Geology Reference
In-Depth Information
Fig. 12.8. Inferred relationships among the major environmental factors of the Great Bahama Bank determining the relations
between substrate and bottom-dwelling organisms. A: All ecologic factors depend on basin geometry and basin energy.
Substrate types are predominantly controlled by water turbulence, circulation and salinity. After Newell et al. 1959.
B: Relationships between feeding types, substrate and water energy in oolitic shoals. Increasing substrate mobility has two
major ecologic consequences: (1) Increase in bottom current velocity results in an increase of substrate mobility, which in
turn causes winnowing of organic matter from the sediment and prevents deposition of organic detritus. Consequently, the
substrate contains only a relatively low percentage of organic matter. The low organic content limits the number of deposit-
feeding individuals and taxa. (2) Shifting bottoms inhibit colonization of the substrate by many marine plants. This limits the
taxonomic and numerical abundance of the vegetal-matter consuming herbivores. The scarceness of organic nutrients in the
substrate and the poorly developed flora on the substrate are reflected by a bottom fauna characterized by low-diversity
suspension feeders represented by vagile forms (e.g. mollusks) on highly mobile substrates (such as oolite shoals), or sessile
forms (e.g. sponges) on moderately mobile substrates (such as fine-grained sands). After Purdy (1964).
or installation of lamination through organic mats), and
(4) water content and fluid movement within intergranu-
lar pores as well as sediment chemistry (thus influenc-
ing early diagenesis; see Meadows 1986).
Bottom-dwelling organisms are deposit feeders and
suspension feeders as well as carnivores and herbivores.
The distribution of the various feeding types depends
on the distribution of the requisite food source.
Deposit feeders, which feed on the organic debris
and detritus accumulated on or within the substrate,
occur predominantly in fine-grained sediments. Sus-
pension-feeders, which filter or strain suspended or-
ganic matter or microscopic organisms out of the wa-
ter are common in medium- to coarse-grained silty and
sandy sediments. Suspension feeders depend on the
amount of food in the surrounding water; deposit feed-
ers depend on the amount of food in and on the sedi-
ment. The distribution of nutrients at sea bottom and in
the water column is controlled by differences in water
energy. Fig. 12.8 summarizes the basic relationships
between water energy, nutrient source and organisms
with different feeding strategies.
The activity of benthic organisms can change
grain size, sorting and shape, usually by digestive
activity,
texture and fabric of the sediment, by mechanical
displacement of the sediment,
sedimentary structures (bedding, lamination, topog-
raphy),
water content and fluid movement in pores, by res-
piration and excretion,
compaction,
shear stress and stability (softness and hardness), and
sediment chemistry.
Substrate and feeding types
Basically four feeding types can be recognized:
(1) suspension feeders feeding on microorganisms and
organic detritus suspended in water,
(2) deposit feeders feeding on the same material in or
on the sediment,
(3) carnivores feeding on other animals, and
(4) herbivores , feeding on plant material. Omnivores
employ two or more of the listed feeding types.
12.1.3.2 Recognizing Substrate Types
Thin-section criteria that allow primary substrate types
to be differentiated are listed in Box 12.5. The main
criteria of soft bottoms are ichnofabric and the compo-
sition of bottom and sediment dwellers. Hard bottoms
can be recognized from borings and encrustations.
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