Organic Matter Optical Properties Research Articles

Organic matter, eutrophication, and increased mass accumulation rates in the Anthropocene are the main drivers of mercury concentrations and historical enrichment in Canadian lake sediments

AbstractMercury (Hg) contamination in the environment is a persistent issue as emissions from industry are ongoing and legacy cycling is prolonged. To quantify total mercury (total Hg) and methylmercury (MeHg) concentrations in lake sediments across a wide suite of temperate to subarctic lakes, measurements and modeling were conducted for 320 sites across Canada. Total Hg varied from 7 to 567 ng g−1 dry sediment with a mean (median) value of 148 (128) ng g−1 dry sediment, with lowest concentrations observed in eutrophic lakes of the Prairies and Boreal Plains. Average total Hg concentrations have increased twofold in contemporary sediments relative to their preindustrial pair. MeHg concentrations were generally low, with a mean of 0.8 ng g−1 dry sediment and this form representing on average 0.6% of the total contemporary mercury concentrations. Water column variables related to eutrophication were among the strongest predictors of contemporary total Hg, MeHg, and total Hg enrichment in the sediments. In lakes where sediment radiometric data were collected, contemporary total Hg accumulation rates were strongly related to longitude(+) and Hg atmospheric deposition(+). Latitude(−) and Hg atmospheric deposition(+) were the best predictors of MeHg accumulation rates. Chromophoric dissolved organic matter optical properties were key predictors of both total Hg concentrations and accumulation rates. Our work highlights the paramount importance of lake trophic state and the nature of organic matter as key predictors of lake sediment mercury metrics.

Nitrogen forms and dissolved organic matter optical properties in bulk rainfall, canopy throughfall, and stormwater in a subtropical urban catchment

The study of nitrogen (N) transformation in urban ecosystems is crucial in the protection of coastal water bodies because excess N may fuel harmful algae blooms (HABs). The purpose of this investigation was to study and identify the forms and concentrations of N in rainfall, throughfall, and stormwater runoff for 4 storm events in a subtropical urban ecosystem and to use fluorescence spectroscopy to evaluate the optical properties and expected lability of dissolved organic matter (DOM) in the same samples. The rainfall contained both inorganic and organic N pools, and organic N as nearly 50 % of total dissolved N in the rainfall. As water moved through the urban water cycle, from rainfall to stormwater and from rainfall to throughfall, it was enriched in total dissolved N, with most of the enrichment coming from dissolved organic N. Throughfall fluxes of total dissolved N were as high as 0.67 kg ha−1, compared to 0.44 kg ha−1 from rainfall, suggesting that the urban tree canopy can facilitate anthropogenic subsidies of N to the urban water cycle. Through analysis of sample optical properties, we saw that the throughfall presented the highest humification index and the lowest biological index when compared to rainfall, suggesting throughfall likely consists of higher molecular weight compounds of greater recalcitrance. This study highlights the importance of the dissolved organic N fraction of urban rainfall, stormwater, and throughfall and shows how the chemical composition of dissolved organic nutrients can change as rainfall is transformed into throughfall in the urban tree canopy.

The influence of sources and solid phase extraction criteria on dissolved organic matter optical properties in aquatic systems

The integrity of studies investigating the composition of dissolved organic matter (DOM) is heavily dependent on the sampling and instrumental techniques employed. In this study, we aimed to assess the impacts of both the source of the DOM and the solid phase extraction (SPE) criteria on changes to the optical properties of DOM. Samples were collected from various aquatic environments, ranging from headwaters to the open ocean. The extraction kinetics of DOM were monitored throughout the SPE process, with alterations to the DOM verified using optical properties derived from chromophoric dissolved organic matter (CDOM), including absorbance and fluorescence measurements. The fluorescence excitation-emission matrices (EEMs) were further analyzed using PARAllel FACtor analysis (PARAFAC) to resolve the underlying chemical components. Results indicate that the reaction of DOM to acidification required for polystyrene-divinylbenzene polymer sorbent SPE varies across samples. And this variability may stem from differences in functional groups and DOM sources. The maximum carbon loading observed was 9.7 mg C per 1 g PPL, which is a widely used styrene-divinylbenzene polymer sorbent, and no overloading was noted. The dissolved organic carbon (DOC) extraction efficiency (EE) ranged from 51.5-61.5% and 27.6-40.0% for post- and extract- calculations, respectively, with no significant differences in DOC EE observed among different initial DOC concentrations, salinities, and flowrates. The differences in DOC EE across diverse sources and changes in optical proxies resulting from PPL extraction were mainly due to the heterogeneity of DOM. PPL extraction altered the overall molecular characteristics of DOM from more labile to more refractory or degraded in the open ocean and headwater source samples. Notably, high and low flowrates yielded identical SPE-DOM extraction and alterations. The results indicate that DOM sources primarily control SPE EE rather than methodological or other environmental factors. PPL SPE results in a convergence of DOM variability. These findings provide crucial insights into inter-study comparisons and the interpretability of SPE-derived DOM data from samples across diverse aquatic environments.

Insights into surface circulation and mixing in James Bay and Hudson Bay from dissolved organic matter optical properties

Hudson Bay and James Bay are fed by numerous rivers, which also deliver large amounts of dissolved organic matter (DOM). The objective of this study is to assess Hudson Bay wide DOM spatial distribution using ultraviolet-visible light absorption and excitation-emission matrix fluorescence spectroscopies. Overall, a five-component PARAFAC model was validated using 1060 discrete samples collected over nine field campaigns between 2009 and 2018 and including the first samples from James Bay ( n = 112). Regional and seasonal variations were observed. Particularly striking was a 9-fold difference in concentration between coastal and marine zones of these bays, which implies limited mixing between those regions. Seasonal variations in absorption coefficient at 355 nm and terrestrial humic-like components were found only in east Hudson Bay, revealing a regional disparity in seasonal DOM behaviour. The summer intensities and relative abundances of DOM revealed that cyclonic circulation transports terrestrial DOM around James Bay following its eastern shores before being transported northward along the western coast of Belcher Islands. The vertical distribution of DOM indicated that fluorescent DOM is delivered to the deeper layer by winter convective mixing in northern Hudson Bay. • Limited mixing between the coastal and marine regions of the bay is observed. • The strong terrestrial signal of east James Bay is found in northern Belcher Island. • DOM is delivered to the deeper layer by winter convection in northern Hudson Bay.

Changes in soil dissolved organic matter optical properties during peatland succession

• Topsoil and subsoil DOM respond differently to aboveground vegetation succession. • Succession increases the content of humic-like substances in topsoil DOM. • The content of protein-like material of the subsoil is stable under succession. • pH is a key factor affecting changes in fluorescence properties of topsoil DOM. Dissolved organic matter (DOM) is the most active component in the soil environment. Understanding the chemical diversity of DOM and the interaction of the physicochemical properties of the soil are key to managing peatland under succession. In this study, we aimed to understand the effects of peatland succession on soil DOM. We collected soil samples (topsoil: 0–10 cm, subsoil: 10–20 cm) from peatland during different stages of peatland succession in mid–high latitude northern regions, determined the changes in quantity and quality of DOM using fluorescence spectroscopy and parallel factor analysis. We found that peatland succession altered the effectiveness of soil nutrients and water content. During the succession, the content of humus-like substances in the DOM of the topsoil increased and the content of protein-like material decreased, whereas the content of substances in the subsoil remained stable. pH was the key factor affecting the change in the composition of the DOM in the topsoil during peatland succession. The variation of DOM in the subsoil may be related to the vegetation composition. The results suggest that fluorescent DOM components respond significantly to changes in peatland succession, and DOM properties are driven by soil pH and vegetation composition during peatland succession. In conclusion, our results reveal the optical changes and factors that influence DOM in peatlands under succession. This suggests that DOM can be modified by simultaneous changes of the physicochemical properties in the soil and the vegetation cover.

Inferring Ecosystem Function from Dissolved Organic Matter Optical Properties: A Critical Review.

Over the last 30 years, the optical property community has shifted from conducting dissolved organic matter (DOM) measurements on new complex mixtures in natural and engineered systems to furthering ecosystem understanding in the context of past, present, and future carbon (C) cycling regimes. However, the appropriate use of optical properties to understand DOM behavior in complex biogeochemical systems is of recent debate. This critical review provides an extensive survey of DOM optical property literature across atmospheric, marine, and terrestrial biospheres using a categorical approach that probes each biosphere and its subdivisions. Using this approach, a rubric of ecosystem variables, such as productive nature, C cycling rate, C inputs, and water quality, sets the foundation for interpreting commonly used optical property DOM metrics such as fluorescence index (FI), humification index (HIX), and specific ultraviolet absorbance at 254 nm (SUVA254). Case studies and a meta-analysis of each biosphere and subdivision found substantial overlap and characteristic distributions corresponding to ecosystem context for FI, HIX, and SUVA254, signifying chromophores and fluorophores from different ecosystems may be more similar than originally thought. This review challenges researchers to consider ecosystem connectivity when applying optical property results rather than making traditional “if this, then that” results-style conclusions.

Ross Sea Dissolved Organic Matter Optical Properties During an Austral Summer: Biophysical Influences

The Ross Sea, one of the most productive regions in the Southern Ocean, plays a significant role in deep water formation and carbon cycling. Dissolved organic carbon (DOC) concentrations and chromophoric dissolved organic matter (CDOM) absorption and fluorescence (FDOM) properties were studied in conjunction with biophysical properties during austral summer. Elevated values of both DOC (mean 47.82 ± 5.70 μM) and CDOM (absorption coefficient at 325 nm, acdom325: mean 0.31 ± 0.18 m–1) observed in the upper shelf waters in the southwest (SW), north of the Ross Ice Shelf (RIS), the northwest and along a transect inward of the shelf break, suggested in situ production and accumulation linked to the productive spring/summer season. However, regional differences were observed in CDOM with acdom325 higher (0.63 ± 0.19 m–1) and its spectral slope S275–295 lower (24.06 ± 2.93 μm–1) in the SW compared to other regions (0.25 ± 0.08 m–1 and 28.92 ± 2.67 μm–1, respectively). Similarly, the specific UV absorption coefficient or SUVA254 determined at 254 nm was greater (1.85 ± 0.55 m2 mg–1 C) compared to other regions (1.07 ± 0.24 m2 mg–1 C), indicating CDOM of greater molecular weight and aromaticity in the SW. Phytoplankton absorption spectra indicated the shallow mixed layer of SW Ross Sea to be dominated by diatoms (e.g., Fragilariopsis spp.), a preferential food source for grazers such as the Antarctic krill, which in large numbers have been shown to enhance CDOM absorption, a likely source in the SW. Excitation-emission matrix (EEM) fluorescence combined with parallel factor analysis (PARAFAC) retrieved one protein-like and two humic-like FDOM fractions commonly observed in the global ocean. In contrast to acdom325 which was uncorrelated to DOC, we observed weak but significant positive correlations between the humic-like FDOM with salinity and DOC, high value of the biological index parameter BIX and an instance of increasing FDOM with depth at a location with sinking organic matter, suggesting autochthonous production of FDOM. The absorption budget showed a relatively higher contribution by CDOM (70.7 ± 18.3%) compared to phytoplankton (22.5 ± 15.2%) absorption coefficients at 443 nm with implications to ocean color remote sensing. This first study of DOM optical properties provides additional insights on carbon cycling in the Ross Sea.

Emerging investigator series: critical review of photophysical models for the optical and photochemical properties of dissolved organic matter.

Optical measurements (absorbance and fluorescence) are widely used to track dissolved organic matter (DOM) quantity and quality in natural and engineered systems. Despite many decades of research on the optical properties of DOM, there is a lack of understanding with regards to the underlying photophysical model that is the basis for these optical properties. This review both summarizes advances to date on the photophysical properties of DOM and seeks to critically evaluate the photophysical models for DOM optical properties. Recent studies have refined the quantitative understanding of DOM photophysical properties such as excited state lifetimes and energies, rates of different photophysical processes, and quantum yields. Considering fundamental models, more clarity is needed on whether DOM photophysical processes are due to a superposition of non-interacting components (superposition model), or whether a portion of optical signals can be ascribed to electronically interacting moieties, for example in the form of electron donor-acceptor complexes (charge transfer model). Multiple studies over more than two decades have provided evidence for the charge transfer model. Questions have been raised, however, about the broad applicability of the charge transfer model. The charge transfer and superposition model are critically reviewed in light of this current research. Recommendations are given for future studies to help clarify the accuracy of these competing photophysical models.

Linking riverine partial pressure of carbon dioxide to dissolved organic matter optical properties in a Dry-hot Valley Region

The mineralization of dissolved organic matter (DOM) can partially explain riverine carbon dioxide (CO2) emissions to the atmosphere. However, little is known about how the DOM origin and composition drive CO2 partial pressures (pCO2). Here, we reveal links between aquatic pCO2, DOM optical parameters (a254, a350 and S275-295 and S350-400) and nutrients in a subtropical river in China’s Dry-hot Valley Region. Biodegradation preferentially decomposed low molecular weight (LMW) DOMs, increasing high molecular weight (HMW) DOMs along the main stem. pCO2 was positively correlated with aromatic and lignin compounds, but negatively correlated with DOM molecular weight. Aquatic respiration of DOMs largely explained the pCO2 levels in the drought period, while terrestrial inputs were a pCO2 source in the initial-wet period. Our results illustrate how both DOM concentrations and speciation can explain pCO2 distribution and sources in rivers.

Dissolved organic carbon and nitrogen release from boreal Holocene permafrost and seasonally frozen soils of Alaska

Permafrost (perennially frozen) soils store vast amounts of organic carbon (C) and nitrogen (N) that are vulnerable to mobilization as dissolved organic carbon (DOC) and dissolved organic and inorganic nitrogen (DON, DIN) upon thaw. Such releases will affect the biogeochemistry of permafrost regions, yet little is known about the chemical composition and source variability of active-layer (seasonally frozen) and permafrost soil DOC, DON and DIN. We quantified DOC, total dissolved N (TDN), DON, and DIN leachate yields from deep active-layer and near-surface boreal Holocene permafrost soils in interior Alaska varying in soil C and N content and radiocarbon age to determine potential release upon thaw. Soil cores were collected at three sites distributed across the Alaska boreal region in late winter, cut in 15 cm thick sections, and deep active-layer and shallow permafrost sections were thawed and leached. Leachates were analyzed for DOC, TDN, nitrate (NO3−), and ammonium (NH4+) concentrations, dissolved organic matter optical properties, and DOC biodegradability. Soils were analyzed for C, N, and radiocarbon (14C) content. Soil DOC, TDN, DON, and DIN yields increased linearly with soil C and N content, and decreased with increasing radiocarbon age. These relationships were significantly different for active-layer and permafrost soils such that for a given soil C or N content, or radiocarbon age, permafrost soils released more DOC and TDN (mostly as DON) per gram soil than active-layer soils. Permafrost soil DOC biodegradability was significantly correlated with soil Δ14C and DOM optical properties. Our results demonstrate that near-surface Holocene permafrost soils preserve greater relative potential DOC and TDN yields than overlying seasonally frozen soils that are exposed to annual leaching and decomposition. While many factors control the fate of DOC and TDN, the greater relative yields from newly thawed Holocene permafrost soils will have the largest potential impact in areas dominated by organic-rich soils.

Response of dissolved organic matter optical properties to net inflow runoff in a large fluvial plain lake and the connecting channels

Fluvial plain lake watersheds are usually highly urbanized and have high concentrations of chromophoric dissolved organic matter (CDOM). CDOM derived from the connecting urban channels usually share strong terrestrial and anthropogenic signals and net inflow runoff (Qnet) to the lake serves as a proxy of residential household sewage input. We investigate how Qnet controls the optical characteristics of CDOM in fluvial plain Lake Taihu and the connecting channels. CDOM absorption coefficient a(350), dissolved organic carbon (DOC), the fluorescence intensity (Fmax) of seven PARAFAC components C1-C7, and δ15N-TDN were higher in the northwestern relative to the other lake regions, and a(250)/a(365), spectral slope S275–295, and δ13C-DOM relative low in the northwestern lake, all indicating strong terrestrial and anthropogenic effects. Conversely, the urban land cover (%Cities), gross domestic product (GDP), and population density were relatively low in the western sub-watersheds and high in the eastern sub-watersheds. This apparent paradox reflects variations in Qnet from different sub-watersheds. Thus, significant positive relationships were found between Qnet and a(350), DOC, chemical oxygen demand (COD), chlorophyll-a (Chl-a), Fmax of C1-C3 and C6-C7, and %C2-%C3 in the five hydraulic sub-watersheds. We revealed significant positive relationships between mean a(350), DOC, COD, Chl-a, C1-C3 and C6, %C2-%C3, and the products of Qnet × %Cities, Qnet × GDP, and Qnet × population density. We further found dominant contribution of lignin to the total number of assigned formulas for the samples collected from the channels in the Huxi watershed and the central lake using high resolution mass spectroscopy. We conclude that Qnet is of key importance for the optical properties of CDOM molecules in the various regions of Lake Taihu and the connecting channels.

Contribution of humic substances to dissolved organic matter optical properties and iron mobilization

Humic substances (HS) are the primary constituents of dissolved organic matter (DOM) and play pivotal roles in aquatic systems. Optical indices of DOM, such as specific UV absorbance (SUVA254), the fluorescence index (FI) and biological index (BIX), have gained wide interest because of their ease of use. In this study, we explored the relationship between HS and the indices in the Trat River Basin (eastern Thailand) from headwaters to the river mouth through the distinct dry and rainy seasons to examine whether changes in index values reflect variability in the relative contribution of HS to DOM, or %HS. The results show that %HS and the indices did not exhibit significant linear relationships (FI and BIX, P > 0.05), or the relationships changed seasonally (SUVA254). However, analyzing the indices versus %HS did show clear DOM composition changes by season with more humic-like or terrestrial material in the rainy season. Relationships between DOM and dissolved iron (dFe) concentrations were also explored. Separating the relationships of DOM versus dFe into HS versus dFe and non-HS versus dFe provides us the opportunity to better understand which fraction contributes more to dFe mobilization. The results indicate stronger positive linear relationships between HS and dFe concentrations independent of river tributary. Overall, this study highlights the importance of quantifying HS for the study of DOM dynamics or compositional changes along a river transect as well as for DOM-induced iron mobilization.

The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics.

The optical properties of dissolved organic matter influence chemical and biological processes in all aquatic ecosystems. Dissolved organic matter optical properties have been attributed to a charge-transfer model in which donor-acceptor complexes play a primary role. This model was evaluated by measuring the absorbance and fluorescence response of organic matter isolates to changes in solvent temperature, viscosity, and polarity, which affect the position and intensity of spectra for known donor-acceptor complexes of organic molecules. Absorbance and fluorescence spectral shape were largely unaffected by these changes, indicating that the distribution of absorbing and emitting species was unchanged. Overall, these results call into question the wide applicability of the charge-transfer model for explaining organic matter optical properties and suggest that future research should explore other models for dissolved organic matter photophysics.

Role of effluent organic matter in the photochemical degradation of compounds of wastewater origin

The photoreactivity of treated wastewater effluent organic matter differs from that of natural organic matter, and the indirect phototransformation rates of micropollutants originating in wastewater are expected to depend on the fractional contribution of wastewater to total stream flow. Photodegradation rates of four common compounds of wastewater origin (sulfamethoxazole, sulfadimethoxine, cimetidine and caffeine) were measured in river water, treated municipal wastewater effluent and mixtures of both to simulate various effluent-stream water mixing conditions that could occur in environmental systems. Compounds were chosen for their unique photodegradation pathways with the photochemically produced reactive intermediates, triplet-state excited organic matter (3OM*), singlet oxygen (1O2), and hydroxyl radicals (OH). For all compounds, higher rates of photodegradation were observed in effluent relative to upstream river water. Sulfamethoxazole degraded primarily via direct photolysis, with some contribution from OH and possibly from carbonate radicals and other unidentified reactive intermediates in effluent-containing samples. Sulfadimethoxine also degraded mainly by direct photolysis, and natural organic matter appeared to inhibit this process to a greater extent than predicted by light screening. In the presence of effluent organic matter, sulfadimethoxine showed additional reactions with OH and 1O2. In all water samples, cimetidine degraded by reaction with 1O2 (>95%) and caffeine by reaction with OH (>95%). In river water mixtures, photodegradation rate constants for all compounds increased with increasing fractions of effluent. A conservative mixing model was able to predict reaction rate constants in the case of hydroxyl radical reactions, but it overestimated rate constants in the case of 3OM* and 1O2 pathways. Finally, compound degradation rate constants normalized to the rate of light absorption by water correlated with E2/E3 ratios (sample absorbance at 254 nm divided by sample absorbance at 365 nm), suggesting that organic matter optical properties may hold promise to predict indirect compound photodegradation rates for various effluent mixing ratios.

Linking optical properties of dissolved organic matter to multiple processes at the coastal plume zone in the East China Sea.

Because of the significance in photosynthesis, nutrient dynamics, trophodynamics and biological activity, dissolved organic matter (DOM) is important to the microbial community in the coastal plume zone. In this study, we investigated the hydrodynamic processes, photodegradation and biodegradation of DOM at the Yangtze River plume in the East China Sea through analyzing water quality and optical properties of DOM. Surface water samples were collected to examine water quality and fluorescence properties of fluorescent dissolved organic matter (FDOM). The results indicated that dilution was the key factor in the multiple processes, and the mixing process gradually increased from nearshore to offshore in coastal water. Four components of FDOM representing humic-like substances (C1 & C4) and protein-like substances (C2 & C3) were identified, and all components showed nearly conservative behaviors. Protein-like substances were more mutable compared to humic-like substances. The photodegradation of humic-like substances caused brown algae blooms to some extent. The molecular weight of humic substances gradually decreased along the mixing process. FDOM in the plume zone was both of terrigenous and autochthonous origins, and the characteristic of terrigenous origin was obvious compared to that of autochthonous origin.

Variability in dissolved organic matter optical properties in surface waters in the Amerasian Basin

Surface absorption and fluorescence measurements of Dissolved Organic Matter (DOM) were conducted along with hydrographic parameters in the Canada and Makarov Basins. Parallel factor analysis of DOM fluorescence identified four humic-like and one protein-like component in all 107 surface samples. Based on strong negative trends observed between the spectral slope in the 275-295 nm range and absorption at 370 nm, and four humic-like components C1-C4, the DOM character was found to be basin-dependent. The Makarov basin surface DOM was largely dominated by high molecular weight and humic-rich material whereas the Canada Basin surface DOM was more heterogeneous with a marked influence of in situ production. This study highlights that absorbing and fluorescing measurements can be used successfully to trace and differentiate DOM from diverse sources and across frontal zones, and as such can be convenient and complementary tools for the better understanding of marine biogeochemical cycling of carbon.

Correlations between Dissolved Organic Matter Optical Properties and Quantum Yields of Singlet Oxygen and Hydrogen Peroxide

Various aquatic dissolved organic matter (DOM) samples produce singlet oxygen (1O2) and hydrogen peroxide (H2O2) with quantum yields of 0.59 to 4.5% (1O2 at 365 nm) and 0.017 to 0.053% (H2O2, 300-400 nm integrated). The two species' yields have opposite pH dependencies and strong, but opposite, correlations with the E2/E3 ratio (A254 divided by A365). Linear regressions allow prediction of both quantum yields from E2/E3 in natural water samples with errors ranging from -3% to 60%. Experimental evidence and kinetic calculations indicate that less than six percent of the H2O2 is produced by reaction between 1O2 and DOM. The inverse relationship between the 1O2 and H2O2 yields is thus best explained by a model in which precursors to these species are populated competitively. A model is presented, which proposes that important precursors to H2O2 may be either charge-transfer or triplet states of DOM.

Spatiotemporal drivers of dissolved organic matter in high alpine lakes: Role of Saharan dust inputs and bacterial activity

The effects of many environmental stressors such as UV radiation are mediated by dissolved organic matter (DOM) properties. Therefore, determining the factors shaping spatial and temporal patterns is particularly essential in the most susceptible, low dissolved organic carbon (DOC) lakes. We analyzed spatiotemporal variations in dissolved organic carbon concentration and dissolved organic matter optical properties (absorption and fluorescence) in 11 transparent lakes located above tree line in the Sierra Nevada Mountains (Spain), and we assessed potential external (evaporation and atmospheric deposition) and internal (bacterial abundance, bacterial production, chlorophyll a, and catchment vegetation) drivers of DOM patterns. At spatial and temporal scales, bacteria were related to chromophoric DOM (CDOM). At the temporal scale, water soluble organic carbon (WSOC) in dust deposition and evaporation were found to have a significant influence on DOC and CDOM in two Sierra Nevada lakes studied during the ice-free periods of 2000-2002. DOC concentrations and absorption coefficients at 320 nm were strongly correlated over the spatial scale (n = 11, R(2) = 0.86; p < 0.01), but inconsistently correlated over time, indicating seasonal and interannual variability in external factors and a differential response of DOC concentration and CDOM to these factors. At the continental scale, higher mean DOC concentrations and more CDOM in lakes of the Sierra Nevada than in lakes of the Pyrenees and Alps may be due to a combination of more extreme evaporation, and greater atmospheric dust deposition.

Maturity studies and source-rock potential in the southern Sverdrup Basin, Arctic Canada

The purpose of the study is to better understand the relationship between organic matter optical properties and the presence of potentially large oil and gas accumulations in Arctic Canada. The type and thermal maturity of the dispersed organic matter of the Mesozoic formations in the southern Sverdrup Basin, Melville Island, have been studied using organic petrology and Rock-Eval pyrolysis. All types of organic matter are present in the strata of Mesozoic age. Hydrogen-rich liptinite is dominated by alginite ( Botryococcus and Tasmanites), dinoflagellate cysts and amorphous fluorescing matrix. Sporinite, cutinite, resinite and liptodetrinite made up the lesser hydrogen-rich exinite. Vitrinite reflectance in Cretaceous sediments ranges from 0.36 to 0.65% R o; in Jurassic sediments it ranges from 0.40 to 1.0% R o and in the Triassic from 0.45 to 1.30% R o, showing an overall increase with depth of burial. Cretaceous sediments of the Deer Bay Formation are thermally immature and contain organic matter of terrestrial origin. The Upper Jurassic shales of the Ringnes Formation contain predominantly organic matter of liptinitic and exinitic origin with a considerable vitrinitic input. At optimum maturation levels, potential source beds of this formation would have a good hydrocarbon-generating potential. The hydrocarbon potential, however, would be limited to the generation of gases due to the leanness of the source rocks. Parts of the Lower Jurassic Jameson Bay Formation are organic-rich and contain a mixed exinitic/vitrinitic organic matter, Botryococcus colonial algae but visible organic matter is dominated by high plant remains (mainly spores). The Schei Point Group shales and siltstones contain organic matter of almost purely marine origin, whereas the predominantly higher plant-derived organic matter found in the Deer Bay, Jameson Bay and partly in the Ringnes formations have higher TOC. Among the Schei Point Group samples, the Cape Richards and Eden Bay members of the Hoyle Bay Formation are richer in TOC (>2.0%) than the Murray Harbour Formation (Cape Caledonia Member). This may reflect differences in the level of maturity or in the depositional environment (more anoxic conditions for the former). Regional variations in the level of thermal maturity of Mesozoic sediments in Sverdrup Basin appear to be a function of burial depth. The Mesozoic formations thicken towards the basin centre (NNE direction), reflecting the general pattern of increasing thermal maturity north of Sabine Peninsula. However, the regional thermal-maturation pattern of the Mesozoic is not solely a reflection of the present-day geothermal gradient, which indicates that anomalous zones of high geothermal gradient may have existed in the past, at least since when the Mesozoic sediments attained maximum burial depth. The contour pattern of the regional variation of maturity at the base of numerous Triassic formations is similar to that of the structural contours of the Sverdrup Basin, indicating that present-day maturation levels are largely controlled by basin subsidence.