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- The climate of the Common Era off the Iberian PeninsulaPublication . Abrantes, Fatima; Rodrigues, Teresa; Rufino, Marta; Salgueiro, Emilia; Oliveira, Dulce; Gomes, Sandra; Oliveira, Paulo; Costa, Ana; Mil-Homens, Mario; Drago, Teresa; Naughton, FilipaThe Mediterranean region is a climate hot spot, sensitive not only to global warming but also to water availability. In this work we document major temperature and precipitation changes in the Iberian Peninsula and margin during the last 2000 years and propose an interplay of the North Atlantic internal variability with the three atmospheric circulation modes (ACMs), (North Atlantic Oscillation (NAO), east atlantic (EA) and Scandinavia (SCAND)) to explain the detected climate variability. We present reconstructions of sea surface temperature (SST derived from alkenones) and on-land precipitation (estimated from higher plant n-alkanes and pollen data) in sedimentary sequences recovered along the Iberian Margin between the south of Portugal (Algarve) and the northwest of Spain (Galiza) (36 to 42 degrees N). A clear long-term cooling trend, from 0 CE to the beginning of the 20th century, emerges in all SST records and is considered to be a reflection of the decrease in the Northern Hemisphere summer insolation that began after the Holocene optimum. Multi-decadal/centennial SST variability follows other records from Spain, Europe and the Northern Hemisphere. Warm SSTs throughout the first 1300 years encompass the Roman period (RP), the Dark Ages (DA) and the Medieval Climate Anomaly (MCA). A cooling initiated at 1300 CE leads to 4 centuries of colder SSTs contemporary with the Little Ice Age (LIA), while a climate warming at 1800 CE marks the beginning of the modern/Industrial Era. Novel results include two distinct phases in the MCA: an early period (900-1100 years) characterized by intense precipitation/flooding and warm winters but a cooler spring-fall season attributed to the interplay of internal oceanic variability with a positive phase in the three modes of atmospheric circulation (NAO, EA and SCAND). The late MCA is marked by cooler and relatively drier winters and a warmer spring-fall season consistent with a shift to a negative mode of the SCAND. The Industrial Era reveals a clear difference between the NW Iberia and the Algarve records. While off NW Iberia variability is low, the Algarve shows large-amplitude decadal variability with an inverse relationship between SST and river input. Such conditions suggest a shift in the EA mode, from negative between 1900 and 1970 CE to positive after 1970, while NAO and SCAND remain in a positive phase. The particularly noticeable rise in SST at the Algarve site by the mid-20th century (+/- 1970), provides evidence for a regional response to the ongoing climate warming. The reported findings have implications for decadal-scale predictions of future climate change in the Iberian Peninsula.
- Change in the North Atlantic circulation associated with the mid-Pleistocene transitionPublication . Martin-Garcia, Gloria M.; Sierro, Francisco; Flores, José A.; Abrantes, FatimaThe 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.
- The Late Pleistocene-Holocene sedimentary evolution of the Sines Contourite Drift (SW Portuguese Margin): A multiproxy approachPublication . Teixeira, Manuel; Terrinha, Pedro; Roque, Cristina; Voelker, Antje; Silva, Pedro; Salgueiro, Emilia; Abrantes, Fatima; Naughton, Filipa; Mena, Anxo; Ercilla, Gemma; Casas, DavidThe Sines Contourite Drift (SCD), located in the Alentejo margin, southwest Iberian continental margin, has been through many depositional phases in result of dimatic variations and bottom current oscillations, which determined a variable depositional pattern and an irregular sedimentary evolution since the Late Pleistocene. The SCD, being in the main path of the Mediterranean Outflow Water (MOW), which greatly constrains the sedimentary building of this drift, constitutes the distal part of the Gulf of Cadiz Contourite Depositional System and its sedimentary evolution therefore reflects the history of MOW variations. In order to investigate this, we report on a multiproxy analysis of grain-size, carbon content and physical, geochemical, and environmental-magnetic properties on the AMS C-14 dated 350-an long gravity core CO14-GC-07, retrieved in the SCD, at 1425 meters water depth (mwd). The main objective is to reconstruct the evolution of the sedimentary package of the SCD from >43 cal kyr BP to Present and bring new insights about the impact of bottom currents' activity on the morphosedimentary evolution of the margin. Results show the existence of four distinct depositional phases in response to climate variations and bottom current oscillations during the Late Pleistocene-Holocene. Phase 1 (350-322 cm: >42.9 cal kyr BP) occurred in Marine Isotope Stage (MIS) 3 and presents silty-muddy sediments with presence of shell fragments and moderate bioturbation. Phase 2 (322-176 cm: >42.9-similar to 30.5 cal kyr BP), which lasted from middle MIS 3 to the onset of MIS 2, reveals sediment coarsening towards the top limit, suggesting climate cooling and strong bottom current winnowing. Phase 3 (176-144 cm: similar to 30.5-17.1 cal kyr BP) covers most of MIS 2 prior to the last deglaciation and shows the sharpest variations on all sediment properties and the coarsest sediment facies, with Fe-rich layers associated with the increase of lerrigenous input and sea-level regression that coincide with the enhancement of MOW. Phase 4 (144-0 cm: <17.1 cal kyr BP) is associated with the degladal sea-level rise and represents a calmer depositional environment with finer sediments and increasing contributions of biogenic material. The MOW is the most prominent bottom current and the most active seabed shaping agent in the study area, whose vertical shifting during glacial (similar to 800-2200 mwd) and interglacial (600-1500 mwd) periods is coincident with the most active morphosedimentary sector of the area, contributing to the development of landslide scars and sediment waves. (C) 2020 Elsevier B.V. All rights reserved.
- Surface and deep water variability in the Western Mediterranean (ODP Site 975) during insolation cycle 74: high-resolution calcareous plankton and molecular biomarker signalsPublication . Quivelli, Ornella; Marino, Maria; Rodrigues, Teresa; Girone, Angela; Maiorano, Patrizia; Abrantes, Fatima; Salgueiro, Emilia; Bassinot, FrankWe reconstructed changes in productivity and surface/subsurface and deep-water dynamics in the Western Mediterranean through a multi-proxy study of Ocean Drilling Program Site 975 between late Marine Isotope Stage (MIS) 20 and early interglacial MIS 19. Our high-resolution study (down to similar to 200-year resolution) combines calcareous plankton assemblages (coccolithophores and foraminifera), biomarkers (C-37-alkenones, n-alkanes, n-alcohols) and elemental proxies (total organic carbon, total nitrogen, calcium carbonate). Surface water conditions are derived (i) from high-resolution delta O-18 and delta C-13 records obtained from the planktonic foraminifer Globigerina bulloides, and (ii) from summer and winter, foraminifera-based sea surface temperature reconstructions (SSTJAS-foram, SSTJFM-foram) achieved through transfer function. The integration of the whole dataset makes it possible to identify in the Balearic Sea, and to accurately characterize for the first time, an Organic Rich Layer (ORL) during latest MIS 20-early MIS 19, close to i-cycle 74. Its presence is marked firstly by higher values of total nitrogen (TN) and an increase of total C-37-alkenone and total organic carbon (TOC) preserved in the sediments. The multi-proxy approach reveals that the deglacial phase played a prominent role for ORL formation that was characterized by centennial scale phases. The alcohol preservation index (API) suggests that the shoaling of the circulation, which boosted marine productivity, started in the deglaciation and, in combination with freshening by Atlantic water inflow/riverine input and surface water buoyancy during sea level rising, culminated during the ORL event. At this time calcareous plankton proliferated on subsurface-surface waters, benefiting from ameliorating conditions, which promoted maximum marine productivity and higher organic matter preservation on the seafloor.
- Studying the past of Mediterranean outflow based on 230th excess inventories and contouritesPublication . Veiga-Pires, C.; Ghaleb, Bassam; Voelker, Antje; Abrantes, FatimaThe Mediterranean Outflow water (MOW) comes out from the Mediterranean Sea and then contours the northern slope of the Cadiz Gulf. Along its way to the southern Portuguese Margin, it divides itself into three levels flowing at different depths, 400 m, 800 m and 1200 m, respectively. These different pathways induce a series of contourites along the Cadiz slope as well as some sedimentary drifts, such as the Faro Drift. Based on the assumption that the sedimentologic characteristics of these contourites should give some light on the history of MOW velocity and intensity variability, two long sedimentary cores collected during the Marion Dufresnes 114/Images cruise in 1999 have been studied. The sampling sites of these two cores, MD99-2336 and MD99-2339, located in the Cadiz Gulf at 690 and 1177 m water column depths respectively, are thus, actually, below the first level and in the main core of the MOW third level. Along time, variations in these current levels, parallel to the slope, should then influence the existence and characteristics of contourites in both sedimentary records. For this purpose, thorium-230 (230Th) as well as granulometric and micropaleontologic analysis have been undergone at high resolution on the 4 uppermost meters spanning MIS1 to LGM times. The referred current prints can be detected by analysing surface and down core sediment for its 230Th content. This radioisotope is produced by the radioactive decay of uranium-234 which content in oceanic waters is known. Therefore, its production rate in the water column can be estimated as a linear function of the water depth (~ 2.6 dpm/cm2.ka for 1 km water depth). As 230Th is almost insoluble, it will sink to the oceanic floor together with the settling particles. This vertical flux to the underlying sediment is considered, in a first order approximation, equal to its production rate in the water column. On this basis, the 230Th excess in the sediment becomes a proxy for sedimentation versus erosion processes accordingly to the sign of the difference between the total and the vertical 230Th flux, i.e. if it is, respectively, positive or negative. With this method it is then possible to extrapolate on the location of the high velocity core area and whether its intensity changed or not looking at the inventory of excess 230Th in the contourite units. We acknowledge FEDER and OE that financed this study through the Portuguese Foundation for Science and Technology (PDCTM/PP/MAR/15297/1999).
- Diatoms as a paleoproductivity proxy in the NW Iberian coastal upwelling system (NE Atlantic)Publication . Zuniga, Diana; Santos, Célia; Frojan, Maria; Salgueiro, Emilia; Rufino, Marta; De la Granda, Francisco; Figueiras, Francisco G.; Castro, Carmen G.; Abrantes, FatimaThe objective of the current work is to improve our understanding of how water column diatom's abundance and assemblage composition is seasonally transferred from the photic zone to seafloor sediments. To address this, we used a dataset derived from water column, sediment trap and surface sediment samples recovered in the NW Iberian coastal upwelling system. Diatom fluxes (2.2 (+/- 5.6) 10(6) valves m(-2) d(-1)) represented the majority of the siliceous microorganisms sinking out from the photic zone during all studied years and showed seasonal variability. Contrasting results between water column and sediment trap diatom abundances were found during downwelling periods, as shown by the unexpectedly high diatom export signals when diatom- derived primary production achieved their minimum levels. They were principally related to surface sediment remobilization and intense Minho and Douro river discharge that constitute an additional source of particulate matter to the inner continental shelf. In fact, contributions of allochthonous particles to the sinking material were confirmed by the significant increase of both benthic and freshwater diatoms in the sediment trap assemblage. In contrast, we found that most of the living diatom species blooming during highly productive upwelling periods were dissolved during sinking, and only those resistant to dissolution and the Chaetoceros and Leptocylindrus spp. resting spores were susceptible to being exported and buried. Fur-thermore, Chaetoceros spp. dominate during spring-early summer, when persistent northerly winds lead to the upwelling of nutrient-rich waters on the shelf, while Leptocylindrus spp. appear associated with late-summer upwelling relaxation, characterized by water column stratification and nutrient depletion. These findings evidence that the contributions of these diatom genera to the sediment's total marine diatom assemblage should allow for the reconstruction of different past upwelling regimes.
- Coral mound development at the Campeche cold-water coral province, southern Gulf of Mexico: Implications of Antarctic Intermediate Water increased influence during interglacialsPublication . Matos, Lelia; Wienberg, Claudia; Titschack, Juergen; Schmiedl, Gerhard; Frank, Norbert; Abrantes, Fatima; Cunha, Marina R.; Hebbeln, DierkCoral mounds formed by framework-forming scleractinian cold-water corals (CWC; mainly Lopheliapertusa) are a common seabed feature along the Atlantic continental margins. While coral mound areas in the NE Atlantic reveal a climate-dependent temporal pattern of CWC occurrence and mound aggradation that is related to distinct environmental conditions (e.g., productivity, water mass properties, hydrodynamics), the long-term development of CWC and coral mounds at the western side of the Atlantic is less well documented and understood. Here, we present a 260-kyr coral record from the recently described Campeche CWC province in the southern Gulf of Mexico, combined with a reconstruction of the paleo-environmental conditions for the last 140 kyr. Uranium-series dating of 26 coral samples reveals that CWC growth predominantly coincided with interglacial periods. Highest vertical mound aggradation rates of 34 to 40 cm kyr(-1) occurred during the Holocene. The reduced occurrence of CWC and the concurrent almost complete stagnation in mound aggradation during glacial periods could be linked to a diminished presence of Antarctic Intermediate Water at those intermediate depths in which the coral mounds occur. Such setting would have caused a less dynamic bottom current regime resulting in a reduced food supply to the CWC along the Campeche Bank.
- The complexity of millennial-scale variability in southwestern Europe during MIS 11Publication . Oliveira, Dulce; Desprat, Stephanie; Rodrigues, Teresa; Naughton, Filipa; Hodell, David; Trigo, Ricardo; Rufino, Marta; Lopes, Cristina; Abrantes, Fatima; Sanchez Goni, Maria FernandaClimatic variability of Marine Isotope Stage (MIS) 11 is examined using a new high-resolution direct land sea comparison from the SW Iberian margin Site U1385. This study, based on pollen and biomarker analyses, documents regional vegetation, terrestrial climate and sea surface temperature (SST) variability. Suborbital climate variability is revealed by a series of forest decline events suggesting repeated cooling and drying episodes in SW Iberia throughout MIS 11. Only the most severe events on land are coeval with SST decreases, under larger ice volume conditions. Our study shows that the diverse expression (magnitude, character and duration) of the millennial-scale cooling events in SW Europe relies on atmospheric and oceanic processes whose predominant role likely depends on baseline climate states. Repeated atmospheric shifts recalling the positive North Atlantic Oscillation mode, inducing dryness in SW Iberia without systematical SST changes, would prevail during low ice volume conditions. In contrast, disruption of the Atlantic meridional overturning circulation (AMOC), related to iceberg discharges, colder SST and increased hydrological regime, would be responsible for the coldest and driest episodes of prolonged duration in SW Europe. (C) 2016 University of Washington. Published by Elsevier Inc. All rights reserved.
- Limited variability in the phytoplankton Emiliania huxleyi since the pre-industrial era in the Subantarctic Southern OceanPublication . Rigual-Hernández, A. S.; Sánchez-Santos, J. M.; Eriksen, R.; Moy, A. D.; Sierro, F. J.; Flores, J. A.; Abrantes, Fatima; Bostock, H.; Nodder, S. D.; González-Lanchas, A.; Trull, T. W.The Southern Ocean is warming faster than the average global ocean and is particularly vulnerable to ocean acidification due to its low temperatures and moderate alkalinity. Coccolithophores are the most productive calcifying phytoplankton and an important component of Southern Ocean ecosystems. Laboratory observations on the most abundant coccolithophore, Emiliania huxleyi, suggest that this species is susceptible to variations in seawater carbonate chemistry, with consequent impacts in the carbon cycle. Whether anthropogenic environmental change during the industrial era has modified coccolithophore populations in the Southern Ocean, however, remains uncertain. This study analysed the coccolithophore assemblage composition and morphometric parameters of E. huxleyi coccoliths of a suite of Holocene-aged sediment samples from south of Tasmania. The analysis suggests that dissolution diminished the mass and length of E. huxleyi coccoliths in the sediments, but the thickness of the coccoliths was decoupled from dissolution allowing direct comparison of samples with different degree of preservation. The latitudinal distribution pattern of coccolith thickness mirrors the latitudinal environmental gradient in the surface layer, highlighting the importance of the geographic distribution of E. huxleyi morphotypes on the control of coccolith morphometrics. Additionally, comparison of the E. huxleyi coccolith assemblages in the sediments with those of annual subantarctic sediment trap records found that modern E. huxleyi coccoliths are 2% thinner than those from the pre-industrial era. The subtle variation in coccolith thickness contrasts sharply with earlier work that documented a pronounced reduction in shell calcification and consequent shell-weight decrease of 30-35% on the planktonic foraminifera Globigerina bulloides induced by ocean acidification. Results of this study underscore the varying sensitivity of different marine calcifying plankton groups to ongoing environmental change.
- Sea-surface temperature, productivity and hydrological changes in the Northern Indian Ocean (Maldives) during the interval similar to 575-175 ka (MIS 14 to 7)Publication . Alonso-Garcia, Montserrat; Rodrigues, Teresa; Abrantes, Fatima; Padilha, M.; Alvarez-Zarikian, C. A.; Kunkelova, T.; Wright, J. D.; Betzler, C.The South Asian Monsoon (SAM) drives seasonal changes in the atmospheric and ocean circulation of the tropical Indian Ocean, affecting precipitation on land and oceanic primary productivity. This work examined sediments from the International Ocean Discover Program (IODP) Site U1467 (IODP Expedition 359) located in the Maldives Inner Sea (Northern Indian Ocean) at a water depth of 487 m. The Maldives Inner Sea is a perfect location to study past changes in tropical climate and ocean circulation related to monsoon dynamics in the Equatorial Indian Ocean. This study focuses on the similar to 575-175 ka interval, from Marine Isotope Stage (MIS) 14 to 7, an important period for human evolution and dispersal to Eurasia. We reconstructed sea-surface temperature (SST), based on alkenone unsaturation index (U-37(K')), hydrological changes, based on terrestrial input of n-alkanes, and past surface ocean productivity, based on total C-37 alkenones concentration. The U-37(K')-SST record shows a difference of about 1.5 degrees C between glacial and interglacial periods, clearly showing all stages and substages from MIS 14 to 7, and revealing a connection between ice sheets extension and SST at the equatorial region. The n-alkanes concentration and average chain length index indicate vegetation changes at the Indian Peninsula with drier conditions generally associated with glacial periods. Precipitation increases abruptly at the end of terminations, lasting for a variable time interval in each interglacial period, except for MIS 13. However, other mechanisms superimposed to the glacial-interglacial forcing, such as the Indian Ocean Dipole (IOD) mode, have been invoked to explain shorter-scale variability in precipitation over India. The total alkenone concentration record indicates that primary productivity at Site U1467 is strongly associated with orbital changes, probably related to the summer inter-tropical insolation gradient (SITIG, 23N-23S on June 21st). High primary productivity occurred during intervals of low SITIG, which resulted in strong inter-monsoon (April-May and October-November) Indian Ocean Equatorial Westerlies (IEW) and reduced precipitation in the equatorial region. This mechanism may also be related to the IOD, which affects the strength of the IEW.