The age of organic material discharged by rivers provides information about its 18 sources and carbon cycling processes within watersheds. While elevated ages in 19 fluvially-transported organic matter are usually explained by erosion of soils and 20 2 sediments deposits 1,2 , it is commonly assumed that mainly young organic material is 21 discharged from flat tropical watersheds due to their extensive plant cover and rapid 22 carbon turnover 3-7 . Here we present compound-specific radiocarbon data of terrigenous 23 organic fractions from a sedimentary archive offshore the Congo River in conjunction 24 with molecular markers for methane-producing land cover reflecting wetland extent. 25We find that the Congo River has been discharging aged organic matter for several 26 thousand years with apparently increasing ages from the Mid-to the Late Holocene. 27This suggests that aged organic matter in modern samples is concealed by radiocarbon 28 from atmospheric nuclear weapons testing. By comparison to indicators for past rainfall 29 changes we detect a systematic control of organic matter sequestration and release by 30 continental hydrology mediating temporary carbon storage in wetlands. As aridification 31 also leads to exposure and rapid remineralization of large amounts of previously stored 32 labile organic matter we infer that this process may cause a profound direct climate 33 feedback currently underestimated in carbon cycle assessments. shelf, extending from the Congo estuary and leading to rapid transport of discharged material 57 to the deep-sea fan. The ages of terrestrial organic material are therefore considered to reflect 58 retention processes within the river basin. We measured the 14 C content of various OM 59 fractions (total organic matter, microscopic wood fragments, leaf-wax n-alkanes, and 60 individual leaf-wax n-alcohols) from several depth intervals of core GeoB6518-1 (Table S2). 61Using the depositional ages based on radiocarbon dating of planktonic foraminifera (Table 62 S1), radiocarbon contents of OM fractions were decay-corrected to derive initial radiocarbon 63 contents. The deviation from the past atmospheric 14 C content (ΔΔ 14 C initial , see Methods) at the 64 time of deposition can be converted to apparent initial radiocarbon ages. The initial ages of 65 the OM fractions range from isochronous up to 3,000 14 C years (Fig. 2a). Notably, initial ages 66 of wood fragments are contemporaneous to or only slightly older than depositional ages in the 67Early to Mid-Holocene. In contrast, the plant-wax fractions are older and show higher but 68 4 coherent age variability (Fig. 3). This is in accordance with both a rapid transfer of plant 69 debris through the river system under high discharge conditions and the refractory nature and 70 persistence of plant waxes in soils 17 . The aged terrestrial organic contributions strongly 71 influence the age of total organic carbon (TOC) deposited off the Congo River despite its 72 mixed terrestrial and aquatic origin (Fig. 2a). From about 5,000 year...
Methane (CH 4) is a strong greenhouse gas known to have perturbed global climate in the past, especially when released in large quantities over short time periods from continental or marine sources. It is therefore crucial to understand and, if possible, quantify the individual and combined response of these variable methane sources to natural climate variability. However, past changes in the stability of greenhouse gas reservoirs remain uncertain and poorly constrained by geological evidence. Here, we present a record from the Congo fan of a highly specific bacteriohopanepolyol (BHP) biomarker for aerobic methane oxidation (AMO), 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol), that identifies discrete periods of increased AMO as far back as 1.2 Ma. Fluctuations in the concentration of aminopentol, and other 35-aminoBHPs, follow a pattern that correlates with late Quaternary glacial-interglacial climate cycles, with highest concentrations during warm periods. We discuss possible sources of aminopentol, and the methane consumed by the precursor methanotrophs, within the context of the Congo River setting, including supply of methane oxidation markers from terrestrial watersheds and/or marine sources (gas hydrate and/or deep subsurface gas reservoir). Compound-specific carbon isotope values of À30& to À40& for BHPs in ODP 1075 and strong similarities between the BHP signature of the core and surface sediments from the Congo estuary and floodplain wetlands from the interior of the Congo River Basin, support a methanotrophic and likely terrigenous origin of the 35-aminoBHPs found in the fan sediments. This new evidence supports a causal connection between marine sediment BHP records of tropical deep sea fans and wetland settings in the feeding river catchments, and thus tropical continental hydrology. Further research is needed to better constrain the different sources and pathways of methane emission. However, this study identifies the large potential of aminoBHPs, in particular aminopentol, to trace and, once better calibrated and understood, quantify past methane sources and fluxes from terrestrial and potentially also marine sources.
a b s t r a c tThe Congo River basin drains the second largest area of tropical rainforest in the world, including a large proportion of pristine wetlands. We present the bacteriohopanepolyol (BHP) inventory of a suite of tropical soils and, from comparison with published data, propose some initial ideas on BHP distribution controls. Strong taxonomic controls on BHP production are evident in wetland sediments. Dominant within the suite were 35-aminobacteriohopane-31,32,33,34-tetrol (aminotetrol) and 35-aminobacterio hopane-30,31,32,33,34-pentol (aminopentol), indicating aerobic methanotrophy. A narrow range and low mean relative abundance of 30-(5 0 -adenosyl)hopane (adenosylhopane) and related compounds, collectively termed ''soil marker" BHPs, were observed in Congo soils (mean 17%, range 7.9-36% of total BHPs, n = 22) compared with literature data from temperate surface soils and Arctic surface soils (mean 36%, range 0-66% of total BHPs, n = 28) suggesting a greater rate of conversion of these BHP precursors to other structures.
UPLC/MS/MS analysis using an ACE Excel C18 column produced superior separation for amine-containing BHPs and reduced run times from 60 to 9 min compared with previous methods. Unexpected variations in fragmentation pathways between structural subgroups must be taken into account when optimising MRM transitions for future quantitative studies. Copyright © 2016 John Wiley & Sons, Ltd.
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