A new study presents high-resolution nitrogen isotope records from the inner-ramp Penglaiba section in the Tarim Basin, documenting a two-stage deoxygenation–reoxygenation redox history of shallow marine environments during the Terreneuvian Epoch. Integrated with regional datasets, the findings demonstrate a persistent depth-dependent redox gradient across the basin, providing cross-basin validation for the hypothesis that stepwise shallow-ocean oxygenation modulated the multi-phase trajectory of the Cambrian Explosion.
The Cambrian Explosion, during which nearly all major animal phyla emerged in a geologically brief interval, stands as one of the most dramatic evolutionary events in Earth history, with marine redox change widely regarded as a key environmental driver of this biological radiation.
Recent nitrogen isotope investigations from the Yangtze Subprovince of South China have revealed that suboxic conditions prevailed in shallow-marine settings during the Terreneuvian, the earliest epoch of the Cambrian. This interval corresponds to the diversification of lophotrochozoans, a group of metazoans with relatively low oxygen requirements, leading to the hypothesis that stepwise oxygenation of shallow oceans underpinned the multi-phase pattern of the Cambrian Explosion. Testing this hypothesis requires independent evidence from other well-preserved early Cambrian paleocontinents.
The Tarim Block, an independent paleocontinent situated at the northwestern margin of Gondwana during the early Cambrian, hosts complete Terreneuvian sedimentary successions in the Aksu area of northwestern China. Previous nitrogen isotope work in the Tarim Basin has characterized redox conditions from middle-ramp to deep-basin settings, but the shallowest inner-ramp environments, the most relevant for early benthic ecosystems, have remained unstudied, leaving a critical gap in understanding the basin’s full redox architecture.
In a study published in Continent and Life Evolution , Chao Chang from Northwest University and colleagues report a high-resolution multi-proxy geochemical dataset from the Penglaiba section, an inner-ramp succession in the Aksu area, to fill this gap. The team analyzed bulk rock nitrogen isotopes (δ 15 N), organic carbon isotopes (δ 13 C org ), total organic carbon (TOC), and total nitrogen (TN) across the complete Yuertusi Formation, and integrated their results with published regional data to reconstruct the spatial redox structure of the Terreneuvian Tarim Basin.
Key findings of the study include:
(1) Two-stage redox evolution in shallow marine environments : The lower Yuertusi Formation records gradual intensification of water-column deoxygenation and denitrification, whereas the upper Yuertusi Formation is marked a phase of partial marine reoxygenation.
(2) Persistent depth-dependent redox gradient : Combined with published data from middle-ramp and deep-basin sections, the new inner-ramp δ 15 N records reveal a systematic depth gradient throughout the Terreneuvian, reflecting progressively weaker denitrification across a redox gradient from anoxic deep-basin to suboxic inner-ramp settings.
(3) Cross-basin support for the stepwise oxygenation hypothesis : The co-occurrence of shallow-marine suboxic conditions and diverse small shelly fossil assemblages in the Tarim Basin matches the pattern previously documented in South China. This consistency across two independent paleocontinents supports the hypothesis that stepwise oxygenation of shallow oceans was a global-scale driver of the multi-phase Cambrian Explosion.
“Our work extends the documented link between shallow-marine redox state and multi-stage early animal evolution from South China to the Tarim Block” explains Dr. Chang.
This paper “Nitrogen isotope constraints on Terreneuvian shallow-marine redox evolution in the Aksu area, Tarim Basin” was published in Continent and Life Evolution .
Liu S, Wang Z, Ju P, Chang C. Nitrogen isotope constraints on Terreneuvian shallow-marine redox evolution in the Aksu area, Tarim Basin. Cont. Life Evol. 2026(2):0008, https://doi.org/10.55092/cle20260008.
Continent & Life Evolution
Observational study
Not applicable
Nitrogen isotope constraints on Terreneuvian shallow-marine redox evolution in the Aksu area, Tarim Basin
4-Aug-2026