Lit

ingt

d

e ofostnsaveoss

the LIA, with colder sea surface temperatures in the Southern Ocean and/or increased sea-ice extent, strongerased snow accumulation. Whilst we nd there was large spatial and temporal

We conclude, that the LIA was either caused by alt

ent climnd varHem

Earth and Planetary Science Letters 308 (2011) 4151

Contents lists available at ScienceDirect

Earth and Planetar

.eclimate pattern with marked regional differences, both in style andtiming of the climate signals (Mann et al., 1999).

For this reason, the causes, timing and geographical extent of theLIA are still debated. However, three major climate modulators havelikely played a role: changes in solar output (Ammann et al., 2007;Bard et al., 2000; Maasch et al., 2005; Mayewski et al., 1997, 2004b,2006; O'Brien et al., 1996), increased volcanic activity (Crowley, 2000;Robock, 2000), and changes in the thermohaline circulation(Broecker, 2000b, 2001; Lund et al., 2006). The reason for thecomplex spatial and temporal expressions is likely due to the LIA's

Oeschger events (Broecker, 2000a), better known for their abruptoccurrences during the last glacial period. For this reason, the LIAprovides an excellent opportunity to evaluate how the climate systemcreates and responds to rapid change.

Here we present new data from the Ross Sea, Antarctica, animportant area of bottom water formation and contributor to thedensitydriven component of the global oceancirculation (Jacobs, 2004).Our data show that LIA climate conditionswere synchronouswith thosein the Northern Hemisphere. We then summarise previously describedconditions in Antarctica during the LIA to discuss spatial and temporalsmall amplitude (Table 1), which competes wof similar or greater amplitude, such asOscillation (Turner, 2004) or the Southern Anand Solomon, 2002).

Corresponding author.E-mail address: Nancy.Bertler@vuw.ac.nz (N.A.N. Be

0012-821X/$ see front matter 2011 Elsevier B.V. Adoi:10.1016/j.epsl.2011.05.021isphere (Grove, 1988).climate event, occurringshow a highly variable

modulations (Kreutz et al., 1997; Mayewski and Maasch, 2006).Moreover, it has been suggested that the LIA is the most recent rapidclimate change (Bond et al., 1999) in a sequence of DansgaardWhilst the term LIA is conducive to a distinctover a distinct time period, reconstructionsMediaeval Warm Periodabrupt climate changesea-saw mechanismAntarcticaSouthern Ocean

1. Introduction

The Little Ice Age (LIA) is a prominbasis of glacier advances in Europe, aconditions throughout the Northerndifferently during warm periods. 2011 Elsevier B.V. All rights reserved.

ate shift dened on theiable but cooler climate

As shown in Table 1, the lower temperature and snow lineassociated with the LIA is about 10% that of the glacial/interglacialchanges, and about 2030% that of the Younger Dryas. However, overthe past 5 kyr, the LIA is one of the most prominent climateith other climate driversthe El Nio-Southernnular Mode (Thompson

differences and i

2. Material and

2.1. Study site

New ZealandScientic Expedirtler).

ll rights reserved.ernative forcings, or that the sea-saw mechanism operatesLittle Ice Age variability, overall Antarctica was cooler and stormier during the LIA. Although temperatures have warmedsince the termination of the LIA, atmospheric circulation strength has remained at the same, elevated level.Keywords: katabatic winds, and decreCold conditions in Antarctica during theclimate change mechanisms

N.A.N. Bertler a,, P.A. Mayewski b, L. Carter c

a Joint Antarctic Research Institute, Victoria University and GNS Science, PO Box 600, Wellb Climate Change Institute, University of Maine, Orono, ME 04469-5790, USAc Antarctic Research Centre, Victoria University, PO Box 600, Wellington 6012, New Zealan

a b s t r a c ta r t i c l e i n f o

Article history:Received 20 October 2010Received in revised form 5 May 2011Accepted 8 May 2011

Editor: P. DeMenocal

The Little Ice Age (LIA) is onthat the LIA might be the mlarge scale climate oscillatio2000b) Antarctica should hpresent new data from the R

j ourna l homepage: wwwtle Ice Age Implications for abrupt

on 6012, New Zealand

the most prominent climate shifts in the past 5000 yrs. It has been suggestedrecent of the DansgaardOeschger events, which are better known as abrupt,during the last glacial period. If the case, then according to Broecker (2000a,warmed during the LIA, when the Northern Hemisphere was cold. Here weSea, Antarctica, that indicates surface temperatures were ~2 C colder during

y Science Letters

l sev ie r.com/ locate /eps lmplications for abrupt climate change mechanisms.

methods

contributes to the International Trans-Antarctiction (ITASE) (Mayewski et al., 2005) by collecting ice

cores from coastal locations in the Ross Sea region (Fig. 1, Map 2).Victoria Lower Glacier (VLG), in the northernmost McMurdo DryValleys, is a small (530 km) valley glacier. It ows from its ice dividewestward into the Victoria Valley and eastward towards the coast,where it feeds the Wilson Piedmont Glacier. The ice of VLG is locallyaccumulated, and lies within 22 km of seasonally open ocean. The icecore came from the highest point of the glacier, the ice divide, whichlies at 626 m above sea level and is underlain by over 600 m of ice.

As characteristic of the McMurdo Dry Valleys, annual snowprecipitation is low. Snow pit data indicate that VLG average annualsnow accumulation is 0.0330.013 mwater equivalent per yr (w.e.a1)for the past ~40 yrs (Bertler et al., 2004a,b). An average annualtemperature of 22 C comes from 15 m-deep temperature mea-surements in a borehole (Bertler et al., 2004a,b). However, the

the core, where detailed analyses with ~2.5 cm resolution werecarried out. The core was processed using a continuous meltersystem (Osterberg et al., 2006). 1912 ice core samples wereanalysed for stable isotope ratios (18O, D, d excess), major ions(Na, Ca, K, Mg, Cl, NO3, SO4) and trace element (Fe, Al, Sr, P, Cu, Si)concentrations. In addition, 144 high-resolution tritium measure-ments were conducted on the top 5 m of the ice core (Pattersonet al., 2005). The analytical methods and precision are described inAppendix A.

2.3. Chronological framework

Due to low snow accumulation rates, annual layer counting isnot a viable option for dating the VLG ice core. The upper 4 m of thecore were dated through correlation with a 4 m-deep snow pitrecord from the same site. The snow pit data were sampled with1 cm resolution and dated using annual layer counting of seasonaluctuations of sodium with a precision of 1 yr for the 36 yrrecord (Bertler et al., 2004a,b). Average annual accumulation overthis time period is 0.0330.013 mw.e. a1. The tritium measure-ments were used to identify major peaks caused by nuclear testingbetween 1957 and 1966 as well as seasonal tritium variations.The dating error of the tritium measurements is b0.3 yrs (Pattersonet al., 2005).

An age model was extrapolated to the remaining core using a

Table 1Comparison of approximate changes in snowline (relative to 1975 AD) and global/hemispherical temperature (relative to preindustrial temperature) during majorclimate shifts. Data for full glacial temperature change are derived from Schneidervon Diemling et al. (2006); all other data from Broecker (2000b).

Lowering of thesnowline (m)

Decrease intemperature (C)

Glacial to interglacial ~900950 5.81.4Younger Dryas ~350 ~34Little Ice Age ~100 0.6

42 N.A.N. Bertler et al. / Earth and Planetary Science Letters 308 (2011) 4151McMurdo Dry Valleys experience some of the largest seasonaltemperature amplitudes on Earth with Victoria Valley recordingsummer maxima of 10 C and winter minima of60 C (Doran et al.,2002a,b). This exceptional range is caused by the ice free areaexperiencing strong solar heating during the summer, and radiativecooling during winter (King and Turner, 1997). The winter cooling isparticularly strong in Victoria Valley as the valley is sheltered fromkatabatic winds and hence creates ideal conditions for a stable winterstratication of the lower troposphere, which enhances effectiveradiative cooling (Doran et al., 2002a,b).

2.2. Data

During the 2001/02 eld season, a 180 m-deep ice core wasrecovered at the ice divide of VLG. This paper focuses on top 50 m ofFig. 1. Location map: 1) map of Antarctica with red square locates Map 2. Yellow numbersSource map: NASA, Radar Image. 2) Map of the Ross Sea region. Black squares indicate coreGradient Programme (Howard-Williams et al., 2006). 3) McMurdo Sound showing the drilstation (yellow star). Source map: NASA Goddard Space Flight Center image frommoderate-rResponse Team).rn decompaction model based on the density regression function:

z = zrpip d pipo dprpo d pipo d

!1

where the density (p) of ice : pi=0.917 g/cm3, measured density atthe surface : po=0.200 g/cm3, density at the reference depth zr:pr=0.600 g/cm3, reference depth : zr=8 m, best t with themeasured density prole: d=1/0.38. From this follows:

p = a bz + c d 2

where: a=pi, b=[(pipr)d(pip0)]/zr, c=(pip0), d=1/0.38.If the accumulation rate (mass of ice per unit area per unit time)

denote locations of ice core records: Taylor Dome, Talos Dome, and Law Dome.locations of the NZ ITASE programme. Square A locates Map 3. Source map: Latitudinall site (red star) on Victoria Lower Glacier and location of Lake Vida automatic weatheresolution imaging spectroradiometre (MODIS) sensor (J. Descloitres, MODIS Land Rapid

is a constant 1/k (Mg m2 a1), then the age t of the ice atdepth z is:

t = kz

0

z x dx 3

From this follows:

t = k az bd

d + 1z +

cd

d+1 c

d

d+1 " #: 4

The density regression is independent of age when constant snowaccumulation rates are assumed. Age benchmarks from the highresolution tritium data, plus the annual snow layer count from thesnow pit data, were used in Eq. (4) to determine k. Using thecalculated value of k=24.9 (or 1/k=0.04 w.e.a1), the age of the iceat layer z can be calculated from Eq. (4).

To adjust the density regression age model for variable snowaccumulation rates, volcanic markers were used. Inventories ofvolcanic eruptions around the world are constantly improved andextended (e.g. Gao et al. (2008)). However, not all volcanic eruptionsare preserved in Antarctic ice. Here, we correlate non-sea-saltsulphate (nss SO4) data, a commonly used proxy of volcanic eruptions(Legrand and Mayewski, 1997) from the VLG record with the nss SO4

a marine ratio of SO4/Na of 0.252 (in ng/g)(Legrand and Delmas,1984):

nssSO24 SO24 0:252Nang=g: 5

The calculated VLG nss SO4 data render less than 2% negativevalues, supporting the assumption that most of the Na is derived fromsea-salt and the calculated nss SO4 contribution is a reasonableestimate.

Nss SO4 peaks are classied as volcanic eruptions if two criteria arefullled: 1) the maximum peak concentration exceeds one standarddeviation above the mean (N1550 ppb) and 2) the nss SO4background increases above the median concentration of 375 ppbfor at least 10 yrs. These conditions are satisedduringve events in therecord. The volcanic eruptions identied in the VLG record are: Agung(1963 AD), Krakatau (1883 AD), Tambora (1816 AD), Huayanaputina(1601 AD), and "Unkown" (1259 AD) (Fig. 2). The 1259 ADeruptionhasbeen recorded in Antarctic and Arctic ice cores and is the largesteruption in Antarctic records in the past millennium (Oppenheimer,2003; Stothers, 2000) and hence provides a particularly well con-strained age benchmark. The EPICA-DML record identied 26 eruptionsover the same time period. Howevermost volcanic peaks, in addition tothe ve identied in the VLG record, are of lower amplitude (withthe exception of the Kuwae eruption, 1458 AD) andmight be present inthe VLG record too, but are masked by the much higher ss SO4 input tothe site.

ons

43N.A.N. Bertler et al. / Earth and Planetary Science Letters 308 (2011) 4151data from EPICA Dronning Maud Land (EDML) reconnaissance icecores (Traufetter et al., 2004) (Fig. 3). We chose the EDML ice core(Traufetter et al., 2004), as the record has an acceptable resolutionowing to (i) an average annual snow accumulation of 0.05 to0.10 mw.e.a1, (ii) good age control (uncertainty of b5 yrs over thestudied time period), and (iii) was drilled at an elevation of 3233 mand ~500 km inland from the coast, which reduces the inuence ofsea-salt sulphate (ss SO4) compared to records from coastal locations.As the VLG ice core comes from a low elevation site that is only 22 kmfrom seasonally open ocean, marine aerosol input to the site is high(Bertler et al., 2004b). We calculated the nss SO4 concentrationby subtracting proportionally ss SO4 contribution as estimated fromNa (assuming all Na is derived from sea salt) from total SO4 assuming

Fig. 2. EDML and VLG nss SO4 records for the past 900 yrs, with identied volcanic erupti

both EDML and VLG nss SO4 records as dated in the EDML record.Using the identied volcanic eruptions, changes in snow accumu-lation can be taken into account. The peak associated with Agung(1963 AD) was adjusted by 3 yrs, Krakatau (1883 AD) by +33 yrs,Tambora (1816 AD) by +29 yrs, Huayanaputina (1601 AD) by 60 yrs, and the 1259 AD eruption by 47 yrs (Fig. 2). The newadjusted age model renders an average annual accumulation of thewhole record of 0.033 mw.e.a1, which is in agreement with thesnow pit snow accumulation data of 0.0330.013 mw.e.a1 (Bertleret al., 2004b). To estimate the upper dating error for any pointbetween age benchmarks, we add a 20% uncertainty to the maximumage adjustment in the original 850 yr long record of 60 yrs, resultingin 72 yrs. This maximum uncertainty is applied proportionally tothe length of the record between age ties. The age uncertainty is

used for tie points of the VLG record. Shaded areas highlight volcanic eruptions found in

smallest nearest an age benchmark (3 yrs) and largest at greatestdistance (midpoint) between age ties. The maximum dating errorwithin each of the now constrained sections is 3 yrs between2000 and 1963 AD, 7 yrs between 1963 and 1883 AD, 24 yrsbetween 1883 and 1601 AD, 29 yrs between 1601 and 1259 AD.

2.4. Isotopetemperature relationship

In the McMurdo Dry Valleys, most precipitation occurs duringsummer (Bromwich, 1988), hence a stable isotope record from localice cores should provide a proxy for summer temperature (Bertleret al., 2004b). In Fig. 3, summer temperatures (December toFebruary), as recorded by Scott Base and Lake Vida automatic weatherstations (AWS), are compared with VLG 18O snow pit data spanningthe past 36 yrs (Bertler et al., 2004a). The Lake Vida station providesthe longest continuous data in Victoria Valley beginning in the1995/1996 austral summer (Doran et al., 1995). However, the overlapbetween our record and the Lake Vida data is only 5 yrs. For thisreason, comparisons are also made with the more distant (~80 km),but longer Scott Base summer temperature data. The two data setsshow signicant similarities, suggesting that the VLG isotope datarepresent the regional summer temperature (Bertler et al., 2004a,b).However, the comparison renders a low, albeit statistically signicant,correlation coefcient of only R2=0.35, (n=30, p=0.0006) indicat-ing that signicant differences also exist. Whilst warmer (colder)summers are coeveal in all three records, the magnitude of changevaries between sites. Scott Base is more exposed to southerly and/orkatabatic wind ow, which could explain some of the differences.Accordingly, we also correlate monthly averaged NCEP/NCAR re-analysis data (Kalnay et al., 1996) for the nearest grid points (77S,166.5E; 77.5S, 162.5E) to VLG (77.2S, 166.5E) and to Scott Base(77.5S, 162.5E) with the VLG isotope and Scott Base DJF data,

respectively, for the 19792000 AD time period. Neither comparisonrenders a statistically signicant correlations (R2=0.01 andR2=0.03, respectively). The limited performance of the isotopetemperature correlation is likely due to: a) the relatively smallisotopic range of inter-annual summer temperature variabilitycompared to, e.g. the seasonal or glacial/interglacial amplitude,which decreases the signal to noise ratio, b) intermittent precipitationat Victoria Lower Glacier which biases the record towards precipita-tion days, c) the lack of long-term weather station data from the icecore site, and d) the 0.5 resolution of the re-analysis data isinsufcient to adequately capture the large geographical variabilitywithin the McMurdo Dry Valleys and the steep relief of the TransAntarctic Mountains.

In the absence of a suitable meteorological record to calculate thelocal temperatureisotope conversion slope, no absolute temperaturesare deferred from the VLG record. Instead, we use previously publishedisotopetemperature conversion slopes to estimate change of temper-ature (T). Masson-Delmotte et al. (2008) provided a comprehensivereview of stable isotope data from Antarctica and their interpretation.Using the entire, quality assessed data set, they obtain a spatialtemperatureisotope conversion slope for 18O of 0.800.01 andfor D of 6.340.09, but note local slopes may vary by 20% or more.For this reason,we discuss three isotopetemperature slopes from sites,which share important similarities with the VLG record: Taylor Domeand Talos Dome (Fig. 1) are the closest deep ice core records to VLG, andLaw Dome (Fig. 1, Map.1, No. 3), a low elevation, coastal ice core recordlocated inWilkes Land. The Taylor Dome ice core was drilled at 2374 mabove sea level (asl) (Steig et al., 1998). The site lies about 120 km fromthe Ross Sea coast and experiences an average annual temperature(derived from 15 m rn cores) of 43 C, and has an average annualsnow accumulation of 0.050.07 mw.e.a1 (Masson et al., 2000). TheTalos Dome ice core was drilled at 2316 m elevation, lies about 250 km

ke Vcien

44 N.A.N. Bertler et al. / Earth and Planetary Science Letters 308 (2011) 4151Fig. 3. VLG 18O snow pit data (red), Scott Base summer temperature (green), and Labetween 18O snow pit data and Scott Base summer temperature with correlation coef

and D data.ida summer temperature (blue) are shown since 1970 AD. Top left inset: Correlationt R2=0.35, n=35, p=0.0006. Top right inset: Correlation between VLG ice core 18O

900) as180

45N.A.N. Bertler et al. / Earth and Planetary Science Letters 308 (2011) 4151from the Ross Sea. The site experiences an average annual temperatureof 41 C, with an average annual snow accumulation of 0.050.11 mw.e.a1 asmeasured from snow stakes and a long-term averageas obtained from the Talos Dome ice core of 0.08 mw.e.a1 (Stenniet al., 2002). The LawDome record is situated in the IndianOcean sectorof Antarctica. The ice core was drilled at 1390 masl, has an averageannual temperature of 22 C, and an average annual snow accumu-lation of 0.66 mw.e.a1 (Delmotte et al., 2000). To convert isotope datainto temperature changes from the Taylor Dome ice core record, atemporal temperatureisotope slope was calculated (Steig et al., 1998).The conversion is based on a 1.5 C cooling that occurred over the past4000 yrs as observed from Taylor Dome borehole temperaturemeasurements and an associated decrease of D=0.6 in the ice

Fig. 4. RawVLG Deuterium isotope (blue) and deuterium excess (green) data for the pastlight blue or light green. Grey shaded area highlights LIA time period (1288 to 1807 ADWarm Period (MWP) (1140 to 1287 AD), and followed by the Modern Era (ME) (sincecore record (Steig et al., 1998), which suggests a temperatureconversion slope of D=4.0 per C. The isotopetemperatureconversion slope for the Talos Dome record was established using thespatial isotopetemperature slope between two sites (Talos Dome andST556), 50 km apart, with an elevation difference of 70 m, and anaverage annual temperature difference of 2.9 C (Stenni et al., 2002).Stenni and colleagues calculated from this relationship a localconversion slope of 18O=0.6 per C (converted for D=4.8 perC). Theexceptionally high snowaccumulation at LawDomeallowed fora temporal isotopetemperature slope to be calculated based on theregression over a ve year monthly mean temperature record from anearby automatic weather station and the mean seasonal isotope cycle(Van Ommen and Morgan, 1997), which provided an isotopetemperature slope of 18O=0.44 per C (converted for D=3.5per C). The difference between conversion slopes from the three sites isD=1.5 per C. None of the three sites matches ideally thecharacteristics of VLG. Whilst the Law Dome record provides the mostcoastal record of the sites and was drilled at a low elevation, acomparison of Holocene climate variability of 11 ice cores from aroundAntarctica (Masson et al., 2000) identied that the Ross Sea Sectorisotope records (e.g. Taylor Dome; Talos Domewas not included in thisstudy) clustered separately from East Antarctic sites (e.g. Law Dome).Both Taylor and Talos Dome records are from the Ross Sea region, butwere drilled at much higher elevations (N2000 m) than VLG (626 m).Annual average snow accumulation rates of 0.68 m w.e.a1 at LawDome allow for exceptionally high resolution data. However, averageannual accumulation at Talos and Taylor Dome are more similar (0.050.10 mw.e.a1) to VLG (0.03 mw.e.a1) and hence are more likely besensitive to post-depositional processes that are likely to occur at VLG.For the above reasons, we chose the most local conversion slope fromTaylor Dome D (4.0 per C) to provide an estimate for T.

3. Results

In Fig. 4 deuterium data (D) are used to reconstruct changessummer temperature in theMcMurdo Dry Valleys for the past 900 yrs.Large multi-decadal and centennial variability is observed throughoutthe record. Abrupt changes from cooler to warmer conditions (andvice versa) of up to 50 occur in less than a decade. Whilst periods ofwarmer than average summers (above 224.0) appear to reach

yrs. Values above average are shown in dark blue or dark green, values below average inidentied by deuterium isotope record, preceded by the last 150 yrs of the Mediaeval8 AD).similar temperatures throughout the record, periods of colder thanaverage summers (below 224.0) are more frequent and reachcolder conditions during 1288 AD to 1807 AD. This period coincidesapproximately with LIA time of 1300 to 1850 AD (Crowley, 2000;Jones and Mann, 2004). Based on the stable isotope data, we denethree distinct time periods in our record: the last 150 yrs of theMediaeval Warm Period (MWP, 1140 to 1287 AD), LIA (1288 to1807 AD), and the Modern Era (ME, 1808 to 2000 AD) (Table 2).During the LIA, 63% (37%) of the summers experienced below (above)average temperatures compared to the ME and MWP with 28% (72%)

Table 2Averages of characteristic parameters of VLG ice core data for ME, the LIA, and the last150 yrs of the MWP.

ME 20001808 AD

LIA 18071288 AD

MWP 12871140 AD

No. of yrs 192 519 147Warm summers % (no. of yrs) 72% (139) 37% (190) 71% (106)Cold summers % (no. of yrs) 28% (53) 63% (328) 29% (42)Average D () 220.1 228.0 218.7 Summer temp (C) relative to ME 0 2.0 +0.35Average annual snow accumulation(m w.e.a1)

0.03 0.04 0.06

d excess() 0.1 0.5 4.4Na/Cl 0.59 0.57 0.52Na(ppb) 749 572 620Fe (ppb) 5.8 5.5 3.7

and 29% (71%), respectively (Table 2). In our characterisation, the MEexperiences one of the coldest episodes, which occurs around1880 AD. Arguably, this could alternatively be interpreted as theconclusion of the LIA. However, a prolonged episode of warming over64 yrs from 1806 to 1870 precedes the cooling from 1870 to 1890. Forthis reason, we argue that it is appropriate to conclude the LIA coolingin the VLG record at 1807 AD. Using the Taylor Dome conversion forice cores from coastal sites of 4 per C (Mayewski et al., 2004a; Steiget al., 1998), reconstructed summer temperatures are on average~2 C cooler during the LIA (average D=228.0) relative to thepast 200 yrs of the ME (D=220.1). In addition, the nal 150 yrsof the MWP were 2.3 C warmer (D=218.7) than the LIA andabout 0.35 C warmer than the ME. The transitions from the MWP tothe LIA and from the LIA to ME occur rapidly in less than a decade.

However, the isotopic signature of snow and ice can be inuencedby (i) changes occurring in the source region, (ii) a different sourcearea, (iii) modulations in the seasonality of snow precipitation, and(iv) post-depositional processes (Masson-Delmotte et al., 2003;Schlosser et al., 2008). Deuterium excess or dexcess (Dansgaard,1964) of precipitated snow is sensitive to humidity (negativecorrelation) and sea surface temperatures (SST) (positive correlation)at the source region (Jouzel et al., 1982) and hence can be used toreconstruct these parameters at a site with one dominant sourceregion, or to trace changes in air mass trajectory between differentsource regions (Masson-Delmotte et al., 2008; Schlosser et al., 2008;

shows overall a more complex relationship but identies an expectedstrong, statistically signicant anti-correlation with dexcess data in thePacic Sector. The correlation patterns for SST and relative humiditysupport the assumption that snow precipitating at VLG over recenttimes originates from the Southern Ocean via the Ross Sea assuggested by Sinclair et al. (2010) based on air mass back trajectoryanalysis, which suggests that synoptic and meso-scale cyclonestypically enter the eastern Ross Sea region and move northward inthe western Ross Sea (Fig. 5a, black arrows).

The VLG dexcess record exhibits high positive values during the nal150 yrs of the MWP, which transitions sharply to negative valuesduring the LIA (Fig. 4). This could reect decreased humidity, warmerSSTs in the Ross Sea and the Pacic Sector of the Southern Ocean, or asignicant change in air mass trajectory between the MWP and LIA.Air mass trajectory reconstructions from NCEP/NCAR and ERA40 re-analysis data (Sinclair et al., 2010) and from snow pit data (Bertleret al., 2004b, 2005; Patterson et al., 2005), identied two distinctsource regions for precipitation in the McMurdo region: a) relativelywarm, humid, marine air masses that are transported by synoptic andmesoscale cyclones from the Ross Sea and b) relatively cold, dry,terrestrial air masses arriving as katabatic wind ow from the interiorof Antarctica. Sinclair et al. (2010) showed that in McMurdo Soundonly ~10% of the snow precipitation occurred during katabatic events,whilst the remaining 90%was derived from cyclones originating in theRoss Sea and beyond. As the VLG site lies near the McMurdo Dry

ndes id

46 N.A.N. Bertler et al. / Earth and Planetary Science Letters 308 (2011) 4151Sodemann et al., 2008; Vimeux et al., 2001). In Fig. 5, the correlationbetween the VLG dexcess record and NCEP/NCAR reanalysis summer(DecFeb) values for a) SSTs and b) relative humidity (Kalnay et al.,1996) are shown from 1991 to 2000 AD. Correlation values 0.60 arestatistically signicant at the 95% level. The correlation with SSTsshows statistically signicant, positive correlations between 50 and70S for the Southern Ocean, with the exception of the Atlantic Sector,where the correlation is negative. In addition, the positive correlationextends north of the Ross Sea into the Pacic to 35S. The opposingtrend in the Atlantic Sector to East Antarctica has been attributed tothe inuence of the Southern Annular Mode and the El Nio-SouthernOscillation (Kwok and Comiso, 2002; Thompson and Solomon, 2002;Turner, 2004). The emerging correlation pattern demonstrates thatwarmer SSTs in the Southern Ocean between 0E and 90W co-varywith higher VLG dexcess data. The correlation with relative humidity

Fig. 5. Correlation between summer (December, January, February) VLG d excess data aCorrelation values0.60 are statistically signicant at the 95% level. Black arrows indicat

provided by the NOAA/ESRL Physical Sciences Division, Boulder Colorado from their Web sValleys the largest ice-free region in Antarctica katabatic ow willcarry terrestrial aerosols and particulates. Thus geochemical tracersfor these terrestrial (e.g. Fe) and marine (e.g. Na) components can beused to identify signicant changes in the relative contribution ofocean and katabatic air masses (Legrand and Mayewski, 1997).

Sodium (Na) and chloride (Cl) are associated with marine air masssources with a marine Na/Cl ratio of 0.56 (Legrand and Delmas, 1988;Legrand and Mayewski, 1997; Wolff et al., 1998a,b). Whilst the VLGNa/Cl data vary, the average is 0.56 (s.d.=0.17) (Fig. 6). Thissuggests that marine air masses are important throughout the recordand their relative contribution remains stable.

Moreover, Na concentrations are lower by 24% during the LIAcompared to ME, whilst Fe concentrations are higher by 34% during theLIA and ME compared to the MWP. In contract to records from theAntarctic interior, the VLG snow pit data show maximum Na peaks

NCEP/NCAR reanalysis data for a) SSTs and b) Relative Humidity for 1991 to 2000 AD.ealised trajectory for synoptic andmeso-scale cycloes after Sinclair et al. (2010). Images

ite at http://www.esrl.noaa.gov/psd/.

ide r

47N.A.N. Bertler et al. / Earth and Planetary Science Letters 308 (2011) 4151associated with summer precipitation (Bertler et al., 2004b). This likelyreects VLG's proximity to seasonally open ocean (Fig. 1). For this reason,the reduction in Na during the LIA time points to reduced open ocean,either spatially (a smaller region of sea-ice break-out), and/or temporally(fewer weeks with open water). The increase in Fe during the LIA alsosupports cooler andwindier conditions. However, increased Fe continuesthrough into the ME. For the past 40 yrs we observe a precipitation biastowards summerprecipitation, it is therefore unlikely that the Fe increaseindicates a seasonal shift in precipitation towards winter precipitation.Alternatively, it could reect a response to a more vigorous atmosphericcirculation, which is capable of transporting more terrestrial particulatesin McMurdo Sound (Dunbar et al., 2009). In Section 4 we note that astrengthening of the Southern Hemisphere westerly winds and adeepening of the Amundsen Sea Low have indeed been reported

Fig. 6. Geochemical data from VLG, including A) deuterium data (blue), B) Sodium/Chlorhighlights the LIA time period as dened by D.(Mayewski et al., 2009). Overall, the geochemical records suggest thatthroughout the past millennium, the VLG site continued to receiveprecipitation predominantly from the Southern Ocean, via the Ross Sea.For this reason, we interpret the abrupt shift in dexcess not as a change inair mass source but a signicant change in the Southern Ocean. Since anabrupt change in SST within less than a decade is unlikely, we propose

Fig. 7. Evolutionary spectrum for annually resampled A) VLG d excess and B) VLG D data. Cocondence level. The chosen window width is 300 yrs, with an implemented step of 2 yrs.that the sharp decrease in dexcess at the onset of the LIA was caused by asignicant increase in sea ice (reduced Na) along with an increase inkatabatic wind strength (increased Fe) over the Ross Sea, and cooler SSTduring summer (reduced open ocean, spatially and/or temporally).

Average annual snow accumulation is highest during the MWP(0.06 mw.e.a1) and lower during ME (0.03 mw.e.a1) and LIA(0.04 mw.e.a1) (Table 2). In addition, the McMurdo Dry Valleyswere 0.35 C warmer during the MWP than during ME, accompaniedby warmer conditions in the Ross Sea, and increased snowprecipitation is perhaps indicative of less sea-ice.

Whether the changes observed in the McMurdo Dry Valleys wereinitiated predominantly by changes in the Ross Sea/Southern Oceanand/or atmosphere may highlight the mechanisms that initiateDansgaardOeschger events. As discussed above, d captures

atio (Na/Cl, purple), C) Sodium (Na, green), and D) Iron (Fe, grey). The grey shaded areaexcess

changes in the Ross Sea/Southern Ocean, whilst D allows recon-struction of summer temperature changes in the McMurdo DryValleys. An evolutionary power spectrum (Fig. 7) graphically displaysthe changes of statistical signicant frequencies (non-black) con-tained in the dexcess and Ddata throughout the VLG record. The dexcessfrequency spectrum shows a clear change at the MWPLIA transition

lours associated with values above 10 (non-black) are statistically signicant at the 95%

48 N.A.N. Bertler et al. / Earth and Planetary Science Letters 308 (2011) 4151from dominant frequencies in the ~60 yr to ~50 and ~40 yrfrequencies, which strengthen as they converge to a ~40 yr frequencyfrom 1600 to 1800 AD. During the LIAME transition, the frequencyweakens and shifts towards a ~45 yr rhythm. In contrast, theevolutionary power spectrum of D changes gradually. Around13001400 AD a ~40 yr frequency changes to ~30 yr and thefrequency band intensies and narrows. At 1450 AD, the frequencysplits into a narrow 30 yr band and a developing ~40 to 50 yr band.Overall the changes around the MWP to LIA and LIA to ME are abruptin dexcess and gradual in the D data. Whilst the dating errorsassociated with these data (329 yrs) limits the use of the actualfrequencies observed, D and dexcess data were measured on the samesample and are therefore co-registered. For this reason, the abruptchange in the dexcess frequency spectrummight suggest that initiationfor the changes observed in the VLG record reside in the ocean via anincrease in sea-ice (dexcess), initated by stronger katabatic wind (Fe),leading to atmospheric cooling (D).

4. Climate conditions during the past millennium across Antarctica

To put our results into a wider context, we review previouslyreported climate conditions across Antarctica. For this comparison,the MWP is broadly dened as occupying 800 to 1300 AD, the LIA1300 to 1850 AD, and the ME since 1850 AD, these time framescapturing a range of published records afliated with the MWP andLIA (e.g. Crowley, 2000). The focus is on ice core records, but includes,where available, ice borehole measurements, marine and terrestrialpaleoenvironmental records, as well as modern data (Fig. 8). Therecords only includes those where the published literature offers aninterpretation with regards to ME, MWP, and LIA trends, thus takingadvantage of the authors' insight regarding any limitations of the data.In making this review, we recognise it is limited by dating un-certainties, regional and hemispherical variability of climate andocean, and imperfect knowledge of the range of climate drivers andtheir interactions (Mayewski et al., 2009; Turner et al., 2009).

4.1. Mediaeval Warm Period

Our results suggest the McMurdo Dry Valleys experienced warmsummers, with increased snow accumulation and higher SSTs in theRoss Sea/Southern Ocean, perhaps accompanied by less sea-ice (thispaper) (Fig. 8a).

Elsewhere, a magnetic susceptibility record from Palmer Deepmarine core (PD92 30MS) also supports warmerMWP conditions, thistime in Drake Passage (Domack and Mayewski, 1999). However, thetemperature amplitude of this warming is comparable to 10 otherwarm events throughout the 2500 yr Palmer Deep record. In addition,Masson et al. (2000) conducted a thorough review of the Holocenerecords contained in seven Antarctic ice cores and identied nineclimate oscillations. The latest oscillation occurred around 1000 yrsago with a warm event followed by a cooling event. The Ca recordfrom the Siple Dome ice core suggests the MWP was windy followinga 4000 yr period of declining wind strength (Yan et al., 2005). Naconcentrations at Siple Dome also highlight theMWP. Na is a sensitiverecorder of the strength of the Amundsen Sea Low (ASL) (Kreutz et al.,2000), a dominant feature of Antarctic meteorology (Mayewski et al.,2009). Kreutz et al. (2000) showed that the ASL gradually intensiedover the past 10,000 yrs, but was weakest during the MWP.

4.2. Little Ice Age

In the Ross Sea region, our data reveal that the LIA (Fig. 8b) startedabruptly, in less than a decade, with colder SSTs in the Ross Sea,perhaps accompanied with more extensive sea-ice, and 67% reducedsnow accumulation. In addition, McMurdo Dry Valleys' summer

temperatures were 2.3 C cooler than during the MWP, whilst higherFe concentrations in the VLG ice core suggest stronger katabatic owto the site. Lower Na concentrations are interpreted to reect perhapsincreased sea-ice and/or a shorter sea-ice break-out season.

As noted earlier, the last of the Holocene climatic oscillationsidentied by Masson et al. (2000) comprised a cooling phasecoincident with LIA time. Likewise, a review by Stenni et al. (2002)suggests a prolonged cooler climate from the 16th century to thebeginning of the 19th century. They note that the cooling varied fromplace to place and is not temporally synchronous between sites.Stenni et al. (2002) also note that snow accumulation at Talos Domedecreased by 11% during the LIA compared to ME. They suggest thatthe cooling in East Antarctic and the Ross Sea could lead to strongerkatabatic ow, which increased the efciency of polynyas to producemore sea-ice as supported by the diatom assemblages (Leventer andDunbar, 1988). This implies more persistent katabatic winds in thesouthwestern Ross Sea during 1600 to 1875 AD. Leventer and Dunbar(1988) also argue that larger and/or more persistent polynas wouldenhance production of High Salinity Shelf Water, a key component inthe production of Antarctic BottomWater. Broecker et al. (1999) alsosuggest that the LIA was likely a time of increased Antarctic BottomWater formation that may explain a ~10 ppm decrease in CO2concentrations between 1550 AD and 1800 AD, as seen in the LawDome (Etheridge et al., 1996) and Taylor Dome (Indermuhle et al.,1999) ice core records. At Palmer Deep, the PD92 30MS magneticsusceptibility record also conrms a prolonged and distinctly cold LIA,superimposed on a longer term cooling trend with 8 similar but lesscool climate oscillations (Domack and Mayewski, 1999). The LIA isalso accompanied by enhanced westerly winds (Yan et al., 2005),whilst the ASL deepens (Kreutz et al., 2000); trends that continuetoday. Furthermore, the East Antarctic High (EAH) decreases from1200 to 1700 AD, out of phase with the ASL (Mayewski et al., 2004a).Mayewski et al. (2004a) relate this trend to the poleward migrationof the circumpolar trough, which can be expressed as a positivetrend in the Southern Annular Mode. Between 1700 and 1850 AD,however, the ASL and EAH are correlated, with large abrupt changes,before they resume their anti-phase behaviour in modern times(Mayewski et al., 2004a). The Atlantic sector of Antarctica has littleevidence of the LIA with perhaps the exception of ice core data fromDronning Maud Land that suggest an 8% decrease in snow accumu-lation (Karlf et al., 2000).

Overall, it appears that Antarctica during the LIA experiencedcooler and drier conditions with higher wind speeds, cooler SSTs,more extensive sea-ice and inferred bottom water increase.

4.3. Modern Era

For the past 200 yrs (Fig. 8c), temperatures in the McMurdo DryValleyswere on average2 Cwarmer than conditionsduring the LIA, but0.35 C cooler than during the nal 150 yrs of MWP. Our dexcess datasuggest that SST in the Ross Sea remained relatively cold, which isaccompanied by a 50% reduction in snow accumulation in comparisonto the MWP (and 24% reduction compared with the LIA). Increased Naconcentrations suggest that sea-ice might be reduced in contrast to theLIA; Fe concentrations similar to LIA data suggest continuing strongkatabaticow.Overall,whilst temperatures inMcMurdo Sound indicatemilder conditions since the termination of the LIA, atmosphericcirculation and oceanic conditions appear to be still in LIA-mode.

With regard to other studies, Schneider et al. (2006) compiled icecore temperatures for the last 200 yrs, which show that Antarctica as awhole has warmed by 0.2 C. However, the warming is not uniform.Law Dome, for example, experienced cooler conditions since 1750 ADpossibly in response to enhanced downwelling of stratospheric airover the site (Mayewski et al., 2004a). In contrast, Siple Dome haswarmed markedly since about 1800 AD, which reect increasedintrusion of (warm) marine air masses into West Antarctica

(Mayewski et al., 2004a). At Talos Dome and in Dronning Maud

Fig. 8. Summary of conditions in Antarctica during the MWP, LIA, and ME. A) MWP: data for McMurdo Dry Valleys and Ross Sea (this paper); data for Law Dome, Dome C, Vostok, Dominion Range, Byrd, and Plateau Remote by Masson et al.(2000); data for strength of the Amundsen Low by Kreutz et al. (2000); data for Southern HemisphereWesterlies by Yan et al. (2005); data for Palmer Deep by Domack andMayewski (1999). B) LIA: data for Ross Sea SST, humidity, and sea-iceextent from geochemical ice core records (this paper); temperature data for McMurdo Dry Valleys from stable isotope records (this paper), temperature data for Law Dome, Dome C, Vostok, Dominion Range, Byrd, and Plateau Remote fromstable isotope records by Masson et al. (2000); temperature data for Talos Dome, EPICA Dome C, South Pole, and Taylor Dome from stable isotope records by Stenni et al. (2002); snow accumulation data for Talos Dome from ice core record byStenni et al. (2002); temperature data for Taylor Dome from borehole measurements by Broecker (2000b); data for Antarctic BottomWater formation from ux calculations by Broecker (2000b); data for katabatic ow, diatom plume, size ofthe polyna, and sea-ice from diatom data by Leventer and Dunbar (1988); data for strength of the Amundsen Low from geochemical ice core data by Kreutz et al. (2000); data for East Antarctic High from geochemical ice core data byMayewskiet al. (2004a); data for Southern Hemisphere Westerlies from geochemical ice core data by Yan et al. (2005); data for SST from magnetic susceptibility record for Palmer Deep by Domack and Mayewski (1999); snow accumulation data forDronning Maud Land from ice core data by Karlf et al. (2000); data for atmospheric CO2 concentration by Etheridge et al. (1996) and Indermuhle et al. (1999). C) ME, which is dened here as the past 150 yrs: data for Ross Sea SST, humidity,and sea-ice extent from geochemical ice core records (this paper); temperature data for McMurdo Dry Valleys from stable isotope records (this paper), temperature data for Talos Dome, EPICA Dome C, and Taylor Dome from stable isotoperecords by Stenni et al. (2002); temperature data for Law Dome from stable isotope data by Mayewski et al. (2004a); snow accumulation data for Talos Dome from ice core record by Stenni et al. (2002); data for Antarctic Bottom Waterformation from ux calculations by Broecker (2000a); data for katabatic ow, diatom plume, size of the polyna, and sea-ice from diatom data by Leventer and Dunbar (1988); data for strength of the Amundsen Low from geochemical ice coredata by Kreutz et al. (2000); data for East Antarctic High from geochemical ice core data by Mayewski et al. (2004a); data for Southern Hemisphere Westerlies from geochemical ice core data by Yan et al. (2005); snow accumulation data forDronning Maud Land from ice core data by Karlf et al. (2000); data for snow accumulation at Gomez by Thomas et al. (2008); data for atmospheric CO2 concentration by Etheridge et al. (1996) and Indermuhle et al. (1999).

49N.A.N.Bertler

etal./

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PlanetaryScience

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(2011)4151

5. Concluding comments

Acknowledgements

air in Antarctic ice and rn. J. Geophys. Res. 101, 41154128.

50 N.A.N. Bertler et al. / Earth and Planetary Science Letters 308 (2011) 4151We would like to thank Antarctica New Zealand and Scott Basefor the logistical support. We are grateful for useful suggestions andcomments by two anonymous reviewers. This project was fundedthe Foundation of Research Science and Technology via contractsawarded to Victoria University of Wellington and GNS Science(contracts VICX0704, CO5X0202, and CO5X0902). Monthly meanNCEP re-analysis time series was obtained from the NOAA, EarthSystem Research Laboratory (http://www.esrl.noaa.gov/psd/cgi-bin/data/timeseries/timeseries1.pl), hourly temperature data forScott Base were obtained from the National Institute for Water andAtmospheric Research (http://clio.niwa.co.nz/index.html), and themonthly mean temperature data for Lake Vida were obtained fromthe Long-Term Ecological Research Programme (http://huey.Broecker (2000b, 2006) and Lund et al. (2006) suggested thatmeridional overturning in the North Atlantic decreased during the LIAthus amplifying cooling in Europe. According to the see-saw hypothesis(Broecker, 1998; Broecker and Denton, 1989; Severinghaus, 2009) thisshould have led to warmer conditions in Antarctica as observed forDansgaardOeschger events (EPICA Community Members, 2006).However, our data from the McMurdo Dry Valleys reveal the Ross Searegion experienced colder conditions, stronger katabatic ow andperhaps increased sea-ice. These data are generally consistent with anAntarctic-wide assessment of LIA climate although there are temporaland spatial differences in climatic responses. Casting the netwider, NewZealand, which has direct climatic (Kidston et al., 2009; Ummenhoferand England, 2007) and oceanic (Carter et al., 2008; Orsi andWhitworth, 2005) links with Antarctica, also displays a cooling thatbegan around 12501350 AD and peaked ~1500 to 1650 AD beforerecovering at the end of the 19th Century (Lorrey et al., 2008). Similarly,southernmost South America underwent a LIA climate shift (Koch andKilian, 2005; Lamy et al., 2001).

In summary it appears, that the timing of the Antarctic LIA wasconcomitant with that in the Northern Hemisphere, inferring thatthe thermohaline circulation changes, as invoked to explain thebipolar see-saw associated with DansgaardOeschger events, wereunlikely to be the prime driver of the LIA climate modulation. Forthis reason we conclude that the LIA was caused a) by alternativeforcings (e.g. solar variability exacerbated by volcanic eruptions),b) a see-saw mechanism that operated differently during warmperiods including perhaps non-linear thresholds (Capron et al.,2010) or inherent lags (Goosse et al., 2004), or c) changes inregional winds that affected oceanic circulation and heat transport(Lozier, 2010; Toggweiler et al., 2006).Land, snow accumulation increased by 11% (Stenni et al., 2002) and8% (Karlf et al., 2000), respectively, whereas the Gomez ice core inthe Antarctic Peninsula recorded a doubling in snow accumulationfrom 1855 to 2006 AD (Thomas et al., 2008). Broecker et al. (1999)note that Antarctic Bottom Water formation decreased, which isconsistent with the longer term, glacialinterglacial cyclicity ofbottom water formation documented by Hall et al. (2001), and withobserved reductions in modern bottom water density (Aoki et al.,2005). Diatom abundance also decreases in the southwestern RossSea (Leventer and Dunbar, 1988), suggesting an easing of katabaticow, decreased size and duration of polynyas and hence a decrease insea-ice. The ASL is stronger than at any time over the past 10,000 yrs,but it may be weakening as the EAH strengthens (Mayewski et al.,2004a).colorado.edu/LTER/datasets/meteorology/vida.html).Gao, C., Robock, A., Ammann, C., 2008. Volcanic forcing of climate over the past1500 years: an improved ice core-based index for climate models. J. Geophys. Res.113, D23111.

Goosse, H., Masson-Delmotte, V., Renssen, H., Delmotte, M., Fichefet, T., Morgan, V., vanOmmen, T., Khim, B.K., Stenni, B., 2004. A late medieval warm period in theSouthern Ocean as a delayed response to external forcing? Geophys. Res. Lett. 31,Appendix A. Supplementary data

Supplementary data to this article can be found online at doi:10.1016/j.epsl.2011.05.021.

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Cold conditions in Antarctica during the Little Ice Age Implications for abrupt climate change mechanisms1. Introduction2. Material and methods2.1. Study site2.2. Data2.3. Chronological framework2.4. Isotopetemperature relationship

3. Results4. Climate conditions during the past millennium across Antarctica4.1. Mediaeval Warm Period4.2. Little Ice Age4.3. Modern Era

5. Concluding commentsAcknowledgementsSupplementary dataReferences