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Guaymas Basin is a young marginal rift basin in the Gulf of California, Mexico, characterized by active seafloor spreading and rapid sediment deposition, including organic-rich sediments. International Ocean Discovery Program (IODP) Expedition 385 drilled two sites near the Sonora margin, Sites U1549 and U1552, that are both close to a gas upflow pipe with Site U1549 being more distal to the corresponding upflow zone than Site U1552. Attenuated cold seepage conditions exist at Site U1549 in the central basin with methane occurrence below 25 meters below seafloor (mbsf), and hydrate was found to be present from ~25 mbsf at Site U1552. These two sites, ~12 km apart, represent an opportunity to study the influence of gas hydrate occurrence and methane seepage in shallow young organic-rich sediments (<170 mbsf; <0.29 Ma). In this data report, we present rock magnetic and geochemical data obtained from Sites U1549 and U1552 to characterize aqueous, solid iron, and sulfur phases present in the sediments, with a focus on iron oxides and iron sulfides. Guaymas Basin sediments are rich in reactive iron, mainly as pyrite. Iron oxides (magnetite and hematite) and authigenic iron sulfides (pyrite and greigite) are ubiquitously found in the sediments, and iron oxides are reduced to form pyrite. Pore water analysis seems to characterize current environmental and diagenetic processes, especially those related to fluid and gas circulation. Differences in methane seepage and hydrate occurrence could be due to spatial variations in methane fluid flow and pathways, leading to dynamic conditions at these sites with an impact on the sulfate reduction and anaerobic oxidation of methane rates. Authigenic magnetic mineralogy, mostly sensitive to biogeochemical processes at the sulfate–methane transition zone, would respond to fluid and gas flow variations, especially of methane. 

International Ocean Discovery Program (IODP) Expedition 385 focused on the impact of sill emplacement and fluid flow on subsurface biogeochemical processes in organic-rich sediments of the Guaymas Basin. It resulted in the drilling of eight sites with contrasted settings; Sites U1545 and U1546 were drilled in the northern basin and allowed the recovery of a deep sill at Site U1546; Sites U1547 and U1548 were drilled on an active hydrothermal vent called Ringvent; Sites U1549 and U1552 were drilled on the Sonora margin where cold seeps and hydrates were identified; Site U1550 was sampled as a replicate of Deep Sea Drilling Project (DSDP) Leg 64 Site 481 on an axial trough; and Site U1551 was drilled ~29 km southeast of the axial graben and was mainly terrigenous. The quantitative X-ray fluorescence data presented here were measured on squeeze cakes retrieved after squeezing of whole-round core segments for pore water extraction. Major elements (Al, Ca, Cl, Fe, K, Mg, Mn, Na, P, Si, Ti, and S) as well as minor/trace elements (As, Ba, Br, Ce, Co, Cr, Cu, La, Mo, Nb, Ni, Pb, Rb, Sc, Sr, V, Y, Zn, and Zr) are provided in this data report.