«To cite this version: Aihua Yuan. Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China. Pa- leontology. Universit´ Pierre et Marie ...»
In order to investigate if the anoxic event occurred in the Dongpan Section, the studies on 115 2008/5 PhD dissertation of University of Pierre Marie Curie & China University of Geosciences (Wuhan) geochemistry, foraminifera assemblage and trace fossils have been already put in practice. In this section, to testify the validity of ostracod faunas in oxygen-level reconstruction, the FF% method, which has been successively applied in neritic ostracod faunas (detailed introduction see below 4.3.2), is firstly attempted here for the deep-water fauna.
4.3.1 Oxygen level reconstruction by other data
The studies on geochemistry, foraminifera assemblage and trace fossils have been conducted in the Dongpan Section. However, the data educed different results for the oxygen level flunctuation. As follows, the different analyses will be presented.
In the Dongpan Section, several redox-sensitive trace elements were chosen for analysis, including Co, Cr, Cu and V. Generally, these elements concentrate in sulphides or become adsorbed by organic compounds. Therefore, the high concentration of these elements indicates low oxygen concentration in the sea water. In the practical interpretation, the ratios of these elements to Al are considered as the indicators for oxygen concentration. The ratio of V/Cr is also indicative to the redox conditions (in anoxic conditions: V/Cr4.25; dysoxic conditions: 2V/Cr4.25; oxic conditions: V/Cr2) (Johns & Manning, 1994). In the Dongpan Section, neither of the ratios of Co, Cr, Cu and V to Al indicate consistent decrease in oxygenation along the section. From the bed 03DP2 to the lower part of 03DP5, the value of V/Cr is between 2.00 to 4.00, indicating dysoxic conditions. During this interval, there is a peak value at the top of the bed 03DP4 to the lower part of bed 03DP5. Above this anoxic peak, the value of V/Cr decreases to lower than 2.00, indicating a normal oxygenation (Fig.4-3-A) (He et al., 2007).
In addition, the negative abnormity of Ce is indicative to the oxygen deficiency. The rare earth element Ce has two variable valencies. In the oxygen conditions, Ce3+ is usually oxygenated to Ce4+ and deposited in the form of unsolvable CeO2. Thus the content of Ce in the sea water will be greatly depleted and appears as the negative anormity (Wright & Holser, 1987). Ceanomy-0.1 indicates the normal oxygenate conditions and Ceanomy-0.1 implies the anoxic environments. In the Dongpan Section, the analysis of Ce indicated two evident negative abnormities which indicate the possible oxygen deficiency, respectively in 03DP2 and 03DP6 to the lower part of 03DP7 (Fig.4-3-A) (Zhang, 2006).
The role of foraminifer assemblage in oxygen level reconstruction is effective in composition of the test. In modern Oxygen Minimum Zone (OMZ), the calcareous foraminifers dominate instead of the agglutinated foraminifer. The ratio of calcareous to agglutinated foraminifers drops with the oxygen increase (Gooday et al., 2000). Thus the calcareous foraminifers seem prefer the low oxygen conditions.
In the Dongpan Section, below the bed 03DP4, the foraminifers have low diversity. In the beds 03DP4 and 03DP5, the foraminifers are dominated by the calcareous species. Above the bed 03DP6, the agglutinated foraminifers increase and become dominant. In the bed 03DP10, there are only agglutinated foraminifers. Consequently, there may be an oxygen deficiency in the beds 03DP4 and 03DP5, above which the oxygen level increased and thus would be suitable for the living of agglutinated foraminifers 116 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 (Zhang, 2006; He et al., 2007).
The trace fossils were collected between 03DP5 and 03DP8. Abundant Chondrites and a few small-sized Zoophycos were recognized (unpublished group work). The Chondrites is considered as a common trace fossil during the change of oxic-anoxic-oxic conditions. In the Dongpan Section, Chondrites was yielded in the whole investigated interval. The other taxa were discovered between the upper part of 03DP5 and 03DP7. Thus this indicated the occurrence of anoxic event between 03DP5 and 03DP7. And the small size of the Zoophycos and other taxa may also imply the oxygen deficiency.
In general, the interpretations based on different data indicate the presence of oxygen deficiency in certain horizons of the Dongpan Section. However, the analysis is not completely accordant with each other. The trace elements and foraminifers indicate that the oxygen deficiengy took place in the horizon of 03DP4 and lower part of 03DP5, whereas the negative abnormity of Ce and trace fossils prefer an anoxic event in 03DP6 and 03DP7. As follows, the interpretation on ostracods will be attempted for oxygen-level reconstruction and then compared with the results above.
4.3.2 Generality on ostracod alimentation mode
respiration during the oxygen reduction. Ostracods lack of the specific respiratory organs (except the Mydocopida with gills). The oxygen interchange takes place in the venter by the brachial plates. The Platycopida possesses larger numbers of branchial plates than the Podocopida. For the alimentation of the Platycopida, the branchial plates circulate the water over their ventral respiratory surface. Then the numerous tufts of setae of the appendages sieve the suspension and transport the fine particles to the mouth. In other words, besides the normal respiration, the Platycopda could also access the oxygen from the circulated water during the feeding. In the anoxic conditions, evidently, this feeding mode greatly enhances the respiration of the Platycopida than the Podocopida with the deposit-feeding mode. In addition, the reproduction of Platycopida as described in 2.1.2, retain the eggs and nauplius in the expanded posterior brood space. This means that the platycopid individual began to benefit the feeding mode since hatch and in the earliest instars and then greatly increases the survival potential in anoxic conditions. The third advantage for the Platycopida is that, during the anoxic conditions, there are more organic suspensions in the water which supplies more food for the filter-feeders and thus enhances their survival potential.
Fig.4-3-B The percentage of filter-feeding ostracod species and approximately calculated oxygen levels in relationship to paleoenvironments and individual abundance proposed by Lethiers & Whatley (1994)∗.
Through the studies on neritic Devonian ostracods, Lethiers & Whatley (1994) demonstrated the availability of the ostracod feeding way in paleo-oxygen level reconstruction. This model was attempted in the area where the environment was well studied. Compared with the post-Paleozoic studies, the Palaezoic filter-feeders contain the Palaeocopida, Platycopida and Metacopida (the Kloedenelloidea and Paraparchioidea were additionally listed in Lethiers & Whatley (1994) and assigned to the Palaeocopida here). The Bairdioidea and Bairdiocypridoidea were regarded as deposit-feeders. Then the relationship between the percentage of filter-feeding species and oxygen level was proposed accordant to the abundance of ostracod fauna and sedimentary facies (Fig.4-3-B). This method is abbreviated as the FF% method herein.
Before applying the model to the studied area, it is necessary to unify the criteria of dividing the oxygen level. Whatley et al. (2003) divided different oxygen levels according to the percentages of the Platycopid specimens. Yan & Liu (2007) proposed the division based on the biofacies. In the modern ocean, the normal oxygen concentration is around 5-6ml/l at the atmosphere pressure and a temperature ∗ The FF% model was firstly applied on modern ostracods and then on Cenozoic-Jurassic ostracods.
118 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 between 10° and 20°and falls to less than 4ml/l in the oxygen maximum zone (3ml/l is typical) (Whatley, 1991; Garrison, 1998). In this work, the concepts proposed by Yan & Liu (2007) are adopted and the division interval is referred to the modern ocean data, i.e. the oxygen concentration of more than 4ml/l is regarded as the oxic condition, between 4ml/l and 0.2ml/l dysoxic condition and less than 0.2ml/l the anoxic conditions (Tab.4-3-A).
4.3.3 Oxygen level reconstruction on ostracods Until now, the FF% method has not been applied in interpretation of deep water ostracod faunas.
In Lethiers & Whatley (1994), the paleopsychrospheric species are excluded from the test of the FF% method since it is difficult to decide to which alimentation mode (deposit-feeding or filter feeding) they belong according to the morphology. It is true that some paleopsychrospheric species belong to the deposit-feeding in higher taxa level (e.g. Spinomicrocheilinella spp.), but some to the filter-feeders (e.g.
Healdia spp.) and the others are ambiguous in alimentation mode (e.g. Polycopidae). There is not certain correlation between paleopsychrospheric species and filter-feeders, two concepts based on different division criteria. And there is neither data of modern species (psychrospheric) and nor of paleopsychrospheric species in geological time.
In this work, I try to reconstruction the oxygen level in studied area with also consideration of paleopsychrospheric species. Following to Lethiers & Whatley (1994), the Palaeocopida are regarded as filter-feeders. The Metacopina ostracods, also belonging to paleopsychrospheric elements in this work, Healdia spp. (with long spines), Healdiopsis, Marginohealdia, Waylandella, Triplacera, Absina, are designated to the filter-feeders. All the bairdiid species including the paleopsychrospheric elements Spinobairdia, Acratia, Cooperuna are assigned as the deposit-feeders by their bow-shaped ventral projection. The Bairdiocypridoidea genera Microcheilinella, Spinomicrocheilinella, Denticupachydomella n. gen., Rectonaria, most species of which are paleopsychrospheric elements, are considered as the deposit-feeding ostracods. The seven undetermined Podocopida species are assigned as deposit-feeders. Ballent & Whatley (2000) demonstrated that the Cytheroidea live in shallow marine conditions with clear and well-oxygenated water. This indicates that the Cytheroidea may be used to oxic conditions. So the cytherids (Bythocytheridae indet., Monoceratina, Bohemina, Paraberounella) 119 2008/5 PhD dissertation of University of Pierre Marie Curie & China University of Geosciences (Wuhan) are considered as the deposit-feeders. As to the Myodocopida, the modern Myodocopa possess the gills/longitudinal canals for oxygen interchange and some lives the filter-feeding life (e.g.
Cylindroleberidoidea, Holmes & Chivas, 2002). In the studies on Devonian ostracods, Lethiers et al.
(1998) indicated that Entomozoidea thrive in dysoxic waters but leave when anoxia takes place. Then it seems be of filter-feeders. However, due to their pelagic life style, the Entomozoidea and Polycopidae are here excluded from the primary statistics and will be taken into account as referrible materials for discussion.
Tab. 4-3-B shows the species number for each feeding mode. Generally, in studied area, the percentage of the filter-feeders is very low with an average value of 28.1%. In the Liuqiao Section, the fauna is composed of 38.5% of filter-feeders, the maximum value in studied area. In the Chaohu fauna, there was not any filter-feeder discovered. According to the FF% model, the dissolved oxygen concentration can be interpretaed as higher than 4.8ml/l. This means that the studied area is not anoxic at all and even not dysoxic. The following statistics are carried out for the bed/sub-beds with more than 2 species and 10 individuals.
The Dongpan fauna, with an average composition of 32% filter-feeders, is analysed for 19 sub-beds (Fig. 4-3-C). The maximum percentage is 62.5% in the bed 03DP4. In the next place, the sub-beds 03DP3-16 yielded 50% and 03DP3-1 composes of 42.9% filter-feeders. In the other sub-beds, the percentages are lower than 33%. Interpreted by the FF% model, the “most drastic dysoxia” in the Dongpan Section occurred in the bed 03DP4 with an oxygen concentration of about 3.2ml/l. The sub-bed 03DP3-16 has an oxygen concentration around 4.1ml/l and 03DP3-1 of 4.6ml/l. In the other sub-beds, the oxygen concentration is over 5ml/l. In other words, in the Dongpan Section, except the bed 03DP4 in which a moderate dysoxic condition is interpreted, the general environment is oxic. In addition, the increase of terrestrial influx may disturb the normal metabolism as indicated by He et al.
(2007). They demonstrated that the input of terrestrial materials was a reason inducing the brachiopod miniaturization. Brachipods are sessile filter-feeders. They use their lophophore to filter organic particles from the sea water. The influx of inorganic terrestrial material may clog the lophophore and then decrease its filter efficiency, which leads to the brachiopod miniaturization. For the ostracod fauna, there is not the phenomenon of general miniaturization was observed in studied sections. However, the 120 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 input of terrestrial materials may have also influenced the living of filter-feeders because the anoxic event in the bed 03DP6, implied by the Ce abnormity and trace fossils, was not reflected in the ostracod fauna (based on applicability of the FF% model in the deep-water fauna).
Fig.4-3-C Diagram showing changes in the percentage of filter-feeders in the Dongpan Section (19 valid sub-beds) (the data of oxygen level according to Lethiers & Whatley, 1994).
Among the studied sections, the Liuqiao Section has the highest general composition of filter-feeders (38.5%) although it also indicates the oxic condition. There are only three beds satisfying the statistic possibility (Tab.4-3-C). The highest percentage of the filter-feeders is 40% in the bed LQ16, which yielded the most diverse fauna in the Liuqiao Section. The bed LQ3 did not comprise filter-feeder and indicate a rather well-oxygenated environment. The general high percentage of the filter-feeders in the Liuqiao Section results from the 100% filter-feeders in some beds with only the single filter-feeding species. Consequently, the oxic conditions are concluded for the Liuqiao Section, whereas several times of oxygen depletions might occurr.