Kong Deng

Modern oceanographic studies demonstrate that marginal seas and semi-restricted marine environments, including epicontinental seas and carbonate platforms, are highly sensitive to changes in circulation, freshwater input, stratification, and redox conditions, allowing climatic perturbations to be recorded with high fidelity. Understanding the behavior of such systems under icehouse conditions is therefore important for interpreting climate variability in both ancient and modern oceans. The Late Paleozoic Ice Age was a prolonged icehouse interval characterized by repeated glacial and interglacial oscillations, yet its climate dynamics are still mainly constrained by Gondwanan glacigenic records and low-latitude carbonate successions. High-resolution climate information from mid-latitude regions remains limited. The purpose of this study is to obtain high-resolution mid-latitude geochemical constraints on climate variability during the Late Paleozoic Ice Age using a semi-restricted marine carbonate succession. Specifically, this study aims to (1) establish high-resolution carbon and oxygen isotope records from well-preserved carbonate samples spanning the Visean to Asselian interval; (2) identify and characterize major glacial to interglacial cycles recorded in the succession; (3) evaluate the extent to which semi-restricted paleogeography amplifies isotopic responses relative to coeval low-latitude open-marine settings and (4) assess the climatic significance of a short-lived negative carbon isotope excursion during the middle Bashkirian. Here we present high-resolution carbon and oxygen isotope records from a Visean to Asselian marine carbonate succession deposited in a semi-restricted basin. Stable isotope analyses of well-preserved carbonate samples document temporal variations in carbonate carbon and oxygen isotopes. The records resolve at least three major glacial to interglacial cycles, with isotope shifts substantially larger than those reported from coeval low-latitude open-marine settings. Carbon isotope variations reach up to 7.7‰, while oxygen isotope variations reach up to 9.2‰. These pronounced responses are attributed to semi-restricted paleogeography, facies heterogeneity, and the sensitivity of marine carbonate systems to stratification, redox variability, and organic carbon cycling. A short-lived negative carbon isotope excursion during the middle Bashkirian may record a Northern Hemisphere deglaciation event superimposed on the broader Gondwanan icehouse background, a signal that is not clearly expressed in other regions. Overall, this study describes new mid-latitude geochemical constraints on Late Paleozoic climate variability and offers valuable analogs for understanding climate responses in modern marginal marine systems.

Investigating how climatic and hydrological conditions in ecological resource-enriched zones of marginal seas respond to external forcing, particularly during past greenhouse climates, holds considerable significance for understanding current environmental and resource challenges driven by global warming. In marginal seas, climatic hydrological states, including salinity, redox conditions, and productivity, are key environmental parameters controlling organic matter production, preservation, and ultimately the formation of high-quality shale. Herein, high-resolution cyclostratigraphic and multi-proxy geochemical analyses were conducted on a continuous core from the upper part of Member 4 of the Eocene Shahejie Formation (Es4cu) in Well NY1, Dongying Sag, Bohai Bay Basin. Based on these data, a refined astronomical timescale was accordingly established for the studied interval. By integrating sedimentological observations with multiple proxy indicators, including elemental geochemistry (e.g., Sr/Ba and Ca/Al ratios), organic geochemistry, and mineralogical data, the evolution of climate and paleo-water mass conditions during the study period was reconstructed. Spectral analyses revealed prominent astronomical periodicities in paleosalinity, productivity, and redox proxies, indicating that sedimentation was modulated by cyclic changes in eccentricity, obliquity, and precession. It was hereby proposed that orbital forcing governed periodic shifts in basin hydrology by regulating the intensity and seasonality of the East Asian monsoon. Intervals of enhanced summer monsoon associated with high eccentricity and obliquity were typically accompanied by increased sediment supply and intensified chemical weathering. Increased precipitation and runoff raised the lake level while promoting stronger connectivity with the ocean. In contrast, during weak seasonal monsoon intervals linked to eccentricity minima, basin conditions shifted from humid to arid, characterized by reduced precipitation, lower lake level, decreased sediment supply, and a concomitant decline in proxies for water salinity. The present results demonstrated orbital forcing as a primary external driver of cyclical changes in conditions favorable for resource formation in the Eocene lacustrine strata of the Bohai Bay Basin. Overall, this study yields critical paleoclimate evidence and a mechanistic framework for predicting the spatial-temporal distribution of high-quality shale under comparable astronomical-climate boundary conditions.

Diatoms are critical indicators in marine paleoecology and contemporary ecosystem studies, yet the accuracy of diatom analysis depends on effective purification methods. Current screening and purification techniques often yield low accuracy. This study introduces a multi-stage screening purification method that integrates both physical and chemical techniques to enhance the extraction of diatom remains from marine sediments, thereby improving the reliability of subsequent analysis. Using surface sediment samples from the Pacific Ocean, we compared the effectiveness of three purification methods: heavy liquid suspension purification, single-layer screening purification, and the newly developed multi-stage screening purification method. The study aimed to evaluate the impact of each method on diatom abundance, valve size distribution, and the accuracy of taxonomic identification. The results revealed that the multi-stage method significantly improved the accuracy of diatom abundance calculation. With this method, diatom abundance was 21.9 times higher than that obtained using the heavy liquid suspension method and 6.5 times higher than that obtained using the single-stage method. This method also proved to be cost-effective, easy to use, and produced samples with low impurity levels, which enhanced microscopic observation, identification, and the reliability of taxonomic statistics. Key factors influencing diatom abundance included sieve pore size, heavy liquid concentration, and valve size and density, while the type of acid and oxidant had minimal effect. Additionally, the multi-stage screening method facilitated the classification of diatoms into various size categories, providing a comprehensive view of diatom size distribution, including nano-sized diatoms (diameter < 20 μm) that are often overlooked in traditional studies. These findings demonstrate that the multi-stage screening purification method is an effective tool for improving the analysis of diatom remains in sediments, potentially refining the accuracy of diatom-based environmental investigations and paleoceanographic reconstructions.

Pore connectivity and ultimate imbibed porosity are two important parameters used to assess the shale oil reservoir property, the proper appraising of which could facilitate the efficient flow of oil from the matrix and an improvement in recovery efficiency. In previous studies, the uncertainty in sample dimensions and the extra-long stable time during imbibition experiments exploring pore connectivity and ultimate imbibed porosity showed a lack of discussion, which influenced the accuracy and efficiency of the SI experiments. In this study, SI experiments with shale samples of different thicknesses are carried out to acquire the two parameters in a short period of time. As a result, the pore connectivity of sample D86-5 from the Qingshankou Formation (Fm) in the Songliao Basin fluctuates with the increase in thicknesses, with an average of 0.265. The water penetrates sample D86-5 of all thicknesses, so the ultimate imbibed porosity fluctuates around 3.7%, and the stable time increases with thicknesses. The pore connectivity of sample Y172 from the Shahejie Fm in the Bohaiwan Basin fluctuates around an average of 0.026, which is much smaller than that of D86-5. The ultimate imbibed porosity of Y172 decreases with thicknesses because the penetration depth is so small that the pores cannot be fully accessed, and the stable time increases before becoming stable with fluctuations. The method is examined using the samples from the Liushagang Fm in the Beibuwan Basin measuring around 400 μm: the ultimate imbibed porosity of BW1-1 and BW1-3 is 5.8% and 18.1%, respectively, the pore connectivity of BW1-1, BW1-2, and BW1-3 is 0.086, 0.117, and 0.142, respectively, and the results can be obtained within a day. In comparison, the average pore connectivity of the 400 μm samples from Qingshankou, Shahejie, and Liushagang Fms is 0.324, 0.033, and 0.097, respectively, and the average ultimate imbibed porosity of these Fms is 3.7%, 3.1%, and 12.0%, respectively. Based on the above results, a quick method for measuring the two parameters with thin samples by spontaneous imbibition is established, providing a fast solution for the evaluation of the sweet spot.