skip to main content

Title: Influence of Different Acid Treatments on the Radiocarbon Content Spectrum of Sedimentary Organic Matter Determined by RPO/Accelerator Mass Spectrometry
ABSTRACT In practice, obtaining radiocarbon ( 14 C) composition of organic matter (OM) in sediments requires first removing inorganic carbon (IC) by acid-treatment. Two common treatments are acid rinsing and fumigation. Resulting 14 C content obtained by different methods can differ, but underlying causes of these differences remain elusive. To assess the influence of different acid-treatments on 14 C content of sedimentary OM, we examine the variability in 14 C content for a range of marine and river sediments. By comparing results for unacidified and acidified sediments [HCl rinsing (Rinse HCl ) and HCl fumigation (Fume HCl )], we demonstrate that the two acid-treatments can affect 14 C content differentially. Our findings suggest that, for low-carbonate samples, Rinse HCl affects the Fm values due to loss of young labile organic carbon (OC). Fume HCl makes the Fm values for labile OC decrease, leaving the residual OC older. High-carbonate samples can lose relatively old organic components during Rinse HCl , causing the Fm values of remaining OC to increase. Fume HCl can remove thermally labile, usually young, OC and reduce the Fm values. We suggest three factors should be taken into account when using acid to remove carbonate from sediments: IC more » abundance, proportions of labile and refractory OC, and environmental matrix. « less
; ; ; ;
Award ID(s):
Publication Date:
Journal Name:
Page Range or eLocation-ID:
395 to 413
Sponsoring Org:
National Science Foundation
More Like this
  1. ABSTRACT Organic carbon (OC) radiocarbon ( 14 C) signatures in marine surface sediments are highly variable and the causes of this heterogeneity remain ambiguous. Here, we present results from a detailed 14 C-based investigation of an Arabian Sea sediment, including measurements on organic matter (OM) in bulk sediment, specific grain size fractions, and OC decomposition products from ramped-pyrolysis-oxidation (RPO). Our results show that 14 C ages of OM increase with increasing grain size, suggesting that grain size is an important factor controlling the 14 C heterogeneity in marine sediments. Analysis of RPO decomposition products from different grain size fractions revealsmore »an overall increase in age of corresponding thermal fractions from finer to coarser fractions. We suggest that hydrodynamic properties of sediment grains exert the important control on the 14 C age distribution of OM among grain size fractions. We propose a conceptual model to account for this dimensionality in 14 C variability that invokes two predominant modes of OM preservation within different grain size fractions of Arabian Sea sediment: finer (<63 µm) fractions are influenced by OM-mineral grain aggregation processes, giving rise to relatively uniform 14 C ages, whereas OM preserved in coarser (>63 µm) fractions includes materials encapsulated within microfossils and/or entrained fossil ( 14 C-depleted) OC hosted in detrital mineral grains. Our findings highlight the value of RPO for assessment of 14 C age variability in sedimentary OC, and for assessing mechanisms of OM preservation in aquatic sediments.« less
  2. Abstract
    Excessive phosphorus (P) applications to croplands can contribute to eutrophication of surface waters through surface runoff and subsurface (leaching) losses. We analyzed leaching losses of total dissolved P (TDP) from no-till corn, hybrid poplar (Populus nigra X P. maximowiczii), switchgrass (Panicum virgatum), miscanthus (Miscanthus giganteus), native grasses, and restored prairie, all planted in 2008 on former cropland in Michigan, USA. All crops except corn (13 kg P ha−1 year−1) were grown without P fertilization. Biomass was harvested at the end of each growing season except for poplar. Soil water at 1.2 m depth was sampled weekly to biweekly for TDP determination during March–November 2009–2016More>>
  3. Abstract Variation in soil organic C (%OC) concentration has been associated with the concentration of reactive Fe- and Al-oxyhydroxide phases and exchangeable Ca, with the relative importance of these two stabilizing components shifting as soil pH moves from acid to alkaline. However, it is currently unknown if this pattern is similar or different with regard to measures of soil C persistence. We sampled soils from 3 horizons (uppermost A, uppermost B, C or lowest B horizons) across a pH gradient of 11 grass-dominated and 13 deciduous/mixed forest-dominated NEON sites to examine similarities and differences in the drivers of C concentrationmore »and persistence. Variation in C concentrations in all soils could be linked to abundances of Fe, Al and Ca, but were not significantly linked to variation in soil C persistence. Though pH was related to variation in Δ 14 OC, higher persistence was associated with more alkaline pH values. In forested soils, depth explained 75% of the variation in Δ 14 OC ( p  < 0.0001), with no significant additional correlations with extractable metal phases. In grasslands, soil organic C persistence was not associated with exchangeable Ca concentrations, but instead was explained by depth and inorganic C concentrations (R 2  = 0.76, p  < 0.0001), implying stabilization of organic C through association with carbonate precipitation. In grasslands, measures of substrate quality suggested greater persistence is also associated with a more advanced degree of decomposition. Results suggest that explanatory variables associated with C concentrations differ from those associated with persistence, and that reactive Fe- and Al-oxyhydroxide phases may not be present in high enough concentrations in most soils to offer any significant protective capacity. These results have significant implications for our understanding of how to model the soil C cycle and may suggest previously unrecognized stabilization mechanisms associated with carbonates and forms of extractable Si.« less
  4. Radiocarbon ages and thermal stability measurements can be used to estimate the stability of soil organic carbon (OC). Soil OC is a complex reservoir that contains a range of compounds with different sources, reactivities, and residence times. This heterogeneity can shift bulk radiocarbon values and impact assessment of OC stability and turnover in soils. Four soil horizons (Oa, Bhs, Bs, Bg) were sampled from highly weathered 350 ka Pololu basaltic volcanics on the Island of Hawaii and analyzed by Ramped PyrOX (RPO) in both the pyrolysis (PY) and oxidation (OX) modes to separate a complex mixture of OC into thermallymore »defined fractions. Fractions were characterized for carbon stable isotope and radiocarbon composition. PY and OX modes yielded similar results. Bulk radiocarbon measurements were modern in the Oa horizon (Fm = 1.013) and got progressively older with depth: the Bg horizon had an Fm value of 0.73. Activation energy distributions (p(E)) calculated using the ‘rampedpyrox’ model yielded consistent mean E values of 140 kJ mol-1 below the Oa horizon. The ‘rampedpyrox’ model outputs showed a mostly bimodal distribution in the p(E) below the Oa, with a primary peak at 135 kJ mol-1 and a secondary peak at 148 kJ mol-1, while the Oa was dominated by a single, higher E peak at 157 kJ mol-1. We suggest that mineral-carbon interaction, either through mineral surface-OC or metal-OC interactions, is the stabilization mechanism contributing to the observed mean E of 140 kJ mol-1 below the Oa horizon. In the Oa horizon, within individual RPO analyses, radiocarbon ages in the individual thermal fractions were indistinguishable (p[0.1). The flat age distributions indicate there is no relationship between age and thermal stability (E) in the upper horizon ([25 cm). Deeper in the soil profile higher lEf values were associated with older radiocarbon ages, with slopes progressively steepening with depth. In the deepest (Bg) horizon, there was the largest, yet modest change in Fm of 0.06 (626 radiocarbon years), indicating that older OC is slightly more thermally stable.« less
  5. Abstract Speleothem organic matter can be a powerful tracer for past environmental conditions and karst processes. Carbon isotope measurements (δ 13 C and 14 C) in particular can provide crucial information on the provenance and age of speleothem organic matter, but are challenging due to low concentrations of organic matter in stalagmites. Here, we present a method development study on extraction and isotopic characterization of speleothem organic matter using a rapid procedure with low laboratory contamination risk. An extensive blank assessment allowed us to quantify possible sources of contamination through the entire method. Although blank contamination is consistently low (1.7more »± 0.34 – 4.3 ± 0.86 μg C for the entire procedure), incomplete sample decarbonation poses a still unresolved problem of the method, but can be detected when considering both δ 13 C and 14 C values. We test the method on five stalagmites, showing reproducible results on samples as small as 7 μg C for δ 13 C and 20 μg C for 14 C. Furthermore, we find consistently lower non-purgeable organic carbon (NPOC) 14 C values compared to the carbonate 14 C over the bomb spike interval in two stalagmites from Yok Balum Cave, Belize, suggesting overprint of a pre-aged or even fossil source of carbon on the organic fraction incorporated by these stalagmites.« less