The impact of Himalayan-Tibetan erosion on silicate weathering and organic carbon burial

Abstract

Cenozoic mountain building in Asia has been proposed as an important control over global climate by atmospheric CO₂ drawdown through silicate weathering and burial of organic carbon (OC) offshore. Because Asian submarine fans represent the most complete record of Asian orogenic erosion and weathering over the Cenozoic, evaluation of sediment major element chemistry and OC content can be used to estimate CO₂ sequestration rates driven by silicate chemical weathering and OC burial. From deep-sea fans in the Indian Ocean and South China Sea, weathering rates are calculated by comparison of weathered sediment to initial bedrock source compositions and then integrated with deposited volumes to derive the first regional weathering budget for India-Asia collision over the last ∼25 m.y. Results indicate the Indus is more important than previously recognized before and during the middle Miocene in sequestering CO₂ (∼76% of Asian total at 16–14 Ma). This in part reflects the more reactive mafic and ultramafic bedrock sources compared to the largest and major east Himalayan drainage, the Ganga-Brahmaputra, but also greater erosional flux from the onshore Indus drainage at that time. This regional synthesis further concludes that OC burial only represented a minority (20–25%) of the regional carbon budget but became more important after 17 Ma, peaking at ∼38% after ∼3 Ma. CO₂ sequestration rates increased from 17 to 15 Ma, coinciding with the Miocene Climatic Optimum, and remained mostly steady in Asia from ∼15–5 Ma as the climate cooled. Only one of three possible sediment flux models for the Bengal Fan predicts increased CO₂ consumption rates after 15 Ma and, even then, only predicts steady rates from 11 to 5 Ma. The timing of changes in CO₂ consumption rates are not consistent with Asian orogenic silicate weathering acting as the dominant control over late Cenozoic atmospheric CO₂.