Enhanced silicate weathering fluxes from tropical shelf sediments exposed during glacial lowstands

Abstract:

Atmospheric CO₂ and global climate are closely coupled. Since 800 ka CO₂ concentrations have been up to 50% higher during interglacial compared to glacial periods. Because of its dependence on temperature, humidity, and erosion rates, chemical weathering of exposed silicate minerals is thought to have dampened these cyclic variations of atmospheric composition. Cooler conditions and lower non-glacial erosion rates suppressed in situ chemical weathering rates during glacial periods. However, using systematic variations in major elements geochemistry, Sr-Nd isotopes and clay mineral records from Ocean Drilling Program Sites 1143 and 1144 in the South China Sea over the last 1.1 Ma, we show that sediment deposited during glacial periods was more weathered than sediment delivered during interglacials. This can be attributed to subaerial exposure and weathering of unconsolidated shelf sediments during glacial sealevel lowstands. An 11-fold increase in the volume of sedimentary deposits available to chemical weathering around the South China Sea during glacial lowstands paired with a minor reduction of tropical weathering rates makes it likely that silicate weathering fluxes from SE Asia and Taiwan were larger during glacial compared to interglacial times. As a result, the effects of increased sediment exposure during lowstands may have outstripped those of reduced weathering rates, and it is possible that tropical weathering of silicate shelf sediments has enhanced rather than dampened glacial-interglacial variations of atmospheric CO₂ and attendant climate change.