Preliminary rock magnetic and geochemical data from cold seepage sites of IODP Expedition 385, Guaymas Basin

Guaymas Basin is a young marginal rift basin in the Gulf of California characterized by active seafloor spreading and rapid sediment deposition, including organic-rich sediments. International Ocean Discovery Program (IODP) Expedition 385 (X385) drilled eight sites in the Guaymas Basin during fall 2019. Drill sites extend from the NW to the SE flanking regions of the northern axial graben of Guaymas Basin, covering an ~81 km long transect.

Two sites on the Sonora margin, U1549 and U1552, have been drilled close to a gas upflow pipe, with U1549 being more distal to the corresponding upflow zone than U1552. Attenuated cold seepage conditions are found at Site U1549 in the central basin with methane appearance below 25 meters below seafloor (mbsf). Hydrate occurrence is found from ~25 mbsf at Site U1552. These two sites, ~12 km apart, represent an opportunity to study the influence of gas hydrate occurrence and different geothermal gradients in shallow young organic-rich sediments (< 170 mbsf, < 0.29 Ma).

Here, we present shipboard rock magnetic and geochemical data obtained from Sites U1549 and U1552. In contrast to the other X385 drilling locations, Sites U1549 and U1552 do not display a drastic decrease of magnetic intensity at the sulfate-methane transition zone (SMTZ), at ~30 and ~10 mbsf respectively. A subsequent coarsening of the magnetic mineral assemblage is inferred from lower mid-destructive field values below the SMTZ at Site U1549. Site U1552, however, does not seem to show this trend, perhaps because of the presence of gas hydrates below the shallower SMTZ.

Recently acquired XRF core scanning data show that both sites are lithologically very similar and do not seem to show any authigenic evidence of a well-marked SMTZ. Downhole variations of major elements and elemental ratios are comparable, differing by the depth where elemental changes happen. By contrast, porewater seems to characterize current environmental and diagenetic processes, especially those related to fluid and gas circulations. Differences in methane and hydrate occurrence could be due to spatial variations in fluid flow and pathways, leading to dynamic conditions at these sites. Authigenic magnetic mineralogy, mostly sensitive to biogeochemical processes at the SMTZ, would respond to variations in fluid and gas flow.