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Minerals Report - International Seabed Authority

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Figure 6: Interpretative plot of hydrogen and carbon isotope composition of methane<br />

in gas hydrate (after SchoeI143).<br />

4.2.4. Pore water redox level<br />

In the marine regime several metals are either particle reactive or<br />

redox sensitive. Manganese is such an element that has been extensively<br />

used in paleo-oceanographic studies for reconstructing paleo-redox levels,<br />

productivity etc. (cf. Mangini et al., 44 ). The crystallization and<br />

decomposition of hydrates are associated with changes in the pore- water<br />

redox levels, which in turn can induce sharp fluctuations in the dissolved<br />

and solid-phase manganese concentration of pore water-sediment.<br />

Microbial methane is generated in the sediment column in anoxic<br />

conditions where sulphate reduction is prominent. In such an<br />

environment, the existence of a solid phase of Mn is highly un- likely.<br />

Therefore, the dissolved pore water Mn pool can develop a strong positive<br />

anomaly at the expense of reduction of solid phase particulate Mn in<br />

sediment. Superposing such redox sensitive-element data on other proxies<br />

may strengthen the gas-hydrate search approach.<br />

INTERNATIONAL SEABED AUTHORITY 532

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