Steven M Bohaty

Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene/Miocene boundary

Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3–4 °C warmer than at present and atmospheric CO2 concentrations were twice as high as today, the Antarctic ice sheets may have been unstable. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1–23.7 Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450 kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40 kyr) and eccentricity (125 kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250 kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7 Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event).

Southern Ocean pliocene paleotemperature variation from high-resolution silicoflagellate biostratigraphy

Abstract Applying the modern biogeographic distribution of silicoflagellate genera Dictyocha and Distephanus and calcareous nannoplankton as an analog, the biostratigraphic record of these phytoplankton can be used qualitatively to examine past changes in surface temperature in the Southern Ocean. The Antarctic Polar Frontal Zone (PFZ) is a major biogeographic barrier, limiting Dictyocha and calcareous nannoplankton to areas north of this zone. Ocean Drilling Project (ODP) holes 748B and 751A on the southern Kerguelen Plateau, 900 km south of the present location of the PFZ, contain expanded Pliocene sections through four time intervals: ∼4.5 to 4.2 Ma (Hole 751A), ∼3.7 to 3.1 Ma (Hole 748B), ∼3.6 to 3.5 Ma (Hole 751A), and ∼3.2 to 3.0 Ma (Hole 751A). Three distinct peaks in Dictyocha and calcareous nannofossil abundance reflect surface-water warming of at least 4°C through either southward migration of the PFZ or a weakening of the thermal gradient across this zone. Peak warming in the intervals studied is centered near ∼4.3 Ma, with other brief, yet significant, warming events near ∼4.5, ∼4.2, and ∼3.6 Ma. Cooling as a result of northward migration of the PFZ, or intensified thermal gradient, is indicated by the absence of Dictyocha and calcareous nannofossils in other stratigraphic intervals. These data support previous studies indicating Pliocene oceanic and climatic variability in the southern high latitudes. Silicoflagellate abundance patterns from Hole 751A identify taxonomic relationships between several morphotypes. The simultaneous appearance and proportional occurrence of five-sided and four-sided Dictyocha specimens suggests a close relationship between these two forms. Distephanus crux s.l. and Dictyocha aspera var. pygmaea also show a near mutually exclusive abundance pattern in Hole 751A and are similar in basal ring size and morphology, indicating they are conspecific. They differ only in apical structure, whereby Distephanus crux s.l. may have formed an apical bar under fluctuating environmental conditions in the Early Pliocene. These variations in silicoflagellate architecture appear to be related to paleoenvironmental changes indicating that silicoflagellate morphology may represent a more useful paleoceanographic tool that has been noted previously.