Pleistocene surface-ocean changes across the Southern subtropical front recorded by cryptic species of Orbulina universa

Abstract

The largest ocean temperature gradients occur in the Southern Ocean, where the Antarctic circumpolar current frontal systems serve as hydrographic barriers with distinctive flora and fauna. During Pleistocene glacial-interglacial cycles, the position and intensity of these frontal systems are thought to have changed. This study examines the occurrence of distinct cryptic species of Orbulina universa during the Mid-Pleistocene Transition at two sites in the South Atlantic, Ocean Drilling Program Holes 1088B and 1090B that are separated by 2° latitude. We discuss abundance records of the Caribbean and Mediterranean species and their relationship to Total Organic Carbon and stable isotope records to further understand the sea-surface changes of this region during the Pleistocene. Data from 1090B, the more southerly Hole, shows the presence of few Mediterranean species together with aberrant and Biorbulina bilobata forms. At ODP Hole1088B, on the other hand, the presence of both cryptic species are observed throughout the Pleistocene, indicating distinct surface water sources and properties. The abnormal forms indicate the possible disappearance of O. universa under stressful environmental conditions in the sub-antarctic zone with reseeding of the species at glacial-interglacial transitions (Marine isotopic stages 23/22, 14/13, 8/7 and 2/1). Although nutrient-rich surface waters and shallow nutricline favours high productivity in this region during glacial intervals, it is possible that Mediterranean species at 1090B would have been restricted by cold sub-polar surface waters. Comparison with published data suggests that the presence of subtropical Caribbean species at 1088B was driven by the prominence of northern sourced waters and interocean exchange south of Africa. Our data point to the presence of sub-tropical waters at 1088B during each glacial-interglacial transition with maxima occurring at three intervals (1200 ka, 700–600 ka and 250–100 ka) that coincide with the inception of the Mid-Pleistocene Transition and sea surface temperatures changes at the subtropical front. The interocean transfer and surface water changes at the subtropical front in the late Pleistocene are consistent with modulation by eccentricity-induced changes from a 41-ky world to a 100-ky world.