A Stratigraphical Basis for the Last Glacial Maximum (LGM)

Quaternary International xxx (2014) 1e12

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A stratigraphical basis for the Last Glacial Maximum (LGM)

* Philip D. Hughes a, , Philip L. Gibbard b a Geography, School of Environment, Education and Development, The University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom b Cambridge Quaternary, Department of Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN, United Kingdom article info abstract

Article history: The Last Glacial Maximum (LGM) is widely used to refer to the episode when global ice volume last Available online xxx reached its maximum and associated sea levels were at their lowest. However, the boundaries of the interval are ill-defined and the term and acronym have no formal stratigraphical basis. This is despite a Keywords: previous proposal to define it as a chronozone in the marine records on the basis of oxygen isotopes and Last Glacial Maximum sea levels, spanning the interval 23e19 or 24e18 ka and centred on 21 ka. In terrestrial records the LGM LGM is poorly represented since many sequences show a diachronous response to global climate changes Greenland Stadial 3 during the last glacial cycle. For example, glaciers and ice sheets reached their maximum extents at Event stratigraphy Last glacial cycle widely differing times in different places. In fact, most terrestrial records display spatial variation in fl MIS 2 response to global climate uctuations, and changes recorded on land are often diachronous, asynchronous or both, leading to difficulties in global correlation. However, variations in the global hydrological system during glacial cycles are recorded by atmospheric dust flux and this provides a signal of terrestrial changes. Whilst regional dust accumulation is recorded in loess deposits, global dust flux is best recorded in high-resolution polar ice-core records, providing an opportunity to define the LGM on land and establish a clear stratigraphical basis for its definition. On this basis, one option is to define the global LGM as an event between the top (end) of Greenland Interstadial 3 and the base (onset) of Greenland Interstadial 2, spanning the interval 27.540e23.340 ka (Greenland Stadial 3). This corresponds closely to the peak dust concentration in both the Greenland and Antarctic ice cores and to records of the global sea-level minima. This suggests that this definition includes not just the coldest and driest part of the last glacial cycle but also the peak in global ice volume. The later part of the LGM event is marked by Heinrich Event 2, which reflects the onset of the collapse of the Laurentide at c. 24 ka, together with other ice sheets in the North Atlantic region. A longer and later span for the LGM may be desirable, although defining this in chronostratigraphical terms is problematic. Whichever formal definition is chosen, this requires the contribution of the wider Quaternary community. © 2014 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction Molnar, 1995) but also for some of the large continental ice sheets. The current status of the LGM is defined not by the terres- The term ‘Last Glacial Maximum’ has attained iconic status in trial record of global environmental change, but by changes Quaternary Science. The often capitalised letters form the acronym recorded in the ocean floor sediments' oxygen isotope sequence. LGM and suggest a formal stratigraphical status. However, the In recognition of this paradox, Mix et al. (2001) used dated search for a common definition for the LGM is not as obvious as the corals and marine oxygen isotope sequences to suggest the LGM as acronym implies. Hughes et al. (2013) argued that the geomor- a chronozone. However, as noted by Walker et al. (2009) when phological record of glaciations does not provide a clear definition defining the base of the Holocene Series, marine records do not for the LGM since the imprint of glaciers in the geological and offer the resolution to provide a formal stratigraphical basis for geomorphological records is asynchronous around the world. This defining short-duration time divisions, exemplified by the LGM. is not only for mid-latitude mountain glaciers (e.g. Gillespie and Mix et al. (2001) argued that the LGM should be centred on the radiocarbon calibrated date of 21 cal. ka BP and should span the period 23e19 ka or 24e18 ka (all further ages in this paper are calibrated or in calendar kilo years, expressed as ka). However, the * Corresponding author. The University of Manchester, Oxford Road, Manchester fi fi M13 9PL, United Kingdom. boundaries for these chronozones are not de ned in a speci c E-mail address: [email protected] (P.D. Hughes). “body of rocks” (cf. Salvador, 1994) but are arbitrary time intervals http://dx.doi.org/10.1016/j.quaint.2014.06.006 1040-6182/© 2014 Elsevier Ltd and INQUA. All rights reserved.

Please cite this article in press as: Hughes, P.D., Gibbard, P.L., A stratigraphical basis for the Last Glacial Maximum (LGM), Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.06.006 2 P.D. Hughes, P.L. Gibbard / Quaternary International xxx (2014) 1e12 spanning events within in a number of different records. Thus the between the different ice-core chronologies and that “some puzzles chronozone status advocated by Mix et al. (2001) and others such remain to be solved regarding ice-core chronologies near the LGM”. as the MARGO Project (2009) can only be considered an informal Since then, new ice-core records have been obtained and used to labelling of the LGM interval. In addition, there is now evidence that define events at the end of the last glacial cycle (e.g. Andersen et al., the timings of maximum global ice volume and the lowest eustatic 2006; Rasmussen et al., 2006; Lowe et al., 2008; Walker et al., 2009) fall are at the older end of the interval 24e18 ka (cf. Thompson and (Fig. 1). Goldstein, 2006) with Peltier and Fairbanks (2006) suggesting that the LGM occurred as early as 26 ka. 3. Defining the LGM in marine records On land, the evidence for an LGM climate signal is even more transient, leading researchers often to lean heavily on the marine 3.1. Oxygen isotopes records for correlation. However, a truly global LGM time division requires definition in both land-based and marine-based environ- Marine oxygen isotopes, determined from the tests of forami- mental proxies in order to provide a stratigraphical unit that can be nifera from deep-sea floor sediments, have been viewed as a proxy used effectively and meaningfully in correlation. This paper pro- for global ice volume since the 1960s (e.g. Shackleton, 1967). Deep- vides a critique of the various Quaternary records, both marine and water temperatures also play a role and this means that d18O terrestrial. In particular, this paper seeks to establish the most variability in benthic foraminifera is not entirely driven by ice appropriate record(s) and a suitable stratigraphical definition for volume (Shackleton, 2000). Nevertheless, this effect can be this iconic label in the Quaternary geological succession. In this accounted for (Shackleton, 2000) and the basic tenet relating respect, it builds on suggestions proposed at the First International variations d18O to shifts in global ice volume stills holds. Since Conference on Stratigraphy (Hughes and Gibbard, 2014). benthic d18O variability is driven by global ice volume, it also provides a globally synchronous record of glacioeustasy (Skinner 2. Current definitions of the LGM and Shackleton, 2005). Whilst this is true when considering changes over longer timescales (>5 ka) and especially 100 ka The term ‘Last Glacial Maximum’ (abbreviated to LGM) refers to glacial cycles (e.g. Shackleton, 2000; Waelbroeck et al., 2002), the the maximum in global ice volume during the last glacial cycle. The marine isotope record does not offer sufficient resolution to LGM was originally described by CLIMAP Project Members (1976, differentiate environmental events at millennial timescales. This 1981) as spanning the interval 23,000e14,000 14C ka BP, with a is, in part, a consequence of the slow sedimentation rates in the mid-point at 18,000 14CkaBP(Shackleton et al., 1977). It is marked deep oceans and especially bioturbation, which “is a virtually by two independent proxies: in the marine isotope record and universal source of degradation for deep-sea records” (Shackleton, changes in global sea level; and, it is on this basis that the LGM was 1987, p. 183; McCave et al., 1995). However, in addition to this, defined (Mix et al., 2001). there are other significant reasons why marine isotopes such as The d18O signal in the marine record is known to lag global ice benthic d18O cannot provide a globally correlative stratigraphical volume (Mix et al., 2001; Thompson and Goldstein, 2006) and, scheme for fine-resolution intervals such as the LGM (Gibbard, consequently, the global sea-level minimum is likely to be closer to in press). the true global Last Glacial Maximum in terms of maximum ice The oxygen isotope signal in the marine record is often assumed volume. Based on evidence of global sea-level change from the to be a proxy for global ice volume. However, it is not a straight- continental margin of northern Australia, Yokoyama et al. (2000) forward as this and, as noted above, Shackleton (2000) highlighted concluded that the global land-based ice volume was at its that a substantial part of the 100 ka glacial climate cycle, recorded maximum from at least 22e19 cal. ka BP. As noted earlier, the age of by d18O in marine foraminiferal records, is a deep-water tempera- 21 ka is now widely used as a time marker for the acme of the ture rather than an ice-volume signal. Skinner and Shackleton global LGM (Mix et al., 2001; MARGO Project Members, 2009). (2005) showed that fluctuations in benthic d18O and MIS bound- The definition of the LGM in terrestrial records depends on the aries from different hydrological settings may be significantly dia- criteria applied. Shakun and Carlson (2010) used 56 records to chronous. The use of benthic d18O as a proxy for global ice volume recognise a climate-defined LGM. They suggested that a global as established by Shackleton (1967) begins to “break down at average age of 22.2 ± 4.0 ka best defines the LGM. However, they millennial time-scales and in particular across glacialeinterglacial recognised that “there is considerable variation in the timing of transitions” (Skinner and Shackleton, 2005, p. 578). Thus, for rela- these extreme climate states in different records with the LGM … tively short intervals such as the LGM, the marine isotope record is spread over more than 10 kyr” (Shakun and Carlson, 2010, p. 1802). inappropriate for defining its span. Shakun and Carlson (2010) found that the LGM within 56% of their The timescale of the deep ocean d18O signal has been con- records fell within the chronozone span of 23e19 ka, defined by structed by assuming that solar forcing paces variations in d18O Mix et al. (2001), and noted that this chronozone does not appear to variations (Hays et al., 1976) and this provides the basis for the capture the length or variability of the LGM. In their dataset the SPECMAP timescale. Thompson and Goldstein (2006) calibrated largest frequency of climate-defined ‘LGM’ events in the northern this timescale using radiometric dating and found significant dis- hemisphere are at 24 ka and 30 ka (Shakun and Carlson, 2010, their crepancies in the orbital tuning with observed U-series ages from Fig. 4), although younger ‘LGM’ events between 23 and 16 ka result corals. They found that for Marine Isotope Stage (MIS) 2, SPECMAP in a global average close to 22 ka. ages were too young, up to 5.2 ka too young at 17.9 ka, the original Whilst Mix et al. (2001) proposed that the LGM should be date assigned to the trough in d18O for the last glacial cycle by defined as a chronozone, the boundaries of such a unit, as deter- Martinson et al. (1987, event 2.2. in their Fig. 18). The newly mined from a particular type-section, remain elusive. Sea-level and adjusted radiometric calibration of SPECMAP places the trough in ice-core evidence, which provided the basis for the LGM chro- d18O at c. 23.1 ka, the lowest sea levels bracketed between 23.1 and nozone in Mix et al. (2001, their section 5.4), provides broad in- 24.6 ka, with the latter age corresponding to the greatest sea-level dications of the glacial maximum event. However, the bracketing lowering of 132.1 m (Thompson and Goldstein, 2006). An ages do not conform to the strict formal requirements of a chro- offset also exists between the high-resolution coral-derived sea- nozone (cf. Hedberg, 1976; Salvador, 1994). Furthermore, Mix et al. level curve of Thompson and Goldstein (2006) and a global syn- (2001) noted at that time that there were some inconsistencies thetic ‘stack’ of 57 marine oxygen isotope records compiled by

Fig. 1. The ice-core records from Greenland (NGRIP) and Antarctica (EPICA). The top two diagrams are the oxygen isotope (Andersen et al., 2006) and dust concentration (Ruth et al., 2007) records from the NGRIP core. The NGRIP core is on the GICC05 age model. The bottom two diagrams are the dust ﬂux (Lambert et al., 2012) and deuterium-derived temperature (Jouzel et al., 2007) records from EPICA, Antarctica. The EPICA records are both on the EDC3 age model.

Lisiecki and Raymo (2005) e (Fig. 2). In their ‘stack’, Lisiecki and 18 ka, as with SPECMAP) than the sea-level minimum of Thompson Raymo (2005) correlated the interval before 22 ka with the and Goldstein (2006). The lag of d18O behind sea-level minima was radiocarbon-dated benthic d18O record of Waelbroeck et al. (2001) noted by Mix et al. (2001, p. 637) who concluded that “the highest and the interval from 22 to 120 ka with the high-resolution benthic value of benthic foraminiferal d18O is not precisely aligned with the d18O record of Shackleton et al. (2000).InLisiecki and Raymo's lowest sea-level stand, and may even be offset in time by as much (2005) LR04 ‘stack’ the d18O trough occurs c. 5 ka earlier (at as a few thousand years”.

idea of an LGM interval spanning the period 26e21 ka, rather than the earlier interval of 24e18 ka suggested by Mix et al. (2001).

4. Deﬁning the LGM in terrestrial records

4.1. Glaciers

The term Last Glacial Maximum is often used to refer to the peak in global ice volume during the last glacial cycle. This is reflected in the marine oxygen isotope record and also global sea levels recorded in corals (Mix et al., 2001). On land, Clark et al. (2009) reviewed 5704 14C, 10Be, and 3He ages that span the interval from 50 to 10 ka to constrain the timing of the LGM during MIS 2. They found that glaciers advanced between 33 and 26.5 ka and reached maximum positions between 26.5 and 20/19 ka, with rapid deglaciation occurring soon after. However, for the longer interval of the entire last glacial cycle the pattern of glacier advances, and in particular, the maximum extent of ice masses, was not synchronous. Hughes et al. (2013) noted that at high, mid- and low latitudes across the world, glaciers reached their maximum extent before MIS 2, in MIS 5, 4 and 3. It is well-established that mid-latitude mountain glaciers were asynchronous with the global record of ice volume (Gillespie and Molnar, 1995) but increasingly, new dating evidence has revealed that even some of the largest ice sheets were out-of-phase with the record of global ice volume determined from the marine isotope record (Hughes et al., 2013). The East Antarctic Ice Sheet, which today represents the largest modern ice mass on Earth, retreated from its maximum position Fig. 2. Comparison of the global oxygen isotope record, based on a stack of 57 records well before the global LGM during MIS 3 (Stolldorf et al., 2012) and from around the world (top diagram from Lisiecki and Raymo, 2005), and the high- resolution coral-derived sea-level record from Thompson and Goldstein (2006).In in parts of the East Antarctic the ice-sheet thickness at the LGM was the marine d18O record the top 22 ka of the stack is correlated with the 14C dated little different from that today (Mackintosh et al., 2007). In New (calibrated calendar years) benthic d18O record of Waelbroeck et al. (2001) whilst the Zealand, glaciers also retreated during MIS 3, with less extensive e d18 22 120 ka is aligned to the high-resolution benthic O record of Shackleton et al. advances occurring in MIS 2, i.e. close to the global LGM (Putnam (2000). The coral-derived sea-level record is directly dated using U-series techniques et al., 2013). In the Northern Hemisphere the Barents-Kara Ice (Thompson and Goldstein, 2006). The interval represented by Greenland Stadial 3 is indicated by the dashed lines. The offset between the d18O and the sea level records is Sheet, the third largest ice mass of this hemisphere, after the Lau- well-known and is also noted in Mix et al. (2001, see their Fig. 4). rentide and Fennoscandinavian, reached its maximum position early in the last glacial cycle, as early as 90 ka. In Asia, most glaciers reached their maximum before MIS 2, with the global LGM being 3.2. Corals recorded by a less extensive advance (e.g. Dortch et al., 2013; Owen and Dortch, 2014), or not recorded all at (Heyman et al., 2011; Whilst the marine oxygen isotope record can be used as a proxy Stauch and Lehmkuhl, 2011). for sea-level changes, it has low resolution. Corals can be used to The Laurentide Ice Sheet was by far the largest ice mass on Earth date low global sea-level stands and calibrate timescales based on during the last glacial cycle. Evidence from its eastern and southern marine isotope records (Thompson and Goldstein, 2006). Sites that margins supports a maximum phase during MIS 2. At Martha's are suited to the study of corals as a proxy for fluctuating global sea Vineyard, in Massachusetts, boulders on a moraine marking the levels are those which are located close to continental shelves, outer limit of the SE sector of the Laurentide Ice Sheet yielded distant from areas of major global glaciation. Examples include the cosmogenic nuclide exposure ages that cluster between 22 and island of Barbados in the Caribbean Sea (Peltier and Fairbanks, 25 ka with a mean age of 23.2 ± 0.5 ka (Balco et al., 2002). New 2006), the Sunda Shelf in Indonesia (Hanebuth et al., 2000), the production rates make these ages older and Balco and Schafer€ Huon Peninsula in New Guinea (Chappell and Shackleton, 1986; (2006) suggest that the ice front started to retreat from the Mar- Yokoyama et al., 2000), and the Bonaparte Gulf, north of Australia tha's Vineyard moraine at c. 24 ka. These authors pointed out that (Yokoyama et al., 2000). Based on evidence from Barbados, Peltier this new age coincides with Heinrich Event 2, a major period of ice and Fairbanks (2006) argued that the global sea-level minimum rafting in the North Atlantic. All later ice-front positions in New occurred at 26 ka and remained low (within 5 m of the lowest England and Connecticut indicate less extensive glaciation (Balco stand) for 5 ka until 21 ka. This contrasts with the findings of et al., 2002; Balco and Schafer,€ 2006). There is also evidence from Yokoyama et al. (2000) who concluded that the lowest sea-level other parts of the Laurentide Ice Sheet margin which suggest an ice stand in the Bonaparte Gulf occurred between 22 and 19 ka. maximum before 23 ka, such as in Ohio (Lowell et al., 1999; Szabo Peltier (2002) argued that Yokoyama et al.'s (2000) mathematical et al., 2011), Illinois (Curry et al., 2011) and North Dakota (Manz, analysis was flawed and this was reiterated by Peltier and Fairbanks 2005). In NW Canada, Zazula et al. (2004) used radiocarbon to (2006). As noted above, Thompson and Goldstein (2006) analysed date pro-glacial lake sediments and suggest that the maximum U-series data from 11 papers and recalculated ages accounting for extent of the Laurentide Ice Sheet in this area occurred between 35 open-system behaviour (e.g. Thompson et al., 2003). Thompson and 22 ka followed by a less extensive readvance at 22e16 ka. These and Goldstein (2006) found that the recalculated ages place the ages, from the outer margins of the last Laurentide Ice Sheet, eustatic low stand between 23.1 and 24.6 ka (Fig. 2). This appears to correspond to global sea-level evidence compiled by Peltier and support the arguments of Peltier and Fairbanks (2006) and their Fairbanks (2006) and Thompson and Goldstein (2006) who place

Fig. 4. Ice volume of the Laurentide Ice Sheet during the last glacial cycle. The shaded parts of the curve represent the margins of error in the modelling (redrawn and adapted based on Stokes et al., 2012, their Fig. 5a). MIS ¼ Marine Isotope Stage. The lower diagram shows the extent of ice at the Last Glacial Maximum over North America.

and driest part of the Late Pleistocene occurred during the interval continental climates during the period around the LGM. Thus, it is 18.5e14.6 ka (Moreno et al., 2012). Elsewhere in the Mediterranean, clear from the small sample of vegetation records included here whilst vegetation records appear to match climatic events recorded that pollen stratigraphy is unlikely to provide a unified signal of in the Greenland Ice Sheet there is no obvious unified LGM global climate change around the world that would facilitate event with vegetation shifts sometimes more pronounced during defining a unified global LGM signal on land. Heinrich stadials (cf. Fletcher et al., 2010). The LGM is also poorly defined in other parts of the world in 4.3. Loess vegetation records. For example, in Central America, Hodell et al. (2008) found that the period around the LGM between 23 and The loess sequences of the world have long been regarded as 19 ka was characterised by temperate pineeoak forest. As in Iberia, some of the best preserved records of continental global change the driest climate phase in lowland Central America occurred later (e.g. Kukla and An, 1989). In the Chinese loess, An et al. (1991) between 18 and 14.7 ka and both Moreno et al. (2012) in Iberia and suggested that the maximum dust flux of the last glacial cycle Hodell et al. (2008) in Central America relate this to the so-called was attained during the global Last Glacial Maximum, close to ‘Mystery Interval’ e a particularly ill-defined and improperly 20 ka. They took the median value of the loess grain size and the named term. However, in both of these areas there is also a cold and aeolian dust flux (bulk density multiplied by a linear deposition arid phase in the period immediately pre-dating the period rate), as well as CaCO3 and magnetic susceptibility variations, to 23e19 ka. In the Southern Hemisphere tropics, the period around infer the general state of the atmosphere. On this basis, and con- the LGM is complicated by a precession-driven insolation strained by two thermoluminescence ages, An et al. (1991) then maximum which caused higher-than-present precipitation as tuned the loess record to the SPECMAP oxygen isotope curve. Porter monsoons strengthened (e.g. Baker et al., 2001a, 2001b; Cruz et al., (2001) noted that “alternating loessepaleosol stratigraphy closely 2005, 2009). Conversely, Whitney et al. (2011) found that vegeta- resembles the marine oxygen-isotope record, implying that tion records indicate that the LGM was drier than today in the episodic dust deposition and pedogenesis are in phase with global Pantanal basin, Paraguay, highlighting the regional complexity of ice-volume fluctuations”. This approach led to numerous other

Please cite this article in press as: Hughes, P.D., Gibbard, P.L., A stratigraphical basis for the Last Glacial Maximum (LGM), Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.06.006 P.D. Hughes, P.L. Gibbard / Quaternary International xxx (2014) 1e12 7 papers, which hailed the loess records as some of the best terres- signal in the Greenland ice-core profile (Andersen et al., 2006; trial records available (cf. review by Porter, 2001). Rasmussen et al., 2006; Svensson et al., 2006, 2008; Lowe et al., Lu et al. (2007) used OSL to date rates of dust accumulation in 2008) and this record is also used to define the base of the Holo- the Chinese loess sequences. They found that high dust accumu- cene Series (Walker et al., 2009). It is logical, therefore, to consider lation rates occurred during MIS 3, whereas the colder intervals of the Greenland Ice-Core Chronology 2005 (Andersen et al., 2006) for MIS 4 and 2 had lower dust accumulation rates. Lu et al. (2007) also the definition of the boundaries of the LGM interval (Figs. 3 and 4) questioned the relationship between loess grain size and dust flux. building on the already well-established event stratigraphy (Bjorck€ Regional variability in dust accumulation and questions over the et al., 1998; Walker et al., 1999; Lowe et al., 2001, 2008; Blockley relationship between loess grain size and atmospheric dust flux et al., 2012). The main basis for defining boundaries in the NGRIP suggest that loess records may not be appropriate for defining ice-core records rests on the counting of annual ice layers. The global climatic events such as the global LGM. This evidence also dating of the period spanning 14.7e41.8 ka is described by highlights the problems in tuning loess records to those from the Andersen et al. (2006) who define the onset of various Greenland marine isotope profiles, which has been widely practised. Interstadials in this interval. The annually-counted d18O signal in the GICC05 record is simply used as a time reference with which 4.4. Speleothems boundaries can be defined and some other criterion is required in order to define the LGM. However, to determine a global reference Speleothems provide a subterranean record of cave water interval that characterises a distinct environmental signal such as chemistry, which can be related to conditions at the surface and in the global Last Glacial Maximum both hemispheres need to be the atmosphere. The advantage of speleothems is that they can be considered. The ice-core records from the Northern and Southern dated with high precision using U-series techniques. This means Hemisphere polar ice sheets provide an obvious target for this that speleothem datasets have an independent chronology which purpose. The Antarctic ice cores can also be used to define events “offers opportunities to critically assess leads and lags in the and correlations between Greenland and Antarctic ice-core records climate system” (McDermott, 2004, p.901). Unlike marine records, have been made by Blunier et al. (1998) and Blunier and Brook this means that speleothem records do not need to rely on ‘tuning’ (2001). isotopic signals to fit pre-existing concepts of climate change It has been suggested that the events preserved in the Antarctic drivers, such as the orbital theory (McDermott, 2004). It has been and Greenland ice-cores are asynchronous, with climate changes in shown that the isotope signal in speleothems has the potential to the former leading the latter by 1e2.5 ka over the period 27e23 ka reflect global climatic changes. For example, at Hulu cave in SE (Blunier et al., 1998). However, Steig and Alley (2003) argue that China, the oxygen isotope record from a speleothem closely whilst the Antarctic and Greenland climate records display anti- matches that recorded in the Greenland ice-cores (Wang et al., phase behaviour about 50% of the time, they are generally in- 2001). However, not all speleothems are suitable for defining the phase during cooling. A later paper by EPICA (2006) showed that global LGM. Water in cave systems is likely to have frozen during there was a one-to-one coupling of climate variability between the coldest part of the last glacial cycle in the mid- to high-latitudes Greenland and Antarctica, with warm events in the latter corre- and at high altitudes in the tropics, and “calcite deposition ceases sponding with cold events in the former. Fig. 3 shows that in both when an area is subjected to glacial conditions” (Gascoyne, 1992,p. Antarctica and Greenland the coldest parts of the period 30e20 ka 614). Thus, the global LGM is often recorded by an hiatus in spe- occurred during Greenland Stadial 2, although the patterns leading leothem sequences in caves. This issue is less of a problem at lower up to this interval are different with a significant cold event latitudes. However, at many mid-latitudes calcite precipitation occurring in Greenland during Heinrich Event 3. However, for ceased during the LGM (e.g. Serefiddin et al., 2004; Constantin Greenland Fig. 3 only presents the oxygen isotope curve (which et al., 2007), whilst in others it continued (e.g. Moreno et al., reaches its lowest point during Greenland Stadial 3, 46.50 d18Oat 2010; Rowe et al., 2012). This limits the application of speleo- 26.560 ka, Andersen et al., 2006). Temperature reconstructions thems for the definition of such an interval. using d15N indicate the lowest temperatures at c. 33 and 44 ka (Kindler et al., 2013). Thus the temperatures over the polar sheets 4.5. Ice cores are not suitable criteria in isolation for defining the global LGM. Given the asynchronous millennial-scale climate changes Ice-core records provide information on the state of the atmo- recorded in the Antarctic and Greenland ice cores, rather than sphere through time. Oxygen isotopes are used as an approximate relying solely on isotopes (such as d18O, d18D, d18N) to match these proxy for air temperatures at the time of snow fall (Johnsen et al., records, a better tool for global correlation is dust flux, especially for 2001). Deuterium and nitrogen isotopes improve the precision of identifying a world-wide hydrological phenomenon such as the temperature reconstructions, in the Antarctic and Greenland ice global LGM. As noted above, the amount of dust in the atmosphere cores, respectively (Jouzel et al., 2007; Kindler et al., 2013). Other is influenced by climate and the extent of dust sources, which is indicators, such as dust concentrations, can also provide insight strongly influenced by vegetation cover (which itself is climatically- into the state of atmosphere through time (e.g. Ruth et al., 2007). driven) (Harrison et al., 2001). Vegetation cover is influenced by The ice-core records from Greenland have been used to develop an atmospheric moisture supply, which is in turn influenced by air and event stratigraphy for the last glacial cycle (Bjorck€ et al., 1998; ocean-surface temperatures. Walker et al., 1999; Lowe et al., 2001, 2008; Blockley et al., 2012). The dust concentration in the NGRIP core can be used as the This existing framework can be explored as one approach available main criterion to define the LGM event in the GICC05 record. These for defining the LGM and this is the focus of the next section. data are provided by Ruth et al. (2007). The peak dust concentrations in the NGRIP core were chosen as a marker for the LGM. Peak 5. Defining the LGM in the ice-core stratigraphy values of >8000 mgkg 1 occur between 25.7955 and 25.4598 ka (in the GICC05 timescale). This peak in dust occurs in Greenland Stadial Gibbard and West (2000, 2014) have argued that marine and 3 between Greenland Interstadials 3 and 2. Thus, based on the terrestrial records both need to be considered in relation to the event stratigraphy of the NGRIP GICC05 record, the global LGM can high-resolution ice-core records. In the Northern Hemisphere the be correlated with Greenland Stadial 3 which spans the interval Late Pleistocene has been subdivided into events based on the d18O 27.540e23.340 ka (Lowe et al., 2008)(Figs. 3 and 4). Given that it is

Please cite this article in press as: Hughes, P.D., Gibbard, P.L., A stratigraphical basis for the Last Glacial Maximum (LGM), Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.06.006 8 P.D. Hughes, P.L. Gibbard / Quaternary International xxx (2014) 1e12 desirable for the LGM to be defined as global signal in both hemi- these reservations regarding the use of isotopic records for global spheres, the dust concentrations from the NGRIP core were correlations, there is evidence that the Greenland ice-core isotopic compared with the record from EPICA (Antarctica) (Figs. 1 and 3). In signal is also represented in tropical cave speleothems, such as in the EPICA ice core record the highest, and most sustained, dust flux Hulu Cave (Wang et al., 2001). Nevertheless, caution is required to of the last glacial cycle also occurs between 27 and 24 ka during a ensure that environmental events in other types of record are not time interval equivalent to Greenland Stadial 3 (Figs. 2 and 3). simply forced into line with the Greenland ice-core isotopic signal. The global LGM can be defined in terms of global dust flux and Correctly defined chronostratigraphical units for the subdivision of recorded by dust concentration or dust flux records in Greenland the Late Pleistocene would avoid this potential pitfall. and Antarctica. In these records this occurred within Greenland Previous definitions of the LGM have attempted to define the Stadial 3 (27.540e23.340 ka). Some peaks in dust flux occurred interval as a chronostratigraphical unit of unspecified rank, a close in time to Heinrich Events (Fig. 3). For example, in both the chronozone spanning the intervals 24e18 or 23e19 ka (Mix et al., NGRIP and the EPICA ice-cores, significant peaks in dust flux occur 2001). However, as discussed in Section 3 above, it is now ques- between 24 and 25 ka, corresponding with Heinrich Event 2 in the tionable whether the criteria originally chosen by Mix et al. still fall North Atlantic. This peak in dust was less sustained than the earlier within their suggested chronozone time span. Applying a chro- dust peak. In the NGRIP core the 24 ka dust peak was smaller than nozone status to the LGM using ice-core records is equally prob- the earlier dust peak at 25.8 ka. In the EPICA core the 24e25 ka dust lematic. According to Salvador (1994, p. 83), “the time span of a peak was larger than the earlier dust peak between 27 and 25 ka, chronozone is the time span of a previously designated strati- although this earlier period saw a more sustained period high levels graphic unit or interval”. This previously designated stratigraphic of dust flux (Fig. 3). It is clear, therefore, that dust peaks in the in- unit or interval could be an event such as Greenland Stadial 3 in the terval 27e24 ka were not simply driven by short-term climate ice-core record. However, as noted in the previous section, there are pertubations caused by Heinrich Events, but were driven by larger- leads and lags between the isotopic signals in ice cores between scale changes in the global hydrological balance. Greenland and Antarctica. Boundaries of chronozones are required Heinrich Event 2 represents one of the most significant pertu- to be isochronous, i.e. time-parallel (Bjorck€ et al., 1998). However, it bations in the global climate system. It is shortly followed by a would not be possible to define globally isochronous boundaries for major accumulation of ice-rafted debris in the North Atlantic ma- a chronozone based on isotopic signals in either Greenland or rine sequences for the last glacial cycle (Bond et al., 1992; Scourse Antarctica. et al., 2009). This corresponds with the fact that Heinrich Event 2 Event stratigraphy offers a suitable alternative, especially since was associated with the largest accumulation of ice on Earth during it is already applied for the Greenland ice-core record by Bjorck€ the last glacial cycle, as indicated by the maximum extents of both et al. (1998). In event stratigraphy the boundaries between events the Laurentide and the BritisheIrish Ice Sheets in the North Atlantic are not specifically designated and problems of time transgression region (Balco and Schafer,€ 2006; Scourse et al., 2009). Heinrich that have arisen in applications of the terrestrial chronostratig- Event 2 is also clearly marked in sea-level records and global dust raphy are no longer encountered (Bjorck€ et al., 1998, p. 289). In flux as well as temperatures over Antarctica (Figs. 1 and 3). Thus, effect, this recognises that ‘events’ can be diachronous. An example the termination of the last glacial cycle may be associated with the of this is where event stratigraphy allows for eustasy transgressions largest ice rafting episode of Heinrich Event 2 and this event could and regressions (Whittaker et al., 1991). This is important because it represent a marker close to the end of the global LGM (Fig. 3). For is apparent from ice-core records from the two hemispheres that some, the suggested window for the LGM may be too short and climate change is not coeval, especially at millennial timescales, exclude important events after 23 ka. An alternative option is to which include distinct leads and lags. Whittaker et al. (1991, p. 820) extend the LGM event through to the onset of Greenland Inter- noted that “perhaps the best prospects for high resolution event stadial 1, taking the span of the interval from 27.5 ka to c. 14.7 ka. stratigraphy come from geochemical techniques, for instance by This would encompass both Heinrich Events 1 and 2 and certainly the use of stable isotopes such as oxygen, carbon, sulphur and encompass the global LGM in most records. However, there is no strontium”. In this respect either marine or ice-core isotope curves clear stratigraphical justification for defining the end of the LGM can be used as a basis for event stratigraphy. However, the impor- between Greenland Interstadials 1 and 2 (within Greenland Stadial tant difference between the ice-core and the marine isotopic re- 2) between 28 and 20 ka, the traditional timing for the end of the cords is that the former has much greater resolution and is most LGM. Thus, correlation with Greenland Stadial 3 offers the best suited to defining fine-resolution stratigraphical intervals for the solution when seeking a formal stratigraphical definition. This late Quaternary. As such, the base of the LGM is defined above using approach also better reflects the transient nature of the global LGM the Greenland ice-core stratigraphy and is based on the mid-point as an event within a complex of millennial scale changes during the isotopic transition between Greenland Interstadial 3 and Stadial 3, last glacial cycle. in a similar way to the practice adopted for the subdivision of marine isotope records. 6. Event versus chronozone for the global LGM If the Late Pleistocene were to be subdivided using chronostratigraphy then this would exist alongside an already estab- The ice-core records provide a useful basis for defining the lished event stratigraphical scheme based on the NGRIP ice-core global LGM within an existing event stratigraphy developed for record. The recent INTIMATE project has defined and correlated Greenland. However, there are limitations to relying solely on event events in the Greenland ice-core stratigraphy with those repre- stratigraphy for global correlation. For example, the global LGM sented in marine and other terrestrial sequences (Bjorck€ et al., until now was not clearly defined in the ice-core records. Relying on 1998; Walker et al., 1999; Lowe et al., 2001, 2008; Blockley et al., Greenland interstadials or stadials for global correlation could lead 2012). This project focused on the last glacialeinterglacial transi- to the same erroneous correlations that have arisen from attempted tion and its remit spanning the past 30 ka. However, the issue of the equation with marine isotope stages. Environmental signals LGM and its stratigraphical significance was not examined although defined by isotopes, as atmospheric temperatures in the case of ice Bjorck€ et al. (1998) do discuss the issue of the chronostratigraphical cores or ice volume in the case of marine sediments, may not problems for the Late Pleistocene in a broader sense. Event stra- translate as an environmental signal in other types of sequences. tigraphy can, and indeed should, reside within a broader longer- Hughes et al. (2013) made this point for glacier records. Despite term chronostratigraphical framework (such as the Late

Pleistocene Subseries) and should not replace locally-defined required for calibrating all dating techniques. Since all dating terrestrial chronostratigraphical frameworks. Indeed, Lowe et al. techniques require independent calibration, it would be erroneous (2008, p. 7) stated that “INTIMATE recommended that all site re- to assume that one dating technique can simply be tested against cords should initially be designated using the appropriate local another to ensure validation. terminology, and that the timing and duration of local or regional As outlined in this paper, definition of the LGM as an event climatic/environmental events be established independently of the within the Greenland ice-core record has the advantage of building ice-core record”. However, in the search for a definition of the on the existing event stratigraphy for the last glacial cycle that is global LGM this has to be made with reference to some globally already in place. However, it is only one possibility. Some may recognisable stratigraphical sequence. If not, then the term ‘global consider that the definition of the LGM as an equivalent within LGM’ or even ‘LGM’ should be abandoned. For stratigraphical Greenland Stadial 3 as too narrow. This is perhaps true when purists this may seem the best course of action, but a more prag- considering the results of Shakun and Carlson (2010) who argued matic approach recognises that this will simply be ignored by the that the climatically-defined LGM should be placed at 22.2 ± 4.0 ka. majority of the Quaternary community. Thus, the compromise is to Thus, broader definitions may be more appropriate. Whichever lean on the ice-core event stratigraphy developed for Greenland formal definition is chosen, this requires the contribution of the and define the global LGM as coeval with Greenland Stadial 3. wider Quaternary community. This paper does not seek to impose Whilst this itself is a regional environmental signal with evidence any definition, simply to raise awareness that the current status of of complexity within this time interval in other types of record the LGM is insufficient and in need of revision and definition. Any (Austin et al., 2012), it does allow for cross-correlation with similar definition will require the integration of marine, ice-core and records (i.e. dust flux) in Antarctica and thus provide an inter- terrestrial records and the collaboration of scientists with a wide hemispheric framework for defining the global LGM. Other local range of specialisms. and regional climatic/environmental events should not be expected to fall within the narrow time span of a global LGM defined by Greenland Stadial 3. In fact, it should be expected that some local 7. Conclusions and regional environmental signals recorded by different types of Quaternary records will not coincide with the global LGM (i.e. The global LGM is not formally defined and this poses problems Hughes et al., 2013). for correlation of global climatic events that are recorded in It is now becoming clear that in many Quaternary records the different types of sediments and landforms. Previous definitions global LGM was not always the most significant time interval proposed the LGM as a chronozone spanning the interval 23e19 ka. marked by geomorphological, biological, climatological or other However, the fact that the LGM (and isotope boundaries in general) signals during the last glacial cycle. Recent advances in geochro- cannot be easily defined by isochronous boundaries within a nological techniques means that Quaternary sediments and land- sediment or ice-core sequence, means that it is difficult to define as forms can be dated with greater accuracy and precision. This a chronostratigraphical unit (Salvador, 1994,p.78e85; Ehlers et al., technological advance has led to fundamental assumptions of time- 2011). Nevertheless, the global LGM can be defined within event equivalence being undermined. For example, in the glacier records stratigraphy comparable to that already adopted for the Greenland what were assumed to be LGM moraines are now known to be ice-core sequence (cf. Bjorck€ et al., 1998; Lowe et al., 2008). The much older in many parts of the world and in some cases (albeit global LGM can be defined as an event coeval with Greenland only a few) evidence of an LGM advance is absent altogether Stadial 3 (27.540e23.340 ka) e as already defined in the NGRIP (Hughes et al., 2013). Greenland ice-core stratigraphy. This event encompasses several Given the statements above, it is tempting to assume too much key environmental global markers including the global sea-level focus on geochronology. To do this over and above a robust strati- low-stand (Peltier and Fairbanks, 2006; Thompson and Goldstein, graphical framework is likely to lead to problems similar to those 2006) and peaks in dust concentrations or dust flux recorded in which geochronological techniques have exposed. Geochronology the Greenland and Antarctic ice-core records (Ruth et al., 2007; should be tied to an independent stratigraphical framework that is Lambert et al., 2012). These environmental effects also coincide both internally consistent (within the same types of record) and with the largest expansion of the Laurentide Ice Sheet to its also externally consistent (allowing timeeequivalent correlations maximum (Fig. 4) and other major ice masses, such as that over the between different types of record). It is becoming apparent that the British Isles and Ireland, which reached maximum extents at c. previous basis for global Quaternary stratigraphy, the marine oxy- 24e25 ka (Balco and Schafer,€ 2006; Stokes et al., 2012; Chiverrell gen isotope record, was neither. This has led to erroneous corre- et al., 2013). lations between many different types of records. There is a risk that This discussion demonstrates the difficulties and potential pit- future advances in Quaternary Science are driven by geochronology falls that can arise when attempting to define a significant global with stratigraphical considerations deemed obsolete. However, event such as the LGM, an event which many would consider as one advances in geochronological techniques are reliant on robust of the most strongly defined markers of the Late Pleistocene. The stratigraphical frameworks. A good example is the current issue clear message is that problems comparable to those outlined in this surrounding production rates in terrestrial cosmogenic nuclide article are very likely to hold for most, if not all, potentially global exposure dating. In order to constrain production rates in surface events that might be identified in the future. The implication, rocks, surfaces of known age are required. In order to have faith in therefore, is that in the majority of cases, it cannot be assumed that the geochronological technique the time-equivalence of the sample a climate signal is likely to have a simple record across the globe. must be independently determined with confidence prior to calibration. Of equal geochronological/stratigraphical importance is that the geomagnetic field instabilities associated with both re- Acknowledgements versals and excursions lead to enhanced production of cosmogenic isotopes. Therefore the application of cosmogenic isotopes for We thank an anonymous reviewer, Thijs van Kolfschoten and surface exposure dating or as chronostratigraphical markers re- Martin Head for their helpful comments on an earlier draft of this quires a complete understanding of geodynamic instabilities paper. We also thank Graham Bowden (The University of Man- (Singer et al., 2009; Singer, 2014). Independent age confirmation is chester) for drawing the figures.

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