Trace Fossils of the Willwood Formation and
the Paleocene-Eocene Thermal Maximum
Early Eocene Global Warming
The Paleocene-Eocene Thermal Maximum (PETM) is recognized worldwide in continental and marine deposits by a 2–6‰ negative C isotopic excursion (CIE) in marine & continental deposits approximately 55.5 Ma (Koch et al., 1992; Magioncalda et al., 2004; Fig. 1). Mean annual temperatures in the Bighorn Basin are estimated to have increased by 3–7°C (Koch et al., 2003; Wing et al. 2005). The CIE and global warming resulted likely from a large release of 13C-depleted carbon to the atmosphere (Zachos et al., 2005); possibly from destabilized methane clathrates on the continental shelf (Dickens et al., 1995), combustion of shallowly buried coal or peat (Kurtz et al., 2003), or volcanogenic methane (Storey et al., 2007). The PETM is an important analog for current climate warming because both are inferred to have similar rates and magnitudes of increased temperatures and atmospheric carbon. Representative Dana Rohrabacher (R-CA) has his own theories explaining the PETM. Please come back after the Representative's lecture!
Biotic Response to PETM
The PETM coincides with a mass extinction of benthic foraminifera (Thomas, 1998) and a dramatic turnover in fossil mammal faunas in North America and Europe (Beard and Dawson, 1999); including the first appearances of artiodactyls, perissodactyls, and primates in North America. In addition, mammals of the Wa-0 North American Land Mammal Age fauna in the Bighorn Basin are up to 50% smaller than their Paleocene and later Eocene congeners (Gingerich, 2003; Fig. 1).
Willwood Formation and Continental Ichnology
The Willwood Formation is an up to 1400-m-thick succession of mudstone and sandstone interpreted as distal- and proximal-overbank alluvial deposits and trunk-channel deposits; all modified by varying degrees of pedogenesis (Kraus, 2001). An ~40 m section of the Willwood Fm. at Polecat Bench northwest of Powell, Wyoming, was deposited during the PETM based on the isotope chemostratigraphy of pedogenic carbonates and dispersed organic material (Fig. 2). The Willwood Formation contains a diverse and abundant assemblage of plant and invertebrate trace fossils (Bown and Kraus, 1983; Hasiotis et al., 1993; Kraus and Hasiotis, 2006; Smith et al., 2008a, 2008b, 2000c); likely produced by soil biota. This study examines if these soil-organism communities, using trace fossils as proxies, show any detectable response to paleoclimatic changes, as is the case with marine- and surface-organisms, to well-documented PETM global warming.
Fig. 1. PETM δ13C isotope stratigraphy and land mammal response; shaded area highlights the PETM and Wa-0 biozone. Modified from Gingerich (2003) & Magioncalda et al. (2004).
Fig. 2. Willwood Formation: red beds interpreted as mature floodplain soils suggesting well-drained episodes of subaerial exposure; interbedded with coarser-grained gray intervals interpreted rapidly deposited and quickly buried alluvium with little or no pedogenic development.
Crayfish-mediated prismatic structures in Willwood Formation paleosols
Coming Soon
Paleopedologic, paleohydrologic, and paleoclimatic implication of Willwood Formation ichnocoenoses
Coming Soon
References
Beard, K.C. and Dawson, M.R., 1999. Intercontinental dispersal of Holarctic land mammals near the Paleocene/Eocene boundary; paleogeographic, paleoclimatic and biostratigraphic implications. Bulletin de la Societe Geologique de France, 170(5): 697-706.
Bown, T.M. and Kraus, M.J., 1983. Ichnofossils of the alluvial Willwood Formation (lower Eocene), Bighorn Basin, northwest Wyoming, U. S. A. Palaeogeography, Palaeoclimatology, Palaeoecology, 43: 95-128.
Dickens, G.R., O'Neil, J.R., Rea, D.K. and Owen, R.M., 1995. Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Paleocene. Paleoceanography, 10: 965-972.
Gingerich, P.D., 2003. Mammalian responses to climate change at the Paleocene-Eocene boundary: Polecat Bench record in the northern Bighorn Basin, Wyoming. In: S.L. Wing, P.D. Gingerich, B. Schmitz and E. Thomas (Editors), Causes and Consequences of Globally Warm Climates in the Early Paleogene. The Geological Society of America, Special Paper 369, Boulder, Colorado, pp. 463-478.
Hasiotis, S.T., Aslan, A. and Bown, T.M., 1993. Origin, architecture, and paleoecology of the early Eocene continental ichnofossil Scaphichnium hamatum - integration of ichnology and paleopedology. Ichnos, 3: 1-9.
Koch, P.L. et al., 2003. Carbon and oxygen isotope records from paleosols spanning the Paleocene-Eocene boundary, Bighorn Basin, Wyoming. In: S.L. Wing, P.D. Gingerich, B. Schmitz and E. Thomas (Editors), Causes and Consequences of Globally Warm Climates in the Early Paleogene. Geological Society of America Special Paper, 369, Boulder, Colorado, pp. 49-64.
Koch, P.L., Zachos, J.C. and Gingerich, P.D., 1992. Correlation between isotope records in marine and continental carbon reservoirs near the Paleocene/Eocene boundary. Nature, 358: 319-322.
Kraus, M.J., 2001. Sedimentology and depositional setting of the Willwood Formation in the Bighorn and Clarks Fork Basins. In: P.D. Gingerich (Editor), Paleocene-Eocene Stratigraphy and Biotic Change in the Bighorn and Clarks Fork Basins, Wyoming. University of Michigan Papers on Paleontology, 33, pp. 15-28.
Kraus, M.J. and Hasiotis, S.T., 2006. Significance of different modes of rhizolith preservation to interpreting paleoenvironmental and paleohydrologic settings: examples from Paleogene paleosols, Bighorn Basin, Wyoming, U.S.A. Journal of Sedimentary Research, 76: 633-646.
Kurtz, A.C., Kump, L.R., Arthur, M.A., Zachos, J.C. and Paytan, A., 2003. Early Cenozoic decoupling of the global carbon and sulfur cycles. Paleoceanography, 18(4): 1090.
Magioncalda, R., Dupuis, C., Smith, T., Steurbaut, E. and Gingerich, P.D., 2004. Paleocene-Eocene carbon isotope excursion in organic carbon and pedogenic carbonate: direct comparison in a continental stratigraphic section. Geology, 32: 553-556.
Storey, M., Duncan, R.A. and Swisher, C.C., 2007. Paleocene-Eocene thermal maximum and the opening of the northeast Atlantic. Science, 316(5824): 587-589.
Thomas, E., 1998, Biogeography of the late Paleocene benthic foraminiferal extinction, in Aubry, M.P., Lucas, S.G., and Berggren, W.A., eds., Late Paleocene-early Eocene Climatic and Biotic Events in the Marine and Terrestrial Records, Columbia University Press, pp. 214-235.
Wing, S.L. et al., 2005. Transient floral change and rapid global warming at the Paleocene-Eocene boundary. Science, 310(5750): 993-996.
Zachos, J.C. et al., 2005. Rapid acidification of the ocean during the Paleocene-Eocene Thermal Maximum. Science, 308(5728): 1611-1615.
updated 03/03/2009