Amann Benjamin
email: benjamin.amann@queensu.ca

Scott F Lamoureux

Maxime P Boreux

Winter temperature conditions (1670 -2010) reconstructed from varved sediments, western Canadian High Arctic

Keywords:

holds great potential for more applied climate research such as data-model comparisons 51 and proxy-data assimilation in climate model simulations.

Abstract

Advances in paleoclimatology from the Arctic have provided insights into long-term climate conditions. However, while past annual and summer temperature have received considerable research attention, comparatively little is known about winter paleoclimate. Arctic winter is of special interest as it is the season with the highest sensitivity to climate change, and because it differs substantially from summer and annual measures. Therefore, information about past changes in winter climate is key to improve our knowledge of past forced climate variability and to reduce uncertainty in climate projections. In this context, Arctic lakes with snowmelt-fed catchments are excellent potential winter climate archives. They respond strongly to snowmelt-induced runoff, and indirectly to winter temperature and snowfall conditions. To date, only a few well-calibrated lake sediment records exist, which appear to reflect site-specific responses with differing reconstructions. This limits the possibility to resolve large-scale winter climate change prior the instrumental period.

Here, we present a well-calibrated quantitative temperature and snowfall record for the extended winter season (November through March; NDJFM) from Chevalier Bay (Melville Island, NWT, Canadian Arctic) back to CE 1670. The coastal embayment has a large catchment influenced by nival terrestrial processes, which leads to high sedimentation rates and annual sedimentary structures (varves). Using detailed microstratigraphic analysis from two sediment cores and supported by µ-XRF data, we separated the nival sedimentary units (spring snowmelt) from the rainfall units (summer) and identified subaqueous slumps. Statistical correlation analysis between the proxy data and monthly climate variables reveals that the thickness of the nival units can be used to predict winter temperature (r= 0.71, pc< 0.01, 5-yr filter) and snowfall (r= 0.65, pc< 0.01, 5-yr filter) for the western Canadian High Arctic over the last ca. 400 years. Results reveal a strong variability in winter temperature back to CE 1670 with the coldest decades reconstructed for the period CE 1800-1880, while the warmest decades and major trends reconstructed for the period CE 1880-1930 (0.26°C/decade) and CE 1970-2010 (0.37°C/decade). Although the first aim of this study was to increase the paleoclimate data coverage for the winter season, the record from Chevalier Bay also

Introduction

The Arctic is recognized as one of the most sensitive regions to climate change (ACIA 2004; [START_REF] Collins | Long-term Climate Change: Projections, Commitments and Irreversibility[END_REF]. Amplified warming of the Arctic climate has long been demonstrated from both observations and simulations [START_REF] Hardy | The climatic signal in varved sediments from Lake C2, northern Ellesmere Island, Canada[END_REF][START_REF] Moritz | Dynamics of recent climate change in the Arctic[END_REF][START_REF] Miller | Arctic Amplification: can the past constrain the future?[END_REF][START_REF] Comiso | Climate trends in the Arctic as observed from space[END_REF]. This warming has led to increases in high-latitude precipitation that are particularly notable during the winter season [START_REF] Kattsov | Twentieth-century trends of Arctic precipitation from observational data and a climate model simulation[END_REF][START_REF] Bintanja | Future increases in Arctic precipitation linked to local evaporation and sea-ice retreat[END_REF]. In this context, Arctic lakes and similar sedimentary settings are excellent potential climate archives owing to their capacity to preserve climate variability through very long times and at a high temporal resolution [START_REF] Hughen | The potential of palaeoclimate records from varved Arctic lake sediments: Baffin Island, Eastern Canadian Arctic[END_REF]. Moreover, these Arctic environments provide very specific and advantageous settings such as a strong sensitivity to extreme seasonal variations in processes (stream flow and sediment delivery), and limited direct human impact [START_REF] Smol | From controversy to consensus: making the case for recent climate change in the Arctic using lake sediments[END_REF][START_REF] Lamoureux | A 750-yr record of autumn snowfall and temperature variability and winter storminess recorded in the varved sediments of Bear Lake, Devon Island, Arctic Canada[END_REF]. In particular, Arctic lakes with nival catchments were shown to respond directly to the length and the intensity of the snowmelt, and indirectly to winter temperature and snowfall conditions on the previous year [START_REF] Forbes | Climatic Controls on Streamflow and Suspended Sediment Transport in Three Large Middle Arctic Catchments, Boothia Peninsula, Nunavut, Canada[END_REF][START_REF] Cockburn | Hydroclimate controls over seasonal sediment yield in two adjacent High Arctic watersheds[END_REF].

Nevertheless, reconstructing past climate from Arctic lakes remains an analytical challenge. For instance, low 210 Pb inventories and the paucity of degradable-terrestrial remains limit 210 Pb and 14 C dating, respectively [START_REF] Wolfe | Geochronology of high latitude lake sediments[END_REF]). Moreover, low sedimentation accumulation rates in some Arctic lakes may restrict the temporal resolution and the length of the paleoclimate records. This is the reason why annually laminated sediments (varves) have received a growing scientific attention over the past two decades. Varved sediments are recognised as a useful tool for Arctic paleoclimatology [START_REF] Hughen | The potential of palaeoclimate records from varved Arctic lake sediments: Baffin Island, Eastern Canadian Arctic[END_REF] [START_REF] Francus | Paleoclimate studies of minerogenic sediments using annually resolved textural parameters[END_REF], and September-to-May temperature and May snow depth [START_REF] Cuven | Mid to Late Holocene hydroclimatic and geochemical records from the varved sediments of East Lake, Cape Bounty, Canadian High Arctic[END_REF]). These differences in the records clearly limit the possibility to resolve large-scale and regional patterns of winter climate change prior the instrumental period.

Here, we present a well-calibrated quantitative temperature record for the extended winter season (November through March; NDJFM), which is representative of a large part of the Canadian High Arctic. Using detailed microstratigraphical analysis supported by an annual chronology, we demonstrate that the relatively thick varves from Chevalier Bay (Melville Island, NWT) contain a winter temperature signal, which can be reconstructed for the past ca. 400 years. With this paleoclimate record, we demonstrate that the indirect information stored in the winter temperature and snowfall conditions holds great potential for more advanced application in climate research.

Study site

Chevalier Bay (75°03'10"N, 111°30'38"W; unofficial name) is a coastal marine setting located on Melville Island, Dundas Peninsula, Northwest Territories in the Canadian High Arctic. The bay is located about 50 km west from the Cape Bounty Arctic Watershed Observatory (CBAWO; Fig. 1a); a research site where hydrological, sediment transport and lake deposition studies have been undertaken since 2003. Chevalier Bay is a small marine embayment that is largely isolated from the ocean and marked by a sedimentary environment similar to a coastal lacustrine basin. The catchment is strongly influenced by terrestrial processes and can be regarded as a nival catchment [START_REF] Cockburn | Hydroclimate controls over seasonal sediment yield in two adjacent High Arctic watersheds[END_REF], enabling the sediment record from Chevalier Bay to be compared with other Arctic nival-lake sediment records. Of particular interest, its catchment has a remarkably large area (350 km²); 152 times larger than the area of the bay (Fig. 1b). This catchment feature leads to high sedimentation rates (mean 2.53 mm yr -1 ), atypical of other studied Arctic sites (common annual rates range from 0.2 to 1 mm; e.g. [START_REF] Tomkins | Autumn snowfall and hydroclimatic variability during the past millennium inferred from the varved sediments of meromictic Lake A, northern Ellesmere Island, Canada[END_REF]Francus et al. 2002, respectively).

The bedrock of the area consists of folded Devonian iron-rich weathered sandstone and siltstone [START_REF] Hodgson | Quaternary geology of central Melville Island, Northwest Territories[END_REF][START_REF] Christie | The geology of Melville Island, Arctic Canada[END_REF]. The landscape features a limited relief (max. 110 m a.s.l.) with the drainage system developing on sparse tundra vegetation and continuous permafrost (Fig. 1b).

The climate of the region is classified as polar desert (tundra climate ETf; Köppen-Geiger classification) characterized by cold winters, cool summers, and limited annual precipitation that falls primarily as snow during the autumn/winter season (Fig. 1c).

Mean summer (JJA) and extended winter (NDJFM) temperatures are 3.5°C and -27.8°C, respectively (CE 1949(CE -1997; Mould Bay meteorological station, 280km to the west; Figure 1). Annual precipitation is dominated by winter snowfall leading to a maximum mean snow accumulation of 300 mm depth in early May, while rare high-intensity rainfall events (> 10 mm/day) occur in summer (Fig. 1c). Total annual precipitation amounts to 98 mm.

Chevalier Bay is at sea level and connected to the Arctic Ocean by an estimated 7-m deep narrow inlet, enabling tidal interaction during the short ice-free period (July-August to early October). Properties of the 16-m deep water-column of Chevalier Bay were measured in June 2010 (Fig 2). It reveals two chemoclines at 3 and 7 m depth, with the hypolimnion characterised by quasi-anoxic conditions (<10 % saturation) and saline water of 35 PSU. In spring, snowmelt-induced freshwater enters the lake under the ice cover (~2-m thick), which is marked by low salinity (4 PSU), a peak in turbidity (58 NTU), warmer temperature (1°C), and high dissolved oxygen content (85% saturation).

Material and methods

The study of snowmelt-induced processes and the relationship between stream discharge and suspended sediment transport could not be conducted at Chevalier Bay.

However, such research was undertaken at nearby West and East Lakes in CBAWO (Fig. 1a), and will be used for comparison and correlation. This nearby site has similar hydroclimate, topography, surface materials, vegetation and geology [START_REF] Cockburn | Hydroclimate controls over seasonal sediment yield in two adjacent High Arctic watersheds[END_REF][START_REF] Lewis | Hydrochemical and sedimentary responses of paired High Arctic watersheds to unusual climate and permafrost disturbance, Cape Bounty, Melville Island, Canada[END_REF].

Sediment coring and processing

Two sediment cores were retrieved from Chevalier Bay in June 2010 with a Universal gravity-percussion corer (Aquatic Research Instruments); one from the middle (10CV03, 86cm long), and another from a more proximal location (10CV04, 46cm long) (Fig. 1b).

The comparison between these two cores was used to: (i) identify anomalous deposits; and (ii) assess whether varve thickness could be consistently used as a proxy through non-localized differences in sedimentation rates (Fig. S1; [START_REF] Tomkins | Autumn snowfall and hydroclimatic variability during the past millennium inferred from the varved sediments of meromictic Lake A, northern Ellesmere Island, Canada[END_REF]). Cores were transported and stored unfrozen to the laboratory. Only data from core 10CV03 are shown here, which offer the longest record.

In the laboratory, the cores were split lengthwise and photographed prior to analysis.

Non-destructive X-Ray Fluorescence (XRF) scans were obtained on the first core half (60kV, 160 mA, and 80 ms exposure time) at INRS Québec. XRF elements were selected following [START_REF] Croudace | ITRAX: description and evaluation of a new sediment core scanner[END_REF] and [START_REF] Kylander | High-resolution X-ray fluorescence core scanning analysis of Les Echets (France) sedimentary sequence: new insights from chemical proxies[END_REF], with a special focus on the ratios Zr/K and Fe/Rb . These two XRF ratios were successfully applied to estimate grain size variability in sediment cores from East Lake (Cape Bounty, Melville Island, 50 km east); a site with bedrock and sedimentary setting comparable to Chevalier Bay [START_REF] Cuven | Estimation of grain size variability with micro X-ray fluorescence in laminated lacustrine sediments, Cape Bounty, Canadian High Arctic[END_REF][START_REF] Cuven | Mid to Late Holocene hydroclimatic and geochemical records from the varved sediments of East Lake, Cape Bounty, Canadian High Arctic[END_REF][START_REF] Lapointe | 1750 years of large rainfall events inferred from particle size at East Lake, Cape Bounty, Melville Island, Canada[END_REF]).

Thin sections: microstratigraphy and sediment chronology

Micro-stratigraphical analysis of the varves and the sediment chronology were generated from merged optical microscopy thin section images (resolution= 0.02 mm; magnification x10). Thin sections were prepared from overlapping sediment slabs (7 x 2 x 1 cm) and dehydrated with acetone prior to embedding with Spurr's epoxy resin [START_REF] Lamoureux | Embedding unfrozen lake sediments for thin section preparation[END_REF]. Two analysts (BA and MB) independently counted the laminae seven months apart and manually measured the thickness of each specific sedimentary unit (nival, rainfall, and clay cap). The graphic software CorelDraw (0.05-mm resolution) was used to delineate sedimentary units from scanned images. Counting uncertainty was derived from the standard deviation of the independent chronologies.

To validate the varve hypothesis (annual character of the layers), 33 samples were extracted from core 10CV03 with depth intervals of 0.5 to 1.0 cm for radioisotope dating [START_REF] Wolf | Paleolimnology of the middle and High Canadian Arctic[END_REF]). Samples were analysed for 210 Pb and 137 Cs using the gamma-ray spectrometry facilities (EGG-Ortec type spectrometer, well germanium detector, capacity 10 cm 3 ) at the PEARL Laboratory (Queen's University, Kingston, ON, Canada). To test the sensitivity of Chevalier Bay to climate variability, varve thickness for each seasonal phase was correlated with individual monthly data and 2-to-12-month windows for temperature (mean) and precipitation (cumulative rainfall and snowfall) from all meteorological stations and the gridded dataset (multiple testing; de Jong and Kamenik, 2011). Both the proxy and the climate data were presented with a 5-year triangular filter prior to calibration, to account for: (i) dating uncertainty from the different varve counts; and (ii) error introduced by sediment sampling. Accordingly, the probability values (p) were corrected for serial autocorrelation (pc). The best correlation from this multiple testing was then used to develop a proxy-climate calibration model, using an Ordinary Least-Squares (OLS) regression model (inverse regression), following the calibration-in-time approach (CIT) described by von Gunten et al. (2012). To validate the model, the root mean squared error of prediction (RMSEP) inferred from the bootstrap cross-validation method was preferred to the split period approach, due to a change in the variance post CE 1900 [START_REF] Birks | Overview of numerical methods in palaeolimnology[END_REF][START_REF] Amann | A millennial-long record of warm season precipitation and flood frequency for the North-western Alps inferred from varved lake sediments: implications for the future[END_REF]. Finally, this model was used to reconstruct climate variability for the length of the chronology.

Climate data and proxy-climate calibration

Results

Sediment properties and varve characterization

Sediments recovered from both locations of Chevalier Bay consist mainly of light-to dark-grey (Munsell colour: 2.5Y-6/1 to 2.5Y-4/1, respectively) laminated clastic material (Fig. 3a). Two primary sedimentary units occur throughout the sediment sequence with: (i) sedimentary couplets interpreted as clastic varves (mean thickness = 2.53 mm); and (ii) thick and coarser-grain deposits interpreted as turbidites. Turbidite layers are characterized by much higher sedimentation rates in the proximal core (10CV04, >0.9 cm) compared to the more central site (10CV03, >0.6 cm; Fig. 1b). On the contrary, laminae interpreted as varves have comparable sedimentation rates at the two locations (core-to-core layer-thickness comparison; R 2 = 0.879 comprising all layers compared with R 2 = 0.928 without turbidites). Fig. 3b shows a typical varve structure that comprises: (i) a light-grey coarse-silt spring deposit characterized by rather high Zr/K values (spring snowmelt = nival unit); (ii) a dark-grey/black coarse-silt to fine-sand deposit characterized by high Zr/K values (summer rainfall = rainfall unit); and (iii) an oxidized clay cap marked by the highest Fe/Rb and the lowest Zr/K values. Six varve facies types were derived from detailed stratigraphic analysis of the thin sections (Fig. 4).

• Facies type 1: simple couplets compose the varve, with a snowmelt-induced deposit (hereafter defined as nival unit) and a winter oxidized clay cap.

• Facies type 2: a rainfall unit characterised by coarser particles and organic remains is found on top of the nival unit.

• Facies type 3: the rainfall unit is intercalated between two nival units, most likely due to early-season rainfall events.

• Facies type 4: anomalously thick nival units (up to 8 mm. yr -1 ) characterize this facies type, which is found at a core depth of 20-30 cm mainly (2.9% occurrence).

• Facies type 5: non-oxidized clay deposits are intercalated between nival units, most likely due to extended periods of calm conditions within the snowmelt season.

• Facies type 6: coarse-grained deposits are randomly intercalated within the varves, interpreted as slumps triggered by internal basin processes.

Although the nival unit and the clay cap are found each year (100% occurrence), the rainfall unit occurring in Facies type 2 and type 3 is only found in 141 of the 343 layers (41.1% occurrence).

Age-depth modelling

The chronology built from the different lamina counts is consistent with the 137 Cs dating technique, which supports the varve hypothesis (Fig. 5). The 137 Cs profile shows a peak at a 14-cm depth (2.74 ± 0.23 dpm g -1 ) and drops to 0 level values below 17 cm. Estimated varve ages for these two depths are 1961 ± 2 yrs (bomb testing peak) and 1952 ± 3 yrs (onset of 137 Cs deposition), respectively.

The counting uncertainty shows a maximum of 1.5 % within the instrumental period (CE 1948 -2010), but reaches 6 % at the lowermost section of the sediment core, as reported on the lower panel of Fig. 5. The greatest increases in the counting error occur during the periods CE 1850 -1825 (1 to 2.7 % counting error, respectively), and CE 1750 -1725 (2.7 to 5.2 %, respectively), indicative of sediment sections with varves that are less well preserved.

Proxy-climate calibration and validation

On the basis of this sedimentological analysis together with knowledge about the regional hydroclimate, we hypothesize that the thickness of the snowmelt-induced deposit from Chevalier Bay sediments is primarily controlled by the snowmelt conditions (intensity and duration), and by previous winter conditions indirectly (snowfall, snow depth, and temperature).

Statistical analysis between sediment proxy and climate data reveals that the thickness of the nival units contains a winter temperature signal, with the best correlation found for the extended winter season; November through March (NDJFM). This result is consistent among all meteorological stations and gridded data with: r= 0.71 (pc< 0.01, n= 59, 5-yr filtered) for CRU TS, r= 0.66 (pc< 0.01, n= 46, 5-yr filtered) for Mould Bay, and r= 0.65 (pc< 0.01, n= 59, 5-yr filtered) for Resolute Bay (Fig. 6, Table 1). Significant negative correlation is also consistently found between the thickness of the nival units and August-through-October (ASO) temperature. The PCA biplot illustrates theses correlation results through the position of the ASO vector aligned with the varve proxy (nival unit), but in the opposite direction (Fig. 6). On the contrary, no significant correlation can be found for summer (JJA) and autumn (SON) temperatures that are associated with eigenvectors positioned orthogonally to the varve proxy. Spring temperature data (MAM) forms an independent cluster, and is not significantly correlated with the thickness of the nival units (Table 1). Statistical tests also revealed positive correlations with winter NDJFM precipitation and snowfall, which are significant for Resolute and CRU TS but not for Mould Bay (Table 1).

Based on these results from multiple testing, a calibration-in-time model was developed between the thickness of the nival unit and NDJFM temperature from CRU TS (Fig. 7a).

The performance of this model is given by the following parameters: r= 0.71, n= 59, pc< 0.01, RMSEP= 0.65°C that corresponds to 19.9% of the peak to peak amplitude and 2.0% of the mean. Although Mould Bay best represents the climate conditions on Melville Island (daily data; Mould Bay vs. CBAWO, R 2 = 0.97, n= 2537, data not shown), the Mould Bay record limits the calibration period to CE 1948 -1997. Therefore, the gridded data from CRU TS 3.23 was preferred, extending the period to CE 2010. The Resolute Bay station records similar long-term temperature variability (Resolute vs. CRU TS; R 2 = 0.83), but shows much warmer conditions (Fig. 7).

Climate reconstruction

Based on the calibration-in-time model, the thickness of the nival units was used to reconstruct NDJFM temperature from Chevalier Bay back to CE 1670 (Fig. 8a). The The quantitative winter temperature reconstruction from Chevalier Bay is representative of a large part of the western Canadian High Arctic (Fig. 8). The correlation map for the NDJFM season reveals a signal extending from the south of Victoria Island to the northernmost point of Ellesmere Island, and eastward (although diminishing in strength) to north-western Greenland (Fig. 8b). Our reconstruction is also consistent with other temperature records from this part of the Arctic (Fig. 8a).

Significant correlation is found between the Chevalier Bay record and temperaturedriven changes in mass balance of South Melville and Devon Ice Caps (r= 0.37, p< 0.01, n= 385; [START_REF] Fisher | Effect of wind scouring on climatic records from ice-core oxygen-isotope profiles[END_REF][START_REF] Boon | Forty-seven Years of Research on the Devon Island Ice Cap, Arctic Canada[END_REF]). Additionally, multi-decadal changes of the NDJFM record (31-yr filtered) agree well with annual temperature anomaly for the entire Arctic region north of 60°N (Fig. 8a).

Discussion

Sedimentology and varve stratigraphy

The clastic varve interpretation was based on the strong seasonality of hydroclimate in the region. Hydrological runoff generally occurs over 3 to 4 months per year from June to August/September [START_REF] Cockburn | Hydroclimate controls over seasonal sediment yield in two adjacent High Arctic watersheds[END_REF], leading to sustained material delivery from surficial sediments and poorly consolidated bedrock [START_REF] Favaro | Antecedent controls on rainfall runoff response and sediment transport in a High Arctic Catchment[END_REF]. In winter, persistent ice cover preserves the primary structure of the sediments in this relatively deep basin and fluvial activity ceases.

The sediments of Chevalier Bay exhibit abnormally thick varves owing to its large catchment size (Fig. 1b). Indeed, the varves found in the bay (mean thickness of 2.53 mm) are thicker than what is commonly reported by studies on nival Arctic lakes.

Among others, mean annual thickness of ~0.2 mm was reported from Lake A (Tomkins et al. 2010), ~0.75 mm from Sanagak Lake (Lamoureux 2006), ~0.8 mm in East Lake [START_REF] Cuven | Mid to Late Holocene hydroclimatic and geochemical records from the varved sediments of East Lake, Cape Bounty, Canadian High Arctic[END_REF], and up to 1 mm in Sawtooth Lake [START_REF] Francus | Paleoclimate studies of minerogenic sediments using annually resolved textural parameters[END_REF]. High annual sedimentation rates from Chevalier Bay enabled us to identify and separate the different sedimentation processes through the entire sediment sequence.

Using detailed stratigraphical analysis (Fig. 3, Fig. 4), the nival units (spring snowmelt) could be clearly distinguished from the rainfall units (summer), from slumps and clay caps (winter). This classification was the cornerstone for the development of: (i) a reliable and precise chronology; and (ii) a good proxy-climate calibration model. In particular, different facies types of varves could be identified based on the absence (facies 1) and the timing of the rainfall unit (facies 2 and 3), the thickness of the nival unit (facies 4), and its internal structure (facies 5). In this exercise, Zr/K ratios from XRF scans were particularly supportive as a visual indicator of particle size, reproducing the Interestingly, the nival unit appeared as the primary sediment deposition unit compared to thinner (average 0.96 mm) and uncommon rainfall events (41 % of years). These small deposits were most likely buffered by the large catchment size and the low relief of the catchment, and/or dampened by antecedent conditions. This pattern is consistent with numerous studies from nival lakes in the Arctic, which showed that most of the total sediment transport occurs with the onset of snowmelt [START_REF] Hardy | The climatic signal in varved sediments from Lake C2, northern Ellesmere Island, Canada[END_REF]

2010).

Varves are known to integrate multiple and complex catchment and lake processes, which can mask the nival signal [START_REF] Hambley | Recent summer climate recorded in complex varved sediments, Nicolay Lake, Cornwall Island, Nunavut, Canada[END_REF]. In a similar study, [START_REF] Francus | Limnological and sedimentary processes at Sawtooth Lake, Canadian High Arctic, and their influence on varve formation[END_REF] combined detailed sedimentological analysis and limnohydroclimatical observations from Sawtooth Lake to disregard varve thickness as a valuable climate proxy on its own. Also, several studies from Arctic lakes have demonstrated the advantage of thicker varves in: (i) differentiating processes that lead to the formation of varves; and (ii) highlighting the prime role of nival runoff. This was the case for Nicolay Lake (Cornwall Island; Lamoureux 2000; [START_REF] Hambley | Recent summer climate recorded in complex varved sediments, Nicolay Lake, Cornwall Island, Nunavut, Canada[END_REF], Bear Lake (Devon Island; [START_REF] Lamoureux | Lacustrine sedimentary environments in High Arctic Proglacial Bear Lake, Devon Island, Nunavut, Canada[END_REF][START_REF] Lamoureux | A 750-yr record of autumn snowfall and temperature variability and winter storminess recorded in the varved sediments of Bear Lake, Devon Island, Arctic Canada[END_REF] and Lake R (Devon Island; Chutko and Lamoureux, 2008).

Varve chronology

Stratigraphical analysis of the thin sections supported by XRF data was used to develop a precise and reliable varve chronology from Chevalier Bay sediments, back to CE 1670 (Fig. 5). Especially, the peak of Fe/Rb in the oxidized clay cap was found to be a consistent marker for the termination of a varve year (Fig. 3, Fig. 4). Iron content associated with the salinity of the hypolimnion of Chevalier Bay (Fig. 2) played a major role in the counting of the varves. The metal, chemically weathered from the iron-rich catchment, is delivered into the Bay by spring (snowmelt) and summer (rainfall) runoff, mainly. Great proportions of highly reactive ferrous iron are present in this saline water, and are then likely to precipitate as ferric hydroxide and to fix to clays [START_REF] Coey | Iron compounds in lake sediments[END_REF]).

Finally, modest oxidizing conditions at the sediment-water interface, such as in Chevalier Bay, are sufficient for the formation of iron-rich clay sediments [START_REF] Haese | The reactivity of iron[END_REF].

Additionally, the water column properties from Chevalier Bay suggest that the sediments are distributed from the river as overflows across the lake, while underflows are likely discouraged by chemical stratification (Fig. 2).

The varve chronology was validated by independent 137 Cs chronomarkers, which captured the known Northern Hemisphere peak of nuclear activity in CE 1963 and the onset of cesium deposition in CE 1954 (Fig. 5). Although no geographical consistency could be derived, cesium counts from Chevalier Bay (peak value of 2.7 dpm g -1 ) were in the same order of magnitude than other Arctic lake studies. For instance, 0.7 dpm g -1 were measured at Upper Soper Lake (Baffin Island; Hughen et al. 2000), 7.5 dpm g -1 in Nicolay Lake (Cornwall Island; Lamoureux 1999), 9.6 dpm g -1 in East Lake (Melville Island; Cuven et al. 2011), and 9.7 dpm g -1 in Skardtjørna (Svalbard archipelago; Holmgren et al. 2009). In contrast, a consistent and interpretable 210 Pb profile could not be developed for our study due to very low lead fallout counts ( 210 Pb excess) in the sediment samples. In a review study, [START_REF] Wolfe | Geochronology of high latitude lake sediments[END_REF] described the influence of developing permafrost and the short ice-free season, which both restrain the transfer of atmospheric 210 Pb to the sediments of High Arctic lakes.

Winter climate signal: correlations and mechanism

The proxy-climate correlations revealed the thickness of the varve nival units as a proxy for winter climate conditions of the Canadian High Arctic (Fig. 6 and7; Table 1). The highest positive correlations were found with CRU TS for NDJFM temperature (r= 0.71, pc< 0.01) and NDJFM snowfall (r= 0.65, pc< 0.01), while significant negative correlations for ASO temperature on the previous year were found (r= -0.47, pc< 0.01). Covariance between these three variables suggests a strong mechanistic relation between winter temperature and snowfall, and subsequent sediment transport and deposition in the Bay.

Warm winters associated with increased winter snowfall and snow water equivalent (SWE) are likely to lead to greater snowmelt runoff in spring, and ultimately to higher spring sedimentation rates (i.e. thicker nival units). This control of winter conditions over sediment transport through SWE in Chevalier Bay is in close agreement with studies on sediment process understanding from nival watersheds at Cape Bounty and the Boothia Peninsula [START_REF] Cockburn | Hydroclimate controls over seasonal sediment yield in two adjacent High Arctic watersheds[END_REF][START_REF] Forbes | Climatic Controls on Streamflow and Suspended Sediment Transport in Three Large Middle Arctic Catchments, Boothia Peninsula, Nunavut, Canada[END_REF]respectively). Additionally, the negative correlations with autumn ASO temperature could be explained by greater rain/snow ratios during warmer autumn seasons, resulting in thinner nival units the following year (distinct from the rainfall units).

Moreover, the reliability of these results is strengthened by the consistency among the three climate datasets considered in this study (Mould Bay, Resolute Bay, and CRU TS; Table 1).

Although the sources of moisture during winter remains under debate, the link between temperature and precipitation is broadly recognized for the Arctic. The longest available record from Resolute Bay reports significant correlation between NDJFM temperature and NDJFM precipitation (r= 0.57, p< 0.01, n= 62, 5-yr filtered), comparable to the spatial correlations over the Canadian Arctic (Fig. S2). Also, [START_REF] Lafrenière | The impact of snow accumulation on the active layer thermal regime in High Arctic soils[END_REF] demonstrated the importance of daily incursion of warm air in winter (with up to 9ºC) to the onset of snow cover on Melville Island. Most of the observed 20 th -and modelled 21 st -century increase of Arctic precipitation has been attributed to the variations of the sea surface temperature, especially in late autumn and winter. Sea-ice boundary conditions are considered a major contributor to the total evaporative response in the Arctic, where the sensible-heat transfer between the warm ocean and the cold atmosphere develops even during cold winters [START_REF] Kattsov | Twentieth-century trends of Arctic precipitation from observational data and a climate model simulation[END_REF][START_REF] Hall | The role of surface albedo feedback in climate[END_REF]; [START_REF] Kattsov | Simulation and projection of Arctic freshwater budget components by the IPCC AR4 global climate models[END_REF]). This is consistent with global climate models that indicate local evaporation changes as the main source of moisture in the Arctic winter [START_REF] Bintanja | Future increases in Arctic precipitation linked to local evaporation and sea-ice retreat[END_REF]. Nevertheless, these results contrast with stable water isotopes used as moisture source tracers, which suggest a greater contribution of moisture transport from lower latitudes towards the end of the winter season [START_REF] Kurita | Origin of Arctic water vapor during the ice-growth season[END_REF]. This switch from local to more distant moisture origin in winter might in part explain why weaker correlations were found with winter precipitation than with winter temperature in Chevalier Bay. Another part of the explanation remains in the difficulty to measure winter precipitation accurately, especially with subsequent snow redistribution by the wind.

Although correlation tests support this mechanistic relation between temperature and precipitation (snowfall) for the winter season, hydroclimate data for the Arctic region should be considered with care. Indeed, precipitation is remarkably heterogeneous in the Arctic, and instrumental observations remain very sparse [START_REF] Kattsov | Simulation and projection of Arctic freshwater budget components by the IPCC AR4 global climate models[END_REF]). This heterogeneity was evidenced by the lower correlations for Mould Bay precipitation and snowfall data, as compared to Resolute and CRU TS (Table 1). Moreover, NDJFM temperature and NDJFM precipitation correlate significantly for Resolute (r= 0.57, p< 0.01, n= 62), while a weaker and non-significant correlation is recorded from Mould Bay (r= 0.22, p> 0.05, n= 40). Also, [START_REF] Cockburn | Hydroclimate controls over seasonal sediment yield in two adjacent High Arctic watersheds[END_REF] and [START_REF] Cuven | Mid to Late Holocene hydroclimatic and geochemical records from the varved sediments of East Lake, Cape Bounty, Canadian High Arctic[END_REF] identified this weak spatial consistency between Mould Bay and Resolute for precipitation. The scarcity of high-quality hydroclimate data in the Arctic remains an important limitation, to the extent that it is still regarded as key knowledge gap by the paleoclimate community [START_REF] Hormes | The quest for temperature and hydroclimate records. Arctic2k Working Group special issue[END_REF][START_REF] Hormes | Spatiotemporal distribution of temperature and hydroclimate proxy data in the Arctic. Arctic2k Working Group special issue[END_REF].

In this context, the best correlation between the thickness of the nival units and NDJFM temperature was used to develop the proxy-climate calibration model (Fig. 7). We rationalize this approach, as many applications in climate research require time series of a single variable and for a specific season of the year, which is typical of data-data comparisons, data-model comparisons or proxy-data assimilation in climate model simulations [START_REF] Goosse | Reconstructing surface temperature changes over the past 600 years using climate model simulations with data assimilation[END_REF]. Two meteorological stations and CRU TS gridded data were used to develop this linear model. This approach strengthens the reproducibility, reliability and spatial representativeness of the climate signal, and differs from the common practice of considering the closest station (e.g. Alert for Lake A, Tomkins et al.

Climate reconstruction and regional representativeness

The varves of Chevalier Bay provide a well-calibrated quantitative winter-temperature record from the Canadian High Arctic, covering the past ca. 400 years (Fig. 8a). Results reveal a strong variability in winter temperature back to CE 1670. The coldest decades are reconstructed for the period CE 1800-1880, while the warmest and most prominent features of the record are reconstructed for the period CE 1880-1930and CE 1970-2010.

The reconstruction provides quantitative temperature estimates for all but the period CE 1920-1940, with values extrapolated outside the range of the calibration period.

Although interpreted qualitatively, these two decades are well known to be among the warmest over the past centuries, especially in the Arctic. This warming has been attributed to a combination of internal-natural and external-forced climate variability [START_REF] Bengtsson | The Early Twentieth-Century Warming in the Arctic-A Possible Mechanism[END_REF][START_REF] Yamanouchi | Early 20th century warming in the Arctic: A review[END_REF][START_REF] Johannessen | Surface air temperature variability and trends in the Arctic: new amplification assessment and regionalisation[END_REF]). However, without validation from other paleoclimate records that this period can be interpreted as an anomalously warm winter period, potential other non-climatic factors might have induced thick nival deposits during this period. Indeed, sedimentary analysis evidences a significant change in varve deposition during this warm period, which might be attributed to different mechanisms such as: (i) a change in the sediment trajectory through temporary delta channel avulsion; (ii) the hydrological influence of antecedent soil moistening during these warm and wet decades; (iii) a change of sediment availability in the catchment; and/or (iv) the occurrence of gullies in the bathymetry of Chevalier Bay (turbidity current pathways). Although further work would be required to resolve the sediment deposition changes for this specific period, other varve records from the Arctic have also captured these warmer decades through greater sediment deposition (e.g. Lake A and Sawtooth Lake, Fig. 9).

The winter climate signal recorded in the varves of Chevalier Bay appears to be representative for a large part of the western Canadian Arctic Archipelago (Fig. 8). [START_REF] Fisher | Effect of wind scouring on climatic records from ice-core oxygen-isotope profiles[END_REF]. The correlation map between winter and annual temperature during the instrumental period supports these similarities among the records (r= 0.4, n= 385), with spatial correlations up to r= 0.5 (Fig. 8c). The similarity of our winter reconstruction to observed and inferred annual temperatures suggests that the Chevalier Bay record contains a more regional signal. But more importantly, differences between the two records (>75 % unexplained variance) underline the potential of the winter reconstruction. Indeed, this study highlights the need to consider the winter season in climate syntheses and in climate models in order to reduce uncertainty in sensitivity tests of the Earth's climate to natural and anthropogenic forcings.

The comparison with other published varve records highlights the challenge of deriving a common picture for the winter season using Arctic lake sediments (Fig. 9). While intraseasonal and geographical climate differences still exist, Arctic lakes with similar nival properties are all very sensitive to specific sedimentation processes. Well-dated varve records from Arctic lakes have previously been used to reconstruct: (i) spring discharge, indirectly through catchment snow water equivalent [START_REF] Lamoureux | Multidecadal variations and decline in spring discharge in the Canadian middle Arctic since 1550 AD[END_REF]); (ii) autumn temperature and snowfall [START_REF] Tomkins | Autumn snowfall and hydroclimatic variability during the past millennium inferred from the varved sediments of meromictic Lake A, northern Ellesmere Island, Canada[END_REF]); (iii) the intensity of the spring snowmelt through median particle-size measured for each varve [START_REF] Francus | Paleoclimate studies of minerogenic sediments using annually resolved textural parameters[END_REF]);

(iv) the maximum snow accumulation prior the first-above 0°C spring temperature [START_REF] Cuven | Mid to Late Holocene hydroclimatic and geochemical records from the varved sediments of East Lake, Cape Bounty, Canadian High Arctic[END_REF]; or (v) more regional melt season temperatures [START_REF] Cook | Five thousand years of sediment transfer in a high arctic watershed recorded in annually laminated sediments from Lower Murray Lake, Ellesmere Island, Nunavut, Canada[END_REF]).

These catchment-specific sedimentation processes and responses led to reconstructed climate records that are substantially different, at both annual and multi-decadal time scales (Fig. 9). This raises important concerns regarding the influence of the size, and the topography of the catchment on the sedimentation, even for lakes that have comparable

Summary and conclusions

We have investigated the varved sediments of Chevalier Bay with the aim to increase the paleoclimate data coverage for winter; the most sensitive season to climate change in the Arctic. We produced a well-calibrated quantitative temperature record for the 'extended' winter season (NDJFM) to CE 1670, which is representative of a large part of the western Canadian High Arctic.

Based on detailed facies analysis of the relatively thick varves of Chevalier Bay, we isolated and separated the nival units (spring snowmelt) from the rainfall units (summer), and from clay caps (winter). This distinction was a basis for the development of a reliable chronology and a good proxy-climate calibration model. Supported by an accurate varve chronology, we demonstrate that the thickness of the nival units can be used as a good predictor for winter climate conditions in this region of the Arctic. We develop a proxy-climate calibration model to predict NDJFM temperature (r= 0.71, pc< 0.01), while significant correlations are also found for NDJFM snowfall (r= 0.65, pc< 0.01). This validates our working hypothesis in which the thickness of the nival units is directly controlled by SWE and spring snowmelt, and ultimately by winter temperature and related snowfall conditions. These results are significant and consistent among all climate datasets considered for this study (Mould Bay, Resolute Bay, and gridded CRU TS), and in agreement with neighbouring sedimentary process studies.

Our ~400-year long reconstruction compares well with annual temperature records for this part of the Arctic, both on decadal and centennial time scales. This clearly suggests a more regional climate signal, while important differences between the annual and winter records underline the importance in considering and separating the winter season in paleoclimate syntheses. On the contrary, the comparison of winter records from lake sediment studies reveals a strong sensitivity to local climate change with sitespecific responses within each catchment. This gives additional interest for the extended winter-season reconstruction from Chevalier Bay.

Our reconstruction reports the highest rate of warming (0.37°C/decade) starting in the late 1960s. This value is in agreement with the 0.40°C/decade for annual means reported for the Arctic over the period CE 1960-2003(ACIA, 2004). Projected increases in winter temperature together with increased snowfall are expected to have prominent effects on the Arctic environment such as for instance: (i) greater permafrost thaw depth, and the alteration in annual carbon balance of northern-latitude ecosystems, towards greater loss of CO2 to the atmosphere [START_REF] Natali | Permafrost degradation stimulates carbon loss from experimentally warmed tundra[END_REF]; (ii) the alteration in the tundra biosphere-atmosphere interactions [START_REF] Morgado | Long-term increase in snow depth leads to compositional changes in arctic ectomycorrhizal fungal communities[END_REF]; and (iii) the acceleration of sea-ice retreat resulting in a greater proportion of Arctic surface evaporative change (Bitanja and Selten, 2014). All these effects contribute to a positive feedback to climate change towards higher winter temperatures in the Arctic. Fig. 8 A Comparison between the winter NDJFM temperature records from Chevalier Bay and the annual temperature records from the Arctic North 60°N (ACIA, 2004), the ice core δ 18 O record from the Devon Ice Cap [START_REF] Fisher | Effect of wind scouring on climatic records from ice-core oxygen-isotope profiles[END_REF]) and the annual mass balance of the South Melville Ice Cap [START_REF] Boon | Forty-seven Years of Research on the Devon Island Ice Cap, Arctic Canada[END_REF]. Note that the black line (our study) and the blue line [START_REF] Fisher | Effect of wind scouring on climatic records from ice-core oxygen-isotope profiles[END_REF]) positively correlate with r=0.37 (p< 0.01, n= 385), B correlation maps of NDJFM temperature for the Canadian High Arctic, C correlation maps between NDJFM temperature at our study site and annual temperature for the rest of the Arctic (for the period CE 1949-1997).

Figure captions

  Arctic warming has been considered unprecedented over the past thousand years based on available paleoclimate data. Information about past climate variability is essential to assess the sensitivity of the Earth's climate to natural and anthropogenic forcings, and to reduce uncertainty in future climate projections (Masson-Delmotte et al. 2013). For the Arctic, the most comprehensive studies of climate change assessment covered the past four centuries (Overpeck et al. 1997), the last two millennia (Kaufman 2009; McKay and Kaufman 2014), and the Holocene (Sundqvist et al. 2014; Briner et al. 2016). However, while most of the paleoclimate records in the Arctic are related to annual and summer temperature (Briner et al. 2016), comparatively little has been reported about winter paleoclimate (e.g. Cuven et al. 2011). As an example, only four winter reconstructions out of over fifty-six records were considered in the extended database used to reconstruct Arctic temperatures by McKay and Kaufman (2014). As a result, seasonality is hardly considered in climate models, especially regarding the winter season (Goosse et al. 2010). Winter climate is of particular interest because it is the season with the highest sensitivity to climate change (Thompson and Wallace, 1998; Rinke et al. 1999; Moritz et al. 2002), which greatly differs from summer and annual measures (Jones et al. 2014). Therefore, information about past changes in winter climate is required in order to reduce uncertainty in large-scale assessments of climate variability and change.

  , which have provided successful paleoclimate examples from the Canadian High Arctic (Wolf and Smith, 2004; Besonen et al. 2008). To date, only a few annually-resolved and well-calibrated sediment records exist for the winter season (e.g. Cuven et al. 2011). Moreover, results from individual records suggest a strong sensitivity to: (i) local changes in winter climate; and (ii) specific catchment properties and snowmelt response. This local focus has resulted in a great diversity among the records, with typical examples such as: June temperature (Hughen et al. 2000), August-to-October temperature and snowfall (Tomkins et al. 2010), snowmelt intensity

  Monthly climate data were obtained from local and more distant meteorological stations to cover a broad geographical representativeness of the climate variability in the region (Fig.1a). Local data for the CBAWO meteorological station were accessed from Polar Data Catalogue (www.polardata.ca; 2003-2015 Climate Data Station, Cape Bounty, Nunavut; CCIN Ref No: 9885; 74.9N/-109.5E). Longer instrumental climate data were accessed from Environment Canada (www.climat.meteo.gc.ca) for the selected stations of Mould Bay A (CE 1949-1997) and Resolute CARS(CE 1947(CE -2015)). Note that Mould Bay measurements were discontinued in 1997 and restarted in 2002 with an automatic station at a different location. We have no means of determining the potential difference between the earlier and later data sets, and hence, we conservatively do not extend the Mould Bay station series after 1997. We also selected the gridded data of observed climate CRU TS 3.23 centered on our study site(Harris et al., 2014; 0.5° resolution), for the calibration with the proxy dataset (CE 1949-2015, grid cell: 11.75-11.25° W; 64.75-75.25° N).

  record shows pronounced sub-and multi-decadal variability, which remains well within the calibration range apart from 11 years of the warm period CE 1920-1940. Relatively cold winters occur from CE 1670 to CE 1800 with mean NDJFM temperature of -32.2°C. This mean value drops to -32.7°C for the period CE 1800-1880; the coldest decades over the past ca. 350 years. Finally, two major warming trends are estimated in the 20 th -21 st centuries for the periods CE 1880-1930 and CE 1970-2010. They are marked by rates of 0.26°C and 0.37°C per decade, respectively.

  sedimentary interpretation model developed by Cuven et al. (2010, 2011) and Lapointe et al. (2012) from East Lake, 50 km away.

  2010; Eureka for Sawtooth Lake, Francus et al. 2008; Alert for Lower Lake Murray, Cook et al. 2008; Taloyoak for Sanagak Lake, Lamoureux et al. 2006; Kimmirut for Upper Soper Lake, Hughen et al. 2000). CRU TS 3.23 was particularly effective, providing a continuous and representative record for winter climate conditions at our study site, and with the longest dataset (calibration period CE 1948-2010).

  However, our past winter reconstruction remains difficult to validate due to a lack of quantitative winter temperature records available for comparison (e.g. McKay and Kaufmann, 2014). The solution presented in this study came from the comparison with annual temperature records and winter-weighted proxies (Fig. 8a). Long-term similarities are evident between our winter temperature reconstruction and: (i) annual temperature anomaly north of 60°N for the period CE 1900 -2003 (ACIA 2004); (ii) annual mass balance from the Melville Ice Cap for CE 1960 -2010 (Boon et al. 2010); and (iii) annual temperature record (δ 18 O) from the Devon Ice Cap extending towards CE 1973

  hydroclimatic and geologic properties. Furthermore, the differences among the varve records underline the importance of understanding sedimentation processes and for climate sensitivity tests from each individual site prior to reconstruct past climate from the Arctic. The range of climatic signals interpreted from High Arctic varve records is diverse (Figure 10). The quantitative record for the extended winter from Chevalier Bay offers a unique opportunity to evaluate climate change in the Arctic. Although this nival catchment appears to respond to the length and the intensity of snowmelt, the indirect information stored in the winter temperature and winter snowfall conditions holds a greater potential. This record could be used in more applied climate research such as data-model comparisons and proxy-data assimilation in climate model simulations over the past c. 400 years (Goosse et al. 2010).

Fig. 1

 1 Fig.[START_REF]Impacts of a Warming Arctic: Arctic Climate Impact Assessment[END_REF] Study site with: A location of Chevalier Bay, Melville Island in the Canadian High Arctic. Orange squares indicate meteorological stations, while the small dashed area shows the grid cell region used from CRU TS 3.23, B catchment properties of Lake Chevalier and its hydrology, C averaged meteorological data for the period CE 1949 -1997 obtained from the station of Mould Bay A.

Fig. 2

 2 Fig. 2 Water column properties of Chevalier Bay on June 20, 2010: salinity (PSU), turbidity (NTU), temperature (°C) and dissolved oxygen (%).

Fig. 3

 3 Fig.[START_REF] Bengtsson | The Early Twentieth-Century Warming in the Arctic-A Possible Mechanism[END_REF] Sediment properties with: A core image and XRF scans, B example of varve years and their associated XRF characteristics. N refers to the nival unit, R for the rainfall unit and Cc to clay cap.

Fig. 4

 4 Fig.[START_REF] Besonen | A record of climate over the last millennium based on varved lake sediments from the Canadian High Arctic[END_REF] Varve facies types occurring in the sedimentary sequence of Chevalier Bay. N stands for the nival unit, R for the rainfall unit, Cc for clay cap, Cci for the intermediate clay cap, and Sp for slump.

Fig. 5

 5 Fig. 5 Varve chronology obtained from thin section measurements. Counting uncertainty (error) is reported by the shaded area around the final age-depth line (visual), and by the dashed line on the lower panel (quantitative values).

Fig. 6

 6 Fig.[START_REF] Birks | Overview of numerical methods in palaeolimnology[END_REF] Principal component analysis (PCA) integrating the thickness of the nival units (indicated as 'varve'), and monthly climate data obtained from Mould Bay (Mould), Resolute Bay (Res.) and the CRU TS grid cell (CRU). The four seasons (MAM, JJA, SON, DJF) and specific seasons of interest (ASO and NDJFM) are considered here.

Fig. 7 A

 7 Fig. 7 A Linear correlations between the different meteorological datasets considered in this study,B time series of NDJFM temperature from each station combined with the calibration-in-time model obtained from the thickness of the nival units in Chevalier Bay.

Fig. 9

 9 Fig. 9 Comparison between the different winter-sensitive varved sediment records published from the Canadian High Arctic with their respective location. Data are shown with an 11-year triangular filter.

Fig. 10

 10 Fig.[START_REF] Christie | The geology of Melville Island, Arctic Canada[END_REF] Illustration of the different correlations between sediment proxy data (mostly varve thickness) and monthly climate variables from nival-catchment lake records located in the Canadian High Arctic. Results are from Sanagak Lake[START_REF] Lamoureux | Multidecadal variations and decline in spring discharge in the Canadian middle Arctic since 1550 AD[END_REF]), Lake A (Tomkins et al. 2010), Sawtooth Lake[START_REF] Francus | Paleoclimate studies of minerogenic sediments using annually resolved textural parameters[END_REF], East Lake[START_REF] Cuven | Mid to Late Holocene hydroclimatic and geochemical records from the varved sediments of East Lake, Cape Bounty, Canadian High Arctic[END_REF], and Lower Lake Murray[START_REF] Cook | Five thousand years of sediment transfer in a high arctic watershed recorded in annually laminated sediments from Lower Murray Lake, Ellesmere Island, Nunavut, Canada[END_REF]. Asterisks are for records that depend on the year-to-year timing of the melting. The thick dark line shows Chevalier Bay record (winter NDJFM temperature, this study).

Fig. 1 Fig. 2 Fig. 3 Fig. 4

 1234 Fig.1

Fig. 5

 5 Fig. 5

  Fig. 6

  ; Braun et al. 2000; Forbes and Lamoureux, 2005; Cockburn and Lamoureux, 2008; Tomkins et al.

Table 1

 1 Correlation table between the thickness of the varve nival units and climate data from Mould Bay, Resolute Bay and CRU TS 3.23. Highest correlation results from multiple testing are reported for temperature, precipitation and snowfall

Acknowledgments

This research was carried out within the Swiss National Science Foundation grant P2BEP2_162029, and supported by NSERC/CRSNS 2015-05276. We thank K. Kathan and E. Kjikjerkovska for their early contributions to research on Chevalier Bay. Thanks to A. Normandeau, J. Fouché, G. King, and A. Rudy for fruitful discussions. Particular thanks also go to the PEARL Lab of J. Smol for access to the gamma counter, and to C. Grooms and B. Sivarajah for their help with sample preparation and dating. We would also like to acknowledge Polar Continental Shelf Program, Natural Resources Canada for field logistics support. Constructive comments from two anonymous reviewers are appreciated.