Introduction

Rocks from the Ediacaran Period (635-539 Ma) record remarkable events in the intertwined histories of Earth and life. Starting with recovery from the last Cryogenian glaciation1, the Ediacaran Period saw the initial radiation of macroscopic animals2; oscillations in the redox state and chemistry of Earth’s oceans and atmosphere3,4,5,6,7; and unusually large magnitude carbon isotope excursions5,8,9. If causal relationships between these events can be demonstrated, then this interval of Earth history may illuminate key controls on the emergence of complex macroorganisms and interrelationships between biological, physical, and chemical processes in the broader Earth system. Assessing these relationships requires the development of an Ediacaran timescale to aid in global correlations and the investigation of cause and effect between biogeochemical cycling, ecosystem complexity and animal evolution. However, the development of such a timescale is difficult in all but the youngest Ediacaran strata. Contributing factors include the paucity of Ediacaran skeletal fossils, making biostratigraphic correlation across lithofacies and basins especially challenging due to taphonomic biases, and uncertainty about the synchroneity of both Ediacaran carbon isotope excursions or climatic events, which might be used as non-biostratigraphic markers10,11. Here, we address these difficulties, developing a robust temporal framework for the Ediacaran Nafun Basin of the Sultanate of Oman, and discussing its implications for the temporal framework of the Ediacaran Period more broadly.

The geological archives of the Ediacaran Nafun and Ara groups (Fig. 1) have been intensively studied, yielding insights into sedimentary environments12,13,14,15,16,17, paleoenvironmental proxies5,18,19,20,21, early biomineralized skeletons22, and biomarker records23,24,25,26. The Shuram excursion, the most negative carbon isotope excursion in Earth history, was first identified in the Nafun Group27. Its origin, cause, consequences, and use as a marker for correlation, remain controversial (Supplementary Table 1)27,28,29.

Fig. 1: Age-depth and depositional rate models and stratigraphic overview of the study area.
figure 1

A Age-depth model for the Ediacaran stratigraphy of Oman built including ages presented here and a Bayesian Markov chain Monte Carlo model. Age for end of the Gaskiers glaciation is a constraint on the end of deposition of the Trinity diamictite95. B Litho- and chemostratigraphy of the South Oman Salt Basin from reference well TM-6 with positions of Re-Os geochronological constraints shown ((H: Hadash Fm; K: Khufai Fm; MB: Masirah Bay Fm; BBC: basal Birba clastics). C Depositional rate history for the succession calculated using the age-depth model shown in A indicates an increase in depositional rates in the middle Ediacaran Period.

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Prior to recent work28 and this study, direct age constraints on the Nafun and Ara groups were limited to a series of volcanic ashes in the uppermost strata, all younger than 547.23 ± 0.96 Ma29 (chemical abrasion-isotope dilution-thermal ionization mass spectrometry U-Pb zircon age; all geochronological uncertainties in this study are reported either as 2σ or 95% confidence intervals). The Hadash Formation, interpreted as a post-Marinoan cap carbonate, provides age control at the base of the Nafun Group, pinning it at c. 635 Ma based on chronostratigraphic correlation with a dated post-Marinoan cap carbonate from the Yangtze Basin30. Such limited age control on a succession spanning nearly ninety million years of Earth history is a major roadblock for efforts to correlate the rich geological archives of the Nafun Basin to global records and test various hypotheses connecting oceanic oxygenation3,7,31,32, biogeochemical cycling19,33,34,35, animal evolution36,37,38,39,40, and solid-Earth processes41,42,43,44. Ultimately, the lack of radioisotopic age constraints has contributed to broader difficulties establishing a timescale for the Ediacaran Period. Here, we leverage six new geochronological constraints on the Ediacaran succession of Oman to deliver a robust temporal framework for the Nafun Basin and the Ediacaran Period.

Results and Discussion

Six new age constraints for the Nafun Basin

Our six new Re-Os ages (Figs. 1 and 2) occur in stratigraphic superposition and are consistent with existing age control on the Nafun Group and overlying units29,45,46. One new date below the Shuram excursion lies within the upper Masirah Bay Formation (L6), yielding an age of 582.0 ± 4.2 Ma (2-sigma uncertainty, number of points used to construct the isochron [n] = 8, mean square of weighted deviates [MSWD] = 1.5; total uncertainties include the uncertainty in the 187Re constant, λ47). The first constraint above the nadir of the Shuram excursion is 567.7 ± 7.4 Ma (M2, n = 7, MSWD = 0.86), within the lower Buah Formation. Two sample sets within the upper Buah Formation yield ages of 560.4 ± 2.3 Ma (L4, n = 7, MSWD = 0.99) and 558.4 ± 2.2 Ma (L3, n = 6, MSWD = 1.4). Within the basal Birba clastics unit, two dates of 555.4 ± 2.4 Ma (L2, n = 8, MSWD = 1.3) and 553.4 ± 4.1 Ma (L1, n = 9, MSWD = 1) were determined. Re concentrations in samples ranged from 1.77 ng/g to 1416 ng/g and Os concentrations from 86.7 pg/g to 30969 pg/g (Supplemental Data). These are high compared to other Neoproterozoic shales, such as those measured from Zambia48 (Re concentrations from 0.83 ng/g to 14 ng/g; Os concentrations from 54.6-431 pg/g), Northwest Canada28,48 (Re concentrations from 0.63 ng/g to 22.2 ng/g; Os concentrations from 43.6 pg/g to 430 pg/g) or China49 (Re concentrations from 16.7 ng/g to 32.8 ng/g; Os concentrations from 249.7 pg/g to 698.32 pg/g).

Fig. 2: Re-Os isochron diagrams for dated horizons.
figure 2

A Sample set L1, Well L. B Sample set L2, Well L. C Sample set L3, Well L. D Sample set L4, Well L. E Sample set M2, Well M. F Sample set L6, Well L.

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Initial 187Os/188Os isotope ratios determined for our samples range from 0.3 ± 0.01 to 1.05 ± 0.01(Supplemental Data), and, in general, are more radiogenic upsection. Given limited data density (8 total initial 187Os/188Os isotope ratio measurements from this study and28 over the nearly 100 million years of the Ediacaran Period), and evidence for tectonic reconfiguration and restriction of the Nafun Basin at this time, we do not necessarily interpret these measurements as indicative of global seawater.

New age-depth model for Ediacaran Oman

Our results, in combination with previously published age constraints28,29 are used to develop a stratigraphic Markov chain Monte Carlo age-depth model in a Bayesian framework for the Ediacaran strata of the Sultanate of Oman (Fig. 1). This age-depth model, comprising nine total age constraints on the succession, allows the interpolation of depositional ages for all strata within the succesion. Applying this model, the Shuram excursion occurs at ({573}_{-6}^{+6}) Ma with recovery at ({568}_{-4}^{+5}) Ma and a duration of ({5}_{-5}^{+6}) million years. Most Ediacaran strata in the Nafun Basin were deposited at or after c. 580 Ma in both shallow and deep-water sections (Figs. 1, 3).

Fig. 3: Age-depth models for key Ediacaran sections.
figure 3

Age depth models for shallower- and deeper-water sections in the Nafun, Nanhua, and Krol basins show that these sections contain relatively thin early Ediacaran strata, with most strata deposited post-c. 580 Ma and post-Shuram excursion. Shaded region is 95% confidence interval. Age constraints, stratigraphic positions of age constraints, and age-depth models are described in Supplementary Data 3-42. Sultantate of Oman: Shallow: Well MQ-1; deep: Well T690. H: Hadash Fm; MB: Masirah Bay Fm; K: Khufai Fm; Shur.: Shuram Fm; BBC: basal Birba clastics; A: Ara Group. Nanhua Basin: Shallow: Jiulongwan–Shipai; deep: Yuanling96,97. Y: Yanjiahe Fm. Krol Basin: Shallow: Mussoorie. Deeper: Nigalidhar98. Bi.: Biani Fm; IK: Infra Krol; A: Krol A; B: Krol B; E: Krol E.

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Thin early Ediacaran records across eastern Gondwana

We build age-depth and depositional rate models for Ediacaran successions worldwide, leveraging chemostratigraphic correlation of the Shuram excursion and published age constraints (Supplemental Notes, Supplemental Data). A consistent pattern across the Nafun Basin (Sultanate of Oman), the Yangtze Basin (South China), and the Krol Basin (India) emerges (Fig. 3). In these basins, most strata are of middle and late Ediacaran age, and early Ediacaran strata are comparatively thin. These successions exhibit similar depositional histories, likely due to their eastern Gondwana affinity50,51. These locations transition from passive margin sedimentation to foreland basin during and just after our study interval, capturing the collision of the Arabian Nubian Shield (later part of western Gondwana) with Neoproterozoic India (later part of eastern Gondwana). For simplicity, we refer to these localities as part of “eastern Gondwana”, while acknowledging the evolving nature of this margin during closure of the Mozambique Ocean51,52,53,54,55,56,57,58,59. Backstripping analysis indicates that accommodation increased more rapidly in these locations after c. 580 Ma (Supplemental Fig. S4). An exception is the deeper Yangtze Basin setting, which is more weakly constrained in time because the strata lack a clear Shuram excursion (Fig. 3), but which still shows a slight increase in accumulation rate in the late Ediacaran Period. Southern Australia also has a Gondwanan affinity50 but shows more steady sediment accumulation over the interval (Supplemental Figs. S2, S3).

Globally, other Ediacaran successions, including the Nama Group60,61,62, Siberian platform63, and South American basins including the Bambui, Arroyo del Soldado and Araras groups64, have extensive mid- to late Ediacaran strata and relatively thin early Ediacaran strata (Supplemental Notes). In contrast, successions along the western margin of Laurentia show relatively steady accumulation rates (Supplemental Figs. S2, S3).

A temporal framework for the middle and late Ediacaran Period

The Nafun Basin is among the best-characterized records of environmental change just prior to and during the diversification of early animals5,12,18,19,20,24,25,26,65,66,67,68,69. This succession is well-exposed in outcrop, with hundreds of meters archived in core repository. The preservation, continuity, richness, and robust age control of the Nafun Basin make it a key global reference section for studies of Earth’s environments during the first recorded diversification of animals. With the age model developed here, the ages of most Ediacaran strata in the Nafun Basin are now constrained to within a few million years (Fig. 1). This provides age control and constraints on sedimentation, which is a critical—but often overlooked—parameter for generating accurate mechanistic geochemical models of Ediacaran Earth system evolution.

These results also bear on the duration and extent of Earth’s largest negative carbon isotope excursion, the Shuram excursion. This excursion was first described in Oman before being identified in many middle Ediacaran basins globally. The origin and environmental or evolutionary significance of the Shuram excursion are debated. These new constraints, combined with our new Bayesian age-depth model, limit the Shuram excursion in the Nafun Basin to ({5}_{-5}^{+6}) million years, a meaningful improvement over a previous estimate, based only on two dates, of <15.5 ± 7.0 million years28. Age control on the onset, recovery, and duration of the excursion in the Nafun Basin are wholly consistent with independent constraints from Northwest Canada28. This buttresses interpretations of the excursion as a globally synchronized event, at least at a timescale of several millions of years or less. This, in turn, strengthens support for models of the excursion as either a primary or globally synchronized, early diagenetic event which postdates the Gaskiers glaciation, in contrast to hypotheses suggesting the excursion is globally diachronous70 (Supplemental Table S1).

Condensation and Ediacaran geochemical records

In several basins across the eastern Gondwanan margin, the record of early Ediacaran sedimentation is either condensed, marked by major hiatus(es), or both (Fig. 3). The Nafun Basin and Yangtze Basin both fall on this margin and are among the most intensively characterized Ediacaran successions in the world. Either condensation or the presence of hiatal surfaces has important implications for the interpretation of their geochemical records. Hiatal surfaces would cause gaps, but in Oman, there is no evidence for major hiatal surfaces in basinal cores. Instead, existing geological data support a condensed Masirah Bay Formation (Fig. 3; though note that in grabens, the Masirah Bay Fm can be >1200 m in thickness66). Condensation is also apparent in the Yangtze Basin of South China71. Direct age control on the Krol Group of India is lacking, but the similarities and correlations between the Krol strata and the Nafun Basin52 support early Ediacaran condensation in the Krol Group, as well (Fig. 3). Low net sedimentation rates in the early Ediacaran records from these locations are not only the result of the Sadler effect, the intrinsic property of the sedimentary record in which the sedimentation rate of strata decreases over longer time intervals due to the non-steady nature of sediment accumulation (Supplemental Notes; Supplemental Figs. S5-S7).

A frequently described hypothesis for the appearance and diversification of animals in the Ediacaran Period is that greater environmental oxygenation permitted the evolution of complex, oxygen-dependent animals (e.g., see discussion in refs. 37,72), with geochemical redox proxies, sedimentary phosphate abundance, and organic carbon concentration all feeding into the argument. However, elemental concentrations alone may be misleading during this critical interval of animal evolution, because sedimentation rate exerts a major influence on a range of concentration data used to draw inferences about pO2 and productivity32,73,74. Phosphate availability, for example, is key to both primary production and oxygenation75,76. The concentrations of phosphorus (P) in Ediacaran rocks are fairly static75 but our results demonstrate that depositional rates in key sections are not. Therefore, geochemical models that infer a constant elemental burial flux from steady geochemical concentrations may well underestimate late Ediacaran oxygenation. Conversely, elevated abundances of redox-sensitive elements in lower Ediacaran shales could result from enrichment due to low sedimentation rates, rather than deposition in an oxic water column32.

Similar concerns apply to isotope proxy records. For example, if uranium is reduced in the water column of an anoxic basin, then systematic changes in sedimentation rate would produce large variations in uranium isotope records unrelated to redox changes, and higher sedimentation rates would produce more positive δ238U values—the same signature as increased oxygenation77. Water column reduction of uranium is not observed in modern anoxic basins, but this illustrative sedimentation rate dependency should motivate caution in considering redox proxy records in the absence of robust sedimentation rate data. Indeed, sulfur isotope records used to infer environmental oxygenation5 have proven to be highly dependent on sedimentation rate73.

We also note that sedimentation rate provides a critical control on the preservation of organisms and their traces in the fossil record78. Diverse early macroscopic Ediacaran fossils occur in strata with high sedimentation rates relative to the Nafun and Nanhua basins, such as the Rackla Group of Northwest Canada79 (Supplemental Fig. S2) or the Conception Group of Newfoundland80. The expansion of the late Ediacaran sedimentary record typically aligns with stratigraphic evidence for transgression and the resulting expansion of shallow marine environments (Supplemental Notes and references therein). The expansion of both the fossil and sedimentary record in the late Ediacaran record of North America has been noted81. The laterally heterogenous nature of shallow marine environments has been hypothesized as an evolutionary driver for greater diversity in both late Ediacaran shallow and, indirectly, deep-water communities82. That said, changing sea levels may also have imposed a strong taphonomic bias to the record by altering the ratio of shallow and deep-water deposits, as may be seen in the paleontological record of other eras83.

Clearly, disentangling the true relationships between changes in life and environments through the Ediacaran Period will require that fossils and geochemical data be interpreted within a detailed framework of space, time, taphonomy, and depositional environment. Our results demonstrate the importance of coupling radiometric ages and age depth modeling to constrain and quantify sedimentation rates at both the basin and regional scale when evaluating ocean redox patterns and concomitant evolutionary innovations.

Conclusion

The expanded nature of the late—and the relative thinness or absence of the early—Ediacaran record seen in multiple basins across eastern Gondwana, as well as in basins across southern Namibia60,61,62, Siberia63, and some in South America64, present a key geological limitation on our understanding of the Ediacaran Period. Our findings draw attention to the importance of basins with expanded early Ediacaran sections, like those in northern Namibia84, Mongolia85, northwest Canada86, the Parecis Basin of Brazil87, for improving our understanding of the period geochemically and biologically. The temporal framework developed here for the Nafun Basin provides us with a robustly dated, geochemically well-characterized, exceptionally well-preserved, high-resolution reference section for studies of the middle and late Ediacaran Period. Our results highlight the need for a critical reappraisal of the role of sedimentation and burial rates for understanding how, and if, environmental changes, including increasing oxygenation, were implicated in the emergence and diversification of animal life. Our results lend geochronological support support to field evidence for condensation and/or hiatus in Ediacaran sedimentary records, with far-reaching implications for those working on paleontology, stratigraphy, sedimentology, geochemistry and the overall evolution of the Earth system during this key time interval.

Online Methods

Re-Os geochronology

Six sample sets of organic-rich rock were taken from Wells L and M, two drill cores in the South Oman Salt Basin drilled by Petroleum Development Oman in the last decade. Each group of samples included between 6 and 9 subsamples. These drill cores sample deep-water environments below storm weather wave base. Subsamples were taken vertically, over intervals spanning 0.81 m (L6), 0.80 m (L5), 0.87 m (L4), 0.92 m (L3), 0.97 m (L2), 0.85 m (L1), 1.38 m (M2), and 1.41 m (M1). Although sampled vertically, sample suites show little internal variation in initial 187Os/188Os values (Table S1). Radioisotopic analyses were performed at the Yale Geochemistry and Geochronology Center; the full methodology is described in the Supplementary Text.

These samples’ stratigraphic positions within a key reference well for the South Oman Salt Basin, well TM-6, shown in Fig. 1, following a correlation model based on outcrop, subsurface, geochronological and geochemical data recently adopted by Petroleum Development Oman (Figure S2)88. Well TM-6 is a key reference well for studies of the South Oman Salt Basin and has been widely studied5,89,90,91.

Age-depth modeling

The age-depth models presented here (Fig. 1) are calculated using a stratigraphic Markov chain Monte Carlo model in a Bayesian framework92,93. Due to the large time gap and lack of constraints between the post-Marinoan cap carbonate and our first dated interval in the Nafun Basin, we exclude this section of stratigraphy from the age-depth model. We correlate the post-Marinoan cap carbonate, the Hadash Fm, with the post-Marinoan cap carbonate from the Yangtze Basin, which has a date constrained by CA-ID-TIMS zircon dating30. We use other published geochronological constraints and assume synchroneity of the Shuram excursion globally to develop age-depth models for other Ediacaran basins; more details for each basin are outlined in the Supplement. Backstripping analysis for selected sections was performed using Backstrip (Supplemental Fig. S4)94.