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

A positive carbonate δ13C excursion (by ~1.5–3.0‰) has been reported across the Mississippian–Pennsylvanian boundary (MPB) from Euramerican epicontinental seas, implying a coeval atmospheric pCO2 decrease and Gondwanan glaciation at that time. This excursion was mainly observed in carbonate platforms which experienced repeated subaerial exposure. The South China Block was located in the middle of the Paleo‐Tethys Ocean and the Panthalassic Ocean during the Carboniferous and contains well‐preserved carbonate slope strata across the MPB. Four lithofacies were defined in the studied Naqing, Narao, and Dianzishang sections, including thin‐bedded lime mudstones, laminated wackestones to packstones, normally‐graded packstones, and slumped limestones. Immediately above the MPB, a bed of normally‐graded packstones or slump masses occurs in the studied sections, which together with coeval subaerial‐exposure features in shallow‐water carbonate platform of South China, suggests a significant sea‐level fall across the MPB. Carbonate δ13C records show a consistent value of ~3.0‰ below the MPB in the Naqing and Narao sections, and both sections show an obvious rise in δ13C across the MPB (by ~0.5–1.0‰). The relatively small‐magnitude rise in δ13C would have resulted from well‐mixed seawater induced by intensified upwelling in the eastern Paleo‐Tethys Ocean during the glacial peak. Hence, the carbonate δ13C values recorded in the South China Block may represent a mean δ13C of the dissolved inorganic carbon in global ocean water at that time. Correlation between carbonate δ13C and previously published conodont δ18O and 87Sr/86Sr records suggests that the MPB glaciation was mainly driven by enhanced continental weathering, rather than increased organic carbon burial, although further quantitative simulation is required to better understand the interlinked processes during the Earth's penultimate icehouse.


| METHODS
We carried out bed-by-bed sedimentologic logging and description for the MPB successions in the three sections (Naqing, Narao, and Dianzishang; Figure 3). In total, 391 hand specimens were collected, slabbed, and thin-sectioned to observe detailed sedimentary and diagenetic features using microscopes. Fine-grained, micritic sediment (50-150 μg) without obvious calcite veins and recrystallization was drilled from the freshly cut surfaces of samples using a dental drill with stainless-steel bits for bulk carbonate analysis. Eighty-six bulk carbonate samples from the Narao and Dianzishang sections were analysed on a MAT 253 mass spectrometer coupled with a Kiel IV carbonate device in the State Key Laboratory of Palaeobiology and Stratigraphy at Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. The carbon isotope values are reported relative to −8.49‰); analytical precision for δ 13 C carb and δ 18 O carb is better than 0.04‰ and 0.08‰, respectively.
In order to evaluate the reliability of δ 13 C values from bulk carbonates, 50 thin sections (70-100 μm thick) from the Naqing section were stained with Alizarin Red-S and Potassium Ferricyanide (Dickson, 1965). Target areas for microsampling and analysis (i.e., well-preserved micrite) were identified using transmitted and cathodoluminescent microscopy. Pink-staining (i.e., low [Fe 2+ ]), non/dully luminescent micrite was microdrilled from thick sections using a fully automated Merchantek microdrilling system with~20μm spatial resolution at the University of California, Davis. Powders (50-100 μg) were collected and roasted for 30 min at 375°C in vacuo in order to remove organic volatiles and analysed using a Fisons Optima isotope ratio mass spectrometer with a 90°C Isocarb common acid bath autocarbonate system in the Stan Margolis Stable Isotope Laboratory,

| Facies interpretation and sea-level change
Thin-bedded lime mudstones were deposited in low-energy, deepwater conditions (most likely below the storm wave-base and photic zone), suggested by lack of current-or wave-induced structures and trace fossils (J. . Laminated wackestones to packstones are interpreted as distal turbidites deposited in moderate-to low-energy conditions. Normally-graded beds were deposited from turbidity currents, interpreted as A-C portions of the Bouma sequence (Korn, 2008;Reijmer, Palmieri, & Groen, 2012).
Slump beds formed by slumping and deformation of sedimentary strata, which are often triggered by earthquakes and relatively sealevel changes (Alsop & Marco, 2011;Hilbrecht, 1989). The chert nodules or irregular beds formed most likely by replacement of preceding calcite by silica during diagenesis given that they show nodular or irregular shapes (primary chert beds are mostly thin-and flat-bedded) and that they occur in all facies and some of the partially chertified bioclastic wackestones to packstones still remain the bioclastic features with unchertified fossil fragments such as crinoids ossicles.  Chen, Sheng, et al., 2018;Mei et al., 2005;Wang et al., 2013).
Integrating these coeval subaerial exposure features in the inner platform of South China, the normally-graded packstones or slump masses that occur immediately above the MPB in the studied sections most likely indicate a significant sea-level fall. The other slump bed in the wackestone-to packstone-dominated interval in the middle part of the studied Serpukhovian succession in the Narao section might also have been triggered by a relative sea-level fall, assuming that they can be correlated with the normally-graded packstone beds of the similar stratigraphic height in the Naqing section (Figure 3; e.g., Martin, Montañez, & Bishop, 2012;Yose & Heller, 1989

| CARBONATE δ 1 3 C ACROSS THE MPB
The δ 13 C values of the slope carbonates from the three sections in South China range largely from 0.5‰ to 3.9‰, with an average value of 2.8‰ (Figure 3). The δ 13 C values from the Naqing section range from 1.8‰ to 3.9‰, with an average value of 3.2‰, and those from the Narao section vary from 1.5‰ to 3.7‰, with an average value of 3.0‰. The δ 13 C values from these two sections are similar in range and average. The δ 13 C values from the Dianzishang section range from 0.5‰ to 3.4‰, with an average value of 2.3‰, which is slightly lower than those from the Naqing and Narao sections. 6 | DISCUSSION 6.1 | Diagenetic evaluation of carbonate δ 13 C Carbon isotopic composition (δ 13 C) recorded in marine carbonates is influenced by both global and local carbon cycling as well as diagenetic alteration (e.g., J. Immenhauser et al., 2003;Panchuk et al., 2005;Patterson & Walter, 1994;Saltzman & Thomas, 2012;Swart, 2015). Before use as a proxy for δ 13 C of depositional seawater and as a tool for stratigraphic correlation, carbonate δ 13 C should first be evaluated with respect to early and late diagenetic alteration.
We utilized petrographically screened, microdrilled samples (n = 50) from the Naqing section in order to evaluate the reliability of bulk carbonate δ 13 C values as being representative of δ 13 C values of the depositional sea water. The results reveal that the microdrilled samples and bulk carbonates have overlapping δ 13 C values ( Figure 3). On the other hand, the studied slope carbonates lack subaerial exposure features (e.g., palaeokarst, palaeosol, and mud cracks), which suggest that the carbonates were barely influenced by meteoric diagenesis. Covariation between the δ 13 C and δ 18 O values is often used to evaluate alteration by either meteoric water diagenesis (Knauth & Kennedy, 2009) or burial diagenesis (Derry, 2010). Lack of the δ 13 C and δ 18 O covariation in the three carbonate slope successions ( Figure 5) indicates carbon isotopic compositions of depositional seawater. The relatively high δ 18 O values (ranging from −7.7‰ to 0.9‰, average of −2.7‰) also suggest minimal diagenetic alteration (e.g., Kaufman & Knoll, 1995).
Degradation of organic matter below the sediment-water interface could shift the carbonate δ 13 C values more negative due to incorporation of light carbon during early diagenesis, especially in stagnant water masses (e.g., Patterson & Walter, 1994). 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, 2005). 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  suggests that the South China data were overall not specifically influenced by organic degradation.
6.2 | Global spatial variation in carbonate δ 13 C across the MPB Carbonate δ 13 C values are influenced by various local to regional factors and thus do not simply reflect the average δ 13 C values of global ocean DIC. The spatial variation in carbonate δ 13 C is obvious across Euramerica either lacked the data across the MPB or likely were altered during meteoric diagenesis when eperic carbonate platforms were subaerially exposed.

The studied sections (except the Dianzishang section) in South
China were deposited near continuously in a carbonate slope setting, which warrants high-resolution carbonate δ 13 C time series across the MPB (Figure 3). The slope carbonates were unaltered by meteoric diagenesis (Buggisch et al., 2011;J. Chen et al., 2016, Chen, Sheng, et al., 2018 and most likely preserved original seawater δ 13 C values across the MPB. Nevertheless, the carbonate δ 13 C values across the MPB in South China sections only record a minor but distinct increase by~0.5-1.0‰, which is significantly smaller in magnitude than that in Europe (~3.0‰) and North America (~1.5‰). What would have caused this large spatial variation in carbonate δ 13 C in low-latitude regions across the globe? The carbonate δ 13 C of which region would more closely represent that of the global ocean DIC?
Upwelling is one type of ocean currents with deep-water swelling upward due to the influence of seaward winds (Parrish, 1987;Tolmacheva, Danelian, & Popov, 2001;Yu & Wei, 2015). The deep water is relatively 13 C depleted (nutrient enriched), which could lower the δ 13 C value of carbonates deposited around the upwelling region.
Upwelling usually occurs on the eastern side of the ocean realm (e.g., Peruvian upwelling that occurs on the eastern side of the Pacific Ocean at the present day) owing to the stable trade winds and Coriolis Effect (Bakun, 1990;Smith, 1981). Heckel (1977Heckel ( , 1999  interpreted as a deep-ocean basin (Feng, Yang, Bao, & Jin, 1998;Jiao, Ma, Deng, Meng, & Li, 2003). Thus, the water mass on the Yangtze Platform could have exchanged freely with open-ocean water.
Furthermore, Carboniferous slope carbonates were all originated from the platforms as calcareous plankton had not yet evolved at that time (Riding, 1993), so they could preserve δ 13 C values affected by upwelling and mixing of ocean water but not influenced by meteoric diagenesis.
In summary, the MPB δ 13 C recorded in slope carbonates in South China would have represented the mean δ 13 C of the global ocean DIC at that time, because the ocean water was well-mixed in the eastern Paleo-Tethys Ocean due to upwelling. The δ 13 C values recorded in North America were also influenced by upwelling processes (e.g., Mii et al., 2001), most likely representing a mean DIC δ 13 C value of the Pathalassic Ocean. In contrast, the large δ 13 C rise in Russia (~3.0‰), which was not affected by upwelling, may have well recorded the epicontinental sea signals that were influenced by various local to regional processes.
Significant atmospheric pCO 2 fall was hypothesized to account for the MPB glaciation, which in turn, was attributed to increased organic carbon burial implied by previously documented large increase (bỹ 3.0‰) in carbonate δ 13 C (e.g., Mii et al., 2001;Popp et al., 1986).
The hypothesis seems largely consistent with radiation of palaeotropical rainforests (Cleal & Thomas, 2005;Montañez, 2016) and increased occurrence of coal deposits in Euramerica (Ferm & Weisenfluh, 1989;Phillips & Peppers, 1984) but lacks support from precise age control for the correlation of these two aspects. Furthermore, the carbonate δ 13 C recorded in Euramerica actually lacks the data immediately across the MPB or might have been altered during meteoric diagenesis (Figure 7). If, however, the carbonate δ 13 C recorded in slope carbonates in South China represents the δ 13 C of global ocean DIC, the relatively small-amplitude (by~0.5-1.0‰) increase in δ 13 C would need less amount of organic carbon burial than previously assumed. Consequently, the atmospheric pCO 2 would not have been lowered enough (due solely to organic carbon burial) to trigger the MPB glaciation, although quantitative modelling is required in further studies.
Alternatively, continental weathering also plays an important role in regulating atmospheric pCO 2 , especially on time scales of more than 10 5 years (Kump & Arthur, 1999 Figure 7). The correlation suggests that the significant cooling across the MPB (or increasing in ice volume proxied by rapid increase in conodont δ 18 O) as a result of decreased pCO 2 was probably caused by enhanced continental weathering (rapid increase in 87 Sr/ 86 Sr) rather than increased organic carbon burial (invariant δ 13 C). analysis of the MPB successions (the Naqing, Narao, and Dianzishang sections) suggests a carbonate slope setting likely below the photic zone. Immediately above the MPB, a bed of either normally-graded packstones or slump deposits occurs in the studied sections, which, when integrated with coeval subaerial exposure signals on shallow-water platform of South China, indicates a distinct sea-level fall likely as a result of the Gondwanan glacial expansion at this time.
2. Carbonate δ 13 C records show a consistent value of~3.0‰ below the MPB in the Naqing and Narao and~2.5‰ in the Dianzishang; all three sections show a rise in δ 13 C by~0.5-1.0‰ across the MPB. The relatively small-magnitude rise in δ 13 C compared to the coeval records in Euramerica was likely due to upwelling in the eastern Paleo-Tethys Ocean. The well-preserved slope carbonates (free of meteoric diagenesis) and well-mixed ocean water by upwelling of deep-ocean water onto shallow-platform seawater would represent an average δ 13 C value of the global ocean DIC during the Carboniferous and thus can be used as a proxy for global biogeochemical cycling.
3. The rapid increase in 87 Sr/ 86 Sr, in contrast to the small-magnitude increase in δ 13 C (by~0.5-1.0‰), suggests that the decreasing pCO 2 and glaciation during the MPB interval were mainly attributed to enhanced continental weathering rather than increased organic carbon burial. The hypothesis is consistent with the Hercynian orogenic uplift and radiated palaeo-tropical rainforests; both of which would have enhanced continental weathering. Further quantitative modelling of the evolution of the late Palaeozoic carbon cycle is required for better understanding of primary driver of pCO 2 during the Earth's penultimate icehouse.