Due to its short residence time in the water column, lateral isopycnal transport (boundary scavenging) of dissolved 230Th may be considered near negligible throughout the ocean with the exception of areas of strong upwelling. Thus a constant flux of 230Thex (ex = derived from the water column) into the sediments may be assumed and variations of the concentration of 230Thex in the sediment may be interpreted as changes of
dilution with vertically settling particles, thus allowing the
calculation of sedimentation rates and ages at high resolution also within fixed oxygen isotope stages for each sample.
The horizontal supply of resedimented particles which are already loaded with 230Thex may be another important source of sediment material in an environment of high bottom current velocities. Normalization to 230Thex facilitates the calculation of accumulation rates directly derived from the water column above each core location (rain rates) and thus a correction for focusing and winnowing effects for accumulation rates of any other sedimentary component assuming that grain size fractionation during bottom water transport was small.
A transect of five sediment cores from 55°S to 43 S° crossing the eastern Atlantic sector of the ACC was investigated. Most of the particles deposited in the Polar Frontal Zone and the Antarctic Zone of the ACC are of biogenic origin. 230Thex normalized sediment rain rates amount to maximum values of 3-8 g cm-2 kyr-1 in the interglacial core sections south of the Antarctic Polar Front (APF) and to 1-4 g cm-2 kyr-1 in the glacial core sections north of the APF. A northward shift of the high particle flux area of 2-5° during glacial periods can be deduced from the rain rate record in accordance to Charles et al. (1991). Following our results, the high particle flux area did not reach a position north of about 47°S during the last 300 kyr
(kyr = 1000 years). The climate optimum at the beginning of interglacial stage 5e (about 127-118 kyrs B.P.) is characterized by an enlargement of the high particle flux area compared to the present. From the rain rates of biogenic barium (Nürnberg, 1995; Frank, 1996), the export paleoproductivity was calculated following Francois et al. (1995), according to which the maximum of biological productivity (about 30 gC . cm-2 . kyr-1) occured south of the APF during isotope stage 5e and during the transition from the last glacial to the Holocene. During glacial stages 3 and 4, the productivity maximum was shifted north to about 47°S, with about the same amount of export productivity.
The rain rates of the particle reactive radionuclides 10Be and 231Pa are controlled by the particle flux in the water column and may be supplied to high particle flux areas via transport along isopycnals (boundary scavenging). The rain rates of 231Paex and, even more pronounced, of 10Be are expected to show increased values in areas of high particle fluxes due to enhanced boundary scavenging. In our cores, the rain rates of 231Paex apparently coincide with the ones of the biogenic particle rain rates,
suggesting that it is a suitable proxy for biogenic particle flux (Kumar et al. 1993). 10Be, however, shows a behaviour just opposite to the expected one: Although the 10Be rain rates have been higher than expected from cosmogenic production throughout the transect, they show minima instead of maxima during periods of increased productivity south of the APF. A positive correlation of the 10Be rain rates to the rain rates of
terrigenous particles suggests that 10Be preferentially adsorbs to clay minerals although high amounts of biogenic opal were available, which renders the 10Be rain rates unsuitable for
tracing biogenic particle fluxes in the Southern Ocean.
Charles, C.D. et al., Paleoceanography 6, 697-728 (1991).
Francois, R. et al., Global Biochemical Cycles 9, 289-303 (1995).
Frank, M., Ber. Polarforsch. 186 (1996).
Kumar. N. et al., Nature 362, 45-48 (1993).
Nürnberg, C.C., GEOMAR Report 38 (1995).