Modeling seasonal and vertical habitats of planktonic foraminifera on a global scale

Abstract

Abstract. Species of planktonic foraminifera exhibit specific seasonal production patterns and different preferred vertical habitats. The seasonality and vertical habitats are not constant throughout the range of the species and changes therein must be considered when interpreting paleoceanographic reconstructions based on fossil foraminifera. Accounting for the effect of vertical and seasonal habitat tracking on foraminifera proxies at times of climate change is difficult because it requires independent fossil evidence. An alternative that could reduce the bias in paleoceanographic reconstructions is to predict species-5 specific habitat shifts under climate change using an ecosystem modeling approach. To this end, we present a new version of a planktonic foraminifera model, PLAFOM2.0, embedded into the ocean component of the Community Earth System Model, version 1.2.2. This model predicts monthly global concentrations of the planktonic foraminiferal species: Neogloboquadrina pachyderma, N. incompta, Globigerina bulloides, Globigerinoides ruber (white), and Trilobatus sacculifer throughout the world ocean, resolved in 24 vertical layers to 250 m depth. The resolution along the vertical dimension has been implemented by 10 applying the previously used spatial parameterization of biomass as a function of temperature, light, nutrition, and competition on depth-resolved parameter fields. This approach alone results in the emergence of species-specific vertical habitats, which are spatially and temporally variable. Although an explicit parameterization of the vertical dimension has not been carried out, the seasonal and vertical distribution patterns predicted by the model are in good agreement with sediment trap data and plankton tow observations. In the simulation, the colder-water species N. pachyderma, N. incompta, and G. bulloides show 15 a pronounced seasonal cycle in their depth habitat in the polar and subpolar regions, which appears to be controlled by food availability. During the warm season, these species preferably occur in the subsurface, while towards the cold season they ascend through the water column and are found closer to the sea surface. The warm-water species G. ruber (white) and T.sacculifer exhibit a less variable shallow depth habitat with highest biomass concentrations within the top 40 m of the water column. Nevertheless, even these species show vertical habitat variability and their seasonal occurrence outside the tropics 20 is limited to the warm surface layer that develops at the end of the warm season. The emergence in PLAFOM2.0 of speciesspecific vertical habitats that are consistent with observations indicates that the population dynamics of planktonic foraminifera species may be driven by the same factors in time, space, and with depth, in which case the model can provide a reliable and robust tool to aid the interpretation of proxy records.

Variable habitat depth of the planktonic foraminifera <i>Neogloboquadrina pachyderma</i> in the northern high latitudes explained by sea-ice and chlorophyll concentration

“…This observation is also in contrast with the modelled relationship of DH with environmental parameters. This is because the strong relationship between DH and MLD in the model reflects a strong link between MLD and the position of the subsurface chlorophyll maximum, as also noted by Kretschmer et al (2018). This strong link likely results from a bias in the ocean component of the Community Earth System Model (CESM1.2) propagated in PLAFOM2.0.…”

“…Finally, we evaluate how PLAFOM2.0 (Kretschmer et al, 2018) captures the observed patterns in N. pachyderma depth habitat. To this end, we assess the relationship between modelled DH of N. pachyderma and SST, SSS, MLD, DCM and chlorophyll concentration for summer months in the geographic area covered by the compilation (Fig.1).…”

Supplementary material to "Sedproxy: a forward model for sediment archived climate proxies"

“…The producers of alkenones are restricted to the photic zone and thus are thus close to the surface. However, for foraminifera, the preferred habitat depth and the depth at which their shells calcify is strongly species dependent and can vary from close to the surface, to the thermocline or deeper (Fairbanks and Wiebe, 1980;Kretschmer et al, 2017). Therefore, the recorded temperature will not necessarily reflect the sea surface temperature (Jonkers and Kučera, 2017).…”

Response to reviewer 2.

“…The producers of alkenones are restricted to the photic zone and thus are thus close to the surface. However, for foraminifera, the preferred habitat depth and the depth at which their shells calcify is strongly species dependent and can vary from close to the surface, to the thermocline or deeper (Fairbanks and Wiebe, 1980;Kretschmer et al, 2017). Therefore, the recorded temperature will not necessarily reflect the sea surface temperature (Jonkers and Kučera, 2017).…”

“…Currently, a constant concentration of the signal carrier is an assumption of sedproxy. Future work will enable climate dependent shifts in habitat and abundance to be modelled by implementing a parametrized response of proxy abundance and export to climate variables (Mix, 1987;Schmidt and Mulitza, 2002;Kretschmer et al, 2017;Jonkers and Kučera, 2017;Roche et al, 2017). An alternative option is to couple sedproxy to the output of ecological models that explicitly resolve the population dynamics of the proxy carrier, such as foraminifera population models (Fraile et al, 2008;15 Lombard et al, 2011).…”