University of Alabama: in Tennessee shale, ocean oxygen depletion preceded Devonian wildfires
Chemical record from one Tennessee outcrop shows oxygen-depleted seawater came first and wildfire traces followed, contradicting the idea that fires triggered the Late Devonian crisis.
Researchers led by The University of Alabama report that widespread wildfires in the Late Devonian were a consequence of ocean oxygen depletion, not its trigger, according to news.ua.edu ua.edu
news.ua.edunews.ua.eduOpen source ↗. The study was published in Science Advances in April. It reconstructs events from a shale deposit in Tennessee dating to about 370 million years ago.
During that period large parts of the coastal ocean lost oxygen, disrupting marine ecosystems worldwide. The interval is considered one of the five great extinction events in Earth's history. One hypothesis held that fires moved nutrients from land into coastal waters and so contributed to the die-off of marine life.
What the rock record shows
The team analysed chemical signals preserved in the shale, including fossil biomarkers, which are organic molecules derived from past organisms and preserved in rocks. Indicators of marine anoxia appear first in the sequence, followed by signals of terrestrial wildfire activity. Earlier work lacked direct evidence, mainly because of uncertainties in age dating and because samples came from different locations or had coarser time resolution, said Dr. Yuehan Lu, who led the study.
The researchers sampled a single location at very high temporal resolution. According to a secondary account, they studied 65 rock samples from one outcrop, 35 of them spaced one centimetre apart.
Proposed mechanism and limits
The authors link the fires to a shift in the carbon cycle. In anoxic events, organic material sinks to the sea floor and its carbon is buried instead of returning to the atmosphere as carbon dioxide, which unbalances atmospheric oxygen ua.edu
news.ua.edunews.ua.eduOpen source ↗. Higher oxygen raises vegetation flammability and promotes more frequent, intense fires. The authors describe the wildfire response as delayed and non-linear, and say it outlasted the mass extinction itself.
In a model based on 1,412 carbon isotope values from 18 rock sections, atmospheric oxygen could have risen from about 17 to up to 20 percent over roughly 405,000 years under certain assumptions. This is a modelled result, not a measured one.
The timing does not by itself show that oxygen alone drove the fires. Lightning, volcanism, drier conditions and the spread of early forests are also possible factors, and the finest-resolution data come from one outcrop.
The work was primarily supported by the American Chemical Society Petroleum Research Fund New Directions. First author Dr. Man Lu carried out it as a doctoral student at the university's Molecular Eco-Geochemistry Laboratory, with contributions from Zhejiang University and the Geological Survey of Alabama ua.edu
news.ua.edunews.ua.eduOpen source ↗.
