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