Reconstructing Indian Summer Monsoon Variability during the Pliocene-Pleistocene

Bokhari Friberg, Yasmin (2022). Reconstructing Indian Summer Monsoon Variability during the Pliocene-Pleistocene. PhD thesis The Open University.



The Indian Summer Monsoon (ISM) affects one of the world’s most densely populated areas, which relies on its seasonal rainfall for their agriculture, economy and industry. Monsoon anomalies can have devastating effects such as crop failure, flooding and droughts, but predictions about the future of the Indian Summer Monsoon are difficult, as it is a complex system with a multitude of variables, especially in light of current anthropogenic climate change. The Pliocene (2.58 to 5.3 Ma) time period displays similar conditions to those projected for climate at the end of the 21st century and is therefore a key time period to study in order to generate more accurate predictions of future monsoon behaviour. Despite this, few studies have covered the Pliocene ISM, since high-resolution deep marine sediment cores have been difficult to obtain from the core convective ISM region Bay of Bengal.

This study uses a multi-proxy approach to reconstruct ISM rainfall, river runoff, and marine productivity, using recently drilled marine sediments across the Plio-Pleistocene recovered during International Ocean Discovery Programme Expedition 353 (Site U1445) in north-western Bay of Bengal. A new high-resolution (~2 kyr) chronology is generated for Site U1445 (2.3 to 3.9 Ma) by measuring benthic oxygen isotope and tuning it with the global benthic stack. High-resolution (~100 years) bulk sediment X-ray fluorescence, physical properties data (1.4 to 4.0 Ma) and planktic foraminiferal stable oxygen and carbon isotope and trace elements data (2.40 to 2.85 Ma) is used to reconstruct ISM rainfall, runoff, and productivity. Additionally, new diatom and phytolith count data (2.2 to 4.0 Ma) are presented as wind and productivity records. Data collected in this study fill spatial and temporal gaps for the ISM and provide a unique insight into its behaviour in context with the East Asian, Australian, East African and West African monsoon systems, global ocean productivity records, tectonic changes and oceanic rearrangements, orbital parameters, global ice volume and atmospheric CO2 during the Pliocene and early Pleistocene.

Indian Summer Monsoon rainfall and wind were found to be decoupled in the late Pliocene and early Pleistocene. A gradual drying of India (4-2.7 Ma) is concurrent with northwest Australian and East African drying, following eastern Indian Ocean sea surface temperature decrease due to the closing of the Indonesian Seaway. ii Reconstructing Indian Summer Monsoon variability during the Pliocene-Pleistocene Simultaneously, Bay of Bengal productivity displays a gradual increase, due to a weakened runoff-induced surface freshwater layer combined with strengthened southwest winds. An opposite relationship between Indian and East Asian Summer Monsoon rainfall and productivity, indicates that the two monsoon systems should be treated separately. On the orbital scale, Indian Summer Monsoon rainfall and runoff indicating stronger precession before 2.8 Ma and strengthened obliquity after, likely as a result of growing Northern Hemisphere ice sheets and high-latitude control. Productivity on the other hand, displays a frequency switch from stronger obliquity to precession at 2.8 Ma, suggesting instead a Southern Hemisphere high-latitude control on productivity.

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