Modeling planktonic foraminiferal seasonality: Implications for sea-surface temperature reconstructions

A global foraminiferal model was used to determine the seasonal imprint of planktonic foraminifera on the sedimentary record. The model provides monthly concentrations of five planktonic foraminiferal species used in paleoceanographic reconstructions including N. pachyderma (sin. and dex.), G. bulloides, G. ruber (white) and G. sacculifer. The temperature imprint in foraminiferal shells varies according to the season of calcification, and the sedimentary records retain this seasonal imprint. Proxy records for a species will therefore be weighted towards the values during the season of maximum production for that species. Our model prediction reveals that, in general, at high latitudes, close to the geographical limit of occurrence of each species, the signal is biased towards summer conditions. In contrast, at lower latitudes the signal is biased towards winter or annual mean conditions. Temperatures derived from G. ruber (white) and G. sacculifer are most suitable for estimating annual mean SST in tropical waters, between 20° N/S, while G. ruber (white) when collected at mid-latitudes, near to 40° latitude, reflects mainly summer conditions. We carried out sensitivity experiments to study the response of planktonic foraminiferal seasonality to changes in temperature. We forced the model decreasing the temperature globally by 2 °C and 6 °C. In most of the regions, due to the cooling, the season of maximum production shifted to a warmer season. Thus, the annual species population recorded little change in the temperature. In tropical waters, where temperature seasonality is low, foraminiferal population recorded the entire temperature variation. These findings highlight the importance of considering changes in seasonality through time, specially for temperature reconstructions based on monospecific samples, such as those derived from planktonic foraminiferal isotopic and trace element composition.