HOLOCENE CLIMATE RECONSTRUCTIONS FROM TANDEM TRACE-ELEMENT AND STABLE-ISOTOPE COMPOSITION OF OSTRACODES FROM COLDWATER LAKE, NORTH-DAKOTA, USA

The geochemistry of ostracode shells and bulk carbonates in a 19-meter sediment core documents at century-scale resolution the evolution of water chemistry in Coldwater Lake, North Dakota, providing a continuou s paleohydrologic record of Holocene climate change in the northern Gr eat Plains. A combination of delta(18)O, delta(13)C, Mg/Ca and Sr/Ca i n ostracode calcite aided by Sr/Ca in bulk carbonates are used to cons train the paleoclimatic reconstructions. A fresh-water phase in the ea rly Holocene, indicated by the absence of Candona rawsoni and low conc entrations of Sr/Ca in bulk carbonate, was followed by a sharp increas e in salinity between 10 800 and 8900 yr B.P. The climate was predomin ately dry during the late part of the early Holocene and most of the m iddle Holocene (8900-5000 yr B.P.), when the lake was very sensitive a nd recorded a series of dry and wet oscillations. Maximum salinity occ urred around 5500 yr B.P. and was followed by a gradual decrease betwe en 5000 and 2400 yr B.P. From 2400 yr B.P. the delta(18)O, Mg/Ca, and Sr/Ca in the ostracodes indicate generally wet conditions interrupted by a series of lesser salinity and temperature oscillations lasting un til 600 yr B.P. Ostracode geochemistry indicates that a warm and dry c limate returned at about the time of the Little Ice Age (600-150 yr B. P.). Ostracode delta(13)C shows a long-term increasing trend during th e Holocene, which suggests that lake productivity and atmospheric CO2 exchange made greater contributions to the hypolimnetic carbon pool as the lake became shallower with time.