Chemical Weathering Proxies Evaluation by Comparative Investigation on Drilled Cores Versus Outcrops and Weathering History During the Permian-Triassic Transition

Continental records of the Permian–Triassic boundary are characterized by intense chemical weathering due to global warming and acid rainfall associated with volcanism. However, chemical weathering throughout the late Permian has not been interrogated in time-equivalent continental successions due to the lack of stratigraphically ‘continuous’ deposits. Here, we conducted geochemistry, petrology, and clay mineralogy investigations on several widely used chemical weathering proxies (i.e., CIA, CIW, PIA, CIX, and CPA) for the upper Permian to Lower Triassic rock samples both from outcrops and drilled cores. Their sediment sources are dominated by mafic weathering products of the Emeishan basalt with little influence by sediment sorting/recycling. However, diagenesis dominates their decreased values through the illitization of clay minerals in samples from the outcrop. For drilled core samples, decreased CIA, CIW, and PIA values partly result from the insufficient subtracting of CaO in secondary carbonates. After eliminating these effects using two modified proxies, the declining trend of CIX and CPA from the lower Xuanwei Fm. through the entire Kayitou Fm. is probably be caused by the cooling climate during the late Permian, which has also been suggested by previous work. However, lower values of CIX and CPA in the base of Dongchuan/Feixianguan Fm. likely indicated weak chemical weathering intensity due to physical denudation and subsequently rapid deposition in the earliest Triassic. This weakly developed chemical weathering is concerned with being accelerated by drier and more developed seasonal conditions in the sediment source area, as well as tectonics related to the closure of the eastern Tethys oceans. Our work is the first comparative study on chemical weathering proxies for drilled core and outcrop samples during the late Permian to the earliest Triassic, which straddled the severest bio-crisis and global environmental change of the Phanerozoic. It highlights the essential evaluation of non-weathering factor alteration on commonly used chemical weathering proxies by the use of multiple techniques. Compared to drilled core, sample collection from subsurface by excavation could not avoid weathering effects or sub-recent rock-water interaction influences