Data Constraints on Glacial Atlantic Water Mass Geometry and Properties. Issue 9 (27th September 2018)

• Record Type: Journal Article
• Authors: Oppo, Delia W.; Gebbie, Geoffrey; Huang, Kuo‐Fang; Curry, William B.; Marchitto, Thomas M.; Pietro, Kathryn R.
• Is Part Of: Paleoceanography and paleoclimatology. Volume 33:Issue 9(2018)
• Page Start: 1013
• Page End: 1034
• Publication Date: 2018-09-27
• Subjects: glacial Atlantic circulation -- preformed phosphate -- remineralized phosphate -- Antarctic Intermediate Water -- nutrient redistribution -- tropical phosphate maximum; Paleoceanography -- Periodicals; Paleoclimatology -- Periodicals; 551.46
• DOI: 10.1029/2018PA003408
• Languages: English
• ISSNs: 2572-4517

Abstract

Abstract: The chemical composition of benthic foraminifera from marine sediment cores provides information on how glacial subsurface water properties differed from modern, but separating the influence of changes in the origin and end‐member properties of subsurface water from changes in flows and mixing is challenging. Spatial gaps in coverage of glacial data add to the uncertainty. Here we present new data from cores collected from the Demerara Rise in the western tropical North Atlantic, including cores from the modern tropical phosphate maximum at Antarctic Intermediate Water (AAIW) depths. The results suggest lower phosphate concentration and higher carbonate saturation state within the phosphate maximum than modern despite similar carbon isotope values, consistent with less accumulation of respired nutrients and carbon, and reduced air‐sea gas exchange in source waters to the region. An inversion of new and published glacial data confirms these inferences and further suggests that lower preformed nutrients in AAIW, and partial replacement of this still relatively high‐nutrient AAIW with nutrient‐depleted, carbonate‐rich waters sourced from the region of the modern‐day northern subtropics, also contributed to the observed changes. The results suggest that glacial preformed and remineralized phosphate were lower throughout the upper Atlantic, but deep phosphate concentration was higher. The inversion, which relies on the fidelity of the paleoceanographic data, suggests that the partial replacement of North Atlantic sourced deep water by Southern Ocean Water was largely responsible for the apparent deep North Atlantic phosphate increase, rather than greater remineralization. Plain Language Summary: The Atlantic circulation system, characterized by northward upper ocean flow and deep southward flow, exerts tremendous influence on the distribution of salt, heat, nutrients, and carbon in the world's oceans, with consequences on surface climate, marine ecosystems, and atmospheric carbon dioxide levels. Reconstructing past ocean circulation and the distribution of ocean properties is challenging, in large part because paleoceanographic proxies may reflect changes in more than one oceanographic variable. We combine new data, published data, and a model to provide new insights into how and why the nutrient distribution of the Atlantic at the peak of the last ice age, approximately 23, 000 to 19, 000 years ago, was different from modern. The results indicate links between glacial properties of waters sinking in the high‐latitude surface ocean, their pathways and subsurface mixtures, and the distribution of nutrients in the glacial ocean. Key Points: Water sinking south of modern North Atlantic subtropical front partly replaced Antarctic Intermediate Water especially in northern tropics Preformed and total phosphate concentrations in glacial Antarctic Intermediate Water and North Atlantic Deep Water were lower than modern A partial replacement of AAIW by subtropical waters and low preformed nutrients in glacial AAIW weakened the tropical phosphate maximum