Geology Reference
In-Depth Information
can raise or lower nutrient levels and primary produc-
tion, and encourage or reduce eutrophic conditions were
summarized by Föllmi et al. (1994) and Brasier (1995).
highly adapted to nutrient-deficient environments.
Changes in nutrient concentration are regarded as a
major control on the demise of coral reefs and drown-
ing of platforms (Schlager 1981; Hallock and Schlager
1986; Föllmi et al. 1994; Dupraz and Strasser 2002)
and on the biotic composition of reefs (Birkeland 1988).
The input of phosphates and nitrates stimulates the
growth of plankton, which reduces water transparency,
limiting depth ranges of zooxanthellate corals and cal-
careous green algae and thereby reducing carbonate pro-
duction. Higher nutrient concentrations and plankton
densities stimulate the growth of fast-growing soft al-
gae and ahermatypic suspension-feeding benthic organ-
isms, and increase bioerosion by microborers.
Many ancient drowned platforms and reefs exhibit
evidence of nondeposition, bioerosion and reduced re-
dox potential, pointing to excess nutrient availability
12.1.8.2 Estimating Paleonutrient Levels
Several factors triggering higher or lower nutrient lev-
els are recorded by facies, paleontological and geo-
chemical criteria (Box 12.8).
12.1.8.3 Effects of Nutrient Excess on Reef
and Platform Carbonates
The primary carbonate sediment producers of coral reef
communities and of shallow carbonate platforms are
Box 12.8. Criteria of carbonates reflecting possible differences in nutrient levels . Note that some of the facies criteria
can also be caused by other environmental constraints. Based on Brasier (1995a, 1995b), Wood (1999) and other sources.
Facies criteria
Basic types of autochthonous limestones. The dominant
reef biota and the resulting carbonate types are related to
nutrient levels. Bindstones and framestones originate in
low-nutrient environments; mud mounds formed by fine-
grained micrite with weakly calcified heterotrophs, and
bafflestones originate in areas of higher and high nutrient
levels (Fig. 8.1; Wood 1993).
Microbial mats . Abundant mats may point to eutrophic
conditions due to increased nitrate fixation by cyanobacte-
ria and other microbes (see Sect. 9.2.).
Bioerosion. Levels of bioerosion are high in eutrophic
areas, and relatively low in oligotrophic environments.An
increase in nutrient supply (C, N, P and trace metals such
as Fe and Mo) in oligotrophic environments influences the
population of bioeroders and may inhibit reef growth
(Peterhänsel and Pratt 2001). The relations between bio-
erosion and micro-encrusters reflect changes from oligo-
trophic to mesotrophic and eutrophic conditions.
Bioturbation. Intensive burrowing of fine-grained car-
bonates may indicate eutrophic conditions caused by the
transport of nutrient elements and organic matter from
deeper sediment layers to the sediment-water interface.
Oncoids and cortoids. Common and autochthonous
cyanobacterial oncoids and cortoids formed by microborers
indicate higher nutrient levels.
Terrigenous grains . Increased fluvial runoff (e.g. dur-
ing regression) will deliver nutrients and lead to higher
nutrient levels than a reduced input of terrigenous material
into shallow-marine carbonate environments (Erlich et al.
1990).
Benthic nutrient types. Suspension feeders are com-
mon in eutrophic environments, symbiotic forms and graz-
ers are more common in oligotrophic environments.
Biotic diversity is often low in eutrophic and higher in
oligotrophic environments.
Density and abundance of sessile benthic fossils may
reflect differences in nutrient levels. An example are Me-
sozoic siliceous sponges exhibiting high densities in low
nutrient areas and low densities in high nutrient environ-
ments.
Changes in ecologic successions and guilds in reefs . A
vertical succession from frame-building to baffling and
from high-growing to low-growing sessile organisms may
indicate an increase in nutrients.
Planktonic organisms. Accumulations of radiolarians,
diatoms, and organic-walled phytoplankton as well as the
occurrence of cold-water planktonic foraminifera may in-
dicate upwelling and high bio-productivity.
Mineralogy
Phosphatic fossils. Accumulations of phosphatic grains.
Formation of phosphorites (see Sect. 13.1.2.4).
Glauconite formation. Phosphogenesis associated with
minimal carbonate accumulation may indicate eutrophic
conditions (Föllmi et al. 1994).
Geochemistry
Organic matter and organic carbon in carbonate rocks.
Intercalation of bedded carbonates and laminated organic-
rich sediments and fluctuations in organic carbon may re-
flect fluctuations in bio-productivity and nutrients or they
reflect preservation differences (Parrish 1995).
Geochemical criteria. Ba/Ca ratios as well as positive
δ 18 O and more negative δ 13 C values in planktonic shells
of upwelling areas are valuable indicators of nutrient con-
ditions (see Sect. 13.2).
Phosphate grains and phosphorites . Both criteria are
used in the context of the discussion on bio-productivity
(Föllmi 1996; see Sect. 13.1.2.4).
Biota
Trophic composition of paleocommunities . A dominance
of deposit-feeders indicates a preferred accumulation of
nutrients within the sediment. A dominance of suspension
feeders points to food particles largely suspended in the
water column.
 
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