Geochemical evidence for widespread euxinia in the Later Cambrian ocean

Widespread anoxia in the ocean is frequently invoked as a primary driver of mass extinction as well as a long-term inhibitor of evolutionary radiation on early Earth. In recent biogeochemical studies it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later Cambrian oceans, possibly influencing evolutionary events during this time. Physical evidence of widespread anoxia in Cambrian oceans has remained elusive and thus its potential relationship to the palaeontological record remains largely unexplored. Here we present sulphur isotope records from six globally distributed stratigraphic sections of later Cambrian marine rocks (about 499 million years old). We find a positive sulphur isotope excursion in phase with the Steptoean Positive Carbon Isotope Excursion (SPICE), a large and rapid excursion in the marine carbon isotope record, which is thought to be indicative of a global carbon cycle perturbation. Numerical box modelling of the paired carbon sulphur isotope data indicates that these isotope shifts reflect transient increases in the burial of organic carbon and pyrite sulphur in sediments deposited under large-scale anoxic and sulphidic (euxinic) conditions. Independently, molybdenum abundances in a coeval black shale point convincingly to the transient spread of anoxia. These results identify the SPICE interval as the best characterized ocean anoxic event in the pre-Mesozoic ocean and an extreme example of oxygen deficiency in the later Cambrian ocean. Thus, a redox structure similar to those in Proterozoic oceans may have persisted or returned in the oceans of the early Phanerozoic eon. Indeed, the environmental challenges presented by widespread anoxia may have been a prevalent if not dominant influence on animal evolution in Cambrian oceans.

Global-scale anoxia in the deep ocean is frequently invoked as a primary driver of mass extinction, as well as a long-term inhibitor of evolutionary radiation on the early Earth. In recent biogeochemical studies, it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later Cambrian oceans 1,2 , possibly influencing evolutionary events during this time 1,2,3 . Physical evidence of widespread anoxia in Cambrian oceans has remained elusive, and thus its potential relationship to the paleontological record remains largely unexplored.
Here, we present sulphur isotope records from six globally distributed stratigraphic Carbonate rocks of Cambrian age preserve large, rapid (of a few million years duration or less) and globally correlated excursions in the marine carbon isotope record (δ 13 C carb ), which indicate perturbations in the global carbon cycle ( Figure S1) 7,8,9,10 . The mechanisms that drove these events, however, are poorly known. What makes these excursions particularly interesting to geobiologists is the observation that many coincide with biological events recorded by fossils, suggesting causal links between biological and environmental history 3,10 . The focus of our work is on the last large excursion of the period, the Steptoean Positive Carbon Isotope Excursion, or SPICE.
The SPICE is recorded as a +4-6‰ shift in δ 13 C carb that occurs globally in later Cambrian successions (at the beginning of the Furongian International Series and Paibian International Stage, 499 Ma); it is thought to have lasted on the order of 2-4 million years 10,11 . A well-documented extinction of trilobites coincides with the onset of the SPICE on the paleocontinent of Laurentia 10,12 , and the isotopic excursion has also been correlated to intervals of biological turnover on other paleocontinents 13 . The SPICE is also coincident with global changes in sea level; its onset coincides with a transgressive event, and its peak is concurrent with a lowstand recorded as the Sauk-II/III hiatus 10,11 . The SPICE sulphur isotope excursion is one of the largest identified in the geologic record and is the first to be correlated globally at this scale of resolution. This excursion occurs in both carbonate-associated sulphate (CAS) and pyrite, which further supports a primary marine signal, and its magnitude indicates a major perturbation in the global sulphur cycle. There are, however, significant differences in the details of the sulphate sulphur isotope trends among basins. In particular, the pre-event δ 34 S CAS baseline differs among the various locations ( Despite overarching similarities, the absolute values and amplitudes of the excursion also differ among the studied basins. The Gondwanan record is the most extreme, with δ 34 S CAS values reaching almost +70‰ and an amplitude of +35‰ ( Figure   2). On the other end of the spectrum, the record in eastern Laurentia shows a peak value of +38‰ and amplitude of only +12‰ ( Figure 2). These isotopic differences support the idea that the sulphur reservoir in the later Cambrian ocean was spatially heterogeneous and that sulphate concentrations were therefore low 14,15 . We also observe that the sulfate isotope excursion peaks stratigraphically slightly before the carbon isotope maximum (see Supplementary Figure S2), which suggests that the sulphate reservoir was relatively more sensitive to change than the marine pool of dissolved inorganic carbon (DIC). This state of sulphate in later Cambrian seawater differs greatly from the modern reservoir, which is relatively homogenous globally with a concentration of 28 mmol/kg (mM) and a sulphur isotope composition of +21‰. This contrast with the modern ocean indicates that the residence time of sulphate in the Cambrian ocean was much shorter.
The parallel behavior between the carbon and sulphur isotope excursions Importantly, our model puts quantitative constraints on the size of the marine sulphate reservoir during the later Cambrian. An assumption of pre-SPICE sulphate concentrations greater than 2.5 mM demands more than 8 million years for recovery of δ 34 S sulphate (i.e., return to the pre-event baseline) following the SPICE ( Figure S13), which is unreasonable in light of the available constraints on the duration of the SPICE 11 . Our simulations suggest, therefore, that the concentration of seawater sulphate was very low -at or below the low end of the 2-12 mM range suggested by previous work 14,15 .
Another important result from the model is that the predicted ratio of carbon-tosulphur (C/S) linked to this transient burial was very low: 1 to 4 moles C/mole S or 0.4-1.5 g C/g S (Figure 4). In younger sediments, similar C/S ratios are only observed in sediments deposited under euxinic conditions 16  The geochemical and stratigraphic framework of the SPICE provides new insight into the pronounced biological turnover associated with this event. Taken together with evidence for sea-level rise, the geochemical data suggest that shoaling of toxic anoxic deep waters onto the shelf led to the extinction of shelf fauna, a situation similar to that envisioned for end-Permian extinctions 23 . Such a scenario was proposed previously to explain recurrent later Cambrian trilobite extinctions 12 but in acknowledged absence of independent constraints for such conditions. Additional oscillations observed in the later Cambrian marine δ 13 C record could reflect environmental perturbations similar to the SPICE. We suggest that anoxic water masses occurred widely in the subsurface of the later Cambrian ocean (i.e., below the wind-mixed surface layer), a view that finds qualitative support in the stratigraphic distribution of organic-rich, pyritic black shales, which peak in abundance in later Cambrian successions 24 . If correct, the high rates of biological turnover 25 169  170  171  172  173  174  175  176  177  178  179  180  181  182  183  184  185  186  187  188  189  190  191  192  193  194  195  196  11  29 Lyons, T. W. & Severmann, S. A critical look at iron paleoredox proxies: New insights from modern euxinic marine basins. Geochimica Cosmochimica Acta 70, 5698-5722 (2006).