Late Quaternary fluctuations of palaeoproductivity in the Murray Canyons area, South Australian continental margin

Abstract

Two sediment cores from the Murray Canyons area, south of Kangaroo Island, South Australia, were investigated for proxy-records to reconstruct past productivity of the surface waters in the area over the last 175 ka. The proxies used included concentrations of aragonite, low- and high-Mg calcite, total carbonate, total organic carbon, sulfur and δ13C of Globigerina bulloides. Cyclic increases in palaeoproductivity were observed to be in tune with insolation minima at 30°S. The atmospheric conditions during insolation minima were comparable to present winter patterns, when strong westerly winds dominate over the area and bring dust from the central desert areas. During the last insolation minimum (last glacial maximum: LGM), the Murray Canyons laid directly under the "Eastern Australian Dust Plume". Dust could have fertilized surface waters and initiated the observed productivity "increases" on a precessional time scale. The core from the more westerly canyons is richer in organic matter; this could be attributed to the influx of organic matter from shallower water, which is carried by an overflow of hypersaline water from the Spencer Gulf to the deep ocean, using the western canyons as conduits. There is no evidence that the "Palaeo-Murray River", which debouches close to the core sites during sealevel lowstands, was a major source of nutrients for surface waters. Although total carbonate concentrations remained high, planktonic foraminifers were fewer in numbers during these periods, possibly due to reduced light penetration caused by suspended river material. A deglacial minimum in δ13C of Globigerina bulloides, observed in other cores from the southern hemisphere and attributed to a major hydrographic change south of the Polar Front, is also visible in our two cores, thus attesting to the global significance of the event.