Weathering and erosion are constant processes on
Earth, and dissolution due to rainwater is one of the primary ways rocks and
minerals break down. But paleontologists have not yet gained a complete
understanding of the rate fossils erode once they are exposed at the surface. South Dakota Mines graduate student Colleen Sullivan set out to
get some answers. Her research has
just been published in the peer-reviewed open-access scientific journal
PLOS ONE.
“I was doing some undergraduate research looking at
how long fossils survive on the surface when exposed to rainfall and there was little
quantitative research on the subject,” says Sullivan. “We often saw qualitative
speculation on this topic, and there was research done on rates of dissolution
for individual minerals. But this is the first time someone has put a numerical
value on the rate of fossil dissolution due to interaction with rainfall.”
Sullivan completed her masters in paleontology at
Mines in 2022. Her research involved mosasaurs – the giant marine reptiles
thrived at the end of the age of dinosaurs in the Western Interior Seaway that
covered much of the Great Plains of the United States. Mosasaurs were common in
this ancient seaway and today their fossilized bones are relatively abundant.
They can often be found protruding from a Cretaceous age rock layer called the
Pierre Shale Formation that was deposited at the bottom of the former seaway
more than 66 million years ago.
During her research, Sullivan used fossil vertebrae
from a late Cretaceous mosasaur. The vertebrae, in the South Dakota Mines Museum
of Geology collections, were too fragmented to be museum display quality but were
still identifiable as mosasaur fossils. These vertebrae were also collected
without detailed location information, so they were less valuable as research
specimens.
“We wanted to look specifically at how water
interactions impact fossils,” says Sullivan. Previous studies show that rainwater
is one of the main causes of erosion and dissolution for several types of
minerals. “One of the goals was to determine how water geochemistry impacts the
long-term stability of fossils in the field,” says Sarah Keenan, Ph.D., an assistant
professor in the Department of Geology and Geological Engineering at Mines.
The fossil vertebrae were tested to determine the
time it would take to dissolve with various levels of acidity that mimic the
chemistry of average rainfall. Sullivan tracked the amount of fossil material
lost to the solution by testing the specimens before and after dissolution
using X-ray diffraction and optical microscopy. The study helped quantify the
various minerals lost during the treatments. Her results showed an expected
outcome: higher rates of acidity lead to faster dissolution. She then used the
data to calculate how long an exposed fossil might stay at the surface before
being lost to weathering and erosion.
“If we just focus on rain, the smaller fossils will
be dissolved and unrecognizable in about a decade,” says Sullivan. “Something
like a skull or larger fossilized bone with more mass is going to vary; bigger
fossils could last longer on the surface maybe a century. But for the smaller
bits, like individual vertebrae, it could be a 10-year scale.”
The paper not only has implications in helping
paleontologists understand how long they have to collect a fossil once it’s
exposed at the surface, but it also adds to the understanding of what types of
fossils are preserved overall. “This is important for addressing biases in our
fossil collections as a whole,” says Keenan.
“Not everything gets fossilized in the first place, and then the limited
selection of fossilized remains are exposed and degraded at the surface,
further reducing the selection. So, when we are talking about the whole fossil
record, we are looking at a snapshot of a snapshot of life on earth.”
Keenan notes that fossils are rare, so anyone who
finds a fossil should leave it in place and contact experts to assess it. She
says those who collect fossils on their own run the risk of destroying
scientific information about their location and context that can’t be
recreated.
The next steps in the research will include analyzing
samples with a new ion-chromatography instrument recently brought to campus
thanks to funding from
the National Science Foundation. “Future research done with this instrument will
help us better constrain what is driving fossil bone dissolution,” says
Keenan.
Sullivan’s success in her master’s research at Mines
helped her land a position in the paleontology Ph.D. program at the University
of Kansas in Lawrence where she is continuing work on geochemistry of Cretaceous
fossils and the surrounding strata that can lend to the understanding of paleoenvironments
and dinosaur evolution into modern birds.