Sonia Sandroni

Antarctic Ice Sheet variability across the Eocene-Oligocene boundary climate transition.

Sensitive ice sheets Why did the Antarctic Ice Sheet begin to grow 34 million years ago, and what does that have to do with us? Galeotti et al. studied a marine sediment core recovered from just off the coast of Antarctica (see the Perspective by Lear and Lunt). The ice sheet did not begin to grow until atmospheric CO2 concentrations had dropped to below around 600 parts per million. Indeed, the ice sheet was unstable when CO2 was higher. As modern atmospheric CO2 concentrations continue their rise, a shift back to an unstable Antarctic Ice Sheet could increase harmful rises in sea level. Science, this issue p. 76; see also p. 34 The growth of the Antarctic Ice Sheet began only when atmospheric levels of carbon dioxide dropped low enough. [Also see Perspective by Lear and Lunt] About 34 million years ago, Earth’s climate cooled and an ice sheet formed on Antarctica as atmospheric carbon dioxide (CO2) fell below ~750 parts per million (ppm). Sedimentary cycles from a drillcore in the western Ross Sea provide direct evidence of orbitally controlled glacial cycles between 34 million and 31 million years ago. Initially, under atmospheric CO2 levels of ≥600 ppm, a smaller Antarctic Ice Sheet (AIS), restricted to the terrestrial continent, was highly responsive to local insolation forcing. A more stable, continental-scale ice sheet calving at the coastline did not form until ~32.8 million years ago, coincident with the earliest time that atmospheric CO2 levels fell below ~600 ppm. Our results provide insight into the potential of the AIS for threshold behavior and have implications for its sensitivity to atmospheric CO2 concentrations above present-day levels.

Amphibole-bearing metamorphic clasts in ANDRILL AND-2A core: A provenance tool to unravel the Miocene glacial history in the Ross Embayment (western Ross Sea, Antarctica)

A petrological investigation of amphibole-bearing metamorphic clasts in the ANDRILL AND-2A core allows a detailed comparison with similar lithologies from potential source regions, leading to the identification of three distinct provenance areas in the present-day segment of the Transantarctic Mountains between the Byrd Glacier and the Blue Glacier (Mulock-Skelton glacier area, the Britannia Range, and the Koettlitz-Blue glacier area in the Royal Society Range). A key role in the comparison is played by the wide range of Ca-amphibole compositions, type of intracrystalline zoning, mineral assemblages, and fabrics, which reflect different bulk rocks and metamorphic conditions. Ca-amphibole compositions and zonations also offer the opportunity for the application of geothermobarometry methods, which, consistent with literature data, provide further evidence that the three provenance regions correspond to distinct metamorphic terrains with pervasive medium-pressure amphibolite-grade conditions restricted to the Britannia Range. The study contributes new insights into the depositional processes in a variety of glacial environments ranging from open marine with icebergs to distal, proximal, and subglacial settings. The results also highlight the record of two distinct glacial scenarios reflecting either short-range (

A sedimentological record of early Miocene ice advance and retreat, AND-2A drill hole, McMurdo Sound, Antarctica

The lowest 501 m (~1139–638 m) of the AND-2A core from southern McMurdo Sound is the most detailed and complete record of early Miocene sediments in Antarctica and indicates substantial variability in Antarctic ice sheet activity during early Miocene time. There are two main pulses of diamictite accumulation recorded in the core, and three significant intervals with almost no coarse clasts. Each diamictite package comprises several sequences consistent with ice advance-retreat episodes. The oldest phase of diamictite deposition, Composite Sequence 1 (CS1), has evidence for grounded ice at the drill site and has been dated around 20.2– 20.1 Ma. It likely coincides with cooling associated with isotope event Mi1aa. This is overlain by a diamictite-free, sandstone-dominated interval, CS2 that includes three coarsening-upward deltaic cycles, is inferred to mark substantial warming, and has an inferred age range between 20.1 and 20.05 Ma. Above this is an interval with variable amounts of diamictite (CS3), with indicators of ice grounding, that is inferred to record ice advance relative to CS2, and is overlain by an ~100-m-thick mud-rich interval (CS4) with no sedimentological evidence for direct glacial influence at the drill site (ca. 19.4–18.7 Ma). A third overlying diamictite-rich interval (CS5) overlies an unconformity spanning 18.7–17.8 Ma (coinciding with isotope event Mi1b), and records a return to more ice-influenced conditions at the drill site in late early Miocene time. The overall picture for the early Miocene (spanning the period 20.2–17.35 Ma) is one of ice advance alternating with periods of ice retreat and hence significant global climate fluctuations after the permanent establishment of the Antarctic ice sheet at the Eocene/Oligocene boundary, and preceding the relative warmth of the middle Miocene climatic optimum (ca. 17.5–14.5 Ma). Sedimentary cyclicity in CS1 and CS2 is consistent with ~21 k.y. precession but in CS3 the frequency is closer to 100 k.y. (consistent with eccentricity), with a possible change to 20 k.y. precession in CS4. CS5 cyclicity is consistent with obliquity forcing. Provenance data are consistent with local Transantarctic Mountains glacial activity under precessional control in CS1 and more southerly ice-cap build up under 100 k.y. eccentricity and obliquity control during CS3 and CS5, respectively.