Minerals Report - International Seabed Authority

4.2.1. Chlorinity anomalies in pore water
During gas-hydrate formation, water molecules crystallize into a
cubic lattice structure. During this process the hydrate crystals exclude
salt ions from the crystal structure. These excluded salt ions increase the
pore-water chlorinity in the host sediment 40 initially. With time, this high
salinity pore waters diffuse back to ambient levels. When the hydrate is
decomposed, the freshwater released from the lattice dilutes the salt
content of the pore-water causing the chlorinity to decrease drastically.
Though the chlorinity anomalies of the- pore-water can be a useful
indicator of hydrate presence, they also can be generated by fluid fiows 30 .
Nevertheless, it should be noted that, such fluid flows generally induce
hydrate formation.
4.2.2. Sediment grain size parameter
In many of the deep-water ODP and DSDP bore well hydrate
records the occurrence of gas-hydrates were generally associated with
coarse textured sediment. Thus, grain-size parameters of the sediment
may be useful to qualify the likelihood of hydrate deposition in an area.
4.2.3. Carbon isotope signature
The carbon isotopic signatures in marine limestone crusts and
nodules can be used as secondary proxies for methane-hydrate presence.
This proxy basically depends upon the relative enrichment/depletion of
12 C and 13 C (Figure 6). The carbonate precipitated due to bacterial
oxidation of methane and subsequent reaction with seawater calcium
ions, exhibits an order of magnitude enrichment of 12C at the cost of
oxidation of isotopically light-methane 41 . In addition the C- and Hisotopes
have been effectively used for differentiating biogenic gashydrates
from thermogenic gas hydrates 42 . Therefore, the isotopic
composition of the carbonate slabs, cement and nodules, and organic
matter may be useful proxies to detect methane discharge due to
decomposition of sub- surface gas-hydrates.
INTERNATIONAL SEABED AUTHORITY 531