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

Abstract

The effect of early diagenesis on Sr/Ca ratios encapsulated in coral skeletons was evaluated by comparing mineralogical, structural and geochemical characteristics of modern and Holocene, branching Acropora colonies. The modern specimens (Acropora danai, Acropora formosa) come from RØunion island (Western 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 range from 3,200 to 10,200 calendar years B.P. Fossil skeletons are 100% aragonite. Earlier diagenesis has occurred in the marine environment; it is expressed by growth of secondary inorganic aragonite over primary skeletal aragonite needles, development of syntaxial aragonite cements within intraskeletal cavities and decrease in size of original 1±1,050-m-wide pores (residual 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 diagenesis has occurred in a mixed marine±freshwater environment. It includes partial dissolution of skeletal and growth of cement aragonite fibres in the form of spherolites, irregular meshes of large squarely terminated laths; 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 variation in Sr/Ca ratio can be linked to the increase in the amount of Sr-enriched cements relative to the volume of the skeletal aragonite and to a more homogeneous distribution of these cements throughout the skeleton. The uncritical use of Sr/Ca ratios as paleothermometers from diagenetically altered skeletons may cause serious misinterpretations. Accordingly, estimate of the degree of diagenetic alteration in skeletons is a prerequisite to any paleoclimatic reconstruction based on coral records.

Preliminary study on the preservation of giant clam (Tridacnidae) shells from the Balobok Rockshelter archaeological site, south Philippines

“…The dissolution-recrystallization patterns observed in our Tridacnidae shells are similar to those observed in coral skeletons (e.g., Hendy et al, 2007;James, 1974;Rabier et al, 2008;Ribaud-Laurenti et al, 2001;Sorauf, 1980). For example, Sorauf (1980) noted that the "contact between aragonite crystals are etched," while Hendy et al (2007) reported that the ".…”

Geochemistry-based coral palaeoclimate studies and the potential of ‘non-traditional’ (non-massive Porites) corals: Recent developments and future progression

“…Although some syntaxial cement becomes coarser than original coral aragonite biocrystals, and is thus identifiable based on texture, cements may mimic the original morphologies of the skeletal ultrastructure (Ribaud-Laurenti et al, 2001;Dalbeck et al, 2011) making it difficult to identify petrographically. Secondary, inorganic aragonite has an increased resistance to early diagenesis , potentially owing to its lack of associated organic matrix, and a reduced Mg/Ca ratio (Enmar et al, 2000;Sayani et al, 2011) compared to primary coral aragonite.…”

“…Significant diagenetic alteration may occur even within the most recently deposited skeleton in the uppermost zone occupied by living tissue . The extent and timing of diagenetic alteration depend on the chemistry and volume of diagenetic fluid as mediated by porosity and permeability of the skeleton (Martin et al, 1986;Ribaud-Laurenti et al, 2001;Hendy et al, 2007) and the setting (e.g., living or dead coral; marine or meteoric environment). Skeletal porosity and permeability vary with taxonomy (Perrin and Cuif, 2001), which in turn controls skeletal ultrastructure (James, 1974;Perrin and Cuif, 2001), composition of intraskeletal organic matrices (Gautret, 2001) and the degree of associated bioerosion .…”

Radiocarbon calibration curve spanning 0 to 50,000 years BP based on paired 230Th/234U/238U and 14C dates on pristine corals

“…Speleothems provide long records and are very useful for identifying general trends and maybe large 14 C production anomalies, but speleothems are potentially limited by the dating errors due to variable groundwater uranium-series and carbon chemistry. 230 Th/ 234 U/ 238 U dating of corals can be reasonably precise and accurate, but samples of appropriate age are hard to acquire and the coral 230 Th/ 234 U/ 238 U ages can be altered by freshwater diagenesis (Hamelin et al, 1991;Henderson et al, 1993;Gallup et al, 1994Gallup et al, , 2002Ribaud-Laurenti et al, 2001), and in some locations by marine cements (Ribaud-Laurenti et al, 2001;Paterne et al, 2004). Even trace amounts of diagenetic calcite deposited in subaerially exposed corals will bias the radiocarbon measurements (Chiu et al, 2005a).…”