Interannual climate variability in the Miocene: High resolution trace element and stable isotope ratios in giant clams

摘要

High resolution stable isotope and trace elemental ratios of a recent Tridacna squamosa from Vietnam and a Middle to Late Miocene (10-13Ma) Tridacna gigas from Indonesia are presented. The seasonal pattern of modern sea surface temperature (SST) variability offshore Vietnam is faithfully recorded in the δ~(18)O of the T. squamosa shell carbonate, confirming the potential of Tridacna shells as sub-annual resolution climate archives. Cultivation of the T. squamosa specimen in controlled conditions after recovery from the natural environment facilitated a quantitative calibration of the δ~(18)O signal to ambient water temperatures. An age model for the Miocene T. gigas shell from Indonesia was therefore constructed on the basis of its δ~(18)O profile, assuming a single-peak annual SST cycle. The magnitude of these oscillations was 5-7°C. Mg/Ca and the growth-banding pattern in the Miocene T. gigas correlates well with shell δ~(18)O during the later part of the organism's lifespan. Ba/Ca is negatively correlated to Mg/Ca, with a lag of several months, suggesting a different phasing of the annual primary productivity cycle from that of SST. Furthermore, δ~(18)O and Mg/Ca show prominent deviations to warmer conditions with a periodicity of ~3years. These shifts demonstrate the existence of substantial interannual sea surface temperature variability in the Miocene, a period with elevated global temperatures compared to the present day.