Provenance of glacial–marine sediments under the McMurdo/Ross Ice Shelf (Windless Bight, Antarctica): Heavy minerals and geochemical data

Abstract Sea floor sediments beneath the McMurdo/Ross Ice Shelf in a 900-m-deep depression behind Ross Island have been cored through access holes at two sites. At Site 1, 31 cm of mud and lesser sand, deposited over the last 22 ka, overlies a 30-cm-thick diamictite of the Last Glacial Maximum. At Site 2, the entire core is mud deposited over the last 12 ka. Analyses of very fine sand-sized, heavy minerals and geochemical analyses of the sand and silt fractions have been carried out on samples from both sites to track variations in sediment provenance through the Late Quaternary. The heavy minerals were identified by optical and electron scanning microscope (SEM-EDAX) analysis. Major and trace elements were determined on fused glass discs by X-ray fluorescence (XRF) spectrometry. Heavy mineral assemblages have high concentrations of pyroxenes (titanoaugite, augite, pigeonite, low-Ca augite and enstatite), olivines and glass grains. Amphiboles (Mg-hornblende, actinolite and kearsutite), oxides (titanite and titanomagnetite) and stable minerals (epidote, garnet, and apatite) are present in minor amounts. Olivine, titanoaugite and glass dominate the heavy mineral assemblages both in upper part of core 1 and in all of core 2. In the diamictite, an increase of augite, pigeonite, low-Ca augite, hornblende and stable minerals are observed. The sediment geochemistry is linked to their mineralogical composition both in the sand and in the silt fractions. K and Rb trends probably represent variations in alkali feldspar and/or mica content. The variation of Si/Al ratio is due to sialic minerals and/or to quartz content variation. Mg, Ca, Ti, Sr are probably comprised in femic minerals. The sets of analyses show that the sediments are a mix of local McMurdo Volcanic Group (MVG) rocks and Transantarctic Mountains (TAM) source rocks to the south and west. The diamicton was a strong TAM provenance, suggesting that the main ice streams drained into the Ross Sea from the West at least prior to 18 ky and later than 25 ky. The overlying mud is dominated by a MVG signal, sediments in the two sampled sites derive from basal melting of the ice sheet which entrapped debris from MVG cropping out to the South of the Ross Embayment. Small quantities of minerals from the TAM can be supplied by marine currents inflowing under the shelf from NW.