Abstract
The delineation of sediment facies provides essential background information for a broad range of investigations in geosciences but is often constrained in quality or quantity. Here we ...leverage improvements in machine learning and X-ray fluorescence core scanning to develop an improved approach to automatic sediment-facies classification. This approach was developed and tested on a regional-scale high-resolution elemental dataset from sediment cores covering various sediment facies typical for the southern North Sea tidal flat, Germany. We use a machine-learning-built classification model involving simple but powerful feature engineering to simulate the observational behavior of sedimentologists and find that approach has 78% accuracy, followed by error analysis. The model classifies the majority of sediment facies and also, importantly, highlights critical sections for further investigation. Research resources can thus be allocated more efficiently. We suggest that our approach could provide a generalizable blueprint that can be applied and adapted for the research question and data type at hand.
Characterization of bulk organic matter (OM) from lacustrine sediments of Frickenhauser See (northern Bavaria, Germany) reveals considerable variation during the last two millennia. Atomic C/N ratios ...and total organic carbon (TOC) content are positively correlated with arboreal pollen percentages which are used as an indicator of land-use intensity. Despite possible alterations of OM during early diagenesis, differences between three major lithological units are large enough to be interpreted as human impact on the sedimentation of OM in the lake. Sediments deposited prior to deforestation in the catchment area (unit C) are characterized by mean C/N ratios of 14.5, indicating a mixed composition of organic matter derived from lacustrine and terrestrial sources (forest litter). During a period of intensive soil erosion (ca. 1000 AD until 1870 AD; unit B), low C/N ratios of around 7.7 suggest that the relative proportion of forest litter decreased in favour of the lacustrine component and soil OM. Terrestrial plant detritus is only transported into the lake through numerous turbidite events. Deforestation and agriculture also coincide with a decreasing TOC content from an average value of 10.7% to 1.5%. This decrease is explained through a dilution effect due to the high input of minerogenic matter. Stable carbon isotopes indicate eutrophication of the lake due to the mobilization of nutrients through soil erosion. Starting around 1870 AD, organic-rich sapropelic sediments are deposited (unit A). A decline of turbidite events and increased wind shelter from trees due to reduced land-use intensity led to meromictic conditions. Consequently, negative excursions in the
δ
13C isotope record at the onset of unit A probably reflect methanogenesis under permanently anoxic conditions.
Environmental changes of the last 9,300 years were reconstructed by geochemical and pollen analyses of a 14-m-long, laminated sediment core from Lago Aleixo, south-eastern Brazil. Fossil pollen ...assemblages indicate open savannah vegetation (campo cerrado) and gallery forests until approximately 6,900 cal. BP. During that time, siderite laminae were deposited under anoxic conditions at the lake bottom. Then, increased rainfall and a shorter annual dry period allowed gallery forests and semi-deciduous forests to expand, leading to more closed cerrado vegetation. High-intensity rainfall events during this period are recorded as peaks in K and Ti concentrations. The sediment facies during this period consists of alternating layers of diatoms and minerogenic matter. C/N ratios imply that algae and perhaps soils, too, were the main contributors to sediment organic matter. Biogenic silica and δ
13
C
org
variations indicate increasing primary productivity, which was related to higher nutrient flux from intensified leaching of soils, as shown by rising K/Al ratios. Around 800 cal. BP, a closed, semi-deciduous forest developed under present-day climate conditions. Slope stabilization diminished erosion processes in the catchment and caused reduced input of minerogenic matter into the basin. Human impact is evident in the topmost homogeneous sediments, as removal of the stabilizing forest cover amplified soil erosion. The continuous trend to more humid conditions during the Holocene probably reflects increased influence of the Amazon Basin as a moisture source. We conclude that the Lago Aleixo sediment archive was a sensitive recorder of environmental dynamics in tropical South America, which were mainly controlled by changes in precipitation patterns.
Sedimentological, geochemical and particle-size analyses were used to reconstruct the evolution of both trophic state and hypolimnetic anoxia in Lake Bourget (French Alps) during the last century. ...Radionuclide dating (
210
Pb,
137
Cs and
241
Am) confirmed the annual rhythm of laminations in the upper sediment profile. In Lake Bourget, biochemical varves are triplets composed of a diatom layer (spring lamina), a bio-precipitated calcite-rich layer (spring/summer lamina), and a layer rich in organic matter and detrital particles (winter lamina). The onset of eutrophication and the first appearance of an anoxic facies occurred simultaneously and were dated by laminae counting to AD 1943±1 year. Persistent anoxic conditions began in AD 1960. Eutrophication is characterised by drastic increases in the flux of biogenic silica (mostly diatoms), lacustrine organic matter, and larger calcite crystals (15–30 μm). The increase of organic matter also represents a marker of the onset of anoxic conditions in the hypolimnion. Our results show that eutrophication was the main factor controlling anoxia in the hypolimnion. This eutrophication was caused mostly by the inflow of untreated sewage effluents, and to a lesser extent, by input of fertilizer-derived phosphorus during floods of the Rhone River and run-off from the lake catchment. The Rhone River, however, can also be a source of re-oxygenation via underflows that originate during flood events. Oxygenation of the hypolimnion is also controlled by low winter temperatures, which enable turnover of the lake. Thus, global warming, associated with a forecasted reduction in precipitation, might reduce the efficiency of hypolimnetic re-oxygenation in Lake Bourget.
The environmental changes and the dynamics of the savanna-forest mosaic, over the last 1050 years, have been reconstructed by pollen, charcoal, radiocarbon dating mineralogical and geochemical ...analyses of sediment cores taken from three different Mauritia flexuosapalm swamps in the northernmost part of the Brazilian Amazon region (northern state of Roraima). Studies on the relationship between the modern pollen rain and the regional vegetation provide additional information for the interpretation of the fossil pollen records. The fossil pollen assemblages and geochemical results indicate relatively wet climatic conditions throughout the recorded period. Despite these moist conditions, fires were frequent and are one of the reasons for the dominance of a grassy savanna instead of forest expansion in the study area. Considering the generally wet climatic conditions, these fires were most likely caused by human activities. Even today, fires hinder forest expansion into savanna areas. Sandy hydromorphic soils may also act as an edaphic control to maintain the current sharp boundary between forest and savanna ecosystems.
Analysis of non-pollen palynomorphs supplemented by pollen analysis, microcharcoal analysis and geochemical data from laminated sediments from Lake Jaczno were used to establish different phases of ...land-use in the catchment between c.a. AD 1840 and AD 2013. The results show that during the first eighty years the vicinity of the lake was heavily deforested. During this period erosional inputs caused accumulation of abundant fungal spores, indicators of pastures and natural fertilizers (manure) as well as of corroded pollen grains and charcoal. Gradual regeneration of forest cover took place after World War II, when expansion of pioneer trees occurred (
). At the same time, a considerable increase in the lake trophy was observed, leading to the changes in phytoplankton and macrophyte communities: a decrease in the proportion of
and an increase in the
population. The non-pollen palynomorphs analyses indicate the substantial human impact that caused changing local environmental conditions, compatible with the results based on pollen analysis and geochemical data.
We reconstructed the changes in the planktonic food web of an oligotrophic subalpine lake over the past century, combining paleoecological methods and historical monitoring data. Analyses of organic ...matter δ¹⁵N from sediment cores show that nutrient enrichment started in the 1910s and intensified from the 1930s. Subsequent changes in sediment organic carbon content and accumulation rates, carotenoid pigment concentrations, organic matter δ¹³C, and cladoceran subfossil remains show that excess nutrient inputs triggered bottom-up-driven increases in lake primary production, as well as in Daphnia abundance. Daphnia size, however, started to decrease in the late 1930s, indicating an increasing size-selective prédation from zooplanktivorous whitefish populations (Coregonus lavaretus) that had been fostered by stocking and increased food availability. Whitefish predation is likely to have indirectly facilitated the establishment of Bosmina longirostris. With decreasing nutrient concentrations, Daphnia abundance decreased, but their size declined, presumably due to an ongoing size-selective predation. At this same time, Bosmina abundance doubled as a likely result of released interspecific competition from smaller Daphnia. δ¹⁵N analyses on subfossil cladoceran remains revealed that these changes in cladoceran community structure were accompanied by major changes in the food web. In spite of successful measures to reduce nutrient inputs from the late 1960s and reduced primary production, anoxia still occurs every summer at the lake bottom. These patterns are the consequence of a still relatively high export of phytoplankton-derived organic matter to the sediment as a result of strong top-down effects on the planktonic food chain.
•Combination of different dating methods provides robust and accurate chronology.•Varve microfacies structure reflects sedimentation conditions.•Rapid ecological shifts were caused mainly by ...anthropogenic pressure.•Lake water level was affected by climate change and human activity.
Sediments from Lake Lubińskie, western Poland, were investigated to determine the limnological responses to environmental changes during the last 3,000 years. To identify the mechanisms driving the eutrophication in the lake, we distinguished six lithozones along with five major varve microfacies. An age-depth model based on varve counting and radiometric measurements was established resulting in final age of 946 +113/-144 before the Common Era at the bottom of the composite profile. Varve microfacies as well as geochemical, pollen, and diatom data was used to determine major phases of landscape and lake ecosystem transformations. Our data shows that until the 4th century of the Common Era, conditions in the lake were stable. Several shifts occurred before the first millennium, when changes were observed in every proxy as a response to the increased agricultural activity in the area as well as climate fluctuations. The highest frequency of shifts was observed from the 17th century onward and mirrored further deforestation (decrease in arboreal pollen), accelerated erosion (increase in Ti), increased nutrient delivery (increased total nitrogen content), and better ventilation of the water body (higher Mn/Fe ratio). Simultaneously, the lake became more eutrophic as a response to changes in the lake catchment area. Our study suggests that inferred lake-level changes are primarily related to human activity in the catchment area.
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