Dimensions of Radiocarbon Variability within Sedimentary Organic Matter

Radiocarbon ◽  
2018 ◽  
Vol 60 (3) ◽  
pp. 775-790 ◽  
Author(s):  
Rui Bao ◽  
Ann P McNichol ◽  
Cameron P McIntyre ◽  
Li Xu ◽  
Timothy I Eglinton

ABSTRACTOrganic 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.

2021 ◽  
Author(s):  
Julian Hülscher ◽  
Edward R. Sobel ◽  
Niklas Kallnik ◽  
J. Elis Hoffmann ◽  
Ian L. Millar ◽  
...  

&lt;p&gt;Sedimentary archives are the main source of information for climatic and tectonic changes in Earth&amp;#8217;s history. We investigate how the tectonically triggered early (28&amp;#177;1 Ma) exhumation of the Tauern Window in the Eastern European Alps is recorded in the Oligocene/Miocene Northern Alpine Foreland Basin (NAFB) in Upper Austria. We combined Nd isotopic compositions on clay-sized bulk sediment and of sand-sized single-grain apatites with additional trace-element geochemistry, fission track (AFT) and U-Pb dating to investigate the timing of when this tectonic signal reaches the sediment archive within these different grain-size fractions.&lt;/p&gt;&lt;p&gt;This well-investigated basin offers an excellent opportunity to investigate environmental signal propagation. From ~27 to 19 Ma, a deep-marine basin-axial, gravity-flow dominated channel controlled the West to East directed sediment transport in the Upper Austrian NAFB. The sediments were sourced in the Eastern and Central Alps. At 19 Ma, channel sedimentation ceased and clinoforms prograded from the southern margin northward into the basin. This change in sediment-routing direction cut off the Central Alpine sediment source.&lt;/p&gt;&lt;p&gt;Drill cuttings of one well on the northern basin slope and drill cores from 12 wells were sampled for clay and the single-grain analysis. Clay eNd values remain stable around -9.7 (&amp;#177;0.5) from 27 to 19 Ma but increase afterwards to -8.7 (&amp;#177;0.2) at 18.3 Ma. In contrast, apatite single-grain results significantly change at 23.3&amp;#177;0.3 Ma from a domination of apatites from low-grade (&lt;upper amphibolite-facies) metamorphic sources, Permo-Mesozoic and late Variscan U-Pb ages and AFT ages &gt;40 Ma to a domination of high-grade metamorphic apatites with late Variscan U-Pb ages and an increasing number of AFT ages &lt;30 Ma. The high-grade metamorphic apatites have slightly more positive eNd values (-2.2 &amp;#177; 3.9) than the low-grade metamorphic apatites (-4.4 &amp;#177; 4.2).&lt;/p&gt;&lt;p&gt;The changes in the single-grain data sets have been previously interpreted to mirror the exposure of a new Upper Austroalpine nappe as a consequence of the ongoing early Tauern Window exhumation. The total signal lag time between the beginning of the exhumation and the arrival of the signal within the apatite assemblage in the sedimentary archive is therefore 3.4 to 6 Myrs. The clay eNd values do not record this change in provenance at 23.3&amp;#177;0.3 Ma as they stayed stable until 19 Ma when they increased slightly. This might point towards a delayed recording of the provenance change revealed at 23.3&amp;#177;0.3 Ma by the apatites. The difference in signal recording is caused by the characteristics of the applied methods. Whereas single-grain distributions of orogen-wide sediment-routing systems can be dominated by geographically small areas with high erosion rates and high mineral fertility, bulk-rock methods integrate over the entire drainage area, a process that diminishes extreme values. The disconnection of the Central Alpine sediment source at 19 Ma, increased the relative proportion of the more positive eNd values of the Upper Austroalpine Nappe in the drainage area, leading to an increase in clay eNd values in our data set. Our results show that different information from the hinterland is recorded in the different grain-size fractions and methods.&lt;/p&gt;


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