Pierre Giresse

Particulate matter and organic carbon budgets for the Gulf of Lions (NW Mediterranean)

An analysis of sedimentological, sediment trap and hydrological data was performed to investigate the transport and fate of particulate matter in the Gulf of Lions. The sedimentological properties outline the major sedimentary units of the shelf (Rhone prodelta, mid-shelf mud belt, outer shelf) and slope. The geometry of these sedimentary units and the southwestwards increase of particulate fluxes on the slope highlight the influence of the general cyclonic circulation on the dispersion of land-derived particulate matter. Considering the known input and output terms, budgets of particulate matter and organic carbon in the Gulf of Lions are proposed. Inputs were river supply, atmospheric deposition and primary production; outputs were sediment burial and advective export on the slope. Degradation of particulate organic carbon in the water column and at the sediment-water interface was also estimated. Mass and POC budgets were balanced within uncertainties. Nevertheless, these results illustrate the difficulty in establishing budgets based on elements estimated at different time scales, for which the system is not necessarily in a steady state.

Late Quaternary palaeoenvironments in the Lake Barombi Mbo (West Cameroon) deduced from pollen and carbon isotopes of organic matter

Abstract A sediment core, 23.5 m long, was recovered from a water depth of ca. 110 m in Lake Barombi Mbo, a maar crater of the Cameroon volcanic chain. This paper presents a carbon isotopic curve of organic matter linked to the main results of the pollen analysis. Interpretation of this carbon isotopic curve is simplified (1) because of the weak role played by the phytoplankton in the balance of the organic particle flux, and (2) because of the presumed absence of diagenetic alteration of the isotopic composition. The carbon isotopic curve exhibits an almost linear correlation with that of total grass pollen which form the main part of the C4 biomass. The major fluctuations of isotopic and pollen curves allow four main phases to be distinguished: • -from ca. 25,000 to 20,000 yr B.P., δ13C values of −25 to −30‰ (PDB) are related to a forest environment associated with a montane floral element; • -from 20,000 to 13,000 yr B.P., the δ13C values are between −23 and −28ℵ. and are linked to openings of landscape with a mosaic of forest and savanna and a fall of lake level with colonization of the shores by Cyperaceae and other aquatic plants; • -from 13,000 to 10,000 yr B.P., the forest extended again reaching a maximum density during the period from 9500 until 3000 yr B.P. and with an average δ13C value of −32‰; • -a new dry phase at around 2500-2000 yr B.P., with temporary openings in the forest, marked by a δ13C excursion above −30‰, but during the last 2 millennia, the forest developed again with δ13C values of −32‰.

Carbon isotope composition and geochemistry of particulate organic matter in the Congo River (Central Africa): Application to the study of Quaternary sediments off the mouth of the river

Organic carbon isotopes and the geochemistry of organic matter have been studied in suspended material from the Congo River and some of its tributaries and in a core located at 2000-m depth, off the mouth of the river. The organic carbon isotopic composition of suspended material from the Congo tributaries identifies the floral cover and the lithologic and pedologic characteristics of their watersheds. Suspended material from the Congo River in the Stanley Pool emphasizes the sole influence of organic matter derived from C3-rich plants. Outside the Congo estuary, differences of isotopic composition between suspended material near the surface and that near the bottom emphasize the respective marine or continental origin of particulate organic carbon. The carbon isotopic composition of the marine deposits records biozonal fluctuations of the recent Quaternary. Shifts in the isotopic composition of organic matter may be explained by an increase of terrigenous organic matter derived from C3 type plants during the wet biozones Z and X whereas during the drier biozone Y the associated organic matter shows a greater influence of the productivity of the oceanic biomass. However, a possible contribution of C4 plant-derived particulate organic matter can be assumed for the very top of the Y biozone, introducing a 13C-enriched signal, indicative of marine planktonic input.

Sedimentation and palaeoenvironment in crater lake Barombi Mbo, Cameroon, during the last 25,000 years

Lake Barombi Mbo is situated in a maar crater of the Cameroon volcanic chain. Sediment cores were recovered with a Livingstone corer. This paper presents data on core BM6, 23.5 m long, from a water depth of 110 m. The sediments are mainly organic matter-rich clays (5 to 10% total organic carbon) showing alternating brown and yellowish horizontal laminae on a cm-scale. The lower part of a couplet (basal sublamina) is rich in siliciclastic (quartz, mica) grains, sponge spicules and documents the deposition of flood detritus. The upper unit (upper sublamina) is composed of grey to bluish clay which becomes green toward the top with frequently yellow, layered concretions of siderite. It corresponds to quiet conditions with settling of fine clayey particles from suspension. The overall recurrence rate of these laminae is about 15 years. This frequency is similar to present-day large flood events in this intertropical zone. From 14,000 to 20,000 yrs BP, the sediment accumulation rate was quite low (15–30 g cm−2 10−3 yrs). With less abundant precipitation, pedogenic clay formed from mica-family minerals. From 12,000 yrs BP throughout the Holocene, the rate of accumulation increased (> 40 g cm−2 10−3 yrs) and increasing precipitation favoured the pedogenic formation of kaolinites. Turbidity currents became less important to the overall sedimentation. Plankton production remained low, documented by organic matter which is more of the exinite than the alginite type. Siderite concretions occur as early diagenetic products, possibly related to alkalinity in anoxic waters at the sediment-water interface which limited the formation of Fe3+. Thin ash layers occur between 11.5 and 18 m depth in core BM6, bearing witness to several volcanic events during the Late Quaternary. Sediments are disturbed below 21 m with folded laminations and inverse ages. These probably result from a slump glide of lower slope sediment but could also signal a possible gas event.

Glauconitic sediments on the continental shelf off Vancouver Island, British Columbia, Canada

ABSTRACT The tectonically active Vancouver Island continental margin is characterized by a very narrow (< 80 km) continental shelf and an extremely steep continental slope. The shelf is marked by high wave and current energies on the inner, nondepositional shelf and very slow rates of sediment accumulation on the outer shelf. Within the muddy sands of the outer shelf and slope, glauconitic grains are frequently an important constituent of the sand fraction, occurring in four principal forms: (a) locally abundant, pale green infillings of benthic foraminifera; (b) very abundant, pale to moderately dark green, dull grains displaying varying degrees of fracturing of the external surfaces; (c) scarce, moderately dark green, altered and expanded mica grains; and (d) scarce, very dark green, small (< 200 µm) grains with polished, highly fractured surfaces. Mineralogically, the grains mainly reflect their origins as detrital sediments. They consist most commonly of quartz, plagioclase minerals, chlorite, smectite, and illite. Also, a mixed-layer mineral, smectite-illite, occurs in significant amounts in glauconitic grams but is absent in unaltered mud fragments and in the clay fraction of sediments from the outer shelf. The appearance of this mineral reflects the first stages of glauconite authigenesis. Bulk analyses of glauconitic grains yielded K2O values of less than 2%, reflecting their very early stage of evolution, but 19-28% Fe2O3, several times the level in the surrounding matrix sediments. Microprobe analyses and scanning electron microscopy show that glauconitic smectite forms in the pores and along fractures in the host grains, commonly as a boxwork arrangement or rosette clusters of blades. Fe2O3 and K2O values at these sites can reach 32.5% and 7.5%, respectively. Several lines of evidence, including radiocarbon age determinations on glauconitized foraminifera tests, indicate a Recent age for the Vancouver Island shelf glauconitic grains.

Comment on “Intensifying Weathering and Land Use in Iron Age Central Africa”

Bayon et al. (Reports, 9 March 2012, p. 1219) interpreted unusually high aluminum-potassium ratio values in an Atlantic sediment core as indicating anthropogenic deforestation around 2500 years before the present (B.P.). We argue that there is no terrestrial evidence for forest destruction by humans and that the third millennium B.P. rainforest crisis can be clearly attributed mostly to climatic change.

Modern pollen deposition in the Rhone delta area (Lagoonal and marine sediments), France

The investigation identifies the two main types of pollen sedimentation in the Rhone delta, and presents synthetic pollen spectra in terms of ecological groups in order to predict the pollen source areas. Two different domains are studied: the lagoonal zone occupied by the Vaccares where atmospheric pollen imput predominates and the Rhone mouth area (river delta and prodelta) where fluvial pollen influx prevails. The reflection of local and regional vegetation in the surface sediment pollen spectra is carefully discussed with reference to atmospheric pollen records and to water sample analyses. This work is designed to help the interpretation of past pollen spectra and paleovegetation and through them also paleoclimate reconstructions.

Lithostratigraphy, volcanism, paleomagnetism and palynology of Quaternary lacustrine deposits from Barombi Mbo (West Cameroon): preliminary results

We present preliminary results from the study of 23.50-m core from Lake Barombi Mbo, representing the last 25,000 years. The lake is in an explosion crater formed during Quaternary time. The very laminated sediment is composed mostly of clay containing 5–10% organic carbon. Each couplet is commonly composed of a basal lamina rich in quartz, plant debris, muscovite and sponge spicules, and of a more clayey upper lamina often with siderite. A perturbed section near the base of the core, before ca. 21,000 yr B.P., could be the result of a violent release of gas, such CO2, comparable to the recent Nyos gas eruption. The paleomagnetic studies exhibit high-frequency oscillations interpreted as paleosecular variations of the local geomagnetic field. This first record obtained on the African continent can be closely compared to the type record obtained in Western Europe. The pollen results demonstrate the presence of a forest refuge in West Cameroon during the last great arid period, ca. 18,000 yr B.P. When equatorial forest was broken up, elements of montane vegetation spread to the lowlands. These phenomena resulted from a drying and cooling climate.

Middle to late Quaternary sediment flux and post-depositional processes between the continental slope off Gabon and the Mid-Guinean margin

Abstract Two cores from the continental slope off Gabon (2330 m of water) and one core from the Mid-Guinean marigin (400 m of water) were studied in order to examine climatic and paleoceanographic fluctuations during the middle and late Quaternary. Oxygen isotope variations in one shallow core were correlated with global isotope stages, and extended to the two other cores on the basis of microfaunal and mineralogic contents. Interglacial stages have been characterized by high carbonate content, low carbonate dissolution effect and moderate flux of organic matter derived from primary oceanic production. Glacial stages have included dissolution processes on carbonate tests, related to increased CO2 content near the seabed due to the sink of organic matter from both planktonic and benthic fauna; at a depth of 4000 m of water, this phenomenon has controlled the vertical carbonate distribution through time. Many interglacial intervals record high terrigenous inputs (increased quartz and kaolinite content), while some glacial intervals contain peaks of both well-crystallized smectites (probably reworked from Mesozoic-Cenozoic outcroppings on the adjacent continental shelf) and poorly crystallized smectites (land derived). At a depth of 4000 m of water, quartz-rich layers deposited during some glacial intervals include surface textures indicative of an aeolian origin. A long-term climatic change is recognized, consisting of a progressive cooling of sea-surface waters and of climatic conditions since 400,000–350,000 yrs B.P.

Present-day rhythmic deposition in the grand Rhone prodelta (NW Mediterranean) according to high-resolution pollen analyses

Abstract A high-resolution pollen study (136 samples) has been performed on the KTR05 core (738 cm long) located in the modern Grand Rhône prodelta. The objectives were (1) to evaluate the palynological and sedimentological record quality of a prodelta in comparison with fluvial inputs (2) and to quantify lost sediment (resuspensions) in this area. The core covers more than the last thirty years. By comparison with the modern pollen rain both in the Rhône delta and in the mid-drainage basin (Lyon), a seasonal forcing in pollen deposition has been demonstrated. Monthly pollen successions can be evidenced in a well preserved sequence, providing an estimate of the true sedimentation rate (60 cm.yr−1). Importance of disturbed or incomplete sequences suggests that most of the sediment first deposited in the prodelta (around ⅔) has been resuspended. Sedimentological and palynological data record not only seasonal but fluvial impact. The progressive shift of the prodelta has been evidenced firstly with sedimentology and secondly with palynology. Despite high fluvial impact, pollen grains in the KTR05 core are representatives of the vegetation of the Rhône drainage basin. The apparent homogeneity of the pollen spectra evidences the high reliability of palynology applied on prodeltaic area, at least in a microtidal environment.