{'fraction_non_alphanumeric': 0.016129032258064516, 'fraction_numerical': 0.0015735641227380016, 'mean_word_length': 5.405541561712846, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Appraisal of sedimentary alkenones for the quantitative reconstruction of phytoplankton biomass

Significance Biomarker proxies, namely alkenones, are commonly used to reconstruct past primary productivity. However, their value has been questioned as they are posited to be controlled by diagenetic processes and often limited to draw qualitative inferences on the carbon cycle. In fact, most proxies have not been globally appraised with ocean biogeochemical data. Here, we use a combination of remote sensing and geochemical data to provide a spatial-based global scale calibration that shows the use of alkenones to quantify past sea surface chlorophyll-a concentration, which ultimately can be used to infer quantitatively past primary productivity in paleorecords. This calibration paves the way to clarify the relative role of the marine carbon cycle in climate variability using field data, and tests biogeochemical models. 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.