Evolution of weathering patterns in the Indo‐Burman Ranges over the last 280 kyr: Effects of sediment provenance on 87Sr/86Sr ratios tracer

Abstract

A high‐resolution study of mineralogy and major element geochemistry combined with Sr and Nd isotopes has been conducted on high sedimentation rate cores collected off the Irrawaddy River mouth in the Andaman Sea and the Bay of Bengal to reconstruct the erosional and weathering history of the Irrawaddy River basin. In both cores, ɛNd(0) values imply that both glacial and interglacial sediments share a common crustal source: the Irrawaddy River. Strong glacial/interglacial cycles are recorded by 87Sr/86Sr: interglacial periods yield values between 0.713 and 0.717, whereas glacial periods show higher values between 0.717 and 0.719. Variations of the pedogenic clays (smectite and kaolinite) to primary mineral (feldspar, quartz, illite, and chlorite) ratios show strong precessional cycles, suggesting a control by past changes in the summer monsoon intensity. Each increase in pedogenic clays content is also associated with a net loss of labile elements (Na, K, and Ca) from the detrital minerals under chemical weathering. Wet periods of summer monsoon reinforcement correspond to an increase in weathering of the Irrawaddy plain soils and a decrease of 87Sr/86Sr ratio. Plotting 87Sr/86Sr versus 87Rb/86Sr gives a pseudo‐isochrons interpreted as a mixing line representing the strength of chemical weathering. During glacial stages, enhanced physical erosion induced by glacier scour and frost action in the highland of the Irrawaddy River basins produced high volumes of unaltered, Rb‐rich minerals. The low sea level of glacial times constricted the river to the main channel in the lower reaches and permitted an efficient transport of unaltered Rb‐rich minerals with high radiogenic Sr composition from the high relief of the Indo‐Burman Ranges and the Tibetan plateau to the Indian Ocean.