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Palaeo-oceanography: Deepwater variability in the Holocene epoch

Holocene Climate Records Palaeo-oceanography: Deepwater variability in the Holocene epoch
Vol. 422, No. 6929, pp. 277, 20 March 2003

Delia W. Oppo and Jerry F. McManus
Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02540, USA
James L. Cullen
Department of Geological Sciences, Salem State College, Salem, Massachusetts 01970, USA

Figure 1. Holocene climate records, top to bottom: benthic d18O and d13C (North Atlantic deepwater contribution) records from site 980; GISP2 sea-salt sodium flux (increasing, calm to windier), and percentage of haematite-stained grains (%HSG) in core V29-191. Triangles, dated levels in sediment cores; arrow marks the onset and intensification of the 5kyr event. Accelerator mass-spectrometer radiocarbon dates converted to calendar age from site 980 provided the chronology for the Holocene. Accumulation rates average about 25 cm/kyr in the Holocene, roughly double that in nearby core V29-191. The average interval between samples is about 100 yr from 9.7 to 1.2 kyr. Shading shows low-13C events (see text) and possible correlative events in the other records; a more tenuous correlation is denoted by the question mark. Rectangles, extreme winter-like conditions deduced from statistical analysis of the full set of palaeochemical indicators from GISP2.

The conversion of surface water to deep water in the North Atlantic results in the release of heat from the ocean to the atmosphere, which may have amplified millennial-scale climate variability during glacial times and could even have contributed to the past 11,700 years of relatively mild climate (known as the Holocene epoch). Here we investigate changes in the carbon-isotope composition of benthic foraminifera throughout the Holocene and find that deep-water production varied on a centennial-millennial timescale. These variations may be linked to surface and atmospheric events that hint at a contribution to climate change over this period.
To read or view the full study, please visit the Nature website.
It was published in Nature, Vol. 422, No. 6929, pp. 277, 20 March 2003.

Download the Data from the WDC Paleo Archive.

During the Holocene, scientists have identified several periods of abrupt change. Some authors have related this to regular 1500 year cycles that may be related to variations in solar output (Bond et al. 2001). While such solar variations would provide only small changes in energy to the Earth's climate system, Bond et al. (2001) proposed that changes in North Atlantic deep water (NADW) formation could have amplified these signals. This work by Oppo et al. (2003) relates a new record of d13C, a stable isotope of carbon, to sea-salt sodium from the GISP2 ice core (O'Brien et al. 1995) and haematite-stained grains in a North Atlantic sediment core (Bond et al. 2001). High d13C values indicate times of greater NADW formation, while low values indicate a slowing or shutdown of NADW formation. This has been implicated as a mechanism to induce cooling in northern latitudes and a return to more glacial-type conditions. Higher sea-salt sodium indicates highly winter-like meteorological conditions in Greenland and higher haematite-stained grains indicate a large influx of cold, fresh, ice-bearing surface water from north of Iceland.

The figure shows major reductions in NADW production around 9,300, 8,000, 5,000, and 2,800 years before present (BP). One of these, a downward trend in NADW around 6,500 BP and ending around 5,000 BP corresponds to greater freshwater influx to the North Atlantic and more winter-like conditions in Greenland. This also corresponds to a wide range of climate-related changes seen over much of the world at that time. Note the arrow on the figure. Similar to the Younger Dryas and some Dansgaard/Oeschger events, this event seemed to have a gradual slowing of NADW formation and cooling in Greenland, but came to a very abrupt end.

References Cited:
O'Brien, S.R., P.A. Mayewski, L.D. Meeker, D.A. Meese, M.S. Twickler, and S.I. Whitlow. 1995. Complexity of Holocene Climate as Reconstructed from a Greenland Ice Core. Science 270: 1962-1964.

Bond, Gerard, Bernd Kromer, Juerg Beer, Raimund Muscheler, Michael N. Evans, William Showers, Sharon Hoffmann, Rusty Lotti-Bond, Irka Hajdas, and Georges Bonani. 2001. Persistent Solar Influence on North Atlantic Climate During the Holocene Science 294: 2130-2136.

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6 May 2004