The geochemistry of the late Cambrian carbonate in North China: the Steptoean Positive Carbon Isotope Excursion (SPICE) record suppressed in a coastal condition?

The Steptoean Positive Carbon Isotope Excursion (SPICE) is globally distributed in late Cambrian sedimentary records but controversially heterogeneous in its magnitudes. Here we use multiple geochemical proxies to investigate the late Cambrian carbonates from the Tangwangzhai section in North China, which were deposited in a shallow coastal environment with three depositional sequences (S1-S3). Each sequence comprises a transgressive systems tract (TST) and a highstand systems tract (HST). The REE + Y and trace element records are consistent with the depositional condition and indicate that terrigenous influence was more significant in the TST than HST. delta C-13(carb) and delta S-34(CAS) are low in the TST relative to HST, consistent with the scenario that terrigenous inputs were profoundly aggressive to seawater by introducing C-13-depleted and S-34-depleted materials. Within the TST of S2, the SPICE excursion shows a scaled-down delta C-13(carb) positive shift (similar to 1.7 parts per thousand) relative to its general records (similar to 4-6 parts per thousand); the corresponding delta S-34(CAS) show no positive excursion. This 'atypical' SPICE record is attributed to enhanced C-13-depleted and S-34-depleted terrigenous influence during the TST, which would reduce the amplitude of delta C-13(carb) excursion, and even obscure delta S-34(CAS) excursion. Meanwhile the subaerial unconformity at the base of TST would also cause a partially missing and a 'snapshot' preservation. Our study confirms significant local influence to the SPICE records, and further supports the heterogeneity and low sulphate concentrations of the late Cambrian seawater, because of which the SPICE records may be vulnerable to specific depositional conditions (e.g. sea-level, terrigenous input).