Jonathan A. Holmes

Ostracod shell chemistry - overview

© 2002 by the American Geophysical Union. Ostracod shells are valuable sources of calcite for geochemical analysis. Over the past two decades, research has provided much insight into the controls on traceelement partitioning and stable-isotope fractionation into ostracod shell calcite both in non-marine and marine settings. This chapter provides an overview of the controls and illustrates the application of the techniques to environmental reconstruction using examples. Trace-element work has concentrated on Mg and Sr uptake, which relate to water chemistry and temperature, both in lakes and the oceans. Oxygen- and carbon-isotope analyses of ostracods have concentrated on lacustrine environments, from which other carbonate microfossils are largely absent. In hydrologically- closed lakes, especially in dryland regions, geochemical signatures provide information on variations in hydrology, and may reflect changes in effective precipitation. In temperate deep lakes with an isothermal hypolimnion and negligible evaporative enrichment, isotopic analyses of benthic ostracod shells reflect the isotopic composition of lake water, which may reflect the regional precipitation signal. In shallow hydrologically-open lakes, geochemical signatures are more complex, although Mg content of ostracod shells may reflect water temperature, provided lake-water ionic composition has not changed. In marginal-marine environments, ostracod shell chemistry may indicate salinity changes that arise from mixing of marine and meteoric waters. Finally, in the deep oceans, the Mg content of benthic ostracods reflects ocean bottom-water temperature. In addition to these well-established approaches, ostracod shells also have potential as indicators of past metal pollution in the aquatic environment through measurements of heavy metal content of the shells, and as material for dating.

Terrestrial impact of abrupt changes in the North Atlantic thermohaline circulation: Early Holocene, UK

Abrupt cooling events are features of Holocene climate and may recur in the future. We use lake records from Hawes Water, NW England, to quantify the impact of two prominent early Holocene climatic events. Subdecadal oxygen isotope records from sedimentary carbonate (18δOc), dated using thermal ionization mass spectrometry (TIMS) U-series analyses, provide evidence for abrupt cold events, lasting ∼50 and ∼150 yr at 9350 and 8380 yr ago, which correlate with the 9.3 ka and 8.2 ka events recognized in Greenland ice cores. At Hawes Water, mean July air temperatures, inferred from chirono-mid assemblages, decreased by ∼1.6 °C during each event. Calculations show that the isotopic excursions were dominantly caused by decreases in the isotopic composition of meteoric precipitation (18δOp) by ∼1.3‰; this is interpreted as a direct downstream response to cooling and freshening of northeast Atlantic surface water by melting ice sheets. Intermediate in magnitude between events observed in Greenland and central Europe, the effects are consistent with a partial shutdown of the North Atlantic thermohaline circulation.

Carbon and oxygen isotope fractionation in non-marine ostracods: Results from a 'natural culture' environment

Abstract Carbon and oxygen isotope analysis of ostracods living in the near-constant conditions of spring-fed ponds in southern England allowed accurate determination of the ostracod’s calcite-water 13C/12C and 18O/16O fractionations. The 13C/12C fractionations of two species, Candona candida and Pseudocandona rostrata, correspond to values expected for isotopic equilibrium with the pond’s dissolved inorganic carbon at the measured temperature (11°C) and pH (6.9), whilst those of a third species, Herpetocypris reptans, would represent equilibrium at a slightly higher pH (7.1). The 18O/16O fractionations confirm two previous studies in being larger, by up to 3‰, than those ‘traditionally’ regarded as representing equilibrium. When the measured fractionations are considered in the context of more recent work, however, they can be explained in terms of equilibrium if the process of calcite formation at the ostracod lamella occurs at a relatively low pH (≤7) irrespective of the pH of the surrounding water. The pH of calcite formation, and therefore the calcite-water 18O/16O fractionation, may be species and stage (adult versus juvenile) specific, and related to the rate of calcification.

Drought and dust deposition in the West African Sahel: a 5500-year record from Kajemarum Oasis, northeastern Nigeria

A high-resolution, multiproxy palaeolimnological record from the Manga Grasslands, northeastern Nigeria, spanning the last 5500 calendar years, reveals the episodic deterioration in Sahelian climate as significant biogeophysical thresholds were crossed. Desert-dust deposition began to increase 4700 cal. BP. Rainfall during the summer-monsoon season declined permanently after 4100 cal. BP. A further significant change in atmospheric circulation, giving rise to multidecadal to centennial-scale droughts and enhanced dust deposition, occurred 1500 cal. BP. Hence, the post-1968 Sahel drought is not unique. The prolonged arid episode that occurred around 1200–1000 cal. BP in Ethiopia, the Sahel and tropical Mexico may have been linked to an abrupt cooling event in the North Atlantic and to a cluster of intense El Niño-Southern Oscillation events in the Eastern Equatorial Pacific.

Paleoclimatic implications of an 850-year oxygen-isotope record from the northern Tibetan Plateau

Oxygen-isotope records from the sediments of hydrologically-closed lakes are commonly interpreted in terms of changing effective precipitation. We compare an 850-year-long oxygen-isotope record derived from ostracode carbonate from the sediments of Sugan Lake, in the northern Tibetan Plateau, with tree-ring and ice core evidence for changing temperature, precipitation and isotopic composition of the lakes inflow. Taking into account all of these independent records, we show that variations in the carbonate delta O-18 values could not have been the result of varying effective precipitation alone: changes in water temperature and in the delta O-18 of source waters also played a significant role. Where independent records of temperature, precipitation or the isotopic composition of input waters are unavailable, care should be taken to avoid simplistic interpretations of carbonate stable isotope records, as these may contribute to incorrect paleoclimatic reconstructions.

Trace-element chemistry of non-marine ostracods as a means of palaeolimnological reconstruction : an example from the quaternary of Kashmir, northern India

Abstract Recent work has demonstrated the use of Sr/Ca and Mg/Ca ratios in the calcite of ostracod shells for reconstructing the palaeosalinity and palaeotemperature of lakes. In this study, the technique was applied to a lacustrine sequence of late Quaternary age (the upper Karewa Formation) in Kashmir, northwest Himalaya. Ostracods are the only fossils found in abundance in the upper Karewa sediments and are thus important for the reconstruction of upper Karewa palaeoenvironments. Trace-element analyses of multiple-shell samples of the ostracod Ilyocypris bradyi Sars 1890 were performed using inductively coupled plasma-atomic emission spectrometry. The results show that the palaeo-lake was even more dilute than the modern residual lakes in Kashmir. Reconstructed molar Sr/Ca ratios for the palaeo-lake waters vary between 30·10−4 and 42·10−4 compared with observed values of (46–110)·10−4 for the modern residual lakes. Changes in the molar Mg/Ca ratios with unchanging Sr/Ca are attributed to variations in water temperature. The palaeo-lake probably receded from the study site at between 116 and 80 ka and lacustrine sedimentation gave way to subaerial loess deposition. The lack of any increasing trend in the reconstructed Sr/Ca values, together with structural and sedimentological evidence, suggests that the lake was drained by tectonic tilting or warping of the basin floor rather than by climatically-induced desiccation.

How the Sahara Became Dry

A continuous lake record elucidates how Saharan climate changed gradually from humid to todays desert conditions.

The effects of diagenesis on the trace element and stable isotope geochemistry of non-marine ostracod valves

The trace-element and stable-isotope geochemistry of non-marine ostracod valves is a valuable tool in palaeolimnology. However, the potential effects of early diagenesis on geochemical composition are poorly documented. In this study, the effect of partial dissolution on the trace-element (Sr and Mg) and isotopic (18O/16O and 13C/12C) composition of late Quaternary non-marine ostracod valves was investigated. Both unaltered and extremely degraded valves of the species Cypretta brevisaepta recovered from the same core level had trace-element and stable-isotope ratios that were drawn from the same statistical population, suggesting that the geochemical effects of early diagenesis are minimal. However, since the possibility remains that diagenesis could under certain circumstances alter valve chemistry, only valves in pristine condition should be used in palaeoenvironmental studies whenever feasible.

Ostracods and the Holocene palaeolimnology of Lake Qarun, with special reference to past human-environment interactions in the Faiyum (Egypt).

We present an ostracod record covering the past two millennia from an 8.25-m core taken from Lake Qarun, in the Faiyum Depression of Egypt. The occurrence of ostracod species in the lake is controlled primarily by variations in solute composition, which are in turn related to shifts in catchment land use. At times when the Faiyum Depression supported thriving agriculture, lake water contained Na+–Cl− brine, and Cyprideis torosa dominated the ostracod assemblage. When the Faiyum Depression experienced periods of environmental and economic decline, lake water contained Na+–HCO3− brine, and Limnocythere inopinata dominated. The relative abundance of other ostracod species provides additional information about past conditions in Lake Qarun including salinity and lake level changes. Overall, the ostracod assemblages provide evidence for human influences in the Faiyum, which extend back before instrumental or detailed observational records began.

Climate and atmospheric circulation changes over the past 1000 years reconstructed from oxygen isotopes in lake-sediment carbonate from Ireland

A 1000 year long subdecadal-resolution record of carbonate oxygen isotopes (δ18Oc) from Lough-na-Shade, Ireland, provides evidence for changing atmospheric circulation over northwest Europe. The total range of δ18Oc values (>5‰) is too large to be explained by changes in water temperature. Moreover, good correlation between the lake record and a previously published δ18O time series from an Irish speleothem indicates that the changes in oxygen isotopes are best explained by variations in the isotopic composition of precipitation. The amplitude of change during this period is too large to be explained by shifts in condensation temperature. Instead we suggest that there have been changes in vapour source and transport paths connected with shifts in atmospheric circulation. Changes from a source area from further south within the North Atlantic to one further to the north could explain the prominent positive shift in oxygen-isotope values between the early eighteenth and early nineteenth centuries, for example. Our results also demonstrate the value of a ‘multiple-archive’ approach to deconvolving lake-based carbonate isotope profiles, which are often complex.