Mediterranean water in the Atlantic Iberian margin reveals early isolation events during the Messinian Salinity Crisis

Recent studies highlight the role of the Mediterranean Outflow Water (MOW), in the intensification of the Atlantic Meridional Overturning Circulation and as source of heat and salty water to high latitudes. During the Late Miocene the MOW suffered major changes and likely a total collapse during the Messinian Salinity Crisis (MSC). In order to study the MOW evolution in the Atlantic margin during the Tortonian-Messinian interval we completed a new high resolution geochemical and stable isotope record for the corresponding interval of the Montemayor-1 and Huelva-1 cores. Both sites are located in the Guadalquivir Basin on the former Atlantic side of the Mediterranean – Atlantic gateways (Iberian Atlantic margin) during the late Miocene. The tuning of this isotope record with astronomical solutions and other global isotope curves has allowed the establishment of an improved chronology and, consequently, to precisely date environmental changes happening on the Atlantic margin of the Iberian peninsula and their link to Mediterranean and global events. At 7.17 Ma, in concomitance with a shallowing of the basin, the residence time, temperature and salinity of the bottom waters increased. These changes were related to a reduction of the MOW reaching the Atlantic side as a consequence of the restriction of the last strand of the Betic corridor that connected the Mediterranean and the Atlantic. This hypothesis is in line with the analogous changes observed in several Mediterranean Sea locations, where from 7.17 Ma onward a reduced Mediterranean – Atlantic connection is observable. Furthermore, the new isotope chronology sheds light, through comparison with other records, on the age of Messinian geomagnetic reversals.</p

Mediterranean water in the Atlantic Iberian margin reveals early isolation events during the Messinian Salinity Crisis

Recent studies highlight the role of the Mediterranean Outflow Water (MOW), in the intensification of the Atlantic Meridional Overturning Circulation and as source of heat and salty water to high latitudes. During the Late Miocene the MOW suffered major changes and likely a total collapse during the Messinian Salinity Crisis (MSC). In order to study the MOW evolution in the Atlantic margin during the Tortonian-Messinian interval we completed a new high resolution geochemical and stable isotope record for the corresponding interval of the Montemayor-1 and Huelva-1 cores. Both sites are located in the Guadalquivir Basin on the former Atlantic side of the Mediterranean – Atlantic gateways (Iberian Atlantic margin) during the late Miocene. The tuning of this isotope record with astronomical solutions and other global isotope curves has allowed the establishment of an improved chronology and, consequently, to precisely date environmental changes happening on the Atlantic margin of the Iberian peninsula and their link to Mediterranean and global events. At 7.17 Ma, in concomitance with a shallowing of the basin, the residence time, temperature and salinity of the bottom waters increased. These changes were related to a reduction of the MOW reaching the Atlantic side as a consequence of the restriction of the last strand of the Betic corridor that connected the Mediterranean and the Atlantic. This hypothesis is in line with the analogous changes observed in several Mediterranean Sea locations, where from 7.17 Ma onward a reduced Mediterranean – Atlantic connection is observable. Furthermore, the new isotope chronology sheds light, through comparison with other records, on the age of Messinian geomagnetic reversals.</p

Paleomagnetic and paleoenvironmental implications of magnetofossil occurrences in late Miocene marine sediments from the Guadalquivir Basin, SW Spain

Although recent studies have revealed more widespread occurrences of magnetofossils in pre-Quaternary sediments than have been previously reported, their significance for paleomagnetic and paleoenvironmental studies is not fully understood. We present a paleo- and rock-magnetic study of late Miocene marine sediments recovered from the Guadalquivir Basin (SW Spain). Well-defined paleomagnetic directions provide a robust magnetostratigraphic chronology for the two studied sediment cores. Rock magnetic results indicate the dominance of intact magnetosome chains throughout the studied sediments. These results provide a link between the highest-quality paleomagnetic directions and higher magnetofossil abundances. We interpret that bacterial magnetite formed in the surface sediment mixed layer and that these magnetic particles gave rise to a paleomagnetic signal in the same way as detrital grains. They, therefore, carry a magnetization that is essentially identical to a post-depositional remanent magnetization, which we term a bio-depositional remanent magnetization. Some studied polarity reversals record paleomagnetic directions with an apparent 60-70 kyr recording delay. Magnetofossils in these cases are interpreted to carry a biogeochemical remanent magnetization that is locked in at greater depth in the sediment column. A sharp decrease in magnetofossil abundance toward the middle of the studied boreholes coincides broadly with a major rise in sediment accumulation rates near the onset of the Messinian salinity crisis (MSC), an event caused by interruption of the connection between the Mediterranean Sea and the Atlantic Ocean. This correlation appears to have resulted from dilution of magnetofossils by enhanced terrigenous inputs that were driven, in turn, by sedimentary changes triggered in the basin at the onset of the MSC. Our results highlight the importance of magnetofossils as carriers of high-quality paleomagnetic and paleoenvironmental signals even in dominantly terrigenous sediments.This study was funded by the Guadaltyc project (MINECO, CGL2012–30875), ARC grant DP120103952, and NSFC grant 41374073

Trace fossil characterization during Termination V and MIS 11 at the western Mediterranean: Connection between surface conditions and deep environment

[EN] Trace fossil assemblages are studied at Ocean Discovery Program (ODP) Site 977 to characterize the response of the macrobenthic trace maker community to deep paleoenvironmental conditions during the Termination V (TV) and interglacial Marine Isotope Stage (MIS) 11 at the western Mediterranean Alboran Sea. An assemblage composed of Chondrites, Planolites, Scolicia, Thalassinoides and Zoophycos is identified, showing notable variations in ichnodiversity, abundance and Bioturbation Index, that were analyzed in detail. The integration of ichnological information with sediment color and high-resolution coccolithophore records from Site 977, evidenced that variations in macrobenthic trace maker community are primarily controlled by oxygen availability and surface organic productivity patterns. During TV, high surface organic productivity by intense Alboran Upwelling System enhanced the deep organic accumulation that, together with reduced deep-water removal, resulted in a decrease of bioturbation and the formation of an Organic Rich Layer. Moderate and stable surface production through MIS 11c reduced deep food availability, resulting in an oligotrophic and stable deep environment. This is reflected by relatively abundant trace fossils in lighter sediments. Intra-interglacial increase in surface organic production at ~405 ka is evidenced by increased organic matter preservation. Minor impact of western Mediterranean circulation on deep-water removal, but a plausible stronger control by Bernoulli aspiration intensities in the region, is, in overall, observed during these intervals. During the Heinrich-type (Ht) events 3 and 2, increased trace fossil diversity and ameliorated oxygenation is driven by limited surface organic production, but intense western Mediterranean deep-water circulation and enhanced regional deep-water removal.Publicación en abierto financiada por la Universidad de Salamanca como participante en el Acuerdo Transformativo CRUE-CSIC con Elsevier, 2021-202

Evolution from Late Miocene to present-day of sandy deposits on mixed depositional systems in the Gulf of Cadiz

Póster presentado en Mixed / hybrid systems (Turbidite, MTDs and Contourites) on continental margin. Lisbon (Portugal), 21-22 June, 2023This work contributes to improve the knowledge of deep marine deposits, specifically those generated by the interaction of gravitational sedimentary processes and bottom currents (mixed processes). The study of these mixed deposits is of great relevance both in the academic world, and in the industry, due to their potential as energy resources and for carbon dioxide (CO2) storage. However, there is still a great lack of knowledge about their diagnostic criteria, how they are generated, how they evolve, and their socio-economic implications. Offshore seismic reflection and logging data analyses from the continental slope of the Gulf of Cadiz reveal Late Miocene and Pliocene-Quaternary contourite and mixed deposits that host sandy bodies of special interest as potential geological storages. In general, it has been observed that the sandier deposits, exhibiting high-amplitude reflections (HARs), are located mainly on the erosional elements of these depositional systems or in the transition between the erosional and the depositional features: a) contourite channels (or moats and furrows); b) in the proximal setting of contourite terraces; and c) at the exits of the Gibraltar Strait. The sandier deposits in these systems are brought into the channels/moats and terraces by gravitational processes; and once inside they are reworked by the bottom currents, being laterally transported and deposited by a higher velocity core of the current along it. The contourite and mixed deposits generated before the full opening of the Gibraltar Strait, during the Late Miocene (~8.2-5.33 Ma), display different depositional and erosional features related to the paleo-Mediterranean Outflow Water (MOW) circulating through the Betic and Rifian corridors prior to the restriction of the Mediterranean-Atlantic gateway. Natural gamma-ray logs from exploration wells in this area show sand deposits up to 50 m thick, except in wells close to the Miocene paleo-shore where alternating deposits of sand and clay are identified. After the opening of the Gibraltar Strait, the Mediterranean Outflow Water (MOW) has generated a complex Pliocene-Quaternary (5.33 Ma-present) contourite depositional system. Sampling of sandy contourites associated with seismic features also suggests the extensive distribution of mature, well-sorted Pliocene¿Quaternary sand about 600 m thick, and showing the following characteristics: a) Early Pliocene deposits (~5.3-3.2 Ma), correspond to sheeted drifts developed mixed with gravitational sedimentary processes and low acoustic response; b) Late Pliocene-early Quaternary deposits (~3.2-2 Ma) displaying sheeted contourite drifts and enhanced acoustic response towards the top, especially in areas adjacent to highs and banks. Borehole logs for these deposits show cyclic swings in amplitude that are generally lower than those observed for the overlying Quaternary deposits; c) Natural gamma-ray logs along the Quaternary sequence show medium-amplitude cyclic swings, varying on decimetre to sub-meter scale, with no major steps in base levels. The main differences of LM sandy deposits respect to the recent succession, consist of higher thicknesses, better-sorted and higher grain size of the sands, and increasing medium-high amplitude on seismic reflections. The Miocene and Pliocene deposits also exhibit cyclic swings in gamma ray logs, but with lower amplitude and at lower frequencies than those observed for the Quaternary ones