Mg/Ca variation within the planktic foraminifer Orbulina universa
Mg/Ca composition and test wall profile characteristics have been measured in a population of Orbulina universa (a symbiotic planktonic foraminifera) obtained from a core-top sediment sample from off the west Australian coast (20º04 S, 112º66 E). Laser ablation-inductively coupled plasma mass spectr...
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| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.614.8444 http://rses.anu.edu.au/research/annrep/ar2004/ee04/Sadekov AR 2004 SME1.pdf |
| Summary: | Mg/Ca composition and test wall profile characteristics have been measured in a population of Orbulina universa (a symbiotic planktonic foraminifera) obtained from a core-top sediment sample from off the west Australian coast (20º04 S, 112º66 E). Laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) and electron microprobe have been used to determine the distribution of Mg/Ca and other elements at micron/submicron-scale resolution within individual foraminifera shells. LA-ICPMS profiles and EMPA maps reveal the development of up to 6 alternating low and high Mg/Ca bands that increase in amplitude and Mg/Ca value toward the outer test surface. These results, gether with the reported duration of test calcification (e.g. Caron et al., 1987), to indicate the Mg/Ca composition of calcite precipitated by O. universa is strongly regulated on a diurnal cycle (Eggins et al., 2004). The development of Mg/Ca banding in O. universa can be linked to the day-time photosynthetic activity of algal symbionts and night-time respiration of both the host foraminifer and symbionts, which modulate the pH and calcite saturation state within the foraminiferal microenvironment (Wolf-Gleadrow and Riebesell, 1997). We attribute the high and low Mg/Ca bands to night-time and day-time calcification respectively based on previously documented increase in bulk test Mg/Ca composition with decreasing seawater pH (Lea et al., 1999). ‘Vital effects ’ similar to those observed in O. universa may affect other symbiont bearing species, and could bias bulk test Mg/Ca compositions and thus derived estimates of palaeo-seawater temperature. Our results provide new insights into the potential importance of such ‘vital effects ’ in controlling the Mg/Ca composition of foraminiferal calcite. They also emphasise the need for a more complete understanding of influences upon the Mg/Ca composition of foraminiferal calcite in order to evaluate both the precision and accuracy of Mg/Ca paleo-seawater thermometry based on bulk test compositions. |
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