Sediments on Bahamian slopes, reef rubble, gully sands, and periplatform carbonate ooze, consist of calcite, magnesian calcite, and aragonite. When not continuously buried by new material, these sediments harden to limestone crusts, 10 cm to several meters thick, on the sea floor. The pathways of sea floor diagenesis vary with depth. (1) Reef wall and talus slope (50 to 400 m) are pervasively cemented by micritic magnesian calcite; botryoidal aragonite fills the large voids. There is no evidence of selective dissolution; primary and diagenetic minerals coexist. (2) On the gullied slopes between 500 and 1,300 m most sediment lithifies to calcite limestone or chalk, with 3 to 7 mol % MgCO₃; however, about 20% of the samples are micritic magnesian calcite limeston s of 8 to 14% MgCO₃. (3) On the gullied slopes from 1,300 to at least 2,500 m, calcite limestones with 2 to 4 mol % MgCO₃ prevail.

The downslope trend from aragonite and magnesian calcite to calcite is explained by the decrease in water temperature and carbonate saturation. The magnesian-calcite limestones between 500 and 1,300 m do not follow this trend even though their oxygen isotopes indicate formation in cold bottom water. Micritic ooids of magnesian calcite with globigerinids as nuclei and coccoliths in the cortex are associated with the limestone crusts and obviously form in this current-swept deep-water environment. This suggests that magnesian calcite precipitates from a pore fluid close to, and derived from, sea water. The microenvironment may be controlled by micro-organisms or by inorganic precipitation of calcium carbonate that increases the Mg:Ca ratio in the pore fluid, thus raising the magnesian c ntent of the precipitating calcite.

End_of_Article - Last_Page 986------------