Tracing submarine hydrothermal inputs into a coastal bay in Baja California using radon

Hydrothermal fluid fluxes into deep ocean environments can be obtained from heat balance approaches. However, in shallow systems, hydrothermal heat fluxes can be masked by solar heating. In this paper, we use radon (²²²Rn) as a naturally occurring geochemical tracer to map the location of hydrothermal fluid inputs, as well as low-temperature groundwater discharges, and quantify fluxes into Concepcion Bay, Baja California, Mexico. This fault-bound bay contains intertidal seeps with salinities ranging from 5.3 to 25.6, temperatures reaching 64 °C, and nitrate reaching 900 μM. The bay is subject to natural eutrophication and frequent red tide events. A detailed ²²²Rn survey around the 100-km perimeter of Concepcion Bay allowed us to map the location of enhanced submarine groundwater inputs. Moorings at three contrasting coastal sites indicated that radon concentrations were higher at low tide and during the winter. Modeled hydrothermal fluid inputs ranged between 0.4 cm/day in the middle of the bay and 43.9 cm/day at the largest hydrothermal coastal seep site. Apparently, faults allow meteoric water to be heated and serve as conduits for its subsequent discharge through permeable marine sediments. When conservatively extrapolated to the entire bay using weighted distributions, these fluxes are estimated at 17.5 m³/s, a flow much higher than local ephemeral rivers. About 42% of the fluxes described consisted of fresh groundwater with the remaining made up of recirculated seawater. New nitrogen inputs associated with groundwater pathways are estimated to directly account for at least 15% of the local primary productivity. Our combined spatial survey/time series strategy can be very useful to quantify hydrothermal fluid inputs in particular at vent sites where a temperature signal in shallow surface waters is difficult to be observed.