Weathering and climate linkage at seasonal to kilo-year timescales: Evidence from water and sediment chemistry

Abstract

Continental erosion plays a dominant role in global biogeochemical and climatic cycles. Intensity of this land-surface (erosion) process is disproportionally higher in the tropics, mainly due to its conducive climate (rainfall and temperature). The main objective of this thesis work is to investigate the interaction between erosion and climate over multiple timescales, ranging from seasonal to kilo-years. Towards this, geochemical and isotopic analyses of paleo-archives (sedimentary records from the Arabian Sea and upper Indus (Zanskar) River basin, and speleothem) and weekly variation of river water chemistry (Brahmaputra River) have been carried out. The chronology of the speleothem (~9200 - 3950 yrs BP), collected from the Gupteswar Cave (Odisha, India), was established using U-Th isotopic systematic, whereas the age ranges for the Zanskar sediment section (~22400 - 1450 yrs BP) and two Arabian Sea cores (SSK40/GC06 (~3560 yrs BP - recent); SSK40/GC10 (~7150 - 800 yrs BP)) were constrained using radiocarbon ages.Seasonality in chemical erosion pattern of the Brahmaputra River basin: Time-series analyses of the Brahmaputra River (Guwahati, India) water chemistry show seasonal changes. The degree of seasonality for the elemental abundances and their ratios, however, is less pronounced at Guwahati when compared to those with other Himalayan (Ganga and Salween) rivers. The monthly averaged Ca/Si ratios for the Brahmaputra show minimal (~6 %) variation compared to that (~17 %) observed for the Ganga outflow, points to the minimal role of runoff in regulating the erosion pattern of this river. Inverse model calculations estimated the silicate-derived fraction of cations and Sr for monsoon and non‐monsoon seasons, respectively, are statistically indifferent. Observations from this study suggest that the erosion intensity in the Brahmaputra basin is not limited by runoff/water‐rock interaction time and is more regulated by rapid mechanical erosion at around eastern syntaxis.Reconstruction of Holocene variability of the erosion patterns using marine records: Geochemical investigation of two sediment cores raised from the Arabian Sea (SSK40/GC06 and SSK40/GC10) was carried out to assess the climatic control on erosion patterns during the Holocene period. The high-resolution geochemical study of these cores shows insignificant temporal change in immobile elemental (Al, Ti, and rare earth elements (REEs)) concentrations and REE patterns, indicating no major change in sedimentary sources. However, trends of multiple weathering indices point to fluctuating chemical erosion patterns during the Meghalayan stage (4200 yr BP to present). The recent intensification of chemical erosion from 1600 yr BP to the present is not synchronous with any major change in the southwest (SW) monsoon during this period. However, this erosion change is consistent with the rise in C4 vegetation, possibly due to sustained agricultural practices in the SW-dominated regions. Human-induced increase in C4 vegetation with shallow root systems may promote efficient sediment transport and hence, intense erosion of young soils with faster dissolution kinetics. Further, down-core variations in the Al-normalized ratios (Ba/Al, TOC/Al, and S/Al) and enrichment factors of redox-sensitive elements (U and Mo) of the SSK40/GC10 core were useful in identifying a shift in oceanic (bottom water) redox state from oxic to anoxic bottom water condition since ~ 4.2 kyr BP. The timing of this change is synchronous to the earlier reported drought phase at the beginning of the Meghalayan stage, underscoring climatic control on the bottom water redox state. Major outcomes from past erosion reconstruction showed modern-day chemical erosion rates are anthropogenically influenced and may not serve as representative riverine value for long-term chemical (e.g., 87Sr/86Sr, 187Os/188Os) budgets.Reconstruction of past erosion patterns using terrestrial records: Chemical and Sr-Nd isotopic investigation of river section sediments from the upper Indus (Zanskar) basin was used to reconstruct the Himalayan erosion pattern during the last glacial-interglacial cycle. The temporal trends of the Sr-Nd isotopic compositions show a sudden shift in sediment provenance at around the Bølling-Allerød climate event. The temporal trends of selected chemical weathering indices (K/Al, CIA) also show an increased chemical erosion rate during this warmer climatic phase. This change in the Himalayan erosion can be linked to glacial retreat and continuous exposure of fresh minerals during this warm and humid phase (Bølling-Allerød climate event), establishing a strong coupling between erosion and climate.Reconstruction of past rainfall using speleothem δ18O record: High-resolution oxygen isotopic data (δ18O) of the speleothem from the core monsoon zone were used to reconstruct the natural variability of the Indian summer monsoon (ISM) during the mid-Holocene period. These data reveal the impact of the Indian Ocean on short-term extreme climate events over the ISM regime. On the contrary, the North Atlantic (NA) climatic changes control the long-term trends of ISM variability. The δ18O data identify a super-drought event spanning ~200 years, at around 6.5 kyr BP, synchronous to the negative Indian Ocean Dipole state. This study also shows that the broad declining nature of ISM during the middle to late Holocene is linked to south-ward migration of the Inter-tropical convergence zone and/or change in NA climate.