Chemical Weathering Indexes: Implication for Paleoclimatic Reconstructions, with the Vendian–Lower Cambrian Section of Podolian Transnistria as Example

Abstract

The article provides an overview of various indexes/indicators (Vogt, Parker, CIA, CIW, PIA, MIA, and others) used for studying weathering profiles/crusts and reconstructing paleoclimatic environments of sedimentary sequence accumulation. Their possibilities are demonstrated with Vendian–Lower Cambrian terrigenous rocks of Podolian Transnistria (southwestern slope of the Ukrainian Shield) as example. Distribution of index ba₁ values in this section indicates the presence of material subjected to intense transformation during the chemical weathering in mudstones of the Nagoryany Formation, the lower part of the Danilovka and the middle part of the Studenitsa formations. For mudstones of the Danilovka–Zbruch interval, the HM values are close to the HM_PAAS. The HM values are slightly higher for rocks of the Yaryshev–Nagoryany interval and comparable to those inherent in hot tropical continental clays for mudstones in the lower part of the Yaryshev Formation. Average value of index SA for mudstones of the entire section is 5.6 ± 0.7. Mudstones of the Grushka–Nagoryany interval, where SA < SA_PAAS, are composed of a more weathered material. The WIP values in mudstones of the Mogilev and Yaryshev formations, as well as in the upper part of the Zbruch Formation, correspond to the interval of their values between the PAAS and average Archean granite. Clay rocks of other formations have WIP ≤ WIP_PAAS. Average CIA value (71 ± 4) for mudstones virtually corresponds to the CIA value (70), which separates the sediments of cold/arid and warm/humid climates. Variations in the index CIW value along the section are oriented similarly as CIA variation. The vast majority of mudstones are characterized by PIA > PIA_PAAS. The average CPA value is 91 ± 4, which is also typical for PAAS. These and other data suggest the following point: based on a “direct” interpretation of the values of various chemical weathering indexes inherent in the fine-grained clastic rocks, paleoclimate in Podolian Transnistria was rather moderate or warm humid in the Vendian‒Early Cambrian. Comparison of the CIA values of mudstones with the values for the particulate suspended matter in modern rivers suggests that the Vendian‒Early Cambrian climate resembled the dry and humid subtropical or dry tropical type with elements of the humid climate.