Princeton Chert

The Princeton Chert is a fossil locality in British Columbia, Canada, that represents one of the best preserved collections of Eocene Epoch floras in the world, with rich species abundance and diversity (Basinger 1976, Cevallos-Ferriz et al. 1991, Stockey 2001). It is located on the east bank of the Similkameen River, approximately 10 km NNE from the town of Princeton (Stockey 2001) in the Regional District of Okanagan-Similkameen.

The area and its fossils have been known since the 1960s, but have recently attracted increased attention. This may be due to the rare type of silica permineralized fossil Lagerstätten found, which has preserved plants and animals in minute 3D detail, with exceptional internal cellular detail. This has meant anatomical descriptions and reconstruction of whole plants from isolated parts has been possible in many species (Basinger 1976, Cevallos-Ferris et al. 1989, Lepage et al. 1994). Few plant fossils elsewhere in the world exhibit such excellence in both preservation and diversity. Similar aged fossil beds in Eocene lake sediments are found elsewhere in British Columbia, including in Driftwood Canyon Provincial Park near Smithers in northern British Columbia, and the McAbee Fossil Beds west of Kamloops, about 160 km NNW of the Princeton Chert beds.

Location and geologic setting

The Princeton Chert is an interbedded sequence consisting of coal, shale, and chert in the Allenby Formation (Basinger 1976, 1984; Moss et al. 2005; Archibald et al. 2011). 49 chert layers, ranging in thickness from 1 to 55 cm have been recognised and described, though each is not unique in organisms preserved. Despite this, trends are evident throughout the outcrop, with certain taxa appearing and disappearing with time (Stockey 1987, 2001). The Princeton Chert was originally considered to be Middle Eocene based on data from mammals (Russell 1935), freshwater fish (Wilson 1980), and potassium-argon dates (Hills & Baadsgaard 1967). Recently, more accurate radiometric techniques provided a date of 48.7mya (Stockey 2001). The Allenby Formation, however is now dated radiometrically as being Early Eocene (Moss et al. 2005; Ickert et al. 2009; Archibald et al. 2011) and it is likely the Princeton Chert is also.

The climate at this time was warm; it had reached a maximum during a series of warming events during the Early Eocene with the Princeton Chert likely deposited after the Eocene Thermal Maximum 2 and during the Early Eocene Climatic Optimum (Bohaty 2003; Moss et al. 2005; Zachos et al. 2008). During this time the sea warmed approximately 4°C and terrestrial temperatures were several degrees warmer than today, meaning little or no ice was present. The temperature difference between poles and equator was small (Pearson 2010). This long term warmth is thought to be due to increased greenhouse gases, particularly CO₂ trapping more heat (Bijl et al. 2010; Zachos et al. 2008). The reason for this sudden increase in CO₂ is unknown, but it is hypothesised that it was due to an increase in ocean floor being recycled via volcanic arcs and metamorphic decarbonation reactions (Pearson 2010). This happened because the ocean between India and Asia was disappearing and being replaced by the Himalayas and the Tibetan plateau due to the collision of tectonic plates. Also at the time, Australia, which was joined to Antarctica, was beginning to move northwards (Scotese 2003).

The Princeton Chert fossils indicate that the area was an aquatic ecosystem, growing in tropical to subtropical conditions (Stockey 2001). Several of the smaller chert layers degrade to volcanic ash, indicating nearby volcanic activity. It is thought that fossils were pervaded with silicic acid due to this volcanic activity. Subsequently, water charged with minerals flowed from springs or geysers into the low lying basin where the Princeton chert was located. Here, the water surrounded organisms as they grew, along with plant debris which had been accumulated (Stockey 2001). Many organisms were preserved in situ, in the lake or small pond environment in which they lived (Cevallos-Ferriz et al. 1991). We know this preservation must have been rapid, due to the minute cellular detail which has been conserved. This sequence of events is thought to have been replicated up to 50 times, as the basin allowed peat to reaccumulate each time (Stockey 2001), producing the multiple layers.

Fossil biota

Sampling into the Princeton Chert has been carried out, but presently the data has not been analysed in detail (Stockey 2001). Across the outcrop, trends in taxa can be seen; in the topmost layers fossil organs of Metasequoia milleri (Basinger 1984) cease to be represented, yet Pinus (pine) and monocotyledons increase in number. There is a huge increase in ferns, such as Dennstaedtiopsis (Fig. 4a.), after a huge ash fall, though few angiosperms occur in these layers. A large number of angiosperms have been found (Table 1) along with several types of conifers, ferns, and several unidentified fossils from various families (Stockey & Wehr 1996).

Fungi

Pathogenic fungi have been recorded on the leaves and other organs of some vascular plants. Fossil Uhlia palms have tar spot fungi on their leaves known as Paleoserenomyces. Interestingly, these fungi are themselves parasitized by a mycoparasite, Cryptodidymosphaerites princetonensis (Currah et al. 1998) Symbiotic mycorrhizal relationships have also been discovered in roots of Pinus and Metasequoia. In Metasequoia these associations have been compared to extant mycorrhizae, and found to be very similar (Stockey 2001).

In situ lacustrine fossils

The array of floral and faunal fossils found in the Princeton Chert has offered unequivocal evidence that it was a lacustrine environment. The plant fossils found show many structural and anatomical adaptations to an aquatic environment, including a reduced vascular system, aerenchyma in tissues (air spaces to provide buoyancy), and protoxylem lacunae surrounded by a ring of cells with thickened inner walls (Stockey 2001, Cevallos-Ferriz et al. 1991). Further evidence is provided by the fossils’ clear affinities with modern aquatic angiosperms. Many extant plants show these adaptations and are similar to the organisms found in the chert. For example Nymphaeaceae (Allenbya), Araceae (Keratosperma) and Juncaceae/Cyperaceae (Ethela) are just some of the angiosperms found both today and in the Princeton Chert. Seeds have also been found which share adaptations with living aquatics (Stockey 2001, Cevallos-Ferriz et al. 1991). On the other hand, terrestrial fossils have rarely been found. The few that are, are represented mainly by seeds, which are likely to have been transported by birds (Stockey & Wehr 1996). Additional support for the aquatic nature of the Princeton Chert deposits comes from animal fossils. Several fossils of a freshwater fish, Amia (bowfin), have been found in the shale overlying the plant deposits (Wilson 1982), along with remains of the freshwater fishes Amyzon and Libotoniusm, and a soft-shelled turtle (Cevallos-Ferriz et al. 1991). Once the lacustrine nature of the fossils has been established, it seems fairly likely they were preserved in situ, especially considering the method of preservation. The growth position, large number of plant organs of the same type, preservation of delicate plant material, and presence of rooted axes all provide further evidence for the preservation of plants where they grew.

References

• Archibald, S. B., Greenwood, D. R., Smith, R. Y., Mathewes, R. W., and Basinger, J. F. (2011) Great Canadian Lagerstätten 1. Early Eocene Lagerstätten of the Okanagan Highlands (British Columbia and Washington State). Geoscience Canada, 38(4), pp. 155–164.
• Basinger, J.F. (1976) Paleorosa similkameenensis, gen. et sp. nov., permineralized flowers (Rosaceae) from the Eocene of British Columbia: Canadian Journal of Botany, 54, pp. 2293–2305.
• Basinger, J.F. (1984) Seed cones of Metasequoia milleri from the Middle Eocene of southern British Columbia: Canadian Journal of Botany, 62, pp. 281–289.
• Bijl, P. K., Houben, A. J. P., Schouten, S., Bohaty, S. M., Sluijs, A., Reichart, G., Damsté, S. & Brinkhuis, H. (2010) Transient Middle Eocene Atmospheric CO₂ and Temperature Variations. Science, 330(6005), pp 819–821.
• Bohaty, S. M. & Zachos, J. C. (2003) Significant Southern Ocean warming event in the late middle Eocene. Geology, 31(11), pp 1017–1020.
• Cevallos-Ferriz, S. & Stockey, R. A. (1988) Permineralized Fruits and Seeds from the Princeton Chert (Middle Eocene) of British Columbia: Araceae. American Journal of Botany, 75(8), pp 1099–1113.
• Cevallos-Ferriz, S. R. S., Stockey, R. A. & Pigg, K.B. (1991) Princeton chert: evidence for in situ aquatic plants. Review of Palaeobotany and Palynology, 70(1-2), pp 173–185.
• Currah, R. A., Stockey, R. A. & LePage, B. A. (1998) An Eocene tar spot on a fossil palm and its fungal hyperparasite. Mycologia, 90(4), pp 667–673.
• Erwin, D. M. & Stockey, R. A. (1989) Permineralized monocotyledons from the Middle Eocene Princeton chert (Allenby Formation) of British Columbia: Alismataceae. Canadian Journal of Botany, 67(9), pp 2636–2645.
• Hills, L. V. Baadsgaard, H. (1967) Potassium-argon dating of some lower Tertiary strata in British Columbia. Bulletin of Canadian Petroleum Geology, 15, pp 138–149.
• Ickert, R.B., Thorkelson, D.J., Marshall, D.D., and Ullrich, T.D. (2009) Eocene adakitic volcanism in southern British Columbia: Remelting of arc basalt above a slab window. Tectonophysics, 464, pp. 164–185.
• Lepage, B. A., Currah, R. S. & Stockey, R. A. (1994) The fossil fungi of the Princeton chert. International Journal of Plant Sciences, 155(6), pp 828–836.
• Moss, P.T., Greenwood, D. R., Archibald, S. B. (2005) Regional and local vegetation community dynamics of the Eocene Okanagan Highlands (British Columbia – Washington State) from palynology. Canadian Journal of Earth Sciences, 42(2), pp. 187–204.
• Pearson P. N. (2010) Increased atmospheric CO₂ during the middle Eocene. Science, 330(6005), pp763–764.
• Phipps, C. J., Osborn, J. M. & Stockey, R. A. (1995) Pinus Pollen Cones from the Middle Eocene Princeton Chert (Allenby Formation) of British Columbia, Canada. International Journal of Plant Sciences, 156(1), pp 117–124.
• Russell, L. S. (1935) A middle Eocene mammal from British Columbia. American Journal of Science, 29(169) pp 54–55.
• Scotese, C. R. (2003) Paleomap Project. [Online]. Available from: http://www.scotese.com/ [Accessed 18/03/2012].
• Smith, S. Y. & Stockey, R. A. (2003) Aroid seeds from the Middle Eocene Princeton chert (Keratosperma allenbyense, Araceae): comparisons with extant Lasioideae. International Journal of Plant Science, 164(2), pp 239–250.
• Stockey, R. A. (1987) A permineralized flower from the middle Eocene of British Columbia. American Journal of Botany, 74(12), 1878-1887.
• Stockey, R. A. (2001) ‘The Princeton Chert’ In: Briggs, D. E. G. and Crowther, P. R., (eds.) Palaeobiology II. Oxford, Blackwell Science pp 359–362.
• Stockey, R. A. & Wehr, W. C. (1996) ‘Flowering plants in and around Eocene lakes of the interior’ In: Ludvigsen, R. (ed.) Life in Stone: A natural history of British Columbia’s Fossils. Vancouver, UBC Press, pp 234–247.
• Wilson, M. V. H. (1980) Eocene lake environments: Depth and distance-from-shore variation in fish, insect, and plant assemblages. Palaeogeography, Palaeoclimatology, Palaeoecology, 32 pp 21–44.
• Wilson, M. V. H. (1982) A new species of the fish Amia from the Middle Eocene of British Columbia. Palaeontology, 25(2), pp 413–424.
• Zachos, J. C., Dickens, G. R. & Zeebe, R. E. (2008) An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics. Nature, 451, pp. 279–283.