Mg ∕ Ca and δ18O in living planktic foraminifers from the Caribbean, Gulf of Mexico and Florida Straits
- 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstrasse 1–3, 24148 Kiel, Germany
- 2Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
Correspondence: Anna Jentzen (email@example.com)
Past ocean temperatures and salinities can be approximated from combined stable oxygen isotopes (δ18O) and Mg ∕ Ca measurements in fossil foraminiferal tests with varying success. To further refine this approach, we collected living planktic foraminifers by net sampling and pumping of sea surface water from the Caribbean Sea, the eastern Gulf of Mexico and the Florida Straits. Analyses of δ18O and Mg ∕ Ca in eight living planktic species (Globigerinoides sacculifer, Orbulina universa, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, Globorotalia menardii, Globorotalia ungulata, Globorotalia truncatulinoides and Globorotalia tumida) were compared to measured in situ properties of the ambient seawater (temperature, salinity and δ18Oseawater) and fossil tests of underlying surface sediments. “Vital effects” such as symbiont activity and test growth cause δ18O disequilibria with respect to the ambient seawater and a large scatter in foraminiferal Mg ∕ Ca. Overall, ocean temperature is the most prominent environmental influence on δ18Ocalcite and Mg ∕ Ca. Enrichment of the heavier 18O isotope in living specimens below the mixed layer and in fossil tests is clearly related to lowered in situ temperatures and gametogenic calcification. Mg ∕ Ca-based temperature estimates of G. sacculifer indicate seasonal maximum accumulation rates on the seafloor in early spring (March) at Caribbean stations and later in the year (May) in the Florida Straits, related to the respective mixed layer temperatures of ∼26 ∘C. Notably, G. sacculifer reveals a weak positive linear relationship between foraminiferal derived δ18Oseawater estimates and both measured in situ δ18Oseawater and salinity. Our results affirm the applicability of existing δ18O and Mg ∕ Ca calibrations for the reconstruction of past ocean temperatures and δ18Oseawater reflecting salinity due to the convincing accordance of proxy data in both living and fossil foraminifers, and in situ environmental parameters. Large vital effects and seasonally varying proxy signals, however, need to be taken into account.
Calcite tests of planktic foraminifers are precipitated from the surrounding seawater and their stable oxygen isotope compositions (δ18Ocalcite) and Mg ∕ Ca ratios are established proxies to reconstruct past ocean conditions (e.g. Erez and Luz, 1983; Nürnberg et al., 2000). The δ18Ocalcite signature depends on the ambient seawater temperatures and oxygen isotopic compositions (δ18Oseawater) the planktic organism is thriving in. Their relationship was defined in several δ18O paleotemperature equations (e.g. Erez and Luz, 1983; Bouvier-Soumagnac and Duplessy, 1985; Bemis et al., 1998). Earlier studies showed that δ18Ocalcite reveals an offset to the equilibrium of the seawater caused by environmental factors (e.g. salinity, carbonate ion concentration , ocean pH) and/or biological controlled processes, so-called vital effects (Weiner and Dove, 2003) (e.g. symbionts photosynthesis, respiration) as influencing factors (Spero and Lea, 1993; Spero et al., 1997; Bemis et al., 1998; Bijma et al., 1999). Symbiont activity, for example, causes a depletion of δ18Ocalcite (e.g. Spero and Lea, 1993). Different ontogenetic stages of the foraminifers result in variable stable isotopes (δ18Ocalcite and δ13Ccalcite), whereas higher δ18Ocalcite and δ13Ccalcite values are measured in tests of adult specimens (e.g. Spero and Lea, 1996). Additionally, encrustation of foraminiferal tests, at the end of the life cycle, results in higher δ18Ocalcite compared to non-encrusted specimens (e.g. Kozdon et al., 2009).
The ratios of Mg ∕ Ca in foraminiferal tests are predominantly controlled by ocean temperature. Meanwhile, robust planktic species-specific calibrations exist (e.g. Nürnberg, 1995; Nürnberg et al., 1996; Lea et al., 1999; Anand et al., 2003; Regenberg et al., 2009), which allow to reconstruct the thermal structure of the entire water column, even on timescales of millions of years. The incorporation of magnesium during calcification is largely driven by physiological processes, which may cause Mg ∕ Ca heterogeneity in single tests with high and low Mg bands in some species (Erez, 2003; Sadekov et al., 2005; Bentov and Erez, 2006; Hathorne et al., 2009; Spero et al., 2015). Further, environmental parameters (e.g. salinity, , ocean pH) may affect foraminiferal Mg ∕ Ca (Nürnberg et al., 1996; Lea et al., 1999; Russell et al., 2004; Kisakürek et al., 2008). Most critical for proxy users are carbonate dissolution processes that considerably lower Mg ∕ Ca in foraminiferal tests (Brown and Elderfield, 1996; Rosenthal et al., 2000; Regenberg et al., 2006). Other influencing aspects, such as variable calcification depths (e.g. vertical migration through the water column during the life cycle of individuals; see Lohmann and Schweitzer, 1990; Schiebel and Hemleben, 2017) or variable seasonal abundances play a major role for the interpretation of stable isotope and Mg ∕ Ca signals of planktic foraminifers (e.g. Tedesco et al., 2007).
Whilst relatively few geochemical studies (e.g. Mg ∕ Ca) have been conducted on recent/living planktic foraminifers, either collected from the water column or cultured under controlled laboratory conditions, these studies are important for assessing different controlling factors on δ18Ocalcite and Mg ∕ Ca during biomineralisation (e.g. Kahn, 1979; Erez and Honjo, 1981; Nürnberg et al., 1996; Lea et al., 1999; Russell et al., 2004; Kisakürek et al., 2008; Spero et al., 2015).
Here, we systematically sampled the upper water column of the Caribbean, the eastern Gulf of Mexico and the Florida Straits for living tropical and subtropical planktic foraminifers using plankton nets and on-board pumping devices. δ18Ocalcite and Mg ∕ Ca analyses within bulk calcite and single chambers of living specimens collected from different depth intervals were (i) related to ocean parameters (temperature, salinity, δ18Oseawater) measured in water samples from conductivity–temperature–depth (CTD) sampling stations nearby and (ii) compared to fossil counterparts from underlying or nearby surface sediments. Our integrated approach aims to evaluate (i) vital effects under natural conditions, (ii) the ontogenetic development in particular test growth and (iii) the impact of environmental conditions on foraminiferal δ18Ocalcite and Mg ∕ Ca to further substantiate their potential as paleoceanographic proxies.
* indicates surface sediment sites close to MSN stations (1) 219, (2) 220, (3) 221 and (4) 211 (Fig. 1).
2.1 Sampling and preparation of planktic foraminifers
Analyses were performed on living foraminifers sampled from plankton nets, pumping from below the ship and fossil foraminifers from surface sediments obtained during cruises SO164 (RV Sonne) in May/June 2002 (Nürnberg et al., 2003) and M78/1 (RV Meteor) in February–March 2009 (Schönfeld et al., 2011) (Fig. 1; Table 1). To collect living planktic foraminifers, the Hydrobios Midi multiple opening–closing plankton net (MSN) with a mesh size of 100 µm was deployed at five stations in different water depth intervals (surface to max. 400 m) (Table 1). Further sampling of living specimens was accomplished by pumping seawater from 3.5 m water depth during ship's transit and subsequent filtering over a 63 µm sieve (PF samples). Immediately after sampling, the plankton samples (MSN and PF) were preserved in a mix of 50 % ethanol and seawater. The MSN samples were stained with rose Bengal (2 g L−1 ethanol). Surface sediment samples were recovered by the multicorer and United States Naval Electronic Laboratory (USNEL) giant box corer at positions close to the MSN stations (Table 1). During cruise M78/1, salinity and temperature were recorded by the RBR XR-420 CTD profiler and by the shipboard thermosalinograph (Fig. 2). For stable isotope analyses in seawater (δ18Oseawater), water samples were collected at different water depths (Table 1) with the shipboard rosette Niskin bottle system connected to the CTD profiler, filled in glass bottles (100 mL) and poisoned with 0.2 mL HgCl2 to prevent biological activity.
In the laboratory (GEOMAR, Kiel), the plankton net samples were rinsed with tap water and all foraminifers were picked wet with a glass pipette. The picked foraminifers were dried on a filter paper at room temperature, fractionated into different mesh sizes (100–125, 125–150, 150–250, 250–300, 300–400, 400–500 and >500 µm) and identified on species level after Bé (1967) and Schiebel and Hemleben (2017). For isotope and geochemical analyses, individual tests from eight different species were selected including Globigerinoides sacculifer (i.e. Trilobatus sacculifer; Spezzaferri et al., 2015) with a spherical last chamber (G. trilobus morphotype), Orbulina universa, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, Globorotalia menardii, Globorotalia ungulata, Globorotalia truncatulinoides dextral and Globorotalia tumida (Sect. S5 in the Supplement). Only test size fractions >250 µm and cytoplasm-bearing specimens with an intact calcite test were considered for analyses, indicating that the foraminifers were still alive when collected. For all species, the weighted average living depth (metres) and habitat temperature (∘C) (temperature at the weighted average living depth) were calculated based on standing stocks (individual m−3) in the water column (Table 2; see Jentzen et al., 2018).
Surface sediment samples were freeze-dried, wet sieved using tap water over a 63 µm sieve and dried at 40 ∘C. Single intact tests were picked from the 355–400 µm size fraction to be directly comparable to published data from similar Caribbean station sites (existing δ18Ocalcite data from Steph et al., 2009 and Mg ∕ Ca data from Regenberg et al., 2006).
2.2 Stable isotope analyses
Depending on the selected species and size fraction, a varying number of specimens were analysed for stable isotopes (δ18Ocalcite and δ13Ccalcite) (see Sect. S1). Prior to the measurements, the foraminiferal tests were cracked and the remaining cytoplasm was removed with a needle. The measurements were run on a Thermo Scientific MAT 253 mass spectrometer connected to an automatic carbonate preparation device Kiel CARBO IV at GEOMAR. The stable isotope results are reported relative to the Vienna Pee Dee Belemnite (V-PDB) in per mil (‰) and calibrated vs. the National Bureau of Standards (NBS) 19. The in-house standard (Solnhofen limestone) was run multiple times and after every 10 measurements with every magazine of samples and gives a long-term analytic precision of <0.06 ‰ (±1σ) for δ18Ocalcite and <0.03 ‰ (±1σ) for δ13Ccalcite, respectively.
a This study (average values). b Jentzen et al. (2018). c G. truncatulinoides dextral. d Gastrich, 1987. e Bé, 1977. f Kučera, 2007. g Facultative symbionts. h Large/thick specimens. i Seasonal variations.
Stable oxygen isotopes in seawater (δ18Oseawater) were analysed by the isotope ratio infrared spectroscopy (IRIS) analyser (Picarro model L1102-i CRDS) at the laboratory of GeoZentrum Nordbayern (Erlangen) (Van Geldern and Barth, 2012). The measurements are expressed in per mil (‰) vs. the Vienna Standard Mean Ocean Water (VSMOW). The analytical precision is better than 0.05 ‰ (±1σ).
The difference between the predicted inorganic calcite δ18O signal of the seawater (calcite formed in thermodynamic equilibrium, δ18Oequilibrium) and the δ18Ocalcite value of the foraminifer is commonly termed the vital effect (δ18Odisequilibrium) (Table 2):
To determine δ18Oequilibrium (Fig. 3a), the temperature equation of Kim and O'Neil (1997) for inorganic precipitation was applied following the relationship after Wilke et al. (2006):
with in situ temperatures (∘C) measured during cruise M78/1 by CTD and measured seawater (δ18Oseawater) values (Schönfeld et al., 2011; Sect. S1). δ18Oseawater was corrected to the V-PDB scale by subtracting 0.27 ‰ after Hut (1987).
2.3 Mg ∕ Ca analyses
The ratios of Mg ∕ Ca in foraminiferal calcite were analysed from both bulk samples comprising numerous of tests (on average ± 25 specimens) of a single species and single specimens, depending on their abundances (see Sect. S1). Prior to analyses, the samples were cleaned with a hydrogen peroxide cleaning step following Barker et al. (2003), which is suggested to be an efficient method to remove the high amount of cytoplasm in live foraminifers (Pak et al., 2004). We omitted a reductive hydrazine cleaning step, as this step is unnecessary for plankton samples. Furthermore, employing only the oxidative cleaning step allows for direct comparison to foraminiferal Mg ∕ Ca from surface sediments, which are treated similarly (Regenberg et al., 2006). For each bulk sample (plankton net and sediment), ∼400–800 µg of G. sacculifer, N. dutertrei and G. ungulata from different size fractions were used for analyses (Sect. S1). The tests were gently crushed between two glass plates, in order to open the chambers, and transferred into a vial. The samples were first rinsed with ultrapure water and ethanol, including an ultrasonic treatment. Then, 250 µL of a NaOH ∕ H2O2 solution (100 µL 30 % H2O2 and 10 mL NaOH) were added to each vial and placed for 20 min in a hot water bath (92 ∘C). For the plankton samples, these steps were repeated one to two times in order to completely remove the cytoplasm. The samples were subsequently rinsed with ultrapure water. Finally, the tests were leached with 250 µL of HNO3 (0.001 M). Prior to the element analyses, the samples were dissolved in HNO3 (0.075 M). The measurements were performed with an axial-viewing VARIAN 720 inductively coupled plasma – optical emission spectrometer (ICP-OES) at GEOMAR. The data of the measurements were normalised and trend-corrected using the ECRM 752-1 standard (3.761 mmol mol−1 Mg ∕ Ca; Greaves et al., 2008). The analytic precision is 0.1 mmol mol−1 (±2σ).
Single chambers of live collected foraminifers were analysed with an Excimer ArF 193 nm laser ablation system, coupled to an inductively coupled plasma – mass spectrometer (ICP-MS Agilent 7500cx) at GEOMAR. Single foraminifers were cleaned with a buffered hydrogen peroxide solution, in a similar way to the bulk samples. Only one specimen was put into a vial to avoid breaking the test during the cleaning process. Each test was rinsed with ultrapure water and ethanol before adding 250 µL of NaOH ∕ H2O2 solution. The samples were then placed in a hot water bath (92 ∘C) for 20 min and rinsed with ultrapure water and ethanol afterwards. Subsequently, the samples were dried at room temperature. The laser ablation technique allowed us to ablate through the test wall from the outer test surface towards the inner side. Its spot size diameter was focused to 50 or 75 µm. Ablation profiles were carried out on the last four chambers (F to F-3) (Sect. S1). The energy density of the laser was 0.9–2.6 J cm−2 and a laser repetition rate of 5 or 7 Hz was selected. The following isotopes were measured: 24Mg, 26Mg, 27Al, 43Ca, 44Ca, 55Mn, 66Zn, 88Sr, 232Th and 238U. The ablation was stopped when the test wall was penetrated. Analyses were calibrated using standard glasses 610 and 612 of the National Institute of Standards and Technology (NIST) using the values of Jochum et al. (2011). The NIST 610 and NIST 612 were ablated with an energy density of 2–3 J cm−2 after every 10 measurements of foraminiferal tests. Raw counts of elements were processed offline and 43Ca was used as internal standard to account for ablation yield. Outliers (average value ±2σ) were rejected from the results. A powder pellet of JCt-1 (giant clam shell) was used as reference and repeatedly analysed (n=15) during the ablation sessions, revealing an average Mg ∕ Ca ratio of 1.21±0.13 mmol mol−1 (standard deviation of 10.6 %, 1σ) being consistent with the consensus of solution analyses in many laboratories (Mg ∕ Ca = 1.289 mmol mol−1; Hathorne et al., 2013).
In situ temperatures (∘C) measured during cruise M78/1 (Schönfeld et al., 2011) were compared to derived Mg ∕ Ca temperature estimates. We applied different calibrations for each species to account for species-specific differences (e.g. Russell et al., 2004; Cléroux et al., 2008; Regenberg et al., 2009; see Sect. S2).
2.4 Calculation of δ18Oseawater
The combination of δ18Ocalcite and Mg ∕ Ca in foraminiferal tests allows us to estimate δ18O of the ambient seawater (Craig and Gordon, 1965; Schmidt, 1999; Fig. 3b), which is used as a proxy for surface seawater salinity. We compared our measured in situ δ18Oseawater to δ18Oseawater estimates derived from combined foraminiferal δ18Ocalcite and Mg ∕ Ca temperatures of G. sacculifer. For the calculation, we used the species-specific δ18O paleotemperature equation for G. sacculifer of Spero et al. (2003) with the species-specific Mg ∕ Ca temperature calibration for G. sacculifer of Regenberg et al. (2009).
2.5 Calcite dissolution
Calcite dissolution can affect foraminiferal Mg ∕ Ca and stable isotopes (δ18O and δ13C) as a function of the regionally different calcite saturation states in the oceans and the sensitivity of the species-specific test structure (Brown and Elderfield, 1996; Spero et al., 1997; Zeebe, 1999; Bijma et al., 1999; Regenberg et al., 2006, 2014). The calcite saturation state is defined as
and decreases from the surface (∼150–200 µmol kg−1) to ∼5000 m water depth (<0 µmol kg−1) in the eastern Caribbean Sea and Gulf of Mexico (Fig. 4). of ∼21 µmol kg−1, which is a critical threshold for the onset of selective Mg2+ ion removal from planktic foraminiferal calcite, is at ∼2500–3000 m water depth in the study area. Below this, the undersaturated water generally lowers foraminiferal Mg ∕ Ca through preferential dissolution (Regenberg et al., 2006, 2014). Furthermore, increasing concentrations and seawater pH cause decreasing δ18Ocalcite and δ13Ccalcite values of the foraminiferal tests (Spero et al., 1997; Zeebe, 1999; Bijma et al., 1999). As all plankton net samples of this study were taken from shallower than 400 m water depth, the studied living foraminifers originate from supersaturated seawater with respect to calcite ( >50 µmol kg−1) and substantial Mg2+ ion removal (loss of higher Mg ∕ Ca calcite) is not to be expected. This is not valid for fossil tests from surface sediments below 2500–3000 m water depth. At these locations (stations M78/1-220/SO164-22-2 and M78/1-219/SO164-02-3), we use the dissolution-corrected Mg ∕ Ca values from Regenberg et al. (2006, 2009) (see Sect. S1).
3.1 Hydrographical setting during sampling
The CTD and thermosalinograph data gathered during cruise M78/1 (February–March 2009) reveal low sea surface temperatures (SSTs) in the Gulf of Mexico (∼20 ∘C) and Florida Straits (∼24 ∘C) (Figs. 1, 2) comparable to the boreal winter situation (Fig. 2; Locarnini et al., 2013). Hydrographic conditions in the Caribbean vary seasonally with a large range of SSTs (range in the Florida Straits up to 5 ∘C) and salinities (SSSs; range in the Caribbean Sea up to 1) (Fig. 2), and are closely linked to the migrating Intertropical Convergence Zone (ITCZ), which is at its northernmost position (6–10∘ N) during summer (Locarnini et al., 2013; Zweng et al., 2013). The surface mixed layer extends to max. 100 m water depth in the Caribbean and is characterised by the relatively fresh Caribbean water (CW; salinity < 36). The lowest salinity is recorded in the southeastern Caribbean during summer and autumn when the Amazon and Orinoco river discharge is most intense and freshwater plumes arrive in the Caribbean Sea (Wüst, 1964; Müller-Karger et al., 1989; Chérubin and Richardson, 2007). Modified CW is transported via anticyclonic eddies (Loop Current) towards the Gulf of Mexico and Florida Straits (Vukovich, 2007). In the upper thermocline, the highly saline Subtropical Underwater (SUW; salinity > 37) prevails. This water mass originates in tropical and subtropical regions (Gallegos, 1996; Blanke et al., 2002) and resides in ∼80–160 m water depth. The 18 ∘C Sargasso Sea water (EDW) prevails in ∼200–400 m water depth entering the Caribbean Sea via the passages of the Greater Antilles (Morrison and Nowlin, 1982). The Gulf Common Water (∼23 ∘C and salinity ∼36.4; Vidal et al., 1994) possibly influences the Florida Straits' hydrography (Station 210/211) in the upper thermocline at 100–150 m, characterised by low salinity (36.5).
Seawater δ18O (δ18Oseawater) averages to ∼0.9 ‰ (VSMOW) in the uppermost 400 m water depth (Fig. 3a). Highest δ18Oseawater values (1.3 ‰) can be found in the salinity maximum at ∼60–150 m water depth, whereas the lowest value (0.3 ‰) is measured in the deepest sample at the lowest salinity. Additionally, the in situ δ18Oseawater and salinity recorded during M78/1 show a positive correlation (linear regression, r=0.81) and yield similar values as earlier datasets from the Caribbean Sea (Schmidt et al., 1999) (Fig. 3b). The δ18Oequilibrium increases with depth from ‰ to 1 ‰ depending on the decreasing ocean temperature (Figs. 2, 3a).
3.2 Vital effects on foraminiferal δ18Ocalcite
In order to address the effects of symbiont activity and life cycle on the foraminiferal oxygen isotopes, δ18Ocalcite values of living foraminifers were compared to the calculated δ18Oequilibrium of the ambient seawater and δ18Ocalcite estimates of fossil tests from underlying surface sediments.
3.2.1 Symbionts and life cycle effect on foraminiferal δ18O and δ13C
Specimens of G. sacculifer and O. universa from the mixed layer are characterised by large negative δ18Odisequilibrium values of −0.35 ‰ and −0.32 ‰, respectively (Table 2). These two species host dinoflagellates as symbionts (Gastrich, 1987), and similarly negative δ18Odisequilibrium values were reported in spinose, symbiont-bearing species caught in plankton tows from various ocean areas (Table 2 and references therein). Laboratory experiments (Spero, 1992; Spero and Lea, 1993; Bemis et al., 1998) revealed a depletion of 0.3 ‰ to 0.6 ‰ in δ18Ocalcite of O. universa and G. sacculifer under high irradiance levels related to algae photosymbiont activity. In particular, a high irradiance in the euphotic zone intensifies the photosynthetic rate in the Caribbean Sea under its prevailing oligotrophic conditions (Spero and Parker, 1985; Morel et al., 2010). Enhanced photosymbiont activity increases the O2 concentration and fosters CO2 fixation, resulting in an elevated pH within the microenvironment around the living foraminifer (Jørgensen et al., 1985; Rink et al., 1998). Both increasing pH and increasing carbonate ion concentration apparently cause a depletion of δ18Ocalcite (Spero et al., 1997; Bijma et al., 1999).
Among all species studied, only G. sacculifer and N. dutertrei reveal a significant positive correlation (Spearman rank correlation, p<0.05) between test size and stable isotopes (δ18Ocalcite and δ13Ccalcite) (Fig. 5, Sect. S3), suggesting that ontogeny affects the isotopic fractionation processes. The species G. ungulata shows lower δ18Ocalcite values in the test size fraction <300 µm and G. menardii indicates no significant ontogenetic effect (p>0.5; Fig. 5). It should be noted that for some species we did not have enough sample material in all test size classes. However, our results are consistent with Kahn (1979), Kahn and Williams (1981), Spero and Lea (1996) and Bemis et al. (1998), who postulated that juvenile foraminifers have a larger vital effect than adult individuals, with their tests being depleted of the heavy 18O and 13C isotopes due to a higher metabolic rate (incorporation of respired CO2) and/or rapid growth rate. Rapidly growing calcitic skeletons result in a stronger kinetic isotope fractionation and cause the depletion of heavier 18O and 13C isotopes (McConnaughey, 1989).
Vertical migration of planktic species to deeper and colder water masses during their life cycle may additionally affect δ18Ocalcite, leading to commonly higher values in adult specimens (Kroon and Darling, 1995; Lončarić et al., 2006; Birch et al., 2013). Samples from the same test size fraction of all species exhibit the enrichment of heavier 18O isotopes at deeper water levels (Fig. 6; Table 3). We speculate that the increasing δ18Ocalcite at deeper water levels is a function of increasing δ18Oequilibrium of the ambient seawater, rather than ontogenetic effects itself. The surface dweller G. sacculifer reveals the largest δ18Odisequilibrium value (∼1 ‰) in the thermocline (Table 2). As a higher rate of photosynthetic processes in deeper water depths can be excluded and specimens were still alive when sampled, we suggest that G. sacculifer completed calcifying in the thermocline before reproduction. Our observation corroborates South Atlantic plankton net studies of Lončarić et al. (2006), who noted that G. sacculifer δ18Ocalcite increased with depth in the upper 60 m water depth and remained constant below the surface mixed layer, even though δ18Oequilibrium increased continuously.
3.2.2 The δ18O offset between living and fossil foraminifers
It becomes evident that almost all fossil tests from surface sediment samples, in particular N. dutertrei, P. obliquiloculata, G. truncatulinoides and G. tumida, are enriched in δ18Ocalcite (>0.5 ‰) compared to their living counterparts from the water column (Fig. 6; Table 3). δ18Ocalcite of fossil shallow dwellers G. sacculifer and O. universa are rather close to those values of specimens caught in the thermocline (average difference of 0.14 ‰ and 0.02 ‰, respectively) (Table 3). Yet, the overall discrepancy in δ18Ocalcite between fossil and living specimens may be best explained by gametogenic calcification processes or calcite crust formation, which take place during the vertical migration through the water column. At the end of the life cycle and prior to gametogenesis, various planktic foraminifer species (including G. sacculifer, O. universa, P. obliquiloculata, G. truncatulinoides, G. tumida) add a calcite crust of variable thickness on the outer surface of the test (Schiebel and Hemleben, 2017, and references therein). Based on calculations of Bouvier-Soumagnac and Duplessy (1985) and Hamilton et al. (2008), up to 25 % (∼4 µg) gametogenic calcite is added by O. universa, which is mainly secreted in colder water prior to reproduction. The tests thereby lose their glassy and transparent appearances (Bé, 1980; Deuser et al., 1981; Duplessy et al., 1981b; Hemleben et al., 1985; Schweitzer and Lohmann, 1991). Specifically, spinose species resorb their spines before releasing their gametes (Bé and Anderson, 1976; Spero, 1988). These processes result in heavier δ18Ocalcite compositions of fossil tests from surface sediments (and even individual foraminifers from sediment traps) (Duplessy et al., 1981b; Bouvier-Soumagnac and Duplessy, 1985; Bouvier-Soumagnac et al., 1986; Lin et al., 2011). Consistently, the heavy δ18Ocalcite values in adult specimens of G. truncatulinoides and G. tumida may be best explained by vertical migration into colder water masses at a late ontogenetic stage (Franco-Fraguas et al., 2011; Birch et al., 2013). Orr (1967) and Vergnaud-Grazzini (1976) recognised that living individuals of G. truncatulinoides with a thick test and pustules on the test surface are more likely to be found in deeper water masses than non-ornamented, thin-shelled specimens. As expected, such tests had δ18Ocalcite values close to those observed in surface sediments. Overall, our proxy database supports the notion that specimens of P. obliquiloculata, G. tumida and G. truncatulinoides add a thick opaque calcite layer or cortex at deeper water depths than ∼400 m. Hence, the fossil tests are enriched in δ18Ocalcite relative to the living foraminifers (up to 0.85 ‰) (Fig. 6; Table 3).
During the sampling campaign in February–March 2009, mainly juvenile specimens of N. dutertrei were found in plankton nets (mode test size fraction 150–250 µm; Jentzen et al., 2018). This finding may additionally explain the large δ18Ocalcite offset between living foraminifers and fossil tests (∼1 ‰) (Fig. 6; Table 3). Kroon and Darling (1995) recognised that small specimens of N. dutertrei have similar δ18Ocalcite values as surface dwellers and lower values than large specimens, supporting the notion on the ontogenetic-related migration to deeper water. Fairbanks et al. (1982) and Bouvier-Soumagnac and Duplessy (1985) also noted increasing δ18Ocalcite values of N. dutertrei with increasing water depth in the Panama Basin and Indian Ocean, suggesting that this species secretes substantial proportions of their tests below the mixed layer. Furthermore, living N. dutertrei from the South China Sea were depleted in δ18Ocalcite compared to individuals from sediment traps (Lin et al., 2011). Our data confirm these assumptions, as we recognised higher δ18Ocalcite values and larger individuals of N. dutertrei in surface sediments compared to the mixed layer (Fig. 6; Table 3; Jentzen et al., 2018).
The species G. menardii show increasing δ18Ocalcite values from the mixed layer to the thermocline (+0.3 ‰) and from the thermocline to the surface sediments (+0.2 ‰), pointing to decreasing ambient seawater temperatures at deeper water levels and migration within the water column (Fig. 2; Table 3). Apparently, G. ungulata is an exception to the rule, as this species does not show the enrichment of δ18Ocalcite in fossil tests compared to living specimens (Fig. 6; Table 3). Yet, the species secreted their calcite tests close to the equilibrium with the ambient seawater (0.01 ‰–0.08 ‰) throughout the water column (Table 2). The average surface sediment δ18Ocalcite value corresponds well with the depth where the highest standing stock was observed during the sampling campaign in February–March 2009 (Fig. 6; Jentzen et al., 2018).
3.3 Mg ∕ Ca-based ocean temperature assessment from living foraminifers
In order to evaluate Mg ∕ Ca as a proxy for seawater temperature, we compared Mg ∕ Ca temperature estimates of living specimens to (i) measured in situ temperatures and (ii) Mg ∕ Ca temperature estimates of fossil tests from surface sediments. Within this study, Mg ∕ Ca analyses were performed on bulk foraminiferal samples measured by ICP-OES and single tests measured by LA-ICP-MS. ICP-OES samples of G. sacculifer, N. dutertrei and G. ungulata yield higher Mg ∕ Ca ratios on average compared to LA-ICP-MS samples from the same MSN sample (Table 3). The data indicate a difference of 0.5±0.5 mmol mol−1 for G. sacculifer (average value of eight MSN sampling intervals), 1.2 mmol mol−1 for N. dutertrei (one MSN sampling interval) and 0.17±0.05 mmol mol−1 for G. ungulata (three MSN sampling intervals). We compare the results of both methods to each other, having in mind the data discrepancy originating from the different analytical techniques. For LA-ICP-MS, only small amounts of foraminiferal calcite from single chambers are analysed and for the ICP-MS the bulk calcite from whole foraminiferal tests are measured.
Our Mg ∕ Ca ratios of eight species collected at specific ocean temperature ranges (corresponding to different water depth intervals) are in good agreement with established species-specific Mg ∕ Ca temperature calibrations (Fig. 7; see Sect. S2) and further support the foraminiferal Mg ∕ Ca dependency on ambient water temperature. Hence, we estimate Mg ∕ Ca temperatures applying the best-fitting calibration for each species (Fig. 8). Overall, all specimens collected in the surface water of the eastern Gulf of Mexico (PF samples) yield low Mg ∕ Ca temperature estimates (averaged ∼20.6 ∘C) according to the low early spring temperatures of ∼20 ∘C prevailing during cruise M78/1 (Fig. 1). Higher Mg ∕ Ca temperature estimates (∼25 ∘C) of shallow dwellers (symbiont-bearing and facultative symbiont-bearing species) in the Florida Straits and Caribbean Sea (MSN samples) point to higher temperatures in the mixed layer (>24 ∘C). Low Mg ∕ Ca ratios of deep dwellers (G. truncatulinoides and G. tumida) in the thermocline follow the decreasing ambient seawater temperatures (Fig. 8).
3.3.1 (Facultative) symbiont-bearing species
Our dataset is most complete for G. sacculifer, allowing for a detailed comparison between Mg ∕ Ca-based temperature estimates from plankton net and surface sediment samples. In the Caribbean Sea, the estimated Mg ∕ Ca temperatures for G. sacculifer (∼26 ∘C) are consistent with in situ temperatures of the mixed layer (∼26.2 ∘C), the average habitat temperature (∼26 ∘C, derived from the standing stock, Table 2) and Mg ∕ Ca temperatures derived from fossil tests (∼26 ∘C) (Fig. 8). Below 150 m water depth, the deviation between Mg ∕ Ca temperature and the ambient seawater temperature increases, which supports the former conclusion based on δ18Ocalcite that G. sacculifer completed calcifying above or within the thermocline. Lower temperature estimates of ∼24 ∘C in the Florida Straits (station 211) (Fig. 7) mirror the generally lower sea surface temperatures of ∼24.6 ∘C at this station during cruise M78/1 (Fig. 2). Here, the fossil tests from surface sediments yield higher Mg ∕ Ca ratios (+0.7 mmol mol−1) than the living specimens. The Mg ∕ Ca temperature of fossil specimens is ∼26.5 ∘C, which is rather comparable to temperatures in the Florida Straits of the mixed layer in May (Locarnini et al., 2013, Fig. 2). Foraminiferal census data from the MSN samples suppose that the highest population density of G. sacculifer, consequently also the highest flux and accumulation rate of empty tests on the seafloor, appears during early spring in the Caribbean Sea, linking this species to the warm and oligotrophic CW (∼26 ∘C) (Jentzen et al., 2018). Furthermore, high frequencies of G. sacculifer are related to the strength of the Loop Current transporting warm CW into the Gulf of Mexico (Poore et al., 2013). Therefore, we presume that a higher flux of G. sacculifer in the Florida Straits is likely to occur later in the year, presumably in May, hence after our sampling, and the fossil tests of G. sacculifer from the Caribbean Sea and Florida Straits thereby reflect different seasonal signals.
Beside the seasonal effect, millennial-scale variabilities further affect the Mg ∕ Ca signal of fossil tests from surface sediments. Regenberg et al. (2006) assumed an age range of 2–3 kyr in surface sediments (∼0–1 cm) of the Caribbean Sea. As such, the surface sediments include the record of earlier climate variations, like the Little Ice Age, when sea surface temperatures in the Caribbean were cooler by ∼2 ∘C (Watanabe et al., 2001). A large scatter of ∼0.9 mmol mol−1 Mg ∕ Ca of fossil tests from Caribbean surface sediments was therefore linked partly to past environmental variabilities (Regenberg et al., 2006). Our study, however, shows a similarly large Mg ∕ Ca scatter in living specimens collected from the same plankton nets (MSN samples, Mg ∕ Ca range up to ∼0.87 mmol mol−1; Fig. 7). Furthermore, LA-ICP-MS profiles across single chamber walls reveal a large Mg ∕ Ca variability, with decreasing Mg ∕ Ca values towards the final chamber (F) (see Sect. S4), which implies that vital effects drive Mg2+ incorporation. Earlier studies on surface sediments and culture experiments indicate an ontogenetic effect on the incorporation of Mg2+ during test growth of G. sacculifer, with lowest Mg ∕ Ca ratios in the final, newly precipitated chambers (Sadekov et al., 2005; Dueñas-Bohórquez et al., 2011). Although lower average Mg ∕ Ca ratios (∼0.3 mmol mol−1) were measured in living specimens than in fossil tests, the bulk foraminiferal samples of living G. sacculifer from the mixed layer show a significant positive correlation between Mg ∕ Ca and in situ temperatures (Pearson linear, r=0.8, p<0.05), with an overall Mg ∕ Ca scatter comparable to that of fossil specimens from surface sediments (Fig. 7).
Our database for the other species is rather limited. Nonetheless, we can derive the following information. The symbiont-bearing species O. universa characteristically yields very high Mg ∕ Ca ratios in single tests (up to ∼10 mmol mol−1 on average) (see Lea et al., 1999; Russell et al., 2004). Mg ∕ Ca temperature estimates of O. universa are on average ∼1 ∘C lower than the measured in situ temperature but show decreasing values in larger depths according to lower in situ temperatures (Fig. 8; Table 3). The offset between Mg ∕ Ca temperatures of P. obliquiloculata and in situ temperatures vary from −3 to 9 ∘C. Both O. universa and P. obliquiloculata show low and high Mg2+ bands across single chambers of the tests (Sect. S4). Those bands are likely caused by physiological processes (Eggins et al., 2004; Kunioka et al., 2006; Sadekov et al., 2009; Spero et al., 2015) and reveal a large Mg ∕ Ca variability in single chambers. Single LA-ICP-MS measurements of N. dutertrei yield lower Mg ∕ Ca ratios than the ICP-OES measurements (Table 3). Here, the high Mg heterogeneity in single chambers (see Fehrenbacher et al., 2017) probably caused the large offset between the two measuring techniques (see above). However, the average derived Mg ∕ Ca temperature of plankton bulk samples (∼26.3 ∘C) at station 221 is in good agreement with the in situ temperature of the seawater at this station (∼26.5 ∘C) (Fig. 8). The difference of 0.71 mmol mol−1 Mg ∕ Ca between the living and fossil bulk samples (Table 3) supports the notion that adult specimens of N. dutertrei dwell at larger depths and continue calcifying (development of a crust; see Steinhardt et al., 2015; Fehrenbacher et al., 2017), as indicated by the lower δ18Ocalcite values and smaller specimens collected in the upper mixed layer (Jentzen et al., 2018). Living specimens of G. menardii yield a Mg ∕ Ca temperature range between ∼18 and 26.5 ∘C, which is larger but covers the temperature range of fossil tests (∼23.2–25 ∘C) and the calculated average habitat temperature (∼24.5 ∘C; Table 2) in the Florida Straits and Caribbean Sea (Fig. 8). The high Mg ∕ Ca temperature estimate in the deep sampling depth interval (Fig. 8) might indicate that G. menardii calcifies in the upper 250 m water depth.
3.3.2 Symbiont barren species
In the Florida Straits, both bulk and single Mg ∕ Ca measurements of G. ungulata yield temperature estimates of ∼24 ∘C in the mixed layer and thermocline (Fig. 8) being congruent to the average habitat temperature of 23.8 ∘C during February–March 2009 (Table 2). The average Mg ∕ Ca temperature estimates of living and fossil G. truncatulinoides (∼19 ∘C) mirror the average habitat temperature of ∼20 ∘C during February–March 2009 (Fig. 8; Table 2). The deep dweller G. tumida shows a decreasing Mg ∕ Ca temperature trend from the mixed layer to the thermocline following the decreasing in situ temperature (Fig. 8). The fossil tests of G. tumida show higher average Mg ∕ Ca ratios than the living individuals (Table 3) and yield higher temperature estimates. However, the Mg ∕ Ca temperature of fossil tests (∼19 ∘C) represents the calculated average habitat temperature (∼21.7 ∘C) far better than the living foraminifers, which show an offset to the prevailing in situ temperature of ∼7 to 17 ∘C (Fig. 8) most likely due to variable crusting of the chambers (see Sect. S4).
3.4 δ18Oseawater relationship
The combination of foraminiferal δ18Ocalcite and Mg ∕ Ca temperatures to estimate δ18Oseawater approximating paleo-salinity is a commonly accepted approach in paleoceanography (e.g. Lea et al., 2000; Schmidt et al., 2004; Nürnberg et al., 2008). Support derived from living foraminifers collected under natural conditions is still sparse. Our unique dataset on living planktic foraminifers in the mixed layer (>125 m water depth) at least allows us to test the abovementioned approach for the surface dweller G. sacculifer from the Caribbean Sea and Florida Straits (Fig. 9). As the δ18Oseawater estimates strongly depend on both the applied δ18O paleotemperature equation and empirical Mg ∕ Ca calibration, we decided to apply the δ18O paleotemperature equation of Spero et al. (2003). This equation is based on G. sacculifer cultured in laboratory settings, which takes the large disequilibrium of δ18Ocalcite in living specimens to the ambient seawater into account (Table 2). For the estimation of Mg ∕ Ca temperature, we applied the species-specific calibration of Regenberg et al. (2009) for G. sacculifer derived from fossil tests of surface sediments in the tropical Atlantic and Caribbean Sea. Our study shows that this calibration reflects our in situ temperatures very closely (Fig. 7). δ18Oseawater estimates of G. sacculifer show a weak positive linear relationship with in situ δ18Oseawater as well as with salinity for stations 211 and 221 (Fig. 9c, d). Station 219 does not show a positive linear relationship between δ18Oseawater estimates of G. sacculifer and in situ δ18Oseawater but with salinity (Fig. 9d). The average dataset of all stations shows a weak positive linear relationship (Fig. 9b); however, it is not statistically significant (p>0.05; p=0.074 for salinity and p=0.069 for δ18Oseawater, respectively). Additionally, different test size fractions influence the δ18Oseawater estimates (see Sect. 3.2.1 for δ18Ocalcite) and should be taken into account for studies reconstructing past ocean parameters (see Metcalfe et al., 2015). However, our study on living foraminifers hence provides compelling evidence that the combination of foraminiferal δ18Ocalcite and Mg ∕ Ca temperature reflecting ambient seawater properties reliably approximates the modern ocean salinity.
Our combined stable isotopes (δ18O and δ13C) and Mg ∕ Ca analyses on living planktic foraminifers, collected by MSN and PF from surface to max. 400 m water depth of the Caribbean Sea, the eastern Gulf of Mexico and the Florida Straits, allow for the following conclusions:
The large negative disequilibrium (between δ18Ocalcite and δ18Oequilibrium) of up to −0.35 ‰ observed for G. sacculifer and O. universa points to a strong photosynthetic activity of the host symbionts (dinoflagellates).
Ontogeny most likely controls δ18Ocalcite and δ13Ccalcite values. In this study, G. sacculifer and N. dutertrei show a significant increase of δ18Ocalcite and δ13Ccalcite with increasing test size.
Vertical migration in the water column and additional secretion of a calcite crust or gametogenic calcite (at the end of the foraminiferal life cycle) likely cause the increase of δ18Ocalcite with water depths and the enrichment of heavier 18O isotopes in fossil tests compared to living specimens.
The large intraspecific scatter of Mg ∕ Ca implies a strong vital effect. Nonetheless, it is evident that the ambient calcification temperature drives the Mg ∕ Ca compositions in foraminiferal tests and causes lowered Mg ∕ Ca derived temperature estimates at lowered in situ temperature.
The various species-specific datasets agree well with published δ18O and Mg ∕ Ca calibrations.
Fossil tests of G. sacculifer from surface sediments in the Caribbean Sea and Florida Straits suggest that the regional Mg ∕ Ca signatures may be seasonally biased. Mg ∕ Ca values indicate that the highest flux/accumulation rate of G. sacculifer occurs during spring (March) in the Caribbean Sea and is delayed by a few months in the Florida Straits (most likely in May) linked to prevailing seawater temperatures of ∼26 ∘C in the mixed layer.
Combined δ18Ocalcite and Mg ∕ Ca temperatures of G. sacculifer yield δ18Osewater estimates, which show a weak positive linear relationship with measured in situ δ18Oseawater and salinity.
The dataset of this article can be found in the Supplement and in Jentzen et al. (2018), Regenberg et al. (2006) and Steph et al. (2009).
The supplement related to this article is available online at: https://doi.org/10.5194/bg-15-7077-2018-supplement.
The authors declare that they have no conflict of interest.
This study was funded by the German Research Foundation DFG (grant
SCHO605/8-1). The authors thank the captain, crew and participants of RV
Sonne cruise SO164 and RV Meteor cruise M78/1. We thank
Nadine Gehre for measuring Mg ∕ Ca on bulk samples
(ICP-OES), and Jan Fietzke and Steffanie Nordhausen for the help during laser
ablation measurements and processing the raw data. We would like to thank
Fynn Wulf and Sebastian Fessler for measuring the stable isotopes of
foraminiferal calcite, Robert van Geldern (GeoZentrum Nordbayern) for
measuring stable isotopes of seawater and Birgit Mohr (Univ. Kiel) for the
support with the preparation of scanning electron microscope photographs. We
gratefully thank Takashi Toyofuku and Brett Metcalfe for their constructive
comments, which helped to improve the manuscript.
Edited by: Lennart de Nooijer
Reviewed by: Takashi Toyofuku and Brett Metcalfe
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