86
Sources and Composition of Marine Sediments
upon precipitation. We shall take up this theme again when discussing the greenhouse
effect (see Sect. 7.7.5).
3.7.2 The Bahamas are ideally suited to test for inorganic precipitation. They are
surrounded by some of the warmest, most alkaline waters in the ocean. There is
hardly any terrigenous input, and pure carbonates are accumulating. The Great Bahama Bank is extremely flat and usually less than 5 m deep (Fig. 3.9a). High rates of
evaporation and low rates of rainfall results in increased salinities, which can attain
over 40 %0. Hence [Ca 2 +] (observed) is increased in Eq. (3.2). Heating of the water
by the tropical sun further increases Dsat. Algae grow abundantly on the sea floor,
removing C02 during the day. Thus, conditions are indeed very favorable for the
precipitation of carbonate.
Two types of calcareous particles on the sea floor of the Great Bank have been
considered as possible products of inorganic precipitation: aragonite needles (length
of a few micrometers) and oolites (consisting of spherical ooids with a diameter near
one third of a millimeter) (Fig. 3.9).
The origin of the aragonite needles has been a puzzle for some time; both direct
precipitation and mechanical breakup (perhaps aided by boring clams, deposit
feeders, etc.) of pre-existing skeletons has been suggested. However, modem investigations, using scanning electron microscopy and stable isotope analysis, indicate that
most of the needles are formed within certain algae.
The oolites are spherical objects formed of concentric layers mostly of aragonite
needles and organic matter. They are quite abundant in the geologic record especially
in subtropical shelf sediments. In the Bahamas they occur especially on the outer rim
of the Great Bank, in the shallowest water. Here, in the zone of growth, they are
altemately moved about by strong tidal currents, or rest just below the sea floor,
buried by other oolites. Recent laboratory experiments suggest that ooids are formed
only if sufficient organic matter is present. According to high-resolution stereos can
investigations, precipitation is largely through biocalcification by unicellular algae.
Hence, carbonate precipitation directly from seawater seems to be negligible under
present-day conditions.
In sum, carbonate platforms and shelves are enormous biological factories generating biogenous carbonates, in competition with the open ocean plankton. The steep
slopes off the carbonate shelves are due to the fact that the carbonate sands and muds
delivered from above can harden quickly by cementation. Coral reefs are responsible
for sharp shelf edges. The alternation from glacial to interglacial sea-level variation,
and between exposure and submergence of carbonate shelves, has important implications for the carbonate budget of the entire ocean, and hence also for the concentration of C02 in the atmosphere (Sect. 8.5).
We have seen that during low sea-level stands more material reaches the deep sea
by turbidity current transport (Chap. 2.10). The reason is an increase of delivery of
terrigenous sediments by continental erosion during glacial time, and a lack of estuaries that could trap the material: it is delivered right to the shelf edge. The reverse
is the case in the region surrounding the Bahama Banks and around similar carbonate
platforms, where biological sediment production dominates. Such platforms deliver
Sources and Composition of Marine Sediments
upon precipitation. We shall take up this theme again when discussing the greenhouse
effect (see Sect. 7.7.5).
3.7.2 The Bahamas are ideally suited to test for inorganic precipitation. They are
surrounded by some of the warmest, most alkaline waters in the ocean. There is
hardly any terrigenous input, and pure carbonates are accumulating. The Great Bahama Bank is extremely flat and usually less than 5 m deep (Fig. 3.9a). High rates of
evaporation and low rates of rainfall results in increased salinities, which can attain
over 40 %0. Hence [Ca 2 +] (observed) is increased in Eq. (3.2). Heating of the water
by the tropical sun further increases Dsat. Algae grow abundantly on the sea floor,
removing C02 during the day. Thus, conditions are indeed very favorable for the
precipitation of carbonate.
Two types of calcareous particles on the sea floor of the Great Bank have been
considered as possible products of inorganic precipitation: aragonite needles (length
of a few micrometers) and oolites (consisting of spherical ooids with a diameter near
one third of a millimeter) (Fig. 3.9).
The origin of the aragonite needles has been a puzzle for some time; both direct
precipitation and mechanical breakup (perhaps aided by boring clams, deposit
feeders, etc.) of pre-existing skeletons has been suggested. However, modem investigations, using scanning electron microscopy and stable isotope analysis, indicate that
most of the needles are formed within certain algae.
The oolites are spherical objects formed of concentric layers mostly of aragonite
needles and organic matter. They are quite abundant in the geologic record especially
in subtropical shelf sediments. In the Bahamas they occur especially on the outer rim
of the Great Bank, in the shallowest water. Here, in the zone of growth, they are
altemately moved about by strong tidal currents, or rest just below the sea floor,
buried by other oolites. Recent laboratory experiments suggest that ooids are formed
only if sufficient organic matter is present. According to high-resolution stereos can
investigations, precipitation is largely through biocalcification by unicellular algae.
Hence, carbonate precipitation directly from seawater seems to be negligible under
present-day conditions.
In sum, carbonate platforms and shelves are enormous biological factories generating biogenous carbonates, in competition with the open ocean plankton. The steep
slopes off the carbonate shelves are due to the fact that the carbonate sands and muds
delivered from above can harden quickly by cementation. Coral reefs are responsible
for sharp shelf edges. The alternation from glacial to interglacial sea-level variation,
and between exposure and submergence of carbonate shelves, has important implications for the carbonate budget of the entire ocean, and hence also for the concentration of C02 in the atmosphere (Sect. 8.5).
We have seen that during low sea-level stands more material reaches the deep sea
by turbidity current transport (Chap. 2.10). The reason is an increase of delivery of
terrigenous sediments by continental erosion during glacial time, and a lack of estuaries that could trap the material: it is delivered right to the shelf edge. The reverse
is the case in the region surrounding the Bahama Banks and around similar carbonate
platforms, where biological sediment production dominates. Such platforms deliver