Boreas

Timing and magnitude of early to middle Holocene warming in East inferred from chironomids

Journal:For Boreas Review Only Manuscript ID BOR-059-2016.R2

Manuscript Type: Original Article

Date Submitted by the Author: n/a

Complete List of Authors: Axford, Yarrow; Northwestern University, Dept. of Earth and Planetary Sciences Levy, Laura; Department of Geoscience, Aarhus University Kelly, Meredith A.; Dartmouth College, Earth Sciences Francis, Donna; Department of Geosciences, University of Massachusetts Hall, Brenda; University of Maine, School of Earth Sciences and the Climate Change Langdon, Peter G.; University of Southampton, Geography Lowell, Thomas; University of Cincinnai, Geology

Arctic, Greenland, Chironomidae, midges, Holocene Thermal Maximum, Keywords: paleotemperatures, paleolimnology, lake sediments

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1 1 2 3 4 1 Timing and magnitude of early to middle Holocene warming in East Greenland 5 2 inferred from chironomids 6 3 7 4 YARROW AXFORD, LAURA B. LEVY, MEREDITH A. KELLY, DONNA R. FRANCIS, 8 9 5 BRENDA L. HALL, PETER G. LANGDON AND THOMAS V. LOWELL 10 6 11 7 12 8 Axford, Y., Levy, L. B., Kelly, M. A., Francis, D. R., Hall, B. L., Langdon, P. G. & Lowell, T. 13 9 V.: Timing and magnitude of early to middle Holocene warming in East Greenland inferred from 14 10 chironomids. 15 16 11 17 18 12 Much of Greenland experiencedFor summersReview warmer than presentOnly during parts of the early to 19 20 13 middle Holocene, during a precession-driven positive anomaly in summer insolation. However, 21 22 23 14 the magnitude of that warmth remains poorly known, and its timing and spatial pattern are 24 25 15 uncertain. Here we describe the first quantitative Holocene palaeotemperature reconstruction 26 27 16 from central East Greenland based upon insect (chironomid) assemblages preserved in lake 28 29 30 17 sediments. We postulate that landscapes like our study site, characterized by minimal soil and 31 32 18 vegetation development through the Holocene and thus less influenced by some important 33 34 35 19 secondary gradients, are especially well-suited to the use of chironomids to reconstruct Holocene 36 37 20 temperatures. The inferred timing of warmth at our study site near Scoresby Sund agrees well 38 39 21 with other nearby evidence, including glacial geological reconstructions and temperatures 40 41 42 22 inferred from precipitation isotopes at ice cap, supporting the use of chironomids to 43 44 23 reconstruct temperatures at this site. We infer highest temperatures from ~10 to 5.5 ka, followed 45 46 24 by gradual cooling after 5.5 ka and progressively colder and less productive conditions after 3.5 47 48 49 25 ka. Models based upon two independent training sets yield similar inferred temperature trends, 50 51 26 and suggest an average summer temperature anomaly from ~10 to 5.5 ka of 3 to 4 °C relative to 52 53 27 the preindustrial last millennium. The estimated overall rate of Neoglacial cooling averaged over 54 55 56 28 the period from 5.5 to 0.5 ka was 0.6 to 0.8 °C per thousand years, more than twice the rate 57 58 59 60 Boreas Page 2 of 30

2 1 2 3 29 previously estimated for the Arctic as a whole. Given strong apparent spatial variability in 4 5 6 30 Holocene climate around the Arctic, and the utility of palaeoclimate data for improving climate 7 8 31 and ice sheet models, it should be a priority to further quantify past temperature changes around 9 10 11 32 the margins of the Greenland Ice Sheet, where few quantitative reconstructions exist and future 12 13 33 warming will affect global sea level. 14 15 34 16 35 Yarrow Axford ([email protected]), Department of Earth and Planetary Sciences, Northwestern University, 17 36 Evanston, IL, USA 60208; Laura B. Levy, Department of Geoscience, Aarhus University, 8000 Aarhus C, Denmark; 18 37 Meredith A. Kelly, DepartmentFor of Earth Sciences,Review Dartmouth College, Only Hanover, NH, USA 03750; Donna R. 19 38 Francis, Department of Geosciences, University of Massachusetts, 233 Morrill Science Center, Amherst, MA, USA 20 39 01003; Brenda L. Hall, School of Earth and Climate Sciences and the Climate Change Institute, University of 21 40 Maine, Orono, ME, USA 04469; Peter G. Langdon, Geography and Environment, University of Southampton, 22 41 Highfield, Southampton SO17 1BJ UK; Thomas V. Lowell, Department of Geology, University of Cincinnati, 23 42 Cincinnati, OH, USA 45221. 24 43 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 3 of 30 Boreas

3 1 2 3 44 Diverse palaeoenvironmental records from Greenland’s ice-free margin and surrounding 4 5 6 45 seas record very different conditions during the late Holocene than in the early to middle 7 8 46 Holocene, when enhanced summer insolation drove higher summer temperatures. During the 9 10 11 47 Holocene Thermal Maximum (HTM), thermophilous species migrated north and glaciers and sea 12 13 48 ice were less extensive in many sectors of Greenland (e.g., Kaufman et al. 2004; Funder et al. 14 15 49 2011; Larsen et al. 2015; Briner et al. 2016). These environmental responses to HTM warming 16 17 18 50 hold clues to what the Forfuture impacts Review of anthropogenic warming Only around Greenland are likely to 19 20 51 be – including changes in the Greenland Ice Sheet and thus global sea level. However, such 21 22 52 assessments are currently hampered by uncertainties regarding the magnitude and spatiotemporal 23 24 25 53 pattern of HTM warmth over Greenland (Lecavalier et al. 2014). 26 27 54 Most quantitative estimates of the HTM temperature anomaly over Greenland thus far 28 29 55 come from the Greenland Ice Sheet itself (Dahl-Jensen et al. 1998; Johnsen et al. 2001; Vinther 30 31 32 56 et al. 2009; Sundqvist et al. 2014), a perspective limited to high-elevation conditions over ice, 33 34 57 some of which experienced large elevation changes through the Holocene (Vinther et al. 2009). 35 36 37 58 A small but growing database of quantitative palaeotemperature reconstructions from Greenland 38 39 59 lakes is adding alternative perspectives (Fréchette & de Vernal 2009; D’Andrea et al. 2011; 40 41 60 Axford et al. 2013; Gajewski 2015). 42 43 44 61 Subfossil chironomid (Insecta: Diptera: Chironomidae, or non-biting midge) assemblages 45 46 62 preserved in lake sediments are increasingly used across the Arctic and subarctic to reconstruct 47 48 63 Holocene temperatures, as indicated for example by the proxy’s representation in recent reviews 49 50 51 64 of Holocene climate records by Briner et al. (2016) and Kaufman et al. (2016). Studies of 52 53 65 chironomid assemblages in surface sediments along spatial and climatic transects have 54 55 66 repeatedly demonstrated that temperature is a dominant predictor of chironomid species 56 57 58 59 60 Boreas Page 4 of 30

4 1 2 3 67 distributions (e.g., Walker et al. 1991; Eggermont & Heiri 2011; Fortin et al. 2015), and 4 5 6 68 chironomid remains are nearly ubiquitous and often abundant in the Arctic’s many small non- 7 8 69 glacial lakes. Pioneering qualitative palaeoclimate interpretations from chironomid assemblages 9 10 11 70 have been published at several sites in eastern and northeastern Greenland (Wagner et al. 2005; 12 13 71 Schmidt et al. 2011; Wagner & Bennike 2015). However, quantitative chironomid-based 14 15 72 temperature reconstructions have previously been published from only two sites on Greenland, 16 17 18 73 both on the island’s westFor coast and Review in warmer climates than Only that of our current study site 19 20 74 (Wooller et al. 2004; Axford et al. 2013). 21 22 75 Here we describe a new quantitative reconstruction of temperature shifts through the 23 24 25 76 Holocene from insect remains preserved in sediments of a lake in central East Greenland near 26 27 77 Scoresby Sund and Renland ice cap (Fig. 1). Bulk sedimentological data from the same lake 28 29 78 sediment cores were previously presented by Levy et al. (2014), who combined geomorphic 30 31 32 79 mapping and cosmogenic surface exposure dating with analysis of lake sediment records to 33 34 80 reconstruct fluctuations of nearby Bregne ice cap through the Holocene. We build upon that 35 36 37 81 work by developing a proxy-based climate reconstruction for the region that is independent of 38 39 82 glacier behavior. 40 41 83 42 43 44 84 Study site and methods 45 46 85 47 48 86 Last Chance Lake (informal name; hereafter LCL) is a small (8.9 m deep, 0.007 km 2) non-glacial 49 50 51 87 lake located at 70.90645 °N 25.56945 °W and 660 m a.s.l. on Milne Land, 60 km southeast of 52 53 88 Renland ice cap and 5 km from the coastline of inner Scoresby Sund (Figs 1, 2). The lake is 54 55 56 89 almost 110 km from the nearest small settlement, Nerlerit Inaat airport at Constable Point, and 57 58 59 60 Page 5 of 30 Boreas

5 1 2 3 90 thus remote from direct human impacts. LCL has been isolated from glacial meltwater since its 4 5 6 91 catchment was deglaciated by 10.7 ka (Levy et al. 2014; ka = thousands of calibrated years BP). 7 8 92 It has seasonal surface inflow and outflow today, and is surrounded by very sparsely vegetated 9 10 11 93 boulder fields (Fig. 2). Surface water temperature in LCL was 8.8 °C on September 5, 2010. 12 13 94 Regional climate is strongly influenced by cold surface waters and frequent pack ice off the outer 14 15 95 coast, associated with the East Greenland Current. At , 140 km to the east at 65 16 17 18 96 m a.s.l. on the outer coast,For mean JulyReview temperature is 7.1 °COnly and mean annual temperature is -6.1 19 20 97 °C for recent years (AD 1986-2011; Carstensen & Jørgensen 2009). 21 22 98 As described by Levy et al . (2014), three overlapping piston cores capture the modern 23 24 25 99 sediment-water interface and the entire Holocene package of 75.5 cm of laminated/stratified 26 27 100 lacustrine mud overlying 8 cm of basal diamicton, the latter interpreted as till dating to regional 28 29 101 deglaciation. For the current study, the ten AMS 14 C ages reported by Levy et al . (2014) on 30 31 32 102 aquatic plants or invertebrates from the lacustrine sediment unit were re-calibrated (updated) 33 34 103 based upon the IntCal13 curve (Reimer et al. 2013). We conducted age-depth modelling using 35 36 37 104 the freely available software clam version 2.2 (Blaauw 2010), with all ages from the lacustrine 38 39 105 mud unit equally weighted. Like Levy et al . (2014), we excluded two inverted ages obtained on 40 41 106 aquatic plants recovered from the basal diamicton which we speculate were dragged down the 42 43 44 107 edges of the tube during coring. Because an intact sediment-water interface was recovered, 0 cm 45 46 108 depth was assigned a modern age (AD 2010, the year of sample collection). All ages before ~10 47 48 109 ka are extrapolated beneath the lowest 14 C sample, thus very uncertain. We do not extrapolate 49 50 51 110 beyond the base of the lacustrine unit at 75.5 cm depth. Throughout this paper, we subdivide the 52 53 111 Holocene according to the suggestions of Walker et al . (2012), who define the middle Holocene 54 55 112 as bounded between 8.2 and 4.2 ka. 56 57 58 59 60 Boreas Page 6 of 30

6 1 2 3 113 Weight percent organic matter (OM) in bulk sediment was estimated by loss-on-ignition 4 5 6 114 at 550 °C for two hours (Heiri et al. 2001). Weight percent biogenic silica (bioSiO 2) was 7 8 115 measured following the methods described by Mortlock & Froelich (1989), using 10% Na 2CO 3. 9 10 11 116 Chironomid (Diptera: Chironomidae) remains >100 µm were analyzed following standard 12 13 117 protocols (Walker 2001). Between 52 and 215 whole head capsules were identified per sample 14 15 118 (median count sum was 109 whole head capsules), except for three samples processed from 16 17 18 119 below the lacustrine unitFor (i.e., within Review the diamicton) which Only yielded no head capsules. Sample 19 20 120 sizes ranged from 0.1 to 0.5 g dry sediment for all except the sample at 75 cm depth (the deepest 21 22 121 sample to yield any head capsules, with low head capsule concentrations that necessitated a 23 24 25 122 sample size of 3.9 g). Head capsules were identified with reference to Wiederholm (1983) and 26 27 123 Brooks et al . (2007). We identified one unknown morphotype as Psectrocladius nr barbimanus 28 29 124 type. P. nr barbimanus type appears similar to P. barbimanus type (Brooks et al. 2007) but with 30 31 32 125 sharply angular median and first lateral teeth, and with median teeth that may have a small apical 33 34 126 projection and that are at least as tall as the first lateral teeth (Fig. 3). 35 36 37 127 July air temperatures were modeled using two transfer functions with independent 38 39 128 training sets: One, a weighted-averaging (WA) model with inverse deshrinking and tolerance 40 41 129 downweighting, based upon a northeastern North American training set including 68 sites 42 43 44 130 representing a July air temperature gradient of 5.0 to 19.0 °C and spanning from the Canadian 45 2 46 131 Arctic islands west of Greenland to Maine, USA (Francis et al. 2006; 44 taxa, r jack = 0.88, 47 48 132 RMSEP = 1.5 °C; hereafter FRA06). Two, a two-component WA partial-least-squares (WA- 49 50 51 133 PLS) model employing a large northern North American training set, comprising 434 sites 52 53 134 representing a July air temperature gradient from 2.0 to 16.3 °C and spanning from Alaska, USA 54 55 135 to eastern Canada and from Boreal to High Arctic environments, including the Canadian Arctic 56 57 58 59 60 Page 7 of 30 Boreas

7 1 2 3 136 islands (Fortin et al. 2015; 78 taxa, r2 = 0.72, RMSEP = 1.9 °C; hereafter FOR15). Neither 4 boot 5 6 137 training set contains calibration data from East Greenland, and no chironomid training set yet 7 8 138 exists for East Greenland or any other Mid- to High Arctic sector of Greenland. The Francis et 9 10 11 139 al . (2006) training set was used in a previous Holocene study in warmer West Greenland (Axford 12 13 140 et al. 2013). Fossil assemblages in East Greenland have much in common with assemblages 14 15 141 from the Canadian Arctic at the genus level (Gajewski et al. 2005; Francis et al. 2006; Fortin et 16 17 18 142 al. 2015), but we did findFor some differences Review at the sub-genus Only level, e.g. within Psectrocladius . To 19 20 143 assess the similarity of modern analogues to our fossil assemblages, we used the modern 21 22 144 analogue technique (MAT) and calculated squared chord distances (SCDs) between each 23 24 25 145 untransformed fossil assemblage and its closest modern analogue in each training set. 26 27 146 Palaeotemperatures were modeled on square-root transformed species data using C2 v. 28 29 147 1.7.6. Most subfossil morphotypes identified to species type level, e.g., the morphotypes of 30 31 32 148 Cricotopus/Orthocladius and Psectrocladius (Psectrocladius) , were lumped at the genus level to 33 34 149 harmonize with the training sets. All subfossil taxa were utilized in temperature modelling, 35 36 37 150 except two minor taxa that are absent or excluded from both models: Trissocladius (which is 38 39 151 absent from both training sets but occurs in six downcore samples at a maximum abundance of 40 41 152 three percent) and Diamesa (of which two head capsules were found in our bottommost gyttja 42 43 44 153 sample). The mix of Psectrocladius species types lumped in the FRA06 training set is not 45 46 154 described, but we know that the training set includes P. sordidellus type, whereas P. barbimanus 47 48 155 and P. nr barbimanus types are most abundant at our study site. In the FRA06 training set, the 49 50 51 156 Tanypodinae are also mostly undifferentiated and (given the broad climate gradient of the 52 53 157 training set sites) likely to include a different mix of species types than our cold, Mid- to High- 54 55 158 Arctic study site. As a means to assess our reconstruction’s sensitivity to possible taxonomic 56 57 58 59 60 Boreas Page 8 of 30

8 1 2 3 159 differences between our study site and the training set sites, temperature modelling was 4 5 6 160 performed both with and without these two groups using the FRA06 training set. 7 8 161 9 10 11 162 Results 12 13 163 14 15 164 Chronology 16 17 18 165 A smooth spline age modelFor (error-weighted, Review with smoothness Only parameter = 0.4) was used to 19 20 166 estimate age-depth relationships down core (Fig. 4). Our updated age model is entirely within 21 22 167 model error of the Levy et al . (2014) model. The age-depth curve below the deepest age (i.e., 23 24 25 168 before 10.2 ka) is extrapolated and very uncertain. Cosmogenic surface exposure dating 26 27 169 indicates that the landscape surrounding the lake was deglaciated by 10.7 ka (Levy et al. 2014), 28 29 170 and may be most consistent with the younger end (i.e., ~11 ka) of the range of ages for onset of 30 31 32 171 lacustrine sedimentation suggested by the age model. Confidence ranges (95% confidence) for 33 34 172 the remainder of the age model vary from 200 years during the middle Holocene, which was 35 36 14 37 173 more densely sampled for C, to 600 years near the bottom-most ages (see the gray envelope in 38 39 174 Fig. 4). Therefore, ages stated below should be considered to have uncertainties of at least ±100 40 41 175 to 300 years, depending upon their position in the record. 42 43 44 176 45 46 177 Palaeoenvironmental proxy data 47 48 178 The early postglacial. 49 50 51 179 The oldest lake sediments at LCL, which overlie diamicton (Levy et al. 2014) and date to 52 53 180 sometime prior to 10 ka, contain assemblages dominated by Oliveridia/Hydrobaenus and by 54 55 181 Tanytarsini including Micropsectra (Fig. 5). Some Tanytarsini may be pioneering postglacial 56 57 58 59 60 Page 9 of 30 Boreas

9 1 2 3 182 colonizers of arctic lakes (Saulnier-Talbot & Pienitz 2010; Axford et al. 2011), so succession 4 5 6 183 may partly explain the composition of these assemblages. At the same time, several lines of 7 8 184 evidence suggest that the period before 10 ka was characterized by low temperatures, dilute 9 10 11 185 waters and low productivity: the cold stenotherm Oliveridia/Hydrobaenus was abundant; 12 13 186 assemblages from this period are similar to those of the late Holocene at LCL and to modern 14 15 187 assemblages from lakes ~650 km north on Store Koldewey (Schmidt et al. 2011); and OM and 16 17 18 188 bioSiO 2 are very low inFor sediments Review from this period (<2% Only dry weight of each; Fig. 6). 19 20 189 21 22 190 The early to middle Holocene. 23 24 25 191 By 10 ka, Oliveridia/Hydrobaenus declined while Tanypodinae and Psectrocladius (both found 26 27 192 in relatively nutrient-rich lakes and absent from more dilute, nutrient-poor lakes in West 28 29 193 Greenland; Brodersen & Anderson 2002) became abundant. Procladius , which is associated in 30 31 32 194 Arctic Canada with both relatively high temperatures and high dissolved organic carbon 33 34 195 (Gajewski 2005) and was found in Duck and Hjort lakes on Store Koldewey only during the 35 36 37 196 inferred local HTM (Schmidt et al. 2011), was consistently present at LCL from 10.1 to 3.6 ka. 38 39 197 Combined with the onset of significant bioSiO 2 production and the jump to 10% OM abundance 40 41 198 after 9.8 ka, this chironomid assemblage shift suggests a transition to warmer and more 42 43 44 199 productive conditions. 45 46 200 The continuing rise of bioSiO 2 and OM throughout the subsequent millennia of inferred 47 48 201 HTM warmth provides evidence for increasing lake and possibly landscape productivity 49 50 51 202 throughout the HTM. Supporting this interpretation, Psectrocladius and Cricotopus , chironomid 52 53 203 genera commonly associated with macrophytes and/or productive lakes (Brodersen et al. 2001; 54 55 204 Langdon et al. 2010), experienced maximum abundances during the period of peak bioSiO and 56 2 57 58 59 60 Boreas Page 10 of 30

10 1 2 3 205 OM from ~7 to 4 ka. Sediment accumulation rates also rose above the overall (presumably 4 5 6 206 compaction-defined) Holocene trend during the period of peak bioSiO 2 and OM percentages 7 8 207 (Fig. 6). 9 10 11 208 12 13 209 Transition to the late Holocene. 14 15 210 Inferred temperatures began to decline in the middle Holocene by ~5.5 ka, followed more than 16 17 18 211 500 years later by a declineFor in bioSiO Review2 and subsequently OnlyOM. An abrupt assemblage shift 19 20 212 occurred at ~3.5 ka, after which Oliveridia/Hydrobaenus , Pseudodiamesa , and Micropsectra 21 22 213 increased and Tanypodinae disappeared. BioSiO 2 and OM percentages were low after 3.5 ka, 23 24 25 214 both having values <10% for much of the past two millennia. 26 27 215 28 29 216 Temperature modelling 30 31 32 217 Quantitative temperature reconstructions generated using the three different models described 33 34 218 above show important differences and similarities (Fig. 5). The tolerance-downweighted WA 35 36 37 219 (FRA06) models are more sensitive to the presence of low abundances of cold stenotherms 38 39 220 (which by definition have narrow temperature tolerances in the training sets), as seen in two 40 41 221 samples ~9 and 7 ka. Reconstructions using FRA06 are indeed sensitive to our treatment of 42 43 44 222 Psectrocladius and Tanypodinae, with inclusion of those taxonomically ambiguous groups 45 46 223 yielding warmer temperature reconstructions in the early and middle Holocene. The FOR15 47 48 224 model yielded more stable inferred temperatures during the early and middle Holocene and 49 50 51 225 overall lower absolute temperatures. The decrease in temperatures inferred with FOR15 in the 52 53 226 last two millennia – not seen in reconstructions from FRA06, but consistent with a rise in the 54 55 227 cold stenotherm Hydrobaenus/Oliveridia and with changes in productivity indicators (bioSiO 56 2 57 58 59 60 Page 11 of 30 Boreas

11 1 2 3 228 and OM) – likely reflects the inclusion of colder training set sites in FOR15 (as cold as 2.0 °C vs. 4 5 6 229 5.0 °C for FRA06). The FOR15 training set may be better suited to reconstructing colder-than- 7 8 230 present temperatures at Mid- to High Arctic sites than the FRA06 training set, which lacks 9 10 11 231 analogues for very cold fossil sites as described previously (Axford et al. 2009). When 12 13 232 compared with the FOR15 training set using MAT, all but two fossil samples from LCL have 14 15 233 good analogues (defined as SCD less than the 5th percentile within the training set), and only 16 17 18 234 one fossil sample is a no-analogueFor Review assemblage (defined asOnly SCD greater than the 10th percentile 19 20 235 within the training set; Fig. 5). In contrast, when compared with the FRA06 training set most 21 22 236 fossil samples have SCDs greater than the 5th percentile, and many greater than the 10th 23 24 25 237 percentile within that smaller training set. This further suggests that FOR15 is a better fit to our 26 27 238 fossil data, presumably due to the colder climate space and more numerous sites represented in 28 29 239 this training set. 30 31 32 240 Despite model differences, reconstructed millennial-scale temperature trends are similar 33 34 241 between models, and most of the absolute temperature reconstructions are within sample-specific 35 36 37 242 errors of one another (Fig. 5). Reflecting the similar trends indicated by the three models, they 38 39 243 yielded similar reconstructed temperature anomalies, especially FOR15 compared with the 40 41 244 FRA06-TR model excluding two taxonomically ambiguous groups (Fig. 6). We henceforth 42 43 44 245 discuss anomalies rather than absolute temperature inferences, and furthermore we give greatest 45 46 246 weight to inferences from the FOR15 training set, which as discussed above contains better 47 48 247 analogues for our fossil data. July temperatures inferred from FOR15 were 3 to 4 °C higher 49 50 51 248 from ~10 to 5.5 ka than in the last millennium, and anomalies inferred using the two FRA06 52 53 249 models generally bracket those estimates. The corresponding inferred overall rate of Neoglacial 54 55 250 cooling, averaged over the period from 5.5 to 0.5 ka, was 0.6 to 0.8 °C per thousand years. 56 57 58 59 60 Boreas Page 12 of 30

12 1 2 3 251 4 5 6 252 Discussion 7 8 253 9 10 11 254 Interpreting chironomid assemblage shifts 12 13 255 Aquatic ecosystems are sensitive to factors other than temperature – for example, vegetation 14 15 256 change and changes in water chemistry that may be associated with watershed ontogeny or 16 17 18 257 eutrophication from externalFor nutrient Review sources (Anderson etOnly al. 2008; Heggen et al. 2010; 19 20 258 Kaufman et al. 2012; Luoto et al. 2015). Such secondary gradients may confound chironomid- 21 22 259 based palaeotemperature reconstructions (Velle et al. 2005, 2010; cf. Brooks et al. 2012), and 23 24 25 260 must be considered as possible alternative drivers of midge assemblage changes. We note that 26 27 261 the sparsely vegetated, boulder-supported landscape surrounding LCL has almost no soil cover, 28 29 262 and that the regional arrival of Betula nana between 9 and 8 ka and regional but likely not local 30 31 32 263 establishment of dwarf shrub tundra (Funder, 1978; Cremer et al. 2001) – the most significant 33 34 264 Holocene vegetation change in the region – does not coincide with a notable chironomid 35 36 37 265 assemblage shift at our study site. Although, as described above, some chironomid taxa 38 39 266 including Psectrocladius and Cricotopus may have responded to changes in macrophyte 40 41 267 abundance or other aspects of lake productivity, shifts in overall productivity and substrate 42 43 44 268 recorded by OM and bioSiO 2 do not predict shifts in inferred temperature. Rather, changes in 45 46 269 these variables consistently lag inferred temperature changes, as would be expected if past 47 48 270 millennial-scale temperature changes drove changes in lake trophic status. Thus we suggest that 49 50 51 271 secondary gradients do not seriously confound temperature reconstructions at this site. 52 53 272 Several additional observations lend confidence in modeled palaeotemperature trends at 54 55 273 LCL: Inferred temperature trends agree well between models (Fig. 6) – a necessary albeit not 56 57 58 59 60 Page 13 of 30 Boreas

13 1 2 3 274 sufficient test of model success. Qualitative interpretations based upon shifts in major taxa, 4 5 6 275 discussed above, agree well with the quantitative reconstructions. Multiple indicators of 7 8 276 relatively warm, productive conditions (e.g., Procladius, Psectrocladius ) are abundant 9 10 11 277 throughout the inferred early to middle Holocene thermal maximum, and multiple cold indicators 12 13 278 (e.g., Pseudodiamesa, Oliveridia/Hydrobaenus ; Fig. 6) replace them in the late Holocene. And 14 15 279 importantly, as summarized below, the timings of inferred temperature changes also agree well 16 17 18 280 with nearby independentFor palaeoclimate Review archives. Only 19 20 281 Evidence for the fidelity of chironomids as indicators of palaeotemperature at LCL agrees 21 22 282 with previous studies concluding that temperature was likely the primary driver (in some cases 23 24 25 283 probably via intermediary effects on lake chemistry and trophic status) of chironomid 26 27 284 assemblage shifts in East Greenland through the Holocene (Wagner et al. 2005; Schmidt et al. 28 29 285 2011). In another relevant study, Medeiros et al . (2015) concluded that secondary gradients are 30 31 32 286 relatively unproblematic in chironomid-based temperature reconstructions from unproductive 33 34 287 Arctic lakes. Brooks et al . (2012) emphasized the importance of identifying a priori which site 35 36 37 288 types are best suited to the use of chironomids as a palaeotemperature proxy. We postulate that 38 39 289 sites like LCL with little soil or vegetation development through the Holocene, and probably 40 41 290 relatively simple food webs, have especially strong potential for the use of chironomid 42 43 44 291 assemblages to infer Holocene temperature shifts by minimizing the effects of many secondary 45 46 292 gradients. 47 48 293 49 50 51 294 Holocene temperature change in East Greenland 52 53 295 We infer the highest temperatures in central East Greenland from ~10 to 5.5 ka, with average 54 55 296 July air temperature anomalies of 3 to 4 °C relative to the last millennium of the Holocene (Fig. 56 57 58 59 60 Boreas Page 14 of 30

14 1 2 3 297 6). Gradual cooling began by 5.5 ka in the middle Holocene, followed by additional assemblage 4 5 6 298 shifts suggesting intensified cooling at ~4 to 3.5 ka and then progressively colder conditions 7 8 299 throughout the late Holocene. Results from Two Move Lake, <0.5 km away, support these 9 10 11 300 climatic interpretations: Influx of meltwater from nearby Bregne Ice Cap ceased at 10 ka when 12 13 301 the ice cap retreated outside the lake’s watershed, supporting the onset of warmer-than-present 14 15 302 conditions at that time. Subsequent cooling is inferred from increasing non-glacial clastic 16 17 18 303 sedimentation beginningFor at 6.5 ka, Review and glacial meltwater Onlyreturned to the watershed initially at 2.6 19 20 304 ka and consistently after 1.9 ka as the ice cap expanded (Fig. 6; Levy et al. 2014). 21 22 305 The timing of peak warmth inferred from these two lakes is very similar to that inferred 23 24 25 306 from sediment geochemistry, diatoms and chironomids at lakes Basaltsø and B1 on Geographical 26 27 307 Society Ø ~230 km to the northeast (Fig. 1; Cremer et al. 2001; Wagner et al. 2005), and aligns 28 29 308 with evidence for extralimital thermophilous marine bivalve species in and near Scoresby Sund 30 31 32 309 ~9.5 to 6 ka (Hjort & Funder, 1974; Bennike & Wagner 2013). These studies add support to 33 34 310 diverse evidence from land and sea for summer warmth in East Greenland during parts of the 35 36 37 311 early and middle Holocene, and that Neoglacial cooling and its environmental impacts 38 39 312 intensified within the late Holocene (e.g., Funder, 1978; Andrews et al. 1997; Jennings et al. 40 41 313 2002; Vinther et al. 2009; Schmidt et al. 2011; Lowell et al. 2013). 42 43 44 314 Evidence that warmer-than-present HTM conditions began as early as 10 ka at LCL and 45 46 315 Two Move Lake contrasts with some records from East Greenland where the inferred onset of 47 48 316 warmth came later, particularly at sites farther north (Schmidt et al. 2011; Wagner & Bennike 49 50 51 317 2015; Gajewski 2015). However, early warmth is supported by local precipitation isotopes over 52 53 318 nearby Renland ice cap, where inferred temperatures consistently exceeded those of the last 54 55 319 millennium for the first time at ~10 ka (Fig. 6; Vinther et al. 2009). Also nearby, 125 km east of 56 57 58 59 60 Page 15 of 30 Boreas

15 1 2 3 320 LCL, lakes adjacent to Istorvet ice cap experienced non-glacial sedimentation from ~9.7 to 0.9 4 5 6 321 ka (Lusas et al. 2011; Lowell et al. 2013). Warmth beginning ~10 ka suggests that by then any 7 8 322 regional factors modulating the climatic response to insolation (e.g., effects of the waning but 9 10 11 323 still proximal Greenland Ice Sheet on local energy balance, or the hypothesized atmospheric or 12 13 324 oceanic effects of continuing Laurentide deglaciation; Kaufman et al. 2004; Kaplan & Wolfe 14 15 325 2006; Renssen et al. 2009) did not override the effects of enhanced insolation forcing around 16 17 18 326 inner Scoresby Sund. For Review Only 19 20 327 The average, overall rate of Neoglacial summer cooling inferred here for central East 21 22 328 Greenland between 5.5 and 0.5 ka (0.6 to 0.8 °C per thousand years) is at least twice that 23 24 25 329 estimated for late Holocene summers across the Arctic as a whole (Kaufman et al. 2009). Recent 26 27 330 palaeodata syntheses and modelling studies alike support the notion that there were strong and 28 29 331 complex spatial variations in the magnitude, seasonality, and probably timing of peak HTM 30 31 32 332 warmth around the Arctic and subarctic (Renssen et al. 2009, 2012; Zhang et al. 2010; Marcott 33 34 333 et al. 2013; Briner et al. 2016; Kaufman et al. 2016). Our results suggest that central East 35 36 37 334 Greenland experienced a larger summer temperature anomaly during the HTM than some other 38 39 335 parts of the Arctic, perhaps due to especially strong positive feedbacks from sea ice or other local 40 41 336 factors. 42 43 44 337 The HTM summer temperature anomaly reconstructed at LCL also exceeds the annually 45 46 338 integrated anomalies inferred from oxygen isotopes in the Renland ice core and from borehole 47 48 339 temperatures at GRIP to the west on the Greenland Ice Sheet (Dahl-Jensen et al. 1998; Vinther et 49 50 51 340 al. 2009), and the overall rate of Neoglacial cooling at LCL exceeds that inferred from argon and 52 53 341 nitrogen isotopes in trapped air bubbles through the late Holocene at GISP2 (Kobashi et al. 54 55 342 2011). This may be explained in whole or part by the differing seasonal biases of these records, 56 57 58 59 60 Boreas Page 16 of 30

16 1 2 3 343 combined with altered seasonality of HTM climate driven by a positive summer (and slightly 4 5 6 344 negative winter) insolation anomaly. 7 8 345 Together, all of these results illustrate the importance of local, seasonally specific 9 10 11 346 temperature reconstructions in accurately characterizing climates of the past. Given the utility of 12 13 347 HTM palaeoclimate data as targets for improving climate and ice sheet models (Simpson et al. 14 15 348 2009; Applegate et al. 2012; Lecavalier et al. 2014), and the important role of summer 16 17 18 349 temperatures at low elevationsFor in drivingReview ice sheet mass balance,Only it should be a priority to further 19 20 350 elucidate the magnitude and seasonality of past warmth around the margins of the Greenland Ice 21 22 351 Sheet. 23 24 25 352 26 27 353 Conclusions 28 29 354 30 31 32 355 Chironomid assemblage shifts track temperature through the Holocene at Last Chance Lake in 33 34 356 central East Greenland. At this site near inner Scoresby Sund and Renland ice cap, the timing of 35 36 37 357 chironomid-inferred temperature shifts is in good agreement with nearby ice core and glacial 38 39 358 geological evidence, and reconstructed temperature trends throughout the Holocene agree well 40 41 359 between our chironomid-based models. We have argued that sites with similarly minimal soil 42 43 44 360 and vegetation development through the Holocene may be especially well suited to the use of 45 46 361 chironomids for inferring Holocene temperatures, because they are less affected by some 47 48 362 important, potentially confounding secondary gradients. 49 50 51 363 Warmest Holocene conditions were registered in central East Greenland ~10 to 5.5 ka, 52 53 364 with July air temperature anomalies of 3 to 4 °C relative to the preindustrial last millennium. 54 55 365 The early onset of intense warming here argues against a cooling effect on local climate from the 56 57 58 59 60 Page 17 of 30 Boreas

17 1 2 3 366 melting Laurentide Ice Sheet, as has been proposed for some sites in the northwestern North 4 5 6 367 Atlantic region. This site experienced gradual cooling after ~5.5 ka, and progressively colder 7 8 368 conditions after ~4 to 3.5 ka. The estimated rate of Neoglacial cooling in central East Greenland, 9 10 11 369 averaged over the period from 5.5 to 0.5 ka, was 0.6 to 0.8 °C per thousand years. Neoglacial 12 13 370 cooling here occurred at more than twice the rate estimated for the Arctic as a whole, suggesting 14 15 371 significant spatial variability in the magnitude of the temperature response to insolation forcing 16 17 18 372 across the Arctic. For Review Only 19 20 373 21 22 374 Acknowledgements. This work was partly funded by the U.S. NSF (ARC-0909270 to Kelly, 23 24 25 375 ARC-0909285 to Lowell, ARC-0908081 to Hall, ARC-1454734 to Axford) and the Institute for 26 27 376 Sustainability and Energy at Northwestern University (ISEN). C. Carrio and G. Schellinger 28 29 377 made chironomid identifications, with input from J. McFarlin. S. Hammer and L. Hempel 30 31 32 378 assisted in the lab. Biogenic silica was measured in the Sedimentary Records of Environmental 33 34 379 Change Laboratory at Northern Arizona University. Staff at the National Lacustrine Core 35 36 14 37 380 Facility (LacCore) assisted with processing sediment cores. C was measured at the National 38 39 381 Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility. B. Honsaker and A. R. 40 41 382 Lusas assisted in the field. This work benefited from use of the training set data of Fortin et al . 42 43 44 383 (2015), made publically available by M.-C. Fortin, A.S. Medeiros and collaborators via the 45 46 384 Canadian Cryospheric Information Network’s Polar Data Catalogue. Anonymous reviewers are 47 48 385 thanked for their thoughtful feedback on our manuscript. 49 50 51 386 52 53 387 REFERENCES 54 388 55 389 56 57 58 59 60 Boreas Page 18 of 30

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22 1 2 3 553 Sundqvist, H. S., Kaufman, D. S., McKay, N. P., Balascio, N. L., Briner, J. P., Cwynar, L. C., 4 5 554 Sejrup, H. P., Seppä, H., Subetto, D. A., Andrews, J. T., Axford, Y., Bakke, J., Birks, H. 6 555 J. B., Brooks, S. J., de Vernal, A., Jennings, A. E., Ljungqvist, F. C., Rühland, K. M., 7 556 Saenger, C., Smol, J. P. & Viau, A. E. 2014: Arctic Holocene proxy climate database – 8 557 New approaches to assessing geochronological accuracy and encoding climate variables. 9 558 Climate of the Past 10 , 1-63. 10 11 559 Velle, G., Brodersen, K. P., Birks, H. J. B. & Willassen, E. 2010: Midges as quantitative 12 560 temperature indicator species: Lessons for palaeoecology. The Holocene 20 , 989-1002. 13 14 561 Velle, G., Brooks, S. J., Birks, H. J. B. & Willassen, E. 2005: Chironomids as a tool for inferring 15 562 Holocene climate: an assessment based on six sites in southern Scandinavia. Quaternary 16 563 Science Reviews 24 , 1429-1462. 17 18 564 Vinther, B. M., Buchardt,For S. L., Clausen,Review H. B., Dahl-Jensen, Only D., Johnsen, S. J., Fisher, D. A., 19 565 Koerner, R. M., Raynaud, D., Lipenkov, V., Andersen, K. K., Blunier, T., Rasmussen, S. 20 566 O., Steffensen, J. P. & Svensson, A. M. 2009: Holocene thinning of the Greenland ice 21 22 567 sheet. Nature 461 , 385-388. 23 568 Wagner, B., Heiri, O. & Hoyer, D. 2005: Chironomids as proxies for palaeoenvironmental 24 569 changes in East Greenland: a Holocene record from Geographical Society Ø. Zeitschrift 25 26 570 der Deutschen Geologischen Gesellschaft 156 , 543–556. 27 571 Wagner, B. & Bennike, O. 2015: Holocene environmental change in the Skallingen area, eastern 28 572 North Greenland, based on a lacustrine record. Boreas 44 , 45–59. 29 30 573 Walker, I. R. 2001: Midges: Chironomidae and related Diptera. In Smol, J. P., Birks, H. J. B. & 31 574 Last, W. M. (eds.): Tracking Environmental Change Using Lake Sediments. Volume 4: 32 33 575 Zoological Indicators , 43-66, Kluwer, Dordrecht. 34 576 Walker, I. R., Smol, J. P., Engstrom, D. R. & Birks, H. J. B. 1991: An assessment of 35 577 Chironomidae as quantitative indicators of past climatic change. Canadian Journal of 36 37 578 Fisheries and Aquatic Sciences 48 , 975-987. 38 579 Walker, M. J. C., Berkelhammer, M., Björck, S., Cwynar, L. C., Fisher, A., Long, A. J., Lowe, J. 39 580 L., Newnham, R. M., Rasmussen, S. O. & Weiss, H. 2012: Formal subdivision of the 40 41 581 Holocene Series/Epoch: a Discussion Paper by a Working Group of INTIMATE 42 582 (Integration of ice-core, marine and terrestrial records) and the Subcommission on 43 583 Quaternary Stratigraphy (International Commission on Stratigraphy). Journal of 44 584 Quaternary Science 27 , 649-659. 45 46 585 Wiederholm, T. 1983: Chironomidae of the Holarctic Region. Keys and Diagnoses: Part 1. 47 586 Larvae. Entomologica Scandinavica Supplement 19 , 457 pp. 48 49 587 Wooller, M. J., Francis, D., Fogel, M. L., Miller, G. H., Walker, I. R. & Wolfe, A. P. 2004: 50 588 Quantitative paleotemperature estimates from δ18 O of chironomid head capsules 51 589 preserved in arctic lake sediments. Journal of Paleolimnology 31 , 267-274. 52 53 590 Zhang, Q., Sundqvist, H. S., Moberg, A., Kornich, H., Nilsson, J. & Holmgren, K. 2010: Climate 54 591 change between the mid and late Holocene in northern high latitudes – Part 2: Model-data 55 592 comparisons. Climate of the Past 6, 609-626. 56 57 593 58 59 60 Page 23 of 30 Boreas

23 1 2 3 594 Figures 4 5 6 595 7 8 596 Fig. 1. Map of Greenland (A) and the Scoresby Sund region (inset, B) showing location of Last 9 10 11 597 Chance Lake and other geographic features mentioned in the text. Gray shading denotes present- 12 13 598 day ice cover. 14 15 599 16 17 18 600 Fig. 2. Photograph of LastFor Chance Review Lake (in foreground, withOnly boat used for coring in August 19 20 601 2010). View is toward the southeast. 21 22 602 23 24 25 603 Fig. 3. Head capsule of Psectrocladius nr barbimanus morphotype (authors’ informal 26 27 604 designation). Photo by G. Schellinger, Northwestern University. 28 29 605 30 31 32 606 Fig. 4. Age-depth model for Last Chance Lake lacustrine sediments. Blue bands show 95% 33 34 607 probability (highest posterior density) ranges of calibrated 14 C ages, and gray zone shows 95% 35 36 37 608 confidence intervals of the smooth spline. The age at 56.25 cm depth was excluded from the 38 39 609 model, as was the underlying diamicton unit (from 83.5-75.5 cm depth, interpreted as till by 40 41 610 Levy et al. 2014). 42 43 44 611 45 46 612 Fig. 5. Chironomid data from Last Chance Lake, including count sums (vertical line indicates 47 48 613 value of 50), chironomid-inferred absolute July air temperatures, percent abundances of 49 50 51 614 chironomid taxa (unfilled lines indicate 5× exaggeration) and squared chord distances (SCD) to 52 53 615 nearest training set analogues. Reconstructions are based upon three different inference models, 54 55 616 as described in the text: the WA model of Francis et al . (2006; FRA06), plus a variant with taxa 56 57 58 59 60 Boreas Page 24 of 30

24 1 2 3 617 removed (FRA06-TR), and the WA-PLS model of Fortin et al . (2015; FOR15). Bootstrapped 4 5 6 618 sample-specific errors for two models are shown. Dotted lines on SCD plots show 5th and 10th 7 8 619 percentiles within the respective training sets. For temperature reconstructions expressed as 9 10 11 620 anomalies, see Fig. 6. 12 13 621 14 622 Fig. 6. (A) Multi-proxy results from Last Chance Lake, including modeled age-depth 15 16 623 relationship (with 14 C ages plotted as squares), sediment accumulation rates (SAR), weight 17 18 For Review Only 19 624 percents organic matter (OM) and biogenic silica (bioSiO 2), and chironomid-inferred July air 20 21 625 temperature anomalies (calculated relative to the preindustrial last millennium). This study’s 22 23 24 626 updated age model is shown as a black line, and the original age model from Levy et al . (2014) 25 26 627 is shown as a green line; the two models are indistinguishable at this figure scale. Note that the 27 28 628 first-order trend in SAR likely reflects sediment compaction, as SAR values are not corrected for 29 30 31 629 density or water content. As discussed in the text, we give greatest weight to the FOR15 32 33 630 reconstruction (thick black line). (B) Palaeoclimate indicators from nearby Renland ice cap 34 35 631 (Vinther et al. 2009) and Two Move Lake (Levy et al. 2014). 36 37 38 632 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 25 of 30 Boreas

A 1 2 3 4 5 6 Geographical o 7 GISP2 Society Ø 21 25'W 8 For Review Only GRIP 9 B 10 11 East Greenland 12 Current 13 14 15 16 N 17 Istorvet ice cap 18 19 Jameson Land Liverpool 20 21 Land 22 23 Bregne ice cap Ittoqqortoormiit 24 Renland Last Chance Lake 25 ice cap 26 Two Move Lake 27 28 29 Milne Land Scoresby Sund 30 31 N 32 33

34 o 25 km 35 70 37'N 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Boreas Page 26 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 For Review Only 19 20 21 22 23 24 25 26 27 28 29 30 31 254x166mm (300 x 300 DPI) 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 27 of 30 Boreas

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 For Review Only 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 169x169mm (300 x 300 DPI) 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Boreas Page 28 of 30

1 0 2 3 4 5 6 7 For Review Only 8 9 10 20 11 12 13 14 15 16

17 Depth (cm) 18 19 40 20 21 22 23 24 25 26 27 28 60 29 30 31 32 33 34 35 36 37 38 12 000 10 000 8000 6000 4000 2000 0 39 Age (cal. a BP) 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 29 of 30 Boreas

1 2 3 4 5 6 7 8 9 10 11 e 12 f. p e if y p d t ty n s 13 e u u For Reviewe s p e e Only n s u y p s e s [cold] p t y p a ri 14 l u y in t y iu e s . . e n t r s s t d im p iu ff f v su f. e a b e ri s a b y d i if / li 15 p if a r o ip t lu l r t a d d s o . d b ig s r s u c a s l n n n ff ca n a [cold]o n n h e o b u c u u e ra i 16 d u s r o lc lv m th s n ro s g e d a e d la c u y e r u a t u e u c n e e y l s p s tr O s i c i a l o ra u SCD 17 h a m H ie iu / iu d im e d iin s n t i R s in ia / r d s s s s d la b s la u y c n d T iu d a d ia fe u u u u a c r p c ad s r e si 18 e - d o s o d e la p p p p cl o a o o r o s r D 5 6 6 a e d ri i c to to to to tr b n tr cl ta p a 5 6 I 1 0 0 l yp u e k o o o o o so c r o c o y o yt 1 0 f R A A c n m e v ra th ic ic ic ic s e M e th n cr n R A 19 o O R R ro a ia s li a r r r r r i s . n . s r i a O R # F F F P T D P O P O C C C C Tr P P P P O Ta M T F F 20 C 21 0 22 1000 23 July air 24 2000 temp 25 3000 (°C) 26 27 4000 28 5000 29 30 6000 31 7000 32 33 8000 34 A g e ( c a l . B P ) 9000 35 36 10 000 37 11 000 38 39 diamicton 40 41 0 200 3 5 7 9 11 13 0 0 0 0 0 20 40 60 0 0 0 0 0 20 0 0 0 0 20 40 0 0 20 0 0 20 40 60 0 20 0 20 40 60 0 0.4 0 0.4 42 taxonomists: C. Carrio and G. Schellinger 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Boreas Page 30 of 30

1 2 3 4 5 6 7 8 9 10 11 A. Last Chance Lake B. Nearby records 12 13 Depth (cm) ForOrganic matterReviewJuly temperature Only Two Move Lake 14 vs. age (weight %) anomaly (°C) 15 sedimentation 0 20 40 60 80 0 10 20 30 -2 0 2 4 6 16 0 17 18 1000 glacial 19

FRA06 20 2000 FOR15

21 FRA06-TR 3000 22 23 ) 4000 P

24 B 25 5000 . a

26 l 6000 non-

27 c a

( glacial

28 e 7000

29 g 30 A 8000 31 9000 32 33 10 000 34 35 11 000 glacial 36 37 0.005 0.010 0 10 20 -30.0 -28.0 -26.0 38 SAR BioSiO2 Renland 39 (cm yr-1) (weight %) uplift-corrected δ18O (‰) 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60