5 Sea Level Processes and Effects of Sea Level Change
5.1 Importance of Sea Level Positions
When studying sedimentary rocks on land, the first question a geologist will ask is
whether the sediment was laid down above or below sea level, that is, whether or not
it is of marine origin. For marine sediments, the next question usually is about the
depth of deposition, that is, about the position of sea level relative to the sedimentary
environment. On the present sea floor, depth of deposition rather dominates the major
facies patterns of the material accumulating on it: the size distributions of clastic
sediments, the chemistry of biogenous and authigenic matter, the distribution of
benthic organisms. For the past, sea level fluctuations, on scales between thousands
and millions of years, dominate the calendar of geologic history (Fig. 5.1).
Sea level fluctuations are of two kinds: global and regional. Global fluctuations
produce contemporaneous transgressions and regressions on the shelves of all continents. These changes in sea level are called eustatic, they originate from changes in
the volume of ocean water or in the average depth of the ocean basin. Examples are
changes in ice volume, or changes in sea-floor spreading rates, as we shall see.
Regional fluctuations consist in transgressions and regressions on one particular
shelf; they are produced by regional sinking or uplift of the shelf. Hence, such sea
level fluctuations are called tectonic, with the understanding that the tectonics are of
regional importance only.
Where the sea level intersects the continental margin, physical, chemical, and
biological processes are of high intensity. Waves, tides, and currents show maximum
activity. The productivity of tidal lands and of the littoral is exceptionally great, and
the sediment is intimately associated with rapid nutrient cycling, gas exchange, and
life processes in general. Furthermore, sea level is the baseline of erosion and deposition: exposed areas erode, submerged areas build up. The erosional and depositional
processes at and near sea level to a large extent determine the coastal mmphology
that we see. They also leave their distinct imprint in the record; they are sea level
indicators.
On a larger scale, the position of sea level with respect to the global hypsographic
curve is of great importance. It determines the degree to which shelves are submerged. Flooded shelves absorb more sunlight than exposed ones, adding heat to the
global budget.
Also, submerged land experiences practically no chemical weathering, which normally keeps down C02 levels in the atmosphere. Thus, C02 rises and climate warms
5.1 Importance of Sea Level Positions
When studying sedimentary rocks on land, the first question a geologist will ask is
whether the sediment was laid down above or below sea level, that is, whether or not
it is of marine origin. For marine sediments, the next question usually is about the
depth of deposition, that is, about the position of sea level relative to the sedimentary
environment. On the present sea floor, depth of deposition rather dominates the major
facies patterns of the material accumulating on it: the size distributions of clastic
sediments, the chemistry of biogenous and authigenic matter, the distribution of
benthic organisms. For the past, sea level fluctuations, on scales between thousands
and millions of years, dominate the calendar of geologic history (Fig. 5.1).
Sea level fluctuations are of two kinds: global and regional. Global fluctuations
produce contemporaneous transgressions and regressions on the shelves of all continents. These changes in sea level are called eustatic, they originate from changes in
the volume of ocean water or in the average depth of the ocean basin. Examples are
changes in ice volume, or changes in sea-floor spreading rates, as we shall see.
Regional fluctuations consist in transgressions and regressions on one particular
shelf; they are produced by regional sinking or uplift of the shelf. Hence, such sea
level fluctuations are called tectonic, with the understanding that the tectonics are of
regional importance only.
Where the sea level intersects the continental margin, physical, chemical, and
biological processes are of high intensity. Waves, tides, and currents show maximum
activity. The productivity of tidal lands and of the littoral is exceptionally great, and
the sediment is intimately associated with rapid nutrient cycling, gas exchange, and
life processes in general. Furthermore, sea level is the baseline of erosion and deposition: exposed areas erode, submerged areas build up. The erosional and depositional
processes at and near sea level to a large extent determine the coastal mmphology
that we see. They also leave their distinct imprint in the record; they are sea level
indicators.
On a larger scale, the position of sea level with respect to the global hypsographic
curve is of great importance. It determines the degree to which shelves are submerged. Flooded shelves absorb more sunlight than exposed ones, adding heat to the
global budget.
Also, submerged land experiences practically no chemical weathering, which normally keeps down C02 levels in the atmosphere. Thus, C02 rises and climate warms