Determinants of Seasonality of Planktonic Foraminifera Shell Flux: Consequences for Paleoproxies

Planktonic foraminifera are widely used proxy carriers in paleoceanography. The flux of foraminiferal shells to the sea floor is not even throughout the year, creating a seasonal bias in the surface conditions recorded in an average fossil sample. This bias, and its changes through time, may account for an important part of the variability in paleoclimate records, but it is often ignored because of limited knowledge on the determinants of flux seasonality. To address this issue we have compiled a global dataset on shell flux seasonality from sediment traps. The database contains 38 globally distributed time series of at least one year and covers >20 species. We use periodic regression to objectively determine peak flux timing and amplitude. Significant seasonality is observed in 80 % of the cases studied and we distinguish three distinct groups of foraminifera with different modes of seasonality. This division is independent of ocean basin or upwelling and appears to reflect three principle patterns of phenology.

Warm-water and symbiont-bearing species change flux seasonality by concentrating a larger proportion of the annual flux in a shorter period in colder water. Peak flux timing appears random at high temperatures and shifts towards autumn at lower temperatures. Seasonal flux variability is small at high temperatures (within their optimal range) resulting in a negligible seasonal bias. In colder waters the timing appear constant and the strength of the peak flux can be predicted by temperature. Temperate and cold-water dwellers adjust their peak timing with average temperature. Peak flux in these species occurs later during the year at lower temperatures and follows chlorophyll maxima by approximately a month. The strength of peak flux is similar across the temperature range, but the association with productivity allows for prediction of the timing of peak flux. Peak flux of deep-dwelling seems to occur in spring independent of temperature, which may agree with a longer reproductive cycle, and show a tendency for increased peak prominence at higher temperatures. These observations shed new light on the factors explaining seasonal variability in the shell flux of planktonic foraminifera and will aid to improve paleoceanographic reconstructions by allowing prediction of temporal changes in seasonality.