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. Reference Seiler, Grootes, Svarva and Nadeau2023). Carbonate precipitates were sampled as described in Nydal and Lövseth (Reference Nydal and Lövseth1983). 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. Reference Seiler, Grootes, Svarva and Nadeau2023). Still, containers were found in a room without climate controls. A thorough 14C investigation conducted by Seiler et al. (Reference Seiler, Grootes, Svarva and Nadeau2023), 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.
Types of containers and lids from the subset of stored CaCO3 powder samples reevaluated in this study and adapted from Seiler et al. (Reference Seiler, Grootes, Svarva and Nadeau2023). 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 (Reference Nydal and Lövseth1983) 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 (Reference Nydal and Lövseth1983) article does not explicitly describe how the CaCO3 was precipitated afterwards, a previous work by Nydal (e.g., Nydal Reference Nydal1966) 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. Reference Seiler, Grootes, Svarva and Nadeau2023 for details). Even though efforts to reproduce Nydal’s historical 14C results (Nydal and Lövseth Reference Nydal and Lövseth1983) after CaCO3 powder chemical cleansing were disappointing (Seiler et al. Reference Seiler, Grootes, Svarva and Nadeau2023), 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. [Reference Currie, Benner, Cachier, Cary, Chow, Urban, Eglinton, Gustafsson, Hartmann, Hedges and Kessler2002] or Szidat et al. [Reference Szidat, Bench, Bernardoni, Calzolai, Czimczik, Derendorp, Dusek, Elder, Fedi and Genberg2013]), 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. (Reference Seiler, Grootes, Svarva and Nadeau2023), 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. Nydal’s 1963 and 1980 set—CaCO3 precipitates of atmospheric CO2 samples sampled by Nydal and Lövseth (Reference Nydal and Lövseth1983) and stored in types 1 and 2 containers, respectively (Figure 1, reproduced from Seiler et al. Reference Seiler, Grootes, Svarva and Nadeau2023).
-
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. (Reference Santos, Southon, Griffin, Beaupre and Druffel2007), Hinger et al. (Reference Hinger, Santos, Druffel and Griffin2010), Bush et al. (Reference Bush, Santos, Xu, Southon, Thiagarajan, Hines and Adkins2013).
-
3. Coral standard—This relatively modern coral sample (F14C is 0.9440 ± 0.0004; Hinger et al. Reference Hinger, Santos, Druffel and Griffin2010) is another in-house reference material that has been repeatedly used in several projects (Bush et al. Reference Bush, Santos, Xu, Southon, Thiagarajan, Hines and Adkins2013, Gao et al. Reference Gao, Xu, Zhou, Pack, Griffin, Santos, Southon and Liu2014).
-
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. Reference Scott, Boaretto, Bryant, Cook, Gulliksen, Harkness, Heinemeier, McGee, Naysmith, Possnert and van der Plicht2004), 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. Reference Bush, Santos, Xu, Southon, Thiagarajan, Hines and Adkins2013). It was chosen for this experiment due to this characteristic.
-
5. USGS coal—Argonne Premium Coal POC#3, collected during the United States Geological survey (Vorres Reference Vorres1990), 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. 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. Reference Huang, Zhang, Santos, Rodríguez, Holden, Vetro and Czimczik2021).
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. (Reference Santos, Alexandre, Coe, Reyerson, Southon and De Carvalho2010) 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. Reference Szidat, Bench, Bernardoni, Calzolai, Czimczik, Derendorp, Dusek, Elder, Fedi and Genberg2013). 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 Reference Santos and Xu2017). 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 Reference Santos and Xu2017).
Radiocarbon measurements were taken on a modified compact AMS system with 13C/12C measurement capabilities (NEC 0.5MV1.5SDH-1) (Beverly et al. Reference Beverly, Beaumont, Tauz, Ormsby, von Reden, Santos and Southon2010). 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 (Reference Stuiver and Polach1977) and Reimer et al (Reference Reimer, Brown and Reimer2004). 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. Reference Beverly, Beaumont, Tauz, Ormsby, von Reden, Santos and Southon2010).
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 Reference Stern1969). Even so, absolute pressure in the calciner environment (Maya et al. Reference Maya, Chejne, Gómez and Bhatia2018), size of the CaO crystallites formed under heat, mineral and organic impurities (Galwey and Brown, Reference Galwey and Brown1999) 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. (Reference Santos, Southon, Griffin, Beaupre and Druffel2007).
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. [Reference Turnbull, Mikaloff Fletcher, Brailsford, Moss, Norris and Steinkamp2017]) due to the high alkalinity of NaOH. In Nydal and Lövseth (Reference Nydal and Lövseth1983), 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 Reference Nydal and Lövseth1996). 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. (Reference Santos, Southon, Griffin, Beaupre and Druffel2007, Reference Santos, Alexandre, Coe, Reyerson, Southon and De Carvalho2010).
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. Reference Santos, Southon, Druffel-Rodriguez, Griffin and Mazon2004) 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 (Reference Nydal and Lövseth1983) and those recently measured at NTNU by Seiler et al. (Reference Seiler, Grootes, Svarva and Nadeau2023). 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. (Reference Seiler, Grootes, Svarva and Nadeau2023).
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. Reference Seiler, Grootes, Svarva and Nadeau2023), Nydal (Nydal and Lövseth Reference Nydal and Lövseth1983), Fruholmen (Nydal and Lövseth Reference Nydal and Lövseth1996) and/or Schauinsland (Levin et al. Reference Levin, Kromer, Schoch-Fischer, Bruns, Münnich, Berdau, Vogel and Münnich1985).
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. (Reference Seiler, Grootes, Svarva and Nadeau2023), 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. (Reference Seiler, Grootes, Svarva and Nadeau2023) approach, atmospheric 14C values of Schauinsland (48ºN) (Levin et al. Reference Levin, Kromer, Schoch-Fischer, Bruns, Münnich, Berdau, Vogel and Münnich1985) 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. Reference Seiler, Grootes, Svarva and Nadeau2023) 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. (Reference Seiler, Grootes, Svarva and Nadeau2023) 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. (Reference Levin, Kromer, Schoch-Fischer, Bruns, Münnich, Berdau, Vogel and Münnich1985) (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 Reference Nydal and Lövseth1996) 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. Reference Seiler, Grootes, Svarva and Nadeau2023). 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. (Reference Seiler, Grootes, Svarva and Nadeau2023) 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. Reference Scott, Boaretto, Bryant, Cook, Gulliksen, Harkness, Heinemeier, McGee, Naysmith, Possnert and van der Plicht2004). This difference is significantly smaller than that reported by Bush et al. (Reference Bush, Santos, Xu, Southon, Thiagarajan, Hines and Adkins2013), 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. (Reference Bush, Santos, Xu, Southon, Thiagarajan, Hines and Adkins2013) 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. (Reference Scott, Boaretto, Bryant, Cook, Gulliksen, Harkness, Heinemeier, McGee, Naysmith, Possnert and van der Plicht2004), 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. (Reference Santos, Alexandre, Coe, Reyerson, Southon and De Carvalho2010) 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. (Reference Scott, Boaretto, Bryant, Cook, Gulliksen, Harkness, Heinemeier, McGee, Naysmith, Possnert and van der Plicht2004), 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.
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. Reference Gupta, Singh, Kumar and Khajuria2015; 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. Reference Liu, Colman, Brown, Minor and Li2013).
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 (Reference Nydal and Lövseth1983), 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. (Reference Seiler, Grootes, Svarva and Nadeau2023) 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 Reference Moore and Wade2013; Yoon et al. Reference Yoon, Major, Dewers and Eichhubl2017). 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. Reference Santos, Alexandre, Coe, Reyerson, Southon and De Carvalho2010). 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. (Reference Seiler, Grootes, Svarva and Nadeau2023). 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. Reference Erans, Nabavi and Manović2020). 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. (Reference Febrida, Cahyanto, Herda, Muthukanan, Djustiana, Faizal, Panatarani and Joni2021) 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 Reference Nydal and Lövseth1983), which can be loaded with several ionic compounds (Carol Reference Carol1962). Therefore, the combination of high air-CO2 pressures found in buildings (as far as 2500 ppm—Erans et al. Reference Erans, Nabavi and Manović2020), CO2 solubility property in liquid water or vapor (Zeman and Lackner Reference Zeman and Lackner2004), 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. Reference Sanz-Pérez, Murdock, Didas and Jones2016; Giacomin et al. Reference Giacomin, Holm and Mérida2020; Toffolo Reference Toffolo2020; Febrida et al. Reference Febrida, Cahyanto, Herda, Muthukanan, Djustiana, Faizal, Panatarani and Joni2021). 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. Reference Yoon, Major, Dewers and Eichhubl2017; Hanein et al. Reference Hanein, Simoni, Woo, Provis and Kinoshita2021; Huang et al. Reference Huang, Zhang, Santos, Rodríguez, Holden, Vetro and Czimczik2021). 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. Reference Seiler, Grootes, Svarva and Nadeau2023). The latter may explain why the 14C results of CaCO3 precipitates in this study corroborate so well with those in Seiler et al. (Reference Seiler, Grootes, Svarva and Nadeau2023). 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. Reference Seiler, Grootes, Svarva and Nadeau2023) 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 Reference Nydal and Lövseth1983, Reference Nydal and Lövseth1996). After 30+ years of storage, those same CaCO3 precipitates are yielding inaccurate 14C values. Neither Seiler et al. (Reference Seiler, Grootes, Svarva and Nadeau2023) 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. Reference Seiler, Grootes, Svarva and Nadeau2023). 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.
SUPPLEMENTARY MATERIAL
To view supplementary material for this article, please visit https://doi.org/10.1017/RDC.2023.106
ACKNOWLEDGMENTS
We thank Jovany Merham, Evan Patrick Garcia, and Dr. Dima Fishman from the Laser Spectroscopy Labs, University of California, Irvine, for assistance on FTIR acquisition. We also wish to express our gratitude to the editors Tim Jull and Quan Hua, and 2 anonymous reviewers for their helpful comments and suggestions.
CONFLICTS OF INTEREST
The authors declare no conflict of interest.
Open access
