²³⁰Th derived glacial to Holocene sediment fluxes in the southern Arabian Sea: insights into carbonate sedimentation

Abstract

We present a record of glacial to Holocene 230Thxs normalized sediment mass fluxes for a site located on the eastern flank of the Carlsberg Ridge, Arabian Sea to evaluate carbonate burial, preservation and surface ocean changes spanning the last 38 ka. Sediment fluxes (0.81–3.70 g cm−2 ka−1) indicate dominant carbonate rain (0.44–1.83 g cm−2 ka−1), followed by 232Th‐based lithogenic flux (0.36–1.42 g cm−2 ka−1), with higher fluxes during the Holocene and the late Marine Isotope Stage (MIS) 3 and extremely low fluxes during MIS 2. Despite large variations in sediment mass fluxes, the average flux of 230Thxs scavenged over the last 38 ka remained nearly constant. In contrast, the large variations in the initial 230Thxs activity display the effect of variable dilution by the carbonate flux. Analysis of 232Th‐based lithogenic flux documents a dominant fluvial source and links its variability to changing SW monsoon precipitation in the Indus source region. Comparison of the last 38 ka carbonate flux record with the present‐day carbonate flux intercepted at the closest mooring at 3915 m depth demonstrates burial efficiency of 80 to 99% to the seabed during the Holocene and late MIS 3 and 53% for the MIS 2. Calculated carbonate preservation efficiency relative to the trapped upper ocean carbonate flux provides evidence for a productivity control of the preserved carbonate flux, which indicates high preservation during the Holocene (63±10%), and MIS 3 (77±24%) and low during MIS 2 (41±15%). Our results suggest that the eastern flank of Carlsberg Ridge experienced enhanced carbonate production during the Holocene and MIS 3 compared to MIS 2. We suggest that changing oceanographic conditions in near‐surface waters have changed the carbonate supply to the sea floor accounting for the glacial to Holocene pattern with increased supply due to the intensified SW monsoon and reduced input associated with the weakened SW monsoon.