Ocean Acidification

Highlights

• Glacial C. pelagicus coccoliths were larger, with broader distal shields.
• Coccolith size variability tracked glacial-Holocene STF productivity changes.
• Dust-driven Fe fertilisation enhanced glacial nutrient utilisation along the STF.
• Coccolith distal shield thinning linked to deglacial Southern Ocean CO₂ outgassing.
• C. pelagicus morphometry records Southern Ocean carbonate chemistry variability.

Abstract

The Southern Ocean plays a key role in regulating atmospheric CO₂ through changes in overturning circulation and is projected to undergo major warming and ocean acidification under future climate scenarios. Coccolithophores, a dominant calcifying phytoplankton group in the northern Southern Ocean, are expected to be particularly sensitive to such changes. Here, we evaluate glacial–Holocene coccolith size variability using morphometric analyses of Coccolithus pelagicus from sediment core ABP S4 located at the Subtropical Front (STF) in the western Indian sector of the Southern Ocean. Mean coccolith size was calculated from statistically robust populations to minimise intra-coccosphere variability. Glacial intervals are characterized by larger coccoliths with broader distal shields (coccolith rim), whereas deglacial and Holocene intervals exhibit smaller coccoliths with thinner distal shield. Coccolith size variability closely corresponds with regional paleoproductivity records, indicating enhanced glacial carbonate production along the STF, while previous studies report increased opal production at the Subantarctic Front (SAF). Together, these findings suggest elevated glacial productivity across the Subantarctic Zone (SAZ) driven by more efficient nutrient utilisation under dust-induced Fe fertilisation. While overall coccolith size reflects changes in productivity and nutrient availability, the distal shield area appears more sensitive to deglacial carbonate chemistry changes. Deglacial reductions in the distal shield area paralleled enhanced Southern Ocean CO₂ outgassing and reinvigorated overturning circulation, indicating strong carbonate chemistry control on coccolith calcification. These findings highlight the potential of coccolith morphometry for tracking Southern Ocean productivity, carbon cycling, and the sensitivity of calcifying phytoplankton to future environmental change.

Graphical abstract

Nair A., Patil S., Mestry N. & Mohan R., 2027. Southern Ocean productivity and carbonate chemistry imprinted on coccolith morphology across the last Glacial-Holocene transition. Global and Planetary Change 268: 105782. doi: 10.1016/j.gloplacha.2026.105782. Article (subscription required).