geosciences

Article An Age Scale for the First Shallow (Sub-)Antarctic Ice Core from Young Island, Northwest Ross Sea

Dorothea Elisabeth Moser 1,2,* , Sarah Jackson 3, Helle Astrid Kjær 4, Bradley Markle 5, Estelle Ngoumtsa 4, Joel B. Pedro 6,7, Delia Segato 8,9 , Andrea Spolaor 8,9, Dieter Tetzner 1,2, Paul Vallelonga 4 and Elizabeth R. Thomas 1

1 Ice Dynamics and Paleoclimate, British Antarctic Survey, Cambridge CB3 0ET, UK; [email protected] (D.T.); [email protected] (E.R.T.) 2 Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK 3 Research School of Earth Science, The Australian National University, Canberra, ACT 2601, Australia; [email protected] 4 Physics of Ice, Climate and Earth (PICE), Niels Bohr Institute, University of Copenhagen, 2200 Copenhagen, Denmark; [email protected] (H.A.K.); [email protected] (E.N.); [email protected] (P.V.) 5 Institute for Arctic and Alpine Research, University of Colorado, Boulder, CO 80309-0399, USA; [email protected] 6 Australian Antarctic Division, Kingston, TAS 7050, Australia; [email protected] 7 Australian Antarctic Program Partnership, Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, TAS 7004, Australia


8 Department of Environmental Sciences, Informatics and Statistics, Ca’ Foscari University of Venice,
 30170 Mestre, Italy; [email protected] (D.S.); [email protected] (A.S.) 9 Citation: Moser, D.E.; Jackson, S.; CNR-Institute of Polar Sciences (ISP-CNR), 30170 Mestre, Italy Kjær, H.A.; Markle, B.; Ngoumtsa, E.; * Correspondence: [email protected] Pedro, J.B.; Segato, D.; Spolaor, A.; Tetzner, D.; Vallelonga, P.; et al. An Abstract: The climate of the sub-Antarctic is important in understanding the environmental con- Age Scale for the First Shallow ditions of Antarctica and the Southern Ocean. However, regional climate proxy records from this (Sub-)Antarctic Ice Core from Young region are scarce. In this study, we present the stable water isotopes, major ion chemistry, and Island, Northwest Ross Sea. dust records from the first ice core from the (sub-)Antarctic Young Island. We present and discuss Geosciences 2021, 11, 368. https:// various dating approaches based on commonly used ice core proxies, such as stable water isotopes doi.org/10.3390/geosciences11090368 and seasonally deposited ions, together with site-specific characteristics such as melt layers. The dating approaches are compared with estimated precipitation rates from reanalysis data (ERA5) and Academic Editors: Juan Pablo Corella volcanic cryptotephra shards likely presenting an absolute tie point from a 2001 CE eruption on and Jesus Martinez-Frias neighboring Sturge Island. The resulting ice core age scale spans the period 2016 to 1995, with an uncertainty of ±2 years. Received: 7 July 2021 Accepted: 25 August 2021 Published: 1 September 2021 Keywords: age scale; (sub-)Antarctic island; shallow ice core; proxies; melting

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- 1. Introduction iations. The sub-Antarctic atmospheric and oceanic circulation patterns play a key role in setting the climate conditions of the Antarctic. They are particularly important to the stability of Antarctica’s outlet glaciers, and for this reason, sub-Antarctic sites are increas- ingly in the spotlight of glaciological research, e.g., [1–4]. However, field-based records Copyright: © 2021 by the authors. of environmental changes in the region are sparse [5], particularly before the satellite era Licensee MDPI, Basel, Switzerland. (since 1979). With the sub-Antarctic highly sensitive to shifts in circulation and potentially This article is an open access article in danger of losing the glaciers on its islands as climate archives, there is urgent need to distributed under the terms and investigate the geochemical proxies preserved in local ice cores to evaluate their potential conditions of the Creative Commons for climate reconstruction, e.g., of position and strength of westerly winds and regional Attribution (CC BY) license (https:// sea ice extent [2,6]. These parameters will help to understand the drivers and anticipate creativecommons.org/licenses/by/ changes in the Antarctic climate at large. 4.0/).

Geosciences 2021, 11, 368. https://doi.org/10.3390/geosciences11090368 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, 368 2 of 18 Geosciences 2021, 11, x FOR PEER REVIEW 2 of 19

ToTo assess assess the the sites’ sites’potential potential forfor longerlonger paleoclimatepaleoclimate records,records, fivefive shallowshallow iceice corescores werewere retrieved retrieved from from Antarctic Antarctic and and sub-Antarcticsub-Antarctic sitessites duringduring thethe Sub-AntarcticSub-Antarctic Ice-CoringIce-Coring ExpeditionExpedition 2016–2017 2016–2017 (SubICE(SubICE [[2]2]).). InIn thisthis study,study, wewe presentpresent thethe firstfirst glacio-chemicalglacio-chemical andand microparticlemicroparticle records records of of a a 17 17 m m ice ice core core from from Young Young Island. Island. YoungYoung Island Island (Figure (Figure1 )1) is is located located ~240 ~240 km km northwest northwest ofof CapeCape Adare,Adare, VictoriaVictoria Land,Land, inin the the Somov Somov Sea, Sea, and and representsrepresents thethe northernmostnorthernmost subaerialsubaerial partpart ofof thethe volcanicvolcanic BallenyBalleny IslandIsland ridgeridge [[7]7].. It It extends extends 30 30 km km N N–S–S and and 6 6km km W W–E–E [8] [8 and] and is isentirely entirely glaciated glaciated [9]. [ 9In]. Inthis this area area NW NW of the of theRoss Ross Sea, Sea,pack pack ice develops ice develops regularly regularly [7,10],[ and7,10 ],a polynya and a polynya is known is knownto reoccur to reoccurannually, annually, especially especially during austral during spring austral and spring summer and [11 summer–13]. Its position [11–13]. and Its positionextent appear and extent to vary appear according to vary to according short-term to atmospheric short-term atmospheric circulation with circulation a tendency with of a tendencythe polynya of the opening polynya up opening leeward, up east leeward, of the eastBalleny of the Islands Balleny (Supplementary Islands (Supplementary Material). Material).Young Island Young is a Islandvaluable is aice valuable core site, ice as coreit lies site, in an as otherwise it lies inan data otherwise-sparse region. data-sparse How- region. However, accessing the site which is positioned in the so-called stormy sixties and ever, accessing the site which is positioned in the so-called stormy sixties and bound by bound by steep cliffs is very challenging. This ice core is further unique due to its position in steep cliffs is very challenging. This ice core is further unique due to its position in the the seasonal sea ice zone at the interface of circumpolar Westerlies and Antarctic Easterlies seasonal sea ice zone at the interface of circumpolar Westerlies and Antarctic Easterlies and could hold novel information about their interplay in the NW Ross Sea region. and could hold novel information about their interplay in the NW Ross Sea region.

Figure 1. Map of (a) the Antarctic including the Balleny Island study area, the September (light blue) Figure 1. Map of (a) the Antarctic including the Balleny Island study area, the September (light blue) and February (dark blue shading) sea ice extent for the long-term median period 1981–2010 and February (dark blue shading) sea ice extent for the long-term median period 1981–2010 (NSIDC); (NSIDC); Subantarctic Front (SAF, outermost anthracite dashed line), Antarctic Polar Front (APF, Subantarcticmiddle light- Frontgrey dash (SAF,ed outermost line), and Southern anthracite Antarctic dashed Circumpolar line), Antarctic Current Polar Front Front (SACCF, (APF, middle inner- light-greymost grey- dasheddashed line),line) given; and Southern (b) detailed Antarctic map of Young Circumpolar Island with Current red and Front blue (SACCF, marker innermostindicating grey-dashedice core drilling line) and given; automatic (b) detailed weather map station of Young (AWS) Island sites, with respectively. red and blue marker indicating ice core drilling and automatic weather station (AWS) sites, respectively. To use the Young Island ice core data for climate reconstructions, we must first define a chronology.To use the YoungA preliminary Island ice estimate core data based for climateon a Herron reconstructions,–Langway wedensification must first definemodel adriven chronology. by the Aannual preliminary average estimate precipitation based from on a ERA5 Herron–Langway reanalysis for densification this site suggests model a drivenbottom by age the of annual 2002 ± average4 years [2] precipitation. However, fromYoung ERA5 Island reanalysis is too small for thisto be site considered suggests aa bottom age of 2002 ± 4 years [2]. However, Young Island is too small to be considered

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a land grid point in ERA5 and thus the precipitation bears significant uncertainty. This approach also fails to consider the impact of wind scouring or post-depositional changes. Thus, a more robust dating approach is needed to establish the time span of this ice core and ascertain if annual average climate information can be extracted. Ice core chronologies from high-accumulation sites can be developed using annual layer counting in stable water isotope and geochemistry records [14,15]. Based on their sensitivity to temperature, stable water isotopes are well-established proxies for past air- temperature variations in Antarctica [16,17], providing seasonal cycles for annual layer counting. Glacio-chemical profiles show distinct seasonality based on the source, transport and preservation history of each aerosol species. Marine aerosols include sodium (Na+) from the open ocean [18,19] and sea–ice surfaces [20]; bromide (Br−), primarily emitted via sea spray [21] and secondarily through autocatalyzed bromine explosions above fresh, 2− acidic sea ice in spring [22]; sulfate (SO4 ), emitted indirectly via sea spray from biogenic dimethylsulfate (~70%, [23]), and to a lesser extent from sulfur dioxide of volcanic and anthropogenic origin [24]; and methanesulfonate (MSA−,[18]), a product of marine algae − and phytoplankton activity. Nitrate (NO3 ) aerosols are produced naturally during soil and ocean denitrification [25], and have an additional extraterrestrial source through nitrogen fixation during lightning [26–28]. Hydrogen peroxide (H2O2), a proxy for identifying annual layers in Antarctic ice cores with high accumulation (>0.22 m a−1,[29]), stems from − 2− − − photolysis during the sunlight season [30]. Br , SO4 , MSA , and NO3 concentrations peak during summer, mainly driven by the biogenic cycle, so that their similar phasing in ice cores can be used for dating. The concentration, size distribution and geochemical composition of insoluble particulate matter in ice cores, which we refer to as dust using the convention of Tetzner et al. [31], give further insight into environmental conditions of the past [32,33]. In the NW Ross Sea, dust is expected to stem largely from Australia and New Zealand [34]. The objectives of this study are to (1) evaluate meteorological information available from a local automatic weather station (AWS) and ERA5 reanalysis; (2) present multiple dating approaches based on stable water isotopes, glacio-chemical data, and physical characteristics; (3) evaluate a potential absolute age marker of volcanic source, and (4) de- termine an age scale for future climate interpretation. Consequently, this study aims to contribute to a first glaciological characterization of Young Island and lay a foundation for future ice core studies at the site.

2. Materials and Methods 2.1. Ice Core Drilling and Processing On 4 February 2017, a shallow ice core was drilled to a depth of 16.92 m on Young Island (66◦31044.300 S, 162◦33021.500 E). The ice core site is an ice saddle at the southern tip of Young Island (Figure1b), 238 m above sea level (a.s.l.). With the drilling location situated in an extensional flow regime, ground-penetrating radar revealed several crevasses near the ice core position covered by 4–5 m of well-stratified snow [2]. The core was drilled using a motorized Kovacs ice-core drill (Mark II) powered by a 4-stroke Honda generator, with core retrieval aided by a sidewinder winch. Ice core sections of 0.20–0.81 m length were retrieved, logged, sealed in ethylene-vinyl-acetate-treated (EVA) polythene bags, and packed in insulating boxes [2]. They were kept at −20 ◦C during the Antarctic Circumpolar Expedition (ACE cruise) and shipped to the British Antarctic Survey (BAS), Cambridge UK, where the cores were stored at −25 ◦C. In August 2017, the Young Island ice core was sampled for various discrete and continuous analyses (Figure2) using a stainless-steel bandsaw in the cold lab facilities at BAS. The weight and length of each bag and ice core fragment were measured, respectively, to calculate density and convert the firn depth scale to a water depth scale (meters water equivalent, m w.eq.) for the presented dating approaches. The positions and thickness of homogeneous, bubble-sparse ice lenses visible to the unaided eye, i.e., >1 mm thick, were recorded and interpreted as melt layers. A strip for ion-chromatographic (IC) and stable Geosciences 2021, 11, x FOR PEER REVIEW 4 of 19

recorded and interpreted as melt layers. A strip for ion-chromatographic (IC) and stable water isotope measurements was discretely cut into 5 cm slices, sealed in tritan copolyes- Geosciences 2021, 11, 368 ter jars, and kept at −23 °C to avoid temperature-induced alteration of the sample chemis-4 of 18 try. The methods and results for physical characteristics, discrete stable water isotopes, and major ion concentrations presented in Sections 3.2.1–3.2.3. were based on previously water isotope measurements was discretely cut into 5 cm slices, sealed in tritan copolyester unpublished work by D.E. Moser [35]. jars, and kept at −23 ◦C to avoid temperature-induced alteration of the sample chemistry.

FigureFigure 2. 2. CuttingCutting scheme scheme of of a a circular circular Young Young Island Island ice ice core core cross cross section, includ includinging square square strips forfor continuous continuous flow flow analysis analysis (CFA) (CFA) at at the the Niels Niels Bohr Bohr Institute Institute (former (former Centre Centre for for Ice Ice and and C Climate,limate, CIC)CIC) and and the the British British Antarctic Antarctic Survey Survey (BAS), (BAS), segments segments for for discrete discrete ion ion chromatography (IC) (IC) and and stablestable water water isotope isotope measurements, measurements, microparticle microparticle analysis analysis (PA), (PA), and and an an archive archive (A); (A); figure figure adapted adapted fromfrom [35] [35]..

2.2. IceThe Core methods Discrete and Chemistry results for physical characteristics, discrete stable water isotopes, andIsotope major ion measurements concentrations (δ presented18O) were in conducted Sections 3.2.1 at BAS–3.2.3 in. were April based–May on 2018 previously using a Picarrounpublished L2130 work-i device, by D.E. using Moser cavity [35 ]. ring down spectroscopy [36,37]. Each sample was measured with seven injections. After removing the first three injections to avoid memory 2.2. Ice Core Discrete Chemistry effects, we used the mean of the final four measurements to calculate δ18O with an accu- δ18 racy ofIsotope 0.3‰. The measurements measurements ( areO) werereported conducted against the at BASinternational in April–May standard 2018 of usingVienna a StandardPicarro L2130-i Mean Ocean device, Water using (V cavity-SMOW), ring and down 2σ spectroscopyprecision lies at [36 0.1‰,37]. for Each δ18 sampleO. was measuredUsing withthe same seven discrete injections. sample After material removing (IC, theFigure first 2), three major injections ion concentrations to avoid memory were effects, we used the mean of the final four measurements to calculate δ18O with an accuracy measured using a high-performance Dionex Integrion ion chromatograph with an injec- of 0.3‰. The measurements are reported against the international standard of Vienna tion volume of 250 μL in a class-100 cleanroom. For the cation chromatograph, we applied Standard Mean Ocean Water (V-SMOW), and 2σ precision lies at 0.1‰ for δ18O. a guard column type CS16-4μm (2 × 50 mm) and a CS16-4μm separator column (2 × 250 Using the same discrete sample material (IC, Figure2), major ion concentrations mm). For the anion chromatograph, we used an AG17-C guard column (2 × 50 mm) to- were measured using a high-performance Dionex Integrion ion chromatograph with an in- gether with an AS17-C analytical column (2 × 250 mm). Here, we specifically focused on jection volume of 250 µL in a class-100 cleanroom. For the cation chromatograph, we SO42−, MSA−, Br−, and NO3− concentrations. Non-sea-salt SO42− (nssSO42−) and sea-salt Na+ applied a guard column type CS16-4µm (2 × 50 mm) and a CS16-4µm separator col- (ssNa+) were calculated using the following equations: umn (2 × 250 mm). For the anion chromatograph, we used an AG17-C guard column (2 × 50 mm) together with[nssSO an AS17-C42−] = [SO analytical42−] − RSO4 column-Na sea water ( 2× ×[Na250+] mm). Here, we specif-(1) 2− − − − 2− ically focused on SO4 , MSA , Br , and NO3 concentrations. Non-sea-salt SO4 2− + [ssNa+ +] = [Na+] – [nssCa2+]/Rcrust (2) (nssSO4 ) and sea-salt Na (ssNa ) were calculated using the following equations:

2+ 2+ + Ca-Na sea water [nssCa2− ] = [Ca2− ] – [ssNa ] × R + (3) [nssSO4 ] = [SO4 ] − RSO4-Na sea water × [Na ] (1) where RSO4-Na sea water correspond to the mean ratio (weight by weight, abbreviated as: w/w) + + − 2+ in bulk sea water (RSO4-Na seawater[ssNa = 0.252;] = [Na [38]).] Rcrust[nssCa and R]/RCa-Nacrust sea water correspond to the mean(2) 2+ crust 2+ + Ca-Na sea water ratios (w/w) in the earth[nssCa crust ](R = [Ca= 1.78)] − [ssNa and in] ×bulkRCa-Na sea seawater water (R = 0.038),(3)

where RSO4-Na sea water correspond to the mean ratio (weight by weight, abbreviated as: w/w) in bulk sea water (RSO4-Na seawater = 0.252; [38]). Rcrust and RCa-Na sea water corre- spond to the mean ratios (w/w) in the earth crust (Rcrust = 1.78) and in bulk sea water 2− + (RCa-Na sea water = 0.038), respectively [39]. The nssSO4 and ssNa were calculated to Geosciences 2021, 11, 368 5 of 18

2− + obtain the nssSO4 :ssNa ratio, which has been shown as a valuable tool to enhance austral summer peaks and winter troughs [40].

2.3. Ice Core Continuous Flow Analysis Data An inner section of the core 32 × 32 mm was shipped to Copenhagen for continuous flow analysis (CFA). The setup resembles the one described by Bigler et al. [41] for the analysis of dust in the particle size range 0.9–15.0 µm and hydrogen peroxide (H2O2,[42]). The core was analysed in May 2018 and melted from the top down. The melt rate was kept at ~5 cm min−1. Standards used for calibration were run for every 3 to 5 m of core, dependent on the core density. At BAS, continuous flow analysis [41] was used to acquire the liquid hydrogen per- oxide concentration based on enzymatic fluorometry, first applied by Lazrus et al. [42], in December 2017. The contamination risk for both CFA sections was considered negligible, because they lay centrally, i.e., ≥0.7 cm inside the ice core rims (Figure2), and the CFA melt head was set up for decontamination [43].

2.4. Microparticle Analysis Microscopy analyses of microparticles were performed to examine the presence of cryptotephra shards, indicative of the 2001 CE eruption on Sturge Island, Balleny Islands, reported by Tetzner et al. [44]. For this, ice samples of ~30 cm were cut below a 10 m snow depth, then melted and filtered. Meltwater was filtered through 13 mm diameter 1.0-µm pore-size Whatman™ polycarbonate membrane filters. Filters were analysed on a Quanta-650F Scanning Electron Microscope (SEM) at the Earth Sciences Department of the University of Cambridge. Filters were imaged using backscattered electrons (BSE) on a low-pressure mode at ×800 magnification for cryptotephra shard identification and characterization, following the method presented by Tetzner et al. [31].

2.5. Meteorological Data An AWS on the northern tip of Young Island (66◦13044.400 S, 162◦16030.000E) at 30 m a.s.l. was operated by the University of Wisconsin-Madison Antarctic Meteorology Program from January 1991 to December 1997. Here, we used 3-hourly, 2-m air temperature that showed a median data coverage of ~88% over this time period after excluding a persistent data gap from November 1994 to February 1996. In addition, we applied the KNMI Climate Explorer to extract monthly 2-m air temper- ature averages and monthly precipitation estimates for the region 66.0–67.5◦ S, 162–165◦ E from ERA5 reanalysis. ERA5 has a spatial grid resolution of 31 km and is available from January 1979 to the present [45].

3. Results 3.1. Meteorological Conditions The annual mean 2-m air temperature at Young Island derived from the low-elevation AWS (209 m below the drilling site) was −7.9 ◦C with 3-h mean temperature values spanning from −35.0 ◦C to +4.2 ◦C (Figure3). Seasonal mean air temperatures varied between −1.6 ◦C in austral summer (November–February) and −13.9 ◦C during austral winter (May–August; [2]). For the larger Balleny Island area defined above, ERA5 reanalysis yielded a slightly cooler annual mean temperature of −8.3 ◦C that was driven by a winter- seasonal mean of −15.0 ◦C. The regional precipitation estimate from ERA5 averaged ~83 cm a−1 with interannual variability 67–103 cm a−1 and a slight seasonal bias towards austral autumn and spring snowfall (Figure3). Geosciences 2021, 11, x FOR PEER REVIEW 6 of 19 Geosciences 2021, 11, 368 6 of 18

Figure 3. Seasonal variability of monthly mean 2-m air temperature (red) and precipitation (blue, in Figure 3. Seasonal variability of monthly mean 2-m air temperature (red) and precipitation (blue, in millimeter water equivalent per month) at Young Island derived from an automatic weather station millimeter water equivalent per month) at Young Island derived from an automatic weather station (AWS)(AWS) inin 1991–19941991–1994 andand 1996–19971996–1997 andand ERA5 reanalysis 1979–2020;1979–2020; figurefigure adapted from [[35]35]..

3.2. Data for Ice Core Dating 3.2.1. Physical Characteristics of the Ice Core 3.2.1. Physical Characteristics of the Ice Core The Young Island ice core was characterized by a substantial densification. The mean The Young Island ice core was characterized by a substantial densification. The mean density derived from ice core pieces rose from ~500 kg m−3 near the surface to ~870 kg m−3 density derived from ice core pieces rose from ~500 kg m−3 near the surface to ~870 kg m−3 at the bottom of the profile (Figure4a). Based on the binary, unaided-eye identification, the at the bottom of the profile (Figure 4a). Based on the binary, unaided-eye identification, ice core contained a total number of 126 melt layers (Figure4). Their thickness averaged the ice core contained a total number of 126 melt layers (Figure 4). Their thickness aver- 6.4 cm and ranged from 0.1 cm to 58 cm [2]. Melt layers occurred throughout the profile but aged 6.4 cm and ranged from 0.1 cm to 58 cm [2]. Melt layers occurred throughout the were distributed unevenly in clusters. Most of the observed melt-affected sections appeared profile but were distributed unevenly in clusters. Most of the observed melt-affected sec- as bubble-containing ice (Figure4b), but a minority of thinner layers were bubble-free or a tions appeared as bubble-containing ice (Figure 4b), but a minority of thinner layers were blending of large crystals with refrozen bubble-containing ice [35]. bubble-free or a blending of large crystals with refrozen bubble-containing ice [35]. Thomas et al. [2] estimated a bottom age for the Young Island ice core of 2002 ± 4 years usingThomas its density et al. at [2] bag estimate resolutiond a bottom (0.2 m toage 0.81 for mthe resolution). Young Island Unfortunately, ice core of 2002 as melt ± 4 phasesyears using were its unequally density distributedat bag resolution throughout (0.2 m the to profile,0.81 m soresolution). was their Unfortunately, accelerating effect as onmelt snow phases metamorphism were unequally [46]. distributed If extensive throughout melting takes the place profile, during so wa as year, their the accelerating respective annualeffect on snow snow layer metamo will thinrphism more [46] quickly. If extensive due to melting facilitated takes densification. place during Consequently, a year, the temporalrespective resolution annual snow in thislayer ice will core thin varies more between quickly non-meltdue to facilitated (higher) densification. and melt-affected Con- sectionssequently, (lower) temporal [35]. resolution in this ice core varies between non-melt (higher) and melt- affected sections (lower) [35].

3.2.2. Stable Water Isotopes (δ18O) δ18O ranged between −15.1‰ and −6.5‰ and exhibited short-term variations over the entire profile (Figure 5a). While the overall mean for δ18O lay at −9.8‰, a slight increase was detected from −10.3‰ (profile top) to −9.3‰ (profile bottom). In the top ~0.75 m w.eq. of the core, δ18O fluctuated with an amplitude of up to ±4.4‰ and at a depth interval of 0.24 ± 0.07 m w.eq. This fit the cyclicity simulated for coastal Adélie Land (±4.5‰, [47]), suggesting that δ18O carries a seasonal signature in this part of the Young Island ice core. At depths >0.75 m w.eq., the amplitude of the local peaks and troughs became more damp- ened, lying within an amplitude of ±2.2‰. Attenuation of amplitude complicated the identification of annual δ18O cycles but was consistent with vapor diffusion in firn [48].

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Figure 4. (a) Density profiles derived from ice core fragments on water equivalent depth scale (w.eq.) Figure 4. (a) Density profiles derived from ice core fragments on water equivalent depth scale (w.eq.) including the overall including the overall trend based on polynomial fit, plotted against the record of melt layers (grey trend based on polynomial fit, plotted against the record of melt layers (grey vertical lines); (b) Alternation of non-melt (darker) andvertical melt-affected lines); (sectionsb) Alternation (lighter) of within non-melt the (darker)Young Island and melt-affectedice core at 1.47 sections–1.63 m (lighter)w.eq. depth within (equals the 2.85–3.15 m snow depth)Young acquired Island from ice core digital at 1.47–1.63 black-and m-white w.eq. photography depth (equals; figure 2.85–3.15 redrawn m snow from depth) [35]. acquired from digital black-and-white photography; figure redrawn from [35]. 3.2.3. Major Ion Concentration Records 3.2.2. Stable Water Isotopes (δ18O) Na+ was the largest contributor to the ion budget at Young Island. Its concentrations δ18O ranged between −15.1‰ and −6.5‰ and exhibited short-term variations over ranged between 394 ppb and 8674 ppb with a profile average of 3362 ppb (Figure 5b). the entire profile (Figure5a). While the overall mean for δ18O lay at −9.8‰, a slight SO42− concentrations ranged between 62 ppb and 2872 ppb, averaging 490 ppb in the increase was detected from −10.3‰ (profile top) to −9.3‰ (profile bottom). In the top ~0.75 m w.eq.upper of the more core, recentδ18O fluctuated part of the with core an (~3.1 amplitude m w.eq.), of upand to 1062±4.4 ppb‰ andin the at deeper a sections 2− depth interval(Figure of 0.24 5c).± 0.07 SO4 m wa w.eq.s the Thissecond fit largest the cyclicity contributor simulated to the for ion coastal budget. Ad Thereélie were no clear 2− Land (±4.5‰,[peaks47]), that suggesting could be that associatedδ18O carries with aa seasonalvolcanic signatureeruption in in the this SO part4 record, of the which would Young Islandotherwise ice core. Atbe depthsuseful as >0.75 an absolute m w.eq., marker the amplitude for ice core of dating the local at Young peaks andIsland. − troughs became moreMean dampened, MSA concentration lying within decline an amplituded from 44 of ppb±2.2 (top)‰. to Attenuation 29 ppb (bottom, of Figure 5d). amplitude complicatedConcentrations the identification of the troughs of mostly annual remaiδ18Oned cycles below but was30 ppb consistent and could with be distinguished vapor diffusionfrom in firn local [48 MSA]. − maxima by sharp transitions. The total MSA− variability spanned a range from 142 ppb (~4 m w.eq. depth) to 2.4 ppb (~8.9 m w.eq. depth). MSA− and SO42− concen- 3.2.3. Major Iontrations Concentration had a correlation Records coefficient of +0.66 (p < 0.05), indicating over 40% shared vari- Na+ was theance largest between contributor these two to records. the ion budget at Young Island. Its concentrations ranged between 394Br ppb− concentration and 8674 ppb range withd between a profile average1.5 ppb and of 3362 59.5 ppbppb (Figure(Figure5 5e).b). Mean Br− concen- 2− − SO4 concentrationstration increase rangedd from between 7.9 ppb 62 (top) ppb to and 31.0 2872 ppb ppb, (bottom). averaging Thereby, 490 ppb Br in evolved analo- the upper moregously recent to part Na+ of concentration the core (~3.1 in m most w.eq.), parts and of 1062 the profile ppb in (r the ≈ +0.89, deeper p sections< 0.05) and the Br−:Na+ 2− (Figure5c). SOratio4 wasaverage thed second 0.0065 largest similar contributor to the standard to the sea ion water budget. ratio There (0.006; were [49] no). An exception 2− + clear peaks thatwa coulds the depth be associated intervalwith between a volcanic 5.1 m w.eq. eruption and in 5.8 the m SOw.eq.4 depth,record, where which the Na profile would otherwiseshow beed useful a high as plateau, an absolute whereas marker the for Br ice− concentration core dating atexhibit Younged Island. stronger fluctuations. − Mean MSA concentrationNO3− concentrations declined range fromd 44 between ppb (top) 7.9 to ppb 29 ppband (bottom,93.0 ppb Figure(Figure5d). 5f). They fluctu- Concentrationsate ofd the strongly troughs with mostly an amplitude remained belowup to 3073 ppbppb andbetween could trough be distinguished and peak positions but − − from local MSAmaintainmaximaed a bystable sharp average transitions. of 34.1 The ppb total throughout MSA variabilitythe profile. spanned Most of athe NO3− peaks − 2− range from 142we ppbre confined (~4 m w.eq. to a depth) narrow to depth 2.4 ppb interval (~8.9 m and w.eq. therefore depth). unambiguous MSA and SO (e.g.4 , ~8.2 m w.eq.). concentrations had a correlation coefficient of +0.66 (p < 0.05), indicating over 40% shared variance between these two records.

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Br− concentration ranged between 1.5 ppb and 59.5 ppb (Figure5e). Mean Br − concen- tration increased from 7.9 ppb (top) to 31.0 ppb (bottom). Thereby, Br− evolved analogously to Na+ concentration in most parts of the profile (r ≈ +0.89, p < 0.05) and the Br−:Na+ ratio averaged 0.0065 similar to the standard sea water ratio (0.006; [49]). An exception was the depth interval between 5.1 m w.eq. and 5.8 m w.eq. depth, where the Na+ profile showed a high plateau, whereas the Br− concentration exhibited stronger fluctuations. − NO3 concentrations ranged between 7.9 ppb and 93.0 ppb (Figure5f). They fluc- Geosciences 2021, 11,tuated x FOR PE stronglyER REVIEW with an amplitude up to 73 ppb between trough and peak positions but 8 of 19 − maintained a stable average of 34.1 ppb throughout the profile. Most of the NO3 peaks were confined to a narrow depth interval and therefore unambiguous (e.g., ~8.2 m w.eq.).

Figure 5. FullFigure glacio 5.-chemicalFull glacio-chemical records acquired records from acquired the Young from Island the ice Young core; Island(a) stable ice water core; (isotopea) stable ratio water δ18O; concen- trations of (bisotope) sodium, ratio (c)δ sulfate,18O; concentrations (d) methanesulfonate, of (b) sodium, (e) bromide, (c) sulfate, (f) nitrate, (d) methanesulfonate, (g) hydrogen peroxide, (e) bromide, and (i) dust, de- fined as insoluble(f) nitrate, particulate (g) hydrogen matter. peroxide, and (i) dust, defined as insoluble particulate matter.

3.2.4. Dust 3.2.4. Dust The dust recordThe of dust the Young record Island of the ice Young core was Island characterized ice core wa bys acharacterized strong down-core by a strong down- ± −1 variability (meancore variability = 3638.5 (mean4031.4 = 3638.5 particles ± 4031.4 mL particles) and mL a−1) three-foldand a three increase-fold increase (mean = ± −1 µ (mean = 326.37326.37191.35 ± 191.35particles particles mL mL) in−1) thein the total total dust dust particle particle concentration concentration (>1 (>1 m)μm) between 7.6 between 7.6 m and 8.2 m w.eq. depth (mean = 11374.8 ± 6712.6 particles mL−1, Figure5i ). m and 8.2 m w.eq. depth (mean = 11374.8 ± 6712.6 particles mL−1, Figure 5i). This major This major increase in dust particle concentration was also identified in the coarser par- increase in dust particle concentration was also identified in the coarser particle concen- ticle concentration (>7 µm), i.e., between 7.83 m and 7.90 m w.eq. depth (see Section 3.5: tration (>7 μm), i.e., between 7.83 m and 7.90 m w.eq. depth (see Chapter 3.5: Figure 12f). Figure 12f). This section was characterized by a short and steep increase, reaching its This section was characterized by a short and steep increase, reaching its highest value highest value (3000.3 particles mL−1) at 7.89 m w.eq. depth. This sharp increase was an (3000.3 particles mL−1) at 7.89 m w.eq. depth. This sharp increase was an order of magni- tude higher than the adjacent coarse particle concentration, standing out as a single fea- ture in the coarse particle record from the Young Island ice core. To obtain the particle size distribution (PSD) of insoluble dust, the volume concen- tration parameter (dV/dlnD) was calculated for the range of particle diameters (Figure 6). The background PSD, excluding the dust event, showed a log-normal distribution with a local peak around 2.5 μm and a mode particle diameter of 6.9 μm. The distribution for the dust event was similar but with a 1-μm rise in the mode particle diameter (7.9 μm). Fur- thermore, the PSD of the dust event exhibited volume concentration values on average five times higher than the background.

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order of magnitude higher than the adjacent coarse particle concentration, standing out as a single feature in the coarse particle record from the Young Island ice core. To obtain the particle size distribution (PSD) of insoluble dust, the volume concen- tration parameter (dV/dlnD) was calculated for the range of particle diameters (Figure6). The background PSD, excluding the dust event, showed a log-normal distribution with a local peak around 2.5 µm and a mode particle diameter of 6.9 µm. The distribution for the dust event was similar but with a 1-µm rise in the mode particle diameter (7.9 µm).

GeosciencesFurthermore, 2021, 11, x FORthe PEER PSD REVIEW of the dust event exhibited volume concentration values on average9 of 19

five times higher than the background.

FigureFigure 6. 6. ParticleParticle size size distribution distribution of dust, of defined dust, as defined insoluble as particulate insoluble matter, particulate in the Young matter, Island in ice the core; Young dV/dlnD Island = derivative of the total dust volume (V) with respect to the natural logarithm of the particle diameter (D) for each bin. ice core; dV/dlnD = derivative of the total dust volume (V) with respect to the natural logarithm of the particle diameter (D)3.2.5. for Hydrogen each bin. Peroxide Record

The H2O2 record for the Young Island ice core was measured continuously at both 3.2.5. Hydrogen PeroxideBAS and Record CIC, and variability was seen in the upper layers with peaks up to 40 ppb as expected for this relative high-temperature site compared to other Antarctic sites [29]. At The H2O2 record for the Young Island ice core was measured continuously at both BAS and CIC, and variabilitydepths >1.5 m was w.eq. seenthe signal in the was uppermostly absent layers with with only peaksa few peaks up toup to 40 ~5 ppb ppb and as thus did not capture seasonal changes (Figure 5g). Whether this is due to dissolution dur- expected for this relativeing melt high-temperature events [30], consumption site during compared reaction to with other the oxidant Antarctic SO42− sites[50], or [ 29with]. dust At depths >1.5 m w.eq.particles the signal [51] demands was mostly further investigation. absent with Until only then, a any few longer peaks-term up trend to ~5of H ppb2O2 is and thus did not capturedifficult seasonal to assess. changes (Figure5g). Whether this is due to dissolution 2− during melt events [303.3.], Dating consumption Approaches during reaction with the oxidant SO4 [50], or with dust particles [51] demands3.3.1. Dating further Approach investigation. A—Based on δ18O Until then, any longer-term trend of H2O2 is difficult to assess.For the Young Island profile, 28 ± 6 winter markers could be set based on δ18O troughs (Figure 7), leading to a 27-year annual record from 2016 to 1989. According to Moser [35], 3.3. Dating Approachesa caveat to this approach is that “distinct troughs in the δ18O profile potentially represent 3.3.1. Dating Approachextreme A—Based cold events on ratherδ18O than seasonal signatures. Using them as primary markers in ice core dating could lead to an under-/overestimation of annual cycles, depending on the For the Young Islandinterannual profile, frequency 28 ± of6 wintersuch events”. markers This is could because be stable set based water isotope on δ18 Orecords troughs in ice (Figure7), leading tocores a 27-year are especially annual dependent record from on the 2016 frequency to 1989. of precipitation According events to Moser [52,53]. [ Further-35], a more, stable water isotope diffusion and amplitude18 reduction prohibit us from using δ18O caveat to this approachas the is foundation that “distinct of an age troughs scale alone. in the δ O profile potentially represent extreme cold events rather than seasonal signatures. Using them as primary markers in ice core dating could lead to an under-/overestimation of annual cycles, depending on the interannual frequency of such events”. This is because stable water isotope records in ice cores are especially dependent on the frequency of precipitation events [52,53]. Furthermore, stable water isotope diffusion and amplitude reduction prohibit us from using δ18O as the foundation of an age scale alone.

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Figure 7. Dating approach A based on the discrete 5 cm δ18O record (light blue line) of the Young Island ice core and its 3 point running average (thin dark blue line) using local troughs as winter markers (vertical purple lines); questionable years are marked as dashed lined.

3.3.2. Dating Approach B—Based on NO3− Record Another approach was based on a continuous annual layer thickness derived from GeosciencesGeosciences 20212021,, 1111,, x 368 FOR PEER REVIEW the nitrate concentration record, which appeared to have a seasonal1010 of of 19signal 18 with peaks

assumed during summer [54] in the upper 2 m w.eq. of the Young Island ice core (Figure 8). Post-depositional processes are known to drive the loss of nitrate from the snowpack, mainly through volatilization and photolysis [55–57]. However, in coastal high-accumu- lation sites such as Young Island this is expected to have only a minor influence on the signal. Annual layers could be counted for the first ~6 years with the distance between peaks ranging 29–45 cm w.eq. with an average of 37 cm w.eq. The seasonality of the signal was less striking further down, so that layer counting bore a larger uncertainty below 2 m w.eq. depth. The annual net accumulation (28–43 cm w.eq.) derived from this approach FigureFigure 7. 7. DatingDating approach approachwas lower A A basedthan based theon on theestimated the discrete discrete regional5 5 cm cm δδ1818O Oprecipitation record record (light (light bluefrom blue line) line)ERA5 of of the the(67 Young– Young103 cm w.eq.), which IslandIsland ice ice core core and anddoes its its 3 3not point point account running running for average average melting (thin (thin and dark dark wind blue blue drift. line) line) Based using using onlocal local the troughs troughs net accumulation as as winter winter derived from markersmarkers (vertical (vertical purpleindividual purple lines); lines); layer questionable questionable counting years years for NOare are marked3 marked− in the as astop dashed dashed 2 m, andlined. lined. assuming an overall stable accumu- lation over the last 30 years, we applied a linear fit to obtain age scale B. The error of this − 3.3.2.3.3.2. Dating Dating Approach Approachage scale B B—Based— waBaseds obtained on on NO NO 3calculating3− RecordRecord a maximum and minimum accumulation using the AnotherAnother approach (average waswa ± sstandard basedbased onon deviation) a a continuous continuous and annual applyingannual layer layer the thickness linear thickness fit. derived On derived this from basis, from the the Young Island thenitrate nitrate concentration concentrationice core record, contains record, which layerswhich appeared until appear to1988 haveed ±to 6 a years. have seasonal aThe seasonal signal largewith uncertaintysignal peaks with assumed shows peaks that an absolute assumedduring summer duringmark summer [54] iner is the [54]essential upper in the 2for mupper developing w.eq. 2 m of w.eq. the an Young ofage the scale Island Young for icethis Island core difficult (Figureice core site.8 (Figure). For Post- this, we calculated 8).depositional Post-depositional processesthe SO processes42 are−:NO known3− areratio known to record, drive to the whichdrive loss theshow of nitratelossed of a fromnitratepeak theat from 9.6 snowpack,– the9.8 snowpack,m w.eq. mainly (Figure 8, vertical mainlythrough through volatilization greyvolatilization arrow) and photolysis that and could photolysis [55 correspond–57]. [55 However,–57] to. However,the in 1991 coastal Pinatubo in high-accumulation coastal eruption, high-accumu- but sites geochemical anal- lationsuch assites Young such Island ysesas Young of this tephra isIsland expected layers this and tois haveexpe volcaniccted only totracers a minorhave are only influence necessary a minor on to theinfluence confirm signal. onthis the hypothesis. signal. Annual layers could be counted for the first ~6 years with the distance between peaks ranging 29–45 cm w.eq. with an average of 37 cm w.eq. The seasonality of the signal was less striking further down, so that layer counting bore a larger uncertainty below 2 m w.eq. depth. The annual net accumulation (28–43 cm w.eq.) derived from this approach was lower than the estimated regional precipitation from ERA5 (67–103 cm w.eq.), which does not account for melting and wind drift. Based on the net accumulation derived from individual layer counting for NO3− in the top 2 m, and assuming an overall stable accumu- lation over the last 30 years, we applied a linear fit to obtain age scale B. The error of this age scale was obtained calculating a maximum and minimum accumulation using the (average ± standard deviation) and applying the linear fit. On this basis, the Young Island ice core contains layers until 1988 ± 6 years. The large uncertainty shows that an absolute marker is essential for developing an age scale for this difficult site. For this, we calculated the SO42−:NO3− ratio record, which showed a peak at 9.6–9.8 m w.eq. (Figure 8, vertical Figure 8. NO3− profile with− manually picked markers (vertical purple lines) and the fully extrapolated age scale B (upper greyFigure arrow) 8. NO 3thatprofile could with correspond manually pickedto the 1991 markers Pinatubo (vertical eruption, purple lines) but andgeochemical the fully extrapo- anal- x-axis) in the context of melt layers (vertical grey bars) and SO42−:NO3− ratio in the lower panel; 2a− grey vertical− arrow lated age scale B (upper x-axis) in the context of melt layers (vertical grey bars) and SO4 :NO3 indicates ysesthe period of tephra of Pinatubo layers eruption.and volcanic tracers are necessary to confirm this hypothesis. ratio in the lower panel; a grey vertical arrow indicates the period of Pinatubo eruption.

Annual layers3.3.3. could Dating be countedApproach for— theBased first on ~6 Marine years with Species the distance and Large between Melt Layers peaks ranging 29–45 cm w.eq. with an average of 37 cm w.eq. The seasonality of the signal was less striking further down, so that layer counting bore a larger uncertainty below 2 m w.eq. depth. The annual net accumulation (28–43 cm w.eq.) derived from this approach was lower than the estimated regional precipitation from ERA5 (67–103 cm w.eq.), which does not account for melting and wind drift. Based on the net accumulation derived − from individual layer counting for NO3 in the top 2 m, and assuming an overall stable accumulation over the last 30 years, we applied a linear fit to obtain age scale B. The error of this age scale was obtained calculating a maximum and minimum accumulation using the (average ± standard deviation) and applying the linear fit. On this basis, the Young Island ice core contains layers until 1988 ± 6 years. The large uncertainty shows that an absolute marker is essential for developing an age scale for this difficult site. For this, we calculated the SO 2−:NO − ratio record, which showed a peak at 9.6–9.8 m w.eq. (Figure8, Figure 8. NO3− profile with manually picked markers4 (vertical3 purple lines) and the fully extrapolated age scale B (upper vertical grey arrow) that could correspond2− − to the 1991 Pinatubo eruption, but geochemical x-axis) in the context of melt layers (vertical grey bars) and SO4 :NO3 ratio in the lower panel; a grey vertical arrow indicates the period of Pinatuboanalyses eruption. of tephra layers and volcanic tracers are necessary to confirm this hypothesis.

3.3.3.3.3.3. Dating Dating Approach Approach—Based—Based on on Marine Marine Species Species and and Large Large Melt Melt Layers Layers − 2− Another approach to dating was based on marine species. MSA and SO4 from marine biogenic activity are assumed to peak during the spring and summer similar to

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Another approach to dating was based on marine species. MSA− and SO42− from ma- rine biogenic activity are assumed to peak during the spring and summer similar to NO3− [54]. They− have provided reliable seasonal signals at several coastal Antarctic ice core sites NO3 [54]. They have provided reliable seasonal signals at several coastal Antarctic ice core [58]sites and [58] on and the on only the only other other annually annually dated dated sub sub-Antarctic-Antarctic island island site site at at Bouvet Bouvet Island Island [6] [6],, −− 22−− − − where the MSAMSA ,, SO SO44 , and, and Br Br concentrationconcentration were were fundamental fundamental in in developing developing the the age scale.scale. In In the the Bouvet Bouvet ice ice core, core, and and in in the the Young Young Island Island ice ice core presented here, the th threeree speciesspecies covary, covary, suggesting a similar marine or sea ice source. Their phasing is supported byby nitrate nitrate in in large large parts parts of of the the profile. profile. Unfortunately, Unfortunately, the solubility of the ions makes them susceptiblesusceptible to migration [[59]59],, asas isis evident evident when when comparing comparing with with the th melte melt layers layers (Figure (Figure9 ). 9).However, However, if weif we assume assume solar solar radiation radiation as as the the major major driver driver of of surfacesurface meltmelt [[60]60],, then at leastleast the the larger larger melt melt layers layers (layers (layers >10 >10 cm cm snow snow depth depth plotted plotted for for comparison) comparison) also also peak peakeded duringduring the the summer summer months. months. Peaks Peaks in in marine marine species species a alsolso occur without the presence of largelarge melt melt layers, layers, suggesting suggesting that that melt melt is is not not the the sole sole driver driver of of variability variability in in the the record. record. − − 22−− −− TheThe approach ofof relyingrelying onon summersummer marine marine proxies proxies (MSA (MSA,, SO SO4 4 and BrBr )) suggest suggesteded a bottombottom age age of of 1995/1994 CE. CE. We We f foundound a a total total o off three three years years within within the the profile, profile, where a peakpeak is is missing in one or more species and thus, we estimate the error to be ±±22 years. years.

− − − 2− FigureFigure 9. 9. DatingDating approach approach C C based based on on MSA MSA−, Br, Br−, NO, NO3−, and3 , andSO42 SO− concentration4 concentration peaks peaks at 5-cm at reso- 5-cm lutionresolution (thick (thick lines) lines) with with a 3- apoint 3-point running running average average (thin (thin lines); lines); purple purple vertical vertical lines lines mark mark assigned assigned years,years, dashed dashed lines lines uncertain; uncertain; light light-grey-grey bars bars in inthe the lowest lowest panel panel represent represent melt melt layers, layers, features features >10 cm>10 thickness cm thickness highlighted highlighted in dark in dark grey; grey; 2011 2011 andand 1997 1997 (arrows) (arrows) included included to highlight to highlight thethe location location of theofthe two two warmest warmest summers summers during during this this period period derived derived from from ERA5. ERA5.

2− + − 3.3.4.3.3.4. Dating Dating Approach Approach D D—Based—Based on on nssSO nssSO442−:ssNa:ssNa+ RatioRatio and and MSA MSA− ConcentrationConcentration 2− + AA fourth approachapproach was conductedconducted usingusing thethe nssSO nssSO442−:ssNa:ssNa+ ratio,ratio, which which was was ana- ana- 2− + lylyzedzed to identify troughs asas winterwinter markers.markers. PotentialPotential winterwinter troughs troughs in in the the nssSO nssSO4 42−:ssNa+ − profileprofile were corroborated using the MSAMSA− minima,minima, commonly commonly used used as as a a winter marker (Figure(Figure 10).10). A A total total of of 23 23 ± ±1 winter1 winter troughs troughs were were identified, identified, resulting resulting in an in age an agescale scale ex- 2− + tendingextending from from austral austral summer summer 2017 2017 to 1993 to 1993 CE. CE.Despite Despite nssSO nssSO42−:ssNa4 +:ssNa ratio beingratio a being useful a 2− tooluseful to toolenhance to enhance peaks and peaks troughs and troughsin the nssSO in the42− nssSOsignal,4 thissignal, parameter this parameteris highly dependent is highly + 2− 2+ ondependent the magnitude on the and magnitude seasonality and of seasonalityNa+, SO42−, and of NaCa2+,. This SO4 makes, and the Ca nssSO. This42−:ssNa makes+ ratio the 2− + nssSO4 :ssNa ratio vulnerable to interannual and/or seasonal variability in the input 2− + of these ions to the ice core site. Therefore, the nssSO4 :ssNa ratio should be used in addition to other dating approaches.

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Geosciences 2021, 11, 368 12 of 18 vulnerable to interannual and/or seasonal variability in the input of these ions to the ice core site. Therefore, the nssSO42−:ssNa+ ratio should be used in addition to other dating approaches.

Figure 10. Dating approach D based on the discrete 5-cm nssSO42−:ssNa+ ratio 2(dark− blue)+ and MSA− (light blue). Purple Figure 10. Dating approach D based on the discrete 5-cm nssSO4 :ssNa ratio (dark blue) and vertical lines indicate ice core horizons that were identified as winters, grey vertical lines show the respective summertime MSA− (light blue). Purple vertical lines indicate ice core horizons that were identified as winters, peaks in between. The dashed vertical line at ~9 m w.eq. depth indicates an uncertain year, which was not considered in the end. grey vertical lines show the respective summertime peaks in between. The dashed vertical line at ~9 m w.eq. depth indicates an uncertain year, which was not considered in the end. 3.4. Summary of Age Scales 3.4. Summary of Age Scales Though each of the dating approaches were based on cyclic variability, they were not Though each of the dating approaches were based on cyclic variability, they were not unequivocal (Table 1, Figure 11) and showed an eight-year spread of bottom ages. Further, unequivocal (Table1, Figure 11) and showed an eight-year spread of bottom ages. Further, the results differed significantly from the preliminary bottom age of 2002 ± 4 based on the results differed significantly from the preliminary bottom age of 2002 ± 4 based on density only [2]. Consequently, both the combination of multiple proxies and a tie point density only [2]. Consequently, both the combination of multiple proxies and a tie point are necessary to reliably date this melt-affected archive. are necessary to reliably date this melt-affected archive. Table 1. Overview of the dating approaches applied in the Young Island ice core, details given in the text (Chapter 3.3.– 3.4.). Table 1. Overview of the dating approaches applied in the Young Island ice core, details given in the text (Sections 3.3 and 3.4). Dating Approach Based on Proposed Bottom Age Uncertainty Proposed Bottom Dating ApproachHerron-Langway densification Based on model driven by the Uncertainty Thomas et al. (2021) Age 2002 ±4 annual average precipitation from ERA5 A Herron-Langwayd18O troughs (winter) 1989 ±6 densification model B NO3− peaks (summer) 1988 ±6 Thomas et al. (2021) driven by the annual 2002 ±4 Marine species MSA−, SO42−, NO3−, and Br− peaks plus C average precipitation 1995/94 ±2 melt layersfrom ERA5>10 cm (summer) 2− + − D A nssSO4 :ssNad18O ratio troughs and (winter) MSA troughs (winter)1989 1993± 6 ±1 NO − peaks B 3 1988 ±6 (summer) Marine species − 2− − MSA , SO4 , NO3 , C and Br− peaks plus 1995/94 ±2 melt layers >10 cm (summer) 2− + nssSO4 :ssNa ratio D and MSA− troughs 1993 ±1 (winter)

Figure 11. Comparison of the four age scales based on δ18O (A), NO3− record (B), marine ion concentrations (C), and nssSO42−:ssNa+ ratio (D); integer years refer to the respective January, winter-based dating approaches point to July and are denoted with a ‘.5’ suffix; 2010 and 2000 highlighted in turquoise as reference years; blue stars point out uncertain years; grey bars indicate horizons of (a) elevated SO42−:NO3− ratio, and (b) cryptotephra discussed in the next Section 3.5.

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vulnerable to interannual and/or seasonal variability in the input of these ions to the ice core site. Therefore, the nssSO42−:ssNa+ ratio should be used in addition to other dating approaches.

Figure 10. Dating approach D based on the discrete 5-cm nssSO42−:ssNa+ ratio (dark blue) and MSA− (light blue). Purple vertical lines indicate ice core horizons that were identified as winters, grey vertical lines show the respective summertime peaks in between. The dashed vertical line at ~9 m w.eq. depth indicates an uncertain year, which was not considered in the end.

3.4. Summary of Age Scales Though each of the dating approaches were based on cyclic variability, they were not unequivocal (Table 1, Figure 11) and showed an eight-year spread of bottom ages. Further, the results differed significantly from the preliminary bottom age of 2002 ± 4 based on density only [2]. Consequently, both the combination of multiple proxies and a tie point are necessary to reliably date this melt-affected archive.

Table 1. Overview of the dating approaches applied in the Young Island ice core, details given in the text (Chapter 3.3.– 3.4.).

Dating Approach Based on Proposed Bottom Age Uncertainty Herron-Langway densification model driven by the Thomas et al. (2021) 2002 ±4 annual average precipitation from ERA5 A d18O troughs (winter) 1989 ±6 B NO3− peaks (summer) 1988 ±6 Marine species MSA−, SO42−, NO3−, and Br− peaks plus Geosciences 2021, 11, 368 C 1995/94 13 of 18 ±2 melt layers >10 cm (summer) D nssSO42−:ssNa+ ratio and MSA− troughs (winter) 1993 ±1

Figure 11. Comparison of the four age scales based on δ18O (A), NO3− 18record (B), marine− ion concentrations (C), and Figure 11. Comparison of the four age scales based on δ O(A), NO3 record (B), marine ion nssSO42−:ssNa+ ratio (D); integer years refer to the respective January, winter-based dating approaches point to July and concentrations (C), and nssSO 2−:ssNa+ ratio (D); integer years refer to the respective January, are denoted with a ‘.5’ suffix; 2010 and 20004 highlighted in turquoise as reference years; blue stars point out uncertain winter-based dating approaches point to July and are denoted with a ‘.5’ suffix; 2010 and 2000 years; grey bars indicate horizons of (a) elevated SO42−:NO3− ratio, and (b) cryptotephra discussed in the next Section 3.5. highlighted in turquoise as reference years; blue stars point out uncertain years; grey bars indicate 2− − horizons of (a) elevated SO4 :NO3 ratio, and (b) cryptotephra discussed in the next Section 3.5. 3.5. Tie Point of the Age Scale Ice core age scales, in addition to annual layer counting, can be constrained by refer- ence horizons such as volcanic acid or tephra layers. In order to constrain the age scale of the Young Island ice core, the dust record was taken into account. Elevated levels of dust input between 7.4 m and 8.1 m w.eq. depth raised the idea of a short-term event increasing dust concentration (Figure 12a). As the particle size distribution (Figure6) exhibited an increase both in total particle concentration and mode particle diameter for this depth interval, an additional, proximal source supplying larger particles to the ice core site should be considered. A large eruption from Buckle Island in 1839 CE increased the mode particle size to ~10 µm in the WAIS Divide core [61]. In the period covered by the Young Island ice core, Tetzner et al. [44] recently presented that a volcanic eruption on nearby Sturge Island occurred in 2001 CE based on increased dust input and tephra shards found in ice cores from the Antarctic Peninsula and West Antarctica. Thus, we filtered the ice core to collect insoluble particles for tephra analysis. We observed large (≤200 µm) and angular cryptotephra shards, with little evidence of corrosion, pointing to a proximal eruption on Sturge Island at ~140 km distance. Among the cryptotephra shards found, many exhibited angular morphologies and concave features (vesicles, Figure 12b–e). Additionally, most of the cryptotephra shards identified presented quenching cracks, indicative of the interaction with water during the eruption [62]. Though geochemical analysis of the tephra could contribute to a clearer allocation to the mentioned Sturge eruption in 2001, the combination of strong dust and tephra input are compelling arguments for this absolute time marker in our age scale. This tie point supports the dating approaches C and D, based on marine ion 2− + species and nssSO4 :ssNa ratio respectively, back to 2001. We propose on this basis that the Young Island ice core contains annual layers from 2016 to 1995 ± 2 years. This is of critical importance for all further climate interpretation in future studies. GeosciencesGeosciences2021 2021, 11, 11, 368, x FOR PEER REVIEW 14 of14 19 of 18

FigureFigure 12. IdentificationIdentification of ofan anage age tie point; tie point; (a) full (a )dust full (>1 dust μm) (>1 concentrationµm) concentration profile derived profile from derived fromthe Young the Young Island Island ice core ice using core CFA using (light CFA grey) (light and grey) the andderived the derived100-point 100-point average (black); average (b (black);–e) (micrographsb–e) micrographs of cryptotephra of cryptotephra shards shardsidentified identified in the ice in sampled the ice sampledbetween 7.7 between and 7.95 7.7 m and w.eq. 7.95 m w.eq.depth, depth, scale bar scale in barbottom in bottom right of right each offrame each represents frame represents 50 μm; ( 50f) detailedµm; (f) detailedview of total view dust of total par- dust particleticle concentration concentration using using the c theolour colour code codeof panel of panel (a) and (a )coarse and coarse dust particle dust particle size fraction size fraction (blue) at (blue) 6.8–8.2 m w.eq. depth. at 6.8–8.2 m w.eq. depth. 4. Discussion 4. Discussion Up to this point, we have presented various approaches for dating the ice core and Up to this point, we have presented various approaches for dating the ice core and identified the 2001 CE Sturge eruption as an absolute time marker. Now, we discuss identified the 2001 CE Sturge eruption as an absolute time marker. Now, we discuss briefly whether our derived age scale with annual layers back until 1995 is probable in the brieflylight of whether ERA5 and our NCEP/NCAR derived age reanalyses, scale with i.e. annual, their layers 2-m air back temperature until 1995 and is probable annual in theprecipitation light of ERA5 estimates. and NCEP/NCAR reanalyses, i.e., their 2-m air temperature and annual precipitationThe top 3 estimates. years with the highest melt content in our ice core were in declining order 2011 The(69%), top 1997 3 years (65%) with, and the 2002 highest (52%). melt These content coincide ind our with ice the core top were two inwarmest declining sum- order 2011mers (69%),(December−February) 1997 (65%), and in ERA5 2002 (52%).[45], i.e., These 1997 and coincided 2011, and with the thetop topthree two warmest warmest summerssummers derived (December from− February)NCEP/NCAR in ERA5[63], i.e. [45, 2011,], i.e., 2002, 1997 and and 1997. 2011, This and overlap the top sup- three warmestported our summers interpretation derived of frommelt layers NCEP/NCAR as predominantly [63], i.e., 2011,summer 2002, features and 1997. and our This sum- overlap supportedmer-based dating our interpretation approaches (e.g. of melt, Dating layers approach as predominantly C, Figure 9). It summer further suggest featuresed andthat our summer-basedboth reanalyses dating can depict approaches important (e.g., aspects Dating of approach the local C, microclimate. Figure9). It further Based suggested on the thatstrong both correlations reanalyses between can depict melt important layer >10 cm aspects and summer of the local temperatures, microclimate. these Based features on the strong correlations between melt layer >10 cm and summer temperatures, these features might even be a valuable hot summer proxy at Young Island, similar to previous studies in the Antarctic [60,64].

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Geosciences 2021, 11, 368 might even be a valuable hot summer proxy at Young Island, similar to previous studies15 of 18 in the Antarctic [60,64]. In a next step, we conducted an interannual comparison of annual layer thickness in the Young Island ice core converted to accumulation (m w.eq.) with annual precipitation In a next step, we conducted an interannual comparison of annual layer thickness in derived from ERA5 and NCEP. It showed that our final dating approach produced accu- the Young Island ice core converted to accumulation (m w.eq.) with annual precipitation mulation rates in the expected range, especially of the NCEP reanalyses (Figure 13), which derived from ERA5 and NCEP. It showed that our final dating approach produced accumu- was important to show its plausibility. It must be noted that the annual layer thickness, lation rates in the expected range, especially of the NCEP reanalyses (Figure 13), which was though ideally representative of the local accumulation, was probably affected by various important to show its plausibility. It must be noted that the annual layer thickness, though factors including glacier flow, topography, wind-scouring, and melting that have not been ideally representative of the local accumulation, was probably affected by various factors quantified and therefore cannot be corrected for this site yet. Further, significant uncer- including glacier flow, topography, wind-scouring, and melting that have not been quanti- fiedtainty and exists therefore in the cannot ability be of corrected the reanalyses for this to site reproduce yet. Further, precipitation significant at uncertainty the island’s exists scale inproperly the ability [65] of, e.g. the, reanalysesthe large grid to reproduce cell overlooks precipitation localized at orographic the island’s effects scale properly on precipita- [65], e.g.,tion. the Resolving large grid the cell constraints overlooks for localized local net orographic accumulation effects requires on precipitation. further investigation Resolving thebeyond constraints the scope for of local this net paper. accumulation For now, requiresannual layer further thickness investigation in the ice beyond core lay the perma- scope ofnently this paper.below Forthe precipitation now, annual layerestimates thickness given inby the ERA5 ice corebut partly lay permanently resembled NCEP below var- the precipitationiability, especially estimates in the given years by 2006−2012 ERA5 but (Figure partly resembled13). We conclude NCEP variability,that ice core especially net accu- inmulation the years lies 2006 in −the2012 lower (Figure yet re 13asonable). We conclude range of that NCEP ice coreestimates net accumulation and that covariance lies in the of lowerlocal precipitation yet reasonable and range accumulation of NCEP estimates should be and investigated that covariance using of a local higher precipitation-resolution anddensity accumulation record. should be investigated using a higher-resolution density record.

FigureFigure 13.13. Interannual comparison comparison of of precipitation precipitation derived derived from from ERA5 ERA5 and and NCEP NCEP reanalyses reanalyses to toice icecore core net net accumulation accumulation derived derived from from annual annual layer layer thickness thickness (based (based on onmarine marine species species dating dating ap- approachproach C). C).

5.5. ConclusionsConclusions Here,Here, wewe have have presented presented the the first first glacio-chemical glacio-chemical records records of the of Youngthe Young Island Island ice core. ice Despitecore. Despite the presence the presence of surface of surface melt at melt this at maritime this maritime location, location, our findings our findings indicate indicate that it isthat possible it is possible to date theto date melt-affected the melt-affected Young Island Young ice Island core. ice We core. have We applied have multiple-proxy applied multi- datingple-proxy approaches dating approaches based on winterbased on and winter summer and signaturessummer signatures and developed and developed an ice core an chronologyice core chronology including including a proximal a proxima volcanicl volcanic tie point tie in point 2001 in CE 2001 [44 ].CE The [44] final. The age final scale age suggestsscale suggests that the that Young the Young Island Island ice core ice covers core covers the period the period from 2016 from to 2016 1995 to± 19952 years. ± 2 years. The ageThe scale age scale gains gains independent independent support support from from reanalysis reanalysis records records and and a strong a strong correlation correlation of meltof melt layers layers >10 >10 cm cm and and warm warm summer summer temperatures. temperatures. By By developing developing a a reliable reliable ice ice corecore chronology,chronology, thisthis studystudy providesprovides thethe crucialcrucial foundationfoundation forfor allall futurefuture studies studies discussing discussing regionalregional climate climate proxies proxies from from the the Young Young Island Island ice ice core. core. Going Going forward, forward, resolving resolving the effectsthe ef- offects surface of surfac melte onmelt chemical on chemical records records and how and these how featuresthese features could could be proxies be proxies themselves them- willselves be will one ofbe theone key of the challenges key challenges at this interestingat this interesting site. site.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/geosciences11090368/s1, Figure S1: Landsat-8 RGB image acquired on (a) 23/10/2019 shows open water prevailing west of the Balleny Islands, and (b) 09/12/2013 open water east of the Geosciences 2021, 11, 368 16 of 18

islands; red marker: Young Island drilling site; blue background: ocean; data available from the U.S. Geological Survey. Author Contributions: When multiple authors have contributed, they are mentioned in alphabet- ical order. Conceptualization, D.E.M. and E.R.T.; data curation—field work, B.M. and J.B.P.; data curation—continuous flow analysis, H.A.K., D.E.M., E.N., E.R.T., P.V.; data curation—discrete data, S.J.; data curation—tephra collection, D.T.; formal analysis, H.A.K., D.E.M., E.N., D.S., A.S., D.T., E.R.T.; writing—original draft preparation, D.E.M.; writing—review and editing, H.A.K., B.M., D.E.M., J.B.P., D.S., D.T., E.R.T.; funding acquisition, J.B.P. and E.R.T. All authors have read and agreed to the published version of the manuscript. Funding: Funding was provided to subICE by École Polytechnique Fédérale de Lausanne, the Swiss Polar Institute, and Ferring Pharmaceuticals Inc (grant no. subICE). ERT received core funding from NERC to the British Antarctic Survey’s Ice Dynamics and Palaeoclimate programme. DEM was supported by BAS, Cambridge, and the NERC C-CLEAR doctoral training programme (grant no. NE/S007164/1). JBP received grant funding from the Australian Government. Data Availability Statement: The data presented in this study will be made openly available through the UK Polar Data Centre. Furthermore, publicly available AWS and reanalysis datasets were analysed in this study. AWS data can be found here: https://amrc.ssec.wisc.edu/aws/index.php? region=Ocean%20Islands&station=Young%20Island&year=1993 (accessed on 15 May 2021). Acknowledgments: The authors would like to acknowledge the coordinators and participants of the Antarctic Circumnavigation Expedition for facilitating collection of the subICE cores, especially Guisella Gacitúa and Julia Schmale. The authors appreciate the support of the University of Wisconsin- Madison Automatic Weather Station program for the data set, data display, and information as well as the NSF (grant no. ANT-1543305) and ECMWF for providing ERA5 reanalysis data. Finally, the authors want to thank the two anonymous reviewers for their critical comments on the manuscript, as they significantly improved the clarity of this paper. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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