Quaternary Science Reviews 222 (2019) 105875
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Quaternary Science Reviews
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Chronology and sedimentology of a new 2.9 ka annually laminated record from South Sawtooth Lake, Ellesmere Island
* Francois Lapointe a, b, , Pierre Francus b, c, Joseph S. Stoner d, Mark B. Abbott e, Nicholas L. Balascio f, Timothty L. Cook a, Raymond S. Bradley a, Steven L. Forman g, Mark Besonen h, Guillaume St-Onge c, i a Climate Science Center, Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA b Centre - Eau Terre Environnement, Institut National de la Recherche Scientifique, Canada Research Chair in Environmental Sedimentology, Quebec, G1K 9A9, Canada c GEOTOP, UniversiteduQuebec a Montreal, Quebec, Montreal, H3C 3P8, Canada d College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA e Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, PA 15260, USA f Department of Geology, College William and Mary, 251 Jamestown Road, Williamsburg VA 23187 USA g Department of Geology, Baylor University, College of Arts and Sciences, One Bear Place #97354, Waco, TX 76798, USA h Harte Research Institute for Gulf of Mexico Studies, Texas A&M University - Corpus Christi, Corpus Christi, TX 78412, USA i Institut des Sciences de la mer de Rimouski, Canada Research Chair in Marine Geology, UniversiteduQuebec a Rimouski, 310 allee des Ursulines, Rimouski, Quebec, G5L 3A1, Canada article info abstract
Article history: Few annually laminated (varved) lacustrine records exist in the Arctic, but these high-resolution climate Received 24 April 2019 archives are needed to better understand abrupt climate change and the natural mode of climate vari- Received in revised form ability of this sensitive region. This paper presents a new high-resolution 2900-year long varved lake 9 August 2019 sediment record from the Fosheim Peninsula, Ellesmere Island. The varve chronology is based on mul- Accepted 9 August 2019 tiple varve counts made on high-resolution scanning electron microscope images of overlapping sedi- Available online xxx ment thin sections, and is supported by several independent dating techniques, including 137Cs and 210Pb analysis, one optically stimulated luminescence age located close to the bottom of the composite Keywords: Holocene sequence, and comparison between paleomagnetic variations of this record and the longest High-Arctic fi Arctic climate varve record, Lower Murray Lake, which con rms the reliability of the Sawtooth chronology. High- Varves resolution backscattered images examined under a scanning electron microscope (SEM) were crucial Paleoclimatology to giving a more detailed view of sedimentation processes in the lake and thus help to delineate varves Chronology more precisely than in conventional image analysis. Fine-scale geochemical analysis reveals that lake Paleomagnetism sedimentation is mainly clastic and that elemental geochemistry is influenced by grain-size. Principal component analysis of multiple proxies and the coarse grain-size fraction of South Sawtooth Lake display similar fluctuations to the nearby Agassiz Ice Cap v18O record, including lower values during the Little Ice Age cold period. These results show this new high-resolution and continuous record has a reliable varve chronology and is sensitive to temperature variability. South Sawtooth Lake's mean sedimentation rate of 1.67 mm a 1 is higher than any other sedimentary sequence in the High Arctic providing a unique op- portunity for extracting new, high-resolution paleoclimatological and paleoenvironmental record in a region where few other records currently exist. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction
The Arctic has undergone substantial warming during the last decade. This trend is expected to increase with further loss of sea- * Corresponding author. Climate Science Center, Department of Geosciences, ice and increased glacier melt, thus accelerating the related positive University of Massachusetts, Amherst, MA 01003, USA. feedback processes. Due to limited instrumental data from the E-mail address: fl[email protected] (F. Lapointe). https://doi.org/10.1016/j.quascirev.2019.105875 0277-3791/© 2019 Elsevier Ltd. All rights reserved. 2 F. Lapointe et al. / Quaternary Science Reviews 222 (2019) 105875 region, there is still a huge lack of understanding of the Arctic including radiometric analysis of the recent record, optically climate system (Cohen et al., 2014). High-resolution climate records stimulated luminescence dating (OSL), and paleomagnetic varia- can provide meaningful information about past temperature and tions over the past 2900 years. This paper is an extension of pre- precipitation (e.g. Lapointe et al., 2017). However, published climate vious work undertaken at SSL (Francus et al., 2002) using new archives from the region are insufficient and their spatial distri- sediment cores to establish a chronology and characterize the bution is limited in extent. Development of reliable paleoclimate stratigraphy. We then extract physical and geochemical properties records from the High Arctic is especially challenging given the of the sediment in order to establish a basis for further paleocli- limited success of radiocarbon dating as a means of constraining matic investigations using this record. the chronology of lacustrine records from Artic lakes. This is largely caused by limited biological productivity and the presence of aged 1.1. Study site carbon stored in watersheds which often leads to unreliable age determinations (Abbott and Stafford, 1996). In contrast, the preci- SSL (79 200N, 83 510W, 280 m a.s.l.) is located on the Fosheim sion and accuracy of varve-count chronologies can provide superior peninsula in the Eastern Canadian High-Arctic (Fig. 1). This site has chronologies compared to those based purely on radiometric been the focus of several studies (Francus et al., 2002, 2008; Perren methods (Zolitschka et al., 2015). Yet new annually laminated re- et al., 2003; Lewis et al., 2005), which indicate that sediments in the cords from the Canadian Arctic must be tested against other inde- lake are varved. The surficial geology of the area is composed of pendent dating methods to demonstrate that they are indeed blanket and veneer tills. Deglaciation started around 5800 BCE varves. This paper provides a new varved record from South (7800 B.P.) with present-day conditions reached around 4300 BCE Sawtooth Lake, Ellesmere Island (hereafter, SSL). The varve chro- (6300 B.P.) (England, 1983). The bedrock geology of the SSL nology is supported by several independent dating techniques, watershed is composed mainly of Triassic sandstones and
Fig. 1. a) Location of South Sawtooth Lake in the Canadian Arctic, Ellesmere Island. Green, red, blue and black stars denote South Sawtooth Lake (SSL) Lower Murray Lake (LML), Agassiz Ice Cap and Eureka weather station, respectively. b) Land digital elevation model (DEM) base map from the ArcticDEM 7 Polar Geoscience Center showing SSL in the Sawtooth Range. Map created in R. c) SSL bathymetry showing SS12-12, the location of the composite sequence (black circles). The gray line crossing the lake corresponds to the seismic profile in d. d) SEG-Y data from the seismic profile were processed using the Kingdom Suite®. Subbottom data were collected in the lake using a Edge Tech 3100 Chirp at a frequency 4e24 kHz. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) F. Lapointe et al. / Quaternary Science Reviews 222 (2019) 105875 3 calcareous siltstones, with minor concentrations of limestone and developed at INRS (Francus and Nobert, 2007), regions of interest shale (Geological Survey of Canada, 1972). The lake and its water- (ROIs) were identified on the digital images. ROIs were labeled with shed surface area are ~2.6 km2 and 47 km2, respectively, with a three character alphabetic codes such as “aag” (see Fig. 2b and c for maximum elevation of ~915 m a.s.l. A single tributary spills into the an example). A Zeiss Evo® 50 scanning electron microscope (SEM) lake from the southeast, while the outlet is situated at the north- was then used to acquire ~8000 images in backscattered mode western end (Francus et al., 2008). SSL is an elongated lake divided following the methods of Lapointe et al. (2012). These high- into a proximal and a distal basin (100 and 82 m deep, respectively) resolution images were used to count varves as they provide su- separated by a 60-m deep sill. This configuration limits erosion in perior contrast relative to optical images and because many the distal basin, where the sediment cores were recovered (e.g. laminae are less than 0.5 mm and hard to delineate using digital Francus et al., 2008). A seismic survey was conducted in 2006 flatbed scan images, or other common techniques. revealing that the distal basin is devoid of major mass movement deposits (Fig. 1b and c), and thus well suited for paleoclimatological 2.1.2. Radiometric dating investigations. The uppermost part (18.25 cm) of a gravity core (undisturbed The geomorphic setting of the study area with its surrounding sediment) were extruded in the field and measured continuously at mountains and highlands limit the incursion of cold Arctic Ocean a 1 cm increment for radionuclides (210Pb, 137Cs) at the University of air masses and cyclonic activity from Baffin Bay (Edlund and Alt, Pittsburgh. 210Pb and 137Cs activities were measured using a Can- 1989). From 1948 to 2016, the average monthly temperature at berra Gamma Spectrometer. The chronology was established using Eureka (the nearest weather station, 84 km to the NW and 10m asl), the constant rate of supply (CRS) model (Appleby, 2002). were 2.5, 5.9, and 3.3 C during June, July and August, respectively. Described as an extreme polar desert, the region's annual precipi- 2.1.3. Paleomagnetic variations tation is 65 mm and ~25 mm falls as rain according to the Eureka Paleomagnetic secular variations were derived from U-channel weather station. However, a temporary weather station at SSL samples that were analyzed through progressive alternating field recorded twice the amount of rain recorded at Eureka (Lewis et al., (AF) demagnetization measured at 1 cm intervals, using a 2G 2005). Precipitation for June and July of 1994 at a nearby site in the EnterprisesTM model 755e1.65UC superconducting rock magne- Sawtooth Range reached 64 mm, similar to the mean annual value tometer at the Oregon State University's Paleo- and Environmental at Eureka (Lewkowicz and Hartshorn, 1998). It is also worth Magnetism Laboratory. The natural remanent magnetization mentioning that great amounts of rain were witnessed during the (NRM) was measured and progressively demagnetized using field season of 2012 at SSL in the end of May, although no rain was stepwise AF up to 70 mT in 5 mT steps to isolate the characteristic recorded at Eureka during that time. Thus, the orographic influence remanent magnetization (ChRM) and collect the inclination of the on climate combined with the steep slopes with elevation reaching ChRM. U-channel results were processed using the UPmag software ~915 m asl (Fig. 1b) promote hillslope processes and sediment (Xuan and Channell, 2009). transfer into the lake, resulting in SSL having one of the highest sedimentation rates in the region (Francus et al., 2008). 2.1.4. Optically stimulated luminescence dating The annual couplets result from seasonal differences in the lake. Optically stimulated luminescence (OSL) dating provides a Clay caps are formed by the settling of fine clay particles during the measure of time since sediment grains were deposited and shiel- winter when the lake is ice-covered (2e3 m thick ice) and the ded from further light exposure, which often effectively resets the turbulence in the water column is low to absent. In turn, deposition luminescence signal to a low defineable level (Murray and Olley, of coarser sediments associated with overflows triggered by early 2002). A time-dependent luminescence signal is acquired upon snow melt and occasional rainfall events occurs later in the sum- buried with exposure to ionizing radiation (a, b and g) from the mer season (Francus et al., 2008). decay of radioisotopes in the surrounding sediments. This exposure to ionizing radiation and to a lesser extent, to cosmic radiation, 2. Methods results in displaced electrons within the quartz crystal lattice, with a proportion of this acquired charge increasing with time, named as 2.1. Chronology a luminescence emission. Single aliquot regeneration (SAR) protocols (Murray and Wintle, 2.1.1. Cores, thin sections and imageries 2003; Olley et al., 2004; Wintle and Murray, 2006) were used in this Overlapping core sections were recovered from three holes study to estimate the apparent equivalent dose of the 44-20 mm drilled in the lake-ice surface at the deepest water depth location quartz fraction for 58 to 86 separate aliquots (Table 1). Each aliquot (82 m) in June 2012. A total of 15 core sections were retrieved using contained approximately 100e500 quartz grains corresponding to a UWITEC percussion corer equipped with a locking piston allowing a 1 mm circular diameter of grains adhered (with silicon) to a 1 cm for multiple drives to be recovered from each hole. Cores were first diameter circular aluminum disc. This aliquot size was chosen to analyzed using a Siemens SOMATOM Definition ASþ 128 CT- maximize light output for the natural emissions with excitation; Scanner at INRS in Quebec City in order to establish the compos- smaller aliquots often yielded insufficient emissions (<400 photon ite sequence. The upper 4.98 m contains finely laminated sedi- counts s 1). ments, which is the focus of this analysis. A total of 35 metal trays The quartz fraction was isolated by density separations using the (each 19 cm long), filled with sediment removed from the cores heavy liquid Naepolytungstate, and a 40-min immersion in HF (Francus and Asikainen, 2001), were first flash frozen by slow im- (40%) was applied to etch the outer ~10 mm of grains, which is mersion in liquid nitrogen and then subsequently freeze dried affected by alpha radiation (Mejdahl and Christiansen, 1994). (Normandeau et al., 2019). These 19 cm-long sediment profiles Finally, quartz grains were rinsed in HCl (10%) to remove any were collected in order to have a 1 cm overlap between them. insoluble fluorides. The purity of quartz separates was evaluated by Dried sediment in the trays was then impregnated with epoxy petrographic inspection, point counting of a representative aliquot, resin (Lamoureux, 1994) and 100 overlapping thin sections were and was tested by exposing aliquots to infrared excitation (1.08 W made to cover the laminated interval. Thin sections (sediment from a laser diode at 845 ± 4 nm), which preferentially excites exposed ~5 2 cm) were digitized using a flatbed scanner at 2400 feldspar minerals. Samples measured showed weak emissions dpi (1 pixel ¼ 10.6 mm). Using the image analysis software package (<200 counts/second), at or close to background counts with 4 F. Lapointe et al. / Quaternary Science Reviews 222 (2019) 105875 infrared excitation, and ratio of emissions from blue to infrared An automated Risø TL/OSLeDAe15 system (Bøtter-Jensen et al., excitation of >20, indicating a spectrally pure quartz extract (Duller, 2000) was used for SAR analyses. Blue light excitation 2003). (470 ± 20 nm) was from an array of 30 light-emitting diodes that
Fig. 2. a) Flat-bed scan of thin section SS12-12-2-1s-C1. b) Enlargement of the black rectangle with Regions of interest (ROIs) aac to aay. C) ROIs aag to aao acquired at the scanning electron microscope are identified on the thin section, red rectangles are selected ROIs (aag to aao). Black lines indicate varve boundaries. d) Section of ss12-12-2-2P showing coarse grain-size (e) and thin varves (f). High-resolution SEM imageries enhance the ability to define varve boundaries. Black rectangles at the right of the SEM image represent the varve boundaries. Note that the sand lamina presented in panel e is a density flow. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) F. Lapointe et al. / Quaternary Science Reviews 222 (2019) 105875 5
Table 1 Optically Stimulated Luminescence (OSL) agesa on quartz from South Sawtooth Lake, Ellesmere Island.
Core/depth Lab Aliquotsb Grain- Equivalent Over- U (ppm)e Th (ppm)e K (%) H2O Cosmic dose rate Dose rate m OSL age number size (mm) dose (Gray)c dispersion (%) m Gray a 1f Gray a 1 (a 1)g (%)d
SS12-10-1-3P 0 BG4055 78/94 20e44 3.21 ± 0.34 100 ± 8 3.51 ± 0.01 13.05 ± 0.01 2.61 ± 0.01 35 ± 5 0.05 ± 0.005 3.31 ± 0.17 960 ± 120 e4cm SS12-10-1-3P BG4057 86/99 20e44 3.94 ± 0.42 116 ± 20 3.43 ± 0.01 13.3 ± 0.01 2.23 ± 0.01 35 ± 5 0.03 ± 0.003 3.03 ± 0.15 1290 ± 165 150e154 cm SS12-10-1-3P BG4058 84/112 20e44 8.84 ± 0.98 72 ± 20 3.20 ± 0.01 12.1¿ 0.01 2.69 ± 0.01 35 ± 5 0.03 ± 0.003 3.25 ± 0.16 2720 ± 355 174e178 cm SS12-12-2-3P BG4056 56/68 20e44 27.62 ± 0.95 22 ± 2 3.22 ± 0.01 13.5 ± 0.01 3.09 ± 0.01 35 ± 5 0.03 ± 0.03 3.62 ± 0.18 7630 ± 595 174e178 cm
a OSL dates are referenced to 2010 BCE. b Aliquots used in equivalent dose calculations versus original aliquots measured. c Equivalent dose calculated on a pure quartz fraction with about 40e100 grains/aliquot and analyzed under blue-light excitation (470 ± 29 nm) by single aliquot regeneration protocols (Murray and Wintle, 2003). The central age model of Galbraith et al. (1999) was used to calculate equivalent dose when overdispersion values are <25% (at one sigma errors; a finite mixture model was used with overdispersion with values > 25% to determine the youngest equivalent dose population. d Values reflect precision beyond instrumental errors; values 25% (at 1 sigma limit) indicate low dispersion in equivalent dose values and an unimodal distribution. e U, Th and K content analyzed by inductively-coupled plasma-mass spectrometry analyzed by ALS Laboratories, Reno, NV; U content includes Rb equivalent. f Cosmic dose rate calculated from parameters in Prescott and Hutton (1994). g Systematic and random errors calculated in a quadrature at one standard deviation. Datum is 2010 CE.