Planktic foraminiferal-based trace element-calcium ratios (TE/Ca) are a cornerstone in paleoceanographic reconstructions. While TE-environment calibrations are often established through culturing experiments, shell growth in culture is not always consistent with growth in a natural setting. For example, many species of planktic foraminifera thicken their shell at the end of their life cycle, producing a distinct ‘gametogenic’ crust. Crust is common in fossil foraminifers, however, shells grown in culture do not often develop a thick crust. Here we investigate potential vital effects associated with the crusting process by comparing the trace element (Mg/Ca, Na/Ca, Ba/Ca, Sr/Ca, Mn/Ca, Zn/Ca) and stable isotope (δ¹³C, δ¹⁸O) composition of alive, fully mature, uncrusted shells to recently deceased, crusted shells of Neogloboquadrina pachyderma collected from the same plankton tows off the Oregon (USA) coast. We find that uncrusted (N = 55) shells yield significantly higher Ba/Ca, Na/Ca, Mn/Ca, and Sr/Ca than crusted (N = 66) shells, and crust calcite records significantly lower TE/Ca values for all elements examined. Isotopic mixing models suggest that the crust calcite accounts for ~40 to 70% of crusted shell volume. Comparison of foraminiferal and seawater isotopes indicate that N. pachyderma lives in the upper 90 m of the water column, and that crust formation occurs slightly deeper than their average living depth habitat. Results highlight the necessity to establish calibrations from crusted shells, as application of calibrations from TE-enriched uncrusted shells may yield attenuated or misleading paleoceanographic reconstructions.

Sampling

Neogloboquadrina pachyderma were collected from two stations along the Newport Hydrographic (NH) Line off the Oregon coast: NH85 (44.652^oN, 126.050^oW) and NH45 (44.652^oN, 125.117^oW) in May 2022. N. pachyderma are an asymbiotic, non-spinose taxon found abundantly in the study region when cooler waters are present (Ortiz and Mix, 1992; Takagi et al., 2019; Lane et al., 2023), and they serve as an important species to reconstruct polar to subpolar paleoceanographic conditions. A 150 μm plankton tow net was used to sample the uppermost 200 m of the water column, sampled as two continuous tows from 200 m depth to the surface. While N. pachyderma have been found at greater depth habitats in other regions (i.e., Greco et al., 2019), the sampling was constrained to the upper 200 m of the water column to ensure that we were sampling populations calcifying from the same water depth and that geochemical differences between alive, uncrusted and dead, crusted could be directly attributed to the process of gametogenic crust development as opposed to any additional geochemical changes associated with the shells sinking through the water column (e.g., scavenging). Previous depth stratified tows conducted in the study region (Ortiz et al., 1996) have found minor concentrations (<0.25 individuals/m³) of N. pachyderma below 200 m water depth. Fully-mature, live specimens and recently deceased, crusted specimens of N. pachyderma were immediately wet-picked from each tow, rinsed with distilled water, and placed in micropaleontological slides. Alive specimens were identified by their distinctive brightly colored cytoplasm and relatively thin shell walls, whereas the shells of recently deceased foraminifers were free of cytoplasm, appeared white under reflective light, and exhibited a thick crust (Fig. 1). A total of 66 dead shells (NH85: N=52; NH45: N=14) and 55 alive shells (NH85: N=44; NH45: N=11) were collected. 

LA-ICP-MS TE/Ca analysis

Sample preparation

Prior to geochemical analyses, specimens were rinsed and oxidatively cleaned to remove remnant organic matter, following previously established protocols, adapted for single shells (Martin and Lea, 2002). To remove any detritus, the shells were rinsed with ultrapure (milliQ) water, followed by methanol, and then given an additional rinse with ultrapure water. Next, shells were submerged in a 1:1 solution of 30% H₂O₂ buffered with 0.1 M NaOH, and vials were placed in a hot (65^oC) water bath for 10 minutes. The solution was removed from the sample vials, and shells were rinsed three times in ultrapure water. Shells were subsequently mounted on a slide covered with strips of carbon tape in preparation for in-situ trace element analysis. 

LA-ICP-MS Analytical Methods

All specimens were analyzed via LA-ICP-MS to evaluate trace element geochemistry. Shells were analyzed using a Thermo Scientific iCAP RQ inductively coupled plasma mass spectrometer coupled to an Applied Spectra RESOlution laser ablation system in the Keck Collaboratory for Plasma Spectrometry at Oregon State University. Ten isotopes were measured with the following seven isotopes evaluated herein: ²³Na, ²⁴Mg, ⁴³Ca, ⁵⁵Mn, ⁶⁶Zn, ⁸⁸Sr, and ¹³⁸Ba. Isotopes were measured using a rapid peak-hopping procedure with the following dwell times for each isotope: ²³Na = 0.04 s, ²⁴Mg = 0.02 s, ⁴³Ca = 0.01 s, ⁵⁵Mn = 0.05 s, ⁶⁶Zn = 0.05 s, ⁸⁸Sr = 0.02 s, and ¹³⁸Ba = 0.05 s. Shells were ablated from the outer shell surface through to the interior of the shell at a repetition rate of 5 Hz and a laser energy of 4.0 mJ attenuated by 75% to 87.5%, focused upon 24–38 μm spot sizes. The last four chambers in the final whorl, F₀ (ultimate chamber) to F₃, were analyzed with repeated analysis on at least one chamber per specimen to assess reproducibility. Shell averages reported herein were calculated by first averaging repeated analyses on single chambers, followed by equal-weight averaging of the chamber averages. In <6% of the specimens, the F3 chamber was too small or not accessible due to the orientation of the specimen on the tape, and thus was not analyzed or included in the calculation of the whole-shell value. Analysis of shells was periodically bracketed by analysis of three standard reference materials (SRM): NIST glasses 610 and 612, and the USGS standard MACS-3. Each standard was ablated for 60 s at an energy of 5.00 mJ using a 50 μm spot size. All element concentrations were determined using NIST 610 and NIST 612, however, due to consistent Na concentrations found in NIST glasses, Na was evaluated using the NIST 610 and MACS-3 SRMs. Elemental calibrations of SRMs measured throughout the analysis session yielded r² ≥ 0.99.

Data Analysis Methods 

LA-ICP-MS data was processed using the Python package LA-Tools (Branson et al., 2019) which follows established data reduction protocols (Longerich et al., 1996). Data processing protocols include despiking and signal smoothing to remove outliers, evaluating and applying any necessary drift correction using the bracketing SRM analyses, and removing average background counts from each data point. The mean TE/Ca for each profile (i.e., individual chamber analysis) is then calculated by normalizing to the known trace element concentrations of the drift-corrected SRM standards (Jochum et al., 2011).