First things to consider are advection and diffusion, and there are old (but not necessarily simple) models for chlorinity profiles in gas hydrate systems. See Davie and Buffett (2001,2003, in JGR and EPSL)

Sedimentation and compaction implies that pore water moves with respect to the solids, even in the absence of an external fluid source. Both hydrate distribution in the sediment and choride concentration are influenced by this process.

Fluid advection from below can occur, and seawater salinity varies during glacial-interglacial cycles. For instance, both processes where observed at Blake Ridge (see Egeberg and Dickens, 1998). Seawater is more saline during glaciations because there is more ice on the continents, and this is reflected in chloride advection-diffusion profiles worldwide (See Adkins and Schrag, 2002).

External fluid sources often invoked are clay and organic matter diagenesis. On the other hand, alteration of volcanic glass (that may be present in the sediment) into clay (dominantly smectites) takes up pore water. Volcanic ash diagenesis in the sediment (as well as basalt alteration in the oceanic crust) can thus increase in situ chlorinity at low temperature while smectite/Illite reaction will lower chlorinity as temperature increase (e.g. Henry and Bourlange, 2004). These processes happen beneath the gas hydrate stability field but can influence pore fluid composition in the shallow sediment by advection-diffusion.

Dear Pierre,

Thanks a lot! Great comment.. Thanks for the references as well..

Matteo

You might also need to send and email to these authors: www.sci-dril.net/3/18/2006/sd-3-18-2006.pdf