The Effect of Radium Partitioning on Hydrothermal Fluid Flow Determination across Guaymas Basin

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 saline environments and changes in activity due to ingrowth/decay. However, the distribution of radium may be influenced by its varying solid:aqueous partitioning coefficients (K_d) and is critical to understand for any study utilizing radium as a tracer. This project explores K_d 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. Sixteen sediment cores across the southern basin were collected on cruise AT42-05 in November 2018, using HOV Alvin (Fig.1). 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 K_d analysis. We compare measured K_d 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 K_d with our rate estimates to examine the effect of partitioning on studies using radium as a tracer of fluid flux. Despite both K_d values and flow rates varying by nearly an order of magnitude across Guaymas Basin, we do not find substantial evidence for any causal relationship between the two parameters, suggesting that K_d exerts a minimal influence on flow rate estimates. Distributions in K_d seem to be controlled by sediment heterogeneity in this setting. Findings of this work will contribute to the foundation of knowledge regarding environmental controls on radium and may be useful for the continued development of this tracer system for estimating flow rates in hydrothermally active deep-sea environments.