Factors controlling the depth habitat of planktonic foraminifera in the subtropical eastern North Atlantic

Abstract

<p><strong>Abstract.</strong> Planktonic foraminifera preserved in marine sediments archive the physical and chemical conditions under which they built their shells. To interpret the paleoceanographic information contained in fossil foraminifera, the proxy signals have to be attributed to the habitat of individual species. Much of our knowledge on habitat depth is based on indirect methods, which reconstruct the depth at which the largest portion of the shell has been calcified. However, habitat depth can be best studied by direct observations in stratified plankton nets. Here we present a synthesis of living planktonic foraminifera abundance data in vertically resolved plankton net hauls taken in the eastern North Atlantic during twelve oceanographic campaigns between 1995 and 2012. Live (cytoplasm-bearing) specimens were counted for each depth interval and the vertical habitat at each station was expressed as average living depth (ALD). This allows us to differentiate species showing an ALD consistently above 100 m (e.g. <i>Globigerinoides ruber</i> white and pink), indicating a shallow habitat; species occurring from the surface to the subsurface (e.g. <i>Globigerina bulloides</i>, <i>Globorotalia inflata</i>, <i>Globorotalia truncatulinoides); and species inhabiting the subsurface (e.g. Globorotalia scitula</i> and <i>Globorotalia hirsuta</i>). For 17 species with variable ALD, we assessed whether their depth habitat at a given station could be predicted by mixed layer (ML) depth, temperature in the ML and chlorophyll <i>a</i> concentration in the ML. The influence of seasonal and lunar cycle on the depth habitat was also tested using periodic regression. In 11 out of the 17 tested species, ALD variation appears to have a predictable component. All of the tested parameters were significant at least in one case, with both seasonal and lunar cyclicity as well as the environmental parameters being able to explain up to > 50 % of the variance. Whereas <i>G. truncatulinoides</i>, <i>G. hirsuta</i> and <i>G. scitula</i> appear to deepen their living depth towards the summer, populations of Trilobatus sacculifer appears to descend in the water column towards the new moon. In all other species, properties of the mixed layer explained more of the observed variance. Chlorophyll <i>a</i> concentration seems least important for ALD, whilst shoaling of the habitat with deepening of the ML is observed most frequently. We observe both shoaling and deepening of species habitat with increasing temperature. Further, we observe that temperature and seawater density at the depth of the ALD were not equally variable among the studied species, and their variability showed no consistent relationship with depth habitat. According to our results, depth habitat of individual species changes in response to different environmental and ontogenetic factors and consequently planktonic foraminifera exhibit not only species-specific mean habitat depths but also species-specific changes in habitat depth.</p>

Influence of surface ocean density on planktonic foraminifera calcification

“…For minor, temporary or short term migrations shallower into the water column, positive buoyancy in plankton may be regulated by alterations in the calcite to protoplasm ratio of the shell 47 or by the lower-density cytoplasmatic entities that can greatly affect their average living depth 48 . The depth regulating mechanism proposed here may support major migrations over wide density ranges of the water column for the acquisition of a maximum depth during the foraminiferal life cycle, and especially during gametogenesis when additional calcite is secreted.…”

A Deep Eastern Equatorial Pacific Thermocline During the Last Glacial Maximum

“…Additionally, to constrain planktic foraminifera calcification depth variability in our Holocene and LGM δ 18 O depth profiles, we summarized modern planktic foraminifera preferred calcification depths for the EEP (Figure 2b). Planktic foraminifera abundance and vertical distribution are influenced by mixed layer depth, temperature, chlorophyll concentrations, and other factors that change seasonally to decadally (Ravelo & Fairbanks, 1992;Rebotim et al, 2016;Rincón-Martínez et al, 2011). These factors may also influence the export of foraminifera to depth and their incorporation into the marine sediment archive (Kawahata et al, 2002;Thunell et al, 1983;Venancio et al, 2017).…”

Single Tests of Thermocline Dwelling Foraminifera Globorotalia inflata as Recorder of Upper Water Column Structure off Mauritania (NW Africa): Methodology and Paleoceanographic Use

“…Calcification likewise takes place over a wide depth range, from the mixed layer to depths below 500 m (Wilke et al, 2009). Rebotim et al (2016) found no correlation between depth habitat of G. inflata and seasonal or lunar cycle or mixed layer depth.…”

“…Population is controlled by food availability and resembles fluorescence and nutrient profiles (Cléroux et al, 2007;Lončarić et al, 2006). Maximum abundance is associated with the base of the seasonal thermocline (Cléroux et al, 2007(Cléroux et al, , 2008 although G. inflata is found throughout the upper water column to depths of several hundred of meters (Erez & Honjo, 1981;Fairbanks et al, 1980;Ganssen & Kroon, 2000;Haarmann et al, 2011;Rebotim et al, 2016;van Raden et al, 2011). Calcification likewise takes place over a wide depth range, from the mixed layer to depths below 500 m (Wilke et al, 2009).…”