Anomalous South Atlantic lithologies confirm global scale of unusual mid-Pleistocene climate excursion

Abstract

Abstract Carbonate-rich nannofossil oozes are the typical sediments deposited under the low-productivity regime of the subtropical gyre in the South Atlantic. Surface productivity has remained virtually constant during the late Pleistocene [Schmiedl and Mackensen, Palaeogeogr. Palaeoclimatol. Palaeoecol. 130 (1997) 43–80], increasing slightly only during the most extreme glacial periods. Recently, the puzzling occurrence of thick monospecific diatom layers ( Ethmodiscus rex ) has been reported within the carbonaceous sequences of two sediment cores in the South Atlantic Gyre [Schmieder et al., Earth Planet. Sci. Lett. 179 (2000) 539–549]. The layers, dated 544–534 ka, are isochronous and record a period of increased organic carbon flux. Above them, at 530–524 ka, unusually high carbonate contents are recorded. This double feature of anomalous lithologies is unique in the sediment record of the last 1500 kyr and can be related to the terminal event of the mid-Pleistocene climate transition (MPT). The MPT is characterized by reduced thermohaline circulation and stronger carbonate dissolution [Schmieder et al., Earth Planet. Sci. Lett. 179 (2000) 539–549]. Reduced circulation and a more stratified water column may have trapped nutrients in deep nitrate pools and may have fostered favorable conditions for the development of deep-dwelling populations of E. rex . The onset of more turbid conditions led to the rapid settling of diatom populations, subsequently introduced nutrients to the surface layer, and improved calcite preservation, thus creating a biogenic silica–carbonate double peak. The carbonate peak is synchronous with an unusual mid-Pleistocene sapropel observed in the Mediterranean, which is attributed to a ‘massive odd monsoon’ over Central Africa [Rossignol-Strick et al., Nature 392 (1998) 269–272]. Since summer insolation was relatively low during this time the formation of this thick sapropel cannot be explained in terms of current understanding of astronomical forcing [Rossignol-Strick et al., Nature 392 (1998) 269–272]. A non-linear response to a 400-kyr low-frequency climatic cycle as discussed for the Mediterranean sapropel is not confirmed by data from our South Atlantic core records.