Editing Cambrian (section)

=== Isotope excursions ===

==== Base of Cambrian ====
The basal Cambrian δ<sup>13</sup>C excursion (BACE), together with low [[Uranium|δ<sup>238</sup>U]] and raised [[Δ34S|δ<sup>34</sup>S]] indicates a period of widespread shallow marine anoxia, which occurs at the same time as the extinction of the Ediacaran acritarchs. It was followed by the rapid appearance and diversification of [[bilateria]]n animals.<ref name="Peng-2020" /><ref name="Pruss-2024" />

==== Cambrian Stages 2 and 3 ====
During the early Cambrian, [[Strontium|<sup>87</sup>Sr/<sup>86</sup>Sr]] rose in response to enhanced continental weathering. This increased the input of nutrients into the oceans and led to higher burial rates of organic matter.<ref name="Zhang-2020">{{Cite journal |last1=Zhang |first1=Yinggang |last2=Yang |first2=Tao |last3=Hohl |first3=Simon V. |last4=Zhu |first4=Bi |last5=He |first5=Tianchen |last6=Pan |first6=Wenqing |last7=Chen |first7=Yongquan |last8=Yao |first8=Xizhu |last9=Jiang |first9=Shaoyong |date=2020 |title=Seawater carbon and strontium isotope variations through the late Ediacaran to late Cambrian in the Tarim Basin |url=https://doi.org/10.1016/j.precamres.2020.105769 |journal=Precambrian Research |volume=345 |pages=105769 |doi=10.1016/j.precamres.2020.105769 |bibcode=2020PreR..34505769Z |issn=0301-9268}}</ref> Over long timescales, the extra oxygen released by organic carbon burial is balanced by a decrease in the rates of [[pyrite]] (FeS<sub>2</sub>) burial (a process which also releases oxygen), leading to stable levels of oxygen in the atmosphere. However, during the early Cambrian, a series of linked δ<sup>13</sup>C and δ<sup>34</sup>S excursions indicate high burial rates of both organic carbon and pyrite in biologically productive yet anoxic ocean floor waters. The oxygen-rich waters produced by these processes spread from the deep ocean into shallow marine environments, extending the habitable regions of the seafloor.<ref name="Peng-2020" /><ref name="He-2019">{{Cite journal |last1=He |first1=Tianchen |last2=Zhu |first2=Maoyan |last3=Mills |first3=Benjamin J. W. |last4=Wynn |first4=Peter M. |last5=Zhuravlev |first5=Andrey Yu |last6=Tostevin |first6=Rosalie |last7=Pogge von Strandmann |first7=Philip A. E. |last8=Yang |first8=Aihua |last9=Poulton |first9=Simon W. |last10=Shields |first10=Graham A. |date=2019 |title=Possible links between extreme oxygen perturbations and the Cambrian radiation of animals |journal=Nature Geoscience |language=en |volume=12 |issue=6 |pages=468–474 |doi=10.1038/s41561-019-0357-z |pmid=31178922 |pmc=6548555 |bibcode=2019NatGe..12..468H |issn=1752-0908}}</ref> These pulses of oxygen are associated with the radiation of the small shelly fossils and the Cambrian [[arthropod]] radiation isotope excursion (CARE).<ref name="Zhang-2020" /> The increase in oxygenated waters in the deep ocean ultimately reduced the levels of organic carbon and pyrite burial, leading to a decrease in oxygen production and the re-establishment of anoxic conditions. This cycle was repeated several times during the early Cambrian.<ref name="Peng-2020" /><ref name="He-2019" />
[[Image:Archeocyathids.JPG|thumb|[[Archeocyathid]]s from the [[Poleta formation]] in the [[Death Valley]] area]]

==== Cambrian Stage 4 to early Miaolingian ====
The beginning of the eruptions of the Kalkarindji LIP basalts during Stage 4 and the early Miaolingian released large quantities of carbon dioxide, methane and sulphur dioxide into the atmosphere. The changes these wrought are reflected by three large and rapid δ<sup>13</sup>C excursions. Increased temperatures led to a global sea level rise that flooded continental shelves and interiors with anoxic waters from the deeper ocean and drowned carbonate platforms of archaeocyathan reefs, resulting in the widespread accumulation of black organic-rich shales. Known as the Sinsk anoxic extinction event, this triggered the first major extinction of the Phanerozoic, the 513 – 508 Ma Botoman-Toyonian Extinction (BTE), which included the loss of the archaeocyathids and [[Hyolitha|hyoliths]] and saw a major drop in biodiversity.<ref name="Myrow-2024" /><ref name="He-2019" /> The rise in sea levels is also evidenced by a global decrease in <sup>87</sup>Sr/<sup>86</sup>Sr. The flooding of continental areas decreased the rates of continental weathering, reducing the input of <sup>87</sup>Sr to the oceans and lowering the <sup>87</sup>Sr/<sup>86</sup>Sr of seawater.<ref name="Zhang-2020" /><ref name="Peng-2020" />

The base of the Miaolingian is marked by the Redlichiid–Olenellid extinction carbon isotope event (ROECE), which coincides with the main phase of Kalkarindji volcanism.<ref name="Myrow-2024" />

During the Miaolingian, orogenic events along the Australian-Antarctic margin of Gondwana led to an increase in weathering and an influx of nutrients into the ocean, raising the level of productivity and organic carbon burial. These can be seen in the steady increase in <sup>87</sup>Sr/<sup>86</sup>Sr and δ<sup>13</sup>C.<ref name="Zhang-2020" />

==== Early Furongian ====
Continued erosion of the deeper levels of the Gondwanan mountain belts led to a peak in <sup>87</sup>Sr/<sup>86</sup>Sr and linked positive δ<sup>13</sup>C and δ<sup>34</sup>S excursions, known as the [[Steptoean positive carbon isotope excursion]] (SPICE).<ref name="Myrow-2024" /> This indicates similar geochemical conditions to Stages 2 and 3 of the early Cambrian existed, with the expansion of seafloor anoxia enhancing the burial rates of organic matter and pyrite.<ref name="Zhang-2020" /> This increase in the extent of anoxic seafloor conditions led to the extinction of the marjumiid and [[Damesellidae|damesellid]] trilobites, whilst the increase in oxygen levels that followed helped drive the radiation of plankton.<ref name="Peng-2020" /><ref name="Pruss-2024" />

<sup>87</sup>Sr/<sup>86</sup>Sr fell sharply near the top of the Jiangshanian Stage, and through Stage 10 as the Gondwanan mountains were eroded down and rates of weathering decreased.<ref name="Peng-2020" /><ref name="Zhang-2020" />