Biogeochemical and physical controls on methane fluxes from two ferruginous meromictic lakes

Abstract

Meromictic lakes with anoxic bottom waters often have active methane cycles whereby methane is generally produced biogenically under anoxic conditions and oxidized in oxic surface waters prior to reaching the atmosphere. Lakes that contain dissolved ferrous iron in their deep waters (i.e., ferruginous) are rare, but valuable, as geochemical analogues of the conditions that dominated the Earth's oceans during the Precambrian when interactions between the iron and methane cycles could have shaped the greenhouse regulation of the planet's climate. Here, we explored controls on the methane fluxes from Brownie Lake and Canyon Lake, two ferruginous meromictic lakes that contain similar concentrations (max. >1 mM) of dissolved methane in their bottom waters. The order Methanobacteriales was the dominant methanogen detected in both lakes. At Brownie Lake, methanogen abundance, an increase in methane concentration with respect to depths closer to the sediment, and isotopic data suggest methanogenesis is an active process in the anoxic water column. At Canyon Lake, methanogenesis occurred primarily in the sediment. The most abundant aerobic methane‐oxidizing bacteria present in both water columns were associated with the Gammaproteobacteria, with little evidence of anaerobic methane oxidizing organisms being present or active. Direct measurements at the surface revealed a methane flux from Brownie Lake that was two orders of magnitude greater than the flux from Canyon Lake. Comparison of measured versus calculated turbulent diffusive fluxes indicates that most of the methane flux at Brownie Lake was non‐diffusive. Although the turbulent diffusive methane flux at Canyon Lake was attenuated by methane oxidizing bacteria, dissolved methane was detected in the epilimnion, suggestive of lateral transport of methane from littoral sediments. These results highlight the importance of direct measurements in estimating the total methane flux from water columns, and that non‐diffusive transport of methane may be important to consider from other ferruginous systems.

Evaluating a primary carbonate pathway for manganese enrichments in reducing environments

“…(4.5 m in May 2017; Figure 2 h inset). The δ 13 CCH4 remained between -60 and -64‰ below the chemocline, and increased as dissolved CH4 concentrations decreased (Lambrecht et al, 2020).…”

“…Brownie Lake (BL) is a small ferruginous lake located in Minnesota, USA. The general biogeochemistry of the site is described in Lambrecht et al (2018), and Lambrecht et al (2020) presented a detailed study of its methane (CH4) cycle. Water column profiling, water sampling and analysis, and geochemical modeling were performed using routine techniques detailed these publications and in our Supplementary Materials.…”

“…(4) CH2O + 2MnO2 + 2H + = CO3 2-+ 2Mn 2+ +2H2O (5) H2S + 4MnO2 + 2H2O = SO4 2-+ 4Mn 2+ + 6OH -(6) CH4 + 4MnO2 + 7H + = HCO3 -+ 4Mn 2+ + 5H2O Of these processes, we consider reactions and 5 most likely to be active at the BL chemocline, based on the concentrations of species present and the eutrophic nature of the lake (Lambrecht et al, 2020), which provides a large reservoir of organic carbon to the system. Similar to reaction 4, iron reduction coupled to organic carbon oxidation is also capable of decreasing acidity (Walter et al, 2014).…”

Carbon isotope evidence for large methane emissions to the Proterozoic atmosphere

“…average value of 123 mmolC m −2 d −1 and 77 mmolC m −2 d −1 , respectively). Interestingly, although the ebullitive contribution in methane degassing is dominant in most modern anoxic lacustrine system 61,62 , in the Dziani Dzaha the average value of its contribution is about 18 ± 15% and the diffusive contribution is predominant (see Supplementary f bur. www.nature.com/scientificreports/ Figure S5).…”

Microbial Processes and Microbial Communities in the Water Column of the Polar Meromictic Lake Bol’shie Khruslomeny at the White Sea Coast

“…Meromictic lakes may act as models for investigation of microbial biogeochemical processes occurring at the oxic-anoxic interface (Koeksoy et al, 2016;Lambrecht et al, 2019). Moreover, investigation of the composition and function of microbial communities involved in the methane biogeochemical cycle is also important for the understanding of climatic changes (Singh et al, 2010;Boetius, 2019).…”