SYNDEPOSITIONAL DIAGENESIS OF MODERN PLATFORM CARBONATES - EVIDENCE FROM ISOTOPIC AND MINOR ELEMENT DATA

Physical, elemental, and stable isotopic evidence documents significan t syndepositional diagenesis of biogenic carbonate in modern sediment pore water. Halimeda (aragonite) and Neogoniolithon (high-Mg calcite) fragments from sediment cores and experimental substrates were compare d with fresh samples to determine the magnitude of minor element (Sr, Mg), and stable isotopic (C and O) compositional shifts. Although no s ignificant shift in bulk sediment mineralogy is apparent, experimental substrates and natural biogenic grains exhibit significant diagenetic changes. These changes include dissolution textures, net loss of mass , changes in minor element composition, and progressive evolution towa rd carbon and oxygen isotopic equilibrium with pore water. Our results demonstrate that carbonate sediment can undergo rapid syndepositional dissolution and reprecipitation. Pore waters likely are buffered chem ically and isotopically by most abundant carbonate grain types. In tur n, compositions of less abundant carbonate grains may shift toward equ ilibrium with respect to pore water. Therefore, many components of she lf limestone have compositions controlled by chemically evolved pore w aters rather than by seawater from which the biotic carbonate original ly precipitated.