Biological and Environmental Research - Earth and Environmental System Sciences
Earth and Environmental System Modeling

Combination of ORP and pH sensors in tow-yo studies to map redox and acid-base anomalies in seawater due to hydrothermal

Thursday, December 15, 2016 - 08:00
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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 certain chemical species. The character of both reduced and acid fluids in the volcanic 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 with these sensors provided the approximate 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. The magnitude of this change is examined by the time derivative of ORP, δ(ORP)/δt. For pH changes, ΔpH, 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.

The submarine volcano of El Hierro, in its degasification stage, provided an excellent opportunity to apply CTD-pH-ORP tow-yo methodology and to study the effect of CO2 emission on the seawater carbonate system, the global carbon flux, and local ocean acidification. Detailed surveys of the volcanic edifice were carried out during VULCANO 0314 and VULCANA0615 cruises using several CTD-pH-ORP tow-yo studies, localizing the redox and acidic changes, which were used to obtain surface maps of anomalies. CTD-pH-ORP yo-yo studies were also conducted that included discrete sampling for carbonate system parameters and total dissolved Fe(II), TDFe(II). The inputs of COalong multiple sections combined with measurements of oceanic currents produced an estimated volcanic CO2 flux = 6.0 105 ± 1.1 105 kg d-1and increases the acidity above the volcano by ~20%.

From the yo-yo studies important anomalies in both pHT and TDFe(II) were observed. The increased TDFe(II) concentrations and the low associated pHT values 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 regeneration of the area.

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