 |
| Titel |
Bottom water production variability in the Ross Sea slope during the Late Pleistocene-Holocene as revealed by benthic foraminifera and sediment geochemistry |
| VerfasserIn |
Leonardo Langone, Alessandra Asioli, Fabio Tateo, Federico Giglio, Domenico Ridente, Vito Summa, Anna Carraro, Maria Luigia Giannossi, Andrea Piva, Fabio Trincardi |
| Konferenz |
EGU General Assembly 2010
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250038489
|
|
|
|
|
|
| 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 are key factors affecting the Global Thermohaline Circulation
during modern and past climate conditions. The western Ross Sea is considered a formation
site for a particularly salty variety of AABW as well as an important area of off-shelf transfer
of water as plumes entraining in Lower CDW and as rapid downhill cascades. The results
here presented were obtained within the frame of the PNRA project 4.8. Among the
goals of the project, the main is to detect a qualitative signal of possible changes
in the rate of bottom water production during the Late Pleistocene-Holocene by
integrating data on foraminifera assemblages with sediment geochemistry (bulk
mineralogy, Total Organic Carbon, biogenic silica, C and N stable isotopes) and
IRD. A gravity core was collected at 2377m water depth off Drygalski Basin on the
slope adjacent the western continental shelf of the Ross Sea, along the pathway of
bottom water spreading. The chronology is based on the best fitting of twelve control
points selected among twenty-two 14C AMS datings performed on the bulk organic
carbon and 210Pb excess data. The trend of the parameters allows the following
observations:
1) two main intervals (15-10 and 7.5-6 cab kyr BP) mark a subsequent enhanced nutrient
supply. Indeed, δ15N variations depend on the utilization degree of nitrates, in turn reflecting
productivity/nutrient supply changes. The concurrent increase of OC and biogenic silica
suggests an increase of the nutrient availability. As the Upper CDW is a water mass rich in
nutrients we interpret these intervals as characterized by a higher efficiency in the Upper
CDW upwelling;
2) around 7.5-7kyr BP (part of the Middle Holocene Climatic Optimum) the IRD content
drops, suggesting the reduction of iceberg production or a change of the iceberg
path.
Within this general context, an oscillatory trend is present from 15 kyr BP to present time.
Two hypotheses are proposed:
a) minima in foraminifera concentrations reflect relatively stronger dissolution, weaker
bottom currents (minima in dry density) and lower nutrient supply (lighter values of δ15N).
These intervals may reflect a lower rate of bottom water formation; the intervals
corresponding to maxima in foraminifers concentration should indicate better preservation,
higher benthic productivity and/or better oxygenation at bottom, stronger bottom currents
(maxima in dry density) and relatively higher nutrient supply reflecting a relatively higher
rate of bottom water formation.
b) alternatively, minima in foraminifers, corresponding to minima in %OC and to
reversal of 14C (relative increase of older carbon), reflect dilution in the sediment
because of rapid accumulation of fine sediment re-suspended at the shelf edge by the
cascading currents. Therefore, the minima represent higher rate of bottom water
formation.
The comparison of the D/H ratio in ice-cores from the Ross Sea sector with the core
AS05-10 record indicates that the foraminifers minima always correspond to colder
condition. This scenario also correlates to the record reported in literature on the slope off
Wilkes-Adelie Land.
At last, a condensed/hiatus interval at ca. 3.5-4 kyr BP does not seems to mark a major
change in the general pattern of our variables, apart from biogenic silica and sheets silicates
showing an increase of the oscillation amplitude. Nevertheless, this feature is coeval to the
base of the Neoglacial and it is time-equivalent to the beginning of major changes in the
Antarctic environment. |
|
|
|
|
|
|