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| Titel |
Bottom water production variability in the Ross Sea slope during the Late-Pleistocene-Holocene as revealed by benthic foraminifera and sediment geochemistry |
| VerfasserIn |
A. Asioli, L. Langone, F. Tateo, M. L. Giannossi, F. Giglio, V Summa, A. Piva, D. Ridente, F. Trincardi |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250022524
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| Zusammenfassung |
| The Antarctic area produces bottom waters that ventilate the vast majority of the deep basins
in the rest of the world ocean. The rate of formation in the source area and the strength of
these cold bottom waters affect their flow toward the equator and are key factors affecting the
Global Thermohaline Circulation during modern and past climate conditions. We present the
results of a multidisciplinary study carried out on a core collected in 2377m of water depth on
the slope off the Drygalski Basin (Ross Sea), along the modern path of the bottom
waters. The goal of this research is to detect a qualitative signal of possible changes
in the rate of bottom water production during the Late Pleistocene-Holocene by
integrating micropaleontological and geochemical proxies. The micropaleontological
signal is represented by the quantitative and qualitative variations of the agglutinated
benthic foraminifera assemblages, while the amount of TOC, nitrogen, δ13C, δ15N,
biogenic silica, CaCO3 in the sediment, along with the bulk rock mineralogy, provide
information on the paleoproductivity and allow reconstruction of changes in the
paleocirculation. The chronology is supported by 14C AMS datings on organic matter.
Although this study is still in progress, the results obtained allow the following
observations:
1) the Holocene sequence includes a major turnover around 8-8.5 calib kyr BP, leading to
reduced nutrient utilization, probably reflecting an increased nutrient supply induced by an
enhanced Upper Circumpolar Deep Water upwelling;
2) within this general context, the total concentration of benthic foraminifera preserved in
the fossil component records millennial scale cycles of variable amplitude after
8.5 calib kyr BP and to present time. This oscillatory trend is paralleled by other
parameters, such as the magnetic susceptibility, the dry density, the sheet silicates and the
δ15N;
3) minima in foraminifera concentration reflect relatively increased dissolution, weaker
bottom currents (minima in dry density=higher amount of fine fraction), and lower nutrient
supply; maxima in foraminifera concentration indicate better preservation, higher benthic
productivity and/or better oxygenation at bottom, stronger bottom currents (maxima in dry
density) and relatively higher nutrient supply;
4) these cycles are interpreted to reflect a relatively higher (maxima in forams
concentration) or lower (minima in forams concentration) rate of bottom water
formation;
5) between 8.5 and 6 kyr BP the amplitude of these cycles (and particularly those with
increased rates of bottom water formation) is higher than the subsequent ones. We equate
this interval with the early part of the Middle Holocene Climatic Optimum of the
literature;
6) the condensed/hiatus interval centred at ca. 3.5-4 kyr BP does not seem to mark a
major change in the general pattern. Nevertheless, this feature is time-equivalent to
a major change in the circulation pattern in other Antarctic regions, such as the
Antarctic Peninsula. This major change consists in oscillations between two contrasting
circulation modes dominated respectively by: a) Upper Circumpolar Deep Water and b)
shelf-water formation. This major change can therefore be ascribed to the southward
migration of the Intertropical Convergence Zone vs. ENSO prevalence, respectively. |
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