Variation in calcification of Reticulofenestra coccoliths over the Oligocene–Early Miocene

[摘要] Coccolithophores are calcifying marine phytoplankton whose intracellularlyproduced calcite plates, coccoliths, have been the dominant source ofcalcium carbonate in open-ocean settings since the Cretaceous. An openquestion is whether their calcification has been affected by changingenvironmental conditions over geological timescales such as variations inthe ocean carbon system. Previous methods using circular polarized lightmicroscopy allowed for only the thickness of small coccoliths thinner than 1.5  µ m to be quantified, but prior to the Pliocene, a significant fractionof the coccoliths exceeded this thickness and have not been quantifiable.Here, we implement a new approach for calibration of circular polarizedlight microscopy enabling us to quantify coccoliths which feature calcite upto 3  µ m thick. We apply this technique to evaluate the evolution ofcalcification in the Reticulofenestra from the early Oligocene to Early Miocene inexceptionally well-preserved sediments from the Newfoundland margin. Throughthis time interval, coccolith thickness and the scale-invariant shape factor k se vary by about 20 % around the mean thickness of0.37  µ mand mean k se of 0.16. Lower shape factors characterize samples witha higher relative abundance of dissolution-resistant nannoliths, suggestingthat dissolution may contribute to thinning of placoliths. We thereforedefine temporal trends in calcification only in samples in which theassemblage suggests minimal dissolution. The lowest k se characterizesthe middle Oligocene, and the highest k se around 18 Ma is in theEarly Miocene. High ocean dissolved inorganic carbon (DIC) concentrations have been proposed forthis period of the Miocene and may be one factor contributing to highcoccolith k se .

[学科分类] 大气科学