G. H. Browne

Obliquity-paced Pliocene West Antarctic ice sheet oscillations

Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth’s orbital geometry control the ice ages, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles. Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the ‘warmer-than-present’ early-Pliocene epoch (∼5–3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming. Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, ∼40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth’s axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to ∼3 °C warmer than today and atmospheric CO2 concentration was as high as ∼400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt under conditions of elevated CO2.

Integrated outcrop, drill core, borehole and seismic stratigraphic architecture of a cyclothemic, shallow‐marine depositional system, Wanganui Basin, New Zealand

Abstract Late Pliocene to mid‐Pleistocene (c. 2.1–0.4 Ma) strata exposed in the, now classical, Nukumaru and Castlecliff coastal cliff sections north‐west of Wanganui comprise 25, 6th (41 ka) order and 5th (100 ka) order, shallow‐marine to marginal marine stratigraphic sequences, deposited during global glacioeustatic sea‐level cycles corresponding to Marine Isotope Stages (MIS) 78–10. Here, we characterise the sequences using: (1) a series of drill cores sited above and behind the coastal outcrops, which recovered a composite record of c. 450 m, (2) a new high resolution multichannel seismic reflection profile acquired along the beach adjacent to the coastal cliffs, and (3) downhole digital logs from the boreholes. This paper integrates the outcrop and subsurface datasets to produce a high resolution model of the stratigraphic signatures and 2D architecture of a cyclical, shallow‐marine deposition system. Such models have significant applications to petroleum exploration, and enable the distribution of reservoir facies and intervening seal rocks within sequences, together with the nature of the connectivity of sandstone facies between sequences, to be evaluated. Similar hydrocarbon‐producing systems within the Eocene Kapuni Group (e.g., Mangahewa and Kaimiro formations) have been, and are still, the focus of intense exploration in Taranaki Basin.