Ocean Deoxygenation Conference | Kiel 2018

The extent and intensity of oxygen miminum zones (OMZs) along productive continental margins are tightly coupled to changes in ocean ventilation and biogeochemical feedbacks. The reconstruction of spatial and temporal changes of these parameters is crucial to understand the mechanisms driving marine redox conditions in the past. The application of redox-sensitive proxies like molybdenum (Mo) and its isotopes can unravel valuable information archived in the sedimentary record. However, the mechanisms by which Mo is delivered to and sequestered by sediments in OMZs are still poorly constrained.
In this study, we compare Mo concentrations and isotope compositions of modern shallow maine sediments as well as paleorecords covering the last 10,000 years from the OMZs in the Gulf of California and the Peruvian Margin [1]. The investigated sites differ in sedimentation rate, export production and ocean circulation.
Shallow sediments from the Gulf of California OMZ have δ98Mo signatures ranging from +1.64 to +2.13 ‰ and Mo concentrations between 3 and 17 µg g-1. The paleorecord has δ98Mo signatures ranging from 1.66 to 1.92 ‰ and Mo concentrations between 5 and 10 µg g-1. In contrast, shallow sediments from the Peruvian OMZ are isotopically lighter (+1.16 to +1.55 ‰) with higher Mo concentrations ( 11 to 101 µg g-1) [1]. Similarly, the paleorecord reveals a large range in δ98Mo signatures (+1.23 to +1.79 ‰) and Mo concentrations (13 to 98 µg g-1). The differences between the two settings can be explained by unique transport mechanisms of Mo to the sediments. The OMZ off Peru is characterized by high organic carbon rain rates and intense water column denitrification. The sedimentary Mo inventory is consequently dominated by particulate supply via Fe-oxides [1] and organic matter. In contrast, the OMZ in the Gulf of California is characterized by a lower organic carbon rain rate and denitrification does not occur in the bottom waters. The Mo flux is dominated by diffusion into the sediments. These findings demonstrate the importance of particulate vs. diffusive Mo delivery in controlling the Mo isotope composition of sediments with important implications for the reconstruction of paleo-redox conditions in the eastern equatorial Pacific.

[1] Scholz et al. (2017) GCA 213, 400-417