Talk1: A multiproxy approach to extract sea surface temperature from coral skeleton

Time: 10:00am June 28 (Tuesday) Venue: A3-206 Zhou Long Quan Bldg

Abstract: Geochemical proxies preserved in scleractinian corals may provide valuable estimates of the past sea surface temperature (SST) at sub-annual resolution over many centuries. The majority of these records rely on the individual proxy-record (e.g. 18O or Sr/Ca) of coral aragonite, which shows a strong correlation with SST. However, these proxies are also subject to biologic and/or kinetic ‘vital effects’ that complicate their interpretation as SSTs. In this talk, I will present multi-proxy approaches todeconvolve the strong ‘vital effects’ (or ‘biological effect’) that affect each individual proxy, including using simultaneous analysis of multiple element ratios (Mg/Ca, Sr/Ca and Ba/Ca), the development of Mg isotope as a temperature proxy, and using combined oxygen and boron isotopes. These approaches not only can yield improved SST estimates, but also help to constrain the biomineralization process in the coral skeleton.

 Talk2: Sr and Mg isotope geochemistry of Marinoan cap carbonates: Implication for Neoproterozoic ocean circulation

Time: 10:00am June 29 (Wednesday) Venue: A3-206 Zhou Long Quan Bldg

Abstract: Neoproterozoic cap carbonates host distinctive geochemical and sedimentological features that reflect prevailing conditions in the aftermath of Snowball Earth. Interpretation of these features has remained contentious, with hypotheses hinging upon timescale and synchronicity of deposition, and whether or not geochemical signatures of cap carbonates represent those of a well-mixed ocean. In this talk,new high-resolution Sr and Mg isotope results are presented from basal Ediacaran cap dolostones in South Australia and Mongolia. Least-altered Sr and Mg isotope compositions of carbonates are identified through a novel incremental leaching technique that monitors the purity of a carbonate sample and the effects of diagenesis.These data can be explained by the formation of these cap dolostones involving two chemically distinct solutions, a glacial meltwater plume enriched in radiogenic Sr, and a saline ocean residue with relatively lower 87Sr/86Sr ratios. Model simulations suggest that these water bodies remained dynamically stratified during part of cap-dolostone deposition, most likely lasting for ～8thousand years. Our results can potentially reconcile previous conflicts between timescales estimated from physical mixing models and paleomagnetic constraints. Geochemical data from cap carbonates used to interpret the nature of Snowball Earth and its aftermath should be recast in terms of a chemically distinct meltwater plume.