| Summary: | The early Pliocene is the most recent time in Earth history when average global temperatures were warmer than today. We present alkenone-derived SST estimates from ODP Site 1022 (40??N, 125??W, 1950m water depth) off northern California from the early Pliocene (4.2-2.8 Ma). The 2.8??C cooling is consistent with previously published records from the southern California margin. In order to determine the cause of the cooling SST trend, diatom assemblages were used to constrain oceanographic conditions. The proportion of subarctic diatoms is unchanged from 4.2 to 3 Ma, suggesting a change in the strength of the California current cannot explain the cooling. The proportion of upwelling-indicative diatoms also shows no trend. Alkenone concentrations, diatom preservation, biosilica, and total organic carbon confirm that the upwelling of nutrientrich subsurface water did not change as SST cooled. Upwelling of nutrient-rich water while SST were warmer can be explained by a deeper early Pliocene thermocline.
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