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Links between Meridional Overturning Circulation and climate changes during the Holocene

Final Activity Report Summary - LIMOCINE (Links between meridional overturning circulation and climate changes during the holocene)

The sedimentary 231Pa/230Th ratio has been proposed as a dynamic proxy to record changes in Atlantic Meridional Overturning Circulation (AMOC). This proxy is based on the difference in particle reactivity of 231Pa and 230th in the water column. However, Pa/Th recorded in the sediments can be affected by changes in particle flux and composition, which can obscure its interpretation in terms of changes in circulation. Therefore this is essential to check the sedimentary conditions of potential study sites, together with preliminary Pa/Th measurements in order to verify the potentiality of those sediment cores to track changes in AMOC. The initial objective of LIMOCINE project is the setting of a new facility allowing the chemical separation of the nuclides prior to analysis by ICP-MS.

The strategy of LIMOCINE was to monitor the changes of the northern boundary of the AMOC along the overflow regions between Scotland-Iceland and Greenland-Iceland during the Holocene. Indeed, supply of sinking cold dense water along this line is a key feature of the MOC. Two drift sites formed by the currents constituting the sinking limb of MOC, where sedimentation during the period of investigation allows high resolution studies, have been investigated to this purpose : Eirik Drift core GS06-144-02GC (57deg 28.71'N 48deg 36.93'W depth 3440m) and Gardar Drift core GS06-144-08GC (60deg 24.20'N 23° 38.43'W depth 2120m). Preliminary results show high Pa/Th, close to the production ratio, suggesting important scavenging of both Pa and Th, related to a strong influence of particle flux and composition at Eirik Drift site. Difficulties during the chemical procedure suggest high biogenic opal abundance at that site. The Gardar drift site presents lower Pa/Th ratios during the earlier part of the Holocene than during the late part of it. This is still work in progress, but this suggests that Gardar Drift site may be a good candidate to track AMOC changes during the Holocene.

Two additional northern sites from the Norwegian sea have been investigated in order to track water masses outflow from the Norwegian sea. At rather shallow depth of 2000 and 1000, preliminary results from Holocene and LGM present the same trend at both sites, with lower ratio during LGM than Holocene, suggesting stronger intermediate outflow during the LGM than the Holocene. These very northern sites appear then to be very promising and will be further investigated.