RESPONSES OF AUSTRAL PLANKTONIC FORAMINIFERA TO THE LATE PALEOCENE THERMAL MAXIMUM: FAUNAL EVIDENCE FOR OCEANIC CHANGE AND AN AFTEREFFECT COLD SNAP

The Late Paleocene Thermal Maximum (LPTM) occurred ~55.5 Ma and is one of the most dramatic episodes of global warming in the geologic record. The initiation of LPTM conditions was abrupt (<10 kyr) and entailed the warming of deep sea and Antarctic sea surface temperatures by ~5 and ~8° C, respectively. A major perturbation to the global carbon cycle, as reflected by a ~3 to 4‰ decrease in the carbon isotopic composition of marine carbonates, demarcates the "onset" of the LPTM. It has been hypothesized that the carbon isotope excursion (CIE) is due to the dissociation of massive amounts (1,200 to 2,800 Gt) of ¹²C-enriched (~-60‰) methane hydrate.

Here, we provide the first report documenting the effects these environmental changes had on high-latitude, planktonic foraminifera. Faunal counts performed on a stratigraphic series of samples spanning the CIE interval preserved at ODP Site 690 reveal a succession of assemblage changes. Assemblages predating the CIE are dominated by the genera Acarinina and Subbotina, containing common A. praepentacamerata. The demise of A. praepentacamerata is roughly coeval with the onset of the CIE, as is the stratigraphic debut of members of the stenothermal, warm-water genus Morozovella. The morozovellids, and "large" (>180 µm) chiloguembelinids, are restricted to the lower half of the CIE during peak, greenhouse warming. In the immediate aftermath of the CIE, assemblages become strongly cold-water in character as indicated by a sharp decline in the relative abundance of acarininids. This inferred, oceanic cooling was transient (~100 kyr) and coincides with an interval of Fe-depleted, chalky, white sediments. Perhaps this post-LPTM "cold snap" reflects a climatic overshoot stemming from the drawdown of CO₂ levels by various feedback mechanisms (i.e. chemical weathering and/or biotic productivity).