Huang Junhua

Main Controlling Factors of Organic Matter Richness in a Permian Section of Guangyuan, Northeast Sichuan

ABSTRACT A complete Permian section in Guangyuan, Northeast Sichuan, has been investigated, to explore the variation in organic matter richness and its main controlling factors. The research results of the detailed lithological description and organic/inorganic geochemical analysis of about 325 samples indicate that a high content of total organic carbon (TOC) occurs in calcareous mudstones and laminated marls or thin-bedded limestones. In carbonate rocks, the TOC content is negatively related to the thickness of the massive beds; the thinner the bed is, the higher the TOC content is. Marine organic matter is enriched in the outer shelf and relatively shallow basin floor environments with the maximum TOC contents of 5.07% and 14.6%, respectively. The main factors that affect the quantity and quality of marine organic matter include primary productivity, depositional processes, and redox conditions during deposition and the early diagenesis stage. Three intervals of marine good quality source rocks are identified in this section; they are lower Chihsia Formation, topmost Maokou Formation, and the middle segment of the Dalong Formation. They are formed in anoxic environments, in association with high primary productivity. Among these, high productivity in the lower Chihsia interval may originate from an upwelling flow area, whereas, in the rest of the intervals, it is inferred to be related to hot fluid activity due to volcanic eruption. The results of this study suggest that good quality marine source rocks in the Permian strata offer significant hydrocarbon potential.

The fluctuating environment associated with the episodic biotic crisis during the Permo/Triassic transition: Evidence from microbial biomarkers in Changxing, Zhejiang Province

The environmental conditions and the biotic crisis during the Permo-Triassic (Tr/P) transition received increasing attention in the past decades. Presented herein are the molecular fossil records of cyanobacteria and green sulfur bacteria, the base of the marine ecosystem, to highlight the episodic nature of both the environment and the biotic crisis during this critical period. At least two episodes of cyanobacterial expansion are documented by 2-methylhopanes ranging from C28 to C32 in carbon number, indicative of the instable marine ecosystem and the fluctuant aquatic nutrients. Meanwhile, the index of 2-alkyl-1,3,4-trimethylbenzenes (biomarkers of green sulfur bacteria) and the ratio of pristane to phytane (Pr/Ph) witness the fluctuation of sedimentary environmental redox conditions. The above molecular evidence suggests the occurrence of highly fluctuating environmental conditions during the Tr/P transition, which is consistent with, and probably the cause of, the multi-phased biotic crisis and the prolonged faunal recovery.

Assessment on Redox Conditions and Organic Burial of Siliciferous Sediments at the Latest Permian Dalong Formation in Shangsi, Sichuan, South China

ABSTRACT The redox sensitive elements, molybdenum (Mo) and uranium (U), in marine sediments from the latest Permian Dalong Formation at the Shangsi Section, Northeast Sichuan, South China, were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) to determine their response to a range of redox conditions, and to estimate the organic carbon burial rate. On the basis of the correlation between authigenic Mo abundance and organic carbon content in modern oceans, the organic carbon burial rates were calculated for the rocks at Dalong Formation, ranging from 0.48–125.83 mmol/(m2·d), which shows a larger range than the mineralization rate of organic carbon at the continental margins (1.6–4.23 mmol/(m2·d)). The Zr-normalized Mo and U abundances show large fluctuations in the entire section. The maxima of Zr-normalized Mo abundance and thus the maxima of the organic carbon burial rates were observed at the interval between the 155th and 156th beds (404–407 m above the base of Middle Permian). A decrease (the minimum) in U/Mo ratios is present in this interval. It is speculated that the oxygen-limited conditions and ultimately anoxia or euxinia may develop within this depth interval. In contrast, an enhanced enrichment of Zr-normalized U abundance is found, in association with less enrichment in Zr-normalized Mo abundance in the interval from the 151st to 154th beds (395–404 m above the base of Middle Permian), inferring the dominance of a suboxic/anoxic depositional condition (denitrifying condition), or without free H2S. The presence of small quantities of dissolved oxygen may have caused the solubilization and loss of Mo from sediments. It is proposed that the multiple cycles of abrupt oxidation and reduction due to the upwelling at this interval lead to the enhanced accumulation of authigenic U, but less enrichment of Mo. A decrease in the contents of U, Mo, and TOC is found above the 157th bed (407 m above the base of Middle Permian), in association with the enhanced U/Mo ratio, suggesting the overall oxic conditions at the end of the Dalong Formation.

Carbon Isotope Records Indicative of Paleoceanographical Events at the Latest Permian Dalong Formation at Shangsi, Northeast Sichuan, China

ABSTRACT Paired organic and carbonate carbon isotope compositions of Late Permian Wujiaping and Dalong formations at Shangsi, Northeast Sichuan were analyzed by MAT 251. An abrupt negative excursion in the two isotope records was observed in the middle part of Dalong Formation, in association with a drop in the carbon isotope difference of the two records and an increase of total organic carbon (TOC) content. The negative drop of the paired carbon isotope records is suggestive of the input of 12C-enriched CO2. The molecular ratios of pristane to phytane and dibenzothiophene to phenanthrene indicate the anoxic condition in this interval. The enhanced TOC content is indicative of the elevated preservation of organic matter due to the anoxic condition. These isotopic and organic geochemical data probably infer the occurrence of the upwelling in this interval. The additional contribution of volcanism activity observed in South China cannot be excluded to the input of 12C-enriched CO2 and the negative shifts in carbon isotope composition of bulk organic matter and carbonate.

Stable isotope and trace element record of a stalagmite in Heshang Cave, Hubei and its palaeoclimatic significance

Through a combined analysis of sedimentary characteristics, C and O isotopes, Mg and Sr trace elements of a stalagmite from Heshang Cave, Hubei, with uranium-series dating, alaeopalaeoclimatic and palaeoenvironmental information from 19.0ka to 6.9ka in the Hubei area is obtained. The Average resolution is 17a, and the local resolution is 7a. The following results are obtained: (1) from 19.0-16.6 kaBP: C and O isotope is lighter, the climate is cold and wet; (2) 16.6-11.1 kaBP: C, O isotope is a little heavy, temperature rises, rainfall is inclined to be little; (3) 11.1-10.3 kaBP: an excursion during dry and hot period responds to Younger Dryas Event; (4) 10.3-6.9 kaBP: C, O isotope is lighter, but Mg/Sr value is bigger, which shows temperature is continuing rising and rainfall is greater. These reflect the variation tends from wet-cold climate, to dry-hot climate, and to wet-warm climate in history, and some climatic variation trends of thousand years and hundred years cycle are got.Through a combined analysis of sedimentary characteristics, C and O isotopes, Mg and Sr trace elements of a stalagmite from Heshang Cave, Hubei, with uranium-series dating, alaeopalaeoclimatic and palaeoenvironmental information from 19.0ka to 6.9ka in the Hubei area is obtained. The Average resolution is 17a, and the local resolution is 7a. The following results are obtained: (1) from 19.0-16.6 kaBP: C and O isotope is lighter, the climate is cold and wet; (2) 16.6-11.1 kaBP: C, O isotope is a little heavy, temperature rises, rainfall is inclined to be little; (3) 11.1-10.3 kaBP: an excursion during dry and hot period responds to Younger Dryas Event; (4) 10.3-6.9 kaBP: C, O isotope is lighter, but Mg/Sr value is bigger, which shows temperature is continuing rising and rainfall is greater. These reflect the variation tends from wet-cold climate, to dry-hot climate, and to wet-warm climate in history, and some climatic variation trends of thousand years and hundred years cycle are got.

Molecular Evidence for Primary Producers and Paleo-environmental Conditions in Mesoproterozoic in the Xuanlong Depression in North China

ABSTRACT The molecular organic compounds have been identified by gas chromatography (GC) and GC-mass spectrometry (GC-MS) from Mesoproterozoic rocks in the Xuanlong depression in North China. The main saturated compounds are n -alkanes, monomethylalkanes, n -alkylcyclohexanes, acyclic isoprenoids, and hopanes. The dominant lower-molecular-weight n -alkanes are indicative of the main contribution of microorganisms, in particular, the chemosynthetic bacteria. The presence of abundant monomethylalkanes (mid- and end-branched) and the long chained (>C 20 ) acyclic isoprenoids indicates the existence of abundant bacteria and/or archaea in ancient oceans. The low abundance of pristane and phytane is suggestive of the relatively low abundance of photosynthetic autotrophs in comparison with chemosynthetic bacteria in the Mesoproterozoic oceans in North China. The sedimentary environmental condition is suboxic/anoxic, as indicated by the low value of the Pr/Ph ratio as well as the presence of abundant sulfur-bearing organic compounds, consistent with the other geochemical data in North China and elsewhere in the world. Both the composition of the primary producers and the sedimentary environmental conditions are favorable for the formation of hydrocarbon source rocks.

Climate history of the middle reach of the Yangtze river over the past 9000 years: A speleothem isotopic record from Za cave, Hubei, China

A stalagmite in Qingjiang, Hubei, has yielded the records of hydrogen and oxygen as well as carbon isotopic changes and provided a climate history of 9000 years. The hydrogen isotopic ratio in fluid inclusions and the oxygen isotopic ratio in stalagmite calcite are controlled by meteoric water, which is in response to ambient temperature or precipitation amount. The carbon isotopic ratio, sensitive to the nature of vegetation, also can be used to reconstruct the climate history. The carbon-14 and U-series dated stalagmite records reveal that climate oscillated in the middle reach of Yangtze river over the past 9000 years: (1) 9.0-6.5 kaBP, a cooler and more humid condition; (2) 6.5-4.0 kaBP, with warmer and drier weather; (3) 4.0-2.0 kaBP, cold weather with heavy precipitation, and (4) 2.0 kaBP to present, tendency to be warmer and drier.A stalagmite in Qingjiang, Hubei, has yielded the records of hydrogen and oxygen as well as carbon isotopic changes and provided a climate history of 9000 years. The hydrogen isotopic ratio in fluid inclusions and the oxygen isotopic ratio in stalagmite calcite are controlled by meteoric water, which is in response to ambient temperature or precipitation amount. The carbon isotopic ratio, sensitive to the nature of vegetation, also can be used to reconstruct the climate history. The carbon-14 and U-series dated stalagmite records reveal that climate oscillated in the middle reach of Yangtze river over the past 9000 years: (1) 9.0-6.5 kaBP, a cooler and more humid condition; (2) 6.5-4.0 kaBP, with warmer and drier weather; (3) 4.0-2.0 kaBP, cold weather with heavy precipitation, and (4) 2.0 kaBP to present, tendency to be warmer and drier.