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

Abstract

International audienceCenozoic 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 (g CaCO3/m2/yr) at five DSDP/ODP sites in the Atlantic, Indian, and Western Pacific oceans. The proportion of nannofossil-dominated fine fraction carbonate (< 38 μm) 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 4 Ma, but this had little or only minor impact on the burial of pelagic carbonate until ~ 4 Ma. After ~ 4 Ma, 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 > 38 μm fraction remained stable over the past 17 Myr. 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 4 Myr, 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

Neogene palaeoceanographic changes recorded in a carbonate contourite drift (Santaren Channel, Bahamas)

“…The data that support this trend are related to the CAS closure successively preventing the flow of nutrient‐rich sub‐surface waters from the Pacific into the Atlantic and diminishing the overall biological activity in the North Atlantic (Jain & Collins, ; Schneider & Schmittner, ). Another reason for lower SDAR values could be the long‐term trend of decreasing silicate weathering and atmospheric CO 2 that could have reduced the burial rates of calcifying organisms (Suchéras‐Marx & Henderiks, ).…”

Absolute nannofossil abundance estimates: Quantifying the pros and cons of different techniques

“…The determination of the absolute abundance of nannofossils in marine sediments is important for the evaluation of nannofossil accumulation rates (and related carbonate fluxes), and is fundamental when comparing datasets collected in different areas and time intervals (e.g., Kinkel et al, 2000;Henderiks et al, 2002;Suchéras-Marx and Henderiks, 2014). Absolute nannofossil abundance can be calculated by:…”

A 15 million-year long record of phenotypic evolution in the heavily calcified coccolithophore &lt;i&gt;Helicosphaera&lt;/i&gt; and its biogeochemical implications

“…Some morphometric features of coccoliths, such as length and thickness, correlate with biogeochemically relevant traits: cell size (Henderiks, 2008), growth rates (Gibbs et al, 2013) and the production rates of organic (POC) and inorganic (PIC) carbon (Bolton et al, 2016;McClelland et al, 2016). Combined with morphospecies-diversity and community composition that is readily inferred from the fossil record (Bown et al, 2004;Knappertsbusch, 2000;Suchéras-Marx and Henderiks, 2014), coccolith morphometry could therefore help elucidate evolutionary processes from the cellular-to the community-level. In this context, the transition from the warm and high-CO2 world of the middle Miocene (~15 Ma) to the colder, low-CO2 conditions of the Pleistocene (Super et al, 2018;Zachos et al, 2001) provides an optimal case-study for investigating the long-term evolutionary adaptation of modern coccolithophore lineages to relevant climate-forcing, and understanding the biogeochemical implications of their evolutionary adaptation.…”

“…To date, studies of coccolithophore evolutionary adaptation have mostly focused on members of the Noelaerhabdaceae family, which dominated coccolithophore communities during the middle Miocene and Pleistocene (Henderiks and Pagani, 2007;Suchéras-Marx and Henderiks, 2014). While their most important modern representative Emiliania huxleyi is highly abundant in today's oceans, this lineage represents only a small fraction of the morphological, ecological and physiological diversity of coccolithophores (Young et al, 2005).…”