Evolution of fluid flow and carbonate recrystallization rates in deep-sea sediments of the Equatorial Pacific

Fluid flow and carbonate recrystallization rates were determined for deep-sea sediments from the Equatorial Eastern Pacific (IODP Exp. 320/321) using δ44/40Ca values of pore water and corresponding sediments. The drill sites are located along a transect of decreasing crustal age, showing different characteristic geochemical pore water depth profiles. An overall isotopic equilibration with the sediment in the upper part of the sedimentary column can be identified, while in the lower part of these Sites, the δ44/40Ca of the pore water increases back to seawater-like values at the sediment/basalt interface, forming a bulge-shaped pore water profile. The magnitude of this bulge decreases with increasing age of the oceanic crust and sediment cover, resulting in seawater-like δ44/40Ca-porewater values throughout the whole sedimentary column of the oldest two Sites. These findings indicate that after sedimentation carbonate recrystallization processes start to enrich the pore water in 40Ca with a subsequent input of a seawater-like fluid from the underlying crust into the sediment. After a time of carbonate recrystallization and cooling of oceanic crust, a flow of seawater-like fluid starts to move through the sedimentary column, shifting porewater back to seawater-like δ44/40Ca. We established a carbonate recrystallization and fluid flow model to quantify these processes. Our determined carbonate recrystallization rates indicate that the fluid flow within the investigated sites of IODP Exp. 320/321 depends on the sedimentary composition and location of the specific site, especially the proximity to a recharge or discharge site of a hydrothermal convection cell, which form the so-called ‘hydrothermal siphon’.