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AbstractInternational Ocean Discovery Program Expedition 355 drilled Sites U1456 and U1457 in Laxmi Basin (eastern Arabian Sea) to document the impact of the South Asian monsoon on weathering and erosion of the Himalaya. We revised the chronostratigraphic framework for these sites using a combination of biostratigraphy, magnetostratigraphy and strontium isotope stratigraphy. The sedimentary section at the two sites is similar and we divided it into six units bounded by unconformities or emplaced as a mass-transport deposit (MTD). Unit 1 underlies the MTD, and is of early–middle Miocene age at Site U1456 and early Paleocene age at Site U1457. An unconformity (U1) created by emplacement of the MTD (unit 2) during the late Miocene Epoch (at c. 9.83–9.69 Ma) separates units 1 and 2 and is identified by a marked change in lithology. Unit 3 consists of hemipelagic sediment with thin interbeds of graded sandstone of late Miocene age, separated from unit 4 by a second unconformity (U2) of 0.5–0.9 Myr duration. Unit 4 consists of upper Miocene interbedded mudstone and sandstone and hemipelagic chalk deposited between c. 8 and 6 Ma. A c. 1.4–1.6 Myr hiatus (U3) encompasses the Miocene–Pliocene boundary and separates unit 4 from unit 5. Unit 5 includes upper Pliocene – lower Pleistocene siliciclastic sediment that is separated from unit 6 by a c. 0.45 Myr hiatus (U4) in the lower Pleistocene sediments. Unit 6 includes a thick package of rapidly deposited Pleistocene sand and mud overlain by predominantly hemipelagic sediment deposited since c. 1.2 Ma.

Upper water column dynamics in the eastern Arabian Sea were reconstructed in order to investigate changes in the activity of the South Asian / Indian monsoon during the early Pleistocene (c. 1.5–2.7 Ma). We used planktic foraminiferal assemblage records combined with isotopic (δ18O and δ13C) data, Mg/Ca-based sea surface temperatures and seawater δ18O records to estimate changes in surface water conditions at International Ocean Discovery Program (IODP) Site U1457. Our records indicate two distinct regimes of monsoon-induced changes in upper water structure during the periods c. 1.55–1.65 Ma and c. 1.85–2.7 Ma. We infer that a more stratified upper water column and oligotrophic mixed layer conditions prevailed during the period 1.85–2.7 Ma, which may be due to overall weaker South Asian / Indian winter (NE) and summer (SW) monsoon circulations. The period 1.55–1.65 Ma was characterized by enhanced eutrophication of the mixed layer, which was probably triggered by intensified winter (NE) monsoonal winds. The long-term trend in hydrographic changes during 1.55–1.65 Ma appears to be superimposed by short-term variations, probably reflecting glacial/interglacial changes. We suggest that an intensification of the South Asian / Indian winter monsoon circulation occurred between ∼1.65 Ma and 1.85 Ma, which is most likely due to the development of strong meridional and zonal atmospheric circulations (i.e. Walker Circulation and Hadley Circulation) because of strong equatorial East–West Pacific temperature gradients. [ABSTRACT FROM AUTHOR]

A 1108.6 m long core was recovered at Site U1457 located on the Indus Fan in the Laxmi Basin of the eastern Arabian Sea during IODP Expedition 355. Shipboard examinations defined five lithologic units (I to V) of the lower Paleocene to Holocene sedimentary sequence. In this study, δ13C values of sedimentary organic matter (SOM) confirm the differentiation of the lithologic units and further divide units III and IV into two subunits (1 and 2). Based on the underlying assumption that the SOM is decided primarily by a mixture of marine and terrestrial origins, δ13CSOM values at Site U1457 provide information on the terrestrial catchment conditions since late Miocene time. Low δ13CSOM values from late Miocene to late Pleistocene times are similar (c. −22.0 ‰) for the most part, reflecting a consistent contribution of terrestrial organic matter from the catchment areas characterized by dominant C3 land plants. Significantly lower δ13CSOM values (c. −24.0 ‰) in Unit III-2 (∼8 to ∼7 Ma) might be due to a greater input of C3 terrestrial organic matter. The increase in δ13CSOM values at ∼7 Ma and the appearance of high δ13CSOM values (c. −18.0 ‰) within Unit III-1 (∼7 to ∼2 Ma) indicate that C4 biomass overwhelmed the terrestrial catchment environment as a result of enhanced terrestrial aridity in the Himalayan foreland. The three-end-member simple mixing model, estimating the relative contributions of SOM from terrestrial C3 and C4 plants and marine phytoplankton, supports our interpretation of the distribution of C3 and C4 land plants in the terrestrial catchment environment. [ABSTRACT FROM AUTHOR]