Documenting the suboxic zone of the Black Sea via high-resolution real-time redox profiling

We coupled an in situ electrochemical analyzer to a CTD pump profiler system to measure redox species across the oxic-anoxic interface of the Black Sea water column. Voltammetry was performed using gold-amalgam working electrodes to measure simultaneously oxygen and dissolved sulfur species (S₈, S_x^{2 -}, HS^-/H₂S) both in situ (at <1 m vertical intervals) and in an on-deck flow cell attached to the outflow of a pump profiler (vertical resolution of about 1.5 m). In situ data agreed with measurements made in the flow cell and with measurements made from samples collected by rosette bottle casts. In situ voltammetry provided undisturbed, high-resolution measurements, and revealed significant yet subtle features not seen by traditional methods because of small spatial separation between the features and the measurements. Layers of oxygen intrusion (<5 m thick, from 10 to 150 μM O₂) were present within the suboxic zone of the southwest Black Sea that are not present in the west-central and northeast Black Sea. Oxygen injection also occurs at other depths throughout the southwest and corresponds with small temperature anomalies, suggesting influence by Bosporus inflow up to 150 km from its entrance to the Black Sea. Such an inflow of oxygen, as well as spatial variations of the halocline, affect both manganese and, subsequently, sulfide oxidation for a large portion of Black Sea intermediate water (H₂S onset occured ∼60 m deeper in the southwest as compared to the west-central). In situ voltammetric analyses provided rapid redox information, thus enabling more accurate targeting of specific geochemical features by the CTD rosette package.