Persistent inflow of warm water onto the central Amundsen shelf

Abstract. During four austral summers (December to January) from 2006 to 2010, we investigated the surface-water carbonate system and its controls in the western Antarctic Ocean. Measurements of total alkalinity (A T ), pH and total inorganic carbon (C T ) were investigated in combination with high-frequency measurements on sea-surface temperature (SST), salinity and Chl a. In all parameters we found large interannual variability due to differences in sea-ice concentration, physical processes and primary production. The main result from our observations suggests that primary production was the major control on the calcium carbonate saturation state ( ) in austral summer for all years. This was mainly reflected in the covariance of pH and Chl a. In the sea-ice-covered parts of the study area, pH and were generally low, coinciding with low Chl a concentrations. The lowest pH in situ and lowest aragonite saturation ( Ar ∼ 1.0) were observed in December 2007 in the coastal Amundsen and Ross seas near marine outflowing glaciers. These low and high pH values were likely influenced by freshwater dilution. Comparing 2007 and 2010, the largest Ar difference was found in the eastern Ross Sea, where Ar was about 1.2 units lower in 2007 than in 2010. This was mainly explained by differences in Chl a (i.e primary production). In 2010 the surface water along the Ross Sea shelf was the warmest and most saline, indicating upwelling of nutrient and CO 2 -rich sub-surface water, likely promoting primary production leading to high and pH. Results from multivariate analysis agree with our observations showing that changes in Chl a had the largest influence on the Ar variability. The future changes of Ar were estimated using reported rates of the oceanic uptake of anthropogenic CO 2 , combined with our data on total alkalinity, SST and salinity (summer situation). Our study suggests that the Amundsen Sea will become undersaturated with regard to aragonite about 40 yr sooner than predicted by models.

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“…Arneborg et al, 2012). The salinity and SST changes entering the Amundsen and Ross seas are shown for each year in Fig.…”

Section: Hydrographymentioning

confidence: 99%

“…Increased melt at the base of the floating ice sheets indicates that the forcing is derived from the oceans. It is suggested that warm Circumpolar Deep Water (CDW) enters the continental shelf and drives the glacier melt of the floating terminus of the Pine Island Glacier (Jacobs et al, 1996;Thoma et al, 2008;Arneborg et al, 2012).…”

Section: Introductionmentioning

confidence: 99%

Ocean acidification state in western Antarctic surface waters: controls and interannual variability

et al. 2014

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“…Specifically, at the regional level, an improved understanding of the processes driving the flux of CDW in the ASE at the shelf edge (30,(52)(53)(54) and of the processes controlling the thermohaline structure in PIB is needed. On a more global scale, a detailed review of factors influencing the wind over the ASE, including but not limited to the tropical Pacific, including synoptic (55), interannual (18,(56)(57)(58)(59), decadal and longer (60) timescales, would be beneficial.…”

Section: Characterization Of the Atmospheric Bridge Between The Ase Amentioning

confidence: 99%

Strong Sensitivity of Pine Island Ice-Shelf Melting to Climatic Variability

Dutrieux

Rydt

Jenkins

et al. 2014

Science

427

686

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Pine Island Glacier has thinned and accelerated over recent decades, significantly contributing to global sea level rise. Increased oceanic melting of its ice shelf is thought to have triggered those changes. Observations and numerical modeling reveal large fluctuations in the ocean heat available in the adjacent bay and enhanced sensitivity of ice shelf melting to water temperatures at intermediate depth, as a seabed ridge blocks the deepest and warmest waters from reaching the thickest ice. Oceanic melting decreased by 50% between January 2010 and 2012, with ocean conditions in 2012 partly attributable to atmospheric forcing associated with a strong La Niña event. Both atmospheric variability and local ice shelf and seabed geometry play fundamental roles in determining the response of the Antarctic Ice Sheet to climate.One Sentence Summary: Ocean melting of the Pine Island Glacier ice shelf was halved in two years as an underlying seabed ridge makes it highly sensitive to climatic forcing. Main Text:Austral summer observations in the Amundsen Sea, West Antarctica, show that lightlymodified, warm (0.5-1.2°C) and saline (>34.6) Circumpolar Deep Water (CDW), 2-4°C above the in-situ freezing point, pervades a network of glacially scoured seabed troughs (1, Fig. 1A).The CDW reaches nearby Antarctic glaciers and delivers heat to the base of their 200-1000 mthick ice shelves (2-4). It is overlain by a 200-300 m-thick layer of cold (-1.5°C) and fresh (salinity<34.4) Winter Water (WW, Fig. 2A) that is seasonally replenished by interaction with the atmosphere and sea ice.Pine Island Glacier (PIG), a major outlet glacier feeding one such ice shelf, has shown apparently continuous thinning (5, 6) and intermittent acceleration (7-9) from 1973 to 2009.During this period, its ice shelf has also thinned (6,(10)(11)(12), and the reduction in buttressing driven by oceanic melting is believed to be responsible for the changes inland. Earlier analysis indicated that a higher CDW volume and temperature in Pine Island Bay (PIB) in January 2009caused an increase in ice-shelf melting and in the associated meltwater-driven circulation, relative to 1994 (2). The lack of sub-annual variability in CDW temperature during one yearlong measurement in PIB (1) and the long-term correlation between the oceanic melting and the mass loss required to sustain thinning of the ice shelf gave the impression that the ice-ocean system had shown progressive change over the last two decades. This is consistent with a positive geometrical feedback, with oceanic melt enlarging the cavity under the ice shelf, allowing stronger circulation and further melting.However, such ice-ocean systems are likely to be more complex. The glacier's rapid change over the last few decades was probably triggered by its ungrounding from a the top of a seabed ridge transverse to the ice flow at some time before the 1970s (4). Subsequent migration of the glacier's grounding line (13) down the seabed slope upstream from the ridge crest was probably an inevitable respon...

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“…Producing proxy records of past CDW incursions from marine sediment cores is still a challenge but has recently been demonstrated to be possible in sediments from the ASE shelf . With recent observations of present CDW advection predominantly through the paleo-ice stream troughs of the ASE (e.g., Arneborg et al, 2012), drilling on the ASE shelf has a good chance to recover sample material suitable for applying benthic foraminifer-based proxies to reconstruct past CDW upwelling onto the shelf and its effect on WAIS dynamics.…”

Section: Hypothesis H2: Ice-proximal Records Of Ice Sheet Dynamics Inmentioning

confidence: 99%

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Gohl¹,

Wellner²,

Klaus³

2017

International Ocean Discovery Program Scientific Prospectus

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The West Antarctic Ice Sheet (WAIS) is largely marine based and thus highly sensitive to both climatic and oceanographic changes. Therefore, the WAIS has likely had a very dynamic history over the last several million years. A complete collapse of the WAIS would result in a global sea level rise of 3.3-4.3 m, yet the world's scientific community is not able to predict its future behavior. Moreover, knowledge about past behavior of the WAIS is poor, in particular during geological times with climatic conditions similar to those expected for the near and distant future. Reconstructions and quantifications of partial or complete WAIS collapses in the past are urgently needed for constraining and testing ice sheet models that aim to predict future WAIS behavior and the potential contribution of the WAIS to global sea level rise. Large uncertainties exist regarding the chronology, extent, rates, and spatial and temporal variability of past advances and retreats of the WAIS across the continental shelves. These uncertainties largely result from the fundamental lack of data from drill cores recovered proximal to the WAIS. The continental shelf and rise of the Amundsen Sea are prime targets for drilling because the records are expected to yield archives of pure WAIS dynamics unaffected by other ice sheets and the WAIS sector draining into the Amundsen Sea Embayment (ASE) currently experiences the largest ice loss in Antarctica (Paolo et al., 2015).We propose a series of drill sites for the ASE shelf where seismic data reveal seaward-dipping sedimentary sequences that span from the preglacial depositional phase to the most recent glacial periods. Our strategy is to drill a transect from the oldest sequences close to the bedrock/basin boundary at the middle-inner shelf transition to the youngest sequences on the outer shelf in the eastern ASE. If the eastern ASE is inaccessible due to sea ice cover, a similar transect of sites can be drilled on the western ASE. The core transect will provide a detailed history of the glacial cycles in the Amundsen Sea region and allow comparison to the glacial history from the Ross Sea sector. In addition, deep-water sites on the continental rise of the Amundsen Sea are selected for recovering continuous records of glacially transported sediments and detailed archives of climatic and oceanographic changes throughout glacial-interglacial cycles. We will apply a broad suite of analytical techniques, including multiproxy analyses, to address our objectives of reconstructing the onset of glaciation in the greenhouse to icehouse transition, processes of dynamic ice sheet behavior during the Neogene and Quaternary, and ocean conditions associated with the glacial cycles.The five principal objectives of Expedition 379 are as follows:1. To reconstruct the glacial history of West Antarctica from the Paleogene to recent times and the dynamic behavior of the WAIS during the Neogene and Quaternary, especially possible partial or full WAIS collapses, and the WAIS contribution to past sea level...

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Persistent inflow of warm water onto the central Amundsen shelf

Cited by 87 publications

References 20 publications

Ocean acidification state in western Antarctic surface waters: controls and interannual variability

Ocean acidification state in western Antarctic surface waters: controls and interannual variability

Strong Sensitivity of Pine Island Ice-Shelf Melting to Climatic Variability

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