Sedimentary biogeochemical provinces in the northern limit of the oxygen minimum zone from the northeastern Mexican Pacific

ABSTRACT Continental margins are reservoirs of materials of terrestrial and marine origin, and they play a crucial role in understanding the spatial and temporal variability of biogeochemical cycles. This, in turn, provides insights into the development and intensity of oxygen minimum zones (OMZs). In the present investigation, a series of geochemical variables was evaluated, including grain size, organic carbon and nitrogen contents, calcium carbonate, biogenic opal, stable isotopes of carbon and nitrogen, and dissolved oxygen concentration. In order to facilitate comprehension of the interactions and relative importance of the source and the state of organic matter degradation, hydrodynamic processes and dissolved oxygen availability, we analysed samples from the northern border of the Mexican Pacific OMZ. Seven perpendicular transects were sampled from the shallow shelf to the continental slope, where surface sediments were collected at 37 stations. Grain size exhibited a monotonic trend, with very fine sands and organic carbon low on the inner shelf and high clay content and organic carbon on the continental slope. The accumulation and enhanced preservation of organic carbon can be attributed to the high clay content and its characteristically large surface area, which reduces exposure time to dissolved oxygen. This organic carbon was derived mainly from marine siliceous plankton, with a minor contribution from terrestrial organic matter. Cluster and principal component analyses defined two biogeochemical provinces on the south‐western margin of the Baja California Peninsula: (1) a coastal province with high dissolved oxygen concentrations, fine sands and low clay and organic carbon contents, and (2) an oceanic province with low dissolved oxygen concentrations, high fine clay and organic carbon contents, located within the northeastern Pacific OMZ. Both provinces have high inputs of marine organic matter derived mainly from siliceous plankton, that is, diatoms and silicoflagellates.