Impact of bottom currents on deep water sedimentary processes of Canada Basin, Arctic Ocean
Graphical abstract
Introduction
Canada Basin is the largest sedimentary basin of the Amerasian Basin within the Arctic Ocean (Fig. 1). Today, the basin is morphologically enclosed; thus, its deepest waters are effectively isolated from thermohaline circulation of the global oceans. Very little is known about how deep waters in Canada Basin circulate, and at present, there are too few near-bottom measurements in deep water to adequately constrain the strength or direction of modern currents that might influence sediment on the seafloor. In their absence, a greater understanding of the sedimentary record of circulation in the deep basin may provide useful context for modern changes as the basin responds to a shifting climate.
Perennial ice cover in Canada Basin has restricted geoscientific and oceanographic investigations in the region. Initial investigations of the hydrography and sedimentology of the basin were conducted from ice camps established on drifting ice floes and ice islands (e.g., Schindler, 1968). It was not until the early 1990's that the first scientific icebreakers managed to successfully acquire data in the region. In the past decade, Canada Basin has been the focus of an international effort to gather geological, geophysical and bathymetric data in support of extended continental shelf mapping to delineate coastal State jurisdictions under the United Nations Convention on the Law of the Sea (UNCLOS). These data, in addition to international datasets acquired for various other scientific purposes, have facilitated a rapid advancement in understanding of the geologic history and sedimentology of the basin (see Coakley et al., 2016, for example).
It is hypothesised that, far from being a stagnant ocean basin due to its geomorphic isolation and perennial sea ice cover, Canada Basin is a dynamic oceanographic environment and deep currents within the basin have reworked and redeposited sedimentary material around its flanks. The Arctic Ocean has undergone pronounced changes during the Neogene due to continued opening of Eurasia Basin, opening and closing of shallow seaways that connect it to the global oceans, sea level fluctuations exposing and submerging its vast continental shelves, and pronounced glaciations that influenced sea ice cover and affected its surrounding landmasses and sources of freshwater input (e.g., Jakobsson et al., 2007, 2014). These factors must have had a dramatic effect on circulation within the basin. This study will investigate sedimentary deposits within Canada Basin to identify deposits related to bottom current activity and assert the nature of these bottom currents and the degree of influence they may have had on the overall Quaternary evolution of the basin. Subbottom profiler, multibeam bathymetric data and a few shallow sediment cores acquired as a result of the aforementioned programs are the principal sources of information for this study.
Canada Basin lies within the larger Amerasia Basin and forms the largest, deepest, and oldest portion of the Arctic Ocean basin (Fig. 1). It lies between about 110o and 160oW longitude and 70o to 85oN latitude. The basin is bound to the south by the Canadian and U.S. Alaska Beaufort margins, to the east by the Canadian Arctic Archipelago (CAA), to the west by Northwind Escarpment of Chukchi Borderland and to the north by Alpha Ridge (Fig. 1). It includes Canada Basin Abyssal Plain (CBAP), and Nautilus and Stefansson basins in the north as well as the continental margins of the adjacent Canadian and U.S. land masses (Fig. 1). It measures 1500 km north to south and 650 km west to east, totaling about 1,000,000 km2 in area.
The geologic setting including geomorphology of Canada Basin is described in detail in Mosher and Hutchinson (2019). Since the initial opening of the Amerasian Basin in the Cretaceous, sediments have infilled Canada Basin from its surrounding continental shelves (Clark et al., 1980; Grantz et al., 1996, 2011; Mosher and Hutchinson, 2019). The earliest basin-fill comprises synrift sediments from the Alaska-Beaufort margin. From the Eocene to Oligocene, the dominant sediment source shifted to the CAA, likely as a result of uplift during the Eurekan Orogeny. Glacial input during Quaternary glaciations also sourced from the CAA along this segment of the margin. The latest phase of sediment input is from the Mackenzie River in the southeast quadrant of the basin. Since Miocene time, the Mackenzie River and its predecessors constructed a large delta and fan complex (>200,000 km2 in area) (Mosher and Hutchinson, 2019). Sedimentary deposits thin away from the fan in a concentric pattern throughout the basin. Resulting deposits are largely thought to be turbidites and plume deposits (Campbell and Clark, 1977; Clark et al., 1980; Grantz et al., 1996, 2011).
The result of such sedimentary processes is that seafloor sediments of the CBAP are remarkably flat, mostly lying between 3800 and 3900 m water depth (Fig. 2). Even shallow reflections on subbottom profiler records within the basin are coherent for 10's to 100's of km. These reflections show onlap against the Alaska-Beaufort slope, Northwind Ridge and the western portion of Alpha Ridge. There is no evidence of syn- or post-depositional deformation of these reflections (i.e. faults, folds), which suggests that the basin has not subsided or endured any active tectonics in its most recent history (Mosher and Hutchinson, 2019).
Water masses are highly stratified throughout Canada Basin (Fig. 3). As all water in the Arctic is less than 2 °C, this density stratification is determined by salinity, not temperature (Rudels et al., 2012). Pacific water enters the Arctic Ocean through the Bering Strait; a fairly narrow (∼ 85 km wide) and shallow passage (∼ 50 m deep), through which about 0.8 Sv of water enters (Woodgate, 2013). This water mass is cold (<-1 °C) and fresh (31.9-33 psu) and forms the top ∼200 m of the ocean surface layer, aside from a shallow (5-10 m) mixed layer (Fig. 3A). Circulation of Pacific Water is complex, driven in part by oscillations in the Beaufort Gyre and prevailing winds as well as constrained by potential vorticity (PV) conservation. Along the Beaufort Shelf, it is also affected by slope/canyon upwelling, mixing and down-welling of Atlantic Water flowing across the Chukchi Plateau and its borderlands (Fig. 3B). Carpenter and Timmermans (2012) introduce the effect of vertical meso-scale eddies that play a role in water mass delivery to the basin, particularly emanating from the Chukchi Plateau region. Eddies also play a significant role in episodic lateral sediment delivery throughout Canada Basin, as discussed by O'Brien et al. (2013).
Atlantic water enters the Arctic Ocean through Fram Strait and the Barents Sea (Woodgate, 2013). This water mass is generally saltier (>34 psu) and warmer (> 0 °C) (Fig. 2A) and about 10 times greater in volume (7 to 10 Sv) than the Pacific inflow (Woodgate, 2013). Atlantic Water flows cyclonically in Eurasia Basin with branches crossing Lomonsov Ridge into the Amerasia Basin via deeper saddles in the ridge (e.g., Björk et al., 2018). In Canada Basin, Atlantic Waters form a temperature maximum (up to ∼1 °C) at depths of around 200 to 400 m and remain clearly identifiable by its temperature anomaly to about 1500 m depth (Fig. 3A). The depth to which the Atlantic Waters extend is not well known. A separate water mass known as Canada Basin Deep Water (CBDW) lies below the Atlantic Water (Timmermans et al., 2003; Rudels et al., 2012). This water mass is colder and saltier than waters above and is remarkably homogenous (see Fig. 2A) (Timmermans et al., 2003). The enhanced salinity of the CBDW relative to other Arctic Ocean deep water indirectly suggests ventilation of the CBDW is primarily driven by cascading brines from the adjacent shelves (Luneva et al., 2020). There is no modern evidence of cascading shelf brines ventilating deep water in the basin; however, radiogenic isotope studies of core tops in Amerasia Basin suggest shelf-basin exchanges were more common in the past, likely sufficient to ventilate the CBDW (Haley and Polyak, 2013). Björk et al. (2010) suggest some CBDW passes across Lomonosov Ridge to circulate in Amundsen Basin. They detect its water mass characteristics at about 2000 m; thus, it presumably passes across Lomonosov Ridge only at its deepest saddles, such as near the North Pole. Since CBDW renewal is greatly restricted, its ventilation age is significantly older than other Arctic Ocean deep water, ranging between ∼360 to more than 450 years old based on tracer and radiocarbon isotope data (Schlosser et al., 1997).
Atlantic Waters (AW) and presumably Canada Basin Deep Waters form part of a pan-Arctic boundary current system known as the Arctic Ocean Boundary Current (AOBC) (Rudels et al., 1999; Woodgate et al., 2001). Temperature-Salinity and tracer data suggest the AOBC follows topographic slopes cyclonically around the basins and along the ocean ridges, with the core of the current lying between the ∼500-3000 m water depth (Fig. 2B) (Aagaard, 1989; Rudels et al., 1999; Woodgate et al., 2001). AOBC is poorly measured in the deep parts of Canada Basin but the implied cyclonic circulation of the boundary current opposes the anticyclonic circulation of surface Pacific Water and ice in the overlying Beaufort Gyre. It is presumed to be weak, but still topographically steered (Jones, 2001; Woodgate et al., 2007).
Section snippets
Methods
The bulk of the subbottom profiler and multibeam bathymetric data used in this study were acquired during multiple international joint icebreaker expeditions in the Arctic Ocean between 2008 and 2016 aboard the CCGS Louis S. St-Laurent and USCGC Healy. Additional data were acquired during expeditions of the R/V Sikuliaq. Subbottom profiler data from the Louis S. St-Laurent and Healy were collected using hull-mounted Knudsen 3260 and 320B/R CHIRP systems, respectively. Sikuliaq uses a Topas
Alaska-Beaufort slope
Alaska-Beaufort Slope forms the southern limit of Canada Basin (Fig. 1). This slope shows a relatively consistent 3 to 4° regional gradient, lessening to 2° along the lower slope. Downslope-trending channels and valleys with relatively straight axes and little sediment infill dominate the morphology of the slope (Fig. 4). These channels incise the slope to 500 m depth. Lobes of sediment deposits are apparent at the base of slope (Fig. 4). These lobes appear to curve eastward and align with
Discussion
Four sedimentation processes predominate along clastic continental margins; 1) pelagic and hemipelagic settling that produce (hemi)pelagites, 2) sediment mass failure produces mass transport deposits (MTD), 3) turbidity currents result in turbidites, and 4) contour currents produce contourites (Rebesco et al., 2017). As there is a continuum between these processes, distinction of them is sometimes difficult, especially where data are limited. Rebesco and Stow (2001) and Stow and Smillie (2020)
Conclusions
Canada Basin is a small, tectonically stable, confined ocean basin with a modern single point source of sediment input. Its shallow passageways to the Pacific and Atlantic oceans restrict inflow and outflow and these corridors were presumably even more restrictive or blocked during lower sea level stands. Additionally, both its oceanography and its sedimentation history were heavily influenced by past glacial epochs. In many ways, this makes it an ideal natural laboratory to investigate
CRediT authorship contribution statement
Dr. Mosher acquired the funding, conceived of the idea for the paper, and led the field acquisition programs. Mr. Boggild conducted the bulk of the data processing and mapping efforts under the supervision of Dr. Mosher. Dr. Mosher conducted the bulk of initial drafting of the text. Otherwise, Dr. Mosher and Mr. Boggild have shared equally in data acquisition, analyses, writing and editing, and figure drafting. This study relies on international data sets and contributions from many individuals
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are indebted to the many people involved in data acquisition in the Arctic Ocean on which this manuscript is based. Many organizations and individuals have worked tremendously hard to pry out the secrets hidden beneath the Arctic ice and water. In particular, the authors express their appreciation to the extended continental shelf mapping programs of Canada and the United States; a highly collaborative affair between the two countries which provided exceptional opportunities for the
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