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| Titel |
Review: geological and experimental evidence for secular variation in seawater Mg/Ca (calcite-aragonite seas) and its effects on marine biological calcification |
| VerfasserIn |
J. B. Ries |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 7, no. 9 ; Nr. 7, no. 9 (2010-09-21), S.2795-2849 |
| Datensatznummer |
250004972
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| Publikation (Nr.) |
 copernicus.org/bg-7-2795-2010.pdf |
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| Zusammenfassung |
Synchronized transitions in the polymorph mineralogy of the major
reef-building and sediment-producing calcareous marine organisms and abiotic
CaCO3 precipitates (ooids, marine cements) throughout Phanerozoic time
are believed to have been caused by tectonically induced variations in the
Mg/Ca ratio of seawater (molar Mg/Ca>2="aragonite seas", <2="calcite seas").
Here, I assess the geological evidence in support of
secular variation in seawater Mg/Ca and its effects on marine calcifiers,
and review a series of recent experiments that investigate the effects of
seawater Mg/Ca (1.0–5.2) on extant representatives of calcifying taxa
that have experienced variations in this ionic ratio of seawater throughout
the geologic past.
Secular variation in seawater Mg/Ca is supported by synchronized secular
variations in (1) the ionic composition of fluid inclusions in primary
marine halite, (2) the mineralogies of late stage marine evaporites,
abiogenic carbonates, and reef- and sediment-forming marine calcifiers, (3)
the Mg/Ca ratios of fossil echinoderms, molluscs, rugose corals, and
abiogenic carbonates, (4) global rates of tectonism that drive the exchange
of Mg2+ and Ca2+ along zones of ocean crust production, and (5)
additional proxies of seawater Mg/Ca including Sr/Mg ratios of abiogenic
carbonates, Sr/Ca ratios of biogenic carbonates, and Br concentrations in
marine halite.
Laboratory experiments have revealed that aragonite-secreting bryopsidalean
algae and scleractinian corals and calcite-secreting coccolithophores
exhibit higher rates of calcification and growth in experimental seawaters
formulated with seawater Mg/Ca ratios that favor their skeletal mineral.
These results support the assertion that seawater Mg/Ca played an important
role in determining which hypercalcifying marine organisms were the major
reef-builders and sediment-producers throughout Earth history. The
observation that primary production increased along with calcification
within the bryopsidalean and coccolithophorid algae in mineralogically
favorable seawater is consistent with the hypothesis that calcification
promotes photosynthesis within some species of these algae through the
liberation of CO2.
The experiments also revealed that aragonite-secreting bryopsidalean algae
and scleractinian corals, and bacterial biofilms that secrete a mixture of
aragonite and high Mg calcite, began secreting an increased proportion of
their calcium carbonate as the calcite polymorph in the lower-Mg/Ca
experimental seawaters. Furthermore, the Mg/Ca ratio of calcite secreted by
the coccolithophores, coralline red algae, reef-dwelling animals (crustacea,
urchins, calcareous tube worms), bacterial biofilms, scleractinian corals,
and bryopsidalean algae declined with reductions in seawater Mg/Ca. Notably,
Mg fractionation in autotrophic organisms was more strongly influenced by
changes in seawater Mg/Ca than in heterotrophic organisms, a probable
consequence of autotrophic organisms inducing a less controlled mode of
calcification simply through the removal of CO2 via photosynthesis.
These results indicate that biomineralogical control can be partially
overridden by ambient seawater Mg/Ca and suggest that modern
aragonite-secreting organisms may have secreted a mixture of aragonite and
low Mg calcite, and that modern high Mg calcite-secreting organisms probably
secreted low Mg calcite, in calcite seas of the past. These effects of
seawater Mg/Ca on the polymorph mineralogy and calcite Mg/Ca ratio of
calcareous skeletons should be accounted for in thermal-chemical
reconstructions of seawater that are based upon skeletal Mg/Ca.
Lastly, by identifying how marine calcifiers respond to changes in seawater
Mg/Ca and absolute Ca2+ concentration, this work should enhance our
interpretation of parallel studies investigating the effects of
anthropogenic CO2-induced ocean acidification on marine calcification. |
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