Geology Reference
In-Depth Information
Plate 4
Modern NonTropical CoolWater Carbonates: HighLatitudinal Temperate and Polar Shelf
and Reef Carbonates from the Northern Atlantic
Marine carbonate production is by no means restricted to tropical warm-water environments. Abundant and
widespread biogenic carbonate production occurs in non-tropical, mid- and high-latitudinal temperate and cool-
water settings with low or missing clastic input, and even in cold water beyond the Arctic Circle. Examples are
known from the southern and northern hemispheres. Cool-water carbonates record climatic and paleoceanographic
information in their sedimentary deposits and architectures. Microfacies and sedimentary architecture of ancient
cool-water carbonates provide basic data for the evaluation of paleoclimate and paleolatitudes. Modern cool-
water carbonates are good analogues for limestones formed in calcite-dominated Phanerozoic seas.
Widely extended carbonate deposits and even reefal frameworks occur in Subarctic and Arctic seas. Most
sediments are bioclastic sands and gravels, formed by balanids, mollusks, foraminifera and coralline red algae.
Many of these organisms are epibionts, living on benthic macroalgae which form extended kelp forests in nearshore
environments. Kelp and coralline algae are adapted to seasonal climatic perturbations. On the Spitsbergen Bank,
the largest open-shelf cold-water carbonate platform of the Arctic region, the carbonate production is centered
around two main factories: (a) barnacle sands, produced within kelp forests growing on the shallowest part of the
platform and transported over the platform, and (b) high biogenic production by infaunal bivalves (-> 6) as well
as balanid-hydrozoan-soft coral-sponge-bryozoan buildups on the flanks of the platform (Henrich et al. 1997).
Marked differences in the composition of bioclastic grain associations reflect specific environmental conditions
on the shelf bottoms; these 'grain association patterns' are powerful indications of the climatic, oceanographic
and latitudinal controls of ancient cool-water limestones (see Sect. 12.2).
Recognizing reefs in deep, aphotic zones is highly important for interpreting ancient analogues. Different reef
types are formed by corals, coralline red algae (-> 1), siliceous sponges, bryozoans, and mollusks. These organisms,
living in stressed ecosystems, are able to tolerate low water temperatures, and seasonal fluctuations of light
intensity, nutrient availability and salinity. Spectacular reef mounds built by the azooxanthellate coral
Lophelia
(-> 3) originate at present e.g. in eastern Atlantic Ocean of northwestern Europe, at the west Florida and the
Bahama Bank slopes, the southeastern Brazilian continental slopes and on eastern Pacific seamounts.
Azooxanthellate deep-water corals have the potential to construct large, biologically zoned framework-supported
reef mounds below the storm wave base in the aphotic zone (Freiwald 1997). The coral reefs of the Nordic Seas
in water depths between 200 and 1200 m can be followed tens of kilometers at lengths and widths of several
hundreds of meters.
1 Coralline red algal reef: The reefal framework covers about 125 000 square meters and grows in water depths of 6 to
16 m.
Lithothamnion
cf.
glaciale
is one of the most common framework builders. Note the high density of branching
common in turbulent waters. The branches are a prominent source for the production of algal gravels and rhodoliths
deposited onshore and offshore and producing rhodolith pavements. Troms District, Northern Norway.
2 Near-coast coralline algal gravel ('maerl') facies. Multiple fragmentation of branched rhodoliths caused by storms result
in the formation of algal fragment accumulations. The maerl, a mixture of carbonate (skeletal) sand and seaweed is used
as agricultural fertilizer. It characterizes cool-water temperate sediments, composed of unattached crusted-forming branched
or nodular algae and algal gravel. In turbid, high-energy waters of the cool-temperate northwestern European Atlantic,
maerl occurs mostly in the photic zone of wave-protected coasts at depths of 1-10 m (Freiwald et al. 1991). In the warm-
temperate Mediterranean Sea, maerl is common in depths down to 40 m. Water depth 5 m. Troms District, Norway.
3 Azooxanthellate reef-forming deep-water coral (
Lophelia pertusa)
. Azooxanthellate corals harbour no photosynthetic
symbionts. The corals feed on suspended particles which are seasonally generated in the high-productivity euphotic zone.
The corallites of
Lophelia
exhibit strong variations in the branching pattern, the shape and thickness of corallites, the
diameter and the intratentacular budding patterns. These variations support the framebuilding potential of the anastomosing
coral. Faroe Island Shelf. Water depth 252 m.
4 Open-shelf skeletal carbonate sediment composed of mollusks, bryozoans and serpulids. Malangsgrunnen Bank, off
Troms, northern Norway. Water depth 95 m.
5 Selected bioclasts (1-2 mm fraction) of the open-shelf skeletal carbonate shown in -> 4. Most bioclasts are bryozoans.
The facies corresponds to the 'bryomol grain association' (see
Pl.
106/2, 3) which is common on modern cool-temperate
shelves. Water depth 90 m. Malangsgrunnen Bank, off Tromso, northern Norway.
6 Arctic-benthic carbonate deposits of the seasonally sea ice-covered Spitsbergenbank (73°30' N and 9°10' W), western
Barents Sea. Coquina-gravel lag deposit colonized by abundant bivalves (
Mya truncata
, - M, note the cylindrical siphones,
arrow;
Chlamys islandica
- C), echinoids (E), hydrozoans, soft corals, balanids, sponges, bryozoans - B) attributed to the
initiation of small-scale mounds. The high density of in situ organisms is due to a permanent high productivity zone at the
boundary of Atlantic and Arctic surface waters over the flanks of the Spitsbergenbank. Water depth 79 m.
-
> 1-5: Courtesy of A. Freiwald (Erlangen), 6: Freiwald et al. 1991
