Variability of the Oxygen Minimum Zone during the Holocene in the Gulf of California, México

Abstract:

Changes in the oxygen minimum zone (OMZ) in the southern portion of the Gulf of California (GoC) can be caused by changes in primary productivity or by advection of water masses with low oxygen concentrations. To discern the acting mechanism, multiple and gravity cores (NH01-22 and NH01-25) comprising the Holocene (~4800 and ~6600 years, respectively) were collected in the OMZ of the GoC during the CALMEX NH-01 cruise. Sediments were collected on the eastern slopes of Pescadero (PB) and Carmen (CB) Basins, intersecting in both cases the GoC’s distinct OMZ. The cores were sectioned at 1-cm intervals and analyzed for total Mo (Mo_Total), total Al and Fe-pyrite. With these values, Mo enrichment factors (EF_Mo) and degrees of pyritization (DOP) were calculated for each sediment interval. Average DOP values for CB and PB were 38%±11% (n=306) and 28.6%±6.5% (n=259), respectively, indicating that the sediments are not limited by the availability of reactive Fe. Higher average Mo_TOTAL values were measured in CB (0.22±0.04 µmol g^-1) relative to PB (0.09±0.02 µmol g^-1). As a result, EF_Mo values were high (14.5±3.4, range of 6.6-35.6) in CB and significantly lower in PB (4.5±1.1, range of 2.7-12.0). For CB, EF_MO values are comparable to regions such as the west coast of Baja California, Mexico (EF_MO=20±9) and the euxinic Black Sea (EF_MO=27±8) and Saanich Inlet, British Columbia (EF_MO=39±35) zones. In PB EF_MO values were similar to those reported for well oxygenated marginal basins (e.g., Japan Sea; EF_MO=2.5±1.5). DOP values for CB and PB are generally <40%, characteristic of oxic sedimentary environments. The DOP and EF_MO conflicting results could indicate that the sediments are not completely pyritized because Fe is also associated with acid volatile sulfides (unlikely), or that values <40% are not necessarily associated with oxic sedimentary environments. However, for both basins, the persistence of laminations, the EF_Mo levels and the DOP values suggest variable oxic, anoxic and even euxinic paleoenvironmental conditions, which were intermittently present during the last ~6.6 ky BP in the GoC.