Expanded marine anoxia at the Cambrian-Ordovician transition: Evidence from lime mudstone I/Ca and 8238U signatures of the GSSP in western Newfoundland, Canada

The early Paleozoic witnessed two spectacular radiations of marine organisms-the Cambrian (CYRILLIC CAPITAL LETTER UKRAINIAN IE) Explosion and the Great Ordovician (O) Biodiversification Event. However, the period between them was characterized by a marine biodiversity plateau attributed to recurring biocrises. In the current study, we present the I/Ca records from slope lime mudstones of the Green Point Formation in western Newfoundland, along with a three-sink Uisotope mass balance model and previously reported limestone 8238Ucarb signals of the same interval, to further explore oceanic redox conditions at the CYRILLIC CAPITAL LETTER UKRAINIAN IE-O boundary. The I/Ca ratios of the lime mudstones, deposited along the eastern Laurentian (western Iapetan) continental slope, exhibit a narrow range between 0.02 and 0.33 & mu;mol/mol. Poor correlations between the I/Ca values and their [Sr], 818O, Mn/Sr, Fe/Sr, Mg/Ca, and 813Corg counterparts, together with near-micritic textures of the limestones, argue against significant influences of postdepositional alterations on the I/Ca signatures. The iodine-depleted lime mudstones, with I/Ca values well below the Proterozoic Eon baseline I/Ca ratios (-0.5-1 & mu;mol/mol), suggest the presence of shallow marine oxic-anoxic interfaces along the regional continental margin. Substantially low limestone I/Ca ratios (<0.5 & mu;mol/mol) and dysoxic to anoxic depositional conditions have also been reported from several other age-equivalent sections deposited along the shelf and slope of ancient Iapetan and Laurentian continental margins. As a result, seawaters surrounding the Iapetan and Laurentian continental margins at the CYRILLIC CAPITAL LETTER UKRAINIAN IE-O transition might have been commonly poorly oxygenated with shallow oxyclines or expanded oxygen minimum zones in the shelf and/or slope areas. This interpretation is further supported by our three-sink U-isotopemass balance modeling, which predicts widespread marine anoxia at the CYRILLIC CAPITAL LETTER UKRAINIAN IE-O boundary with anoxic to euxinic water covering 1.0-21.1% of the ocean floor, significantly higher than the modern day (-0.2%). Furthermore, the wide range of the oceanic 8238UOC (-1.22 to - 0.25 & PTSTHOUSND;) values, estimated from the limestone 8238Ucarb signals, might reflect oceanic redox oscillations during this period. However, the 8238UOC fluctuations could also be attributed to variable accumulations of the 238U-enriched authigenic U phases during early diagenesis. Overall, evidence from the lime mudstone I/Ca ratios and the estimated extent of marine anoxia at the CYRILLIC CAPITAL LETTER UKRAINIAN IE-O boundary in this study aligns with earlier viewpoints that the slowness of marine biodiversity accumulation during the late Cambrian and the Early Ordovician was linked to widespread oceanic anoxia.

Automated summary

In this study, the I/Ca records from slope lime mudstones of the Green Point Formation in western Newfoundland were analyzed to explore the oceanic redox conditions at the CYRILLIC CAPITAL LETTER UKRAINIAN IE-O boundary. The results suggest the presence of shallow marine oxic-anoxic interfaces in the region, which may have contributed to the slow marine biodiversity accumulation during the early Paleozoic.