Background & Summary

Element concentrations and stable isotope ratios in the calcareous shells (tests) of Foraminifera provide information on paleoceanography and past climates1,2,3. The trigonal polymorph of CaCO3 allows inclusion of a wide range of trace elements including Li, B, Na, Mg, Mn, Sr, Cd, Ba, Nd, and U3,4. Changes in environmental conditions such as temperature and seawater carbonate ion concentration ([CO32−]) can produce predictable variations in trace element composition of foraminifer shell calcite3,5,6.

Calibrations of ratios between incorporated elements or fractionated isotopes and environmental parameters obtained from foraminiferal calcite provide the basis for paleoclimate reconstruction7,8. With the help of culture experiments, plankton tows, sediment traps, and core-top studies, such calibrations were established for several elements3,9,10, and the Mg/Ca of planktic foraminifer tests has been established as a proxy of seawater temperature5,11,12,13,14,15,16. Under ideal conditions, these proxies are affected by a single environmental parameter only12. Because this is rarely the case, application of these proxies tends to be challenging, and needs to be based on a wide range of empirical data. Since the Mg/Ca is primarily temperature dependent, comparison with δ18O from the same tests provides estimates of δ18O of the water δ18Osw, a proxy of ambient seawater temperature and salinity, and global ice volume17,18. Here, we provide new high-resolution Mg/Ca data produced with femtosecond Laser Ablation ICP-MS (fs-LA-ICP-MS) on the last three chambers of adult tests of the warm-water planktic foraminifer species Globigerinoides ruber albus (syn. G. ruber, white) and G. ruber ruber (syn. G. ruber red/pink), both sensu stricto morphotypes, to assess the applicability of data from single chambers as paleo-temperature proxies of seawater of the low-latitude surface oceans in addition to wet-chemical methods applied to entire tests, as proposed by Nürnberg et al.12.

Foraminifers are major marine archives for climate reconstruction and paleoceanography, owing to the high fossilization potential of their calcareous shell and omnipresence in calcareous sediments of the ocean basins over the past 100 Myrs. The chemical composition of their chambered test is affected by the chemical composition and physical parameters of the environment they live in1. These marine protozoans live in the surface to sub-thermocline layer of the open ocean and deep marginal seas1. Their shell usually consists of low-Mg calcite layers orientated in a radial structure (Arns et al., 2022, and references therein)19.

The temperature dependence of the partitioning of Mg into ontogenetic foraminifer calcite is the basic principle of Mg/Ca palaeothermometry20. Mg/Ca is affected by temperature and other factors such as salinity, pH, and ∆CO32− 12,21,22. Mg/Ca tends to increase with increasing salinity12,21 and decreasing pH3,21. In addition, alternating high and low Mg/Ca bands within the chamber walls of single specimens are assumed to result from diurnal changes in Mg uptake23,24.

Traditionally, Mg/Ca thermometry relies on the bulk analysis of samples comprising 10–30 foraminifer tests of the same species providing averaged temperature signals incorporated at different habitat depths5,14,16,25. Recently new techniques such as LA-ICP-MS were developed and facilitate investigation of the underlying mechanism of differences in Mg/Ca composition by profiling Mg/Ca distribution across individual chambers of the same test over a month-long (on average) life cycle26,27,28.

In this study, fs-LA-ICP-MS was employed to determine and compare the Mg/Ca composition of different chambers of the spinose, symbiont-bearing planktic foraminifer G. ruber at high resolution across the walls of the last three chambers (final, F0; penultimate, F-1 (final minus one); and antepenultimate, F-2) of individual foraminifer tests. Due to the shallow dwelling depth, G. ruber constitutes an excellent archive for studying past hydrological changes in tropical and subtropical surface oceans1,5,15. Globigerinoides ruber were collected onboard S/Y Eugen Seibold during three research cruises in 2019 in the Madeira Basin. Finally, new chamber-specific equations for the relationship between Mg/Ca and ambient water temperature are presented and compared to existing Mg/Ca-temperature equations.

Methods

Sampling and sample processing

Samples were collected in the subtropical eastern North Atlantic with the S/Y Eugen Seibold during cruises ES19C08, ES19C12, and ES19C14 in 2019 (Table 1) with a bongonet and a multinet (both Hydrobios®, Germany) from different water depth intervals (Table 2). The samples were preserved in a hexamine buffered 3% formalin solution at pH > 8.2 immediately after sampling. Ambient temperature was measured using Temperature-Depth (TD48, Sea&Sun Technology®, Germany) and Conductivity-Temperature-Depth (CTD75M, Sea&Sun Technology®, Germany) probes attached to the bongonet and multinet, respectively (Table 2).

Table 1 Scientific expeditions of S/Y Eugen Seibold in 2019.
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Table 2 Mg/Ca of Globigerinoides ruber. F0, F-1, and F-2 are the final, penultimate, and antepenultimate chambers, respectively.
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In the micropaleontology laboratory at the Max Planck Institute for Chemistry (MPIC, Mainz), the plankton net samples were rinsed with one liter of tap water. The planktic foraminifers were picked from the solution with a glass pipette, dried at room temperature, and identified at the species level according to Schiebel and Hemleben (2017). For Mg/Ca analyses, 21 clean individuals of the planktic foraminifer G. ruber were selected from the >100 μm size fraction. Additional chemical (reductive and oxidative) cleaning procedures were not applied to the tests to not alter the original and pristine shell material. The foraminifer tests were glued (Tylopur MOBS 4000, ShinEtsu, 1:100 additive-free methylhydroxypropylcellulose (MHPC), Wiesbaden, Germany) onto a glass slide with the spiral side, with the umbilical side facing up and exposing the final three chambers for LA-ICP-MS analyses29.

Laser ablation, mass spectrometry, and Mg/Ca temperature calculation

Magnesium and calcium of 19 specimens of G. ruber albus and two G. ruber ruber tests were measured at high-resolution on either two or three chambers of each test using a 200 nm wavelength NWR femtosecond laser ablation system (NWRFemto) from Electro Scientific Industries (ESI, New Wave Research Division, Portland, USA) combined with a ThermoFinnigan high-resolution sector-field ICP-MS Element2 mass spectrometer27,30,31.

Femtosecond LA-ICP-MS (fs-LA-ICP-MS) analyses were performed on 45 μm diameter laser spots on each chamber with a pulse repetition rate (PRR) of 1 Hz at low fluence of 0.1 J/cm2 27. MACS-3 was used as reference material for calibration at the beginning of analyses of the objects and every 2 hours in the following, i.e., after each 27 to 30 measurements of the analyzed objects32. MACS-3 is a homogeneous pressed powder pellet consisting of synthetic calcium carbonate powder provided by the United States Geological Survey (USGS), used for calibration with the mass fractions of Mg = 1756 μg/g and Ca = 37.69% m/m33.

The ablated material was analyzed with a single collector sector-field SF mass spectrometer (ThermoFinnigan Element2), with electric and magnetic fields, operated at low mass resolution mode. Employing fast (<0.001 s) electric jump, the double-charged 44Ca2+, which is within a similar mass range (m/2e = 22) as 25Mg+ (m/e = 25) was measured27. This adjustment shortens measurement time and improves the measurement precision. The disadvantage of this technique is the difference between the masses of interest, which must not be more than 30% apart27. Accordingly, 44Ca and 25Mg measurements were carried out on the less abundant double-charged 44Ca rather than on the single charged Ca ions27.

Raw fs-LA-ICP-MS data were evaluated with an automated Microsoft Excel application, at a minimum of 44Ca2+ and 25Mg+ count rates per second27. The washout time of the ESI Large Format Cell is one second, thus the individual peaks were separated due to their shorter lengths of about 0.9 seconds. Data calibration was conducted on the MACS-3 reference material during each session. Obvious outliers and unusually high trace element abundances at the beginning of the ablation (1–3 seconds) were identified and rejected by the Microsoft Excel routine aiming at excluding data from potential surface contamination27,34. Ablation profiles (see Supplement) were optically inspected for verification of the different ablation intervals and to only interpret data from the ontogenetic calcite. In order to correct for interferences, background data were subtracted from the individual ion intensities. The background-corrected count rates of 25Mg were divided by the background-corrected count rates of 44Ca for each scan34. The trace element concentration CEl (μg g−1) is given by

$${{\rm{C}}}_{{\rm{El}}}={{\rm{C}}}_{{\rm{El,uncorr}}}\ast 1/{\rm{RSF}}$$
(1)

with CEl,uncorr being the apparent (uncorrected) concentration, and RSF being the relative sensitivity factor34. CEl,uncorr is determined by:

$${{\rm{C}}}_{{\rm{El,uncorr}}}={{\rm{C}}}_{{\rm{IS}}}\ast {{\rm{R}}}_{{\rm{ik}}}\ast {{\rm{A}}}_{{\rm{k}}}/{{\rm{A}}}_{{\rm{i}}}\ast {{\rm{M}}}_{{\rm{EL}}}/{{\rm{M}}}_{{\rm{IS}}}$$
(2)

with CIS being the concentration (μg g−1) of an internal standard element (IS), Rik being the ratio of the ion intensities of the isotope i of the element of interest (EL), and of isotope k of the internal standard element. Ai and Ak are the isotopic abundances of isotope i and k, respectively34. MEL and MIS are the relative atomic masses of the element of interest and the internal standard element, respectively34. The RSF is determined by:

$${\rm{RSF}}={{\rm{C}}}_{{\rm{El,uncorr}}}/{{\rm{C}}}_{{\rm{El,true}}}$$
(3)

with CEl,uncorr being the uncorrected concentration of the element El obtained by reference to the internal standard element, and CEl,true being the ‘true’ concentration in a reference material34, with values (mass fractions in mg kg−1) for MACS-3 of Mg = 1756, and Ca = 37690033.

The measurement precision (1 relative standard deviation in percent, 1RSD, 1 s) of the Mg/Ca, is determined by repeated measurements of homogeneous calcium carbonate reference materials. Six to 12 independent analyses of the pressed power pellets of MACS-3 yield uncertainties (1 RSD) in Mg/Ca between 4.96% and 7.34%.

The 3-standard deviation and the mean of the blank estimates the limit of detection (LOD)27. For calcareous samples, such as foraminifer tests, the Ca content is high and uniform at about 40% m/m27. Therefore, the LOD of Mg/Ca mainly depends on the LOD of Mg ranging at 0.4–1.2 mg kg−1 23.

To test data reliability, Mg/Ca values were converted into ambient water temperature estimates according to

$${\rm{Mg}}/{\rm{Ca}}={be}^{{\rm{(}}aT{\rm{)}}}$$
(4)

with a = 0.38 and b = 0.09 for G. ruber5,15.

Data Records

The raw (MgCa_per_laser_spot.tab) and averaged (MgCa_avg.tab) Mg/Ca data are available at https://doi.org/10.17617/3.D9FSSN35 on the Edmond Open Research Data Repository of the Max Planck Society, Germany. All column headings of the metadata (sample ID, latitude, longitude, time of sampling), measured Mg/Ca data, and statistical data, i.e., 1σ (1RSD, ‘Error’) are detailed in the headers of the data files. Temperature (°C) of ambient seawater at the time of sampling (also Table 2, T in-situ) is given for comparison with the calculated temperature of the measured Mg/Ca of the shell carbonate of the respective planktic foraminifer species G. ruber (white) or G. ruber (red).

Sample identification (ID) in the data tables numbers provide information per expedition of the sailing yacht Eugen Seibold (ES), and year of sampling (e.g., ES19 in 2019). Cruise numbers are consecutive and start with cruise number one each year (e.g., C08 for cruise ES19C08). The cruise number is followed by the number for station, cast, and sample with a consistent format. Station numbers have three digits (e.g., 008 in ES19C08_008). Cast labels have two digits (e.g., 01 as in ES19C08_008_01). Sample labels have one digit (e.g., 1 as in ES19C08_008_01_1). Hence, the label ES19C08_008_01_1 identifies a sample taken onboard the S/Y Eugen Seibold, in 2019, on Cruise 8, at Station 8 in the same year, obtained with the first cast at station, and subsampled from the first multinet employed at the same station (Table 2).

The hydrological conditions including ambient seawater temperature have been determined from CTD48 and CTD75M data (Table 2) gathered during cruises ES19C08, ES19C12, and ES19C14 (Tables 1, 2).

Technical Validation

Relationship of Mg/Ca data between the different chamber F0, F-1, and F-2 of G. ruber

The majority of Mg/Ca data (MgCa_per_laser_spot.tab) range at 1–6 mmol/mol and are considered reasonable (Fig. 1). The high Mg/Ca values above 20 mmol/mol of the three chambers of one G. ruber (pink) individual from ES19C12_012_06 (Table 2) are considered not realistic and may not be used for further analyses. F-test and t-test show that the remaining Mg/Ca data from the F-1 (\(\overline{x}\) = 3.74 ± 0.58, n = 16) and F-2 (\(\overline{x}\) = 3.79 ± 1.00, n = 17) chambers are statistically similar, and significantly different from F0 (\(\overline{x}\) = 2.71 ± 0.99, n = 17).

Fig. 1
figure 1

Mg/Ca data of planktic foraminifer shell calcite from fs-LA-ICP-MS analyses. Comparison of the Mg/Ca values obtained for different chambers in 20 G. ruber tests. The final chamber (F0), penultimate chamber (F-1), and antepenultimate chamber (F-2) are shown by red dots, green triangles, and blue squares, respectively. Data sorted according to Mg/Ca in F-1. Error bars are in 1σ.

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Relationship between single chamber-Mg/Ca values of G. ruber and measured water temperature

The Mg/Ca of the penultimate (F-1; Fig. 2) and antepenultimate chambers (F-2; Fig. 3) of G. ruber increases with the SST, temperature of the surface mixed layer, and in-situ temperature. The Mg/Ca of the penultimate (F-1) and antepenultimate chambers (F-2) show statistically significant relationships with SST (Figs. 2, 3). In comparison, the relationships of Mg/Ca of both penultimate (F-1) and antepenultimate (F-2) chambers with the mixed layer temperatures are rather weak (Figs. 2, 3). The Mg/Ca relationships to in-situ temperatures are weakest among the three different temperature measures (Figs. 2, 3).

Fig. 2
figure 2

Mg/Ca of the penultimate chamber (F-1) calcite of G. ruber per measured temperature. Relationship between measured Mg/Ca (ln scale) of the penultimate chamber (F-1) and SST, over the surface mixed layer (MLD, mixed layer depth), and in-situ temperatures. Error bars are in 1σ. Sensitivities of Mg/Ca vs. SST, average surface mixed layer temperature, and in-situ temperature are 5 ± 2%, 4 ± 3%, and 7 ± 5% per 1 °C, respectively.

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Fig. 3
figure 3

Mg/Ca of the antepenultimate chamber (F-2) calcite of G. ruber per measured temperature. Relationship between measured Mg/Ca (ln scale) of the antepenultimate chamber (F-2) and SST, surface mixed layer (MLD, mixed layer depth), and in-situ temperatures. Error bars are in 1σ. Sensitivities of Mg/Ca vs. SST, average surface mixed layer temperature, and in-situ temperature are 11 ± 4%, 10 ± 4%, and 4 ± 7% per 1 °C, respectively.

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The relatively poor numerical relationship of the Mg/Ca of the antepenultimate chamber (F-2) with the in-situ temperature may be explained by the temporal distance of chamber formation and sampling date of the foraminifer shell in the respective water body, i.e., typically a couple of days to weeks between the formation of the F-2 and the final chamber36,37,38. The question remains, why -on average- SST does best explain the Mg/Ca of the penultimate and antepenultimate chambers (Figs. 2, 3). The fact that the Mg/Ca of final chamber (F0) shows weak relationships with any measure of seawater temperature recorded at the time of sampling may be explained by the potentially incomplete calcification of the chamber wall1,39.

Relationship between single chamber-Mg/Ca temperature of G. ruber and measured water temperature

Mean Mg/Ca from each chamber of the analyzed G. ruber (Table 2) are used to assess the relationship with the sea surface temperature (SST), ambient water temperature (in-situ), and average mixed layer temperature, ranging from the sea surface to the upper limit of the thermocline, by applying the relationship formulated by Dekens et al.15 and Anand et al.5 (Table 2).

Comparing the calculated temperatures (°C) of penultimate chamber (F-1) to the measured SST a statistically significant correlation is revealed at r = 0.83, and p < 0.02 (Fig. 4). With the average mixed layer temperature and in-situ temperature being statistically not significant, only positive trends can be detected with r = 0.59 and r = 0.49, respectively.

Fig. 4
figure 4

Calculated Mg/Ca temperatures of the penultimate chamber (F-1) G. ruber per measured temperatures. Comparison of calculated Mg/Ca temperature (°C) after Dekens et al.15 and Anand et al.5 versus measured temperatures, SST, surface mixed layer (MLD), and in-situ. Black line marks 1:1 relationship.

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Comparing the calculated temperatures (°C) of antepenultimate chamber (F-2) to the measured temperatures, no statistically significant relationship exists with SST (r = 0.64) and average mixed layer temperature (r = 0.71). Calculated Mg/Ca temperatures show no relationship (r = 0.22) or trends in-situ temperatures (Fig. 5).

Fig. 5
figure 5

Calculated Mg/Ca temperatures of the antepenultimate chamber (F-2) G. ruber per measured temperatures. Comparison of calculated Mg/Ca temperature (°C) after Dekens et al.15 and Anand et al.5 versus measured temperatures, SST, surface mixed layer (MLD), and in-situ. Black line marks 1:1 relationship.

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Finally, the Mg/Ca temperatures produced here range at the upper limit of the Mg/Ca temperature of G. ruber of earlier analyses, which have analyzed various types of samples and species with different methodologies (Fig. 6, and references therein). In particular, absolute values and slope of the regression of the data from the penultimate chambers (Fig. 6, Line 7) are close to the values of Bolton et al. (2011; LA-ICP-MS of single chambers)39, and the salinity and pH corrected values of Gray et al.21, whereas the slope of the regression of the data from the antepenultimate chambers (Fig. 6, Line 8) is steeper than in Gray et al.21. The high-resolution data set provided here is based on a limited number of individual G. ruber representative of the 19–22 °C temperature range, and may be used together with other data of similar nature and quality for a broader application of the Mg/Ca thermometer at the basin to global scale.

Fig. 6
figure 6

Paleothermometers developed in the present study compared to other Mg/Ca-paleothermometers. Two thermometers (Lines 7 and 8) derived here from Mg/Ca data of the penultimate chamber and SST data (7) as well as from the antepenultimate chamber (F-2) and mixed layer temperature data (8) of G. ruber, compared to the findings of (1) Sadekov et al. (2009; core top samples, LA-ICP-MS)26, (2) Dekens et al. (2002; core top samples, wet chemistry)15 and Anand et al. (2003; sediment trap samples, wet chemistry)5, (3) Mohtadi et al. (2009; sediment trap samples, wet chemistry)40, (4) Bolton et al. (2011; LA-ICP-MS, F-1)39, (5) Bolton et al. (2011; LA-ICP-MS, F-2)39, and (6) Gray et al. (2018; sediment trap samples, wet chemistry)21.

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