| Abstract: | Temperatures at the tropical end of greenhouse latitudinal transects have posed as difficult a paleoclimatic puzzle as polar warmth. Simply stated, for greenhouse high latitudes to have been as warm as data suggest, the tropics should have been much warmer than most published estimates. Recent oxygen isotopic data from exquisitely preserved material at several tropical sites (e.g., Wilson and Opdyke, 1996; Pearson et al., 2001) may resolve much of the problem. These studies report low planktic  values suggesting hot greenhouse tropics and implying that early alteration of tropical planktic specimens in relatively cool bottom waters/pore waters was pervasive even in the well preserved samples used in most paleotropical studies. Further, evidence for hot tropics supports a proposition that temperature increased across all latitudes during warming intervals and decreased during cooling intervals. Results from the Maastrichtian at Blake Nose, though, show that the subtropical western Atlantic was warming at the same time high latitudes were cooling and demonstrate that controls on low latitude temperatures remain a problem for understanding greenhouse climates regardless of the absolute temperature in tropical/subtropical settings.
At three Blake Nose ODP/DSDP sites (390, 1050, and 1052) values of various planktic foraminifera and fine fraction carbonate decrease by 0.5 to 1.5 from the early to the late Maastrichtian. Superimposed upon the long-term negative trend is a short-term negative shift that likely represents a brief pulse of globally recognized, late Maastrichtian warming. Burial depths at the Blake Nose sites range from < 150 m at 390 to > 450 m for the oldest samples from 1052. Preservation ranges from excellent to good with the poorest preservation occurring at the deepest levels of Site 1052. These most deeply buried samples have values ~0.5 lower than values in correlative samples from the other two sites, and, consequently, the extent of apparent warming is least at 1052. We attribute the differences between 1052 and the other two sites to burial diagenesis. These burial effects, though, act in the opposite direction to the long-term decrease in we observe. Similarly, diagenesis has not obscured the record of the independently documented late Maastrichtian warming event. Thus, we are confident that the trend of decreasing planktic values across the Maastrichtian at Blake Nose represents a paleoenvironmental signal. The size of the shift suggests ~4-6°C of warming or 3-4 ppt freshening of surface waters.
Apparent warming from the early to late Maastrichtian at Blake Nose is provocative because this 6 million year interval is generally considered to be a time of global cooling and, perhaps, includes high latitude glaciation. We can not rule out the possibility that Blake Nose data reflect a regional, western North Atlantic phenomenon rather than global, subtropical phenomenon, but our data suggest that poleward heat transport decreased during the Maastrichtian. |