The Last Glacial Maximum Clark, Peter U.; Dyke, Arthur S.; Shakun, Jeremy D. ...
Science (American Association for the Advancement of Science),
08/2009, Letnik:
325, Številka:
5941
Journal Article
Recenzirano
We used 5704 ¹⁴C, ¹⁰Be, and ³He ages that span the interval from 10,000 to 50,000 years ago (10 to 50 ka) to constrain the timing of the Last Glacial Maximum (LGM) in terms of global ice-sheet and ...mountain-glacier extent. Growth of the ice sheets to their maximum positions occurred between 33.0 and 26.5 ka in response to climate forcing from decreases in northern summer insolation, tropical Pacific sea surface temperatures, and atmospheric CO₂. Nearly all ice sheets were at their LGM positions from 26.5 ka to 19 to 20 ka, corresponding to minima in these forcings. The onset of Northern Hemisphere deglaciation 19 to 20 ka was induced by an increase in northern summer insolation, providing the source for an abrupt rise in sea level. The onset of deglaciation of the West Antarctic Ice Sheet occurred between 14 and 15 ka, consistent with evidence that this was the primary source for an abrupt rise in sea level ∼14.5 ka.
We present a revised and extended high Arctic air temperature reconstruction from a single proxy that spans the past ∼12,000 y (up to 2009 CE). Our reconstruction from the Agassiz ice cap (Ellesmere ...Island, Canada) indicates an earlier and warmer Holocene thermal maximum with early Holocene temperatures that are 4–5 °C warmer compared with a previous reconstruction, and regularly exceed contemporary values for a period of ∼3,000 y. Our results show that air temperatures in this region are now at their warmest in the past 6,800–7,800 y, and that the recent rate of temperature change is unprecedented over the entire Holocene. The warmer early Holocene inferred from the Agassiz ice core leads to an estimated ∼1 km of ice thinning in northwest Greenland during the early Holocene using the Camp Century ice core. Ice modeling results show that this large thinning is consistent with our air temperature reconstruction. The modeling results also demonstrate the broader significance of the enhanced warming, with a retreat of the northern ice margin behind its present position in the mid Holocene and a ∼25% increase in total Greenland ice sheet mass loss (∼1.4 m sea-level equivalent) during the last deglaciation, both of which have implications for interpreting geodetic measurements of land uplift and gravity changes in northern Greenland.
The North American Ice Sheet Complex played a major role in global sea level fluctuations during the Late Quaternary but our knowledge of its dynamics is based mostly on its demise from the Last ...Glacial Maximum (LGM), a period characterised by non-linear behaviour in the form of punctuated ice margin recession, episodic ice streaming and major shifts in the location of ice divides. In comparison, knowledge of the pre-LGM ice complex is poorly constrained, largely because of the fragmentary nature of the evidence relating to ice sheet build-up. In this paper, we explore the inception and growth of ice (120–20 ka) using a glacial systems model which has been calibrated against a large and diverse set of data relating to the deglacial interval. We make use of calibration data prior to the LGM but its scarcity introduces greater uncertainty, which is partly alleviated by our large ensemble analysis. Results suggest that, following the last interglaciation (Oxygen Isotope Stage: OIS 5e), the ice complex initiated over the north-eastern Canadian Arctic and in the Cordillera within a few thousand years. It then underwent rapid growth to an OIS 5 maximum at ∼110 ka (5d) and covered ∼70% of the area occupied by the LGM ice cover (although only 30% by volume). An OIS 5 minimum is modelled at ∼80 ka (5a), before a second phase of rapid growth at the start of OIS 4, which culminated in a large ice complex at ∼65 ka (almost as large as at the LGM). Subsequent deglaciation was rapid (maximum modelled sea level contribution of >16 cm per century) and resulted in an OIS 3 minimum between ca 55–60 ka. Thereafter, the ice complex grew towards its LGM configuration, interrupted by several phases of successively less significant mass loss. Our results support and extend previous inferences based on geological evidence and reinforce the notion of a highly dynamic pre-LGM ice complex (e.g. with episodes of ±10 s m of eustatic sea level equivalent in <5 ka). Consistent with previous modelling, the fraction of warm-based ice increases towards the LGM from <20 to >50%, but even the thin OIS 5 ice sheets exhibit fast flow features (several 1000 m a−1) in major topographic troughs. Notwithstanding the severe limitations imposed by the use of the ‘shallow-ice approximation’, we note that most fast flow-features generated prior to the LGM correspond to the location of ‘known’ ice streams during deglaciation, i.e. in major topographic troughs and over soft sediments at the southern and western margins. Moreover, the modelled flux of these ‘ice streams’ (sensu lato), appears to be non-linearly scaled to ice sheet volume, i.e. there is no evidence that decay phases were associated with significantly increased ice stream activity. This hypothesis requires testing using a model with higher-order physics and future modelling would also benefit from additional pre-LGM constraints (e.g. dated ice free/margin positions) to help reduce and quantify uncertainties.
► We present numerical modelling of the North American Ice Sheet from 120 to 20 ka. ► Early inception in the north-eastern Canadian Arctic from ca 118 ka. ► Rapid growth to a large but thin OIS 5d ice sheet at 110 ka, followed by shrinkage. ► Rapid growth and decay of major OIS 4 ice sheet, almost as large as LGM ice. ► Find a scaling between overall ice stream flux and ice volume from 120 to 20 ka.
Past deglacial ice sheet reconstructions have generally relied upon discipline-specific constraints with no attention given to the determination of objective confidence intervals. Reconstructions ...based on geophysical inversion of relative sea level (RSL) data have the advantage of large sets of proxy data but lack ice-mechanical constraints. Conversely, reconstructions based on dynamical ice sheet models are glaciologically self-consistent, but depend on poorly constrained climate forcings and sub-glacial processes.
As an example of a much better constrained methodology that computes explicit error bars, we present a distribution of high-resolution glaciologically-self-consistent deglacial histories for the North American ice complex calibrated against a large set of RSL, marine limit, and geodetic data. The history is derived from ensemble-based analyses using the 3D MUN glacial systems model and a high-resolution ice-margin chronology derived from geological and geomorphological observations. Isostatic response is computed with the VM5a viscosity structure. Bayesian calibration of the model is carried out using Markov Chain Monte Carlo methods in combination with artificial neural networks trained to the model results. The calibration provides a posterior distribution for model parameters (and thereby modeled glacial histories) given the observational data sets that takes data uncertainty into account. Final ensemble results also account for fits between computed and observed strandlines and marine limits.
Given the model (including choice of calibration parameters), input and constraint data sets, and VM5a earth rheology, we find the North American contribution to mwp1a was likely between 9.4 and 13.2m eustatic over a 500year interval. This is more than half of the total 16 to 26m meltwater pulse over 500 to 700years (with lower values being more probable) indicated by the Barbados coral record (Fairbanks, 1989; Peltier and Fairbanks, 2006) if one assumes a 5meter living range for the Acropora Palmata coral. 20ka ice volume for North America was likely 70.1±2.0m eustatic, or about 60% of the total contribution to eustatic sea level change. We suspect that the potentially most critical unquantified uncertainties in our analyses are those related to model structure (especially climate forcing), deglacial ice margin chronology, and earth rheology.
► New data-calibrated glaciological reconstruction of last North American deglaciation. ► Bayesian calibration of glaciological model against large and diverse set of data. ► Detailed uncertainty estimates are computed. ► Calibrated North American contribution to meltwater pulse 1a is 9.4 to 13.2m. ► Calibrated North American 20ka ice volume is 68.1 to 72.1m eustatic equivalent.
Long-term rates of subaerial and episodic subglacial erosion by predominately cold-based ice cover are determined for tors on weathered plateaus on Cumberland Peninsula. By measuring terrestrial ...cosmogenic nuclide concentrations in differentially weathered surfaces on a given tor, we reconstruct the complex exposure and erosion history involving recurring cold-based ice cover. The style and rate of subaerial and subglacial erosion at multiple tor sites on Cumberland Peninsula are assessed with a Monte Carlo approach that computes plausible exposure histories based on a proxy record of global ice volume. Constant subaerial erosion rates by weathering are low (<2 mm ka−1), corroborated by nuclide concentrations measured on two tors located on coastal ridge crests that have likely never been glaciated. Summit plateaus intermittently covered by cold-based ice throughout the Quaternary have experienced episodic subglacial erosion by plucking of fractured bedrock with rates between 1 and 16 mm ka−1. Variation of episodic erosion rates is associated with topographic position of the sampled tors and bedrock fracture density. Most of the tors were last glacially plucked in pre-ultimate glaciations, not during the Wisconsinan glaciation. Furthermore, the new approach provides evidence for the extent of ice coverage during the late Wisconsinan, which is significant if no erratics are available for exposure dating. Despite late Pleistocene intervals of ice cover and glacial plucking, tor-studded landscapes of Cumberland Peninsula are of considerable antiquity.
•Protocol for determining rates of episodic erosion by glacial plucking.•Modelling of complex exposure histories with recurring cold-based ice cover.•Timing of last plucking provides maximum-limiting constrains on last ice cover.•Total landscape lowering rates comparable to long-term exhumation rates.
An ice sheet model was constrained to reconstruct the evolution of the Greenland Ice Sheet (GrIS) from the Last Glacial Maximum (LGM) to present to improve our understanding of its response to ...climate change. The study involved applying a glaciological model in series with a glacial isostatic adjustment and relative sea-level (RSL) model. The model reconstruction builds upon the work of Simpson et al. (2009) through four main extensions: (1) a larger constraint database consisting of RSL and ice extent data; model improvements to the (2) climate and (3) sea-level forcing components; (4) accounting for uncertainties in non-Greenland ice. The research was conducted primarily to address data-model misfits and to quantify inherent model uncertainties with the Earth structure and non-Greenland ice. Our new model (termed Huy3) fits the majority of observations and is characterised by a number of defining features. During the LGM, the ice sheet had an excess of 4.7 m ice-equivalent sea-level (IESL), which reached a maximum volume of 5.1 m IESL at 16.5 cal ka BP. Modelled retreat of ice from the continental shelf progressed at different rates and timings in different sectors. Southwest and Southeast Greenland began to retreat from the continental shelf by ∼16 to 14 cal ka BP, thus responding in part to the Bølling-Allerød warm event (c. 14.5 cal ka BP); subsequently ice at the southern tip of Greenland readvanced during the Younger Dryas cold event. In northern Greenland the ice retreated rapidly from the continental shelf upon the climatic recovery out of the Younger Dryas to present-day conditions. Upon entering the Holocene (11.7 cal ka BP), the ice sheet soon became land-based. During the Holocene Thermal Maximum (HTM; 9-5 cal ka BP), air temperatures across Greenland were marginally higher than those at present and the GrIS margin retreated inland of its present-day southwest position by 40–60 km at 4 cal ka BP which produced a deficit volume of 0.16 m IESL relative to present. In response to the HTM warmth, our optimal model reconstruction lost mass at a maximum centennial rate of c. 103.4 Gt/yr. Our results suggest that remaining data-model discrepancies are affiliated with missing physics and sub-grid processes of the glaciological model, uncertainties in the climate forcing, lateral Earth structure, and non-Greenland ice (particularly the North American component). Finally, applying the Huy3 Greenland reconstruction with our optimal Earth model we generate present-day uplift rates across Greenland due to past changes in the ocean and ice loads with explicit error bars due to uncertainties in the Earth structure. Present-day uplift rates due to past changes are spatially variable and range from 3.5 to −7 mm/a (including Earth model uncertainty).
•We build on the Simpson et al. (2009) study and model the Greenland Deglaciation.•A constraint dataset of RSL, ice extent, and ice-core observations is established.•We data-constrain a glaciological model and GIA model of RSL change.•We identify a best fitting Greenland reconstruction with respect to the observations.•We quantify the impact of NAIC uncertainties on the Greenland chronology.
History of sea ice in the Arctic Polyak, Leonid; Alley, Richard B.; Andrews, John T. ...
Quaternary science reviews,
07/2010, Letnik:
29, Številka:
15
Journal Article
Recenzirano
Arctic sea-ice extent and volume are declining rapidly. Several studies project that the Arctic Ocean may become seasonally ice-free by the year 2040 or even earlier. Putting this into perspective ...requires information on the history of Arctic sea-ice conditions through the geologic past. This information can be provided by proxy records from the Arctic Ocean floor and from the surrounding coasts. Although existing records are far from complete, they indicate that sea ice became a feature of the Arctic by 47
Ma, following a pronounced decline in atmospheric pCO
2 after the Paleocene–Eocene Thermal Optimum, and consistently covered at least part of the Arctic Ocean for no less than the last 13–14 million years. Ice was apparently most widespread during the last 2–3 million years, in accordance with Earth's overall cooler climate. Nevertheless, episodes of considerably reduced sea ice or even seasonally ice-free conditions occurred during warmer periods linked to orbital variations. The last low-ice event related to orbital forcing (high insolation) was in the early Holocene, after which the northern high latitudes cooled overall, with some superimposed shorter-term (multidecadal to millennial-scale) and lower-magnitude variability. The current reduction in Arctic ice cover started in the late 19th century, consistent with the rapidly warming climate, and became very pronounced over the last three decades. This ice loss appears to be unmatched over at least the last few thousand years and unexplainable by any of the known natural variabilities.
In 2008, four decades since Meldgaard's work at Alarniq—the type site for Dorset culture—Savelle and Dyke returned to resurvey the site. Archaeological investigations continued in 2015 and 2017 as ...part of the Foxe Basin Archaeological Project, when Howse conducted further surveys, excavated six semi-subterranean dwellings and two associated middens, and tested five additional features. The new site map and radiocarbon sequence have significantly changed our understanding of site use and beach-level chronology at Alarniq. The number of dwellings varies across the beach ridges, suggesting populations fluctuated throughout the site's use (2,700–800 cal BP). However, the new radiocarbon analyses also indicate that dwellings between 14.5 and 21.5 m above sea level are the same general age and that paleodemography at Alarniq is less straightforward than suggested by the number of features per beach ridge. It appears that ideal house construction location is a stronger indicator of the placement of winter houses at the site than proximity to the shoreline. We suggest this is largely related to site seasonality. These new data have significant implications for our understanding of current Dorset artifact typologies that have largely been developed using the material Meldgaard recovered at the site. En nous appuyant sur les précédentes recherches paléodémographiques régionales dans l'aire centrale du bassin de Foxe, nous présentons ici les données des niveaux d'occupation dorsétiens du site-clé d'Alarniq. Le site a de nouveau été prospecté en 2015 et en 2017. Six habitations semi-souterraines et deux dépotoirs associés ont été fouillés et d'autres éléments ont été sondés. L'occupation dorsétienne s’étend sur 5,5 km à travers une succession de paléoplages localisées entre 8 et 24 mètres au-dessus du niveau de la mer. Elle est presque exclusivement composée d'habitations semi-souterraines qui furent vraisemblablement occupées durant la saison froide. La variation quantitative des occupations selon les paléoplages à différentes élévations semblait indiquer jusqu’à maintenant des fluctuations dans l'utilisation du site à travers le temps. Cependant, de nouvelles datations radiométriques indiquent plutôt que les habitations entre 14,5 et 21,5 mètres d'altitude sont en fait approximativement du même âge. Ainsi l'interprétation paléodémographique d'Alarniq n'est pas aussi simple que ce qui avait été suggéré auparavant en calculant le nombre de structures par paléoplages. Nous présentons ici une nouvelle série de dates radiométriques pour Alarniq et discutons du fait que la proximité du rivage comme critère pour l'emplacement idéal de construction ne prédomine pas lorsqu'il s'agit d'occupations hivernales.
Radiocarbon dating of fossil flora and fauna collected along receding cold-based ice caps and cold-based sections of polythermal glaciers on Cumberland Peninsula reveal insights into Neoglacial ice ...expansion and late Holocene ice dynamics. The taphonomic advantages of subfossilized moss were exploited to precisely document regional expansions of ice caps through the late Holocene. When compared with climate proxies and records of volcanic eruptions, the moss radiocarbon age distributions indicate i) onset of Neoglaciation shortly after 5 ka, concomitant with increased sea ice cover, ii) intensification of ice expansion between 1.9 and 1.1 ka, followed by halt of ice growth, or ice recession during the Medieval Warm Period, and iii) renewed ice expansion after 0.8 ka, in response to cooling related to a combination of large volcanic eruptions and low solar activity. Overall, the observations support a model of near-instantaneous glacial response to regional climate controls and that these responses were synchronous throughout eastern Canadian Arctic and possibly eastern Greenland.
•Radiocarbon dating of fossil flora and fauna reveal late Holocene ice expansions.•Onset of Neoglaciation after 5 ka BP is concordant with increased sea ice cover.•Two millennial to centennial hiatuses, the last corresponding to MWP.•Ice expansion during LIA correlates with large volcanic eruptions and low solar activity.
This paper presents the first detailed record of Paleoeskimo occupation history of Foxe Basin, Nunavut, Arctic Canada, the traditional Paleoeskimo “core area.” Rather than continuous, stable ...occupations from approximately 4000–1000 B.P. traditionally assumed for the core area, the region has undergone a series of demographic oscillations, including several instances of abandonment of key areas, most notably Igloolik. The Foxe Basin demographic trends are reminiscent of Paleoeskimo “boom and bust” cycles recognized elsewhere, but show no consistent chronological pattern either within Foxe Basin or inter-regionally. Equally important, our results bear on the critical question of the Pre-Dorset to Dorset transition. Rather than having been a gradual in situ process centered within the core area, the demographic patterns, including the abrupt and widespread appearance of semi-subterranean dwellings during earliest Dorset, are consistent with newly arrived populations from outside of Foxe Basin. While there is no obvious “parent” culture to Dorset within the Eastern Arctic, it is suggested that a Western Arctic origin, specifically Norton Culture, invoking to some extent Jorgen Meldgaard’s “smell of the forest”, may have played a significant role.
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