ALBERT

Description: Quantification of ice-rafted debris (IRD) abundances in deep-sea records using three independent methodologies of obtaining IRD abundances and how different approaches will affect determinations of mass accumulation rates (MARs). The three methodologies for this cross comparison of methods include: counting clasts 〉2 mm in x-radiograph images; the sieved weight percentage of the medium-to-coarse sand fraction (250 μm-2 mm); and volumetric estimates of the 〉125 μm sand fraction using Laser diffraction Particle Size Analysis (LPSA) methods to determine particle size. The data are collected from the Wilkes Land and Ross Sea region of Antarctica, using cores RS15-LC42,RS15-LC48, IODP sites 1361 and ODP site 1165.

Description: Earth is heading towards a climate that was last experienced more than 3 Myr during the “mid-Pliocene warm period”1. Atmospheric carbon dioxide (pCO2) concentrations were ~400 ppm, global sea level oscillated in response to orbital forcing2,3 and peak global mean sea level (GMSL) may have reached ~20 m above present4,5. For sea-level rise of this magnitude extensive retreat or collapse of the Greenland, West Antarctic and marine based sectors of the East Antarctic ice sheets are required. Yet the relative amplitude of sea-level variations within glacial-interglacial cycles remains poorly-constrained. Here, we show sea-level varied on average by 13 ± 5 m over glacial-interglacial cycles during the mid- to late Pliocene, ~3.3 - 2.5 Myrs. We calibrated a theoretical relationship between modern sediment transport by waves and water depth and then applied the technique to Pliocene grain size in shallow-marine sediments from Whanganui Basin, New Zealand, thereby estimating past sea level variation. The resulting PlioSeaNZ record is independent of the deep ocean oxygen isotope (δ18O) record for global ice volume3, and in phase with ~20 kyr duration cycles of insolation over Antarctica, paced by eccentricity-modulated orbital precession between 3.3 and 2.7 Ma6. Thereafter, sea-level fluctuations are paced by ~41 kyr cycles in Earth's axial tilt as ice sheets stabilise on Antarctica and intensify in the northern hemisphere3,6. Sensu stricto, we provide the amplitude of relative sea-level (RSL) change, rather than absolute GMSL change. However, glacio-isostatic adjustment (GIA) simulations of RSL change, show that the PlioSeaNZ record approximates eustatic sea level (ESL), defined here as GMSL unregistered to the centre of the Earth. Nonetheless, under conservative assumptions, our estimates limit maximum Pliocene sea level to less than +25 m and provide new constraints on polar ice-volume variability under climate conditions Earth is on track to experience this century.

Description: The Pliocene and Early Pleistocene, between 5.3 and 0.8 million years ago, span a transition from a global climate state that was 2-3 °C warmer than present with limited ice sheets in the Northern Hemisphere to one that was characterized by continental-scale glaciations at both poles. Growth and decay of these ice sheets was paced by variations in the Earth's orbit around the Sun. However, the nature of the influence of orbital forcing on the ice sheets is unclear, particularly in light of the absence of a strong 20,000-year precession signal in geologic records of global ice volume and sea level. Here we present a record of the rate of accumulation of iceberg-rafted debris offshore from the East Antarctic ice sheet, adjacent to the Wilkes Subglacial Basin, between 4.3 and 2.2 million years ago. We infer that maximum iceberg debris accumulation is associated with the enhanced calving of icebergs during ice-sheet margin retreat. In the warmer part of the record, between 4.3 and 3.5 million years ago, spectral analyses show a dominant periodicity of about 40,000 years. Subsequently, the powers of the 100,000-year and 20,000-year signals strengthen. We suggest that, as the Southern Ocean cooled between 3.5 and 2.5 million years ago, the development of a perennial sea-ice field limited the oceanic forcing of the ice sheet. After this threshold was crossed, substantial retreat of the East Antarctic ice sheet occurred only during austral summer insolation maxima, as controlled by the precession cycle.

Description: Warm intervals within the Pliocene epoch (5.33-2.58 million years ago) were characterized by global temperatures comparable to those predicted for the end of this century (Haywood and Valdes, doi:10.1016/S0012-821X(03)00685-X) and atmospheric CO2 concentrations similar to today (Seki et al., 2010, doi:10.1016/j.epsl.2010.01.037; Bartoli et al., 2011, doi:10.1029/2010PA002055; Pagani et al., 2010, doi:10.1038/ngeo724). Estimates for global sea level highstands during these times (Miller et al., 2012, doi:10.1130/G32869.1) imply possible retreat of the East Antarctic ice sheet, but ice-proximal evidence from the Antarctic margin is scarce. Here we present new data from Pliocene marine sediments recovered offshore of Adélie Land, East Antarctica, that reveal dynamic behaviour of the East Antarctic ice sheet in the vicinity of the low-lying Wilkes Subglacial Basin during times of past climatic warmth. Sedimentary sequences deposited between 5.3 and 3.3 million years ago indicate increases in Southern Ocean surface water productivity, associated with elevated circum-Antarctic temperatures. The geochemical provenance of detrital material deposited during these warm intervals suggests active erosion of continental bedrock from within the Wilkes Subglacial Basin, an area today buried beneath the East Antarctic ice sheet. We interpret this erosion to be associated with retreat of the ice sheet margin several hundreds of kilometres inland and conclude that the East Antarctic ice sheet was sensitive to climatic warmth during the Pliocene.

Description: We examine a single interglacial during the late Pliocene (KM5c, ca. 3.205 +/- 0.01 Ma) when atmospheric CO2 concentrations were higher than pre-industrial, but similar to today and to the lowest emission scenarios for this century. As orbital forcing and continental configurations were also similar to today, we are able to focus on equilibrium climate system response to modern and near-future CO2. We have synthesised largely published datasets, scrutinised their age models, and generated mean absolute SSTs and their anomaly relative to the pre-industrial. Here we present those values for both alkenone and foraminifera-Mg/Ca proxies.

Description: The Pliocene Epoch (~2.6–5.3 million years ago, Ma) was characterized by a warmer than present climate with smaller Northern Hemisphere ice sheets, and therefore offers an example of a climate system in long-term equilibrium with current or predicted near-future carbon dioxide concentrations. The end of the Pliocene (~2.6 Ma) is marked by further ice-sheet expansion and intensification of glacial (cold) stages, referred to as the "intensification of Northern Hemisphere Glaciation" (iNHG). Here we present the data used to assess the spatial and temporal variability of ocean temperatures and ice-volume indicators through the late Pliocene and early Pleistocene (from 3.3 to 2.4 Ma) to determine the character of this climate transition. The data come from the Atlantic, Pacific, Indian and Southern Ocean, as well as some marginal seas. Here we present the synthesized alkenone sea-surface temperature, Mg/Ca sea-surface temperature, planktonic foraminifera d18O and benthic foraminifera d18O data which were used in our synthesis. Although the original data sets are largely published, here we present the alkenone SST records calculated using the BAYSPLINE calibration where these were not part of the original publication; the Mg/Ca-SST records where we revised the absolute SSTs; any data sets where we revised the age model.