Variation in calcification of Reticulofenestra coccoliths over the Oligocene–Early Miocene
Coccolithophores are calcifying marine phytoplankton whose intracellularly produced calcite plates, coccoliths, have been the dominant source of calcium carbonate in open-ocean settings since the Cretaceous. An open question is whether their calcification has been affected by changing environmental...
| Published in: | Biogeosciences |
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| Main Authors: | , , , , |
| Format: | Text |
| Language: | English |
| Published: |
2022
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| Subjects: | |
| Online Access: | https://doi.org/10.5194/bg-19-5007-2022 https://bg.copernicus.org/articles/19/5007/2022/ |
| Summary: | Coccolithophores are calcifying marine phytoplankton whose intracellularly produced calcite plates, coccoliths, have been the dominant source of calcium carbonate in open-ocean settings since the Cretaceous. An open question is whether their calcification has been affected by changing environmental conditions over geological timescales such as variations in the ocean carbon system. Previous methods using circular polarized light microscopy allowed for only the thickness of small coccoliths thinner than 1.5 µ m to be quantified, but prior to the Pliocene, a significant fraction of the coccoliths exceeded this thickness and have not been quantifiable. Here, we implement a new approach for calibration of circular polarized light microscopy enabling us to quantify coccoliths which feature calcite up to 3 µ m thick. We apply this technique to evaluate the evolution of calcification in the Reticulofenestra from the early Oligocene to Early Miocene in exceptionally well-preserved sediments from the Newfoundland margin. Through this time interval, coccolith thickness and the scale-invariant shape factor k se vary by about 20 % around the mean thickness of 0.37 µ m and mean k se of 0.16. Lower shape factors characterize samples with a higher relative abundance of dissolution-resistant nannoliths, suggesting that dissolution may contribute to thinning of placoliths. We therefore define temporal trends in calcification only in samples in which the assemblage suggests minimal dissolution. The lowest k se characterizes the middle Oligocene, and the highest k se around 18 Ma is in the Early Miocene. High ocean dissolved inorganic carbon (DIC) concentrations have been proposed for this period of the Miocene and may be one factor contributing to high coccolith k se . |
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