Results and Discussion of the Carboniferous Ganigobis Shale Member 87

The Ganigobis Shale Member belongs to Dwyka Group sediments, which represent the lowest stratigraphic unit of the Karoo Supergroup (Bangert et al., 1999; Himmler et al., 2008). Inside the Ganigobis Shale Member 24 ash fall horizons are recorded, from which 2 tuff horizons near Ganigobis (southern Namibia) were used for age determination on juvenile magmatic zircons (Bangert et al., 1999). They yield ²⁰⁶Pb/²³⁸U ages of 302.0 ± 3.0 Ma to 299.2 ± 3.2 Ma designating the Ganigobis Shale Member as Carboniferous (Bangert et al., 1999). The upper part of the formation is dominated by columnar and lenticular limestone deposits which contain four authigenic carbonate phases, microspar, banded/botryoidal cement, yellow calcite and spheroidal calcite, representing fossil hydrocarbon seep carbonates (Himmler et al., 2008).

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Raman spectroscopy can give additional information about the carbonate phase. Mainly 3 different kinds of spectra can be distinguished identifying the respective carbonate phase (Fig. 33). The spheroidal calcite (sc) shows clear spectra of a normal calcite with the main symmetric stretching vibration of the CO3

molecule at 1084 cm^-1, the translational modes at 153 cm^-1, the librational modes at 279 cm^-1 and the in-plane bend at 710 cm^-1 (cf. Rutt & Nicola, 1974;

Bischoff et al., 1985; Urmos et al., 1991) (Fig. 33A, B). Also the weaker bands of the antisymmetric stretch at 1433 cm^-1 can be observed.

The yellow calcites show the same bands, but the spectra are dominated by very intense bands of stretching vibrations of carbon bonds, centered at 1600 cm^-1 (G band) and 1350 cm^-1 (D band). Remarkable is a clear shoulder on the D band centered at 1270 cm^-1 (Fig. 33A, C, D). Also the second order bands in the higher wavenumber region at 2690, 2940 and 3195 cm^-1 appear. Here, it should be noted that the band at 2690 cm^-1 is not the dominating one. Both, the shoulder on the D band and the only small band at 2690 cm^-1 can be interpreted as a clear indication that these signals are derived from complex organic molecules (Kudryavtsev et al., 2001).

The other carbonate phases give also spectra with both calcite and carbon signal. But here, either no clear domination of one of the two signals is evident, or vibrational bands of the calcite are dominating (Fig. 33E, F, G, H).

That means the microbial influence during the formation of the yellow calcite phase can be demonstrated by the domination of carbon bonds over the calcite signals in the Raman spectra. In the mixed phases which microscopically cannot be allocated to the pure yellow calcite phases, a similar microbial influence is visible, with both calcite and carbon signals. However, the spheroidal calcite phase obviously is a later phase which formed on top or in cracks between the yellow calcite. This is in concordance to the δ¹³C data, which are reported to have higher values for the spheroidal calcite and the lowest values in the yellow calcite (Himmler et al., 2008).

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Figure 33: Raman spectra of the different carbonate cements of the Carboniferous Ganigobis Shale Member. In the spheroidal calcite (sc) only calcite vibrations occur. The yellow calcite (yc) is dominated by carbon vibrations, with additional bands of the calcite. In the other carbonate cements (cc) no clear domination of the carbon signal is visible. The marked points in the transmitted light images (A, E) indicate the area where the Raman spectra were recorded.

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6.3 Results and Discussion from the Oligocene Lincoln Creek Formation and the Holocene cold seeps from the Black Sea

From the Oligocene cold-seep limestone of the Lincoln Creek Formation (Washington State, USA) similar authigenic carbonate phases are reported as the following common paragenetic sequence: (I) micrite, (II) yellow aragonite and clear aragonite, (III) brownish calcite and (IV) equant calcite spar (Peckmann et al., 2002; Hagemann et al., 2013). Here, the Raman spectra are in many cases highly masked by fluorescence, especially the calcite phases.

But most surprisingly the carbonate phases do not show additional carbon bands in their Raman spectra (Fig. 34). In this case that suggests a formation of the carbonate phases due to microbial activity and associated increase of alkalinity. It is likely, that the carbonate phases showing high fluorescence are associated to the microbial activity, which remains are causing the fluorescence itself. Furthermore, small aggregates of framboid pyrite occur, which is partly oxidized to hematite-like iron oxide (Fig. 34C).

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Figure 34: Raman spectra of the cement facies of the Oligocene Lincoln Creek carbonates.

Surprisingly no difference between the clear and the yellow phases can be detected. But the spectra are heavily influenced by fluorescence, which can significantly mask the Raman signal.

The numbered dots in the transmitted light images (A, D) indicate the area where the Raman spectra were recorded.

Similar results could be obtained for cold-seep carbonates from the Black Sea.

The main carbonate phases at this location are: (I) microcrystalline carbonate dominated by High-Mg-calcite and (II) aragonitic cement (Peckmann et al., 2001). Here again only signals from the respective carbonate phase could be observed, or the spectra were completely masked by fluorescence. The strong autofluorescence of the carbonate phases was also reported by Peckmann et al. (2001), even when excited with UV-light. Nevertheless, excitation in this range (244 nm) for Raman spectroscopy could reduce the fluorescent behavior, and even additional carbon and organic bands could be detected (Fig. 35). The

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assignment of these bands is not definite, however, it is proposed that the band at 1412 cm^-1 belongs to vibrations of aromatic units and the bands between 1700 and 1800 cm^-1 arise due to carboxylic acid (cf. Smith & Dent, 2005). As already mentioned in chapter 1 a differentiation between the carbonate phases (calcite and aragonite) normally is not straightforward with UV excitation.

However, the carbonate signal in this example is rather strong and also the band in the higher wavenumber region is clearly visible. As this band occurs at 1462 cm^-1 and there are two bands visible at 706 and 853 cm^-1, it can be assumed that the carbonate phase is aragonite rather than calcite (cf. Frech et al., 1980; Urmos et al., 1991).

Figure 35: Raman spectra of the cement facies of the Holocene Black Sea cold seep. With 488 nm excitation wavelength aragonite and calcite phases, but no additional carbon could be detected (A, B). Additionally, a typical spectrum of pyrite is shown (C). With excitation in the UV (244 nm) the fluorescence could be reduced and several organic signatures could be detected (D). (arom. = aromatic units; carbox. acid = carboxylic acid).

Interestingly, for the younger cold seep structures of Lincoln Creek and the Black Sea the yellow carbonate phase is aragonite, whereas in older samples it is calcite. Also a much stronger fluorescence behavior can be observed.

However, the use of excitation in the UV seems to be a promising tool for gaining additional information on the nature of the organic signatures in the yellow carbonates.

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6.4 Results and Discussion for the Ediacaran Doushantou cap carbonates,