Olivier Weber

Zooming in on Heinrich layers

Theories explaining the origin of the abrupt, massive discharges of ice-rafted detritus (IRD) into the glacial North Atlantic (the Heinrich layers (HLs)) generally point to the Laurentide ice sheet as the sole source of these events, until it was found that the IRDs also originated from Icelandic and European ice sheets [Bond and Lotti, 1995; Snoeckx et al., 1999; Grousset et al., 2000]. This apparent contradiction must be reconciled as it raises fundamental questions about the mechanism(s) of HL origin. We have analyzed two ∼12 cm thick HLs in an ultrahigh-resolution mode (1–2 century intervals) in a mid-Atlantic ridge piston core. The δ18O record (N. pachyderma left coiling) reveals strong excursions induced by the melting of the icebergs; these excursions are associated with a strong decrease in the amount of planktic foraminafersand with a 3°C cooling of the surface waters. Counts of coarse detrital grains reveal that IRD are deposited according to a typical sequence (1) volcanic glass, (2) quartz and feldspars, (3) detrital carbonate, that implies a chronology in the melting of the differentpan-Atlantic ice sheets. Sr and Nd isotopic composition confirm that in both Heinrich layers H1 and H2, “precursor” IRD came from first Europe/Iceland, followed then by Laurentide-derived IRD. An internal cyclicity can be identified: during H1 and H2, about four to six major, abrupt discharges occurred roughly on a century timescale. The δ13C and δ15N records reveal that dominant inputs of continent-derived organic matter are associated with IRD within the HLs, hiding the plankton productivity signal.

Clay minerals from the sedimentary cover from the Northwest Iberian shelf

Abstract The Northern Iberian margin is a typical example of a continental margin subjected to seasonal highly energetic regime (waves and tides) and receiving inputs of continental sediments via riverine discharges. The principal goal of this study has been to use clay minerals as indicators of sedimentary dynamics in the open shelf system. The distributions of clay mineral in the top layer of the sedimentary cover are shown to be related to their continental sources, but also reflect the influences of winter storms and longshore currents in determining the pathways of sediment transport. The mineralogical composition of the material issuing from the rivers is very similar to the general mineralogical composition of the fine fractions of the seabed sediments. Those deposits that are directly influenced by riverine discharges have higher contents of kaolinite (>20%), whereas those that are not have higher contents of illite (>80%). The available data indicate no significant quantities of terrigenous particles are being discharged from the Spanish rias. Therefore, we conclude that physical processes are controlling the clay mineral distributions and that, despite contributions from the Minho River, the main source of fine detrital particles to the shelf region is the Douro River discharge. These particles settle on the middle shelf, below the 60 m isobath. During storm events these particles are re-suspended and advected northwards to the Galician shelf or into deeper domains. Thus the distributions of the clays indicate there is a net transport of fine sediments both northwards and off-shelf.

Dispersal and deposition of suspended sediment on the shelf off the Tagus and Sado estuaries, S.W. Portugal

Abstract The Portuguese margin in front of the Tagus and Sado rivers is characterized by a narrow shelf incised by numerous canyons and by a large mud deposit. The two estuaries that feed this continental margin have distinct impact. The suspended particulate matter concentration values in the mouth of the Tagus are four times higher than in the Sado. During the summer the surface nepheloid layer is always larger than during the winter when it is restricted near the mouth of the estuary. This nepheloid layer may reach 30 km in length extending westward. The bottom nepheloid layer usually shows higher nephelometer values, and has a typical distribution: it is usually diverted southward in the direction of the Lisbon Submarine Canyon. We estimate the amount of suspended matter being discharged annually from the Tagus estuary to be between 0.4 and 1×10 6 t. The area covered by fine deposits is about 560 km 2 . Hence the thickness of sediments deposited annually should be between 0.07 and 0.18 cm. The sedimentation rates calculated from the 210 Pb excess vary between 0.16 and 2.13 cm y −1 which correspond to the maximum rate. For a layer of 1 cm thick, 81,000 t of particulate organic carbon (POC) should be trapped. That would represent, with a minimum sedimentation rate between 0.07 and 0.18 cm y −1 , an entrapment of 6000–15,000 t POC y −1 . The trace metals content of box core samples clearly shows the anthropogenic impact in the uppermost level (5 cm thick) in the Tagus estuary and in all the sedimentary deposits (15 cm thick) on the shelf muddy area. Despite the narrowness of the shelf, a significant part of continental fluxes fails to reach the deep ocean.

Present day sedimentary processes on the northern Iberian shelf

Abstract The Galicia-Minho Shelf features two large mud patches, the Douro and the Galicia Mud patches. These are recent sediment bodies that have accumulated under a combination of conditions including: (1) abundant supplies of sediment; (2) morphological barriers that act as sediment traps; and (3) hydrographic conditions that favour the accumulation of fine sediment in these sinks. This paper describes the mechanisms controlling the deposition of the fine-grained sediment depositions and the processes that result in resuspension processes on the Galicia-Minho Shelf. Fine-grained sediments are provided from discharges from the river basins on the southern sector of the shelf, mainly the Douro and Minho rivers. Sediments are exported from river estuaries onto the shelf during episodic flood events. In contrast, most of the sediments originating from the Galician hinterland fail to contribute significantly to sedimentation on the shelf, because they are retained in the Galician Rias, which function as sediment traps. Sediments deposited on the shelf are frequently remobilized, particularly during southwesterly storms that coincide with downwelling conditions. Once in suspension the fine-grained sediments are transported northwards by the poleward flowing bottom currents and are eventually deposited on the Douro and Galicia Mud patches after a series of resuspension events. The locations of the two mud patches are strongly influenced by the shelf morphology. Fines already deposited on the mud patches are occasionally reintroduced into the system by large storm events. Some material from the Douro Mud patch and adjacent areas is re-deposited in the Galicia Mud patch. It is probable that sediments re-suspended from the Galicia Mud patch are carried off the shelf when storm events coincide with downwelling conditions.

Recent sedimentation and sedimentary budgets on the western Iberian shelf

Abstract Thus study is based on 31 box-core samples collected mainly from the mud-patches of the Galician and North-Portuguese shelf. Each of these cores, some decimetres in length, have been described, radiographied and sampled each centimetre for grain-size analysis and measurement of the particulate organic carbon (Corg) content. 210Pb excess profiles have been measured using gamma spectrometry, which, combined with the analysis of the vertical variations of the sedimentary facies, have allowed estimates of sedimentation rates. These rates were found to vary between 0.05 and 0.40 cm yr−1. Considering the latitudinal variation of sedimentation rates around the 100m contour, we note that the maximum rates occurred off the Douro estuary in the southern sector and also near the Galician rias to the north. The organic content (Corg) of sediments varied between 0.4 and 2%. Latitudinally Corg showed a decrease from the South to the North. These results coupled with the sedimentary map (see Dias, Gonzalez, Garcia & Diaz del Rio, 2001 ) have permitted us to establish the detrital mass budget and an inventory of Corg burial.

Geochemical tracers of northern Portuguese estuarine sediments on the shelf

The northern Portuguese coastal zone is drained by large Iberian rivers, along which there is intensive industrial, agricultural and urban activity. Offshore of the two main river basins of the Douro and Minho, two fine-grained sedimentary formations can be characterised. A geochemical study of this sediment cover has been performed focussing in particular on the fine deposits and the adjacent estuaries. The chemical composition of superficial sediments collected from the northern Portuguese shelf and from the river basins of the Minho and Douro rivers was determined for major, minor and trace elements; the rare earth elements (REE) composition was also determined in selected samples. Statistical analyses of these data for the shelf sediments allow us to identify elements with similar behaviours and the different sediment types along the shelf, and to evaluate of their origins. Downcore profiles of the chemical composition of the fine sediments reveal no evidence for changes induced by recent human activities. To assess the influence of the rivers on the fine sedimentary formations, the elemental distribution patterns of selected sediments have been compared with the estuarine sediments. Lithogenic elements composition normalised to Al and the shale normalised REE distributions showed there are similarities between the Douro River sediments and the sediment composition of the Douro and Galicia mud patches. However, the Galicia mud field showed no relationship in its chemical composition to sediments derived from the Minho River. Nor was any evidence for heavy metal contamination originating from the Douro estuary detected in anthropogenic element/Al ratios in the shelf sediments. This may be because of processes associated with resuspension, complexation and bioturbation that occur during sediment transport and deposition. Nevertheless, the Douro River seems to be the main continental source of fine sediments being deposited on the northern Portuguese shelf.

Nepheloid layer dynamics in the northern Portuguese shelf

Abstract A general hydrographic and nephelometric survey of the NW Portuguese continental shelf and upper slope was undertaken, under winter and spring conditions. The nepheloid layer dynamics along the shelf during three cruises were controlled, principally by the following factors: (a) the hydrography of the shelf-slope waters, i.e. the nepheloid layers followed isopycnals and water masses; (b) prevalence of upwelling or downwelling circulation over the shelf; (c) dispersion of material by river discharge (mainly the Douro river); (d) resuspension of mid-shelf fine deposits induced by swell; (e) peculiar morphology with the presence of Porto Canyon and outer shelf rock outcrops. The general circulation controls the seaward extension of the nepheloid layers. In winter, under dominant downwelling conditions, an intense bottom nepheloid layer (BNL) was observed on the shelf, due to river borne particle supply and remobilization of mid-shelf muddy sediments (depth ≈100 m). In this period the BNL increased in thickness to the top of the slope, following the isopycnals. Near the shelf-break the BNL detached to form intermediate nepheloid layers (INL). Small INLs appear deeper in some areas of the slope. A surface nepheloid layer (SNL) appears in the surface water over the shelf and slope. In spring, biological particles mainly contribute to the SNL, which is separated from the BNL by a zone of clear water. In winter the SNL is restricted to the inner shelf where there are high inputs of fluvial particles. Downwelling circulation probably induces transport of shelf particles to deeper waters in the BNL. In spring, the predominant circulation was southward (upwelling), the water column was highly stratified, and dispersion of particles in the SNL was offshore.

Grain-size relationship between suspended matter originating in the Gironde estuary and shelf mud-patch deposits

Abstract The grain-size variations of the elementary fraction (without aggregates) of Gironde suspended matter during its transfer to the shelf were analyzed and compared with those of the surficial sediment of the west Gironde mud patch, located between water depths of 30 and 70 m, which is considered as a preferential deposition area for estuarine suspended matter (SM). The results made it possible to show that two processes complicate the relationships between the estuary and the shelf: a biogenic contribution identified as early as at the Gironde outlet, corresponding to a mode at 15–20 μm in the SM of the turbid plume, and an inorganic contribution of a fine-grained fraction coming from sandy shelf deposits corresponding to micas with a mode at 50–60 μm and settling on the inner part of the mud patch. The nourishment of the western and deeper part of this mud patch by estuarine suspended matter is derived from the benthic nepheloid layer; in terms of grain-size distribution this supply is characterized by the 7.5 μm mode, which is typical of particulate matter originating in the Gironde estuary.

Late Holocene pollution in the Gernika estuary (southern Bay of Biscay) evidenced by the study of Foraminifera and Ostracoda

The Arketas pier in the Gernika estuary (southern Bay of Biscay) has during summer periods waters with strong hypoxia (≤1 ml l−1) but curiously bears the richest assemblages of foraminifers and ostracods in the whole estuary. The most abundant species in Arketas are: Ammonia beccarii, Cribroephidium williamsoni, Haynesina germanica and Lobatula lobatula (foraminifers) Loxoconcha elliptica and Leptocythere castanea (ostracods). The benthic foraminifer sub-species A. beccarii tepida exhibits in this estuary three different morphotypes, whose distributions are controlled by changes in the salinity, dissolved oxygen and nutrient content of the waters, and thus could be used as a good indicator of environmental alteration caused by those parameters. To determine whether this environmental alteration is due to natural causes or is anthropogenic, micropalaeontologic and sedimentologic contents of a sediment core 140 cm thick have been analysed. The study of Foraminifera and Ostracoda assemblages, species diversity, and A. beccarii tepida morphotypes allows us to reconstruct the evolution of this part of the estuary over the last 3800 years. To examine anthropogenic effects at this site, we compared the occurrence of heavy metals Pb, As, with the record of A. beccarii tepida morph C in Arketas. The maximum concentration of heavy metals coincides with the maximum occurrence of A. b. tepida morph C, in the 1970s (date calculated according to the estimated rate of sedimentation in this area). This indicates that distribution of A. b. tepida morph C is, at least partially, caused by heavy metal pollution. Nevertheless, the fact that we have found no deformed tests of this taxa in Arketas induces us to think that another factor, such as dysaerobia caused by eutrophization of nutrient inputs in several areas of this estuary, might have been responsible for at least of part of the anomalous shift of A. b. tepida morph C at that time. In recent times, an increase in this anthropogenic activity was evident, reaching its maximum in the 1970s and decreasing from the 1980s. During the late Holocene period (up to 3500 years BP), this site was a middle-outer estuarine settlement, euryhaline with low to moderate salinity. This episode ends with a marine transgressive pulsation. After depositional hiatus of unknown duration, recent sediments suggest a modern palaeogeographic position of Arketas located in a lower estuary environment. The study of A. beccarii through the Holocene sediments of Arketas indicates that dysaerobic/pollution conditions were not a singular event. At the same time, the increase of A. b. tepida morph C were particularly strong over several years of the 20th Century, at precisely the time that a noticeable decrease of richness and diversity of calcareous microfauna is detected in the estuary.

Evidence of bioturbation in the Cap-Ferret Canyon in the deep northeastern Atlantic

In the sedimentary column, a combined quantification of burrows and macrobenthic community provides evidence of bioturbation features in the submarine canyon of Cap-Ferret between 2000 and 3000 m depth. An image-processing technique allows accurate quantification of burrow volumes with depth in the sedimentary column. The major bioturbation mode seems to be different in the channel compared to the interfluve. Macrobenthic activity is more inclined to mix the sediment in the channel in response to increased organic matter supplies. Sediment mixing leads to burrow destruction in the upper mixed layer of sediment in the canyon. Burrows are better preserved on the interfluve where mixing is slower. Under the mixed zone, the volume of recorded burrows is higher when sedimentation rate increases, as in the upper canyon. In this transition layer, the burrow volume is estimated to be between 3 and 64% of the total sediment volume depending on the sediment depth. The fill-down of numerous burrows with surface sediment by bioregeneration suggests that anaerobic degradation of fresh organic matter is dominant in this canyon. In the sedimentary column, the negative relationship between carbonate content and macrobenthic abundance confirms that carbonate dissolution is largely influenced by bioturbation.