The Last Glacial Termination Denton, G. H.; Anderson, R. F.; Toggweiler, J. R. ...
Science (American Association for the Advancement of Science),
06/2010, Letnik:
328, Številka:
5986
Journal Article
Recenzirano
A major puzzle of paleoclimatology is why, after a long interval of cooling climate, each late Quaternary ice age ended with a relatively short warming leg called a termination. We here offer a ...comprehensive hypothesis of how Earth emerged from the last global ice age. A prerequisite was the growth of very large Northern Hemisphere ice sheets, whose subsequent collapse created stadial conditions that disrupted global patterns of ocean and atmospheric circulation. The Southern Hemisphere westerlies shifted poleward during each northern stadial, producing pulses of ocean upwelling and warming that together accounted for much of the termination in the Southern Ocean and Antarctica. Rising atmospheric CO₂ during southern upwelling pulses augmented warming during the last termination in both polar hemispheres.
A case is made that seasonality switches dominated by wintertime were instrumental in abrupt climate changes in the North Atlantic region during the last glaciation and into the Holocene. The primary ...evidence comes from mismatches between mean annual temperatures from Greenland ice cores in comparison with snowline changes in East Greenland, northern Europe, and North America. The most likely explanation is a shutdown (or reduction in strength) of the conveyor. This allows the spread of winter sea ice across the North Atlantic, thus causing the northern region to experience much colder winters. Because they mimic the Greenland temperature rather than the snowline signal, changes in the Atlantic Intertropical Convergence Zone and the Asian monsoon may also share a winter linkage with Greenland. Thus the paleoclimate record is consistent with the notion that a huge continental sector of the Northern Hemisphere, stretching from Greenland to Asia, was close to an extreme winter threshold during much of the last glaciation. Winter climate crossed this threshold repeatedly, with marked changes in seasonality that may well have amplified and propagated a signal of abrupt change throughout the hemisphere and into the tropics.
The history of deglaciation of the West Antarctic Ice Sheet (WAIS) gives clues about its future. Southward grounding-line migration was dated past three locations in the Ross Sea Embayment. Results ...indicate that most recession occurred during the middle to late Holocene in the absence of substantial sea level or climate forcing. Current grounding-line retreat may reflect ongoing ice recession that has been under way since the early Holocene. If so, the WAIS could continue to retreat even in the absence of further external forcing.
We present a comprehensive 10Be chronology for Holocene moraines in the Lago Argentino basin, on the east side of the South Patagonian Icefield. We focus on three different areas, where prior studies ...show ample glacier moraine records exist because they were formed by outlet glaciers sensitive to climate change. The 10Be dated records are from the Lago Pearson, Herminita Península-Brazo Upsala, and Lago Frías areas, which span a distance of almost 100 km adjacent to the modern Icefield. New 10Be ages show that expanded glaciers and moraine building events occurred at least at 6120 ± 390 (n = 13), 4450 ± 220 (n = 7), 1450 or 1410 ± 110 (n = 18), 360 ± 30 (n = 5), and 240 ± 20 (n = 8) years ago. Furthermore, other less well-dated glacier expansions of the Upsala Glacier occurred between ~1400 and ∼1000 and ∼2300 and ∼2000 years ago. The most extensive glaciers occurred over the interval from ∼6100 to ∼4500 years ago, and their margins over the last ∼600 years were well within and lower than those in the middle Holocene. The 10Be ages agree with 14C-limiting data for the glacier histories in this area.
We then link southern South American, adjacent South Atlantic, and other Southern Hemisphere records to elucidate broader regional patterns of climate and their possible causes. In the early Holocene, a far southward position of the westerly winds fostered warmth, small Patagonian glaciers, and reduced sea ice coverage over the South Atlantic. Although we infer a pronounced southward displacement of the westerlies during the early Holocene, these conditions did not occur throughout the southern mid-high latitudes, an important exception being over the southwest Pacific sector. Subsequently, a northward locus and/or expansion of the winds over the Patagonia-South Atlantic sector promoted the largest glaciers between ∼6100 and ∼4500 years ago and greatest sea ice coverage. Over the last few millennia, the South Patagonian Icefield has experienced successive century-scale advances superimposed on a long-term net decrease in size. Our findings indicate that glaciers and sea ice in the Patagonian-South Atlantic sector of the Southern Hemisphere did not achieve their largest Holocene extents over the last millennium. We conclude that a pattern of more extensive Holocene ice prior to the last millennium is characteristic of the Southern Hemisphere middle latitudes, which differs from the glacier history traditionally thought for the Northern Hemisphere.
•10Be dating of Holocene moraines formed by the South Patagonian Icefield.•The most extensive outlet glaciers occurred from ∼6100 to ∼4500 years ago.•Linking Patagonian and South Atlantic records reveals broad paleoclimate patterns.•Extensive Holocene before 1000 years ago is characteristic of the Southern Hemisphere.
We report radiocarbon dates that constrain the timing of the deposition of the late-glacial Puerto Bandera moraine system alongside the western reaches of Lago Argentino adjacent to the Southern ...Patagonian Icefield. Close maximum-limiting radiocarbon ages (
n = 11) for glacier advance into the outer moraines, with a mean value of 11,100 ± 60
14C yrs BP (12,990 ± 80 cal yrs BP), were obtained from wood in deformation (soft) till exposed beneath flow and lodgment till in Bahía del Quemado on the northeast side of Brazo Norte (North Branch) of western Lago Argentino. Other exposures of this basal deformation till in Bahía del Quemado reveal incorporated clasts of peat, along with larger inclusions of deformed glaciofluvial and lacustrine deposits. Radiocarbon dates of wood included in these reworked peat clasts range from 11,450 ± 45
14C yrs BP to 13,450 ± 150
14C yrs BP (13,315 ± 60 to 16,440 ± 340 cal yrs BP). The implication is that, during this interval, glacier fronts were situated inboard of the Puerto Bandera moraines, with the peat clasts and larger proglacial deposits being eroded and then included in the basal till during the Puerto Bandera advance.
Minimum-limiting radiocarbon ages for ice retreat come from basal peat in cores sampled in spillways and depressions generated during abandonment of the Puerto Bandera moraines. Glacier recession and subsequent plant colonization were initiated close behind different frontal sectors of these moraines prior to: 10,750 ± 75
14C yrs BP (12,660 ± 70 cal yrs BP) east of Brazo Rico, 10,550 ± 55
14C yrs BP (12,490 ± 80 cal yrs BP) in Peninsula Avellaneda, and 10,400 ± 50
14C yrs BP (12,280 ± 110 cal yrs BP) in Bahía Catalana. In addition, a radiocarbon date indicates that by 10,350 ± 45
14C yrs BP (12,220 ± 110 cal yrs BP), the Brazo Norte lobe (or former Upsala Glacier) had receded well up the northern branch of Lago Argentino, to a position behind the Herminita moraines. Furthermore, glacier termini had receded to just outboard of the outer Holocene moraines at Lago Frías and Lago Pearson (Anita) prior to 10,400 ± 40
14C yrs BP (12,270 ± 100 cal yrs BP) and 9040 ± 45
14C yrs BP (10,210 ± 50 cal yrs BP), respectively. The most extensive recession registered during the early Holocene was in Agassiz Este Valley, where the Upsala Glacier had pulled back behind the outer Holocene moraine, reaching close to the present-day glacier terminus before 8290 ± 40
14C yrs BP (9300 ± 80 cal yrs BP).
The radiocarbon-dated fluctuations of the Lago Argentino glacier in late-glacial time, given here, are in accord with changes in ocean mixed layer properties, predominately temperature, derived from the isotopic record given here of ODP Core 1233, taken a short distance off shore of the Chilean Lake District. It also matches recently published chronologies of late-glacial moraines in the Southern Alps of New Zealand on the opposite side of the Pacific Ocean from Lago Argentino. Finally, the timing of the late-glacial reversal of the Lago Argentino glacier fits the most recent chronology for the culmination of the Antarctic Cold Reversal (ACR) in the deuterium record of the EPICA Dome C ice core from high on the East Antarctic Plateau. Therefore, we conclude that the climate signature of the ACR was widespread in both the ocean and the atmosphere over at least the southern quarter of the globe.
► We present a radiocarbon chronology of late-glacial glacier behavior in Patagonia. ► Glacier advance occurred ∼13,000 yrs ago. ► Ice receded to near-present-day margins by ∼12,000 yrs ago. ► South Patagonian glaciers registered Antarctic Cold Reversal.
A reconstruction of the Antarctic Ice Sheet at the Last Glacial Maximum (LGM), constrained by geologic data, shows expansion across continental shelves accompanied by little inland change. This added ...peripheral ice volume contributed 14
m to the lowering of eustatic sea level at the LGM. Of the total expansion, one-third took place in East Antarctica and two-thirds in West Antarctica.
We present chironomid and pollen records from the Huelmo site (∼41°30′S), NW Patagonia, to examine in detail the timing and structure of climate changes during the Last Glacial Termination in the ...southern mid-latitudes. The chironomid record has the highest temporal and taxonomic resolution for this critical interval, and constitutes the first account of midge faunas at the culmination of the Last Glacial Maximum (LGM) for the region. The chironomid record suggests cold and wet conditions during the LGM, followed by deglacial warming between 17.6 and 16.8
cal
kyr
BP. Relatively warm conditions prevailed between ∼15–14
cal
kyr
BP, followed by a reversal in trend with cooling pulses at ∼14 and 13.5
cal
kyr
BP, and warming at the beginning of the Holocene. Cool-temperate conditions prevailed during the Huelmo Mascardi Cold Reversal (HMCR) which, according to chironomid data, exhibits a wet phase (13.5–12.8
cal
kyr
BP) followed by a conspicuous drier phase (12.8–11.5
cal
kyr
BP). The chironomid and pollen records from the Huelmo site indicate step-wise deglacial warming beginning at 17.6
cal
kyr
BP, in agreement with other paleoclimate records from NW Patagonia and isotopic signals from Antarctic ice cores. Peak warmth during the Last Glacial Termination was achieved by ∼14.5
cal
kyr
BP, followed by a cooling trend that commenced during the Antarctic Cold Reversal, which later intensified and persisted during the HMCR (13.5–11.5
cal
kyr
BP). We observe a shift toward drier conditions at ∼12.8
cal
kyr
BP superimposed upon the HMCR, coeval with intense fire activity and vegetation disturbance during Younger Dryas time.
A corrected radiocarbon age of 11,050 ± 14 years before present for an advance of the Franz Josef Glacier to the Waiho Loop terminal moraine on the western flank of New Zealand's Southern Alps shows ...that glacier advance on a South Pacific island was synchronous with initiation of the Younger Dryas in the North Atlantic region. Hence, cooling at the beginning of the Younger Dryas probably reflects global rather than regional forcing. The source for Younger Dryas climatic cooling may thus lie in the atmosphere rather than in a North Atlantic thermohaline switch.
The chronology of late-glacial and Holocene ice fluctuations in East Greenland is important for documenting glacier response to climate change and for determining the mechanisms behind abrupt climate ...switches. Raised marine sediments and landforms suggest culmination of the late-glacial Milne Land Stade in Schuchert Dal alongside Scoresby Sund before ∼11,900–12,300
cal
yr B.P. Ice in Schuchert Dal apparently receded during the later part of the Younger Dryas, consistent with the idea that this period was characterized by strong seasonality. A stillstand or slight readvance probably interrupted overall recession just prior to 11,000
cal
yr B.P. In many locations, ice was back close to present positions by the start of the Holocene. The most extensive glacier advance of the Holocene occurred during the Little Ice Age and was restricted to within a few kilometers of present-day ice margins.
Records of past variability afford context for evaluating present-day glacier behavior and for testing hypotheses of climate change. Here, we use 69 radiocarbon dates of wood and other organic ...materials in association with glacial deposits to document the behavior of Ventisqueros Marinelli and Brooks in Cordillera Darwin over the past ∼17,000 years. Recession from the last glacial maximum was early, with most occurring prior to 17,000 yr BP. Any glacial resurgence during the Antarctic Cold Reversal must have terminated within the bounds of Holocene ice fluctuations. By early Holocene time, Ventisquero Marinelli had retreated such that it was no more extensive than it was in AD 1992. We identify several subsequent glacier readvances, but also note long periods of restricted ice extent, particularly in the mid-Holocene. There were times when Holocene glaciers must have been smaller than at present. Our new record from Marinelli shows similarities to other reconstructions of Holocene glacier variation from southern South America, such as at Mt. Sarmiento and the South Patagonian Icefield, suggesting an underlying climate signal. The overall implication is of substantial early Holocene deglaciation followed by repeated advances interspersed with periods of recession when ice extent was smaller than at present. This general pattern of glacier behavior appears to differ from that of New Zealand's Southern Alps and points to the value of a geographic spread of datasets to elucidate the pattern of Southern Hemisphere climate during the Holocene.
•69 14C dates of organic remains document Holocene history of Cordillera Darwin.•Ice was within range of Holocene fluctuations by the Antarctic Cold Reversal.•Advances occurred at ≤6650, ∼≤2430–3800, ∼1300, ∼750, and ∼480 yr BP.•There were long periods of restricted ice in the early and middle Holocene.•Fluctuations of southern South American glaciers differ from those in New Zealand.
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