Climate Modulation of Pb Isotopes in the Deep Indian Ocean Linked to the Himalayan Chemical Weathering

Abstract:

Leaching of sediments from the Eastern flank of the Chagos-Laccadive ridge, the 90°E ridge and the distal part of the Bengal Fan have extracted authigenic lead (Pb). This allowed the reconstruction of the Pb isotope evolution of the deep central Indian Ocean over the past 250 thousand years at ∼3 kyr resolution and over the past 20 Ma at 2-3Myr resolution. High frequency temporal variations recorded close to the ridges define a binary mixing line that records the variable admixture of radiogenic Pb with a signature characteristic of the composition of Ganges–Brahmaputra river sediments to the stable unradiogenic widely-distributed Pb source, from mid-ocean ridges or possibly volcanic aerosols. The temporal variations suggest an enhancement of Himalayan contributions by two to three times during interglacial periods, indicating that climate modulates the supply of dissolved elements to the ocean. While these changes could accurately record variations in the continental chemical weathering flux in response to warmer and wetter conditions during interglacials, the relative proportions of Pb derived from the Ganges and Brahmaputra appear to have been constant through time. This observation may point towards particulate-dissolved interactions in the estuary or pro-delta as a buffer of short timescale variability of the fluvial inputs. The changes recorded directly in the turbiditic fan during the Neogene are more difficult to interpret and will be discussed in lengh. If the last Ma data points are consistent with the 2 records from either side of the sedimentary basin, the input from the weathering of the Himalaya could have been impacted by 1) the uplift of more radiogenic terrane consistent with the onset of the Main Boundary Thrust around 10 Ma, and 2) changes in the weathering style pointing toward a more uncongruent weathering between 7 and 1 Ma.