Geology Reference
In-Depth Information
12.1.6.1 Seawater Temperature: Biotic Proxies
assemblages. Warm-water planktonic foraminifera tend
to be more susceptible to solution than cold water fora-
minifera; this may lead to an enrichment in cold water
forms (Miao et al. 1994). The method is successfully
applied to Cenozoic paleoenvironments.
• Reef-building corals: Warm sea-surface temperature
is a key factor controlling the distribution of modern
scleractinian corals and coral reefs in the tropics and
subtropics owing to physiological limitations to meta-
bolic activity (calcification) and growth (larval survival)
and ecological limitations (e.g. competitive interactions
between corals and soft algae). These relationships in-
volve only zooxanthellate corals. Scleractinian corals
without endosymbiotic algae also construct reefs in cold
waters. The distribution patterns of Mesozoic and also
Early Tertiary coral reefs exhibit many exceptions as
compared with modern temperature-controlled patterns
(see papers in Kiessling et al. 2002). The question of
climate controls on reefs built by organisms other than
corals is strongly debated (Kiessling 2001).
• Skeletal mineralogy. The mineral composition of
invertebrate skeletons exhibits temperature-dependent
relations (Dodd and Stanton 1990). The relation be-
tween the aragonite/calcite ratio and temperature is not
always obvious, however, because of the effects of other
environmental factors, e.g. salinity, and because the
mineralogy-temperature effect can differ from species
to species.
• Morphological criteria of skeletal elements, e.g. the
thickness and size of shells.
• Rhythmic growth patterns of bivalve shells or other
invertebrates, e.g. banding of corals, generally associ-
ated with seasonality. Thickness, density and spacing
of increments as well as trace element data and stable
isotopes reflect seasonal fluctuations of water tempera-
ture. Examples are known from bivalves, corals and
corallinacean red algae. However, temperature fluctua-
tions are just one factor in the formation of growth in-
crements.
• Functional criteria of skeletal elements, e.g. the num-
ber and length of spines, size of pores, and coiling di-
rections of planktonic foraminifera.
The interpretation of paleotemperature conditions is
predominantly based on paleontological features that
are only sometimes recorded in thin sections. The only
thin-section criteria exhibiting significant temperature
controls are skeletal grain associations (see Sect. 12.2)
and ooids (Sect. 4.2.5). The occurrence of abundant
ooids is often used as a proxy for warm water environ-
ments of tropical settings (Opdyke and Wilkinson
1990). The main assumption is that many carbonate
ooids form in warm waters of high salinities (Lees
1975).
Common paleontological criteria used in the evalu-
ation of seawater temperature are
• Distribution patterns of benthic associations. Warm-
water and cool-water associations exhibit marked taxo-
nomic differences.
• Transfer of the temperature range tolerated by ex-
tant taxa to ancient taxa with close taxonomic relation-
ships. This method can be successfully applied to Qua-
ternary and sometimes also to Tertiary taxa, but be-
comes increasingly more hazardous for Mesozoic and
Paleozoic taxa.
• Transfer of temperature ranges and abundance pat-
terns of defined recent organism groups to fossil groups.
Many modern marine invertebrates are limited to warm
waters (e.g. larger foraminifera, hermatypic corals);
others occur predominantly in temperate and cold wa-
ters (e.g. brachiopods). This approach has many pit-
falls.
• Diversity of benthic organisms expressed by spe-
cies number and abundance. The general rule, that di-
versity is high in warm water and low in cold water,
has many exceptions.
• Evaluation of organisms exhibiting strong tempera-
ture controls. Commonly used groups are algae, fora-
minifera, corals, mollusks and ostracods.
• Benthic foraminifera. Many agglutinated foramin-
ifera (Textulariina) prefer cold-temperate and brackish
waters. Miliolina are abundant in warm waters of nor-
mal and high salinity. Rotaliina occur in waters of in-
termediate temperatures and normal salinity. The 'tem-
perature index', using the relative proportions of warm
water and cold water genera of benthic foraminifera, is
valuable in evaluating paleotemperatures of shelf en-
vironments back to the Mid-Cretaceous.
• Planktonic foraminifera. The sea surface tempera-
ture (SST) can be estimated using the transfer function
of planktonic foraminifera and other calcareous plank-
ton (see Murray 1995). The success of the method de-
pends on the quality of preservation of foraminiferal
12.1.6.2 Geochemical Proxies of Seawater
Temperatures
Carbonate oxygen isotope palaeothermometry
The most widely used geochemical technique to
determine paleotemperatures is based on the ratio of
oxygen isotopes in calcite and aragonite. The method
is applied to modern organisms and fossils as well as
to carbonate sediments and limestones. The fraction-
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