Discussion Reply to comment on: bGas hydrate growth, methane transport and chloride enrichment at the southern summit of Hydrate Ridge, Cascadia Margin off OregonQ M.E. Torres a, * , K. Wallmann b,1 , A.M. Tre ´hu a,2 , G. Bohrmann c,3 , W.S. Borowski d , H. Tomaru e a College of Oceanic and Atmospheric Sciences, Oregon State University, 104 COAS Admin. Building, Corvallis, OR 97331-5503, United States b GEOMAR, Wischhofstrasse 1-3, D-24148 Kiel, Germany c Research Center Ocean Margins, University of Bremen, Post Box 330 440, D-28334 Bremen, Germany d Department of Earth Sciences, Eastern Kentucky University, 512 Lancaster Avenue, Roark 103, Richmond KY 40475-3102, USA e Department of Earth and Planetary Science, University of Tokyo, Science Building 5, 7-3-1 Hong, Bunkyo-ku, Tokyo 113-0033, Japan Received 19 July 2005; accepted 3 August 2005 Available online 23 September 2005 Editor: E. Boyle 1. Introduction In the Torres et al. [1] paper we present a quantita- tive 1-D model for gas hydrate formation in the pres- ence of abundant gas and conclude that: 1) transport of gaseous methane within the gas hydrate stability zone (GHSZ) is necessary to sustain the observed massive methane hydrate deposits and chloride enrichment in the pore fluids; and 2) hydrate in the shallow subsurface must be forming very fast. We also present a possible scenario for gas migration in this setting, based on the work of Clennell et al. [2,3], in which we suggest that capillary effects inhibit hydrate formation from the base of the GHSZ to the shallow subsurface. We show that the onset of massive hydrate and pore water brines corresponds to the shallowest depth at which the inter- nal pressures of gas bubbles and hydrate crystals can overcome the effective overburden stress. We postulate that at this depth (~30 mbsf), internal pressures are large enough to push aside the sediment grains, thus reducing methane supersaturation and enhancing meth- ane hydrate formation. Milkov and Xu [4] postulate an alternate conceptual scenario (Fig. 1E). They suggest that as solid hydrate forms it generates a high-salinity fluid, which shifts the hydrate stability field sufficiently to preclude additional gas hydrate formation. A similar model has been pro- posed by Liu and Flemings (2004, in review). This brine supports vertical methane transport as a gas phase from the bottom-simulating reflector (BSR) to the shallow surface beneath a carbonate structure known as the Pinnacle. Here, impermeable carbonates cause lateral deflection of gas flow towards the summit, resulting in the observed change in hydrate and chloride distribution at 30 mbsf beneath the summit. 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.08.012 DOI of original article: 10.1016/j.epsl.2005.05.044. * Corresponding author. Tel.: +1 541 737 2902; fax: +1 541 737 2064. E-mail addresses: jrs_torres@ship.iodp.tamu.edu (M.E. Torres), kwallmann@geomar.de (K. Wallmann), trehu@coas.oregonstate.edu (A.M. Tre ´hu), gbohrmann@uni-bremen.de (G. Bohrmann), Wborowski@eku.edu (W.S. Borowski), tomaru@gbs.eps.s.u-tokyo.ac.j (H. Tomaru). 1 Tel.: +49 431 600 2287. 2 Tel.: +1 541 737 2655; fax: +1 541 737 2064. 3 Tel.: +49 421 218 8639. Earth and Planetary Science Letters 239 (2005) 168 – 175 www.elsevier.com/locate/epsl