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Home , Antarctic Cold Reversal, Last Glacial Maximum

The early rise and late demise of New Zealand’s

Henrik Rothera,1, David Finkb, James Shulmeisterc, Charles Mifsudb, Michael Evansd, and Jeremy Pughe,2

aInstitute for Geography and Geology, University of Greifswald, 17489 Greifswald, Germany; bAustralian Nuclear Science and Technology Organisation, Menai, NSW 2234, ; cSchool of Geography, Planning and Environmental Management, The University of Queensland, St. Lucia Campus, Brisbane, QLD 4072, Australia; dMoultrie Geology, Banyo, QLD 4014, Australia; and eDepartment of Geological Sciences, University of Canterbury, Christchurch, New Zealand

Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved June 13, 2014 (received for review February 3, 2014) Recent debate on records of southern midlatitude glaciation has second aspect in the debate centers on determining the precise focused on reconstructing dynamics during the last glacial onset of last glacial maximum (LGM) retreat (a period referred termination, with different results supporting both in-phase and to as Termination 1 and dated to ∼19–10 ka) and the mode of out-of-phase correlations with glacial sig- this (i.e., models of collapse versus slow recession) nals. A continuing major weakness in this debate is the lack of in NZ. Together, these issues are responsible for ongoing con- robust data, particularly from the early and maximum phase of troversy about the evolution of the last glacial-to- southern midlatitude glaciation (∼30–20 ka), to verify the compet- transition in the SH midlatitudes and the relative importance of ing models. Here we present a suite of 58 cosmogenic exposure interhemispheric forcing versus regional insolation and/ ages from 17 last-glacial ice limits in the Rangitata Valley of New or synoptic forcing of SH glaciation. Zealand, capturing an extensive record of glacial oscillations be- tween 28–16 ka. The sequence shows that the local last glacial Previous Work maximum in this region occurred shortly before 28 ka, followed NZ represents one of only three areas in the SH midlatitudes by several successively less extensive ice readvances between – that experienced extensive late glaciation. During 26 19 ka. The onset of Termination 1 and the ensuing glacial retreat the LGM, valley 70–100 km in length reached the alpine is preserved in exceptional detail through numerous recessional

forelands of the eastern Southern Alps, whereas shorter and , ATMOSPHERIC, , indicating that ice retreat between 19–16 ka was very steeper glaciers in the western Alps typically terminated into the AND PLANETARY SCIENCES gradual. Extensive valley glaciers survived in the Rangitata catch- ment until at least 15.8 ka. These findings preclude the previously Tasman Sea. According to cosmogenic exposure ages from Lake inferred rapid climate-driven ice retreat in the Southern Alps after Pukaki terminal moraines in the central Southern Alps, it has the onset of Termination 1. Our record documents an early last been suggested that large-scale glacial retreat in NZ commenced “ glacial maximum, an overall trend of diminishing ice volume in at 17.4 ka, representing the start of the termination of the New Zealand between 28–20 ka, and gradual deglaciation until LGM” (16). This appeared to be virtually indistinguishable from at least 15 ka. the onset of the final LGM retreat at similar NH and SH mid- latitude sites, supporting the idea of a global mechanism re- surface exposure dating | global climate linkages sponsible for triggering a “near-synchronous” ice retreat at these far-field sites. Conversely, evidence for millennial-scale climate ccording to the Milankovitch orbital theory of glaciation, reversals in NZ at the end of Termination 1, during the Antarctic – Avariations in northern high-latitude summer insolation are Cold Reversal (14 15 ka) and the chronozone responsible for glacial–interglacial cycles (1, 2). On this basis, it (11–12 ka), has been interpreted to indicate late-glacial climatic is commonly assumed that climatic changes in the Northern antiphasing with NH events (17, 18). Hemisphere (NH) constitute the principle forcing mechanism for glaciation in the Southern Hemisphere (SH) (3). However, in Significance recent times, this view has been challenged by studies showing that at least some glacial signals in the SH have no NH correl- We present here a comprehensive record of glaciation from ative or precede events in the NH by up to 1.5 ka (4–7). Al- a New Zealand valley glacier system covering the critical though some of these patterns can be explained by an extended 15,000-y period from the local last glacial maximum (LGM) to bipolar seesaw model, other features, including the decreasing near the end of the last . This record from a key site in expression of hemispheric antiphasing away from the polar the midlatitude Southern Hemisphere shows that the largest regions, indicate that additional forcing mechanisms must be glacial advance did not coincide with the coldest temperatures involved (8). during this phase. We also show that the regional post-LGM ice A compilation of glacial records from the NH indicates that retreat was very gradual, contrary to the rapid ice collapse midlatitude ice sheets in and northern Eurasia widely inferred. This demonstrates that glacial records from began a major expansion phase between 33–29 ka (9). From this New Zealand are neither synchronous with nor simply lag or time onward, relative reconstructions indicate a steady lead Northern Hemisphere records, which has im- increase in global ice volume, until most ice sheets had reached portant implications for the reconstruction of past inter- a near-maximum extent by 26.5 ka, followed by 6–7 ky of equi- hemispheric climate linkages and mechanisms. librium conditions until the onset of final retreat around 20–19 Author contributions: H.R. and J.S. designed research; H.R., M.E., and J.P. performed re- ka (9). For the midlatitude SH, specifically New Zealand (NZ), search; D.F. and C.M. contributed new reagents/analytic tools; H.R. and D.F. analyzed paleoecologic data (10), supported by dated glaciofluvial aggra- data; and H.R., D.F., and J.S. wrote the paper. dation sequences (11), have also suggested an onset of mountain The authors declare no conflict of interest. glacier growth around 30–28 ka (12). Contrary to NH ice volume 1To whom correspondence should be addressed. Email: [email protected]. trends, however, recent records from NZ may indicate that 2Deceased February 2012. maximum glaciation in the Southern Alps occurred before 28 ka, This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. several thousand before the NH ice maximum (13–15). A 1073/pnas.1401547111/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1401547111 PNAS Early Edition | 1of6 Downloaded by guest on September 28, 2021 Compounding these apparently incompatible interpretations high-latitude spallation 10Be production rate for NZ of 3.87 ± 0.08 atoms/ 26 ± of the NZ glacial history using cosmogenic exposure dating is the gquartz/a (19) and a Al production rate of 26.08 0.55 atoms/gquartz/a 26 10 introduction of a local 10Be reference production rate for the NZ corresponding to a Al/ Be production ratio of 6.727 (20). All exposure region (19). This new production rate is significantly lower (by ages were calculated using the CRONUS Web-based calculator. Both spall- 16%) than the statistically weighted mean global production rate ation and muon production rates were then scaled to geographic location and altitude of the individual sample sites, using the scaling methods of obtained from a collection of geographic calibration data sets T. Dunai (ref. 23; for age results based on other scaling schemes, see (20). As a consequence, any intercomparison of glacial chro- SI Appendix). nologies, both relative and absolute, across the globe, as well as from within NZ itself, requires great care to ensure a consistent Results synthesis of the published data. Specifically, exposure ages from Of the 73 zero-erosion ages (58 10Be and 15 26Al ages; Fig. 1 and NZ calculated using the global production rate (20) are now SI Appendix) presented in this study, 48 are from the Clearwater considered to underestimate landform ages by anywhere from and 25 from the Heron area. Apart from two sites (site 10, site – – 1.8 4.5 ky across the period of interest (i.e., 15 30 ka). This 11 ∼600 m above the valley floor, all in-valley terminal and lateral draws into question the previously implied synchronicity of the weighted average ages fall between 27.7 and 15.8 ka, onset of the final LGM retreat in NZ and NH midlatitude sites providing one of the most extensive glacial sequences from NZ “ ” (16) as the rescaled NZ timing for glacial collapse increases to track ice extent from the local LGM until well after the onset (by 2.8 ky) to 20.2 ka. Recent studies by Putnam and colleagues of the last termination. We bracket this sequence into four dis- (15, 21) report that at Lake Ohau and the Rakaia Valley in the tinct periods consistent with an early local LGM ice extent, an central Southern Alps, fast retreat from LGM moraines com- initial phase of ice volume reduction followed by stabilization menced around 17.8 ka, followed by a substantial reduction in − ∼ (or readvance) of the ice front at 25 ka, further recession overall glacier length ( 50%) over a period of 1,000 y. A and renewed ice front stabilization (or readvance) at 19 ka, and similar rapid post-LGM ice decay has also been proposed for a gradual final retreat phase from 19 to 15 ka (Fig. 2 and SI Ap- overall glaciation across NZ, with an estimated 60% reduction in pendix, Fig. S4). glaciated area soon after the onset of the final retreat (12). Two ice-overridden hills in the Heron Basin yield mean site However, a different view is provided by Shulmeister and col- ages of 27.7 ± 0.5 ka (site 12) and 27.6 ± 0.8 ka (site 13). On the leagues (14), who reconstruct a much lower rate of ice retreat basis of the required ice thickness to overflow these sites (site-13 during the same period (10–20% glacier reduction) on the basis surface ∼230 m above valley floor), we infer that during its of exposure dating of terminal and recessional moraines in the maximum extent, the Heron ice lobe coalesced with the glacier Rakaia Valley (renormalized using the lower local 10Be pro- tongue occupying the Clearwater area. The cumulative weighted duction rate; see ref. 19). mean age for site 12, and site 13, representing the minimum local Despite significant progress in reconstructing NZ glacial dy- LGM age, is 27.7 ± 0.4 ka (n = 6, SI Appendix, Fig. S4A). After namics, a common deficiency in the available glacial chronolo- gies is the relatively short-term nature of the investigated the initiation of ice retreat from this local LGM ice position, the moraine sequences. Missing are moraine records spanning the two lobes separated again, with an independent terminal mo- raine position established in the Heron area soon thereafter at whole sequence from the ice maximum to the subsequent retreat ± stages pertaining to the whole LGM cold period of 30–15 ka. In 27.6 0.8 ka (site 14). Evidence for a comparable down-valley NZ, such records are rare: in the western Alps because most major ice extent is also indicated by an elevated lateral moraine NW of Lake Heron, giving an age of 25.0 ± 0.6 ka (site 17; SI Appendix, LGM glacier systems were marine terminated, and in the eastern A Alps because many cross-valley terminal and retreat moraines Fig. S4 ), suggesting only minor changes in ice volume during this time. Interestingly, the next up-valley moraines (site 15, site were buried during ice retreat in deep proglacial troughs or – ∼ destroyed by extensive postglacial fluvial processes. Here we 16), located no further than 3 4 km from the 27.6 ka ice po- ± present an extended glacial reconstruction from the Rangitata sition (site 14), yield significantly younger site ages of 19.0 0.4 ka ± Valley, a major drainage system of the central Southern Alps in (site 15) and 18.6 0.3 ka (site 16). which an extensive moraine record survived in a stranded posi- Ice recession from these younger moraine positions and tion on the interfluves between the entrenched Rangitata and through Termination 1 is preserved on the Clearwater interfluve, Rakaia trunk valleys. This allows the assessment of glacier dy- where nine moraine sites were investigated. Two Clearwater ± ± namics within a single catchment from the local LGM, through moraines (site 1, 18.7 0.7 ka, and site 8, 18.5 0.4 ka) and the ± ± Termination 1, and into the last glacial–interglacial transition. two Heron moraines at site 15 (19.0 0.4 ka) and site 16 (18.6 0.3 ka) clearly show that a stable ice front was established by Study Site and Methods ∼19 ka in both areas (site 1, site 8, site 15, and site 16; weighted mean The targeted moraines are located in the Lake Clearwater and Lake Heron age, 18.7 ± 0.2 ka; n = 17). During the next 3–4 ky, ice recession areas of the Rangitata Valley, ∼50 km downstream from the modern glacier from these positions was gradual, with the ice front moving at margins. At its maximum extent, the Rangitata Glacier comprised two major most 8–10 km up-valley, leaving several nested clusters of closely lobes: a southern ice tongue that followed the trunk Rangitata Valley spaced moraines. The largest of these recessional moraine (Rangitata lobe), and a northern lobe that advanced onto a higher-lying clusters (site 2, site 3, and site 4) is located southeast of Spider area north of Harper Range (Clearwater lobe). A smaller distributary lobe Lakes, yielding overlapping ages of 17.0 ± 0.6 ka (site 2), 16.6 ± from the main Rakaia Glacier entered the Lake Heron area from the north 0.5 ka (site 3), and 17.9 ± 0.6 ka (site 4), with a weighted mean via the Prospect Hill saddle (Heron lobe; Fig. 1). Ice marginal landforms ± n = SI consist primarily of lobate laterofrontal dump moraines, lateral moraines, age of this moraine group of 17.1 ka 0.3 ( 10, one reject; and hummocky terrain that formed during the disintegration of the ice Appendix,Fig.S4C). Coeval retreat of the main Rangitata Valley lobes. These landforms are present over a considerable down-valley distance glacier during this time is observed at site 9 (17.2 ± 0.4 ka). The and altitudinal range and have formed the basis for an earlier geomorphic final stages of retreat from the Clearwater area are preserved differentiation of five late associations (22). through three moraines ∼8 km upstream (site 6, 16.8 ± 0.4 ka; Until now, no absolute ages have been available to constrain the timings of site 7, 15.8 ± 0.4 ka; site 5, 15.8 ± 0.3 ka), with a weighted mean these glacial advances in the area. age of 16.1 ± 0.2 (n = 11, one reject; SI Appendix,Fig.S4D). Covering a valley distance of ∼12 km in-board from terminal ice positions of the Heron and Clearwater lobes, we sampled 56 quartzo-feldspathic Collectively, the moraines show a broad up-valley decrease in sandstone (greywacke) erratics from 17 moraine sites and two ice overridden age consistent with a retreating ice margin, which was punctu- bedrock surfaces (SI Appendix, Fig. S3). Five samples of this total were ated by repeated ice still stands. Ice thickness reduction within rejected as outliers. We calculate exposure ages on the basis of a sea-level the ∼12 ky of retreat from the local LGM limit to our youngest

2of6 | www.pnas.org/cgi/doi/10.1073/pnas.1401547111 Rother et al. Downloaded by guest on September 28, 2021 moraine at about 16 ka amounts to 400–500 m. The oldest ages of the ages requires a reliable measure of long-term surface were obtained from six glacial boulders from two sites in the weathering of greywacke rock. An upper limit estimate of grey- Dogs Hill Range (∼600 m above valley floor; Fig. 1C), yielding wacke erosion rate of 1–2 mm/ky would yield site-11 erosion- individual boulder ages of 113.2 ± 4.0, 104.5 ± 3.5, and 84.6 ± 2.5 ka corrected exposure ages between 141–114 ka and site-10 ages (site 11) from the higher surface and 54.6 ± 3.4, 48.6 ± 1.9, between 53–32 ka, which are effectively coeval with glaciation and 31.4 ± 1.1 ka (site 10) from the lower of these surfaces. during late marine isotope stage (MIS) 6/MIS-5 and MIS-3, Despite their broad age range, these erratics clearly relate to respectively. We report erosion-uncorrected ages but note that previous, larger-scale glacier advances in the central Southern in some cases, thin quartz veins were found to be ∼5 mm proud of Alps. Removing the youngest boulder age from each site, their the sampled surface (SI Appendix,Fig.S3D), giving a minimum weighted mean site ages (site 11, 108.3 ± 2.4 ka; site 10, 50.1 ± erosion rate of ∼0.3 mm/ky. This is almost identical to the amount 1.7 ka) fall into the last glacial cycle. A more robust interpretation of postemplacement erosion on greywacke surfaces reported from EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES

Fig. 1. Location of the Rangitata catchment in the South Island, NZ (A), study areas (B), and positions of all sampled moraines and resulting cosmogenic exposure ages (C and D). The shaded relief map (B) shows the topographic context and the ice outlet routes of the major Rangitata and Rakaia glaciers. Inset rectangles denote regions of sampled moraines, with their relative locations detailed for the Lake Clearwater (C) and Lake Heron (D) interfluves. Multiple terminal and lateral moraine sequences span the period from maximum ice extent during the LGM (29–28 ka) to subsequent retreat phases in the Clearwater and Heron areas (19–15 ka). Sites selected for 10Be and 26Al exposure dating are indicated by numbered black circles and comprise from two to five boulders per site. All ages shown are derived from in situ 10Be with associated analytical errors (for 26Al ages, see SI Appendix). Bold ages represent site-weighted mean ages with their standard mean error (ages using scaling schemes calculated via the CRONUS on-line web-calculator are given in SI Appendix, Tables S4 and S5). Five sample ages denoted with an asterisk are interpreted to be outliers (two of these relate to prelocal LGM sites). Sample ages denoted with a b were derived from bedrock.

Rother et al. PNAS Early Edition | 3of6 Downloaded by guest on September 28, 2021 Fig. 2. Summary of the Rangitata glacial chronology based on 49 10Be exposure ages (excluding three outliers) that characterize four phases of last glacial– interglacial activity (A–D in graph). The width of the bars represents the timing of moraine formation during each glacial phase according to the weighed mean (with standard mean error) for moraines grouped according to valley position, and thus indicating similar ice extent. The y axis denotes relative glacial extent, with 1.0 representing the approximate maximum ice extent during the LGM (total Rangitata glacier length, 65–70 km). Ice recession from the local LGM position commenced at 18.7 ± 0.3 ka. At 15.8 ka, the Rangitata glacier terminated ∼11 km up-valley from the first inbound recessional moraines, in- dicating the survival of more than 80% of the overall glacier length (and nearly 70% of the full LGM glacier length) through to 15.8 ka.

NZ by Kaplan and colleagues (17). Assuming a much higher ero- Heinrich 1 in the North Atlantic (21). Considering the sion rate of 1–2 mm/ky, the ages would increase by 2–4% (i.e., ca. time evolution of the presented Clearwater–Heron glacial se- 300 y at 15-ka and 1 ky at 30-ka exposure), showing that within quence, we also document recession during this time, but sig- these limits, the uncertainties resulting from potential surface nificantly, we find no evidence for a rapid pace of deglaciation in erosion would have no effect on our main conclusions. NZ (12). In contrast, we see that retreat of the Rangitata Glacier between 18.7–15.8 ka was very gradual, with ∼70–80% of the Discussion and Conclusion local LGM glacier length surviving until 15.8 ka. This outcome Our detailed chronology of the Rangitata glacial system es- differs markedly from other NZ deglaciation studies, specifically tablishes four distinct phases of the LGM and last deglaciation from Lake Pukaki (16), Lake Ohau (15), and the Rakaia Valley in NZ (Fig. 2): a maximum local LGM ice extent reached (21), which all report recession or collapse of major ice tongues shortly before 28 ka (phase A), an initial phase of ice volume over tens of kilometers during a short period. However, we note reduction followed by stabilization or readvance of the ice until that in these areas, the expression of rapid local ice retreat was 25 ka (phase B), further ice volume reduction after 25 ka and heavily influenced by the formation of large and deep proglacial renewed stabilization of the ice at 18.7 ka at moraine positions lakes during deglaciation (27, 28). These proglacial lakes estab- only a few kilometers (∼3–4 km) distant from the local LGM lished heat advection and wave impact as significant additional limit (phase C), and a gradual and final glacier retreat com- mechanisms of ice loss, probably leading to locally enhanced mencing shortly after 18.7 ka (and lasting to about 15.8 ka), rates of ice recession. This mode of post-LGM retreat was producing numerous closely spaced recessional moraines rep- a common feature of glacier systems across the Southern Alps, as resenting the regional onset of Termination 1 (phase D). Co- evidenced by widespread glaciolacustrine deposits, particularly in eval with this last phase, Antarctic records indicate the Eastern Alps, that have been documented by numerous cessation of severe cold conditions that had persisted for the glacial workers (e.g., refs. 29–35). Within this type of retreat previous 10 ky and commencement at 18–19 ka of continuous setting (i.e., early transition to lake-terminated glaciers after the warming proxied via increasing δ18O and a synchronous rise in onset of retreat), ice-loss dynamics frequently decouple from atmospheric CO2 (24–26). climatic influences, implying that these glacier systems may not The exposure age results show that recession of the Rangitata present reliable indicators for inferring overall regional degla- Glacier from the innermost LGM moraine (site 1) commenced ciation trends and/or reconstructing climatic signals. at about 18.7 ± 0.7 ka, which is similar, although slightly earlier, In this context, we suggest that the Rangitata record offers than the 17.8 ± 0.2 ka onset of major LGM retreat found a distinct advantage because this glacier retreated predomi- by Putnam and colleagues (21) in the nearby Rakaia Valley. nately via a grounded glacier margin with little to no influence There, dating of recessional moraines points to extensive local from lake calving on ice retreat dynamics. Our results indicate deglaciation between 17.8–15.6 ka, interpreted to coincide with that the Rangitata Glacier reached its largest LGM ice extent

4of6 | www.pnas.org/cgi/doi/10.1073/pnas.1401547111 Rother et al. Downloaded by guest on September 28, 2021 about 28 ka ago and maintained these glacial limits until 25 ka, maximum was less extensive, suggesting an overall trend of dimin- with only a minor reduction in ice volume during this time. This ishing regional glaciation in NZ between 28–20 ka. timing corresponds well with the first major LGM cooling event The available terrestrial and marine evidence from the NZ (28.8–25.4 ka), as defined by the composite NZ climate event region, supported by regional paleoclimatic modeling (45), stratigraphy (36), and is also broadly contemporaneous with indicates that the persistence of relatively high levels of pre- glacial maxima recorded at other midlatitude Southern Hemi- cipitation against a background of moderate cooling during the sphere sites (37–39) and the onset of major cooling observed early LGM (∼30–28 ka) are responsible for generating the in the Talos Dome and European project for ice coring in largest LGM ice advance in NZ; conversely, the subsequent less- -Dome C ice cores of Antarctica (24, 40, 41). How- extensive ice advances reflect a trend of decreasing ever, we note that within the NZ pollen-based climate stratig- that limited glaciation during the remainder of the LGM in raphy, the largest Rangitata ice advance occurred near the NZ, despite intensifying cooling. Although global ice volume transition from mild interstadial conditions [New Zealand cli- remained stable or even slightly increased between 26–20 ka, mate event (NZce)-11 phase ∼30–28.8 ka: pollen flora 81–95% glacier systems in NZ were in slow but continuous retreat. The trees and shrubs] to a progressively cooler climate (NZce-10 pattern is repeated at ∼19 ka, when our record shows a region- phase 28.8–25.5 ka: pollen flora 49–71% trees and shrubs). wide hiatus in warming or ice-stillstand that is coeval with the Compared with the last glacial coldest period in NZ (NZce-6 dated onset of major retreat in the NH. This implies an overall scenario to 21.7–18.3 ka; ref. 36), we infer that the ice maximum coincided in which increasing NH summer insolation initiates the slow with less-severe but probably moister climatic conditions. retreat of NH ice sheets (from 20 ka onward), whereas Antarctic Recent records from both hemispheres have emphasized that ice extent remains stable at this time. It is now also evident that similar to many continental alpine glaciers, various parts of the the Southern Ocean plays an important role in the further de- major ice sheets in Eurasia and Antarctica were not in synchrony mise of global ice volume by invoking major changes in oceanic with the global LGM (42). More important, although most sec- CO2 discharge. Once CO2 levels rise sharply (from 18 ka on- tors of the largest ice sheet, the , along ward), Antarctic temperatures increase synchronously (26), with the , and the southeast sector of the triggering the onset of retreat in Antarctica and the midlatitude Scandinavian Ice Sheet appear to have reached their maximum SH (NZ). extent synchronous with the global LGM (43, 44). We find that contrary to these NH trends, in which where ice sheets expanded ACKNOWLEDGMENTS. Cosmogenic surface exposure dating was funded strongly between 29–26 ka (9) and then remained at near- by Australian Institute of Nuclear Science and Engineering Grant 05/219 (Australian Nuclear Science and Technology Organisation) and the Austra- EARTH, ATMOSPHERIC,

– AND PLANETARY SCIENCES maximum limits until the end of the LGM (19 18 ka), our lian Nuclear Science and Technology Organisation’s Cosmogenic Climate findings show that in NZ each ice advance subsequent to the 28-ka Archives of the Southern Hemisphere Project 0203v.

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