RADIOCARBON STEP-COMBUSTION OXIDATION METHOD AND FTIR ANALYSIS OF TRONDHEIM CaCO₃ PRECIPITATES OF ATMOSPHERIC CO₂ SAMPLES: FURTHER INVESTIGATIONS AND INSIGHTS

Abstract

Eight atmospheric carbon dioxide samples (as calcium carbonate—CaCO₃—precipitates) from Lindesnes site (58ºN, 7ºE), belonging to 1963 and 1980 (four samples from each year) and stored at the National Laboratory for Age Determination (NTNU), have been reevaluated through radiocarbon (¹⁴C) analysis. Previous ¹⁴C results indicated the presence of a contaminant, which was not removed through different chemical cleansing procedures (e.g., hydrochloric acid—HCl and/or hydrogen peroxide—H₂O₂). Here, we present a follow up investigation using ¹⁴C step-combustion and Fourier-transform infrared spectroscopy (FTIR) analysis. Results from ¹⁴C data indicate unsuccessful removal of the contaminant, while further FTIR analysis displayed the presence of moisture. This finding alludes to the possibility that the contaminant is of ambient air-CO₂ deeply embedded in CaCO₃ powders (within clogged CaCO₃ pores and/or bonded to the lattice). Samples were found exposed to air-CO₂ and humidity. These conditions may have lasted for years, possibly even decades, leading to the ¹⁴C offsets detected here.

1. INTRODUCTION

An archive of atmospheric CO₂ (as calcium carbonate—CaCO₃—precipitates) from Lindesnes site (58°N, 7°E), stored at the National Laboratory for Age Determination (NTNU), formerly Trondheim Radiocarbon Laboratory has been evaluated through radiocarbon (¹⁴C) analysis for its reliability (Seiler et al. 2023). Carbonate precipitates were sampled as described in Nydal and Lövseth (1983). The CaCO₃ powders have been stored since the 1960s in distinct glass vials with closures (Figure 1). The 1980s jars and lids (type B1 and B2) seem to offer better seal (Seiler et al. 2023). Still, containers were found in a room without climate controls. A thorough ¹⁴C investigation conducted by Seiler et al. (2023), commenced by using different chemical cleansing procedures (e.g., hydrochloric acid—HCl and/or hydrogen peroxide—H₂O₂), indicated the presence of a contaminant. Regardless of multiple chemical attempts for contamination removal and different methods to evolve its carbon content, offsets from expected atmospheric ¹⁴C values were as high as 160‰. This finding suggested that the pollutant was somewhat embedded in the carbonate precipitates.

Figure 1 Types of containers and lids from the subset of stored CaCO₃ powder samples reevaluated in this study and adapted from Seiler et al. (2023). Early 1960s samples were stored in type 1 container (panels A1 and A2), while 1980s used type 2 container (panels B1 and B2). Nydal and Lövseth (1983) used a 0.5 N sodium hydroxide (NaOH) solution in an open dish to absorb atmospheric CO₂ over a 4-to-7-day period. While the Nydal and Lövseth (1983) article does not explicitly describe how the CaCO₃ was precipitated afterwards, a previous work by Nydal (e.g., Nydal 1966) describes the usage of CaCl₂ [CaCl₂ reacts with Na₂CO₃ to produce NaCl and a CaCO₃ precipitate].

There is a vast array of atmospheric CO₂ samples in CaCO₃ powder stored at NTNU that has not been measured yet (> 1000 samples; see Seiler et al. 2023 for details). Even though efforts to reproduce Nydal’s historical ¹⁴C results (Nydal and Lövseth 1983) after CaCO₃ powder chemical cleansing were disappointing (Seiler et al. 2023), further investigations were performed here to gain insight on possible contamination sources, and maybe secure its removal. Assuming that the contaminant was somewhat sensitive to heat treatments at lower temperatures (e.g., lower than 375ºC, a temperature setting that is high enough to remove labile organic carbon, but sufficiently low to avoid charring, Currie et al. [2002] or Szidat et al. [2013]), additional analyses were carried out. These included (i) a ¹⁴C two-step thermal oxidation to remove adsorbed/absorbed CO₂ and/or labile organic carbon (OC) from CaCO₃ precipitate powders, and (ii) Fourier-transform infrared spectroscopy (FTIR) analysis of the step-combustion treated and untreated CaCO₃ powders. While our new efforts did not produce Nydal’s historical ¹⁴C results, offsets were still similar to those obtained by Seiler et al. (2023), we provided new insights on the carbon contaminant of Nydal’s CaCO₃ precipitate archive.

2. SAMPLES AND METHODS

2.1. Sample Selection

Besides the subset of the CaCO₃ samples from NTNU’s archive (addressed below), we also selected reference materials of carbonate and organic sources to be subjected to the same sample processing steps as carbonate precipitates. Samples are briefly described below.

1. 1. Nydal’s 1963 and 1980 set—CaCO₃ precipitates of atmospheric CO₂ samples sampled by Nydal and Lövseth (1983) and stored in types 1 and 2 containers, respectively (Figure 1, reproduced from Seiler et al. 2023).

2. 2. Calcite—Clear calcite crystal used at the Keck Carbon Cycle Accelerator Mass Spectrometer facility of the University of California, Irvine (KCCAMS/UCI) as an in-house blank for several years (F¹⁴C =0 or ¹⁴C-free), e.g., Santos et al. (2007), Hinger et al. (2010), Bush et al. (2013).

3. 3. Coral standard—This relatively modern coral sample (F¹⁴C is 0.9440 ± 0.0004; Hinger et al. 2010) is another in-house reference material that has been repeatedly used in several projects (Bush et al. 2013, Gao et al. 2014).

4. 4. FIRI-C—Marine turbidite samples from the Fourth International Radiocarbon Intercomparison (FIRI). The consensus ¹⁴C age of FIRI-C has been reported as 18,176 ± 10.5 yrs BP (Scott et al. 2004), when CO₂ evolved has been produced by acid hydrolysis. This material is a carbonate/clay mixture with < 50% carbonate, several minerals and a younger organic carbon fraction (Bush et al. 2013). It was chosen for this experiment due to this characteristic.

5. 5. USGS coal—Argonne Premium Coal POC#3, collected during the United States Geological survey (Vorres 1990), was used as an independent reference blank material (F¹⁴C =0 or ¹⁴C-free). This highly recalcitrant organic sample was added to this study to evaluate the background processing of treatments, during removal of surface carbon by heat.

6. 6. Rice Char—Rice charcoal from University of Zurich containing both organic and elemental/recalcitrant carbon. It was used in our experiment to evaluate the effectiveness of the removal of OC during heat treatment. Consensus of its recalcitrant fraction (also termed EC—elemental-carbon fraction), after organic carbon removal, is F¹⁴C = 1.0675 ± 0.0007 (n=3) (Huang et al. 2021).

2.2. Sample Preparation and Handling

2.2.1. Radiocarbon Sample Processing and Measurements

For carbonate samples (CaCO₃ precipitates, calcite and coral standard), various amounts between 8.0 and 12 mg of chemically untreated material were loaded into prebaked quartz tubes of about 15 cm long with 40 to 50 mg of copper oxide (CuO), used as a catalyst to oxidize the samples (Table S1). Loaded tubes with samples were placed upright on an in-house modified reaction heat block from Corning PC-400D set to a maximum of 285°C for 24 hr. This temperature setting was thoroughly tested for its stability. Moreover, this temperature is higher than that reported by Santos et al. (2010) when removing adsorbed/absorbed CO₂ and carbon embedded in porous powders (i.e., 160ºC), and slightly lower than the typical temperatures reported to evolve OC without charring, when heating is conducted under air or pure oxygen (i.e., 340º–375ºC during < 1 to 24 hr; Szidat et al. 2013). Thus, 285ºC for 24 hr was chosen for our first-step combustion oxidation method.

About 5–6 cm of the lower end of each quartz tube with loaded materials was inserted into the heating element holes, while the remaining portions of the tube received heat transferred from below. To avoid contaminants falling into quartz tube openings during the course of the treatment, and while OC was being removed, a large heavy-duty aluminum foil tent was set up over the heat block set. Precise maximum heating of the heat block set was checked during, and after treatment by an independent temperature probe (Precision RTD Handheld Data Logger Thermometer). Upon 24 hr, quartz tubes with loaded samples were transferred still hot to the vacuum line (to avoid reabsorption of CO₂ from air), evacuated and sealed off with a flame torch for combustion. Samples were then heated to 1000°C per 6 hr (or 900ºC over 3 hr) to extract CO₂ (details in Tables S1 and S2). Quartz tubes were carefully laid down horizontally after powders and CuO were well distributed within, as some etching from inside out was expected (Santos and Xu 2017). Several of the tubes combusted at 1000°C per 6 hr ruptured during the procedure, especially those loaded with calcite, coral standard or FIRI-C. For CaCO₃ precipitates, higher CO₂ yields were obtained from those with > 10 mg, combusted at 1000ºC over 6 hr, although some variability was detected (Table S1). The organic reference standards (USGS coal and Rice char), which were selected for comparisons, were handled under the same methods. Their weights were adjusted according to their EC content, e.g., 1 mg for USGS coal (80–100% EC) and about 6.5 mg for Rice Char (15–30% EC). Sealed tube combustion evolved CO₂ were cryogenically cleaned using a vacuum line, and later transferred to a graphitization vessel to produce filamentous graphite following specific protocols (Santos and Xu 2017).

Radiocarbon measurements were taken on a modified compact AMS system with ¹³C/¹²C measurement capabilities (NEC 0.5MV1.5SDH-1) (Beverly et al. 2010). For normalization and quality assessment of spectrometer, 6 oxalic acid I (OX-I) targets, an oxalic acid II (OX-II) from NIST and a sucrose from ANU were also measured with samples. Sample preparation backgrounds have been subtracted, based on measurements of ¹⁴C-free calcite and USGS coal. Radiocarbon concentrations are given as fractions of the Modern standard and/or conventional radiocarbon age (Tables S1 and S2), following the conventions of Stuiver and Polach (1977) and Reimer et al (2004). All ¹⁴C results have been corrected for isotopic fractionation, based on spectrometer AMS online-δ¹³C values derived from ¹²C and ¹³C loop-by-loop measured from the same graphite targets (Beverly et al. 2010).

2.2.2. Fourier Transform Infrared Spectroscopy (FTIR) Analysis

As for FTIR measurements, just CaCO₃ precipitates and FIRI-C were analyzed. Samples were processed as followed—one set was kept untreated, while another was heated to a maximum of 285°C for 24 hr in air (as described above—section 2.2.1) in shell glass vials. Sample weights (in mg) before and after heating were carefully recorded. Mass percentage variation was higher among the CaCO₃ precipitates (0 to 15%), while FIRI-C showed a difference of 9%.

Vials of untreated and step-combustion treated samples were kept closed by a tight insert cap until measurements were performed at the Laser Spectroscopy Labs at the University of California, Irvine. Heat treated samples were measured within less than 24 hr of treatment, in order to preserve freshness. Analyses were performed using a JASCO FT/IR-4700 spectrometer with measuring transmittance values set between the spectral range of 4000 to 400 cm⁻¹ and with data intervals at 0.482117 cm⁻¹, as standard conditions for all measurements. For clarity, the wavenumber window shown here starts at 600 cm⁻¹ to avoid the noise area at the low wavenumber end of each spectrum.

3. RESULTS AND DISCUSSIONS

3.1. Radiocarbon Results

The complete set of CaCO₃ precipitate ¹⁴C results as well as those of reference materials are reported in Tables S1 and S2 (supplementary material). Target sizes (in mg C) vary significantly, e.g., 0.22 to 0.97 mg C. Notably, minimum CO₂ yields were obtained for pure calcite and coral standard, where particles loaded in quartz tubes were significantly coarser than the fine powders of Nydal’s CaCO₃ precipitates.

Literature searches show that carbonate decomposition should occur at temperatures as low as 850°C (Stern 1969). Even so, absolute pressure in the calciner environment (Maya et al. 2018), size of the CaO crystallites formed under heat, mineral and organic impurities (Galwey and Brown, 1999) can also play a role in CaO reactivity and CO₂ release. Here, carbonate samples were loaded in an evacuated quartz tube with no partial pressure, until CuO was finally activated. Reactivity of CaO and its sintering against inner quartz tubes walls may have diminished the access to carbon during the CaCO₃ decomposition reaction. Plus, we cannot rule out CO₂ recombination with the CaO leftover and/or CuO, once quartz tubes returned to room temperature after combustion. We also used two temperature settings for sealed quartz tubes combustion and time durations (1000°C per 6 hr and/or 900°C over 3 hr), and distinct sample weights (8–12 mg; see Tables S1 and S2). All of the above, may have affected the CaCO₃ decomposition response during sealed tube combustion step.

Nonetheless, lower graphite mass (as mg C) and its impact on ¹⁴C results were addressed by background mass balance correction, using blanks treated in the same fashion as the other samples. For that, we used 7 blanks (2 calcites and 5 USGS coals) of different masses (Table S2). Their ¹⁴C concentration (relative to modern standard) and masses (as mg C) were plotted to derive the constant mass of contamination introduced during the entire sample processing (step-combustion, seal-off combustion, graphitization, and pressing) plus measurement at the spectrometer (Figure S1). A consistent value of 1 μg C modern blank was found, as ¹⁴C results of calcites and coals overlapped. Thus, this constant blank mass was used for ¹⁴C results background correction, following the equations in Santos et al. (2007).

As mentioned earlier, we also determined AMS online-δ¹³C values derived from ¹²C and ¹³C loop-by-loop measured from the same graphite targets (not shown here). The AMS online-δ¹³C values based on Nydal’s CaCO₃ precipitates were significantly lighter (e.g., δ¹³C = –20‰ in average) than what one would expect for a CaCO₃ matrix. The NaOH-static method, the one used by Nydal to capture atmospheric CO₂ during several days, tend to introduce large isotope fractionation in ¹³C (e.g., –15 to –25‰, according to Turnbull et al. [2017]) due to the high alkalinity of NaOH. In Nydal and Lövseth (1983), the CaCO₃ precipitates δ¹³C values reported ranged from –25 to –27‰ and were used for the intended purpose of correcting ¹⁴C data. When δ¹³C was not measured, its value was then estimated based on multiple samples (see details in Nydal and Lövseth 1996). Since at KCCAMS/UCI, isotopic fractionation correction to ¹⁴C data is performed by using the AMS online-δ¹³C values, the effect of chemical reaction shifts, machine and size dependence (if any) were completely addressed and removed. For more details on data analysis, see Santos et al. (2007, 2010).

Overall, duplicated ¹⁴C results as well as those of reference materials support the reliability of the measurements. The carbonate samples, L26 and FIRI-C, overlap in ±2σ of each other (Table S1). Paired ¹⁴C results of the samples L22B, L24, and L28 showed larger differences, e.g., more than ±2σ. Note that carbonate samples did not undergo chemical leaching by a weak HCl treatment (Santos et al. 2004) before step-combustion, and this may have played a role. Lower temperature step-combustion is expected to remove just OCs, and not secondary carbonates. Another possibility is that the CaCO₃ precipitates contamination is not homogeneous, and distinct initial combusted masses resulted in uneven ¹⁴C signatures (see further discussion in 3.4 section). Replicated ¹⁴C results of the recalcitrant fraction of the Rice char overlap with each other as well as accurately reproduce its expected consensus ¹⁴C value (Table S2). Paired FIRI-C turbidite ¹⁴C results also overlapped with each other.

In figure 2 we compared UCI ¹⁴C results of CaCO₃ precipitates as F¹⁴C with those reported by Nydal and Lövseth (1983) and those recently measured at NTNU by Seiler et al. (2023). In the latter, we chose the set of ¹⁴C results from CaCO₃ powders subjected to just flash-combustion using an elemental analyzer for comparisons. Both NTNU and Nydal’s values were also reproduced in Table S1 to serve as reference. For a complete overview of chemical treatments attempts to remove contaminants from the CaCO₃ powders stored at NTNU, please refer to Seiler et al. (2023).

Figure 2 Radiocarbon results as F¹⁴C values for both the 1963 (panel A) and 1980 (panel B) series from different works are shown. Note that panels show very distinctive y-axis scales. Color labels inserted in distinct plot areas discriminate the results from UCI (this study), NTNU (Seiler et al. 2023), Nydal (Nydal and Lövseth 1983), Fruholmen (Nydal and Lövseth 1996) and/or Schauinsland (Levin et al. 1985).

For simplicity’s sake, in Figure 2 we averaged the UCI ¹⁴C results of CaCO₃ precipitates produced in duplication and used the greater error of the two, individual error or standard deviation of the paired ¹⁴C results, as error bar. According to Nydal’s runlog books, the 1963 samples measured here, and by Seiler et al. (2023), were indeed measured by ¹⁴C before moving to storage in vial type 1 (Figure 1). Regarding the CaCO₃ precipitate 1980 series, they were not measured by Nydal in the past. Following Seiler et al. (2023) approach, atmospheric ¹⁴C values of Schauinsland (48ºN) (Levin et al. 1985) are loosely used here, as reference for the ¹⁴C results of the CaCO₃ precipitate 1980 series. We also added data from Fruholmen (71ºN) (Nydal and Lovseth 1996) during the same period, as a way to bracket the ¹⁴C results of Lindesnes (58ºN) site.

For the most part, current CaCO₃ precipitate ¹⁴C results obtained at UCI (this study) and at NTNU (Seiler et al. 2023) are in agreement with each other, especially those belonging to the 1963 series (Figure 2A). For the CaCO₃ precipitates 1980 series (Figure 2B), L358 ¹⁴C results between UCI and NTNU are visibly apart from each other (standard deviation = 0.6%). For detailed differences in ¹⁴C data and assemblages, see Table S1. Radiocarbon data agreements between UCI and at NTNU sets are remarkably good for samples that undergo different handling procedures, spectrometer measurement setups, and data analyses, especially when the contamination embedded into CaCO₃ powders was clearly not removed. As Seiler et al. (2023) already pointed out, the ¹⁴C results of CaCO₃ powders from the 1980 series are higher than those from the Schauinsland (48ºN) site reported by Levin et al. (1985) (Figure 2B). The Schauinsland (48ºN) site is located 10º further south than Lindesnes (58ºN), and therefore, the geographical provenance of the air parcels transporting ¹⁴CO₂ reaching those sites may explain the differences observed here. As aforesaid, we also plotted the dataset of Fruholmen (Nydal and Lövseth 1996) for comparisons (Figure 2B). This geographic location is approximately 10º north of the Lindesnes (58º N) site. The fact that our ¹⁴C results fell between ¹⁴CO₂ signatures of both Schauinsland (48ºN) and Fruholmen (71ºN) could be somewhat promising. Yet, the results of a series of archaeological samples, archived in the same type 2 containers (Figure 1), indicate potential atmospheric contamination (Seiler et al. 2023). Without further atmospheric ¹⁴C results in very close proximity to the Lindesnes (58ºN) site, it cannot be substantiated whether the ¹⁴C results of the CaCO₃ precipitates of the 1980 series of either Seiler et al. (2023) or this study are correct.

As per the reference materials (Table S2), sealed tube combusted step-combustion carbonates FIRI-C turbidite yielded a ¹⁴C age of 18,380 ± 57 (n=2) yrs BP, while coral standard a F¹⁴C of 0.9435 ± 0.0015 (Table S2). Both are in reasonable agreement with expected consensus values (reported in section 2.1), especially coral standard. FIRI-C showed a small ¹⁴C-age difference of about 200 yrs between sealed tube step-combustion at 285ºC (this study) and its consensus value, which is based on the standard acid hydrolyze procedure (Scott et al. 2004). This difference is significantly smaller than that reported by Bush et al. (2013), e.g., 2–4 kyrs offsets, when mixed carbon fractions of this turbidite were directly measured by ¹⁴C-AMS when loaded into aluminum target holders. Bush et al. (2013) also attempted to remove the ¹⁴C effect of the FIRI-C organic fraction by heating this turbidite at 500°C in air. While the authors failed to reproduce expected ¹⁴C results of Scott et al. (2004), they demonstrated that the FIRI-C turbidite powder is highly active, and can reabsorb CO₂ from ambient, once powders are allowed to cool off. Santos et al. (2010) demonstrated a similar effect when heating fine powders at 160°C. Thus, this issue seems to be related to absorption properties of materials in particulate form, their porosity level, and particulate surface area available, rather than just temperature settings for the purpose of cleansing.

Here, all samples (in the form of coarse particulates or powders, carbonates or organics) were not allowed to cool off after 285ºC treatment per 24 hr. Thus, we have no knowledge if reabsorption effects would be different between distinct types of particulate samples. While our FIRI-C two step-combustion oxidation age-value of 18,380 ± 57 (n=2) yrs BP is somewhat older than that reported in Scott et al. (2004), it is not significantly different. Radiocarbon result of the recalcitrant fraction of organic Rice char yielded F¹⁴C of 1.0655 ± 0.0020 (n=4), and is in perfect alignment with its expected ¹⁴C value (section 2.1). We can then conclude that nearly all OC have been removed from both FIRI-C and Rice char upon step-combustion treatment, as described in section 2.2.1.

3.2. FTIR Results

FTIR spectroscopy was performed to help determine possible CaCO₃ precipitate impurities (Figure 3). FIRI-C, a carbonate/clay mixture, was also analyzed in view of its heterogeneous properties. Typical FTIR of simple molecules, such as calcium carbonate (CaCO₃), water (H₂O), sodium hydroxide (NaOH) and sodium chloride (NaCl) are shown in the supplementary material (Figure S2) to assist in discussions.

Figure 3 FTIR spectrum of untreated (panel A) and heat-treated (panel B) CaCO₃ powders from 1963 and 1980, as well as FIRI-C turbidite (carbonate/clay mixture). The window was set between 4000 to 600 cm⁻¹.

All FTIR CaCO₃ precipitate profiles of untreated (Figure 3A) and step-combustion treated (Figure 3B) samples show the presence of the usual CaCO₃ profile peaks between 1500 and 700 cm⁻¹ (Figure S1), e.g., the pronounced and broad peaks (1400, 865, and 714 cm⁻¹). Those are characteristic peaks of CaCO₃ molecules and calcite crystals (Gupta et al. 2015; NIST webbook online database—Figure S2). The FIRI-C untreated and heated-treated samples showed the IR band at 1022 cm⁻¹ due to Si-O-Si stretching vibration characteristic of sediments (Liu et al. 2013).

FTIR spectrum indicate that three of the 1963 untreated CaCO₃ powders were still slightly wet, possibly due to CaCO₃ powders exposed to moist air during storage. While no post-treatment procedures are detailed in Nydal and Lövseth (1983), we assume that carbonate precipitate washing was sufficient to remove Na residues from incomplete removal of supernatant (NaOH) and reaction byproducts (NaCl). Nonetheless, we added here the transmittance peak profiles of pure H₂O as well as NaOH and NaCl (Figure S2). Their transmittance peaks most likely overlap in the same fingerprint broad region (2500–3750 cm⁻¹), and therefore, they would be difficult to disentangle in any FTIR profile, especially if they are subtle.

The 1980 series had little to no peaks within the 2500–3750 cm⁻¹ region before and after heat-treatment (Figure 3B), indicating that these samples were somewhat drier, than those of the 1963 series (Figure 3A). The rationality behind peak differences between 1963 and 1980 series samples may be explained by types of containers and lids (Figure 1). As Seiler et al. (2023) noted, cap type 2 seems to provide better closure than cap type 1.

The FIRI-C untreated and heated-treated samples also showed FTIR profile differences by the reduction of the broad area of the single bond between 2500–4000 cm⁻¹. This area is known to contain aromatic C-H and C-C bonds, as well as overlapping νH-O (H₂O). A reduction of this broad band, but not its complete removal, would imply that some of the recalcitrant C-H and C-C bonds are still present after heat treatment. Unremoved recalcitrant fraction of FIRI-C may corroborate the small ¹⁴C-age difference of about 200 years from sealed tube step-combustion (this study). Even though in our procedure samples were transferred hot to vacuum line for evacuation and sealing (so that ambient-air CO₂ reabsorption over fine powders could be avoided), any unremoved elemental/refractory carbon would be combusted in conjunction with CaCO₃ in the presence of CuO. The evolved CO₂ (and its associate ¹⁴C value) would therefore be biased by this sediment-like carbon contribution.

3.3. Nydal’s Stored CaCO₃ Precipitates and Possible Sources Exogenous C Contamination

Nydal’s CaCO₃ precipitates have been stored in a room at NTNU without climatic condition controls (variable temperature and relative humidity). While containers with CaCO₃ precipitates (retested in this study) appeared to be properly closed at first glance, there is no guarantee that this was the case for the whole storage period. Moreover, the lid from the type 1 container appears to be less air-sealing effective than that of type 2 (Figure 1). The possibility of decades of exposure to moisture, increased air-CO₂ and variable temperatures, leads us to consider some theories on how CaCO₃ precipitates have been contaminated.

First, ambient CO₂ that seeped into containers could be adsorbed as such in CaCO₃ precipitate inter- and intra-particle voids. While we have not conducted porosimetry analysis of Nydal’s CaCO₃ precipitate, it is fair to assume that the powder is highly porous, as judged by bio- and industrially produced CaCO₃ precipitates that have been heavily studied for their permeability characteristics and CO₂ storage capabilities (e.g., Moore and Wade 2013; Yoon et al. 2017). Moreover, the large specific surface area of all types of finer powders and the interstitial gaps created by particles of different sizes tend to naturally entrap gasses. Nonetheless, adsorbed ambient CO₂ within and between particles is normally very volatile and sensitive to heat treatments as low as 160ºC (Santos et al. 2010). If the main contaminants of Nydal’s stored CaCO₃ precipitates are interparticle OCs and/or other gasses, they should have been mostly removed upon step-combustion treatment, as we demonstrated by ¹⁴C measurements of step-combustion Rice char and FIRI-C samples (Table S2). Moreover, partial OC removal of Nydal’s stored CaCO₃ precipitates by step-combustion would at least lead to measurable differences between ¹⁴C step-combustion treated CaCO₃ results and those of Seiler et al. (2023). We have not observed these differences, thus solo ambient air-CO₂ contamination does not seem to explain the mismatch between current and Nydal’s ¹⁴C results.

A second hypothesis involves re-carbonation of ambient-CO₂ and formation of new layers of carbonate over and within intraparticle voids of the existing CaCO₃ powders, a type of CO₂-water-rock interaction at room-temperature (small amounts of hydrate lime—Ca(OH)₂, for example). For a significant secondary formation of CO₂ to CaCO₃ microspheres into more stable CaCO₃ crystalline forms at room temperature, pre-calcination of CaCO₃ to CaO at higher temperatures is required (Erans et al. 2020). While we do not believe Nydal’s CaCO₃ precipitates undergo calcination, there is very little information on how they were handled. Among the numerous factors that can affect the CaCO₃ precipitation process and secondary species formation, Febrida et al. (2021) stress the presence of various foreign ions or molecules depending on the aqueous solution used from which the carbonate precipitates. For sample collection, Nydal used rainwater (Nydal and Lövseth 1983), which can be loaded with several ionic compounds (Carol 1962). Therefore, the combination of high air-CO₂ pressures found in buildings (as far as 2500 ppm—Erans et al. 2020), CO₂ solubility property in liquid water or vapor (Zeman and Lackner 2004), and the fact that the Nydal’s CaCO₃ powder containers were found in a room with moisture variability for 30+ years of storage may have played a role in further air-CO₂ entrapment, soluble calcium bicarbonate formation (H₂CO₃), and CaCO₃ recrystallization filling up CaCO₃ pores, especially if foreign ions were present in precipitates (Sanz-Pérez et al. 2016; Giacomin et al. 2020; Toffolo 2020; Febrida et al. 2021). Thus, ambient CO₂ adsorbed to CaCO₃ powders may just have promoted a dynamic and continuous gas-solid exchange process with CO₂ bonding to the lattice. Dissolved CO₂ reacts with crystals through a CO₂-water-rock interaction, where H₂CO₃ and HCO₃ ^– secondary species are formed (Yoon et al. 2017; Hanein et al. 2021; Huang et al. 2021). Once in the interior of minute particles, this exchanged CO₂ may no longer be easily removed by either heating at lower temperatures (this study) or selective leaching (Seiler et al. 2023). The latter may explain why the ¹⁴C results of CaCO₃ precipitates in this study corroborate so well with those in Seiler et al. (2023). Finally, a simpler mechanism may just involve adsorption of air CO₂ to surface of powder crystals followed by desorption of a CO₂ molecule of the original crystal lattice, followed by subsequent crystal lattice diffusion. In every case, gas-solid exchange equilibrium followed by significant ¹⁴C changes to CaCO₃ original signal would require at least (a) the presence of very small crystals, and (b) longer duration of exposure to CO₂.

Even though we detected a difference in overall CaCO₃ precipitate mass before and after heat treatments, our oxidative decomposition of carbonate by step-combustion (possibly more active on powder surfaces, than their interior) most likely removed just H₂O and interparticle adsorbed CO₂. Any other adsorbed CO₂, either deeply trapped in CaCO₃ pores or bonded to the lattice, would contribute to the final ¹⁴C results we obtained.

We may never know for certain what is the mechanistic process of how ambient CO₂ has altered Nydal’s CaCO₃ precipitates, and when this occurred during the 30+ years storage. Either way, several attempts to remove this exogeneous carbon, e.g., after HCl and H₂O₂ treatments, or chemically untreated flash-combustion (Seiler et al. 2023) as well as ¹⁴C two-step combustion oxidation method (this study) were insufficient to bring the 1963 and the 1980 series to expected ¹⁴C values.

As far as one can tell, all of Nydal’s CaCO₃ precipitates published in the literature yielded correct ¹⁴C values once measurements were completed and corrections were applied (Nydal and Lövseth 1983, 1996). After 30+ years of storage, those same CaCO₃ precipitates are yielding inaccurate ¹⁴C values. Neither Seiler et al. (2023) nor this study introduced artifacts to data to justify the differences detected. Hence, we can only assume that the current Nydal’s CaCO₃ precipitate archive is of no use to reproduce atmospheric ¹⁴CO₂ signatures, until the contamination issue can be effectively resolved.

Carbon dioxide sequestration in the form of CaCO₃ is a useful way to store atmospheric CO₂ for further analyses. But it requires proper storage conditions, ideally within a hermetically sealed vial, such as evacuated flame-sealed glass ampoules.

4. CONCLUSION

Samples from the Lindesnes site (58ºN), a small fraction of the large archive of atmospheric CO₂ (as CaCO₃) samples stored at NTNU since the 1960s, have been tested in this study. These samples have been previously ¹⁴C measured at NTNU after chemical cleansing procedures (e.g., HCl and H₂O₂) that attempted to remove surface contaminants.

Here, we applied a two-step oxidation treatment from room temperature to 285ºC with air standard pressure. Our recent ¹⁴C results from a total of eight CaCO₃ samples, associated with the atmospheric CO₂ values of 1963 and 1980, did not differ significantly from those obtained by NTNU (Seiler et al. 2023). Like the NTNU values, they do not match with expected atmospheric ¹⁴C values. Our heating treatment worked well on reference materials of carbonate (FIRI-C) and organic (Rice char), mixed matrixes known to contain organic labile compounds, implying that the contaminant in CaCO₃ samples is not OC and cannot be readily removed by low temperature heating. FTIR spectrum results indicate the presence of moisture. While their removal by heat did not improve ¹⁴C results per se, it gave insight to the current conditions of those CaCO₃ samples.

To provide a new perspective on the elusive carbon contaminant of the Nydal’s stored CaCO₃ samples, we relied on notions of CaCO₃ formation, growth and recrystallization. While numerous works have shown that CO₂ to CaCO₃ by NaOH reaction does, in principle, follow a straight pathway, calcium carbonate equilibria can be rather complex and influenced by several factors. However, in the case of CaCO₃ precipitates that need to be stored for future ¹⁴C analysis, the use of a hermetically sealed vial for storage purposes would be the best practice.