Sedimentary phosphorus dynamics in response to lake trophy and mixing regime changes during the Late-Glacial, Holocene and the Anthropocene: three case studies from deep lakes in Switzerland

Phosphorus (P) released from sediments into surface waters, termed internal P loadings, has been widely recognized as a key P source contributing to delayed recovery from human-induced eutrophication in lakes where external P loadings have been reduced. It is critically important to evaluate the availability of sediment-P and its release risks into lake water by studying P fractions in sediment profiles. However, previous studies focused only on well-mixed shallow lakes and did not examine the relationships between sediment records of P fractions and lake trophic status in seasonally stratified, deep lakes. Additionally, records of P fractions spanning longer than a few decades have not been studied. Hypolimnetic waters in temperate eutrophic deep lakes tend to become anoxic during stratification periods, which can lead to high internal P loadings and sustain eutrophication. It is not yet fully understood whether, and how, lake trophic levels and hypolimnetic redox conditions can influence the long-term behavior of sedimentary P fractions and potentials of internal P loadings in deep lakes. Our research questions were: (i) How have P fractions in sediment profiles and potentials of internal P loadings varied with different lake trophic levels over the past few decades? (ii) What was the role of hypolimnetic anoxia and lake trophic state in affecting P cycling, sediment P fractions stratigraphy, and potentials of internal P loadings in the last century prior to cultural eutrophication? (iii) Is the P cycling under natural or pre-anthropogenic anoxic conditions comparable with the P cycling under anthropogenic eutrophication and anoxia in recent times? In order to address these questions, we selected three deep lakes in Switzerland: the Ponte Tresa basin of Lake Lugano, Lake Burgäschi, and Soppensee. We investigated sediment cores from these lakes to produce records of P fractions in sediments, lake trophy, and anoxia history covering the past few decades, more than one century, and the Late-glacial and Holocene periods, respectively. The Ponte Tresa basin was selected because it is one of several deep lakes in Switzerland that have not yet recovered from human-induced eutrophication after large reductions of external P loadings, and the lake’s eutrophication history since the mid-20th century is already well documented. Lake Burgäschi was selected because there were substantial changes in trophic levels and possibly lake mixing regimes during the last century, and the lake has exceptionally long historical and limnological survey data available for most of the last 50 years. We selected Soppensee because it is a deep, eutrophic lake featuring a varved sediment record, its sediments have an exceptionally good chronology, and it has a record of diatom-inferred epilimnetic total P (DI-TP) concentrations available for the entire Holocene. Hyperspectral imaging (HSI) was applied for high-resolution analysis of sedimentary pigments, combined with geochemical analyses, which allows reconstructions of lake primary production (eutrophication history) and hypolimnetic redox conditions. XRF core scanning was used to determine the elemental composition of sediments and these geochemical variables were related to in-lake and catchment processes (e.g. XRF-Mn and Fe/Mn ratios as redox proxies). In this study, different P forms in sediments were characterized by P-fractionation schemes, mainly by sequential P extraction procedures and the standards, measurements and testing (SMT) protocol. The results of P-fractionation showed that, in each lake, labile P fractions (mainly the redox sensitive Fe-bound P or Fe/Al metal oxides bound P) were the dominant P form in anoxic sediments during most of the periods studied. This phenomenon suggests high potentials of internal P loadings in the three lakes. Hypolimnetic redox conditions appear to control contents of redox-sensitive Fe and Mn in sediments, which in turn influences P retention in sediment profiles of the lakes. In the Ponte Tresa basin of Lake Lugano, we find that net burial rates of total P and the labile P fraction (mainly redox sensitive Fe-bound P) in sediments showed significant decreasing trends from 1959 to 2017, when the lake underwent higher eutrophic levels and severe anoxia. This finding suggests that, in the Ponte Tresa basin, higher eutrophication conditions increased internal P loadings, thus reducing net burial rates of P in sediments. This case study highlights the concern that eutrophication restoration might be hindered in deep, seasonally stratified lakes by extensive internal P cycling and reduced capacity of P-trapping in surface sediments. In Lake Burgäschi, the results highlight the importance of hypolimnetic redox conditions in controlling the long-term P cycling and P retention in sediments since the 1900s. We found relatively high total P and labile P fractions in Fe- and Mn enriched layers when the hypolimnion was seasonally oxygenated. The results also imply that hypolimnetic water withdrawal in Lake Burgäschi can effectively reduce P retention in sediments and potentials of internal P loadings. In Soppensee, we estimated long-term qualitative internal P loadings by comparing the Holocene record of DI-TP concentrations with the labile P fraction (Fe/Al-P) concentrations in sediments under changing trophic, redox, and lake mixing regimes. The results demonstrate that enhanced internal P loadings acted as a positive feedback to promote and maintain natural eutrophication process in Soppensee from ~9000 to 6000 cal BP. However, such a positive feedback was not inferred for other eutrophic phases. For example, ferromagnetic minerals from magnetotactic bacteria preserved in sediments from ~6000 to 2000 cal BP and Fe-rich layers formed from ~2000 to 200 cal BP appear to have prevented internal P loadings in these two periods, resulting in high labile P fraction in sediments. In summary, this project provides new insights about the influence of lake primary production and hypolimnetic redox conditions on P cycling and the record of sediment P fractions on short and long timescales in seasonally stratified, deep lakes. We conclude that, in stratified deep lakes, sedimentary total P and P fractions may not reflect lake trophic evolution and the history of external P loadings. However, comparisons of the lake trophic history and the record of sediment P fractions can shed light on in-lake P cycling in the past

The varved sediments of Lake Żabińskie, Poland as a high-resolution archive of environmental change

Human impacts to the global environment have accelerated in the past several decades resulting in global warming, pollution, modification of biogeochemical cycles and other major impacts to terrestrial and aquatic ecosystems. The diverse and growing field of paleoenvironmental reconstruction offers a long-term perspective of environmental change that is necessary to understand natural processes and what conditions were like prior to human impacts. Lake sediments are a particularly important archive of paleoenvironmental information because of their wide distribution on the continents, the potential for well-constrained chronologies, and the vast array of information that can be obtained from lake sediments. In this thesis, a 10,800-year record of environmental change is presented from Lake Żabińskie, Poland. This site is relatively unique because annual laminations (varves) are preserved over the majority of the past 10,800 years and a high sedimentation rate enables analyses at high temporal resolution. The site has been investigated extensively to understand modern processes as well as the past 2,000 years of the sedimentary record, but the full Holocene record had not been investigated until this thesis. The first case study of the thesis (Chapter 3) investigates the application of cutting-edge techniques in radiocarbon measurements for dating lake sediments. Samples of plant macrofossils containing as little as 11 μg of carbon were measured for radiocarbon using a gas-source input to the mass spectrometer. Multiple samples were taken from the same stratigraphic levels in order to assess the accuracy and precision of these miniature radiocarbon samples. Additionally, the impact of miniature samples on age-depth models was assessed using simulations of different sampling scenarios. We found no evidence of age bias from miniature radiocarbon samples, and the precision of ages was not strongly affected by the technique used (gas-source or graphitized input). Simulated sampling scenarios indicate that increasing the number of ages can compensate for increased uncertainty produced by samples with small masses. The second study (Chapter 4) focuses on reconstructing primary production and water column anoxia over the past 10,800 years. This was accomplished primarily by the use of ultra-high-resolution pigment measurements from hyperspectral imaging (HSI) core scanning, with complimentary evidence provided by elemental data from X-ray fluorescence scanning and pigment data from high-performance liquid chromatography. Reconstructions of primary productivity and anoxia were compared with local and regional climate and vegetation data to understand how these factors influenced conditions in the lake. The presence of bacteriopheopigments, a specific biomarker for water column anoxia, indicates that an extensive anoxic zone was present in the lake nearly continuously prior to approximately 2,800 cal yr BP. Several shifts in lake mixing regime occurred after that time and lead to variable water column oxygen concentrations. These shifts were linked to human modifications of forest cover, leading to the conclusion that forest cover was a primary control on lake mixing over the Holocene. Since the 17th century, intensive agricultural development and deforestation surrounding the lake caused major increases in erosional and nutrient inputs leading to major increases in primary production and biogeochemical cycling. The third study (Chapter 5) investigates relationships between meteorological conditions and the structure and composition of varves over the period 1966-2019. This period was selected because a varve chronology could be established with no uncertainty over this interval. Ultra-high resolution spatially resolved biogeochemical data was obtained using micro X-ray fluorescence and hyperspectral imaging. Sub-varve scale patterns of geochemical data were used to classify varves into four varve types. The years associated with these varve types were shown to experience differing seasonal weather conditions, providing evidence that weather affects the composition of varves at sub-varve scale. Statistical analysis showed that total C and Ti could be used to reconstruct spring and summer temperature, and mass accumulation rate and Si could be used to reconstruct the frequency of windy days. This study shows the potential of bulk geochemical data from biochemical varves for paleoclimate reconstructions. In the fourth study (Chapter 6), the relationship between spring temperatures sedimentary chloropigments was investigated, building upon previously published work that indicated chloropigments could be used as a proxy for spring temperature at Lake Żabińskie. Although a significant relationship between these two variables was found for the period 1779-2016 CE, there is no significant correlation over the period 1779-1906 CE. This result suggests that chloropigments are not a reliable temperature indicator at this site, and highlights the potential problems inherent to quantitative climate reconstructions based on the assumption that climate - proxy relationships remained stable through time. Together, these studies demonstrate the variety of insights that can be gained from high quality lake sediments records. High-resolution scanning techniques, such as micro X-ray fluorescence (μXRF) and hyperspectral imaging (HSI), applied to varved sediments represent the cutting-edge of paleolimnological research in terms of measurement resolution. The feasibility of using hyperspectral imaging on resin-embedded sediment slabs was assessed; this would enable alignment of μXRF and HSI data with the greatest possible precision. However, for Lake Żabińskie sediments, resin-embedded sediments absorbed too much light, and it was not possible to apply HSI to resin-embedded slabs. Nonetheless, these high-resolution measurements enable investigation of sub-annual processes, and make it possible to detect rapid changes that cannot be observed with conventional proxy techniques. Additionally, the speed and low-cost of these non-destructive techniques make it possible to study long records in a relatively short amount of time. Therefore, these methods should be increasingly applied to cover sites across a wide variety of environments

Using microcomputed tomography (μCT) to count varves in lake sediment sequences: Application to Lake Sagtjernet, Eastern Norway

Varved lake sediments are one of the most important natural archives that allow annual resolution paleoclimate reconstructions. Conventional varve counting techniques use thin sections to manually identify lamina. However, this technique is destructive, labour intensive and limited to a 2D representation of complex 3D features which may lead to misidentification of varve boundaries. This study presents the use of microcomputed tomography (μCT) scans in constructing varve chronologies, utilizing scanning resolutions of ∼50 μm (binned to ∼200 μm) for core sections up to 150 cm long. To evaluate this method, we cored and analysed Lake Sagtjernet in Eastern Norway — revealing a 593 cm-long sediment record of the past 10274 (+220/-329) years, with continuous laminations from 84 to 533 cm depth (75% of the sediments). Through limnological monitoring and microfacies analyses we demonstrate that the laminations are rare ferrogenic varves, with an annual deposition pattern comprised of seasonal changes in biogenic production superimposed on seasonal precipitates of iron and manganese hydroxides. The floating μCT-counted varve chronology presented here is the first non-glacial varve chronology in Norway and covers 4023 +113/-185 years. We find that μCT scans allow for a very fast and non-destructive way of counting varves with sufficient detail of varve boundaries. In the few sections where varve boundaries are too vague to resolve, we recommend using complimentary techniques such as thin sections in parallel. The varve chronology is in good agreement with the 95% confidence interval of the independent radiocarbon chronology based on 16 14C dates, and 210Pb and 137Cs activity peaks, indicating that the varve chronology can be equated to calendar age.publishedVersio

The past : a compass for future earth

Antarctic sea ice impacts on the ocean-atmosphere heat and gas fluxes, the formation of deep and intermediate waters, the nutrient distribution and primary productivity, the so-called &#8216;biological carbon pump&#8217;, one of the most active in the global ocean. In this study, we explore the link between sea ice dynamic, biological production and nutrient cycling during the late Holocene (the last 2,000 yrs) in the Adélie Basin, East Antarctica, from the well-dated sediments of the Ocean Drilling Program (ODP) Site U1357. This archive, composed from ~32 meters of seasonal to annual laminated diatomaceous sequences, allows reconstructions at an unprecedented time resolution (5-10 yrs). Our study combines records of diatom census counts and diatom-specific biomarkers (a ratio (D/T) of di- and tri-unsaturated Highly Branched Isoprenoid lipids (HBI)) as indicators of sea ice and biological production changes, XRF data as markers for terrigenous inputs and bulk nitrogen isotopes (d15N) and d15N on chlorins as proxies for reconstructing nitrogen cycle. The diatom and HBI records reveal five distinct periods. From 0 to 350 yrs AD, decreasing occurrences of sea ice-related diatom species (e.g. Fragilariopsis curta + F. cylindrus) together with low D/T values and increasing open ocean diatom species (large centrics, Chaetoceros Resting Spores (CRS)) document a progressive decline of sea ice presence during the year (>9 months per year) with spring melting occurring earlier in the year and autumn sea ice formation appearing later. In contrast, between 350 and 750 yrs AD, high production of open ocean diatom species and low low D/T values and sea ice related species indicate a short duration of sea ice cover (~10 months per year) is illustrated by a pronounced increase of sea ice-associated diatom species and high D/T values. Between ~1400 and 1850 yrs AD, seasonal sea ice strongly declines (<~7 months per year) as a result of early spring melting (increasing CRS production) and late autumn waxing (high occurrences of Thalassiosira antarctica). Longer growing seasons promoted a substantial development of phytoplankton communities (especially large centric diatoms) that conducted to lower D/T values. Consistent with diatom and HBI reconstructions, XRF data show higher Fe/Al and Zr/Al ratios values during inferred warmer periods and lower ratio values during inferred cooler and icier periods, thus supporting a strong impact of the sea ice seasonal cycle on glacial runoffs. The link between sea ice conditions, biological production and nutrient cycling is still being explored and we will discuss its relationship by combining all the cited records cited above with the d15N records that we are currently generated. Based on our results, we find that sea ice dynamic and associated diatom production in the Adélie Basin revealed an opposite climatic trend than that identified in the Northern Hemisphere for the last 2000 years. The 'Little Ice Age' (1400-1850 yrs AD) or the 'Dark Ages' (400-750 yrs AD) corresponded to warmer climate conditions in the Adélie Basin, while the 'Roman Warm Period' (0-350 yrs AD) or the 'Medieval Warm Period' (900-1200 yrs AD) were associated to colder conditions. We therefore emphasize that Northern and Southern Hemisphere climate evolved in anti-phase seesaw pattern during the late Holocene

Paleolimnology of Lake Iznik (NW Turkey) during the past ~ 31 ka cal BP

Lake Iznik, situated in the Marmara region (NW Turkey), is an alkaline lake with about 300 km2 surface area, inserted in an area of typical Mediterranean climate. During the dry summer season, carbonates are precipitating from the water column. The endogen carbonate accumulation, e.g. aragonite, is expected to hold past climate information. A detailed understanding of the limnological system is required to differentiate site specific signals and responses to climatic forcing. Geochemical and mineralogical evidence from a continuous composite profile was documented in a decadal to centennial time scale. A novel improved age model shows that the sediment record reaches up to ~ 31.5 ka cal BP. Compositional Data (CoDa) analysis allowed the identification of three groups of elements, and particular elements with distinct geochemical behavior, which indicate specific geochemical processes. Also, CoDa analysis allowed clear geochemical characterization of litho-stratigraphic units. Grain size measurements were undertaken to determine the energy levels of the physical deposition environment. Detailed grain size analysis allowed amorphous silica to be quantified. From siliciclastic grain size analysis, the ratio coarse silt/clay was identified to be a sensitive indicator to infer changes in the depth of the water column. This ratio was a calibrated to the modern depositional regime. The endogen carbonate production proved to be a sensitive climatic indicator. Changes in carbonate concentration are timely associated to the inferred fluctuations in water column depth. The aragonite concentrations are most likely related to regional temperature, catchment hydrology and the mixing dynamics of the lake. The physical mixing dynamics of the lake is reflected in (a) behavior of elements mobile under different oxic/anoxic conditions, and (b) geochemical patterns for carbonate bound elements in hardwater lakes, and (c) stability of various minerals. This study established the current knowledge of the geochemical evolution of Lake Iznik. It further adds to the understanding of paleoclimate evolution in the Marmara region on a millennial time scale. From ~31 ka cal BP until the deglaciation at ~18 ka cal BP, Lake Iznik is characterized by low productivity and higher detrital load, in association to a low carbonate accumulation. Thicker epilimnion and lower supersaturation states are inferred in association to a deeper water column. During the last glacial, i.e. from ~ 26 ka cal BP to ~18 ka cal BP, Lake Iznik passes through prolonged stages of incomplete mixing of the water column, whereas the lake level is most likely maintained during that period. In addition, the crystal structure of carbonates reflects mineral instability. At the Last Glacial Maximum (~ 22 ka cal BP) carbonate accumulation in the lake is nearly absent. During the deglaciation, starting at ~18 ka cal BP, dynamic and pronounced lake level variations occur. A shallow water column is inferred at ~ 16.5 ka calBP, and a possible low stand is identified for the period between ~ 14 and ~ 9 ka cal BP. Generally Marine Isotope Stage (MIS) 1 is marked by increased aragonite concentrations, and enhanced chemical weathering. The terrestric organic load increases gradually, and is accompanied by lake trophic conditions. Lake Iznik climate event stratigraphy highly correlates with the regional geological record. The endogen carbonate accumulation seems to occur in phase to Northern Hemisphere climate variability, for instance warm interstadials and cold stadials are depicted. In synchrony with Dansgaard-Oeschger interstadials Iznik sediments point to a deep water column, which decreases in time. After a short time lag, geochemistry reacts with increasing aragonite concentrations. In synchrony with Heinrich stadials, the Iznik sediments underwent a short phase of magnesian calcite preservation, after that generally more elevated calcite concentrations prevail. In general the cold phases are associated to a higher input of detrital calcite, likewise during the Younger Dryas cold event (~ 12 ka cal BP). The early Holocene (from ~ 12 to ~ 9 ka cal BP) is characterized by pronounced summer stratification of the water column and higher epilimnion carbonate supersaturation. Recurrent stages of good lake mixing are accompanying a shallow water column. The middle Holocene is generally more humid, as indicated by enhanced chemical weathering and by two distinct lake level increases. The first of such increases occured at ~ 9.3 ka cal BP and is related to the reconnection of the Black Sea to Mediterranean water ways. This geological event is followed by the settlement of the first farming communities in the Iznik basin. Moreover, at circa 5 ka cal BP a change occurs in delivery of grain size proportions – in relation to what is expected according to the inferred water column depth. This is most likely resulting from anthropic land-use within the basin.Der alkalische Iznik See ist in der Marmara Region (NW-Türkei), in der ein typisches mediterranes Klima vorherrscht, gelegen. Er besitzt eine Oberfläche von ca. 300 km2. Im See selbst fallen aus der Wassersäule Karbonate hauptsächlich während des Sommers aus, wobei die hydro-geochemischen Bedingungen die Karbonatkonservierung in den Sedimenten begünstigen. Es wird erwartet, dass endogene Karbonate, besonders Aragonit, Informationen über die vergangenen klimatischen Bedingungen speichern. Die geochemische und mineralogische Zusammensetzung der Sedimente wurde für ein kontinuierliches Kompositprofil untersucht. Ein neu erstelltes Altersmodell zeigt dass dieses Kompositprofil ~31.5 ka der Klimageschichte der Marmara Region aufzeichnet. Somit ergeben die Untersuchungen eine zeitliche Auflösung von zehn bis zu hundert Jahren. Die statistische Analyse der Daten (Compositional Data Analysis, CoDa) identifiziert drei geochemische Hauptgruppen aus Elementen, die hydro-geochemisch ähnlich auf Umweltveränderungen reagieren. Entsprechend konnten die fünf lithostratigraphischen Einheiten auch eindeutig geochemisch erfasst werden und lassen sich im Kontext vergangener Klimazustände interpretieren. Die siliziklastischen Korngrößenverteilungen der Sedimente deuten das Energieniveau der Ablagerungsbedingungen der Iznik Sedimente an. Besondere Einblicke hierzu liefert das Verhältnis von Grobsilt zu Ton. Der Zusammenhang zur Wassersäulentiefe wurde anhand der rezenten Sedimentation modelliert. Somit konnten die Änderungen der Seetiefe bzw. der Seespiegel für die Vergangenheit rekonstruiert werden. Die endogene Karbonatausfällung erweist sich als äußerst klimasensitiv. Änderungen in der Karbonatakkumulation zeigen Änderungen in der Mischungsdynamik innerhalb der Wassersäule an, da der vertikale Wasseraustausch sensitiv auf Klimaveränderungen reagiert, was wiederum die Karbonatausfällung im Wasser beeinflusst. Entsprechend spiegelt sich die Dynamik der Seemischung (a) im Verhalten von redox-sensitiven Elementen (oxische versus anoxische Bedingungen), (b) in geochemischen Mustern von Elementen die sich an Karbonate binden, und (c) in der geochemischen Stabilität diverser Minerale wider. Aus den abgeleiteten Veränderungen der Seespiegelschwankungen und Mischungsdynamik im Iznik-See lassen sich Aussagen über die regionale Klimaentwicklung der Marmara Region gewinnen, da das Klima letztlich das Mischverhalten des Sees maßgebend bestimmt. Somit war der Iznik See in der Zeitspanne zwischen ~31.5 bis ~18 ka cal BP nur wenig produktiv und nur wenig Karbonate sind in den Sedimenten erhalten. Entsprechend war die Sedimentation zu dieser Zeit durch terrestrischen Eintrag stark geprägt. Während des letzten Glazials (~26 bis ~18 ka cal BP) traten verschiedene Phasen auf, in denen die Wassersäule nur unvollständig durchmischte. Während dieser Zeitspanne variierte die Seetiefe kaum und Karbonate sind geochemisch nur wenig stabil. Somit fehlen Karbonate während des letzten glazialen Maximums (LGM) beinahe vollständig. Mit Beginn des finalen Rückzugs der Vereisung (~18 ka cal BP) zeigt der Iznik See ausgeprägte Seespiegelschwankungen. Tiefstände finden sich bei ~16.5 ka cal BP und zwischen ~14 und ~9 ka cal BP. Das marine Isotopenstadium 1 (MIS 1) ist im Allgemeinen durch erhöhte Aragoniterhaltung und eine höhere chemische Verwitterung charakterisiert. Der terrestrisch-organische Eintrag erhöht sich graduell und entsprechend nimmt die Trophie des Sees zu. Die Klima-Eventstratigraphie der Sedimente des Iznik Sees lässt sich gut und eindeutig in den Kontext der regionalen Klimaentwicklung der Marmara Region integrieren. So scheint die endogene Karbonatakkumulation in Phase mit der Klimavariabilität der Nordhemisphäre zu sein. Zudem lassen sich beispielsweise anhand der Karbonatphasen die warmen Interstadiale deutlich von den kälteren Stadialen unterscheiden. Dansgaard-Oeschger-Interstadiale sind in den Iznik Sedimenten durch Aragoniterhaltung gekennzeichnet. Während der Heinrich- Stadiale wird in den Sedimenten für kurze Phasen hoch Mg-Kalzit erhalten. Außerdem, sind im Allgemeinen die kälteren Phasen mit höherem Eintrag von terrestrischem Kalzit verknüpft, so auch während der Jüngeren Dryas (~12 ka cal BP). Zu Beginn des Holozäns zeigt die Sedimentgeochemie ein gutes Mischverhältnis des Sees an, was mit einer niedrigen Wassersäule einhergeht. Das mittlere Holozän ist generell feuchter, was anhand einer höheren chemischen Verwitterung und zwei ausgeprägten Seespiegelanstiegen zu erkennen ist. Der Erste dieser Anstiege ist zeitgleich mit der ‚Wieder' -Verbindung der Wasserstraße zwischen dem Schwarzen Meer und dem Mittelmeer, um ~9 ka cal BP. Im Anschluss an dieses geologische Ereignis folgte im Einzugsgebiet des Iznik Sees die menschliche Erstbesiedlung die durch Ackerbau gekennzeichnet ist. Ein zweiter ausgeprägte Seespiegelanstieg im Holozän ist bei ~6.5 ka cal BP dokumentiert. Um ~5 ka cal BP ist eine Veränderung der Verhältnisse der Korngrößen zu beobachten die auf anthropogenen Einfluss im Einzugsgebiet in Folge höherer Erosion durch Bodennutzung hindeutet.O lago Iznik, localizado na região de Marmara (NE, Turquia), possui águas alcalinas, área superficial de cerca 300 km2, e está inserido em uma área de clima tipicamente Mediterrâneo. Durante o verão, carbonatos são precipitados na coluna d'água. Espera-se que a acumulação de carbonatos endógenos, e.g. aragonita, retenha informações sobre o clima do passado. A fim de diferenciar entre sinais específicos do lago e respostas à forçantes climáticas, é necessário um entendimento em detalhe sobre o sistema limnológico. Registros geoquímicos e mineralógicos foram documentados para um perfil contínuo e composto em escala de tempo decadal a secular. Um novo modelo de idades mostra que o perfil sedimentar alcança até ~31.5 ka cal BP. O uso da análise de dados composicionais (Compositional Data – CoDa) permitiu a identificação de três grupos de elementos que reagem de maneira similar às condições hidrogeoquímcias. O uso da análise CoDa também permitiu a caracterização geoquímica das unidades lito-estratigráficas. Análises granulométricas foram realizadas a fim de determinar o nível de energia do ambiente físico deposicional. O detalhamento das análises de granulometria permitiu estimar um parâmetro que mede variações relativas da concentração de sílica amorfa. A partir da análise granulométrica siliciclástica, a razão silte coarso/argila foi identificada como indicator sensível para inferir mudanças na profundidade da coluna d'água. Esta razão foi calibrada para o regime deposicional moderno. A produção de carbonatos endógenos se mostrou um indicator sensível para variações climáticas. Mudanças na concentração de carbonatos estão associadas temporalmente com as mudanças inferidas para a profundidade da coluna d'água. As concentrações de aragonita estão associadas à temperatura regional, hidrologia da bacia e à dinâmica de mistura do lago. A dinâmica dos processos físicos de mistura lacustre se reflete (a) no comportamento de elementos móveis em diferentes condições de oxidação/anoxia, e (b) padrões geoquímicos para elementos ligados à carbonatos em lagos alcalinos, e (c) estabilidade de diversos minerais. Este estudo estabelece o conhecimento atual sobre a evolução geoquímica do lago Iznik. Além disso, este trabalho é uma contribuição para o entendimento da evolução paleoclimática na região de Marmara em escala temporal milenar. A partir de ~31 ka cal BP, até a deglaciação em ~18 ka cal BP, o lago Iznik é caracterizado por baixa produtividade e grande aporte terrígeno, em associação com baixa acumulação de carbonatos. Maior profundidade de epilímnio e menores estados de supersaturação podem ser inferidos em conjunto com uma maior profundidade geral da coluna d'água do lago. Durante o último glacial, i.e. de ~26 ka cal BP até ~18 ka cal BP, o lago Iznik passa por prolongados estágios de mistura incompleta da coluna d'água, sendo que o nível do lago provavelmente é mantido durante este período. Em adição, a estrutura cristalina dos carbonatos reflete instabilidade mineral. Durante o último máximo glacial (~22 ka cal BP) a acumulação de carbonatos no lago é praticamente ausente. Durante a deglaciação, começando em ~18 ka calBP, se inferem variações dinâmicas e pronunciadas do nível do lago. Uma queda do nível de água é inferida em ~ 16.5 ka cal BP, e uma coluna d'água rasa é identificada para o período entre ~14 e ~9 ka cal BP. Em geral, o Estágio isotópico marinho 1 (MIS 1) é marcado por um aumento nas concentrações de aragonita, e intemperismo químico elevado. O aporte orgânico terrígeno aumenta gradualmente, e é acompanhado pelas condições tróficas do lago. A estratigrafia de eventos climáticos do lago Iznik pode ser claramente correlacionada com o registro geológico regional. A acumulação de carbonatos endógenos ocorre em fase com a variabilidade climática do Hemisfério Norte, por exemplo é possivel diferenciar entre os interstadials quentes e os stadials frios. Em sincronia com os interstadials Dansgaard-Oeshger, os sedimentos do lago Iznik apontam para uma coluna d'água profunda, que descresce com o tempo. Após um curto intervalo de tempo, a geoquímica reage com aumento das concentrações de aragonita. Em sincronia com os stadials Heinrich, os sedimentos do lago Iznik passam por uma curta fase de preservação de calcita magnesiana, após a qual em geral teores mais elevados de calcita prevalecem. Em geral as fases frias estão associadas à maior aporte de calcita detrítica, da mesma maneira durante o evento Younger Dryas (~12 ka cal BP). O início do Holoceno (de ~12 até ~9 ka cal BP) é caracterizado por pronunciada estratificação da coluna d'água durante o verão e elevada supersaturação do epilímnio. Estágios recorrentes de boas condições de mistura do lago são acompanhados por uma coluna d'água rasa. O Holoceno médio é geralmente mais úmido, como indicado por um aumento do intemperismo químico e por dois aumentos marcantes do nível do lago. O primeiro destes aumentos ocorreu em ~9.3 ka cal BP, e está relacionado à reconexão da via aquática entre os mares Negro e Mediterrâneo. Este evento geológico foi seguido pelo estabelecimento das primeiras comunidades na bacia do lago Iznik. Além disso, em ~5 ka cal BP nas proporções granulométricas do aporte sedimentar se deve ao uso antrópico do solo na bacia.İznik Gölü Marmara Bölgesinde (KB Türkiye) konumlanmış, tipik olarak Akdeniz iklim kuşagi içerisinde yer alan, 300 km2'lik yüzey alanına sahip bir alkali göldür. Kuru yaz sezonu süresince su kolununda karbonat çökelmektedir. Endojenik karbonat çökelimi (örneğin, aragonit oluşumu), eski iklim hakkında bilgiler barındırır. Limnolojik sistemin iyi bir şekilde anlaşılması için, yöresel, kendine özgü sinyaller ve iklimsel etkiye olan tepkinin ayırt edilmesi gereklidir. Sürekli, birleşik bir çökel profili boyunca bin yıllık ve yüzyıllık zaman ölçeğinde jeokimyasal ve mineralojik bulgular belgelenmiştir. Yeni geliştirilmiş yaş modeli, sediment kaydının G.Ö. 31.5 bin kalibre edilmiş yıla kadar ulaştığını göstermektedir. Çökel bileşimi (CoDa) analizleri belirgin jeokimyasal süreçleri gösteren farklı jeokimyasal davranıştaki üç element gurubu ve özel elementlerin tanımlanmasını sağlamıştır. Ayrıca, CoDa analizleri litostratigrafik birimlerin anlaşılır jeokimyasal tanımlamasını sağlamıştır. Tane boyu analizleri çökelme ortamının fiziksel enerji düzeyinin saptanması için yürütülmüştür. Detaylı tane boyu analizleri amorf silika miktarının belirlenmesini sağlamıştır. Silisiklastik tane boyu analizlerinden elde edilen iri silt/kil oranınin su sütunu derinliğindeki değişimlerin anlaşılması için hassas bir gösterge oldugu saptanmistir. Bu oran güncel çökelme ortamlarına göre kalibre edilmiştir. Endojen karbonat üretiminin hassas iklim göstergesi olduğu kanıtlanmıştır. Karbonat bileşimindeki değişimler, su sütunu derinliğindeki salınımların zamana bağlı değişimleri ile ilişkilidir. Aragonit konsantrasyonu büyük olasılıkla yersel sıcaklık, havza hidrolojisi ve gölün karışım dinamiği ile ilgilidir. Gölün fiziksel karışım dinamiği; (a) oksik/anoksik şartlar altında hareketli elementlerin davranışını, ve (b) sert sulu göllerde karbonat oluşturan elementlerin jeokimyasal motiflerini, ve (c) çeşitli minerallerin duraylılığını yansıtmaktadır. Bu çalışma İznik Gölü'nün jeokimyasal evrimi ile ilgili güncel bilgilere ulaşmamızı ve ayrıca, Marmara bölgesinin bin yıl zaman ölçeğindeki eski iklimsel evriminin anlaşılmasını sağlamıştır. G.Ö. ~31 – ~18 bin yıllari arasinda, İznik Gölü düşük karbonat birikimi ile ilişkili olarak düşük üretkenlik ve yüksek detritik girdi ile karakterize edilir. Kalın epilimnion ve daha düşük doygunluk hali daha derin su kolonu ile ilişkilidir. Son buzul döneminde, örneğin G.Ö. ~26 – ~18 bin yıl, İznik Gölü uzunyarı-karışım dönemleri geçirirken, göl seviyesi muhtemelen buzul döneminin büyük bir kısmında değişmeden kalmıştır. Buna ilaveten, karbonatların kristal yapıları mineral duraysızlığını göstermektedir. Son Maksimum Buzul döneminde (G.Ö. ~22 bin yil) gölde karbonat birikimi neredeyse yoktur. G.Ö. ~18 bin yılında başlayan buzul erime döneminde, dinamik ve belirgin göl seviyesi değişimi yaşanır. Sığ su derinliği, G.Ö. ~16.5 bin yılında ortaya çıkar. G.Ö. ~14 – ~9 bin yılları arasındaki dönemde, düşük su seviyesi tespit edilmiştir. Genel olarak 1. Denizel İzotop Dönemi (MIS) hızlanan kimyasal ayrışma ve aragonit konsantrasyonu ile belirgindir. Gölün trofik şartlarına uyumlu olarak karasal organik girdi kademeli olarak artar. İznik Gölü'nün iklimsel olaylara bağlı stratigrafisi bölgesel jeoloji kayıtları ile büyük bir oranda örtüşmektedir. Endojen karbonat birikiminin Kuzey Yarım Küre iklim değişimi ile uyumlu olarak gerçekleştiği gözlenmekte, örneğin sıcak buzullararası ve soğuk buzullaşmalar tanımlanmaktadır. Dansgaard-Oeschger ısınmaları ile uyumlu olarak İznik çökelleri zaman içinde azalan derin su kolonuna işaret eder. Kısa bir zamansal gecikmeden sonra artan aragonit konsantrasyonu ile jeokimyasal tepkiyi gösterir. Heinrich soğumaları ile uyumlu olarak, İznik çökelleri kısa bir mağnezyum kalsit korunmasına maruz kalır, daha sonra genellikle daha yüksek konsantrasyonda kalsit yaygınlaşır. Genç Dryas soğuk olayında (G.Ö. ~12 bin yıl) olduğu gibi genel olarak soğuk evreler daha yüksek detritik kalsit girdisi ile bağdaştırılırlar. Erken Holosen (G.Ö. ~12 – ~9 bin yil) bariz yaz tabakalanması ve daha yüksek epilimnion karbonat doygunluğu ile temsil edilir. Tekrarlayan iyi göl karışımı evreleri sığ su kolonu ile birlikte gelişir. Orta Holosen artan kimyasal ayrışma ve iki ayrı göl seviyesi yükselimi ile belirtildiği gibi genellikle daha fazla nemlidir. Bunun gibi ilk evre yaklaşık G.Ö. ~9.3 bin yılında meydana gelir ve bu Karadeniz'in Akdeniz ile su geçişinin sağlanması ile ilgilidir. Bu jeolojik olayı, İznik havzasında ilk tarım topluluğunun yerleşmesi takip eder. Bunun dışında yaklaşık G.Ö. ~5 bin yılında, su kolonu derinliğine bağlı tane boyu oranlarında beklenen bir değişim meydana gelir. Bu değişim büyük olasılıkla havza içinde antropojenik arazi kullanımından kaynaklanmıştır

How magnetics and granulometry of continental margin sediments reflect terrestrial and marine environments of South America and West Africa

Continental margins are supplied by terrigenous clastic, as well as by biogenic marine sediments and, thus, act as natural archives for various environmental conditions. This thesis delineates sediment-distribution patterns off SE South America (20-55 deg. S) and NW Africa (14-17 deg. N) mainly based on rock-magnetic properties supplemented by clastic grain-size distributions, major-element concentrations, planktic and benthic foraminiferal assemblages, as well as stable-isotope signatures obtained from surface and down-core sediment samples. These investigations, spanning the last 19 kyr, reveal insights about prevalent climatic conditions on land (i.e., intensities of South American Monsoon System & Southern Westerly Wind Belt, or North-African Harmattan & trade winds) with impacts on major terrestrial sediment sources and transport paths. However, they also enable to draw conclusions about marine environmental conditions linked to ocean-current systems (i.e., Brazil & Malvinas Currents)

Exploring the Quaternary mercury cycle

Exploring how climate-driven processes impact biogeochemical cycles is critical for understanding how discrete components of the Earth system have interacted through time. Mercury (Hg) is a toxic metal released into the atmosphere as a result of natural processes and more recently by human activities, yet whose long-term (>10-kyr) processing remains in many ways poorly understood in the terrestrial realm. More specifically, there is a need for closer examination of which mechanisms govern the transport, accumulation, and cycling of Hg in this domain across broad temporal and spatial scales, and how long-term changes in the terrestrial Hg cycle may translate to the sedimentary record. This thesis presents three new Hg datasets from Lake Ohrid (Southeast Europe; 1360–0 ka), Lake Prespa (Southeast Europe; 92–0 ka), and Lake Bosumtwi (West Africa; 96–0 ka). Each are assessed relative to high-resolution geophysical, sedimentological and micropaleontological measurements taken from their corresponding cores, in order to better understand which mechanisms are most significant for transport, accumulation, and cycling of Hg in these environments over multiple millennia. These successions offer new and valuable perspectives on the terrestrial Hg cycle, and demonstrate that climate-driven changes in the Hg cycle can be effectively recorded by lacustrine sediments over many hundreds, even thousands of years. However, they are also separated by the extent to which different sedimentary fractions can (or cannot) explain time-varying patterns in Hg, and the extent to which different environmental processes can (or cannot) explain links between Hg cycle perturbations and millennial-scale climate changes. These observations suggest that identical stratigraphic signals are unlikely to be recorded in separate lakes subject to the same climate perturbations, processes, or phenomena, and that local-scale differences in depositional setting, lake bathymetry, Hg sources, hydrology, and/or catchment structure may cause shifts in terrestrial Hg cycling that do not always scale, at least linearly, with the severity and/or expression of the underlying climate perturbations. It is evident that a comprehensive understanding of a lake system and its history is critical in order to accurately distinguish the Hg signals that are climate-mediated, from those that may emerge as a function of local, independent processes. Nonetheless, this work highlights how there is great scope for continued development of millennial-scale Hg archives spanning a diverse range of spatiotemporal settings: archives that will permit better characterization of the processes impacting Hg cycling in lacustrine sediments on long timescales, and provide a solid foundation for exploring how this cycle has evolved throughout the Quaternary

Corrigendum to: "Variations of sedimentary Fe and Mn fractions under changing lake mixing regimes, oxygenation and land surface processes during Late-glacial and Holocene times" [Sci. Total Environ. 755 (2021) 143418].

ISSN:0048-9697ISSN:1879-102