Michael J. Bentley

A Reconciled Estimate of Ice-Sheet Mass Balance

Warming and Melting Mass loss from the ice sheets of Greenland and Antarctica account for a large fraction of global sea-level rise. Part of this loss is because of the effects of warmer air temperatures, and another because of the rising ocean temperatures to which they are being exposed. Joughin et al. (p. 1172) review how ocean-ice interactions are impacting ice sheets and discuss the possible ways that exposure of floating ice shelves and grounded ice margins are subject to the influences of warming ocean currents. Estimates of the mass balance of the ice sheets of Greenland and Antarctica have differed greatly—in some cases, not even agreeing about whether there is a net loss or a net gain—making it more difficult to project accurately future sea-level change. Shepherd et al. (p. 1183) combined data sets produced by satellite altimetry, interferometry, and gravimetry to construct a more robust ice-sheet mass balance for the period between 1992 and 2011. All major regions of the two ice sheets appear to be losing mass, except for East Antarctica. All told, mass loss from the polar ice sheets is contributing about 0.6 millimeters per year (roughly 20% of the total) to the current rate of global sea-level rise. The mass balance of the polar ice sheets is estimated by combining the results of existing independent techniques. We combined an ensemble of satellite altimetry, interferometry, and gravimetry data sets using common geographical regions, time intervals, and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of Earth’s polar ice sheets. We find that there is good agreement between different satellite methods—especially in Greenland and West Antarctica—and that combining satellite data sets leads to greater certainty. Between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by –142 ± 49, +14 ± 43, –65 ± 26, and –20 ± 14 gigatonnes year−1, respectively. Since 1992, the polar ice sheets have contributed, on average, 0.59 ± 0.20 millimeter year−1 to the rate of global sea-level rise.

Chronology of the last glaciation in central Strait of Magellan and Bahia Inutil, southernmost South America

ABSTRACT. Glacier fluctuations in the Strait of Magellan tell of the climatic changes that affected southern latitudes at c. 53–55°S during the Last Glacial Maximum (LGM) and Late‐glacial/Holocene transition. Here we present a revised chronology based on cosmogenic isotope analysis, 14C assays, amino acid racemisation and tephrochronology. We unpick the effect of bedrock‐derived lignite which has affected many 14C dates in the past and synthesise new and revised dates that constrain five glacier advances (A to E). Advance A is prior to the LGM. LGM is represented by Advance B that reached and largely formed the arcuate peninsula Juan Mazia. Carbon‐14and 10Be dating show it occurred after 31 250 cal yrs BP and culminated at 25 200–23 100 cal yrs BP and was then followed by the slightly less extensive advance C sometime before 22 400–20 300 cal yrs BP. This pattern of an early maximum is found elsewhere in South America and more widely. Stage D, considerably less extensive, culminated sometime before 17 700–17 600 cal yrs BP and was followed by rapid and widespread glacier retreat. Advance E, which dammed a lake, spanned 15 500–11770 cal yrs BP. This latter advance overlaps the Bølling‐Allerød interstadials and the glacier retreat occurs during the peak of the Younger Dryas stadial in the northern hemisphere. However, the stage E advance coincides with the Antarctic Cold Reversal (c. 14800–12700 cal yrs BP) and may indicate that some millennial‐scale climatic fluctuations in the Late‐glacial period are out of phase between the northern and southern hemispheres.

Lower satellite-gravimetry estimates of Antarctic sea-level contribution

Recent estimates of Antarctica’s present-day rate of ice-mass contribution to changes in sea level range from 31 gigatonnes a year (Gt yr−1; ref. 1) to 246 Gt yr−1 (ref. 2), a range that cannot be reconciled within formal errors. Time-varying rates of mass loss contribute to this, but substantial technique-specific systematic errors also exist. In particular, estimates of secular ice-mass change derived from Gravity Recovery and Climate Experiment (GRACE) satellite data are dominated by significant uncertainty in the accuracy of models of mass change due to glacial isostatic adjustment (GIA). Here we adopt a new model of GIA, developed from geological constraints, which produces GIA rates systematically lower than those of previous models, and an improved fit to independent uplift data. After applying the model to 99 months (from August 2002 to December 2010) of GRACE data, we estimate a continent-wide ice-mass change of −69 ± 18 Gt yr−1 (+0.19 ± 0.05 mm yr−1 sea-level equivalent). This is about a third to a half of the most recently published GRACE estimates, which cover a similar time period but are based on older GIA models. Plausible GIA model uncertainties, and errors relating to removing longitudinal GRACE artefacts (‘destriping’), confine our estimate to the range −126 Gt yr−1 to −29 Gt yr−1 (0.08–0.35 mm yr−1 sea-level equivalent). We resolve 26 independent drainage basins and find that Antarctic mass loss, and its acceleration, is concentrated in basins along the Amundsen Sea coast. Outside this region, we find that West Antarctica is nearly in balance and that East Antarctica is gaining substantial mass.

Late-glacial glacier events in southernmost South America : A blend of 'northern' and 'southern' hemispheric climatic signals?

ABSTRACT. This paper examines new geomorphological, chronological and modelling data on glacier fluctuations in southernmost South America in latitudes 46–55°S during the last glacial–interglacial transition. Establishing leads and lags between the northern and southern hemispheres and between southern mid‐latitudes and Antarctica is key to an appreciation of the mechanisms and resilience of global climate. This is particularly important in the southern hemisphere where there is a paucity of empirical data. The overall structure of the last glacial cycle in Patagonia has a northern hemisphere signal. Glaciers reached or approached their Last Glacial Maxima on two or more occasions at 25–23 ka (calendar) and there was a third less extensive advance at 17.5 ka. Deglaciation occurred in two steps at 17.5 ka and at 11.4 ka. This structure is the same as that recognized in the northern hemisphere and taking place in spite of glacier advances occurring at a time of high southern hemisphere summer insolation and deglaciation at a time of decreasing summer insolation. The implication is that at orbital time scales the‘northern’ signal dominates any southern hemisphere signal. During deglaciation, at a millennial scale, the glacier fluctuations mirror an antiphase southern’ climatic signal as revealed in Antarctic ice cores. There is a glacier advance coincident with the Antarctic Cold Reversal at 15.3–12.2 ka. Furthermore, deglaciation begins in the middle of the Younger Dryas. The implication is that, during the last glacial–interglacial transition, southernmost South America was under the influence of sea surface temperatures, sea ice and southern westerlies responding to conditions in the southern’ Antarctic domain. Such asynchrony may reflect a situation whereby, during deglaciation, the world is more sensitized to fluctuations in the oceanic thermohaline circulation, perhaps related to the bipolar seesaw, than at orbital timescales.

Deglacial history of the West Antarctic Ice Sheet in the Weddell Sea embayment: Constraints on past ice volume change

The retreat history of the West Antarctic Ice Sheet (WAIS) since the Last Glacial Maximum is important for understanding the process of rapid deglaciation, constraining models that seek to predict the future trajectory of the ice sheet, and for estimating rates of sea-level change. Here we report new glacial geologic data from the southwestern Weddell Sea embayment that demonstrate that this part of the WAIS was thinner than previously suggested, and that there was progressive thinning of the ice sheet by 230–480 m since ca. 15 ka. We use geomorphological data and a numerical ice sheet model to reconstruct the ice sheet in the Weddell Sea at the Last Glacial Maximum. The volume of this ice would have added between 1.4 and 2.0 m to postglacial sea-level rise and would not have been sufficient to contribute significantly to meltwater pulse 1A, a rapid rise in sea level ∼14,200 yr ago.

Widespread low rates of Antarctic glacial isostatic adjustment revealed by GPS observations

Bedrock uplift in Antarctica is dominated by a combination of glacial isostatic adjustment (GIA) and elastic response to contemporary mass change. Here, we present spatially extensive GPS observations of Antarctic bedrock uplift, using 52% more stations than previous studies, giving enhanced coverage, and with improved precision. We observe rapid elastic uplift in the northern Antarctic Peninsula. After considering elastic rebound, the GPS data suggests that modeled or empirical GIA uplift signals are often over?estimated, par t icularly the magnitudes of the signal maxima. Our observation that GIA uplift is misrepresented by modeling (weighted root?meansquares of observation?model differences: 4.9–5.0 mm/yr) suggests that, apart from a few regions where large ice mass loss is occurring, the spatial pattern of secular ice mass change derived from Gravity Recovery and Climate Experiment (GRACE) data and GIA models may be unreliable, and that several recent secular Antarctic ice mass loss estimates are systematically biased, mainly too high.

Early Holocene retreat of the George VI Ice Shelf, Antarctic Peninsula

The recent collapse of several Antarctic Peninsula ice shelves has been linked to rapid regional atmospheric warming during the twentieth century. New high-resolution lake sediment records of Holocene ice-shelf behavior show that the George VI Ice Shelf was absent beginning ca. 9595 calibrated (cal.) yr B.P., but reformed by ca. 7945 cal. yr B.P. This retreat immediately followed a period of maximum Holocene warmth that is recorded in some ice cores and occurred at the same time as an influx of warmer ocean water onto the Antarctic Peninsula shelf. The absence of the ice shelf suggests that early Holocene ocean-atmosphere variability in the Antarctic Peninsula was greater than that measured in recent decades.

Geomorphological evidence and cosmogenic 10Be/26Al exposure ages for the Last Glacial Maximum and deglaciation of the Antarctic Peninsula Ice Sheet

This paper presents the first systematic attempt to map the Last Glacial Maximum (LGM) configuration of the southern and central parts of the Antarctic Peninsula Ice Sheet, and to determine the timing of onshore ice-sheet retreat. Geomorphologic evidence shows that the LGM ice sheet expanded to form two ice domes in Palmer Land and merged with an expanded and thicker West Antarctic Ice Sheet in the Weddell Sea. Ice from the Antarctic Peninsula merged with Alexander Island ice in George VI Sound. Cosmogenic 10 Be and 26 Al data from 29 erratics on nunataks yield model ages between 7.2 ka and older than 1 Ma. The data set contains a high proportion of erratics with evidence of nuclide inheritance. Once these ages have been excluded, the cosmogenic ages suggest that thinning of the west side of the Antarctic Peninsula Ice Sheet to near-present configuration was almost complete by the early Holocene. These data, combined with previously published 14 C data, exclude the possibility that the west side of the Antarctic Peninsula Ice Sheet has been thinning throughout the Holocene, as has been demonstrated for some other sectors of the West Antarctic Ice Sheet. On the east side of the Antarctic Peninsula, ice-sheet thinning was under way prior to the early Holocene, but our data do not constrain the ice-sheet behavior more recently than 7.2 ka.

EVIDENCE FOR LATE‐GLACIAL ICE DAMMED LAKES IN THE CENTRAL STRAIT OF MAGELLAN AND BAHÍA INÚTIL, SOUTHERNMOST SOUTH AMERICA

ABSTRACT. This paper critically appraises the evidence for a succession of ice‐dammed lakes in the central Strait of Magellan (c. 53°S) c. 17 000–12 250 cal. yr BP. The topographic configuration of islands and channels in the southern Strait of Magellan means that the presence of lakes provides compelling constraints on the position of former ice margins. Lake shorelines and glacio‐lacustrine sediments have been dated by their association with a key tephra layer from Volcan Reclús (c. 15 510–14 350 cal. years bp) and by 14C‐dated peats. The timing of glacial lake formation and associated glacier readvances is at odds with the rapid and widespread glacier retreat of the Patagonian ice fields further north after c. 17 000 cal. yr bp, suggesting rather that the lakes were coeval with the Antarctic Cold Reversal and persisted to the Late‐glacial/Holocene transition. This apparent asymmetrical latitudinal response in glacier behaviour may reflect overlapping spheres of northern hemisphere and Antarctic climatic influence in the Magellan region.

Geological and geomorphological insights into Antarctic ice sheet evolution

Technical advances in the study of ice-free parts of Antarctica can provide quantitative records that are useful for constraining and refining models of ice sheet evolution and behaviour. Such records improve our understanding of system trajectory, influence the questions we ask about system stability and help to define the ice-sheet processes that are relevant on different time-scales. Here, we illustrate the contribution of cosmogenic isotope analysis of exposed bedrock surfaces and marine geophysical surveying to the understanding of Antarctic ice sheet evolution on a range of time-scales. In the Dry Valleys of East Antarctica, 3He dating of subglacial flood deposits that are now exposed on mountain summits provide evidence of an expanded and thicker Mid-Miocene ice sheet. The survival of surface boulders for approximately 14 Myr, the oldest yet measured, demonstrates exceptionally low rates of subsequent erosion and points to the persistence and stability of the dry polar desert climate since that time. Increasingly, there are constraints on West Antarctic ice sheet fluctuations during Quaternary glacial cycles. In the Sarnoff Mountains of Marie Byrd Land in West Antarctica, 10Be and 26Al cosmogenic isotope analysis of glacial erratics and bedrock reveal steady thinning of the ice sheet from 10 400 years ago to the present, probably as a result of grounding line retreat. In the Antarctic Peninsula, offshore analysis reveals an extensive ice sheet at the last glacial maximum. Based on radiocarbon dating, deglaciation began by 17 000 cal yr BP and was complete by 9500 cal yr BP. Deglaciation of the west and east sides of the Antarctic Peninsula ice sheet occurred at different times and rates, but was largely complete by the Early Holocene. At that time ice shelves were less extensive on the west side of the Antarctic Peninsula than they are today. The message from the past is that individual glacier drainage basins in Antarctica respond in different and distinctive ways to global climate change, depending on the link between regional topography and climate setting.