14C Bomb Spike Signal Constrains Microbial Mat Growth Rates

Microbial mats are considered among the earliest forms of life to have inhabited our planet and occur in early Archean sedimentary sequences. Living mats consist of a coherent network of metabolically diverse microorganisms that produce extracellular polymeric substances, which allows them to thrive in harsh environments. Despite their importance for the study of early life on Earth and their relevance in the search for life on Mars, growth rates as well as carbon cycling of microbial mats remain unclear. Past radiocarbon (14C) studies of microbial mats show the presence of allochthonous organic particles trapped within the autochthonous biomass which complicate reconstruction of the mat growth rates (Bahniuk Rumbelsperger, 2013). In this study, millimeter-scale sampling and radiocarbon analysis was conducted on vertical profiles of microbial mats and lagoon water samples from two modern coastal sabkhas located on the Northwest and South coast of Qatar, respectively. The 14C measurements of mats from both studied sabkhas show increasing Δ14C values from the surface downwards, from an average of -25 ‰ up to a maximum 180 ‰. At the base of the mat there is an abrupt ~150 ‰ decrease in Δ14C values. This clear and shared trend between different sites and profiles shows that in both sites organic matter is predominantly being produced in situ via photosynthesis. Moreover, this 14C trend correlates with the global bomb spike carbon signal (Graven, 2015) and implies that these 45-55 mm thick mats are relatively modern structures and cannot be more than 60 - 70 years old. Therefore, the microbial mat growth rate is higher than previously estimated, suggesting that stromatolites, possibly those from earlier in Earth history, may have formed in a relatively short time. The trend also shows that most organic bound carbon is not extensively recycled within the mat. Instead, the primary source of in-situ produced organic carbon is the dissolved inorganic carbon of the lagoon water. These findings shed new light on the development of one of the earliest life forms on our planet.

REFERENCES
Bahniuk Rumbelsperger A.M., 2013. Coupling organic and inorganic methods to study growth and diagenesis of modern microbial carbonates, Rio de Janeiro State, Brazil: Implications for interpreting ancient microbialite facies development, PhD Thesis ETH Zurich, ETH No. 20984.
Graven, H.D., 2015. Impact of fossil fuel emissions on atmospheric radiocarbon and various applications of radiocarbon over this century. Proceedings of the National Academy of Sciences, 112(31): 9542-9545.