Diagenesis and its impact on Sr/Ca ratio in Holocene Acropora corals

The effect of early diagenesis on Sr/Ca ratios encapsulated in coral skelet ons was evaluated by comparing mineralogical, structural and geochemical ch aracteristics of modern and Holocene, branching Acropora colonies. The mode rn specimens (Acropora danai, Acropora formosa) come from Reunion island (W estern Indian Ocean) and the Great Barrier Reef of Australia respectively. The Sr/Ca ratios of modern specimens range from 9.08 to 9.37 mmol/mol. The fossil acroporids (Acropora group danai-robusta) were collected from a 50-m core drilled through a barrier reef in Tahiti island; their C-14 ages rang e from 3,200 to 10,200 calendar years B.P. Fossil skeletons are 100% aragon ite. Earlier diagenesis has occurred in the marine environment; it is expre ssed by growth of secondary inorganic aragonite over primary skeletal arago nite needles, development of syntaxial aragonite cements within intraskelet al cavities and decrease in size of original 1-1,050-mum-wide pores (residu al porosity ranges from 25 to 28%), which results in a volume reduction by 34 to 49%. Cementation increases with increasing age of the corals. Later d iagenesis has occurred in a mixed marine-freshwater environment. It include s partial dissolution of skeletal and growth of cement aragonite fibres in the form of spherolites, irregular meshes of large squarely terminated lath s; this results in an increase in porosity from 30 to 59%. By reference to modern well-preserved acroporids, this diagenetic alteration has led to an increase of Sr/Ca values (from 9.08-9.37 to 8.89-10.55 mmol/mol). This vari ation in Sr/Ca ratio can be linked to the increase in the amount of Sr-enri ched cements relative to the volume of the skeletal aragonite and to a more homogeneous distribution of these cements throughout the skeleton. The unc ritical use of Sr/Ca ratios as paleothermometers from diagenetically altere d skeletons may cause serious misinterpretations. Accordingly, estimate of the degree of diagenetic alteration in skeletons is a prerequisite to any p aleoclimatic reconstruction based on coral records.