Molluscs isotopic compositions

Zooplankton population samples for isotope analysis were composites of 50-200 individuals. Population samples are less variable in isotope composition than are samples of individuals. Replicate isotope analyses of composite samples of zooplankton or POM collected at different locations within the lake varied by no more than 0.5%. Larger organisms such as molluscs, insects, and fish were analyzed individually. Molluscs were soaked in dilute HCl to remove carbonates and then rinsed copiously with distilled water. Fish muscle was analyzed. Sediment trap material was collected in replicate cylinders (11.4-cm diameter, 76.2-cm length) suspended at 4.5-m depth. All isotope samples were dried at 60 °C before analysis. 

Keith, M. L., Anderson, G. M., and Eichler, R. Carbon and oxygen isotopic composition of mollusc shells from marine and fresh-water environments. Geochim. Cosmochim. Acta 28, 1757-1786 (1964). 

The isotopic composition of strontium of mollusc shells in the early Miocene section of the CRP-1 core (46-62 mbsf) indicated dates of 16.6 0.4 and 18.7 0.3 Ma in agreement with the diatom biostratigraphy of Harwood et al. (1998). The Ar/ Ar date of a volcanic clast (18.4 1.1 Ma) measured by McIntosh (1998) confirms the early Miocene (Burdigalian) age of the sediment. 

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