Rethinking the Use of Seabed Sediment Temperature Profiles to Trace Submarine Groundwater Flow

Abstract

Submarine groundwater fluxes across the seafloor facilitate important hydrological and biogeochemical exchanges between oceans and seabed sediment, yet few studies have investigated spatially distributed groundwater fluxes in deep‐ocean environments such as continental slopes. Heat has been previously applied as a submarine groundwater tracer using an analytical solution to a heat flow equation assuming steady state conditions and homogeneous thermal conductivity. These assumptions are often violated in shallow seabeds due to ocean bottom temperature changes or sediment property variations. Here heat tracing analysis techniques recently developed for terrestrial settings are applied in concert to examine the influences of groundwater flow, ocean temperature changes, and seabed thermal conductivity variations on deep‐ocean sediment temperature profiles. Temperature observations from the sediment and bottom ocean water on the Scotian Slope off eastern Canada are used to demonstrate how simple thermal methods for tracing groundwater can be employed if more comprehensive techniques indicate that the simplifying assumptions are valid. The spatial distribution of the inferred groundwater fluxes on the slope suggests a downward groundwater flow system with recharge occurring over the upper‐middle slope and discharge on the lower slope. We speculate that the downward groundwater flow inferred on the Scotian Slope is due to density‐driven processes arising from underlying salt domes, in contrast with upward slope systems driven by geothermal convection. Improvements in the design of future submarine hydrogeological studies are proposed for thermal data collection and groundwater flow analysis, including new equations that quantify the minimum detectable flux magnitude for a given sensor accuracy and profile length.

Mixed-Convective Processes Within Seafloor Sediments Arising From Fresh Groundwater Discharge

“…Conversely, the upward movement of lower-density groundwater (e.g., as expected to arise during SFGD) is rarely explored in the solute transport context, although upward, buoyancy-driven groundwater flow has received significant attention in the field of heat transport and geothermal phenomena. For example, Kurylyk et al (2018) utilized temperature-based methods to quantify submarine groundwater fluxes in seafloor sediments offshore of eastern Canada. However, their study suggested that the flow patterns inferred from seafloor temperature-depth profiles appear to be influenced by both density-driven and geothermal processes.…”

Theory, tools, and multidisciplinary applications for tracing groundwater fluxes from temperature profiles

“…Seabed TD profiles in these settings are often curved and thus not linear as expected under conduction‐dominated, homogeneous, and steady‐state conditions. Several studies have attributed this curvature to the impacts of groundwater flow (Figure b) and have quantified submarine groundwater fluxes or geothermal circulation based on steady‐state thermal groundwater tracing methods described in Section 3 (Abbott & Menke, , ; R. N. Anderson, Hobart, & Langseth, ; Fisher & Becker, ; Geller, Weissel, & Anderson, ; Kurylyk et al, ; Wheat et al, ). However, such analyses can yield highly uncertain vertical groundwater fluxes because TD curvature may also be attributed to other factors such as vertical variations in seabed thermal conductivity and low‐frequency bottom water temperature changes (Noel, ).…”

“…The estimation of q using Equation relies on the deviation of a TD profile from a linear relationship. The maximum deviation ( Δ max ) is (Kurylyk et al, ): …”

“…Thermal probes used to record seabed TD profiles are normally lowered from marine vessels and slowly driven into the seabed sediment with large weights (Wright & Louden, ). In this way, seabed thermal data can be recorded even with overlying ocean depths of several kilometers (Kurylyk et al, ). Instruments to thermally monitor shallow coastal zones are often similar to those applied in streambed applications (e.g., Wilson et al, ).…”

Using Heat to Trace Vertical Water Fluxes in Sediment Experiencing Concurrent Tidal Pumping and Groundwater Discharge

“…Heat has been widely applied to trace groundwater-surface water exchanges in inland surface water bodies such as streams, rivers, and lakes as detailed in prior reviews (Constantz, 2008;Irvine et al, 2017;Kurylyk et al, 2019;Rau et al, 2014). However, these methods have only rarely been applied in tidal estuaries (Befus et al, 2013;Henderson et al, 2009), lagoons (Swain & Prinos, 2018;Tirado-Conde et al, 2019) or offshore environments (Goto et al, 2005;Kurylyk et al, 2018;Wilson et al, 2016). To our knowledge, previous studies have not directly addressed the influence of tide-driven oscillations in vertical water fluxes when using conventional heat as a tracer techniques.…”