User:Diagenesis/Shuram excursion

The Shuram excursion, or Shuram-Wonoka excursion, is a change in δ 13 C, or in the ratio of carbon-13 to carbon-12, taking place between around 573 and 562 million years ago, during the Ediacaran Period. It is the largest negative δ13C excursion in Earth history, and recovery took 50 million years, although the apparent magnitude of the excursion may be distorted due to meteoric water diagenesis. It was first noticed in the Wonoka Formation in South Australia in 1990 and later in the Shuram Formation in Oman in 1993. It can also be found in other places in the world such as Northwest Canada, Peru, Death Valley in California, Siberia and South China.  { "type": "FeatureCollection", "features": [ {     "type": "Feature", "properties": {}, "geometry": { "type": "Point", "coordinates": [ -127.96875, 62.593341 ] }   },    {      "type": "Feature", "properties": {}, "geometry": { "type": "Point", "coordinates": [ 131.484375, 69.162558 ] }   },    {      "type": "Feature", "properties": {}, "geometry": { "type": "Point", "coordinates": [ -116.806641, 36.173357 ] }   },    {      "type": "Feature", "properties": {}, "geometry": { "type": "Point", "coordinates": [ -78.530273, -7.577605 ] }   },    {      "type": "Feature", "properties": {}, "geometry": { "type": "Point", "coordinates": [ 56.975098, 21.371244 ] }   },    {      "type": "Feature", "properties": {}, "geometry": { "type": "Point", "coordinates": [ 111.917725, 31.381779 ] }   },    {      "type": "Feature", "properties": {}, "geometry": { "type": "Point", "coordinates": [ 140.317383, -34.849875 ] }   }  ] }

Age Determination
The approximate age is ambiguous but two main age models for the excursion have emerged both presuming the event ends around 550 million years ago based on ash dated to 551.1 ± 0.7 million yaers above the Shuram Carbon Isotope Excursion (CIE) in the Doushantuo Formation in South China. This also associated the CIE and the extinction of the White Sea assemblage. The models differ when potentially linking the Shuram CIE with the Gaskiers glaciation which places the onset of the Shuram CIE at 580 million years ago. Based on the rhenium-osmium (Re-Os) dates from mixed carbonate and sedimentary rocks bracketing the excursion on two different paleocontinents, the data suggests the event lasted no longer than 6.7 ± 5.6 million years. Table 1: Table showing formation names and their respective locations.

Stratigraphic Expression
The Shuram CIE was preserved on four different paleocontinents and reflect Ediacaran sections over a range of of water depths between inner ramp and upper slope depositional environments. The stratigraphic sections created based on the formations globally range in thicknesses of hundreds of meters to over a kilometer. The Khufai formation in Oman is an approximately 300 to 320 meter thick section that captures the onset of the Shuram CIE and is primarily composed of black, sulfurous limestone with gradual upper and lower contacts. The Bunyeroo formation in South Australia is approximately 350 meters thick and consists of red brown shale and siltstone with a presence of minor fine sandy layers which displays the Shuram CIE. The Gametrail formation is an approximately 75 meter thick limestone unit that also captures of Shuram CIE.

Anomaly magnitude
The magnitude of the anomaly reached approximately a carbonate δ13C nadir of -12‰ and varies slightly between the different localities where the assemblages are preserved. The δ13C nadir reaches -15‰ in the Nadaleen formation in northwest Canada and -11‰ in the San Juan formation in Peru. Table 2: Table showing measured anomaly magnitudes at the various locations.

Controversies and potential mechanisms
It is still not exactly known what caused the excursion. The exact timeframe of the excursion is still controversial due to the large error present in the geochronology results. The magnitude of the anomaly could also have been distorted due to meteoric water diagenesis. It was suggested that this excursion represents a global diagenetic event with possible links to global transgressions and local DIC changes. It has also been linked to the simultaneous switching from an anoxic ocean to a near-modern ocean using δ238U data. Although there have been a lot of studies done on the excursion it is difficult to come to a conclusion based on today’s data due the many ways the assemblages have been altered or simply did not preserve.

Conclusion
The Shuram negative carbon isotope excursion is the largest in earth history and reaches a higher magnitude than other negative carbon isotope excursions such as the Paleocene–Eocene Thermal Maximum which happened approximately 55.5 million years ago a reached a nadir of -7‰. The Shuram excursion may have played a role in sparking the rise of animals that resulted later in the Cambrian explosion. The oxygen-consuming Ediacara biota experienced a radiation during the isotopic excursion as a response to the transient surplus of oxidants. Microbial blooms of oxygenic phototrophs regulated the recovery of the carbon cycle from the isotopic excursion. Table 3: Table showing the comparison between the Shuram CIE and other negative CIE's.