Greenhouse gases are vital in trapping heat within our atmosphere. One significant contributor found beneath the ocean floor is methane. Present as an icy substance on the ocean floor known as methane hydrate, this gas can be released into the ocean once it decomposes or melts, potentially exacerbating global warming. Similar phenomena, including thawing permafrost, active tectonic movements, daily tidal patterns, and changes in sea level, can trigger methane release from sediments. However, scientists remain uncertain about how these factors will respond to future climate change.
Researchers hypothesize that future global warming may accelerate the flow of methane into the ocean. To investigate this, they examined an ancient global warming event that occurred approximately 56 million years ago known as the Paleocene-Eocene Thermal Maximum (PETM). During this period, temperatures in the Arctic Ocean soared above 20°C (68°F) at times, serving as a relevant analogy for today’s rapidly warming conditions.
Upon entering seawater, methane’s fate is influenced primarily by two biological processes. Currently, it is estimated that 90% of the methane released from the ocean floor is consumed by tiny organisms called microorganisms through a process known as anaerobic methane oxidation. This process involves microorganisms consuming methane along with sulfate, leading to the formation of solid iron-sulfur minerals known as pyrite. Anaerobic methane oxidation effectively traps methane in these minerals, preventing its escape into the atmosphere. Hence, the ocean acts as a reservoir, or sink for methane.
However, an excess of methane can overwhelm the sulfate-dependent cycle. In such cases, another set of microorganisms consumes methane along with oxygen through a process called aerobic methane oxidation. This process converts methane into carbon dioxide, a powerful greenhouse gas. Presently, 10% of methane consumption in the ocean results from aerobic oxidation, though this may have varied historically.
To explore the balance of anaerobic versus aerobic methane oxidation during the PETM, the research team analyzed sediment samples from the Arctic Ocean floor. As sediment accumulates, it becomes compacted, allowing scientists to drill and extract cylindrical samples known as cores.
The age of the sediment increases with depth: younger sediments reside at the top while older layers lie below. For this project, the research team leveraged cores previously retrieved from the Arctic Ocean, which contained sediments dating back 100 million years. They identified deposits from the PETM at a depth of 386 meters (1,266 feet).
Researchers noted that microbes leave behind distinctive carbon-based molecules known as organic biomarkers during decomposition, which accumulate in seafloor sediments. Different types of methane-consuming microorganisms produce unique biomarkers—one for anaerobic methane oxidation and another for aerobic methane oxidation. By measuring these biomarkers in sediment cores, the team could deduce the predominant microorganisms during the PETM.
The biomarkers indicative of aerobic methane oxidation are Hop(17)21-en. The research team observed a fourfold increase in these biomarkers during the PETM, while those associated with anaerobic methane oxidation, Glycerol dialkyl tetraether, decreased by half. This trend suggests an increase in aerobic methane circulation accompanied by a decline in anaerobic activity, likely triggered by sufficient methane release under warming conditions to disrupt the sulfate-dependent methane cycle.
To estimate the carbon dioxide produced during aerobic methane oxidation in the PETM, researchers identified an additional biomarker within the sediment core: Phytan. Phytan is produced by organisms that utilize carbon dioxide during photosynthesis, and its structure provides clues about carbon dioxide levels at the time. Researchers found that during and long after the PETM, carbon dioxide concentrations in the Arctic Ocean were four times higher than today, indicating that the Arctic Ocean remained a significant carbon dioxide source to the atmosphere.
The team posits that increased aerobic methane oxidation during the PETM serves as a model for the contemporary Arctic Ocean, which is undergoing rapid warming amidst modern climate change. Their findings underscore how the transformation of methane into carbon dioxide poses a significant threat, as more carbon dioxide in the atmosphere warms the air, heats ocean waters, and leads to additional methane release from the ocean floor, perpetuating this cycle. Once initiated, this feedback loop can be self-reinforcing, complicating recovery efforts.
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Source: sciworthy.com