Downsizing the pelagic carbonate factory: Impacts of calcareous nannoplankton evolution on carbonate burial over the past 17 million years

Abstract

Cenozoic deep-sea carbonates (“calcareous oozes”) are predominantly biogenic in origin and offer detailed records of the evolution of calcifying plankton groups, such as coccolithophores and foraminifera. The size and abundance of calcifying plankton determine the strength of the calcium carbonate “pump” in the open ocean, which acts as a short-term source of CO2, while the burial of pelagic carbonates serves as a long-term sink of carbon. Here, we show how the macroevolutionary size decrease in calcareous nannoplankton (coccoliths and calcareous nannoliths) has affected burial rates of calcareous ooze over the past 17 million years. We quantified nannofossil carbonate burial rates (gCaCO3/m2/yr) at five DSDP/ODP sites in the Atlantic, Indian, and Western Pacific oceans. The proportion of nannofossil-dominated fine fraction carbonate (<38μm) and its mass accumulation rates were regionally and temporally variable, but our combined data reveal globally consistent long-term trends. Mean nannofossil carbonate mass decreased about four-fold between 9 and 4Ma, but this had little or only minor impact on the burial of pelagic carbonate until ~4Ma. After ~4Ma, when small-sized coccolith-bearing species prevailed, nannofossil carbonate burial rates decreased by one order of magnitude to the lowest values during the Pleistocene. In contrast, mass accumulation rates of the foraminifera-dominated >38μm fraction remained stable over the past 17Myr. This suggests that changes in the deposition of calcareous ooze were primarily driven by calcareous nannoplankton, and that foraminifera did not compensate for the lower nannofossil-carbonate accumulation rates since the Pliocene. Despite a deepening of the lysocline over the past 4Myr, global pelagic carbonate mass accumulation likely decreased. Whether, or how, this may relate to changes in weathering or other components within the long-term carbonate cycle remains unclear. Explanations for the macroevolutionary size decrease in calcareous nannoplankton focus on the physiological and ecological advantages of small, lightly calcified algal cells in a low-CO2 and more stratified marine environment.