SW Iberian Margin Research Articles

New Insights Into the Surface‐Ocean Dynamics of the Northeastern Atlantic Ocean Across the Marine Isotope Stage 7

AbstractThe upper water‐column dynamics and surface productivity variability in the Northeastern Atlantic Ocean across the Marine Isotope Stage (MIS) 7 interglacial complex is not well understood. Here, we present high‐resolution planktic foraminiferal proxies combined with Artificial Neural Network based‐sea‐surface temperature (SST) and ice‐rafted detritus records from International Ocean Discovery Program Site U1385, SW Iberian Margin for the intervals representing MIS 8 deglaciation, MIS 7 interglacial complex and MIS 6 glacial inception. The long‐term SST pattern is modulated by insolation and precession parameters across the MIS 7 interglacial complex and is superimposed by the millennial‐scale variability (stadials at ∼250, ∼243, ∼230, ∼221, ∼203, ∼196 and ∼192 ka). The regional SST records indicate high temperature gradient (∼6°C) between the sub‐polar North Atlantic Ocean and the SW Iberian Margin during MIS 7d which enhanced the moisture transport from mid‐to‐high latitudes. Further, low obliquity with low insolation induced cooling at high latitudes and promoted the expansion of ice‐sheets during MIS 7d. Comparison of our faunal proxies with the published marine and terrestrial records from SW Europe and western Mediterranean Sea suggested a weakening and southward shift of Azores High (AH) pressure system, similar to the present‐day (−) NAO‐like atmospheric configuration during the early phases of MIS 7e, MIS 7c and MIS 7a, resulting reduced surface productivity of SW Iberian Margin. During the late phases of MIS 7e, MIS 7c and MIS 7a, the AH pressure system strengthened and shifted northward causing the intensification of the trade winds, a scenario similar to present‐day (+) NAO‐like atmospheric configuration, which resulted in high surface productivity of SW Iberian Margin.

Paleoenvironmental Controls on the Abundances of Magnetofossils in the Southwestern Iberian Margin

AbstractMagnetofossils are nanosized magnetic fossil remnants of magnetotactic bacteria (MTB) that are widely distributed in marine and freshwater sediments. Past studies have revealed that changes in the morphology and abundance of magnetofossils are linked with diverse paleoenvironmental changes, such as glacial‐interglacial variation, redox conditions associated with primary productivity and organic matter supply, chemical weathering and diagenesis. Besides, it has long been speculated that changes in geomagnetic field intensity could also affect the abundance of magnetofossils. However, it is not yet clear, how these environmental factors controlled past MTB populations in combination. To answer this question, we studied magnetofossils‐rich sediments from cores MD01‐2443 and 2444 off the Southwest Iberian Margin. The paleomagnetic records of our samples reveal geomagnetic excursions corresponding to the Laschamp, Blake, Iceland Basin, and Pringle Falls events. We estimated the relative abundance of magnetofossils in the samples based on isothermal remanent magnetization unmixing and found it to increase during warmer periods and decrease during colder periods, while not covary with changes in the geomagnetic field intensity. To further distinguish the role of different paleoenvironmental factors, we compared magnetofossil abundance with planktonic δ18O, δ13C of organic matter, K/Al and Si/Al, which indicate continental ice volume (paleotemperature), organic matter origin, run‐off intensity and productivity, respectively. The results indicate that less continental ice volume (higher paleotemperature), enhanced productivity and run‐off intensity are closely related and jointly promoted magnetofossil production and preservation. Our study offers a comprehensive approach to understanding how magnetofossil assemblages relate to paleoenvironmental changes off the SW Iberian Margin.

Planktonic foraminifera response to orbital and millennial-scale climate variability at the southern Iberian Margin (IODP Site U1385) during Marine Isotope Stages 20 and 19

The orbital configuration of Marine Isotope Stage (MIS) 19 provides a good analogue to our present interglacial, and it is marked by millennial-scale climate variability. To evaluate how orbital and millennial-scale climate changes interact during this interglacial we have conducted a high-resolution study of planktonic foraminifera assemblages at International Ocean Discovery Program (IODP) Site U1385, SW Iberian Margin, encompassing late MIS 20 and MIS 19. Estimates of sea surface temperature (SST), using transfer-functions, are compared with geochemical, XRF elemental data, biomarker and pollen records from the same site. These results are then compared with SST and ice-rafted debris records from North Atlantic IODP Site U1314 to reconstruct basin-wide palaeoceanographic conditions. During MIS 20, the large size of the ice sheets imparted a strong cooling in the subpolar gyre whereas the Iberian Margin remained under the influence of the subtropical gyre with the development of a high meridional thermal gradient in the North Atlantic. During MIS 20 terminal stadial (794–789 ka) and MIS 19b-a stadial events, the meltwater discharges related to the instability of ice sheets affected both the North Atlantic and the Iberian Margin leading to a weaker thermal gradient. Our data indicate that Termination IX (TIX, ∼795–788 ka) was punctuated by short-term warming/cooling phases in the ocean coupled with warming/cooling events on land. A short-term cooling episode, recognized in the ocean and on land, occurred on Termination IX and preceded the beginning of MIS 19c. During MIS 19c, the planktonic foraminifera SST indicates warm and almost stable conditions in the sea decoupling the cool and dry millennial-scale events on land inferred by pollen assemblages. Starting from 780 ka towards MIS 18, the planktonic foraminifera assemblages and SSTs indicate a cooling trend along the Iberian Margin related to a progressive southward migration of the polar front.

Sensitivity of Tsunami Scenarios to Complex Fault Geometry and Heterogeneous Slip Distribution: Case‐Studies for SW Iberia and NW Morocco

AbstractThe SW Iberian margin is one of the most seismogenic and tsunamigenic areas in W‐Europe, where large historical and instrumental destructive events occurred. To evaluate the sensitivity of the tsunami impact on the coast of SW Iberia and NW Morocco to the fault geometry and slip distribution for local earthquakes, we carried out a set of tsunami simulations considering some of the main known active crustal faults in the region: the Gorringe Bank (GBF), Marquês de Pombal (MPF), Horseshoe (HF), North Coral Patch (NCPF) and South Coral Patch (SCPF) thrust faults, and the Lineament South strike‐slip fault. We started by considering for all of them relatively simple planar faults featuring with uniform slip distribution; we then used a more complex 3D fault geometry for the faults constrained with a large 2D multichannel seismic dataset (MPF, HF, NCPF, and SCPF); and finally, we used various heterogeneous slip distributions for the HF. Our results show that using more complex 3D fault geometries and slip distributions, the peak wave height at the coastline can double compared to simpler tsunami source scenarios from planar fault geometries. Existing tsunami hazard models in the region use homogeneous slip distributions on planar faults as initial conditions for tsunami simulations and therefore underestimate tsunami hazard. Complex 3D fault geometries and non‐uniform slip distribution should be considered in future tsunami hazard updates. The tsunami simulations also support the finding that submarine canyons attenuate the wave height reaching the coastline, while submarine ridges and shallow shelves have the opposite effect.

A database of submarine landslides offshore West and Southwest Iberia

Submarine landslides are major geohazards occurring on distinct seabed domains ranging from shallow coastal areas to the deeper points of the ocean. The nature and relief of the seabed are key factors influencing the location and size of submarine landslides. Efforts have recently been made to compile databases of submarine landslide distribution and morphometry, a crucial task to assess submarine geohazards. The MAGICLAND (Marine Geo-hazards Induced by underwater Landslides in the SW Iberian Margin) database here presented contributed to that assessment offshore Portugal. Based on EMODnet bathymetric DEMs and GIS analysis, the morphometric properties of 1552 submarine landslides were analysed and wealth of 40 parameters was obtained. This dataset is now made available for the free use and benefit of the international marine community. Further contributions or analysis based on, and complementing the MAGICLAND database will be welcome.

Control Mechanisms of Primary Productivity Revealed by Calcareous Nannoplankton From Marine Isotope Stages 12 to 9 at the Shackleton Site (IODP Site U1385)

AbstractNowadays, primary productivity variations at the SW Iberian Margin (IbM) are primarily controlled by wind‐driven upwelling. Thus, major changes in atmospheric circulation and wind regimes between the Marine Isotope Stages (MIS) 12 and 9 could have driven substantial changes in phytoplankton productivity which remains poorly understood. We present a high‐resolution calcareous nannofossil record from the Shackleton Site Integrated Ocean Discovery Program Site U1385 that allow the assessment of primary productivity and changing surface conditions on orbital and suborbital timescales over the SW IbM. These records are directly compared and integrated with terrestrial – Mediterranean forest pollen – and marine – benthic and planktic oxygen stable isotopes (δ18O), alkenone concentration [C37], Uk´37‐Sea Surface Temperature and % C37:4 – proxy records from Site U1385. Our results indicate intra‐interglacial increase in primary productivity together with intensification of the Azores anticyclonic high‐pressure cell beyond the summer that suggests a two‐phase upwelling behavior during the full interglacial MIS 11c (∼420–397 ka), potentially drived by atmospheric NAO‐like variability. Primary productivity is largely enhanced during the inception of glacial MIS 10 and the early MIS 10 (∼392–356 ka), linked to intensified upwelling and associated processes during a period of strengthened atmospheric circulation. In agreement with the conditions observed during Heinrich events of the last glacial cycle, primary productivity reductions during abrupt cold episodes, including the Heinrich‐type (Ht) events 4 to 1 (∼436, 392, 384 and 339 ka) and the Terminations V and IV, seems to be the result of halocline formation induced by meltwater arrival, reducing the regional upward nutrient transference.

Probabilistic mapping of earthquake-induced submarine landslide susceptibility in the South-West Iberian margin

The SW Iberian continental margin is well recognized as a tectonically active area, where major canyons and mass wasting events are both present. Earthquake triggered submarine landslides may cause tsunami and result in catastrophic damage to bordering coastal areas. In this setting, submarine landslide susceptibility mapping represents a major step towards a regional risk mitigation strategy. Landslide susceptibility mapping in large offshore areas presents significant challenges as a result of the limited information on controlling variables, large uncertainties in triggering mechanisms and limited geotechnical data. In this study, a geotechnical model-based approach has been followed that narrows the range of controlling factors and, within a probabilistic framework, allows a systematic treatment of parameter uncertainties. This model-based analysis covers the whole SW Iberian margin increasing by three orders of magnitude the areal extent of precedent offshore slope stability susceptibility studies. This jump in spatial scale is facilitated by application of a systematic Bayesian updating procedure, to combine geotechnical information from global databases and that available from regional sites. Seismic shaking is estimated using an available regional database of seismogenic faults. These tools are implemented within a GIS to generate, via Montecarlo simulations, probabilistic landslide susceptibility maps based on two different analytical seismic infinite slope stability models. These models differ mainly in the form of their final results, either as distributions of slope stability safety factors or as distributions of seismic-triggered slope displacements. Receiving Operator Curves are used to assess the different landslide susceptibility predictions obtained against a comprehensive regional database of submarine landslides. It turns out that the models analyzed correctly predict 92% and 82% of the mapped landslide subset chosen for validation for pseudo-static and displacement-based method respectively. This suggests that, within the limits of the currently available databases, seismic events are the dominant factor at the origin of the submarine landslides mapped in the study area. An advantage of the framework presented is that it can quickly incorporate new regional geotechnical information or better regional landslide databases, as they become available.

Paleogeographic evolution of a segmented oblique passive margin: the case of the SW Iberian margin

The SW Iberian margin is an oblique passive margin developed during the Mesozoic, as part of the opening of the westernmost Ligurian Tethys. The Algarve Basin developed on the SW Iberian margin and its sedimentary record was strongly controlled by tectonics. The Algarve Basin was dominated during the Mesozoic by WSW–ENE trending extensional faults and their associated depocenters, segmented by oblique NW–SE transfer zones. Two major NW–SE trending transfer zones connected the Algarve Basin to the Gorringe Bank in the west and to the Betic Basin in the east. The SW Iberian margin was later inverted during Late Cretaceous to Cenozoic times, due to convergence between Africa and Iberia. Inversion of the Algarve Basin and the SW Iberian margin was controlled by both the WSW–ENE and NW–SE trending structures formed during the passive margin phase. The evolution of the Algarve Basin during the Mesozoic and up to the Paleogene is documented in this paper through paleogeographic reconstructions based on field geology and reinterpretation of offshore seismic and well data.

Late Quaternary multi-genetic processes and products on the northern Gulf of Cadiz upper continental slope (SW Iberian Peninsula)

On continental margins, the upper slope to shelf break environment forms a critical region where sediment supply, hydrographic activity and gravitational processes determine how and when sediments are partitioned between the shallow- and deep-marine realm. On the SW Iberian margin, relatively few studies have addressed the dynamics of this region, although it holds key information regarding the link between the sedimentary evolution of the continental shelf and the contourite depositional system on the middle slope. This work therefore presents a high-resolution analysis of the morphological and stratigraphic expressions of late Quaternary (dominantly last glacial and present-day interglacial) sedimentary processes on the upper slope and shelf margin sector between 7° and 7°30’ W.The integration of seismic, bathymetric and hydrographic data reveals the presence of alongslope processes and products (a bottom current-related plastered drift, moat, erosional surface and terrace, an internal wave-/tide-controlled sediment wave field), downslope (gravitational) processes and products (an upper slope – shelf margin valley system, slumps, debrites, gullies), neotectonic elements (diapirs) and fluid flow features (pockmarks, bright spots). The spatial distribution of these features indicates that the study area becomes increasingly alongslope-dominated towards the W, and oppositely, more downslope-dominated towards the E, because sediment supply to the latter area is enhanced under the dominant eastward dispersal of fluvially supplied sediments on the shelf. In addition, glacial-interglacial variations in the amount of sediments supplied to the shelf edge and the intensity of oceanographic processes in the study area also generate a distinct temporal variability, with glacial and interglacial intervals respectively recording principally downslope- and alongslope-controlled morphological elements. Finally, regardless of these overall spatial and temporal patterns, diapirism and fluid flow are inferred to locally destabilize sediments and induce small-scale mass wasting in the study area. These findings are not only relevant to the northern Gulf of Cadiz, but also to the general understanding of sedimentary dynamics and controls in mixed downslope- and alongslope-controlled upper slope to shelf margin settings worldwide.

Tectonic evolution, geomorphology and influence of bottom currents along a large submarine canyon system: The São Vicente Canyon (SW Iberian margin)

A multi-scale dataset consisting of multi-beam echo-sounder, 2D multi-channel seismic and sidescan sonar (TOBI) data allows us to identify a large variety of morphologies originating from sedimentary and tectonic processes along the São Vicente Canyon (SVC), which is the largest submarine canyon developed in the external part of the Gulf of Cadiz. The SVC is located in one of the most seismogenic areas of Western Europe. The convergence between the Eurasian and African plates has controlled the formation and evolution of the canyon. The SVC is tectonically controlled by three main thrust faults: the Marquês de Pombal Fault, the São Vicente Fault and the Horseshoe Fault. No major rivers feed sediment to the canyon head, but the main sediment source is related to the dismantling of canyon flanks and the MOW (Mediterranean Overflow Water). This current contributes sediments by two different processes: a) conturites deposition at the head and flanks of the SVC that periodically fail into the canyon; and b) the coarser-grained and denser sediment of the MOW might be trapped at the head of the canyon and could develops into hyperpycnal flows. The SVC is characterized by retrogressive erosion being submarine landslide deposits and scars the main seafloor morphologies. The tectonic and stratigraphic interpretation of seismic profiles indicate that the SVC is a clear example of a diachronous and segmented canyon developed since the Late Miocene in an area of present-day active plate tectonics. This study investigates the interaction between active tectonics, the dynamics of submarine canyons and the resulting geomorphologies.

The impact of abrupt deglacial climate variability on productivity and upwelling on the southwestern Iberian margin

This study combines high-resolution records of nannofossil abundances, oxygen and carbon stable isotopes, core scanning X-ray fluorescence (XRF), and ice rafted debris (IRD) to assess the paleoceanographic changes that occurred during the last deglaciation on the SW Iberian Margin. Our results reveal parallel centennial-scale oscillations in coccolithophore productivity, nutricline depth and upwelling phenomena not previously observed, explained by means of arrival of iceberg-melting waters, iceberg-induced turbulent conditions, SST changes and riverine discharges. On millennial time-scales, higher primary productivity (PP), shallower nutricline, and upwelling occurrence/invigoration are observed for the Last Glacial Maximum (LGM) and Bølling-Allerød (B/A). The opposite scenario (i.e., lower productivity, deeper nutricline and upwelling weakening/absence) is linked to cold spells such as Heinrich Stadials 2 and 1 (HS2 and HS1) and the Younger Dryas (YD). Such paleoproductivity variations are attributed to latitudinal migrations of the thermal fronts associated with oceanic gyres in the North Atlantic, in parallel to oscillations in the strength of the Atlantic Meridional Overturning Circulation (AMOC). Moderate-to-high PP during the Holocene is ascribed to the development of the modern seasonal surface hydrography, with a more persistent Iberian Poleward Current (IPC) and seasonal wind-induced upwelling.

A new 3-D P-wave velocity model for the Gulf of Cadiz and adjacent areas derived from controlled-source seismic data: application to nonlinear probabilistic relocation of moderate earthquakes

SUMMARYThe SW Iberian margin is well known for its complex tectonic setting and crustal structure and by the occurrence of moderate magnitude earthquakes and some great tsunamigenic earthquakes. Fortunately, many seismic reflection and refraction profiles have been carried out, providing detailed information about the crustal structure of the main geologic domains in this region. These studies show a first-order variation due to the transition from oceanic to continental domain, large-scale heterogeneities within the crust and an irregular Moho topography. Routine earthquake locations in this area have been usually computed using a general 1-D velocity model which is clear that cannot account for such a heterogeneous structure. In addition, regional seismic stations used to locate the Gulf of Cadiz seismicity are on land and far away to the east, implying large azimuthal gaps and distances. In this context, a 3-D approach seems necessary to properly solve the crustal velocity field and improve earthquake location in this area. With this purpose, we present a new digital 3-D P-wave velocity distribution for the crust and uppermost mantle derived from previously published controlled-source seismic experiments carried out in SW Iberia and the Gulf of Cadiz over the last 40 yr. We have reviewed more than 50 wide-angle and multichannel seismic reflection and refraction profiles and digitized the most significant published 2-D seismic velocity models, performing an updated compilation of crustal parameters (P-wave velocities and geometry and depth of the main crustal interfaces). These velocities as a function of position and depth have been interpolated using ordinary kriging algorithm to obtain, in the form of a regular georeferenced 20 × 20 × 1 km grid spacing, a high-resolution 3-D P-wave velocity distribution for the crust and uppermost mantle and a continuous Moho depth map of the whole area of this study (33°N–41°N latitude and 15°W–5°W longitude). Since current seismic location tools allow the implementation of 3-D grid structures, we have applied our 3-D model to relocate a selection of moderate earthquakes occurred in the studied region using a probabilistic nonlinear method. In the Gulf of Cadiz area the probabilistic approximation provides maximum likelihood hypocentres located within the uppermost mantle with the majority of depths ranging between 20 and 45 km. This model would subsequently be implemented at the Spanish Seismic Network for the routine relocation of the seismicity of the area.

Radiocarbon Age Offsets Between Two Surface Dwelling Planktonic Foraminifera Species During Abrupt Climate Events in the SW Iberian Margin.

This study identifies temporal biases in the radiocarbon ages of the planktonic foraminifera species Globigerina bulloides and Globigerinoides ruber (white) in a sediment core from the SW Iberian margin (so‐called Shackleton site). Leaching of the outer shell and measurement of the radiocarbon content of both the leachate and leached sample enabled us to identify surface contamination of the tests and its impact on their 14C ages. Incorporation of younger radiocarbon on the outer shell affected both species and had a larger impact downcore. Interspecies comparison of the 14C ages of the leached samples reveal systematic offsets with 14C ages for G. ruber being younger than G. bulloides ages during the last deglaciation and part of the Early and mid‐Holocene. The greatest offsets (up to 1,030 years) were found during Heinrich Stadial 1, the Younger Dryas, and part of the Holocene. The potential factors differentially affecting these two planktonic species were assessed by complementary 14C, oxygen and carbon isotopes, and species abundance determinations. The coupled effect of bioturbation with changes in the abundance of G. ruber is invoked to account for the large age offsets. Our results highlight that 14C ages of planktonic foraminifera might be largely compromised even in settings characterized by high sediment accumulation rates. Thus, a careful assessment of potential temporal biases must be performed prior to using 14C ages for paleoclimate investigations or radiocarbon calibrations (e.g., marine calibration curve Marine13, Reimer et al., 2013, https://doi.org/10.2458/azu_js_rc.55.16947).

Genesis of mud volcano fluids in the Gulf of Cadiz using a novel basin-scale model approach

Mud Volcanism and fluid seepage are widespread phenomena in the Gulf of Cadiz (SW Iberian Margin). In this seismically active region located at the boundary between the African and Eurasian plates, fluid flow is typically focused on deeply rooted active strike-slip faults. The geochemical signature of emanating fluids from various mud volcanoes (MVs) has been interpreted as being largely affected by clay mineral dehydration and recrystallization of Upper Jurassic carbonates. Here we present the results of a novel, fully-coupled 1D basin-scale reactive-transport model capable of simulating major fluid forming processes and related geochemical signatures by considering the growth of the sediment column over time, compaction of sediments, diffusion and advection of fluids, as well as convective and conductive heat flow. The outcome of the model is a realistic approximation to the development of the sediment pore water system over geological time scales in the Gulf of Cadiz. Combined with a geochemical reaction transport model for clay mineral dehydration and calcium carbonate recrystallization, we were able to reproduce measured concentrations of Cl, strontium and 87Sr/86Sr of emanating mud volcano fluids. These results support previously made qualitative interpretations and add further constraints on fluid forming processes, reaction rates and source depths. The geochemical signature at Porto MV posed a specific problem, because of insufficient constraints on non-radiogenic 87Sr/86Sr sources at this location. We favour a scenario of basement-derived fluid injection into basal Upper Jurassic carbonate deposits (Hensen et al., 2015). Although the mechanism behind such basement-derived flow, e.g. along permeable faults, remains speculative at this stage, it provides an additional source of low 87Sr/86Sr fluids and offers an idea on how formation water from the deepest sedimentary strata above the basement can be mobilized and eventually initiate the advection of fluids feeding MVs at the seafloor.The dynamic reactive-transport model presented in this study provides a new tool addressing the combined simulation of complex physical-geochemical processes in sedimentary systems. The model can easily be extended and applied to similar geological settings, and thus help us to provide a fundamental understanding of fluid dynamics and element recycling in sedimentary basins.

Relative paleointensity (RPI) in the latest Pleistocene (10–45 ka) and implications for deglacial atmospheric radiocarbon

We report magnetic properties and relative paleointensity (RPI) proxies from a suite of 10 conventional piston cores and Kasten cores from the SW Iberian Margin collected during cruise JC089 of the RSS James Cook in August 2013. Mean sedimentation rates are in the 10–20 cm/kyr range. Age models were acquired by correlation of Ca/Ti and Zr/Sr XRF core-scanning data to L* reflectance from the Cariaco Basin that is, in turn, tied to the Greenland ice-core chronology. The natural remanent magnetization (NRM) is represented by a single magnetization component carried by a low-coercivity mineral (magnetite), although reflectance and bulk magnetic properties indicate the presence of a high-coercivity (hematitic) magnetic phase, possibly from eolian dust. The presence of fine-grained hematite means that the sediments are not ideal for RPI studies, however the detrital hematite does not appear to contribute to the NRM or anhysteretic remanent magnetization (ARM). In order to test the usefulness of the RPI data, we construct a stack of 12 RPI records from the SW Iberian Margin for the 0–45 ka interval and compare it with a stack of 12 globally distributed marine and lake records, chosen on the basis of mean sedimentation rates (>15 cm/kyr) and superior age models. The two stacks are similar, but different from published RPI stacks, particularly for the 10–30 ka interval, and imply a virtual axial dipole moment (VADM) high at ∼15–18 ka followed by a drop in field strength from ∼15 to 13 ka. A revised VADM estimate calculated from Greenland 10Be ice-core flux using a contemporary age model is remarkably consistent with the new overall RPI stack, based on Iberian Margin and global RPI records. The elevated atmospheric 14C levels of the last ice age cannot, however, be fully explained by this RPI stack although relative changes such as the long-term drop in atmospheric 14C from 30 to 15 ka are reproduced, supporting the hypothesis of a combined influence of production rate and ocean ventilation on 14C during the last ice age.

The nature of crustal reflectivity at the southwest Iberian margin

Reprocessing of multi-channel seismic reflection data acquired over the northern margin of the Gulf of Cádiz (SW Iberian margin) places new constraints on the upper crustal structure of the Guadalquivir-Portimão Bank. The data presented have been processed with optimized stacking and interval velocity models, a better approach to multiple attenuation, preserved amplitude information to derive the nature of seismic reflectivity, and accurate time-to-depth conversion after migration. The reprocessed data reveal a bright upper crustal reflector just underneath the Paleozoic basement that spatially coincides with the local positive free-air gravity high called the Gulf of Cádiz Gravity High. To investigate the nature of this reflector and to decipher whether it could be associated with pieces of mantle material emplaced at upper crustal levels, we calculated its reflection coefficient and compared it to a buried high-density ultramafic body (serpentinized peridotite) at the Gorringe Bank. Its reflection coefficient ratio with respect to the sea floor differs by only 4.6% with that calculated for the high-density ultramafic body of the Gorringe Bank, while it differs by 35.8% compared to a drilled Miocene limestone unconformity. This means that the Gulf of Cádiz reflector has a velocity and/or density contrast similar to the peridotite at the Gorringe Bank. However, considering the depth at which it is found (between 2.0 and 4.0km) and the available geological information, it seems unlikely that the estimated shortening from the Oligocene to present is sufficient to emplace pieces of mantle material at these shallow levels. Therefore, and despite the similarity in its reflection coefficient with the peridotites of the Gorringe Bank, our preferred interpretation is that the upper crustal Gulf of Cádiz reflector represents the seismic response of high-density intracrustal magmatic intrusions that may partially contribute to the Gulf of Cádiz Gravity High.

Impact of basin structure and evaporite distribution on salt tectonics in the Algarve Basin, Southwest Iberian margin

The SW Iberian margin developed as a passive margin during Mesozoic times and was later inverted during the mainly Cenozoic Alpine orogeny. The initial syn-rift deposits include a Lower Jurassic evaporite unit of variable thickness. In the onshore, this unit is observed to thicken basinward (i.e., southward), in fault-controlled depocenters, and salt-related structures are only present in areas of thick initial evaporites. In the offshore, multiple salt-structures cored by the Lower Jurassic evaporites are interpreted on seismic reflection data and from exploratory drilling. Offshore salt structures include the allochthonous Esperança Salt Nappe, which extends over an area roughly 40 × 60 km. The abundance of salt-related structures and their geometry is observed to be controlled by the distribution of evaporite facies, which is in turn controlled by the structure of rift-related faulting. This paper presents a comprehensive study of salt tectonics over the entire onshore and offshore SW Iberian passive margin (southern Portugal and Gulf of Cadiz), covering all aspects from initial evaporite composition and thickness to the evolution of salt-related structures through Mesozoic extension and Cenozoic basin inversion.

Marine sedimentary lipidomics of the glacial-interglacial changes during the lower Pleistocene (SW Iberian Margin).

Analyses of trimethylsilyl-derivatized neutral lipids by gas chromatography coupled to mass spectrometry afford a wealth of information contained in the marine sediments. This information is useful for unraveling the processes associated with past climate changes such as those from glacial and interglacial periods. Studies of samples from the Iberian Margin deposited about 1,300,000-1,400,000years ago afforded the qualitative identification and quantification of n-alkanes, n-alkan-1-ols, isoprenoids, triterpenoids, sterols, hopanols, alkenones, alkenols, and polycyclic aromatic hydrocarbons. Using this information, the lipidomic approach has allowed to identifying differences in sea surface temperatures, marine productivity, microbial degradation processes, and inputs from continental plants between these periods. Differences in predominant higher plant species related to climate changes have also been observed.

Unexpected weak seasonal climate in the western Mediterranean region during MIS 31, a high-insolation forced interglacial

Marine Isotope Stage 31 (MIS 31) is an important analogue for ongoing and projected global warming, yet key questions remain about the regional signature of its extreme orbital forcing and intra-interglacial variability. Based on a new direct land-sea comparison in SW Iberian margin IODP Site U1385 we examine the climatic variability between 1100 and 1050 ka including the “super interglacial” MIS 31, a period dominated by the 41-ky obliquity periodicity. Pollen and biomarker analyses at centennial-scale-resolution provide new insights into the regional vegetation, precipitation regime and atmospheric and oceanic temperature variability on orbital and suborbital timescales. Our study reveals that atmospheric and SST warmth during MIS 31 was not exceptional in this region highly sensitive to precession. Unexpectedly, this warm stage stands out as a prolonged interval of a temperate and humid climate regime with reduced seasonality, despite the high insolation (precession minima values) forcing. We find that the dominant forcing on the long-term temperate forest development was obliquity, which may have induced a decrease in summer dryness and associated reduction in seasonal precipitation contrast. Moreover, this study provides the first evidence for persistent atmospheric millennial-scale variability during this interval with multiple forest decline events reflecting repeated cooling and drying episodes in SW Iberia. Our direct land-sea comparison shows that the expression of the suborbital cooling events on SW Iberian ecosystems is modulated by the predominance of high or low-latitude forcing depending on the glacial/interglacial baseline climate states. Severe dryness and air-sea cooling is detected under the larger ice volume during glacial MIS 32 and MIS 30. The extreme episodes, which in their climatic imprint are similar to the Heinrich events, are likely related to northern latitude ice-sheet instability and a disruption of the Atlantic Meridional Overturning Circulation (AMOC). In contrast, forest declines during MIS 31 are associated to neither SST cooling nor high-latitude freshwater forcing. Time-series analysis reveals a dominant cyclicity of about 6 ky in the temperate forest record, which points to a potential link with the fourth harmonic of precession and thus low-latitude insolation forcing.

Morphostructure, tectono‐sedimentary evolution and seismic potential of the Horseshoe Fault, SW Iberian Margin

AbstractHigh‐resolution acoustic and seismic data acquired 100 km offshore Cape São Vicente, image with unprecedented detail one of the largest active reverse faults of the SW Iberian Margin, the Horseshoe Fault (HF). The HF region is an area seismogenically active, source of the largest magnitude instrumental and historical earthquake (Mw > 6) occurred in the SW Iberian Margin. The HF corresponds to a N40 trending, 110 km long, and NW‐verging active thrust that affects the whole sedimentary sequence and reaches up to the seafloor, generating a relief of more than 1 km. The along‐strike structural variability as well as fault trend suggests that the HF is composed by three main sub‐segments: North (N25), Central (N50) and South (N45). Swath‐bathymetry, TOBI sidescan sonar backscatter and parametric echosounder TOPAS profiles reveal the surface morphology of the HF block, characterized by several, steep (20°) small scarps located on the hangingwall, and a succession of mass transport deposits (i.e. turbidites) on its footwall, located in the Horseshoe Abyssal Plain. A succession of pre‐stack depth‐migrated multichannel seismic reflection profiles across the HF and neighbouring areas allowed us to constrain their seismo‐stratigraphy, structural geometry, tectono‐sedimentary evolution from Upper Jurassic to present‐day, and to calculate their fault parameters. Finally, on the basis of segment length, surface fault area and seismogenic depth we evaluated the seismic potential of the HF, which in the worst‐case scenario may generate an earthquake of magnitude Mw 7.8 ± 0.1. Thus, considering the tectonic behaviour and near‐shore location, the HF should be recognized in seismic and tsunami hazard assessment models of Western Europe and North Africa.