Global analysis of seasonality in the shell flux of extant planktonic Foraminifera

[摘要] Shell fluxes of planktonic Foraminifera species vary intra-annually in apattern that appears to follow the seasonal cycle. However, the variation inthe timing and prominence of seasonal flux maxima in space and among speciesremains poorly constrained. Thus, although changing seasonality may result ina flux-weighted temperature offset of more than 5° C within aspecies, this effect is often ignored in the interpretation ofForaminifera-based paleoceanographic records. To address this issue we present an analysis ofthe intra-annual pattern of shell flux variability in 37 globally distributedtime series. The existence of a seasonal component in flux variability wasobjectively characterised using periodic regression. This analysis yieldedestimates of the number, timing and prominence of seasonal flux maxima. Over80% of the flux series across all species showed a statisticallysignificant periodic component, indicating that a considerable part of theintra-annual flux variability is predictable. Temperature appears to be apowerful predictor of flux seasonality, but its effect differs among species.Three different modes of seasonality are distinguishable. Tropical andsubtropical species (Globigerinoides ruber (white and pink varieties),Neogloboquadrina dutertrei, Globigerinoides sacculifer, Orbulina universa,Globigerinella siphonifera, Pulleniatina obliquiloculata, Globorotalia menardii, Globoturborotalita rubescens, Globoturborotalita tenella and Globigerinoides conglobatus) appear to have a less predictable fluxpattern, with random peak timing in warm waters. In colder waters,seasonality is more prevalent: peak fluxes occur shortly after summertemperature maxima and peak prominence increases. This tendency is strongerin species with a narrower temperature range, implying that warm-adaptedspecies find it increasingly difficult to reproduce outside their optimumtemperature range and that, with decreasing mean temperature, their flux isprogressively more focussed in the warm season. The second group includes thetemperate to cold-water species Globigerina bulloides,Globigerinita glutinata, Turborotalita quinqueloba, Neogloboquadrina incompta, Neogloboquadrina pachyderma,Globorotalia scitula, Globigerinella calida, Globigerina falconensis, Globorotalia theyeriand Globigerinita uvula. These species show a highly predictable seasonalpattern, with one to two peaks a year, which occur earlier in warmer waters.Peak prominence in this group is independent of temperature. Theearlier-when-warmer pattern in this group is related to the timing ofproductivity maxima. Finally, the deep-dwelling Globorotalia truncatulinoidesand Globorotalia inflata show a regular and pronounced peak in winter andspring. The remarkably low flux outside the main pulse may indicate a longreproductive cycle of these species. Overall, our analysis indicates that theseasonality of planktonic Foraminifera shell flux is predictable and revealsthe existence of distinct modes of phenology among species. We evaluate theeffect of changing seasonality on paleoceanographic reconstructions and findthat, irrespective of the seasonality mode, the actual magnitude ofenvironmental change will be underestimated. The observed constraints on fluxseasonality can serve as the basis for predictive modelling of flux pattern.As long as the diversity of species seasonality is accounted for in suchmodels, the results can be used to improve reconstructions of the magnitudeof environmental change in paleoceanographic records.

[学科分类] 地球化学与岩石