A high-resolution elemental record of post-glacial lithic sedimentation in Upernavik Trough, western Greenland: History of ice-sheet dynamics and ocean circulation changes over the last 9100 years
Introduction
Compared to other sectors of the Greenland Ice Sheet (GIS), western and northwestern Greenland from 69°N to 76°N mainly hosts fast moving glaciers with speeds commonly exceeding 3 km/yr (Rignot and Mouginot, 2012; MacGregor et al., 2016). This spatial heterogeneity in ice flow speed illustrates the role on ice sheet dynamics of marine terminating glaciers which drain most of west and northwest Greenland. The cumulative drainage area of western and northwestern glaciers is in the order of 460,000 km2 and represents 30% of the total GIS surface (Rignot and Mouginot, 2012). The northward transport of warm Atlantic Water (AW) carried by the West Greenland Current (WGC) in eastern Baffin Bay and its role on the direct melting of major tidewater glacier termini as well as on regional atmospheric temperatures and precipitations explain most of the modern changes in ice flow and mass loss of outlet glaciers in this sector (Chauché et al., 2014; Sakakibara and Sugiyama, 2018). Marine geological studies of sediment cores collected over the slope and within cross-shelf troughs off western Greenland have demonstrated that ocean warming linked to the initiation of the WGC ca. 15 cal. Kyrs BP was pivotal in driving, together with the orbitally-induced increase in boreal summer temperature, the retreat of the grounded GIS from the shelf edge to its near-modern position during the last deglaciation (Sheldon et al., 2016; Jennings et al., 2017; Jennings et al., 2018). This ocean – ice-sheet connection is seen as a persistent feature explaining mid to late Holocene glacier changes (advances, retreats) in local sectors of central West Greenland (Schweinsberg et al., 2017). Both ice-sheet model simulations (Lecavalier et al., 2014) and 10Be and 14C dating of ice-marginal systems (Young and Briner, 2015; Sinclair et al., 2016; and references therein) suggest that the GIS retreat from the west Greenland shelf (WGS) was completed by ca. 10 cal. Kyrs BP. This extensive deglacial retreat of the western GIS was accompanied by a massive delivery of terrigenous material to the shelf and slope via iceberg calving, meltwater plumes and sediment gravity flows. These reached the shelf edge through a series of bathymetric troughs interpreted as paleo ice-streams (Fig. 1), namely from south to north Disko Trough, Uummannaq Trough, Upernavik Trough, and Melville Bay (Cofaigh and Dowdeswell, 2001; Hogan et al., 2016; Sheldon et al., 2016; Newton et al., 2017). Following the last deglaciation, sedimentation over the WGS is assumed to derive mainly from iceberg rafting and sediment plumes from proximal and southerly sources and is subjected to subsequent redistribution under the influence of the northward flowing WGC (Andrews et al., 2018). Holocene changes in primary productivity and sedimentation of marine organic matter over the west Greenland shelf are intimately linked to sea ice dynamics. Sea ice and surface conditions in eastern Baffin Bay are mainly constrained by temperature and salinity variations of the WGC, which are themselves controlled by the changing contribution of Irminger water to this current and the variable input of freshwater from the bordering GIS (e.g. Myers et al., 2007). Baffin Bay ice cover is also controlled by the North Atlantic Oscillation (NAO) through its effect on regional atmospheric temperatures. Accordingly, cooler air temperature and greater sea ice extent in Baffin Bay generally occur during positive phase of the NAO (Wang et al., 1994; Stern and Heide-Jørgensen, 2003).
High resolution, continuous biotic proxy records obtained from marine sediment cores off western Greenland, especially in the Disko Bay region, have been widely used in the past 10–15 years to shed light on the intricate connection between hydrographic conditions and the behavior of the western GIS over the Holocene (Lloyd et al., 2005; Lloyd et al., 2007; Moros et al., 2006; Perner et al., 2012; Perner et al., 2013; Sha et al., 2014; Krawczyk et al., 2016; among others). More recently, mineralogy has been used to assess the provenance of sediment from the various ice-sheets draining into Baffin Bay over the last glacial cycle (Andrews et al., 2012; Simon et al., 2013) by applying a sediment-unmixing program to the bulk mineralogy of marine sediments and taking into account the bedrock geology underlying major ice-streams (Andrews and Eberl, 2011). This methodological approach was pivotal in distinguishing eastern (Greenland), northern (Ellesmere Isl.), and western (Baffin Isl.) contributions to the late Pleistocene sedimentation in Baffin Bay, and hence to discuss the synchroneity in the behaviors of the ice-sheets which drained these regions (Andrews et al., 2018; and references therein).
A thorough compilation of chemical analyses of stream sediment samples undertaken by the Geological Survey of Denmark and Greenland (Steenfelt, 2001) showed that the distribution patterns of many major and trace elements closely reflects the various rock complexes of western Greenland, and discriminates local bedrock formations drained by distinct, major marine terminating glaciers between 68°N and 73°N. The present study investigates the geochemical (elemental) composition of a marine sediment core collected over the shelf of western Greenland in order to track potential changes in the delivery of local and distal glacial sources over the last 9100 years. The reconstructed continuous, high resolution patterns in lithogenic provenance might unravel distinct post-glacial behaviors of major marine-terminating glaciers of western Greenland, as well as Holocene changes in the strength of the main process (WGC) of transport and redistribution of lithic material over the west Greenland shelf.
Section snippets
Environmental and geological setting
The main features of the onshore geology of western Greenland as well as the hydrography and bathymetry of the adjoining shelf are summarized in Fig. 1. We restricted this short summary to the region extending from ca. 68°N to 75°N which includes some of the main proximal and distal glacial tributaries which are thought to contribute to the lithic sedimentation at the studied core location (Upernavik Trough).
The bedrock of western Greenland varies in age from Archaean to Paleogene (Escher and
Material and methods
The 734-cm long giant, square, gravity core AMD 204 CQ (hereafter referred to as 204 CQ) studied here was collected at the head of Upernavik Trough (73°15.66′N, 57°53.98′W, 987 m. water depth) as part of the 2014 ArcticNet expedition of the CCGS Amundsen. The core was immediately sampled onboard using large U-channels. The lithology is homogeneous and consists of bioturbated clayey silts (Caron et al., 2018).
Sedimentological data
Core 204 CQ consists of homogeneous bioturbated hemipelagic clayey silts with very rare and scattered occurrence of ice-rafted debris (IRD > 2 mm) (Caron et al., 2018). The sediment median grain size (D50) which splits the distribution with half above and half below this diameter, ranges from ca. 3 to 6 μm and contains a short interval of dominantly silty clay centered at 6.8 cal. Kyrs BP (Fig. 4). Highest grain sizes recorded in core 204 CQ fall within coarse and very coarse silts (31–63 μm)
Discussion
The delivery of lithic material to the WGS is assumed to be essentially triggered by processes which are common to such high latitude settings: (1) ice loss from summer surface melting and runoff at the margin of the ice sheet, a process which is further accelerated in marine terminating glaciers under the influence of ocean melting of basal or frontal ice, and (2) the erosion of exposed (unglaciated) coastal formation as well as remobilization of shallow shelf sediment. Sea ice cover and
Conclusions
The high resolution geochemical (elemental) proxy records presented in this study reveal major changes in the provenance of lithogenic material in the Upernavik Trough, northern sector of the WGS, over the last 9100 years. Our records reflect the distinct dynamics of the main proximal and distal (southern) marine terminating glaciers of Western Greenland as well as changes in the efficiency of sediment transport and redistribution by the WGC.
Contributions from the proximal Melville Bay glaciers
Declaration of Competing Interest
None.
Acknowledgments
This work is a contribution to the GREENEDGE project funded by the French Agence Nationale de la Recherche and the Fondation Total, and coordinated by Marcel Babin (Takuvik, CNRS-University Laval, Canada). We acknowledge additional funding provided by the Initiative d'Excellence (IdEx) programme of the University of Bordeaux and by the Natural Science and Engineering Research Council of Canada (NSERC), as well as support from the Network of Centres of Excellence ArcticNet. Kerstin Perner,
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