Variations of biogenic particle flux in the southern Atlantic section of the Subantarctic Zone during the late Quaternary:: Evidence from sedimentary 231Paex and 230Thex

Variations of intensity and composition of biogenic particle flux at the northern boundary of the present Polar Frontal Zone in the Atlantic sector of the Southern Ocean are indicators of major changes of paleoenvironmental conditions on glacial/interglacial time scales during the Late Quaternary. In order to estimate those past changes, sediment accumulation patterns of two piston cores, one from just north and one just south of the present day position of the Subantarctic Front were reconstructed. Using the Th-230(ex) method large contributions of laterally supplied material were quantified and used to correct sediment accumulation rates. During the last glacial focussing of biogenic opal-dominated material exceeded the original contribution from the surface water above by a maximum factor of 8.7. The initial activity ratio of Pa-231(ex)/Th-230(ex) was used as tracer for biogenic particle flux and composition and indicates that during the glacial stages 2 and 4 the area of high opal productivity was situated above the location of the southern core whereas the northern core has not been reached by this northward shift during the last 130 kyr as shown by the pattern of focussing-corrected bulk accumulation rates. If the position of the Antarctic Polar Front has remained at the northern boundary of the high opal productivity area during the last 130 kyr, the results suggest that was located exactly between the two core sites during glacial stages 2 and 4. A two-box modeling approach involving particle flux and boundary scavenging intensity of Pa-231 was applied to estimate the possible range of the Pa-231(ex)/Th-230(ex) ratio recorded in Southern Ocean sediments. Previous estimates on the export of Pa-231 from the Atlantic into the Southern Ocean are corroborated but the model suggests a low sensitivity of the Pa-231(ex)/Th-230(ex) ratio in Southern Ocean sediments to variations of the residence time of North Atlantic Deep Water in the Atlantic Ocean. (C) 1999 Elsevier Science B.V. All rights reserved.