Characterizing the Unique Chemical Imprint of On‐Axis, Lower‐Temperature Hydrothermal Flow to the Deep Ocean (Southern East Pacific Rise, 16.5°–18.0°S)

Abstract In addition to high‐temperature vents, lower‐temperature flow (LTF) (<300°C) is abundant along mid‐ocean ridges and contributes globally‐important fluxes of heat and water along with largely‐unconstrained geochemical influences on the ocean. We examined the impact of on‐axis LTF on the chemical composition of the overlying water column (<40 m above seafloor) along the 16.5°–18.0°S sector of the ultrafast‐spreading southern East Pacific Rise using autonomous underwater vehicle Sentry surveys and conductivity‐temperature‐depth rosette casts. LTF sites were typically spatially isolated from high‐temperature systems and imparted unique chemical signatures to the overlying ocean. Water column samples impacted by LTF exhibited low particulate iron:sulfur ratios and high methane: total‐dissolvable manganese ratios, whereas samples influenced by high‐temperature venting exhibited opposite trends. We confirmed that LTF imparts a distinct and measurable chemical signature to the water column, independent from high‐temperature vents. Isolated, on‐axis LTF will be important to consider when assessing hydrothermal circulation impacts upon ocean biogeochemistry.