Radiogenic isotope composition of Carboniferous seawater from North American epicontinental seas

摘要

Abundant marine sedimentary deposits accumulated in the epeiric seas of North America during the Carboniferous Period (359-299Ma) when the Earth's climate shifted from a global "greenhouse" to "icehouse". Geochemical records extracted from these deposits are often assumed to reflect global ocean processes and are increasingly being used to infer relationships between tectonism, carbon cycling and climate change. We analyzed radiogenic isotopes (Sr, Nd) in biogenic apatite along a North American transect to constrain the degree of coupling between the epicontinental seas and the open ocean. In many samples the primary Sr signal is altered, however the original Nd isotopic composition of the biogenic apatite seems to be preserved. The ε_(Nd)(t) values range from -8.6 to -4.5 in the US midcontinent basins and from -11.4 to -5.7 at Arrow Canyon (Nevada, USA), indicating the development of an east-west geochemical gradient throughout the Pennsylvanian. This apparent decoupling of the epicontinental seas from the open ocean was established by the latest Mississippian. Thus, circulation of waters between epicontinental seaways of western US and the basins to the east and south was periodically restricted. Shifts toward unradiogenic Nd at Arrow Canyon seem to correspond with glacio-eustatic changes across the mid-Carboniferous boundary. These ε_(Nd)(t) variations may have been caused by changes in weathering rates/sources, marine circulation or a combination of both.