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Expedition 361: Site U1479

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      Abstract

      Site U1479 is located on a 30 km wide morphological high rising ~200 m above the regional seafloor on the mid-to-lower western slope of the Agulhas Bank in Cape Basin (35°03.53′S; 17°24.06′E) ~85 nmi southwest of Cape Town, South Africa, at a water depth of 2615 m below sea level (mbsl). The Site U1479 primary objectives are to (1) Recover a complete Pliocene–Pleistocene sedimentary succession, including the early Pliocene warm period, mid-Pliocene expansion of Northern Hemisphere ice sheets, and the mid-Pleistocene transition, from a high-accumulation site located within the Agulhas ring corridor; (2) Reconstruct Agulhas Current warm-water transports over the course of both orbitally modulated and more abrupt climate changes; (3) Assess the linking between Antarctic climate variations, circumpolar ocean front instability, and connections with Agulhas leakage into the South Atlantic; (4) Assess the vigor and hydrography of NADW (or its precursors) exported to Circumpolar Deep Water and the southwest Indian Ocean at a location proximal to the entrance of NADW to the Southern Ocean and southern Indian Ocean; and (5) Evaluate the possibility of advective salinity feedbacks between Agulhas leakage and Atlantic Meridional Overturning Circulation variability, notably the possible role of the leakage in modulating surface-to-deep-ocean coupling in the North Atlantic during the transition between climatic states.

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      Most cited references 35

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      On the role of the Agulhas system in ocean circulation and climate.

       ,  Arne Biastoch,  Rainer Zahn (2011)
      The Atlantic Ocean receives warm, saline water from the Indo-Pacific Ocean through Agulhas leakage around the southern tip of Africa. Recent findings suggest that Agulhas leakage is a crucial component of the climate system and that ongoing increases in leakage under anthropogenic warming could strengthen the Atlantic overturning circulation at a time when warming and accelerated meltwater input in the North Atlantic is predicted to weaken it. Yet in comparison with processes in the North Atlantic, the overall Agulhas system is largely overlooked as a potential climate trigger or feedback mechanism. Detailed modelling experiments--backed by palaeoceanographic and sustained modern observations--are required to establish firmly the role of the Agulhas system in a warming climate.
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        Metabolic activity of subsurface life in deep-sea sediments.

        Global maps of sulfate and methane in marine sediments reveal two provinces of subsurface metabolic activity: a sulfate-rich open-ocean province, and an ocean-margin province where sulfate is limited to shallow sediments. Methane is produced in both regions but is abundant only in sulfate-depleted sediments. Metabolic activity is greatest in narrow zones of sulfate-reducing methane oxidation along ocean margins. The metabolic rates of subseafloor life are orders of magnitude lower than those of life on Earth's surface. Most microorganisms in subseafloor sediments are either inactive or adapted for extraordinarily low metabolic activity.
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          Vigorous exchange between the Indian and Atlantic oceans at the end of the past five glacial periods.

          The magnitude of heat and salt transfer between the Indian and Atlantic oceans through 'Agulhas leakage' is considered important for balancing the global thermohaline circulation. Increases or reductions of this leakage lead to strengthening or weakening of the Atlantic meridional overturning and associated variation of North Atlantic Deep Water formation. Here we show that modern Agulhas waters, which migrate into the south Atlantic Ocean in the form of an Agulhas ring, contain a characteristic assemblage of planktic foraminifera. We use this assemblage as a modern analogue to investigate the Agulhas leakage history over the past 550,000 years from a sediment record in the Cape basin. Our reconstruction indicates that Indian-Atlantic water exchange was highly variable: enhanced during present and past interglacials and largely reduced during glacial intervals. Coherent variability of Agulhas leakage with northern summer insolation suggests a teleconnection to the monsoon system. The onset of increased Agulhas leakage during late glacial conditions took place when glacial ice volume was maximal, suggesting a crucial role for Agulhas leakage in glacial terminations, timing of interhemispheric climate change and the resulting resumption of the Atlantic meridional overturning circulation.
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            Author and article information

            Journal
            10.14379/iodp.proc.361.2017
            Proceedings of the International Ocean Discovery Program
            International Ocean Discovery Program
            2377-3189
            30 September 2017
            10.14379/iodp.proc.361.108.2017

            This work is licensed under a Creative Commons Attribution 4.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

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            Self URI (journal page): http://publications.iodp.org/

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