Search

The Antarctic Circumpolar Current (ACC) is the world's largest current system connecting all three major basins of the global ocean. Our knowledge of glacial‐interglacial changes in ACC dynamics in the southeast Pacific is not well constrained and presently only based on reconstructions covering the last glacial cycle. Here we use a combination of mean sortable silt grain size of the terrigenous sediment fraction (10–63 μm, SS¯) and X‐ray fluorescence scanner‐derived Zr/Rb ratios as flow strength proxies to examine ACC variations at the Pacific entrance to the Drake Passage (DP) in the vicinity of the Subantarctic Front. Our results indicate that at the DP entrance, ACC strength varied by ~6–16% on glacial‐interglacial time scales, yielding higher current speeds during interglacial times and reduced current speeds during glacials. We provide evidence that previous observations of a reduction in DP throughflow during the last glacial period are part of a consistent pattern extending for at least the last 1.3 Ma. The orbital‐scale cyclicity follows well‐known global climate changes from prevailing ca. 41‐kyr cycles in the early part of the record (1.3 Ma to 850 ka; marine isotope stage 21) across the mid‐Pleistocene transition into the middle and late Pleistocene 100‐kyr world. A comparison to a bottom water flow record from the deep western boundary current off New Zealand (Ocean Drilling Program Site 1123) reveals anti‐phased changes between the two sites. The enhanced supply of deep water along the DP and into the Atlantic Ocean during interglacials corresponds to a weakened flow of the SW Pacific deep western boundary current. Key Points: The strength of ACC flow in the SE Pacific varied strongly on glacial‐interglacial time scalesOver the past 1.3 Ma, our results show a consistent pattern of stronger (weaker) flow during interglacials (glacials)The sortable silt record largely follows orbital‐scale cyclicity, changing from ca. 41‐kyr cycles into a 100‐kyr world after the MPT [ABSTRACT FROM AUTHOR]

The Indian monsoon drives seasonal changes in precipitation and weathering across India as well as circulation and productivity in the northern Indian Ocean. Variation in paleo-monsoon intensity and its effect on productivity and lithogenic fluxes is poorly constrained in the Bay of Bengal. In this paper, we present analysis of a sediment record from the offshore Mahanadi Basin recovered during the Indian National Gas Hydrate Program Expedition 01 (Site NGHP-01-19B). We reconstruct variation in biogenic and lithogenic components during the last 110 kyr using measurements of total organic carbon (TOC), total nitrogen (TN), TOC/TN, CaCO 3 , biogenic silica (BSi), δ 13 TOC, δ 15 TN, bulk mineralogy from X-ray diffraction, bulk and lithogenic grain size distribution, magnetic susceptibility, bulk density, and Ca, Br, and Zr/Rb from x-ray fluorescence (XRF). The mass-accumulation rate (MAR) of CaCO 3 , a function of marine productivity, drastically increased between 70 and 10 ka and is correlated to previously-documented elevated Asian dust fluxes and increased Bay of Bengal salinity during a weakened southwest monsoon. Decreased freshwater input over this period likely diminished stratification, allowing for increased mixing and nutrient availability, thus enhancing productivity despite weaker southwest monsoon winds. The MAR of lithogenic material is highest during the Holocene suggesting that sediment supply driven by monsoon intensity is a stronger control on margin sedimentation than sea level at the Mahanadi Basin. Over the entire record, magnetic susceptibility and XRF Zr/Rb are strongly correlated with CaCO 3 , suggesting higher primary mineral input under a weakened southwest monsoon. TOC/TN and δ 13 TOC also increase under glacial conditions, suggesting higher relative input of terrestrial C4 organic matter. These results highlight the Mahanadi Basin as a supply-dominated margin where terrigenous sedimentation is strongly influenced by monsoon intensity, and that productivity is limited by variation in monsoon-driven stratification on glacial-interglacial timescales rather than a direct response to monsoon winds. [ABSTRACT FROM AUTHOR]