The Younger Dryas: Relevant in the Australian region?

Quaternary International 253 (2012) 47e54

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The Younger Dryas: Relevant in the Australian region? John Tibby Geography, Environment and Population, University of Adelaide, North Terrace, Adelaide, South Australia 5005, Australia

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 6 January 2012

An assessment of Australian climate during the Younger Dryas Chronozone (YDC) is presented. This review focuses on securely dated records from sites of continuous deposition, placing greatest emphasis on temperature reconstructions, with records of effective precipitation (i.e. the combined effect of precipitation minus evapotranspiration) also considered. While there is a paucity of Australian records covering the last glacial interglacial transition, particularly those which directly infer temperature, sufficient data exist to examine YDC climate from southern and eastern Australia. Temperature reconstructions from Tasmania, based on both chironomid and pollen data, show no evidence for Younger Dryas cooling. By contrast, there is evidence for cooling associated with the Antarctic Cold Reversal, from Tullabardine Dam pollen data and the sediment organic content from Eagle and Platypus Tarns in Tasmania. Records from a number of eastern Australian mainland sites provide no evidence of effective precipitation shifts concurrent with the Younger Dryas Chronozone. Similarly, reconstructions of discharge from the Murray-Darling Basin, which covers a large proportion (14%) of the Australian continent, and dust transport from a larger portion of the continent also show no evidence of climate shifts concomitant with the Younger Dryas. Of research published in the past decade, only one study, located in the Great Australian Bight, claims evidence of a YDC cooling (Andres et al., 2003). By contrast, this review suggests that there is no conclusive evidence for cooling, or indeed any distinctive climate patterning, during the Younger Dryas Chronozone in Australia. Ó 2012 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction Multiple lines of evidence indicate that, in addition to the orbitally forced patterns of glacial-interglacial climate fluctuations operating on Milankovitch timescales, late Quaternary climates have experienced a number of shorter term climatic fluctuations (e.g. Bond et al., 1995). In terms of its magnitude and duration, the Younger Dryas, is foremost amongst the millennial-scale variations in Northern Hemisphere climates to occur since the Last Glacial Maximum. During the Younger Dryas Chronozone (or YDC defined as the period 12,850e11,650 cal a before 1950 sensu Rasmussen et al., 2006) temperatures in central Greenland cooled by approximately 8
C (Alley, 2000) and there were widespread declines in temperature across much of the Northern Hemisphere, with generally reduced magnitude at lower latitudes (Shakun and Carlson, 2010). However, in a recent consideration of last glacial interglacial transition (LGIT) climates, Menviel et al. (2011) noted that the effect of the Younger Dryas in the Southern Hemisphere remained “somewhat elusive.” Understanding the presence and extent, or absence, of YDrelated climate variability in Australia is important since it can E-mail address: [email protected]. 1040-6182/$ e see front matter Ó 2012 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2012.01.003

improve understanding of the Earth’s variable ocean and atmospheric systems. Specifically, it enables assessment of the relative influence of climate forcing in different regions. More particularly, given that high latitude climates in the two hemispheres are argued to operate in a see-saw like manner (Broecker, 1998; Pedro et al., 2011), consideration of the LGIT in Australia allows the influence of “North Atlantic” and “Antarctic” climate patterns to be evaluated over a broad range of Southern Hemisphere latitudes. At a finer spatial scale, reconstructions through the LGIT in Australia allow testing of important hypotheses about key climate processes. These include the latitudinal extent of the ITCZ and westerly wind belts (and their influence on global CO2 dynamics, Hodgson and Sime, 2010) and the post glacial pattern and development of both ENSO (Turney et al., 2004) and monsoonal systems (Spooner et al., 2005; Williams et al., 2010). Recent reviews of Australian late Quaternary climates have considered the LGIT more broadly, though with only two sites from Australia (Turney et al., 2006b) or longer time periods (Williams et al., 2010). By contrasts, this study focuses specifically on the Younger Dryas since it is the dominant non-orbitally forced feature of the LGIT in much of the Northern Hemisphere and because its influence on Southern Hemisphere climates is strongly debated,

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though more so in locations other than Australia (Markgraf, 1993; Turney et al., 2003), arguably due to a lack of suitable sites with which to test hypotheses. This evaluation of the Younger Dryas in Australia focuses on palaeoenvironmental archives with continuous deposition, in particular those from lake and marine sediments which are the dominant source of LGIT palaeoclimate data from Australia. Hence, geomorphological records which are discontinuous in nature (Williams et al., 2010) are not considered. In addition, emphasis is placed on records that have age with centennial scale accuracy. In essence, this means a focus on sites with multiple age determinations through the LGIT and where the provenance of the dated material provides confidence in the derived ages (Section 1.2). As such, the review is biased towards recently published studies. However, some consideration is given to older studies where the YD has been specifically considered (e.g. De Deckker et al., 1990; Goede et al., 1996). Also proffered are key criteria required to properly consider the nature of the YD in Australia. 1.1. The Younger Dryas: causal mechanisms and Southern Hemisphere linkages Although a number of causal mechanisms are purported for the YD (Carlson, 2010), including most controversially the impact of an extra terrestrial object (Pinter et al., 2011), this review takes, as its starting point, the notion that the YD was caused by a large scale reduction in Atlantic Ocean meridional overturning without identifying the precise causal mechanism. Following from this, this review then specifically assesses whether this cooling of the North Atlantic resulted in cooling of Australian climates or any other notable expression climatic variation during the Younger Dryas Chronozone (YDC). In particular, it considers whether there is any evidence associated with the operation of a bipolar climate see-saw during the YDC. 1.2. Ideal criteria for evaluating the YD in Australia Records used to evaluate the importance of the YD in Australia (or indeed anywhere) should fulfil a number of criteria. These criteria are: 1. They should span the Younger Dryas Chronozone, 2. be contiguous, of high resolution and 3. be securely dated. In addition, they should: 4. cover a period sufficient to allow the climate patterns observed during the YDC to be placed in context, and 5. lastly, the reconstructions should provide estimates of temperature. As is observed in Section 2.3, there are no studies where all criteria are met in a single site. However, through a consideration of the accumulated evidence from a number of sites, it is possible to consider the influence of the Younger Dryas in Australia. In relation to criterion 3, since the YDC is of relatively short duration, then climate records used to evaluate the extent of Younger Dryas related climate variability must be adequately dated. Ideally, this occurs through dating of material with a known origin where any sources of error can be rigorously controlled (e.g. through calibrated 14C dating of terrestrial plant macrofossils) and where sufficient chronological markers exist to estimate the position of the YDC in records based on the overall pattern of the ages

(e.g. through Bayesian modelling), rather than via simple interpolation between individual points (Blaauw et al., 2007). Records that assess the YD should, ideally, span the full range of conditions from the Last Glacial Maximum to Holocene to allow climates in the YDC to be placed in the context of the full LGIT (criterion 4). Explicitly, this allows for any climate “reversals” (or other trends) to be assessed relative to the full range of LGM to Holocene climate. This approach is demonstrated by Shakun and Carlson (2010) who explicitly express YDC temperature and climate changes as a proportion of the total differences between the last glacial maximum and the Holocene altithermal. Furthermore, records of this length from a specific site can be “calibrated” against well documented deglacial climate patterns, thus testing the record’s skill in recording large scale climate trends. Finally, since the Younger Dryas is, primarily, a temperature phenomenon, then any strict assessment of the global expression of the YD, including in Australia, should consider the record of temperatures during the YDC (criterion 5). Ideally, such reconstructions should be quantitative, since this allows for an explicit examination of the magnitude of climate shifts through the YDC. However, given that regional temperature patterns also affect those of rainfall, this review also considers the changing nature of precipitation through the YDC. 2. Results and discussion 2.1. Overview Shakun and Carlson (2010) synthesise global temperature records of the transition from the LGM to The Holocene. In their review, no Australian sites are included, presumably as no reconstructions extend to 25 ka (a criterion for inclusion in Shakun and Carlson, 2010). While the resolution of many sea surface temperature (SST) records is too low to permit an assessment of the Younger Dryas in Australia, some SST reconstructions from the continental margin have a resolution high enough to permit consideration of YDC climates (e.g. Andres et al., 2005; Calvo et al., 2007 see Fig. 1 for locations). Furthermore in continental locations, chironomid and pollen based temperature reconstructions have recently been derived in Tasmania (Fletcher and Thomas, 2010; Rees and Cwynar, 2010; Fig. 1). The Tasmanian reconstructions are valuable since their southerly setting is likely to be sensitive to global temperature change, as higher latitudes are most responsive to such change at present (Cai and Lu, 2007) and in the past (i.e. LGIT increases, Barrows et al., 2007a; Shakun and Carlson, 2010). Apart from these limited temperature based estimates, most evidence for Australian climate in the YDC is derived from data sources that reflect parameters other than temperature, or temperature in combination with other variables. Specifically, most proxies reflect “effective precipitation” (Williams et al., 2010), the net outcome of precipitation minus evapotranspiration. Hence, records not specifically reflective of temperature constitute the bulk of this review. There is an implicit tendency to suggest that any YD related effects on effective precipitation would result in greater aridity in Australia (e.g. Black and Mooney, 1996; Gingele et al., 2007; Petherick et al., 2009). This is presumably due to hypothesised reductions in sea surface temperature resulting in reduced convective moisture generation. However, while this review takes this possibility as a starting point it recognises that, increased effective precipitation in Australia associated with the YD is a theoretical possibility. Such an occurrence would result from increased Southern Hemisphere sea surface temperatures to the south of the continent resulting from a bipolar see-saw. Herein, LGIT records from temperate Australia are presented first, followed by those from the tropics. These records are discussed by

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Fig. 1. Locations of sites mentioned in the text. Letters correspond to the records illustrated in Fig. 2. Also shown is the approximate boundary of dust provenance in Petherick et al. (2009) and the catchment of the Murray Darling Basin that discharges terrigenous material examined from core MD03-2611 (location f; Gingele et al., 2007).

decreasing latitude, given the sensitivity of high latitudes to global cooling. Some of these records, specifically the dust flux record from North Stradbroke Island (Petherick et al., 2009) and Murray River discharge record (Gingele et al., 2009), source material from both the temperate and tropical zones though their catchments are predominantly located in temperate climate zones (see Fig. 1). Also considered is a composite charcoal record from mainland Australia which is dominated by sites in the temperate zone (Mooney et al., 2011). 2.2. Temperate and arid Australia New quantitative temperature estimates through the LGIT have recently been published from four Tasmanian sites. Two are derived from chironomid-based reconstructions of the temperature of the warmest quarter (Rees and Cwynar, 2010), with the others reanalyses of published pollen records with a new pollen-annual temperature transfer function (Fletcher and Thomas, 2010). The pollen and chironomid transfer functions have similar precision with root mean squared errors of prediction of 1.0
C and 0.94
C, respectively. It is likely that the chironomid-based reconstruction is more sensitive to short-term temperature shifts, since vegetation, and therefore pollen, may take centuries to equilibrate with climate (Ritchie, 1995). Although limited by both a low sampling resolution and radiocarbon dates determined on bulk sediment, Fletcher and

Thomas (2010) conclude, from their pollen-temperature reconstructions, there is no evidence for a YD related temperature decline in western Tasmanian pollen records. By contrast, they argue for a reversal in temperatures (<1
C Fig. 2d) that is concomitant with the Antarctic Cold Reversal (ACR) from Tullabardine Dam, with no evidence of either YD or ACR temperature reversals from Lake Selina. An absence of Younger Dryas cooling in Tasmania is also indicated by chironomid-based reconstructions of warm season temperatures from Eagle and Platypus Tarns in south central Tasmania (Rees and Cwynar, 2010; see Fig. 1 for location). In this study, age control is provided by 14C dates on terrestrial macrofossils, with two and three ages covering the period from 16,000 to 10,000 cal yr BP in Eagle and Platypus Tarns, respectively (Rees and Cwynar, 2010). Unfortunately, there is little YDC data from Platypus Tarn, with no chironomids preserved between approximately 14 and 12 ka BP (a situation that may reflect inferred lower lake levels at the site, Rees and Cwynar, 2010). The record from Eagle Tarn commences at approximately 15 ka BP and temperature reconstructions at the site are above their current value (9.5
C) and average 10.2
C for the period up until 12.7 ka BP (Rees and Cwynar, 2010 and Fig. 2c). While reconstructed temperatures decline after 12.7 ka BP (thus largely coinciding with the onset of the Younger Dryas Chronozone), this inferred temperature reduction extends well into the Holocene (to 9.6 ka BP) and

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a g

b h c

i

j d

k e

l f

Fig. 2. LGIT palaeoclimate records from Australia and surrounding oceans from 10 to 20 ka BP. a) d18O from the EPICA Dronning Maud Land core (EPICA Community Members, 2006); b) Sea-surface temperature (SST) estimates from the Southern Ocean derived from an average of SST records from cores MD97-2120 and MD88-770 (Barrows et al., 2007a); c) Chironomid-based estimates of temperature of the warmest quarter (TWARM) and sediment organic content estimates (loss on ignition: LOI) from Eagle and Platypus Tarns, Tasmania (Rees and Cwynar, 2010); d) Pollen based annual average temperature (AAT) estimates from Lake Selina and Tullarbardine Dam (Fletcher and Thomas, 2010); e) sea-surface temperature reconstructions from core MD03-2611 (Calvo et al., 2007); f) illite concentrations from core MD03-2611 that reflect discharge from the Murray

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thus does not have a Younger Dryas-type expression (Rees and Cwynar, 2010). Interestingly, Rees and Cwynar (2010) suggest that patterns in the sediment organic content of Eagle and Platypus Tarns correspond with the timing of the ACR. They argue that reduced concentrations of organic matter (at a time of reconstructed warming) may be indicative of increased precipitation delivering minerogenic material from a catchment with little terrestrial vegetation. They furthermore attribute this increased precipitation due to a stronger temperature gradient between Antarctica (cooler) and the mid-latitudes (warmer) which increased the strength of the Westerlies and delivery of moisture to Tasmania (Rees and Cwynar, 2010). Research on Tasmanian glacial deposits adjacent to Eagle and Platypus Tarns and a number of other sites also suggests an absence of YD cooling (Barrows et al., 2002). This absence is reflected in the glacial records from the Snowy Mountains in mainland Australia (Barrows et al., 2002; see Fig. 1 for location). Indeed, the youngest 10 Be dated glacial advances in Tasmania and The Snowy Mountains are 16.0  1.6 (Poets Hill) and 16.3  1.6 ka (Lake Cootapatamba), respectively, with the correspondence of late Pleistocene advances across a large latitudinal range, indicative of temperature being the primary driver (Barrows et al., 2002). The absence of Australian glacial advances during the YDC is important since 14C dating of the terminal moraine from the Waiho Loop of the Franz Joseph Glacier in the southern New Zealand Alps formed the basis of claims of a global YD climate (Denton and Hendy, 1994), although new cosmogenic dating indicates the Franz Joseph re-advance postdates the YDC (Barrows et al., 2007a, 2007b). In assessing the YDC from mainland Australia, the best dated LGIT record (with 21 14C ages in the period 10e18 ka BP) is that from the north-west crater at Tower Hill, western Victoria. It is the only temperate Australian site considered in Turney et al.’s (2006b) evaluation of the LGIT where a combination of summary pollen, diatom, ostracod and geochemical data are used. The Tower Hill record, like others in this review, is highly variable through the LGIT. The pollen record from Tower Hill is indicative of deglacial climate “amelioration” with the expansion of sclerophyll tree taxa: Eucalypt from approximately 17 ka BP, followed by Casuarinaceae at approximately 14 ka BP. The latter increase is likely to be associated with climate warming, since values of Casuarinaceae >20%, which predominate after 14 ka BP, are only associated with modern day temperatures >10
C (D’Costa and Kershaw, 1997). The warming implied by the expansion of these tree taxa may have resulted in reductions in effective moisture due to increased evapotranspiration since lake water salinity inferred from ostracod composition increased, while no diatoms were preserved during much of this time (Turney et al., 2006b). At Tower Hill, some changes in the pollen record approximately correspond to the beginning of the YDC. Most notably there are increases in grass pollen (Poaceae) that commence at approximately 12.6 ka. Such increases are generally interpreted as indicative of drying, however the ostracod-based salinity reconstruction from within the basin displays the opposite trend. Moreover, the increase in grass pollen continues beyond the end of the YDC, almost to 10.9 ka. The long period of Poaceae increase, in combination with the contrasting nature of other indicators, indicates there is no coherent expression of the Younger Dryas in the Tower Hill record.

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Contrasting the interpreted absence of YD cooling from Tasmania and Western Victoria is Goede et al.’s (1996) assessment of the d18O content of stalagmites from Buchan Caves in eastern Victoria. Age control was provided by two U series ages constraining the period from 13,343 to 11,510 years ago (with the second being the average of two dates). From the dated d18O record, Goede et al. (1996) argue there was a substantial decline in temperature between 12.3 and 11.4 ka. They suggest that the degree of isotopic change in the Buchan record is indicative of a minimum of 2.25
C of cooling. However, their analysis assumed that there are no effects associated with changes in precipitation source. However, given the isotopic differences in rainwater noted in Goede et al. (1996) and more recent analysis which shows a greater dependence of southern Australian d18O on precipitation than temperature (Treble et al., 2005; Liu et al., 2010), this assumption appears invalid. In addition, the study covered a relatively short time period meaning that it was not possible to “calibrate” the purported YD climate reversal against an LGM pattern (cf. criterion 4 in Section 1.2). Given these caveats, Goede et al.’s (1996) claim of a YD cooling must be considered questionable. Apart from De Deckker et al. (1990) and Goede et al. (1996) (who argued from a low resolution Gulf of Carpentaria dust record that YD related aridity, while present, was not unusual), Andres et al. (2003) is one of few studies that claim a YD related climate shift in Australia. Andres et al.’s (2003) record of d18O from two species of planktonic foraminifera deposited in the Great Australian Bight (GAB, Fig. 1) shows strong evidence for cooling during the LGIT (Fig. 2g) and is constrained by eight 14C ages between 18 and 10 ka BP. Andres et al. (2003) argue for two cold reversals, one between 13.1 and 12.8 ka BP and the other from 12.3 to 11.1 ka BP. These are argued to “correspond” with the YDC (which they argue to spans 12.9e11.5 ka BP). The second sea surface temperature reversal is in the order of 2
C, with a possible additional 0.5
C cooling if changes in ice volume are taken into consideration. Andres et al. (2003) argue, in particular, that the pattern of the two cooling events matches the pattern of the Intra-Allerød Cold Period and YD record from GRIP, with the offset of w400 years between their GAB record and the GRIP record due to the imprecision in the ages of the latter. However, in considering Andres et al. (2003) claims of YD cooling in the context of the broader LGIT, the patterning of the d18O record from 18 to 13.1 ka BP follows neither an “Antarctic” or “North Atlantic” patterning, with generally stable sea surface temperatures inferred from w16 ka BP to 13.2 ka BP (Fig. 2g). Given the offset in ages between cooling recorded in the GAB record and Greenland, the purported Younger Dryas cooling of SST in the GAB must be viewed with caution. In this context, it is noteworthy that a (albeit lower resolution) planktonic foraminifera d18O and alkenone record of sea-surface temperature to the east of the GAB (Fig. 1) indicates warming through the YDC (Calvo et al., 2007; Fig. 2e). Several other studies document LGIT Australian climates but from broad geographic areas. These are a dust record from North Stradbroke Island (Petherick et al., 2009), a record of discharge from the Murray Darling Basin (Gingele et al., 2007) and a compilation of Australian charcoal records (Mooney et al., 2011). Although these records lack spatial, and in the case of Mooney et al. (2011), temporal resolution, they provide insights that would otherwise be unavailable. The record from North Stradbroke Island is important

Darling Basin (higher illite ¼ higher discharge); g) d18O records from planktonic foraminifera in core (ODP) 1127, Great Australian Bight (Andres et al., 2003); h) standardised composite charcoal records from the Australian mainland (Mooney et al., 2011); i) Aeolian dust content, Native Companion Lagoon, North Stradbroke Island (Petherick et al., 2009); j) peat humification record from Lynch’s crater (Turney et al., 2004) (higher absorption ¼ greater dryness); k) sediment organic content estimates (LOI) from Lake Euramoo (Haberle, 2005 and unpublished data); l) d18O from the NGRIP Greenland ice core plotted on the GICC05 timescale (Rasmussen et al., 2006). The definition of the Younger Dryas Chronozone (YDC) follows Rasmussen et al. (2006), while the Antarctic Cold Reversal (ACR) definition utilises the “earliest start” and “latest finish” dates from Pedro et al.’s (2011) composite Antarctic record.

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since, according to Petherick et al. (2009), it allows the assessment of dust flux from a large proportion of the Australian continent, including much of the arid zone which, because of a lack of preservation (see Figure 1 in Lynch et al., 2007 and Mooney et al., 2011) would otherwise be largely unaccounted for in this review. Petherick et al. (2009) have geochemically fingerprinted the long travelled dust component and utilised a mixing model to identify the likely sources from a widely distributed series of samples from the centre, east and south east of Australia (see Fig. 1). For the period covering the YDC, the dominant source of dust is argued to be the Lake Frome region in central eastern South Australia. Petherick et al. (2009), using a chronology based on ten 14C ages on organic sediment, identify a 12.2e10.2 ka “event” of higher dust flux noting that due to large (300 year) errors in their dating model overlaps the boundary of the YDC. Importantly, however, the size of the dust flux in the YDC is almost an order of magnitude lower than that which occurs during the LGM and lower than through the majority of the Holocene (Fig. 2i; Petherick et al., 2009). Hence, this record does not support the notion of marked YD related aridity in Australia. Gingele et al.’s (2007) qualitative reconstruction of discharge from Australia’s largest river catchment, the Murray River, provides useful insights from this critical period. Gingele et al. (2007) use the proportion of illite and other clay minerals from the same core as analysed by Calvo et al. (2007) to infer past discharge of The Murray River, a record constrained by six 14C ages between 10 and 20 ka BP. This record shows no evidence of aridity that is restricted to the YDC (Fig. 2f). Rather, there is a discharge decline commencing w13.5 ka BP that continues to the early Holocene (10 ka BP). Lastly composite charcoal records for the Australian continent (Fig. 2h) also suggest that the YDC was not distinctive relative to late glacial and indeed the Holocene in terms of fire history (Mooney et al., 2011). Rather, the Australian mainland composite fire record exhibits a variable increase through the LGIT (Mooney et al., 2011 and Fig. 2h). 2.3. Tropical Australia Whilst, as argued above, high latitude climate records are likely to be more sensitive to global cooling than those at low latitudes, the record from northern Australia is nevertheless important in considering the YD in Australia. Climate phenomena with a Northern Hemisphere origin may propagate into the tropics (for example with evidence of Heinrich events recorded in sediments from Lynch’s crater, Turney et al., 2004; Muller et al., 2008 see Fig. 1 for location). In contrast to the, albeit sparse, record of LGIT temperatures from southern Australia, there are no high resolution temperature reconstructions from tropical mainland Australia. Hence, information is drawn from indicators that, for the most part, reflect effective precipitation. In this assessment, the information is drawn entirely from the Atherton Tableland in northeast Queensland (Fig. 1). Lynch’s crater has long been identified as a site that is strongly linked to Northern Hemisphere climate variability (Kershaw, 1986; Kershaw et al., 2007). Hence it is pertinent to examine the record from this site for any link to the YD. Chronology for an LGIT study from Lynch’s crater was based on seven 14C ages on peat deposits for the period 20e10 ka BP (Turney et al., 2006b). Turney et al. (2006b) note that, while this number of dates is less than ideal, it allows an “approximation” of the timing of events. Notably, the resolution of this dating is as good as or better than the majority of records considered herein. There is a long history of study of the Lynch’s crater deposit, focussed mainly on the pollen record (Kershaw, 1974). Recent interpretations of the LGIT from Lynch’s crater records have

focussed on moisture availability, particularly in regard to the degree of dryness on the peat surface (see Fig. 2j), the contrasting proportions of Cyperaceae versus Poaceae pollen (Turney et al., 2004, 2006b) or the composition and origin of deposited sediments (Muller et al., 2008). This has come about partly because much of the LGIT pollen record is dominated by taxa (Casuarinaceae, Eucalyptus and Poaceae) that lack ecological specificity. At Lynch’s crater, there are no notable changes in the terrestrial vegetation that correspond with the YDC. For example, there is a variable increase in Eucalypt pollen and low proportions of grasses through the YDC (Turney et al., 2006b). However, as noted, the generalised nature of the dominant pollen taxa may not fully record changes in climate. In this context, it is notable that biomass burning (charcoal), surface moisture (peat humification) and sediment composition records also have no correspondence to the YD (Fig. 2j and Turney et al., 2004, 2006b; Muller et al., 2008). The other LGIT record from the Atherton Tableland is the extended and well dated pollen record from Lake Euramoo (Haberle, 2005). Nine calibrated 14C ages on organic detritus from 20 to 10 ka cal yr BP provide the age control for this record although the YDC covers a small section of the Lake Euramoo record (approx. 20 cm, Tibby and Haberle, 2007). There is no evidence from the Lake Euramoo pollen record of a YD related signal, with no correspondence between the statistically determined pollen zones, principal component analysis summary of the pollen data or notable changes in ecological indicator taxa (Haberle, 2005). For example, grasses (Poaceae) continue a general decline from a late glacial maximum that commenced at 20 ka cal yr BP through the YDC. Importantly there is a contiguous organic content record from Lake Euramoo (Haberle, 2005). Tibby and Haberle (2007) argue that the sediment organic content (see Fig. 2k), in combination with much lower resolution diatom data, suggest that between 13.8 and 11.5 ka BP conditions became drier at Lake Euramoo, noting that this drying spans both the ACR and YDC. In contrast, Williams et al. (2010) suggest, based on interpreting the respective pollen records from Lake Euramoo and Lynch’s crater, that at Lake Euramoo a reversal to drier conditions occurred between 11.4 and 9.5 ka BP which largely corresponds to similar conditions at Lynch’s crater from 11.6 to 10.9 ka BP. The differences in the above interpretations serve to highlight the complex nature of the LGIT in eastern Australia (sensu Turney et al., 2006b) and, importantly, the absence of an overarching imprint of the YD on climates in this region. The very sparse records from the Australian tropics suggest there is no evidence for YD related climate reversals, either of temperature or moisture availability. In this context, the results contrast to those from the very low latitude Liang Luar stalagmite record from Flores that documents substantial YD related temperature lowering (w5
C relative to modern values) and a southerly migration of the ITCZ (Griffiths et al., 2010), neither of which is registered in the Atherton Tableland records. 2.4. Challenges in understanding the influence of the YD in Australia It is clear that the fundamental challenge to understanding the influence of the YD on Australia, and the LGIT more generally, is the lack of well dated contiguous palaeoclimate records. Indeed, there are, essentially, no high resolution records from the western half of the continent or the arid zone. Accentuating this difficulty is the fact that all the records are in some ways inadequate (in terms of the criteria outlined in Section 1.2). In particular, few records quantitatively reconstruct temperature (these being Andres et al., 2003; Calvo et al., 2007; Fletcher and Thomas, 2010; Rees and Cwynar, 2010) and these are all hampered by a lack of resolution, with a maximum of eleven temperature estimates covering the

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YDC (from the d18O data from Andres et al., 2003). Moreover, reconstructions reliant on using modern climate-proxy calibration sets are limited by the fact that the modern climate space in Australia has poor or no analogues for glacial climates (Porch, 2010). This problem is particularly related to an absence of contemporary cold climates that can be used to define the full niche breadth of cold adapted taxa (Porch, 2010). When proxies other than temperature are considered, contiguous higher resolution records of the LGIT generally, and the YDC specifically, exist from The Great Australian Bight (Fe inferred from core scanning: Andres et al., 2003), Native Companion Lagoon (Petherick et al., 2009 and Fig. 2i), Lynch’s crater (Turney et al., 2004 and Fig. 2j) and Lake Euramoo (Haberle, 2005 and Fig. 2k). These records reflect effective moisture which, due to the variable influences of temperature and precipitation, are by no means simple to interpret (Turney et al., 2006a). The longer Lynch’s crater record, with evidence of anthropogenic biomass burning from 45 ka BP, indicates that the influence of people on the landscape and, therefore, on many of the indicators considered in this review has been both lengthy and fundamental (Kershaw et al., 2007). Given the paucity of archaeological sites that date to the first arrival of people (Gillespie et al., 2006), it is clear that it is unlikely to be possible to uncouple these influences at more than a small number of sites (though see Black et al., 2008 for a useful approach). Hence, it is quite possible if not probable, that there will be a human imprint on many of the indicators used to reconstruct LGIT climate in Australia. 3. Conclusion This review has highlighted that, apart from basic sedimentary information and a very limited number of charcoal records (e.g. Haberle, 2005), there are no continuous proxy records from Australia through the LGIT. To adequately address important questions about climate processes through this period, it is essential that more contiguous records are derived. As such records are produced, “false positive” conclusions must be avoided. In this context, it is important to note that a number of palaeoclimate records from the Southern Hemisphere have emphasised the complexity of deglaciation (e.g. Petherick et al., 2008; Williams et al., 2009). Notably, temperature minima in New Zealand precede the maximum global extent of glaciers, with evidence for multiple cold periods between 20 and 30 ka BP (Alloway et al., 2007 and references therein). This complexity, combined with climate systems that have multiple ocean and atmospheric drivers and proxy records influenced by local and stochastic phenomena means that, as the number of LGIT records increases, so does the chance that Younger Dryas-like “reversals” may be inferred. Bennett (2002) makes a similar observation in relation to correlations drawn between sediment records and the 8.2 ka event recorded in Greenland ice core records. It is important, therefore, that assessments of the Younger Dryas in Australia are made in the light of Bennett’s recommendations for avoiding falsely attributing causality to correlated events. Despite the lack of contiguous, quantitative, high resolution Australian palaeoclimate records, it is possible to make an assessment of climates during the Younger Dryas Chronozone. In doing so, it should be noted recent research on Australian palaeoclimate has highlighted strong links to millennial-scale Northern Hemisphere climate variability, rather than that exhibited at high latitudes in the Southern Hemisphere. For example, there is an apparent correspondence between DangsaardeOescher events and biomass burning in Australasia (Mooney et al., 2011) and between Heinrich events and moisture recorded at Lynch’s crater (Turney et al., 2004; Muller et al., 2008). Given this observation, it is clear

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that the available proxy records have sufficient sensitivity to record millennial-scale climatic excursions. However, with the possible exception of Andres et al. (2003), and consistent with recent findings from New Zealand (Barrows et al., 2007b), there is little or no strong evidence of cooling in Australia that is restricted to the YDC or indeed any YD related climatic “reversal” (related to a variable other than temperature). Furthermore, there are few, if any, records that record any climatic shifts that correspond to the YDC. Notably, this includes an absence of documented warming restricted to the YDC that may be associated with a simple operation of the bipolar see-saw. However, there is evidence that the Atlantic Cold Reversal did influence LGIT Australian climates at least in the southern temperate zone. Acknowledgements Simon Haberle kindly provided the Lake Euramoo loss on ignition data. Christine Crothers drew Figs. 1 and 2. Peter Kershaw is thanked for his input, enthusiasm and encouragement. Discussions with colleagues at The Environmental Change Research Centre, University College London helped to clarify some of the ideas presented. References Alley, R.B., 2000. The Younger Dryas cold interval as viewed from central Greenland. Quaternary Science Reviews 19 (1e5), 213e226. Alloway, B.V., Lowe, D.J., Barrell, D.J.A., Newnham, R.M., Almond, P.C., Augustinus, P.C., Bertler, N.A.N., Carter, L., Litchfield, N.J., McGlone, M.S., Shulmeister, J., Vandergoes, M.J., Williams, P.W., Anderson, B., Brackley, H., Burge, P., Carter, J., Cochran, U., Cooke, P., Crampton, J., Crouch, E., Crundwell, M., Deng, Y., Drost, F., Graham, I., Harper, M., Hayward, B., Hendy, C., Hollis, C., Hughes, M., Kennedy, D., Kennedy, L., King, D., Mackintosh, A., Manighetti, B., Marra, M., Mildenhall, D., Morgenstern, U., Naish, T., Neil, H., Nobes, D., Page, M., Palmer, A., Prior, C., Rieser, U., Rother, H., Shane, P., Strong, P., Suggate, P., Thomson, J., Tonkin, P., Trustrum, N., Van Dissen, R., Vucetich, C., Wilmshurst, J., Woodward, C., Zondervan, A., 2007. Towards a climate event stratigraphy for New Zealand over the past 30 000 years (NZ-INTIMATE project). Journal of Quaternary Science 22 (1), 9e35. Andres, M.S., Bernasconi, S.M., McKenzie, J.A., Röhl, U., 2003. Southern Ocean deglacial record supports global Younger Dryas. Earth and Planetary Science Letters 216 (4), 515e524. Barrows, T.T., Stone, J.O., Fifield, L.K., Cresswell, R.G., 2002. The timing of the last glacial maximum in Australia. Quaternary Science Reviews 21 (1e3), 159e173. Barrows, T.T., Juggins, S., De Deckker, P., Calvo, E., Pelejero, C., 2007a. Long-term sea surface temperature and climate change in the AustralianeNew Zealand region. Paleoceanography 22 (2), PA2215. Barrows, T.T., Lehman, S.J., Fifield, L.K., De Deckker, P., 2007b. Absence of cooling in New Zealand and the adjacent ocean during the Younger Dryas Chronozone. Science 318 (5847), 86e89. Bennett, K.D., 2002. Comment: the Greenland 8200 cal. yr BP event detected in loss-on-ignition profiles in Norwegian lacustrine sediment sequences. Journal of Quaternary Science 17 (1), 97e99. Black, M.P., Mooney, S.D., Attenbrow, V., 2008. Implications of a 14 200 year contiguous fire record for understanding human-climate relationships at Goochs Swamp, New South Wales, Australia. The Holocene 18 (3), 437e447. Blaauw, M., Christen, J.A., Mauquoy, D., Van Der Plicht, J., Bennett, K.D., 2007. Testing the timing of radiocarbon-dated events between proxy archives. The Holocene 17 (2), 283e288. Broecker, W.S., 1998. Paleo-ocean circulation during the last deglaciation: a bipolar seesaw? Paleoceanography 13, 119e121. Cai, M., Lu, J., 2007. Dynamical greenhouse-plus feedback and polar warming amplification. Part II: Meridional and vertical asymmetries of the global warming. Climate Dynamics 29 (4), 375e391. Carlson, A.E., 2010. What caused the Younger Dryas cold event? Geology 38 (4), 383e384. Denton, G.H., Hendy, C.H., 1994. Younger Dryas age advance of Franz Josef Glacier in the Southern Alps of New Zealand. Science 264, 1434e1437. D’Costa, D., Kershaw, A.P., 1997. An expanded pollen data base from south-eastern Australia and its potential for refinement of palaeoclimatic estimates. Australian Journal of Botany 45, 583e605. EPICA Community Members, 2006. One-to-one coupling of glacial climate variability in Greenland and Antarctica. Nature 444 (7116), 195e198. Gillespie, R., Brook, B.W., Baynes, A., 2006. Short overlap of humans and megafauna in Pleistocene Australia. Alcheringa 31, 163e186. Goede, A., McDermott, F., Hawkesworth, C., Webb, J., Finlayson, B., 1996. Evidence of Younger Dryas and Neoglacial cooling in a Late Quaternary palaeotemperature

54

J. Tibby / Quaternary International 253 (2012) 47e54

record from a speleothem in eastern Victoria, Australia. Journal of Quaternary Science 11 (1), 1e7. Griffiths, M.L., Drysdale, R.N., Vonhof, H.B., Gagan, M.K., Zhao, J.-x., Ayliffe, L.K., Hantoro, W.S., Hellstrom, J.C., Cartwright, I., Frisia, S., Suwargadi, B.W., 2010. Younger Dryas-Holocene temperature and rainfall history of southern Indonesia from d18O in speleothem calcite and fluid inclusions. Earth and Planetary Science Letters 295 (1e2), 30e36. Haberle, S.G., 2005. A 23,000-yr pollen record from Lake Euramoo, Wet Tropics of NE Queensland, Australia. Quaternary Research 64, 343e356. Hodgson, D.A., Sime, D.C., 2010. Southern westerlies and CO2. Nature Geoscience 3, 666e667. Kershaw, A.P., 1974. A long continuous pollen sequence from north-eastern Australia. Nature 251 (5472), 222e223. Kershaw, A.P., 1986. Climatic change and Aboriginal burning in north-east Australia during the last two glacial/interglacial cycles. Nature 322 (6074), 47e49. Kershaw, A.P., Bretherton, S.C., van der Kaars, S., 2007. A complete pollen record of the last 230 ka from Lynch’s Crater, north-eastern Australia. Palaeogeography, Palaeoclimatology, Palaeoecology 251 (1), 23e45. Liu, J., Fu, G., Song, X., Charles, S.P., Zhang, Y., Han, D., Wang, S., 2010. Stable isotopic compositions in Australian precipitation. Journal of Geophysical Research 115 (D23), D23307. Lynch, A.H., Beringer, J., Kershaw, P., Marshall, A., Mooney, S., Tapper, N., Turney, C., Van Der Kaars, S., 2007. Using the paleorecord to evaluate climate and fire interactions in Australia. Annual Review of Earth and Planetary Sciences 35, 215e239. Markgraf, V., 1993. Younger Dryas in southernmost South America e an update. Quaternary Science Reviews 12 (5), 351e355. Menviel, L., Timmermann, A., Timm, O.E., Mouchet, A., 2011. Deconstructing the Last Glacial termination: the role of millennial and orbital-scale forcings. Quaternary Science Reviews 30 (9e10), 1155e1172. Mooney, S.D., Harrison, S.P., Bartlein, P.J., Daniau, A.L., Stevenson, J., Brownlie, K.C., Buckman, S., Cupper, M., Luly, J., Black, M., Colhoun, E., D’Costa, D., Dodson, J., Haberle, S., Hope, G.S., Kershaw, P., Kenyon, C., McKenzie, M., Williams, N., 2011. Late Quaternary fire regimes of Australasia. Quaternary Science Reviews 30 (1e2), 28e46. Muller, J., Kylander, M., Wüst, R.A.J., Weiss, D., Martinez-Cortizas, A., LeGrande, A.N., Jennerjahn, T., Behling, H., Anderson, W.T., Jacobson, G., 2008. Possible evidence for wet Heinrich phases in tropical NE Australia: the Lynch’s Crater deposit. Quaternary Science Reviews 27 (5e6), 468e475. Pedro, J.B., Van Ommen, T.D., Rasmussen, S.O., Morgan, V.I., Chappellaz, J., Moy, A.D., Masson-Delmotte, V., Delmotte, M., 2011. The last deglaciation: timing the bipolar seesaw. Climates of the Past 7, 671e683. Pinter, N., Scott, A.C., Daulton, T.L., Podoll, A., Koeberl, C., Anderson, R.S., Ishman, S.E., 2011. The Younger Dryas impact hypothesis: a requiem. EarthScience Reviews 106 (3e4), 247e264. Petherick, L., McGowan, H., Moss, P., 2008. Climate variability during the Last Glacial Maximum in eastern Australia: evidence of two stadials? Journal of Quaternary Science 23 (8), 787e802. Petherick, L.M., McGowan, H.A., Kamber, B.S., 2009. Reconstructing transport pathways for late Quaternary dust from eastern Australia using the

composition of trace elements of long travelled dusts. Geomorphology 105 (1e2), 67e79. Porch, N., 2010. Climate space, bioclimatic envelopes and coexistence methods for the reconstruction of past climates: a method using Australian beetles and significance for Quaternary reconstruction. Quaternary Science Reviews 29 (5e6), 633e647. Rasmussen, S.O., Andersen, K.K., Svensson, A.M., Steffensen, J.P., Vinther, B.M., Clausen, H.B., Siggaard-Andersen, M.-L., Johnsen, S.J., Larsen, L.B., DahlJensen, D., Bigler, M., Rothlisberger, R., Fischer, H., Goto-Azuma, K., Hansson, M., Ruth, U., 2006. A new Greenland ice core chronology of the last glacial termination. Journal of Geophysical Research 111. doi:10.1029/2005JD006079. Rees, A.B.H., Cwynar, L.C., 2010. Evidence for early postglacial warming in Mount Field National Park, Tasmania. Quaternary Science Reviews 29 (3e4), 443e454. Ritchie, J.C., 1995. Current trends in studies of long-term plant community dynamics. New Phytologist 130 (4), 469e494. Shakun, J.D., Carlson, A.E., 2010. A global perspective on Last Glacial Maximum to Holocene climate change. Quaternary Science Reviews 29 (15e16), 1801e1816. Spooner, M.I., Barrows, T.T., De Deckker, P., Paterne, M., 2005. Palaeoceanography of the Banda Sea, and Late Pleistocene initiation of the Northwest Monsoon. Global and Planetary Change 49 (1e2), 28e46. Tibby, J., Haberle, S.G., 2007. A late glacial to present diatom record from Lake Euramoo, wet tropics of Queensland, Australia. Palaeogeography, Palaeoclimatology, Palaeoecology 251 (1), 46e56. Treble, P.C., Budd, W.F., Hope, P.K., Rustomji, P.K., 2005. Synoptic-scale climate patterns associated with rainfall d18O in southern Australia. Journal of Hydrology 302 (1e4), 270e282. Turney, C.S.M., McGlone, M.S., Wilmshurst, J.M., 2003. Asynchronous climate change between New Zealand and the North Atlantic during the last deglaciation. Geology 31 (3), 223e226. Turney, C.S.M., Kershaw, A.P., Clemens, S.C., Branch, N., Moss, P.T., Keith Fifield, L., 2004. Millennial and orbital variations of El Nino/Southern Oscillation and high-latitude climate in the last glacial period. Nature 428 (6980), 306e310. Turney, C.S.M., Haberle, S., Fink, D., Kershaw, A.P., Barbetti, M., Barrows, T.T., Black, M., Cohen, T.J., Corrège, T., Hesse, P.P., Hua, Q., Johnston, R., Morgan, V., Moss, P., Nanson, G., Van Ommen, T., Rule, S., Williams, N.J., Zhao, J.X., D’Costa, D., Feng, Y.X., Gagan, M., Mooney, S., Xia, Q., 2006a. Integration of icecore, marine and terrestrial records for the Australian last glacial maximum and termination: a contribution from the OZ INTIMATE group. Journal of Quaternary Science 21 (7), 751e761. Turney, C.S.M., Kershaw, A.P., Lowe, J.J., van der Kaars, S., Johnston, R., Rule, S., Moss, P., Radke, L., Tibby, J., McGlone, M.S., Wilmshurst, J.M., Vandergoes, M.J., Fitzsimons, S.J., Bryant, C., James, S., Branch, N.P., Cowley, J., Kalin, R.M., Ogle, N., Jacobsen, G., Fifield, L.K., 2006b. Climatic variability in the southwest Pacific during the Last Termination (20e10 kyr BP). Quaternary Science Reviews 25, 886e903. Williams, M., Cook, E., van der Kaars, S., Barrows, T., Shulmeister, J., Kershaw, P., 2009. Glacial and deglacial climatic patterns in Australia and surrounding regions from 35 000 to 10 000 years ago reconstructed from terrestrial and near-shore proxy data. Quaternary Science Reviews 28 (23e24), 2398e2419.