Cap carbonate

Cap carbonates are layers of distinctively textured carbonate rocks (either limestone or dolomite) that occur at the uppermost layer of sedimentary sequences reflecting major glaciations in the geological record.[1][2][3]

Characteristics and occurrence

Cap carbonates are found on most continents.[4] They are typically 3–30 meters thick, laminated structures. They are depleted in 13C compared to other carbonates. The progression of late Neoproterozoic glaciations portrayed by substantial δ13C deviations in cap carbonates suggest out of control ice albedo.[1]

Formation theories

There are several different hypotheses for cap carbonate formation.

Physical stratification theory

Physical stratification results in a strong carbon isotopic gradient in the ocean.[5] Massive carbonates will precipitate when the postglacial upwelling carries the alkalinity and isotopically light carbon to the continents. In this model, cap carbonates is the by-product of continental flooding.[6]

Snowball formation theory

The short-lived change in carbon isotopic composition is the foundation for this theory. In the snowball Earth episode, the surface ocean of Earth is covered by the sea ice that separates the ocean and the atmospheric CO2 reservoirs.[1] The atmospheric CO2 then built up to ~100,000 ppm and triggered the rapid deglaciation and melting of the sea ice, which reconnects the ocean and the atmosphere and provides access alkalinity to the ocean. The transport of carbon dioxide from that atmosphere to the ocean will lead to carbonate precipitation. This is caused caused by mixing upwelling, isotopically-depleted, alkaline bottom water and calcium-rich surface water.[7]

Methane clathrate formation theory

A third theory for cap carbonate formation is that methane hydrate destabilization results in the formation of cap carbonate and strongly negative carbon anomalies[8] The unusual fabrics within the cap carbonate is similar to carbonate fabrics as from cold methane seeps.

Experiments

Experiments have been performed to see if the massive abiotic carbonate is possible in extreme environments.[9]

See also

References

  1. ^ a b c Hoffman, P. F. (28 August 1998). "A Neoproterozoic Snowball Earth". Science. 281 (5381): 1342–6. Bibcode:1998Sci...281.1342H. doi:10.1126/science.281.5381.1342. PMID 9721097.
  2. ^ Kennedy, Martin J.; Christie-Blick, Nicholas; Sohl, Linda E. (2001). "Are Proterozoic cap carbonates and isotopic excursions a record of gas hydrate destabilization following Earth's coldest intervals?". Geology. 29 (5): 443–6. Bibcode:2001Geo....29..443K. doi:10.1130/0091-7613(2001)029<0443:APCCAI>2.0.CO;2.
  3. ^ Shields, Graham A. (August 2005). "Neoproterozoic cap carbonates: a critical appraisal of existing models and the plumeworld hypothesis". Terra Nova. 17 (4): 299–310. Bibcode:2005TeNov..17..299S. doi:10.1111/j.1365-3121.2005.00638.x.
  4. ^ Kennedy, M. J. (1 November 1996). "Stratigraphy, sedimentology, and isotopic geochemistry of Australian Neoproterozoic postglacial cap dolomites; deglaciation, delta 13 C excursions, and carbonate precipitation". Journal of Sedimentary Research. 66 (6): 1050–64. Bibcode:1996JSedR..66.1050K. doi:10.2110/jsr.66.1050.
  5. ^ Knoll, A. H.; Hayes, J. M.; Kaufman, A. J.; Swett, K.; Lambert, I. B. (June 1986). "Secular variation in carbon isotope ratios from Upper Proterozoic successions of Svalbard and East Greenland". Nature. 321 (6073): 832–838. Bibcode:1986Natur.321..832K. doi:10.1038/321832a0. PMID 11540872.
  6. ^ Kennedy, M. J.; Christie-Blick, N. (8 March 2011). "Condensation origin for Neoproterozoic cap carbonates during deglaciation" (PDF). Geology. 39 (4): 319–322. Bibcode:2011Geo....39..319K. doi:10.1130/G31348.1.
  7. ^ Grotzinger, JP; Knoll, AH (December 1995). "Anomalous carbonate precipitates: is the Precambrian the key to the Permian?". PALAIOS. 10 (6): 578–96. Bibcode:1995Palai..10..578G. doi:10.2307/3515096. JSTOR 3515096. PMID 11542266.
  8. ^ Jiang, Ganqing; Kennedy, Martin J.; Christie-Blick, Nicholas (December 2003). "Stable isotopic evidence for methane seeps in Neoproterozoic postglacial cap carbonates". Nature. 426 (6968): 822–6. Bibcode:2003Natur.426..822J. doi:10.1038/nature02201. PMID 14685234.
  9. ^ Fabre, Sébastien; Berger, Gilles; Chavagnac, Valérie; Besson, Philippe (December 2013). "Origin of cap carbonates: An experimental approach". Palaeogeography, Palaeoclimatology, Palaeoecology. 392: 524–533. Bibcode:2013PPP...392..524F. doi:10.1016/j.palaeo.2013.10.006.

Further reading

What are Cap Carbonates? at www.snowballearth.org

24-Isopropylcholestane

24-isopropyl cholestane is an organic molecule produced by specific sponges, protists and marine algae. The identification of this molecule at high abundances in Neoproterozoic rocks has been interpreted to reflect the presence of multicellular life prior to the rapid diversification and radiation of life during the Cambrian explosion. In this transitional period at the start of the Phanerozoic, single-celled organisms evolved to produce many of the evolutionary lineages present on Earth today. Interpreting 24-isopropyl cholestane in ancient rocks as indicating the presence of sponges before this rapid diversification event alters the traditional understanding of the evolution of multicellular life and the coupling of biology to changes in end-Neoproterozoic climate. However, there are several arguments against causally linking 24-isopropyl cholestane to sponges based on considerations of marine algae and the potential alteration of organic molecules over geologic time. In particular the discovery of 24-isopropyl cholestane in rhizarian protists implies that this biomarker cannot be used on its own to trace sponges. Interpreting the presence of 24-isopropyl cholestane in the context of changingglobal biogeochemical cycles at the Proterozoic-Phanerozoic transition remains an area of active research.

Clumped isotopes

Clumped isotopes are heavy isotopes that are bonded to other heavy isotopes. The relative abundance of clumped isotopes (and multiply-substituted isotopologues) in molecules such as methane, nitrous oxide, and carbonate is an area of active investigation. The carbonate clumped-isotope thermometer, or "13C–18O order/disorder carbonate thermometer", is a new approach for paleoclimate reconstruction, based on the temperature dependence of the clumping of 13C and 18O into bonds within the carbonate mineral lattice. This approach has the advantage that the 18O ratio in water is not necessary (different from the δ18O approach), but for precise paleotemperature estimation, it also needs very large and uncontaminated samples, long analytical runs, and extensive replication. Commonly used sample sources for paleoclimatological work include corals, otoliths, gastropods, tufa, bivalves, and foraminifera. Results are usually expressed as Δ47 (said as "cap 47"), which is the deviation of the ratio of isotopologues of CO2 with a molecular weight of 47 to those with a weight of 44 from the ratio expected if they were randomly distributed.

Ediacaran

The Ediacaran Period ( ee-dee-AK-ə-rən), spans 94 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 541 Mya. It marks the end of the Proterozoic Eon, and the beginning of the Phanerozoic Eon. It is named after the Ediacara Hills of South Australia.

The Ediacaran Period's status as an official geological period was ratified in 2004 by the International Union of Geological Sciences (IUGS), making it the first new geological period declared in 120 years.

Although the period takes its name from the Ediacara Hills where geologist Reg Sprigg first discovered fossils of the eponymous Ediacara biota in 1946, the type section is located in the bed of the Enorama Creek within Brachina Gorge in the Flinders Ranges of South Australia, at 31°19′53.8″S 138°38′0.1″E.

Katanga Supergroup

The Katanga Supergroup is a Neoproterozoic sequence of geological formations found in central Africa. The formation is well-studied for its rich stratiform copper-cobalt deposits mined extensively in from the Central African Copperbelt in Zambia and the Democratic Republic of the Congo. Particularly rich outcrops of the Roan Group of the supergroup occur in eastern Katanga Province of the Democratic Republic of the Congo where open-pit copper mining has occurred.

The Katanga Supergroup nonconformably overlies the 883 Ma Nchanga Granite. The Katangan Supergroup is divided into four metasedimentary series, from the oldest siliclastic and dolomitic Roan Group conglomerates, sandstones, and shales, to Nguba Group of mostly carbonates and carbon-rich shales, to the youngest, upper most Kundelungu Group including glacial metasediments and a cap carbonate.The Katanga Supergroup correlates with rocks of the Makuti Group in other parts of the Democratic Republic of Congo.

Marinoan glaciation

The Marinoan glaciation was a period of worldwide glaciation that lasted from approximately 650 to 635 Ma (million years ago) during the Cryogenian period. The glaciation may have covered the entire planet, in an event called the Snowball Earth. The end of the glaciation may have been sped by the release of methane from equatorial permafrost.

Snowball Earth

The Snowball Earth hypothesis proposes that during one or more of Earth's icehouse climates, Earth's surface became entirely or nearly entirely frozen at least once, sometime earlier than 650 Mya (million years ago). Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical palaeolatitudes and other enigmatic features in the geological record. Opponents of the hypothesis contest the implications of the geological evidence for global glaciation and the geophysical feasibility of an ice- or slush-covered ocean and emphasize the difficulty of escaping an all-frozen condition. A number of unanswered questions remain, including whether the Earth was a full snowball, or a "slushball" with a thin equatorial band of open (or seasonally open) water.

The snowball-Earth episodes are proposed to have occurred before the sudden radiation of multicellular bioforms, known as the Cambrian explosion. The most recent snowball episode may have triggered the evolution of multicellularity. Another, much earlier and longer snowball episode, the Huronian glaciation, which would have occurred 2400 to 2100 Mya, may have been triggered by the first appearance of oxygen in the atmosphere, the "Great Oxygenation Event".

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