Michael L. Prentice

Major features and forcing of high‐latitude northern hemisphere atmospheric circulation using a 110,000‐year‐long glaciochemical series

The Greenland Ice Sheet Project 2 glaciochemical series (sodium, potassium, ammonium, calcium, magnesium, sulfate, nitrate, and chloride) provides a unique view of the chemistry of the atmosphere and the history of atmospheric circulation over both the high latitudes and mid-low latitudes of the northern hemisphere. Interpretation of this record reveals a diverse array of environmental signatures that include the documentation of anthropogenically derived pollutants, volcanic and biomass burning events, storminess over marine surfaces, continental aridity and biogenic source strength plus information related to the controls on both high- and low-frequency climate events of the last 110,000 years. Climate forcings investigated include changes in insolation of the order of the major orbital cycles that control the long-term behavior of atmospheric circulation patterns through changes in ice volume (sea level), events such as the Heinrich events (massive discharges of icebergs first identified in the marine record) that are found to operate on a 6100-year cycle due largely to the lagged response of ice sheets to changes in insolation and consequent glacier dynamics, and rapid climate change events (massive reorganizations of atmospheric circulation) that are demonstrated to operate on 1450-year cycles. Changes in insolation and associated positive feedbacks related to ice sheets may assist in explaining favorable time periods and controls on the amplitude of massive rapid climate change events. Explanation for the exact timing and global synchroneity of these events is, however, more complicated. Preliminary evidence points to possible solar variability-climate associations for these events and perhaps others that are embedded in our ice-core-derived atmospheric circulation records.

Stability of the Larsen B ice shelf on the Antarctic Peninsula during the Holocene epoch

The stability of the Antarctic ice shelves in a warming climate has long been discussed, and the recent collapse of a significant part, over 12,500 km2 in area, of the Larsen ice shelf off the Antarctic Peninsula has led to a refocus toward the implications of ice shelf decay for the stability of Antarcticas grounded ice. Some smaller Antarctic ice shelves have undergone periodic growth and decay over the past 11,000 yr (refs 7–11), but these ice shelves are at the climatic limit of ice shelf viability and are therefore expected to respond rapidly to natural climate variability at century to millennial scales. Here we use records of diatoms, detrital material and geochemical parameters from six marine sediment cores in the vicinity of the Larsen ice shelf to demonstrate that the recent collapse of the Larsen B ice shelf is unprecedented during the Holocene. We infer from our oxygen isotope measurements in planktonic foraminifera that the Larsen B ice shelf has been thinning throughout the Holocene, and we suggest that the recent prolonged period of warming in the Antarctic Peninsula region, in combination with the long-term thinning, has led to collapse of the ice shelf.

Antarctic Glacial History Since the Last Glacial Maximum: An Overview of the Record on Land

This overview examines available circum-Antarctic glacial history archives on land, related to developments after the Last Glacial Maximum (LGM). It considers the glacial-stratigraphic and morphologic records and also biostratigraphical information from moss banks, lake sediments and penguin rookeries, with some reference to relevant glacial marine records. It is concluded that Holocene environmental development in Antarctica differed from that in the Northern Hemisphere. The initial deglaciation of the shelf areas surrounding Antarctica took place before 10 000 14 C yrs before present( BP ), and was controlled by rising global sea level. This was followed by the deglaciation of some presently ice-free inner shelf and land areas between 10 000 and 8000 yr BP . Continued deglaciation occurred gradually between 8000 yr BP and 5000 yr BP . Mid-Holocene glacial readvances are recorded from various sites around Antarctica. There are strong indications of a circum-Antarctic climate warmer than today 4700–2000 yr BP . The best dated records from the Antarctic Peninsula and coastal Victoria Land suggest climatic optimums there from 4000–3000 yr BP and 3600–2600 yr BP , respectively. Thereafter Neoglacial readvances are recorded. Relatively limited glacial expansions in Antarctica during the past few hundred years correlate with the Little Ice Age in the Northern Hemisphere.

On marine microfossil transport and pathways in Antarctica during the late Neogene: Evidence from the Sirius Group at Mount Fleming

There are two extreme views of the evolution of the Pliocene Antarctic Ice Sheet. Dynamicists argue for ice-sheet reduction and reexpansion on the basis of Pliocene marine diatoms in a glacial deposit, the Sirius Group, that is widespread in the Transantarctic Mountains. Stabilists argue from other evidence that the Antarctic cryosphere remained essentially constant in area and volume; they propose marine diatom transport by eolian processes and emplacement into terrestrial glacial strata. Hence, the inferred source area and transport mechanism of marine diatoms are of critical importance. We tested the reduction hypothesis on an important outcrop of the Sirius Group at Mount Fleming, South Victoria Land. We observed very few, unidentifiable, marine diatom fragments in Sirius Group strata. In contrast, marine diatoms enclosed in a surface diamicton covering the Sirius Group were more abundant and identifiable. Our study further indicates that the Sirius Group at Mount Fleming was not deposited by the East Antarctic Ice Sheet but rather by alpine ice originating on the Transantarctic Mountains. On the basis of both data sets, we infer that marine diatoms postdate Sirius Group deposition at Mount Fleming and that transport was by wind, and we advance alternative scenarios for their source and transport pathways.

A case for Sirius Group alpine glaciation at Mount Fleming, South Victoria Land, Antarctica: A case against Pliocene East Antarctic Ice Sheet reduction

The Sirius Group comprises a suite of consolidated glacial deposits that are widespread in the Transantarctic Mountains; some of the deposits contain Pliocene marine diatoms. Because of the interpretation that the Sirius Group tills and the marine diatoms were deposited by the East Antarctic Ice Sheet, it has been inferred that the East Antarctic Ice Sheet was much reduced in area and volume during the Pliocene. These interpretations were evaluated by studying Sirius Group lodgment tills on Mount Fleming in the Dry Valleys sector of the Transantarctic Mountains. We infer that the Sirius Group lodgment tills at Mount Fleming do not support the hypothesis that the East Antarctic Ice Sheet was much reduced during the Pliocene. We demonstrate that Sirius Group lodgment tills on Mount Fleming were deposited by alpine ice. This conclusion is founded on ice-flow directions inferred from rattails, gravel fabric, and the orientation of bullet boulders, as well as on sediment and gravel characteristics of the tills. Only a very few unidentifiable diatom fragments were recovered. Better preserved, identifiable, Pliocene marine diatoms occur in moderate abundances in a 5–10-cm-thick unconsolidated and discontinuous drift that caps the lodgment tills. On the basis of the decline in diatom preservation with depth into the till, we dismiss transport of the better preserved diatoms by the ice that deposited the underlying Sirius Group lodgment till. Instead, we attribute diatom presence to eolian deposition and recycling into lodgment tills at a later stage.