Carbonate recrystallization in basal sediments: evidence for convective fluid flow on a ridge flank

Abstract

In many Deep Sea Drilling Project (DSDP) drill holes, chalks and/or limestones and even dolomites overlie oceanic basement rocks. These occurrences have often been interpreted as the result of increased calcium carbonate dissolution and reprecipitation as cement and overgrowths (recrystallization) at higher prevailing temperatures, either near an oceanic ridge system or at greater burial depths (burial diagnesis). We report here detailed chemical and isotopic analyses of carbonate sediments recovered from a drill hole on the western flank of the East Pacific Rise during Leg 92 of DSDP. This hole was drilled on 4.6-Myr old oceanic crust and recovered chalks directly overlying oceanic basement at a depth as shallow as 8 m below the sediment/water interface. Oceanic basement was not reached in this hole because of the hardness of deeper chalks, or, more likely, limestones. The chemical and isotopical evidence, detailed below, shows that considerable calcite recrystallization did occur in an aqueous medium similar to normal seawater and at relatively low temperature. This suggests that the observed recrystallization of calcareous ooze to chalk and limestone took place not as a result of burial diagenesis at higher temperatures, but as a result of extensive advective pore water flow through the sediments, which would allow recrystallization of calcium carbonate to take place. This process was, and apparently still is, widespread in the south-east Pacific Ocean.