Fate Of Terrestrial Organic Matter Research Articles

Nunataryuk field campaigns: understanding the origin and fate of terrestrial organic matter in the coastal waters of the Mackenzie Delta region

Abstract. Climate warming and related drivers of soil thermal change in the Arctic are expected to modify the distribution and dynamics of carbon contained in perennially frozen grounds. Thawing of permafrost in the Mackenzie River watershed of northwestern Canada, coupled with increases in river discharge and coastal erosion, triggers the release of terrestrial organic matter (OMt) from the largest Arctic drainage basin in North America into the Arctic Ocean. While this process is ongoing and its rate is accelerating, the fate of the newly mobilized organic matter as it transits from the watershed through the delta and into the marine system remains poorly understood. In the framework of the European Horizon 2020 Nunataryuk programme, and as part of the Work Package 4 (WP4) Coastal Waters theme, four field expeditions were conducted in the Mackenzie Delta region and southern Beaufort Sea from April to September 2019. The temporal sampling design allowed the survey of ambient conditions in the coastal waters under full ice cover prior to the spring freshet, during ice breakup in summer, and anterior to the freeze-up period in fall. To capture the fluvial–marine transition zone, and with distinct challenges related to shallow waters and changing seasonal and meteorological conditions, the field sampling was conducted in close partnership with members of the communities of Aklavik, Inuvik and Tuktoyaktuk, using several platforms, namely helicopters, snowmobiles, and small boats. Water column profiles of physical and optical variables were measured in situ, while surface water, groundwater, and sediment samples were collected and preserved for the determination of the composition and sources of OMt, including particulate and dissolved organic carbon (POC and DOC), and colored dissolved organic matter (CDOM), as well as a suite of physical, chemical, and biological variables. Here we present an overview of the standardized datasets, including hydrographic profiles, remote sensing reflectance, temperature and salinity, particle absorption, nutrients, dissolved organic carbon, particulate organic carbon, particulate organic nitrogen, CDOM absorption, fluorescent dissolved organic matter intensity, suspended particulate matter, total particulate carbon, total particulate nitrogen, stable water isotopes, radon in water, bacterial abundance, and a string of phytoplankton pigments including total chlorophyll. Datasets and related metadata can be found in Juhls et al. (2021) (https://doi.org/10.1594/PANGAEA.937587).

Anthropogenic perturbations to the fate of terrestrial organic matter in a river-dominated marginal sea

River-dominated ocean margins (RiOMar) are major terrestrial organic carbon (OCterr) repositories that play an important role in the global carbon cycle. However, riverine inputs of sediment and associated OCterr have decreased substantially due to intensifying human activity. Thus, studying OCterr processing during transport to and within RiOMar is crucial. The East China Sea (ECS) receives reduced sediment loads from the Changjiang River (CJR), which is compounded further by the impoundment of the Three Gorges Reservoir. Here, we examined the CJR-ECS source-to-sink system to understand the fate of OCterr in a typical RiOMar. We analyzed the sedimentary properties (grain size and the specific surface area), bulk organic carbon properties (C/N ratio and δ13C), and molecular biomarkers (n-alkanes and GDGTs) of riverine suspended particulate matter (SPM) and surficial marine sediment collected in 2006 and 2018. The C/N ratio, δ13C value, and short-chain n-alkanes abundances in the riverine SPM indicate that phytoplankton productivity could be an important source of OCterr in the basin due to the reduced sediment loads. Seabed erosion occurred in the estuarine–inner shelf areas of the ECS as a result of reduced riverine inputs, which resulted in a change in the distribution of sedimentary OCterr. The reduced total organic carbon content and n-alkanes abundance are indicative of the low sedimentary OCterr content. Variations in GDGT-based indices are further manifestation of reduced sediment load, as they show increased in situ production of GDGTs. Spearman correlations indicate that the biomarker abundances were positively correlated with silt, which is in accordance with silt being easily shaped by hydrodynamic sorting. Biomarker abundances along four estuarine–inner shelf transects also exhibited pronounced alongshore or cross-shelf OCterr transport. Furthermore, the relationship between the biomarkers and sedimentary properties indicates that mineral protection plays an important role in regulating the fate of OCterr on continental margins. Therefore, variations in riverine inputs and the sedimentary environment caused by anthropogenic perturbations have affected the abundance, distribution, composition, and transport of sedimentary OCterr in the study area. These findings can be applicable to other RiOMar, as humans continue to shape riverine inputs globally.

Stream metabolism controls diel patterns and evasion of CO2 in Arctic streams.

Streams play an important role in the global carbon (C) cycle, accounting for a large portion of CO2 evaded from inland waters despite their small areal coverage. However, the relative importance of different terrestrial and aquatic processes driving CO2 production and evasion from streams remains poorly understood. In this study, we measured O2 and CO2 continuously in streams draining tundra‐dominated catchments in northern Sweden, during the summers of 2015 and 2016. From this, we estimated daily metabolic rates and CO2 evasion simultaneously and thus provide insight into the role of stream metabolism as a driver of C dynamics in Arctic streams. Our results show that aquatic biological processes regulate CO2 concentrations and evasion at multiple timescales. Photosynthesis caused CO2 concentrations to decrease by as much as 900 ppm during the day, with the magnitude of this diel variation being strongest at the low‐turbulence streams. Diel patterns in CO2 concentrations in turn influenced evasion, with up to 45% higher rates at night. Throughout the summer, CO2 evasion was sustained by aquatic ecosystem respiration, which was one order of magnitude higher than gross primary production. Furthermore, in most cases, the contribution of stream respiration exceeded CO2 evasion, suggesting that some stream reaches serve as net sources of CO2, thus creating longitudinal heterogeneity in C production and loss within this stream network. Overall, our results provide the first link between stream metabolism and CO2 evasion in the Arctic and demonstrate that stream metabolic processes are key drivers of the transformation and fate of terrestrial organic matter exported from these landscapes.

Tracing terrestrial DOC in the Baltic Sea—A 3‐D model study

AbstractThe fate of terrestrial organic matter brought to the coastal seas by rivers and its role in the global carbon cycle are still not very well known. Here the degradation rate of terrestrial dissolved organic carbon (DOCter) is studied in the Baltic Sea, a subarctic semienclosed sea, by releasing it as a tracer in a 3‐D circulation model and applying linear decay constants. A good agreement with available observational data is obtained by parameterizing the degradation in two rather different ways: one by applying a decay time on the order of 10 years to the whole pool of DOCter and one by dividing the DOCter into one refractory pool and one pool subject to a decay time on the order of 1 year. The choice of parameterization has a significant effect on where in the Baltic Sea the removal takes place, which can be of importance when modeling the full carbon cycle and the CO2 exchange with the atmosphere. In both cases the biogeochemical decay operates on time scales less than the water residence time. Therefore, only a minor fraction of the DOCter reaches the North Sea, whereas approximately 80% is removed by internal sinks within the Baltic Sea. This further implies that DOCter mineralization is an important link in land‐sea‐atmosphere cycling of carbon in coastal and shelf seas that are heavily influenced by riverine DOC.

Identifying the drivers of the pelagic ecosystem of an oligotrophic bight (KwaZulu–Natal, South Africa) using stable isotopes (δ13C, δ15N) and C : N ratio analysis

The international literature highlights the importance of understanding the role and fate of terrestrial organic matter (OM) in the marine system, yet little is known about this on the oligotrophic east coast of South Africa. Within the Bight, three important processes make OM biologically available: (1) a topographically induced upwelling cell (north), (2) a Cyclonic Eddy (south), and (3) riverine input. We hypothesise that: (i) these OM sources differ substantially in their isotope ratios, and (ii) zooplankton at the inshore region has an isotope signature linked to localised fluvial fluxes. Particulate OM samples were collected at five locations along the Bight and in three main estuaries during austral summer and winter seasons in 2010 and subjected to isotopic and mixing model analysis. Riverine input played an important role in this ecosystem inshore during the wet season. Only marine OM played a role as a biological driver offshore and at all sites during the dry season. Although there were indications that the upwelling cell did occur, it was not confirmed as active in either season. The current study provides valuable insights into the Bight's biological functioning and principle insights into the fate of terrestrial organic carbon on the east coast of southern Africa.

Comparative study of isotopic trends in two coastal ecosystems of North Biscay: A multitrophic spatial gradient approach

In coastal estuarine embayments, retention of water masses due to coastal topography may result in an increased contribution of continental organic matter in food webs. However, in megatidal embayments, the effect of topography can be counterbalanced by the process of tidal mixing. Large amounts of continental organic matter are exported each year by rivers to the oceans. The fate of terrestrial organic matter in food webs of coastal areas and on neighboring coastal benthic communities was therefore evaluated, at multi-trophic levels, from primary producers to primary consumers and predators. Two coastal areas of the French Atlantic coast, differing in the contributions from their watershed, tidal range and aperture degree, were compared using carbon and nitrogen stable isotopes (δ13C and δ15N) during two contrasted periods. The Bay of Vilaine receives large inputs of freshwater from the Vilaine River, displaying 15N enriched and 13C depleted benthic communities, emphasizing the important role played by allochtonous inputs and anthropogenic impact on terrestrial organic matter in the food web. In contrast, the Bay of Brest which is largely affected by tidal mixing, showed a lack of agreement between isotopic gradients displayed by suspended particulate organic matter (SPOM) and suspension-feeders. Discrepancy between SPOM and suspension-feeders is not surprising due to differences in isotopes integration times. We suggest further that such a discrepancy may result from water replenishment due to coastal inputs, nutrient depletion by phytoplankton production, as well as efficient selection of highly nutritive phytoplanktonic particles by primary consumers.

Biomarkers reveal the effects of hydrography on the sources and fate of marine and terrestrial organic matter in the western Irish Sea

A suite of lipid biomarkers were investigated from surface sediments and particulate matter across hydrographically distinct zones associated with the western Irish Sea gyre and the seasonal bloom. The aim was to assess the variation of organic matter (OM) composition, production, distribution and fate associated with coastal and southern mixed regions and also the summer stratified region. Based on the distribution of a suite of diagnostic biomarkers, including phospholipid fatty acids, source-specific sterols, wax esters and C25 highly branched isoprenoids, diatoms, dinoflagellates and green algae were identified as major contributors of marine organic matter (MOM) in this setting. The distribution of cholesterol, wax esters and C20 and C22 polyunsaturated fatty acids indicate that copepod grazing represents an important process for mineralising this primary production. Net tow data from 2010 revealed much greater phytoplankton and zooplankton biomass in well-mixed waters compared to stratified waters. This appears to be largely reflected in MOM input to surface sediments. Terrestrial organic matter (TOM), derived from higher plants, was identified as a major source of OM regionally, but was concentrated in proximity to major riverine input at the Boyne Estuary and Dundalk Bay. Near-bottom residual circulation and the seasonal gyre also likely play a role in the fate of TOM in the western Irish Sea.

Mechanistic controls on diverse fates of terrestrial organic components in the East China Sea

Terrestrial carbon transferred from the land to sea is a critical component of the global carbon cycle. A range of geochemical proxies has been developed to fingerprint the fate of terrestrial organic matter (TOM) in marine sediments. However, discrepancies among different proxies limit our ability to quantify and interpret the terrestrial signals in marine sediments, with consequences for the investigation of both the modern carbon cycle and past environmental change. To mechanistically understand these discrepancies, we examined the distributions of a range of terrestrial proxies and their aquatic counterparts (i.e. marine proxies) in the Yangtze river-East China Sea (YR-ECS) shelf system, where TOM experiences extensive modification during transport and burial. TOM proxies in the YR-ECS system collectively fit a power-law model but with distinct attenuation rates (the a∗ values) for individual molecular proxy groups. Among a range of TOM proxies, the modeled a∗ values decrease in the order: soil-marker BHPs>triterpenols>lignin>HMW n-alkanols>branched GDGTs>HMW n-alkanes for biomarkers; and Rsoil>BIT>%TOMiso for proxies tracing %TOM. Rapid loss of TOM components through dissociation in the narrow estuary, followed by oxidation over the wide open shelf, are best described by power curves. Inherent chemical reactivity (i.e. the number of functional groups), responses to hydraulic sorting, and in situ production regulate the individual attenuation rates. Of them, chemical reactivity plays the most important role on proxy behavior, supported by a strong correlation between a∗ values and standard molal Gibbs energies. Both, physical protection and chemical reactivity fundamentally control the overall behavior of TOM components, with the relative importance being setting-dependant: The former is relatively important in the estuary, whereas the later is the primary control over the open shelf. Moreover, regional variation of different marine-counterparts is also significant over the river-ECS shelf system, seemingly regulated by regional nutrient distributions. Therefore, for %TOM estimates using molecular ratio approaches, the specific behavior of individual terrestrial components and marine-counterparts and the physical, biological and chemical characteristics of depositional settings all need to be considered.

Organic matter distribution in the modern sediments of the Pearl River Estuary

We determined biomarker concentrations and distributions for surface sediments from 54 sites in the Pearl River Estuary, China. We focus on a suite of four biomarker-based indicators for relative terrestrial to marine organic matter (OM) source: the branched-isoprenoid tetraether (BIT) index, the ratio of high/low molecular weight n-alcohols [(ΣC26–34/(ΣC16+18+ΣC26–34)], an analogous ratio for n-fatty acids and the ΣC29-steroids/(ΣC29-steroids+brassicasterol) ratio. All four exhibit the same terrestrial to marine transition seen in previous bulk δ13C studies, but with an abrupt decrease in the relative terrestrial contribution across the delta front to pro-delta transition. Concentrations of terrestrially-derived biomarkers show no systematic decrease across the transition. Instead, the decrease in the proportion of terrestrial OM is due to a decrease in the sedimentation rate and associated terrestrial OM burial across the delta toe. This suggests that diagenetic controls on the fate of terrestrial OM, such as increased biodegradation where sedimentation rate is low, are subordinate to sedimentological processes. Biomarker-derived temperature values are cooler than expected for the lower Pearl River catchment, suggesting that the dominant component of the terrestrial OM is derived from the cooler upland regions of the catchment. The dominance of input from more distal terrain with greater topographic relief is evidence for the importance of geomorphological control on terrigenous OM transport. Collectively, the results demonstrate the importance of sedimentological processes in the supply, deposition and transport of terrestrial OM.

Altered microbial community structure and organic matter composition under elevated CO2 and N fertilization in the duke forest

AbstractThe dynamics and fate of terrestrial organic matter (OM) under elevated atmospheric CO2 and nitrogen (N) fertilization are important aspects of long‐term carbon sequestration. Despite numerous studies, questions still remain as to whether the chemical composition of OM may alter with these environmental changes. In this study, we employed molecular‐level methods to investigate the composition and degradation of various OM components in the forest floor (O horizon) and mineral soil (0–15 cm) from the Duke forest free air CO2 enrichment (FACE) experiment. We measured microbial responses to elevated CO2 and N fertilization in the mineral soil using phospholipid fatty acid (PLFA) profiles. Increased fresh carbon inputs into the forest floor under elevated CO2 were observed at the molecular‐level by two degradation parameters of plant‐derived steroids and cutin‐derived compounds. The ratios of fungal to bacterial PLFAs and Gram‐negative to Gram‐positive bacterial PLFAs decreased in the mineral soil with N fertilization, indicating an altered soil microbial community composition. Moreover, the acid to aldehyde ratios of lignin‐derived phenols increased with N fertilization, suggesting enhanced lignin degradation in the mineral soil. 1H nuclear magnetic resonance (NMR) spectra of soil humic substances revealed an enrichment of leaf‐derived alkyl structures with both elevated CO2 and N fertilization. We suggest that microbial decomposition of SOM constituents such as lignin and hydrolysable lipids was promoted under both elevated CO2 and N fertilization, which led to the enrichment of plant‐derived recalcitrant structures (such as alkyl carbon) in the soil.

Optimised recovery of lignin-derived phenols in a Scottish fjord by the CuO oxidation method

Lignin is found only in vascular plant tissues, hence monitoring of lignin in aquatic environments is important in the determination of the sources and fate of terrestrial organic matter (OM). Concentrations of lignin-derived phenols provide an estimate of the amount of terrestrial OM in a system. Other lignin parameters such as the ratios of syringyl to vanillyl (S/V) and cinnamyl to vanillyl (C/V) phenols provide information on vegetation sources; and the ratios of vanillic acid to vanillin, (Ad/Al)v, and syringic acid to syringaldehyde, (Ad/Al)s, indicate the degradation stage of lignin materials. Concentrations of lignin-derived phenols were determined for the surface and subsurface sediments of a Scottish sea loch, Loch Creran, using a CuO oxidation method. In order to achieve the highest methodological efficiency, a series of validation experiments for each procedural step were carried out. As a result, several procedural steps were optimized. It was found that a lower oxidation temperature resulted in higher product yield and the duration and temperature of the silylation step have no effect on the outcome of the process. Studies of seasonal variations of lignin parameters showed that the occasional increase in lignin content in sediment trap samples was attributable to materials discharged from River Creran and the incoming water from the direction of Creagan bridge. The flow regime caused resuspension of surface sediments and entrainment of lignin materials into the water column. Lignin parameters exhibited more significant trends across an axial transect of the loch (from the head to mouth). Total lignin content decreased and the C/V ratios increased from upper to lower Loch Creran as these lignin materials were transported further down the loch and were subjected to hydrodynamic sorting processes.

The fate of terrestrial organic matter in two Scottish sea lochs

Sea lochs are zones of rapid organic matter (OM) turnover. Most of this OM is of allochthonous origin, being introduced into the lochs via freshwater input. In this study the behaviour of terrestrially derived OM was elucidated using a combination of parameters which indicate OM diagenesis in the near surface sediments from two Scottish sea lochs, Loch Creran and Loch Etive. Alkaline CuO oxidation was used to determine lignin phenols which serve as biomarkers for terrestrial OM in sediments. Stable carbon isotope, total carbon and nitrogen and total OM (including the labile and refractory fractions) compositions were also determined. Lignin materials in the lochs were generally highly degraded and undergo little degradation further seaward. The vanillic acid to vanillin ratio, (Ad/Al)v in the lochs ranged from 0.52 to 2.69. However, there was a fraction of relatively fresh, land-derived OM, still undergoing degradation adding to the carbon cycling in the lochs, as indicated by the Rp values (ratio of refractory to total OM) and OC/N ratios in the surface sediments. The hydrological and hydrodynamic regimes in Loch Creran result in several phenomena such as the transportation of terrestrial debris via hydrodynamic sorting processes and the promotion of surface sediment diagenesis by bioturbation. Frequent water renewal results in better water circulation and oxygenation which facilitate OM decomposition. In Loch Etive the less frequent renewal gives rise to a more constant OM diagenesis along the loch.