Change in the North Atlantic circulation associated with the mid-Pleistocene transition

The southwestern Iberian margin is highly sensitive to changes in the distribution of North Atlantic currents and to the position of oceanic fronts. In this work, the evolution of oceanographic parameters from 812 to 530 ka (MIS20-MIS14) is studied based on the analysis of planktonic foraminifer assemblages from site IODP-U1385 (37 degrees 34.285' N, 10 degrees 7.562' W; 2585m b.s.l.). By comparing the obtained results with published records from other North Atlantic sites between 41 and 55 degrees N, basin-wide paleoceano-graphic conditions are reconstructed. Variations of assemblages dwelling in different water masses indicate a major change in the general North Atlantic circulation during MIS16, coinciding with the definite establishment of the 100 ky cyclicity associated with the mid-Pleistocene transition. At the surface, this change consisted in the redistribution of water masses, with the subsequent thermal variation, and occurred linked to the northwestward migration of the Arctic Front (AF), and the increase in the North Atlantic Deep Water (NADW) formation with respect to previous glacials. During glacials prior to MIS16, the NADW formation was very weak, which drastically slowed down the surface circulation; the AF was at a southerly position and the North Atlantic Current (NAC) diverted southeastwards, developing steep south-north, and east-west, thermal gradients and blocking the arrival of warm water, with associated moisture, to high latitudes. During MIS16, the increase in the meridional overturning circulation, in combination with the northwestward AF shift, allowed the arrival of the NAC to subpolar latitudes, multiplying the moisture availability for ice-sheet growth, which could have worked as a positive feedback to prolong the glacials towards 100 ky cycles.info:eu-repo/semantics/publishedVersio

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

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

Differential impact of anthropogenic noise during the acoustic development of begging calls in Blue Tits (Cyanistes caeruleus)

In many bird species, nestling begging signals play a key role in the interaction between parents and their offspring during development. The information conveyed by begging calls can be disrupted by anthropogenic noise, which is one of the major threats to biodiversity in increasingly urbanized landscapes. Here, we describe the developmental change in acoustic structure of begging calls in nestling Eurasian Blue Tits Cyanistes caeruleus; begging calls are pure-tone, low-frequency, soft calls during the first days of development and gradually turn into white-noise, hiss-like, powerful calls of broadband frequency. This strong developmental variation highlights the importance of an extended sampling scheme in developmental studies. Furthermore, we pinpoint two phases where begging calls could be most vulnerable to masking by anthropogenic noise. First, during early development, begging calls are very soft and low-pitched, closer to high-intensity noise bands of traffic noise. Secondly, around day 11, begging calls show reduced tonality, which implies higher degradation, and relatively low amplitude, which implies reduced signal range. We encourage future research to describe acoustic development of begging calls in other species, to provide a robust foundation that will make noise mitigation policies more effective