ISOTOPIC TEMPERATURES FROM THE EARLY AND MID-PLIOCENE OF THE US MIDDLE ATLANTIC COASTAL PLAIN, AND THEIR IMPLICATIONS FOR THE CAUSE OF REGIONAL MARINE CLIMATE CHANGE

Mean seasonal extreme temperatures on the seafloor calculated from the shell d 18 Oofthescallop Placopecten clintonius from the basal part of the early Pliocene Sunken Meadow Member (Yorktown Formation) in Virginia are very similar to those from the same horizon at the latitude of Cape Hatteras in North Carolina ( ~ 210 km to the south). The lowest and highest temperatures calculated from each shell (using d 18 O seawater ¼þ 0.7 % )givemean values for winter and summer of 8.4 6 1.1 8 C( 6 1 r ) and 18.2 6 0.6 8 CinVirginia,and8.6 6 0.4 8 Cand16.5 6 1.1 8 C in North Carolina (respective median temperatures: 13.3 8 C and 12.6 8 C). Patterns of ontogenetic variation in d 18 O, d 13 C and microgrowth increment size indicate summer water-column stratification in both areas, with summer surface temperatures perhaps 6 8 C higher than on the seafloor. The low winter paleotemperatures in both areas are most simply explained by the greater southward penetration of cool northern waters in the absence of a feature equivalent to Cape Hatteras. The same current configuration but a warmer general climate can account for the high benthic seasonal range (over 15.0 8 C in some cases) but warmer median temperatures (15.7–21.3 8 C) derived from existing d 18 O data from scallops of the higher Yorktown Formation (using d 18 O seawater ¼þ 0.7 % for the upper Sunken Meadow Member and d 18 O seawater ¼þ 1.1 % for the mid-Pliocene Rushmere, Morgarts Beach, and Moore House members). Existing d 18 O data from the infaunal bivalve Mercenaria of the Rushmere Member yields a similarly high median temperature (21.6 8 C) but a low seasonal range (9.2 8 C), pointing to the periodic influence of warm currents, possibly at times when the Gulf Stream was exceptionally vigorous.