Quaternary warming and cooling trends in the Bransfield Basin, Antarctic Peninsula, based on gamma-ray spectrometry

Abstract

Spectral gamma-ray measurements were carried out on eight cores collected in the Bransfield Strait at points distributed over a cross section connecting the South Shetland Islands (NW) and the Antarctic Peninsula (SE). These cores were acquired during the cruises OPERANTAR XXXIII, XXXIV, and XXXV, aboard the P/V Almirante Maximiano, from the Brazilian Navy, within the scope of the project “Use of Sedimentary and Biogeochemical Records as Indicative of Climate and Environmental Changes in the South Shetland Islands and Antarctic Peninsula”. The energy spectrum was measured in counts per period through a portable spectrometer which displays the total gamma-ray (total GR) counts, in addition to the individual concentrations of K (%), U (ppm), and Th (ppm), internally converted. The results, integrated with core sedimentological descriptions, enabled us to characterize the different geochemical environments and to recognize paleoenvironmental cycles and stratigraphic surfaces, proving to be a powerful tool for high-resolution stratigraphic studies. The Th/U and U/K ratios reveal six well-marked cycles (identified from A to F) interpreted as transitions from warm to cold periods. The increase in Th/U ratio was used as a proxy for higher clastic meltwater sedimentary input during warm periods, while the increase in U/K ratio indicates higher amounts of organic matter, and therefore higher U content, during cold periods, when clastic sedimentary input is smaller. The concomitant decrease in the Th/U ratio with the increase in the U/K ratio suggests a gradual change from a warm to a cold period. Otherwise, the gradual increase in the Th/U ratio and decrease in the U/K ratio indicates the transition from cold to warm periods. In addition, this study observed different behaviors between proximal and distal gamma-ray spectral logs.