Carbon isotope geochemistry of the Precambrian Lomagundi carbonate province, Rhodesia

115

139

The oxygen content of Earth's atmosphere has varied greatly through time, progressing from exceptionally low levels before about 2.3 billion years ago, to much higher levels afterward. In the absence of better information, we usually view the progress in Earth's oxygenation as a series of steps followed by periods of relative stasis. In contrast to this view, and as reported here, a dynamic evolution of Earth's oxygenation is recorded in ancient sediments from the Republic of Gabon from between about 2,150 and 2,080 million years ago. The oldest sediments in this sequence were deposited in well-oxygenated deep waters whereas the youngest were deposited in euxinic waters, which were globally extensive. These fluctuations in oxygenation were likely driven by the comings and goings of the Lomagundi carbon isotope excursion, the longest-lived positive δ 13 C excursion in Earth history, generating a huge oxygen source to the atmosphere. As the Lomagundi event waned, the oxygen source became a net oxygen sink as Lomagundi organic matter became oxidized, driving oxygen to low levels; this state may have persisted for 200 million years.GOE | Paleoproterozoic | marine chemistry | Mo isotope | trace metal

Section: Resultsmentioning

confidence: 92%

mentioning

confidence: 99%

Oxygen dynamics in the aftermath of the Great Oxidation of Earth’s atmosphere

Canfield

Ngombi-Pemba

Hammarlund

et al. 2013

115

139

“…Carbonates of this age have markedly positive δ 13 C values often reaching þ10‰ and peaking above þ20‰ (4)(5)(6)(7)(8). These markedly positive values-found in over 15 formations worldwide-are traditionally assumed to track the marine dissolved inorganic carbon (DIC) reservoir and to reflect greatly enhanced organic carbon burial (4)(5)(6)(7)(8). It has been estimated that 12-22 times the present atmospheric inventory of oxygen was released because of organic carbon burial during this event, commonly referred to as the Lomagundi excursion (LE) (5).…”

mentioning

confidence: 99%

Sulfur record of rising and falling marine oxygen and sulfate levels during the Lomagundi event

Planavsky

Bekker

Hofmann

et al. 2012

180

130

Carbonates from approximately 2.3-2.1 billion years ago show markedly positive δ 13 C values commonly reaching and sometimes exceeding þ10‰. Traditional interpretation of these positive δ 13 C values favors greatly enhanced organic carbon burial on a global scale, although other researchers have invoked widespread methanogenesis within the sediments. To resolve between these competing models and, more generally, among the mechanisms behind Earth's most dramatic carbon isotope event, we obtained coupled stable isotope data for carbonate carbon and carbonate-associated sulfate (CAS). CAS from the Lomagundi interval shows a narrow range of δ 34 S values and concentrations much like those of Phanerozoic and modern marine carbonate rocks. The δ 34 S values are a close match to those of coeval sulfate evaporites and likely reflect seawater composition. These observations are inconsistent with the idea of diagenetic carbonate formation in the methanic zone. Toward the end of the carbon isotope excursion there is an increase in the δ 34 S values of CAS. We propose that these trends in C and S isotope values track the isotopic evolution of seawater sulfate and reflect an increase in pyrite burial and a crash in the marine sulfate reservoir during ocean deoxygenation in the waning stages of the positive carbon isotope excursion.Lomagundi excursion | Great Oxidation Event | Precambrian

“…1). Limestones and dolostones with extreme carbon isotopic values of + 8‰ to greater than + 15‰ occur globally (6)(7)(8), and a duration of between 128 million years (m.y.) and 249 m.y.…”

mentioning

confidence: 99%

The rise of oxygen and siderite oxidation during the Lomagundi Event

Bachan

Kump

2015

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 O 2 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 O 2 as well as a source of acidity. However, sulfide oxidation consumes more O 2 than it can supply CO 2 , leading to insurmountable imbalances in both carbon and oxygen. In contrast, the oxidation of siderite (FeCO 3 proper, as well as other Fe 2+ -bearing carbonate minerals), produces 4 times more CO 2 than it consumes O 2 and is a common-although often overlooked-constituent of Archean and Early Proterozoic sedimentary successions. Here we propose that following the initial rise of O 2 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 O 2 . 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 O 2 .carbon isotopes | oxygen | siderite | carbon cycle | Great Oxidation Event R econstructing the geologic history of atmospheric oxygen is among the foremost scientific challenges of our time (1). The level of atmospheric oxygen (pO 2 ) without doubt played a key role in the evolution of the Earth System (2), exerting a major influence on the biosphere, especially the evolution of metazoans (3). With no direct way of measuring oxygen concentrations in deep geologic time, the stable isotopes of carbon recorded in marine limestones provide key constraints (4). Carbon enters the ocean−atmosphere system through volcanoes and weathering of carbon-bearing sedimentary rocks and can exit in one of two ways: (i) uptake during photosynthesis and burial of organic carbon leading to O 2 production and (ii) reaction during weathering and formation of CaCO 3 in the ocean. The carbon isotopic record tells us how carbon was partitioned between these two sinks: A δ 13 C value of 0‰ indicates that ∼80% of incoming carbon was buried as carbonate carbon and 20% as organic carbon. Positive excursions in δ 13 C are unusual and indicate that a larger fraction of carbon was fixed and buried as organic carbon and, with it, a larger amount of O 2 was produced.Following the indications for the first rise of O 2 in the atmosphere (5, 6) is the largest and most protracted period of 13 C enrichment in the geologic record, known as the Lomagundi Event (Fig. 1). Limestones and dolostones with extreme carbon isotopic values of + 8‰ to greater than + 15‰ occur g...