ENHANCED SILICATE WEATHERING OF TROPICAL SHELF SEDIMENTS EXPOSED DURING GLACIAL LOWSTANDS: A SINK FOR ATMOSPHERIC CO2

Atmospheric CO₂ and global climate are closely coupled over millennial and longer timescales because of the role that CO₂ plays as a greenhouse gas. Chemical weathering of silicate minerals partly modulates the climate through its dependence on temperature, humidity, and erosion rates. Cooler and drier conditions reduce chemical weathering rates during glacial periods. At high latitudes this is balanced by the exposure of continental shelves during sealevel low stands. However, at low latitudes the situation may be different. Major element geochemistry, Sr-Nd isotopes and clay mineral records from Ocean Drilling Program Sites 1143 and 1144 in the South China Sea spanning the last 1.1 m.y. show that sediment deposited during glacial periods was more weathered than sediment delivered during interglacials. We attribute this to subaerial exposure and weathering of unconsolidated shelf sediments during glacial sealevel lowstands. We estimate that enhanced weathering of tropical silicate shelf sediments exposed during glacial lowstands can account for ~7% of the CO₂ removed from the atmosphere during the glacial and thus represent a significant part of the observed glacial-interglacial variation of ~80 ppmv. The effects of increased sediment exposure and subsequent silicate weathering during lowstands could significantly enhance the drawdown of atmospheric CO₂ during cold stages of the Quaternary.