The paleoenvironmental evolution of the Cambrian Longwangmiao Formation (Stage 4, Toyonian) on the Yangtze Platform, South China: Petrographic and geochemical constrains

Abstract

Abstract During the Cambrian Period Stage 4, biodiversity and biomass declined rapidly and globally in an event known as the Botomian-Toyonian crisis. To gain a better understanding of the paleoenvironment in South China during this stage, we present and interpret complex multi-parameter carbonate analyses (petrography, carbon and oxygen isotopic composition, and major, trace, and rare earth elements) from the Toyonian Longwangmiao Formation in the Bandenggou section. The Longwangmiao Formation comprises primarily carbonate deposits, with massive detrital mineral occurrences in its lower-middle sequence. Principal component analysis of the minerals and oxides leads to the definition of two intervals, I and II. Interval I is composed mainly of dolostones interbedded with sandstones and contains exceedingly high amounts of terrestrial original minerals and oxides, as well as high ΣREE + Y, Th, and Sc concentrations. Interval II, which is composed primarily of limestones, has trace amounts of terrestrial original minerals and oxides and low ΣREE + Y, Th, and Sc concentrations. The δ13C and δ18O values show a significant positive shift from Interval I to Interval II. After eliminating the effects of diagenesis contributions through the analyses of diagenesis sensitive indexes (δ18O values, MgO/CaO, Mn/Sr and Fe/Sr ratios, and the correlations of δ13C-δ18O, δ18O-Eu/Eu*, Eu/Eu*-Ce/Ce* and Eu/Eu*-FeO), the Sr and Na concentrations of calcites in Interval I are extremely high, exceeding even the corresponding ranges of marine aragonites, while calcites from Interval II have low Sr and Na concentrations that are within the range of marine low-Mg calcite; however, both show strong positive correlations between Ca-Sr, Ca-Na, and Sr-Na. The different calcite elemental behaviors in Intervals I and II suggest different carbonate precursors associated with seawater temperature and climate. Furthermore, the Sr/Ca ratios are negatively correlated with δ13C and δ18O values, and the δ13C values also negatively correlated with SiO2 content. These features indicate a significant change in the depositional environment during the Cambrian Toyonian Stage from an early restricted marine environment under a warm and arid climate with massive terrestrial influxes to a later relatively open marine environment under a cool and humid climate with negligible terrestrial influxes; this shift likely resulted in the observed changes in biomass and biodiversity.