M. Canals

Impact of iceberg melting on Mediterranean thermohaline circulation during Heinrich events

Down-core samples of planktonic and benthic foraminifera were analyzed for oxygen and carbon isotopes in International Marine Past Global Changes Study (IMAGES) core MD99-2343 in order to study the interactions between climate change in the Northern Hemisphere and the western Mediterranean thermohaline circulation at times of Heinrich events (HE). Our results confirm the antiphase correlation between enhanced North Atlantic Deep Water formation and low ventilation in the Mediterranean. However, this study reveals that this antiphase relationship in deepwater formation between the North Atlantic and Mediterranean was interrupted during times of HE when the injection of large volumes of water from melting icebergs reached the entrance to the Mediterranean. These events, which lasted less than 1000 years, are represented by pronounced decreases in both planktonic d18O and benthic d13C signals. Lower salinities of Mediterranean surface water resulted in a slowdown of western Mediterranean deepwater overturn even though cold sea surface temperatures and drier climate should have resulted in enhanced deepwater formation.

Response of the Western Mediterranean Sea to rapid climatic variability during the last 50,000 years: a molecular biomarker approach

Abstract The present paper is a synopsis of the research on the climatic evolution of the Western Mediterranean Sea developed within the MATER programme. The sea surface temperature (SST) evolution during the last glacial period, deglaciation and present interglacial have been examined in detail. Special attention has been focussed to millennial–centennial scale changes related to rapid global climatic oscillations. The results have shown the extreme sensitivity of the Western Mediterranean oceanography to this rapid climatic variability giving rise to amplified climatic signals, e.g. strong SST oscillation, that follow the changes recorded in the North Atlantic Ocean or in Greenland ice. Overall, the Western Mediterranean Sea appears to be an ideal environment for the study of the climatic processes occurring at high and intermediate latitudes.

Flow variability in the Gulf of Lions during the MATER HFF experiment (March-May 1997)

Abstract Hydrological and current meter data were gathered during the High Frequency Flux Experiment that took place in the Gulf of Lions from March to May 1997, within a 20×40-km experimental box over the shelf edge and the continental slope offshore of Marseille. The data set has been supplemented and jointly analysed with sea surface temperature images to characterise flow variability with a particular regard on the mesoscale effects associated to the regional circulation. The current meter observations showed a southwestward flow characteristic of the Northern Current. It is strongest near the shelf break, constrained to flow along topography near the bottom but much less polarised over the upper and mid-slope. Mesoscale variability shows up as fluctuations around 3.5- and 7-day periods at 250- and 650-m depth, and around 7-day periods at 1230-m depth. The Northern Current, as observed on satellite images, forms a 30-km-wide stream with meanders displaying length scales longer than 60 km. These meanders are observed to embrace smaller structures, which are responsible for the intense mesoscale activity recorded in current meter data. Both the hydrological observations and current meter data down to 650-m depth match the superficial structures. Two major flow patterns are observed during the experiment: (i) the core of the Northern current flows south of the experimental site during March and early April, with transitory eddies moving over the experimental site; (ii) following the traverse of a westward-propagating large meander on mid-April, the Northern Current remains over the experimental site. We provide evidence that the synoptic current variability observed in the upper layer may be related to baroclinic instability. A distinct near-bottom current variability is also documented on the mid-slope. Bottom trapped topographic waves are proposed as the mechanism which produces the along-slope deep flow variability.

Holocene climate variability in the western Mediterranean region from a deepwater sediment record

[1] The detailed analysis of the International Marine Past Global Changes Study core MD99-2343 recovered from a sediment drift at 2391 m water depth north of the island of Minorca illustrates the effects of climate variability on thermohaline circulation in the western Mediterranean during the last 12 kyr. Geochemical ratios associated with terrigenous input resulted in the identification of four phases representing different climatic and deepwater overturning conditions in the Western Mediterranean Basin during the Holocene. Superimposed on the general trend, eight centennial- to millennial-scale abrupt events appear consistently in both grain size and geochemical records, which supports the occurrence of episodes of deepwater overturning reinforcement in the Western Mediterranean Basin. The observed periodicity for these abrupt events is in agreement with the previously defined Holocene cooling events of the North Atlantic region, thus supporting a strong AtlanticMediterranean climatic link at high-frequency time intervals during the last 12 kyr. The rapid response of the Mediterranean thermohaline circulation to climate change in the North Atlantic stresses the importance of atmospheric teleconnections in transferring climate variability from high latitudes to midlatitudes.

Composition and spatio-temporal variability of particle fluxes in the Western Alboran Gyre, Mediterranean Sea

Total mass and main constituents (carbonate, organic matter, biogenic opal and lithogenic fraction) flux series were obtained in the northern part of the Western Alboran Gyre during an annual cycle from July 1997 to May 1998, at 10 days sampling interval. Two mooring lines equipped with sediment trap–current meter pairs were deployed across the continental slope off Malaga. CTD, SeaWiFS and watershed fluvial discharge data sets were also obtained from several sources to establish their relations with particle flux data. Time-averaged fluxes for the whole period, including fluxes of organic carbon, increased slightly with depth in both locations as the result of the input of particulate matter by near-bottom nepheloid layers. Furthermore, time-averaged fluxes at mid waters were higher in the centre of the gyre than at its periphery, suggesting some kind of particle funnelling from the gyre periphery towards its centre. Temporal evolution of fluxes was highly variable throughout the year. All series showed an overall common evolution with a first peak at the beginning of summer 1997, a second broad composite peak during late autumn/winter and a third one in spring 1998. Composition of settling particles showed that summer and spring peaks were richer in biogenic constituents compared to the late autumn/winter peak. Particle fluxes to mid water depths in the northern part of the Alboran Sea seem to be mainly controlled by fluvial discharge and primary production. Fluvial discharge could be responsible for the higher lithogenic flux during autumn and winter, while high primary production could play a key role in generating biogenic particles during spring and summer. Regarding near-bottom fluxes, the temporal evolution was controlled both by the downward transfer of particulate matter from mid waters and the advective input of mostly lithogenic and carbonate matter. The increased advective input during maximum near-bottom fluxes is tentatively related to intense eddy-like activity recorded in the deep-water masses.

Accumulation rates of major constituents of hemipelagic sediments in the deep Alboran Sea: a centennial perspective of sedimentary dynamics

The accumulation rates of sediment and major constituents in three different geographic areas of the Alboran Sea have been assessed by means of 210Pb and 137Cs concentration profiles. Mixing is present in the top layer of the sedimentary column, with mixing coefficients estimated to range from 0.2 to 15 cm2 yr−1. Overall, apparent sedimentation rates for the last 100 years range from 0.014 to 0.182 g cm−2 yr−1 and show a tendency to decrease with distance to the coast and water column depth. However, specific characteristics of the sea floor for each physiographic environment govern the actual patterns of sediment accumulation. Particularly noticeable are the feeding role of submarine canyons, the trapping effect of slope terraces and the isolation caused by ridges. Sediment accumulation induced by turbiditic flows is also observed north of the Almeria–Oran area. Excess 210Pb inventories and surface concentrations reveal a net effect of sediment focussing and point to the role of bottom nepheloid layers in supplying a significant fraction of sediments to the deep areas. This is also evidenced by comparison of bottom sediment with the sediment trap data available from the Malaga area. We estimate that the particle advective input below 30 m above the bottom accounts for as much as 50–70% of the material that is ultimately deposited onto the sea floor. The greater abundance of lithogenic material in the Malaga zone (∼80%) reflects its larger input in the western Alboran Sea. Conversely, carbonate contents increase from less than 20% in this area to about 30% in the Almeria–Oran zone, reflecting the lesser importance of the dilution by lithogenic components. Biogenic silica was only detected in some surface samples, and no significant preservation was observed below the Eh boundary. Abundance and accumulation rates of organic matter are in accordance with the spatial patterns of primary production in the Alboran Sea: higher in the western part, due to the presence of the Western Alboran Gyre. However, near-bottom redistribution leads to the homogenisation of organic matter concentrations in bottom sediments of any given area. Finally, from comparisons with sediment trap data, the degradation of organic matter has been estimated to be approximately 30–40% of what reaches the bottom.

The Gebra Slide: a submarine slide on the Trinity Peninsula Margin, Antarctica

A large submarine slide – the Gebra Slide – has been discovered on the continental margin of Trinity Peninsula, Central Bransfield Basin, Antarctic Peninsula. The slide scar is clearly expressed in the bathymetry and is cut into the toe of the glacial-period slope-prograding strata on the lower continental slope. Seismic data give evidence of an associated debris-flow deposit embedded in the interglacial-period basin-fill strata of the basin floor. The total volume of sediment involved in the mass movement is about 20 km3. Indirect dating of the mass-wasting event, based on seismic–stratigraphic relationships of the slide scar and associated debris-flow deposit with underlying glacial-period slope units and the overlying interglacial-period basin-floor units, suggests that it took place at the transition between the last glacial period and the present-day interglacial. The initiation of the Gebra Slide is attributed to a combination of several factors, such as high sedimentation rates during the last glacial period, the unloading effect of a retreating ice sheet during deglaciation, pre-existing tectonic fabric and high seismicity in the area. This is the first recent submarine slide of this size identified on the glacial, continental margins of Antarctica. In morphology and general characteristics it is quite similar to the well-known large-scale submarine slides from the northern hemisphere glacial margins, although it is smaller. Its most striking characteristic is its lower-slope position (at 1500–2000 m of water depth), which remains up to now difficult to explain.

A dynamic explanation for the origin of the western Mediterranean organic‐rich layers

The eastern Mediterranean sapropels are among the most intensively investigated phenomena in the paleoceanographic record, but relatively little has been written regarding the origin of the equivalent of the sapropels in the western Mediterranean, the organic-rich layers (ORLs). ORLs are recognized as sediment layers containing enhanced total organic carbon that extend throughout the deep basins of the western Mediterranean and are associated with enhanced total barium concentration and a reduced diversity (dysoxic but not anoxic) benthic foraminiferal assemblage. Consequently, it has been suggested that ORLs represent periods of enhanced productivity coupled with reduced deep ventilation, presumably related to increased continental runoff, in close analogy to the sapropels. We demonstrate that despite their superficial similarity, the timing of the deposition of the most recent ORL in the Alboran Sea is different than that of the approximately coincident sapropel, indicating that there are important differences between their modes of formation. We go on to demonstrate, through physical arguments, that a likely explanation for the origin of the Alboran ORLs lies in the response of the western Mediterranean basin to a strong reduction in surface water density and a shoaling of the interface between intermediate and deep water during the deglacial period. Furthermore, we provide evidence that deep convection had already slowed by the time of Heinrich Event 1 and explore this event as a potential agent for preconditioning deep convection collapse. Important differences between Heinrich-like and deglacial-like influences are highlighted, giving new insights into the response of the western Mediterranean system to external forcing.

Subseafloor stratigraphic profiling and soil classification from piezocone tests: A case study in the Gulf of Lion (NW Mediterranean Sea)

We show the results provided by piezocone tests in determining the stratigraphic profile and the soil classification of two drilling sites in the outer shelf and the upper slope of the Gulf of Lion, PRGL2 and PRGL1, respectively. Correlations with grain-size data indicate that sleeve friction can be used for profiling fine-grained sediments (site PRGL1), whereas cone tip resistance is the most adequate for sequences made of alternations of coarse- and fine-grained intervals (site PRGL2). Normalized cone resistance and friction ratio proved to be also appropriate for soil stratigraphy as it depicts trends in the coarse fraction of the tested soil. Silts and clays present in similar proportions at site PRGL1 responded to piezocone testing as pure clays usually do. Consequently, classical soil classification methods resulted in erroneous interpretation of these sediments as clays, whereas classification of the heterogeneous deposits at PRGL2 was consistent with the grain size. When tied to a high-resolution seismic reflection profile, the stratigraphy interpreted from the piezocone profile matches with the main seismic sequences and discontinuities defined from seismic stratigraphy analysis. Graded bedding also matches with cone tip resistance and sleeve friction data.

Bransfield Basin fine-grained sediments: late-Holocene sedimentary processes and Antarctic oceanographic conditions

The Antarctic Peninsula is sensitive to climatic change due to its northerly position and to the relatively reduced volume and character of its ice cover. High-resolution palaeoclimatic records from the Ant arctic Peninsula ice cores extend back only 500 years. A climatic record of 2850 years in the Bransfield Basin is investigated through the analysis of sediment gravity cores from the floor of the central subbasin (core GEBRA-1) and the slope of the eastern subbasin (core GEBRA-2). Sedimentological, mineralogical and geo chemical properties have been systematically measured, together with Accelerator Mass Spectrometry (AMS) radiocarbon dating. The fine-grained sediments result from two main processes: hemipelagic settling from resuspensions and primary productivity, and turbidity currents. Hemipelagic sediments were selected to investi gate the oceanographic and climatic conditions of the northern Antarctic Peninsula region during the last three millennia. Cold climatic periods are characterized by millimetric laminations and/or black layers with higher organic carbon, nitrogen and opal contents. Warm periods are recorded as massive to diffuse laminated facies with lower biogenic contents. The results include the ‘Little Ice Age’ (LIA) cold pulse as well as several 200– 300 year long fluctuations within the LIA and before this major climatic event of the Holocene.