The basis for ostracod shell chemistry in palaeoclimate reconstruction
This project examined the trace-element and stable-isotope composition of non-marine ostracod shells and their use in Holocene palaeoclimate reconstruction, with particular reference to mid-latitude shallow carbonate lakes. Controls on the chemical composition of shells from such environments were studied. Results were used to test the application of ostracod shell chemistry data to palaeoclimate reconstruction. Selected techniques were applied to a high resolution study of Holocene lake marl sequence. The project investigated influences on shell chemistry during formation, sedimentary deposition and extraction. A core covering approximately 9,450 to 5,800 Cal. Yr BP was taken from Cunswick Tarn, Cumbria, yielding ostracod faunal and shell chemistry data used to produce a palaeoclimate interpretation for the early Holocene. This interpretation agreed with studies showing a warm, dry early Holocene and a cooler, wetter mid-Holocene. Seasonal sampling of water and ostracods at Cunswick Tarn provided a modern analogue for the core material. Species-specific fractionations of stable isotopes were similar to findings from ponds in Greywell, Hampshire. There were seasonal variations in the trace-element composition of shells, which may reflect: seasonal fluctuations; shell growth following initial calcification; and cleaning problems. Monthly sampling of water and ostracods was performed at carbonate ponds near Greywell. Species-specific fractionations of oxygen isotopes were observed. Compared to theoretical equilibrium calcite, all species exhibit [delta][sup]18 O values more positive than expected, while [delta][sup]13 C values equal equilibrium values. Variations in shell trace-element composition was greater than that of the water. This result may be related to animal physiology and the water’s low trace-element content. Ostracods collected from additional sites with elevated Mg/Ca[sub](water) ratios showed the best correlation between the Sr/Ca ratio of the water and the Sr/Ca of the shell. Effects of established cleaning techniques on shell chemistry were investigated. Whenever possible, only brush cleaning with deionised water should be used. Sodium hypochlorite is recommended for any further cleaning. An investigation of the effect of partial dissolution on shell chemistry demonstrated that even extremely degraded Cypretta brevisaepta shells have similar trace-element and stable-isotope ratios to unaltered shalls. However, dissolution may alter shell chemistry, and whenever possible, pristine shells should be used. Pseudocandona rostrata specimens from Greywell were grown in vitro. Adults had low weight with low M/Ca ratios. The Sr/Ca[sub](ostracod) ratios were negatively correlated with weight, while M/Ca[sub](ostracod) ratios were weakly positively correlated with M/Ca[sub](water), Sr/Ca[sub](water), and temperature. The research shows that a knowledge of the life-history and ecology of ostracods, together with the chemical signals recorded in their shells, can provide an effective tool for the interpretation of palaeoclimatic events.