Elisabeth Lallier-Vergès

Lacustrine Sedimentary Organic Matter Records of Late Quaternary Paleoclimates

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.

The glacial ocean productivity hypothesis: the importance of regional temporal and spatial studies

Abstract Higher ocean productivity has often been proposed to explain lower atmospheric carbon dioxide during the last glacial episodes. But recent consideration of marine cores from different areas show that higher local productivity can be postulated for interglacials as well as for glacial periods. Based on the detailed study of two piston cores from the northwest Africa upwelling system, the results presented here, including δ18O stratigraphy, organic carbon contents and fluxes, Ti Al ratios and grain size measurements, clearly indicate that the two cases of sedimentary records can even co-exist within a single upwelling system. This regional heterogeneity is presumably attributed to combined wind stress and sea-level changes that would induce different sedimentary records in the northern and in the southern part of the system. These results emphasize the importance to understand and to model the response of the main kinds of highly productive oceanographic regional systems which are spatially heterogeneous due to complex continent-ocean interactions, or to the presence of mobile hydrodynamic heterogeneities. For such an understanding it is not necessary to acquire a huge amount of core data throughout the world ocean, but to increase the density of cores as well as the regional-scale modelling efforts in systems such as coastal and equatorial upwelling areas, and the migration areas of the southern polar front.

The composition of sedimentary organic matter in relation to the dynamic features of a mangrove-fringed coast in French Guiana

The sedimentary organic matter content of a series of 2-m-deep cores was examined in relation to the evolution of mangrove forest, on the basis of geochemical analyses and optical observations. Avicennia-dominated forest deposits, developing along the highly dynamic coastline of French Guiana, were collected in five stations based on stage of forest evolution. The sedimentary organic matter in the upper sediment of the youngest mangrove swamp is mainly derived from algal mats with low carbon:nitrogen ratios (C:N ratio, from 6 to 8) and typical greyish amorphous organic flakes as observed in optical studies. Indeed, rare young Avicennia trees are present, and effectively, geochemical parameters do not give evidence of a litter made up of higher plant debris, these rare debris being probably exported by the tides. A slight increase with depth in the first decimetres of both total organic carbon (TOC) content and C:N ratio results from the development of the radial cable root-system of the pioneer Avicennia germinans. Early diagenetic conditions of this young forest are rather controlled by dominant suboxic processes, as suggested by high Eh values (range, 200–400 mV) and local anoxic processes (occurrence of pyrite) in micro-environments: this is mainly due to the oxygen available by roots and crab bioturbation. The organic content of the senescent mangrove sediment is mainly derived from higher plant debris in the uppermost 30 cm, as indicated by relatively high C:N ratios and the predominance of ligno-cellulosic debris. The strong decrease in hydrogen index values results from the degradation of the higher plant debris, losing hydrogen bounds through decay processes. Moderately acidic pH values, low Ehs and the presence of pyrite framboids point towards the reducing decay processes in surficial layers of the senescent mangrove mediated by sulphate-reducing bacteria. Whatever the stage of evolution of the forest, the geochemical characteristics of the sediment below 30 cm are those of the shoreface one, with opaque refractory debris derived from the Amazon river detritus discharge. The sediment collected from dead mangrove forests, subsequently recolonized by pioneer mangroves, contains organic markers, which predate recolonization, recording previous phases of erosion and accretion.

Electron microscopy and pyrolysis of kerogens from the Kimmeridge Clay Formation, UK: Source organisms, preservation processes, and origin of microcycles

Abstract Recent studies revealed short-term cyclic variations (microcycles) in total organic carbon (TOC) and the hydrogen index (HI) in the Kimmeridge Clay Formation, an organic-rich deposit considered to be a lateral equivalent of the main source rocks of the North Sea. In addition, three different types of organic matter that all appear to be amorphous when observed by light microscopy (AOM) were recognized. Together, these AOM types account for over 80% of total kerogen and their relative abundances show large variations along each microcycle. In the present work, transmission electron microscopy (TEM) observations were carried out on samples (whole kerogens, kerogen subfractions only comprising a single type of AOM, selected rock fragments) corresponding to typical points within a microcycle and obtained via high resolution sampling. The nature and the relative abundances of the products generated by Curie-point Py-GC-MS and off-line pyrolyses of isolated kerogens were also determined for two selected samples corresponding to the beginning and the top of the microcycle. Combination of such ultrastructural observations, including some semiquantitative studies, and the analysis of pyrolysis products allowed (1) determination of the ultrastructural features of the three AOM types thus providing what we believe to be the first example of correlations between light microscopy (palynofacies, in situ maceral analysis) and TEM observations on “amorphous” fossil materials; (2) identification of the source organisms and elucidation of the mode of formation of the different AOM types in the Kimmeridge Clay; (3) explanation of the variations in their relative abundances taking place along a microcycle and establishment of tight correlations with TOC and HI changes; and (4) explanation of the origin of the microcyclic variations in kerogen quantity (TOC) and quality (III) occurring in the Kimmeridge Clay Formation. Interrelationships between primary productivity, sulphate reduction intensity, and lipid “vulcanization” likely played a major role in the control of such variations.

Protection of organic matter by mineral matrix in a Cenomanian black shale

Abstract Three types of pathways (degradation–recondensation, natural sulphurization and selective preservation) are commonly considered for the formation of kerogen dispersed in sedimentary rocks. A fourth pathway has been recently put forward, however, from studies on Recent marine sediments, the so-called sorptive protection mechanism. This pathway is based on the adsorption of otherwise labile organic compounds onto minerals, thus preventing their diagenetic degradation and promoting their subsequent condensation into kerogen. The main results of the present study are derived from a combination of microscopic and pyrolytic methods applied on a Cenomanian kerogen. They provide (i) evidence, on an ancient material, for a crucial role of the mineral matrix both in organic matter (OM) preservation during kerogen formation and in kerogen stability once formed, (ii) indications that the dominant protective process likely involves physical protection by minerals, resulting from alternation of organic and clay nanolayers of approximately 100 nm in thickness, rather than OM adsorption as molecular monolayers and (iii) observations of the relatively poor stability of an isolated kerogen, contrary to the inertness commonly assumed for fossil macromolecular organic matter.

Geochemical study of organic-matter rich cycles from the Kimmeridge Clay Formation of Yorkshire (UK): productivity versus anoxia

Abstract In this contribution, we study two meter-scale cycles from the Kimmeridge Clay Formation (cored near Marton, Yorkshire) which shows cyclic organic matter (OM) distribution. Our aim is to try to understand the factors responsile for (OM) accumulation. The first cycle, called lower cycle, shows a total organic carbon (TOC) content between 1 and 10% whereas the second, called upper cycle, shows a TOC content varying between 5 and 35%. The geochemical composition (major elements and trace elements), the organic geochemistry (TOC, HI, palynofacies) and mineralogy of the sediments have been determined. In both cycles, the cyclicity is expressed through variations in the nature and in the relative abundance of the various types of organic-matter constituents. Furthermore, dilution effects by inorganic components of the sediment cannot account for the TOC cyclicity. For lower cycle, the Mo, V and U content is low and little variable as is the intensity of the oxidation which OM suffered from. This indicates that variations in phytoplanktonic productivity may be held responsible for the cyclicity in steady and mildly reducing redox conditions. In the upper cycle, cyclicity also appears to depend on productivity but variations in the concentration of Mo, V and U and in the oxidation state of the OM suggest the environment was temporarily more reducing. It is proposed that larger amounts of H 2 S were released into marine bottom waters as a result of initial OM decomposition, forced the oxic-anoxic boundary to rise in the water column and thus favoured OM storage. The main driving force for variations in the OM concentration was the productivity of organic-matter-walled phytoplankton. Redox conditions of the depositional environment could have had a positive action, but only by acting as a positive feedback effect.

Rapid ecosystem response to abrupt climate changes during the last glacial period in western Europe, 40-16 ka

We present a high-resolution and independently dated multiproxy lake sediment record from the paleolake at Les Echets in southeastern France that displays synchronous changes in independent limnic and terrestrial ecosystem proxies, in concert with millennial-scale climate oscillations during the last glacial period. Distinct lake-level fluctuations, low lake organic productivity, and open, treeless vegetation indicate cold and dry conditions in response to Heinrich events. Alternating phases of higher and low lake organic productivity, stratified surface waters and long-lasting lake ice cover, decreased or increased catchment erosion, and tree-dominated or herb-dominated vegetation resemble Dansgaard-Oeschger interstadialstadial variability. Transitions between different ecological states occurred in as little as 40–230 yr and seem to have been controlled by the position of the Polar Front. Ecosystem response after 30 ka suggests that local climate conditions became more important. Our results demonstrate that all parts of the terrestrial system responded to the abrupt and dramatic climatic changes associated with Dansgaard-Oeschger and Heinrich events, and that regional factors modulated ecosystem response.

Comparative study of organic matter preservation in immature sediments along the continental margins of Peru and Oman. Part I: Results of petrographical and bulk geochemical data

Abstract Detailed petrographical and bulk geochemical investigations of organic matter (OM) have been performed on sediments deposited below or close to upwelling areas offshore Peru (ODP-Leg 112; Sites 679, 681, 688) and Oman (ODP-Leg 117; Sites 720, 723, 724) in order to obtain a quantitative understanding of its accumulation and degradation. Microscopical as well as nanoscopical investigations reveal that the OM in sediments affected by upwelling mechanisms mainly (up to 98%) consists of unstructured (amorphous) organic aggregates without any apparent biological structures. In sediments which are not or to a lesser extent affected by upwelling (Site 720) terrestrial OM predominates. Organic carbon (TOC) contents are highly variable and range between 9.8% in sediments deposited below upwelling cells and 0.2% in sediments outside the upwelling zone. The TOC/sulphur ratios of the sediments scatter widely. The samples from the deep-water locations (Sites 688 and 720), show C S - ratios of “normal” marine sediments, whereas at the other locations no correlation or even a negative correlation between sulphur and TOC concentration exists. In most of the upwelling-influenced sediments OM contains a significant amount of sulphur. The incorporation of sulphur into the OM followed microbial sulphate reduction and occurred in the upper meters of the sedimentary column. Below, OM is still present in vast amounts and relatively hydrogen-rich, but is nevertheless non-metabolizable and becomes the limiting factor for bacterial sulphate reduction. According to mass balance calculations 90–99% of the OM produced in the photic zone was remineralized and 1–3% was consumed by microbial sulphate reduction. The aerobic and anaerobic processes have greatly affected degradation and conservation of OM.

Human impact and soil erosion during the last 5000 yrs as recorded in lacustrine sedimentary organic matter at Lac d'Annecy, the French Alps

Sedimentary organic matter has been systematically studied in an eight-metre long core from the centre of the Petit Lac (Annecy, French Alps). The palynofacies composition identifies different terrestrial organic sources including forest floors, soil-horizons and geological substratum. The amount of recycled organic matter derived from the geological substratum is estimated and subtracted from the other contributions from the catchment area. The palynological record indicates that the relative variations in organic sources are directly dependent on human land-use. From ca. 5000 to 1700 BP, the human impact on soil cohesion is very low and organic matter is mainly exported from the surficial forest floor. The Roman invasion (ca. 1700 BP) marks the most important ecological and hydrological change. From 1700 to 900 BP, the clearing of forests released deeper-soil components. This trend is increased after 900 BP with agriculture intensification, which resulted in a higher sedimentation rate. In all the periods, extreme events such as flood or intensive run-off are characterised by notable increases of organic matter from surface (5000 to 1700 BP) and deep (1700 BP to now) soils.

Organic matter composition and sulfate reduction intensity in Oman Margin sediments

Abstract Petrographical and geochemical studies of Neogene marine sediments from the Oman Sea (Leg 117, Sites 720, 724, 726 and 730), show a close relationship between the nature and amount of the organic matter, and the degree of degradation of organic matter by sulfate reduction, i.e. pyritization. Petrographically, three major pyritization types were observed: (1) Finely dispersed pyrite framboids in sediments from Oman Margin and Indus Fan, enriched in autochthonous marine organic matter. (2) Infilling of pores by massive pyrite crystals in Oman Margin sediments with a low TOC and a high microfossil content. (3) Pyrite mineralization of lignaceous fragments in organic-depleted sediments from the Indus Fan leading to more massive pyrite. Geochemically, we can define a sulfate reduction index (SRI) as the percentage of initial organic carbon versus that of residual organic carbon. Finely laminated Pliocene-Pleistocene sediments from the Oman Margin exclusively contain organic matter deriving from organic phytoplankton, for which the quantity (TOC) positively correlates with the geochemical quality (Hydrogen Index). We think that the occurrence of this residual organic matter is linked mainly to a high primary paleo-productivity. The intensity of sulfate reduction is constant for sediments with TOC up to 2% and becomes more important when organic input decreases. This degradation process can destroy up to 50% of the initial organic matter, but is not sufficient to explain some of the encountered very low TOC values. It can be seen that sharp increases of certain plankton species (with mineral skeletons) are responsible for a pronounced degradation of organic matter, due to increased sulfate reduction. In that case, the organic matter may be strongly degraded (high SRI), although deposited in an oxygen-depleted environment. Conversely, Miocene-Pliocene sediments contain an autochthonous organic matter that is typical of both low productivity and oxic processes; their very low sulfate reduction index indicates that very little metabolizable organic matter was initially present.