During the middle Miocene, Earth's climate changed from a global warm period (Miocene Climatic Optimum) into a colder one with the expansion of the Antarctic ice sheet. This prominent climate transition was also a period of drastic changes in global atmospheric circulation. The development of the South Asian Monsoon (SAM) is not well understood and mainly derived from proxy records of wind strength. Proxy records for middle Miocene changes in rainfall are virtually non-existent for India and the Arabian Sea prior to 11 Ma. This study presents planktic foraminiferal trace element (Mg/Ca and Ba/Ca) and stable oxygen isotope records from NGHP-01 Site 01A off the coast of West India in the Eastern Arabian Sea (EAS) to reconstruct the regional surface hydrography and hydroclimate in the SAM region during the middle Miocene. The Ba/Ca and local seawater d18O (d18Osw) changes reveal a notable gradual increase in SAM rainfall intensity during the middle Miocene. Additionally to this long term increase in precipitation, the seawater δ18O is punctuated by a prominent decrease, i.e. freshening, at ~14 Ma contemporary with Antarctic glaciation. This suggests that Southern Ocean Intermediate Waters (SOIW) transmitted Antarctic salinity changes into the Arabian Sea via an “oceanic tunnel” mechanism. The middle Miocene increase in SAM rainfall is consistent with climate model simulations of an overall strengthening Asian monsoon from the Eocene to the middle/late Miocene with a further acceleration after the Middle Miocene climate transition.

• δ18O [‰ VPDB]. Benthic foraminifera Planulina wuellerstorfi and Cibicidoides mundulus, measured at the State Key Laboratory of Marine Geology, Tongji University, Shanghai, China using a Finnigan-MAT253 mass spectrometer connected with an automatic line for carbonate preparation (type “Kiel III”).- δ18O [‰ VPDB] adjusted. The benthic foraminiferal δ18O values are presented in “Uvigerina-scale” by adding 0.64‰ (Shackleton and Opdyke, 1973).- δ18O [‰ VPDB]. Planktic foraminifera Trilobatus sacculifer, measured at MARUM, University of Bremen, Germany, using a Finnigan MAT 251 mass spectrometer connected with an automatic line for carbonate preparation (type “Kiel III”)- Mg/Ca [mmol/mol]. Planktic foraminifera Trilobatus sacculifer. Measured with an ICP-MS (Thermo X7) at the State Key Laboratory of Marine Geology, Tongji University Shanghai, China. - Ba/Ca [µmol/mol]. Planktic foraminifera Trilobatus sacculifer. Measured with an ICP-MS (Thermo X7) at the State Key Laboratory of Marine Geology, Tongji University Shanghai, China.- Mn/Ca [µmol/mol]. Planktic foraminifera Trilobatus sacculifer. Measured with an ICP-MS (Thermo X7) at the State Key Laboratory of Marine Geology, Tongji University Shanghai, China.- SST [˚C]. Obtained by using the Anand et al. (2003); B = 0.347; A = 0.09- SST [˚C] adjusted. To account for past seawater Mg/Ca changes, we adjusted our Mg/Ca SST estimates for changes in Cenozoic seawater Mg/Ca by following the approach of Sosdian et al. (2020). Adjusted SST estimates for changes in seawater Mg/Ca were calculated from the following equation: Mg/Ca (mmol/mol) = 0.293 exp (0.090 x T (°C). - local seawater δ18O [SMOW, ‰] - To calculate the δ18O of seawater (δ18Osw), firstly, the temperature driven component of changes in the planktic foraminiferal δ18O record was removed by using the temperature-δ18O relationship of Bemis et al. (1998): δ18Osw (VSMOW ‰) = (T(°C) - 16.5 + 4.8* δ18Ocalcite VPDB ‰))/4.8 + 0.27. For the temperature correction we used the SST estimates.- Secondly, an estimate of the Middle-Late Miocene (16-11 Ma) ice-volume signal was subtracted from the δ18Osw record in order to examine relative variations in local seawater. Changes in ice-volume were estimated by subtracting 70% (which is related to ice volume changes) of a benthic foraminifera isotope record after normalizing the benthic record to its average values.

Supplement to: Yang, Xueping; Groeneveld, Jeroen; Jian, Zhimin; Steinke, Stephan; Giosan, Liviu (2020): Middle Miocene Intensification of South Asian Monsoonal Rainfall. Paleoceanography and Paleoclimatology, 35(12), e2020PA003853