Dimensions of Radiocarbon Variability within Sedimentary Organic Matter

Abstract

Organic carbon (OC) radiocarbon (¹⁴C) signatures in marine surface sediments are highly variable and the causes of this heterogeneity remain ambiguous. Here, we present results from a detailed ¹⁴C-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 ¹⁴C ages of OM increase with increasing grain size, suggesting that grain size is an important factor controlling the ¹⁴C heterogeneity in marine sediments. Analysis of RPO decomposition products from different grain size fractions reveals 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 ¹⁴C age distribution of OM among grain size fractions. We propose a conceptual model to account for this dimensionality in ¹⁴C 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 ¹⁴C ages, whereas OM preserved in coarser (>63 µm) fractions includes materials encapsulated within microfossils and/or entrained fossil (¹⁴C-depleted) OC hosted in detrital mineral grains. Our findings highlight the value of RPO for assessment of ¹⁴C age variability in sedimentary OC, and for assessing mechanisms of OM preservation in aquatic sediments.