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Glacier Retreat in New Zealand During the Younger Dryas Stadial

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Glacier Retreat in New Zealand During the Younger Dryas Stadial Vol 467 | 9 September 2010 | doi:10.1038/nature09313 LETTERS Glacier retreat in New Zealand during the Younger Dryas stadial Michael R. Kaplan1, Joerg M. Schaefer1,2, George H. Denton3, David J. A. Barrell4, Trevor J. H. Chinn5, Aaron E. Putnam3, Bjørn G. Andersen6, Robert C. Finkel7,8, Roseanne Schwartz1 & Alice M. Doughty9 Millennial-scale cold reversals in the high latitudes of both hemi- weakened intensity of the Asian monsoon15, cooler sea surface tem- spheres interrupted the last transition from full glacial to inter- peratures in the tropical Atlantic16 and increased precipitation in glacial climate conditions. The presence of the Younger Dryas Brazil as far south as 28 uS17. stadial ( 12.9 to 11.7 kyr ago) is established throughout much Thus, the sea surface temperature signature of the YDS can be of the Northern Hemisphere, but the global timing, nature and traced south to at least the northern tropics, and affected precipita- extent of the event are not well established. Evidence in mid to low tion patterns even in the southernmost tropics17. Outstanding ques- latitudes of the Southern Hemisphere, in particular, has remained tions include determining the location of the southern boundary of perplexing1–6. The debate has in part focused on the behaviour of the YDS climate imprint, the nature of the transition to the ‘ACR- mountain glaciers in New Zealand, where previous research has type’ climate that is recorded in southern polar latitudes and the place found equivocal evidence for the precise timing of increased or where this transition occurred. We investigate past atmospheric con- reduced ice extent1–3. The interhemispheric behaviour of the ditions in the southern mid latitudes to find a missing piece of the climate system during the Younger Dryas thus remains an open jigsaw puzzle of late-glacial climate. New Zealand’s oceanic island question, fundamentally limiting our ability to formulate realistic setting in the southwest Pacific Ocean is ideal for testing hypotheses models of global climate dynamics for this time period. Here we concerning late-glacial climate change, far from the influence of show that New Zealand’s glaciers retreated after 13 kyr BP, at the Northern Hemisphere ice sheets and the North Atlantic deep-water onset of the Younger Dryas, and in general over the subsequent convection. So far, studies in the New Zealand region have shown 1.5-kyr period. Our evidence is based on detailed landform ambiguous signatures of late-glacial climate. Some records show a mapping, a high-precision 10Be chronology7 and reconstruction late-glacial reversal spanning both the ACR and the YDS time interval, of former ice extents and snow lines from well-preserved cirque and others display dominant imprints of the ACR (see, for example moraines. Our late-glacial glacier chronology matches climatic refs 1–4, 18–20). trends in Antarctica, Southern Ocean behaviour and variations We report here on a cirque basin, of about 7-km2 extent, at the in atmospheric CO2. The evidence points to a distinct warming head of Irishman Stream in the Ben Ohau range on the eastern side of of the southern mid-latitude atmosphere during the Younger the Southern Alps of South Island, New Zealand (Fig. 1). This basin Dryas and a close coupling between New Zealand’s cryosphere was purposely chosen for study because of its outstanding suitability and southern high-latitude climate. These findings support the for quantifying palaeoclimate (Supplementary Information). The hypothesis that extensive winter sea ice and curtailed meridional moraines of the Irishman basin are exceptionally well preserved ocean overturning in the North Atlantic led to a strong inter- and the topographic setting allows the former glaciers to be recon- hemispheric thermal gradient8 during late-glacial times, in turn structed to a high degree of certainty. As a result, precise limits can leading to increased upwelling and CO2 release from the Southern be placed on the dimensions and snow lines associated with these Ocean9, thereby triggering Southern Hemisphere warming during glaciers. Furthermore, the basin floor gradients are such that the the northern Younger Dryas. former glacier was particularly sensitive to small climate fluctuations. The transition of Earth’s climate from the last ice age to the warm To track late-glacial glacier dynamics in detail, we produced a geo- conditions of the Holocene epoch (the past ,11.5 kyr) is not yet fully morphic map (,1:10,000 scale), which forms the foundation for a explained. Polar ice-core records show that during this transition a comprehensive, precise and accurate 10Be chronology7 based on a period of general warming was interrupted in each hemisphere. The nearby production rate calibration of high precision21. To evaluate Antarctic cold reversal (ACR), as defined by a distinct levelling off or the amplitude of late-glacial climate changes, we reconstructed the slight reversal of warming over Antarctica between ,14.5 and former glacier extents and estimated their commensurate equilib- ,12.9 kyr ago, is mirrored by parallel deglacial changes in atmo- rium line altitudes (ELAs). spheric CO2 (ref. 10). Subsequently, Antarctic temperatures and South Island glaciers are very responsive to climate fluctuations, atmospheric CO2 concentrations increased and reached full intergla- particularly variations in summer temperatures. Consequently, res- cial values by ,11.5 kyr ago. In the North Atlantic region, the ponse times for glaciers of the sizes that existed in Irishman basin are Younger Dryas stadial (YDS) period between ,12.9 and ,11.7 kyr on the order of years22,23. The glacier in the Irishman basin recorded ago11,12 saw annual temperatures cool by ,15 uC in Greenland (and atmospheric changes well above sea level, between ,1,700 and perhaps farther afield from there) despite steadily increasing global ,2,300 m. Our chronology of glacier and snow-line changes is thus 13,14 atmospheric CO2 concentrations . The YDS coincided with interpreted to represent a centennial-scale proxy record of regional 1Lamont-Doherty Earth Observatory, Geochemistry, Palisades, New York 10964, USA. 2Department of Earth and Environmental Sciences, Columbia University, New York, New York 10027, USA. 3Department of Earth Sciences and Climate Change Institute, University of Maine, Orono, Maine 04469, USA. 4GNS Science, Private Bag 1930, Dunedin 9054, New Zealand. 5Alpine and Polar Processes Consultancy, Lake Hawea, Otago 9382, New Zealand. 6Department of Geosciences, University of Oslo, 0316-Oslo, Norway. 7Department of Earth and Planetary Sciences, University of California, Berkeley, California 95064, USA. 8CEREGE, 13545 Aix-en-Provence, Cedex 4, France. 9Antarctic Research Centre and School of Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand. 194 ©2010 Macmillan Publishers Limited. All rights reserved NATURE | Vol 467 | 9 September 2010 LETTERS 170º E 170º E Regional geomorphology Glacier Late-glacial moraine (LGM) Lake LGM outwash plain 40º S River LGM moraine Holocene fan or terrace Pre-LGM outwash plain Holocene moraine Pre-LGM moraine Study arareaea 45º S 11.9 ± 0.3 06-27 44º S Irishman 11.4 ± 0.3 06-26 StrStreameam 11.1 ± 0.3 06-23 11.4 ± 0.3 06-22 18.1 ± 0.4 06-20 11.8 ± 0.3 06-21 10.4 ± 0.2 06-17 0 10 km N 12.2 ± 0.4 06-18 12.0 ± 0.3 06-19 12.6 ± 0.3 06-24 12.1 ± 0.3 06-25 12.1 ± 0.3 06-47 11.9 ± 0.3 06-28 11.7 ± 0.3 06-49 12.5 ± 0.3 06-15 9.7 ± 0.2 06-02 13.3 ± 0.3 06-03 11.4 ± 0.3 06-01 7.3 ± 0.2 06-16a 7.3 ± 0.2 06-16b 12.7 ± 0.3 06-38 13.0 ± 0.3 06-39 13.2 ± 0.2 06-44 12.7 ± 0.3 06-40 12.0 ± 0.4 06-43 13.1 ± 0.3 06-33 12.8 ± 0.3 06-42 13.5 ± 0.4 06-32 12.8 ± 0.3 06-41 12.8 ± 0.3 06-37 100-m-interval topographic contour Stream 12.6 ± 0.3 06-36 Geomorphology Geomorphologic features 12.9 ± 0.3 06-35 Lake or pond Geomorphologic boundary 13.7 ± 0.3 06-31 Holocene scree Scree slope formline Holocene alluvial fan Rock glacier ridge 12.7 ± 0.3 06-30 Holocene rock glacier Crest of ice-contact slope 12.6 ± 0.3 06-29 Holocene moraine Sample locations eam Irishman Holocene moraine ridge Inner moraine belt (east) StreamStr Late-glacial moraine Inner moraine belt B (west) Late-glacial moraine ridge Inner moraine belt A (west) Outboard moraine Intermediate moraines Outboard moraine ridge 0 250 500 m Outer moraine belt Bedrock terrain Outboard boulders Figure 1 | Glacial geomorphology map of the moraines in the Irishman analytical errors. Outliers are in non-bold. Systematic uncertainties, such as basin, showing locations of sample sites and measured 10Be ages. The map that associated with the production rate, are minimal when comparing ages (,1:10,000 scale) differentiates between discrete moraine ridges and areas of of adjacent moraines. Inset maps show location on South Island (top right) more diffuse moraine28. Individual ages are shown in kiloyears with their 1s and in relation to adjacent major valley systems (top left). atmospheric conditions, especially summer conditions, for South the analytical uncertainty and the uncertainty in the local production Island. rate (Supplementary Fig. 4). The data set is of high internal consist- Well-defined moraine ridges are preserved in the outer and inner ency and only three out of 37 ages are considered outliers (IS-06-16, parts of the Irishman basin. Most striking is a prominent, very well- IS-06-17 and IS-06-20; Fig. 1). All nine 10Be ages from the outer preserved, arcuate moraine that was a focus of our study (Fig.
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