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Influence of inherited paleotopography and water level rise on the sedimentary infill of Lake Ossa (S Cameroon) inferred by continuous color and bulk organic matter analyses
... The Rock-Eval pyrolysis has been used to study OM-rich sediments from tropical environments (e.g., Sifeddine et al. 1995;Biscara et al., 2010;Fujisaki et al., 2015). In Africa, a variety of Holocene lacustrine and palustrine environments from Niger, Cameroon, and Gabon have been investigated using Rock-Eval pyrolysis Debret et al., 2014;Mabicka Obame et al., 2014). Although this method provides valuable information regarding the sedimentary dynamics and the sediment sources in the studied lake basins, the common presence of siderite, particularly in tropical lake sediments, can alter the primary signal of the SOM Sifeddine et al., 2001;Jacob et al., 2004;Debret et al., 2014). ...
... In Africa, a variety of Holocene lacustrine and palustrine environments from Niger, Cameroon, and Gabon have been investigated using Rock-Eval pyrolysis Debret et al., 2014;Mabicka Obame et al., 2014). Although this method provides valuable information regarding the sedimentary dynamics and the sediment sources in the studied lake basins, the common presence of siderite, particularly in tropical lake sediments, can alter the primary signal of the SOM Sifeddine et al., 2001;Jacob et al., 2004;Debret et al., 2014). ...
... In summary, the sector with HI:OI > 1.25 indicates aquatic OM and biological tissues rich in lipids (Algae in Fig. 5), the sector with 1.25 > HI:OI > 0.5 indicates biogenic OM from terrestrial organic sources, such as fresh plant tissues and soil litters (Litter in Fig. 5), and the sector with HI:OI <0.5 indicates detrital soil OM from organomineral and mineral horizons (Soil in Fig. 5). These observations are in line with other published dataset (e.g., Disnar et al., 2003;Boussafir et al., 2012;Saenger et al., 2013;Sebag et al., 2013;Mabicka Obame et al., 2014;Debret et al., 2014;Sebag et al., 2016) and support the use of the Van Krevelen pseudo-diagram for the interpretation of the siderite-effect-free results from Lake Barombi sediments. ...
Originally developed for use in the petroleum industry, Rock-Eval pyrolysis is a technique commonly applied to lake sediments to infer paleoenvironmental reconstructions. The standard Rock-Eval parameters provide information on the amount of total organic and inorganic carbon (TOC and MinC, respectively), and are usually interpreted as proxies for the source (aquatic or terrestrial) of the primary production of organic matter (Hydrogen Index vs Oxygen Index). Although this method usually provides valuable evidence, the common presence of siderite in tropical lake sediments can alter the primary signal of the sedimentary organic matter (SOM). Indeed, the CO2 and CO released by the pyrolysis of siderite are integral to the calculation of the SOM-related standard Rock-Eval parameters. In this study, we analyze sediments from a core collected in the Lake Barombi (southwest Cameroon) and describe the impact of siderite on standard Rock-Eval parameters. We propose a workflow that allows standard Rock-Eval parameters to be corrected, based on the analysis of thermograms. The proposed corrections provide siderite-effect-free parameters, accurately reflecting the changes in sedimentary organic matter composition.
... Lakes in floodplains are complex open-water systems (Crisman et al., 2005). For example, paleotopography and the shape of the basin in shallow lakes can influence the infill pattern, which is clearly not captured by an average depth approximation (e.g., Debret et al., 2014;Bennett and Buck, 2016). The complex spatiotemporal distribution of sediments can vary with depth and distance to the nearest major river channel, such that completion of infill is best reported as a range (e.g., Kossoni and Giresse, 2010). ...
Shallow lakes in tropical floodplains provide significant ecosystem services that can be altered by natural and anthropogenic forces. Despite their importance, little is known about the infill patterns and timescales and the magnitude of these changes in tropical floodplain lakes. Here, we present a global meta-analysis of sediment core-derived accumulation rate data for shallow floodplain lakes in tropical lowlands to quantify the timescales of basin infill. Environmental variables (e.g., sediment accumulation rates, bathymetry, surface area) were compiled from the literature or derived from remote sensing imagery, resulting in a database (n = 76 lakes) that includes various lake morphologies. Our results show an exponential increase in sediment accumulation rates in many of these lakes over the past 50 years, which we interpret as a response to growing human populations and deforestation, particularly in topographically steep watersheds with pronounced seasonal rainfall. Over centennial periods, tropical floodplain lakes accumulate sediment faster than many other extratropical lakes. The dataset suggests that complete infill of some tropical floodplain lakes will occur in as little as a few centuries. Our findings also reveal the critical environmental and human factors that influence sediment accumulation patterns and affect ecosystem services in shallow tropical floodplain lakes. These findings have important implications for water and sediment management in low latitude watersheds, many of which are located in densely populated and/or developing nations.
... In any case, both approaches produce OI and OI RE6 values that are exceptionally high and exceed by far those of previously reported values (up to $500 mg CO 2 g À1 TOC) in studies carried out in diverse environments (Meyers and Lallier-Vergès, 1999;Li et al., 2000;Steinmann et al., 2003;Vannière et al., 2008;Debret et al., 2014;Omodeo-Salé et al., 2016). These studies have similar and coherent OI values, suggesting a specific type of OM even though their settings are different. ...
Qualitative and quantitative changes of organic and carbonate carbon in sedimentary records are frequently used to reconstruct past environments, paleoproductivity and sediment provenance. Amongst the most commonly used proxies are Total organic carbon (TOC), Mineral carbon (MinC), as well as Hydrogen (HI) and Oxygen Indices (OI) of organic matter (OM). Rock Eval pyrolysis enables the assessment of these quantitative and qualitative parameters with a single analysis. This is achieved through transient pyrolysis of the samples up to 650°C followed by combustion up to 850°C, with hydrocarbons, CO and CO2 measured during the thermal decomposition of both OM and carbonate minerals.
Carbonate minerals with low thermal cracking temperatures such as siderite (<400°C) can induce significant matrix effects, which bias the TOC, MinC and OI Rock-Eval parameters. Here we assess the applicability of End-Member Analysis (EMA) as a means to correct Rock-Eval thermograms for siderite matrix effects. For this, we performed Rock-Eval pyrolysis on sideritic sediments of Lake Towuti (Indonesia). New thermal boundaries were constrained in Rock-Eval thermograms using EMA to limit siderite matrix effects and improve TOC, MinC, and OI calculations. Our approach allowed us to: 1) evaluate the influence of siderite matrix effects on Rock-Eval thermograms; 2) properly exploit a Rock-Eval dataset to characterize the type and sources of OM in siderite-rich sediments; and 3) identify the OM behind degradation and mineralization processes. The Rock-Eval dataset revealed sediments with a substantial amount of refractory OM, especially in those where TOC is high and HI characteristic of autochthonous biomass. These results, associated to alternative indices used to assess OM preservation, suggest that refractory OM is residually enriched following strong degradation of labile compounds. Finally, relatively labile and refractory organic fractions may be consumed in the formation of siderite during this sequential process of OM mineralization.
... Based on a detailed investigation of a regional archaeological database, we present evidence that humans altered the rainforest ecosystem and left detectable traces in the sediments deposited in Lake Barombi. control of sedimentation by local forcing factors rather than regional climate, which have cast doubt on this assessment (11). Although the LHRC is contemporaneous with a marked increase in human settlements and activity in the region (7,(12)(13)(14)(15)(16), it is generally assumed that humans at that time were not yet major agents of vegetation disturbances (4,13,15,(17)(18)(19)(20). A recent study (21) suggesting that human land-use intensification associated with the southward migration of Bantu-speaking farmers across Central Africa contributed to forest destruction has, however, fueled a fierce debate in the paleoecology and archaeology communities (22,23). ...
A potential human footprint on Western Central African rainforests before the Common Era has become the focus of an ongoing controversy. Between 3,000 y ago and 2,000 y ago, regional pollen sequences indicate a replacement of mature rainforests by a forest-savannah mosaic including pioneer trees. Although some studies suggested an anthropogenic influence on this forest fragmentation, current interpretations based on pollen data attribute the "rainforest crisis" to climate change toward a drier, more seasonal climate. A rigorous test of this hypothesis, however , requires climate proxies independent of vegetation changes. Here we resolve this controversy through a continuous 10,500-y record of both vegetation and hydrological changes from Lake Barombi in Southwest Cameroon based on changes in carbon and hydrogen isotope compositions of plant waxes. δ 13 C-inferred vegetation changes confirm a prominent and abrupt appearance of C 4 plants in the Lake Barombi catchment, at 2,600 calendar years before AD 1950 (cal y BP), followed by an equally sudden return to rainforest vegetation at 2,020 cal y BP. δD values from the same plant wax compounds, however, show no simultaneous hydrological change. Based on the combination of these data with a comprehensive regional archaeological database we provide evidence that humans triggered the rainforest fragmentation 2,600 y ago. Our findings suggest that technological developments, including agricultural practices and iron metallurgy, possibly related to the large-scale Bantu expansion, significantly impacted the ecosystems before the Common Era. Western Central Africa | late Holocene | rainforest crisis | paleohydrology | human activity
... Based on a detailed investigation of a regional archaeological database, we present evidence that humans altered the rainforest ecosystem and left detectable traces in the sediments deposited in Lake Barombi. control of sedimentation by local forcing factors rather than regional climate, which have cast doubt on this assessment (11). Although the LHRC is contemporaneous with a marked increase in human settlements and activity in the region (7,(12)(13)(14)(15)(16), it is generally assumed that humans at that time were not yet major agents of vegetation disturbances (4,13,15,(17)(18)(19)(20). A recent study (21) suggesting that human land-use intensification associated with the southward migration of Bantu-speaking farmers across Central Africa contributed to forest destruction has, however, fueled a fierce debate in the paleoecology and archaeology communities (22,23). ...
Significance
Modern human societies live in strongly altered ecosystems. However, anthropogenic environmental disturbances occurred long before the industrial revolution. About 2,600 y ago, a forest–savannah mosaic replaced dense rainforests in Western Central Africa. This rainforest crisis was previously attributed either to the impact of climate change or, to a lesser extent, to the expansion of Bantu peoples through Central Africa. A 10,500-y sedimentary record from Lake Barombi, Southwest Cameroon, demonstrates that the rainforest crisis was not associated with any significant hydrological change. Based on a detailed investigation of a regional archaeological database, we present evidence that humans altered the rainforest ecosystem and left detectable traces in the sediments deposited in Lake Barombi.
... Visible spectroscopic data interpretations for sediment analyses can use the Q7/4 diagram (Debret et al. 2011(Debret et al. , 2014 and the First derivative spectra (FDS) signatures (Barranco et al. 1989;Deaton and Balsam 1991). Here, we also used the I-band index method (Rein and Sirocko 2002). ...
Lacustrine sediments contain a wide range of proxies that enable paleoenvironmental reconstructions. For instance, charcoal can be used to document past fire regime changes. In order to analyse high-temporal- and spatial-resolution records, however, it is necessary to develop fast, low-cost and high-stratigraphic-resolution methods. We developed a new paleo-fire proxy by studying a lacustrine core from the Esterel Massif, SE France, an area affected by two recent fire events, in AD 1987 and 2003. For this purpose, we searched for charcoal deposited and preserved in the lake sediments by combining a number of complementary methods, including: classic macrocharcoal tallying, scanning spectrophotometry, scanning hyperspectral imaging and high pressure liquid chromatography analyses. Macrocharcoal quantification is efficient, but time-consuming, and only provides intermediate-resolution data (cm scale). Spectrophotometry, used classically to quantify colour, is very fast, provides high-resolution data (4 mm) and is non-destructive (core preservation). Hyperspectral data have the same advantages as spectrophotometry, but offer higher spatial resolution (64-µm pixel size) and higher spectral resolution (6 nm) for core logging applications. The main result of this research is based on hyperspectral analysis at very high stratigraphic resolution using the I-band index. This index usually measures reflectance values at [660, 670 nm] corresponding to the trough in red reflectance produced by Chlorophyll a and its diagenetic products. This [660, 670 nm] reflectance trough, however, is also affected by the presence of altered organic matter and decreases with altered organic matter such as charcoal particles. Charcoal effect on the reflectance of Chlorophyll a and its diagenetic products is identified on first derivative spectra by a characteristic pattern around 675 nm, which is also in agreement with the Chlorophyll a concentrations measured by high-pressure liquid chromatography and charcoal particles. The I-band index is hence suitable for detecting burned organic matter, by quantifying the dilution of the chlorophyll signal by the charcoal signal. Thus, this adaptation of the I-band index can be applied in fire reconstruction studies.
... This complementary data acquisition enables determination of various standardized parameters, such as Total Organic Carbon (TOC) and Mineral Carbon (Cmin) contents, thermal maturity (Tmax or TpS2), and Hydrogen (HI) and Oxygen (OI) Indices correlated to elemental H/C and O/C values, respectively (Tissot and Welte, 1984). Initially designed for petroleum applications, these parameters were used to study mature and immature OM found in sedimentary rocks (Tyson, 1995;Disnar, 1994), but other works have demonstrated that they can also be used to characterize the organic fraction of recent sediments and peats (e.g., Sifeddine et al., 1995;Patience et al., 1996;Di Giovanni et al., 1998;Meyers and Lallier-Vergès, 1999;Marchand et al., 2003;Sanei et al., 2005;Debret et al., 2014;Hare et al., 2014). The use of Rock-Eval pyrolysis has also been performed in soil and sediment contamination studies (Lafargue et al., 1998;Poot et al., 2014) and to analytically survey profiles from various ecosystems (Disnar et al., 2003). ...
... This complementary data acquisition enables determination of various standardized parameters, such as Total Organic Carbon (TOC) and Mineral Carbon (Cmin) contents, thermal maturity (Tmax or TpS2), and Hydrogen (HI) and Oxygen (OI) Indices correlated to elemental H/C and O/C values, respectively (Tissot and Welte, 1984). Initially designed for petroleum applications, these parameters were used to study mature and immature OM found in sedimentary rocks (Tyson, 1995;Disnar, 1994), but other works have demonstrated that they can also be used to characterize the organic fraction of recent sediments and peats (e.g., Sifeddine et al., 1995;Patience et al., 1996;Di Giovanni et al., 1998;Meyers and Lallier-Vergès, 1999;Marchand et al., 2003;Sanei et al., 2005;Debret et al., 2014;Hare et al., 2014). The use of Rock-Eval pyrolysis has also been performed in soil and sediment contamination studies ( Lafargue et al., 1998;Poot et al., 2014) and to analytically survey profiles from various ecosystems ( Disnar et al., 2003). ...
Application of Rock-Eval pyrolysis to soil organic matter (SOM) quantitation and characterization has been explored by the study of about 100 soil samples taken from a variety of soil profiles from different ecosystems at different latitudes. A straightforward illustration of these possibilities can be obtained from a Hydrogen Index (HI in mg hydrocarbons g−1 TOC) vs. Total Organic Carbon (TOC) diagram that effectively allows one to follow simultaneously the main qualitative (SOM hydrogen richness given by HI values) and quantitative (TOC) changes that affect SOM with increasing depth and humification, in the soil profiles. In addition, abnormally high Oxygen Index (OI in mg CO, CO2 or O2 g−1 TOC) values are fully diagnostic of extensive SOM alteration, as frequently observed in podzol B horizons. More detailed information on the heterogeneity of SOM and on its degree of evolution, can be gained from the shape of the pyrolysis S2 peak recorded in the course of programmed pyrolysis in an inert atmosphere (N2) and/or from its maximum temperature "Tpeak". All these parameters and others, all determined rapidly and automatically, are particularly useful to screen major SOM variations within large sets of samples
The vast tropical rainforests of West and Central Africa were impacted by marked climatic deterioration, which mainly developed between 2500 and 2000 cal yr BP. However, the incidence of human intervention in these rainforests appears all the more speculative, although this was the period when the first stages of Bantu migration occurred towards the south. On the scale of the last millennium, which has seen increasing penetration and colonization of these Bantu peoples, one can envisage a priori a more accentuated anthropogenic pressure on the forest landscape.
The combined observations of the environmental processes during the last 1000 years, as preserved in sediments from lakes, swamps, flooded forests and even oceanic environments, are presented on the basis of 24 records. The dominant number of sites in the humid forest or wooded savannas recorded only processes of relatively small size and local effects influencing vegetation cover almost simultaneously around 1000 and 500 cal yr BP; nothing is comparable with the devastating effect on a very large scale of the 2600 cal yr BP forest crisis. Conversely, other sites located in more perennial forest or savanna did not reveal any discontinuity. In all the examples of sedimentary archives considered, no out-of-phase or unusual event, possibly linked to human population and its impact, was observed. However, in more recent times, in intensive iron-ore mining/smelting areas, changes in the vegetal cover could be a legacy of charcoal production impacting forest biomass. Nevertheless, despite the clear cultural and socio-political importance of iron, its role in shaping vegetation communities in the Central African forest is generally presumed to be negligible.
The oceanic causes of these climatic oscillations are more widely accepted. On a larger scale, the Medieval Warm Period and Little Ice Age are climatic anomalies visible in a number of palaeoenvironmental records worldwide and date to ~1600–1000 cal yr BP and ~ 700–100 cal yr BP, respectively. These events are less visible or sometimes not present in some palynological records of tropical Africa, whereas reconstructions from lacustrine records in Atlantic equatorial Africa show overall reductions in lake levels and increases in pollen belonging to light-demanding and pioneer vegetation formations in relation to these two oscillations.
To reply to what was more important «the potential influence of human activities on regional climate, or the vulnerability of societies to environmental variability», it appears clearly to date, that the second paradigm is most likely. However, today, the ongoing combined efffects of climate change and human impact, including the decimation of the diversity of seed dispersers necessary to regenerate the rainforests of West and Central Africa, may have irreparable and unforeseen consequences.
Soil organic matter (OM) is a complex heterogeneous mixture: resulting from decomposition and organo-mineral interactions, it challenges characterization in terms of composition and biogeochemical stability. From this perspective, the Rock-Eval® method is a rapid and efficient thermal analysis, which combines quantitative and qualitative information on soil OM, including several parameters related to thermal stability. This approach has already been used to monitor changes in OM properties at landscape, cropland, and profile scales. This study aims to assess the stability of soil organic matter pools by characterizing grain-size fractions from forest litters and topsoils using Rock-Eval® thermal analyses. Organic and topsoil samples were selected from a beech forest located in Normandy (France), whose management has been documented for the last 200 years. Fractionation by wet sieving was used to separate large debris (>2000 μm), coarse (200–2000 μm), and fine particulate organic matter (50–200 μm) in organic samples, and coarse (200–2000 μm), medium (50–200 μm), and fine (<50 μm) fractions in topsoils. Rock-Eval® was able to provide thermal parameters sensitive enough to study fine-scale soil processes. In organic layers, quantitative and qualitative changes are all explained by progressive decomposition of labile organic compounds from plant debris to the finest organic particles. On the other hand, the grain size fractions of the topsoil display different characteristics: indeed, the coarse organo-mineral fractions show high C contents, but with a different composition and a higher thermal stability and degree of decomposition than the plant debris forming the organic layers. These results are consistent with previous studies concluding that the microbial activity is more effective in this fraction. The finest fractions of topsoil reveal low C contents and the highest thermal stability, but also a low degree of decomposition, which can be explained by stronger interactions with the mineral matrix. Therefore, it is suggested that the dynamics of OM present in the different size fractions be interpreted in the light of a plant-microbes-soil continuum. Finally, three distinct thermal stability C pools are highlighted through the grain-size heterogeneity of soil OM: free-coarse organic matter (large debris, coarse and fine particles), weakly-protected organic matter in (bio)aggregates (coarse fraction of topsoil), and stabilized organic matter in fine fractions of topsoil, the latter resulting from interactions inside organo-mineral complexes. These results allow Rock-Eval® thermal parameters to be used in order to empirically illustrate the conceptual models emphasizing the role of drivers played by the gradual decomposition and protection of the most thermally labile organic constituents.
Ferruginous conditions were a prominent feature of the oceans throughout the Precambrian Eons and thus throughout much of Earth’s history. Organic matter mineralization and diagenesis within the ferruginous sediments that deposited from Earth’s early oceans likely played a key role in global biogeochemical cycling. Knowledge of organic matter mineralization in ferruginous sediments, however, remains almost entirely conceptual, as modern analogue environments are extremely rare and largely unstudied, to date. Lake Towuti on the island of Sulawesi, Indonesia is such an analogue environment and the purpose of this PhD project was to investigate the rates and pathways of organic matter mineralization in its ferruginous sediments. Lake Towuti is the largest tectonic lake in Southeast Asia and is hosted in the mafic and ultramafic rocks of the East Sulawesi Ophiolite. It has a maximum water depth of 203 m and is weakly thermally stratified. A well-oygenated surface layer extends to 70 m depth, while waters below 130 m are persistently anoxic. Intensive weathering of the ultramafic catchment feeds the lake with large amounts of iron(oxy)hydroxides while the runoff contains only little sulfate, leading to sulfate-poor (< 20 µM) lake water and anoxic ferruginous conditions below 130 m. Such conditions are analogous to the ferruginous water columns that persisted throughout much of the Archean and Proterozoic eons. Short (< 35 cm) sediment cores were collected from different water depths corresponding to different bottom water redox conditions. Also, a drilling campaign of the International Continental Scientific Drilling Program (ICDP) retrieved a 114 m long sediment core dedicated for geomicrobiological investigations from a water depth of 153 m, well below the depth of oxygen penetration at the time of sampling. Samples collected from these sediment cores form the fundament of this thesis and were used to perform a suite of biogeochemical and microbiological analyses. Geomirobiological investigations depend on uncontaminated samples. However, exploration of subsurface environments relies on drilling, which requires the use of a drilling fluid. Drilling fluid infiltration during drilling can not be avoided. Thus, in order to trace contamination of the sediment core and to identify uncontaminated samples for further analyses a simple and inexpensive technique for assessing contamination during drilling operations was developed and applied during the ICDP drilling campaign. This approach uses an aqeous fluorescent pigment dispersion commonly used in the paint industry as a particulate tracer. It has the same physical properties as conventionally used particulate tracers. However, the price is nearly four orders of magnitude lower solving the main problem of particulate tracer approaches. The approach requires only a minimum of equipment and allows for a rapid contamination assessment potentially even directly on site, while the senstitivity is in the range of already established approaches. Contaminated samples in the drill core were identified and not included for further geomicrobiological investigations. Biogeochemical analyses of short sediment cores showed that Lake Towutis sediments are strongly depleted in electron acceptors commonly used in microbial organic matter mineralization (i.e. oxygen, nitrate, sulfate). Still, the sediments harbor high microbial cell densities, which are a function of redox conditions of Lake Towuti’s bottom water. In shallow water depths bottom water oxygenation leads to a higher input of labile organic matter and electron acceptors like sulfate and iron, which promotes a higher microbial abundance. Microbial analyses showed that a versatile microbial community with a potential to perform metabolisms related to iron and sulfate reduction, fermentation as well as methanogenesis inhabits Lake Towuti’s surface sediments. Biogeochemical investigations of the upper 12 m of the 114 m sediment core showed that Lake Towuti’s sediment is extremely rich in iron with total concentrations up to 2500 µmol cm-3 (20 wt. %), which makes it the natural sedimentary environment with the highest total iron concentrations studied to date. In the complete or near absence of oxygen, nitrate and sulfate, organic matter mineralization in ferruginous sediments would be expected to proceed anaerobically via the energetically most favorable terminal electron acceptors available - in this case ferric iron. Astonishingly, however, methanogenesis is the dominant (>85 %) organic matter mineralization process in Lake Towuti’s sediment. Reactive ferric iron known to be available for microbial iron reduction is highly abundant throughout the upper 12 m and thus remained stable for at least 60.000 years. The produced methane is not oxidized anaerobically and diffuses out of the sediment into the water column. The proclivity towards methanogenesis, in these very iron-rich modern sediments, implies that methanogenesis may have played a more important role in organic matter mineralization thoughout the Precambrian than previously thought and thus could have been a key contributor to Earth’s early climate dynamics. Over the whole sequence of the 114 m long sediment core siderites were identified and characterized using high-resolution microscopic and spectroscopic imaging together with microchemical and geochemical analyses. The data show early diagenetic growth of siderite crystals as a response to sedimentary organic matter mineralization. Microchemical zoning was identified in all siderite crystals. Siderite thus likely forms during diagenesis through growth on primary existing phases and the mineralogical and chemical features of these siderites are a function of changes in redox conditions of the pore water and sediment over time. Identification of microchemical zoning in ancient siderites deposited in the Precambrian may thus also be used to infer siderite growth histories in ancient sedimentary rocks including sedimentary iron formations.
Dans le bassin versant du lac Ossa, situé dans la zone littorale du Cameroun, nous avons mis à jour deux sites d'occupation humaine : des fragments de poteries et des scories de réduction du fer sont associés à des charbons de bois datés entre 1290 et 1647 ans corrigés en A.D. (Résumé d'auteur)
Successful petroleum exploration relies on detailed analysis of the petroleum system in a given area. Identification of potential source rocks, their maturity and kinetic parameters, and their regional distribution are best accomplished by rapid screening of rock samples (cores and/or cuttings) using the Rock-Eval apparatus. The technique has been routinely used for about fifteen years and has become a standard tool for hydrocarbon exploration. This paper describes how the new functions of the latest version of Rock-Eval (Rock-Eval 6) have expanded applications of the method in petroleum geoscience. Examples of new applications are illustrated for source rock characterization, reservoir geochemistry, and environmental studies, including quantification and typing of hydrocarbons in contaminated soils.
In this study we report on accelerator mass spectrometry (AMS) wiggle-match dating of selected macrofossils from organic deposits ca. 800 cal bc (ca. 2650 bp). Based on paleological, archaeological and geological evidence, we found that the sharp rise of atmospheric 14C between 850 and 760 cal bc corresponds to the following related phenomena:
1. In European raised bog deposits, the changing spectrum of peat forming mosses and a sharp decline in decomposition of the peat indicate a sudden change from relatively dry and warm to cool, moist climatic conditions.2. As a consequence of climate change, there was a fast and considerable rise of the groundwater table so that peat growth started in areas that were already marginal from a hydrological point of view.3. The rise of the groundwater table in low-lying areas of the Netherlands resulted in the abandonment of settlement sites.4. The contemporaneous earliest human colonization of newly emerged salt marshes in the northern Netherlands (after loss of cultivated land) may have been related to thermal contraction of ocean water, causing a temporary stagnation in the relative sea-level rise.Furthermore, there is evidence for synchronous climatic change in Europe and on other continents (climatic teleconnections on both hemispheres) ca. 2650 bp. We discuss reduced solar activity and the related increase of cosmic rays as a cause for the observed climatological phenomena and the contemporaneous rise in the 14C-content of the atmosphere. Cosmic rays may have been a factor in the formation of clouds and precipitation, and in that way changes in solar wind were amplified and the effects induced abrupt climate change.
The late Quaternary sedimentary architecture of the Guadiana estuary (southwestern Iberian Peninsula), a narrow, bedrock-controlled estuary with moderate sediment supply, was studied by applying concepts of high-resolution seismic stratigraphy. The estuarine sedimentary infill consists of a discontinuous basal interval overlain by five seismic units bounded by laterally continuous seismic horizons. The correlation between seismic facies and a stratigraphic section of the Guadiana valley enables the proposal of a detailed sequence stratigraphic interpretation of the estuarine infill.
During the last glacial lowstand, the Guadiana was a subaerial valley with no significant accumulation of fluvial deposits because of increased sediment bypass towards the present-day middle and outer shelf. Towards the end of the postglacial transgression, the subaerial valley was transformed into an estuary, and sediments began preferentially accumulating in structural depressions. Furthermore, flood tidal currents constrained by basement highs enhanced sand deposition in the upper part of the estuary. Wave influence was reduced and confined to the lower estuarine system. Here, the narrow morphology of the valley led to an increased sediment export to the shelf during the Holocene highstand period.
The lower part of the estuarine infill consists of four fifth-order depositional sequences, composed of regressive deposits (HST). The last glacial maximum is recorded by a distinct stratigraphic surface, representing simultaneously the sequence boundary and the transgressive surface. A tidal ravinement surface is characterized by strong erosion and channel formation in the outer estuarine zones. The maximum flooding surface is identified by change of stratal patterns between landward-prograding transgressive deposits and downlapping highstand deposits. Both transgressive (TST) and regressive tracts (HST) were deposited during the final part of the postglacial transgression and subsequent highstand.
Reflectance spectrophotometry potentially provides a rapid method of investigating the changing characteristics and com-position of marine sediments. Recognizing this, the Ocean Drilling Program has permanently deployed a commercially avail-able, handheld spectrophotometer, the Minolta CM-2002, aboard the JOIDES Resolution. The present study evaluated shipboard spectral data obtained from Amazon Fan cores during Leg 155 using the Minolta instrument. These data were com-pared to spectra measured from comparable Leg 155 core samples using a shore-based, research-grade, Perkin-Elmer Lambda 6 spectrophotometer. The spectral signal is muted in the percent-reflectance curves from the wet sediments analyzed aboard ship when compared to percent-reflectance curves generated from dry core samples on shore. However, when both sets of reflectance curves are processed using a first-derivative transformation, shipboard and shore-based analyses are quite similar and suggest that useful, accurate spectral data can be obtained from wet core sediments at sea. This observation is further sup-ported by factor analysis of parallel (shipboard vs. shore-based) data sets (400–700 nm) produced by the two instruments. The same four factors are present in both data sets, but do not necessarily explain a similar amount of variance. These factors are related to hematite plus goethite, clay minerals, organic matter, and carbonate content. The shore-based spectral data set was subjected to further factor analysis to determine if additional compositional informa-tion could be extracted using the increased accuracy and extended wavelength range (250–850 nm) of the laboratory instru-ment. This analysis produced one additional factor, probably chlorite; separated hematite and goethite into two factors; and clarified the nature of the clay mineral factor as probably a combination of illite and montmorillonite. By calculating factor scores, semiquantitative estimates of concentrations of variations of these sediment components were calculated downhole for several Leg 155 sites. The Amazon Fan sediments represent a rigorous test of the spectral technique because most of the sedi-ments are dark and show little variation in the visible region of the electromagnetic spectrum. Nevertheless, it was possible to extract compositional information from the spectra. Our investigations also lead us to make the following important recommendations for using the Minolta CM-2002 spectro-photometer at sea. First, if wet core surfaces must be covered with plastic film during spectral measurements to protect the instrument, only Glad Cling Wrap (a brand of clear plastic food wrap) should be used because this brand does not significantly distort the spectra of the sediments. Second, when calibrating the Minolta instrument, the white calibration cap should not be covered by the clear plastic wrap used to cover the cores. Third, the Minolta instrument should be set to exclude the specular component (SCE setting) for all measurements, and the optional granular-materials cover for the instrument (CM-A40) should not be used. These recommendations will ensure that the spectral data can be compared directly to data generated by labora-tory-grade spectrophotometers.
Visible reflectance spectroscopy (VRS) has been used to reconstruct lake sediment chlorophyll a concentrations. Despite good concordance between inferred and measured chlorophyll a values, questions remain as to whether this spectral technique is tracking past changes in aquatic primary production, or
simply recording a diagenetic signal. In this study, we critically evaluate how well VRS chlorophyll a determinations track past trends in aquatic primary production using sediment cores from several lake systems with well-known
trophic histories. Our study sites include Arctic, boreal and prairie lakes that encompass a gradient of trophic states. In
general, our spectrally inferred chlorophyll a values tracked past trends in lake trophic status consistent with historical measurements of production, or as inferred by
independent proxies of primary production. We conclude that VRS chlorophyll a inferences indeed track histories of lake production trends and that this method is widely applicable as a rapid, inexpensive
and non-destructive alternative to wet-chemical analyses of sediment chlorophyll a concentrations.
Chlorophyll a preserved in lake sediments reflects, in part, past primary production. This study assesses the spectral properties of sedimentary
chlorophyll a using visible-near infrared reflectance (VNIR) spectroscopy, with the objective of establishing a new, non-destructive paleolimnological
proxy. Reflectance spectra were determined from a dilution series (n = 10) involving incremental additions of pulverized modern algae to a lake sediment matrix of low organic content. This enabled
an assessment of the development of sediment reflectance spectra in relation to different sediment chlorophyll a concentrations, and subsequent regression of spectral features against measured concentrations of chlorophyll a and derivatives obtained by high performance liquid chromatography (HPLC). The experiment demonstrates that ubiquitous troughs
in sediment reflectance near 675nm are attributable to chlorophyll a and derivative compounds. A significant correlation (r
2 = 0.98, P < 0.01) was obtained between the area of the reflectance trough in the 650–700nm interval and summed concentrations of chlorophyll
a, all derivative isomers, and degradational pheopigments. A simple linear inference model derived from this experiment was
applied to a down-core sequence of VNIR spectra from a productive prairie lake (Alberta, Canada), where it produced inferred
sediment chlorophyll a concentrations in concordance with HPLC measurements. Although a larger training set is desirable to further refine the inference
model, the analyses reported here demonstrate that reflectance spectroscopy provides a rapid, semi-quantitative method for
assessing the chlorophyll a content of lake sediments.
The Holocene environmental evolution of the shallow Ossa lacustrine complex in Cameroon is analysed on the basis of the present-day sedimentary dynamics and, principally, by the study of five well-dated cores. Sediment accumulation rates, lithofacies and mineral content changes and various tracers of flux provenance are considered in connection with the pollen data. At the shallower sites (1–2-m-water depth during the dry season), a moderate rise in lake level may have induced the transition from hydromorphic marshy conditions (gleys) to lacustrine conditions (muds) ca. 8000 cal BP at the easternmost site. A moderate fall may have generated the opposite process, ca. 7000 cal BP in the near northern part. A marked decrease in sediment accumulation rate between 5000 and 3000 cal BP is apparent in some cores, coinciding with an increasing discontinuity of the particulate supply. During this interval, the weakening of monsoon rainfall is associated to an increasingly seasonal regime characterized by lower and sporadic fluxes. This decrease corresponds to a northern shift of “equatorial” climatic conditions with the main dry season occurring during the boreal summer. During the time interval between ca. 2800 and 2000 cal BP, the return of “tropical” climatic conditions induced the opening of the vegetation associated with more intensive erosion. Increasing Cyperaceae may indicate flooding of areas suitable for colonisation until 2800 cal BP. The first overflow of the Sanaga River into Lake Ossa occurred after ca. 5800 cal BP with a NE–SW depositional axis of coarse material. Transport of suspended matter in the surface waters northward was probably induced by prevailing SW winds.The timing of this climatic evolution is independently supported by other lake sediment records from the forest domain of West Africa and can be compared with other cyclical phenomena, such as Bond and Dansgaard-Oeschger, as well as solar oscillations.
Oxygen-isotope ratios of precipitation (δ18O(p)) inferred from deep- lake ostracods from the Ammersee (southern Germany) provide a climate record with decadal resolution. The record in detail shows many of the rapid climate shifts seen in central Greenland ice cores between 15,000 and 5000 years before the present (B.P.). Negative excursions in the estimated δ18O(p) from both of these records likely reflect short weakenings of the thermohaline circulation caused by episodic discharges of continental freshwater into the North Atlantic. Deviating millennial-scale trends, however, indicate that climate gradients between Europe and Greenland changed systematically, reflecting a gradual rearrangement of North Atlantic circulation during deglaciation.
Résumé.
Cet article présente une compilation des données publiées dans la littérature et des premières observations de terrain relatives
au lac Ossa, lac peu profond mais étendu, localisé à 'ouest d'Édéa dans le département de la Sanaga maritime. Cette présentation de
I'écosystème (climat, végétation, géologie, influence de l'anthropisation, paléoenvironnement récent) a pour but d'établir la base de
donnée nécessaire à l'interprétation des travaux de recherche menés dans le cadre du programme interdisciphaire ÉCOFIT et dont
l'objectif est la reconstitution des variations de l'environnement forestier au cours des demiers millénaires. Mots clés :Littoral camerounais, lac Ossa, écosystème actuel.
Abstract
This paper is a compilation of the literature search and recent field observations about lake Ossa (westem Cameroon, coastal Sanaga department), a shallow and extended bodywater, for the purpose of presenting an up to date review of the ecosystem (climate, vegetation, geology, human settlement, paleoenvironment), base of comparison for future research relative to recent quaternary palaenvironmental evolution, developed through the interdisciplinary program ECOFIT.
Key words : Littoral Cameroon, lake Ossa, present-day environment.
Petrographic (palynofacies) and geochemical (Rock Eval pyrolysis) studies of sedimentary organic matter of a core from a crater lake in Madagascar show a peaty sedimentation before 36 ky and between ca 28 and 15 ky, and between 6 and 0 ky. Between 36 and 28 ky, the organic sedimentation is characterized by alternate phytoplanktonic sedimentation and migrations of vegetation from the basin slope to the lake itself. Contributions of oxidized allochthonous organic matter also prove the presence of vegetation on the surrounding basin and soil leaching. The variation of the ecosystems and their development reveal the climatic changes during the last 36 kyrs, in terms of arid and humid periods. -English summary
Production and Accumulation of Organic Matter: A Geological Perspective.- Production and Accumulation of Organic Matter The Organic Carbon Cycle.- Evolution of the Biosphere.- Biological Productivity of Modern Aquatic Environments.- Chemical Composition of the Biomass: Bacteria, Phytoplankton, Zooplankton, Higher Plants.- Sedimentary Processes and the Accumulation of Organic Matter.- The Fate of Organic Matter in Sedimentary Basins: Generation of Oil and Gas.- Diagenesis, Catagenesis and Metagenesis of Organic Matter.- Early Transformation of Organic Matter: The Diagenetic Pathway from Organisms to Geochemical Fossils and Kerogen.- Geochemical Fossils and Their Significance in Petroleum Formation.- Kerogen: Composition and Classification.- From Kerogen to Petroleum.- Formation of Gas.- Formation of Petroleum in Relation to Geological Processes. Timing of Oil and Gas Generation.- Coal and its Relation to Oil and Gas.- Oil Shales: A Kerogen-Rich Sediment with Potential Economic Value.- The Migration and Accumulation of Oil and Gas.- An Introduction to Migration and Accumulation of Oil and Gas.- Physicochemical Aspects of Primary Migration.- Geological and Geochemical Aspects of Primary Migration.- Secondary Migration and Accumulation.- Reservoir Rocks and Traps, the Sites of Oil and Gas Pools.- The Composition and Classification of Crude Oils and the Influence of Geological Factors.- Composition of Crude Oils.- Classification of Crude Oils.- Geochemical Fossils in Crude Oils and Sediments as Indicators of Depositional Environment and Geological History.- Geological Control of Petroleum Type.- Petroleum Alteration.- Heavy Oils and Tar Sands.- Oil and Gas Exploration: Application of the Principles of Petroleum Generation and Migration.- Identification of Source Rocks.- Oil and Source Rock Correlation.- Locating Petroleum Prospects: Application of Principle of Petroleum Generation and Migration - Geological Modeling.- Geochemical Modeling: A Quantitative Approach to the Evaluation of Oil and Gas Prospects.- Habitat of Petroleum.- The Distribution of World Oil and Gas Reserves and Geological-Geochemical Implications.
The Rock-Eval pyrolysis method was designed to meet the needs of petroleum prospection and has now gained widespread use. It quickly provides different data on the organic content of rocks, such as the petroleum potential of series encountered, the nature of kerogens and their state of maturity. The vertical representation of results in the form of geochemical logs leads to an interpretation that is both effective and practical. Reference charts and diagrams are used to characterize source rocks (petroleum potentials, types of organic matter, degree of evolution, weathering, etc.) as well as migration phenomena. The compiling of geochemical maps on a basin-wide scale then becomes possible. Likewise, the method has left the field of petroleum prospection per se and found applications in the analyzing of coals, bituminous rocks, recent sediments and even techniques of the refining and secondary recovery of crude oils.-from English summary
Sediment samples collected along a 6-m core, drilled in the deepest part of the Lagoa do Caçó (NE Brazil), have been investigated in order to determine source(s) and degradation conditions of the organic matter (OM) with special emphasis on paleoenvironmental implications. Bulk organic geochemistry (Rock-Eval pyrolysis, C/N determination, δ13C and δ15N measurement) and petrography combined with sedimentological evidence and radiocarbon dates allowed to identify four major intervals documenting major environmental changes that occurred during the last 20,000 years. The first interval, dating back to the end of the Last Glacial Maximum (LGM), contains well-preserved OM derived from higher plants. This material was most probably produced in an ephemeral palustrine system and rapidly buried by sands. This level is thought to have been deposited under relatively arid climate conditions associated with strong but episodic rainfalls. Between 19,240 and 17,250 Cal years BP, the climate appears to have been more humid and seasonality more pronounced as suggested by the presence of a permanent lake. After a drastic environmental change dating back to 17,250 Cal years BP, the sediment became truly lacustrine with restricted mineral input and highly degraded higher plant-derived organic matter. After that, a stepwise improvement in the preservation of OM occurred, as revealed by several pronounced shifts in the Rock-Eval TpS2 signal. These changes could document abrupt climatically driven changes during the Late Glacial. Finally, around 5610 Cal years BP, environmental conditions, approaching those prevailing today were established. Minor climatic changes during the Holocene were probably buffered by a high water table which might explain the lack of paleoenvironmental fluctuations.
Biogenic silica profiles in varved sediments from northern Lake Malawi (Nyasa), East Africa, span the past 700 yr and reflect past primary productivity in the overlying waters. On a centennial scale this has been influenced by lake level and a consequent shift in the location of high diatom productivity within the lake basin. Primary production was higher during the Little Ice Age, an arid period from about A.D. 1570 to 1850, when lake level was about 120 m lower than during the previous three centuries or the past 150 yr.
The Balazuc 1 borehole drilled in the frame of the “Géologie Profonde de la France” programme and the work carried out by various specialists provided the samples and the independent paleotemperature assessment data both needed to check a model aiming to the straightforward determination of maximum paleotemperatures of burial (MPTB) from Rock-Eval® pyrolysis data. Basically, the model supposes that laboratory (e.g., Rock-Eval®) pyrolysis resumes the successive elimination of kerogen moieties of increasing thermal stability where it had stopped during burial diagenesis. Then, the temperature Tmin determined at the onset of the S2 Rock-Eval® pyrolysis peak — supposed to correspond to a peculiar kerogen moiety which was just undergoing cracking when maximum burial depth was reached — might be used to calculate the corresponding MPTB, taking into account the different values of the experimental and natural thermal gradients. The good agreement between calculated MPTB-values and results of fission-track and fluid-inclusion studies carried out on Balazuc 1 borehole samples supports the validity of the model. According to fundamental considerations the application of this approach is limited to samples having at least reached the onset of the metagenetic stage. Other possible limitations and the causes of uncertainty on the obtained results are discussed. A detailed operating procedure is presented.
Architecture of late Quaternary incised-valley fills is commonly attributed to the interplay between sea-level rise, sediment supply, and hydrodynamic processes. Inundation of fluvial terraces is commonly overlooked as an autocyclic mechanism for formation of parasequences. If the rate of sea-level rise and sediment supply is constant, architecture of terraced incised-valley fills will likely show backstepping parasequences. The control that variable antecedent topography has on architecture of incised-valley fills is examined in the Trinity incised valley, Texas. The Trinity valley is characterized by a series of downward-stepping terraces, and the Galveston Estuary formed above this irregular antecedent topography. Flooding surfaces, recognized in core by a decrease in sedimentation rates and a change from delta-plain to central-basin facies, formed at similar to -14 m, 8,200 cal. yr BP and similar to -10 m, 7,700 cal. yr BP, matching depths of the relatively flat fluvial terraces. Flooding surfaces formed rapidly and represent entire reorganization of the estuarine complex. Across the -10 m flooding surface, the river mouth and bay-head delta shifted landward at a rate of similar to 6.5 km per century and the associated barrier shoreline was stranded on the inner continental shelf, forming Heald Bank. Flooding surfaces formed as the rate of sea-level rise was decreasing, and are not associated with a decrease in sediment delivery to the estuary. As sea level inundates relatively flat fluvial terraces, rates of transgression rapidly increase, resulting in a sudden increase in accommodation space and an associated landward shift in coastal facies. Backstepping parasequences are inherent to the architecture of terraced incised-valley fills.
The response of coastal systems to allogenic forcing factors is of interest to diverse research communities, including those interested in global change, sequence stratigraphy and modelling. Quaternary systems are of particular interest because they provide analogues for ancient rock records. To understand the processes responsible for the sedimentary evolution of estuarine systems, it is necessary to study as many fluvial systems as possible. The objective of this review of the sedimentary evolution of a coastal marsh is to describe the influence of glacial paleotopography on the record of climatic and sea-level changes. The Marais Vernier, located at the interface between the marine and fluvial parts of the estuary, is a part of the Lower Seine Valley wetland network, which formed after the Last Glacial Maximum. Previous studies have described the Holocene filling, which is composed of peat and detrital material deposited following climatic and sea-level changes. To understand the sedimentary evolution, a paleotopographical (based on drillings and electromagnetic surveys) and a chronological framework (based on radiocarbon dates) for the southern peat marsh were defined. The peat marsh paleotopography has three erosional surfaces. The S1 surface is the oldest and also the highest, topographically; the S2 surface is younger, wider, and incised below the S1 surface; the S3 surface, the youngest of the three, is narrow and deeply incised. Radiometric ages were considered on the basis of their geographical position in relation to the S3 surface. Prior to 7.5 ka cal BP, sediments accumulated only above the narrow area described by the S3 surface, at a rate of 5.5 mm yr− 1. After 7.5 ka cal BP, shortly after the flooding of the Seine estuary, sediments accumulated as peat deposits over the entire peat marsh at a rate of 3 mm yr− 1 in response to the sea-level rise. The paleotopography delimits the area of deposition during the Holocene, and thus plays a critical role in determining the vertical accretion rate expressed as a thickness: prior to 7.5 ka cal BP, the vertical accretion rate (5.5 mm yr− 1) was less than that observed for the Seine estuary (6.8 mm yr− 1). However, rate of sea-level rise and sediment supply, which also affects sediment accumulation rates, vary in northwestern Europe during the Holocene. Therefore, although the Marais Vernier is a good illustration of paleotopographic influence, the effects of autocompaction, sea level and sediment supply complicate efforts to quantify the degree to which it controls sediment accumulation.
Colour is a fundamental property of sediment and is often used for lithographic description to determine sed-imentological structures, facies etc. However, the sedimentary information contained in this parameter is dif-ficult to extract because it is difficult to quantify. Colour can be quantified by spectrocolorimetry which provides very high resolution data quickly and non-destructively. When adapted to sedimentology, spectro-colorimeters prove to be powerful tools due to their low purchase and maintenance costs, and some are por-table and easily used in-the-field. Several methods have been used to extract sedimentological data from colorimetric spectra (first derivatives, factorial analysis, etc.). In the present study, we first provide a review of the sedimentological application of spectrophotometers and, after having described these methods, their advantages and disadvantages, we then describe a new tool called the Q7/4 diagram (abscissa L*; Ordinates 700/400 ratio). This new technique permits sedimentological units to be defined, allows the identification of dif-ferent sediment components and provides 5 distinct poles: Clayey deposits, organic rich deposits (chlorophyll a and by products), altered organic matter deposits, iron rich deposits, carbonated deposits. Coupled with the analysis of first derivative spectra, it is possible to distinguish different pigments linked to the degradation and/or nature of the organic material (Chlorophyll a, melanoidin, etc.), the state of iron oxidation (for example, hematite and goethite-like signatures) and the nature of clays. The Q7/4 diagram permits rapid acquisition of high resolution data on changes of sediment dynamics in geosystems that have been subjected to highly varied climatic/environmental conditions. The instrument is non destructive, easy to use and maintain, portable for use in the field, fast to implement, is capable of high resolution, and has a vast range of possible applications. Spectro-colorimetry appears to provide many advantages and could become an essential and robust tool for preliminary sedimentological studies.
Investigation of the sedimentary record of pre-Alpine Lake Mondsee (Upper Austria) focused on the environmental reaction to rapid Lateglacial climatic changes. Results of this study reveal complex proxy responses that are variable in time and influenced by the long-term evolution of the lake and its catchment. A new field sampling approach facilitated continuous and precisely controlled parallel sampling at decadal to sub-annual resolution for µ-XRF element scanning, carbon geochemistry, stable isotope measurements on ostracods, pollen analyses and large-scale thin sections for microfacies analysis. The Holocene chronology is established through microscopic varve counting and supported by accelerator mass spectrometry 14C dating of terrestrial plant macrofossils, whereas the Lateglacial age model is based on δ18O wiggle matching with the Greenland NGRIP record, using the GICC05 chronology. Microfacies analysis enables the detection of subtle sedimentological changes, proving that depositional processes even in rather large lake systems are highly sensitive to climate forcing. Comparing periods of major warming at the onset of the Lateglacial and Holocene and of major cooling at the onset of the Younger Dryas reveals differences in proxy responses, reflecting threshold effects and ecosystem inertia. Temperature increase, vegetation recovery, decrease of detrital flux and intensification of biochemical calcite precipitation at the onset of the Holocene took place with only decadal leads and lags over a ca. 100 a period, whereas the spread of woodlands and the reduction of detrital flux lagged the warming at the onset of the Lateglacial Interstadial by ca. 500–750 a. Cooling at the onset of the Younger Dryas is reflected by the simultaneous reaction of δ18O and vegetation, but sedimentological changes (reduction of endogenic calcite content, increase in detrital flux) were delayed by about 150–300 a. Three short-term Lateglacial cold intervals, corresponding to Greenland isotope substages GI-1d, GI-1c2 and GI-1b, also show complex proxy responses that vary in time. Copyright © 2011 John Wiley & Sons, Ltd.
The reconstruction of paleoenvironmental changes from diatoms in lake sediments is based on the usual assumption that, in each studied sample, the dominant species reflect the environment that prevailed during the time of deposition. If the environment changed significantly during the period of deposition (several years) one can expect a mixture of species having different or contradictory ecological affinities. In this paper we present analyses of diatoms in surface sediment samples collected in the Lake Ossa area (3°50′N, 9°36E) and fossil diatoms from a mid-late Holocene core retrieved in the deepest part of the lake. The Lake Ossa area alternates between a short dry season centered at around the northern winter and a long rainy season during the rest of the year entailing significant changes in water level and pH. Based on multivariate analyses, we will show here that mean annual water depth is the most significant variable explaining the distribution of diatoms in the entire Lake. However, seasonal changes of water level are poorly recorded by diatom assemblages, except in some flat areas on the borders of the lakes where a mixing between species with different affinities to water depth is likely due to seasonal changes in water level. Inferred water depth based on a quantitative transfer function reflected essentially secular to millennial changes in the studied core. The relationship between pH and diatoms is not statistically significant but seasonal to multi-annual pH variations mainly observed in the central parts of the lake are reflected by a mixing of acidophilous and alkaliphilous species. Hierarchical ascending cluster analysis (HAC) considered as the most efficient mean of describing diatom mixing shows that seasonal to multi-annual changes in pH are recorded both in the modern and fossil assemblages. According to the degree of mixing between diatoms with different pH affinities we conclude that short-term pH variability was weaker than today between 5200 and 2700 cal yr BP, stronger between 2700 and 2000 cal yr BP, weaker again between 2000 and 600 cal yr BP and similar to present from 400 cal yr BP onwards. Short-term changes were thus superimposed on secular to millennium trends recorded by modifications in the abundance of alkaliphilous diatoms. All these changes are interpreted as variations in precipitation according to a previous model showing that pH is strongly controlled by acidic meteoric water. Inferred water depth slightly changed over the last 5500 years showing weak variations of precipitation minus evaporation balance at secular to millennial time scales. These results will be used to refine previously published paleoclimatic interpretations, which explained changes in precipitation and P–E balance by modifications in the vertical structure of the atmosphere and subsequently by changes in cloud cover, convective or stratiform.
The valley of the Sanaga River, notably in its lower course, experienced a marked alluvial accumulation (7–12 m thick) when the Holocene sea-level and thus the base level of the river approached the present zero. This resulted in an increased height of the floor of the streambed, which facilitated the overflowing of the floods over the top of the river banks. The sediments of Lake Ossa, situated approximately 35 km from the Atlantic shoreline, but the margins of which extend to 1–2 km from the Sanaga River, were studied on the basis of five sediment cores. This lake basin remained isolated from the river until 6500 cal. BP, from when we note the first alluvial accumulation, especially in its southern part. A more easily submersible segment of the lake shoreline about 2 km long was identified. Several specific mineralogical markers of the sandy fraction and the clayey fraction allow us to distinguish autochthonous materials emanating from proximal hillsides upstream of the river, and then to follow the increase of these allochthonous particles during the second half of the Holocene. The first important introduction of alluvial materials in the lake, notably sands, was associated with an acceleration of the accumulation rate. These sedimentary processes seem to have intervened independently of the climate change of the region; they began in a dense forest wet phase and continued during a period of relative opening of the landscape corresponding to more contrasted seasons. The accumulation of coarse alluviums in the lake occurred according to an approximately south–north axis, while that of the suspended load developed with a wider fan-like geometry. These sedimentary processes are diachronous according to the distance from the Sanaga River and so lead us to reconsider partly the palaeoenvironmental meaning of the assemblages of pollen and possibly also of diatoms from previous studies. These vegetal markers were previously considered to have been deposited in the lacustrine system, in restricted isolation.
The Holocene pollen record from Lago Grande di Monticchio reveals environmental changes throughout the Holocene. Comparison of this record with other records of Holocene environmental variability indicates that there is a temporal correlation between these changes and north Atlantic events. However, the nature of this relationship and the underlying mechanism(s) is yet to be determined. It is evidently complex with the apparent coincidence of sea-surface temperature cooling events with both species expansions and disappearances. The picture is further complicated by the interaction between human activity and natural environmental changes.
Twelve gravity cores from various settings within the Mayotte barrier reef–lagoon complex were studied to determine the sedimentology of the sequence stratigraphic systems tracts that formed during the Holocene transgression. Our studies focussed on the determination of physical, chemical, mineralogical and biological parameters of the sediments from specific systems tracts. These parameters determine the thickness and facies of each systems tracts and are controlled by the rate and amplitude of sea-level rise, lagoonal topography and environmental changes. The lowstand systems tract (LST) (before 11.5 ka BP) comprises ferralitic or organic-rich paleosoils in the proximal and middle lagoon and karstified Pleistocene reefal carbonates in the distal lagoon. The transgressive systems tract (TST) (11.5–7 ka BP) consists of a lower terrigenous and an upper mixed terrigenous–carbonate or carbonate-dominated unit. Locally, mangrove muds were deposited. The highstand systems tract (HST) can be divided into an early highstand (eHST) (7–1 ka BP) and a late highstand systems tract (lHST) (after 1 ka BP). In the proximal lagoonal wedge, the early highstand systems tract consists of terrigenous or mixed terrigenous–carbonate muds to sandy muds. In the middle lagoon, it shows carbonate mud to sandy mud and carbonate gravel to reefal carbonates in the distal lagoons. Terrigenous muds dominate the late highstand systems tract in the proximal lagoonal wedge. In the mid-lagoonal plain, mixed terrigenous–carbonate or carbonate mud to sandy mud dominates, while carbonate gravel to reefal carbonate prevails in the distal lagoon.
A Rock-Eval pyrolysis study was carried out on sedimentary cores and leaf and woody tissue of vascular plant species from the mangroves of French Guiana. These forests develop on moving mudbanks and have a lifetime limited to few decades before being eroded. Our main purpose was to complete the understanding of carbon cycling in this specific environment using a method that allows monitoring the depth evolution of sources and transformation of organic matter (OM) within a tight depth span. Leaf and woody tissue showed clearly distinct Rock-Eval parameters. However, these parameters strongly varied with OM decomposition, rapidly limiting their efficiency as mangrove plant tracers. The decrease in values of TPS2 (pyrolysis temperature corresponding to the maximum release of hydrocarbons) with depth clearly showed a line between a pedogenetic layer enriched in autochthonous OM and marine sediments enriched in allochthonous OM. This allochthonous OM originated from the particulate discharge of the Amazon River, which is partly deflected northwestwards by the current of the Guianas. Within the soil layer, decay processes induced a strong dehydrogenation and slight oxidation of OM, the two processes occurring simultaneously. The deeper layers were characterized by a very strong increase in OICO2 (quantity of pyrolysed CO2 relative to total organic carbon), beneath both the young and the senescent forests. This trend resulted rather from an increasing content of authigenic carbonate than from increasing concentration of oxidized allochthonous debris. Carbonate can form as a result of the more anoxic conditions prevailing in buried layers vs. the upper suboxic ones. Beneath the senescent mangrove, a greater increase in OICO2 probably reflected greater carbonate precipitation, resulting from the decomposition of higher amounts of OM.
Minerals derived from catchment soils were determined using FTIR spectroscopy in the well-dated core OW4 from Lake Ossa, a lowland rainforest area in Cameroon. This quantification provides a hydrologic record indicating that the magnitude of runoff events, and by inference, rainfall pattern, has varied during the Late Holocene. The comparison between minerogenic inputs and vegetation changes improves the understanding of the inferred climate dynamics. Since at least 5400 cal yr B.P., the paleomonsoon rainfall intensity decreased, as shown by a general decrease in mineral fluxes. This observation is consistent with a gradual weakening of the boreal summer insolation in tropical latitudes. However, the major vegetational change lags behind the onset of the decrease in mineral fluxes. From 2800 to ca 1000 cal yr B.P., the forest receded: the amount of rainforest taxa decreased and is replaced by pioneer trees and Poaceae, when the mineral fluxes attained their lowest values. This episode of maximum dryness is attributed to an abrupt climatic event of global significance which is superimposed onto the paleomonsoon variability. It is related to a cold event, which in turn produced a change in the lower atmospheric circulation that was characterized by a strengthening of northern trade winds, probably correlated with sea-surface temperature variations in the eastern tropical Atlantic area.
A borehole core through the Holocene sediments of Saanich Inlet (British Columbia, Canada) was analyzed using high-resolution spectrophotometry to determine whether spectral–signal variations can be used as environmental/climatic proxies. The effects of post-drilling oxidation and water loss on the core sample were assessed by comparing spectral measurements taken onboard the research ship immediately after the core was opened with shore-based measurements taken one year later. For Saanich Inlet sediments, the shore-based measurements provided more reliable indications of the degree of basin anoxia. Our spectral analyses allowed us to identify all the major features of the Saanich Inlet sediment sequence (Mazama ash and Fraser River Drainage stratigraphic markers, oxygenation state of the basin). By using the first derivative value at 675 nm as a proxy for changes in organic matter content and the colorimetric parameter b⁎ as an indicator of diatom content, we were able to recognize changes in terrestrially derived organic matter (OM) and diatom concentrations in the sediment. We suggest that these variations are due to changes in upwelling intensity and fluctuations in sea level during the Holocene. Our results confirm the potential value of spectrophotometry in identifying sediment composition and in reconstructing paleoenvironments.
This work relates to the debate on the fossil organic carbon (FOC) input in modern environments and its possible implication for the carbon cycle, and suggests the use of Rock-Eval 6 pyrolysis as a relevant tool for tracking FOC in such environments. Considering that such a delivery is mainly due to supergene processes affecting the continental surface, we studied organic matter in different reservoirs such as bedrocks, alterites, soils and rivers in two experimental catchments at Draix (Alpes de Haute Provence, France). Samples were subjected to geochemical (Rock-Eval 6 pyrolysis) investigations and artificial bacterial degradations. After comparing the geochemical fingerprint of samples, geochemical markers of FOC were defined and tracked in the different reservoirs. Our results confirm the contribution of FOC in modern soils and rivers and display the various influences of weathering and erosional processes on the fate of FOC during its exchange between these pools. In addition, the contrasting behaviour of these markers upon the supergene processes has also highlighted the refractory or labile characters of the fossil organic matter (FOM). Bedrock to river fluxes, controlled by gully erosion, are characterized by a qualitative and quantitative preservation of FOM. Bedrock to alterite fluxes, governed by chemical weathering, are characterized by FOC mineralization without qualitative changes in deeper alterites. Alterite to soils fluxes, controlled by (bio)chemical weathering, are characterized by strong FOC mineralization and qualitative changes of FOM. Thus weathering and erosional processes induce different FOM evolution and affect the fate of FOC towards the global carbon cycle. In this study, gully erosion would involve maintenance of an ancient sink for the global carbon cycle, while (bio)chemical processes provide a source of CO2. Finally, this study suggests that Rock-Eval 6 pyrolysis can be considered as a relevant tool for tracking FOC in modern environments.
Rock-Eval pyrolysis was designed for petroleum exploration to determine the type and quality of organic matter in rock samples. Nevertheless, this technique can be used for bulk characterization of the immature organic matter in soil samples and recent sediments. We studied 76 samples from seven soil classes and showed that their pyrograms can be described by a combination of four elementary Gaussian components: F1, F2, F3 and F4. These four components are related to major classes of organic constituents differing in origin and their resistance to pyrolysis: labile biological constituents (F1), resistant biological constituents (F2), immature non-biotic constituents (F3) and a mature refractory fraction (F4). We discriminated the relative contributions of these components and used them to derive two indices: (i) to quantify the relative contributions of labile and resistant biological constituents and (ii) to quantify the degradation stage of the soil organic matter. The practical applications are illustrated via the influence of vegetal cover on soil organic matter dynamics and peat development in a Holocene sedimentary sequence, but we suggest that the approach is of much wider application.
Application of Rock-Eval pyrolysis to soil organic matter (SOM) quantitation and characterization has been explored by the study of about 100 soil samples taken from a variety of soil profiles from different ecosystems at different latitudes. A straightforward illustration of these possibilities can be obtained from a Hydrogen Index (HI in mg hydrocarbons g−1 TOC) vs. Total Organic Carbon (TOC) diagram that effectively allows one to follow simultaneously the main qualitative (SOM hydrogen richness given by HI values) and quantitative (TOC) changes that affect SOM with increasing depth and humification, in the soil profiles. In addition, abnormally high Oxygen Index (OI in mg CO, CO2 or O2 g−1 TOC) values are fully diagnostic of extensive SOM alteration, as frequently observed in podzol B horizons. More detailed information on the heterogeneity of SOM and on its degree of evolution, can be gained from the shape of the pyrolysis S2 peak recorded in the course of programmed pyrolysis in an inert atmosphere (N2) and/or from its maximum temperature "Tpeak". All these parameters and others, all determined rapidly and automatically, are particularly useful to screen major SOM variations within large sets of samples
Identification of the sources of organic matter in sedimentary records provides important paleolimnologic information. As the types and abundances of plant life in and around lakes change, the composition and amount of organic matter delivered to lake sediments changes. Despite the extensive early diagenetic losses of organic matter in general and of some of its important biomarker compounds in particular, bulk identifiers of organic matter sources appear to undergo minimal alteration after sedimentation. Age-related changes in the elemental, isotopic, and petrographic compositions of bulk sedimentary organic matter therefore preserve evidence of past environmental changes. Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/43082/1/10933_2004_Article_186773.pdf
Conclusions et recommandations du Groupe d'Interprétation Bassin de Douala
- Elf-Serepca
Elf-Serepca, 1987. Conclusions et recommandations du Groupe d'Interprétation Bassin de
Douala. Rapport multigraphié 341-DE N ± 6/160 (Douala, 30 pp.).
Petroleum formation and Occurrence The sharp rise of 14C ca.800 cal BC: possible causes, related climatic teleconnections and the impact on human environ-ments
- J B Suchel
- Springer
- Heidelberg
- Van
- B Geel
- Van Plicht
- J Kilian
- Z M R Klaver
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