Green bands as markers of deep ocean ventilation in marine sediments

Abstract

Abstract Ocean ventilation of "young" surface waters, which have recently been in contact with the atmosphere, in the high-latitude North Atlantic Ocean, serves as a major source of oxygen-rich deep waters (> 2.5 km) to the world's oceans 1,2. Both models and observations suggest deep water production and the oxygenation of deep water are declining, threatening valuable marine ecosystems and the crucial role the ocean plays in carbon storage 3–6. Widely observed diagenetic banding in marine sediments is believed to be connected to the availability of oxygen in pore waters and the remineralization of buried organic matter. This study combines a comprehensive spatial survey of distinctive green banding in near-surface sediments with an analysis of the temporal variability in banding at International Ocean Discovery Program (IODP) Sites U1313 and U1474 to document fluctuations in bottom water oxygen across the mid-late Pleistocene (1 million years to present). The survey of near surface sediments (core-tops) indicates that the green bands have their origin in the post-glacial resurgence in the production of oxygen-rich North Atlantic Deep Water (NADW). The stratigraphic survey shows the synchronous development of banding numerous times during the glacial marine isotope stages (MIS) of the Pleistocene, indicating the timing of regional deoxygenation events as a consequence of Northern Component Water (NCW; the glacial equivalent of NADW) slowdowns. This glacial-interglacial pattern in band abundance is superimposed on a lower-frequency cycle corresponding to the 400-thousand-year (ka) band in orbital eccentricity and a globally prevailing pattern in benthic carbon isotopes. We link green band formation and benthic carbon isotopes to sapropel barren intervals from the Mediterranean Sea and productivity records from the Southern Ocean and propose that long eccentricity cycles influenced bottom water oxygen across the Late Pleistocene.