Distinctive Sources of Fluids in Different Environments of the Beaufort Sea Seafloor

Abstract:

In the Canadian Arctic shelf and upper slope, the thermal disturbance caused by sea level rise at the end of the last glacial period, is still propagating into the subsurface and heating shelf sediments, where submerged terrestrial permafrost and gas hydrate, and marine gas hydrate occur in close proximity. Studies conducted on the Canadian sector of the Beaufort Sea shelf and slope show evidence of gas venting in three distinct environments: in Pingo-Like-Features (PLF) on the mid-shelf, along the shelf edge, and in ~1 km wide circular topographic highs in 290, 420 and 790 m water depths on the continental slope. The chemical and stable isotopic composition of fluids found in these environments and in background sediments where determined on pore waters obtained from piston and gravity cores. Isotopic and major ion chemistry results from the shelf and shelf edge indicate downcore mixing between seawater and a freshwater end-member with a Mackenzie River-like composition. In contrast, water from the PLFs display an isotopic composition to the left of the Local Meteroric Water Line (LMWL) suggesting a source different from current surface and ground waters found onshore. Waters from sediments of the slope topographic highs are isotopically and chemically evolved: They display different degrees of mixing between seawater and a Na-Cl-HCO₃ type brackish component, plotting to the right of the LMWL, and possibly reflect extensive interaction with clays. Pore waters collected in background sediments of the slope down to 750 m, also show freshening downcore, but as opposed to the topographic highs found at the same water depths, their isotopic composition indicate the contribution of a Mackenzie River-type fresh water end-member. Overall, results presented here indicate that the shelf and slope of the Beaufort sea experience vertical fluid advection, but the sources of these waters are different: The background shelf and slope show influence of freshwater similar to the present meteoric water, the topographic highs in the slope probably tap a much deeper source with waters that experienced a large degree of chemical evolution, and the PLFs in the mid shelf have isotopic characteristics that may suggest meteoric water that precipitated under different climatic conditions from the present.