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The magnesium to calcium ratio (Mg/Ca) of benthic foraminiferal calcite serves as an important tool for reconstructing past deep water temperature. Application of this proxy relies upon accurate calibrations and an understanding of the factors that may influence the Mg/Ca ratios of foraminifer tests. Core-top calibrations are a method of assessing the temperature sensitivity of deep-dwelling benthic taxa which are difficult to raise in culture. This study contributes a new set of Mg/Ca core-top measurements for the infaunal species Uvigerina peregrina derived from a suite of sediment cores in the Southwest Pacific spanning water depths of 600 to 4400 m. Results agreed with previous calibrations for samples shallower than 2000 m, but unexpectedly high Mg/Ca values were found between the depths of 2400 and 3300 m, necessitating further investigation into potential non-temperature influences. Specimens of different morphotypes were analyzed separately, but variations between hispid and costate samples failed to account for the high-Mg anomaly observed. Lack of correlation between Mg/Ca and the contaminant indicators Mn/Ca, Al/Ca, Fe/Ca, and Ti/Ca suggests contaminant phases are not the source of excess Mg. Laser ablation ICP-MS analysis of chamber cross-sections revealed that the high-Mg signature is located within the interior of test walls, rather than contained in an external coating or contaminant phase. The high- Mg anomaly observed in mid-depth New Zealand waters is likely related to a secondary, non-temperature control on Mg incorporation. Samples with excess Mg are those most strongly influenced by carbon-rich (high dissolved inorganic carbon, high alkalinity) waters flowing south from the northern Pacific, suggesting that inorganic carbonate chemistry plays a role.
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The Mg/Ca proxy for temperature: A Uvigerina core-top study in the Southwest Pacific
The Mg/Ca ratio of the infaunal benthic foraminifer Uvigerina peregrina is a commonly used proxy for reconstructing bottom water paleotemperatures because it is hypothesized to be insensitive to changes in bottom water carbonate chemistry. This study presents core-top U. peregrina samples from the southwest Pacific from 0.6 to 4.4 km water depth, corresponding to water temperatures of 1–9 C. Samples from New Zealand’s Bay of Plenty and Chatham Rise were compared to previous calibrations to assess Mg/Ca temperature sensitivity. Published core-top temperature sensitivies can explain U. peregrina Mg/Ca at a majority of our study sites, with the exception of sites at intermediate depths of ~2.4–3.3 km, where the Mg/Ca ratios are substantially higher than expected from these calibrations, possibly reflecting diagenetic or non-thermal effects. Stable isotope measurements (d18O and d13C), laser ablation analyses, and scanning electron microscope imagery complement trace element measurements to assess variables potentially affecting U. peregrina Mg/Ca at these sites. Morphotype variability, contamination, dissolution, and recrystallization all failed to provide satisfactory explanations for anomalously high Mg/Ca observations. We infer that a non-temperature control, perhaps related to carbonate chemistry, may be affecting the Mg incorporation in some U. peregrina specimens, though no factor has yet been clearly identified. It is unclear whether the factor affecting these sites is constant through time or could vary. For this reason, we recommend that Mg/Ca of recent specimens at each core site should be checked against established calibration curves prior to pursuing down-core paleotemperature reconstructions using U. peregrina. Where possible, paleotemperature estimates should also be validated using other independent proxies. Existing core-top calibrations effectively predict U. peregrina Mg/Ca at the majority of our study sites, but our findings underscore the need for a more thorough understanding of non-temperature factors that can influence Mg/Ca in U. peregrina.
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- PAR ID:
- 10308595
- Date Published:
- Journal Name:
- Geochimica et cosmochimica acta
- Volume:
- 309
- ISSN:
- 0016-7037
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/https://doi.org/10.1016/j.gca.2021.06.015
