Geochemical characteristics and paleoenvironmental implications of carbonate rocks at the Givertian and Eifelian boundary in northeast Guangxi

The Devonian period, marking the initial phase of the Late Paleozoic era, has garnered considerable attention in recent decades. This heightened interest stems from its intricate climatic fluctuations, recurrent global shifts in sea levels, and a series of consequential biological catastrophes. Of particular captivation is the upper Devonian-lower Carboniferous in northeastern Guangxi, characterized by a multifaceted paleogeographic configuration known as platform-trench-basin facies. Notably, the stratigraphic progression from basin facies to open platform facies displays a notable continuity, culminating in the formation of a substantial kilometer-thick carbonate rock layer. This unique geological foundation underpins the development of a renowned karst geomorphic landscape, which is globally significant.Furthermore, this distinctive karst landscape has facilitated the establishment of the international Devonian-Carboniferous boundary auxiliary layer profile, adding another layer of importance to the study of the Devonian system in this region. While prior investigations have primarily concentrated on sedimentary facies, paleontology, pivotal organisms, and event layers like the 'F-F' biological catastrophe event and the D/C boundary layer during the Late Devonian, certain critical events, notably the Kačak-Otomari event in the late middle Devonian, have received comparatively less attention. Moreover, a comprehensive global-scale comparative analysis has been lacking.Given these gaps, the present study investigates a critical juncture, the boundary between the Eiffelian and Givertian systems, situated in Longyankou village, Yangshuo county. A meticulous examination was carried out, involving the collection of ten carbonate rock samples from this specific section. Through a comprehensive analysis encompassing microscopic features, elemental compositions, ratios, as well as carbon and oxygen isotope data, the study aimed to elucidate and discuss the ancient oceanic sedimentary environments in proximity to this location. Our findings can be summarized as follows:(1) The presence of Polygnathus intermedius, a conodont fossil, is observed within the YS3 sample layer, attributed to the late Eifelian period. This fossil aids in establishing a precise temporal boundary for both the carbon isotope migration event and the Kačák event within this stratum. (2) By employing Wilson's established microfacies sedimentary model in conjunction with lithological traits, four distinct microfacies types, SMF23, SMF16, SMF4, and SMF15, have been successfully distinguished. (3) Through meticulous geochemical analysis, we draw several significant conclusions regarding the sedimentary environment during various stages: The sedimentary environment during the YS1 to YS2 transition demonstrates rapid deepening of sedimentary water and a consistent material source supply. This mirrors the geological process of swift transformation from limited platform facies to slope facies during the initial transgression phase; the low terrigenous component content during YS3 and YS4 stages indicates a distant shoreline sedimentary environment with deep waters and relatively elevated sea levels. Notably, this stage is characterized by widespread hypoxia, displaying global uniformity; Geochemical indicators for YS5 to YS10 stages exhibit minor fluctuations. Notably, the terrigenous component content gradually increases, while the appearance of radial oolitic limestone signifies a low-energy environment. Simultaneously, the sedimentary area witnesses a significant sea level decline, characterized by thicker upward progradational sequences that provide excellent sedimentary responses. (4) Reconstructing the paleoseawater temperature evolution curve based on oxygen isotope data reveals an average seawater temperature of 21 ℃ during the studied period. This temperature profile reflects a warm subtropical climate, aligning with the paleogeographic proximity to the equator. (5) The observed negative migration of carbon isotopes corresponds with the E/G boundary stratotype profile in Morocco and the carbon isotope curve characteristics observed in Canada within the same horizon as sample YS3. In summary, these findings emphasize the prevalence of a deep-water and anoxic sedimentary environment amidst a global-scale transgressive backdrop.