CHANGES IN OCEAN VENTILATION RATES OVER THE LAST 7000 YEARS BASED ON C-14 VARIATIONS IN THE ATMOSPHERE AND OCEANS

Changes in the ocean ventilation rate may be one of the causes for a net decrease of 100 parts per thousand DELTA-C-14 in atmospheric CO2 over the last 8000 years. Ocean ventilation rates of the past can be derived from the C-14 record preserved in planktonic and benthic foraminifera in deep-sea sediments. Results of C-14 dating using accelerator mass spectrometry on deep sea sediments from the South China Sea show that the age differences between planktonic (G sacculifer) and benthic foraminifera increase from 1350 yr ca 7000 yr ago to 1590 yr at present. An 11-box geochemical model of global ocean circulation was used for this study. Both tree-ring-determined atmospheric C-14 values and foraminifera C-14 age differences are used as constraints to place limits on patterns of changes in ocean ventilation rates and in atmospheric C-14 production rates. Results indicate: 1) C-14 production rates in the atmosphere may have decreased by as much as 30% between 7000 and 3000 yr ago, and may have increased again by ca 15% in the past 2000 yr, and 2) the global ocean ventilation rate may not have been at steady state over the last 7000 yr, but may have slowed by as much as 35%.