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generally, the pteropod link in food webs, and the resulting sequestration of carbon in the
form of carbonate (in excess of the Redfield ratio by at least 10% and possibly up to 100%)
should be favoured by conditions under which preexisting organic matter is being recycled by
microheterotrophs. Overall, it has been estimated (Berner and Honjo, 1981) that the pelagic
aragonite flux (contributed to mainly by pteropods), largely from shallow waters but with
significant lateral transport, accounts for at least 12 % of the worldwide carbonate flux to the
deep ocean.
An interesting aspect of carbonate export from the surface layer into deep waters is the
dissolution of calcareous tests with depth, so that calcite (produced, among pelagic organisms,
by coccolithophores and foraminiferans) is not found in sediments deeper than 4000-5000 m
and aragonite (produced by pteropods) in those deeper than 1000-2500 m. In deep-water
areas, carbonate-carbon is thus progressively released into the water column, so that its fate
(i.e. rapid release to the atmosphere vs. sequestration for tens to hundreds of years in the deep
waters) depends on sinking velocity and on the deep circulation. In shallow waters, on the
other hand, carbonate sediments may sequester carbon for millions of years (e.g. sediment
accumulation up to several kilometres deep near the continents and on the flank of ridges,
limestone deposits from epicontinental seas accumulated on the continents). Areas shallower
than 3000 m and 4000 m represent more than 20% and 40%, respectively, of the total surface
of the oceans. These, and especially the continental margins, should therefore be carefully
investigated when assessing the global flux and sequestration of carbon in the oceans. Another
reason for taking the continental shelves into account is the possible significant export of both
dissolved and particulate organic matter from the ocean margins into the deep sea (e.g. Walsh,
1989).
Concerning the biogeochemical cycle of carbon, the production of plankton organisms with
calcareous tests leads to rather complex results. Limestone deposits were derived, through
biological processes, from atmospheric CO2 but it has been postulated (e.g. Yolk, 1989) that,
at some geological time scales (e.g. glacial/interglacial oscillations), the precipitation of
carbonate by marine organisms results in higher atmospheric CO 2 through changes in the
carbonate chemistry of surface waters (i.e. lower alkalinity and concentration of total
dissolved CO2), This effect has been discussed for coccolithophores (e.g. Dymond and Lyle,
generally, the pteropod link in food webs, and the resulting sequestration of carbon in the
form of carbonate (in excess of the Redfield ratio by at least 10% and possibly up to 100%)
should be favoured by conditions under which preexisting organic matter is being recycled by
microheterotrophs. Overall, it has been estimated (Berner and Honjo, 1981) that the pelagic
aragonite flux (contributed to mainly by pteropods), largely from shallow waters but with
significant lateral transport, accounts for at least 12 % of the worldwide carbonate flux to the
deep ocean.
An interesting aspect of carbonate export from the surface layer into deep waters is the
dissolution of calcareous tests with depth, so that calcite (produced, among pelagic organisms,
by coccolithophores and foraminiferans) is not found in sediments deeper than 4000-5000 m
and aragonite (produced by pteropods) in those deeper than 1000-2500 m. In deep-water
areas, carbonate-carbon is thus progressively released into the water column, so that its fate
(i.e. rapid release to the atmosphere vs. sequestration for tens to hundreds of years in the deep
waters) depends on sinking velocity and on the deep circulation. In shallow waters, on the
other hand, carbonate sediments may sequester carbon for millions of years (e.g. sediment
accumulation up to several kilometres deep near the continents and on the flank of ridges,
limestone deposits from epicontinental seas accumulated on the continents). Areas shallower
than 3000 m and 4000 m represent more than 20% and 40%, respectively, of the total surface
of the oceans. These, and especially the continental margins, should therefore be carefully
investigated when assessing the global flux and sequestration of carbon in the oceans. Another
reason for taking the continental shelves into account is the possible significant export of both
dissolved and particulate organic matter from the ocean margins into the deep sea (e.g. Walsh,
1989).
Concerning the biogeochemical cycle of carbon, the production of plankton organisms with
calcareous tests leads to rather complex results. Limestone deposits were derived, through
biological processes, from atmospheric CO2 but it has been postulated (e.g. Yolk, 1989) that,
at some geological time scales (e.g. glacial/interglacial oscillations), the precipitation of
carbonate by marine organisms results in higher atmospheric CO 2 through changes in the
carbonate chemistry of surface waters (i.e. lower alkalinity and concentration of total
dissolved CO2), This effect has been discussed for coccolithophores (e.g. Dymond and Lyle,