General paleoenvironmental interpretations with ostracod tool

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Chaohu Section (CH)

The Chaohu Section (i.e.West Pingdingshan Section, the GSSP candidate of the Induan-Olenekian boundary) is situated 5km northwest of Chaohu City, Anhui Province. There are railway and highway to connect with other big cities, e.g. about 60km southeast to Hefei City, the capital of Anhui Province (Fig.1-1-C (a)) (Tong & Zhao, 2005; Gui et al., submitted).
During the Late Permian, the Chaohu area was situated in the northern margin of the Lower Yangtze basin (Fig.1-B). The stratigraphical sequence is exposed from the Upper Precambrian (Sinian) to Middle Triassic except for the absence of the Lower-Middle Devonian. This area began to receive
14 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5
terrestrial instead of marine sediments or be eroded since the Middle Triassic. All strata were folded as the Mt.Majiashan-Mt.Pingdingshan Syncline during the Indosinian Movement in the Late Triassic (Tong & Zhao, 2005; Tong et al., 2005; Li et al., 2007; Zhao et al., 2008).

Permian-Triassic Boundary

This study is focused on the latest Changhsingian strata. Thus the determination of Permian-Triassic boundary (PTB) is the first important problem.
In the Dongpan Section, the presence of the Neoalbaillella optima radiolarian zone in the beds from 03DP2 to 03DP 6 and ammonoids Huananoceras cf. perornatum Chao and Liang, Laibinoceras cf. compressum Yang, Qianjiangoceras sp. at the top of the bed 03DP12 indicates the Changhsingian age of the Dalong Formation. The first occurrence of the typical Triassic ammonoids Ophiceras tingi Tien and bivalve Claraia dieneri Nakazawa at the base of the bed 03DP13 gives the Luolou Formation Early Triassic age. Consequently, the PTB in DP Section is placed between the beds 03DP12 and 03DP13 (Feng et al., 2004, 2006, 2007a; He et al., 2005; Meng, 2005; Jin et al., 2007; Zhang et al, 2007a, 2007b) (Fig. 1-2).
In the Liuqiao Section, the radiolarian Neoalbaillella optima zone assigns the strata to the Changhsingian.Albaillellaria is present from the bottom up along the whole section, which disappears above the bed 03DP6 of the DP Section. Furtherly, Albaillellaria flabellata was recognized in the top of the bed 03DP2 of the Dongpan Section but not found in the Liuqiao Section. Thus the Liuqiao Section should be underlying the top of the bed 03DP2 of the Dongpan Section. In addition, our investigation in the studied area (group work, unpublished data) indicates that there are still at least 5m strata between the bottom of Dongpan Section and the top of the Liuqiao Section. In another word, all the strata of the Liuqiao Section underlie the Dongpan Section and belong to the Changhsingian (Fig. 1-2).
The Member IV of the Shaiwa Section is constrained to the late to latest Changhsingian based on biostratigraphical studies. The presence of the ammonoids Ophiceras sp. and the bivalve Claraia sp. determines the PTB between the Shaiwa Group and the Luolou Formation (Yang et al., 2000; Gao et al., 2001, 2005; Feng & Gu, 2002; Chen et al., 2006).

Siliceous rocks & calcareous/silicified/pyritized ostracods

The ostracods studied here were obtained from the hydrofluoric acid (HF) technique, which is a special method for extracting the radiolarians from the cherts (Pessagno and Newport, 1972). Some detail procedures are improved in our own practical operation depending on the laboratory conditions and nature of rocks. In fact, the use of hydrofluoric acid in micropaleontology is not new, Wetzel (1921) recommended it as a preparatory technique to extract calcareous microfossils from non-calcareous rocks. The process, which transforms the opaque calcium carbonate into translucent calcium fluoride, was termed as “fluoridization” by Upshaw et al. (1957). The pseudomorphs of the fossils are believed to be a molecule-by-molecule replacement and reveal the calcareous skeletal and fine sculptures very well (Schallreuter, 1982).
Equipments: ventilated cabinet, hammer, plastic beakers, meshy bags, funnel, filter paper, large plastic container for acid recycling, large container for rinsing (e.g. basin), 300 mesh sieve, rubber gloves. Chemicals: hydrofluoric acid, blue methylene powder, acetone.
The same as the calcareous rocks, the siliceous samples should be crushed into fragments (several cubic centimetres, like the size of walnut) so as to increase the surface area and further accelerate the reaction. After crushed, each sample was put into a meshy bag (made from nylon window screening or something like that. It provides convenience of changing the acid) and then place in a plastic beaker (beaker 1) labelled with the sample number. At the same time, for every sample, another plastic beaker (beaker 2) should be prepared and labelled for containing the residua.
All beakers 1, in which all samples are settled, are placed in the ventilate cabinet. According to the content of silica, generally 2% to 5% concentration of HF water solution was confected. During the process, the concentration can be changed according to the disaggregation status of samples. Second day: The HF solution is very muddy and generally will not be recycled. If the sample contains much mud, it is necessary to gently rinse the meshy bag under the water. Then in the beaker 1, new diluted HF solution is poured to immerge the samples.
After about 12 hours: The excess HF solution is decanted to another container for recycling (considered as lower concentration e.g. half concentration in the next use). The meshy bag with samples is transferred to the beaker 2. The beaker 1 with the deposit is then filled with water (or add some soda ash) to reduce acidity so as to prevent destroying the fossils (water inpouring gently). The aggregate samples in beaker 2 are again immerged with new diluted HF solution. After another 12 hours: The deposit in beaker 1 has sunk to the bottom. Pour out the water on the top of the deposit. Transfer the new deposits in beaker 2 to the beaker 1 and again fill with water. The aggregate samples in beaker 2 are immerged with the new diluted HF solution. Every 12 hours, this step is repeated until enough deposit is collected.
The deposits are sieved through 300 meshes. Gently transfer the residua in beaker 1 onto the sieve. Immerge the bottom of the sieve into a large container (e.g. basin) filled with water. Shake the sieve gently with hand and accelerate the diffusion of the mud until the water flowing is clean. Transfer gently the residua into beaker 1. Place the beaker 1 in the heating-store or let it air-dry for picking. According to our experiment, if not used for next sample rinsing, the sieve with residua was directly put in the heating-store. This way effectively avoids the conglutination between residua and beaker/pot. The residua can be completely and readily collected. The more important is that the destroy to fossils due to multi-transmit was greatly reduced.

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Dongpan Section (DP)

a84 samples were collected from the Bed 2 to Bed 12 of the Dongpan Section (Fig.1-B). 41 samples from mudstones, siliceous mudtones, muddy siliceous rocks and bedded siliceous rocks yielded more than 1,600 ostracods (remarks: some samples were combined or added to samples in Yuan et al. (2007)). In general, ostracods were abundantly yielded and diverse in some samples but majority of them were in very poor preservation. 28 genera and 50 species with 2 new species Bairdia dongpanensis Yuan & Crasquin-Soleau, 2007 and Spinomicrocheilinella anterocompressa Yuan & Crasquin-Soleau, 2007 distributed in 26 samples (mainly below the Bed 6) were recognized (Fig.3-1-A). There are still many undetermined specimens excluded in this work.

Liuqiao Section (LQ)

38 samples were collected along the Liuqiao Section (Fig.1-B) and processed. About 480 ostracods were yielded from 20 samples of siliceous mudstones and muddy siliceous rocks. The ostracod fauna has not so high abundance as in the Dongpan Section, but has rather high diversity. 21 genera and 39 species (1 new genus Denticupachydomella n.gen. and 1 new species Denticupachydomella spinosa n.sp. are proposed) have been recognized (Fig.3-1-B).

Shaiwa Section (SW)

The Shaiwa section (Fig.1-B) was sampled according to each sub-bed, 87 samples collected in total. Only 16 samples from siliceous mudstones yielded few ostracods (about 250 individuals) in very poorly preservation. 14 genera and 37 species (1 new species Pseudobythocypris guiqianensis n.sp. is proposed) were identified (Fig.3-1-C). Some of them are still in open nomenclature due to the poor 90 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 preservation and too rare material. In all studied sections, the Shaiwa fauna has the lowest abundance.

Table of contents :

Chapter 1 Regional Geology and Stratigraphic Correlation
§1.1 Description of sections
1.1.1 Dongpan Section (DP) & Liuqiao Section (LQ)
1.1.2 Shaiwa Section (SW)
1.1.3 Chaohu Section (CH)
§1.2 Stratigraphic correlation
1.2.1 Permian-Triassic Boundary
1.2.2 Section Correlation
Chapter 2 Taxonomy of Ostracoda
§2.1 Generality on ostracods
2.1.1 Soft parts of living ostracods
2.1.2 Reproduction and ontogeny
2.1.3 Shell morphology
2.1.4 Principles for orientation and measurement
2.1.5 Adopted taxonomic classification (Fossil Ostracods)
§2.2 Methodology
2.2.1 Sampling
2.2.2 Extracting
2.2.3 Picking, scanning and identifying
§2.3 Systematic descriptions
Chapter 3 General features and distribution of ostracod faunas
§3.1 Ostracod distribution in studied sections
3.1.1 Dongpan Section (DP)
3.1.2 Liuqiao Section (LQ)
3.1.3 Shaiwa Section (SW)
3.1.4 Chaohu Section (CH)
§3.2 General features and correlation with previous studies
Chapter 4 Paleoecological analysis
§4.1 General paleoenvironmental interpretations with ostracod tool
§4.2 Paleobathymetric variation
4.2.1 Introduction of concept
4.2.2 Adopted model
4.2.3 Paleobathymetric evaluation on ostracod faunas
4.2.4 Integration with other data
§4.3 Oxygen level reconstruction
4.3.1 Oxygen level reconstruction by other data
4.3.2 Generality on ostracod alimentation mode
4.3.3 Oxygen level reconstruction on ostracods
4.3.4 Discussion
Chapter 5 Was there an « Extinction Event » in Deep Water Ostracod Faunas?
§5.1 Review on ostracod extinction during Permian-Triassic events
§5.2 “Extinction event” in the studied ostracod faunas
5.2.1 Discussion on several hotspots related to the extinction event
5.2.2 Concrete discussion on Dongpan and Liuqiao faunas
5.2.3 Comparison and Discussion
Chapter 6 Conclusions and Perspectives
§6.1 Conclusions
§6.2 Perspectives
Appendix 1: Index of figures and tables
Appendix 2: Alphabetic index of identified species in this dissertation
Appendix 3: Published paper
Plate & Plate Captions


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