Environmental Engineering Reference
In-Depth Information
LATE QUATERNARY ISOSTATIC CRUSTAL ADJUSTMENTS
applications
Northern Britain, at the centre of the last British ice sheet 18,000 years ago, is still in
glacio-isostatic rebound
of 3 mm a
−1
. By comparison, the Thames/Rhine estuary region
is subsiding at 1-2 mm a
−1
, through
flexural isostasy
beyond the margins of the ice sheet
and crustal loading through sedimentation and sea level rise in the shallow, epicontinental
North Sea basin. Zero isobase (the neutral line) in Britain passes through Anglesey-
Manchester-Middlesbrough. Rebound rates are three to five times higher in northern
Scandinavia and the Hudson Bay area of eastern Canada, beneath the respective former
centres of the
Scandinavian
and
Laurentian
ice sheets. In addition to isostatic rebound
after deglaciation, glacial erosion further unloads continental crust. The corresponding
transfer of sediment directly to the sea, or to the deltas of major rivers such as the
Mississippi, Ganges and Amazon, has depressed sea floors by an average 1-3 mm a
−1
throughout the 10,000 years of
Holocene
(postglacial) time. Human agency is introduced
to crustal unloading (by mining) and loading (by engineering structures such as large
reservoir construction), although their effects can occur inland and do not always have
coastline impacts. Eustatic and isostatic effects overlap each other, leading to complex,
fluctuating levels which may be particularly rapid at passive, low-angled continental
margins and shallow epicontinental seas - with consequences for ocean circulation.
Concern is also growing about the possible impact of future volcanic activity at active
plate margins. Since most volcanoes are located in island arcs or coastal orogenic belts,
they have been subjected to crustal loading by Holocene sea-level rise, with a
corresponding rise in seismo-volcanic activity. Further loading leading to the potential
collapse of volcanic structures is also fuelling fears of enhanced
tsunami
activity.
at particular points of major
influx
or
efflux
of water and/or minerals such as the ocean-
atmosphere boundary, estuaries, tidewater glaciers and human pollution sources. We note
that increased fresh water or decreased mineral flux
dilutes
and decreased fresh water or
increased mineral flux
concentrates
the solution. Density varies inversely with
temperature but is complicated by changes in salinity, outlined below. In addition to
mineral solutions and suspensions, oceans are also reservoirs of dissolved atmospheric
gases, incorporated by diffusion from the atmosphere and in sea spray. Concentrations
are usually related directly to pressure and inversely to temperature. Average
concentrations of N
2
and O
2
are about 1·1 parts and 0·5 parts per thousand respectively,
but at 1·3 parts per thousand, CO
2
is much more abundant, given its low atmospheric
mass. Levels of both oxygen and carbon dioxide vary considerably in the photic zone
(see below) as a result of biosynthesis.
SALINITY
The common expression of solute content is
salinity
, or the quantity of solutes in parts
per thousand (‰) by weight and irrespective of composition. NaCl (sodium chloride) is
by far the commonest species at over 90 per cent by mass and thus the principal
constituent of
brine
or 'salt water'. Global surface values range from about one part per
