Carbon Cycle Perturbations at the Frasnian-Famennian Boundary in the Illinois Basin, USA

The Late Devonian extinction occurred from ~372 million years ago (Ma) to the Devonian-Carboniferous boundary ~359 Ma. The extinction is characterized as one of five major mass extinctions, as it killed 70-82% of Earth’s species. After years of research, debate for the event’s trigger continues. Positive carbon isotope excursions, measured using δ13C, are seen in many of the strata across the world from Late Devonian extinctions. Thus, this study aims to address this intricacy by providing a more complete record of carbon isotopes in the Illinois Basin to determine if the Upper Kellwasser Event is preserved in the Illinois Basin.

In this study, the samples analyzed were from the Storey Core, housed at the Illinois Geological Survey. The Storey Core is from the southeast depocenter of the Illinois Basin, and covers the Selmier and Grassy Creek members of the New Albany Shale. 30 samples were chosen from ranging from depths of 1518 – 1580 meters. Samples were powdered with a ball mill and acidified multiple times, before being analyzed for organic carbon values of δ13C (δ13C-TOC) using mass spectrometry. At ~1550m, there is a significant +2‰ excursion, similar to other positive δ13C excursions recorded for the Upper Kellwasser Event. Trace element data measured using X-ray Fluorescence support these results. δ13C-TOC values from the Kavanaugh Core – also in the southeast depocenter (~100 km northeast) – show striking similarities. These data indicate a high likelihood that the Upper Kellwasser Event is preserved in the southeast depocenter of the Illinois Basin. Molybdenum and Uranium concentrations increase, and Zirconium decreases, at the same depth as the +2‰ δ13C-TOC excursion. These may indicate expanding marine anoxia and potential marine transgression during this time. In the future, research could continue to investigate other parts of the Storey Core and their relation to Late Devonian events.

Hello, my name is Catherine Lau I’m a geology major and today I’m here to talk about our project which is titled “Carbon Cycle Perturbations at the Frasnian-Famennian Boundary in the Illinois Basin”.

Since the Cambrian Explosion began about 540 million years ago, there have been many extinction events, five of which have been deemed as the most severe. Unfortunately, some scientists have suggested that we are at the beginning of another mass extinction. This is mainly due to rapid biodiversity loss which is linked to anthropogenic climate change. So clearly it’s very important to continue developing an understanding of HOW these types of events can happen, and this leads to me to our project, which is about the Late Devonian mass extinction.

The Devonian was a period in Earth’s history that lasted from about 420 to 360 million years ago, during which many physical and biological changes happened on Earth. It was a really interesting time, with some of these changes including the appearance of forests and soils*, as well as the evolution and expansion of marine invertebrates such as corals.

In the Late Devonian, however, there was a mass extinction which killed about 70% species, and this included a lot of marine life on Earth. However, this extinction is unique because rather than occurring as a single event (which is more common), it’s been shown that the extinction happened in multiple pulses. This has led to debate over what may have triggered these events. Many recent studies on the Late Devonian extinction include marine anoxia as a part of their explanation.

Black shales are sedimentary rocks that have a high organic carbon content. Organic carbon is derived from dead animals and plants, and that’s helpful for learning about what environmental conditions may have been like at the time the rock formed.

This is measured by analyzing delta C 13, which is a ratio comparing two stable isotopes of carbon, namely Carbon 13 to Carbon 12. Carbon 13 is called the heavier isotope because it has an extra neutron. A Carbon isotope excursion is a term that’s used to describe when there is a change in the delta C 13 values. These excursions are usually linked to large changes in the environment that affect where and how carbon is stored in the Earth system. So it makes sense that a lot of the extinction events in Earth’s history coincide with observations of carbon isotope excursions, because mass extinctions tend to involve substantial changes in the composition of the ocean and atmosphere.

One way these substantial changes are observed is through the presence of marine anoxia. When a marine environment is anoxic, the absence of oxygen means the organic matter may be buried without fully decomposing. That burial of organic matter removes the lighter carbon isotopes, Carbon 12, and enriches the oceans with Carbon 13, which over time can be preserved in sediments.

There are two main events in the Late Devonian extinction, known as the Kellwasser Event and the Hangenberg Event. Within the Kellwasser Event there are two parts, but in this project, we only looked at the Upper Kellwasser Event which happened around 372 million years ago at a transition known as the Frasnian Famennian boundary.

Specifically, black shale samples taken from the Southeast depocenter of the Illinois Basin, and they were analyzed to see if the levels of Carbon 13 in those samples changed similarly to other recordings of the Upper Kellwasser Event from studies done in North America and around the world. One of the units in the Illinois Basin is the New Albany Shale, and the samples in this study were taken from a part of that unit known as the Storey Core.

The Illinois Basin was a shallow sea during the Devonian, so if the delta C 13 values changed in the samples, it may indicate that the environment was anoxic when the sediments were deposited.

To analyze this, first the samples had to be crushed and then powdered in a ball mill. After being acidified with Hydrochloric acid multiple times, the samples were rinsed and dried. They were then sent to UC Santa Cruz where mass spectrometry was used to get the data. 30 samples were used here, from depths of 1518 to 1580 meters.

From looking at the results on the graph, we can see that there’s a big change between the two points that are highlighted in the orange bar, around 1550 meters. Based on how quickly that amount of change is happening, as well as similar changes seen in samples from the nearby Kavanaugh Core, we think it is highly likely that these data are in fact recording the Upper Kellwasser Event.

This is also supported by trace element data for Molybdenum, Uranium, and Zirconium from the Storey Core and from the Kavanaugh Core. Additionally, the results here are also supported by international studies on Late Devonian rocks, which show similar trends in delta C 13 values.

To conclude, the results from this study show a high likelihood that the Upper Kellwasser Event is preserved in the Storey Core samples from the Illinois Basin. The data are supported by trace element data and observations from other studies, which show that this positive carbon isotope excursion may have happened during anoxic conditions in the Illinois Basin, as well as a potential rise in sea level. This is really great because not many carbon isotope excursions have been recorded in North America, so hopefully we can continue to examine Storey Core samples to learn more about the variations seen on this graph, and whether the other ones relate to a different part of the Kellwasser Event.

To wrap up, I’d like to thank my mentor Dr. Gilleaudeau, our collaborators listed here, and GMU’s Office of Student Scholarship, Creative Activities, & Research for their generous support. Thank you for watching!

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*Please note “appearance of forests and soils” refers to emergence of soils similar to what exist on Earth today. Other types of soils existed before the development of terrestrial (land) plants.