Carbon isotopic variations on glacial‐to‐interglacial time scales in the ocean: Modeling and implications

Downcore variations in the deep‐sea δ13C signal as recorded in benthic foraminifera are a function of past changes in global organic carbon inventory and ocean circulation. Thus the carbon isotopic composition of fossil benthic foraminifera constitutes a valuable geochemical proxy for reconstructing paleooceanographic conditions. This study examines the mean ocean 13C/12C variations due to the climatically induced changes in the size of the two organic carbon pools: terrestrial biomass (forest‐soil) and continental shelf sediment. Through modeling the paired foraminiferal δ13C‐δ18O data in the literature, it is estimated that during the past several hundred thousand years in late Quaternary, transfer of organic carbon between these two pools and the ocean in response to glacial‐interglacial climate and sea level changes has resulted in global oceanic δ13C fluctuations with a mean amplitude of about 0.32‰. The transfer has accounted for about two thirds of the observed carbon isotopic shift in the deep eastern Pacific (ocean circulation accounting for the remaining one‐third of δ13C amplitude variations). In the deep North Atlantic Ocean, it has accounted for about one‐third of the shift. Of the two organic carbon reservoirs, forest‐soil is roughly 2.5 times more responsible than shelf sediment in altering the mean carbon isotopic composition of the Quaternary oceans. Destruction of organic matter from the shelf sediment reservoir is responsible for raising the oceanic nutrient inventory during glacial times by about 4% relative to the present (interglacial) level, a level which is difficult to be assessed by the cadmium proxy. Marine deposition of organic carbon in glacial oceans increased by 3–10%, in spite of a reduction in terrestrial biomass during ice ages. Isolating out the global climatic signals in δ13C records shows that since about 300 kyr ago, there has been a trend of increasing organic carbon accumulation toward the present, at least in the equatorial E. Pacific. It also shows that the contrast between the δ13C values (hence nutrient content) of deep water in the Atlantic and Pacific observed in today's ocean was smaller during glacial periods. In the northwestern Indian Ocean, the global climate‐corrected δ13C results reveal interglacial intensification of the monsoon‐driven upwelling and productivity, back to at least oxygen isotopic stage 11 circa 400 kyr B.P.