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Fieldwork: Context matters

I recently returned from fieldwork in Bolivia, and although I am a paleontologist, our team’s focus wasn’t fossils. Rather, we were collecting other types of data to allow us to understand the exact ages of fossils from the site and the environments in which those fossils were preserved. This contextual information is just as important as the fossils themselves, as it makes it possible to integrate information from many fossil sites to answer far-reaching questions like how ecological communities change over millions of years. This year’s trip was also a great learning opportunity for me because I was able to see firsthand how researchers in other specialties collect their field data.

Radiometric Dating

One of the most common questions I get asked is “How do you know how old a fossil is?” The short answer is that we use principles of radioactive decay to date volcanic layers of rock. And when I say “we,” I mean people like our team’s geochronologist, Alan Deino. Al usually dates layers of volcanic ash (commonly referred to as tuffs) because these are much more common and widespread than things like lava flows, at least places where we look for fossils. This page provides a succinct overview of how radiometric dating works, but it doesn’t tell you what geochronologists like Al actually do in the field: spend all day walking up and down through rock outcrops looking for ash layers that can be reliably dated. This requires a combination of knowledge, experience, and educated guesswork, and even then you never really know what type of results you will get until the sample is analyzed.


A typical layer of volcanic ash. Its bright white color is only evident after exposing a fresh surface. Photo by D. Croft. Reuse permitted under CC-BY-NC-SA.

Stratigraphy and Sedimentology

The most productive fossil sites come from rock layers that are broadly exposed, because more rock equals more fossils. However, this creates challenges when dealing with thick layers of rock that were deposited over millions of years. Without precise information about the layer in which each fossil is collected, specimens of different ages can get lumped together. This can lead to errors in interpretation, such as thinking that two species were living at the site at the same time when they were actually living there million of years apart. Working with a geologist who specializes in stratigraphy (how rock layers are positioned relative to one another) and sedimentology (the study of the of the particles that make up a sedimentary rock) can help ensure that this doesn’t happen. Our team’s stratigrapher and sedimentologist, Beverly Saylor, spends her time studying all of the exposed rock in the area so she can create stratigraphic columns (vertical diagrams of rock layers) showing the relationships of the rocks (and fossils) at different sites. Correlating rocks at different sites can be relatively straightforward if outcrops are continuous, but if they aren’t, radiometric dates, the chemistry of ash layers, and particularly unusual rock layers can be used to determine how layers relate to one another.


Each layer of rock at a fossil site such as this has its own characteristics and records a different interval in the history of the area. Photo by D. Croft. Reuse permitted under CC-BY-NC-SA.

Paleomagnetism

A compass points to the North Pole today, but 800,000 years ago, that same compass would have pointed south. A million years ago, it would have pointed north, and 1.1 million years ago, it would have pointed south. (What causes these changes? Check out this brief synopsis.) Scientists have created a master time scale of these flips known as the Geomagnetic Polarity Time Scale (GPTS) that can be used to date fossils more precisely than would be possible using only radiometric dating. One reason for this is that many types of rocks contain paleomagnetic data, whereas only a few types of rocks can be dated radiometrically. As a consequence, you might only be able to obtain two radiometric dates at a site, which may only constrain the age of a fossil to between 13 and 12 million years. But if you can place that fossil in a particular paleomagnetic interval (technically known as a chron), it might be possible to determine it is between 12.27 and 12.17 million years old, a ten-fold increase in precision. Our team’s paleomagnetist, Luis Gibert, spends much of his time collecting rock samples that enable him to construct a detailed paleomagnetic profile of each fossil site.


Extracting a rock sample for paleomagnetic analysis. The numbers on the sample indicate the how the sample is positioned in the outcrop. Photo by D. Croft. Reuse permitted under CC-BY-NC-SA.

Paleosols and ichnofossils

Many of the fossils we find are preserved in ancient soils (paleosols) that also preserve a diversity of information about past climate, vegetation, and ecosystems. The structure and characteristics of a paleosol can be used to determine if it formed in a relatively moist or dry environment, whether the precipitation varied by season, and how quickly new material was piled on top of it to create a new soil. They also preserve tracks and traces of past life, which are known as ichnofossils. Soil ichnofossils can include tiny invertebrate burrows, natural casts of tree roots, nests of ground-dwelling wasps, and even brood balls from dung beetles. These all provide information about the ancient environment in which these animals and plants were living. Our team’s paleosol and ichnofossil experts, Dan Hembree and Angeline Catena, spend their time digging trenches that allow them to study fresh rock faces and take samples for microscopic and chemical analyses in the lab.


A fossilized wasp nest (left arrow) and dung beetle brood ball (right arrow) preserved in an ancient soil (paleosol). Photo by D. Croft. Reuse permitted under CC-BY-NC-SA.

Phytoliths

Plant leaves aren’t usually preserved in the same types of rocks as fossil bones and teeth, but small remains of plant cells known as phytoliths (literally “plant stones”) often are. Like leaves, these can be used to determine whether an ancient habitat was a forest, grassland, or something in between. However, because phytoliths are microscopic, it isn’t possible to know whether a rock sample contains phytoliths without taking it back to the laboratory, putting it through a multi-step process involving strong acids and small sieves, and then examining the remains under a microscope. Thus, sampling for phytoliths in the field can be a little like looking for good rock layers to date. Our team’s phytolith expert, Caroline Strömberg, hasn’t been able to visit Bolivia yet, but  her students have been able to collect samples from throughout the area.

Of course, fieldwork is only the first step in the process. The really interesting part will take place over the next year as we analyze all of our data!

-Dr. Darin Croft is an Associate Professor at Case Western Reserve University in Cleveland, Ohio
Posted: 6/19/2016 7:16:37 AM by croftdarinadmin | with 0 comments
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