Microbiotic Replacement And Eukaryote Evolution In Mesoproterozoic Carbonate Series Of North China

Numerous basins have been formed because of Colombia breakup during1600-1400Ma. The Columbia supercontinant hasn’t fully opened, then joined together into Rodinia. So, basins formed in this interval mostly were rift basins. The Yanshan basin from North China was one of these rift basins. Dozens of microbiotas have been reported from these rift basins, like the microbiotas from the Roper Group, Dismal Lakes Group, Beck Spring dolomite, Bangemall Group, Amelia dolomite, Balbirini dolomite, Semri Group and Jixian Group. But regional correlation among these microbiotas is still problematic because of lacking precise age data. Meanwhile, biotic variation in the time longitudinal direction is also not clear due to the limitation of fossil preservation. Although the study of sedimentology and geochemistry has already rebuilt the environment of the Proterozoic, coevolution of environment and biota is still a shortage for this period.Being one of the rift basins formed during the Colombia breakup, the Yanshan basin has the most continuous and unmetamorphosed Proterozoic successions in the world. In the past decades, abundant of fossils have been reported from here, especially in the Jixian Group which is dominated by carbonate sediments. The Jixian Group contains the most abundant microfossils in the Yanshan basin, and these fossils can be used for statistics in the level of sub-formation (or member). Meanwhile, chronologic studies these years provided high-precision age restriction. Combined these two conditions, fossil statistical and biota succession pattern analysis is feasible. This study will not only can help to understand the biotic replacement during the Columbia breakup, but also provide important information for studying the coevolution of biota and environment.Although reported biotas from the Yanshan basin have high continuity, there are still some weak links in some horizons. We selected six horizons from five sections, including the Tuanshanzi Formation of the Guzifang section, Dahongyu Formation and Gaoyuzhuang Formation of the Jixian section, Gaoyuzhuang Formation of the Kuancheng section, Gaoyuzhuang Formation of the Qianxi section and Wumishan Formation of the Lingyuan section. Through the systematically investigation, which made a lot of fossil data, a total of25genera and38species have been described(including10undetermided species and3new genera and new species) and20undetermined types. Via this work, the synonyms fossils have been discussed and revised. These information provided a great supplement for the latter statistical research. During this investigation, four well-preserved and meaningful microbiotas have been discovered.1) Silicified microbiota from the Dahongyu Formation (Jixian section):The Dahongyu Formation of the Jixian section contained well-preserved silicified microfossils, mostly are spherical microfossils and filamentous microfossils. Non-filament microfossils were colorful, besides coccoid-dominated mat, including palmella, coenobium and single cell fossils. Some of these microfossils have complex morphology and ornaments, such as multilayer wall, rough surface, reticulate ornaments, spiny ornaments, open texture and fusiform morphology and so on. Many of these complex microfossils may be eukaryotes. Leiosphaeridia sp. which is smooth-surfaced spherical fossil is very rich. Among them greater than100μm ones are quite common, and some are up to600μm in diameter. Generally speaking, microfossils in carbonate rocks mostly are small. Thriving of large individuals in carbonate rocks is a very rare phenomenon in the Yanshan basin and other Proterozoic carbonate strata. Filaments are preserved very well, including Siphonophycus, Oscillatoriopsis and so on. They mostly appeared in colony and filament-dominated mat was also abundant. The Dahongyu microbiota is comparable to other silicified microbiotas from the Jixian Group. It is distinct from the Dahongyu microbiota from the Pangjiapu section, because there is no gunflint element has been found in the Jixian section.2) Nanofossils from the Gaoyuzhuang Formation (Qianxi section):Via field observation and sample collection of carbonaceous compressions in the Gayuzhuang Formation of the Qianxi section, we found that most of the carbonaceous compressions are irregular and unstable in morphology. We observed these samples under scanning electron microscope and found that the carbonaceous macrofossils are mostly composed of EPS (Extraceller Polymer Substances) and filaments. No evidence of metaphyte has been found. Thus, indicating that the Gaoyuzhuang carbonaceous compressions are not multicellular fossils but mat fragments. During the observation of these carbonaceous compressions, we found a new microbiota which is composed of nannofossils. Most nanofossils are smaller than10μm. There are smooth-surfaced spherical microfossils, Tappania-like mcirofossils, cauliferous-bearing spherical microfossil, acanthomorphic microfossil, vase-shaped microfossil and several unknown types. Most microfossils are tiny, and some of them maybe eukaryotes.3) Silicified microbiota from the Gaoyuzhuang formation (Jixian section):Biological sedimentary structures are development in Mesoproterozoic strata. However, biota study from most of these sedimentary structures is blank except for stromatolites. We got a microbiota which preserved in one of these Proterozoic sedimentary structures. There are some silicious globular nodules which occurred accompanied with Molar-tooth structures in the third member of the Gaoyuzhuang Formation. Some scientists interpreted them as spherical Molar-tooth structures, and some scientists believe that they are macro-algae body fossils. By field study and thin section observation, we presume that they are formed because of the undersaturation of silica. Colloid silica deposited on the depositional interface and they are not sufficient to form bedded layer and collapsed into dispersed spherical or ellipsoidal nodular. When colloid silica was more, they can form prolate ellipsoid or small blanket. We found a diversified microbiota inside these silicious globular nodules. There are spherical fossils, acanthomorphic microfossils, tubular ornamentated spherical microfossils, fusiform microfossils, vase-shaped microfossils, multicellular filaments and some other complex types. Many of them maybe are eukaryotic. Highly diversified filaments are characteristic elements of this microbiota. Planktonic taxa are flourished and benthic microbial mats are not developed. These phenomenons may be caused by deep water environment.4) Silicified microbiota from the Wumishan Formation (Lingyuan section):The Wumishan biota in the Lingyuan section is a microbiota with highly diversified plankton. It contained a variety of spherical, oval and fusiform microfossils. However, compared to other microbiotas of the Jixian Group, this microbiota has fewer filaments, and filament dominated mat was very rare in the Lingyuan section. In order to dissect microbial mats biomes and explore the significance of allochthonous, we compared the proportions of plankton and benthos with other Wumishan microbiotas from four different localities. Proportion of plankton diversity were71%in the Lingyuan section,44%in the Kuancheng section,41%in the Jixian section,38%in the Xishan section and27%in the Ming Tombs section. By analysis the relation of lithic faces and proportion of plankton diversity, we found that the proportion of plankton diversity is related to the water depth. Deep water environment is appropriate for flourishing of plankton. Stable and deep water is suitable for planktonic microfossils. According to the study of microfossils, and combining with the water depth information which is drawn from stromatolites, we built a model of the relationship between microfossils and water depth.Via biota investigations, we got a lot of fossil data. Combined with reported informations, we summarize all fossil record from the Changzhougou to Changlongshan Formation, and found that the microbiotic replacement experienced three different developmental stages. We discussed the biotic faces of the three main stages of the Yanshan basin development. Three stages are:stage A (from the Changzhougou Formation to the Chuanlinggou Formation which is dominated of clastic sediments), stage B (from the Tuanshanzi Formation to the Wumishan Formation which is dominated of carbonate sediments) and stage C (from the Hongshuizhuang Formation to the Changlongshan Formation which is dominated of clastic sediments). Through analysis and discussion of fossil records, we found that microbiota was very monotonous in the stage A. Although big individuals are rich, most fossils are smooth-surfaced, only a few of them have concentric striation ornaments. In the past, stage B is considered that lack of evolved organisms, but this study suggests that there are a lot of complex forms and the biota have a high degree of diversification.Biota of stage B is the focus of our study. The following understanding have been obtained in this study:Biota of stage B has a high diversity and abundance; Silicified microbiota is not like the traditional understanding that lack of evolved types, it contains a variety of plankton with complex morphology and ornamentation; Microfossils in stage B are generally small, and lack of big individuals (except the Dahongyu Formation).Microbial microbiota, which are dominant elements of the stage B, are lacking of changes on their morphology. So their replacement characters haven’t been attached importance to. Based on the dissection of the biocommunity, we made statistic and analysis of the abundance, diversity, plankton diversity/benthic diversity, average diameter of benthic spherical fossils, maximum diameter of spherical fossils, average diameter of single-celled microfossils and distribution of diameters. The results show that from the Tuanshanzi to the Wumishan Formation, the biota community structure, dominated elements and character elements all obviously changed. This study is the highest resolution of the biota changes of the Mesoproterozoic. Through the analysis of statistical indicators, a number of new understandings have been obtained, and two biota succession events have been preliminarily raised.The first event occurred in the third member of the Dahongyu Formation. Within this period big plankton individuals are very prosperous. And maximum diameter of this microbiota was significantly increased. Plankton fossils are highly diversified. All these changes are presumably associated to volcanic activity.The second events occurred in the Gaoyuzhuang third sub-formation (the third member). The Gaoyuzhuang third subformation is a key period for microbiotic change. In this period average diameter of benthic organisms (non-filaments) significantly rise, indicated that the dominant benthic taxa changed. Bigger individual began to flourish. Plankton diversity proportion is considerably higher and biota faces replacement occurred here. These changes are congruent with stromatolites decline event, change of carbon isotope and ocean geochemical indicators in the same horizon. All information indicated that important geological events occurred in the third member of the Gaoyuzhuang Formation.Another important aspect of the biota study is to explore the eukaryote’s occurrence and development. Mesoproterozoic was an important period for biological evolution. From the evolutionary history showed by the existing fossil record, we knew that many important evolutionary events, including eukaryotes diversification, sexual reproduction, multicellular body, animal and plant differentiation maybe all occurred in Mesoproterozoic. And the most direct and effective way of this exploration is to look for fossil evidence.Mesoproterozoic was time for prokaryotes and stromatolite reefs are very common in carbonate sediments of this time. However, most possible eukaryotic fossils were found in black shales, mainly are acritarchs and carbonaceous compression. Although carbonates were not deposited in a suitable environment for eukaryotes, we still found a lot of possible eukaryotes in silicified carbonates these years. In carbonate rocks, in the case of most fossils are single cell, extract new feature for identifying eukaryotes is essential. In our investigation, we have found chlorophyta, ornamented fossils, acanthomorphic microfossils, fungi, vase-shaped microfossils and fusiform microfossils. Among them, the acanthomorphic microfossil, fungi and vase-shaped microfossils are the earliest fossil record. There are two new kinds of nanofossils have been found. One is microfossils with flagella and the other is microfossils with reticulate ornaments. Different generation specimens of the reticulate ornamented ones are preserved very well and three layers of their wall have been observed. These microfossils are confident eukaryotes.We analyzed and discussed these eukaryotes which have been found in our investigations, and made statistics of all possible eukaryotes from carbonates. There are up to20types of eukaryotes in carbonates from the Tuanshanzi Formation to the Wumishan Formation. These fossils record displayed that eukaryotes are diversified in the1600-1400Ma interval.Based on the study of the morphology and growth pattern of microfossils, we recognized several filamentous microfossils that are not phototropism. These filaments are evidence of layering of ancient microbial mat. They represent of different chemoautotrophic microfossils. We discussed their affinity and significance, and consider that they are important for revealing ancient biological functional groups.