Lead isotopes in Southern Ocean marine sediments: implications for paleoclimatic provenance and seawater studies

Abstract:

Lead (Pb) isotopes in detrital marine sediments and authigenic oxide coatings have proven useful in reconstructing past changes in continental weathering of glaciated continents and ocean circulation. This approach is under-utilised in the Southern Ocean, despite uncertainties about the evolution of Antarctica’s ice sheets and oceanographic changes in this region throughout the Cenozoic.

Lead isotopes in sediments are controlled by the mineralogy and age of their bedrock sources, making them an ideal tool to trace changes in Southern Ocean sediment provenance associated with onland erosion patterns (and ice sheet variability), and ocean circulation. However, core-top mapping of Pb isotope signatures of detrital marine sediments in this region is non-existent. We present new Pb isotope data for acid-digested <63µm and bulk Late Holocene sediments from 40 cores located in the East Antarctic sector of Southern Ocean. Results demonstrate that, similar to existing Nd and Sr core-top surveys, distinct provenance sectors can be seen in sediments proximal to the continent, due to the geological characteristics of bedrock sources along the margin. A striking N-S trend is also evident, with distal sediments characterised by less radiogenic isotopic signatures, likely associated with a decrease in radiogenic Pb-bearing minerals with increasing distance from the continental margin. This pattern suggests that Pb isotopes in marine sediments may be useful in reconstructing past provenance trends associated with ice sheet change, and migrations of Southern Ocean frontal positions.

In addition, we will present new seawater Pb isotope data from reductive leaching of authigenic coatings of these same sediments, permitting for Late Holocene Pb isotope seawater mapping of the Southern Ocean. To ensure we measured the true seawater signal, we adapted established sequential leaching procedures to minimise detrital contamination, and monitored leachate compositions with Nd isotopes, and major and trace elements. Preliminary results, along with those of parallel detrital studies, suggest Pb isotopes are a viable tool for paleoclimatic studies in areas of the Southern Ocean proximal to Antarctica, and our new data forms an important framework for future studies in the region.