Observations collected by AUV Sentry, ROV Jason-II (Dec 2013, AT26-09) and submersible ALVIN (Dec 2014, AT26-24) on Dorado Outcrop; chemical composition of rock samples, discrete temperature data and long-term temperature and dissolved oxygen data

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 (~15 °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 °C or more. Instantaneous fluid discharge rates (0.16 to 0.19 L/s) were determined resulting in a calculated discharge of ~200 L/s when integrated over the area defined by the most robust fluid discharge. Such discharge has a power output of 10-12 MW. Hydrothermal seepage through thin sediment adjacent to the outcrop accounts for