AN ORGANIC-CARBON ISOTOPE RECORD OF LATE ORDOVICIAN TO EARLY SILURIANMARINE SEDIMENTARY-ROCKS, YANGTZE SEA, SOUTH CHINA - IMPLICATIONS FORCO2 CHANGES DURING THE HIRNANTIAN GLACIATION

Studies of delta(13)C of marine sedimentary organic carbon in six elas tic sections with sediments deposited from the latest Ordovician to ea rliest Silurian in the Yangtze Sea on the South China Plate, show a po sitive delta(13)C(org) excursion of up to 4 parts per thousand in the latest Ordovician Hirnantian Stage, corresponding in time to a major c ontinental glaciation on Gondwana. Various factors that might be respo nsible for the delta(13)C(org) increase are considered, including inpu t and contribution from exogenic carbon sources, secondary processes ( such as heterotrophy and thermal maturation), and changes in Yangtze S ea surface-water temperature, productivity, primary producers, dissolv ed CO2 concentrations and the isotopic composition of dissolved inorga nic carbon. Current data are consistent with the interpretation that t he observed delta(13)C(org) increase may have resulted from a decrease in the surface-water dissolved CO2 in the Yangtze Sea during the Hirn antian glaciation, probably reflecting a significant decrease in glaci al ocean/atmosphere p(CO2) levels at that time. (C) 1997 Elsevier Scie nce B.V.