Aragonite Lability During Very Early Burial and Its Effects on Sediment Composition, Budgets and Diagenetic Potentials

Wright, V. Paul¹, Lesley Cherns¹ (1) Cardiff University, Cardiff, United Kingdom

Whereas geochemical evidence for extensive very shallow burial aragonite dissolution has been known for some years, physical evidence for the extent of this effect has only recently been compiled. In certain, mainly low energy settings where fine grained sediments with high levels of organic metter can accumulate, effectively syn-sedimentary, microbially-mediated aragonite dissolution appears to be a major process skewing the sediment record. Molluscs are especially prone to this effect and evidence from Quaternary sediments and especially from a number of "skeletal lagerstatten" shows that shallow burrowing bivalves and small gastropods are the main victims. If these examples are representative the composition of many ancient carbonates has been radically altered, making paleoecological reconstructions woefully incomplete. Many other aragonite grains are important paleo-depth or nutrient-level indicators and these may also have been removed by early dissolution. Much of the carbonate released is likely to have contributed towards the formation of diagenetic bedding. One implication of these discoveries is that offshore mud-grade carbonate may represent the in situ transformation and translocation of coarse skeletal material.This dissolution is likely to affect depth-productivity relationships and sediment budgets, but no forward models currently include these processes, making them flawed. Another implication is that the diagenetic potentials for secondary porosity development of many carbonates were lost within centimetres of burial, making early aragonite lability probably the most important process preconditioning later diagenesis. We understand little of the distribution and extent of early aragonite dissolution but we can no longer take for granted that what we see in a rock is a reasonable representation of its original grain composition; for the carbonate sedimentologist, life has just become more uncertain.