Abstract
The trace element (Sr/Ca and Mg/Ca) and stable isotope (δ18O and δ13C) geochemistry of fossil ostracod valves provide valuable information, particularly in lacustrine settings, on palaeo-water composition and palaeotemperature. The removal of sedimentary and organic contamination prior to geochemical analysis is essential to avoid bias of the results. Previous stable isotope and trace element work on ostracod shells has, however, employed different treatments for the removal of contamination beyond simple ‘manual’ cleaning using a paint brush and methanol under a low-power binocular microscope. For isotopic work pre-treatments include chemical oxidation, vacuum roasting and plasma ashing, and for trace element work sonication, chemical oxidation and reductive cleaning. The impact of different treatments on the geochemical composition of the valve calcite has not been evaluated in full, and a universal protocol has not been established. Here, a systematic investigation of the cleaning methods is undertaken using specimens of the ubiquitous euryhaline species, Cyprideis torosa. Cleaning methods are evaluated by undertaking paired analyses on a single carapace (comprising two valves); in modern ostracods, whose valves are assumed to be unaltered, the two valves should have identical geochemical and isotopic composition. Hence, when one valve is subjected to the chosen treatment and the other to simple manual cleaning any difference in composition can confidently be assigned to the treatment method. We show that certain cleaning methods have the potential to cause alteration to the geochemical signal, particularly Mg/Ca and δ18O, and hence have implications for palaeoenvironmental reconstructions. For trace–element determinations we recommend cleaning by sonication and for stable isotope analysis, oxidation by hydrogen peroxide. These methods remove contamination, yet do not significantly alter the geochemical signal.
Highlights
Ostracods are small bivalved crustaceans that occur in almost all aquatic environments
We have systematically evaluated the effect of preetreatment upon the preservation of the isotopic and trace element signature within ostracod valves
Gaffey and Bronnimann (1993), Ito (2001) and Keatings et al (2006) have all suggested that hydrogen peroxide does little to remove organic material adhered to the shell, and instead weak organic acid produced during the oxidation process may cause partial dissolution of the calcite surface
Summary
Ostracods are small (generally 0.5e3 mm long) bivalved crustaceans that occur in almost all aquatic environments. Their low Mg-calcite carapaces calcify using elements taken solely from the host water (Turpen and Angell, 1971), and are secreted within a short period providing a snapshot of conditions at the time of calcification. Often abundant and well preserved in Quaternary sediments, ostracods have become a popular proxy for palaeoenvironmental studies. The trace element (Sr/Ca and Mg/Ca) and stable isotope (d18O and d13C) geochemistry of fossil ostracod valves. L.R. Roberts et al / Quaternary Science Reviews 189 (2018) 197e209 composition of the water, along with any vital effects, which are species-specific for ostracod shells. Carbon isotope ratios predominantly reflect the isotope composition of the total dissolved inorganic carbon (TDIC) within the water
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