The trophic history of Myall Lakes,New South Wales,Australia: interpretations using δ<Superscript>13</Superscript>C and δ<Superscript>15</Superscript>N of the sedimentary record

In an attempt to determine the trophic history of the Myall Lakes complex (New South Wales, Australia) δ¹³C_org, δ¹⁵N and C_org:N profiles were determined for bulk organic matter of two short sediment cores from Bombah Broadwater and Myall Lake. ²¹⁰Pb profiles and sediment types indicate significantly different trophic trajectories during the time periods examined. δ¹³C_org and C_org:N indicate Bombah Broadwater has been dominated by increasing inputs of terrestrial organic material over the last century, thought to be related to watershed disturbance including agricultural activity. Primary production appears to be dominated by phytoplankton. δ¹⁵N remained relatively stable at around 1‰ until the mid–1970s when there was a sharp increase to 4.7‰, interpreted as an influx of sewage-derived material. These observations offer an insight into the recent trophic changes at the site. Sedimentation rates are noticeably lower in Myall Lake and the most recent sediment is a flocculent organic rich deposit overlying mineral clay. δ¹³C_org and C_org:N values indicate a transition from plankton to macrophyte dominated primary production around 1800AD. δ¹⁵N values become increasingly negative from approximately 1900AD. This is interpreted as being due to increasing reliance by macrophytes on nitrogen recycled from decomposing sediments driven by natural infilling and eutrophication in this basin. The contrasting sedimentation rates, sediment types and geochemical profiles suggest the different basins of this water body are subject to substantially different internal and external influences which should be considered in management decisions.     

Relative sea‐level change regulates organic carbon accumulation in coastal habitats

Because coastal habitats store large amounts of organic carbon (C_org), the conservation and restoration of these habitats are considered to be important measures for mitigating global climate change. Although future sea‐level rise is predicted to change the characteristics of these habitats, its impact on their rate of C_org sequestration is highly uncertain. Here we used historical depositional records to show that relative sea‐level (RSL) changes regulated C_org accumulation rates in boreal contiguous seagrass–saltmarsh habitats. Age–depth modeling and geological and biogeochemical approaches indicated that C_org accumulation rates varied as a function of changes in depositional environments and habitat relocations. In particular, C_org accumulation rates were enhanced in subtidal seagrass meadows during times of RSL rise, which were caused by postseismic land subsidence and climate change. Our findings identify historical analogs for the future impact of RSL rise driven by global climate change on rates of C_org sequestration in coastal habitats.     

Distributed GIS for estimation of soil carbon stock of West Siberia boreal zone

Distributed GIS developed to assess the soil carbon stocks (SOC) includes digital maps “Vegetation” and “Soil” of boreal zone of West Siberia, middle-scale soil maps of the key sites of landscape provinces that have been created on the basis of interpretation of satellite images, and the software for updating and management of the database “Carbon in soils of Siberia”. According to preliminary estimates the resources of C_org in the soil of the boreal zone is 83.6 × 10⁸ t, which is 28.2% of the resources of C_org in soils of Russia or 15.2% of global carbon pool.     

Blue Carbon Storage in Tropical Seagrass Meadows Relates to Carbonate Stock Dynamics,Plant–Sediment Processes,and Landscape Context: Insights from the Western Indian Ocean

Globally, seagrass ecosystems are considered major blue carbon sinks and thus indirect contributors to climate change mitigation. Quantitative estimates and multi-scale appraisals of sources that underlie long-term storage of sedimentary carbon are vital for understanding coastal carbon dynamics. Across a tropical–subtropical coastal continuum in the Western Indian Ocean, we estimated organic (C_org) and inorganic (C_carb) carbon stocks in seagrass sediment. Quantified levels and variability of the two carbon stocks were evaluated with regard to the relative importance of environmental attributes in terms of plant–sediment properties and landscape configuration. The explored seagrass habitats encompassed low to moderate levels of sedimentary C_org (ranging from 0.20 to 1.44% on average depending on species- and site-specific variability) but higher than unvegetated areas (ranging from 0.09 to 0.33% depending on site-specific variability), suggesting that some of the seagrass areas (at tropical Zanzibar in particular) are potentially important as carbon sinks. The amount of sedimentary inorganic carbon as carbonate (C_carb) clearly corresponded to C_org levels, and as carbonates may represent a carbon source, this could diminish the strength of seagrass sediments as carbon sinks in the region. Partial least squares modelling indicated that variations in sedimentary C_org and C_carb stocks in seagrass habitats were primarily predicted by sediment density (indicating a negative relationship with the content of carbon stocks) and landscape configuration (indicating a positive effect of seagrass meadow area, relative to the area of other major coastal habitats, on carbon stocks), while seagrass structural complexity also contributed, though to a lesser extent, to model performance. The findings suggest that accurate carbon sink assessments require an understanding of plant–sediment processes as well as better knowledge of how sedimentary carbon dynamics are driven by cross-habitat links and sink–source relationships in a scale-dependent landscape context, which should be a priority for carbon sink conservation.     

Characterizing niche differentiation among marine consumers with amino acid δ13C fingerprinting

Marine food webs are highly compartmentalized, and characterizing the trophic niches among consumers is important for predicting how impact from human activities affects the structuring and functioning of marine food webs. Biomarkers such as bulk stable isotopes have proven to be powerful tools to elucidate trophic niches, but they may lack in resolution, particularly when spatiotemporal variability in a system is high. To close this gap, we investigated whether carbon isotope (δ¹³C) patterns of essential amino acids (EAAs), also termed δ¹³C_AA fingerprints, can characterize niche differentiation in a highly dynamic marine system. Specifically, we tested the ability of δ¹³C_AA fingerprints to differentiate trophic niches among six functional groups and ten individual species in the Baltic Sea. We also tested whether fingerprints of the common zooplanktivorous fishes, herring and sprat, differ among four Baltic Sea regions with different biochemical conditions and phytoplankton assemblages. Additionally, we investigated how these results compared to bulk C and N isotope data for the same sample set. We found significantly different δ¹³C_AA fingerprints among all six functional groups. Species differentiation was in comparison less distinct, due to partial convergence of the species' fingerprints within functional groups. Herring and sprat displayed region‐specific δ¹³C_AA fingerprints indicating that this approach could be used as a migratory marker. Niche metrics analyses showed that bulk isotope data had a lower power to differentiate between trophic niches than δ¹³C_AA fingerprinting. We conclude that δ¹³C_AA fingerprinting has a strong potential to advance our understanding of ecological niches, and trophic linkages from producers to higher trophic levels in dynamic marine systems. Given how management practices of marine resources and habitats are reshaping the structure and function of marine food webs, implementing new and powerful tracer methods are urgently needed to improve the knowledge base for policy makers.     

Relationship between soil organic carbon and microbial biomass on chronosequences of reclamation sites

The interrelationship between soil microorganisms and soil organic carbon was studied on an agricultural and on a forest chronosequence of open-pit mine reclamation soils. Thirty years after reclamation, soil carbon levels of 0.8% on the agricultural sites and 1.7% on the forest sites (A-horizon) were reached. Microbial biomass rose very fast to levels characteristic of undisturbed soils. Microbial carbon (C_mier) was 57 mg·100 g^–1 soil after 15 years on the agricultural sites and 43 mg·100 g^–1 on the forest sites. The contribution of C_mier to the total organic carbon (C_org) decreased with time, more rapidly on the forest sites than on the agricultural ones. From the C_mierr/C_org ratio it became evident that both chronosequences had not yet reached a steady state within the 50 years of reclamation. A significant decrease of the metabolic quotient qCO₂ (microbial respiration per unit biomass) with time was observed on the agricultural sites but not on the forest sites. The C_mier/C_org ratio proved to be a reliable soil microbial parameter for describing changes in man-made ecosystems. For evaluating reclamation efforts, the C_mier/C_org ratio can be considered superior to its single components (C_mier or C_org) and to other parameters.     

Seagrass losses since mid‐20th century fuelled CO2 emissions from soil carbon stocks

Seagrass meadows store globally significant organic carbon (C_org) stocks which, if disturbed, can lead to CO₂ emissions, contributing to climate change. Eutrophication and thermal stress continue to be a major cause of seagrass decline worldwide, but the associated CO₂ emissions remain poorly understood. This study presents comprehensive estimates of seagrass soil C_org erosion following eutrophication‐driven seagrass loss in Cockburn Sound (23 km² between 1960s and 1990s) and identifies the main drivers. We estimate that shallow seagrass meadows (<5 m depth) had significantly higher C_org stocks in 50 cm thick soils (4.5 ± 0.7 kg C_org/m²) than previously vegetated counterparts (0.5 ± 0.1 kg C_org/m²). In deeper areas (>5 m), however, soil C_org stocks in seagrass and bare but previously vegetated areas were not significantly different (2.6 ± 0.3 and 3.0 ± 0.6 kg C_org/m², respectively). The soil C_org sequestration capacity prevailed in shallow and deep vegetated areas (55 ± 11 and 21 ± 7 g C_org m^?2 year^?1, respectively), but was lost in bare areas. We identified that seagrass canopy loss alone does not necessarily drive changes in soil C_org but, when combined with high hydrodynamic energy, significant erosion occurred. Our estimates point at ~0.20 m/s as the critical shear velocity threshold causing soil C_org erosion. We estimate, from field studies and satellite imagery, that soil C_org erosion (within the top 50 cm) following seagrass loss likely resulted in cumulative emissions of 0.06–0.14 Tg CO_2‐eq over the last 40 years in Cockburn Sound. We estimated that indirect impacts (i.e. eutrophication, thermal stress and light stress) causing the loss of ~161,150 ha of seagrasses in Australia, likely resulted in the release of 11–21 Tg CO_2‐eq since the 1950s, increasing cumulative CO₂ emissions from land‐use change in Australia by 1.1%–2.3% per annum. The patterns described serve as a baseline to estimate potential CO₂ emissions following disturbance of seagrass meadows.     

The effect of afforestation with Scots pine (Pinus silvestris L.) of sandy post-arable soils on their selected properties. II. Reaction, carbon, nitrogen and phosphorus

Despite the extensive literature on the effect on soil properties of afforestation of former arable land, we still lack full understanding of whether the changes proceed in the same direction and at the same rate, and of how long is required to achieve a state of soil equilibrium typical of a natural forest ecosystem. Therefore, as part of a study comparing post-arable sandy soils (Dystric Arenosols) afforested with Scots pine (Pinus silvestris L.) with arable soils and soils of continuous coniferous forests, the range and direction of changes in pH, organic carbon (C_org), total nitrogen (N_tot), ammonium (N-NH₄) and nitrates (N-NO₃) in soil solution, total (P_tot) and available (P_av) phosphorus were determined. The studies were carried out in south-east Poland (51°30′-51°37′N, 22°20′-22°35′E). Ten paired sites of afforested soils (five with 14- to 17-year-old stands and five with 32- to 36-year-old stands) with adjacent cultivated fields, and five sites of continuous forest with present stands of ca. 130–150 years old were selected. Soil samples were taken from the whole thickness of master horizons and, in the case of the A horizon of the afforested soils, from three layers: 0–5 (A_0–5), 5–10 (A_5–10) and 10–20 cm (A_10–20). The cultivated soils in the Ap horizon showed higher pH (by ca. 1.0 unit), lower C_org and C:N, similar N_tot, lower N-NH₄, higher N-NO₃, higher P_tot and P_av contents compared with the Ah horizon of continuous forest soils. The results indicated decreased soil pH in the former plough layer of the afforested soils, with the greatest decrease observed in the 0–5 cm layer. In these soils, the C_org content was considerably higher in the A_0–5 layer, but lower in the two deeper layers and in the whole A horizon (0–20 cm) compared with the Ap horizon of the arable soils. The results indicate that the C_org content, after an initial phase of decline, again achieved a level characteristic of arable soils. The N_tot content in all layers of the A horizon of the afforested soils was lower than in the Ap horizon of the arable soils, and showed a reduction with stand age, especially in deeper layers. The C:N ratios in the mineral topsoil increased with stand age. N-NH₄ content increased and N-NO₃ decreased after afforestation. The P_tot and P_av contents in all layers and in the whole A horizon of the afforested soils, on stands of both ages, was lower than in the Ap of the cultivated soils. From the results, it could be concluded that, after more than 30 years of tree growth, the soils of the A horizon were still more similar to arable than to continuous forest soils with respect to C_org, P_tot and P_av. With respect to pH, N-NH₄ and N-NO₃, especially in the 0–5 cm layer, they were more similar to continuous forest soils than to cultivated soils, but with respect to N_tot and C:N ratio they were somewhere in between.     

Relative importance of the land‐use composition and intensity for the bird community composition in anthropogenic landscapes

Humans are changing the biosphere by exerting pressure on land via different land uses with variable intensities. Quantifying the relative importance of the land‐use composition and intensity for communities may provide valuable insights for understanding community dynamics in human‐dominated landscapes. Here, we evaluate the relative importance of the land‐use composition versus land‐use intensity on the bird community structure in the highly human‐dominated region surrounding Paris, France. The land‐use composition was calculated from a land cover map, whereas the land‐use intensity (reverse intensity) was represented by the primary productivity remaining after human appropriation (NPP_remaining), which was estimated using remote sensing imagery. We used variance partitioning to evaluate the relative importance of the land‐use composition versus intensity for explaining bird community species richness, total abundance, trophic levels, and habitat specialization in urban, farmland, and woodland habitats. The land‐use composition and intensity affected specialization and richness more than trophic levels and abundance. The importance of the land‐use intensity was slightly higher than that of the composition for richness, specialization, and trophic levels in farmland and urban areas, while the land‐use composition was a stronger predictor of abundance. The intensity contributed more to the community indices in anthropogenic habitats (farmland and urban areas) than to those in woodlands. Richness, trophic levels, and specialization in woodlands tended to increase with the NPP_remaining value. The heterogeneity of land uses and intensity levels in the landscape consistently promoted species richness but reduced habitat specialization and trophic levels. This study demonstrates the complementarity of NPP_remaining to the land‐use composition for understanding community structure in anthropogenic landscapes. Our results show, for the first time, that the productivity remaining after human appropriation is a determinant driver of animal community patterns, independent of the type of land use.     

Twentieth century human and climate impacts on a large mountain lake in southwest China

Subfossil chironomid and sediment geochemistry data from Lugu Lake, a large high-elevation lake in southwest China, were used to assess the influences of climate warming and direct human impacts on the lake through time. A 95-year-long sediment record was recovered from the lake. Principal components analysis (PCA) and redundancy analysis of fossil chironomid data were performed to determine the controlling factors on the chironomid community. The four prominent environmental controls were summer air temperature, organic matter C:N ratio (C_org:N), dry mass accumulation rate (DMAR), and total nitrogen. C_org:N proved to be the most important controlling factor through time. However, C_org:N and summer air temperature were highly co-linear, possibly due to temperature directly impacting lake productivity (and thus C:N ratios) through increased stratification and a longer growing season. PCA Axis 1 scores were a strong predictor of summer temperatures even after DMAR was factored out to account for direct human influences. The strong temperature–chironomid relationship over the last 50 years could be due to the lake becoming more responsive to climate warming after cultural eutrophication of the 1950s, as lakes with higher nutrient loads are shown to be more responsive to the effects of climate warming.     

The influence of acid irrigation and liming on the soil microbial biomass in a Norway spruce (Picea abies [L.] K.) stand

Over a period of three years (1990–1992) microbial biomass-C (C_mic), CO₂ evolution, the C_mic:C_org ratio and the metabolic quotient for CO₂ (qCO₂) were determined in a Norway spruce stand (Höglwald) with experimentally acid-irrigated and limed plots since 1984. A clear relationship between soil pH and the level of microbial biomass-(C_mic) was noted, C_mic increasing with increasing soil pH in O_h or A_h horizons. More microbial biomass-C per unit C_{org} (C_mic:C_org ratio) was detected in limed plots with elevated pH of O_h or A_h horizons as compared to unlimed plots with almost 3 times more C_mic per unit C_org in the limed O_h horizon. Differences here are significant at least at the p=0.05 level. The positive effects of liming (higher pH) on the C_mic:C_org ratio was more pronounced in the upper horizon (O_h)). The total CO₂ evolution rate of unlimed plots was only half of that noted for limed plots which corresponded to the low microbial biomass levels of unlimed plots. The specific respiratory activity, qCO₂, was similar and not significantly different between the unlimed control plot and the limed plot.Acid irrigation of plots with already low pH did not significantly affect the level of microbial biomass, the C_mic:C_org ratio or qCO₂. An elevated qCO₂ could be seen, however, for the limed + acid irrigated plot. The biomass seemed extremely stressed, showing with 3.8 g CO₂-C mg_-1 C_mic h_-1 (O_h) the highest qCO₂ value of all treatments. This was interpreted as a reflection of the continuous adaptation processes to the H⁺ ions by the microflora. The negative effect of acid irrigation of limed plots was also manifested in a decreased C_mic:C_org ratio.     

Knowledge-based classification of remote sensing data for the estimation of below- and above-ground organic carbon stocks in riparian forests

Floodplain forests play a crucial role in the storage of organic carbon (C_org). However, modeling of carbon stocks in these dynamic ecosystems remains inherently difficult. Here, we present the spatial estimation of C_org stocks in riparian woody vegetation and soils (to a depth of 1 m) in a Central European floodplain using very high spatial resolution remote sensing data and auxiliary geodata. The research area is the Danube Floodplain National Park in Austria, one of the last remaining wetlands with near-natural vegetation in Central Europe. Different vegetation types within the floodplain show distinct capacities to store C_org. We used remote sensing to distinguish the following vegetation types: meadow, reed bed and hardwood, softwood, and cottonwood forests. Spectral and knowledge-based classification was performed with object-based image analysis. Additional knowledge rules included distances to the river, object area, and slope information. Five different classification schemes based on spectral values and additional knowledge rules were compared and validated. Validation data for the classification accuracy were derived from forest inventories and topographical maps. Overall accuracy for vegetation types was higher for a combination of spectral- and knowledge-based classification than for spectral values alone. While water, reed beds and meadows were clearly detectable, it remained challenging to distinguish the different forest types. The total carbon storage of soils and vegetation was quantified using a Monte Carlo simulation for all classified vegetation types, and the spatial distribution was mapped. The average storage of the study site is 428.9 Mg C ha⁻¹. Despite certain difficulties in vegetation classification this method allows an indirect estimation of C_org stocks in Central European floodplains.     

Altered sediment biota and lagoon habitat carbonate dynamics due to sea cucumber bioturbation in a high‐pCO2 environment

The effects of global change on biological systems and functioning are already measurable, but how ecological interactions are being altered is poorly understood. Ecosystem resilience is strengthened by ecological functionality, which depends on trophic interactions between key species and resilience generated through biogenic buffering. Climate‐driven alterations to coral reef metabolism, structural complexity and biodiversity are well documented, but the feedbacks between ocean change and trophic interactions of non‐coral invertebrates are understudied. Sea cucumbers, some of the largest benthic inhabitants of tropical lagoon systems, can influence diel changes in reef carbonate dynamics. Whether they have the potential to exacerbate or buffer ocean acidification over diel cycles depends on their relative production of total alkalinity (A_T) through the dissolution of ingested calcium carbonate (CaCO₃) sediments and release of dissolved inorganic carbon (C_T) through respiration and trophic interactions. In this study, the potential for the sea cucumber, Stichopus herrmanni, a bêche‐de‐mer (fished) species listed as vulnerable to extinction, to buffer the impacts of ocean acidification on reef carbonate chemistry was investigated in lagoon sediment mesocosms across diel cycles. Stichopus herrmanni directly reduced the abundance of meiofauna and benthic primary producers through its deposit‐feeding activity under present‐day and near‐future pCO₂. These changes in benthic community structure, as well as A_T (sediment dissolution) and C_T (respiration) production by S. herrmanni, played a significant role in modifying seawater carbonate dynamics night and day. This previously unappreciated role of tropical sea cucumbers, in support of ecosystem resilience in the face of global change, is an important consideration with respect to the bêche‐de‐mer trade to ensure sea cucumber populations are sustained in a future ocean.     

Variables of Microbial Response in Natural Soil Aggregates for Soil Characterization in Different Fluvial Land Shapes

The main objective of this study was to determine changes in microbial response in natural soil aggregates for soil characterization in different fluvial land shapes. This study was carried out in fluvial lands formed on accumulated sediment depositions carried by K?z?l?rmak River. The majority soils of the study area were classified as Typic Ustifluvent and Typic Haplustept in Soil Taxonomy. It was found that macroaggregates (especially >6300 μm and 2000–4750 μm diameters) of all soil samples were higher than microaggregate of soils. In addition, it was determined that the C_org content varies between 0.41–0.91% in soil samples. C_mic content was also found higher level in aggregates involved <250 and 250–425 μm diameters as compared to other aggregate size classes. Moreover, we detected that C_org:C_mic ratio was much higher in macroaggregates than in microaggregate fractions. BR levels were also greater in macroaggregates of >6,300, 4,750–6,300 and 2,000–4,750 μm than in the other macroaggregates sizes and microaggregates. Consequently, macroaggregates have relatively more C_org level than the C_org level in microaggregates, even if the absolute values of C_mic were the lower. This study thus evidenced contrasting microbial habitats and their response in different soil aggregate size formed in various developed soils.     

Plant diversity shapes microbe‐rhizosphere effects on P mobilisation from organic matter in soil

Plant species richness (PSR) increases nutrient uptake which depletes bioavailable nutrient pools in soil. No such relationship between plant uptake and availability in soil was found for phosphorus (P). We explored PSR effects on P mobilisation [phosphatase activity (PA)] in soil. PA increased with PSR. The positive PSR effect was not solely due to an increase in C_org concentrations because PSR remained significant if related to PA:C_org. An increase in PA per unit C_org increases the probability of the temporal and spatial match between substrate, enzyme and microorganism potentially serving as an adaption to competition. Carbon use efficiency of microorganisms (C_mic:C_org) increased with increasing PSR while enzyme exudation efficiency (PA:C_mic) remained constant. These findings suggest the need for efficient C rather than P cycling underlying the relationship between PSR and PA. Our results indicate that the coupling between C and P cycling in soil becomes tighter with increasing PSR.     

Metagenomic investigation of the geologically unique Hellenic Volcanic Arc reveals a distinctive ecosystem with unexpected physiology

Hydrothermal vents represent a deep, hot, aphotic biosphere where chemosynthetic primary producers, fuelled by chemicals from Earth's subsurface, form the basis of life. In this study, we examined microbial mats from two distinct volcanic sites within the Hellenic Volcanic Arc (HVA). The HVA is geologically and ecologically unique, with reported emissions of CO₂‐saturated fluids at temperatures up to 220°C and a notable absence of macrofauna. Metagenomic data reveals highly complex prokaryotic communities composed of chemolithoautotrophs, some methanotrophs, and to our surprise, heterotrophs capable of anaerobic degradation of aromatic hydrocarbons. Our data suggest that aromatic hydrocarbons may indeed be a significant source of carbon in these sites, and instigate additional research into the nature and origin of these compounds in the HVA. Novel physiology was assigned to several uncultured prokaryotic lineages; most notably, a SAR406 representative is attributed with a role in anaerobic hydrocarbon degradation. This dataset, the largest to date from submarine volcanic ecosystems, constitutes a significant resource of novel genes and pathways with potential biotechnological applications.     

Patterns and controls of seasonal variability of carbon stable isotopes of particulate organic matter in lakes

Carbon stable isotopes (δ¹³C) of particulate organic matter (POM) have been used as indicators for energy flow, primary productivity and carbon dioxide concentration in individual lakes. Here, we provide a synthesis of literature data from 32 freshwater lakes around the world to assess the variability of δ¹³C_POM along latitudinal, morphometric and biogeochemical gradients. Seasonal mean δ¹³C_POM, a temporally integrated measure of the δ¹³C_POM, displayed weak relationships with all trophic state indices [total phosphorus (TP), total nitrogen (TN), and chlorophyll a (Chl a)], but decreased significantly with the increase in latitude, presumably in response to the corresponding decrease in water temperature and increase in CO₂ concentration. The seasonal minimum δ¹³C_POM also correlated negatively with latitude while seasonal maximum δ¹³C_POM correlated positively with all trophic state indices, pH, and δ¹³C of dissolved inorganic carbon (DIC). Seasonal amplitude of δ¹³C_POM (the difference between seasonal maximum and minimum values) correlated significantly with pH, TP and Chl a concentrations and displayed small variations in oligotrophic, mesotrophic and low latitude eutrophic lakes, which is attributed to low primary productivity and abundant non-living POM in the low trophic state lakes and relatively stable environmental conditions in the subtropics. Seasonal amplitude of δ¹³C_POM was the greatest in high latitude eutrophic lakes. Greater seasonal changes in solar energy and light regime may be responsible for the large seasonal variability in high latitude productive lakes. This synthesis provides new insights on the factors controlling variations in stable carbon isotopes of POM among lakes on the global scale.     

Free and kerogen‐bound biomarkers from late Tonian sedimentary rocks record abundant eukaryotes in mid‐Neoproterozoic marine communities

Lipid biomarker assemblages preserved within the bitumen and kerogen phases of sedimentary rocks from the ca. 780–729 Ma Chuar and Visingsö Groups facilitate paleoenvironmental reconstructions and reveal fundamental aspects of emerging mid‐Neoproterozoic marine communities. The Chuar and Visingsö Groups were deposited offshore of two distinct paleocontinents (Laurentia and Baltica, respectively) during the Tonian Period, and the rock samples used had not undergone excessive metamorphism. The major polycyclic alkane biomarkers detected in the rock bitumens and kerogen hydropyrolysates consist of tricyclic terpanes, hopanes, methylhopanes, and steranes. Major features of the biomarker assemblages include detectable and significant contribution from eukaryotes, encompassing the first robust occurrences of kerogen‐bound regular steranes from Tonian rocks, including 21‐norcholestane, 27‐norcholestane, cholestane, ergostane, and cryostane, along with a novel unidentified C₃₀ sterane series from our least thermally mature Chuar Group samples. Appreciable values for the sterane/hopane (S/H) ratio are found for both the free and kerogen‐bound biomarker pools for both the Chuar Group rocks (S/H between 0.09 and 1.26) and the Visingsö Group samples (S/H between 0.03 and 0.37). The more organic‐rich rock samples generally yield higher S/H ratios than for organic‐lean substrates, which suggests a marine nutrient control on eukaryotic abundance relative to bacteria. A C₂₇ sterane (cholestane) predominance among total C₂₆–C₃₀ steranes is a common feature found for all samples investigated, with lower amounts of C₂₈ steranes (ergostane and crysotane) also present. No traces of known ancient C₃₀ sterane compounds; including 24‐isopropylcholestanes, 24‐n‐propylcholestanes, or 26‐methylstigmastanes, are detectable in any of these pre‐Sturtian rocks. These biomarker characteristics support the view that the Tonian Period was a key interval in the history of life on our planet since it marked the transition from a bacterially dominated marine biosphere to an ocean system which became progressively enriched with eukaryotes. The eukaryotic source organisms likely encompassed photosynthetic primary producers, marking a rise in red algae, and consumers in a revamped trophic structure predating the Sturtian glaciation.     

Methanotrophy in a Paleoproterozoic oil field ecosystem,Zaonega Formation,Karelia, Russia

Organic carbon rich rocks in the c. 2.0 Ga Zaonega Formation (ZF), Karelia, Russia, preserve isotopic characteristics of a Paleoproterozoic ecosystem and record some of the oldest known oil generation and migration. Isotopic data derived from drill core material from the ZF show a shift in δ¹³C_org from c. ?25‰ in the lower part of the succession to c. ?40‰ in the upper part. This stratigraphic shift is a primary feature and cannot be explained by oil migration, maturation effects, or metamorphic overprints. The shift toward ¹³C‐depleted organic matter (δ¹³C_org < ?25‰) broadly coincides with lithological evidence for the generation of oil and gas in the underlying sediments and seepage onto the sea floor. We propose that the availability of thermogenic CH₄ triggered the activity of methanotrophic organisms, resulting in the production of anomalously ¹³C‐depleted biomass. The stratigraphic shift in δ¹³C_org records the change from CO₂‐fixing autotrophic biomass to biomass containing a significant contribution from methanotrophy. It has been suggested recently that this shift in δ¹³C_org reflects global forcing and progressive oxidation of the Earth. However, the lithologic indication for local thermogenic CH₄, sourced within the oil field, is consistent with basinal methanotrophy. This indicates that regional/basinal processes can also explain the δ¹³C_org negative isotopic shift observed in the ZF.     

Unravelling climate change impacts from other anthropogenic influences in a subalpine lake: a multi-proxy sediment study from Oberer Soiernsee (Northern Alps,Germany)

Mountain lakes are increasingly impacted by a series of both local and global disturbances. The present study reveals the eutrophication history of a remote subalpine lake (Oberer Soiernsee, Northern Alps, Germany), triggered by deforestation, alpine pasturing, hut construction, tourism and atmospheric deposition, and identifies the intertwined consequences of on-going global warming on the lake’s ecosystem. The primary objective was to disentangle the various direct and indirect impacts of these multiple stressors via down-core analyses. Our multi-proxy approach included subfossil diatom assemblages, carbon and nitrogen stable isotope ratios and subfossil pigments from dated sediments. Shifts within the diatom assemblages were related to variations in trophic state, lake transparency, water temperature and thermal stratification. The organic carbon isotope (δ¹³C_org) records, the diatom valve density and the pigment concentrations documented the development of primary production and composition. Total nitrogen isotope values (δ¹⁵N) are more likely to reflect the history of atmospheric nitrogen pollution than lake-internal processes, also mirrored by the decoupling of δ¹⁵N and δ¹³C_org trends. The composition of sedimentary pigments allowed a differentiation between planktonic and benthic primary production. Concordant trends of all indicators suggested that the lake ecosystem passed a climatic threshold promoted by local and long-distance atmospheric nutrient loadings.