Chemical, biological and hydrological controls on the 14C content of cold seep carbonate crusts:: numerical modeling and implications for convection at cold seeps

Understanding the hydrology of cold seep environments is crucial to perform accurate estimates of fluid and chemical fixes at sedimentary wedges. Shallow convection processes may affect fluid flux estimates and could favor the destabilization of gas hydrate accumulations, increasing the sediment-ocean methane flux. Evidence for the occurrence of convection at cold seeps, however, is still limited. We use the concentration of C-14 ((DC)-C-14) in carbonate crusts formed at cold seeps of the eastern Mediterranean Sea as a tracer for convective recirculation of seawater-derived fluids. A numerical model is applied to investigate the controls on C-14 incorporation in cold seep carbonates. Our simulations show that increased amounts of CH4 in the expelled fluids result in elevated crust (DC)-C-14, while high Ca2+ and HCO3- concentrations produce the opposite effect. Convection is the only transport process that can significantly increase crust (DC)-C-14. Advection, b oirrigation, eddy diffusion and bioturbation instead, have little effect on, or produce a decrease of, crust (DC)-C-14. In addition, the presence of old or modem carbon (MC) in host sediments prior to cementation and the C-14-decay associated to the time needed to form the crust contribute in defining the (DC)-C-14 of carbonate crusts. We then use the model to reproduce the C-14 content of the eastern Mediterranean Sea crusts to constrain the chemical and hydrological conditions that led to their formation. Some crusts contain relatively low amounts of C-14 (-945.0<(DC)-C-14 parts per thousand<-930.2) which, assuming no ageing after crust formation, can be reproduced without considering convection. Other crusts from two sites (the Amsterdam and Napoli mud volcanoes), instead, have a very high C-14-content (-899.0< C-14 parts per thousand<-838.4) which can only be reproduced by the model if convection mixes deep fluids with seawater. Order-of-magnitude calculations using the Rayleigh criterion for convection suggest that the slow seepage (about 10 cm year(-1)) of low salinity (20parts per thousand) fluids at the Amsterdam sites could trigger haline convection there. On the Napoli mud volcano, where high-density brines are expelled, density-driven convection cannot take place and other processes, possibly involving the rapid movement of free gas in the sediment, could be important. (C) 2004 Elsevier B.V. All rights reserved.