Date of Award

7-5-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Marine Science: Coastal and Marine Systems Science (PhD)

Department

Coastal and Marine Systems Science

College

College of Science

First Advisor

Paul T. Gayes

Second Advisor

Erin E. Hackett

Third Advisor

Samantha B. Joye

Additional Advisors

Christof D. Meile; Willard S. Moore; Richard F. Viso

Abstract

The permanently dark deep-sea, located at oceanic water depths greater than 200 m, represents the largest potential habitat space on Earth. The physicochemical conditions of the planet’s largest biome are tightly coupled to the exchange of matter and energy from terrestrial and sea-floor end-members. In fact, global ocean and climate systems are significantly impacted by deep-sea processes. Seafloor vents and seeps appear to act as geologic exchange conduits, returning recycled materials to the hydrosphere to sustain another generation of life. Despite submarine seepage having control on global elemental cycling, it is estimated that less than 1% of the deep-sea has been mapped in detail sufficient to truly understand the spatial extent of regions of especially active material and energy exchange at regions of seafloor venting and seepage. Slow-flow discharge occurring at elevated temperatures (hydrothermal seepage) is suspected to exchange ~ 90% of the water required to balance heat budgets as compared to energetic vents. Deep-sea seepage occurring at ambient ocean temperatures (cold seeps), first discovered in the Gulf of Mexico, represents a second seepage environment where chemosynthetic primary production supports some of the most diverse biomes in the bathypelagic zones. However, methods and research directly applicable for understanding the rate of fluid discharge at low-flow submarine seepage sites are lacking, resulting in poorly constrained global chemical cycling estimates. This Dissertation provides a vertical exchange model designed to determine an effective fluid flux of porefluid from deep-sea environments. The vertical exchange model utilizes vertical distributions of aqueous 224Ra in porefluid recovered from regions impacted by hydrothermal and cold seepage and determines porefluid residence time related to radiogenic changes attributable to production and decay. The vertical exchange model is qualitatively tested whereby isotope proxy estimates confirm seepage in areas where seepage is indicated by ancillary evidence and suggest porefluid transport into the sediments best explains vertical isotope distributions observed for Control core. The vertical exchange model is applied to a hydrothermal site in Guaymas Basin to test whether spatial associations between microbial mats and seepage rates exist. We identify spatial relationships between subsurface temperature range and fluid flux where white colored microbial colonies exist; however, fluid flux appears unrelated to subsurface temperature range where orange filaments are found. Fluid flux estimates for sampled regions within both sites were observed to be similar despite the unique thermal source present only at Guaymas Basin. This work offers a novel approach to quantify fluid flow both into and out of the sediments across a variety of deep-sea habitats where seepage moderates the success of unique benthic ecosystems.

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