A field-based study on variation in Mg/Ca and Sr/Ca in larger benthic foraminifera

The Mg/Ca and Sr/Ca ratios of foraminifera are used as proxies of paleotemperature. Although the Mg/Ca is found to be related with temperature in planktonic foraminifera, the reliability of this proxy in benthic foraminifera has yet to be established. It is essential therefore to understand the mino...

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Bibliographic Details
Published in:Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2007-10, Vol.8 (10), p.Q10012-n/a
Main Authors: Raja, Ramalingam, Saraswati, Pratul Kumar, Iwao, Kenji
Format: Article
Language:English
Subjects:
Algae
Biological and Chemical
Geochemistry
larger foraminifera
Life span
Magnesium
Marine Geology and Geophysics
Mg/Ca
Micropaleontology
Oceanography
Paleoceanography
paleotemperature
Proxy client servers
Salinity
Sea surface temperature
Seasons
Sr/Ca
Strontium
Symbiosis
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Summary:The Mg/Ca and Sr/Ca ratios of foraminifera are used as proxies of paleotemperature. Although the Mg/Ca is found to be related with temperature in planktonic foraminifera, the reliability of this proxy in benthic foraminifera has yet to be established. It is essential therefore to understand the minor elements' composition and their relation to ambient water parameters in different benthic foraminiferal species. In this study, a systematic field‐based study was carried out on larger benthic foraminifera (LBF) of Okinawa (Japan) to assess if Mg/Ca and Sr/Ca in their tests could be used as proxies of seawater temperature. Five seasonal samples were collected from the reef flat to pick live specimens of Amphistegina lessonii, Neorotalia calcar, Calcarina hispida, Calcarina gaudichaudii, Baculogypsina sphaerulata, and Marginopora kudakajimaensis. The Mg/Ca in A. lessonii was ∼60 mmol/mol, while in the remaining five species the values were >200 mmol/mol, substantially higher than that recorded in planktonic and smaller benthic foraminifera. Interspecies and intraspecies Mg/Ca variations were fairly large even in samples of the same season. The Sr/Ca were quite uniform (∼2 mmol/mol) across the species and seasons. The correlations between test Mg/Ca and Sr/Ca and ambient water temperature and salinity were found to be poor. The possible reasons are (1) the influence of biological activities, particularly the photosynthesis‐respiration cycle of algal symbionts, and (2) masking of the true relationship due to an “averaging” effect in whole‐test analysis of species having longer a life span. In view of this it is suggested that the whole‐shell Mg/Ca and Sr/Ca of LBF is not an apt proxy of paleotemperature.
ISSN:1525-2027
1525-2027
DOI:10.1029/2006GC001478