The Application Of Cd Isotopes In Paleoceanography

The Late Devonian Frasnian-Famenian mass extinction event is characterized by massive demises of marine f-aunal species at low latitude.Despite the extensive studies,the ultimate causes of this extinction event remain in debate due to the lack of direct evidences for variations in oceanic primary productivity,as changes in phytoplankton communities would have proportionally large repercussions for ocean ecosystem healthPrevious reconstructions of primary productivity were all based on simultaneous positive shift in carbon isotopes of both cairbonate and organic matter.However,such an assumption was in contrast with the recently established dramatic cooling during this period.Cadmium(Cd)isotope ratios are tightly related to nutrient utilization and primary productivity in the surface seawater.The Cd isotope signal can be well preserved in marine carbonates and thus serve as a powerful tool to reconstruct primary productivity in the past.We collect carbonates samples from the Yangdi section,South China,in the same intervals with recently established high-resolution conodont stratigraphic units,and apply Cd isotope to reconstruct primary productivity in order to link the variations of primary productivity with the faunal extinction events.In the Frasnian,salinity-corrected ?114Cd(?114CdCOR)values were relatively heavy(0.21?0.77‰),similar to those in the surface water of North Atlantic Ocean,indicating high primary productivity in this period.During the F-F interval,abrupt declines of?114CdCOR(-0.17?0.49‰)happened 3 times,accompanied by the decrease of Fe and Zn concentrations;The first abrupt decline of primary productivity was followed by a massive decrease in the total num ber of conodonts,and the second one by the demise of both pelagic and benthic faunas.No more extinction event occurred following the 3rd decline as the whole ecosystem already collapsed.In the Famennian after F-F biotic crisis,?114CdCOR(-0.04-0.13‰)increased,indicating a gradual recovery in primary productivity.We propose that the F-F mass extinction event was initiated with the abrupt decline of primary productivity and the subsequent destruction of the food chain,while climate cooling and possibly associated nutrient depletion might trigger abrupt decline of primary productivity.