13000 cal years upwelling variation in southwestern Atlantic (Brazil): continental paleoclima implications

Ana Luiza ALBUQUERQUE(1); Bruno TURCq(2); Abdel SIFEDDINE(1,2). (1) Departamento de Geoquímica, Universidade Federal Fluminense, Niterói, Brazil. (2) LOCEAN, IRD/UPMC/CNRS/MNHN, Bondy, France. The Cabo Frio region as indicated by its name is a place of low SST due to a local upwelling triggered by the Northeast trade winds, the northward flow of cool South Atlantic Central Water and vortex of the Brazilian current in the upper warm Tropical Water. Paleoceanographic conditions during the last 13000 years have been reconstructed based on two cores collected on the outer shelf. The studied proxies included mineral and heavy metal quantification, bulk organic matter characteristics and planktonic foraminifera. A first phase of sedimentation between 13000 and 7000 cal BP is characterized by high mineral content probably due to the lower sea level. SST reconstruction indicate cool and highly variable temperatures that were probably not related to upwelling events made difficult by the low sea level but to lower regional SSTs. This is in good agreement with observations of continental climate dryer in southwest Brazil with intense events of precipitation. A second phase between 7000 and 3000 cal BP shows higher SST indicating few occurrences of upwelling. Its may be due to the decrease of South Atlantic Convergence Zone (ZCAS) intensity linked to the lower summer insolation and the reduced monsoonal flux at that time. On the adjacent continent the decrease monsoon is evidenced by low lake levels and poorly developed forests. The third and last phase, post 3000 cal BP, is characterized by the onset of upwelling events that may be related to an intensification of the South American Monsoon and of the ZCAS activity leading to an increase of Northeast winds during summer which is typically the upwelling season. On the continent this period was marked by forest development. The transition at 3000 yrs BP is very late compared to other Holocene Record. Paleoclimate model simulations suggest that this later change would be a characteristic of the southern tropics.