Decreased carbonate pump during the Oligocene-Miocene Transition: Regulating the oceanic buffering capacity

The oceanic carbonate cycle plays a crucial role in buffering anthopogenic CO2 emmision by regulating the total alkalinity (TA) and dissolved inorganic carbon (DIC) in the seawater. There is a growing interest in comprehending the role of biogenic calcification, the carbonate (counter) pump. The Early Miocene glaciation is thought to be triggered by the declining pCO2 potentially through a threshold effect of ~400 ppm (Greenop et al., 2019), a level we are approaching today. The mechanism(s) behind the long-term pCO2 decline during this period is still an open question, with little discussion on fluctuations in the carbonate burial. We estimated the changes in volume and flux of pelagic carbonate, specifically using coccoliths (calcite scales produced by coccolithophores). Our investigation spanned the transition from the Paleogene to the Neogene (~27-20 Ma), using marine calcareous nannofossil ooze retrieved from the IODP Site U1501 and U1505 located in the western tropical Pacific Ocean. The circular-polarized light microscope is used to measure the thickness (and volume) of the coccolith crystals. Integrating the linear sedimentation rates, we estimated that coccolith carbonate burial varied between 2-8×104 mol·yr-1·km-2. Our result aligns with the modeled alkalinity removal through pelagic carbonate burial (van der Ploeg et al., 2019). Moreover, scanning electron microscope (SEM) observations revealed calcite carbonate dissolution effects in the water column, with ~5-30% of coccolith carbonate dissolving during sinking, releasing additional alkalinity to the sea-water. A negative correlation between Ks and bulk TOC suggests that the organic and inorganic carbon burial were decoupled during the studied period. While further constraints are needed to improve our estimation (e.g., considering assemblage changes in coccolith and planktonic foraminifera), we tentatively conclude that the decline in carbonate production together with the increased dissolution weakened the carbonate pump. As a result, enhanced buffering capacity of the ocean likely played a role in the drawdown of pCO2 from the Late Oligocene to the Early Miocene.