CARBON AND OXYGEN ISOTOPES IN PENNSYLVANIAN BIOGENIC AND ABIOGENIC ARAGONITE (OTERO COUNTY, NEW-MEXICO) - A LASER MICROPROBE STUDY

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 delta-C-13 and delta-O-18 multiple analyses within algal thalli and cement crusts, and allowed discrimination of the effects of later calcitization. The alga Eugonophyllum gives delta-C-13 = + 1.2 PDB and delta-O-18 = -3.2 PDB. Botryoidal aragonite cement crusts (delta-C-13 = +4.2 parts per thousand; delta-O-18 = -0.5 parts per thousand) have been patchily converted to calcite (delta-C-13 = +4.5 parts per thousand; delta-O-18 = -6.0 parts per thousand), yielding an approximate 5 parts per thousand shift in delta-O-18 values. This calcitization occurred in a system closed with respect to carbon. A delta-C-13 and delta-O-18 depletion of algal aragonite compared to cement aragonite exists. The biogenic fractionation effect is -3.0 parts per thousand for carbon and -2.7 parts per thousand for oxygen. Delta-C-13 values proposed previously for Pennsylvanian ocean water, based principally on data derived from brachiopod shells, usually diverge 2-3 parts per thousand from modern seawater. The Dry Canyon aragonite cements have delta-13-C values the same, and delta-O-18 values 1 to 1.7 parts per thousand 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 modern 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.