Erosion is a fundamental player of the interactions existing between internal geodynamics and climate, in particular through its influence on the carbon dioxide budget. However, long term (>
1
Ma) ...erosion rates, estimated indirectly from sediment budget, remain poorly constrained. While some studies suggest that worldwide erosion rates increased at the Plio-Pleistocene climatic transition (~
4–2
Ma), the validity of this observation and its significance is a matter of debate due to potential biases of the sedimentary record and to the influence of sea level fall on the global sedimentary flux to marginal basins. In the present study, we estimate erosion rates over the last ~
9
Ma using in situ produced cosmogenic
10Be concentrations measured in magnetostratigraphically dated continental sediments. We focus on an intracontinental endorheic watershed draining the northern Tianshan in Central Asia, a key region regarding the ongoing debate. While erosion rates between 0.1 and 1
mm·yr
−
1
are derived from most of our record, they reach values as high as ~
2.5
mm·yr
−
1
from 2.5 to 1.7
Ma. Then, after 1.7
Ma, recent and modern erosion rates fell below 1
mm·yr
−
1
. This temporary increase is correlated with the onset of Quaternary ice ages and suggests that global climate had a significant and transient impact on erosion.
►
10Be was analyzed in 9-0 Ma old continental quartz sediments from the Tianshan piedmont. ► Paleo-erosion rates were derived from
10Be concentration and show an abrupt and transient increase at 3 Ma. ► This suggests that Quaternary glaciations had a strong impact on erosion rates.
Mangroves of New Caledonia act as a buffer between a lagoon of more than 20,000
km
2 and the Island, which suffers intense processes of erosion resulting from urbanization and natural resources ...exploitation. This preliminary study aims at determining how heavy metals are distributed in mangrove sediments and pore-waters in relationship with their organic content. To reach our goal, a series of 50-cm deep cores were collected in the mangrove of Conception Bay. The various coring sites are representative of live forests (
Avicennia marina,
Rhizophora stylosa), dead forest, and intertidal unvegetated area. The ranges of concentrations in sediments were the following (μmol
g
−1): Cu (0.08 to 0.51), Co (0.01 to 0.38), Ni (0.03 to 3.55), Cr (0.36 to 3.11), Zn (0.68 to 2.36), Mn (1.13 to 5.0) and Fe (22.64 to 721.69). Heavy metals distribution within sediments and pore-water appear to result from diagenetic processes linked to OM decomposition. Beneath the dead
Avicennia forest and in the
unvegetated sediments, the higher the organic content, the higher the metal concentration in the solid phase. Beneath living mangrove stands, despite a higher organic content, as well as higher sulphur content, heavy metals concentrations were not higher than those measured in the other sampling sites. Beneath these forests, redox conditions were mainly controlled by the length of waterlogging and the activity of root system. We suggest that, because of the specificity of the
Avicennia root system and its position in the intertidal zone, heavy metals are more bioavailable and potentially more mobile than beneath
Rhizophora stand.
►Organic content is higher beneath
Rhizophora than beneath
Avicennia. ►Organic sources differ beneath the two species. ►Redox conditions are mainly controlled by length of waterlogging and root activity. ►Metals distribution results from diagenetic processes linked to OM decomposition. ►Heavy metals are more potentially more mobile beneath
Avicennia than
Rhizophora.
We report on the detection, in a sediment core drilled in Lake Le Bourget (French Alps), of a fossil molecule (miliacin) that was synthesized by broomcorn millet cultivated in the watershed, and then ...exported to the sediment. The variation in abundance of this molecule allows us reconstructing the history of millet cultivation around Lake Le Bourget. Our results support the introduction of millet around −1700 BC in the region. After an intensive cultivation during the Late Bronze Age, the failure of millet cropping during the Hallstatt period coincides with a phase of climatic deterioration. Millet cultivation recovers during the Roman and Mediaeval periods before falling most probably due to the introduction of more productive cereals. These pioneering results constitute the first continuous record of an agrarian activity covering the last 6000
years and emphasize the close relationships between local hydrology, land use and agro-pastoral activities around Lake Le Bourget.
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. We review different bulk organic matter proxies of climate change in tropical and temperate sedimentary records ranging in age from 10-500 ka. Times of wetter climate result in enhanced algal productivity in lakes as a consequence of greater wash-in of soil nutrients, and these periods are recorded as elevated Rock-Eval hydrogen indices, lowered organic C/N ratios, less negative organic δ13C values, and increased organic carbon mass accumulation rates. Lowering of lake water levels, which typically depresses algal productivity, can also cause an apparent increase in organic carbon mass accumulation rates through suspension of sediments from lake margins and redeposition in deeper basins. Alternations between C3 and C4 watershed plants accompany climate changes such as glacial/interglacial transitions and wet/dry cycles, and these changes in land-plant types are evident in δ13C values of organic matter in lake sediments. Changes in climate-driven hydrologic balances of lakes are recorded in δD values of sedimentary organic matter. Visual microscopic examination of organic matter detritus is particularly useful in identifying changes in bulk organic matter delivery to lake sediments and therefore is important as an indicator of climate changes.PUBLICATION ABSTRACT
•Miliacin is a promising biomarker for broomcorn millet (Panicum miliaceum).•Other less agriculturally important Panicum spp. and some Pennisetum spp. also synthesize it.•Seeds are the main ...miliacin-contributing organ in P. miliaceum.•Miliacin is not present in the Italian millet, Setaria italica. It is absent from the fungus, Chaetomium olivaceum.
Using sedimentary miliacin (olean-18-en-3β-ol methyl ether) as a molecular tracer of the history of Panicum miliaceum (broomcorn millet) cultivation depends upon broomcorn millet being sedimentary miliacin’s dominant source. It also requires knowledge of the variability in miliacin concentration in broomcorn millet. Finally, it is affected by the presence of other pentacyclic triterpene methyl ethers (PTMEs) that may exist in conjunction with miliacin in other sources but not in broomcorn millet.
Miliacin biosynthesis has been proposed for other Panicum species, Setaria italica (Italian or foxtail millet), Pennisetum sp. and Chaetomium olivaceum (an olive green mold). We found miliacin concentration in seeds of different varieties of P. miliaceum to be similarly high (with trace amounts of β- and α-amyrin methyl ethers). It was absent from hulls and roots, and nominally present in leaves and stems. The transfer of miliacin from plant to sediments is therefore mostly from seeds. It was abundant (often with larger amounts of β- and α-amyrin methyl ethers) in all other Panicum species studied, but only in some species of the genus Pennisetum and was absent from S. italica. Neither C. olivaceum nor its growth medium (rice) showed any trace of miliacin. Our results, with miliacin absent from S. italica and C. olivaceum, its high concentration in the seed of P. miliaceum relative to other PTMEs and to other grasses and, considering the high biomass that cultivated broomcorn millet has relative to other potential plant sources, support the use of sedimentary records of miliacin in some contexts to track past millet agricultural dynamics.
The accumulation of nine heavy metals in fine-grained sediments from the mangrove fringed coast of French Guiana is evaluated. The dynamic features of the South American tropical coastline, from the ...Amazon to the Orinoco Rivers, result in mangrove sediments being alternately submitted to phases of erosion and net sedimentation a few tens of years long. This process influences the distribution of the heavy metals associated with these frequently re-mobilized deposits. Sedimentary cores and mangrove plant samples were collected, at different seasons, in various swamps characterized by different properties (content of sedimentary organic matter, distance from sea water and fresh water). The ranges of measured concentrations expressed in μmol g
−
1 were the following: Cu (0.06 to 0.61), Co (0.12 to 0.68), Pb (0.08 to 0.18), Ni (0.32 to 0.76), Cr (0.61 to 1.40), Zn (1.25 to 5.94), Mn (4.36 to 45.4) and Fe (441 to 1128). No differences were found between sediments from mangroves developing upstream and downstream of urban areas, i.e. Cayenne and Kourou. This suggests that the content of mangrove sediments in heavy metals along the coastline of French Guiana is essentially the result of the continuous alternation of accumulation and transport phases occurring upstream after departing from the Amazon watershed. The sources of this heavy metals content are thus difficult to identify. However it is well known that the alluvium produced by the natural erosion of the Amazonian soils is naturally enriched in mercury. Also, the run-off from gold mining activities is known to contribute to mercury pollution. Ranges in total Hg were between 0.15 and 2.57 nmol g
−
1, with mean values close to 0.41 nmol g
−
1, and were clearly correlated with total organic carbon except for some outstanding high values, which may be a result of rapid geochemical changes. Heavy metal concentrations showed variations with depth. The redox conditions and the decay processes affecting the organic matter control the cycling of iron and manganese, which in turn control the concentrations and associations of heavy metals. These preliminary results suggest that the variations in heavy metal content with depth or between mangrove areas result largely from diagenetic processes rather than changes in metal input resulting from local human activities.
A new set of geochemical data obtained from a long sediment sequence from the Les Echets basin provides a new record of palaeoenvironmental change for western Europe. The sequence covers the late ...part of Marine Isotope Stage (MIS) 3 and MIS 2 (c. 46.1–15.0 cal. kyr BP) and extends over 30 m of sediment, allowing for high sampling resolution. Distinct cyclic fluctuations in aquatic productivity proxies suggest a tempo of environmental change at Les Echets that resembles the one established for Dansgaard‐Oeschger cycles and Heinrich events. Increases in organic carbon are paralleled by high hydrogenous contents and enriched δ13Corg and Corg/Ntot ratios, implying a mixture of aquatic and land‐derived constituents. The stable isotope values are directly linked to primary aquatic productivity and the input of terrestrial organic matter, enriched during warm, productive, periods, and drastically depleted during cold periods, particularly Heinrich events. Phases of relatively high productivity correlate with Dansgaard‐Oeschger interstadials 8–2, while Heinrich events 3 and 2 are also clearly delineated by distinct lithological horizons with very low organic contents, high dry densities and enhanced organic matter oxidation. A large hiatus occurring about the time of Heinrich event 4 suggests that the cold and dry climate reconstructed for this event in the North Atlantic region also greatly affected Les Echets and its catchment.
Purpose
The mangroves of New Caledonia, in the south Pacific, act as a buffer between a lagoon of more than 20,000 km
2
and the island, which is characterized by ultramafic rocks and lateritic soils ...that are exploited for their richness in heavy metals. We will provide a better understanding of the redox conditions, and of heavy metal distributions in mangroves receiving shrimp farm effluents.
Materials and methods
Samples were collected from four areas defined in terms of vegetation composition: a salt flat, an
Avicennia marina
forest, in which effluents are released; a
Rhizophora stylosa
forest, and a dead
Rhizophora
forest. They were collected during times of maximum effluent release. Some measurements on pore water were also done during a period without effluent. Cores (70 cm deep) were collected at low tide with an Eijkelkamp gouge auger, and pore waters were extracted using soil moisture samplers. Physico-chemical parameters (pH, Eh, salinity) were measured by directly inserting the probes into cores. Total nitrogen and total sulfur were also determined. The sedimentary organic content was studied using a Rock-Eval 6 pyrolysis. Finally, heavy metal concentrations were determined, both in the solid and the dissolved phases, using an HR-ICP-AES.
Results and discussion
The distribution of heavy metals in the core collected in the salt flat is mainly controlled, on the one hand, by the sedimentary organic content, and on the other hand, by the elevation of the area, which induced dessication. The release of effluent within the
Avicennia
stand induced anoxic conditions on the whole depth profile, while these conditions are suboxic without effluent release, probably inducing different metal speciation. The
Rhizophora
forests, located at 100 m from the release point, do not seem to show any impact from the effluent phase, as indicated by the redox profiles which show similar results with and without effluent release. Beneath these two stands, conditions are mainly anoxic and sulfidic, as a result of the decomposition of high organic matter content.
Conclusions
The release of effluent within the
Avicennia
stand, by modifying the length of waterlogging, clearly modifies the redox conditions. We suggest that the differences in redox conditions between the two periods modify the carrier phase of heavy metals, being mainly associated with sulfides during effluent release. Metals are thus less mobile, and consequently when mangrove receives effluents, they act as a sink for trace metals.
Colour is a fundamental property of sediment and is often used for lithographic description to determine sedimentological structures, facies etc. However, the sedimentary information contained in ...this parameter is difficult 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, spectrocolorimeters prove to be powerful tools due to their low purchase and maintenance costs, and some are portable 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 different 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. Spectrocolorimetry appears to provide many advantages and could become an essential and robust tool for preliminary sedimentological studies.
► We first provide a review of the sedimentological application of spectrophotometry. ► In a second part, it’s described a new tool called the Q7/4 diagram. ► This diagram permits to identify high-resolution changes of sedimentary dynamics. ► This technique allows the identification of 5 sediment components. ► Clayey, organic rich, altered organic matter, iron rich and carbonated deposits.
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