Kuntz L B, Laakso T A, Schrag D P, Crowe S A
Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA.
Departments of Microbiology & Immunology and Earth, Ocean, & Atmospheric Sciences, University of British Columbia, Vancouver, Canada.
Geobiology. 2015 Sep;13(5):454-61. doi: 10.1111/gbi.12141. Epub 2015 Apr 29.
Lake Matano, Indonesia, is a stratified anoxic lake with iron-rich waters that has been used as an analogue for the Archean and early Proterozoic oceans. Past studies of Lake Matano report large amounts of methane production, with as much as 80% of primary production degraded via methanogenesis. Low δ(13)C values of DIC in the lake are difficult to reconcile with this notion, as fractionation during methanogenesis produces isotopically heavy CO2. To help reconcile these observations, we develop a box model of the carbon cycle in ferruginous Lake Matano, Indonesia, that satisfies the constraints of CH4 and DIC isotopic profiles, sediment composition, and alkalinity. We estimate methane fluxes smaller than originally proposed, with about 9% of organic carbon export to the deep waters degraded via methanogenesis. In addition, despite the abundance of Fe within the waters, anoxic ferric iron respiration of organic matter degrades <3% of organic carbon export, leaving methanogenesis as the largest contributor to anaerobic organic matter remineralization, while indicating a relatively minor role for iron as an electron acceptor. As the majority of carbon exported is buried in the sediments, we suggest that the role of methane in the Archean and early Proterozoic oceans is less significant than presumed in other studies.
印度尼西亚的马塔诺湖是一个分层缺氧湖，湖水富含铁，一直被用作太古宙和元古宙早期海洋的模拟对象。过去对马塔诺湖的研究报告称甲烷产量很高，初级生产力中高达80%通过甲烷生成作用被降解。湖泊中溶解无机碳（DIC）的低δ(13)C值很难与这一观点相协调，因为甲烷生成过程中的分馏作用会产生同位素较重的二氧化碳。为了帮助协调这些观测结果，我们构建了一个印度尼西亚含铁质的马塔诺湖碳循环箱式模型，该模型满足了甲烷和DIC同位素剖面、沉积物组成及碱度的限制条件。我们估计甲烷通量比最初提出的要小，约9%的有机碳输出到深水层后通过甲烷生成作用被降解。此外，尽管水中铁含量丰富，但有机物质的缺氧三价铁呼吸作用降解的有机碳输出量不到3%，这使得甲烷生成成为厌氧有机物质再矿化的最大贡献者，同时表明铁作为电子受体的作用相对较小。由于大部分输出的碳被埋藏在沉积物中，我们认为甲烷在太古宙和元古宙早期海洋中的作用不如其他研究中所推测的那么重要。
