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Geological History: It’s Written in Stone—Mudstone

Some people stop playing in the mud by age ten—not Alex Washburn. In August 2016, the geology graduate student found himself across the world steeped in mud, collecting samples.

Of course, the specific mud Washburn works with is now stone, having hardened after millions of years; and finding the right samples takes diligently hammering into geochemically undisturbed rocks in the right interval of terrain.

“It’s a dirty science,” Washburn said about geology. “Since we study things that are only in nature, and nature—by nature—is really complicated, dirty, and doesn’t behave the way you want it to.”

For Washburn, it’s all about finding the right sample—a sample that he and his faculty advisor Sam Hudson hope will tell a snapshot of Azerbaijan’s geological history. Specifically, Washburn hopes to discover at what point millions of years ago the Paratethys Sea lost its connection to the ocean, resulting in the isolation of the South Caspian and Kura basins.

“That’s a topic that is really highly debated about this area: when that closure happened,” Washburn said.

Washburn hopes to resolve some of the debate about the age of the basins using emerging geochemical techniques on the samples.

“It’s called rhenium-osmium geochronology,” Washburn said. “It’s a technique recently developed for organic-rich mudstones, but it has never been done on mudstones as young or organically diverse as mine.”

The technique looks at the amount of rhenium and osmium in the rock. The isotope rhenium-187 decays into osmium-187. Rhenium and osmium, which occur naturally in water, bind to organic matter.

The team knows when, in a broad geologic timescale, the basins were isolated because of the appearance of thin, discreet intervals of organic-rich mudstone, which are related to basin closure. Performing rhenium-osmium geochronology on those organic-rich intervals of mudstone will reveal with much greater accuracy when the basins were isolated, according to Washburn.

“When we see high total organic carbon in the mudstone, that means the basin was restricted. In rock that we don’t have as much organic carbon, that means it was open,” Washburn said.

Water column stratification, a process that can result when a basin is closed off from seaways, determines how organically rich the mudstone is. Water column stratification occurs when differences in water density stop the water on the top from mixing with the water at the bottom of the sea. Because of this separation, the water at the top is allowed to replenish its dissolved oxygen, but the water on the bottom is not.

Eventually, all dissolved oxygen at the bottom of the water column is consumed. When organisms living in the oxygenated zone die and fall to the bottom of the sea, their organic matter is preserved because microorganisms at the bottom lack the oxygen needed to break down the organic matter to produce chemical energy.

The organic matter, and the subsequent abundances of rhenium and osmium that bind to it, is buried and lithified, allowing Washburn and Hudson to dig it up and analyze it millions of years later.

“We take the rock [sample] and crush it into powder. We dissolve it in this really powerful acid, and then we put that into an analyzer that can analyze rhenium and osmium up to parts per trillion, which are insanely small abundances,” Washburn said.

The team also looks at the trace metals in the samples. The team took samples from several different locations and performed chemostratigraphic analysis, taking the abundances of redox-sensitive metals and providing data for future geologists.

“Nature isn’t just like a beautiful layer cake,” Washburn said. “It’s got all kinds of crazy stuff going on, and so the more data you have the better you can figure out what’s going on in between [each sample layer].”

Washburn said the data he collects can both satiate many scientists’ curiosity about the region and help the petroleum industry.

“The better we understand [the formation of the basin], the better the industry will be able to model hydrocarbon generation and migration,” Washburn said.

The research also helps geologists better understand the events that lead to the basin closure and a variety of things—like the paleoclimate—that were affected by the closure.

When Washburn isn’t off collecting and analyzing samples, he’s home with his wife and six-year-old stepdaughter, who’s just learning to ride a bike.

“She loves learning about science, any kind of science,” Washburn said about his daughter.

Like Washburn, she’ll collect her own rocks. Washburn will cut and polish them for her.

By Jessica Grimaud Posted on