North Atlantic Deep Water and Antarctic Bottom Water variability during the last 200 ka recorded in an abyssal sediment core off South Africa
Introduction
In the Late Quaternary, there are two main deep water production sites, which replenish the deep and bottom waters in the world's oceans and thus play a key role in controlling the thermohaline circulation. These sites are i) the Greenland, Iceland, Norwegian, and Labrador seas where North Atlantic Deep Water (NADW) is produced (Reid, 1994) and ii) the Weddell Sea, where the majority of Antarctic Bottom Water (AABW) is formed (Carmack and Foster, 1975, Foster and Carmack, 1976, Foldvik and Gammelsrod, 1988).
Deep and bottom water proportions at these two sites is of major importance to the Atlantic Meridional Overturning Circulation (AMOC) and the associated meridional heat flux in the Atlantic Ocean. Reconstructing the changes in relative portions between northern (NADW) and southern (AABW) source deep and bottom water thus is necessary to interpret Quaternary climate change. For assessing changes in the relative portion between NADW and AABW in the Agulhas Basin during the last 200 ka, we use benthic foraminiferal δ13C, a proxy for the NADW/AABW composition of the bottom water, Kaolinite/Chlorite ratio (K/C), a proxy for the NADW/AABW composition of the deep water and the bottom water, and the median of the sortable silt (SS), a proxy for the flow speed of the bottom water mass.
Delta 13C of F. wuellerstorfi is known to be a generally good representation of bottom water δ13CDIC (Woodruff et al., 1980, Zahn et al., 1986, Curry et al., 1988, McCorkle and Keigwin, 1994). NADW, representing a young and nutrient-depleted deep water mass, originally carries a δ13C signal > 1‰ in the North Atlantic (Kroopnick, 1985), which is progressively lowered on its way south. In contrast, AABW, has negative δ13C values. Thus, increasing values in the benthic δ13C record in the South Atlantic indicate increasing NADW influence, whereas decreasing values represent a shift to a larger AABW portion (Oppo and Fairbanks, 1987, Charles and Fairbanks, 1992, Bickert and Wefer, 1999, Mackensen, 2001). 143Nd/144Nd isotope ratios of Mn-Fe crusts (Piotrowski et al., 2004, Piotrowski et al., 2005) and benthic foraminiferal δ13C variability in the South Atlantic (Oppo and Fairbanks, 1987, Charles and Fairbanks, 1992, Mackensen et al., 2001, Mackensen, 2004) indicate a northward retreat and shallowing of NADW during glacials as a result of reduced deep water formation in the North Atlantic.
The K/C in South Atlantic surface sediments displays a strong latitudinal zonality with values between 5 and 10 at the equator, and values < 0.15 at 50° to 60°S, under the Antarctic Circumpolar Current (ACC) (Petschick et al., 1996, Diekmann et al., 2003). Accordingly, NADW is showing higher K/C than AABW. In warm periods, a stronger influence of NADW in the Agulhas Basin is evidenct from high values of δ13C and K/C (Diekmann et al., 1996, Diekmann et al., 1999, Krueger et al., 2008.) In contrast, Cd/Ca and 231Pa/230Th measurements on sediments from the Southern Ocean indicate that NADW influx did not change much over the last glacial/interglacial cycles (Lea and Boyle, 1990, Boyle, 1992, Yu et al., 1996, Marchal et al., 2000).
The AABW response to global climate change is also under debate. The Weddell Sea in the Atlantic sector of the Southern Ocean is considered to be the primary production site for AABW (Foster et al., 1987, Foldvik and Gammelsrod, 1988). In this paper we use the term AABW in a broader sense, also comprising Weddell Sea Deep Water (WSDW), Weddell Sea Bottom Water (WSBW), Warm Deep Water (WDW) which contribute to the AABW sensu strictu (Mackensen et al., 2001; Foldvik et al., 2004). Two principle processes contribute to AABW formation. The first process is based on the enhanced atmosphere–sea-ice–ocean interaction on the large continental shelf of the southwestern Weddell Sea (Gill, 1973, Carmack and Foster, 1977, Schodlok et al., 2002, Nicholls et al., 2009). There surface water is cooled to freezing point by contact with the atmosphere. Its salinity and thus density increases due to sea ice formation, which causes concentration of brine. The resulting dense brine water sinks down the continental slope and is incorporated into the Weddell Gyre (Gill, 1973, Carmack and Foster, 1975, Killworth, 1977). The brine formation is especially effective in the perimeter of the seasonal sea-ice zone (SIZ, Mackensen, 2001a) and in polynyas, stretches of open water surrounded by ice, which form when cold catabatic winds blow newly formed sea ice offshore, allowing new sea ice to form. We refer to this mode as Polynya Mode. The second mode depends on the existence of large ice shelves. Water drawn underneath these ice shelves is super-cooled below the freezing temperature of water at surface pressure. The resulting dense Ice Shelf Water (ISW) flows down the continental slope in plumes and forms WSBW (Foldvik et al., 1985, Foldvik and Gammelsrod, 1988). We refer to this mode of contribution to AABW formation as Ice Shelf Mode (ISM).
Sediments from the Weddell Sea are in general carbonate free, which severely limits age control and makes changes in AABW formation difficult to assess (Kuhn and Diekmann, 2002). Reconstructions range from a reduced over unchanged to increased bottom water formation in the Weddell Sea during glacials. All three scenarios discussed in the literature are plausible in their own right and supported by proxy data: (i) Deep and Bottom Water formation in the Weddell Sea was reduced during glacial periods as the sea level was lower and the Antarctic shelves were largely covered with grounded ice. Together with a decreased supply of warm, saline northern-borne deep water into the Weddell Sea, this prevented substantial ISW formation (Grobe and Mackensen, 1992, Pudsey, 1992, Mackensen et al., 1994). (ii) Curry and Lohmann, 1982, Weber et al., 1994 and others propose that the rate of bottom water production stayed at approximately the same level during glacials and intergalcials, but the dominant mode changed from ISM during interglacials to the Polynya Mode during glacials (Pudsey et al., 1988). In this paper we try to enhance the understanding of the processes controlling the AABW production in the Weddell Sea. (iii), Deep and Bottom Water formation in the Weddell Sea was enhanced during glacials because the relative amount of bottom water formed by brine rejection in polynyas and the seasonal SIZ increased greatly. This is corroborated by the dominance of Epistominella exigua in the benthic foraminiferal faunas at the Antarctic continental margin throughout most of the glacial stages, which depends on the existence of large polynyas with at least seasonally significant primary production (Mackensen et al., 1989, Mackensen et al., 1994) and also fossil evidence for sustained existence of petrel colonies in the Weddell Sea's hinterland throughout the last glacial period (Thatje et al., 2008).
Section snippets
Study area
The investigated core site PS2561-2 is located at the south-eastern flank of the Agulhas Plateau in the Agulhas Basin (Fig. 1). The Agulhas Basin lies beneath the confluence of water masses from the South Atlantic, the Southern Ocean and the Indian Ocean. It is flanked by the Cape Basin in the Northwest, from which it is separated by the Agulhas Ridge, the Enderby Basin in the South, from which it is separated through the Southwestern Indian Ridge, and the Mozambique Basin in the Northeast.
Material and methods
Core PS2561-2 was recovered from the south-eastern flank of the Agulhas Plateau (41°51.5'S, 28°32.5'E) at a water depth of 4465 m (Fig. 1). The sediment core is 11.5 m long and comprises calcareous nannofossil bearing mud. It includes two intervals rich in siliceous microfossils (8.10 m to 9.30 m and 10.40 m to 11.15 m). The stable oxygen and carbon isotope composition of the epibenthic foraminifer species Fontbotia wuellerstorfi was determined at the Alfred Wegener Institute for Polar and Marine
Stable isotopes
The δ18O values of core PS2561-2 range between 2.75‰ to 3.75‰ during interglacial MIS 1, 5 and 7 and between 4.0‰ to 4.5‰ during glacial MIS 2, 4, 6 and the transitional stage 3. Glacial/Interglacial shifts (G-I) are approximately 1.5‰. Also the δ13 C values generally show a clear glacial/interglacial variability (Fig. 2). Ratios are reduced during glacial MIS 2, 4, and 6 and cold substages 5.2 and 5.4 while ratios are higher during MIS 7, interglacial substages 5.1, 5.3, 5.5, during MIS 3 and
Characteristics of deep water masses in the Agulhas Basin
For discussion and a more comprehensive interpretation we correlate our data from core PS2561-2 with those of other cores from the region, namely MD02-2589 from intermediate water depth at the Agulhas Plateau (Molyneux et al., 2007), RC11-83 from the abyssal Cape Basin (Charles et al., 1996), and ODP Leg 177 Site 1089 from the southern abyssal Cape Basin (Kuhn and Diekmann, 2002; Fig.1). All benthic stable isotope records were measured on F. wuellerstorfi.
The good correlation of the δ18O record
Conclusions
The water mass proxies δ13C and K/C, as well as SS of abyssal core PS2561-2 allowed to reconstruct the changes in the influence of NADW and AABW in the deep and bottom water composition at the Agulhas Plateau during the last 200 ka. The K/C and δ13C records show that NADW influence in the Agulhas Basin varies on a glacial/interglacial pattern with increased export during interglacials and reduced export during glacials. Variations in the SS values record changes in the bottom water flow speed.
Acknowledgements
We thank the ship crews and scientists of R.V. Polarstern for good collaboration during various expeditions and S. Dorn for assisting with the laboratory work. This study was supported by the Deutsche Forschungsgemeinschaft. We also thank C.-D. Hillenbrand and and Pierre Giresse for their very helpful suggestions and recomendations for improvement of the manuscript.
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