User:BIFChick/Lomagundi–Jatuli Carbon Isotope Excursion

The Lomagundi-Jatuli event (LJE) was a carbon isotope excursion that occurred in the Paleoproterozoic between ≈2.3-2.1 Ga, possessing the largest magnitude and longest duration of positive δ13C values found in marine carbonate rocks through time. The  δ13C values range from +5 to + 30‰. These values are an extreme deviation from typical carbon isotope compositions in marine carbonates which usually fluctuate around zero per mil (‰) through time. While the LJE's high δ13Ccarb values were first thought to show a substantial local increase in organic carbon (forg) in the localities in which the elevated values were found, marine carbonate outcrops with similarly elevated values have since been found around the world, shifting consideration that this event reflects a global increase. It has been argued that to account for global δ13Ccarb values across the LJE, the amount of buried organic carbon would have needed to double or triple, and this increase of burial would have had to have lasted over millions of years.

Location and duration
Under an assumption that the LJE was globally synchronous in its commencement and termination, the duration has been dated to range from a maximum of 249 ± 9 Myr (2306 ± 9 Ma to 2057 ± 1 Ma) to a minimum of 128 ± 9.4 Myr (2221 ± 5 Ma to 2106 ± 8 Ma). The extremely positive carbon isotope values expressed across the LJE  can be seen on all continents, with the notable exception of Antarctica, having stratigraphic thicknesses ranging from tens of meters to over a kilometer. The highly elevated δ13C values were first found in the Lomagundi Group in Zimbabwe and the Jatuli group in Fennoscandia at a time when the LJE was first hypothesized to have been a local event. Table 2: Carbonate lithology within global formations, including associated δ13Ccarb variation (‰) values, and stratigraphic thicknesses of each.

Synchronous, global-scale disturbance
The global view argues that during the LJE carbonates were deposited world-wide with large 13C enrichments. The hypothesis that the LJE is related to the Great Oxidation Event (GOE), with the LJE causing a large deviation in the global carbon reservoir, infers a disequilibrium of the carbon cycle and substantial release of oxygen.

To explain this global ẟ13C enrichment, the oxidation of siderite, (FeCO3 with other Fe2+ carbonate minerals), was proposed as a hypothesis because it produces 4 times the amount of CO2 than it consumes O2. The oxidation of siderite was the driver for the carbon needed in burial and further oxidation, as well as the accumulation of O2, making the length of the LJE dependent on the size of the Archean siderite reservoir.

Another hypothesis to explain the global nature of the LJE, is large tectonic changes leading to increased degassing of volcanic CO2, which could have increased deposition of carbonates and organic matter, due to higher weathering rates and nutrients to the ocean. Similar to a tectonic change, the formation of subaerial continents or global glaciations could have also enhanced volcanic CO2 leading to the same outcome of CO2 and O2 in carbonates and atmosphere.

Localized, facies driven process
This hypothesis acknowledges that there is a global change to the carbon cycle, and agrees that it was a globally synchronous event, but with the idea that different environments selectively preserve high C-isotope values, meaning that only specific carbonate facies record the LJE. This means that the values of ẟ13C and changes in the values are because of processes in individual basins, depending on where the locality is along a carbonate platform/slope environment together with the regional tectonic environment. Using ẟ13C carbonate data from locations worldwide and detailed stratigraphic descriptions, carbon isotope values can be organized into open marine, nearshore marine-inner shelf and intertidal-coastal-sabka, with a notable correlation between facies and ẟ13C carbonate values. For open marine the mean ẟ13C carbonate value was +1.5 ± 2.4‰, +6.2 ± 2.0‰ for inner-shelf, and +8.1 ± 3.8‰ for intertidal settings. Using this hypothesis, the extremely positive ẟ13C values can be explained by changes in local dissolved inorganic carbon (DIC) pools, influenced individual basins, not a global change in ocean DIC.

Localized, diagenesis or methanogenesis origin
For a global carbon isotope excursion, sedimentary organic carbon (shales) tend to show a trend as well, as an excursion would affect the ẟ13C value of the biosphere and therefore sedimentary organic matter. Between 2.60 and 1.60 Ga there is no trend within organic carbon. Fluctuations in ẟ13C can be linked to isotopic alteration from the breakdown of organic matter due to diagenesis and metamorphism.

A process in sediment columns that can contribute to carbonates with high ẟ13C values is methanogenesis. Methanogenesis creates methane that is low in carbon-13, leaving behind dissolved inorganic carbon which is 13C-enriched, which can then form carbonate minerals. During the Great Oxidation Event (GOE), deeper methanogenesis could have made porewater richer in 13C, which would be seen in early carbonate minerals.

However, this process is puzzling because methane production usually stops carbonate formation, and there has yet to be found widespread evidence of carbon-13-depleted carbonates from methane oxidation. Methanogenesis could have caused carbonates enriched in ẟ13C, creating an explanation for the ẟ13C values reaching +28‰. To explain the LJE, carbonate formation would have had to occur within the methanic zone in oceans during the start of ocean oxygenation (the GOE) where DIC ẟ13C values would be pushed to extremely positive values.