Editing Cambrian (section)

=== Magnesium/calcium isotope ratios in seawater ===
The mineralogy of inorganic marine carbonates has varied through the Phanerozoic, controlled by the Mg<sup>2+</sup>/Ca<sup>2+</sup> values of seawater. High Mg<sup>2+</sup>/Ca<sup>2+</sup> result in [[calcium carbonate]] precipitation dominated by [[aragonite]] and high-magnesium [[calcite]], known as [[aragonite sea]]s, and low ratios result in [[calcite sea]]s where low-magnesium calcite is the primary calcium carbonate precipitate.<ref name="Wei-2022">{{Cite journal |last1=Wei |first1=Guang-Yi |last2=Hood |first2=Ashleigh v. S. |last3=Planavsky |first3=Noah J. |last4=Li |first4=Da |last5=Ling |first5=Hong-Fei |last6=Tarhan |first6=Lidya G. |date=2022 |title=Calcium Isotopic Constraints on the Transition From Aragonite Seas to Calcite Seas in the Cambrian |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GB007235 |journal=Global Biogeochemical Cycles |language=en |volume=36 |issue=5 |doi=10.1029/2021GB007235 |bibcode=2022GBioC..3607235W |issn=0886-6236}}</ref> The shells and skeletons of biomineralising organisms reflect the dominant form of calcite.<ref name="Xiong-2023">{{Cite journal |last1=Xiong |first1=Yi |last2=Wood |first2=Rachel |last3=Pichevin |first3=Laetitia |date=2023 |title=The record of sea water chemistry evolution during the Ediacaran–Cambrian from early marine cements |url=https://onlinelibrary.wiley.com/doi/10.1002/dep2.211 |journal=The Depositional Record |language=en |volume=9 |issue=3 |pages=508–525 |doi=10.1002/dep2.211 |bibcode=2023DepRe...9..508X |issn=2055-4877|hdl=20.500.11820/3e22be46-182d-421a-a791-8a20a96c9814 |hdl-access=free }}</ref>

During the late Ediacaran to early Cambrian increasing oxygen levels led to a decrease in ocean acidity and an increase in the concentration of calcium in sea water. However, there was not a simple transition from aragonite to calcite seas, rather a protracted and variable change through the Cambrian. Aragonite and high-magnesium precipitation continued from the Ediacaran into Cambrian Stage 2. Low-magnesium calcite skeletal hard parts appear in Cambrian Age 2, but inorganic precipitation of aragonite also occurred at this time.<ref name="Xiong-2023" /> Mixed aragonite–calcite seas continued through the middle and late Cambrian, with fully calcite seas not established until the early Ordovician.<ref name="Xiong-2023" />

These variations and slow decrease in Mg<sup>2+</sup>/Ca<sup>2+</sup> of seawater were due to low oxygen levels, high continental weathering rates and the geochemistry of the Cambrian seas. In conditions of low oxygen and high iron levels, iron substitutes for magnesium in [[Authigenesis|authigenic clay minerals]] deposited on the ocean floor, slowing the removal rates of magnesium from seawater. The enrichment of ocean waters in silica, prior to the radiation of siliceous organisms, and the limited bioturbation of the anoxic ocean floor increased the rates of deposition, relative to the rest of the Phanerozoic, of these clays. This, together with the high input of magnesium into the oceans via enhanced continental weathering, delayed the reduction in Mg<sup>2+</sup>/Ca<sup>2+</sup> and facilitated continued aragonite precipitation.<ref name="Wei-2022" />

The conditions that favoured the deposition of authigenic clays were also ideal for the formation of ''[[lagerstätten]]'', with the minerals in the clays replacing the soft body parts of Cambrian organisms.<ref name="Pruss-2024" />