Surface and deep water variability on the Agulhas Plateau over the past 170 ka

Molyneux, Elizabeth Grace 2007. Surface and deep water variability on the Agulhas Plateau over the past 170 ka. PhD Thesis, Cardiff University.

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Abstract

High resolution multi-proxy analysis was undertaken on core MD02-2589 (41 26.03'S, 25 15.30'E, 2660 m water depth) from the Agulhas Plateau, Indian to Atlantic throughflow region, for the past 170 ka in order to provide a detailed record of both deep and surface ocean-climate interactions in the region. Particular focus was on Terminations I and II and Marine Isotope Stage (MIS) 5a-4 transition. Planktonic foraminifera assemblage counts and inferred sea surface temperature calculations combined with planktonic stable carbon and oxygen isotopes and ice rafted debris counts were used to show that towards the end of glacial periods MIS 6 and 2 there was a pooling of heat and salt in the throughflow region, transported to the area by the Agulhas Current. These waters were unable to be transported into the Atlantic Ocean due to the northwards position of the frontal systems associated with the Antarctic Circumpolar Current (ACC). Wider Southern Ocean warming towards the end of MIS 6, during maximum global ice volume, also shown by the benthic proxies, is seen to release a pulse of the pooled waters through the Agulhas throughflow region, some 6 ka before a Southern Ocean wide cold reversal linked to the resumption of North Atlantic Deep Water (NADW) convection to interglacial levels. This is significantly longer than ages suggested through modelling for the resumption of the thermohaline circulation interglacial mode following the recommencement of Agulhas leakage. Additional planktonic carbon isotope data from core MD02-2588 (41 19.90'S, 25 49.40'E, 2907 m water depth) show the development of a Carbon Isotope Minima Event during Terminations I and II, probably linked to the breakdown of stratification of the water column following glacial periods. Benthic stable carbon and oxygen isotope measurements combined with the sortable silt mean grain size sedimentological proxy are used to show the phasing of bottom water changes over terminations, with changes in chemical ventilation being largely decoupled from near-bottom physical flow speeds. Flow speeds record Southern Ocean variability and the chemical ventilation Northern Component Water variability. The results suggest the persistent presence of a high 5,3C water mass, similar to present day NADW, during glacial periods, with a rapid shoaling of NADW in the water column and replacement by Southern Component Water (SCW) over the MIS5a-4 transition. The benthic 8,3C record of MD02-2589, combined with that of MD95-2042 (Shackleton et al., 2000) from the Iberian Margin, show 8 periods in the past 150 ka where the mid-depth South Atlantic was bathed by a better ventilated water mass than the mid-depth North Atlantic, corresponding with glacial stages, substages and Heinrich events. The high 8,3C water mass present at MD02-2589 must have originated from outside the North Atlantic. Productivity and air-sea gas exchange overprints on both records cannot explain the full offsets seen with the most likely mechanism being the formation of a separate high 8I3C SCW mass, different to Antarctic Intermediate Water (AAIW) during glacial periods. It is possible that increased depth penetration of AAIW can explain the offset during Heinrich events. A suite of cores forming a latitudinal and depth transect over the modern day position of the Polar, Subantarctic and Subtropical Fronts (PF, SAF, STF) are investigated for MIS 2, with a chemocline, seen by Hodell et al. (2003), being identified. This study however shows the presence of a mid-depth divide north and south of the SAF, with very depleted values south and enriched values north, with deep depths (>3750 m water depth) both north and south of the SAF showing depleted values. This is used to suggest that the high 8,3C SCW mass seen during glacial periods is forming around the SAF in the southeastern Atlantic Ocean due to air-sea gas exchange and upwelling linked to wind forcing and sea ice extent. This study suggests that previous work using benthic 8 C data looking at the extent of glacial water masses in the world's oceans may have been over-reconstructing the extent of glacial Northern Component Water as it is likely to have a very similar 8 C and 8I3Ca$ signature as this SCW mass forming around the SAF and migrating northwards.

Item Type:	Thesis (PhD)
Status:	Unpublished
Schools:	Earth and Environmental Sciences
Subjects:	G Geography. Anthropology. Recreation > GC Oceanography
ISBN:	9781303213571
Date of First Compliant Deposit:	30 March 2016
Last Modified:	12 Feb 2016 23:12
URI:	https://orca.cardiff.ac.uk/id/eprint/54751

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