Oxygen isotope ecology of recent planktic foraminifera at the central Walvis Ridge (SE Atlantic)

Above the Walvis Ridge, in the SE Atlantic Ocean, we collected living plantkic foraminifera from the upper water column using depth stratified plantkon tows. The oxygen isotope composition (δ18Oc) in shells of foraminifera and shell concentration profiles show seasonal and depth habitats of individual species. The tow results are compared with the average annual deposition δ18Oc from sediment traps and the interannual average δ18Oc of fossil specimens in top sediments at the same site. The species Globigerinita glutinata best reflects the austral winter/spring sea surface temperature (SST). Its δ18Oc signal in top sediments remains pristine. In contrast, tow results also show that Globigerinoides ruber continues to calcify below the surface mixed layer (SML), i.e., down to the deep chlorophyll maximum (DCM); hence its δ18Oc signature of exported specimens reflects the SST only when SML incorporates the DCM. Deep tow and sediment trap results show that both Globorotalia truncatulinoides and Globorotalia inflata record the temperature between 150 and 350 m, depending on the season and the shell size. However, for all fossil taxa in sediments apart from Globigerinita glutinata, we observe a positive δ18Oc shift with respect to the sediment trap and plankton tow values, likely related to the interannual flux changes and deep encrustation.