Hydrothermal scavenging of ²³⁰Th on the Southern East Pacific Rise during the last deglaciation

Abstract

Thorium-230 (²³⁰Th) is a fundamental tool for estimating sediment fluxes in the open ocean. Because ²³⁰Th is rapidly scavenged by particles falling through the water column, the flux of ²³⁰Th to underlying sediments is typically equal to its water column production rate. However, recent surveys suggest hydrothermal plumes are unusually efficient scavengers of ²³⁰Th. Here we show that hydrothermal scavenging on the Southern East Pacific Rise (SEPR) resulted in ²³⁰Th fluxes several times higher than the water column production rate during the last deglaciation. Elevated fluxes likely require diffusive transport of dissolved ²³⁰Th from the ridge flanks towards the ridge crest. Depending on the length-scale of ²³⁰Th transport, the resulting deficits in ²³⁰Th may yield overestimates of sediment flux to ridge flank sediments. We also show that Fe fluxes at 19°S on the SEPR lag those at 11°S and 6°S by several thousand years, inconsistent with a signal driven by changes in deep water pH and oxygen levels. Instead, variable hydrothermal activity is the simplest explanation of the observed signals in the Pacific, Indian, and Atlantic basins.

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