Bachan Aviv, Kump Lee R
Department of Geosciences, The Pennsylvania State University, University Park, PA 16802
Department of Geosciences, The Pennsylvania State University, University Park, PA 16802.
Proc Natl Acad Sci U S A. 2015 May 26;112(21):6562-7. doi: 10.1073/pnas.1422319112. Epub 2015 May 11.
The Paleoproterozoic Lomagundi Event is an interval of 130-250 million years, ca. 2.3-2.1 billion years ago, in which extraordinarily (13)C enriched (>10‰) limestones and dolostones occur globally. The high levels of organic carbon burial implied by the positive δ(13)C values suggest the production of vast quantities of O2 as well as an alkalinity imbalance demanding extremely low levels of weathering. The oxidation of sulfides has been proposed as a mechanism capable of ameliorating these imbalances: It is a potent sink for O2 as well as a source of acidity. However, sulfide oxidation consumes more O2 than it can supply CO2, leading to insurmountable imbalances in both carbon and oxygen. In contrast, the oxidation of siderite (FeCO3 proper, as well as other Fe(2+)-bearing carbonate minerals), produces 4 times more CO2 than it consumes O2 and is a common--although often overlooked--constituent of Archean and Early Proterozoic sedimentary successions. Here we propose that following the initial rise of O2 in the atmosphere, oxidation of siderite provided the necessary carbon for the continued oxidation of sulfides, burial of organic carbon, and, most importantly, accumulation of free O2. The duration and magnitude of the Lomagundi Event were determined by the size of the preexisting Archean siderite reservoir, which was consumed through oxidative weathering. Our proposal helps resolve a long-standing conundrum and advances our understanding of the geologic history of atmospheric O2.
古元古代洛马贡迪事件是一个发生在约23亿至21亿年前、持续1.3亿至2.5亿年的时期，在此期间全球出现了异常富集（>10‰）碳-13的石灰岩和白云岩。δ(13)C正值所暗示的高有机碳埋藏水平表明产生了大量氧气，以及存在碱度失衡，这需要极低水平的风化作用。硫化物氧化被认为是一种能够缓解这些失衡的机制：它是氧气的强大消耗者以及酸度的来源。然而，硫化物氧化消耗的氧气比它能供应的二氧化碳更多，导致碳和氧都出现无法克服的失衡。相比之下，菱铁矿（FeCO3本身以及其他含Fe(2+)的碳酸盐矿物）的氧化产生的二氧化碳比消耗的氧气多4倍，并且是太古宙和早元古代沉积层序中常见的成分——尽管常常被忽视。在此我们提出，在大气中氧气最初上升之后，菱铁矿的氧化为硫化物的持续氧化、有机碳的埋藏，以及最重要的自由氧的积累提供了必要的碳。洛马贡迪事件的持续时间和规模由太古宙时期预先存在的菱铁矿储库的规模决定，该储库通过氧化风化作用被消耗。我们的提议有助于解决一个长期存在的难题，并推进我们对大气氧气地质历史的理解。
