Geology and Fluid Discharge at Dorado Outcrop, a Low Temperature Ridge-Flank Hydrothermal System

Two expeditions to Dorado Outcrop on the eastern flank of the East Pacific Rise and west of the Middle America Trench collected images, video, rocks, and sediment samples and measured temperature and fluid discharge rates to document the physical and biogeochemical characteristics of a regional, low-temperature (similar to 15 degrees C) hydrothermal system. Analysis of video and images identified lava morphologies: pillow, lobate, and sheet flows. Glasses from collected lavas were consistent with an off-axis formation. Hydrothermal discharge generally occurs through pillow lavas but is patchy, sporadic, and sometimes ceases at particular sites of discharge. Yearlong temperature measurements at five of these discharge sites show daily ranges that oscillate with tidal frequencies by 6 degrees C or more. Instantaneous fluid discharge rates (0.16 to 0.19L/s) were determined resulting in a calculated discharge of similar to 200 L/s when integrated over the area defined by the most robust fluid discharge. Such discharge has a power output of 10-12MW. Hydrothermal seepage through thin sediment adjacent to the outcrop accounts for <3% of this discharge, but seepage may support an oxic sediment column. High extractable Mn concentrations and depleted delta C-13 in the low but variable organic solid phase suggest that hydrothermal fluids provide a source for manganese accumulation and likely enhance the oxidation of organic carbon. Comparisons of the physical and geochemical characteristics at Dorado and Baby Bare Outcrops, the latter being the only other site of ridge-flank hydrothermal discharge that has been sampled directly, suggest commonalities and differences that have implications for future discoveries.