Computer Science
Scientific paper
Sep 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55.2607d&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 9, pp.2607-2613
Computer Science
Scientific paper
Phylloid algal plates and encrusting botryoidal marine cements from the Holder Formation (Pennsylvanian), Dry Canyon in Otero County, New Mexico, USA, have been discovered that are largely preserved as aragonite. Laser microsampling enabled 13 C and 18 O multiple analyses within algal thalli and cement crusts, and allowed discrimination of the effects of later calcitization. The alga Eugonophyllum gives 13 C = +1.2 PDB and 18 O = -3.2 PDB. Botryoidal aragonite cement crusts ( 13 C = +4.2%.; 18 0 = -0.5%.) have been patchily converted to calcite ( 13 C = +4.5%.; n 18 O = -6.0%.), yielding an approximate 5%. shift in 18 O values. This calcitization occurred in a system closed with respect to carbon. A 13 C and 18 O depletion of algal aragonite compared to cement aragonite exists. The biogenic fractionation effect is -3.0%. for carbon and -2.7%. for oxygen. 13 C values proposed previously for Pennsylvanian ocean water, based principally on data derived from brachiopod shells, usually diverge 2-3%. from modem seawater. The Dry Canyon aragonite cements have 13 C values the same, and 18 O values 1 to 1.7% lighter, than modern equivalent material. This indicates that either previous estimates of Pennsylvanian ocean water were made from fractionated material, a temperature difference exists between Pennsylvanian and modem oceans, or the single Dry Canyon sample was precipitated in anomalous Pennsylvanian seawater. Only the fine scale of laser microsampling has allowed the special preservation of coexisting biogenic and abiogenic aragonite and its partial alteration to be unravelled.
Dickson A. D. J.
Kirkland B. L.
Smalley P. C.
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