AN ORGANIC-CARBON ISOTOPIC RECORD OF GLACIAL-POSTGLACIAL CHANGE IN ATMOSPHERIC PCO(2) IN THE SEDIMENTS OF LAKE BIWA, JAPAN

A 12 m piston core of sediment from Lake Biwa, Japan, provides a high- resolution record of sedimentary organic matter accumulation over the Pleistocene-Holocene transition. Organic carbon concentrations are gen erally 1-2% in sediments deposited since 15 kyr ago. C/N ratios indica te that the organic matter is predominantly from aquatic production. A shift in organic carbon isotopic composition from -21 parts per thous and to -25 parts per thousand occurs over the glacial-postglacial boun dary. Changes in lake productivity, organic matter preservation, sourc es of organic matter, and water temperature can be eliminated as proba ble causes of the observed organic carbon isotopic shift. Annual overt urn of lake waters and rapid dissolution of biogenic carbonates mainta in equilibrium between atmospheric CO2 and dissolved CO2 in this lake. The isotopic composition of organic matter buried in the sediments of Lake Biwa consequently appears to record the shift in atmospheric pCO (2) from 200 ppm to 280 ppm which occurred at the Pleistocene-Holocene transition.