Geology and Fluid Discharge at Dorado Outcrop, a Low Temperature Ridge‐Flank Hydrothermal System. (29th January 2019)

• Record Type: Journal Article
• Authors: Wheat, C. Geoffrey; Hartwell, Anne M.; McManus, James; Fisher, Andrew T.; Orcutt, Beth N.; Schlicht, Lucy E. M.; Niedenzu, Sara; Bach, Wolfgang
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 20:Number 1(2019)
• Page Start: 487
• Page End: 504
• Publication Date: 2019-01-29
• Subjects: ridge flank -- Dorado Outcrop -- seafloor geology -- hydrothermal -- fluid discharge -- sediment; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2018GC007933
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: 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 <3% of this discharge, but seepage may support an oxic sediment column. High extractable Mn concentrations and depleted δ 13 C 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. Plain Language Summary: Below the seafloor in the upper volcanic crust (underneath the sediment) seawater circulates through permeable rocks much like groundwater flows through aquifers on land. This circulating flow of seawater through the oceanic crust transports 25% of the global heat loss, most of it as cool hydrothermal seawater on the flanks of mid‐ocean ridges. However, only two sites of this low‐temperature hydrothermal discharge have been discovered, even though many such sites must exist. This study presents and links geologic, chemical, and hydrologic observations from one of these two discharge sites. We show that discharge is patchy and sporadic, and it occurs through pillow lava formations that were formed by off‐axis volcanism. Measured hydrothermal fluid discharge rates are consistent with regional models. The low‐temperature discharge does not alter basaltic rocks at the seafloor even those rocks that were collected from areas of fluid discharge. However, the hydrothermal flow does affect the chemistry of the thin sediment next to the outcrop. This work provides the foundation for understanding fundamental processes that have shaped Earth's history. Key Points: Dorado Outcrop is a midplate outcrop formed off axis and is a site of low‐temperature hydrothermal discharge focused through pillow lavas Observations from one 25‐m 2 area indicate discharge of ~200 L/s with a power output of 10–12 MW, equivalent to three to four high‐temperature vents Exposed basalts are altered by oxygenated seawater, and fluid seepage through thin sediment mobilizes Mn and promotes carbon consumption