1
STRATI 2013
Evaluating the concept of a global ‘Last Glacial Maximum’ (LGM): a terrestrial perspective
Philip D. Hughes 1, Philip L. Gibbard 2
1 Geography, School of Environment and Development, The University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom
2 Cambridge Quaternary, Department of Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN, United Kingdom
SUMMARY
The concept of a Last Glacial Maximum (LGM) dominates the Quaternary literature and ideas associated with the last glacial cycle (Weichselian, Wisconsinan, Valdaian Stage, Marine Isotope Stages [MIS] c. 5d-2). However, its meaning and stratigraphical definition is not well-defined. This is despite recent efforts to formalise the term and to define the event or period within time (e.g. Mix et al., 2001). The prevailing view associates the LGM with the maximum extent of ice on land and a corresponding low-stand in global eustatic sea levels – the lowest of the last glacial cycle. However, the global ice signal is based on the marine isotope record and on land the LGM is not clearly represented in glacier records (Hughes et al., 2014). The former is a composite signal and as such provides no indication of spatial and temporal variability of glaciers on the Earth’s surface.
KEYWORDS
LGM; Last Glacial Maximum; Stratigraphy; chronozone; chron; event; climatostratigraphy; asynchronous
The term ‘Last Glacial Maximum’ is widely accepted to refer to the maximum global ice volume during the last glacial cycle corresponding to the trough in the marine isotope record at ca. 18 14C ka BP (Martinson et al., 1987) and the associated global eustatic sea-level low also dated to 18 14C ka BP or c. 21 ka cal. BP (Clark and Mix, 2002; Yokoyama et al., 2004) during MIS 2. Mix et al. (2001) considered the event should be centred on the calibrated date of 21 ka cal. BP, and should span the period 23-19 or 24-18 cal. ka BP dependent on the dating applied (e.g. MARGO project members, 2009). However, other research on the global sea-level minima places the global ice maximum slightly earlier at between 26-21 ka (Peltier and Fairbanks, 2006). Thus, the actual definition of the term 'LGM' is open to debate depending on what criteria are used to define it and today it has no formal stratigraphical status despite attempts to assign the it chronozone status (Mix et al. 2001). Nevertheless, it is clear that a major global glaciation did occur during the broader definition of MIS 2 – this is not in doubt (e.g. Clark et al., 2009). That said, there is increasing evidence to suggest that glacier advances were more extensive earlier in the last glacial cycle, not only in mid-latitude mountain areas (e.g. Gillespie and Molnar, 1995), but also in some of the large continental ice sheets (Ehlers et al., 2011).
The Barents-Kara Ice Sheet in northern Eurasia reached a maximum extent early in the last glacial cycle, together with a majority of ice masses throughout Asia and Australasia. The East Antarctic Ice Sheet also reached its maximum extent several millennia before the global LGM. In numerous mountainous regions at high-, mid- and low-latitudes across the world glaciers reached their maximum extent before MIS 2, often during MIS 4. This is in contrast to most sectors of the Laurentide Ice Sheet, the Cordilleran Ice Sheet, the SE sector of the Fennoscandian Ice Sheet and the Alpine Ice Sheet in central Europe, which appear to have reached their maximum close to the global LGM in MIS 2. The diachronous maximum extents of both mountain glaciers and continental ice sheets during the last glacial cycle, means that the term and acronym Last Glacial Maximum (LGM) may be potentially problematic chronostratigraphically when correlating glacial deposits and landforms.
Which records then, best define the global LGM event on land? This should be determined by records which reflect a change that can be mappable in the stratigraphical record across the globe. Whilst the LGM limit is clearly are not mappable in glacier records across the globe, the concept of peak global ice volume stills holds – with the largest volume locked up in the Laurentide Ice Sheet. This is a major event in the global hydrological system. If the LGM is considered in hydrological terms, then the best records to define this period are those where positive or negative changes are driven largely by global hydrology. This is not the case for glaciers because global hydrology (precipitation) is only one of two major drivers of glaciation (the other being air temperatures). Terrestrial records where positive or negative changes are driven largely by moisture supply include atmospheric dust flux (Ruth et al., 2007; Lambert et al., 2008) and arboreal vegetation densities (e.g. Fletcher et al, 2010). Close inspection of these proxies at sites around the world (recorded in both terrestrial and in marine records) indicates much clearer synchronicity compared with the terrestrial record left by glaciers. However, no matter how the LGM is defined it does not have fixed time boundaries and cannot be given chronozone or chron status. Instead the boundaries defining the LGM need to be flexible and is better defined as an ‘event’ without any chronostratigraphical implication. In many respects the LGM also suits climatostratigraphical classification but is not currently defined as such. The current lack of satisfactory stratigraphical status for the LGM is an issue which is symptomatic of other parts of the Quaternary record. This causes significant problems, especially for correlation, and undermines our understanding of global climate changes during glacial cycles.
References
Clark P.U. & Mix A.C. (2002) – Ice Sheets and sea level of the Last Glacial Maximum. Quaternary Science Reviews 21, 1-7.
Clark, P.U. Dyke A.S., Shakun J.D., Carlson A.E., Clark J., Wohlfarth B., Mitrovica J.X., Hostetler S.W., McCabe A.M. (2009) – The Last Glacial Maximum. Science 325, 710-714.
Ehlers J., Gibbard P.L. & Hughes P.D. (2011) – Introduction. In Ehlers J., Gibbard P.L. & Hughes, P.D. (Eds.), Quaternary Glaciations – Extent and Chronology: A Closer Look. Developments in Quaternary Science 15. Elsevier, Amsterdam, pp. 1-14.
Fletcher W., Sanchez-Goñi M.F., Allen J.R.M., Cheddadi R., Combourieu Nebout N., Huntley B., Lawson I., Londeix L., Magri D., Margari V., Müller U.C., Naughton F., Novenko E., Roucoux K., Tzedakis P.C. (2010) – Millennial-scale variability during the last glacial in vegetation records from Europe. Quaternary Science Reviews 29, 2839-2864.
Gillespie A. & Molnar, P. (1995) – Asynchronous maximum advances of mountain and continental glaciers. Reviews of Geophysics 33, 311-364.
Hughes P.D., Gibbard P.L. & Ehlers, J. (2014) – Evaluating the meaning and significance of the ‘Last Glacial Maximum’ (LGM): a glacial perspective. Earth Science Reviews. [Submitted and currently under review]
Lambert F., Delmonte B., Petit J.R., Bigler M., Kaufmann P.R., Hutterli M.A., Stockler T.F., Ruth U., Steffensen J.P. & Maggi V. (2008) – Dust–climate couplings over the past 800,000 years from the EPICA Dome C ice core. Nature 452, 616-619.
MARGO Project Members (2009) – Constraints on the magnitude and patterns of ocean cooling at the Last Glacial Maximum. Nature Geoscience 2, 127-132.
Mix A.C., Bard E. & Schneider, R. (2001) – Environmental processes of the ice age: land, oceans, glaciers (EPILOG). Quaternary Science Reviews 20, 627–657.
Ruth U., Bigler M., Rothlisberger R., Siggaard-Andersen M.L., Kipfstuhl S., Goto- Azuma K., Hansson M.E., Johnsen S.J., Lu H.Y. & Steffensen J.P. (2007) – Ice core evidence for a very tight link between North Atlantic and east Asian glacial climate. Geophysical Research Letters 34 (3), L03706. doi:10.1029/2006GL027876.
Peltier W.R. & Fairbanks R.G. (2006) – Global glacial ice volume and Last Glacial Maximum duration from an extended Barbados sea level record. Quaternary Science Reviews 26, 862–875.
Yokoyama Y., Lambeck K., DeDeckker P., Johnston P. & Fifield, K. (2000) – Timing of the Last Glacial Maximum from observed sea-level minima. Nature 406, 713-716.