Geology Reference
In-Depth Information
Solar incident light at the water surface is closely
dependent on latitude. Once the light has entered the
water, its rate of penetration depends on the extinction
coefficient of the water. Since this varies, subsurface
illumination shows little correlation with a simple lati-
tudinal pattern. To broadly generalized, light penetra-
tion decreases from the open ocean to the coasts, and
from the tropics toward the poles. The amount of light
depends on the angle at which the light strikes the wa-
ter surface. A maximum angle producing a maximum
penetration of light occurs in the tropics where light
conditions are optimal all year long and photosynthe-
sis works in all seasons. Approaching the poles, the
amount of light penetrating the water surface and pho-
tosynthesis rates differ significantly with the season.
About 50% of the light penetrating clear water is
absorbed in the upper tens of meters. The depth to which
light penetrates depends on the absorption of light by
the water, wavelength, transparency, reflection from the
surface of the water, reflection from suspended particles,
season of the year and latitude. Large numbers of sus-
pended particles in the water, e.g. in coastal waters,
severly reduce light penetration; photosynthesis is pos-
sible only within a range of a few meters. In clear tropi-
cal waters, where interfering particles are rare, light
intensity may be sufficient for photosynthesis down to
100-150 m.
Sunlight is composed of radiation in a spectrum of
wavelengths. As these wavelengths enter the seawater,
violet and red components are quickly absorbed within
the first few meters. Green and blue components are
absorbed less rapidly and hence penetrate more deeply,
although the intensity of light decreases. These differ-
ences control the distribution of photosynthetic organ-
isms.
prises the supratidal, the intertidal and parts of the sub-
tidal zones. The zone can be further subdivided into a
shallow and a deeper euphotic zone using differences
in light intensity and wavelength recorded by specific
assemblages of endolithic cyanobacteria and algae
(Budd and Perkins 1980; Glaub 1994)
• The
dysphotic zone
is the twilight zone character-
ized by dim light and little photosynthesis. The bound-
ary between the dysphotic zone and the aphotic zone is
defined by the existence of obligate photoautotroph
multicellular organisms. This boundary corresponds to
the lower limit of the rhodocline, which receives 0.01
to 0.001% of the surface light and corresponds to the
occurrence of crustose coralline algae. The zone is
sometimes regarded as the lower part of the euphotic
zone. Twilight is common in shadowy places, e.g. sub-
marine caves and reefs.
• In the
aphotic zone
there is not enough light for pho-
tosynthesis. Endolithic microborers are restricted to
chemoheterotroph taxa. The zone comprises parts of
the deep shelf and the deep-sea bottoms.
These zones can be further differentiated into verti-
cally arranged units whose boundaries are defined by
the distributional limits for specific organisms (Liebau
1984):
The limit of the uppermost unit, the
chlorocline,
is
characterized by the last occurrence of green algae. The
hermacline
is defined by the lower limit of reef-build-
ing organisms with photosymbiotic algae. The
rhodo-
cline
can be recognized by the lower limit of encrust-
ing calcareous red algae. The lower limit of the
chryso-
cline
is defined by the last occurrence of autotroph
single-celled algae, particularly diatoms. The deep-ly-
ing
ophthalmocline
zone is typified by blind arthro-
pods, particularly ostracods.
The depths of these boundaries depend on the opti-
cal quality of the water and the latitudinal position (see
Sect. 12.3).
12.1.4.1 Zonation and Light Conditions
Distribution patterns of light-dependent organisms
are used in ecological and bathymetric zonations of the
oceans (Liebau 1984; Lüning 1985). One common
method distinguishes the illuminated photic and the
non-illuminated aphotic zone. Other concepts differ-
entiate three zones:
12.1.4.2 Recognition of Photic and Aphotic
Conditions
The following criteria are useful for estimating paleo-
light conditions.
• In the
euphotic zone
or
photic zone
there is suffi-
cient penetration of light to support photosynthesis. Its
lower boundary is the water depth at which the visible
light is reduced to 1% of the light touching the water
surface. This boundary corresponds approximately to
the maximum compensation depth of algae and the
lower limit of the chlorocline. The euphotic zone com-
Microfacies criteria
• The formation of some carbonate grain types is in-
fluenced by light conditions. Most cortoids (Sect. 4.2.3)
and porostromate skeletal oncoids (cyanoids; Sect.
4.2.4.1) originate in the well-lighted euphotic zone.
