Abstract
To help understanding the potential relationship between chemical weathering and Indian summer monsoon (ISM) since the last glacial period a gravity core (BoB-56) was retrieved from the central Bay of Bengal (BoB). The data of chemical weathering indexes (CIA, WIP, and αAlNa) used in this study showed general synchronicity with the regional monsoon precipitation and temperature record on precessional scale, indicating existence of control from the ISM on weathering. Corresponding to alteration of warm/cold period during the last deglaciation, obvious simultaneously alteration of higher/lower values of the chemical weathering and terrestrial input proxies’ record support our hypothesis that the ISM driving chemical weathering on the millennial scale. However, a contradiction occurred during the Holocene period, when the ISM precipitation and temperature rose to a higher level, while the alternative indexes unanimously reflected a weaker chemical weathering conditions. In this study, we discussed the applicability of chemical weathering indexes in the BoB during the Holocene period. Besides the possible weakened monsoon during 6–3 ka, recorded by the stalagmite δ18O and Sea Surface Temperature (SST) reconstruction results in the northeastern Indian Ocean, other factors were responsible for this phenomenon, including the grain size effect and distinction between the mountain high land and floodplain low land. The chemical weathering records, during the last glaciation, indicated the presence of control from the ISM on weathering at precessional and millennial scales. While, during the Holocene, they failed to reflect the actual chemical weathering dynamics of the source area. Indeed, a mixture of physical erosion and chemical weathering seems to be representative of the chemical weathering dynamics in the area. Our findings emphasized on the tight connections between the chemical weathering evolution and global-regional climate conditions around the BoB, implying possible ISM-controlled mechanisms during different time scales.
Highlights
Chemical weathering is one of the most important processes of the earth’s elemental cycle, especially for the carbon cycle, and is closely related to tectonic, climatic, and environmental factors (Walker et al, 1981; Berner, 1992; Gaillardet et al, 1999; Yang et al, 2004a; Eiriksdottir et al, 2011; Miriyala et al, 2017; He et al, 2020; Liu et al, 2020)
Higher contents of CaO are observed during the Holocene period and lower contents during the last glaciation
This is due to the fact that the short-cycle climate changes on the millennial scale break up such long-cycle changes on the orbital scale and special circumstances during the Holocene period, which will be discussed in depth later
Summary
Chemical weathering is one of the most important processes of the earth’s elemental cycle, especially for the carbon cycle, and is closely related to tectonic, climatic, and environmental factors (Walker et al, 1981; Berner, 1992; Gaillardet et al, 1999; Yang et al, 2004a; Eiriksdottir et al, 2011; Miriyala et al, 2017; He et al, 2020; Liu et al, 2020). Deciphering the chemical weathering intensity records in stratigraphic signals can provide us with information on the environmental evolution in source regions at different time scales, helping to understand the different control mechanisms of chemical weathering. From the transportation characteristics of environmental signals in the process of “source-sink,” the stronger the signal in the source area, the shorter the transformation time, and the more stable and continuous the strata are, the more conducive to the decoding of environmental signals (Romans et al, 2016). The obvious tectonic and climatic signals in the source area are likely to be masked, modified, or damaged by other signals during transportation (Jerolmack and Paola, 2010; Romans et al, 2016), such as fluvial-floodplain processes, which are not affected by the original upstream signals. The drilling results show that except for the early Eocene and late Miocene, the remaining strata are relatively continuous (Curray and Moore, 1971), providing us with a good target for deciphering regional chemical weathering evolution and its control mechanism, and recorded in the sediment strata
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