Cooling upwelling regions along the California margin during the early Pliocene: evidence for a shoaling thermocline

摘要

The early Pliocene is the most recent time in Earth history when average global temperatures were warmer than they are today. Previous work has shown that coastal upwelling regions had a particularly strong response to early Pliocene warmth, with sea sur-face temperature records in the southern California margin indicating SST was 9-10°C warmer compared to today. Warmer SST in upwelling regions could have been an important factor in maintaining Pliocene warmth. However, the cause for the warmer conditions in upwelling regions and the transition to colder SST has remained elusive. We present alkenone derived SST estimates from a northern California margin location, ODP Site 1022 (40°N, 125°W, 1950m water depth), from the early Pliocene (4.2-2.8 Ma). The 2.8°C cooling during this time interval is similar to that observed at two southern California margin sites (ODP Sites 1014 and 1012), indicating the three sites were responding to the same forcing. However, SST was only 3°C warmer than today at ODP Site 1022 while the southern sites were 9-10°C wanner than today, thereby increasing the latitudinal temperature gradient along the California margin dramatically during the early Pliocene compared to today. This indicates that the tropical warm pool expansion during the early Pliocene was not felt along, the Northern California margin. We quantified diatom assemblages at ODP Site 1022 to constrain changes in oceanographic conditions during early to middle Pliocene. The percent of diatoms indicative of subarctic waters is unchanged from 4.2 to 3 Ma, suggesting that a change in the strength of the California current cannot explain the cooling SST trend. We also found no trend in the percent of upwelling-indicative diatoms, which is consistent with records of alkenone mass accumulation rate, diatom preservation, biosilica, and total organic carbon, none of which show evidence that the upwelling of nutrient-rich subsurface water increased as SST cooled. The fact that upwelling of nutrient-rich water was occurring even when SST were warmer can be explained by a deeper early Pliocene thermocline.