A different kind of swamp gas
The vast, waterlogged peat swamp forests of Kalimantan, Borneo, are some of the most carbon-dense ecosystems on the planet. For decades, scientists have studied the methane produced by the slow, anaerobic decay of plant matter in the peat—a process known as biogenic methane production. This process is a standard component of global climate models. Research has uncovered a surprising and unaccounted-for source of this potent greenhouse gas: ancient, 'fossil' methane seeping up from deep within the Earth's crust.
This is not the familiar swamp gas from decomposing surface vegetation. This is thermogenic methane, formed from organic matter cooked at high temperatures and pressures deep underground over geological time. This ancient gas, some of which is radiocarbon-dated to be thousands of years old, migrates upwards through networks of geological faults. These faults channel the gas from reservoirs deep in the Earth's crust to the surface. The emissions are concentrated at the apex of the peat domes, which are massive, convex lenses of peat that can be over 10 meters thick and have been accumulating for millennia. Some inland peat deposits in Kalimantan have been found to be up to 18 meters deep and began forming at least 47,800 years ago.
The climate model blind spot
The discovery of these geological seeps fundamentally changes the understanding of the greenhouse gas budget of these ecosystems. Climate models that calculate methane emissions from peatlands are calibrated to account only for the methane produced by modern biological activity. They are missing this entire geological component. This thermogenic methane is 'fossil' and radiocarbon-free, meaning it contains no Carbon-14, which confirms its ancient origin from deep hydrocarbon reservoirs.
Scientists identify the origin of the methane using isotopic analysis. The stable carbon isotope signature (δ¹³C) of thermogenic methane is distinct from that of biogenic methane. This allows researchers to fingerprint the gas and trace it back to its deep source. The discovery means that pristine, undrained peat swamp forests, long considered to be significant carbon sinks, may be natural and persistent sources of this powerful fossil greenhouse gas. While the total volume of this geological seepage is still under investigation, its existence reveals a previously unknown natural pathway for ancient carbon to enter the modern atmosphere, a pathway that has operated for millennia but is only now being understood. The peatlands in this region are immense, with the Sebangau peat dome in Central Kalimantan alone covering an area of approximately 7,347 square kilometers.