ALBERT

Description: As major calcifiers in the open ocean, coccolithophores play a key role in the marine carbon cycle. Because they may be sensitive to changing CO2 and ocean acidification, there is significant interest in quantifying past and present variations in their cellular calcification by quantifying the thickness of the coccoliths or calcite plates that cover their cells. Polarized light microscopy has emerged as a key tool for quantifying the thickness of these calcite plates, but the reproducibility and accuracy of such determinations has been limited by the absence of suitable calibration materials in the thickness range of coccoliths (0–4 microns). Here, we describe the fabrication of a calcite wedge with a constant slope over 15 this thickness range, and the independent determination of calcite thickness along the wedge profile. We show how the calcite wedge provides more robust calibrations in the 0 to 1.55 μm range than previous approaches using rhabdoliths. We show the particular advantages of the calcite wedge approach for developing equations to relate thickness to the interference colors that arise in calcite in the thickness range between 1.55 and 4 μm. The calcite wedge approach can be applied to develop equations relevant to the particular light spectra and intensity of any polarized light microscope system and could significantly improve within and inter-laboratory data comparability.

Description: Quantification sinking velocities of individual coccoliths will contribute to optimizing laboratory methods for separating coccoliths of different sizes and species for geochemical analysis. The repeated settling–decanting method was the earliest method proposed to separate coccoliths from sediments and is still widely used. However, in the absence of estimates of settling velocity for nonspherical coccoliths, previous implementations have depended mainly on time-consuming empirical method development by trial and error. In this study, the sinking velocities of coccoliths belonging to different species were carefully measured in a series of settling experiments for the first time. Settling velocities of modern coccoliths range from 0.154 to 10.67 cm h−1. We found that a quadratic relationship between coccolith length and sinking velocity fits well, and coccolith sinking velocity can be estimated by measuring the coccolith length and using the length–velocity factor, kv. We found a negligible difference in sinking velocities measured in different vessels. However, an appropriate choice of vessel must be made to avoid “hindered settling” in coccolith separations. The experimental data and theoretical calculations presented here support and improve the repeated settling–decanting method.

Description: The sinking velocities of individual coccoliths are relevant for export of their CaCO3 from the surface ocean, and for laboratory methods to separate coccoliths of different sizes and species for geochemical analysis. In the laboratory, the repeat settling/decanting method was the earliest method to separate coccolith from sediments for geochemical analyses, and is still widely used. However, in the absence of estimates of settling velocity for non-spherical coccoliths, previous implementations have depended mainly on time consuming empirical method development by trial and error. In this study, the sinking velocities of coccoliths belonging to different species were carefully measured in a series of settling experiments for the first time. Settling velocities of modern coccoliths range from 0.154 to 10.67 cm h−1. We found that a quadratic relationship between coccolith length and sinking velocity fits well and coccolith sinking velocity can be estimated by measuring the coccolith length and using the length-velocity factor, ksv. We found a negligible difference in sinking velocities measured in different vessels. However, an appropriate choice of vessel must be made to avoid hindered settling in coccolith separations. The experimental data and theoretical calculations presented here will support and improve the repeat settling/decanting method.

Description: As major calcifiers in the open ocean, coccolithophores play a key role in the marine carbon cycle. Because they may be sensitive to changing CO2 and ocean acidification, there is significant interest in quantifying past and present variations in their cellular calcification by quantifying the thickness of the coccoliths or calcite plates that cover their cells. Polarized light microscopy has emerged as a key tool for quantifying the thickness of these calcite plates, but the reproducibility and accuracy of such determinations has been limited by the absence of suitable calibration materials in the thickness range of coccoliths (0–4 µm). Here, we describe the fabrication of a calcite wedge with a constant slope over this thickness range, and the independent determination of calcite thickness along the wedge profile. We show how the calcite wedge provides more robust calibrations in the 0 to 1.55 µm range than previous approaches using rhabdoliths. We show the particular advantages of the calcite wedge approach for developing equations to relate thickness to the interference colors that arise in calcite in the thickness range between 1.55 and 4 µm. The calcite wedge approach can be applied to develop equations relevant to the particular light spectra and intensity of any polarized light microscope system and could significantly improve inter-laboratory data comparability.

Description: Coccolithophores play a key role in the marine carbon cycle and ecosystem. The carbonate shells produced by coccolithophore, named as coccolith, could be well preserved in the marine sediment for millions of years and become an excellent archive for paleoclimate studies. The micro-filtering and sinking–decanting methods have been successfully designed for coccolith separation and promoted the development of geochemistry studies on coccolith, such as the stable isotopes and Sr / Ca ratio. However, these two methods are still not efficient enough for the sample-consuming methods. In this study, the trajectory of coccolith movement during a centrifugation process was calculated in theory and carefully tested by separations in practice. We offer a MATLAB code to estimate the appropriate parameter, angular velocity at a fixed centrifugation duration, for separating certain coccolith size fractions from bulk sediment. This work could improve the efficiency of coccolith separation, especially for the finest size fraction, and make it possible to carry out the clumped isotope and radio carbon analyses on coccoliths in sediment.

Description: Coccolithophores contribute significantly to marine primary productivity and play a unique role in ocean biogeochemistry by using carbon for photosynthesis (soft-tissue pump) and for calcification (carbonate counter pump). Despite the importance of including coccolithophores in Earth system models to allow better predictions of the climate system's responses to planetary change, the reconstruction of coccolithophore productivity mostly relied on proxies dependent on accumulation and sedimentation rates and preservation conditions. In this study we used an independent proxy, based on the coccolith fraction (CF) Sr∕Ca ratio, to reconstruct coccolithophore productivity. We studied the marine sediment core MD03-2699 from the western Iberian margin (IbM), concentrating on glacial–interglacial cycles of Marine Isotopic Stage (MIS) 12 to MIS 9. We found that IbM coccolithophore productivity was controlled by changes in the oceanographic conditions, such as in sea surface temperature (SST) and nutrient availability, and by competition with other phytoplankton groups. Long-term coccolithophore productivity was primarily affected by variations in the dominant surface water mass. Polar and subpolar surface waters during glacial substages were associated with decreased coccolithophore productivity, with the strongest productivity minima concomitant with Heinrich-type events (HtEs). Subtropical, nutrient-poorer waters, increased terrigenous input, and moderate to strong upwelling during the deglaciation and early MIS11 are hypothesized to have attributed a competitive advantage to diatoms to the detriment of coccolithophores, resulting in intermediate coccolithophore productivity levels. During the progression towards full glacial conditions an increasing presence of nutrient-richer waters, related to the growing influence of transitional surface waters and/or intensified upwelling, probably stimulated coccolithophore productivity to maxima following the rapid depletion of silica by diatoms. We present conceptual models of the carbon and carbonate cycle components for the IbM in different time slices that might serve as a basis for further investigation and modelling experiments.