By Ignacio Jara
The issue of whether the last glacial-interglacial transition was synchronous between the Northern Hemisphere (NH) and Southern Hemisphere (NH) has been an ongoing and often controversial debate amongst Quaternarists. There is now considerable amounts of proxy data for the Last Glacial Maximum (LGM; 22-19 ka) and the Last Glacial Termination (19-11 ka) and yet there are still conflicting opinions about the phase -or anti-phase- of warming and cooling events between the hemispheres. The dominant view for many years was that the orbital-driven variations of the NH summer insolation were the main factor behind the waxing and waning of the continental ice sheets at global scales. However, conspicuous climate oscillations at timescales shorter than the orbital cycles have widely been recognized in Antarctica, Greenland and marine records from different latitudes, suggesting that a more complex –and fascinating- dynamic has been driving the climate. Despite all these new findings, the paradigm has remained more or less the same, i.e. that the SH glaciers have been ultimately responding to a NH signal. However, new evidence might suggest that the LGM in the SH may have occurred several thousand years earlier than the NH.
New Zealand is one of the few landmasses in the SH mid-latitudes that experienced extensive glacier advances during the last ice age, and therefore it is arguably a key region to test some of these ideas. Not surprisingly, this region has not been absent of the debate about the timing and structure of the New Zealand LGM and the Termination, with different –and sometimes opposite- interpretations being common.
Trends in New Zealand cosmogenic studies
Glacial reconstructions based on cosmogenic chronologies in New Zealand are a good example of this debate. Numerous well-preserved glacial landforms on both sides of the Southern Alps have provided a great opportunity to develop detailed cosmogenic glacial chronologies. Nevertheless, the relative novelty of this method, the lack of compelling datasets and the documenting of new cosmogenic production rates have all contributed to a wide range of different interpretation and hypotheses.
The first attempt to date New Zealand glacial landforms using cosmogenic dating was in 1999 (Ivy-Ochs et al., 1999). In 2006 a set of cosmogenic dates (n=7) from the Lake Pukaki area (44°S) showed that the timing for the final LGM ice retreat in New Zealand was centred at 18 ka (Schaefer et al., 2006), coincident with the onset of other glacial retreat in mid-latitude regions from both the NH and the SH. Based on similar results, but with a much more extensive cosmogenic dataset (n=39) from the Rakaia valley (43°S), Putnam et al., (2013) also proposed 18 ka as the onset of widespread glacial retreat in the Southern Alps. Additionally they showed that the rate of glacial retreat was very fast, with almost half of the total LGM ice lost in less than 2000 years.
An alternative view to this “fast and furious” ice retreatment was presented by Schulmeister et al (2010) and supported by a new article published this year (Rother et al., 2014). In this recent publication, Rother et al. (2014) presents a new set of cosmogenic ages (n=48) also from the Rakaia valley and suggests a series of glacial advances and retreatments from 28 to 16 ka, comprising the whole LGM interval. Based on their chronology the authors divide the New Zealand LGM into four main stages, with two periods of sustained ice retreat between 28-25 ka and 25-19 ka punctuated by two shorter phases of ice re-advance or more stable ice positions. While these results support a date of 18-19 ka for final LGM ice retreat, the authors also argue for a longer and more gradual glacial recession that lasted until 16 ka. According to Rother et al., (2014) the critical factor that could have misleadingly led previous cosmogenic researchers to argue for a “fast and furious” ice withdrawal is the formation of large glacial lakes at their base. Glacial lakes are a common feature in glacial valleys on the eastern side of the Southern Alps, and in that type of lacustrine environment, ice melting at the lake base is enhanced by water heat advection and not necessary by a regional climate (temperature) signal.
Rother et al., (2014) also suggest that the largest glacial extent was between 28-25 ka, several thousand years earlier than any other glacial reconstruction, and earlier than the LGM maximum extension in the NH. This time frame is more or less contemporaneous with an interval of cold conditions from the recently published NZ climate stratigraphy (NZCS; Barrell et al., 2013), however, it does not match the coldest interval proposed by the NZCS (occurring between 22-18 ka). Interestingly, the authors overcome this disparity by suggesting that their reported period of maximum ice extension did not occur during the coldest interval of the last glacial termination, but instead as a result of the combination of “not-the coldest-but-still-cold” conditions combined with higher than normal precipitation.
Regional climate drivers
The date of 18 ka for the beginning of the LGM termination in the Southern Alps (Schaefer et al., 2006) was in good agreement with other reported ages for glacial terminations in both hemisphere mid-latitudes. Based on this inter-hemispheric consistency the authors postulated that NH summer temperatures were the main driver for the global glacial recessions at the end of the LGM. By comparing their cosmogenic ages with other glacial chronologies and marine proxies from the SH, Putnam et al (2013) argued for a southward displacement of the Southern Westerly Winds (SWW) and their associated ocean fronts as the direct driver responsible for the final LGM termination in New Zealand.
This interpretation fits with the “bipolar seesaw” hypothesis. While the LGM glacial recession in the SH mid-latitudes showed a coherent timing that matches a gradual increase in Antarctic temperatures and rise in CO2, the NH showed a much more complex picture with the final LGM warming also triggering a massive discharge of icebergs into the North Atlantic Ocean during the so-called Heinrich Event 1, reducing the Atlantic meridional overturning circulation, and finally pushing the NH into a new cooling period for the next couple of thousand years. Moreover, it seems clear now that this bipolar seesaw switches between a “south-warm” and a “north-warm” phase several times over the last 60 ka. A new cosmogenic dataset (n=44) from Lake Pukaki supports this anti-phase relationship by dating an extensive Southern Alps glacial advance to 42 ka, a time coeval not only with a prominent cold episode in Antarctica and the Southwest Pacific, but also a warm interval in the NH (Kelley et al., 2014).
But which hemisphere started the last termination, driving the seesaw and opposite response in its counterpart?
The fact that the NH did not experience any long-lasting or prominent warming trend until 15 ka and that the SH experienced a much more coordinated warming response starting at least 3000 years before might be interpreted as evidence towards the SH as the “leading end” of the seesaw. However, a more detailed observation of the Greenland temperature record reveals that the initial warming in the NH occurred around 24 ka, at the same time that summer insolation in this hemisphere started to increased. Denton et al., (2010) suggest that the main condition for triggering the last glacial Termination was the orbitally-driven widespread collapse of Laurentide ice sheet after it reached an LGM maximum. Paradoxically, this collapse may have triggered a succession of large-scale changes that ultimately led to intense cooling in the NH, while in the SH these changes may have been associated with a more gradual warming via the southward shift of the SWW and the Subtropical front.
It is clear that the discussion about the timing and structure of the LGM and the Termination in New Zealand is still ongoing more than 15 years after the pioneering cosmogenic work. Although in some respects the publications over the last few years look more controversial than ever before, there seems to be a general consensus on at least two things: (1) a widespread ice retreat in the Southern Alps started about 18 ka, and (2) there was a close link between the glacial activity, temperature changes and the SWW during the Termination. Future cosmogenic studies from this part of the globe should test whether the retreat was “fast and furious” despite the relatively gradual changes in Antarctica temperatures, as well as other relevant topics such as the potential links between ice dynamics and atmospheric CO2 (something that is undoubtedly relevant to future climate change scenarios). Get out your rock hammers Quaternarist!
References
Barrell, D. J. A., Almond, P. C., Vandergoes, M. J., Lowe, D. J., and Newnham, R. M., 2013, A composite pollen-based stratotype for inter-regional evaluation of climatic events in New Zealand over the past 30,000 years (NZ-INTIMATE project): Quaternary Science Reviews, v. 74, no. 0, p. 4-20.
Denton, G. H., Anderson, R. F., Toggweiler, J. R., Edwards, R. L., Schaefer, J. M., and Putnam, A. E., 2010, The Last Glacial Termination: Science, v. 328, no. 5986, p. 1652-1656.
Ivy-Ochs, S., Schlüchter, C., Kubik, P. W., and Denton, G. H., 1999, Moraine Exposure Dates Imply Synchronous Younger Dryas Glacier Advances in the European Alps and in the Southern Alps of New Zealand: Geografiska Annaler: Series A, Physical Geography, v. 81, no. 2, p. 313-323.
Kelley, S. E., Kaplan, M. R., Schaefer, J. M., Andersen, B. G., Barrell, D. J. A., Putnam, A. E., Denton, G. H., Schwartz, R., Finkel, R. C., and Doughty, A. M., 2014, High-precision 10Be chronology of moraines in the Southern Alps indicates synchronous cooling in Antarctica and New Zealand 42,000 years ago: Earth and Planetary Science Letters, v. 405, no. 0, p. 194-206.
Putnam, A. E., Schaefer, J. M., Denton, G. H., Barrell, D. J. A., Andersen, B. G., Koffman, T. N. B., Rowan, A. V., Finkel, R. C., Rood, D. H., Schwartz, R., Vandergoes, M. J., Plummer, M. A., Brocklehurst, S. H., Kelley, S. E., and Ladig, K. L., 2013, Warming and glacier recession in the Rakaia valley, Southern Alps of New Zealand, during Heinrich Stadial 1: Earth and Planetary Science Letters, v. 382, no. 0, p. 98-110.
Rother, H., Fink, D., Shulmeister, J., Mifsud, C., Evans, M., and Pugh, J., 2014, The early rise and late demise of New Zealand’s last glacial maximum: Proceedings of the National Academy of Sciences, v. 111, no. 32, p. 11630-11635.
Schaefer, J. M., Denton, G. H., Barrell, D. J. A., Ivy-Ochs, S., Kubik, P. W., Andersen, B. G., Phillips, F. M., Lowell, T. V., and Schlüchter, C., 2006, Near-Synchronous Interhemispheric Termination of the Last Glacial Maximum in Mid-Latitudes: Science, v. 312, no. 5779, p. 1510-1513.
Shulmeister, J., Fink, D., Hyatt, O. M., Thackray, G. D., and Rother, H., 2010, Cosmogenic 10Be and 26Al exposure ages of moraines in the Rakaia Valley, New Zealand and the nature of the last termination in New Zealand glacial systems: Earth and Planetary Science Letters, v. 297, no. 3–4, p. 558-566.