Abstract
Continental margins are transitional areas between the land and the deep ocean where large amounts of particulate matter are supplied, transported, and ultimately deposited in the deep sea. High-energy hydrodynamic processes such as storms, ocean currents, or internal waves and tides, as well as bottom trawling activities contribute to the resuspension and remobilization of these particles. These mechanisms favor their transference from the continental shelf to the continental slope and maintain high concentrations of particulate matter in the water column forming nepheloid layers. The temporal evolution of the hydrographic and nepheloid structure in the upper slope off Vancouver Island (British Columbia, Canada, NE Pacific) was assessed by analyzing 4 months (from mid-August to late-November 2018) of sensor data from Ocean Networks Canada (ONC) NEPTUNE cabled seafloor observatory. The distribution of particulate matter during the study period illustrated a well-defined turbidity structure consisting of surface nepheloid layers (SNLs) at <100 m depth associated to primary and secondary productivity, intermediate nepheloid layers (INLs) between 150 and 300 m depth at the shelf-break and upper slope domain, and bottom nepheloid layers (BNLs) developed at >400 m depth. Moderate storm events occurred during fall when the more intense INLs were recorded at shelf-break depths. However, not all the INLs recorded during this period occurred in coincidence with these storms, indicating that these INL detachments were modulated by a different sediment resuspension mechanism. Analyses of fishing vessel activity during the study period revealed that trawlers operated over the same depth range as these INLs. Our results suggest that, in combination with the regional currents, the presence of continuous fishing along the continental slope off Vancouver Island contributes to the advection of suspended sediment particles, playing a major role in their transfer as nepheloid layers.
Highlights
Continental margins are the continuum between the markedly different domains of land masses and deep-ocean basins, and form a transitional area where the oceanic, continental, and atmospheric processes interact (Levin and Sibuet, 2012)
Sediment resuspension by natural and human-induced processes play a major role in the shaping of continental margins and maintain high concentrations of material in the water column in the so-called nepheloid layers (NLs) (i.e., cloudy layers within the water column containing higher concentrations of suspended particulate material (SPM) compared to the surrounding clear waters)
The aim of the present study is to investigate the hydrographic and nepheloid layer distribution through the water column in the upper slope region of Vancouver Island (British Columbia, Canada, NE Pacific), as well as to assess and discuss the potential resuspension mechanisms -natural and anthropogenic- that contribute to present-day sedimentary dynamics in this region
Summary
Continental margins are the continuum between the markedly different domains of land masses and deep-ocean basins, and form a transitional area where the oceanic, continental, and atmospheric processes interact (Levin and Sibuet, 2012). Large amounts of organic and inorganic particulate material from both terrestrial and highly productive coastal waters are supplied to continental margins (Blair and Aller, 2012; Milliman and Farnsworth, 2013; Liu et al, 2016; Kwon et al, 2021) Much of these particles do not accumulate on continental shelf regions, where intense hydrodynamic processes (i.e., storms, ocean currents, internal waves, and tides) contribute to the resuspension and remobilization of ephemerally deposited sediments, favoring their transference to the continental slope and deeper areas of the ocean (Walsh and Nittrouer, 2009). Many studies dealing with particulate matter dispersal in the oceans have noted the presence of NLs at continental margins worldwide (Gardner et al, 2018), including upwelling areas off California (Cacchione et al, 1999) and off Namibia (Inthorn et al, 2006), as well as associated with the presence of submarine canyons, for example in the NW Mediterranean (Puig and Palanques, 1998), Portuguese margin (Oliveira et al, 2002; Quaresma et al, 2007), NE Atlantic (Wilson et al, 2015a,b; Hall et al, 2017), and the US margin (Carson et al, 1986; Gardner, 1989)
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