Marine heat flow data from the South West Indian Ridge during POLARSTERN cruise ANT-XXIX/8

The relation of tectonics and geochemical cycling at magma-starved ultraslow spreading ridges are hardly understood. Here we present yield-strength profiles, heat flow determinations, geochemical- and microbiological results from the axial valley of the Oblique Supersegment at the Southwest Indian Ridge. Our results report a rheologically weak lithosphere down to 18 km and a very low heat flow of 63.4 - 82.5 mW/m² at the brittle-ductile transition. This is best explained by serpentinization possibly focused in shear zones of deep reaching boundary faults. The axial valley sediments, especially in the deepest areas, are enriched in organic carbon due to high primary productivity and efficient downslope sediment transportation. The microbial activity is comparable inside and outside the valley and appears strongest at a site where we discovered a bivalve that is endemic to hydrothermal- or reducing habitats. We did not find any site of active hydrothermal discharge. Pore water geochemical profiles are contrary to the local temperature and heat flow values and show high diffusive upward fluxes at sites of low heat flow and vice versa. Biogeochemical processes in the axial valley sediments appear strongly influenced by the accumulation and subsequent remineralization of organic matter. Increasing flux rates towards the boundary faults indicate a diffuse, sluggish fluid circulation in these fault zones.