The spatial footprint of hydrothermal scavenging on 230ThXS-derived mass accumulation rates


Middleton JL, Mukhopadhyay S, Costa KM, Pavia FJ, Winckler G, McManus JF, D'Almeida M, Langmuir CH, Huybers PJ. The spatial footprint of hydrothermal scavenging on 230ThXS-derived mass accumulation rates. Geochimica et Cosmochimica Acta [Internet]. 2020.
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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 (3HeET) and excess thorium-230 (230ThXS), 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 230ThXS deposition and complicate its application as a constant flux proxy in near-ridge environments. We characterize the footprint of hydrothermal scavenging on sedimentary 230ThXS using coupled 3HeET-230ThXS 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 230ThXS relative to its production rate. These deficits contrast with sedimentary 230ThXS 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 (typically within ∼5 km, but variable by ridge) receive relatively high 230ThXS deposition rates as a direct result of hydrothermal particle scavenging, while more distal sediments receive relatively low 230ThXS deposition rates due to diffusive loss of overlying seawater 230ThXS towards the vent. Aside from the East Pacific Rise, far-field sediments are likely to exhibit typical 230ThXS deposition rates at distances greater than ∼10 km of the ridge axis. However, 230ThXS systematics within the axial valleys of slow-spreading ridges may be complicated by other factors. Combined 3HeET-230ThXS studies at multiple ridges are needed to further characterize the nature of hydrothermal scavenging and to evaluate the potential of sedimentary 230ThXS anomalies to record large-scale variability in past hydrothermal activity.

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