Changes in monsoon and ocean circulation and the vegetation cover of southwest Sumatra through the last 83,000 years: The record from marine core BAR94-42

We present the palynological record from deep-sea core BAR94-42 taken offshore of southwest Sumatra in the Indian Ocean. Refinement of a previous age model for the core has been made using 7 additional radiocarbon ages and 29 additional oxygen-isotope measurements. These analyses have substantially improved the previously published chronology, which has enabled revision of the results of sedimentological studies of the core. The pollen and re-interpreted biogenic and terrigeneous data were used to reconstruct monsoon circulation and vegetation of southwest Sumatra over the last 83,000 years (83 kyr). During marine isotope stage (MIS) 5a, southwest Sumatra was covered by rainforest, with open herbaceous swamps lining river courses and surrounding lakes. The SE component of the monsoon was stronger than the NW component, resulting in a humid climate with a short, dry season. During MIS 4, conditions were drier, fire activity increased and the monsoon was generally weaker. This latter pattern persisted until MIS1. The vegetation was most open during MIS 3, between ~ 52 and 43 kyr; identifying this phase as the driest of the last glacial. An increase in montane trees from ~ 52 kyr indicates the onset of cooler conditions, which lasted until the transition to MIS 1 at about 11.9 kyr. After ~ 43 kyr, an everwet climate gradually developed as monsoonal circulation intensified and the SE monsoon component became stronger. During this time, closed-canopy rainforest became dominant regionally over southwest Sumatra. Increased monsoonal activity during the early Holocene resulted in increased precipitation, river runoff, sediment discharge and offshore sediment transport from the continental shelf.