Clarke Andrew

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Clarke
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  • Article
    The freshwater system west of the Antarctic Peninsula : spatial and temporal changes
    (American Meteorological Society, 2013-03-01) Meredith, Michael P. ; Venables, Hugh J. ; Clarke, Andrew ; Ducklow, Hugh W. ; Erickson, Matthew ; Leng, Melanie J. ; Lenaerts, Jan T. M. ; van den Broeke, Michiel R.
    Climate change west of the Antarctic Peninsula is the most rapid of anywhere in the Southern Hemisphere, with associated changes in the rates and distributions of freshwater inputs to the ocean. Here, results from the first comprehensive survey of oxygen isotopes in seawater in this region are used to quantify spatial patterns of meteoric water (glacial discharge and precipitation) separately from sea ice melt. High levels of meteoric water are found close to the coast, due to orographic effects on precipitation and strong glacial discharge. Concentrations decrease offshore, driving significant southward geostrophic flows (up to ~30 cm s−1). These produce high meteoric water concentrations at the southern end of the sampling grid, where collapse of the Wilkins Ice Shelf may also have contributed. Sea ice melt concentrations are lower than meteoric water and patchier because of the mobile nature of the sea ice itself. Nonetheless, net sea ice production in the northern part of the sampling grid is inferred; combined with net sea ice melt in the south, this indicates an overall southward ice motion. The survey is contextualized temporally using a decade-long series of isotope data from a coastal Antarctic Peninsula site. This shows a temporal decline in meteoric water in the upper ocean, contrary to expectations based on increasing precipitation and accelerating deglaciation. This is driven by the increasing occurrence of deeper winter mixed layers and has potential implications for concentrations of trace metals supplied to the euphotic zone by glacial discharge. As the regional freshwater system evolves, the continuing isotope monitoring described here will elucidate the ongoing impacts on climate and the ecosystem.
  • Article
    The discovery of new deep-sea hydrothermal vent communities in the Southern Ocean and implications for biogeography
    (Public Library of Science, 2012-01-03) Rogers, Alex D. ; Tyler, Paul A. ; Connelly, Douglas P. ; Copley, Jonathan T. ; James, Rachael H. ; Larter, Robert D. ; Linse, Katrin ; Mills, Rachel A. ; Naveira Garabato, Alberto C. ; Pancost, Richard D. ; Pearce, David A. ; Polunin, Nicholas V. C. ; German, Christopher R. ; Shank, Timothy M. ; Boersch-Supan, Philipp H. ; Alker, Belinda J. ; Aquilina, Alfred ; Bennett, Sarah A. ; Clarke, Andrew ; Dinley, Robert J. J. ; Graham, Alastair G. C. ; Green, Darryl R. H. ; Hawkes, Jeffrey A. ; Hepburn, Laura ; Hilario, Ana ; Huvenne, Veerle A. I. ; Marsh, Leigh ; Ramirez-Llodra, Eva ; Reid, William D. K. ; Roterman, Christopher N. ; Sweeting, Christopher J. ; Thatje, Sven ; Zwirglmaier, Katrin
    Since the first discovery of deep-sea hydrothermal vents along the Galápagos Rift in 1977, numerous vent sites and endemic faunal assemblages have been found along mid-ocean ridges and back-arc basins at low to mid latitudes. These discoveries have suggested the existence of separate biogeographic provinces in the Atlantic and the North West Pacific, the existence of a province including the South West Pacific and Indian Ocean, and a separation of the North East Pacific, North East Pacific Rise, and South East Pacific Rise. The Southern Ocean is known to be a region of high deep-sea species diversity and centre of origin for the global deep-sea fauna. It has also been proposed as a gateway connecting hydrothermal vents in different oceans but is little explored because of extreme conditions. Since 2009 we have explored two segments of the East Scotia Ridge (ESR) in the Southern Ocean using a remotely operated vehicle. In each segment we located deep-sea hydrothermal vents hosting high-temperature black smokers up to 382.8°C and diffuse venting. The chemosynthetic ecosystems hosted by these vents are dominated by a new yeti crab (Kiwa n. sp.), stalked barnacles, limpets, peltospiroid gastropods, anemones, and a predatory sea star. Taxa abundant in vent ecosystems in other oceans, including polychaete worms (Siboglinidae), bathymodiolid mussels, and alvinocaridid shrimps, are absent from the ESR vents. These groups, except the Siboglinidae, possess planktotrophic larvae, rare in Antarctic marine invertebrates, suggesting that the environmental conditions of the Southern Ocean may act as a dispersal filter for vent taxa. Evidence from the distinctive fauna, the unique community structure, and multivariate analyses suggest that the Antarctic vent ecosystems represent a new vent biogeographic province. However, multivariate analyses of species present at the ESR and at other deep-sea hydrothermal vents globally indicate that vent biogeography is more complex than previously recognised.