Compound-Specific Radiocarbon Analysis by Elemental Analyzer-Accelerator Mass Spectrometry: Precision and Limitations

A methodological review of compound-specific radiocarbon analysis for polycyclic aromatic hydrocarbons in environmental matrices

Identifying the sources of polycyclic aromatic hydrocarbons (PAHs) in complex environmental matrices is essential for understanding the impact of combustion-related human activities on the environment. Since the turn of the century, advances in analytical capability and accuracy of accelerator mass spectrometry (AMS) have made it possible to accurately determine the source apportionment of PAHs based on their radiocarbon (14C) mass conservation. This also allows us to trace the environmental transport processes of PAHs from the perspective of molecular 14C. However, natural environmental matrices have very low concentrations of PAHs (ppb to ppm level). To meet the requirements of carbon weight for 14C measurement by AMS, trace PAHs in complex environmental matrices must be enriched thousands of times, and then higher purity individual PAH molecules should be obtained through a series of complex purification procedures. Therefore, the technical difficulty is the main challenge in expanding the application of compound-specific 14C analysis in environmental science. This article reviews the detailed pretreatment procedures for 14C measurement of specific PAHs, including sample enrichment, extraction and purification of aromatic components, preparation of compound-specific PAHs by preparative capillary gas chromatography, graphitization of samples with ultra-small carbon content, and relevant quality control and assurance procedures. This study aims to help environmental geoscientists understand the technical process of 14C analysis of PAHs and inspire new scientific questions related to environmental science. To our knowledge, this is the first comprehensive review of the technical method of compound-specific 14C analysis for PAHs.

Multi-molecular 14C evidence for mineral control on terrestrial carbon storage and export

Compound- and compound class-specific radiocarbon analysis of source-diagnostic 'biomarker' molecules has emerged as a powerful tool to gain insights into terrestrial carbon cycling. While most studies thus far have focused on higher plant biomarkers (i.e. plant leaf-wax n-alkanoic acids and n-alkanes, lignin-derived phenols), tracing paedogenic carbon is crucial given the pivotal role of soils in modulating ecosystem carbon turnover and organic carbon (OC) export. Here, we determine the radiocarbon (14C) ages of glycerol dialkyl glycerol tetraethers (GDGTs) in riverine sediments and compare them to those of higher plant biomarkers as well as markers of pyrogenic (fire-derived) carbon (benzene polycarboxylic acids, BPCAs) to assess their potential as tracers of soil turnover and export. GDGT Δ14C follows similar relationships with basin properties as vegetation-derived lignin phenols and leaf-wax n-alkanoic acids, suggesting that the radiocarbon ages of these compounds are significantly impacted by intermittent soil storage. Systematic radiocarbon age offsets are observable between the studied biomarkers, which are likely caused by different mobilization pathways and/or stabilization by mineral association. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.

Molecular 14 C evidence for contrasting turnover and temperature sensitivity of soil organic matter components

Climate projection requires an accurate understanding for soil organic carbon (SOC) decomposition and its response to warming. An emergent view considers that environmental constraints rather than chemical structure alone control SOC turnover and its temperature sensitivity (i.e., Q₁₀ ), but direct long-term evidence is lacking. Here, using compound-specific radiocarbon analysis of soil profiles along a 3300-km grassland transect, we provide direct evidence for the rapid turnover of lignin-derived phenols compared with slower-cycling molecular components of SOC (i.e., long-chain lipids and black carbon). Furthermore, in contrast to the slow-cycling components whose turnover is strongly modulated by mineral association and exhibits low Q₁₀ , lignin turnover is mainly regulated by temperature and has a high Q₁₀ . Such contrasts resemble those between fast-cycling (i.e., light) and mineral-associated slow-cycling fractions from globally distributed soils. Collectively, our results suggest that warming may greatly accelerate the decomposition of lignin, especially in soils with relatively weak mineral associations.

Development of graphitization method for low carbon aerosol filter samples with Automated Graphitization System AGE-3

The wide applications of the radiocarbon (¹⁴C) approach in environmental, archeological, and geological research often necessitates the analysis of microgram-sized samples. The ability to measure low carbon samples is particularly relevant for aerosol particle filters, especially for samples from pristine environments. For this purpose, we investigated the sample dilution method for graphitization of low-carbon samples (20-200 μg C) with an Automated Graphitization System (AGE-3), and applied a mass balance equation for the calculation of ¹⁴C values. Materials with known ¹⁴C values (standards NIST-OXII and IAEA-C7) were diluted with ¹⁴C-free phthalic anhydride (PhA) until sufficient mass (500 μg C) for graphitization with the AGE-3 system was acquired. Reliable ¹⁴C values were obtained for samples with carbon amount in the range of 40-200 μg. Next, we adapted the dilution method for estimation of aerosol sample ¹⁴C values. Using it, we attained a precision of 0.71 ± 0.83 pMC for ¹⁴C measurements of aerosol samples containing 40-200 μg C. A shift of radiocarbon values to 5.07 pMC (average 3.08 ± 1.7 pMC) was observed for samples with low carbon content (<20 μg C). We determined that a precision of 2-3 pMC is acceptable for aerosol particle source apportionment studies. Using the sample dilution method, graphitization with AGE-3 of aerosol samples with carbon content >40 μg becomes a viable and efficient way of sample preparation for ¹⁴C analysis.

Vegetal Undercurrents-Obscured Riverine Dynamics of Plant Debris

Much attention has been focused on fine-grained sediments carried as suspended load in rivers due to their potential to transport, disperse, and preserve organic carbon (OC), while the transfer and fate of OC associated with coarser-grained sediments in fluvial systems have been less extensively studied. Here, sedimentological, geochemical, and biomolecular characteristics of sediments from river depth profiles reveal distinct hydrodynamic behavior for different pools of OC within the Mackenzie River system. Higher radiocarbon (¹⁴C) contents, low N/OC ratios, and elevated plant-derived biomarker loadings suggest a systematic transport of submerged vascular plant debris above the active riverbed in large channels both upstream of and within the delta. Subzero temperatures hinder OC degradation promoting the accumulation and waterlogging of plant detritus within the watershed. Once entrained into a channel, sustained flow strength and buoyancy prevent plant debris from settling and keep it suspended in the water column above the riverbed. Helical flow motions within meandering river segments concentrate lithogenic and organic debris near the inner river bends forming a sediment-laden plume. Moving offshore, we observe a lack of discrete, particulate OC in continental shelf sediments, suggesting preferential trapping of coarse debris within deltaic and neritic environments. The delivery of waterlogged plant detritus transport and high sediment loads during the spring flood may reduce oxygen exposure times and microbial decomposition, leading to enhanced sequestration of biospheric OC. Undercurrents enriched in coarse, relatively fresh plant fragments appear to be reoccurring features, highlighting a poorly understood yet significant mechanism operating within the terrestrial carbon cycle.

Degradation and Aging of Terrestrial Organic Carbon within Estuaries: Biogeochemical and Environmental Implications

Estuaries are action zones for organic carbon (OC) degradation and aging. These processes influence the nature of terrestrial OC (OC_terr) export and the magnitude of OC_terr burial in marginal seas, with important environmental implications such as CO₂ release and hypoxia. In this study, we determined the contents and carbon isotopic compositions (¹³C and ¹⁴C) of bulk OC and fatty acids (FAs) as well as the sedimentological characteristics of suspended particulate matter (SPM) samples collected from two sites over four seasons and of surface sediment samples from three sites in the Pearl River estuary (PRE) to evaluate processes controlling OC_terr degradation and aging along an estuarine gradient. We found that the abundance-weighted average C_24-32FA ¹⁴C ages increased by an average of ∼1170 years for SPM and by an average of ∼3440 years in PR/PRE sediments, along the ∼60 km PRE transect. These increases in the FA age coincided with an 86% decrease in the corresponding mineral surface area-normalized FA loading along the sediment transport pathway, implying that selective degradation of labile and younger OC resulted in apparent OC aging. These measurements reveal an important shift in the nature of OC, with implications for biogeochemical cycling within estuaries and for regional environmental changes.

Climate control on terrestrial biospheric carbon turnover

Terrestrial organic-carbon reservoirs (vegetation, soils) currently consume more than a third of anthropogenic carbon emitted to the atmosphere, but the response of this “terrestrial sink” to future climate change is widely debated. Rivers export organic carbon sourced over their watersheds, offering an opportunity to assess controls on land carbon cycling on broad spatial scales. Using radiocarbon ages of biomolecular tracer compounds exported by rivers, we show that temperature and precipitation exert primary controls on biospheric-carbon turnover within river basins. These findings reveal large-scale climate control on soil carbon stocks, and they provide a framework to quantify responses of terrestrial organic-carbon reservoirs to past and future change.

Terrestrial vegetation and soils hold three times more carbon than the atmosphere. Much debate concerns how anthropogenic activity will perturb these surface reservoirs, potentially exacerbating ongoing changes to the climate system. Uncertainties specifically persist in extrapolating point-source observations to ecosystem-scale budgets and fluxes, which require consideration of vertical and lateral processes on multiple temporal and spatial scales. To explore controls on organic carbon (OC) turnover at the river basin scale, we present radiocarbon (¹⁴C) ages on two groups of molecular tracers of plant-derived carbon—leaf-wax lipids and lignin phenols—from a globally distributed suite of rivers. We find significant negative relationships between the ¹⁴C age of these biomarkers and mean annual temperature and precipitation. Moreover, riverine biospheric-carbon ages scale proportionally with basin-wide soil carbon turnover times and soil ¹⁴C ages, implicating OC cycling within soils as a primary control on exported biomarker ages and revealing a broad distribution of soil OC reactivities. The ubiquitous occurrence of a long-lived soil OC pool suggests soil OC is globally vulnerable to perturbations by future temperature and precipitation increase. Scaling of riverine biospheric-carbon ages with soil OC turnover shows the former can constrain the sensitivity of carbon dynamics to environmental controls on broad spatial scales. Extracting this information from fluvially dominated sedimentary sequences may inform past variations in soil OC turnover in response to anthropogenic and/or climate perturbations. In turn, monitoring riverine OC composition may help detect future climate-change–induced perturbations of soil OC turnover and stocks.

Radiocarbon dating of lead white: novel application in the study of polychrome sculpture

Recently, radiocarbon dating underwent considerable technological advances allowing unprecedented sample size downscaling. These achievements introduced novel opportunities in dating cultural heritage objects. Within this pioneering research, the possibility of a direct ¹⁴C dating of lead white pigment and organic binder in paint samples was investigated on polychrome sculptures, a foremost artistic expression in human history. The polychromy, an indivisible part of polychrome sculpture, holds a key role in the interpretation and understanding of these artworks. Unlike in other painted artworks, the study of polychromies is repeatedly hampered by repaints and degradation. The omnipresence of lead white within the original polychromy was thus pursued as dating proxy. Thermal decomposition allowed bypassing geologic carbonate interferences caused by the object's support material, while an added solvent extraction successfully removed conservation products. This radiocarbon dating survey of the polychromy from 16 Portuguese medieval limestone sculptures confirmed that some were produced within the proposed chronologies while others were revised. Within this multidisciplinary study, the potential of radiocarbon dating as a complementary source of information about these complex paint systems guiding their interpretation is demonstrated. The challenges of this innovative approach are highlighted and improvements on sampling and sample preparation are discussed.

An Abrupt Aging of Dissolved Organic Carbon in Large Arctic Rivers

Permafrost thaw in Arctic watersheds threatens to mobilize hitherto sequestered carbon. We examine the radiocarbon activity (F14C) of dissolved organic carbon (DOC) in the northern Mackenzie River basin. From 2003-2017, DOC-F14C signatures (1.00 ± 0.04; n = 39) tracked atmospheric 14CO2, indicating export of "modern" carbon. This trend was interrupted in June 2018 by the widespread release of aged DOC (0.85 ± 0.16, n = 28) measured across three separate catchment areas. Increased nitrate concentrations in June 2018 lead us to attribute this pulse of 14C-depleted DOC to mobilization of previously frozen soil organic matter. We propose export through lateral perennial thaw zones that occurred at the base of the active layer weakened by preceding warm summer and winter seasons. Although we are not yet able to ascertain the broader significance of this "anomalous" mobilization event, it highlights the potential for rapid and large-scale release of aged carbon from permafrost.

Pretreatment and gaseous radiocarbon dating of 40-100 mg archaeological bone

Radiocarbon dating archaeological bone typically requires 300–1000 mg material using standard protocols. We report the results of reducing sample size at both the pretreatment and ¹⁴C measurement stages for eight archaeological bones spanning the radiocarbon timescale at different levels of preservation. We adapted our standard collagen extraction protocol specifically for <100 mg bone material. Collagen was extracted at least twice (from 37–100 mg material) from each bone. Collagen aliquots containing <100 μg carbon were measured in replicate using the gas ion source of the AixMICADAS. The effect of sample size reduction in the EA-GIS-AMS system was explored by measuring ¹⁴C of collagen containing either ca. 30 μg carbon or ca. 90 μg carbon. The gas dates were compared to standard-sized graphite dates extracted from large amounts (500–700 mg) of bone material pretreated with our standard protocol. The results reported here demonstrate that we are able to reproduce accurate radiocarbon dates from <100 mg archaeological bone material back to 40,000 BP.