428 - Hydrothermal vents, are they the most important natural source of mercury to the Ocean? A study from the Mid-Atlantic Ridge.

Hydrothermal vents are potentially one of the main natural sources of mercury (Hg) to the ocean. During hydrothermal vent activity, acidic hot fluids ascend through the ocean crust leaching high amounts of metals and other elements, including Hg, from the surrounding rocks. The leached elements will enter the seawater column potentially affecting the biogeochemistry of the surrounding environments. Since Hg can bioaccumulate in the food web, quantifying Hg release from hydrothermal vents is crucial. However, Hg release from hydrothermal vents has been measured at only four locations, three of them located in the northeast Pacific Ocean and all of them showing results that vary by several orders of magnitude. In the present study, Hg concentrations of focused fluids and ambient seawater at the hydrothermal vents from TAG, Lucky Strike and Rainbow hydrothermal vent sites of the Mid-Atlantic Ridge are reported. Mercury concentrations above the vent sites (focused fluids) vary from 367 to 4,206 pM in TAG, 4 to 16,930 pM at Lucky Strike and 56 to 4,830 pM at Rainbow. Once these fluids are released into the water column, the high Hg concentrations decrease rapidly in proximity of the vent. Dilute fluids have Hg concentrations as high as 2 pM near TAG, 26 pM at Lucky Strike and 1.2 pM at Rainbow. Our findings indicate that rapid Hg scavenging is one of the main processes occurring in these hydrothermal sites, suggesting that not all deep hydrothermal vents are important Hg sources to the ocean, and previous budget may have overestimated this source.