Iodine and bromine concentrations were determined at all nine drill sites; results are listed in Table T1, together with the chloride results from the shipboard measurements (Tréhu, Bohrmann, Rack, Torres, et al., 2003). Except for samples very close to the seafloor, concentrations for iodine fall into a range between 0.5 and 2.3 mM, constituting enrichments by factors up to 5000 compared to seawater (0.0004 mM) (Geochemical Reference Model, earthref.org/GERM). A strong increase in iodine concentrations with depth is visible in all profiles at Hydrate Ridge (Fig. F3). Although some sites reach rather constant values quickly with depth, several of the sites display maxima in iodine concentrations at depths between 100 and 150 mbsf (i.e., in the layer above the BSR). This maximum is clearly visible at the flank sites, most pronounced at Site 1245, and is probably also present at the summit sites, although the drilling depths there are not sufficient to make a definitive statement. Together with generally lower concentrations, the two sites in the slope basin do not have maxima comparable to those at the flank sites.
Bromine concentrations follow patterns similar to that of iodine concentrations, with concentrations falling into a range only slightly higher than that of iodine. Because the bromine concentration in seawater (0.8 mM) (Geochemical Reference Model, earthref.org/GERM) is considerably higher than that of iodine, enrichment factors are much lower for Br than for I. Maxima in bromine concentrations are also present at depths just above the BSR but are not as strongly pronounced as in iodine. Chlorine concentrations follow a very different pattern from that of the other two halogens. For most of the sections, Cl– concentrations are close to that of seawater (550 mM), with some scatter in the sections just above the BSR. Several of the cores display a distinct decrease in Cl– concentrations, particularly discernable at the slope basin sites. At a few summit sites, Cl– concentrations are greatly enhanced very close to the sediment/seawater interface, reaching values >1000 mM at Site 1249. No comparable increase is observed for iodine or bromine concentrations in this depth range, but it is worth mentioning that, because of the goals of our investigation, coverage of this depth range was very limited in our sample set.
The general observations for halogen concentrations are in good agreement with those found for other gas hydrate locations, such as investigations at the Peru margin (Martin et al., 1993), Blake Ridge (Egeberg and Dickens, 1999), or Nankai Trough (Fehn et al., 2003). At all of these locations, very strong enrichment of iodine is observed together with parallel enrichment of bromine. Whereas all of these sites have iodine concentrations of 0.2 mM or higher (i.e., are enriched by factors of 500 or more compared to seawater), pore water in sediments without gas hydrates have iodine concentrations of ~0.001 mM (Martin et al., 1993). Although the general enrichment in iodine at Hydrate Ridge is similar to other gas hydrate sites, the specific characteristics differ both in degree of enrichment and shape of depth profiles from the other sites. The maximum concentration measured at Site 1245 (2.3 mM) is the highest iodine concentration found at gas hydrate sites so far. Although concentrations at the other Leg 204 sites are somewhat lower, they still are among the highest values observed at gas hydrate locations. Maxima in iodine concentrations comparable to those found at the flank sites have, so far, not been observed at sites other than Hydrate Ridge. As at other gas hydrate sites, chlorine concentrations show very different behavior from that of the other two elements; specifically, concentrations of Cl– are at or below that of seawater and show considerable scatter in the layers above the BSR.
Although the general characteristics of halogens at Hydrate Ridge sites are within the range of observations found at other gas hydrate sites, there are important differences among the sites from Leg 204, specifically between the flank sites and the slope basin sites. The degree of enrichment in iodine, as well as the presence of maxima at the flank sites (and perhaps at the summit sites), is different from the observations for the slope basin sites. Profiles at the slope basin sites and Site 1244 of the flank sites show a consistent decrease in Cl– concentrations with depth, in considerable contrast to the other sites on the flank and summit. Because a decrease of this type might be related to the influx of deep fluids, as suggested for other gas hydrate sites (e.g., Hesse, 2003), this observation might suggest different hydrologic regimes for separate parts of Hydrate Ridge.
As is typical for gas hydrate locations, I/Br ratios change rapidly from seawater values (I/Br = 0.0005) to values between 0.3 and 0.5 but remain quite constant for the sections where methane is present, demonstrating the parallel behavior of these two halogens. In contrast to most other hydrate sites, in the sections associated with iodine concentration maxima, iodine and bromine do not follow similar enrichment paths, resulting in strong increases in the I/Br ratios to values up to 0.8 at Site 1245 just above the BSR. A comparison between I/Br ratios at the site with the most pronounced maximum (Site 1245) to that with the least developed one (Site 1251) illustrates this observation (Fig. F4).
A major goal of this investigation was the determination of 129I/I ratios in the pore water. Because of the high concentrations in most of the samples, we were able to make AMS measurements on individual samples for this study. Although work on the halogen concentrations in pore water is completed, work on the iodine isotope ratios is still ongoing. We report here results for an initial set of samples, tabulated in Table T2; the study will be continued and further results will be reported in a separate paper. So far, we have measured 41 samples, with >1 sample from all sites except Site 1249. Although we have results for all of the sites, we have a somewhat complete profile only from Site 1245 at this time.
All of the ratios are below the marine input ratio (Ri = [1500 ± 150] x 10–15) (Moran et al., 1998), demonstrating the absence of contamination from anthropogenic sources (Fig. F5). The highest ratio (Rm = 760 x 10–15; tmin = 15 Ma) was found for a shallow sample from Site 1244, but the majority of samples have ratios between 150 and 250 x 10–15. Using equation 1, this range corresponds to minimum ages between 40 and 52 Ma for iodine in the pore water. For three sites (Sites 1244, 1245, and 1251), a sufficient number of samples was measured to construct depth profiles (Fig. F6). Error margins for individual samples are generally <20%, with some being as high as 78%. Although the error margins are somewhat larger than obtained for similar investigations (Fehn et al., 2000, 2003), the overall trends in these profiles are nevertheless clearly visible. In general, a decrease in ratios with depth is observed for the individual cores, although data coverage is too limited so far to indicate whether this decrease is monotonous or not. There are several samples at depths beyond 100 mbsf that have ratios higher than Rm = 400 x 10–15 (tmin = 30 Ma). The most complete depth profile available so far is for Site 1245. It indicates that there is at least one clearly defined maximum in the 129I/I ratios, which reaches a value of Rm = 650 x 10–15 at a depth of 120 mbsf. The presence of this maximum is supported by the systematic increase of values above and below this depth. Although several of the other cores also show the presence of higher 129I/I values, because of the limited data coverage it is too early to decide whether these higher values indicate systematic maxima as well.