Influence of temperature and CO<sub>2</sub> on the strontium and magnesium composition of coccolithophore calcite
Marine calcareous sediments provide a fundamental basis for palaeoceanographic studies aiming to reconstruct past oceanic conditions and understand key biogeochemical element cycles. Calcifying unicellular phytoplankton (coccolithophores) are a major contributor to both carbon and calcium cycling by...
| Main Authors: | , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2014-02-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/11/1065/2014/bg-11-1065-2014.pdf |
| Summary: | Marine calcareous sediments provide a fundamental basis for palaeoceanographic
studies aiming to reconstruct past oceanic conditions and understand key
biogeochemical element cycles. Calcifying unicellular phytoplankton
(coccolithophores) are a major contributor to both carbon and calcium cycling
by photosynthesis and the production of calcite (coccoliths) in the euphotic
zone, and the subsequent long-term deposition and burial into marine
sediments. Here we present data from controlled laboratory experiments on
four coccolithophore species and elucidate the relation between the divalent
cation (Sr, Mg and Ca) partitioning in coccoliths and cellular physiology
(growth, calcification and photosynthesis). Coccolithophores were cultured
under different seawater temperature and carbonate chemistry conditions. The
partition coefficient of strontium (<i>D</i><sub>Sr</sub>) was positively
correlated with both carbon dioxide (<i>p</i>CO<sub>2</sub>) and temperature but
displayed no coherent relation to particulate organic and inorganic carbon
production rates. Furthermore, <i>D</i><sub>Sr</sub> correlated positively with
cellular growth rates when driven by temperature but no correlation was
present when changes in growth rates were <i>p</i>CO<sub>2</sub>-induced. Our results
demonstrate the complex interaction between environmental forcing and
physiological control on the strontium partitioning in coccolithophore
calcite and challenge interpretations of the coccolith Sr / Ca ratio from
high-<i>p</i>CO<sub>2</sub> environments (e.g. Palaeocene–Eocene thermal maximum). The
partition coefficient of magnesium (<i>D</i><sub>Mg</sub>) displayed
species-specific differences and elevated values under nutrient limitation.
No conclusive correlation between coccolith <i>D</i><sub>Mg</sub> and temperature
was observed but <i>p</i>CO<sub>2</sub> induced a rising trend in coccolith
<i>D</i><sub>Mg</sub>. Interestingly, the best correlation was found between
coccolith <i>D</i><sub>Mg</sub> and chlorophyll <i>a</i> production, suggesting
that chlorophyll <i>a</i> and calcite associated Mg originate from the same
intracellular pool. These and previous findings indicate that Mg is
transported into the cell and to the site of calcification via different
pathways than Ca and Sr. Consequently, the coccolith Mg / Ca ratio should be
decoupled from the seawater Mg / Ca ratio. This study gives an extended
insight into the driving factors influencing the coccolith Mg / Ca ratio and
should be considered for future palaeoproxy calibrations. |
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| ISSN: | 1726-4170 1726-4189 |