Comparison of Clumped Isotope, δ18 ${\boldsymbol{\delta }}^{\mathbf{18}}$ O, and Alkenone Proxies for Sea‐Surface Temperature off Coastal California Over the Past 4.2 Million Years

In the Northern Hemisphere, Eastern Pacific coastal upwelling systems host zones of high marine productivity and moderate the western United States' coastal climate by bringing cold, nutrient‐rich waters to the surface. Reconstructions of regional sea‐surface temperatures (SST) indicate that upwelling of cold waters began during the Pliocene. At ocean drilling program (ODP) Site 1014 in the Tanner Basin, southern California, this cooling is recorded by the U37K′ ${{U}_{37}^{K}}^{\prime }$ unsaturation index, which finds cooling of ∼11 ° ${}^{\circ}$ C, compared to only 3 ° $\mathit{{}^{\circ}}$ C in surrounding regions. Here, we used this record to test the suitability of reconstructing coastal California SSTs with oxygen and clumped‐isotope paleothermometry. We measured carbonate‐based isotope proxies (δ18O and clumped isotopes) in bulk sediment and mixed‐foraminifera fractions from ODP Site 1014, the core originally used for the U37K′ ${{U}_{37}^{K}}^{\prime }$ measurements. Carbonate‐based SST reconstructions in both bulk and foraminifera fractions show no cooling trend with time. Clumped‐isotope temperatures, which do not rely on assumptions about seawater δ18 ${\delta }^{18}$ O, are on average 11 ° ${}^{\circ}$ C colder than alkenone‐derived estimates, calculated as the difference between mean clumped‐isotope (∼9 ° $\mathit{{}^{\circ}}$ C) and alkenone (∼20 ° $\mathit{{}^{\circ}}$ C) temperatures over 4,200 ka. To investigate this discrepancy, we evaluated three potential sources: (a) contamination; (b) disequilibrium isotope effects during carbonate precipitation; and (c) carbonate diagenesis during burial. In SEM images of foraminifera, we find visually apparent diagenetic alteration across all depths, including the 1000‐year‐old core top sample, where about two‐thirds of specimens are severely altered. We propose that the discrepancy between the alkenone‐ and carbonate‐stable‐isotope‐based temperatures dominantly reflects carbonate diagenesis occurring within millennia of deposition.