Variability of the Indian Monsoon in the Andaman Sea Across the Miocene‐Pliocene Transition

Janika Jöhnck,Ann Holbourn,Wolfgang Kuhnt,Nils Andersen

doi:10.1029/2020pa003923

Abstract

AbstractWe reconstructed the variability of the Earth's strongest hydrological system, the Indian monsoon, over the interval 6.24 to 4.91 Ma at International Ocean Discovery Program (IODP) Expedition 353 Site U1448 in the Andaman Sea. We integrated high‐resolution benthic and planktic foraminiferal carbon and oxygen isotopes with Mg/Ca measurements of the mixed layer foraminifer Trilobatus sacculifer to reconstruct the isotopic composition of seawater (δ18Osw) and the gradient between planktic and benthic foraminiferal δ13C. A prominent increase in mixed layer temperatures of ~4°C occurred between 5.55 and 5.28 Ma, accompanied by a change from precession‐ to obliquity‐driven variability in planktic δ18O and δ18Osw. We suggest that an intensified cross‐equatorial transport of heat and moisture, paced by obliquity, led to increased summer monsoon precipitation during warm stages after 5.55 Ma. Transient cold stages were characterized by reduced mixed layer temperatures and summer monsoon failure, thus resembling late Pleistocene stadials. In contrast, an overall cooler background climate state with a strengthened biological pump prevailed prior to 5.55 Ma. These findings highlight the importance of internal feedback processes for the long‐term evolution of the Indian monsoon.

Highlights

Monsoonal circulation in the Bay of Bengal and Andaman Sea is primarily induced by seasonal changes in the thermal gradient between the southern Indian Ocean and Asian continental landmasses, in particular the southern Tibetan Plateau (e.g., Webster et al, 1998)
The age model is derived by correlation of the U1448 benthic foraminiferal stable isotope data to the well dated records of ODP Site 1146 (Holbourn et al, 2018) and ODP Site 982 (Drury et al, 2018) (Figures 2, S1, S2a, and S2b and supporting information Text S1)
At Andaman Sea Site U1448, planktic δ18O shows increased amplitude variability and a shift from a dominant precession (19 and 23 kyr) to obliquity (41 kyr) signal, which coincides with the onset of global warming at 5.5 Ma (Herbert et al, 2016; Holbourn et al, 2018)

Summary

Introduction

Monsoonal circulation in the Bay of Bengal and Andaman Sea is primarily induced by seasonal changes in the thermal gradient between the southern Indian Ocean and Asian continental landmasses, in particular the southern Tibetan Plateau (e.g., Webster et al, 1998). Long‐term variations in monsoonal intensity are influenced by insolation changes induced by variations in the Earth's precession. Changes in the Earth's obliquity control the seasonality of radiation and the interhemispheric insolation gradient, driving interhemispheric heat and moisture transfer and enhancing summer monsoon rainfall (Bosmans et al, 2015; Mantsis et al, 2014). The sensitivity of the East Asian and Indian monsoon systems to internal feedback processes such as changes in atmospheric greenhouse gases and ice volume, which affect zonal and meridional temperature gradients and moisture transport, remains a matter of intense debate (e.g., Clemens et al, 2018; Gebregiorgis et al, 2018).

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