The anatomy of long-term warming since 15 ka in New Zealand based on net glacier snowline rise Michael R. Kaplan1, Joerg M. Schaefer1,2, George H. Denton3, Alice M. Doughty4, David J.A. Barrell5, Trevor J.H. Chinn6, Aaron E. Putnam1,3, Bjørn G. Andersen7†, Andrew Mackintosh4, Robert C. Finkel8, Roseanne Schwartz1, and Brian Anderson4 1Division of Geochemistry, Lamont-Doherty Earth Observatory, 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 4Antarctic Research Centre and School of Geography, Environment and Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand 5GNS Science, Private Bag 1930, Dunedin 9054, New Zealand 6Alpine and Polar Processes Consultancy, Lake Hawea 9382, New Zealand 7Department of Geosciences, University of Oslo, 0316 Oslo, Norway 8Department of Earth and Planetary Sciences, University of California–Berkeley, Berkeley, California 95064, USA ABSTRACT The timing and magnitude of postglacial climatic changes around the globe provide glacial geomorphology map of both branches insights into the underlying drivers of natural climate change. Using geomorphologic map- (Fig. 1) is based on the regional-scale approach ping of moraines, 10Be surface-exposure dating, snowline reconstructions, and numerical in Barrell et al. (2011). Emanating from the modeling, we quantifi ed glacier behavior during Late Glacial (15–11.5 ka) and Holocene east branch and abutting the mouth of the (the past ~11.5 k.y.) time in the Ben Ohau Range, New Zealand. Glaciers were more exten- west branch is a prominent, well-preserved, sive during the Antarctic Cold Reversal (ACR), than subsequently, and the margins under- 60–80-m-tall moraine loop (Fig. 1). The east went a punctuated net withdrawal over the Holocene. Numerical modeling experiments branch moraine loop, the crest of which ranges that achieve the best fi t to the moraines suggest that air temperature during the ACR was in altitude from ~1200 to ~1500 m above sea between 1.8 °C and 2.6 °C cooler than today, with similar (±20%) prescribed precipitation. level (asl), comprises several adjoining frontal After the ACR, a net snowline rise of ~100 m through the Younger Dryas stadial (12.9– and lateral ridges. Inside this loop, several low 11.7 ka) was succeeded by a further “long-term,” or net, rise of ~100 m between ~11 k.y. and moraine ridges are on the valley fl oor. A glacier ~500 yr ago. Glacier snowline records in New Zealand show generally coherent Late Glacial fl owing from the west branch abutted the outer and Holocene climate trends. However, the paleoclimate record in the southwest Pacifi c margin of the east branch moraine loop (Fig. 1). region shows important differences from that in the Northern Hemisphere. As the west branch glacier withdrew upvalley, it formed moraines ranging from ~1300 to 1600 INTRODUCTION maximum Holocene extents (e.g., Grove, 2004; masl. Subsequently, moraine sets formed in the An important goal of paleoclimatology is Holzhauser et al., 2005; Schimmelpfennig et north and south cirques of the west branch. In to understand the reasons behind climate con- al., 2012). It remains unclear, however, whether contrast, there is no succession of moraines in trasts between the Northern and Southern Northern Hemisphere expressions of the YD, the upper east branch, but rather a single large Hemispheres. Changes since the Late Glacial altithermal, and neoglaciation are applicable in moraine complex that was not investigated in are particularly important because they pro- the Southern Hemisphere. this study. The general geological and climatic vide a recent geologic context for present cli- The glacier records of New Zealand’s Southern setting of the Ben Ohau Range was described in mate behavior. General features of Northern Alps, situated in the southwestern Pacifi c region Kaplan et al. (2010) and Putnam et al. (2010a). Hemisphere Late Glacial to Holocene climate on the opposite side of Earth from the North Our chronology is based on 42 samples from are the cold Younger Dryas (YD), followed by Atlantic Ocean, are ideal for testing hypoth- boulders composed of hard quartzofeldspathic an interval when conditions were mostly unfa- eses of regional and interhemispheric paleocli- graywacke sandstone (Tables DR1–DR3 in the vorable for mountain glaciers (ca. 11–7 ka; the mate patterns. To deepen our understanding of GSA Data Repository1). Of the samples, 18 warm altithermal concept of Porter and Denton, past climate change in this region, we studied a came from the lower reaches of the east and 1967), which was succeeded by the colder neo- well-preserved sequence of moraines at Whale west branches (Fig. 1), while 24 samples were glaciation. From a broad perspective, although Stream, in the Ben Ohau Range. Glaciers in the collected from the west branch cirque moraines, the early Holocene is inferred to be a period mid-latitude temperate maritime setting of South 14 from the north cirque and 10 from the south of time not favorable for mountain glaciers in Island respond relatively quickly to variations cirque. Samples were processed in the Lamont- the Northern Hemisphere, climate changes dur- in temperature and precipitation (Oerlemans, Doherty Earth Observatory Cosmogenic ing this period exhibited diversity and at least 2010). Both theoretical and empirical climate- Nuclide Laboratory (New York) and measured some regions underwent cold episodes and ice glacier studies have shown that New Zealand at the Center for Accelerator Mass Spectrome- growth (Kaufman et al., 2004; Davis et al., glacier mass depends more upon atmospheric try at Lawrence Livermore National Laboratory 2009; Schimmelpfennig et al., 2012). After the temperature than on amounts of precipitation, on (California). For details of the cosmogenic dat- early Holocene, some Northern Hemisphere time scales greater than decades (Anderson and ing method employed in this study, see Schaefer regions underwent a relatively marked, or Mackintosh, 2006; Anderson et al., 2010). et al. (2009) and Putnam et al. (2010b). gradual, transition into the neoglaciation, which culminated in the Little Ice Age (LIA). During SETTING AND METHODS 1GSA Data Repository item 2013245, supple- mental text, fi gures, and tables, is available online at the LIA, glaciers were at, or very close to, their Whale Stream drains from two main head- www.geosociety.org/pubs/ft2013.htm, or on request water valleys, which we refer to informally as from [email protected] or Documents Secre- †Deceased the east and west branch (Fig. 1). Our detailed tary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. GEOLOGY, August 2013; v. 41; no. 8; p. 1–4; Data Repository item 2013245 | doi:10.1130/G34288.1 | Published online XX Month 2013 GEOLOGY© 2013 Geological | August Society 2013 of | America.www.gsapubs.org For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 1 170° CameronCameron to rest on an older deposit; we do not include GlacierGlacier their ages in further discussion. 40° Aoraki/Aoraki/ In the west branch south cirque, the southern Mt.Mt. CCookook Study sector of the outer moraine ridge returned 10Be 45° area BirchBirch HHillill ages from 9230 ± 320 yr to 8150 ± 210 yr old (n = 3), while one boulder on the northeastern sec- tor of this ridge yielded an age of 6410 ± 210 yr. The ridge’s northeastern sector is lighter in color easte branch a (i.e., less weathered) than the southern sector s t 390 ± 20 06-20 b (Fig. DR2B). The color contrasts and range of r 13 020 ± 300 925 northnorth 540 ± 10 06-19 a n 15 290 ± 430 525 ages indicate that this ridge may be of compos- cirquecirque 1670 ± 40 06-25 c 12 910 ± 520 526 1760 ± 60 06-26 h ite age. Inboard of the outermost ridge are two 790 ± 20 06-21 11 790 ± 310 921 distinct crests with 10Be ages of ~1400 yr (n = 860 ± 30 06-23 7940 ± 190 06-28 11 560 ± 300 920 690 ± 20 06-22 8120 ± 270 06-29 12 270 ± 330 529 2) and 1140 yr (n = 1), respectively; these boul- 8160 ± 190 06-27 11 940 ± 280 530 1360 ± 40 06-04 8010 ± 150 06-24 12 410 ± 330 922 ders have little or no surface oxidation. Since 1410 ± 50 06-03 12 520 ± 490 528 collection, we realized that boulders WS-06–02, 14 340 ± 360 923 1110 ± 40 06-01a 11 610 ± 270 06-18 WS-06–06, and WS-06–07 are on a rock ava- 1180 ± 40 06-01b 10 730 ± 250 06-17 10 810 ± 210 06-16 southsouth 15 080 ± 320 527 lanche runout track that overprints and postdates 2960 ± 70 06-02 15 430 ± 360 919 850 ± 30 06-06 cirquecirque the moraine, and we exclude their ages from fur- 350 ± 10 06-07 6410 ± 210 06-05 14 210 ± 620 06-12 8150 ± 200 06-10 14 820 ± 320 06-11 ther discussion (Figs. DR3F and DR2B). 9230 ± 320 06-09 8840 ± 210 06-08 13 430 ± 420 06-15 Geomorphology 13 560 ± 490 06-14 SNOWLINES AND MODELING westw branch ice front west branch river glacier est branc 14 350 ± 350 917 late Holocene moraine h 14 680 ± 360 918 We reconstructed glacier geometries and late Holocene moraine ridge landslide terrain Holocene fan or scree map boundary AAR-based snowlines for the following dated Holocene alluvium terrace scarp Late Glacial and west branch north cirque Holocene moraine eroded ice contact slope Holocene moraine ridge crest of ice contact slope moraines: ca. 15–14 ka, ca. 12 ka, ca. 11 ka, Late Glacial moraine 100 m contour stream Late Glacial moraine ridge ca. 8 ka, ca.