Editing Carboniferous (section)

=== Oxygen and carbon isotope ratios in seawater ===
Unlike Mg<sup>2+</sup>/Ca<sup>2+</sup> and <sup>87</sup>Sr/<sup>86</sup>Sr isotope ratios, which are consistent across the world's oceans at any one time, [[Δ18O|δ<sup>18</sup>O]] and [[Δ13C|δ<sup>13</sup>C]] preserved in the fossil record can be affected by regional factors.<ref name="Chen-2022" /> Carboniferous δ<sup>18</sup>O and δ<sup>13</sup>C records show regional differences between the South China open-water setting and the epicontinental seas of Laurussia. These differences are due to variations in seawater salinity and evaporation between epicontinental seas relative to the more open waters.<ref name="Chen-2022" /> However, large scale trends can still be determined. δ<sup>13</sup>C rose rapidly from c. 0 to 1‰ (parts per thousand) to c. 5 to 7‰ in the Early Mississippian and remained high for the duration of the Late Paleozoic Ice Age (c. 3–6‰) into the early Permian.<ref name="Chen-2022" /> Similarly from the Early Mississippian there was a long-term increase in δ<sup>18</sup>O values as the climate cooled.<ref name="Montañez-2022" />

Both δ<sup>13</sup>C and δ<sup>18</sup>O records show significant global isotope changes (known as excursions) during the Carboniferous.<ref name="Chen-2022" /> The mid-Tournaisian positive δ<sup>13</sup>C and δ<sup>18</sup>O excursions lasted between 6 and 10 million years and were also accompanied by c. 6‰ positive excursion in organic matter [[Δ15N|δ<sup>15</sup>N]] values,<ref name="Montañez-2022" /> a negative excursion in carbonate δ[[Uranium-238|<sup>238</sup>U]] and a positive excursion in carbonate-associated sulphate [[Δ34S|δ<sup>34</sup>S]].<ref name="Chen-2022" /> These changes in seawater geochemistry are interpreted as a decrease in atmospheric CO<sub>2</sub> due to increased [[Total organic carbon|organic matter]] burial and widespread ocean anoxia triggering climate cooling and onset of glaciation.<ref name="Chen-2022" />

The Mississippian-Pennsylvanian boundary positive δ<sup>18</sup>O excursion occurred at the same time as global sea level falls and widespread glacial deposits across southern Gondwana, indicating climate cooling and ice build-up. The rise in <sup>87</sup>Sr/<sup>86</sup>Sr just before the δ<sup>18</sup>O excursion suggests climate cooling in this case was caused by increased continental weathering of the growing Central Pangean Mountains and the influence of the orogeny on precipitation and surface water flow rather than increased burial of organic matter. δ<sup>13</sup>C values show more regional variation, and it is unclear whether there is a positive δ<sup>13</sup>C excursion or a readjustment from previous lower values.<ref name="Chen-2022" />

During the early Kasimovian there was a short (<1myr), intense glacial period, which came to a sudden end as atmospheric CO<sub>2</sub> concentrations rapidly rose.<ref name="Montañez-2022" /> There was a steady increase in arid conditions across tropical regions and a major reduction in the extent of tropical rainforests, as shown by the widespread loss of coal deposits from this time.<ref name="Chen-2018" /> The resulting reduction in productivity and burial of organic matter led to increasing atmospheric CO<sub>2</sub> levels, which were recorded by a negative δ<sup>13</sup>C excursion and an accompanying, but smaller decrease in δ<sup>18</sup>O values.<ref name="Montañez-2022" />