Strontium and carbon isotopic evidence for decoupling of pCO₂ from continental weathering at the apex of the late Paleozoic glaciation Open Access

Seawater ⁸⁷Sr/⁸⁶Sr has long been used as a tool for chronostratigraphic correlation (e.g., McArthur et al., 2012), and, in combination with global seawater δ¹³C, to constrain the timing and magnitude of tectonic events, continental weathering, and paleoclimate change (e.g., Kump and Arthur, 1997; Goddéris et al., 2017). For the middle to late Cenozoic, the high-resolution seawater ⁸⁷Sr/⁸⁶Sr curve has provided robust chronostratigraphic constraints and insight into the interlinked processes of the Earth system during our modern icehouse (e.g., Zachos et al., 1999).

The late Paleozoic ice age (LPIA, ca. 340–285 Ma) is one of two major icehouses of the Phanerozoic, and records the only greenhouse gas–forced transition from an icehouse with complex terrestrial ecosystems to a fully greenhouse world (Montañez and Poulsen, 2013). The LPIA was a time of very low atmospheric pCO₂ (Montañez et al., 2016) and high pO₂ (Glasspool et al., 2015), dynamic glaciation on Gondwana (Isbell et al., 2012), global tectonic reconfiguration (Veevers, 2013), and the evolution and radiation of the oldest tropical rainforests (DiMichele, 2014). The fingerprints of these events should have been recorded in seawater ⁸⁷Sr/⁸⁶Sr given they collectively influenced continental weathering, and thus Sr flux to the late Paleozoic oceans. The existing Carboniferous–early Permian seawater ⁸⁷Sr/⁸⁶Sr record derived using calcitic brachiopods (Bruckschen et al., 1999; Korte et al., 2006) remains only moderately resolved, reflecting stratigraphic uncertainties, relatively low temporal resolution, and possible diagenetic alteration.

Here, we present a ⁸⁷Sr/⁸⁶Sr record of unprecedented temporal resolution (10⁵ yr) for ∼38 m.y. of the LPIA, built using conodont bioapatite from an open-water carbonate slope succession (Naqing, south China) of the eastern Paleo-Tethys Ocean (Fig. DR1 in the GSA Data Repository^¹). Our record refines the structure of the middle Mississippian to early Permian seawater ⁸⁷Sr/⁸⁶Sr curve and places more precise temporal constraints on the timing of major shifts and rates of change. Integrated conodont apatite ⁸⁷Sr/⁸⁶Sr and carbonate δ¹³C records provide insight into the relative roles of orogenic uplift, pan-tropical aridification, and the evolution of the paleo-tropical wetland rainforests on continental weathering and atmospheric pCO₂ during Earth’s penultimate icehouse.

During the Carboniferous–Permian, the South China Block was a nearly isolated terrain located at the interface of the Paleo-Tethys Ocean (west) and Panthalassic Ocean (east) (Fig. DR1). The Carboniferous–Permian Naqing succession in the Guizhou Province consists of thin-bedded lime mudstones intercalated with intraclast-bearing bioclastic wackestones to packstones (Fig. 1), and contains abundant conodonts with complete evolutionary lineages (Qi et al., 2014). The succession records near-continuous, hemipelagic deposition on a carbonate slope episodically punctuated by turbidity currents and debris flows in the Qian-Gui Basin that defined an open-water seaway to the Paleo-Tethys Ocean (Buggisch et al., 2011; Chen et al., 2016).

Sr was isolated from conodonts (n = 99) and carbonates (n = 22), collected from the Naqing section, using Eichrom exchange resin (50–100 µm) in pipette-tip columns attached to a Watson Marlow 205U Peristaltic pump (detailed methods are provided in the Data Repository). The ⁸⁷Sr/⁸⁶Sr ratios were measured on a Nu Plasma HR (Nu032) multicollector–inductively coupled plasma–mass spectrometer (MC-ICPMS) at the University of California–Davis (USA). Analytical precision (2 standard deviations [SD] = ±0.000026) is based on repeated ⁸⁷Sr/⁸⁶Sr analysis of strontium carbonate isotopic standard SRM 987 (avg. of 0.710251; n = 44) over the study period. All data are normalized to a SRM 987 value of 0.710249.

Conodont apatite ⁸⁷Sr/⁸⁶Sr values from the Naqing section delineate three phases during the middle Mississippian to early Permian (Fig. 1; Fig. DR2). First, after a brief decline from 0.70780 to 0.70769 during the Middle Mississippian (ca. 336–334 Ma), ⁸⁷Sr/⁸⁶Sr values increase rapidly (avg. of 0.000035/m.y.) over a 16 m.y. period (334–318 Ma) to 0.70827. Second, the ⁸⁷Sr/⁸⁶Sr values define an ∼15 m.y. plateau throughout much of the Pennsylvanian (318–303 Ma). Third, ⁸⁷Sr/⁸⁶Sr values decline, at an average rate of 0.000043/m.y., from ca. 303 Ma through to the end of the record in the early Permian (ca. 298 Ma).

The ⁸⁷Sr/⁸⁶Sr values of diagenetically screened micrite from the Naqing section are overall higher, by up to 0.00029, than co-existing conodonts and exhibit greater scatter (Fig. 1). The Naqing conodont ⁸⁷Sr/⁸⁶Sr record largely agrees with a published first-order ⁸⁷Sr/⁸⁶Sr trend (Bruckschen et al., 1999; Korte et al., 2006), but with significantly less scatter and greater continuity (Fig. 2B). The Naqing data, with minimal stratigraphic uncertainty and higher temporal resolution (10⁵ yr), refine the trend and fill in existing gaps. Notably, the Naqing ⁸⁷Sr/⁸⁶Sr values are comparable, within analytical uncertainty, with those of brachiopods from Panthalassic open-ocean settings (Brand et al., 2009) and of conodonts from the high-precision U-Pb calibrated Russian succession (Henderson et al., 2012).

Seawater ⁸⁷Sr/⁸⁶Sr represents a mixture of two main sources: a continent-derived, more-radiogenic weathering flux, and mantle-derived, less-radiogenic volcanic and hydrothermal fluxes. The rise in ⁸⁷Sr/⁸⁶Sr during ca. 334–318 Ma likely records the increased ⁸⁷Sr/⁸⁶Sr ratio of the riverine flux due to exposure and weathering of uplifted radiogenic basement rocks (Goddéris et al., 2017) driven by the Hercynian orogeny (ca. 340–260 Ma; Hatcher, 2002; Veevers, 2013). The subsequent protracted (15 m.y.) ⁸⁷Sr/⁸⁶Sr plateau (318–303 Ma) is interpreted to record sustained high-⁸⁷Sr/⁸⁶Sr riverine flux due to westward progression of maximum elevations and subsequent rapid denudation of the highlands in the paleo-tropics. The new ⁸⁷Sr/⁸⁶Sr record indicates a rate of rise comparable to that of last 34 m.y. of the Cenozoic icehouse (0.000040/m.y.; McArthur et al., 2012, and references therein), suggesting that increased global weatherability due to orogenic uplift may be a common driver of these icehouses (cf. Kump and Arthur, 1997).

Conversely, the rapid, near-linear decline in ⁸⁷Sr/⁸⁶Sr ca. 303–285 Ma likely records decreased continental (silicate) weathering. This raises a paradox, as the potential for tectonically driven weatherability most likely remained unchanged through the early Permian (cf. Goddéris et al., 2017) with continued orogenesis to ca. 260 Ma (Hatcher, 2002). We hypothesize that continental weathering likely decreased during this time based on two other factors. First, the onset of widespread aridification in pan-tropical regions that began in the late Moscovian and intensified with time eastward across Pangaea through to the early Permian (Tabor and Poulsen, 2008; Michel et al., 2015) would have dramatically decreased silicate weathering. Second, the Euramerican tropical wetland forests underwent permanent turnover toward the close of the Carboniferous to dryland forests with less weathering potential (Wilson et al., 2017). Moreover, weathering of less-radiogenic basaltic provinces, which were emplaced initially in the latest Carboniferous and throughout the early Permian opening of the Neo-Tethys (e.g., Liao et al., 2015), may have contributed to declining ⁸⁷Sr/⁸⁶Sr. The relative contribution of basalt weathering on global seawater ⁸⁷Sr/⁸⁶Sr, however, was likely small in the latest Carboniferous–earliest Permian, as early basaltic province emplacement was limited in volume and occurred primarily in mid-latitude regions (Liao et al., 2015) where weathering rates would have been lower, in particular during the earliest Permian apex of glaciation.

In order to evaluate the relative roles of silicate weathering and organic carbon (C_org) burial in regulating pCO₂, we couple the ⁸⁷Sr/⁸⁶Sr and carbonate δ¹³C records and compare them to proxy-based pCO₂ estimates (Fig. 2). We present a consensus seawater δ¹³C curve for the late Paleozoic (Fig. 2C) developed using published values of diagenetically screened brachiopods from Euramerican epeiric platforms (Grossman et al., 2008) and slope carbonates, argued to be unaltered by meteoric diagenesis (Buggisch et al., 2011).

Integrated isotopic records and published pCO₂ estimates delineate four intervals (Fig. 2). First, the long-term rise (ca. 334–318 Ma) and earlier portion of the ⁸⁷Sr/⁸⁶Sr plateau (318–309 Ma) correspond to a long-term decline in pCO₂ (∼1200 to ∼500 ppm) and to relatively stable δ¹³C values (∼3‰) between 340 Ma and 324 Ma, followed by a subsequent rise to a δ¹³C maximum (>5‰) at ca. 309 Ma. Second, the later portion of the ⁸⁷Sr/⁸⁶Sr plateau (309–303 Ma) corresponds to a decline in δ¹³C to its nadir (<3‰) at ca. 304 Ma and overall low pCO₂ values (∼500–200 ppm), with a short-lived rise in pCO₂ at the end of the ⁸⁷Sr/⁸⁶Sr plateau. Third, a decline in ⁸⁷Sr/⁸⁶Sr from 303 to 297 Ma is coincident with a renewed rise in δ¹³C to ∼5‰ and a drop in pCO₂ to its nadir (≤200 ppm). Fourth, continued decline in ⁸⁷Sr/⁸⁶Sr from 297 to 285 Ma corresponds to a decrease in δ¹³C and a long-term rise in pCO₂.

Perturbations in global carbon cycling and atmospheric pCO₂ are thought to have been the primary driver of the initiation and demise of the LPIA (e.g., Montañez et al., 2007, 2016; Goddéris et al., 2017). Increased tectonically driven weatherability of silicate rocks inferred from the long-term rise in ⁸⁷Sr/⁸⁶Sr would have drawn down pCO₂ and initiated major glaciation (Goddéris et al., 2017). Radiation of the tropical forests in the latest Mississippian to their apex in the Middle Pennsylvanian (Fig. 2A; Cleal and Thomas, 2005; DiMichele, 2014) would have further contributed to lowering pCO₂ through enhanced silicate weathering and C_org sequestration in tropical wetlands (Nelsen et al., 2016). Evidence for this exists in the long-term δ¹³C rise (Fig. 2C) and overall falling pCO₂ (Fig. 2D) through the Early and Middle Pennsylvanian. Major contraction of the Carboniferous tropical wetland rainforests during the Late Pennsylvanian (Kasimovian; Fig. 2A), however, would have decreased the terrestrial C_org sink (Cleal and Thomas, 2005; Montañez et al., 2016) as recorded in the widespread loss of coals, decreased δ¹³C values (Fig. 2C), and increased pCO₂ (Fig. 2D).

While the coupled ⁸⁷Sr/⁸⁶Sr and δ¹³C records provide support for the collective influence of orogenic uplift and expansion of the terrestrial tropical ecosystems on both accelerated weathering and overall decreasing pCO₂ through the Pennsylvanian, the records also reveal a paradoxical relationship with regard to the driver of the pCO₂ minimum across the Carboniferous–Permian transition. That is, reduced continental weathering driven by the aforementioned climatic and biotic factors and coincident with the rapid decline in ⁸⁷Sr/⁸⁶Sr between 303 Ma and 297 Ma hypothetically should have led to decreased consumption of atmospheric CO₂ and a consequent rise in pCO₂ (Goddéris et al., 2017). However, the proxy-based pCO₂ estimates decrease to a nadir across the Carboniferous–Permian transition (Fig. 2). While the low pCO₂ is consistent with the apex of late Paleozoic glaciation (Fig. 2A; Fielding et al., 2008; Isbell et al., 2012; Montañez and Poulsen, 2013) and eustatic fall of ∼120 m (Rygel et al., 2008), it requires an additional driver given the aforementioned hypothesized decrease in continental weathering rates at this time.

One mechanism that could account for the pCO₂ minima coincident with peak pO₂ in the earliest Permian (Glasspool et al., 2015) is an increase in the magnitude of the C_org burial flux, which is supported by a return to more positive carbonate δ¹³C values (Fig. 2C). An increase in terrestrial C_org burial rates is precluded by the loss of wetland forests and peat burial throughout Euramerica in the Late Pennsylvanian (Nelsen et al., 2016). We hypothesize that a major shift in the predominant C_org sink from land to the oceans occurred across the Carboniferous–Permian transition. Reconstructed paleo-plant physiology and process-based ecosystem modeling support this hypothesis and suggest a 2- to 6-fold increase in water-use efficiencies (WUE) of early Permian tropical plants relative to the Carboniferous wetland floral dominants (Wilson et al., 2017). This shift in WUE suggests an up to 50% decrease in canopy transpiration and a similar magnitude increase in surface runoff. Increased surface runoff would have resulted in greater delivery of nutrients and organic matter to coastal waterways, increasing the potential for increased primary productivity and C_org burial in the oceans. This hypothesized shift in the loci of late Paleozoic C_org burial sinks requires further evaluation through integrated biogeochemical and ecosystem modeling. It is further possible that hypothesized enhanced eolian delivery of reactive iron during the apex of the LPIA (Sur et al., 2015) would have further stimulated marine primary productivity and increased marine C_org burial.

In summary, coupled conodont apatite ⁸⁷Sr/⁸⁶Sr and carbonate δ¹³C records indicate predominant roles for both continental weathering and C_org burial in regulating LPIA climate. A decoupling of pCO₂ from continental weathering across the Carboniferous–Permian transition (303–297 Ma), however, suggests that the coincident pCO₂ minimum and pO₂ maximum during the earliest Permian apex glaciation may record a previously unrecognized unidirectional shift in the primary loci of C_org burial from land to sea. This shift was not a response to tectonically driven changes in weathering intensity or source, but rather to pantropical climate change and major ecosystem restructuring.

We thank Q. Wang for picking conodont elements for Sr isotope analysis, and J. Glessner for assistance in Sr isotope analysis. We are grateful to J. Parrish for editorial handling of the manuscript and to E. Nardin, G. Gianniny, and A. Sedlacek for their constructive reviews. This work was supported by the Chinese Academy of Sciences (grants XDB18030400 and XDPB05), the National Natural Science Foundation of China (grants 41630101, 41290260, and 41672101), and U.S. National Science Foundation funding to Montañez (grant EAR1338281).