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

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 Pleist ocene [Schmiedl and Mackensen, Palaeogeogr. Palaeoclimatol. Palaeoecol. 130 (1997) 43-80], increasing slightly only during the most extreme glacial pe riods. Recently, the puzzling occurrence of thick monospecific diatom layer s (Ethmodiscus rex) has been reported within the carbonaceous sequences of two Sediment cores in the South Atlantic Gyre [Schmieder et al., Earth Plan et. Sci. Lett. 179 (2000) 539-549]. The layers, dated 544-534 ka, are isoch ronous and record a period of increased organic carbon flux. Above them, at 530-524 ka, unusually high carbonate contents are recorded. This double fe ature 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 c limate transition (MPT). The MPT is characterized by reduced thermohaline c irculation and stronger carbonate dissolution [Schmieder et al., Earth Plan et. Sci. Lett. 179 (2000) 539-549]. Reduced circulation and a more stratifi ed water column may have trapped nutrients in deep nitrate pools and may ha ve fostered favorable conditions for the development of deep-dwelling popul ations of E. re,:. The onset of more turbid conditions led to the rapid set tling of diatom populations, subsequently introduced nutrients to the surfa ce layer, and improved calcite preservation, thus creating a biogenic silic a-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., N ature 392 (1998) 269-272]. Since summer insolation was relatively low durin g this time the formation of this thick sapropel cannot be explained in ter ms of current understanding of astronomical forcing [Rossignol-Strick et al ., Nature 392 (1998) 269-272]. ii non-linear response to a 400-kyr low-freq uency climatic cycle as discussed for the Mediterranean sapropel is not con firmed by data from our South Atlantic core records. (C) 2001 Elsevier Scie nce B.V. All rights reserved.