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and “transect physical oceanographic structures”. As in the case of the
physical oceanography measurements, frequency and duration need to be
specified. In addition, some important depths for chemical measurements
need to be delineated, e.g. within the oxygen-minimum zone and around
the depth of forecasted discharge.
2.3. Sediment properties
The LTC recommendations for sediment-property baseline data are
as follows (paragraph 8(c)):
“determine the basic properties of the sediment, including
measurement of soil mechanics, to adequately characterize
the surficial sediment deposits and the potential source of
deep-water plume; sample the sediment taking into account
the variability of the sediment distribution”.
The Sanya Workshop recommended the following guidelines to
meet these requirements:
At no fewer than four stations, these sediment properties should be
measured: water content, specific gravity, bulk density, shear strength,
grain size and the sediment depth of change from oxic to suboxic
conditions. In addition, sediment profiles of organic and inorganic carbon,
and pore-water profiles of phosphate, nitrate, silicate, alkalinity and the
“redox system” should be measured to at least 20 centimetres or to below
the sub-oxic layer, whichever is deeper. Measurements of the
“geochemistry of pore water” down to at least 20 cm, or below the sub-oxic
layer (whichever is deeper), are also recommended.
The distribution of these measurements in space and time needs to
be specified, as does the type of grain-size analysis. For modelling
suspended sediment dispersion and redeposition, grain-size analyses of
natural sediments are much more useful than analyses of disaggregated
sediments (i.e., sediments dispersed by treatment with H2O2 to remove
organic matter, and sonication). In addition, the term “geochemistry of
pore water” is so vague that it is not clear which geochemical parameters
should be measured (most likely pore-water concentrations of Fe, Mn, SO4,
H2S and other redox-sensitive substances important in microbial
metabolism).
INTERNATIONAL SEABED AUTHORITY 58