F. J. Lobo

Natural and human-induced Holocene paleoenvironmental changes on the Guadiana shelf (northern Gulf of Cadiz)

Three contrasting sedimentary environments on the continental shelf off the Guadiana River (northern Gulf of Cadiz) were integrated in a chronological framework and analysed in terms of sedimentology and benthic foraminiferal assemblages to understand the Holocene paleoenvironmental evolution. The analysed environments differ in terms of their depositional regimes and benthic foraminiferal assemblages. However, a dominant fluvial origin of the sand fraction was observed in all three environments. Holocene sedimentary processes were mainly controlled by natural (sea level changes and climate variations) and human-induced processes (e.g. deforestation, agriculture) along four evolutionary stages. The three older stages were mainly influenced by natural processes, such as sea level variations and fluvial inputs, whereas the most recent stage reflects a combination of climatic- and human-induced processes. A deepening of sedimentary environments related to a period of rapid sea level rise, strongly influenced by river discharges occurred from c. 11,500 to c. 10,000 cal. yr BP. A reduction in sediment export to the shelf, as a result of the continuous and rapid sea level rise and enhanced estuary infilling reflects the second stage, from c. 10,000 to c. 5000 cal. yr BP. The beginning of the third stage, from c. 5000 to c. 1500–1000 cal. yr BP, is marked by a sea-level slowdown and the relatively stable climate and environmental conditions. The fourth stage, from c. 1500–1000 cal. yr BP to Recent times, reflects the intensification of human-induced processes and climatic variability in the Guadiana River basin. This stage also reflects modern depositional conditions, with the formation of a proximal prodeltaic wedge and a distal muddy body.

Chapter 11 The Iberian Mediterranean shelves

Abstract The Iberian Mediterranean shelves are divided into three different geographical segments (the Northeastern Shelf, the Southeastern Shelf and the Northern Alboran Sea Shelf), the understanding of which has evolved over the years. The best known sector is the Northeastern Shelf, comprising the narrow, abrupt and prograding Catalonia Shelf and the wider, prograding Ebro Shelf–Gulf of Valencia, where pioneering Spanish marine geology studies have been conducted since the 1970s. The knowledge of the Quaternary stratigraphic architecture of the Northeastern Shelf is very detailed, and provides an outstanding example of regressive–transgressive cycles leading to shelf build-up with various margin configurations. The Southeastern Shelf exhibits a change of margin configuration from intermediate to abrupt in response to declining fluvial influence. The knowledge of this shelf is limited in comparison with the rest of the Iberian Mediterranean shelves. Abundant studies have also been performed on the Northern Alboran Sea Shelf, which, in contrast to the Northeastern Shelf, does not have a major fluvial source but numerous short, mountain rivers draining from the Betic Cordillera. For this shelf, a high-resolution sequence stratigraphy model has been proposed for the most recent Late Quaternary depositional sequence.

Subaqueous Dunes Over Sand Ridges on the Murcia Outer Shelf

Multibeam swath bathymetry, high-resolution seismic data and sediment samples were used to characterize a field of sand ridges and subaqueous dunes on the outer Murcia continental shelf (western Mediterranean Sea). Sand ridges are 1.5–3 m high and show a predominant E-W orientation oblique to the present-day shoreline. High-resolution seismic data reveal a backstepping stacking pattern of high-angle clinoforms dipping towards the southwest, interpreted as buried sand bodies. Subaqueous dunes have a mean height of 0.3 m and appear superimposed on the sand ridges showing a NW-SE orientation oblique to the ridges. They are composed of sandy sediments and display asymmetric morphology, with the lee side towards the southwest. Ridge and dune asymmetry and internal structure are indicative of long-term sediment transport towards the southwest. At present, dune migration rates deduced from repeated bathymetric surveys indicate that the dunes remain stationary or migrate at very low rates on a decadal scale.

Seismic Stratigraphy of Miocene to Recent Sedimentary Deposits in the Central Scotia Sea and Northern Weddell Sea: Influence of Bottom Flows (Antarctica)

Multichannel and high resolution seismic profiles from the central Scotia Sea and northern Weddell Sea show a sequence of seismic units interpreted to be the result of high-energy bottom currents. The seismic character of the units is indicative of active bottom flows, which developed extensive drifts under the influence of the Weddell Sea Bottom Water (WSBW) and the Antarctic Circumpolar Current (ACC). The opening of the connection between Jane Basin and the Scotia Sea is marked by a major regional unconformity that recorded a reorganization of bottom flows. The uppermost deposits are characterized by intensified bottom currents, which may reflect increased production of WSBW.

Hazard potential of widespread but hidden historic offshore heavy metal (Pb, Zn) contamination (Gulf of Cadiz, Spain)

Natural and human-induced seabed sediment disturbances affect wide areas of the global coastal ocean. These recurrent to chronic disturbances mobilize significant amounts of material, including substances that have the potential to significantly harm the environment once re-released. This very challenging issue is difficult to deal with if sub-surface contaminant concentrations are unknown. Based on the analysis of 11 new, up to 5-m long sediment cores taken offshore in the Gulf of Cadiz, the contamination history (using the trace elements lead and zinc) is well documented over major parts of the gulf. Ore mining and metal processing industries on the southwestern Iberian Peninsula started five thousand years ago and experienced a first peak during the Roman Period, which can be detected over the entire gulf. The Industrial Era added a massive, shelf-wide heavy metal excursion of unprecedented dimension. This metal contamination to the coastal ocean decreased in the 1990s and appears to be today limited to larger areas off the Tinto/Odiel and Guadiana River mouths. The unforeseen, significant finding of this study is that the gulf-wide, peak heavy metal concentration, stemming from the Industrial Era, is widely overlain by a modern sediment veneer just thick enough to cover the contaminant horizon, but thin enough to have this layer within the reach of natural or human-induced sediment mobilization events.

Sedimentary infilling of bedrock-controlled palaeo-embayments off Cape Trafalgar, Strait of Gibraltar (Gulf of Cadiz)

This study investigates two bedrock-controlled palaeo-coastal embayments on the Barbate Platform off Cape Trafalgar near the Strait of Gibraltar (Gulf of Cadiz shelf, SW Iberian Peninsula), aiming to reveal their infilling dynamics and the influence of rocky outcrops on shallow-water hydrodynamics and sediment transport. The approach relies on detailed multibeam bathymetric data, high-resolution seismic profiles and tidal current simulations. Elongated rocky outcrops formed a palaeo-coast when sea level was approximately 35 to 20 m below that of the present day, and bound a relatively flat area. However, the seismic profiles enabled to distinguish two main troughs (A and B) that were infilled following a distinctive evolution during the last transgression. Five seismic units were identified (I to V, from base to top). Deposit A is composed of seismic units II to V and is interpreted as a marine embayment infill, here termed the Barbate palaeo-embayment (BPE). Deposit B is composed of seismic units I to IV and is interpreted as a palaeo-valley infill, here termed the Barbate palaeo-valley (BPV). The complex internal stratigraphic architecture depicts an overall evolution from tidal/fluvial deposits to shallow-water marine deposits. Most significant is the occurrence of coupled tidal flats/estuarine sand bars constituting the infilling of the BPE; this suggests the persistence of a high-energy current in a shallow, confined embayment, which was amplified by the rocky outcrop constrictions and possibly facilitated by the episodic movement of a normal fault. In contrast to this active setting, the nearby straight and narrow BPV was subjected to lower-energy infilling dynamics during its initial flooding phases. There, tidal activity was possibly reduced by the straight morphology of the valley and the occurrence of a topographic constriction (i.e. the Barbate Passage) at the mouth of the valley.

Structure of Subsurface Sediments in the Scan Basin (Scotia Sea)

The structure of sediments in the Scotia Sea is used as a basis for reconstructing the geological history of its bottom in the Late Quaternary. The Scan Basin is one of the main elements of the topography of the southern Scotia Sea. Its formation played a considerable role in the fragmentation of the continent, which included the Bruce and Discovery banks. The main parameters of the sediment layer in the Scan Basin have been reconstructed by the present time, but its top part has not been studied. In this work, we analyze the first data obtained on the R/V Gesperidas with the use of a TOPAS PS 18/40 high-resolution seismic profilograph in 2012. Three layers in the subsurface sediments on the bottom of the Scan Basin were specified for the first time. The mean periods of their deposition in the Late Quaternary were determined as 115000 years for the first, 76000 years for the second, and 59 000 years for the third layer from the surface of the bottom. The duration of the total accumulation period of the three layers is about 250000 years.