CALIFORNIA MARGIN PRODUCTIVITY RESPONSE TO EARLY PLIOCENE WARMTH

The modern California Current marine ecosystem is primarily driven by strong seasonal upwelling, yet the response of this important system to future global warmth is largely unknown. While upwelled water that sustains productivity is usually cold and nutrient-rich, modern studies indicate the relationship between sea surface temperatures (SST) and productivity is not always clear, as recent observations show little correlation between increasing SST and productivity. Instead, productivity appears to be more tightly linked to offshore wind dynamics and the resulting nutrient availability of upwelled waters.

The early Pliocene (~3-5 Ma) is the most recent time in Earth history when average global temperatures were 3-4°C warmer than today for a sustained period of time, continent locations, ocean currents, and atmospheric CO₂levels were all similar to today. Global upwelling regions were 3-9°C warmer in the early Pliocene compared to today, but productivity records from several regions show inconsistent patterns. Along the California margin, there is no clear correlation between SST and alkenone mass accumulation rates (MAR). ODP Site 1016 (34.0˚N, 122.0˚W, depth 3835 m) smear slide analysis shows coccolith MAR increase from 4.5 to 2.5 Ma, with a 3.3-4 Ma data gap, while biogenic silica shows variable but no trend during the Pliocene. Higher coccolith MAR in the earlier Pliocene (~4 Ma) indicates lower productivity, as coccoliths are outcompeted for nutrients when productivity is high.

We take a multi-proxy approach to test the hypothesis that the California margin experienced enhanced productivity during the early Pliocene, when warmer SST existed. Specifically, we focus on ODP Site 1016, and fill in the 3.3-4 Ma data gap. This builds on John Barron’s work along the California margin, which showed diatoms were very sparse at ODP Site 1016, as the site is seaward of the coastal zone of Pliocene diatom productivity. Since diatom assemblage counts are not possible at this site, we use biogenic silica in conjunction with smear slide analysis as a proxy for diatom productivity. Using an updated biostratigraphic age model, these analyses, in addition to new alkenone MAR and SST estimates, will help resolve the question of how productivity along the California margin responded to Pliocene warmth.