Geogenic Organic Carbon - Traceable in deep subsoils?

As the largest active terrestrial reservoir of organic carbon (OC), soils play an essential role inthe global carbon cycle. Their response to rising global temperatures is of uttermost interest,having the potential to move and store an extensive amount of carbon within the climatesystem. One of the critical aspects of soil OC is the turnover rate, which has been reported todecrease rapidly below the A horizon. While most studies have focused on processes thatpromote higher ages of organic matter including various stabilization mechanisms, recentobservations suggest that part of the soil OC might already have been deposited with thesediment parent material, i.e. geogenic carbon and thus has turnover rates of millennia orlonger. This could lead to underestimations of turnover rates and needs to be taken intoaccount in biogeochemical models. This study aims to characterize geogenic OC and quantifyits contribution to subsoils. Deep soil cores (10 m) were collected from three different parent materials incentral Germany: Pleistocene loess, Paleogene sand and red sandstone. These wereanalyzed using a set of geochemical methods, including bulk 14C, selected lipidbiomarkers, pyrolysis-GCMS and 13C-CPMAS spectroscopy in order to characterizeorganic matter at multiple depth intervals and identify contributions of geogenicorigin. In the Pleistocene loess, apparent bulk 14C ages increased with depth from 2,200 to30,700 years BP, reflecting the depositional history of this relatively young sediment. Thelipid analysis revealed multiple sections with high OC content and Carbon Preference Index(CPI) values (>13), suggesting the presence of paleosols. The oldest part yielded lower CPIvalues (<3.0), evidently phases where deposition was too rapid for pedologicalprocesses to take place. This was supported by 13C-CPMAS NMR spectroscopy,demonstrating a highly degraded (alkyl-C rich) but soil like chemical composition of theOM. In these sections the organic matter is scarce and characterized by aliphaticcompounds such as n-alkanes, possibly deposited along with the mineral material,and methyl ketones as well as monoaromatic compounds including benzene andtoluene. Apparent bulk 14C ages of the Paleogene sand varied between 6,750 and 12,000 years,significantly younger than the deposition of the sediment (23-34 Mio. years BP). The sampleswere absent of n-alkanes and only small traces of fatty acids (2.67-0.59 μg/g dry weight)were detected, dominated by even long chain homologues. Young ages were also displayedby the red sandstone (about 245 Mio. years old), ranging between 12,760 and 17,400 yearsBP. In summary, the hitherto investigations of the deep sediment cores of differentparent material show a significant variance in age and composition of the OC fordifferent parent material at various depths, owing to OC components deposited alongwith the sediment parent material mixed with more recently deposited biogeniccarbon. [ABSTRACT FROM AUTHOR]