Sunday, December 5, 2010

Florissant’s Sequoias: Redwood Giants of the Eocene

During the late Eocene large Sequoia (redwood) trees grew on terraces along an ancient river valley near present-day Florissant. Among these trees was the extinct Sequoia affinis, closely related to the modern redwoods (Sequoia sempervirens) that are now restricted to a narrow coastal region in northern Californa.

The “Big Stump” is one of the largest petrified redwood stumps exposed in
the Monument.  It measures 3.6 m tall and is 3.7 m in diameter at breast height. 
This stump is all that remains of a tree that was more than 90 m tall
when the mudflow buried its base.  Image date Nov 2003 by S. Veatch.
 More than 34 million years ago a lahar or volcanic mudflow, coming from a nearby volcanic center, flowed into the Florissant valley. The mudflow was up to 5 meters deep and quickly surrounded these giant trees. The upper section of the redwoods decayed and rotted away while the lower 5 meters were protected by the silica-rich mudflow. The unhurried progression of petrification—through a process known as permineralization—preserved the remains of these ancient trees with incredible detail, including tree rings.

Tree rings, which record annual growth, also reveal the growing conditions of trees. A researcher at the Florissant Fossil Beds National Monument carefully measured tree rings in fossil wood (Sequoioxylon pearsallii) and noted a larger (40%) average tree-ring width than the modern redwoods (Sequoia sempervirens) and giant Sequoia (Sequoiadendron giganteum) growing along the northern California coast. The significant difference in mean ring width between modern and extinct redwoods suggests the ancient redwoods in the Florissant Valley were growing under more favorable conditions than their modern counterparts.
It is thought that there was more precipitation, from moist Pacific Ocean air,that reached the western interior as the Sierra Nevada mountains of California had not yet been uplifted to block the flow of moist air. More precipitation, resulting from these moist air masses, would have fallen during the growing season.

Precipitation at Florissant during the late Eocene is estimated at about 50 – 80 centimeters of annual rainfall, greater than the modern precipitation of 38 centimeters. A higher level of atmospheric CO2, perhaps twice that of modern levels, may have contributed to the favorable growing conditions in the Florissant Valley of the late Eocene.

A subsequent volcanic mudflow moved across the ancient paleovalley, forming a dam of mud, rocks, boulders, and associated debris. A stream, running over earlier mudflows, began to back up behind the mud dam, eventually forming a large lake—Lake Florissant. Leaf and insect fossils were preserved in this lake.

Not only are the Sequoia stumps preserved, but Sequoia cones and foliage are also represented in the fossil record at Florissant. The cones and foliage are found separated from the tree, and have a separate classification from the fossil wood. Sequoia affinis cones are found as fossils in the thinly bedded shales of ancient Lake Florissant. The ovoid cones are made of spirally arranged scales and tend to be smaller (about two-thirds the size) than the modern coast redwood cones.

Fossil branches of the Florissant redwood, Sequoia affinis. 
Specimen FLFO-4858 from the collection of Florissant Fossil Beds National Monument. 
Image date Oct 2003 by S. Veatch.

Sequoia affinis cones. Specimen FLFO-4717 from
the collection of Florissant Fossil Beds National Monument. 
Image date Oct 2003 by S. Veatch.

6 comments:

  1. Very interesting! I am a new resident to the area, and was wondering the history of the fossil beds. This article was very informative. Now I MUST visit the Florissant Fossil Beds to see for myself.

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  2. You're trying to tell me that redwoods evolved and grew at 8,000 feet with winter temps as low as -15 C?

    You're theorizing that a large lake sporadically formed in this area and grew to support marine life, fish, and mollusks?

    You're hypothesizing that Colorado's vast sub-arid climate, rocky terrain, and elevation variances supported sub-tropical dwelling plants and animals?

    And please draw on a map the extent of this 'lake'?

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  3. The elevation and temperatures were different here during the Eocene. The petrified wood and fossils support the discussion about the flora and fauna that once lived in Florissant 34 MYA.

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  4. You seem very sure of yourself on that last statement.

    Are you trying to say that the petrified trees I see in Florissant were once close to sea level but are now magically 1 mile in the air?

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    1. It isn't magic, it's plate tectonics. Mobile land masses collided with the Pacific coast causing large thrust faults to move huge slabs of rock eastward, resulting in significant thickening of the crust (and thereby, increased the elevation). This has studied, deciphered, argued, and documented by many geologist in excruciating detail.

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  5. There is ongoing scientific debate about the elevation of the trees when they were petrified. What do you have to say about all of this?

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