The spatial footprint of hydrothermal scavenging on ²³⁰Th_(XS)-derived mass accumulation rates

Abstract

Mid-ocean ridges are valuable archives of sedimentary flux records used to investigate atmospheric, oceanographic, and solid Earth responses to climate variability. Constant flux proxies, such as extraterrestrial helium-3 (³He_(ET)) and excess thorium-230 (²³⁰Th_(XS)), constrain vertical mass accumulation rates independent of the biases associated with lateral sediment transport and age model resolution. However, thorium scavenging by hydrothermal particles can perturb local ²³⁰Th_(XS) deposition and complicate its application as a constant flux proxy in near-ridge environments. We characterize the footprint of hydrothermal scavenging on sedimentary ²³⁰Th_(XS) using coupled ³He_(ET)-²³⁰Th_(XS) analyses in cores from the Mid-Atlantic Ridge and the Juan de Fuca Ridge. Samples deposited >10 km from the Juan de Fuca Ridge indicate reliable off-axis behavior of both constant flux proxies. In contrast, samples deposited <10 km from the Juan de Fuca Ridge axis and within the axial valley of the Mid-Atlantic ridge suggest 50-80% deficits in sedimentary ²³⁰Th_(XS) relative to its production rate. These deficits contrast with sedimentary ²³⁰Th_(XS) surpluses recently observed on the East Pacific Rise. The spatial footprint of hydrothermal scavenging varies globally and temporally, likely as a function of the intensity of local hydrothermal activity. The combined ridge data suggest that near-vent sediments (within ∼5 km, but variable by ridge) receive relatively high ²³⁰Th_(XS) deposition rates as a direct result of hydrothermal particle scavenging, while more distal sediments receive relatively low ²³⁰Th_(XS) deposition rates due to diffusive loss of overlying seawater ²³⁰Th_(XS) towards the vent. Aside from the East Pacific Rise, far-field sediments are likely to exhibit typical ²³⁰Th_(XS) deposition rates at distances greater than ∼10 km of the ridge axis. However, ²³⁰Th_(XS) systematics within the axial valleys of slow-spreading ridges may be complicated by other factors. Combined ³HeET-²³⁰Th_(XS) studies at multiple ridges are needed to further characterize the nature of hydrothermal scavenging and to evaluate the potential of sedimentary ²³⁰Th_(XS) anomalies to record large-scale variability in past hydrothermal activity.

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