«To cite this version: Aihua Yuan. Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China. Pa- leontology. Universit´ Pierre et Marie ...»
14 sub-beds are here considered as the valid horizons according to the statistic standard for the triangular model (Tab.4-2-E, Fig.4-2-B, C). With the highest 80% and lowest 17% of paleopsychrospheric species, the triangular diagram indicates that the Dongpan Section was situated in the outer shelf to bathyal environments. The sub-bed 03DP2-25, in the lowest horizon of the section, presents a rather shallow environment of outer shelf with normal salinity. Then the environment became deeper to the top of the bed 03DP2. In this course, the percentage of the Bairdiidae altered between 0% and 34%, which may imply the variation of the oxygen level and salinity. There is a shoaling between 03DP2-4 and 03DP3-1 before the paleobathymetry attained the maximum among the statistic horizons in 03DP3-7, in which the bairdiid species were not discovered. And then the general shoaling occurred until 03DP5-8, comprising high percentage of bairdiid species and reprenting an outer shelf environment with normal salinity and rich oxygen. From this horizon up, the percentage of paleopsychrospheric species increased and bairdiid species rapidly decreased until 03DP5-19, which may indicate an unsuitable environment for the living of bairdiids. Then, the paleobathymetry became shallower in 03DP10 and the bairdiids re-occurred.
Fig.4-2-B Proportions of paleopsychrospheric, bairdiid and other neritic species by sub-bed (14 valid horizons according to the statistics standard of triangular model) along the Dongpan Section.
Fig.4-2-C Projecting based on the Dongpan ostracod compositions by sub-bed (14 valid horizons according to the statistics standard of triangular model) in the triangular diagram.
110 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 Liuqiao Section Only 5 out of the total 39 species are considered as the paleopsychrospheric species. These species are distributed in eight beds of the whole section. However, there are only three beds could reach the statistic standard and thus discussed here (Tab. 4-2-E). The other beds have 1-3 species or less than 10 individuals. The percentages of the paleopsychrospheric species show a deepening from the bottom up along the Liuqiao Section, shifting from the inner shelf to the outer shelf (Fig.4-2-D). The impressively high proportions of the bairdiid species are mostly ornamented and thick-shelled, whereas mainly acuminate or thin-shelled species in the Dongpan Section. Both the triangular analysis and the morphological characters attribute a shallower environment than the Dongpan Section. And the environment should have normal salinity and high oxygen level by virtue of yielding abundant bairdiids.
Chaohu Section The Chaohu fauna is characterized by the absolute dominance of bairdiids (9 species), accompanied by 2 polycopid species and 3 other neritic species. As mentioned in 4.2.1 and the Tab.4-2-D, the polycopid species are usually found with the paleopsychrospheric species, whereas themselves have not the bathymetric significance. However, the bairdiids in the Chaohu Section usually have very thin and elongated shells which may indicate a relatively deep environment, maybe open-marine. And the composition of simple taxa (Bairdiidae and Bairdiocyprididae) and no-evident variation along the section imply that the environment was stable in the studied area. The composition and species morphology support that the Chaohu Section represented shallower environment than the Dongpan Section and Shaiwa Section. The paleobathymetric relationship between the Chaohu Section and the Liuqiao Section seems difficult to determine. According to the analysis based on the families/superfamilies in 4.1, the Liuqiao Section should be shallower than the Chaohu Section. And the dominance of thin-shelled bairdiids in the Chaohu Section may also indicate deeper environment than the Liuqiao Section yielding most of the ornamented bairdiids.
4.2.4 Integration with other data In the section above, the paleobathymetric variation is interpreted and presumed on ostracod faunas.
As is known, the organism is the direct reflection of its habitat controlled by complex and interdependent factors. However, the interpretation resulted in one aspect is sometimes hard to exclude or distinguish the influence caused by another aspect. In addition, different groups of organism have variable extent of 112 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 response to different environmental factors. In the paleoecological studies, the inevitable loss of original information calls in more risk of the interpretation based on a single group of organism. Thus a multi-proxy analysis is necessary for better understanding the paleoenvironmental features. In this section, the evaluation by ostracoda and other fossils, geochemistry and sedimentology is compared. The integration is evolved mainly in the Dongpan Section which is the most comprehensively studied section in this work.
Sedimentary facies In the Dongpan Section, the paleobathymetry evaluated by ostracod analysis reaches mostly external platform to the upper part of slope. For consummating the evaluation, analysis on sedimentary facies, clay mineralogy and other fossils are discussed here.
The strata of the Dongpan Section have an evidently lithological shift in the bed 03DP6. From the bed 03DP2 to 03DP5, the strata are dominanted by muddy siliceous rocks and siliceous mudstones with bearing abundant radiolarians. The latest Permian strata above 03DP7 changed to silty and calcareous mudstones intercalated with siliceous mudstones (Meng et al., 2005a). This means that the siliceous content descreases from the bottom up along the Dongpan Section and announces the end of the chert event. The bed 03DP6 is considered as deposit during a regression and the the transitional horizon of the environments from being bathyal to neritic (He et al., 2007).
It is noticeable that there are two layers of turbid limestones with the Bauma sequence in the sub-beds 03DP3-18 and 03DP5-16. The turbid current should have origined from shallow and high-energy environment, which can be inferred from the shallow water foraminiferas and the ceratoid intraclastics in the turbiditic limestones. The overlying sub-beds (i.e. 03DP3-16, 03DP3-14 and 03DP 5-17) yielded more diverse and abundant ostracods than the other horizons (Fig.3-1-A). And the proportion of the neritic species increases (Fig. 4-2-A). Especially in the sub-beds 03DP3-16 and 03DP3-14, many neritic species gave their first occurrences along the Dongpan Section, e.g. Cavellina cf.
nebrascensis, Kirkbya sp.1, Libumella spp. and Bairdia spp.. The “new occurring” species are only discovered in the two beds or disappeared until the top of the bed 03DP5 where the presumable regression began. Thus it seems probable that the new neritic comers in the sub-beds 03DP16 and 03DP3-14 were brought by the turbid current from the shallow water environment. Their coming elevated the percentage of neritic species between 03DP3 and 03DP5. In other words, the turbid current may have transported the allochthonous faunas and thus distort the original composition of the assemblage. So it could be inferred that the paleobathymetry of the beds 03DP3 and 03DP5 should have been deeper than that evaluated by the triangular model and may shift to the bathyal stage from the present interpreted depth.
200-500m and thus indicate bathyal to abyssal settings (Kozur, 1993). The Latentifistularia preferred outer shelf or bathyal environment (Fang & Feng, 1996).
In the Dongpan Section, the radiolarians were abundantly yielded and assemblage alterations present the paleobathymetric variation. The following interpretation is based on the published group work and personal communication (Feng et al., 2007a; He et al., 2007; Jin et al., 2007). The radiolarian fauna from beds 03DP2 to 03DP5, named as “Albaillellaria assemblage”, is dominanted by the Spumellaria, Latentifistularia and Entactinaria, accompanied by abundant Albaillellaria. This composition indicates a bathyal environment. A rapidly decline of the Albaillellaria occurred in the bed 03DP6, which implies a drastic change of sea-level. The radiolarian fauna became to be composed of Latentifistularia, Spumellaria and Entactinaria. With a percentage of 39% Latentifistularia, this fauna also presents an outer shelf environment. In the bed 03DP7, the Albaillellaria completely disappeared and the Latentifistularia occupied 50% of all species in the fauna, which may suggest a water depth around 200m.
Then the Latentifistularia disappeared in the bed 03DP8, above which the fauna is only comprised of the Entactinaria and Spumellaria. Thus the upper part of the Dongpan Section was deposited in the inner shelf environment. He et al. (2007) proposed that “at least 150 m of sea-level fall” during the deposition of beds 03DP5 to 03DP12 by comparing the other radiolarian faunas in South China (see p.154 in He et al., 2007).
Then turning back to the results from ostracods, the good consistency can be observed between interpretations by the two groups. Both of the ostracod and radiolarian assemblages indicate a bathyal environment below the bed 03DP6 (if the influence of turbidite is considered for the ostracods) and much shallower settings in the upper horizons. However, there are also some differences between the two interpretations. (1) The interpretation by ostracod assemblage displayed the environments of external platform to the upper part of slope in the bed 03DP10, whereas the radiolarian assemblage presented the characters of inner shelf fauna. The divarication between the two results doubts about the opinion of "150 m of sea-level fall” based on radiolarian assemblage. (2) The ostracod assemblage recorded more detailed fluctuation of paleobathymetry (? sea-level), although some of which should be used very cautiously.
For better integrating, the comparision with radiolarian fauns from the Chaohu Section will be briefly described herein. The ostracod faunas in the Chaohu Section, as mentioned above (Fig.3-1-D & Fig.4-2-H), nearly 90% of species are neritic. Only 2 indetermined polycopid species were yielded. No typical deep water indicators were discovered. The general environment is presumed to be shallow open-marine according to the dominance of thin-shelled and elongated bairdiids. The radiolarian faunas in the Chaohu Section are dominated by Entactinaria and Spumullaria, associated with a few Latentifistularia. This radiolarian fauna is distinguished with the Dongpan radiolarian fauna by the absence of the Albaillellaria and low diversity of the Latentifistularia. Apparently, without yielding the bathyal indicator, the Chaohu radiolarian faunas indicate shallower environments than the Dongpan fauna. Anyhow, the thin-shelled microfossils including ostracods and lithology of siliceous mudstones still suggest a deeper environment than the Meishan Section. Thus the water depth of 60-200m was 114 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 proposed by Gui et al. (submitted). The analysis from radiolarians agrees the interpretations of ostracods.
Mineralogy & Geochemistry The sea-level flunctuation may result in more exposure of the shallow area and then enhance the weathering in the terrestrial region. Thus the increase of terrestrial materials in the sediment can indirectly imply the sea-level change.
In the Dongpan Section, the common minerals are argillaceous minerals (illite, smectite and illite/smectite) and detritus (quartz and feldspar) (Meng et al., 2005b). The X-Ray diffraction analysis indicated that the content of feldspar increases discontinuously in the lower part and evidently above the bed 03DP7 along the section. Generally, the feldspar is transported by the terrestrial influx, although it also possibly results from the volcanic activity. However, in the Dongpan Section, the feldspar was rarely observed in the claystones with volcanic origin and thus considered as the terrestrial material. Then, the increase of the feldspar exactly implies the influx increase of terrestrial material, which means the weathering was enhanced in the terrestrial domain of the studied area.
The influx increase of terrestrial material was also suggested by the geochemistrical analysis. The elements Ti and Al are generally not soluble in the water so that their abundance is not easily affected by post-depositional processes (Brookins, 1988). Thus Ti and Al are considered as excellent indicators to diagnose the source of argillaceous materials. In the Dongpan Section, the concentrations of Al2O3 and TiO2 show a negative correlation with that of SiO2 and increase evidently from the bed 03DP7 upwards.
This implies that the increasing of the terrestrial material influx (He et al., 2007).
In general, the paleobathymetric evaluation interpreted by ostracods is accordant to that presumed through other evidences. Especially in the Dongpan Section, the research on ostracods, radiolarians, sedimentary facies and mineralogy imply the presence of regression. In the Chaohu Section, the ostracod fauna suggest a shallower environment, which is well reflected on radiolarian assemblages.
However, during the interpretation of the triangular model, the necessity of integrating multi-disciplinary research is outstanding. The data may not represent the characters of the original fauna, which could have been disturbed and changed by some geoevents, e.g. the influx of the turbid current. Thus, the interpretation should be made cautiously.
§4.3 Oxygen level reconstruction
The anoxic events have been widely considered as an important reason resulting in the Permian-Triassic mass extinction (e.g. Wignall and Hallam, 1992; Isozaki, 1997). However, the anoxic duration and extent are various in different regions (Wignall & Twitchett, 2002). Thus, there is not a completely unified rule for defining the anoxic events in a section before the related analysis is carried out.