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

The uranium content of sediments and pore fluids along a ridge axis to ridge flank transect perpendicular tothe 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 Uenrichments occur and (2) the ridge flank (1.9–4.6 Ma crust) with uniform U association with hydrothermalFe oxide phases in the solid phases. Upward advection and diffusion of cool, U?depleted basement fluidsoccurs at many coring sites. At the 0.36 Ma site the oxic basal?fluids strip the plume derived sediment ofthe excess U, effectively migrating the U upwards through the sediment into the overlying water column. Atthe ridge flank sites the pore fluid advection rates are highest at bathymetric/basement highs and advectionvelocities of up to 7.5 mm yr?1 are inferred from the pore fluid profiles. These estimates are consistent withprevious calculations based on fluoride pore fluid distributions. The basal?fluid U depletions are in the range10–70% relative to seawater depending on temperature and redox state. Low?temperature uptake of U duringbasalt alteration is a significant sink from seawater in the global seawater U budget (6.7–29 Mmol yr?1). Porefluid 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 forbasement alteration history where low ratios indicate extensive oxic alteration of basal?sediments duringfluid flow