CRITICAL INTERVALSWe have known for many years that the early Paleogene, particularly the early Eocene (~53 to 50 Ma), represents the most extreme long-lived interval of global warming witnessed on Earth since the well-documented mid-Cretaceous 'greenhouse' (e.g., Shackleton and Kennett, 1975). Yet, little is known about the number of constituent hyperthermals, the range of temperatures, or their effects on biotic evolution (Thomas and Zachos 1999). Similarly, whereas we know that the Eocene greenhouse period was followed by a long shift toward lower temperatures and ice sheet growth into the late Eocene and early Oligocene, the detailed history of these events and consequences for ocean circulation, carbon cycling, and biotic evolution are only vaguely understood. The late Eocene is also interesting from the perspective of the response of global climate and biodiversity to the history of large extraterrestrial impact events on Earth. Thus, the Palaeogene can be thought of as containing numerous 'critical intervals' that provide an excellent opportunity to improve our understanding of important paleoceanographic problems involving the dynamics of greenhouse gas release, warm climate stability, biotic turnover associated with extreme climate transitions, and extraterrestrial impacts. Below, we discuss three such intervals that have a particularly high chance of being recovered during Leg 199 drilling.
The Late Paleocene Thermal Maximum
It is now well accepted that the Late Paleocene Thermal Maximum (LPTM) involved a substantial (~5°-7°C) warming in the Southern Ocean and subtropics, a 35%-50% extinction of deep-sea benthic foraminifers, and rapid perturbation to the global geochemical carbon cycle (e.g., Zachos et al., 1993; Koch et al., 1992). A growing body of evidence attributes these events to the massive release and oxidation of methane from the marine gas hydrate reservoir (e.g., Dickens et al., 1997; Katz et al., 1999). High-resolution stable isotope analyses (Bains et al., 1999) and orbitally tuned chronologies from sites in the Atlantic and off Antarctica (Norris and Röhl, 1999) suggest that carbon release occurred extremely rapidly (a fraction of a precession cycle). Thus, the LPTM may represent the best example in the geologic record of the response of the Earth ocean-atmosphere climate system to greenhouse warming on a time scale approaching that of the ongoing global anthropogenic experiment. Recent evidence in favor of elevated biogenic barium accumulation rates in deep-sea sites suggests that enhanced deposition of organic matter in deep-sea sediments may have acted as a negative feedback on atmospheric CO2 levels and global temperatures to return Earth to average late Paleocene conditions (Bains et al., 2000). Whereas these new data support the methane hydrate hypothesis, considerable uncertainty remains about the mechanism and location of carbon release, the response of the CCD, and biotic overturn. Leg 199 presents a major opportunity to help improve our understanding of the chain of events. Results from the leg should prove particularly useful given the volumetric significance of the Pacific Ocean to geochemical mass balance simulations (e.g., Dickens, et al., 1997) and the current paucity of LPTM records from the basin.
The Eocene-Oligocene boundary represents an important point in the transition from the greenhouse world of the Cretaceous and early Paleogene into the late Paleogene-Neogene 'icehouse.' Attempts to estimate global ice volumes from deep-sea benthic delta18O records have prompted very different conclusions as to the timing of the onset of the accumulation of continental-scale ice sheets. These range from the Early Cretaceous (Matthews and Poore, 1980) to the middle Miocene (Shackleton and Kennett, 1975). However, recent improvements in the stratigraphic resolution of the delta18O record have led to suggestions that either the late Middle Eocene (~43 Ma) or the earliest Oligocene (~34 Ma) are better estimates of the greenhouse to icehouse transition (Shackleton, 1986; Miller et al., 1987, 1991; Zachos et al., 1994). Supporting evidence for this interpretation comes from oceanic records of ice-rafted debris, weathered clay mineral compositions, microfossil assemblages, and sequence stratigraphic analyses (Browning et al., 1996). Yet, the rarity of complete deep-sea sections across these intervals has limited our understanding of the dynamics of this important step to the modern icehouse world.
The Eocene-Oligocene transition is marked by a large rapid increase in the benthic
foraminiferal calcite delta18O record in earliest Oligocene time (Oi-1). This excursion was first
ascribed to a 5°C temperature drop associated with the onset of thermohaline circulation but,
more recently, Oi-1 has been associated with the onset of continental ice accumulation on
Antarctica. Such confusion reflects the long-standing difficulty of separating the effects of
temperature and ice on benthic delta18O. Recent application of an independent paleothermometry
technique based on Mg/Ca in benthic foraminifers shows no significant change corresponding
to Oi-1 (Lear et al., 2000). This result suggests that all of the delta18O increase associated with Oi
1 can be ascribed to ice growth with no concomitant decrease in polar temperatures. This
finding implies that the trigger for continental glaciation lay in the hydrological cycle rather than
the carbon cycle. Specifically, it has been proposed that the opening of the Australian-Antarctic
seaway in earliest Oligocene time might have enhanced the supply of moisture as snow to the
Antarctica interior (Lear et al., 2000). Yet, our most complete records of the Eocene/Oligocene
boundary come from only two mid-latitude sites (DSDP Site 522, ODP Site 744). Leg 199
offers an excellent opportunity to generate low-latitude records of the Eocene-Oligocene
transition and thereby fully evaluate the competing roles played by global cooling and ice
growth in the transition from the Cretaceous greenhouse into the Neogene icehouse.
Late Eocene Impact Events
Widespread evidence now exists to support the occurrence of at least two large closely spaced extraterrestrial impact events on Earth during early late Eocene time. In particular, two large craters (order ~100 km diameter; Chesapeake Bay, North America and Popigai, Northern Siberia) have been proposed to explain impact-ejecta strewn fields that are documented in deep-sea sediments from around the world (e.g., Koeberl et al., 1996; Bottomley et al., 1997). Proxy records for fine-grained extraterrestrial dust (Helium-3 measurements) in correlative marine carbonate strata have been interpreted as evidence for a comet shower triggered by an impulsive perturbation of the Oort cloud (Farley et al., 1998). Intriguingly, unlike the more famous and pronounced precursor extraterrestrial impact event at Cretaceous/Paleogene boundary time, biostratigraphic studies indicate that the late Eocene impact horizons do not correspond to major extinctions among marine organisms. Only five radiolarian species appear to disappear from the record, accompanied by modest compositional changes in planktonic foraminifers and organic-walled dinoflagellate cysts (Sanfilippo et al., 1985; Keller, 1986). On the other hand, whereas little evidence exists in the literature for climate change across the Cretaceous/Paleogene transition, recent work has suggested that the late Eocene impact event was associated with a short-term (~100 ka), albeit modest (maximum 2°C) cooling event at high latitude (Vonhof et al., 2000). Leg 199 presents an ideal opportunity to study the climatic and biotic effects of impacts that were too small to precipitate global mass extinctions but were apparently large enough to have engendered global changes in climate.
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