Use of ORP and pH sensors in two-yo studies to map redox and acid-base anomalies in the seawater properties due to hydrothermal activity in shallow areas
Monday, 30 January 2017
Marina/Gretel (Hobart Function and Conference Centre)
J. Magdalena Santana-Casiano1, Melchor González-Dávila1, Eugenio Fraile-Nuez2 and Carolina Santana-González1, (1)IOCAG. Universidad de Las Palmas de Gran Canaria, Las Palmas de Gran Canaria, Spain, (2)Spanish Institute of Oceanography, Santa Cruz de Tenerife, Spain
Abstract:
The detection of activity from low-temperature hydrothermal vents, where the increase in temperature is not evident, requires the utilization of alternative sensors that respond to emissions of specific chemical species. The character of both reduced and acid of the hydrothermal emissions in the El Hierro submarine volcano allowed us to detect anomalies related with changes in the chemical potential and the proton concentration using ORP and pH sensors, respectively. Tow-yos using a CTD-rosette with these two sensors provided the locations of the emissions plotting δ(ORP)/δt and ΔpH versus the latitude or longitude. The ORP sensor responds very quickly to the presence of reduced chemicals in the water column. Changes in potential are proportional to the amount of reduced chemical species present in the water. The magnitude of these changes are examined by the time derivative of ORP, δ(ORP)/δt. To detect changes in the pH, the mean pH for each depth at a reference station in an area not affected by the vent emission is subtracted from each point measured near the volcanic edifice, defining in this way ΔpH.
A detailed survey of the volcanic edifice was carried out using seven CTD-pH-ORP tow-yo studies, localizing the redox and acidic changes, which were used to obtain surface maps of anomalies. In order to investigate the temporal variability of the system, CTD-pH-ORP yo-yo studies were also conducted that included discrete sampling for carbonate system parameters and total dissolved Fe(II), TDFe(II). Meridional tow-yos were used to calculate the amount of volcanic CO2 added to the water column for each surveyed section. The inputs of CO2 along multiple sections combined with measurements of oceanic currents produced an estimated volcanic CO2 flux = 6.0 105 ± 1.1 105 kg d-1 which increases the acidity above the volcano by ~20%.
Moreover, important anomalies in both pHT and TDFe(II) were observed from the yo-yo studies. The increased TDFe(II) concentrations and the low associated pHTvalues may be acting as an important fertilization event in the seawater around the volcano at the Island of El Hierro providing optimal conditions for the biological regeneration of the area.
Figure 1. Distribution map of ΔpH showing the areas affected by diffuse venting.
