Kinshasa, Democratic Republic of the Congo
Kinshasa, Democratic Republic of the Congo

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Stubbins A.,Old Dominion University | Spencer R.G.M.,University of California at Davis | Chen H.,Old Dominion University | Hatcher P.G.,Old Dominion University | And 6 more authors.
Limnology and Oceanography | Year: 2010

Congo River water was filtered and then irradiated for 57 d in a solar simulator, resulting in extensive photodegradation of dissolved organic matter (DOM). Whole-water (i.e., unfractionated) DOM was analyzed pre- and post-irradiation using ultrahigh resolution Fourier transform ion cyclotron mass spectrometry (FT-ICR MS), revealing the following three pools of DOM classified based upon their photoreactivity: (1) photo-resistant, (2) photo-labile, and (3) photo-produced. Photo-resistant DOM was heterogeneous, with most molecular classes represented, although only a small number of aromatics and no condensed aromatics were identified. The photo-produced pool was dominated by aliphatic compounds, although it included a small number of aromatics, including condensed aromatics. Aromatic compounds were the most photoreactive, with > 90% being lost upon irradiation. Photochemistry also resulted in a significant drop in the number of molecules identified and a decrease in their structural diversity. The FT-ICR MS signatures of two classes of refractory organic matter, black carbon and carboxylic-rich alicyclic molecules (CRAM), were present in the sample prior to irradiation, indicating that the Congo River could be a significant exporter of recalcitrant DOM to the ocean. All black carbon-like molecules identified in the initial sample were lost during irradiation. Molecular signatures consistent with CRAM were also highly photo-labile, demonstrating that environmental solar irradiation levels are capable of removing these refractory compounds from aquatic systems. Irradiation also shifted the molecular signature of terrestrial DOM toward that of marine DOM, thereby complicating the task of tracking terrestrial DOM in the ocean. © 2010, by the American Society of Limnology and Oceanography, Inc.

Spencer R.G.M.,Woods Hole Oceanographic Institution | Spencer R.G.M.,University of California at Davis | Hernes P.J.,University of California at Davis | Aufdenkampe A.K.,Stroud Water Research Center | And 10 more authors.
Geochimica et Cosmochimica Acta | Year: 2012

The Congo River, which drains pristine tropical forest and savannah and is the second largest exporter of terrestrial carbon to the ocean, was sampled in early 2008 to investigate organic matter (OM) dynamics in this historically understudied river basin. We examined the elemental (%OC, %N, C:N), isotopic (δ 13C, Δ 14C, δ 15N) and biochemical composition (lignin phenols) of coarse particulate (>63μm; CPOM) and fine particulate (0.7-63μm; FPOM) OM and DOC, δ 13C, Δ 14C and lignin phenol composition with respect to dissolved OM (<0.7μm; DOM) from five sites in the Congo River Basin. At all sample locations the organic carbon load was dominated by the dissolved phase (~82-89% of total organic carbon) and the total suspended sediment load was principally fine particulate material (~81-91% fine suspended sediment). Distinct compositional and isotopic differences were observed between all fractions. Congo CPOM, FPOM and DOM all originated from vegetation and soil inputs as evidenced by elemental, isotopic and lignin phenol data, however FPOM was derived from much older carbon pools (mean Δ 14C=-62.2±-13.2‰, n=5) compared to CPOM and DOM (mean Δ 14C=55.7±30.6‰, n=4 and 73.4±16.1‰, n=5 respectively). The modern radiocarbon ages for DOM belie a degraded lignin compositional signature (i.e. elevated acid:aldehyde ratios (Ad:Al) relative to CPOM and FPOM), and indicate that the application of OM degradation patterns derived from particulate phase studies to dissolved samples needs to be reassessed: these elevated ratios are likely attributable to fractionation processes during solubilization of plant material. The relatively low DOM carbon-normalized lignin yields (Λ 8; 0.67-1.12 (mg(100mg OC) -1)) could also reflect fractionation processes, however, they have also been interpreted as an indication of significant microbial or algal sources of DOM. CPOM appears to be well preserved higher vascular plant material as evidenced by its modern radiocarbon age, elevated C:N (17.2-27.1) and Λ 8 values (4.56-7.59 (mg(100mg OC) -1)). In relation to CPOM, the aged FPOM fraction (320-580ybp 14C ages) was comparatively degraded, as demonstrated by its nitrogen enrichment (C:N 11.4-14.3), lower Λ 8 (2.80-4.31 (mg(100mg OC) -1)) and elevated lignin Ad:Al values similar to soil derived OM. In this study we observed little modification of the OM signature from sample sites near the cities of Brazzaville and Kinshasa to the head of the estuary (~350km) highlighting the potential for future studies to assess seasonal and long-term OM dynamics from this logistically feasible location and derive relevant information with respect to OM exported to the Atlantic Ocean. The relative lack of OM data for the Congo River Basin highlights the importance of studies such as this for establishing baselines upon which to gauge future change. © 2012 Elsevier Ltd.

Spencer R.G.M.,Florida State University | Hernes P.J.,University of California at Davis | Dinga B.,Groupe de Recerche en science Exactes et Naturelles GRSEN DGRST | Wabakanghanzi J.N.,Congo Atomic Energy Commission | And 2 more authors.
Global Biogeochemical Cycles | Year: 2016

The Congo River in central Africa represents a major source of organic matter (OM) to the Atlantic Ocean. This study examined elemental (%OC, %N, and C:N), stable isotopic (δ13C and δ15N), and biomarker composition (lignin phenols) of particulate OM (POM) and dissolved OM (DOM) across the seasonal hydrograph. Even though the Congo exhibits an extremely stable intra-annual discharge regime, seasonal variability in OM composition was evident. DOM appears predominantly derived from vascular plant inputs with greater relative contribution during the rising limb and peak in discharge associated with the major November–December discharge maximum. Generally, POM appears to be sourced from soil-derived mineral-associated OM (low C:N, low Λ8, and higher (Ad:Al)v) but the relative proportion of fresh vascular plant material (higher C:N, higher Λ8, and lower (Ad:Al)v) increases with higher discharge. During the study period (September 2009 to November 2010) the Congo exported 29.21 Tg yr−1 of total suspended sediment (TSS), 1.96 Tg yr−1 of particulate organic carbon (POC), and 12.48 Tg yr−1 of dissolved organic carbon. The Congo exports an order of magnitude lower TSS load in comparison to other major riverine sources of TSS (e.g., Ganges and Brahmaputra), but due to its OM-rich character it actually exports a comparable amount of POC. The Congo is also 2.5 times more efficient at exporting dissolved lignin per unit volume compared to the Amazon. Including Congo dissolved lignin data in residence time calculations for lignin in the Atlantic Ocean results in an approximately 10% reduction from the existing estimate, suggesting that this material is more reactive than previously thought. ©2016. American Geophysical Union. All Rights Reserved.

Spencer-Jones C.L.,Northumbria University | Wagner T.,Northumbria University | Dinga B.J.,Groupe de Recherche en science Exactes et Naturelles GRSEN DGRST | Schefuss E.,University of Bremen | And 5 more authors.
Organic Geochemistry | Year: 2015

The 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-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol), indicating aerobic methanotrophy. A narrow range and low mean relative abundance of 30-(5'-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. © 2015 Published by Elsevier Ltd.

Talbot H.M.,Northumbria University | Handley L.,Northumbria University | Spencer-Jones C.L.,Northumbria University | Bienvenu D.J.,Groupe de Recherche en science Exactes et Naturelles GRSEN DGRST | And 6 more authors.
Geochimica et Cosmochimica Acta | Year: 2014

Methane (CH4) 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 1.2Ma. 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 -30‰ to -40‰ 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. © 2014 Elsevier Ltd.

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