideis torosa calcification, and thus our understanding of the potential seasonality of signals in palaeotemperature datasets. We suggest seasonal calcification in spring and autumn, with persistence at the adult life stage for up to 12–18 months. Sr/Ca values of C. torosa appear to reflect a Sr/Cawater control on calcification timing and have no temperature dependence. For Mg/Ca, we show a minimum temperature control on calcification of 7 °C, with C. torosa Mg/Ca-inferred temperatures broadly tracking spring and autumn temperatures.

Holocene hydrologic and hydrochemical changes of the South Basin of Lake Manitoba, Canada, inferred from ostracode shell chemistry and autoecology

Holocene record of ostracode shell chemistry from the South Basin of Lake Manitoba shows how isostatically and climatically induced changes in the surface and groundwater input to the overall hydrologic budget affected the hydrochemistry. The shallow South Basin experienced periods of extensive shell transport from isolated pools and groundwater seeps, as suggested by disparate chemistry of Fabaerformiscandona rawsoni shells from the same 1-cm-thick sediment sample. Temporal changes reconstructed from ostracode species and F. rawsoni shell chemistry include, for some depth intervals, shells of individuals that were not living at the same time or were not living at the core site. Shell chemistry before the South and North Basins became connected at 1850 BP broadly show a hydrologically closed-basin behavior with alternating periods of a more and less stressful environment to ostracodes. F. rawsoni with high δ 18O and low Mg/Ca or low δ 18O and high Mg/Ca were likely transported from marginal evaporated pools or groundwater seeps. Overall δ 18O and δ 13C values increase over time, but at levels for which multiple analyses were made, the values show a wide range, and indicate a highly variable hydrochemical environment both spatially and temporally (over one summer or multiple summers).

Ostracod shell chemistry as proxy for paleoenvironmental change

The application of ostracod shell chemistry as a paleoenvironmental tool has grown within the past decades. Most studies have investigated Mg and Sr in ostracod shells as proxies for temperature and salinity, and the use of a wide range of trace elements as prospective paleoenvironmental indicators has yet to be developed for lacustrine systems. Only a few preliminary studies have used trace metals in paleolimnological studies such as Cd, Ba and Zn as paleonutrient indicators, or Mn, Fe and U as redox and oxygenation indicators. This paper reviews the state of the art of geochemical analyses in microfossils such as ostracods, foraminifera, and corals, and provides insights in new trace element proxies with the goal to promote the use of trace elements in ostracod shells as paleoenvironmental proxies.In paleoceanography, foraminifera and corals are most prominently used to reconstruct past climate conditions. Well-established proxies such as δ18O, Mg/Ca and Sr/Ca provide information about changes in sea surface temperatures. In addition, a great number of new proxies have been developed recently, such as radiogenic isotopes and redox sensitive trace elements. In paleolimnology, ostracod shell chemistry is widely used to assess paleohydrological changes. Reconstruction of temperature and salinity changes in lake environments is often achieved by oxygen isotopes as well as Mg/Ca and Sr/Ca ratios, but depending on the hydrological and geological settings of the lake system, local calibrations are needed to assess which proxy is suited to reflect which processes.New proxies need to be tested by novel techniques that recently have become available. Compared to conventional instrumentation used in ostracod shell chemistry, methods such as Laser Ablation ICP-MS and NanoSIMS allow single shell analysis and provide high-resolution data. The potential of ostracods in paleolimnology is not yet fully assessed, but can be developed by learning from paleoceanographic studies.

Changes in continental ostracode shell chemistry; uncertainty of cause

Ostracode shell chemistry is a powerful tool for recording changes of certain dissolved ions in their environment. Such data are commonly interpreted in terms of change in temperature and salinity, which are then used to offer insights into paleohydrology and from that paleoclimate. In particular, the changes in Sr/Cashell are believed to reflect changes in salinity, and the changes in Mg/Cashell to reflect both salinity and water temperature. However, the established application of this proxy too often ignores the complexities involved in the chemical changes accompanying solute evolution, hydrologic change, physical chemistry, and the autoecology and biology of ostracodes. Chemical changes occur: (1) in the dissolved Mg/Ca and Sr/Ca ratios during solute evolution, (2) as a consequence of multiple sources of water, but especially ground and surface water interchange as well as evaporation, (3) as a consequence of different dissolved cation–anion pair formation at various concentrations, and (4) because of bio/ecologic effects, including seasonality of molting and taxa-specific ionic regulation capabilities. We suggest that changes in shell chemistry in a stratigraphic sequence tell us when changes occurred in the solutes, but cannot identify what caused those change(s) in the chemical environment. Inclusion of information from other paleoenvironmental and paleoclimatic proxies that co-occur with ostracode shells may help to limit the choices amongst possible causes.

Comment on G. Wansard and F. Mezquita, The response of ostracode shell chemistry to seasonal change in a Mediterranean freshwater spring environment

In a study of the minor element chemistry of ostracode shells, Wansard and Mezquita (2001) concluded that the Mg/Ca and Sr/Ca of the ambient water controlled the respective ratios in ostracode calcite. Contrary to their conclusion that minor element chemistry is not influenced by temperature, we find a very strong statistical correlation (r = 0.97) between temperature and shell chemistry in their data. This discrepancy apparently arises due to their use of a partition coefficient model of shell chemistry that masks a number of strong correlations in their data. We argue here that the partition coefficient model is not appropriate for this biologically-mediated carbonate, and that a broader range of possible controls on shell chemistry needs to be tested.

The response of ostracod shell chemistry to seasonal change in a Mediterranean freshwater spring environment

We established the relationships between water chemistry changes in a pool fed by a permanent spring and seasonal variations in trace-element contents (Sr & Mg) in the shells of the ostracod species Herpetocypris intermedia, based on monthly collections of ostracod and water samples. The water chemistry of the investigated pool (Maiques, Valencia, Spain) was dominated by calcium and bicarbonate, and showed marked seasonal variation in alkalinity, Ca2+ content, Sr/Ca and Mg/Ca ratios. Although the variability in the water chemistry was relatively low (similar to 10% relative standard deviation over the entire period), the trace-clement contents: in the ostracod shells tracked the seasonal change in the water chemistry of Maiques pool. Moreover, due to the rapid renewal of H. intermedia population, this species is able to record in its shells the evolution of the water chemistry at a monthly time scale. Our results also showed that, in the Maiques pool system, ostracod Sr/Ca and Mg/Ca ratios increased with the decrease in water salinity. To our knowledge, this is the first geochemical study of ostracods dwelling in spring environments. The results of this study may be applied to paleohydrological reconstruction using ostracods preserved in sediments deposited around springs (i.e., travertine and tufa deposits).

Hydrologic Variation in the Northern Great Plains During the Last Two Millennia

Reconstructions of lake-water salinity at decadal resolution for the last 2000 yr are compared among three lakes in North Dakota to infer regional patterns of drought. The intersite comparisons are used to distinguish local variation in climate or hydrology from regional patterns of change. At one lake, diatom-inferred salinity and lake-water Mg/Ca inferred from ostracode shell chemistry are coherent, both in terms of direction and magnitude of change, indicating that each is a robust technique for reconstructing lake-water chemistry. The data show that the last 2000 yr have been characterized by frequent shifts between high and low salinity, suggesting shifts between dry and moist periods. Long intervals of high salinity suggest periods of multiple decades when droughts were intense and frequent, thus indicating times when drought was more persistent than in the 20th century. Both the Medieval Period and Little Ice Age were hydrologically complex, and there is no clear evidence to suggest that either interval was coherent or unusual in effective moisture relative to long-term patterns. Differences among the three sites may be attributed to variation in local hydrology, and these differences emphasize the need for multiple sites in deriving regional climate interpretations from paleoecological data.

La géochimie des éléments en traces dans la calcite des ostracodes: Principes, limites et perspectives de la méthode pour une reconstitution quantifiée des paléoenvironnements

Ostracods are aquatic microcrustaceans with calcitic, bivalved carapaces. Recent studies high-lighted relationships between trace-element (mainly Sr and Mg) contents in ostracod valves and the temperature and chemistry of the host water. These relationships were successfully applied by several authors for quantification of paleoenvironmental reconstructions. This paper reviews the basics of the geochemical study of the ostracod shells and briefly describes new factors that play a role in the uptake of trace-elements and make the interpretation of the trace-element contents in fossil valves more complex. More geochemical studies through monitored laboratory cultures will provide better controls on ostracod shell chemistry and thus a better efficiency of the method for the quantification of paleoenvironmental and paleoclimate changes.

Paleoenvironmental reconstruction of human activity in Central Arizona using shell chemistry of Hohokam canal ostracodes

The prehistoric Hohokam are the only known culture to develop large-scale irrigation systems before the arrival of Europeans to the United States. Hohokam social evolution was based, in part, on intensive agriculture. To date, most models of Hohokam irrigation agriculture (i.e., hydrologic and sedimentologic models) fail to address dynamics like flow regime, seasonality, canal use and abandonment (Henderson and Hackbarth, 1939). Biota recovered from Hohokam canals often include ostracodes, carbonate-secreting microcrustaceans that are sensitive to hydrochemical changes. m(Mg/Ca)v and m(Sr/Ca)v ratios in the ostracode Limnocythere staplini can be converted into approximate temperature and salinity values at the time of calcification. Ostracode shell chemistry is becoming a powerful tool for understanding the history of canal water chemistry, intensity of land use, and human impact on soil. This study uses multiple regression models to generate the first paleoenvironmental estimates of temperature and salinity from ostracode shell chemistry. Temperature estimates suggest that most canals were used between late winter (February–March) and the premonsoonal season (May–June). Salinity estimates apparently vary over time. Both paleoenvironmental estimates are in good agreement with history records for the Phoenix area. © 1997 John Wiley & Sons, Inc.

Trace-element and stable-isotope geochemistry of non-marine ostracod shells in Quaternary palaeoenvironmental reconstruction

Ostracods are small bivalved aquatic crustancean. They secrete shells of low-Mg calcite that are often preserved in lake sediments. Recent work has shown that the uptake of trace elements (especially Mg and Sr) into the shell may be a function of the salinity and temperature of the host water. Furthermore, ostracod shells are a source of carbonate for stable-isotope analysis. This paper reviews the application of ostracod shell chemistry to Quaternary palaeolimnology. Although such work has revealed the excellent potential of these techniques to provide quantitative palaeolimnological reconstructions, a number of problems have also emerged. These problems relate to (1) methods used for extraction of ostracod shells from sediment and their subsequent cleaning (2) post-mortem diagenesis and alteration of the shell (3) complications with the calcification mechanism (4) spatial and temporal variability in shell composition (5) the ecological tolerances of individual species and (6) the relationships between shell chemistry and palaeohydrology. To some extent, these problems are an inevitable outcome of the diversity of lacustrine systems: they may be overcome by developing a thorough understanding of the physiology, life-cycle and ecology of the species concerned, together with the modern limnology of the study site. Overall, these techniques have excellent potential in Quaternary palaeolimnology, especially when used with other palaeoenvironmental indicators.

Deep-sea ostracode shell chemistry (Mg:Ca ratios) and Late Quaternary Arctic Ocean history

Abstract The magnesium:calcium (Mg:Ca) and strontium:calcium (Sr:Ca) ratios were investigated in shells of the benthic ostracode genus Krithe obtained from 64 core-tops from water depths of 73 to 4411 m in the Arctic Ocean and Nordic seas to determine the potential of ostracode shell chemistry for palaeoceanographic study. Shells from the Polar Surface Water (−1 to −1.5°C) had Mg:Ca molar ratios of about 0.006–0.008; shells from Arctic Intermediate Water (+0.3 to +2.0°C) ranged from 0.09 to 0.013. Shells from the abyssal plain and ridges of the Nansen, Amundsen and Makarov basins and the Norwegian and Greenland seas had a wide scatter of Mg:Ca ratios ranging from 0.007 to 0.012 that may signify post-mortem chemical alteration of the shells from Arctic deep-sea environments below about 1000 m water depth. There is a positive correlation ( r 2 = 0.59) between Mg:Ca ratios and bottom-water temperature in Krithe shells from Arctic and Nordic seas from water depths <900 m. Late Quaternary Krithe Mg:Ca ratios were analysed downcore using material from the Gakkel Ridge (water depths 3047 and 3899 m), the Lomonosov Ridge (water depth 1051 m) and the Amundsen Basin (water depth 4226 m) to test the core-top Mg:Ca temperature calibration. Cores from the Gakkel and Lomonosov ridges display a decrease in Mg:Ca ratios during the interval spanning the last glacial/deglacial transition and the Holocene, perhaps related to a decrease in bottom water temperatures or other changes in benthic environments.

Nonmarine ostracode shell chemistry from ancient hohokam irrigation canals in central Arizona: A paleohydrochemical tool for the interpretation of prehistoric human occupation in the North American Southwest

AbstractEnvironmental archaeology studies on samples from an agricultural context provide information that may be used to answer questions of climatic vs. human impact on Hohokam prehistory. Hohokam irrigation systems were fragile and subject to episodic damage and destruction. Ostracodes are sensitive to hydrochemical changes. Their fossil record in Hohokam canals can provide useful data to understand human/environment relationships in the past. Low‐Mg calcite ostracode carapaces form in thermal and chemical equilibrium with the host water. Magnesium and strontium are commonly incorporated into the shell as trace elements, which can be used to track paleotemperature and paleosalinity variations. Thus, fossil ostracodes reflect the water chemistry in which the valves were precipitated. Valves of Limnocythere staplini, Candona patzcuaro, and Cypridopsis vidua were analyzed using inductively coupled plasma mass spectrometry (ICP‐MS) to determine the Mg/Ca and Sr/Ca ratios. Trace element data were used to reconstruct the canal water chemistry and to propose paleoclimatic and human‐impact hypotheses for environmental change. The ostracode record shows some evidence of a progressive salinization process between A. D. 1025 and 1425 that may be the result of human disturbance; this inference is supported by independent evidence from geographic, palynologic, and dendroclimatologic data. In addition, two major climatic events, large enough not to be masked by human activity, were also recorded by ostracode geochemistry: a flood between A. D. 855 and 910, another flood prior to A. D. 1350, and a drought between A. D. 1365 and 1425. © 1994 John Wiley & Sons, Inc.

Nonmarine ostracods as Quaternary palaeoenvironmental indicators

Ostracods are small, bivalved crustaceans with calcitic shells that are often well preserved in Quaternary sediment sequences. Although ostracods are found in most types of water body, work on Quaternary faunas has focused on marine environments. This paper, however, reviews the significant contribution that ostracods have also made to the study of nonmarine environments, particularly lakes. Although much of the early work on ostracods was taxonomic, recent studies have dramatically improved our understanding of ostracod ecology. Aquatic chemistry, in particular, has been shown to be an important control on the occurrence of nonmarine species. However, data are still lacking from many parts of the world and for many species. A knowledge of such controls is vital if ostracods are to be used as palaeoenvironmental indicators. An exciting development over the past decade has been the use of ostracod shell chemistry as a palaeoenvironmental tool. The stable isotope composition of the shell has been used in a number of studies to provide important information on palaeohydrology and palaeoproductiv ity of lakes. Furthermore, the trace-element geochemistry of ostracod shells has been used most successfully as a palaeosalinity and palaeotemperature indicator. Although nonmarine ostracods have yet to be used routinely as Quaternary palaeoenvironmental indicators, it is clear that they will play an increasingly important role with the collection of more ecological data and the development of new techniques.

Ostracod shell chemistry: A new palaeoenvironmental indicator applied to a regressive/transgressive record from the gulf of Carpentaria, Australia

We demonstrate the feasibility of determining a continuous record of chemical changes occurring during marine transgressive/regressive phases, using the Mg/Ca and Sr/Ca ratios measured in some 300 single valves of the euryhaline ostracod Cyprideis from two cores from the Gulf of Carpentaria, Australia. The molar Mg and Sr distribution coefficients K d [Mg] and K D[Sr] for modern Australian Cyprideis specimens (Chivas et al. 1986b), K D[ Mg] at 25°C=( Mg/Ca) shell /( Mg/Ca) water =0.00458±0.00072 K D[ Sr]=( Sr/Ca) water /( Sr/Ca) water =0.475±0.057 are used here to reconstruct the palaeoenvironmental record of the gulf. The two cores studied show similar patterns during the past 40,000 yr for equivalent hydrologic phases recognized in the cores. The Mg/Ca ratios of the water inferred from ostracod shell chemistry indicate that Carpentaria was a fresh or slightly saline lake between ≈ 40,000 and ≈ 13,000 yr B.P., before being transgressed by the sea during sea level rise. Around 26,000 yr B.P., a phase of substantial precipitation of authigenic calcite caused a dramatic increase in the Sr/Ca and Mg/Ca ratios of the lake water, and this was registered by the ostracod shell chemistry. From ≈ 26,000 to ≈ 13,000 yr B.P., the lake remained fresh or slightly saline despite the Ca depletion of the water which caused the Sr/Ca and Mg/Ca of the ostracods to be unusually high.