Revised chronostratigraphy of DSDP Site 270 and late Oligocene to early Miocene paleoecology of the Ross Sea sector of Antarctica

Abstract

Deep Sea Drilling Project (DSDP) Site 270, located in the central high of the Ross Sea, was cored to 422.5 m below seafloor (mbsf) and recovered a thick Oligocene to lower Miocene sequence of mudstone with varying amounts of ice rafted debris (IRD), overlain by ~20 m of Pliocene to Recent diatom silty clay with IRD. This site provides important temporal constraints on regional stratigraphy and insights into late Oligocene to early Miocene ice sheet dynamics; however, previous age models were based on limited data. Here we provide a revised age model using a combination of biostratigraphy (dinoflagellate cysts, pollen, calcareous nannofossils, foraminifers, and diatoms), magnetostratigraphy, Sr-isotope stratigraphy, and KAr dating of glauconite. We divide the sequence between 386 and 20 mbsf into four chronostratigraphic intervals (CSIs). CSI 1 (386–352 mbsf) is dated to between ~26 and 25 Ma based on glauconite KAr dating, the highest occurrence (HO) of the dinoflagellate Lejeunecysta rotunda (24.5 Ma), and a paleomagnetic reversal tied to the C8r/C8n.2r boundary (25.987 Ma). A distinct change in the benthic foraminifer assemblage at 352 mbsf marks an unconformity and the base of CSI 2. CSI 2 (352 to 149–146 mbsf) is dated to between 25.44 Ma and 23.13 Ma based on nannofossil biostratigraphy. Within this 200 m interval we correlate seven magnetic reversals to Chrons C8n.2n to C6Cr (25.3–23.3 Ma) and our line of correlation suggests rapid sedimentation (~80 m/m.y.). Microfossil and lithologic evidence indicate a distal marine setting with a paleo-water depth of ~200 m at 345 mbsf, with deepening above. Within CSI 2 is a diamictite overlain by grounding line proximal sandstone and laminated mudstone indicating glaciomarine deposition in a grounding-line proximal setting between 245 and 230 mbsf, followed by ice sheet grounding-line retreat into a more distal setting from 230 to 146 mbsf (~24.5–23.5 Ma), likely due to basin subsidence and incursion of relatively warm deep water onto the shelf. Another unconformity between 149 and 146 mbsf marks the boundary between CSI 2 and CSI 3. The thin CSI 3 (149–146 to 121/112 mbsf) is dated to ~23 Ma and we place the Oligocene/Miocene boundary at the base of the sequence, although we cannot rule out that the boundary is represented by an unconformity. Benthic foraminifers indicate continued deepening in the earliest Miocene and finer-grained mudstone with less IRD suggests that the site remained in a grounding line distal setting, although a decrease in abundance of calcareous plankton indicates cooling climatic conditions at that time. The boundary between CSI 3 and CSI 4 is marked by an unconformity spanning at least 2.5 m.y. based on the lowest occurrence of the dinoflagellate Batiacasphaera cooperi (<20.7 Ma) at 111 mbsf. Biostratigraphic data constrain CSI 4 to the early Miocene. We tentatively identify three magnetic reversals within CSI 4 (121/112–20 mbsf) that may tie this interval to Chrons C6An.2n to C6r (~20.6–19.7 Ma). A major unconformity at 20 mbsf separates the lower Miocene sediments from the overlying Pliocene and younger sediments. Late Oligocene to early Miocene pollen assemblages from Site 270 suggest a tundra landscape with low-growing Nothofagaceae, Podocarpaceae, and Proteaceae scrub in warmer locations with a relatively stable terrestrial environment during that time.