Date of Award

4-30-2020

Document Type

Thesis

Degree Name

Master of Science in Coastal Marine and Wetland Studies

Department

Coastal and Marine Systems Science

College

College of Science

First Advisor

Richard N. Peterson

Second Advisor

Richard F. Viso

Third Advisor

Angelos Hannides

Additional Advisors

Andreas Teske

Abstract

Analysis of the distribution of radium through sedimentary porewater profiles is a promising method for constraining diffuse discharge/recharge flow rates in hydrothermal settings; these parameters are critical for evaluating hydrothermal circulation and its chemical and biological implications. The short-lived 224Ra (T1/2 = 3.54 days) is an effective tracer due to its behavior in marine environments and changes in activity due to ingrowth/decay. However, the distribution of radium may be influenced by its varying solid:aqueous partitioning coefficients (Kd) and is critical to understand for any study utilizing radium as a tracer. This project explores Kd across the hydrothermally active Guaymas Basin in the Gulf of California to understand the potential influence of partitioning by various environmental conditions, such as the rate of flow through hydrothermal sediments. A secondary goal of this project was to detect a measurable radium signature within the water column over areas of hydrothermal venting, such that this tracer may ultimately be useful to constrain mixing of the hydrothermal plume with ambient waters. Sixteen sediment cores across the southern basin were collected on cruise AT42-05 in November 2018, using HOV Alvin. Differing flow conditions were targeted in background sediments, areas of suspected diffuse flow marked by microbial mats, and areas of visual discharge. Porewater from cores sectioned in 4 cm intervals was analyzed for 224Ra. Sediments were then returned to the lab for Kd analysis. We compare measured Kd values across the basin for statistically significant relationships between partitioning, biological sediment overgrowth, and porewater temperature. We also employ a vertical exchange model that utilizes the gradient of 224Ra through porewater to estimate flow conditions (direction and magnitude) and couple Kd with our rate estimates to examine the effect of partitioning on studies using radium as a tracer of fluid flux. Our results conclude only 4% of the variation in radium partitioning across the basin is attributed to venting site. We also found no correlation between effective fluid flux measurements and Kd. Distributions in Kd seem to be controlled by sediment heterogeneity in this setting, and are not necessary for calculating flux estimates via 224Ra isotopes through hydrothermal sediments in Guaymas Basin. Initial results to constrain hydrothermal plumes were promising, and additional sample collection/analysis is certainly warranted to better understand the potential of utilizing 224Ra as tracer of dispersed, ambient plume water into the surrounding ocean.

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