Table 2 Summary of redox evolution in Kazhdumi intrashelf basin. Carbon-isotope segments C2–C6 after³.

Segment	δ¹³C	I/(Ca + Mg)	Ce/Ce*	Mn/Ca, Fe/Ca	RSTE, PSTE, TOC¹	Interpreted Redox Conditions
Upper C5 + C6	Relatively constant	Stepwise increase (~ 0.2 to ~ 1.3 µmol/mol)	Slight increase	Increase	Varied TOC, low RSTE and PSTE; intervals with high Mo and P	Improved oxygenation (suboxic to oxic). Oxygenation linked to sea-level fall and enhanced weathering.
Lower C5 + C6	Rel. constant	Similar to C4	Similar to C4	Low	High, decreasing	Low oxygenation. No significant redox change yet.
C4	Stepwise positive excursion	Relatively constant (~ 0.23 µmol/mol)	Highly variable. Maximum (> 1.1) in upper Radiolarian Flood Zone.	Low	High	Persistently low oxygenation. Continued suboxic to anoxic conditions.
C3	Abrupt negative excursion	Decreasing	Decreasing	Rel. high, decreasing	Low	Oxic. Onset of OAE1a; major carbon injection into oceans.
Upper C2	Stepwise increase	Stepwise increasing (low overall)	Relatively high (> 0.9)	Relatively high	Low, high Mo	Suboxic. Pre-OAE1a oxygen depletion; enhanced continental weathering.

¹ RSTE = redox-sensitive trace elements (U, V, Mo), PSTE = productive-sensitive trace elements (Cu, Ni, P), TOC = total organic content (data adopted from⁴⁴).

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