H. J. Walter

Enhanced scavenging of 231Pa relative to 230Th in the South Atlantic south of the Polar Front: Implications for the use of the 231Pa/230Th ratio as a paleoproductivity proxy

Abstract The fractionation of 230Th and 231Pa was investigated throughout the Atlantic sector of the Southern Ocean. Published scavenging models generally assume that the 231Pa/230T ratio of surface sediments is primarily determined by the mass flux of particles. This relationship holds north of the Polar Front, where low primary productivity coincides with ratios of unsupported 231Pa/230Th—xs(231Pa/230Th) — in surface sediments below the production ratio of both radionuclides in the water column. However, we observed high xs231Pa/230Th ratios, conventionally interpreted as a high-productivity signal, in surface sediments south of the Polar Front, especially throughout the Weddell Sea, in contradiction with the low particle flux of this region. Measurements of both dissolved and particulate fractions of 231Pa and 230Th in the water column revealed a strong N S decrease in the Th/Pa fractionation factor, from typical open ocean values around 10 north of the Polar Front to values between 1 and 2 south of 60°S. This observation clearly indicates that the high xs231Pa/230Th ratios in surface sediments south of the Antarctic Circumpolar Current are produced by a N S increase in the relative scavenging efficiency of 231Pa relative to 230Th, most probably due to a change in the chemical composition of particulate matter, and not by a high mass flux. It is speculated that biogenic opal, suggested not to significantly fractionate231Pa and 230Th, may explain the enhanced scavenging of 231Pa to the south. This assumption is further supported by extremely high 231Pa/230Th ratios up to 0.34 in material collected with sediment traps south of the Polar Front, where fluxes are primarily determined by biogenic opal. Based on these results we conclude that, in regions where the sedimenting flux is dominated by biogenic opal, the 231Pa/230Th ratio is not a reliable indicator for the mass flux of particles, thus limiting its use as a paleoproductivity proxy in the Southern Ocean.

Provenance and transport of terrigenous sediment in the south Atlantic Ocean and their relations to glacial and interglacial cycles: Nd and Sr isotopic evidence

Sr and Nd isotopic compositions of Late Quaternary surface sediment and sediment cores from the south Atlantic and southeast Pacific sectors of the Southern Ocean are used to constrain the provenance and transport mechanisms of their terrigenous component. We report isotopic and mineralogical data for core samples from three localities, the Mid-Atlantic Ridge at 41°S and the northern and southern Scotia Sea. In addition, data for surface sediment samples from the south Atlantic and southeast Pacific sectors of the Southern Ocean are presented. The variations of Sr and Nd isotopic compositions of the bulk sediment samples in all cores were correlated with the magnetic susceptibility of the sediment and with the inferred glacial–interglacial stages. The isotopic data indicate that, during glacial periods, sediment was delivered from continental crust with a shorter residence time than that supplying material during interglacial periods. At the core site near the Mid-Atlantic Ridge, Nd isotopic, combined with mineralogical evidence indicates interglacial period deposition of a relatively high amount of kaolinite and silt with low ϵNd values −4.5, probably derived from southern South America, was indicated. The glacial–interglacial shift in sources may be due to either a decreasing influence of North Atlantic Deep Water during glacial times or by a larger contribution of glaciogenic detritus from southern South America. At the core site in the northern Scotia Sea, sediment of interglacial periods is dominated by smectite with ϵNd −4. We suggest that smectite was derived from the Falkland shelf and silt was derived from the Argentinian shelf. During glacial periods, the Argentinian shelf was an important source for silt and chlorite with ϵNd > −4. The contribution from the Falkland shelf seems to have remained similar during glacial and interglacial periods. Hydrographic transport by bottom currents and turbidites could account for the high glacial detrital flux. An evaluation of the significance of an aeolian contribution to deep sea sediment suggests that it plays only a minor role. In the southern Scotia Sea, the Antarctic Peninsula is considered an important source for young material with ϵNd > −4, in particular during glacial periods. During interglacial periods, sediment supply from the Antarctic Peninsula was lower than during glacial times, resulting in a relatively high contribution of old material (ϵNd < −8) from East Antarctica. Deep water currents and icebergs could account for the transport of the old component to the southern Scotia Sea. The accumulation rates of material from the various source regions for glacial times are in agreement with an increase in the strength of the Antarctic Circumpolar Current. The production rate and the circulation pattern of bottom water in the Weddell Sea appear to have remained similar over most of the last 150 kyr.

Shallow vs. deep-water scavenging of 231Pa and 230Th in radionuclide enriched waters of the Atlantic sector of the Southern Ocean

Abstract The scavenging of 231 Pa and 230 Th was investigated in the Atlantic Sector of the Southern Ocean by combining results from sediment trap and in situ filtration studies. We present the first high-resolution profile of dissolved 230 Th and 231 Pa in surface waters across the ACC, showing a dramatic southward increase of both radionuclides around the southern ACC Front at 53°S. High dissolved 231 Pa / 230 Th ratios combined with low 230 Th / 231 Pa fractionation factors (F) in these surface waters result in extremely high 231 Pa xs / 230 Th xs ratios of material collected in the shallow traps. Particulate 231 Pa xs / 230 Th xs ratios in a shallow trap near Bouvet Island increase continuously during the productive period in austral summer, and drop back in the low flux period. This behavior, following the Rayleigh fractionation principle, is interpreted to be due to an increase in the dissolved 231 Pa / 230 Th ratio in the euphotic zone resulting from preferential scavenging of 230 Th relative to 231 Pa , even in opal-dominated regions. In the post-bloom stage, the depleted radionuclide concentrations are replenished by upwelling of Circumpolar Deep Water. The high particulate 231 Pa xs / 230 Th xs signal is weakened during downward transport of the bloom particles in the water column by incorporation of deep suspended particles, which have a lower 231 Pa xs / 230 Th xs ratio. It is shown that under the special hydrographic conditions in the Southern Ocean scavenging from the upper water column significantly influences the budgets of 230 Th and 231 Pa in the sediment. Nevertheless, the budgets are still made up primarily by scavenging from the large standing stock of deep suspended particles.

Reliability of the 231Pa /230Th Activity Ratio as a Tracer for Bioproductivity of the Ocean

In large areas of the world’s oceans, there is a relationship between the mass flux of particulate matter and the unsupported 231Pa/230Th (xs 231Pa/xs 230Th) activity ratio of recent sediments. This observation forms the basis for using the xs 231Pa/xs 230Th ratio as a proxy for past changes in export productivity. However, a simple relationship between xs 231Pa/xs 230Th ratio and particle flux requires that the water residence time in an ocean basin is far in excess of the scavenging residence time of 231Pa, and that the composition of sinking particles maintains a strong preference for the adsorption of 230Th over 231Pa with a constant 230Th/231Pa fractionation factor (F). The best correlation between 231Pa/ 230Th ratio and mass flux is found in the Pacific Ocean. In the Atlantic, the contrast xs xs’ in the xs 231Pa/xs 230Th ratios between open ocean (low flux regions) and ocean margins (high flux regions) is much less pronounced due to the shorter residence time of deep water, resulting in less effective boundary scavenging of 231Pa. In the Southern Ocean, south of the Polar Front, there is no more a simple relationship between xs 231Pa/xs 230Th and particle flux. This is a result of a southward decrease in F, probably reflecting the increased opal content of sinking particles. Opal does not fractionate 231Pa and 230Th significantly. This lack of fractionation results in high xs 231Pa/xs 230Th ratios in opal-dominated regions, even in areas of very low particle fluxes such as the Weddell Sea. The xs 231Pa/xs 230Th ratio can therefore only be used as a paleoproductivity proxy if, in the time interval of interest, changes in the basin ventilation rate and differential scavenging of both radionuclides due to changes in the chemical composition of particulate matter can be excluded.