Tracing low-temperature fluid flow on ridge flanks with sedimentary uranium distribution

Abstract

The uranium content of sediments and pore fluids along a ridge axis to ridge flank transect perpendicular to
the southern East?Pacific Rise is used to assess the impact of low?temperature fluid flow on U distribution.
Two distinct regions are identified: (1) a young crustal site (0.36 Ma crust) where significant solid phase U
enrichments occur and (2) the ridge flank (1.9–4.6 Ma crust) with uniform U association with hydrothermal
Fe oxide phases in the solid phases. Upward advection and diffusion of cool, U?depleted basement fluids
occurs at many coring sites. At the 0.36 Ma site the oxic basal?fluids strip the plume derived sediment of
the excess U, effectively migrating the U upwards through the sediment into the overlying water column. At
the ridge flank sites the pore fluid advection rates are highest at bathymetric/basement highs and advection
velocities of up to 7.5 mm yr?1 are inferred from the pore fluid profiles. These estimates are consistent with
previous calculations based on fluoride pore fluid distributions. The basal?fluid U depletions are in the range
10–70% relative to seawater depending on temperature and redox state. Low?temperature uptake of U during
basalt alteration is a significant sink from seawater in the global seawater U budget (6.7–29 Mmol yr?1). Pore
fluid U content is a sensitive tracer of extremely low?temperature (<5°C) and low?velocity (<10 mm yr?1)
advection through ridge flank systems and the basal sediment U/Fe ratio is potentially a useful proxy for
basement alteration history where low ratios indicate extensive oxic alteration of basal?sediments during
fluid flow.

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Identifiers

ORCID for Rachel A. Mills: orcid.org/0000-0002-9811-246X

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