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THE PETM: AN EXTREME Global Biogeochemical PERTURBATION

At the PETM, within a very short time interval, at least 2000 Gt of carbon were added to the ocean-atmosphere system, and polar temperatures soared by as much as 8°C (Kennett et al., 1991; Thomas & Shackleton, 1996; Katz et al., 1999). The primary evidence for these changes comes from high-resolution isotope records (e.g., Fig. 1) which show dramatic negative excursions in d13C and d18O. As highlighted throughout this proposal, many other records across the PETM generally support massive carbon input (e.g., pronounced carbonate dissolution on the seafloor) or extreme warmth (e.g., pronounced increase in warm water nannoplankton in open ocean sites, and migration of low-latitude biota to high latitudes).

As for the carbon source, both empirical and theoretical evidence implicate a massive and rapid release of marine methane hydrates. The primary geochemical evidence is a coeval >3‰ negative carbon isotope excursion in both marine and terrestrial reservoirs at the PETM (Fig. 1) . The presence of a regional seismic discontinuity indicative of a massive slump along the eastern continental margin of the US further supports this hypothesis . The primary theoretical evidence comes from numerical consideration of carbon isotopes, fluxes, and mass balance constraints, which require a very 12C enriched carbon source (i.e., one that is produced by bacteria).

Coupled to the carbon cycle and temperature perturbations are large-scale and widespread changes in physical, chemical, geological, and hydrological systems including changes in ocean and atmosphere circulation, precipitation patterns and intensity, and the global sedimentation patterns (Sloan et al. 1997). Deep-sea sediment cores are characterized by pronounced, widespread carbonate dissolution at the start of the PETM followed by gradual increases in carbonate and barium accumulation at some locations . In near shore and shallow marine environments, increased accumulation of carbonates and clastics occurs, particularly kaolinite, a chemical weathering byproduct .

These climatic changes had profound effects on global ecosystems. In the ocean, they include the extinction of between 30 and 50% of deep-sea benthic foraminiferal species, including many large, heavily calcified epifaunal taxa (Thomas, 1990), An unusual proliferation of exotic "excursion" species of planktonic foraminifera, calcareous nannoplankton, and dinocysts (Apectodinium) in the low to mid-latitude oceans at the event is indicative of massive shifts in oceanic fertility (Kelly et al., 1996; Bujak & Brinkhuis, 1998; Aubry, 1998; Crouch et al., 2001; Bralower et al., in press). Terrestrial ecosystems experienced dramatic changes in faunal diversity and composition (including the first appearance of many important groups of "modern" mammals, such as primates, artiodactyls, and perissodactyls), body size, and guild structure, as well as a pronounced pulse of higher speciation rates in the million years following the event (Gingerich, 1989; . Terrestrial floras exhibit a smaller pulse of changes, including the dispersal of low-latitude, warm climate floras into mid- and high-latitudes . Although some responses were transient (e.g., temporary latitudinal displacement), others were permanent and altered the course of evolution of Holarctic terrestrial ecosystems.

The PETM occurred during a gradual warming that began in the late Paleocene ~58 Ma and culminated at the early Eocene Climatic Optimum (EECO) ~51 Ma ; . This warming trend, although interrupted by at least one "cool" interval between 54.3 and 53.7 Ma , was probably caused by elevated greenhouse gas concentrations . Within the context of conventional carbon cycle models, enhanced volcanic emissions provide the most probable source for this gas . However, given the current framework for understanding the PETM, slow release of CH4 from the seafloor presents an interesting, unexplored alternative. An overall decline in the d13C of the ocean between 58 and 53 Ma is consistent with either interpretation (Fig. 1). Regardless, the long-term temperature rise may have brought the climate system to a threshold that, once crossed, set in motion the perturbations seen at the PETM.

As with the PETM, the biotic consequences of the EECO were pronounced, particularly in middle and high latitudes. In mid-latitude areas temperate floras were supplemented and replaced by subtropical and tropical forms (Wing 1987, 1998). The presence the same warm-adapted plant genera in Asia, North America and Europe suggests that some of these lineages were able to expand their ranges across high-latitude land bridges (Tiffney 1984, Manchester 1999). Above the Arctic Circle the very warm climates of this period permitted the growth of moderately diverse and productive deciduous forests (McIver and Basinger 1999, Basinger 1991).. Tropical to sub-tropical marine organisms similarly expanded their ranges into higher latitudes. Benthic organisms fared better during this period, possibly because forms susceptible to the environmental pressures associated with the extreme warming became extinct at the PETM, leaving opportunistic taxa and post-disaster faunas . Nonetheless, benthic faunal data indicate several episodes of stress during the EECO with an ecological response similar to that of the PETM (Thomas et al. 1999).