Page 25 - Carbonate Sedimentology and Sequence Stratigraphy

16 WOLFGANG SCHLAGER

Indo-Pacific Caribbean
Great barrier reef Great barrier reef Coral Sea Papua New Guinea Philippines Singapore Pacific atolls Maladives Persian Gulf gulf of Aqaba Red sea Aldabra Bahamas Barbados Bermuda Curaçao Grand Cayman Jamaica Panama W. Caribbean Yucatan

0 Bikini Tahiti Belize Florida

50

depth (m)

100

active reef growth
strongly reduced growth
150 maximum depth

Fig. 2.6.— Depth of the euphotic zone in the Indo-Pacific and the Caribbean, constrained by the limits of reef growth. The base is
gradational and varies regionally by tens of meters. After Vijn and Bosscher (written communication).

cific data, mainly from reef environments. The growth- Temperature rivals light in its effect on skeletal carbonate
depth curve displays a shallow zone of light saturation fol- production. Generally, warmer is better, but there exist up-
lowed by a zone of rapid decrease and a third, deep zone per temperature limits for the various carbonate-secreting
where growth asymptotically approaches zero. This growth organisms. Thus, the temperature window of calcifying
curve can be derived, via a hyperbolic function, from the benthos is different for different organisms. Most her-
well-established exponential decrease of light intensity with matypic (i.e. symbiotic) corals function in the range of 20-30
depth(Fig. 2.4). The growth-depth patterns of most other ◦ C. The upper temperature boundary sets important limits
carbonate-producing organisms are less well known but to carbonate production, particularly in restricted lagoons
◦
seem to follow similar trends. where temperatures frequently exceed 30 C. The most im-
Characterization of growth-depth curves requires defini- portant effect of temperature, however, is the global zona-
tion of two parameters – the light saturation zone and the tion of carbonate deposits by latitude (Figs 2.8, 2.9). In spite
euphotic zone. For both exist stringent biological definitions of what has just been said about the importance of light,
(Fig. 1.15). Geologists are normally not able to measure the the boundary northern and southern limit of coral reefs, and
required variables and have to resort to proxy indicators. thus the boundary of tropical and cool-water carbonates, in
The euphotic zone is defined in the geologic record as the in- the modern oceans seems to be controlled by winter temper-
terval where abundant growth of photosynthetic, carbonate- ature rather than radiation. This indicates that as one moves
secreting benthos is possible. The zone of light saturation poleward in the modern oceans, the temperature limit for
has not been defined in geological terms. Loosely speak- hermatypic coral growth is reached before the light limit. In
ing, it is the interval where light has no recognizable control the past, this need not always have been the case. The tem-
on the rates of growth and calcification of organisms. The perature limit and the light limit may have shifted relative to
growth forms of corals indicate the zone of severe light lim- one another during geologic history. The fairly stable posi-
◦
itation by a change from massive to platy colonies (Fig. 2.7). tion of 30-35 latitude for the boundary of Phanerozoic trop-
In the light-saturated zone, corals indicate the presence of ical carbonates may reflect the joint control by temperature
a very turbulent surface layer of the sea by dominance of and light.
branching growth forms.