2018/05/30 by Rui Bao, Ann P McNichol, Ann P. McNichol +5 · 1 citation
Environmental Science · Earth and Planetary Sciences · #Isotope Analysis in Ecology #Geology and Paleoclimatology Research #Marine and coastal ecosystems
paper · pdf · doi:10.1017/rdc.2018.22
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 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 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.