No 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

The Visean–Serpukhovian boundary is not yet defined by a Global Stratotype Section and Point (GSSP) but it is recognizable operationally by the appearance of the conodont Lochriea ziegleri in the L. nodosa–L. ziegleri chronocline. Foraminiferal successions across this boundary in the type area of the Serpukhovian Stage (Moscow Basin, Russia), elsewhere in Russia and in the central United States suggest that the appearances of Asteroarchaediscus postrugosus, Janischewskina delicata, Eolasiodiscus donbassicus, and specimens controversially referred to “Millerella tortula” are reliable, auxiliary indices to the base of the Serpukhovian. In southern Guizhou Province, China, Visean–Serpukhovian rock sequences from slope and platform settings have yielded rich associations of conodonts and foraminifers, respectively. The Nashui section is a leading candidate for the Serpukhovian GSSP because its slope deposits contain an uninterrupted record of conodont occurrences including the L. nodosa–L. ziegleri transition. Foraminifers recovered from the Nashui section are comparatively rare and include none of the basal Serpukhovian indices. In contrast, the nearby Yashui section represents a platform interior setting in which foraminifers flourished and conodonts were nearly absent. The base of the Serpukhovian at Yashui is marked approximately by the appearance of “tortula-like” specimens. Although it is not possible to correlate biostratigraphically between the Nashui and Yashui sections, the occurrence of “tortula-like” specimens at the Yashui section allows correlation with the mid-Venevian Substage of the Moscow Basin at a level coinciding with the appearance of L. ziegleri. Together, the slope and platform sections comprise an informative biostratigraphic reference area for micropaleontologic characterization of the Visean–Serpukhovian boundary in southern Guizhou.