Preliminary Integrated Chronostratigraphy of the AND-1B Core, ANDRILL McMurdo Ice Shelf Project, Antarctica

Chronostratigraphic data available for the preliminary age model for the upper 700 m for the AND-1B drill core include diatom biostratigraphy, magnetostratigraphy, 40Ar/39Ar ages on volcanic material, 87Sr/86Sr ages on calcareous fossil material, and surfaces of erosion identifi ed from physical appearance and facies relationships recognised in the AND-1B drill core. The available age data allow a relatively well-constrained age model to be constructed for the upper 700 m of the drill core. Available diatom biostratigraphic constraints and 40Ar/39Ar ages allow a unique correlation of ~70% of the AND- 1B magnetic polarity stratigraphy with the Geomagnetic Polarity Time Scale (GPTS). Unique correlation is not possible in several coarse diamictite intervals with closely spaced glacial surfaces of erosion and sparse microfl ora. However, the age model indicates relatively rapid (up to 1 m/k.y.) and continuous accumulation of intervening fi ner grained diatomaceous intervals punctuated by several half- to millionyear hiatuses representing more than half of the last 7 m.y. in the AND-1B record. The mid- to late Pleistocene is represented by superimposed diamictite units separated from upper Pliocene alternating diamictites/diatomites by a ~1 m.y. hiatus co-incident with a regionally correlated seismic reflection surface. A c. 100 m-thick diatomite represents a signifi cant portion of the early Pliocene record in the AND-1B drill core. Strata below ~620 m are late Miocene in age; however, biostratigraphic constraints are absent below 586 m and correlation with the GPTS is relatively unconstrained. At the time of writing, the only chronostratigraphic data available below 700 mbsf include three 40Ar/39Ar ages on volcanic clasts from near 1280 mbsf affording a maximum depositional age of 13.57 Ma for the base of the AND-1B drill core

Temporal controls on silicic acid utilisation along the West Antarctic Peninsula

The impact of climatic change along the Antarctica Peninsula has been widely debated in light of atmospheric/oceanic warming and increases in glacial melt over the past half century. Particular concern exists over the impact of these changes on marine ecosystems, not only on primary producers but also on higher trophic levels. Here we present a record detailing the historical controls on the biogeochemical cycling of silicic acid [Si(OH)4] on the west Antarctica Peninsula margin, a region in which the modern phytoplankton environment is constrained by seasonal sea-ice. We demonstrate that Si(OH)4 cycling through the Holocene alternates between being primarily regulated by sea-ice or glacial discharge from the surrounding grounded ice-sheet. With further climate-driven change and melting forecast for the 21st Century, our findings document the potential for biogeochemical cycling and multi-trophic interactions along the peninsula to be increasingly regulated by glacial discharge, altering food-web interactions

Palaeontologic Characterisation and Analysis of the AND-1B Core, ANDRILL McMurdo Ice Shelf Project, Antarctica

Fossils provide key data sets for the interpretation of the AND-1B core. Calcareous plankton and benthos provide the basis for palaeoenvironmental interpretations of both surface and bottom waters. Calcareous fossils are rare throughout, but occurrences noted are significant. Some calcareous fossils provide potential for age control via 86Sr/87Sr, and palaeoenvironmental information may come from Mg/Ca ratios as well as oxygen and carbon isotopes. Organic-walled microfossils provide an index of reworking and transport, as well as the identifi cation of a possible in situ Pliocene assemblage of previously unknown marine palynomorphs. Diatoms are abundant in the core, with diatom-rich sediments constituting nearly half the upper 600 m of core, subdivided into 13 diatomaceous units, ranging in thickness from under 1 m to nearly 100 m. Diatoms provide biostatigraphic age control, but calibration to the Southern Ocean zonation is limited by ecologic exclusion of many taxa and previously undocumented diachrony among other taxa. A new, high-resolution diatom biostratigraphy for the Antarctic continental shelf is now under development. Diatoms provide the basis for numerous palaeoenvironmental applications, including providing a proxy for palaeotemperature and palaeo-sea ice, as well as palaeoproductivity. All results presented here are preliminary, and interpretations should be considered tentative, pending quantitative and more detailed qualitative follow-up analyses

Late Pleistocene to Holocene Strata from Soft-Sediment Coring at the AND-1B Site, ANDRILL McMurdo Ice Shelf Project, Antarctica

Prior to rotary coring, a range of soft-sediment coring tools were deployed to recover the sediment-water interface and the upper few metres of strata, whose integrity was threatened by embedment of the sea riser for drilling of the ANDRILL (AND)-1A/1B holes. These coring options included (1) a sediment gravity corer deployed through the ice-shelf hole, and (2) a push corer deployed through the sea riser suspended a few metres above the seabed. Within the AND-1A hole (during an attempt at sea-riser embedment) an extended-nose corer was advanced in front of the sea riser with limited success. The hydraulic piston corer was not deployed as a consequence of the firmness of the Last Glacial Maximum (LGM) diamicton and the occurrence of outsized clasts that could damage the drill string and compromise the deeper coring options. Successive attempts at gravity and push coring recovered 12 cores up to a maximum of 1.56 metres below sea-floor (mbsf). The longest core was dedicated to sampling for microbial life and pore-water geochemical studies which were expected to show the greatest gradients in the upper few metres of the sediment column. All cores sampled Holocene sub-iceshelf sediments above the LGM diamicton, and displayed a similar stratigraphy to a previously obtained site survey core, 250 m to the southeast (McKay et al., in press), with an unconsolidated diamicton passing upwards into muddy sub-ice-shelf facies. The sediment cores record the retreat of the grounding line through the region about 10 000 years ago and a transition from grounding line proximal, through sub-ice-shelf to calving-line proximal environments

Preliminary chronostratigraphy for the upper 700 m (upper Miocene – Pleistocene) of the AND-1B drillcore recovered from beneath the McMurdo Ice Shelf, Antarctica

Chronostratigraphic data available for the preliminary age model for the upper 700 m for the AND-1B drill core include diatom biostratigraphy, magnetostratigraphy, 40Ar/39Ar ages on volcanic material, 87Sr/86Sr ages on calcareous fossil material, and surfaces of erosion identified from physical appearance and facies relationships recognized in the AND-1B drill core. The available age data allow a relatively well-constrained age model to be constructed for the upper 700 m of the drill core. Available diatom biostratigraphic constraints and 40Ar/39Ar ages allow a unique correlation of ~70% of the AND-1B magnetic polarity stratigraphy with the Geomagnetic Polarity Time Scale (GPTS). Unique correlation is not possible in several coarse diamictite intervals with closely spaced glacial surfaces of erosion and sparse microflora. However, the age model indicates relatively rapid (up to 1m / k.y.) and continuous accumulation of intervening finer grained diatomaceous intervals punctuated by several half to million year hiatuses representing more than half of the last 7 m.y. in the AND-1B record. The mid-late Pleistocene is represented by superimposed diamictite units separated from upper Pliocene alternating diamictites / diatomites by a ~ 1 m.y. hiatus co-incident with a regionally correlated seismic reflection surface. A c. 100 m thick diatomite represents a significant portion of the early Pliocene record in the AND-1B drillcore. Strata below ~620 m are late Miocene in age, however biostratigraphic constraints are absent below 586 m and correlation with the GPTS is relatively unconstrained

Antarctic and Southern Ocean influences on Late Pliocene global cooling

The influence of Antarctica and the Southern Ocean on Late Pliocene global climate reconstructions has remained ambiguous due to a lack of well-dated Antarctic-proximal, paleoenvironmental records. Here we present ice sheet, sea-surface temperature, and sea ice reconstructions from the ANDRILL AND-1B sediment core recovered from beneath the Ross Ice Shelf. We provide evidence for a major expansion of an ice sheet in the Ross Sea that began at ~3.3 Ma, followed by a coastal sea surface temperature cooling of ~2.5 °C, a stepwise expansion of sea ice, and polynya-style deep mixing in the Ross Sea between 3.3 and 2.5 Ma. The intensification of Antarctic cooling resulted in strengthened westerly winds and invigorated ocean circulation. The associated northward migration of Southern Ocean fronts has been linked with reduced Atlantic Meridional Overturning Circulation by restricting surface water connectivity between the ocean basins, with implications for heat transport to the high latitudes of the North Atlantic. While our results do not exclude low-latitude mechanisms as drivers for Pliocene cooling, they indicate an additional role played by southern high-latitude cooling during development of the bipolar world