Elemental and Isotopic Chloride Geochemistry and Fluid Flow in the Nankai Trough

Abstract

[1] The first transect of pore fluid Cl À profiles across the deformation front of an accretionary prism and underthrust section, Nankai Trough, and the first Cl isotopic profile in this environment are presented. These demonstrate that there is channelized horizontal episodic flow through most of the section arcward of the deformation front and in the lowermost section seaward of the deformation front. In the underthrust sediments, channels appear to be continuous. Fluid velocities, within the channels, based on Cl À gradients, are 13 ± 5cm/year. The near balance of depth-integrated flow and the subduction of pore fluid indicates that flow episodes are frequent. d37 Cl values are lower than seawater throughout the section, averaging À6.1 per mil. This demonstrates fluid/mineral exchange of Cl and suggests that subducted solids carry enriched d 37 Cl that is efficiently recycled from deeper levels in the subduction zone back into seawater and may be responsible for the seawater/ mantle d 37 Cl fractionation.

Distribution of B, Cl and Their Isotopes in Pore Waters Separated from Gas Hydrate Potential Areas, Offshore Southwestern Taiwan

“…Cl as low as -6 0 00 are reported in marine mud volcanoes, Barbados (Goden et al 2004) and similar observations are reported from the decollement zone fluids at ODP Site 808, Nankai Trough (Ransom et al 1995;Spivack et al 2002). Interestingly, the seismic profile at site G23 is indeed a mud diapiric structure, indicating possible involvement of deep generated fluids and the most depleted isotopic composition of -6 0 00 is similar to fluids emitted from mud volcanoes near the deformation front at Barbados.…”

“…In particular, the distribution of B and δ 11 B in pore waters separated from drilling sites at various accretionary wedges (i.e., Nankai Trough, the Barbados Ridge Complex, and Hydrate Ridges) have provided important information on fluid migration along porous sandy layers and the decollement zones (Gieskes et al 1989;You et al 1993;Spivack et al 2002;Teichert et al 2005). Ransom et al (1995) also reported large δ 37 Cl depletions in deep generated fluids separated from the decollement zones during ODP Leg 131 at Site 808.…”

“…The observed large depletion and shallow occurrence ruled out any possibility due to molecular diffusion and/or halite precipitation. Deep generated fluids reported were depleted in δ 37 Cl (Ransom et al 1995;Spivack et al 2002) and this has been used as a sensitive tracer for fluid migration in accretionary wedges.…”

“…The above mentioned mechanisms, however, will cause only a small degree of isotopic fractionation (less then 2 0 00). A rather large depletion of δ 37 Cl, -8 0 00, observed at the decollement zone at the Nankai Trough, was explained in terms of fluid-rock reactions at depth (Ransom et al 1995;Spivack et al 2002). Since gas hydrate dissolution and fresh water dilution will not cause any change in Cl isotopes in fluids, the Cl isotopic ratio has become an interesting tracer to distinguish the origin of fluids at shallow circulation or depth (Ransom et al 1995).…”

Comparison of Rhizon Sampling and Whole Round Squeezing for Marine Sediment Porewater

“…Ammonium is critical for understanding microbial respiration and the nitrogen cycle (Blackburn, 1988). Chloride is used to reconstruct ocean salinity variations, constrain flow rates, and estimate gas hydrate concentrations (Paull et al, 1996;Adkins et al, 2002;Spivack et al, 2002). Each of these studies requires the recovery of porewater that is not compromised by sampling artifacts.…”