Appraisal of sedimentary alkenones for the quantitative reconstruction of phytoplankton biomass

Marine primary productivity (PP) is the driving factor in the global marine carbon cycle. Its reconstruction in past climates relies on biogeochemical proxies that are not considered to provide an unequivocal signal. These are often based on the water column flux of biogenic components to sediments (organic carbon, biogenic opal, biomarkers), although other factors than productivity are posited to control the sedimentary contents of the components, and their flux is related to the fraction of export production buried in sediments. Moreover, most flux proxies have not been globally appraised. Here, we assess a proxy to quantify past phytoplankton biomass by correlating the concentration of C₃₇ alkenones in a global suite of core-top sediments with sea surface chlorophyll-a (SSchla) estimates over the last 20 y. SSchla is the central metric to calculate phytoplankton biomass and is directly related to PP. We show that the global spatial distribution of sedimentary alkenones is primarily correlated to SSchla rather than diagenetic factors such as the oxygen concentration in bottom waters, which challenges previous assumptions on the role of preservation on driving concentrations of sedimentary organic compounds. Moreover, our results suggest that the rate of global carbon export to sediments is not regionally constrained, and that alkenones producers play a dominant role in the global export of carbon buried in the seafloor. This study shows the potential of using sedimentary alkenones to estimate past phytoplankton biomass, which in turn can be used to infer past PP in the global ocean.

Global carbon distribution between the ocean and the atmosphere regulates global climate on Earth. This distribution is primarily controlled by marine primary productivity (PP) and phytoplanktonic organisms, which transforms atmospheric CO₂ into organic matter. Only a fraction of this produced organic matter is exported to the deep ocean. Global models estimate that 48 PgC·y⁻¹ are produced in ocean surface waters (1), while 6 PgC·y⁻¹ (2) are exported out from the photic zone and 0.15 PgC·y⁻¹ are buried in sediments (3). Exported organic carbon is out of contact with the atmosphere on decadal-to-millennial timescales or longer once is buried in the seafloor, which exerts a major control on global climate by regulating the partial pressure of atmospheric CO₂ (4). Hence, estimating marine PP, export, and burial productivity changes during past key climatic periods (e.g., glacial–interglacial transitions) is essential to understand our present climate and predict its evolution in the future.To infer past PP, a range of proxies based on the fluxes of biogenic components are available (5–7). As flux proxies, they are related to changes in past export productivity, which are assumed to be proportional to surface PP in paleoreconstructions. However, depositional factors such as oxygen or ballasting effect are thought to be important in controlling organic matter export from the upper water column to sediments (8–10), and thus, sedimentary organic proxies concentration. The relative weights of the factors that control the spatial variability of organic matter concentration in sediments are still unconstrained, which leads to some uncertainty on the applicability of organic matter proxies to infer PP (7). Consequently, available proxies are sometimes interpreted to infer either changes in PP or depositional conditions (11).One of the common approaches to reconstruct PP relies on the measurement of C₃₇ di- and tri- unsaturated methyl ketones (i.e., C₃₇ alkenones) concentrations or fluxes in sediments (12–18). These organic molecules are biomarkers of the ubiquitous coccolithophore Emiliania huxleyi, which is the principal source of alkenones and the most abundant coccolithophore in the modern pelagic ocean (19–24). Geophyrocapsa oceanica and other coccolithophoral species from the same genera are also considered important alkenones producers nowadays (20).In this study, we evaluate the potential use of sedimentary C₃₇ alkenones content to infer past phytoplankton biomass at a global scale through the comparison of their spatial variability in a global compilation of core-top sediments with sea surface chlorophyll-a (SSchla) (Fig. 1). This is the primary pigment of photosynthesis and is present in all photosynthetic phytoplankton species. Its concentration in surface waters is commonly used as an indicator of phytoplankton biomass and to infer PP (25, 26). On a global scale, its concentration in surface waters is estimated by remote sensing (27). We also assess the effect of oxygen on the spatial accumulation of alkenones in sediments by comparing its concentration in bottom waters with alkenones abundance on a global scale.Open in a separate windowFig. 1.Global core-top sediments distribution. Lines delineate distinct biogeochemical regions defined on the basis of phytoplankton community, temperature, and nutrient concentration (44). SA, subarctic; SO, Southern Ocean; ST, subtropics; T, tropics.