Distinct pathways of redox changes from settling to burial in contrasting marine environments of the Gulf of California: Alfonso, La Paz and Pescadero basins

The oxygenation fluctuations of marine basins are controlled by complex interplay between circulation, climate, productivity and sea level variability, also enhanced by anthropogenic influence. These variations can cause OMZ expansions and increase coastal hypoxia, thus leading to unforeseen consequences of ecosystem alteration, marine life mortality and release of potentially-toxic trace elements previously sequestered within marine sediments. Despite many findings by the scientific community, accurate past reconstructions and future change predictions are far from perfect. This is mainly due to gaps in the understanding of redox processes occurring during settling and burial especially in coastal, yet iron-limited areas of the oceans. Hereby, we tackle this problem by following the transformation pathways of various paleoproductivity and paleoredox indicators (Mo, Ni, V, U, detailed Fe speciation, C and S concentrations and isotope ratios) from their sources to settling and burial. We emphasize sediment trap, core and pore water geochemistry of contrasting oxic, suboxic and anoxic marine settings of the southern Gulf of California: semi-closed Alfonso Basin, OMZ-influenced La Paz Basin and upper-OMZ Pescadero Basin with cyclic dust and fluvial inputs. Our data show evident physicochemical and microbiological transformations during settling with several event-related U enrichments in the marine snow aggregates, probably related to redox microniche processes. The sediment core solid fraction geochemistry, on the other hand, reveals a diagenetic downcore decrease of most highly reactive Fe phases (including the labile ascorbate-extracted Fe), correlating with trace element trends and dissolved Fe in pore waters. The comparison among basins exhibits an expected Fe_HR/Fe_T ratio trend for oxic (less than 0.1) and suboxic/anoxic (0.2 and higher) sediments. Although, Mn, S, Mo, U trends varied from core to core suggesting chemical zone and metabolic pathway differences, tightly related to specifics of each dispositional setting (organic matter type, availability of electron acceptors and sedimentation rate). Finally, both the water column and early diagenetic redox biogeochemistry discussed here point towards a strong influence of microbially-mediated processes in the Gulf of California.