Assessing Regional Variability in the Eastern Tropical North Pacific Oxygen Minimum Zone through a Geochemical Comparison of Pescadero Slope and Soledad Basin Sediments

The marine nitrogen cycle is dynamic and experiences episodic variations that are not well constrained. This study investigates changes in the intensity of denitrification and the oxygen minimum zone (OMZ) in the eastern tropical north Pacific through the measurement of δ¹⁵N from bulk laminated sediments between the Pescadero Slope, Gulf of California, and the Soledad Basin, in the Pacific Ocean 45 km from Baja California Sur, to assess variation in nitrogen cycling in these environments. Similar trends in OMZ expansion are observed in both records, however, the expansions are offset in time with events first occurring in Soledad Basin and then at Pescadero Slope. This temporal offset is not in a direction that would be expected if changes were propagated with a slow moving current. The discrepancy is hypothesized to be a product of age model uncertainty, rather than a time lag in the events themselves. This study utilizes x-ray fluorescence (XRF) data to help constrain the age discrepancies between the two age models independently from δ¹⁵N_sed records. XRF correlation ‘stratigraphy’ revealed a 110-year offset in the age models, which is likely related to inherent difficulties associated with reservoir age corrections. After applying an increased reservoir age to the Soledad Basin age model the δ¹⁵N_sed records show greater synchronicity. XRF data analysis also reveals differences in the geochemistry of the sediments at the two sites with Pescadero Slope showing increased Ti, K, and Sr values and with Soledad Basin exhibited high Ca abundance. The increased Ti, K values are likely associated with increased detrital input from the volcanic Sierra Madre Occidental. High Sr values are hypothesized to relate to hydrothermal activity in the Gulf of California. Elevated Ca in the Soledad Basin sediment may relate to higher carbonate content, which is likely due to coccolithophore blooms.