Glacial-interglacial productivity changes recorded by alkenones and microfossils in late Pliocene eastern equatorial Pacific and Atlantic upwelling zones

摘要

Oceanic upwelling regions are highly productive systems that dominate global ocean new production, with important implications for cycling of nitrogen and carbon as well as for climate. Many proxy methods have been proposed to reconstruct past changes in ocean productivity but uncertainties are associated with the interpretation of each. Here we report new records of calcareous nannofossil assemblages and siliceous fragment abundance from the late Pliocene eastern equatorial Pacific and Atlantic Oceans and compare them with records of total C_(37) alkenone content from the same sediment samples. Our data show prominent coherent glacial-interglacial timescale variability among all three of these time series with remarkable inter-correlation, lending support to their interpretation as records of climate-coupled changes in productivity. We infer that, prior to the dominance of the relatively well-understood modern alkenone-synthesising coccolithophores, Reticulofenestra spp. were the principal calcifying alkenone-synthesisers during the late Pliocene. Our records suggest that, in upwelling areas on orbital timescales, total C_(37) alkenone concentration tracks productivity not only of the alkenone-synthesising coccolithophores, but also of the wider phytoplankton community, including siliceous diatoms. These results lend new significance to pre-Pleistocene records of total C_(37) alkenone concentration from upwelling regions, particularly those from the eastern equatorial Pacific. Today this region accounts for the majority of global ocean-to-atmosphere CO_2 efflux and, through biological pump invigoration, has the capacity to have contributed significantly to oceanic opal burial and drawdown of atmospheric carbon dioxide. The strong correlations that we demonstrate between C_(37) alkenone concentration and other palaeoproductivity proxies suggest that alkenone accumulation may represent a useful means of inferring pre-Pleistocene carbon cycle changes.