Editing Carboniferous (section)

== Geochemistry ==
As the continents assembled to form Pangea, the growth of the Central Pangean Mountains led to increased [[weathering]] and carbonate sedimentation on the ocean floor,<ref name="Turchyn-2019">{{Cite journal |last1=Turchyn |first1=Alexandra V. |last2=DePaolo |first2=Donald J. |date=2019-05-30 |title=Seawater Chemistry Through Phanerozoic Time |journal=Annual Review of Earth and Planetary Sciences |language=en |volume=47 |issue=1 |pages=197–224 |doi=10.1146/annurev-earth-082517-010305 |bibcode=2019AREPS..47..197T |issn=0084-6597|doi-access=free }}</ref> whilst the distribution of continents across the paleo-tropics meant vast areas of land were available for the spread of tropical rainforests.<ref name="Stanley-2015" /> Together these two factors significantly increased [[Carbon dioxide|CO<sub>2</sub>]] [[Drawdown (climate)|drawdown]] from the atmosphere, lowering global temperatures, [[Ocean|increasing ocean pH]] and triggering the Late Paleozoic Ice Age.<ref name="Turchyn-2019" /> The growth of the supercontinent also changed [[seafloor spreading]] rates and led to a decrease in the length and volume of [[mid-ocean ridge]] systems.<ref name="Stanley-2015" />

=== Magnesium/calcium isotope ratios in seawater ===
During the early Carboniferous, the Mg<sup>2+</sup>/Ca<sup>2+</sup> ratio in seawater began to rise and by the Middle Mississippian [[aragonite sea]]s had replaced [[calcite sea]]s.<ref name="Stanley-2015" /> The concentration of calcium in seawater is largely controlled by ocean pH, and as this increased the calcium concentration was reduced. At the same time, the increase in weathering, increased the amount of magnesium entering the marine environment. As magnesium is removed from seawater and calcium added along mid-ocean ridges where seawater reacts with the newly formed lithosphere, the reduction in length of mid-ocean ridge systems increased the Mg<sup>2+</sup>/Ca<sup>2+</sup> ratio further.<ref name="Stanley-2015" /> The Mg<sup>2+</sup>/Ca<sup>2+</sup> ratio of the seas also affects the ability of organisms to [[Biomineralization|biomineralize]]. The Carboniferous aragonite seas favoured those that secreted [[aragonite]] and the dominant reef builders of the time were aragonitic sponges and corals.<ref name="Stanley-2015" />

=== Strontium isotopic composition of seawater ===
The [[strontium]] isotopic composition (<sup>87</sup>Sr/<sup>86</sup>Sr) of seawater represents a mix of strontium derived from continental weathering which is rich in <sup>87</sup>Sr and from mantle sources e.g. mid-ocean ridges, which are relatively depleted in <sup>87</sup>Sr. <sup>87</sup>Sr/<sup>86</sup>Sr ratios above 0.7075 indicate continental weathering is the main source of <sup>87</sup>Sr, whilst ratios below indicate mantle-derived sources are the principal contributor.<ref name="Woodcock-2012" />

<sup>87</sup>Sr/<sup>86</sup>Sr values varied through the Carboniferous, although they remained above 0.775, indicating continental weathering dominated as the source of <sup>87</sup>Sr throughout. The <sup>87</sup>Sr/<sup>86</sup>Sr during the Tournaisian was c. 0.70840, it decreased through the Visean to 0.70771 before increasing during the Serpukhovian to the lowermost Gzhelian where it plateaued at 0.70827, before decreasing again to 0.70814 at the Carboniferous-Permian boundary.<ref name="Chen-2022">{{Cite journal |last1=Chen |first1=Jitao |last2=Chen |first2=Bo |last3=Montañez |first3=Isabel P. |date=2022 |title=Carboniferous isotope stratigraphy |url=https://www.lyellcollection.org/doi/10.1144/SP512-2020-72 |journal=Geological Society, London, Special Publications |language=en |volume=512 |issue=1 |pages=197–211 |doi=10.1144/SP512-2020-72 |bibcode=2022GSLSP.512..197C |s2cid=229459593 |issn=0305-8719}}</ref> These variations reflect the changing influence of weathering and sediment supply to the oceans of the growing Central Pangean Mountains. By the Serpukhovian [[Basement (geology)|basement]] rocks, such as [[granite]], had been uplifted and exposed to weathering. The decline towards the end of the Carboniferous is interpreted as a decrease in continental weathering due to the more arid conditions.<ref name="Chen-2018">{{Cite journal |last1=Chen |first1=Jitao |last2=Montañez |first2=Isabel P. |last3=Qi |first3=Yuping |last4=Shen |first4=Shuzhong |last5=Wang |first5=Xiangdong |date=2018-05-01 |title=Strontium and carbon isotopic evidence for decoupling of pCO2 from continental weathering at the apex of the late Paleozoic glaciation |journal=Geology |language=en |volume=46 |issue=5 |pages=395–398 |doi=10.1130/G40093.1 |bibcode=2018Geo....46..395C |issn=0091-7613|doi-access=free }}</ref>

=== 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" />