Evaluation of chemical weathering proxies by comparing drilled cores versus outcrops and weathering history during the Permian–Triassic transition

Abstract

Continental records of the Permian–Triassic boundary are characterized by intense chemical weathering, likely due to global warming and acid rainfall associated with volcanism. However, the weathering history throughout the late Permian has not been interrogated in time-equivalent continental successions because of the lack of stratigraphically continuous deposits. Here, we present comparative investigations of outcrops and drilled cores from upper Permian to lowest Triassic terrestrial deposits from the Taoshujing Section, South China. These sediments have a mafic-dominated source with insignificant sorting and recycling during deposition, as documented by the invariable TiO2/Al2O3, Zr/Ti, Th/Sc, Zr/Sc, and Cr/Th ratios, and low values of Th/Sc and Cr/Th ratios. However, different diagenetic effects on outcrops and drilled cores have generated counterintuitively low values of commonly used chemical weathering proxies. Potassium enrichment, recognized from the A-CN-K diagram produced for portions of the outcrop, is also verified by the strongly negative correlations between CIA, CIX, and the molar contents of K2O (r = −0.90 and −0.92, respectively), as well as clay mineralogy analysis. However, the reduced CIA, CIW, and PIA values for the drilled core samples primarily result from an insufficient subtracting of CaO in the secondary carbonates. Using two modified proxies (CIX and CPA), the decline observed in the lower Xuanwei Fm. was probably caused by the cooling climate during the early Wuchiapingian. Nevertheless, the lower values from the Kayitou Fm. to the basal Dongchuan/Feixianguan Fm. likely indicated weak chemical weathering intensity due to physical denudation and subsequent rapid deposition under rapid warming and short fluid-rock reaction time conditions. This weak chemical weathering might have been intensified by drier conditions and increased seasonality in the sediment source area and tectonics related to the closure of the eastern Tethys Ocean. Our work is the first comparative study on chemical weathering proxies for drilled core and outcrop samples from the Late Permian to Early Triassic, a time period which included the most severe bio-crisis and global environmental change in the Phanerozoic. These results highlight that non-weathering factor alterations on commonly used chemical weathering proxies are essential to evaluate with multiple techniques. Compared to the drilled cores, samples collected from the subsurface by excavation could not avoid later weathering alteration and should be used with caution to uncover the past chemical weathering history of an area.