Minerals Report - International Seabed Authority

alter the gas content of a sediment sample, and methane analyses
techniques have to consider these influences. The most common method
for the determination of hydrocarbons in sediments is the headspace
technique 55 , where an aqueous solution of the sediment is equilibrated
with an overlying gas phase that is then analysed by gas chromatography.
Degassing of sediments has been shown to deliver reliable information on
both the gas quantities and the stable isotopic composition of methane in
sediments. Methane extraction is done using ultrasonic extraction unit for
headspace methane gas collection. This headspace detection helps to
identify the higher hydrocarbon (methane) concentrations in the
sediments.
Pore water chemistry is studied at various intervals of the core
from samples collected using pore water squeezers or centrifuges, and
analysed for proxy elements such as sulphate etc., using CNS analysers
and other titrimetric methods. The entire core sample can be utilized for
understanding both textural (grain size history, porosity and permeability
etc.) parameters and microbial signatures. Both water column and near
bottom water samples may also be collected using rosette water sampler
equipped with CTD, and Niskin sampling bottles, for identifying methane
proxies, if any are present.
Recently, carbonate crusts have been found on the continental
margin seafloor in association with methane-bearing cold water
seepages 56,57 , and are suspected to be related to dissociation of gas
hydrates 58 . Carbonate lenses encountered within the sedimentary
sequences may indicate methane seepages to the seafloor in response to
massive decomposition of gas hydrates.
Bacteria too play a dominant role in the degradation of organic
matter within the sediments and, as a consequence drive chemical changes
and early diagenesis. The estimation of total bacterial numbers and
bacterial activity rates may help to investigate methane cycling in deep-sea
sediments 59 which probably contain gas hydrates.
INTERNATIONAL SEABED AUTHORITY 539