AUTHIGENIC CLAY VERSUS CARBONATE AS PRODUCTS OF MARINE SILICATE WEATHERING (MSIW) IN THE INPUT SEQUENCE TO THE SUMATRA SUBDUCTION ZONE

Analyses of pore fluid, sediment and carbonate-cements recovered during IODP Expedition 362 document the weathering of reactive silicates in the input sequence to the Sumatra subduction zone. Reaction products result in formation of authigenic clays and carbonates, each with significant consequences for the carbon cycle and atmospheric CO₂.

A marked increase in the ⁸⁷Sr/⁸⁶Sr ratio in pore fluids of Sites U1480 (to 0.71376) and U1481 (to 0.71296) shows the radiogenic strontium contribution from alteration of the continentally-derived minerals shed from the Himalayas. Peaks in the pore fluid isotope data coincide with zones of methane presence, consistent with marine silicate weathering (MSiW) reactions driven by CO₂ generation during methanogenesis.

Imaging via scanning electron microscopy (SEM) in the upper 200 mbsf of Site U1480 reveal partially altered feldspar showing dissolution pits and authigenic clay minerals nucleating upon feldspar surfaces. In contrast, conventional polarizing light microscopy and SEM-EDS elemental mapping of the carbonate-cemented zones deeper than 300 mbsf, display crystallographically-controlled replacement of feldspars and dense minerals by carbonate that range in volume from a few percent of the grain to near total obliteration of the silicate grain. Consistent with the petrographic data, the ⁸⁷Sr/⁸⁶Sr ratios in carbonates from the upper 230 mbsf deviate slightly from the co-eval seawater values (0.70920 to 0.70930), in contrast with authigenic carbonates recovered between 780 and 1250 mbsf that are significantly enriched in radiogenic ⁸⁷Sr (0.71136 to 0.71328). These results illustrate how MSiW leads to distinct products, likely in response to a supply of silica in the modern setting and calcium enrichment via diffusion from oceanic basement in the older sequences.

The resulting texture and porosity changes associated with cements and their associated grain replacements play a major role on the physical properties of the sediment. Nearly constant in situ porosity and overconsolidation behavior of samples from 800 and 1250 mbsf support a cemented/strengthened horizon resisting mechanical pore space reduction. Our results highlight the importance of MSiW in rock physical property evolution, which may affect fluid budgets and the seismogenic behavior within the subduction zone.