Implications for sedimentation changes on the Iberian margin over the last two glacial/interglacial transitions from ( 230Th excess) 0 systematics

Abstract

The Portuguese margin is well-suited for studies of the contrasts in North Atlantic circulation during glacial and interglacial times because of its rapid sediment accumulation rate. This paper reports a ( 230Th excess) 0-based study of sediment accumulation over the past 140 ky, a period which includes the last two glacial/interglacial transitions, in two cores at 2.4 and 3.5 km water depth on a slope transect at 40°N. Although the independently-determined mean sediment accumulation fluxes over the past 140 ky are unequivocally high with means of 13.2 and 10.5 g cm −2 ky −1 in the two cores, conventional application of the ( 230Th excess) 0 method yields consistently lower fluxes with means of 3.5 and 3.8 g cm −2 ky −1. These ( 230Th excess) 0 estimates are interpreted as representations of the regional depositional flux through time. This ( 230Th excess) 0 regional flux is composed of a carbonate flux of <1 g cm −2 ky −1 and a larger and variable clay input which indicates the importance of the sea level control on the clay input to the basin. Clay flux dominates the regional sediment accumulation which has a total flux of ∼2 g cm −2 ky −1 and a CaCO 3 content up to 50% in interglacial times, and a total flux up to ∼5 g cm −2 ky −1 and a CaCO 3 content down to 10% in glacial times. This pattern of change of sediment composition through time is also typical of the NE Atlantic, and sediment focusing (contourite formation) appears responsible for the high actual fluxes observed in glacial compared with interglacial times on the Iberian margin. The inventories of ( 230Th excess) 0 exceed those which could have been supplied from the overlying water column alone over 140 ky by factors of ×3.2 and ×4.0 in the two cores. This is ascribed to preferential current focusing of fine sedimentary material in glacial times as a result of a systematic change in deep ocean circulation in response to climatic forcing. The presence of Heinrich events in the sediments is clearly evident, but at this southerly latitude they produce muted regional increases in accumulation flux (<2 g cm −2 ky −1), apart from the large Heinrich event 4 which introduced an additional 10–20 g cm −2 ky −1 over ambient background levels.