Evaluating the utility of rare earth elements as a redox proxy: a case study from the c. 1.57 Ga Gaoyuzhuang Formation, North China Craton

Rare Earth Elements (REE) in carbonate rocks are widely used as palaeo-ocean redox proxies partly due to their perceived resistance to diagenetic alteration. However, REE can also be mobilised at low fluid/rock ratios in anoxic porewaters, potentially modifying original patterns during early diagenesis. This study assesses REE preservation in marine carbonates across four sections of the ∼1.57 Ga Gaoyuzhuang Formation (Member III), spanning a negative carbon isotope excursion linked to ocean oxygenation. Shale-normalised REE patterns show a systematic transition from light REE enrichment below and within the excursion to a more seawater-like pattern above. Our findings suggest that this shift is likely related to Fe-Mn oxide dissolution during early diagenesis. The Ce anomaly trend, identified exclusively in samples exhibiting seawater-like patterns, aligns closely with the carbon isotope excursion and the emergence of multicellular eukaryotic macrofossils. We note that samples with near-seawater Sr isotope compositions do not consistently align with those exhibiting seawater-like REE patterns, with only the latter preserving a stratigraphic trend towards more negative Ce anomalies. We therefore recommend considering normalised REE patterns when interpreting REE proxies and Ce anomalies.