Introduction

The end of the Late Palaeozoic Ice Age (LPIA) represents one of the most extreme climate transformations in geological history, transitioning from icehouse to greenhouse conditions1,2,3. This global event is critical to understanding changes in the carbon cycle associated with the highest rates of global organic carbon burial (up to 6.5 × 1018 mol/Myr) in the past half billion years4, and coal formation across the terrestrial lithosphere.

The terminal deglaciation of the LPIA was an irregularly distributed, asynchronous event occurring from the Late Palaeozoic, where polar ice melted due to continental scale warming at high-latitudes over Gondwana1,5,6. The multiple ice centres of the LPIA across Gondwana are evidenced by widespread glacial deposits in Palaeozoic-aged basins of Australia, Antarctica, South America, Arabia, India and Africa1,2,7. Recently, growing evidence suggests that the LPIA may have been triggered, and subsequently terminated, by uplift and erosion of the Hercynian Mountains8.

δ13C data has been increasingly utilised to understand both palaeoclimatic and palaeoenvironmental changes associated with the LPIA5,6,9,10,11. Recently, a negative isotopic shift in δ13C was observed in both inorganic and organic carbon sources, in sediments from high and low latitude Palaeozoic basins11. This δ13C excursion has been proposed as the Kungurian Carbon Isotopic Excursion (KCIE), and hypothesised to record the release of extremely depleted (ca. −35‰) methane clathrates during glacial melting11.

Despite coal occurrence recording the high rates of carbon burial during this period, high-resolution δ13Corg records of coals deposited during the LPIA have not yet been used to demonstrate changes in the global carbon cycle. The slow accumulation of peat, ~0.9 mm/yr in modern high-latitude settings12, allows subsequently formed coals to continuously record palaeoclimate and palaeoenvironmental change at high-temporal resolutions.

This study investigates if the proposed KCIE is evidenced in δ13Corg of Early Permian coals of the Moatize Basin, Mozambique. These coals were selected for this investigation as they are associated with the final occurrence of widespread glacial deposits, and interbedded with deglacial lacustrine sediments, noted throughout other stratigraphically equivalent basins in Southern Africa13,14,15,16.

Samples from core recovered by Vale Moçambique were utilised from available core across eight (8) locations in the Moatize Basin, Tete Province, central Mozambique (Fig. 1A). These cores intersect target stratigraphy for the Vale Moçambique mine, that include thick (>10 m) coal accumulations. Coals occur within both the Permian Matinde and Moatize formations (Fig. 1B), although the thickest accumulation of coals is within the lower Moatize Formation (net coal ~52.9 m). Coals were sampled from the lower Moatize Formation that conformably overlies the glacial sediments of the Vúzi Formation marking local glacial to deglacial transition15. The glacial sediments and the lacustrine sediments above the first correlatable coal seam, Sousa Pinto seam, to the base of the Chipanga seam, aided in the correlations between drill cores across the basin15, in the absence of better chronostratigraphic markers (e.g., volcanic ash).

Figure 1
figure 1

(A) Map of Southern Africa36, Palaeozoic (Karoo Basin) and stratigraphically equivalent sediments coloured in dark grey, subcrop in light grey, location of Moatize Basin marked by arrow (longitude 16.172772S, latitude 33.806066 E), (B) stratigraphy of the Moatize Basin, with inset of simplified stratigraphy of the sampled interval from the Early Permian, Moatize Formation; black indicates coal seams, diamond indicates glacial sediments, grey horizontal pattern indicates lacustrine black shales, grey indicates clastic interburden.

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Results

The range of δ13Corg for the coals of the Moatize Formation falls within the typical range of C3 plant organic matter17, ranging from an absolute maximum of −20.0‰, to an absolute minimum of −26.9‰. The data were collected across all locations domained by each respective ply within the Bananeiras, Chipanga, and Sousa Pinto seams (Fig. 2A). From observing the range of data within these domains, three distinctive ply-domained stages are interpreted.

Figure 2
figure 2

(A) Compiled data of Moatize Formation coals by ply domain (SPB - Sousa Pinto base, SPM - Sousa Pinto middle, SPU - Sousa Pinto upper, BCB - basal Chipanga, LUCB - lower Chipanga base, LUCT - lower Chipanga top, MCM - middle Chipanga, UCB - Upper Chipanga base, UCT - Upper Chipanga top, BNL - Bananeiras lower, BNU - Bananeiras upper). (B) compiled data of Moatize Formation coals normalised by sample distribution within each ply domain, grey shading highlighting Stage 1/3 δ13C cycling and Stage 2 negative δ13C excursion.

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Stage 1 – Initiation of peat accumulation

Stage 1 coals, encapsulate the Sousa Pinto seam (plys SPB, SPM, SPU). The average δ13Corg value for this stage is −23.5‰ (σ = 0.8‰, n = 75), exhibiting a shift to more positive δ13Corg with time.

Stage 1, Artinskian coals of the Sousa Pinto seam exhibit variable ranges of δ13Corg, suggesting some variation in palaeoenvironmental factors controlling low-magnitude (~±1‰) δ13C cycling. A weak (~1.5‰) positive shift in δ13Corg of Stage 1 coals suggests a more long-lived change in atmospheric CO2 concentrations and δ13C. These changes are concurrent with the development of widespread peat deposits, resulting in increased rates of carbon burial coincident with the Artinskian6,8.

Stage 2 – Terminal deglaciation

Stage 2 coals, encapsulate the basal Chipanga seam ply only (BCB). The mean δ13Corg value for this stage is −24.7‰, with a high standard deviation (σ = 1.5‰, n = 15). Stage 2 coals have the most negative δ13Corg of all the data domains. A striking, high-magnitude (~3.5‰) negative excursion is observed δ13Corg in Stage 2, coincident with the base of the Chipanga seam in the early Kungurian. This negative excursion is relatively short-lived compared to smaller-scale δ13Corg cycles (~±1‰) in Stage 1 coals of the Artinskian, and Stage 3 coals of the Kungurian. The compiled δ13Corg record from the Moatize Formation coals is time equivalent to other, continuous δ13Corg records from sediments in both low and high-latitude sediments (Fig. 3), suggesting the observed negative carbon shift may be the globally recorded KCIE.

Figure 3
figure 3

Comparison of geochemical data from other studies5,11,37, with this study; grey shading highlighting the proposed Kungurian Carbon Isotopic Excursion (KCIE) interval.

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Stage 3 – Cyclic pluvials

Stage 3 coals, encapsulate the lower Chipanga seam, through to the upper Bananeiras seam (plys LUCB, LUCT, MCM, UCB, UCT, BNL and BNU). The average δ13Corg value for this stage is −22.6‰ (σ = 0.6‰, n = 222), and remains stable throughout each ply domain, regardless of seam and age distribution. When the total sample set (excluding statistical outliers, n = 305) is normalised for each ply domain, the cyclic variation of δ13Corg within these coals can be observed (Fig. 2B). Mechanisms for these δ13Corg cycles are further discussed below.

Discussion

The Early Permian coals of the Moatize Formation exhibit a three-stage evolution in atmospheric δ13C from the Artinskian to the Kungurian. In this study, δ13Corg cycles (particularly striking in Stage 3, see Fig. 2B), indicate a ~±1‰ shift in δ13Corg, over discrete, regular spacing at normalised depths, from which time intervals may be estimated.

Cyclic variation of δ13Corg in coal at similar scales has been previously observed in high-resolution isotopic studies from Eastern Australia18,19,20. In these works, the primary control on the distribution of δ13C cycles within coal is attributed to palaeoenvironmental factors controlling peat accumulation, including water availability, salinity, pH and atmospheric temperature21. However, the timescales over which these cycles occur have not yet been addressed.

The accumulation rates of peat are dependent on both depositional environment, and biological productivity, often genetically linked with peat-forming plant communities22. In the Moatize Formation coals of Stage 3, it is demonstrated that both the plant community, and depositional environment controlling peat distribution remained temporally stable to be able to preserve these δ13Corg cycles. This also implies a state of atmospheric δ13C equilibrium, with no significant injections of isotopically heavy or light carbon, nor major changes in CO2 concentrations, to disrupt δ13Corg cycling.

By assuming a relatively constant rate of peat deposition, similar to modern rates of high latitude peat accumulation12,23, with peat-to-coal compaction ratios sourced from literature22,24,25,26, a range of potential time scales for each δ13C cycle may be calculated (Table 1).

Table 1 Approximation of the duration (yrs) of a single δ13C cycle observed over ~10 m of coal in Stage 3.
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From these calculations, it is likely that δ13Corg cycling occurs on a similar scale to short-term (103–105 years) trends inferred from Palaeozoic palaeosol development27, and in palaeofloral communities28. These short-term changes observed in low-latitude sediments, attributed to pluvials, result in changes in base level. These base-level changes, coincident with δ13Corg cycling, are also observed in coals from Eastern Australia18,19,20, and the 103–105 year time-frame is coincident with Milankovitch-scale orbital frequencies28, also observed in Mesozoic and Cenozoic coals29,30.

The observed KCIE is equivalent to the duration of a 103–105 year cycle. The short-lived nature of this isotopic excursion suggests the rapid injection of 13C-depleted carbon into the atmosphere, rather than any relatively long-lived changes in CO2 concentration. It is possible that this negative carbon isotopic shift is due to the release of methane clathrates (CH4) into the atmosphere during terminal deglaciation. Furthermore, the contribution of deep soil organic carbon (SOC) loss and CH4 from terrestrial permafrost may also have contributed to widespread δ13C perturbation31.

The timing of this rapid CH4 release is equivalent to the development of euxinic lake deposits across Southern Africa as a result of deglaciation marking the end of the LPIA13,14,15. The stratigraphic equivalent of these euxinic lacustrine deposits is represented by organic rich black shale separating the Sousa Pinto and Chipanga seams, at variable thickness at each sample location (Fig. 1B).

The accumulation of peat, evidenced by the occurrence of the Sousa Pinto seam during Stage 1, implies that more gradual global scale warming and glacial retreat resulting in base-level rise had initiated in the Artinskian, prior to evidence of any catastrophic CH4 release. Furthermore, atmospheric CH4 injection indicated by the KCIE seems to have little to no observable effect on peat accumulation subsequent to the ultimate terminus of the LPIA, suggesting peat-forming terrestrial ecosystems remained relatively stable during this period.

This estimated time-frame of carbon cycle perturbation during the KCIE is relatively short lived, corresponding to the short residence time of CH4 in the atmosphere32. This brief time-period of potential methane clathrate release, and subsequently rapid oxidation to CO2, is not accompanied by any known mass extinctions, or terrestrial ecosystem catastrophe during the Early Permian33.

These observations suggest that whilst CH4 release may have contributed to enhanced global warming during the terminus of the Late Palaeozoic Ice Age, the proposed effects of continental weathering and organic carbon burial linked with uplift and subsequent erosion of the Hercynian range demonstrate what maybe a more profound, and long-lived impact on global climate8. Additionally, the lack of observable effects on land plant communities despite significant carbon cycle perturbation during the KCIE event further supports the resilience of terrestrial flora to the effects of global scale atmospheric perturbation34.

The authors suggest an understanding of the global carbon cycle across geological time may greatly benefit from further research into δ13Corg from coals.

Methods

Samples were taken from plys of the Bananeiras (n = 62, average seam thickness = 8.5 m), Chipanga (n = 175, average seam thickness = 31.3 m) and Sousa Pinto (n = 75, average seam thickness = 13.1 m) coal seams (ntotal = 312). Great care was taken to only sample bright (vitrain) bands from coals, as to minimise δ13Corg variation with coal lithotype or biochemical composition18,21,35. Vitrains were hand-picked at a millimetre scale to avoid any potential carbonate contamination from mineralised cleats. The typical low taphonomic diversity of peat-forming ecosystems28, minimises the likelihood of δ13Corg variation dependent on taxa9.

The δ13Corg values were determined in the Stable Isotope Geochemistry Laboratory (SIGL) at the University of Queensland using a stable isotope ratio mass spectrometer (Isoprime), coupled in continuous flow mode with an elemental analyser (Elementar Cube) (EA-CF-IRMS). Calibration was performed by use of two standards, USGS24 (−16.1‰ δ13CPDB) and NAT76H (−29.26‰ δ13CPDB), interspersed throughout analytical runs. Each sample was analysed in duplicate, using 50–200 μg of concentrate combusted at 1020 °C in 3.5 mm × 5 mm tin capsules. Any sample with a beam size outside the working range of 1 × 10−9 to 9 × 10−9 Å, or with a δ13Corg result variation between duplicates of >0.4‰, was re-analysed, in accordance with laboratory quality control practices. Final data values were normalised and are reported in ‰ VPDB.