Teaching Slides: Carbon isotopes and stratigraphic correlation
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5.2: Steady-state variation in carbon isotopic composition of calcium carbonate (δ13C) and organic matter (δ13Corg) as a function of their relative proportion in the sediment burial flux, assuming carbon input into the ocean-atmosphere is constant at -6‰ VPDB. δ13C from the present ocean is ~0‰, implying a burial flux ƒorg = ~0.2, assuming steady state. Observed pre-Marinoan δ13Ccarb values ~5‰ for long periods (107-108 years) implies ƒorg = ~0.4.

5.3: Published δ13Ccarb values over geological time (Shields & Veizer, 2002). Note persistent mean values 0-3‰, and anomalous variability 2.3-2.0 and 0.8-0.6 Ga, broadly correlative with snowball earth episodes.

5.4: Composite δ13Ccarb record from 820 to 490 Ma, based on carbonate-dominated successions in Australia, Canada, Namibia, Morocco and the western U.S. (modified after Halverson et al., 2005). Cambrian “explosion” in faunal (and floral) diversity from Knoll & Carroll (1999).

5.5: Stratigraphic architecture, generalized sedimentary facies, U-Pb ages of volcanic zircons and composite carbon isotope profile for the Otavi Group, projected onto a north-south cross-section of the carbonate platform and its southern foreslope. Note rift-drift transition at the base of the Ombaatjie Formation. The 635-Ma Ghaub glaciation occurred after the rift-drift transition and the older Chuos glaciation before it.

5.6: Comparison of δ13Ccarb profiles for the Abenab Subgroup, deposited between the older and younger Cryogenian glaciations, on the outer, middle and inner parts of the Otavi Group platform. Note consistent and progressive increase in values from outer to inner areas, and inflation of the isotopic gradient at the time of the rift-to-drift transition (base of Ombaatjie Formation).

5.7: Columnar sections of the Ombaatjie Formation from south to north across the Otavi carbonate platform in Namibia (after Halverson et al., 2002). Datum (red line) is the 0 (VPDB) crossing in the Trezona δ13C anomaly. Note the local relief (<60 m) on the sub-Ghaub glacial erosion surface but the regional-scale paraconformity between the erosion surface and the datum. There is no evidence of structural rotation coincident with the Ghaub glaciation.

5.8: Large (>10‰) downward gradients in δ13Ccarb are observed beneath the Stelfox (Canada), lower Polarisbreen (Svalbard), Ghaub (Namibia), Elatina (Australia) and Port Askaig (Scotland, not shown) glacials. They occur beneath the younger glacial in Australia (type Trezona isotope anomaly), Canada and Namibia, but beneath the older glacial in Svalbard and possibly Scotland. Is there one Trezona anomaly that can thereby be used for global correlation, or did Trezona-like anomalies (variably preserved) precede both Cryogenian glaciations? As the anomalies occur in relatively shallow-water platformal sections, they must lead any large sea-level fall associated with ice-sheet buildup.

5.9: Younger Cryogenian (Marinoan) cap-carbonate sequence (Maieberg Formation) near Ombaatjie on the Otavi Group carbonate platform, NW Namibia. Top of the depositional sequence is the first subaerial exposure surface above the glacial horizon. At 400 m, this sequence is 16x and 80x thicker than those in the underlying and overlying formations, respectively.

5.10: Lithologic column with corresponding carbon and oxygen isotopic records for the upper Ombaatjie through lower Elandshoek Formations, near Ombaatjie, Otavi Group platform, Namibia.

5.11: Regional variation in thickness, lithology and carbon isotopic record of the Keilberg cap dolostone and overlying limestone across the Otavi Group platform (above) and southern foreslope (below), Namibia.

5.12: Carbon isotopic record of the Keilberg cap dolostone on the Otavi Group platform, middle foreslope and deep foreslope. Is the apparent offset in values related to water depth or secular change (and diachronous deposition)?

5.13: Carbon and oxygen isotopic records (scaled for thickness) of the younger Cryogenian post-glacial cap dolostones in Australia, Canada, Namibia and Svalbard. Similarity of carbon isotopic profiles within and between regions suggests that they record seawater values and were not grossly altered during diagenesis, as proposed by different research groups.

5.14: Carbon and oxygen isotopic records for the younger Cryogenian (Marinoan) cap-carbonate sequence (Tsabisis Formation) in the Naukluft allochthon, NW Kalahari craton, Namibia. .

5.15: Older Cryogenian (Sturtian) post-glacial cap-carbonate sequence (Rasthof Formation near Omukutu, upper Hoanib River, Otavi Group platform, Namibia. The thin glacials (Chuos Formation) at this location consist of proglacial quiet-water deposits with ice-rafted debris.

5.16: Regional variation in lithology and carbon isotopes of the older Cryogenian (Sturtian) Rasthof cap-carbonate sequence in the Otavi Group, NW Namibia. Platform-foreslope differentiation was not well-established in Rasthof time.

5.17: Geochemical model of the carbon isotopic record (δ13Ccarb) of seawater in the aftermath of a snowball earth by Higgins & Schrag (2003). Cap dolostone deposition is driven by alkalinity input from carbonate weathering and by temperature rise. Limestone deposition is driven by carbonate and silicate weathering, with comensurate lowering of atmospheric pCO2.