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Measurements of cosmic-ray-produced 10 Be and 26 Al in a bedrock core from beneath the summit of the Greenland Ice Sheet show that Greenland was nearly ice-free for extended periods under Pleistocene climate forcing. A nearly ice-free Greenland in the Pleistocene The Greenland Ice Sheet is a main contributor to modern sea-level rise, but its stability during past warm periods is uncertain, thus compromising our ability to robustly predict future rates and magnitudes of ice loss. Joerg Schaefer and colleagues present cosmogenic isotope evidence, from bedrock samples from beneath an ice core near the summit of the ice sheet, to show that the Greenland Ice Sheet was largely ice-free at some point during the Pleistocene. The data cannot constrain the time or duration of ice-free conditions, and seem incompatible with our current understanding of ice-sheet behaviour. Nevertheless, alternative explanations for the cosmogenic data are lacking, suggesting that our understanding of the response of the Greenland Ice Sheet to past warmth remains incomplete. The Greenland Ice Sheet (GIS) contains the equivalent of 7.4 metres of global sea-level rise 1 . Its stability in our warming climate is therefore a pressing concern. However, the sparse proxy evidence of the palaeo-stability of the GIS means that its history is controversial (compare refs 2 and 3 to ref. 4 ). Here we show that Greenland was deglaciated for extended periods during the Pleistocene epoch (from 2.6 million years ago to 11,700 years ago), based on new measurements of cosmic-ray-produced beryllium and aluminium isotopes ( 10 Be and 26 Al) in a bedrock core from beneath an ice core near the GIS summit. Models indicate that when this bedrock site is ice-free, any remaining ice is concentrated in the eastern Greenland highlands and the GIS is reduced to less than ten per cent of its current volume. Our results narrow the spectrum of possible GIS histories: the longest period of stability of the present ice sheet that is consistent with the measurements is 1.1 million years, assuming that this was preceded by more than 280,000 years of ice-free conditions. Other scenarios, in which Greenland was ice-free during any or all Pleistocene interglacials, may be more realistic. Our observations are incompatible with most existing model simulations that present a continuously existing Pleistocene GIS. Future simulations of the GIS should take into account that Greenland was nearly ice-free for extended periods under Pleistocene climate forcing.

Constraining the response time of the climate system to changes in North Atlantic Deep Water (NADW) formation is fundamental to improving climate and Atlantic Meridional Overturning Circulation predictability. Here we report a new synchronization of terrestrial, marine, and ice-core records, which allows the first quantitative determination of the response time of North Atlantic climate to changes in high-latitude NADW formation rate during the last deglaciation. Using a continuous record of deep water ventilation from the Nordic Seas, we identify a ∼400-year lead of changes in high-latitude NADW formation ahead of abrupt climate changes recorded in Greenland ice cores at the onset and end of the Younger Dryas stadial, which likely occurred in response to gradual changes in temperature- and wind-driven freshwater transport. We suggest that variations in Nordic Seas deep-water circulation are precursors to abrupt climate changes and that future model studies should address this phasing. The response time of North Atlantic climate to changes in high-latitude deep-water formation during the last deglaciation is still unclear. Here the authors show that gradual changes in Nordic Seas deep-water circulation systematically lead ahead of abrupt regional climate shifts by ~400 years.

Understanding the history of the Greenland Ice Sheet (GrIS) is critical for determining its sensitivity to warming and contribution to sea level; however, that history is poorly known before the last interglacial. Most knowledge comes from interpretation of marine sediment, an indirect record of past ice-sheet extent and behavior. Subglacial sediment and rock, retrieved at the base of ice cores, provide terrestrial evidence for GrIS behavior during the Pleistocene. Here, we use multiple methods to determine GrIS history from subglacial sediment at the base of the Camp Century ice core collected in 1966. This material contains a stratigraphic record of glaciation and vegetation in northwestern Greenland spanning the Pleistocene. Enriched stable isotopes of pore-ice suggest precipitation at lower elevations implying ice-sheet absence. Plant macrofossils and biomarkers in the sediment indicate that paleo-ecosystems from previous interglacial periods are preserved beneath the GrIS. Cosmogenic 26Al/10Be and luminescence data bracket the burial of the lower-most sediment between <3.2 ± 0.4 Ma and >0.7 to 1.4 Ma. In the upper-most sediment, cosmogenic 26Al/10Be data require exposure within the last 1.0 ± 0.1 My. The unique subglacial sedimentary record from Camp Century documents at least two episodes of ice-free, vegetated conditions, each followed by glaciation. The lower sediment derives from an Early Pleistocene GrIS advance. 26Al/10Be ratios in the upper-most sediment match those in subglacial bedrock from central Greenland, suggesting similar ice-cover histories across the GrIS. We conclude that the GrIS persisted through much of the Pleistocene but melted and reformed at least once since 1.1 Ma.

We present the WD2014 chronology for the upper part (0–2850 m; 31.2 ka BP) of the West Antarctic Ice Sheet (WAIS) Divide (WD) ice core. The chronology is based on counting of annual layers observed in the chemical, dust and electrical conductivity records. These layers are caused by seasonal changes in the source, transport, and deposition of aerosols. The measurements were interpreted manually and with the aid of two automated methods. We validated the chronology by comparing to two high-accuracy, absolutely dated chronologies. For the Holocene, the cosmogenic isotope records of 10Be from WAIS Divide and 14C for IntCal13 demonstrated that WD2014 was consistently accurate to better than 0.5 % of the age. For the glacial period, comparisons to the Hulu Cave chronology demonstrated that WD2014 had an accuracy of better than 1 % of the age at three abrupt climate change events between 27 and 31 ka. WD2014 has consistently younger ages than Greenland ice core chronologies during most of the Holocene. For the Younger Dryas–Preboreal transition (11.595 ka; 24 years younger) and the Bølling–Allerød Warming (14.621 ka; 7 years younger), WD2014 ages are within the combined uncertainties of the timescales. Given its high accuracy, WD2014 can become a reference chronology for the Southern Hemisphere, with synchronization to other chronologies feasible using high-quality proxies of volcanism, solar activity, atmospheric mineral dust, and atmospheric methane concentrations.

The geologic record of mountain glaciations is a robust indicator of terrestrial paleoclimate change. During the last glaciation, mountain ranges across the western US hosted glaciers while the Cordilleran and Laurentide ice sheets flowed to the west and east of the continental divide, respectively. Records detailing the chronologies and paleoclimate significance of these ice advances have been developed for many sites across North America. However, relatively few glacial records have been developed for mountain glaciers in the northern Rocky Mountains near former ice sheet margins. Here, we report cosmogenic beryllium-10 surface exposure ages and numerical glacier modeling results, which show that mountain glaciers in the northern Rockies abandoned terminal moraines after the end of the global Last Glacial Maximum around 17–18 ka and could have been sustained by −10 to −8.5 ∘C temperature depressions relative to modern assuming similar or less than modern precipitation. Additionally, we present a deglacial chronology from the northern Rocky Mountains that indicates while there is considerable variability in initial moraine abandonment ages across the Rocky Mountains, the pace of subsequent ice retreat through the late glacial exhibits some regional coherence. Our results provide insight on potential regional mechanisms driving the initiation of and sustained deglaciation in the western US, including rising atmospheric CO2 and ice sheet collapse.

Sometime during the middle to late Holocene (8.2 ka to ∼ 1850–1900 CE), the Greenland Ice Sheet (GrIS) was smaller than its current configuration. Determining the exact dimensions of the Holocene ice-sheet minimum and the duration that the ice margin rested inboard of its current position remains challenging. Contemporary retreat of the GrIS from its historical maximum extent in southwestern Greenland is exposing a landscape that holds clues regarding the configuration and timing of past ice-sheet minima. To quantify the duration of the time the GrIS margin was near its modern extent we develop a new technique for Greenland that utilizes in situ cosmogenic 10Be–14C–26Al in bedrock samples that have become ice-free only in the last few decades due to the retreating ice-sheet margin at Kangiata Nunaata Sermia (n=12 sites, 36 measurements; KNS), southwest Greenland. To maximize the utility of this approach, we refine the deglaciation history of the region with stand-alone 10Be measurements (n=49) and traditional 14C ages from sedimentary deposits contained in proglacial–threshold lakes. We combine our reconstructed ice-margin history in the KNS region with additional geologic records from southwestern Greenland and recent model simulations of GrIS change to constrain the timing of the GrIS minimum in southwest Greenland and the magnitude of Holocene inland GrIS retreat, as well as to explore the regional climate history influencing Holocene ice-sheet behavior. Our10Be–14C–26Al measurements reveal that (1) KNS retreated behind its modern margin just before 10 ka, but it likely stabilized near the present GrIS margin for several thousand years before retreating farther inland, and (2) pre-Holocene 10Be detected in several of our sample sites is most easily explained by several thousand years of surface exposure during the last interglaciation. Moreover, our new results indicate that the minimum extent of the GrIS likely occurred after ∼5 ka, and the GrIS margin may have approached its eventual historical maximum extent as early as∼2 ka. Recent simulations of GrIS change are able to match the geologic record of ice-sheet change in regions dominated by surface mass balance, but they produce a poorer model–data fit in areas influenced by oceanic and dynamic processes. Simulations that achieve the best model–data fit suggest that inland retreat of the ice margin driven by early to middle Holocene warmth may have been mitigated by increased precipitation. Triple10Be–14C–26Al measurements in recently deglaciated bedrock provide a new tool to help decipher the duration of smaller-than-present ice over multiple timescales. Modern retreat of the GrIS margin in southwest Greenland is revealing a bedrock landscape that was also exposed during the migration of the GrIS margin towards its Holocene minimum extent, but it has yet to tap into a landscape that remained ice-covered throughout the entire Holocene.

Isochron burial dating with cosmogenic nuclides 26 Al and 10 Be shows that the skeleton of the australopithecine individual known as ‘Little Foot’ is around 3.67 million years old, coeval with early Australopithecus from East Africa; a manuport dated to 2.18 million years ago from the Oldowan tool assemblage conforms with the oldest age previously suggested by fauna. An early date for 'Little Foot' australopithecine The cave infillings at Sterkfontein in South Africa contain some of the richest assemblages of fossil hominins in the world. The problem with Sterkfontein and many caves like it is that it is notoriously difficult to date such sediments : they accumulate in a haphazard way with many episodes of deposition, erosion and reworking. Darryl Granger et al . use isochron burial dating with cosmogenic nuclides 26 Al and 10 Be to show that the breccia containing the substantially complete skeleton of the australopithecine individual known as 'Little Foot' is around 3.67 million years old, coeval with Australopithecus afarensis ('Lucy') from East Africa. The earliest stone tools from Sterkfontein are dated to around 2.18 million years ago, a similar age to tools from nearby sites such as Swartkrans. The cave infills at Sterkfontein contain one of the richest assemblages of Australopithecus fossils in the world, including the nearly complete skeleton StW 573 (‘Little Foot’) 1 , 2 , 3 , 4 in its lower section, as well as early stone tools 5 , 6 , 7 in higher sections. However, the chronology of the site remains controversial 8 , 9 , 10 , 11 , 12 , 13 , 14 owing to the complex history of cave infilling. Much of the existing chronology based on uranium–lead dating 10 , 11 and palaeomagnetic stratigraphy 8 , 12 has recently been called into question by the recognition that dated flowstones fill cavities formed within previously cemented breccias and therefore do not form a stratigraphic sequence 4 , 14 . Earlier dating with cosmogenic nuclides 9 suffered a high degree of uncertainty and has been questioned on grounds of sediment reworking 10 , 11 , 13 . Here we use isochron burial dating with cosmogenic aluminium-26 and beryllium-10 to show that the breccia containing StW 573 did not undergo significant reworking, and that it was deposited 3.67 ± 0.16 million years ago, far earlier than the 2.2 million year flowstones found within it 10 , 11 . The skeleton is thus coeval with early Australopithecus afarensis in eastern Africa 15 , 16 . We also date the earliest stone tools at Sterkfontein to 2.18 ± 0.21 million years ago, placing them in the Oldowan at a time similar to that found elsewhere in South Africa at Swartkans 17 and Wonderwerk 18 .

Constraining the timing and rate of Laurentide Ice Sheet (LIS) retreat through the northeastern United States is important for understanding the co-evolution of complex climatic and glaciologic events that characterized the end of the Pleistocene epoch. However, no in situ cosmogenic 10Be exposure age estimates for LIS retreat exist through large parts of Connecticut or Massachusetts. Due to the large disagreement between radiocarbon and 10Be ages constraining LIS retreat at the maximum southern margin and the paucity of data in central New England, the timing of LIS retreat through this region is uncertain. Here, we date LIS retreat through south-central New England using 14 new in situ cosmogenic 10Be exposure ages measured in samples collected from bedrock and boulders. Our results suggest ice retreated entirely from Connecticut by 18.3 ± 0.3 ka (n = 3). In Massachusetts, exposure ages from similar latitudes suggest ice may have occupied the Hudson River Valley up to 2 kyr longer (15.2 ± 0.3 ka, average, n = 2) than the Connecticut River Valley (17.4 ± 1.0 ka, average, n = 5). We use these new ages to provide insight about LIS retreat timing during the early deglacial period and to explore the mismatch between radiocarbon and cosmogenic deglacial age chronologies in this region.

The upper Yangtze River flows southward on the southeastern Tibet characterizing by uniquely low and continuous relief. The river makes a sharp turn at Shigu, heading northeast, and forms the first bend of the Yangtze River. Many previous studies assumed southward flow of the ancestral Yangtze River from Shigu to the South China Sea. However, field evidence of southward flow of the paleo‐Yangtze is lacking. In this paper we report our identification, based on detrital zircon U‐Pb age distributions, of a range of fluvial sands left by the paleo‐Yangtze in Tongdian, Madeng and Nanjian basins. Cosmogenic10Be and 26Al burial dating provides burial ages for these fluvial sands from 1.7 to over 8.7 Ma. Rerouting of the Yangtze River therefore occurred within the last 1.7 Ma, postdating the major uplift of the central Tibet. We attribute the rerouting of the Yangtze River to response to activation of the Dali fault system, and in a larger scale, initiation of crustal deformation by clockwise rotation around eastern Himalayan syntaxis 2–4 Ma ago. Reorganization of the Yangtze drainage pattern most likely reflects regional uplift and displacement due to lower crust flowing beneath major faults in the southeastern Tibet and Yunnan. Key Points Fluvial sands found from Tongdian to Nanjian are left by the paleo‐Yangtze River Formation of the first bend of the Yangtze River occurred within the last 1.7 Ma Rerouting of the Yangtze River resulted from activation of the Dali fault system

Surface exposure ages of glacial deposits in the Ford Ranges of western Marie Byrd Land indicate continuous thinning of the West Antarctic Ice Sheet by more than 700 meters near the coast throughout the past 10,000 years. Deglaciation lagged the disappearance of ice sheets in the Northern Hemisphere by thousands of years and may still be under way. These results provide further evidence that parts of the West Antarctic Ice Sheet are on a long-term trajectory of decline. West Antarctic melting contributed water to the oceans in the late Holocene and may continue to do so in the future.