A new study in Communications Earth & Environment reports that late Cambrian life suffered a dramatic blow during the Marjuman extinction, and that the catastrophe may have unfolded in two phases. According to authors led by Wang, Zhao, and Grasby, the key drivers were enhanced ocean upwelling and the spread of “photic zone euxinia,” a form of oxygen-poor, sulfide-rich seawater that reaches the sunlit layer where photosynthesis occurs.
To investigate the timing and mechanisms of the die-off, the researchers integrated geochemical evidence preserved in late Cambrian marine sediments with a broader interpretation of ocean circulation and nutrient delivery. Their approach links changes in the cycling of deep and surface waters to shifts in seawater chemistry across the extinction interval.
The study argues that stronger upwelling would have transported nutrient-rich waters upward, fueling high primary productivity. However, this same nutrient surge likely also intensified microbial decomposition of organic matter after blooms collapsed, drawing down dissolved oxygen in surface waters.
As oxygen levels declined, the ocean would have shifted toward euxinic conditions—characterized by pervasive hydrogen sulfide. Importantly, the researchers propose that sulfide did not remain confined to deep basins. Instead, it spread into the photic zone, producing a hostile “deadly light” environment where sunlight could still penetrate but photosynthetic ecosystems could not function safely.
The two-phase pattern of the Marjuman extinction is explained as the consequence of escalating and then partially restructuring oceanographic stress. In the first phase, upwelling-enhanced productivity and oxygen drawdown may have begun to destabilize food webs and respiratory tolerance thresholds. In the second, sustained anoxia and sulfidic chemistry likely expanded, producing more widespread ecosystem collapse.
The authors emphasize that photic zone euxinia provides a direct pathway from physical ocean changes to biological failure. Even organisms adapted to low oxygen would face additional stress from sulfide toxicity and habitat degradation in surface waters.
Overall, the work connects regional or global circulation shifts to chemical feedbacks that transform ocean surface conditions. If the same chain of processes occurred during the Marjuman interval, it would help explain why the extinction was severe and temporally structured rather than a single, uniform event.
By tying enhanced upwelling to sulfidic surface-water expansion, the study adds a mechanistic template for understanding how ocean physics and biogeochemistry can combine to trigger mass mortality. The findings also offer a cautionary parallel: modern oceans experiencing stratification, nutrient imbalance, and oxygen loss could, under the right circumstances, approach similar self-reinforcing states.
Subject of Research: Late Cambrian Marjuman extinction; ocean circulation, upwelling, and photic zone euxinia
Article Title: Enhanced upwelling and subsequent photic zone euxinia linked to the two-phase late Cambrian Marjuman extinction
Article References: Wang, X., Zhao, H., Grasby, S.E. et al. Enhanced upwelling and subsequent photic zone euxinia linked to the two-phase late Cambrian Marjuman extinction. Commun Earth Environ (2026). https://doi.org/10.1038/s43247-026-03846-9
Image Credits: AI Generated
