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Aragonitic algae in calcite seas; effect of seawater Mg/Ca ratio on algal sediment production

Justin B. Ries
Aragonitic algae in calcite seas; effect of seawater Mg/Ca ratio on algal sediment production
Journal of Sedimentary Research (April 2006) 76 (3-4): 515-523


Secular variation in seawater Mg/Ca would have subjected the aragonite-producing codiacean algae to two intervals of low-Mg calcite seas (mMg/Ca <2) and two intervals of aragonite+high-Mg calcite seas (mMg/Ca >2) seas since their origin in the Ordovician. It is asserted that the aragonite+high-Mg calcite affinity of modern and Recent tropical seas (mMg/Ca >2) has enabled the aragonitic Codiaceae to act as major carbonate sediment producers in these seas. To test this assertion, specimens of the sediment-producing codiacean alga Udotea flabellum were grown in artificial seawaters formulated at mMg/Ca ratios (1.0, 2.5, 5.2) that have occurred in the oceans throughout the geologic history of the Codiaceae. While the U. flabellum algae produced exclusively aragonite in both the artificial boundary seawater (mMg/Ca = 2.5) and the artificial aragonite+high-Mg calcite seawater (modern seawater; mMg/Ca = 5.2), the algae produced a portion (25+ or -3%) of their CaCO (sub 3) as low-Mg calcite (< or =4 mol% MgCO (sub 3) ) in the artificial calcite seawater (mMg/Ca = 1.0), suggesting that their CaCO (sub 3) polymorph control can be partially overridden by ambient seawater chemistry. Primary productivity, calcification, and linear growth of the algae decreased with reductions in ambient Mg/Ca, despite increases in absolute Ca concentration of the seawater. Reduced calcification in the lower Mg/Ca seawaters is probably due to kinetic difficulties of producing aragonite from seawater that favors the nucleation of calcite. The decreased linear growth and primary production are likely associated with reductions in the ambient CO (sub 2) available for photosynthesis, resulting from the reduced calcification. These findings support the assertion that the dominant ecological and sedimentological roles of codiacean algae in Recent tropical seas is facilitated by a seawater Mg/Ca ratio that favors the nucleation of aragonite over calcite. Furthermore, producing aragonite in seawater that favors the nucleation of low-Mg calcite (mMg/Ca <2) may have reduced the rates of growth and calcification of codiacean algae, making them less competitive for space on the shallow tropical seafloor and more susceptible to predatory grazing, thereby reducing their contribution to carbonate sedimentation.

ISSN: 1527-1404
EISSN: 1938-3681
Serial Title: Journal of Sedimentary Research
Serial Volume: 76
Serial Issue: 3-4
Title: Aragonitic algae in calcite seas; effect of seawater Mg/Ca ratio on algal sediment production
Author(s): Ries, Justin B.
Affiliation: Johns Hopkins University, Morton K. Blaustein Department of Earth and Planetary Sciences, Baltimore, MD, United States
Pages: 515-523
Published: 200604
Text Language: English
Publisher: Society for Sedimentary Geology, Tulsa, OK, United States
References: 78
Accession Number: 2006-056462
Categories: Geochemistry of rocks, soils, and sedimentsSedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data supplied by SEPM (Society for Sedimentary Geology), Tulsa, OK, United States
Update Code: 200631
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