Temperature as a Tracer for Calculating Advective Porewater Fluxes in Permeable Nearshore Sediments

Temperature measurements in the upper sediment and bottom water allowed for the calculation of in-situ porewater velocities in two nearshore permeable sediments. We calculated vertical nutrient fluxes in the upper sediment and across the sediment water interface (SWI) by coupling calculated porewater velocities with discrete porewater nutrient measurements. Also, convergences/divergences of fluxes allow for calculation of in-situ consumption/production rates. We buried a miniature thermistor chain (mTc), an array of 16 thermistors (0.002°C accuracy) attached to a stainless steel backbone, in the upper 20 cm of the sediment - an approach that minimized sampling effects on permeable sediment hydrodynamics. By measuring temperature, a conservative tracer of water velocities at the SWI and within the uppermost sediment, the advective propagation of the fluctuating temperature of the overlying water was recorded by monitoring the attenuation and phase lag of that temperature signal in the upper sediment. The overlying water temperature is largely dominated by diel heating and cooling. Because thermal transport in surficial permeable sediments is dominated by advection, the mTc can be used to calculate advective transport as much as 20 cm into the sediment (i.e., over depths where advection dominates). We found that calculated porewater velocities are sensitive to the method chosen for analysis; therefore, a variety of analytical methods were evaluated. Lastly, flow velocities in a fine-sand sediment and in a more permeable, coarse-sand sediment (both on Oahu, Hawaii) were compared over a variety of bottom-current conditions.