Fluid and Solute Fluxes from the Deformation Front to the Upper Slope at the Cascadia Margin

Abstract:

Fluid expulsion from accretionary convergent margins may be an important factor in global geochemical cycling and biogeochemical processes. However, the rates and distribution of fluid flow at these margins are not well known. To better understand these processes at the Cascadia margin, we collected 35 short (<1m) sediment cores and 23 Mosquito fluid flow meter measurements along a transect from the deformation front to the upper slope offshore of the Washington coast as part of a coupled heat and fluid flow survey. We identified two active seep areas, one emergent at 1990 mbsl, and one long-lived at 1050 mbsl. At both sites we observed carbonate deposits several meters thick and hundreds of meters in horizontal dimension. Thermogenic hydrocarbons measured in pore waters at the long-lived seep site indicate deeply-sourced fluids originating at >80^oC, likely migrating along faults. In addition, pore water solute profiles from the emergent seep site suggest active shallow circulation in the upper sediment column, with implications for the seep biological community and fluid budget of the margin. Pore fluid advection rates along the transect are used to characterize the geographic distribution and geologic controls on active fluid pathways. Pore water solute profiles from the sediment cores are integrated with the measured fluid advection rates to calculate solute fluxes out of the margin. Our transect of fluid flow and pore water chemistry measurements from the Cascadia margin will help to better understand fluid and geochemical cycling at accretionary convergent margins.