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

The Portuguese margin is well-suited for studies of the contrasts in North Atlantic circulation during glacial and interglacial times because of its r apid sediment accumulation rate. This paper reports a (Th-230(excess))(0)-b ased study of sediment accumulation over the past 140 ky, a period which in cludes the last two glacial/interglacial transitions, in two cores at 2.4 a nd 3.5 km water depth on a slope transect at 40 degrees N. Although the ind ependently-determined mean sediment accumulation fluxes over the past 140 k y are unequivocally high with means of 13.2 and 10.5 g cm(-2) ky(-1) in the two cores, conventional application of the (Th-230(excess))(0) method yiel ds consistently lower fluxes with means of 3.5 and 3.8 g cm(-2) ky(-1). The se (Th-230(excess))(0) estimates are interpreted as representations of the regional depositional flux through time. This (Th-230(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 contro l on the clay input to the basin. Clay flux dominates the regional sediment accumulation which has a total flux of similar to 2 g cm(-2) ky-l and a Ca CO3 content up to 50% in interglacial times, and a total flux up to similar to 5 g cm(-2) ky(-1) and a CaCO3 content down to 10% in glacial times. Thi s pattern of change of sediment composition through time is also typical of the NE Atlantic, and sediment focusing (contourite formation) appears resp onsible for the high actual fluxes observed in glacial compared with interg lacial times on the Iberian margin. The inventories of (Th-230(excess))(0) exceed those which could have been supplied from the overlying water column alone over 140 ky by factors of x3.2 and x4.0 in the two cores. This is as cribed to preferential current focusing of fine sedimentary material in gla cial limes as a result of a systematic change in deep ocean circulation in response to climatic forcing. The presence of Heinrich events in the sedime nts is clearly evident, but at this southerly latitude they produce muted r egional 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. (C) 1999 Elsevier Science B.V. All rights reserved.