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

Abstract

Earth's penultimate icehouse (ca. 340-285 Ma) was a time of low atmospheric pCO 2 and high pO 2 , formation of the supercontinent Pangaea, dynamic glaciation in the Southern Hemisphere, and radiation of the oldest tropical rainforests. Although it has been long appreciated that these major tectonic, climatic, and biotic events left their signature on seawater 87 Sr/ 86 Sr through their influence on Sr fluxes to the ocean, the temporal resolution and precision of the late Paleozoic seawater 87 Sr/ 86 Sr record remain relatively low. Here we present a high-temporal-resolution and high-fidelity record of Carboniferous-early Permian seawater 87 Sr/ 86 Sr based on conodont bioapatite from an open-water carbonate slope succession in south China. The new data define a rate of long-term rise in 87 Sr/ 86 Sr (0.000035/m.y.) from ca. 334-318 Ma comparable to that of the middle to late Cenozoic. The onset of the rapid decline in 87 Sr/ 86 Sr (0.000043/m.y.), following a prolonged plateau (318-303 Ma), is constrained to ca. 303 Ma. A major decoupling of 87 Sr/ 86 Sr and pCO 2 during 303-297 Ma, coincident with the Paleozoic peak in pO 2 , widespread low-latitude aridification, and demise of the pan-tropical wetland forests, suggests a major shift in the dominant influence on pCO 2 from continental weathering and organic carbon sequestration (as coals) on land to organic carbon burial in the ocean. 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr analysis of 1 GSA Data Repository item 2018128, analytical methods, age calibration, revision of pCO 2 estimates, Figures DR1 and DR2, and Tables DR1 and DR2

Glacio‐eustasy and δ13C across the Mississippian–Pennsylvanian boundary in the eastern Paleo‐Tethys Ocean (South China): Implications for mid‐Carboniferous major glaciation

“…The Carboniferous slope carbonate δ 13 C values from South China were most likely not influenced by such processes given that the ocean water was most likely well mixed during the MPB maximum glacial interval because vertical ventilation is commonly stronger during glacial period than during the warmer period (e.g., Saltzman, ). Furthermore, the fact that the Carboniferous δ 13 C values from the Naqing section in South China largely represent the mean of the Euramerican brachiopod data (Chen, Montañez, et al, ) suggests that the South China data were overall not specifically influenced by organic degradation.…”

“…Alternatively, continental weathering also plays an important role in regulating atmospheric p CO 2 , especially on time scales of more than 10 5 years (Kump & Arthur, ). Strontium isotopes ( 87 Sr/ 86 Sr) are one of the important geochemical proxies for continental weathering, and when combined with carbon isotopes, they can provide better constraints on palaeoclimate changes (e.g., Chen, Montañez, et al, ; Goddéris et al, ; Kump & Arthur, ). Chen, Montañez, et al () presented a high‐resolution seawater (conodont apatite) 87 Sr/ 86 Sr record of the late Mississippian to early Permian, which delineates a rapid increase during the MPB interval from 0.70792 at ~326 Ma to 0.70823 at ~317 Ma (Figure ).…”

“…Strontium isotopes ( 87 Sr/ 86 Sr) are one of the important geochemical proxies for continental weathering, and when combined with carbon isotopes, they can provide better constraints on palaeoclimate changes (e.g., Chen, Montañez, et al, ; Goddéris et al, ; Kump & Arthur, ). Chen, Montañez, et al () presented a high‐resolution seawater (conodont apatite) 87 Sr/ 86 Sr record of the late Mississippian to early Permian, which delineates a rapid increase during the MPB interval from 0.70792 at ~326 Ma to 0.70823 at ~317 Ma (Figure ). The rapid increase in seawater 87 Sr/ 86 Sr indicates enhanced continental weathering that resulted mainly from increased Hercynian orogenic uplift during formation of the supercontinent Pangaea by collision between Gondwana and Euramerica (e.g., Chen, Montañez, et al, ; Goddéris et al, ).…”

“…The late Palaeozoic ice age (LPIA) started in the latest Devonian and ended in the middle Permian, when global palaeogeography (closure of Rheic Ocean and formation of supercontinent Pangaea) and land ecosystems (radiation and evolution of the oldest rainforests) experienced major changes (Chen, Montañez, Qi, Shen, & Wang, ; DiMichele, Cecil, & Montañez, ; Isbell et al, ; Montañez, ; Montañez & Poulsen, ; Veevers, ). Sedimentological and geochemical studies suggest that major continental glaciation on Gondwana was most likely initiated in the late Visean (e.g., Barham, Murray, Joachimski, & Williams, ; Fielding & Frank, ; Chen, Sheng, et al, ; Smith & Read, ; Wright & Vanstone, ) and reached the first peak in the Mississippian–Pennsylvanian boundary (MPB) interval (B. Chen et al, ; Montañez & Poulsen, ).…”

“…Global carbon perturbation and atmospheric p CO 2 decrease have been hypothesized as the driver of the LPIA (e.g., Algeo, Berner, Maynard, & Scheckler, ; Chen, Montañez, et al, ; Goddéris et al, ; Montañez, ). The δ 13 C values of global ocean dissolved inorganic carbon (DIC) recorded in marine carbonates can be used as a proxy for organic carbon burial and global carbon perturbations (e.g., Kump & Arthur, ; Saltzman, ).…”