LA‐ICP‐MS depth profiling perspective on cleaning protocols for elemental analyses in planktic foraminifers

Abstract

[1] Measurements of trace metal ratios in foraminiferal calcite are routinely used to reconstruct paleoceanographic conditions. Analyses using solution-based inductively coupled plasma mass spectrometry (ICP-MS) require dissolution of the entire foraminifer shell. The potential exists for contamination from adherent clays, mineralized coatings, and other diagenetic components that confound the biogenic trace metal signal. We present results from a cleaning experiment on fossil specimens of the planktic foraminifer Orbulina universa that were cracked into several shell fragments and subjected to different cleaning protocols. We use laser ablation ICP-MS (LA-ICP-MS) depth profiling to evaluate the effects of reductive, oxidative, and chelating (diethylene-triamine pentaacetic acid) cleaning protocols on shell Mg/Ca and Ba/Ca ratios. Using the natural pattern of intrashell Mg/Ca heterogeneity exhibited by O. universa, we demonstrate that reductive and oxidative cleaning can dissolve shell calcite from available surfaces, although intrashell Mg/Ca minima and maxima are unaffected. High-resolution depth profiles can be used to identify areas of heterogeneous intrashell Ba/Ca, which can be excluded from computations of whole-shell Ba/Ca. The size and density of shell pores plays a major role in the degree of contamination from sedimentary material. We demonstrate an approach for computing whole-shell Me/ Ca ratios from LA-ICP-MS depth profiles that accounts for potential contamination and diagenetic overprinting.

Oxygen isotope geochemistry of Laurentide ice-sheet meltwater across Termination I

“…Quality control on ablated profiles included screening and exclusion of initial high counts of Al/Ca from surface contamination, and an observed initial anomalous associated high metal/calcium (Me/Ca) ratio ( Fig. S-3; Vetter et al, 2013b). This initial high Me/Ca appears on depth profiles of both uncleaned shells and shells that have undergone reductive and oxidative cleaning, and is interpreted to be an artifact of the laser ablation process (Vetter et al, 2013b).…”

“…S-3; Vetter et al, 2013b). This initial high Me/Ca appears on depth profiles of both uncleaned shells and shells that have undergone reductive and oxidative cleaning, and is interpreted to be an artifact of the laser ablation process (Vetter et al, 2013b). A total of 6% of individual shells analyzed were culled from the full dataset (41 out of 668 specimens).…”

“…Thus, the uncertainty for LA-ICP-MS measurements is different for each individual foraminifera. Measured values and uncertainties are provided in Supplemental Data (Vetter et al, 2013b), because we did not use reagent cleaning on shells prior to laser ablation analyses. We convert the resulting Ba/Ca shell to salinity (S) using eq.…”

Optimizing LA-ICP-MS analytical procedures for elemental depth profiling of foraminifera shells

“…In many samples, the U concentration was below our detection limit (5 ppb) and shell Ba content is invariant through the shell of this species (Vetter et al, 2013b). The Mg/Ca ratios for the foraminifera were calculated offline in MS Excel, following data reduction protocols detailed in Longerich et al (1996), that include screening for outliers, drift correcting by bracketing samples with NIST SRM 610 analyses, and subtracting background count rates from each data point.…”

“…Cleaned fragments were inspected under a microscope and fragments that appeared dirty were rinsed again. We did not subject the fossil foraminifer to the full cleaning method that includes oxidative and reductive cleaning steps because initial analyses indicated the foraminifers subjected to the rinse step only did not contain high levels of manganese, aluminum, or barium, which would have indicated further cleaning steps were necessary (Vetter et al, 2013b). The cultured N. dutertrei shell was subjected to an oxidative cleaning in a buffered hydrogen peroxide solution (1:1 mixture of 30% H 2 O 2 and 0.1 N NaOH) for 30 min at 65°C to remove remnant organic matter, followed by 3 rinses in ultra-pure water (N18 MΩ).…”