Thursday 17 April 2014

Preserved wood from an Early Eocene kimberlite pipe in northwestern Canada’s Slave Province.

Kimberlite pipes are produced by rapid volcanic intrusions carrying magma from the Earth’s mantle rapidly to the surface, often resulting in explosive phreatomagmatic eruptions (explosions caused by hot magma coming into contact with water). These pipes are considered high value economic resources due to the common occurrence of diamonds within them. Surprisingly kimberlite pipes also often contain fossil material. This can come from two separate sources; organisms can fall directly into the erupting lava and be entombed within it as it cools (such intrusions of non-volcanic material, organic or otherwise, are known as xenoliths by volcanologists), alternatively material can be preserved in volcanic craters after the eruption, as organisms are buried in fine-grained volcanic ash and clay (volcanic maar), which has high preservational potential.

In a paper published in the journal PLoS One on 19 September 2012, Alexander Wolfe of the Department of Earth and Atmospheric Sciences at the University of Alberta, Adam Csank of the Environment and Natural Resources Institute at the University of Alaska, Anchorage, Alberto Reyes of the Department of Geoscience at the University of Wisconsin, Madison, Ryan McKellar also of the Department of Earth and Atmospheric Sciences at the University of Alberta, Ralf Tappert of the Institute of Mineralogy and Petrography at the University of Innsbruck and Karlis Muehlenbachs, again of the Department of Earth and Atmospheric Sciences at the University of Alberta, describe the discovery of a number of large wood fragments from the Panda Kimberlite Pipe, a volcanic intrusion which forms part of the  Ekati diamond mining concession worked by BHP Billiton in Canada’s Great Slave Province, which has been calculated to be about 53.3 million years old (Early Eocene). The Panda Kimberlite Pipe forms part of the Lac de Gras Field, which contains about 150 such pipes, emplaced between 45 and 78 million years ago. The Panda Pipe is a simple 200 m diameter cylinder, apparently produced by a single eruption.

Wolfe et al. provide a detailed description of a single piece of wood, a large wood fragment which had fallen into the lava and been mummified. The wood is excellently preserved, with only the outermost millimetre having been fusinized (burned), suggesting an absence of free oxygen when it was entombed. The preserved structure of the wood allows the specimen to be assigned to a tree of the genus Metasequoia, a form of Giant Redwood now restricted to central China, but known to have been common in Alaska during the late Palaeocene and Early Eocene, and therefore not a great surprise in Slave Province.

(D) Fossil wood encrusted in olivine-rich volcaniclastic kimberlite. (E) Photograph of the specimen characterized in this study. The wood was split when removed from the ore, revealing a sliver of opaque amber (9.5 cm long by 0.5 cm wide) in the xylem. Wolfe et al. (2012).

Metasequoia requires a high level of humidity to survive, with a minimum of around 1000 mm of rainfall per year. The area where the fossil was recovered has around 280 mm of rainfall per year, suggesting that the climate was much wetter during the Early Eocene (it is possible that this 53 million year old specimen comes from a tree of the same species as the modern Chinese trees, since these are exceptionally long lived organisms). Since the tree was living close to the Paleocene-Eocene Thermal Maximum (about 55.5 million years ago), when the climate is predicted to have been substantially warmer and wetter in this region, this confirms the climatic predictions. Isotopic data obtained from both the cellulose of the wood and an amber (tree resin) inclusion within the fossil suggests that temperatures would have been around 7-12˚C warmer than at present while the tree was living, again tending to confirm the climatic predictions.

(F) RLS in transmitted light showing uniseriate and biseriate bordered pits and cross-fields. (G) TLS showing rays stacked 3–26 cells high. (H). SEM (TS) of ring boundary with earlywood (left) and latewood (right). (I) Close-up of tracheids in TS and calcite crystals within cells (arrows). J. Cross-section of ray with cross-field pits. (K and L) Close-ups of cross-field pits. (M) TLS close-up of rays. (N). Radial longitudinal section showing four contiguous rows of ray parenchyma cells with smooth end walls and no separation between the individual rows of cells. Wolfe et al. (2012).

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