Auxiliary Material for "Western Arctic Ocean temperature variability during the last 8000 years" S1. Chronostratigraphy Chronostratigraphy for core HLY0205-GGC19 was established using four accelerator mass spectrometry (AMS) 14C dates on benthic foraminifera given by Keigwin et al. [2006], and five additional 14C dates were obtained from mollusc (Portlandia arctica) and gastropod (Neptunea lyrata) carbonate analyzed at the National Ocean Sciences AMS facility (NOSAMS) (Figure S1a and Table S1). Planktic foraminifers are rare to absent in GGC-19 and were thus unavailable for dating. Radiocarbon years were calibrated to calendar years using Calib 6.0 (http://calib.qub.ac.uk/calib/calib.html) using the Marine09 calibration dataset [Reimer et al., 2009]. A constant ĆR of 506 years was used to account for measurements of surface water 14C reservoir ages in bivalves from the vicinity of Pt. Barrow [McNeely et al., 2006]. This choice for reservoir age is supported by correlation of radiocarbon-independent paleomagnetic features (inclination, declination, and relative intensity) in GGC-19 with nearby core JPC16 [Darby et al., pers. comm.]. These nine calibrated ages yield a mean sedimentation rate of 65 cm kyr-1 for the last 7 kyr for GGC-19. An age model for cores P1-92AR-P1/B3 was constructed from seven AMS 14C dates in P1 and four AMS 14C dates in B3 [Darby et al., 2001] (Figure S1b). A ĆR=400±60 years was used, accounting for variations in analyses of the modern western Arctic Ocean reservoir effect [Bauch et al., 2001; Darby et al., 2009] and measurements of surface water 14C reservoir ages from the Chukchi Sea [McNeely et al., 2006]. A composite depth scale for P1 and B3 was created by accounting for approximately 20 cm of overpenetration by the piston core and 5 cm of overpenetration by the box corer [de Vernal et al., 2005]. Sedimentation rates average 30 cm kyr-1 from 8 ka to 5.5 ka and 1.5 ka to present, and ~ 10 cm kyr-1 between 5.5 and 1.5 ka. Reservoir corrections for 14C measurements in the Arctic Ocean are not well constrained [Darby et al., 2009], but we note that using a different correction would result in a small shift of absolute ages, but would not change the patterns observed within, and between, our records. S2. Methodology for Bottom Water Temperature Analyses Two-cm thick samples were taken every 2.5 to 5 cm from HLY0205-GGC19 for calcareous micofaunal analyses. Micropaleontological samples were wet weighed and washed through a 63 um sieve with the coarse fraction dried at 50ˇC overnight. Due to the small amount of coarse material (generally 0.1 gram or less), all benthic foraminifera in the >63 um fraction were counted for each sample. Benthic foraminifera were identified through a binocular light microscope using taxonomy of Feyling-Hanssen and Buzas [1976], Ishman and Foley [1996], Polyak et al. [2002], Jennings et al. [2004], and references therein. The benthic foraminiferal assemblages in GGC-19 were dominated by four taxa: Nonion labradoricum, Islandiella helenae, Elphidium excavatum forma clavata, and Cassidulina reniforme (Figure S2). Although many factors influence the relative abundance of benthic foraminiferal taxa, N. labradoricum and I. helenae are generally preferential toward warmer conditions while C. reniforme and E. excavatum prefer colder (<0ˇC) conditions. Well-preserved specimens of N. labradoricum and I. helenae were picked from the >125 um size fraction for stable isotopic composition. Between five and ten specimens were analyzed for each sample, with smaller counts for the larger N. labradoricum. Stable isotope analyses were conducted at the University of South Carolina stable isotope laboratory in a VG Optima stable isotope ratio mass spectrometer (IRMS) equipped with an Isocarb single acid bath carbonate preparation system. All isotope data are presented in per mil delta-notation relative to the Vienna Peedee belemnite (VPDB, NBS-19). Replicate analyses of this standard yielded an analytical precision of ±0.08 per mil. Where available, replicate analyses of samples returned a sample error of ±0.04 per mil, yielding a total error of ±0.12 per mil. Oxygen isotopes for both species were converted to temperature using the derivation of O'Neil et al. [1969] by Shackleton [1974]; this equation was chosen over more recent calibrations in consideration of the low bottom water temperatures at our two locations (see discussions of low-temperature oxygen isotope thermometry in Lubinski et al. [2001] and Bauch et al. [2004]). We assumed constant d18OSW for our records. A ±0.12 per mil d18O error corresponds to an approximate single-sample temperature error of ±0.5ˇC. For 3-point smoothed records, this temperature error drops to ±0.3ˇC by accounting for a ~ 40% error reduction when using three samples to calculate mean d18O (see discussion of multisample error reduction in Dwyer et al. [1995]). Ostracode specimens of Krithe glacialis were brush-picked from the >150 um fraction in samples from P1-92AR-P1/B3 for trace metal analysis. Each specimen was assigned a preservation index ranging from 1 (transparent) to 7 (opaque white) [Dwyer et al., 1995]. Specimens were soaked in 5% NaOCl for 16-24 h to oxidize any remaining organic matter and assist in removal of any adhering particles. Each sample was then triple-rinsed with deionized water, visually inspected under light microscope, and then twice more rinsed with deionized water under light sonication. Individual ostracode valves were then dissolved in 3-30mL of 0.05N nitric acid and the resulting aqueous solution analyzed for Mg and Ca on a Fisons Instruments Spectraspan 7 direct current plasma atomic emission spectrometer (DCP) at Duke University using ultra-pure plasma-grade SPEX standard solutions. Analytical precision is approximately 2% based on replicate analyses of standards. Replicate analyses of samples show a sample error of ±0.4 mmol mol-1, yielding a total error of approximately ±0.5 mmol mol-1. Mg/Ca values were converted to temperature using the calibration in Cronin et al. [1996], giving an approximate temperature error of ±0.5ˇC. S3. Uncertainty in Bottom Water Temperature Analyses The d18O of foraminiferal calcite is also a function of the isotopic composition of ambient water (d18Osw). For this study area, the formation of isotopically light brines associated with enhanced sea-ice formation [Bauch and Bauch, 2001; Hillaire-Marcel and de Vernal, 2008] or the intrusion of an 18O-depleted water mass could impact the foraminiferal d18O and thus obscure the temperature signal. However, higher brine production seems unlikely, as intervals of lower d18O correspond with significantly reduced sea-ice cover (Figure 2 in manuscript). Likewise, downwelling of isotopically light (< -1.0 per mil) Chukchi shelf water [Cooper et al., 1997] is ruled out because temperature, salinity, and nutrient profiles suggest that dense outflows of Chukchi shelf water in Barrow Canyon do not exceed 200m in depth [Weingartner et al., 1998; Cooper et al., 2005; Shimada et al., 2005] and downwelling shelf water is not apparent at the P1/B3 site. Mg/Ca ratios in Krithe from deep sea and shallow Arctic environments reflect a direct or indirect (secretion rate or metabolism) temperature dependence of the uptake of Mg into its calcitic shell [Cronin et al., 1996; Dwyer et al., 2002]. Although other factors such as carbonate ion concentration can affect shell geochemistry of deep-sea benthic fauna [e.g. Elderfield et al., 2006], such factors are not relevant to the shallow water P1/B3 site, and thus temperature likely exerts a primary control on the shell geochemistry of shallow-water Krithe. Temperature calibrations for both d18O and Mg/Ca likely add a significant amount of additional error to each temperature estimate. Although we are unable to quantify this additional error, the consistency of both the Mg/Ca-derived temperature record and the d18O-derived temperature record with modern BWT at the P1/B3 and GGC-19 sites, respectively, gives us confidence that these derived temperatures are representative of past BWT changes. S3. Dinocyst Assemblages from GGC-19 Approximately 5 g of wet sediment was taken every 10 cm for dinocyst assemblages from GGC-19; samples were prepared using standardized palynological procedures [de Vernal et al., 1999]. Dinoflagellate cysts are abundant through the core with generally excellent preservation. Dinoflagellate cyst assemblages are converted to months of sea-ice cover >50% per year based on the modern analogue technique, with a calibration root mean square error of ±1.1 months/year [de Vernal et al., 2008]. Cysts of two dinoflagellates common in North Pacific surface sediment (P. kofoidii and S. nephroides) compose up to 4% of the dinocyst assemblage at GGC-19 for the past 7 ka (Figure S3). 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Baker, (2002), Trace elements in ostracodes, in Applications of the Ostracoda to Quaternary Research, AGU Monograph 131, edited by J.A. Holmes and A.R. Chivas, pp. 205-225. Elderfield, H., J. Yu, P. Anand, T. Kiefer and B. Nyland. 2006. Calibrations for benthic foraminiferal Mg/Ca paleothermometry and the carbonate ion hypothesis. Earth and Planetary Science Letters 250: 633-649, doi:10.1016/j.epsl.2006.07.041. Feyling-Hanssen, R.W., and M.A. Buzas (1976), Emendation of Cassidulina and Islandiella helenae new species, J. Foram. Res, 6(2), 154-158. Hillaire-Marcel, C. and A. de Vernal, (2008), Stable isotope clue to episodic sea ice formation in the glacial North Atlantic, Earth Planet. Sc. Lett., 268, 143-150, doi:10.1016/j.epsl.2008.01.012. Ishman, S.E., and K.M. Foley (1996), Modern benthic foraminifer distribution in the Amerasian Basin, Arctic Ocean, Micropaleontology, 42(2), 206-220. Jennings, A.E., N.J. Weiner, G. Helgadottir, and J.T. 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Lubinski (2002), Benthic foraminiferal assemblages from the southern Kara Sea, a river-influence Arctic marine environment, J. Foram. Res., 32(3), 252-273. Reimer, P.J., M.G.L. Baillie, E. Bard, A. Bayliss, J.W. Beck, P.G. Blackwell, C. Bronk Ramsey, C.E. Buck, G. Burr, R.L. Edwards, M. Friedrich, P.M. Grootes, T.P. Guilderson, I. Hajdas, T.J. Heaton, A.G. Hogg, K.A. Hughen, K.F. Kaiser, B. Kromer, F.G. McCormac, S.W. Manning, R.W. Reimer, D.A. Richards, J. Southon, C.S.M. Turney, J. van der Plicht, and C. Weyhenmeyer (2009), IntCal09 and Marine09 radiocarbon age calibration curves, 0 - 50,000 years cal BP, Radiocarbon, 51(4), 1111-1150. Shackleton, N.J. (1974), Attainment of isotopic equilibrium between ocean water and the benthonic foraminifera Uvigerina: isotopic changes in the ocean during the last glacial, Colloque CNRS, 219, Centre National de la Recherche Scientifique, Paris, 203-210. Shimada, K., M. Itoh, S. Nishino, F. McLaughlin, E. Carmack, and A. Proshutinsky (2005), Halocline structure in the Canada Basin of the Arctic Ocean, Geophys. Res. Lett., 33, L03605, doi:10.1029/2004GL021358. Weingartner, T.J., D.J. Cavalieri, K. Aagaard, and Y. Sasaki (1998), Circulation, dense water formation, and outflow on the northeast Chukchi shelf, J. Geophys. Res., 103 (C4), 7647-7661. Supplementary Figure Captions Figure S1. Age-depth profiles for cores HLY0205-GGC19 (A) and P1-92AR-P1/B3 (B). Box core B3 dates are denoted by blue squares; piston core P1 dates are denoted by black circles. Figure S2. Benthic foraminiferal assemblages in GGC-19, expressed as number of specimens per gram wet sediment. (a) N. labradoricum (blue) and I. helenae (red) representing the warm end-members, (b) E. excavatum (purple) and C. reniforme (yellow) representing the cold end-members. (c) Total benthic foraminiferal counts (black) per gram wet sediment. Benthic foraminiferal abundance is generally highest from 7 to 6 ka, 5 to 3 ka, and around 1 ka, generally consistent with times of reduced surface sea-ice cover (see Figure 2 in manuscript). Figure S3. Percentage of identified dinoflagellate cysts versus core depth for GGC-19. Two of the identified dinocysts, Selenopemphix nephroides and Polykrikos kofoidii, are rare in Arctic coretops but common from the North Pacific. Their low abundance (? 2%) may represent transport by strong, erosive currents in Barrow Canyon.