The Seawater Sr Record and Processes of Erosion and Weathering in Himalaya

Christian France-Lanord CRPG-CNRS, BP20 54501 Vandoeuvre, France

Louis A. Derry Cornell Univ., Dept. of Geol. Sciences, Ithaca, NY 14853-1504, USA

Himalayan erosion is currently one of the major vectors of chemical transfer from continents to ocean. Particulate and dissolved fluxes from the Ganges and Brahmaputra (GB) rivers represent ca. 8 and 4 %, respectively, of the global riverine fluxes. GB rivers are characterized by unusually high dissolved Sr concentrations and 87Sr/86Sr ratios. While there is no doubt that they represent a significant source of 87Sr for seawater, the weathering processes involved and their exact impact on the global Sr isotopic budget remain highly debated. We present data from the modern Himalayan river system to determine the processes which control the Sr isotopic ratio of GB rivers, and from pedogenic minerals from the Bengal Fan which document the past variations of the Sr isotopic ratio of GB rivers.

Weathering in the modern Himalaya

In the Himalaya, 87Sr/86Sr ratios of major rivers progressively increase from 0.715 in the North of the high range to 0.735 - 0.750 in the foot of the south flank in the Lesser Himalaya. Cation budgets show that carbonate dissolution increases toward the south where it represents ca. 80% of the total cations. This is confirmed by the unusually high values of *13C of dissolved inorganic carbon (around -8”) of south flank rivers. Cation and isotopic mass balance calculations including composition of the main Himalayan terranes imply that weathering of the High Himalaya Crystalline formation, which is the main source of detrital material, cannot be the single source for radiogenic Sr. In situ weathering of schist and carbonates of the Lesser Himalaya, which are highly radiogenic (0.78 to 1.50), is necessary to satisfy the high 87Sr/86Sr and Ca/(Na+K) ratios of these rivers. In the Indogangetic plain, progressive dissolution of non-
radiogenic carbonate from the sediment load and contribution from rivers draining the south of the basin progressively lower the 87Sr/86Sr of the Ganges to 0.720 while weathering of Himalayan silicates does not represent a major contribution to the river chemistry.

Past Sr isotopic composition of the GB rivers

The Bengal Fan documents past variations of the erosion and weathering regime in the GB basin. The Fan records a clear change near 7.4 Ma from a physically dominated
erosion regime to one dominated by chemical weathering. The pattern is reversed near 1 Ma.. Pedogenic minerals such as smectites and vermiculite record the 87Sr/86Sr of surface waters in the Indogangetic plain. The 87Sr/86Sr ratios change from values close to modern GB rivers before 7.4 Ma to much more radiogenic values around 0.740-0.760 between 7.4 and 1 Ma. Modelling the impact of this change on the global seawater Sr budget implies a contemporaneous decrease of the net Sr flux of the GB rivers which is best explained by reduced physical erosion of the Himalaya and consequent reduced carbonate dissolution.

The implication of these results is that the interpretation of the seawater Sr record in term of erosion rates or global weathering requires detailed study of the processes involved and the eroded terrane. During the Mio-Pliocene, increased 87Sr/86Sr of the oceans corresponds paradoxically to an interval of reduced erosion of the Himalaya The variation in the relative importance of physically dominated erosion, which mainly dissolves Sr from carbonates, and chemical
weathering, which releases Sr from radiogenic silicates, exerts a primary long term control on the Himalayan input of Sr to the oceans.