Geoscience Reference
In-Depth Information
mineral extraction, disposal of dredged mater-
ial and changed supply of material from rivers.
Sediment supply might be either reduced be-
cause of damming of large rivers or increased as
a result of deforestation in catchments leading
to higher sediment yields, or by the canalization
of some rivers, which leads to increased bypass-
ing of sediment through the coastal zone. For
many types of human impact on the sediment-
ary environment, the main method of measur-
ing change is restricted to historical surveys,
but consideration of the significance of human
impacts means that it is necessary to measure
change on time-scales of decades to centuries
(Berger & Iams 1996; Sarnthein et al. 2002). Of
particular interest are those radiometric tech-
niques that allow delineation of chronologies
over a few decades (e.g.
210
Pb,
137
Cs and others).
On continental shelves themselves, the best re-
cords of past change are generally formed at
sites with high rates of sediment accumulation
(e.g. the Amazon shelf, Kuehl et al. 1986) or
on shelves that have biogenic features with a
high preservation potential, from which can be
derived a proxy record of environmental change
(e.g. Schöne et al. 2004).
Sediments form habitats for marine fauna and
flora, and one view of environmental manage-
ment is that the overall mangement aim should
be habitat retention, on the basis that if the
habitats themselves are present, then it is likely
that the structure and function of the associated
ecosystem components will also be retained. Shelf
sediments may accumulate contaminants to the
extent that the biology is negatively impacted,
and these surface accumulations may them-
selves become sources of pollution, with regard
to transport across the shelf and/or bioturbation
down into the sediment column (e.g. Kershaw
et al. 1988; MacKenzie et al. 1999). Trace metal
elements are often of environmental concern,
background concentrations of which are con-
trolled by the geological history of a continental
shelf and its catchments (e.g. Chapter 1). There
are few texts regarding human management of
shelf environments, and much information
derives from reports of past or ongoing activ-
ities. The websites of Government departments
and associated regulatory bodies are increas-
ingly useful. Examples include the US Environ-
mental Protection Agency (EPA), Environment
Canada, the UK Department of Environment,
Food and Rural Affairs (DEFRA), the Nether-
lands National Institute for Coastal and Marine
Management, the Australian Department of
Environment and Heritage, the Great Barrier
Reef Marine Park Authority (GBRMPA), the
New Zealand Department of Conservation and
National Institute for Water and Atmosphere.
10.2
SHELF SETTINGS AND SEDIMENT SOURCES
10.2.1 Shelf settings
Continental shelves receive sediments that have
been transported from the terrestrial realm by
rivers, are the sites of biogenic sediment produc-
tion, store sediments in various environments
and for various time-scales, and transport some
sediments off-shelf down the continental slope
towards the abyssal plains. There are two main
morphological types of continental shelf ( Johnson
& Baldwin 1996):
1
pericontinental shelves - which occur on con-
tinental margins and equate to those modern
continental shelves with the classic division
and profile of shoreline, shelf and slope (e.g. the
Amazon and Californian shelves);
2
epicontinental shelves - which are partially
enclosed seas within continental areas usually
with a uniform shallow-dipping ramp profile
(e.g. the Gulf of Mexico and the North Sea).
Tectonically, the largest continental shelves are
located at passive margins, where they generally
develop seaward-thickening accumulations of
sediment, supplied by large continental drain-
age systems. At active plate margins, shelves may
develop at the convergent margin itself, upon
which shelf areas are relatively small and which
may have zones with high sediment accumula-
tion rates, or in foreland basins, where extensive
areas of continental shelf may develop and sedi-
ment accumulation rates may be high.
Understanding the processes and features of
active shelf sedimentary systems involves long
