Sedimentology, Carbon And Sulfur Cycles In The Mesoproterozoic Jixian Group On The Northern Margin Of North China

The Mesoproterozoic has recorded a critical evolution of the palaeoceanic chemistry from completely anoxic, iron-rich for the Archean to fully oxic for most of the Phanerozoic eon, meantime, it is also a critical geological stage when the Earth’s biosphere evolution from dominated by microbes into macro-organisms. Therefore, it is very important to study the palaeoceanic chemisty and the interaction between the organisms and environments during this time interval, and also is one of important topics of the Precambrian researches. But so far, due to so long-time geological courses, and special environmental conditions, microbes are very hard to preserve their calcified outerwalls, leading to incomplete fossil records. Therefore, the study on the eary Mesoproterozoic oceanic chemistry, dominant microbial functional groups and interactions with natural environments has been extremely weak, that strongly hampered further understanding for the organisms and environments.The Mesoproteozoic, especially the Jixian Group dominated by carbonate strata developed very well at the Yanshan Basin, North China, they were relatively complete, continuous and have resisted a high degree of regional metamorphisms and deformations, holding the potential to record their primary information about marine organisms and environments. This study mainly focuses on the bio-sedimentary construction, typical microfacies (micro-fabrics), and the carbon and sulfur stable isotopic compositions, combined with distribution of the redox-sensitive trace elements from a～80m thick sequence of the Gaoyuzhuang Formation, to disscuss the evolutionary patterns of the early Mesoproterozoic oceanic chemistry and dominant biogeochemistry processes under the special environmental conditions. The main results related to this study are given as follows.1, Microbialites are much more abundant in the well-preserved Mesproterozoic Jixian successions, including stromatolites, thrombolites, biolaminites, oncolites (oncolite-like carbonate concretions) and various types of microbially induced sedimentary structures (MISS), indicating diversified microbial activities in the early Mesoproterozoic ocean. On the basis of this study and previous achievements, microbialites that mainly occurred in shallw water environments, such as stromatolites, thrombolites, biolaminites and oncolites are believed to be probably related with fiamentous and/or spherical cyanobacteria metabolizing processes under distinct environmental conditions. The differences in external morphology and internal microfabrics for these shallow-water microbialites are mainly controlled by changes of environment physical/chemical conditions. However, the oncolite-like carbonate concretions, mainly appeared in relatively deeper water environments, are closely associated with anaerobic microbes such as sulfate-reducing bacteria (SRB), and methanogenic archaea (ANME) et al. The various microbial fuctional groups and their biogeochemical processes might have put significant influences on the early Mesoproterozoic climate and oceanic environments.2, The abundance of acicular aragonites are recognized in the thin-sections under microscopes, these argonites generally occur as fibrous cladding at organic-rich micro-pellets edges or as cements filling in some "mini-environments" protected by intensive microbial mats layers, suggesting dysoxic/anoxic conditions are prevalent in the Mesoproterozoic deep ocean, and also, some special shallow water environments protected by microbial mat layers are probably dysoxic due to oxygen consumption during the processes of organic matter decompositions. In addition, the appearence of abundant acicular aragonites indicates HCO3-supersaturated seawater conditions, which is probably related with much higher PCO2during the early Mesoproteorozic.3, To improve our knowledge about the evolutional processes of the early Mesoproterozoic surface system including atmosphere, hydrosphere and biosphere, the Mesoproterozoic Jixian depositional successions composed of five formations from three study sections (Gaoyuzhuang and Yangzhuang formations from the Pingquan Section, Wumishan and Tieling formations from the Lingyuan Section and Hongshuizhuang formation from the Huailai Section) are systematically ananlyzed on high-resolution carbonate and organic carbon isotopic compositions, including623δ13Ccarb and252δ13Corg data. The dataset presented here currently reprsents the highest resolution chemostratigraphic dataset from the early Mesoproterozoic Calymmian period (1.6～1.4Ga) and, as such, permits unprecedented exploration of the isotopic patterns and origin of isotopic variation in the early Mesoproterozoic carbon cycle. In the Jixian succession,623carbonate carbon isotope data points reveal values within a narrow range from-2‰to+2‰, oscillating in a repeated succession of positive and negative excursions around values near0%o. Data presented here show strong isotopic similarity to the previously reported that from other stratigraphic successions elsewhere, however, compared to them, strata within the Yanshan area have a series of superiority in strata continuity, integrity, low degree of regional metamorphisms and deformations and superb age constraints, thus, so high-resolution carbon isotope dataset presented here have the potcntical to serve as a reference for the entire early Mesoproterozoic, and provide important data materials for stratigraphic comparision, subdivision and definition of geological events in the further.4, The carbon isotope data from the early Mesoproterozoic Jixian succession record an increase in both the average isotopic composition and an increase in the magnitude of isotopic excursions. Data from the Gaoyuzhuang Formation, which record-with the exception of a single50 m thick interval-only minimal isotopic variation from-1‰to+1‰, with an average isotope composition of-0.3‰. While data shift to more variable from-1.8‰to+1.8‰, with an increased average value at-0.1‰in the Wumishan Formation. Despite relatively differences in carbon isotopes for the two formations, the degree of organic carbon burial (forg) calculated here remained within a fairly narrow range (0.15～0.25), which suggests that the increased isotopic variation observed in marine carbonate between the two successions must have been driven, instead by a decrease in the buffering capacity of the marine DIC system, probably related to a long-term decrease in PCO2through the early Mesoproterozoic and perhaps until the whole Mesoproteorzoic.5, A vertical depth-gradient in the isotopic composition of organic carbon is identified from the studied Jixian succession. δ13Corg vary from-26‰to-30‰, with values averaging-28‰, in the upper Gaoyuzhuang Formation and the entire Wumishan Formation, and the δ13Corg is coupled with the coeval δ13Ccarb, reflecting a microbial community dominated by autotrophic organisms in shallow water peritidal environments, and the burial or oxidative decomposition of benthic microbial material put much influence on carbon isotopic compositions. By constrast, the lower and middle Gaoyuzhuang Formation representing relatively deeper water environments shows substantially lighter isotopic compositions for organic carbon (varying from-26.3‰～-34.4‰, and averaging-31.1‰) and correspoding higher AC values (≥32‰), which suggests a substantially enhanced heterotrophic remineralization of benthic microbial mats. These differences in organic and inorganic carbon isotopes under different depositional environment most likely reflect generally low oxygen conditions and a dynamically maintained stratified ocean, where anoxic conditions likely occurred close to oxygenated, well-mixed surface oceans, this conclusion is consistent with that derived from sedimentary features.6, The analyses on redox-sensitive trace elements, such U, V, Mo, Cr and Co et al., are performed in a～80m thick carbonate succession from the middle Gaoyuzhuang Formation. This succession spans an interval from relatively queit and deeper subtidal zone to high energic shallower subtidal/intertidal environments and therefore is particularly well-suited to explore the early Mesoproterozoic ocean redox states. The distributions of redox-sensitive trace elements suggest a relatively shallow chemocline in the ocean, transitions from anoxic/euxinic to oxic seawater conditions may occur in lower subtidal zone.7, Based on high-resolution carbon isotope records,110carbonate samples are selected from the Gaoyuzhuang and Wumishan formations for CAS sulfur isotope analyses. Unlike the δ13Ccarb, CAS-sulfur isotopes show significant stratigraphic variations, with values ranging from+3.7‰～+38.6%o, indicating a very small sulfate reservoir size, which are more easily affected by environmental and biological changes. The oceanic sulfate concentration is speculated to be very low during the early Mesoproterozoic, probably near or less than1mM. The extremely low sulfate level is compatible with low atmospheric oxygen concentrations and low weathering ratios in that time period. More than two episodes of enrichments of534SCAS exceeding+35‰were present in the middle Gaoyuzhuang and top Wumishan formations, indicating multiple-episodes of ocean anoxic aggravation on a local or global scale during the early Mesoproterozoic. A combined study of C-S cycles indicates a huge DOC reservoir below the chemocline, heterotrophic and secondary chemoautotrophic microbes (eg. methanogen) except for sulfate-reducing bacteria probably exists and contributes widespread organic carbon remineralization in the defined anoxic deep ocean.8, The declined oceanic sulfate concentration would also have reduced anaerobic oxidation of methane (AOM) dominated by a consortia of methanogenic archaea and sulfate-reducing bacteria, but in turn facilitated the development of methanogenic archaea and enhanced fluxes of CH4release into the surface ocean and the atmosphere. Being an important greenhouse gas, the release of abundant CH4would have intensified the global warming at that time period, and maintained as long as1.6Ga "no ice" ages in the geological period. The abundance of well preserved sedimentary signatures, such as gas blister structures et al., provide evidences for CH4generation and release under some proper conditions.