Dataset: BASIN 2019 Benthic Fluxes

Benthic fluxes of solutes where determined with three in-situ benthic flux chambers (BFC) according to Treude et al. (2009). The BFCs consisted of a frame equipped with a cylindrical polycarbonate chamber (inner diameter = 19 centimeters) with its lower portion sticking out of the frame. The upper side of the chamber was closed by a lid containing a stirrer (Type K/MT 11, K.U.M., Kiel, Germany), oxygen optodes (Type 4330, Aanderaa Data Instruments, Bergen Norway and Hydroflash, Contros/Kongsberg Maritime, Kongsberg, Norway), a conductivity sensor (type 5860, Aanderaa Data Instruments), and a valve for water to escape when the chamber is inserted into the sediments.

Prior to deployment by the ROV Jason, the chambers were held upside down by the ROV manipulating arms within approximately 10 meters of the seafloor and moved back and forth to make sure water from shallower depths that may have been trapped was replaced by bottom water. Chamber incubations lasted between 240 and 390 minutes. Each BFC was outfitted with a custom-built syringe sampler containing seven 50-milliliter (mL) syringes that were connected by tubes to sampling ports in the upper portion of the wall of the chambers: one injection syringe and six sampling syringes that were fired at approximately 60-minute increments, depending on the total duration of the incubation. The injection syringe contained deionized water that was purged with helium to remove oxygen and the volume of the overlaying water (typically 2.5 to 4 liters depending on sediment characteristics and chamber handling by ROV) was determined based on the observed reduction in conductivity after sensor readings stabilized (i.e., full mixing was achieved) 10-30 minutes after injection (Kononets et al., 2021). Samples obtained from the overlaying water of the BFC were examined for NH4+, PO43-, Fe2+ (according to Grasshoff et al., 1999), and NO3- (according to García-Robledo et al., 2014). Benthic fluxes of the solutes were calculated as follows:

J=Δc/Δt*V/A

Where J is the diffusive flux in millimoles per square meter per day (mmol/m²/d), Δc is the concentration change in millimoles per cubic meter (mmol/m³), Δt is the time interval in days (d), V is the overlying water volume in cubic meters (m³), and A is the surface area of the sediment covered by the benthic flux chamber in square meters (m²). One chamber per site contained 200 micromoles 5N-NO3- in the injection syringe for in-situ nitrogen cycling experiments. The injection of 15N-NO3- roughly doubled the concentration of NO3- in the overlying chamber water compared to the natural concentration. The effect of the nitrate addition on conductivity and, hence, volume determination was negligible (<1%).