Variations in ?13C values of sedimentary organic matter since late Miocene time in the Indus Fan (IODP Site 1457) of the eastern Arabian Sea

Abstract

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, delta13C 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, delta13CSOM values at Site U1457 provide information on the terrestrial catchment conditions since late Miocene time. Low delta13CSOM 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 delta13CSOM 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 delta13CSOM values at ~7 Ma and the appearance of high delta13CSOM 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.