Tagus Abyssal Plain

Tagus Abyssal Plain
Name	Tagus Abyssal Plain
Location	North Atlantic Ocean, east of the Iberian Peninsula
Type	Abyssal plain
Basin countries	Portugal, Spain

Tagus Abyssal Plain is an extensive bathymetric feature on the abyssal seafloor east of the Iberian Peninsula, occupying part of the eastern North Atlantic Ocean basin near the mouth of the Tagus River. The plain lies seaward of the Portuguese Continental Shelf and south of the Iberian Abyssal Plain margin, forming a relatively flat depositional surface influenced by distal inputs from continental sources and by regional tectonics associated with the Iberian Plate, the Eurasian Plate and the African Plate. Its setting makes it relevant to studies linked to the Gulf of Cadiz, the Iberian Margin, and transatlantic circulation connecting to the Mid-Atlantic Ridge.

Geography and Geomorphology

The plain extends across the continental rise between the Gulf of Cadiz escarpment and abyssal basins adjacent to the Mid-Atlantic Ridge, with bathymetric gradients controlled by inherited structures such as the Torres Vedras Fault system and other fracture zones familiar to practitioners of marine geology in the Azores–Gibraltar Fault Zone. Morphological elements include elongate sediment waves, buried channel-levee complexes linked to past turbidity current pathways, and locally incised canyons that connect to the Lisbon Canyon and smaller gullies mapped by multinational programs including teams from the Instituto Português do Mar e da Atmosfera, the Spanish Instituto Español de Oceanografía, and the National Oceanography Centre. The seafloor hosts contourite drifts that reflect persistent bottom current interaction similar to drifts observed on the Northeast Atlantic Margin, and its geomorphology has been imaged by multibeam echosounder surveys conducted using vessels such as RRS James Cook, RV Pelagia, and RV Sarmiento de Gamboa.

Geological History and Sedimentation

Sedimentation on the plain records Pleistocene to Holocene variations of the Tagus River discharge, eustatic sea-level changes from events like the Last Glacial Maximum, and episodic mass transport deposits correlated with Mediterranean outflow events studied in the context of the Messinian Salinity Crisis and post‑Messinian readjustments. Sediment provenance studies using detrital zircon geochronology, heavy-mineral suites, and isotopic tracers tie sediments to upland source terranes such as the Central System (Iberia), the Trás-os-Montes region, and southern Iberian river catchments explored by researchers affiliated with University of Lisbon, University of Porto, CSIC, and international groups from Georgia Tech and Lamont–Doherty Earth Observatory. Stratigraphic records show alternation of hemipelagic clays, turbidites, and contourites consistent with models of slope instability like those investigated in the Storegga Slide and Cap Blanc analogs. Chronostratigraphy has been constrained by radiocarbon dating, optically stimulated luminescence performed in collaboration with CEA, and biostratigraphic markers such as planktonic foraminifera assemblages cataloged by scientists from the Natural History Museum, London.

Oceanography and Water Masses

Bottom and intermediate waters bathing the plain are influenced by the eastward extension of the North Atlantic Current and by modified waters from the Mediterranean Outflow Water that traverse the Gibraltar Strait; these interactions are monitored by programs like Argo, GO-SHIP, and regional moored arrays maintained by EuroGOOS members. Thermohaline structure exhibits seasonal and interannual variability tied to forcing from atmospheric systems including the Azores High, the North Atlantic Oscillation, and episodic storms cataloged by ECMWF reanalyses. Bottom currents produce nepheloid layers similar to those described off the Bay of Biscay and contribute to sediment redistribution measured with current meters and acoustic Doppler current profilers aboard platforms operated by IFREMER and the Woods Hole Oceanographic Institution. Biogeochemical cycling on the plain couples to oxygen minimum dynamics explored in the framework of GEOTRACES and carbon sequestration processes relevant to the IPCC carbon budget.

Biological Communities and Ecology

The abyssal plain supports assemblages typical of North Atlantic deep-sea plains, including benthic invertebrates such as holothurians, echinoderms, ophiuroids, and polychaetes recorded by surveys using remotely operated vehicles like ROV Victor 6000 and towed cameras deployed by teams from Ifremer, NOCS, and the Portuguese Navy research vessels. Megafaunal distributions correlate with organic carbon fluxes linked to surface productivity in regions influenced by the Iberian Upwelling and seasonal blooms driven by the North Atlantic spring bloom. Microbial communities in sediments exhibit chemosynthetic and heterotrophic metabolisms characterized in metagenomic studies by laboratories at Max Planck Institute for Marine Microbiology and Marine Biological Laboratory (MBL), while benthic-pelagic coupling has been investigated through time-series experiments in collaboration with ICOS infrastructure. Conservation concerns raise connections with regional designations under frameworks like the Convention for the Protection of the Marine Environment of the North-East Atlantic.

Human Activities and Impact

Human pressures include deep-sea fisheries historically prosecuted by fleets from Portugal, Spain, France, and Morocco targeting demersal species on the upper slope, as reported by agencies such as the European Commission and the Food and Agriculture Organization. Hydrocarbon exploration and pipeline routing in the nearby Portuguese EEZ prompted environmental assessments by companies headquartered in cities such as Lisbon, Madrid, and London, with permitting overseen by national ministries. Seabed mining interest in polymetallic nodules and possible aggregates has prompted stakeholder engagement by institutions like the International Seabed Authority and regional regulators. Anthropogenic pollution, including microplastic deposition, radionuclide fallout traced to testing sites cataloged by the Comprehensive Nuclear-Test-Ban Treaty Organization, and litter recorded by deep-sea expeditions, has been quantified by collaborative projects with NOAA and UNEP.

Research and Survey Methods

Investigations employ multibeam echosounder mapping, sub-bottom profiling, seismic reflection surveys run by contractors such as PGS and academic groups from ETH Zurich, combined with sediment coring using gravity corers and piston corers developed by Kongsberg Maritime partners. In situ experiments utilize benthic landers, autonomous underwater vehicles like AUV Remus variants, and ROV deployments supported by institutions including Woods Hole Oceanographic Institution, Scripps Institution of Oceanography, and Ifremer. Data synthesis integrates geochemical assays at facilities including WHOI and Lamont–Doherty Earth Observatory, molecular taxonomy at the Smithsonian Institution and computational modeling using tools developed at NCAR and MPI for Meteorology. Long-term monitoring benefits from international programs like Eurofleets and open-data repositories maintained by EMODnet and PANGAEA.

Category:Abyssal plains