Lomagundi-Jatuli Carbon Isotope Excursion

The Lomagundi-Jatuli Carbon Isotope Excursion or Lomagundi-Jatuli Event (LJE) was a carbon isotope excursion that occurred in the Paleoproterozoic between 2.3 and 2.1 Ga, possessing the largest magnitude and longest duration of positive δ¹³C values found in marine carbonate rocks. The  δ¹³C values range from +5 to + 30‰. Carbon isotope compositions in marine carbonates typically fluctuate around zero per mil (‰) through time. To coincide with the LJE global δ¹³C_carb levels, the amount of buried organic carbon would have needed to double or triple, and over millions of years.

Measuring δ¹³C_carb values within marine carbonate rocks provides scientists with a window into the history of fluxes in the global carbon cycle over the course of Earth history.

In the context of the global carbon cycle, "flux" refers to the movement or flow of carbon between different reservoirs or components of the Earth system. This includes the atmosphere, oceans, terrestrial biosphere (plants and soil), and geosphere (rocks and sediments). These carbon fluxes are driven by various processes, including photosynthesis (which removes CO₂ from the atmosphere and incorporates it into plant biomass), respiration and decay (which return CO₂ to the atmosphere), weathering of rocks (which can transfer carbon from the geosphere to the hydrosphere and atmosphere), and the dissolution and precipitation of carbonate minerals in the ocean

Understanding these fluxes, especially within the LJE, is crucial for studying the global carbon cycle. They determine the concentration of carbon dioxide in the Earth's atmosphere, which in turn influences the planet's climate. While the LJE's high δ¹³C_carb values were first thought to show a substantial local increase in organic carbon (f_org) 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.

Changes in carbon fluxes can lead to significant variations in atmospheric CO₂ levels and thus have been a major focus of research, and debate, especially in the context of anthropogenic climate change.

Locations and duration

Assuming this excursion is 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).

Table 1: Lomagundi-Jatuli Event localities presenting similarly elevated δ13 values, formations of occurrence, dated age of formation, and procedural method of δ¹³C value analysis.

The extremely positive carbon isotope values having occurred during the LJE  can be seen on all continents, with the notable exception of Antarctica, having stratigraphic thickness ranging from several to tens of meters 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 δ¹³C_carb variation (‰) values, and stratigraphic thicknesses of each.

Methods

Scientists choose which geochronology method is best suited for the types of rocks and sediments they work with. Attempting to date marine carbonate rocks is a challenge due to use of uranium (U) and lead (Pb). These rocks do not have a high composition of uranium, but contain a lot of lead. Both of these can experience diagenetic overprinting, a modification over geologic time. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) has been able to work around this challenge, but is less successful when applying Isotope Dilution-Thermal Ionization Mass Spectrometry (ID-TIMS) . The spatial resolution makes this a preferred method, although testing continues. Secondary ion mass spectrometry (SIMS) is a common method of dating.

Genesis of the LJE

Synchronous, global-scale disturbance

The global view concludes that during the Lomagundi carbon isotope excursion carbonates were deposited world-wide with large amounts of ¹³C enrichment. 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, which in turn led to disequilibrium of the carbon cycle and released oxygen.

To explain this global ¹³C enrichment, the oxidation of siderite (FeCO₃, with other Fe²⁺ carbonate minerals), was proposed as a hypothesis because it produces four 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 change 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. Supporting this, there is evidence for the first large continental plates around 2.2-2.1 Gyr experiencing rifting and a global orogeny. During this time frame there was an increase in seawater ⁸⁷Sr/ ⁸⁶Sr, which indicates there was higher levels of continental erosion. To reinforce the effect of high ⁸⁷Sr/ ⁸⁶Sr, the first known glaciations occurred during 2.2-2.1 Gyr as well which favours weathering rates by lowering sea level.

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 facies environments drive the high C-isotope values. This meaning that the values of δ¹³C and changes in the values are because of processes in individual basins, depending on where the locality is along a carbonate platform/slope. Using ¹³C carbonate data from locations worldwide and the stratigraphic descriptions, the values can be organized into open marine, nearshore marine-inner shelf and intertidal-coastal-sabka, with a noticed correlation between facies and ¹³C carbonate values. For open marine the mean ¹³C 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 δ¹³C values can be explained by changes in local dissolved inorganic carbon (DIC) pools, influenced in individual basins, not representative of world-wide change in ocean DIC.

Localized, Diagenesis or Methanogenesis

For a global carbon isotope excursion, sedimentary organic carbon (shales) tend to show a trend as well, as an excursion would affect the δ¹³C 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 δ¹³Ccan be linked to isotopic alteration from breakdown of organic matter due to diagenesis and metamorphism.

A process in sediment columns that can contribute to carbonates with high levels of ¹³C, methanogenesis, could have caused carbonates enriched in ¹³C, creating an explanation for the δ¹³C values reaching +28‰. To explain the LJE, a deeper methanic zone in oceans during the start of ocean oxygenation (the GOE) would push pore water DIC to higher δ¹³C values. The carbonates forming at this time would record δ¹³C enrichment.

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