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 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]

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See also

References
  1. 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


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