ABSTRACT: Carbon and Oxygen Isotopic Composition of Carbonate Fluorapatite in the Monterey Formation
J. M. Leather, G. Emanuele, M. Kastner
Studies from organic-rich continental margins have indicated that carbonate fluorapatite (CFA) forms at or just below the sediment surface and, therefore, may record paleoceanography. To investigate this possibility, CFA from the Miocene Monterey Formation in the Santa Maria (SM) and Santa Barbara (SB) basins was analyzed for ^dgr13Cc and ^dgr18Oc in the lattice-bound carbonate ion and for ^dgr18Op in the phosphate ion. In contrast to modern continental margins, these sections are characterized by low sedimentation rates, but contain high (<5 to >20%) organic carbon contents (possibly related to anoxic bottom-water conditions?). The CFA isotopic data display a more restricted range than the data for d lomites from the same sections. The SM CFA ^dgr13Cc (-6.7 to -3.4 ^pmil PDB) and ^dgr18Oc (-7.8 to -3.5 ^pmil PDB) values are more depleted than the SB CFA ^dgr13Cc (-2.5 to 1.8 ^pmil PDB) and ^dgr18Oc (-2.9 to 0.9 ^pmil PDB) values. The ^dgr18Op values (15.4 to 20.9 ^pmil SMOW) are strongly correlated with the ^dgr18Oc values and suggest that both record the same diagenetic environment. The content of lattice-bound carbonate in SM CFA (2±1%) is also lower than in SB CFA (3.5±1%).
The CFA data suggest an early paleoenvironmental or diagenetic signal overprinted by later deeper burial diagenesis, especially in the SM basin where deeper burial and/or greater geothermal gradients were experienced. In addition, within each site there are inverse relationships between ^dgr13Cc and ^dgr18Oc and between ^dgr13Cc and the amount of lattice-bound carbonate that allow various diagenetic environments to be distinguished.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990