PP14A-01 16:00h
Model Simulations of the Global Carbon and Sulfur Cycles: Implications for the Paleocene-Eocene Thermal Maximum
Extreme global warmth and an abrupt negative carbon isotope excursion during the Paleocene-Eocene Thermal Maximum (PETM) have been attributed to a rapid addition of isotopically depleted carbon to the ocean-atmosphere system. Potential carbon sources include the abrupt release of 1000-2000 Gt C as methane hydrate (\delta$^{13}$C ~-60\permil) from sediments on the continental slope (Dickens et al., 1995) and the oxidation of 8000-9000 Gt of organic carbon (\delta$^{13}$C ~-25\permil) in rampant global wildfires (Kurtz et al., 2003). Using a simple geochemical model of the global carbon and sulfur cycles, we investigate whether these hypotheses are consistent with estimates of climate warming during the PETM by considering the effects of atmospheric composition and climate in the Paleocene and feedbacks driven by changes in sulfur cycling and seawater chemistry. Modest increases in atmospheric CO$_{2}$ (70-150 ppm) associated with methane hydrate release cannot, without additional feedbacks in the climate system, account for a $5-6\deg$ C increase in global sea surface temperature during the PETM. In contrast, a significant increase in atmospheric CO$_{2}$ (600-700 ppm) is observed following the oxidation of 8000-9000 Gt of organic carbon. However, constraints on the size and extent of the Paleocene terrestrial carbon pool and the absence of geologic evidence indicative of vast wildfires argue against a global conflagration as an important source of depleted carbon. Instead, we interpret the PETM and its associated negative carbon isotope excursion as representing the oxidation of 8000-9000 Gt C as organic matter in shallow marine and near shore terrestrial sediments following the retreat of major epicontinental seaways in the Paleocene. This hypothesis is also consistent with large changes in the sulfur cycle in the early Eocene inferred from the \delta$^{34}$S of seawater sulfate. References: Dickens G.R., et al., (1995) Paleoceanography, 10, 965-971. Kurtz, A.C., et al., (2003) Paleoceanography, 18, 1090-1104.
PP14A-02 16:15h
Multiple Early Eocene Thermal Maximums
Periodic dissolution horizons signifying abrupt shoaling of the lysocline and CCD are characteristic features of deep-sea sections and often attributed to Milankovitch forcing via their diagnostic frequencies. Prominent dissolution horizons also correspond to abrupt climate events, such as the Paleocene-Eocene thermal maximum (PETM), as a result of input of significant CH$_{4}$ - CO$_{2}$ into the ocean-atmosphere system. The question arises whether other significant dissolution horizons identified in sediments of late Paleocene and early Eocene age similar to the recently identified ELMO (Lourens et al., 2004) were formed as a result of greenhouse gas input, or whether they were related to cumulative effects of periodic changes in ocean chemistry and circulation. Here we report the discovery of a 3$^{rd}$ thermal maximum in early Eocene (about 52 Ma) sediments recovered from the South Atlantic during ODP Leg 208. The prominent clay layer was named the "X" event and was identified within planktonic foraminifer zone P7 and calcareous nannofossil zone CP10 at four Walvis Ridge Transect sites with a water depth range of 2000 m (Sites 1262 to 1267). Benthics assemblages are composed of small individuals, have low diversity and high dominance. Dominant taxa are {\it Nuttallides truempyi} and various abyssaminids, resembling the post PETM extinction assemblages. High-resolution bulk carbonate \delta$^{13}$C measurements of one of the more shallow Sites 1265 reveal a rapid about 0.6 per mill drop in \delta$^{13}$C and \delta$^{18}$O followed by an exponential recovery to pre-excursion \delta$^{13}$C values well known for the PETM and also observed for the ELMO. The planktonic foraminiferal \delta$^{13}$C records of {\it Morozovella subbotina} and {\it Acaranina soldadoensis} in the deepest Site 1262 show a 0.8 to 0.9 per mill drop, whereas the \delta$^{13}$C drop of benthic foraminifera {\it Nuttallides truempyi} is slightly larger (about 1 per mill). We are evaluating mechanisms for the widespread change in deep-water chemistry, its connection to the surface-water response, and the relationship of the event, as well as the PETM and ELMO, with current astronomical solutions (Laskar et al., 2004; Varadi et al., 2003). References 1. Lourens, L.J., Sluijs, A., Kroon, D., Zachos, J.C., Thomas, E., Roehl, U., and the ODP Leg 208 Shipboard Scientific Party, 2004. An early Eocene transient warming (~53 Ma): Implications for astronomically-paced early Eocene hyperthermal events.- Abstract, 8th International Conference on Paleoceanography (ICP), 5-10 September 2004, Biarritz, France. 2. F. Varadi, B. Bunnegar, M. Ghil, Astrophysical J. 592, 620-630 (2003). 3. J. Laskar et al., Astronomy and Astrophysics (2004).
PP14A-03 16:30h
An early Eocene transient warming ($\sim $53 Ma): Implications for astronomically-paced early Eocene hyperthermal events.
Here we report on a pronounced hyperthermal event approximately 2 Myrs after the Paleocene-Eocene thermal maximum (PETM) and place both events in a new orbitally-tuned framework. This previously unrecognized transient event and the underlying PETM were both recovered at five sites along a 2km depth transect on the Walvis Ridge (southeast Atlantic) during Ocean Drilling Program Leg 208. Similar to the PETM, this event, \textit{Elmo}, is marked by a red clay layer, associated with severe drop in CaCO$_{3}$ concentrations, suggesting a $\sim $2 km rise in the lysocline. High-resolution (1cm) bulk carbonate $\delta ^{13}$C measurements of the shallowest, and hence most complete Site 1263 revealed a $\sim $1.5% drop in $\delta ^{13}$C and $\delta ^{18}$O. The negative $\delta ^{13}$C excursion is composed of three steep steps of which the last one corresponds to the base of the red clay layer. The post-\textit{Elmo} interval mirrors the typical PETM signature with an exponential recovery to pre-excursion $\delta ^{13}$C values. The planktonic foraminiferal $\delta ^{13}$C record (measured on single specimens of the surface dweller \textit{Acaranina soldadoensis}) confirms the bulk pattern, although the amplitude of the excursion, $\sim $-2.5, is larger. The planktonic foraminiferal $\delta ^{18}$O record shows the same $\sim $-1% excursion as the bulk, suggesting a fast increase in sea surface temperature of $\sim $4 degrees C. The case of conditions similar to the PETM during the ELMO event is further strengthened by the occurrence of low diversity, diminutive benthic foraminifer assemblages, and a decrease in planktonic foraminifer diversity. Examination of published isotope, X-ray fluorescence and magnetic susceptibility records further indicates that this event is global in nature and recorded in other marine and terrestrial basins. Orbital tuning of the deepest Sites 1262 magnetic susceptibility and Sites 1262 and 1267 color reflectance records to the La2004 and R7 astronomical solutions shows that the \textit{Elmo} is five 400-kyr eccentricity cycles younger than the PETM and that both events are linked to eccentricity-maxima. This, and the subtle resemblance between the \textit{Elmo} and PETM mentioned above, suggests that similar, astronomically modulated, mechanisms are at the root of them. The leading hypothesis to explain the PETM climatic event and its $\delta ^{13}$C excursion is the dissociation of submarine methane hydrates. If the \textit{Elmo} shares a similar origin, its less extreme appearance may be associated with the ability of the methane hydrate reservoir to recharge after the PETM, especially under the warm conditions that prevailed in the interval spanning the two events.
PP14A-04 16:45h
Revisiting ODP Site 690 to Assess the Responses of Marine Carbonate Chemistry to the Paleocene-Eocene Thermal Maximum
The close of the Paleocene epoch (ca. 55 Ma) is punctuated by a transient ($ < $100 kyr) global warming event referred to as the Paleocene-Eocene Thermal Maximum (PETM). In the marine realm, hallmark signatures of the PETM are a negative carbon isotope excursion (CIE) on the order of 3 per mil, widespread carbonate dissolution, and a benthic foraminiferal mass extinction. The rapid onset and anomalous magnitude of the CIE has been attributed to a sudden release of some 2000 Gt of methane into earth's surficial carbon reservoir. Presumably, oxidation of this methane elevated pCO2 levels in the atmosphere/ocean system, fueling global greenhouse warmth and carbonate dissolution. Here we revisit what is arguably the most complete deep-sea record of the PETM recovered from ODP Site 690 to explore the dynamic coupling between atmospheric CO2 levels, marine carbonate chemistry, continental weathering and global climate. The abrupt onset of the CIE is accompanied by a sharp decline in wt.% carbonate, yet wt.% coarse-fraction ($ > $63 microns, foraminiferal shells) values remain fairly constant. These sedimentological shifts collectively point toward the selective removal of fine-fraction ($ < $63 microns) carbonate produced by calcareous nannoplankton during peak oceanic warmth. We believe this selective pattern of "dissolution" actually reflects, in part, reduced calcification among some calcareous nannofossil taxa. An important corollary of this interpretation is that rising pCO2 levels attained a critical threshold that inhibited nannoplankton calcification. Decreased surface-ocean carbonate production triggered a shoaling of the local lysocline and concomitantly enhanced the ocean's carbon-storage capacity providing an important sink for atmospheric CO2. The character of carbonate sedimentation is reversed during the later, recovery stages of the CIE. It is within this stratigraphically expanded portion of the CIE that wt.% carbonate values and the relative proportion of wt.% fine-fraction increase markedly. This secondary shift coincides with a 5 degrees C cooling of intermediate waters and a sharp influx of kaolinite. Concurrent cooling of sea-surface temperatures is also suggested by the disappearance of warm-water microplankton. We believe these changes to be interrelated. The kaolinite spike likely reflects intensified silicate weathering on Antarctica as well as increased continental runoff. Thus, enhanced silicate weathering reactions (CaSiCO3 + CO2 -$ > $ SiO2 + CaCO3) may have served as an added sink for atmospheric CO2 and a source of oceanic Ca2+ and HCO3- that drove an alkalinity overshoot thereby fostering increased carbonate sedimentation within the Weddell Sea region. The remarkable sequence of oceanic changes preserved in the Site 690 PETM record is consistent with the hypothesis that both marine carbonate chemistry and continental weathering acted as negative feedbacks to curb PETM warmth.
PP14A-05 17:00h
Barium Cycling During the Paleocene-Eocene Thermal Maximum: Evidence From Ba/Ca in Foraminifera
The Paleocene-Eocene thermal maximum (PETM) around 55 Ma reflects short-term, rapid climate change during a period of intense greenhouse climate. This interval is characterized by a negative carbon isotopic shift, interpreted as a release of methane from seafloor gas hydrate reservoirs. This perturbation of the carbon cycle is accompanied by significantly greater rates of euhedral barite accumulation in deep sea sediment commonly believed to be a reflection of elevated primary productivity in surface waters. An interpretation of higher productivity during the PETM, however, is contrary to microfossil assemblage data which indicates a decrease in primary productivity. It has also been suggested that the increase in barite accumulation during the PETM may have been the result of an increase in dissolved barium concentrations in the deep ocean coeval with methane release. This supposition has support from the fact that modern gas hydrate reservoirs are surrounded by pore waters with dissolved barium concentrations considerably higher than that of seawater. This investigation utilizes the barium content of foraminifera as a proxy to reconstruct changes in the barium concentration of the ocean. At 55 Ma, Ba/Ca decreases between 25 to 28% in the planktic foraminifer \textit{Morozovella velascoensis}, indicating a decrease in the barium concentration of the surface ocean. These results bolster the theory that there was increased biogenic barite precipitation during the PETM. Changes in surface water temperature and pH may have altered species assemblages such that celestite (SrSO$_4$) precipitating organisms enriched in barium as BaSO$_4$ (possibly acantharia) were dominant, modifying the barite precipitation pathways, which affected water column barite cycling. Ba/Ca and Cd/Ca measurements on benthic foraminifera show a positive correlation with Mn/Ca, indicating contamination of manganese oxide coatings. This contamination is in part due to the greater surface to volume ratio of benthic foraminifera, which are smaller than planktic species. Additionally, smaller quantities of benthics were available for analysis, which limited the intensity of pre-treatment to remove these coatings. After correcting for this coating contamination, no significant change in benthic Ba/Ca could be discerned through this interval. Taken together, the planktic and benthic foraminiferal Ba/Ca data suggests that enhanced biogenic barite accumulation during the PETM was primarily due to increased export from the surface waters.
PP14A-06 17:15h
Coccolithophorid Productivity Response to Paleocene-Eocene Thermal Maximum
The response of marine plankton to the Paleocene-Eocene Thermal Maximum may have influenced the rate of uptake of greenhouse gases and recovery of the climate but indicators of marine productivity in the open ocean are sparse and have yielded conflicting results at some sites. Here we report on the productivity response of coccolithophorids during the PETM at sites from the Southern Ocean, Tropical Pacific, and Tropical Atlantic. A novel technique for picking individual coccoliths for analysis using secondary ion mass spectrometery (SIMS) allows us to apply the coccolith Sr/Ca productivity proxy to individual genera and avoid biases from changing species assemblages and noncoccolith carbonate in sediments. In the Southern Ocean site, Chiasmolithus experiences a brief surge in productivity in the first thousand years following the event onset, Zygrhablithus experiences an increase in productivity beginning about 20,000 years following the event onset and lasting about 50,000 years. Toweius, the dominant genera, experiences increased productivity beginning about 20,000 years following the event onset and lasting about 100,000 years and likely contributing to the high carbonate accumulation rates at this site during the carbon isotope recovery. Discoaster shows a slight decrease in productivity. In the Equatorial Pacific Site, Toweius experiences a dramatic decline in productivity after the event onset, and recovery to pre-event levels about 50,000 years later. Discoaster shows a slight increase in productivity during the PETM. In the Equatorial Atlantic site, Toweius shows a slight decrease in productivity during the PETM whereas Coccolithus is unaffected. Consequently, while published Os isotopic data are inferred to indicate increased global average weathering rates, the largest effect of such an increase may have been in higher latitude areas which were previously weathering-limited. Hence, the effect of nutrient fertilization may have been confined to these regions.
PP14A-07 17:30h
Major Transient Floral Change During the Paleocene-Eocene Thermal Maximum
New continental sections representing the Paleocene-Eocene Thermal Maximum (PETM) have been discovered in the southern Bighorn Basin, Wyoming. Three localities preserving leaves and fruits are the first megafossil record of plants from this geologically short (about 150ky) period of intense warming. The localities produce fossil pollen and spores as well. The plant remains are in sections that preserve vertebrate fossils characteristic of the Wa-0 faunal zone that elsewhere is restricted to the period of negative carbon isotopic values that occurred during the PETM. The megaflora and palynoflora from the three localities contain a small number of long-ranging taxa that are common in many late Paleocene and early Eocene localities from the Bighorn Basin (e.g., {\it Macginitiea}, {\it Caryapollenites}, {\it Ulmipollenites}, {\it Alnipollenites}), however they also produce up to 10 taxa that have not been seen before in either the Paleocene or Eocene of this area. In the palynoflora, several of these previously unrecorded taxa (e.g, {\it Labropollis}, cf. {\it Bombax}, and a distinctive large tricolpate grain) are common in the Gulf Coastal Plain Paleogene. Although the number of dicot leaf species so far is too small to produce a robust physiognomic estimate of temperature or precipitation, it is notable that one locality near the base of the PETM is dominated by small-leaved legumes, and another near the top by an undetermined dicot leaf with an extremely long drip-tip. Preliminary analysis of the floral data strongly suggests that some types of plants extended their ranges up to 1500 km northward during the PETM, where they became sympatric with native mid-latitude taxa. These range extensions are restricted to the PETM. Furthermore, PETM floral change was essentially synchronous with the better-known mammalian turnover event. The unusual physiognomy of PETM fossil leaves raises the possibility of major changes in seasonality or amount of precipitation. We plan to measure stable carbon isotope ratios in organic matter and soil carbonates from the new sections in order to establish more precisely the relationship between biotic and geochemical change during the PETM.
PP14A-08 17:45h
New Sections and Fossils From the Southern Bighorn Basin, Wyoming Document Faunal Turnover During the PETM
Though earliest Eocene (Wa-0) mammals are known from the southern Bighorn Basin, late Paleocene mammals are not. Recent discovery of latest Paleocene mammals in section with new Wa-0 faunas and floras at Cabin Fork allows for the first studies of terrestrial biotic change across the Paleocene-Eocene boundary interval outside of the northern Bighorn Basin. A differential GPS was used to map the area and provide a framework for high-resolution biostratigraphy. Least squares interpolation of bedding planes from points marking outcrop of beds reveals high r$^{2}$ coefficients (0.97-0.98). This indicates that small scale folding is minimal and bed traces are smoothly planar. Beds in the study area strike N-NW ($355\deg$) and dip shallowly W-SW ($ < $$1.0\deg$). Smaller scale undulations are present: to the NE beds strike NW and dip to the SW ($342\deg$/$1.0\deg$) whereas those to the SW strike NE and dip NW ($5\deg$/$1.0\deg$). Shallow dips allow us to approximate stratigraphic thickness with elevation. Paleocene mammals, including diagnostic Clarkforkian land-mammal age indicators, {\it Aletodon gunnelli}, {\it Apheliscus nitidus}, and {\it Haplomylus simpsoni}, were found in a ferruginous, grit-pebble conglomerate at the base of a channel sand at the top of the Fort Union Fm. The fossiliferous horizon is extensive and has produced over 200 specimens from more than 60 sites for which positions have been determined with sub-meter accuracy. Absence of {\it Plesiadapis cookei} and {\it Hyracotherium} spp., together with high relative abundance of {\it Phenacodus} and {\it Ectocion}, indicate this fauna is latest Clarkforkian ({\it Phenacodus-Ectocion} Range Zone, Cf-3). Earliest Eocene mammals, including diagnostic Wa-0 taxa {\it Arfia junnei}, {\it Copecion davisi}, {\it Hyracotherium sandrae}, and {\it Diacodexis ilicis}, are represented by more than 233 specimens from 70 sites at three levels in the lowest Willwood Formation. The lowest fossils come from paleosols and claygall accumulations in stringer sands approximately 3 meters above the top of the channel sand that is the top of the Fort Union Formation, and 8 meters above the Clarkforkian localities. The second level is a fossiliferous paleosol 6-8 meters above the top of the channel sand. The third level is a paleosol fossil accumulation approximately 22 meters above the top of the channel sand and 15 meters below a large and persistent red paleosol informally called ?Big Red.? In the Cabin Fork area, Big Red produces mammal fossils, including {\it Cardiolophus}, that have been used to define the succeeding Wa-1 faunal zone (although some Wa-0 taxa have been recovered from Big Red approximately 6 km to the east). The Wa-0 interval in the Cabin Fork area is 38-40 meters thick and bounded by distinct Cf-3 and Wa-1 faunas. Preliminary analyses of fossils from the Cabin Fork section show that the faunal shift marking the PETM was very similar to that seen in the northern Bighorn Basin. This suggests that previously documented differences between Wa-0 mammalian faunas in the southern and northern Bighorn Basin, including the absence of certain diagnostic Wa-0 taxa in the south, may reflect local sampling variation or artifacts rather than regional variation in faunal composition.