Barron (1995) used a diatom paleoclimatic ratio ("Tdf") to estimate mid-Pliocene SSTs at northwest Pacific ODP Sites 881, 883, and 480. This ratio compared relatively small, finely silicified pennate diatoms with different ecological preferences (Neodenticula = subarctic; Thalassionema = temperate; Alveus marinus (and Nitzschia reinholdii) = tropical to subtropical). Barron (1995) applied the Tdf ratio to Constance Sancetta's (pers. comm., 1994) core-top data for the Pacific and found strong linear relationships with February and August SSTs (r2 values of 0.888 and 0.736, respectively) in the northwest Pacific.
The Tdf ratio was applied to the Hole 1022A diatom counts. The resultant SSTs (Fig. 1) appear to be anomalously cold for February (7°-1°C colder than modern values) and anomalously warm for August (6.0°-1.5°C warmer than modern values) based on PRISM reconstructions for the mid-Pliocene of the North Pacific (Dowsett et al., 1996; J.A. Barron, unpubl. data), which suggests that February and August SSTs were not more than 2°C warmer than modern values during the mid-Pliocene. Presumably, this discrepancy reflects the scarcity of Sancetta core-top data in the offshore California region and the resultant failure of the Tdf ratio to estimate paleotemperatures. In addition, the transitional zone between subarctic and subtropical waters off California is rather broad and oceanographically more complex compared to the more geographically constrained transition across the North Pacific Front in the northwest Pacific. This suggests that estimation of paleotemperatures off California using diatoms would require a detailed core-top data set for the region. Lateral transport of diatoms off the continental shelf is another problem in the offshore northern California region that complicates paleoenvironmental interpretation (Sancetta, 1992). Efforts are under way to compile modern core-top data in the offshore California region that may be more useful for diatom paleoclimatic studies.
The Tdf curves (Fig. 1) suggest that the main cooling event occurred between 2.8 and 2.7 Ma, whereas isotope records demonstrate that major cooling (and/or increased global ice volume) began later, at ~2.7 Ma (Shackleton et al., 1995). The earlier cooling recorded by diatoms reflects the use of the extinct diatom, Neodenticula kamtschatica, as a cold-water component equivalent in ecological preference to its descendant, N. koizumii. An increasing dominance of N. koizumii over N. kamtschatica beginning at ~2.7 Ma is more likely reflective of major cooling of surface waters.