Evolution of the Optical Properties of Seawater Influenced by the Deepwater Horizon Oil Spill in the Gulf of Mexico

The fluorescence excitation-emission matrix (EEM) technique coupled with parallel factor (PARAFAC) modeling and measurements of bulk organic carbon and other optical properties were used to characterize the oil components released from the Deepwater Horizon oil spill in the Gulf of Mexico and to examine the chemical evolution and transformation of oil in the water column. Seawater samples were collected from the Gulf of Mexico during October 2010 and October 2011, three months and fifteen months, respectively, after the oil spill was stopped. Together with previous results from samples collected during the oil spill in May/June 2010, these time series samples allow us to elucidate changes in the optical properties of dissolved organic matter (DOM) from the time of maximum oil impact to its recovery, 15 months after the spill. Although the oil had profoundly altered the optical properties of the DOM in the entire water column during the oil spill, naturally occurring DOM became predominant in surface waters by October 2010, three months after the spill. Anomalous DOM with high optical yields, however, still resided in deep waters even 15 months after the oil spill in October 2011, showing a persistent influence of the oil in deep waters. Based on fluorescence EEM data and PARAFAC modeling, three oil components and one natural humic-like DOM could be readily identified. The most prominent oil component had its maximum fluorescence intensity at Ex/Em 224/328 nm, and the other two centered on Ex/Em 264/324 and 232/346 nm, respectively. The humic-like DOM component had its wide emission peak from 390 to 460 nm over the excitation wavelength at similar to 248 nm. We hypothesized that component-2 (264/324 nm) was mostly derived from photochemical degradation and the component-3 (232/346 nm) could be a degradation product from both microbial and photochemical degradation, although both C2 and C3 are subject to degradation at different rates. The oil component ratios, such as C2/C1 and C3/C1, were closely related to degradation states of oil and can be used as a sensitive index to track the fate, transport and transformation of oil in the water column

Source and Transport of Terrigenous Organic Matter In the Upper Yukon River: Evidence From Isotope (C-13, ΔC-14, and N-15) Composition of Dissolved, Colloidal, and Particulate Phases

Natural organic matter was collected from the upper Yukon River and size fractionated into the (LMW-DOC), colloidal (COC, 1 kDa to 0.45 mu m) and particulate organic carbon (POC, \u3e 0.45 mu m) phases for characterization of elemental (C and N) and isotopic (C-13, C-14 and N-15) composition to examine their sources and transport. Concentrations of total organic carbon (TOC) decreased from 3010 mu M in mid-May to 608 mu M in September, accompanying an increase in river water delta O-18 from the snowmelt to summer and early fall. COC was the predominant OC species, comprising, on average, 63 +/- 8% of the TOC, with 23 +/- 5% partitioned in the LMW-DOC and 14 +/- 5% in the POC fraction. Annual riverine export flux to the ocean was 2.02 +/- 10(12) g-C for TOC, 7.66 x 10(10) g-N for total organic nitrogen (TON), and 3.53 x 10(12) g-C for dissolved inorganic carbon (DIC), respectively. The C/N molar ratios were distinctly different between colloidal organic matter (COM, 46 +/- 3) and particulate organic matter (POM, 15 +/- 1.4). Similar delta C-13 values were found for LMW-DOM (-27.9 +/- 0.5 parts per thousand), COM (-27.4 +/- 0.2 parts per thousand), and POM (-26.2 +/- 0.7 parts per thousand), although there was a general increase with increasing size, suggesting a common terrigenous organic source. In contrast, distinct D 14 C values were found for LMW-DOC (-155 to +91 parts per thousand), COC (40 to 140 parts per thousand), and POC (-467 to -253 parts per thousand) with a decreasing trend from snowmelt to ice-open season, suggesting that turnover pathways and transport mechanisms vary with organic matter size fractions. The high abundance of COC and its contemporary C-14 ages points to a predominant source from modern terrestrial primary production, likely from the leaching/decomposition of fresh plant litter in the upper soil horizon. The predominately old POC (average 3698 +/- 902 years B. P.), in contrast, was largely derived from riverbank erosion and melting of permafrost. These results imply that ice-opening Yukon River flows are dominated by snowmelt (low delta O-18) with high DOC (high Delta C-14) but low DIC and Si(OH)(4) concentrations, whereas late summer flows contain more products of permafrost or ice melt and rain (high delta O-18), with low DOC (low Delta C-14) but high DIC and Si(OH)(4) concentrations. A warming climate with a deeper permafrost active layer in the Yukon River watershed would enhance the mobilization and export of old terrestrial OC, but largely in the particulate form into the Bering Sea and Arctic Ocean

Particulate Organic Carbon Export Fluxes in The Canada Basin and Bering Sea as Derived from 234Th/238U Disequilibria

Seawater samples were collected in the water column of the Canada Basin and the Bering Sea from aboard the R/V Xue Long during August 1999. Activity concentrations of dissolved and particulate 234Th were measured using beta counting techniques to quantify the scavenging and residence time of 234Th and organic carbon export fluxes. Primary production (PP) and bacterial production were also determined in the study areas through in situ incubation experiments. Significant 234Th scavenging was observed in the upper 100 m of the water column in both study areas, with up to 40% of 234Th deficit found at Bering Sea stations and ~15% of 234Th deficit at the Canada Basin station. Measured PP decreased from ~12.5 µmol C/m³/h in surface water to near zero at ~100 m depth, with an integrated PP of 3.83 mmol C/m²/d in the Canada Basin. Bacterial production, on the other hand, was on the order of 2.0 mmol C/m²/d, which is up to 52% of the integrated PP. Particulate organic carbon (POC) export fluxes derived from 234Th/238U disequilibrium were ~1 mmol C/m²/d in the Canada Basin and ~10 mmol C/m²/d in the Bering Sea, with fluxes in the latter area being 5 to 10 times higher than those found in the Canada Basin. These export fluxes correspond to a ThE ratio (the ratio of 234Th-derived POC export to primary production) of 0.26 for the Canada Basin and 0.7 for the Bering Sea. The higher ThE ratios in the study areas suggest a decoupling of production and particulate export in the high-latitude ocean. Ratios of POC to particulate 234Th (µmol C/dpm) decreased consistently with increasing depth, suggesting that organic carbon is preferentially remineralized relative to 234Th. Interestingly, the profile of particulate 234Th in the Canada Basin showed a unique characteristic: particulate 234Th activities increased with increasing depth, suggesting a continuous scavenging of 234Th and a rapid settling rate of the particles.Au cours du mois d'août 1999, on a prélevé des échantillons d'eau de mer dans la colonne d'eau du bassin Canada et de la mer de Béring depuis le vaisseau RV Xue-Long. L'activité volumique du 234Th dissous et particulaire a été mesurée à l'aide de techniques de comptage au rayonnement bêta afin de quantifier le temps de balayage et le temps de séjour du 234Th et les flux d'exportation du carbone organique. On a aussi établi la production primaire (PP) et la production bactérienne dans les zones d'étude en réalisant des expériences d'incubation in situ. Dans les deux zones d'étude, on a observé un balayage marqué du 234Th dans les 100 m supérieurs de la colonne d'eau, avec jusqu'à 40 p. cent de déficit de 234Th aux postes de la mer de Béring et environ 15 p. cent au poste du bassin Canada. La PP mesurée baissait d'environ 12,5 µmol C/m³/h dans l'eau de surface à près de zéro à environ 100 m de profondeur, avec une PP intégrée de 3,83 mmol C/m²/j dans le bassin Canada. D'autre part, la production bactérienne était de l'ordre de 2,0 mmol C/m²/j, ce qui représente jusqu'à 52 p. cent de la PP intégrée. Les flux d'exportation du carbone organique particulaire (COP) calculés à partir du déséquilibre 234Th /238U étaient d'environ 1 mmol C/m²/j dans le bassin Canada et d'environ 10 mmol C/m²/j dans la mer de Béring, les flux dans cette région étant de 5 à 10 fois plus élevés que ceux trouvés dans le bassin Canada. Ces flux d'exportation correspondent à un rapport ThE (le rapport de l'exportation du COP dérivé du 234Th à la production primaire) de 0,26 pour le bassin Canada et de 0,7 pour la mer de Béring. Les rapports plus élevés de ThE dans les zones d'étude suggèrent un découplage de la production et de l'exportation de particules dans l'océan septentrional. Les rapports de COP au 234Th particulaire (µmol C/dpm) diminuaient de façon uniforme avec l'augmentation de la profondeur, ce qui suggère que le carbone organique est reminéralisé préférentiellement par rapport au 234Th. Il est à noter que le profil du 234Th particulaire dans le bassin Canada affichait une caractéristique unique: l'activité du 234Th particulaire augmentait avec la profondeur, ce qui suggère un balayage continu du 234Th et un taux rapide de sédimentation des particules

Abundance, Stable Isotopic Composition, and Export Fluxes of DOC, POC, and DIC From the Lower Mississippi River During 2006-2008

Sources, abundance, isotopic compositions, and export fluxes of dissolved inorganic carbon (DIC), dissolved and colloidal organic carbon (DOC and COC), and particulate organic carbon (POC), and their response to hydrologic regimes were examined through monthly sampling from the Lower Mississippi River during 2006–2008. DIC was the most abundant carbon species, followed by POC and DOC. Concentration and δ13C of DIC decreased with increasing river discharge, while those of DOC remained fairly stable. COC comprised 61 ± 3% of the bulk DOC with similar δ13C abundances but higher percentages of hydrophobic organic acids than DOC, suggesting its aromatic and diagenetically younger status. POC showed peak concentrations during medium flooding events and at the rising limb of large flooding events. While δ13C-POC increased, δ15N of particulate nitrogen decreased with increasing discharge. Overall, the differences in δ13C between DOC or DIC and POC show an inverse correlation with river discharge. The higher input of soil organic matter and respired CO2 during wet seasons was likely the main driver for the convergence of δ13C between DIC and DOC or POC, whereas enhanced in situ primary production and respiration during dry seasons might be responsible for their isotopic divergence. Carbon export fluxes from the Mississippi River were estimated to be 13.6 Tg C yr−1 for DIC, 1.88 Tg C yr−1 for DOC, and 2.30 Tg C yr−1 for POC during 2006–2008. The discharge-normalized DIC yield decreased during wet seasons, while those of POC and DOC increased and remained constant, respectively, implying variable responses in carbon export to the increasing discharge

Pan-Arctic Patterns in Black Carbon Sources and Fluvial Discharges Deduced From Radiocarbon and PAH Source Apportionment Markers In Estuarine Surface Sediments

A pan-arctic geospatial picture of black carbon (BC) characteristics was obtained from the seven largest arctic rivers by combining with molecular combustion markers (polycyclic aromatic hydrocarbons) and radiocarbon ((14)C) analysis. The results suggested that the contribution from modern biomass burning to BC ranged from low in the Yukon (8%) and Lena (5%) Rivers to high in the Yenisey River (88%). The Mackenzie River contributed almost half of the total arctic fluvial BC export of 202 kton a(-1) (kton = 10(9) g), with the five Russian-Arctic rivers contributing 10-36 kton a(-1) each. The (14)C-based source estimate of fluvially exported BC to the Arctic Ocean, weighted by the riverine BC fluxes, amount to about 20% from vegetation/biofuel burning and 80% from (14)C-extinct sources such as fossil fuel combustion and relict BC in uplifted source rocks. Combining these pan-arctic data with available estimates of BC export from other rivers gave a revised estimate of global riverine BC export flux of 26 x 10(3) kton a(-1). This is twice higher than a single previous estimate and confirms that river export of BC is a more important pathway of BC to the oceans than direct atmospheric deposition

Do invasive quagga mussels alter CO2 dynamics in the Laurentian Great Lakes?

The Laurentian Great Lakes have experienced unprecedented ecological and environmental changes, especially after the introduction of invasive quagga mussel (Dreissena rostriformis bugensis). While impacts on ecological functions have been widely recognized, the response of carbon dynamics to invasive species remains largely unknown. We report new CO(2) data showing significant increases in pCO(2) (up to 800 μatm in Lake Michigan) and CO(2) emission fluxes in most of the Great Lakes compared to those prior to or during the early stage of the colonization of invasive quagga mussels. The increased CO(2) supersaturation is most prominent in Lakes Huron and Michigan, followed by Lakes Ontario and Erie, but no evident change was observed in Lake Superior. This trend mirrors the infestation extent of invasive quagga mussels in the Great Lakes and is consistent with the decline in primary production and increase in water clarity observed pre- and post-Dreissena introduction, revealing a close linkage between invasive species and carbon dynamics. The Great Lakes have become a significant CO(2) source to the atmosphere, emitting >7.7 ± 1.0 Tg-C annually, which is higher than the organic carbon burial rate in global inland-seas and attesting to the significant role of the Laurentian Great Lakes in regional/global CO(2) budget and cycling

Seasonal Variations In Nutrient Concentrations and Speciation in the Chena River, Alaska

To better understand the seasonal controls on nutrient abundances, speciation, and fluxes in a watershed underlain by discontinuous permafrost, we collected water samples biweekly from the Chena River during 2005-2006 to measure inorganic and organic N, P, and Si in dissolved and particulate phases. Nitrate concentrations were low (8-14 mu M) during the winter and summer dry seasons but were elevated during the spring freshet (15-24 mu M). Ammonium varied from 8 to 13 mu M during the winter but dropped dramatically during the ice-open season to 0.1-3 mM. Phosphate was very low throughout the year (ranging from 0.03 to 0.3 mu M), reflecting the pristine condition of the watershed. Dissolved silica was high in the winter and reached its minimum during the spring freshet. DIN was the dominant species in the total N pool (60%), followed by DON (30%) and PN (10%). Most of the phosphorous was present in the particulate phase (74%), with phosphate and DOP only comprising 19% and 7%, respectively. Seasonal variations in nutrient concentrations and speciation were mostly controlled by the hydrological flow regime and biological activity in the river. Annual nutrient export fluxes from the Chena River during 2005-2006 were 51.1 x 10(6) mole-N, 1.4 x 10(6) mole-P, and 197 x 10(6) mole-Si, corresponding to an annual yield of 9.8 x 10(3) mol-N km(-2), 0.28 x 10(3) mol-P km(-2), and 37.9 x 10(3) mol-Si km(-2), respectively. Within the annual export fluxes, the spring freshet contributed about 18% of TN, 27% of TP, and 10% of Si, while the winter season contributed 11% of TN, 12% of TP, and 20% of Si. Continued climatic warming in northern watersheds will likely increase the export of nutrient species from watersheds

Chemical and Isotopic Characterization of Size-Fractionated Organic Matter From Cryoturbated Tundra Soils, Northern Alaska

Recent studies indicate a second layer of organic matter often accumulates in the lower active layer and upper permafrost in arctic tundra soils as a result of cryoturbation. In this study, cryoturbated organic matter was characterized using a combination of physical size fractionation and modern analytical techniques for elemental composition (C and N), stable isotopes (delta(13) C and delta(15)N), radiocarbon content (Delta(14)C), and molecular fingerprinting (pyrolysis-gas chromatography/mass spectrometry, Py-GC/MS). The results indicated that cryoturbated organic matter could be highly bioavailable. Soil organic matter (SOM) associated with fine sand particles was considered to be the organic carbon pool most sensitive to the changing climate. More organic matter is stabilized on clay minerals in arctic tundra soils compared to those in temperate and tropical soils. The bioavailable soluble organic matter extracted from cryoturbated soil was found to have significant long-term effects on carbon cycling. The similar molecular composition between cryoturbated and surface soil organic matter suggests that the vegetation cover has not significantly changed since the early Holocene. Furthermore, the SOM quality in moist acidic tundra was found to be higher than that of wet nonacidic tundra. With thawing permafrost and a deepening of the active layer, cryotrubated organic matter could reenter the biogeochemical cycles in the Arctic, resulting in a positive feedback to climate change