Isotopic Imprints of Coccolithophore Blooms Overthe Past Million Years

Coccolithophores, calcifying marine phytoplankton, play a dual role in the oceanic
carbon cycle by contributing to carbon fixation through photosynthesis and to carbon
release via calcification (uptake of bicarbonate and release of CO2). To evaluate the net
effect of coccolithophore long-term evolutionary and productivity dynamics on the car-
bon cycle, we analyzed two sediment cores, MD96-2060 (Mozambique Channel) and MD97-
2125 (Coral Sea), spanning the past 1 Myr. Using automated light microscopy and im-
age recognition, we quantified coccolithophore assemblages, morphology, and calcite mass.
These data were complemented by stable isotope analyses (δ13C and δ18O) of coccolith-
dominated the fine fraction (< 30 µm,) sediment samples. Our results reveal pronounced
coccolithophore bloom phases, characterized by high abundances of Gephyrocapsa caribbean-
ica and Emiliania huxleyi, and sharp increases in total Noelaerhabdaceae mass accumu-
lation rate. The Morphological Divergence Index, a proxy for evolutionary divergence,
exhibits similar long-term trends at both sites, in phase with orbital eccentricity cycles.
Fine-fraction δ13C records display long-term patterns that are absent in benthic and plank-
tonic foraminiferal δ13C records, indicating a persistent coccolithophore-driven isotopic
signal. We interpret this signal as the result of species-specific vital effects in dominant
blooming taxa, particularly during periods of low eccentricity, when reduced ecological
niche partitioning may have favored the proliferation of smaller more cosmopolitan species.
This, in turn, may have led to a significant depletion in δ13C values of the fine fraction
during low eccentricity phases, thereby influencing the marine carbon cycle on orbital
timescales.