Day after day, over the course of two years, the massive tunnel borer worked its way through the sedimentary rock layers of the Berkeley Hills during the construction of the fourth bore of the Caldecott Tunnel, grinding up the rocks in the process into fist-size pieces that were later deposited outside the entrance of the tunnel. At the end of each work day, paleontologists sifted through these piles, referred to as the day’s "spoils." They were not only on the lookout for fossils of plants and animals; each day they also collected samples of the rocks for later testing for microfossils.
These samples eventually made their way into one of the prep labs of the UC Museum of Paleontology, a room that has become my second home during the spring semester of 2013. One of my jobs as a graduate student researcher on the CalTrans project is to break down and process these rock samples to look for evidence of ancient microscopic life.
Here I am in the UCMP prep lab. In the foreground are some of the microfossil samples to be processed. Photo by Pat Holroyd.
Looking at forams
Microfossils are by definition too small to be studied with the naked eye. A group of microfossils that we are particularly interested in are the Foraminifera, commonly referred to as “forams.” These single-celled amoeboid-like organisms, which are usually about the size of a sand grain, have shells, known as “tests,” often consisting of multiple chambers, arranged in a myriad of configurations. Living specimens extend strands of protoplasm from their tests in order to “communicate” with their ambient environment. This enables benthic (bottom-dwelling) forms to crawl and the planktonic (floating) forms to remain in suspension, while providing both with a means of obtaining food. Forams are common in marine environments all over the world, and their tests are often a major component of marine sediments.
Left: Drawing of the living foram Polystomella strigillata, from John H. Finley ed. Nelson's Perpetual Loose-Leaf Encyclopaedia (vol. 5) (New York, NY: Thomas Nelson and Sons, 1917); Right: © Creative Commons, Mihai Dragos
Foram tests are important fossils because they are paleoenvironmental indicators. As the tiny fossils accumulate in marine sediments they leave records that are often continuous for long geological stretches of time. By comparing the fossils to modern species, we can infer a great deal about the temperature, ocean depth, and depositional conditions that existed at the time that the organisms were living millions of years ago.
Processing the samples
In order to separate the microfossils from the shale and mudstone matrix, we first gently disaggregate the rocks by soaking them in water and adding Calgon water softener to prevent the finer sediments from clumping. If the rocks don’t readily start to disaggregate, heat and hydrogen peroxide are added. Because the shells of forams and other creatures often contain calcium carbonate we do not use acids to break down the rocks or we will dissolve the fossils at the same time!
Left: First stages of the process; Right: Some of the rocks in this sample are already starting to break down.
Once the rocks have completely broken down, the sediment is rinsed through a sieve with 63 micron (1 micron =0.001 mm) openings to remove silt and clay. After the residue is filtered and dried, it is ready to examine for forams under the stereomicroscope.
Left: Sieving to remove the smaller silt and clay particles; Right: Filtered samples drying in the oven.
So far the process sounds pretty straightforward, but the reality of doing science doesn’t always live up to our expectations. The first batch of samples were from the Orinda Formation; these broke down readily but revealed only a few charcoal fragments. The absence of forams was not surprising, as this unit was deposited in freshwater! I am hoping the Orinda will yield some ostracodes (another kind of microfossil), but none have been observed in the material processed thus far.
I next turned my attention to the samples collected from the definitely marine Sobrante Formation. While a few forams were noted on the surface of some partially broken-down rocks, most of the rocks did not break down at all. While experimenting with some alternative treatments on these samples, including soaking them in kerosene, I have begun to process the tunnel samples of the Claremont Formation, which is stratigraphically between the younger Orinda and older Sobrante formation, and represents the final sequence of marine deposition before emergence of the sea floor.
The first batch broke down readily with our gentle treatments and, when the results were viewed under the microscope, the sediment sample contained not only tiny pieces of coalified plants but a fair number of foraminifera shells.
Left: Examining the dried residue under the stereomicroscope; Right: The view through the eyepiece. Each square in the grid is about 4 mm wide.
UCMP’s foram expert Ken Finger identified the three most common taxa as Martinotiella communis, Pyramidulina acuminata, and Lenticulina sp. Today this benthic association occurs on the continental slope, no shallower than 500 meters. Try to identify the three genera in the close up of the microscope photo on the left, below, based on the reference drawings on the right.
Read other blog posts about the Caldecott Tunnel fossils:
— Fossil neighbors, posted September 12, 2012
— The arrival of the fossils, posted October 1, 2012
— Prepping the fossils from the Caldecott Tunnel, posted May 16, 2013
All photos by Susan Tremblay except where indicated.