Grant: Discovery Grants Program - Individual

During the earth's early history, the sun was only ~70-80% bright as it is today but the earth remained unfrozen or even warmer-a phenomenon termed as the faint young sun problem (FYSP). Geological evidence suggests that CO2 concentrations in the Archean (4.0-2.5 billion years ago) atmosphere were not high enough to generate sufficient greenhouse effect to keep the earth warm. One of the hypotheses to explain the FYSP is the strong greenhouse effect provided by high atmospheric methane (CH4) concentrations (dubbed as the " CH4 solution") that originated from CH4 production by a special group of microbes, called methanogens, in the Archean ocean (AO). The methanogen hypothesis is supported by strong evidence that the AO was devoid of oxygen, leading to anoxic conditions required by the growth of methanogens. Studies in the present ocean have proven that CH4 is produced not only by biotic processes but also by a few abiotic pathways, including sunlight-induced photochemical degradation of organic matter, in oxygenated surface seawater. Preliminary estimates suggest that the photochemical source can account for up to 60% of the CH4 emission from the present open ocean to the atmosphere. Notably, it has also been demonstrated that elimination of oxygen (i.e., anoxic conditions) can increase the rate of CH4 photoproduction from natural organic matter and model methyl compounds by one to two orders of magnitude. Moreover, although the overall insolation was lower during the Archean eon, the ultraviolet (UV) radiation flux (250-400 nm) reaching the AO was orders of magnitude higher than today due to the absence of stratospheric ozone during that time. This is particularly true for UVC (250-280 nm), which is nonexistent on today's earth surface. As UV radiation is primarily responsible for CH4 photoproduction from organic matter, the larger UV flux would have greatly enhanced CH4 photoproduction in the AO as well. The warmer temperatures of the AO and higher concentrations of reduced forms of organic compounds due to the absence of oxygen could have also boosted CH4 photoproduction. This research program aims to determine the rates of CH4 photoproduction from natural organic matter under simulated AO conditions; to scale up the lab-determined CH4 photoproduction results to the AO; and to elucidate the photochemical mechanisms underlying CH4 photoproduction. Results from this research program may profoundly change our view regarding the nature of the source of CH4 for the CH4 solution to solving the FYSP: i.e., from biotic to abiotic or more likely to a combination of both. Furthermore, understanding the role of CH4 in regulating the earth's early climate helps better appreciate the potential contributions of ongoing global changes, such as ocean deoxygenation and CH4-rich permafrost thawing, to the present earth's climate warming by which Canada is one of the countries most impacted.