Browsing by Author "Oliveira, Dulce"
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- A 12,000-yr pollen record off Cape Hatteras — Pollen sources and mechanisms of pollen dispersionPublication . Naughton, F.; Keigwin, L.; Peteet, D.; Costas, S.; Desprat, S.; Oliveira, Dulce; de Vernal, A.; H L Voelker, Antje; Abrantes, FatimaIntegrating both marine and terrestrial signals from the same sediment core is one of the primary challenges for understanding the role of ocean-atmosphere coupling throughout past climate changes. It is therefore vital to understand how the pollen signal of a given marine record reflects the vegetation changes of the neighboring continent. The comparison between the pollen record of marine core JPC32 (KNR178JPC32) and available terrestrial pollen sequences from eastern North America over the last 12,170 years indicates that the pollen signature off Cape Hatteras gives an integrated image of the regional vegetation encompassing the Pee Dee river, Chesapeake and Delaware hydrographic basins and is reliable in reconstructing the past climate of the adjacent continent. Extremely high quantities of pollen grains included in the marine sediments off Cape Hatteras were transferred from the continent to the sea, at intervals 10,100-8800 cal yr BP, 8300-7500 cal yr BP, 5800-4300 cal yr BP and 2100-730 cal yr BP, during storm events favored by episodes of rapid sea-level rise in the eastern coast of US. In contrast, pollen grains export was reduced during 12,170-10,150 cal yr BP and 4200-2200 cal yr BP, during episodes of intense continental dryness and slow sea level rise episodes or lowstands in the eastern coast of US. The near absence of reworked pollen grains in core JPC32 contrasts with the high quantity of reworked material in nearby but deeper located marine sites, suggesting that the JPC32 record was not affected by the Deep Western Boundary Current (DWBC) since the end of the Younger Dryas and should be considered a key site for studying past climate changes in the western North Atlantic.
- Combination of insolation and ice-sheet forcing drive enhanced humidity in northern subtropical regions during MIS 13Publication . Oliveira, Dulce; Desprat, Stéphanie; Yin, Qiuzhen; Rodrigues, Teresa; Naughton, Filipa; Trigo, Ricardo M.; Su, Qianqian; Grimalt, Joan O.; Alonso-Garcia, Montserrat; H L Voelker, Antje; Abrantes, Fatima; Sánchez Goñi, Maria FernandaMarine Isotope Stage (MIS) 13, similar to 533-478 ka, has received particular attention due to the unexpected enhancement of monsoon systems under a cool climate characterized by lower atmospheric CO2 and larger ice volume than many other interglacials. Key questions remain about its regional expression (intensity, climate variability, length), and underlying forcing factors, in particular at the mid-latitudes. Here we examine the SW Iberian vegetation, terrestrial climate and sea surface temperature (SST) variability during MIS 13 by combining pollen and biomarker data from IODP Site U1385 with climate-model experiments. We show, for the first time, that despite strong precessional forcing, MIS 13 stands out for its large forest expansions with a reduced Mediterranean character alternating with muted forest contractions, indicating that this stage is marked by a cool-temperate climate regime with high levels of humidity. Results of our data-model comparison reveal that MIS 13 orbitally driven SW Iberian climate and vegetation changes are modulated by the relatively strong ice-sheet forcing. We find that the Northern Hemisphere ice-sheets prescribed at the MIS 13 climate optimum reinforce the insolation effect by increasing the tree fraction and both winter and summer precipitation. We propose that the interactions between ice-sheets and major atmospheric circulation systems may have resulted in the persistent influence of the mid-latitude cells over the SW Iberian region, which led to intensified moisture availability and reduced seasonality, and, in turn, to a pronounced expansion of the temperate forest.
- Control mechanisms of primary productivity revealed by Calcareous Nannoplankton from marine isotope stages 12 to 9 at the Shackleton Site (IODP Site U1385)Publication . González‐Lanchas, A.; Flores, J.‐A.; Sierro, F. J.; Sánchez Goñi, M. F.; Rodrigues, Teresa; Ausín, B.; Oliveira, Dulce; Naughton, F.; Marino, M.; Maiorano, P.; Balestra, B.Nowadays, 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 (420–397ka), potentially drived by atmospheric NAO-like variability. Primary productivity is largely enhanced during the inception of glacial MIS 10 and the early MIS 10 (392–356ka), 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 339ka) and the Terminations V and IV, seems to be the result of halocline formation induced by meltwater arrival, reducing the regional upward nutrient transference
- Coupled ocean and atmospheric changes during Greenland stadial 1 in southwestern EuropePublication . Naughton, F.; Costas, S.; Comes, S. D.; Desprat, S.; Rodrigues, Teresa; Goni, M. F. Sanchez; Renssen, H.; Trigo, R.; Bronk-Ramsey, C.; Oliveira, Dulce; Salgueiro, Emilia; Voelker, Antje H L; Abrantes, FatimaPaleoclimate reconstructions suggest that the complex variability within the Greenland stadial 1 (GS-1) over western Europe was governed by coupled ocean and atmospheric changes. However, few works from the North Atlantic mid-latitudes document both the GS-1 onset and its termination, which are often considered as single abrupt transition events. Here, we present a direct comparison between marine (alkenone-based sea surface temperatures) and terrestrial (pollen) data, at very high resolution (28 years mean), from the southwestern Iberian shelf record D13882. Our results reveal a rather complex climatic period with internally changing conditions. The GS-1 onset (GS-1a: 12890-12720 yr BP) is marked by a progressive cooling and drying GS-1b (12720-12390 yr BP) is the coldest and driest phase; GS-1c (12390-12030 yr BP) is marked by a progressive warming and increase in moisture conditions; GS-1 termination (GS-1d: 12030-11770 yr BP) is marked by rapid switches between cool wet, cold dry and cool wet conditions. Although hydroclimate response was very unsteady throughout the GS-1 and in particular during its termination phase, the persistence of an open temperate and Mediterranean forest in southwestern Iberia during the entire episode suggests that at least some moisture was delivered via the Westerlies. We propose coupled ocean and atmospheric mechanisms to reproduce these scenario. Changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) as well as variations in the North Atlantic sea-ice growth have favoured the displacement of the polar jet stream's latitudinal position and contributed to a complex spatial pattern and strength of the Westerlies across western Europe. (C) 2019 Elsevier Ltd. All rights reserved.
- Dinoflagellate cyst population evolution throughout past interglacials: key features along the Iberian margin and insights from the new IODP Site U1385 (Exp 339)Publication . Eynaud, Frédérique; Londeix, Laurent; Penaud, Aurélie; Sanchez-Goni, Maria-Fernanda; Oliveira, Dulce; Desprat, Stéphanie; Turon, Jean-LouisIODP 339 Site U1385 ("Shackleton site", e.g. Hodell et al., 2013a), from the SW Iberian margin, offers the opportunity to study marine microfossil population dynamics by comparing several past interglacials and to test natural shifts of species that occurred across these warm periods, in a subtropical context. Here, more specifically, we present results obtained for the dinoflagellate cyst (dinocyst) population integrated at a regional scale thanks to the addition of data from proximal sites from southern Iberian margin. When possible, observations made using the dinocyst bio-indicator are compared to additional proxies from the same records in order to test the synchronicity of the marine biota response. Pollen data available for some of the compiled marine sequences also offer the opportunity to directly compare marine biota with terrestrial ecosystem responses. This spatio-temporal compilation reveals that, over the last 800 ka, surface waters around Iberia were tightly coupled to (rapid) climate changes and were characterised by coherent dinocyst assemblage patterns, highlighting a permanent connection between Atlantic and Mediterranean waters as evidenced through a continuous exchange of dinocyst populations. Some index species well illustrate the evolution of the regional hydrographic context along time, as for instance Spiniferites and Impagidinium species, together with Lingulodinium machaerophorum, Bitectatodinium tepikiense and heterotrophic brown cysts. They constitute key bio-indicators in context of natural environmental shifts at long and short timescales.
- Leaf wax biomarkers of a high-mountain lake area in western iberia—Implications for environmental reconstructionsPublication . Santos, Ricardo N.; Schefuß, Enno; Cordeiro, Lívia; Oliveira, Dulce; Hernández, Armand; Ramos, Alexandre M.; Rodrigues, TeresaLeaf wax n-alkane biomarkers are widely used to infer past vegetation dynamics and hydroclimate changes. The use of these compounds strongly relies on the characterization of modern plants. However, few studies have explored leaf waxes of modern plants and their application to reconstructing climate and environmental changes in the Iberian Peninsula, a region known for its high vulnerability to climate change. In this study, we characterize the distributions and compound-specific isotopic compositions of the leaf waxes of dominant plants in the vegetation cover, soil, and surface sediment of the Lake Peixao area, a high-mountain glacial lake in Serra da Estrela (central Portugal). Our results show that the modern oro-Mediterranean (subalpine) vegetation of the study area is dominated by C-3 grasses/herbs and shrubs that preferentially produce long-chain leaf waxes (& GE;C-27). The C-31 n-alkane display the overall highest concentration, produced by some grasses and shrubs, but especially Erica sp (heather), which is highlighted as a major source for the total n-alkane pool in the lake sediments. C-29 is the second-most abundant and the most equally produced n-alkane of the vegetation cover; C-25 and C-27 homologs are mainly associated with aquatic-related grasses/herbs, while C-33 and C-35 are particularly linked to cold-drought tolerant Juniperus sp. shrubs. Shrubs show higher but proportional values than grasses/herbs in the isotopic space, suggesting a directly proportional physiological adaptation of the two ecological forms to the prevailing climatic and environmental factors of the study area. C-29 is pointed as the most representative (or less plant-biased) leaf wax n-alkane in the lake sediments. Thus, delta D of C-29 n-alkane is interpreted as a robust terrestrial hydrological indicator (delta D-terr), which signal is believed to be strongly influenced by the mean air temperature and/or precipitation amount. Despite the sparse vegetation and small catchment area, the apparent hydrogen fractionation factor, determined from delta D-terr of the lake surface sediment, is in line with the modeled global mean values for the latitude of the study area. The different molecular and compound-specific signatures of the studied oro-Mediterranean species have the potential to support future interpretations of leaf wax biomarkers in the Iberian Peninsula.
- Marine Isotope Stage 11c in Europe: recent advances in marine–terrestrial correlations and their implications for interglacial stratigraphy – a reviewPublication . Candy, Ian; Oliveira, Dulce; Parkes, Daniel; Sherriff, Jennifer; Thornalley, DavidThe interglacial known as MIS 11c (c. 426 000-396 000 years ago) receives intensive international interest because of its perceived role as an analogue for the current interglacial and its importance for understanding future climate change. Here we review the current understanding of the stratigraphy of this interglacial in Europe. This study considers (i) the evidence for the environmental history of this interglacial as reconstructed from the varved lake records from northern Europe, (ii) the climate history of MIS 11c as preserved in the long pollen records of southern Europe and (iii) a comparison of both of these with marine records from the North Atlantic. The result of this review is a discussion of the evidence for millennial and centennial scale climate change found in European records of MIS 11c, the patterns of warming that are seen across this interglacial and the discrepancy in aspects of the duration of this interglacial that seems to exist between the marine and terrestrial records of this warm period. A review of the recent advances in the study of MIS 11c in Europe confirms its importance for understanding both the past evolution of the Holocene and the future patterns of long-term climate change.
- A new perspective of the Alboran Upwelling System reconstruction during the Marine Isotope Stage 11: a high-resolution coccolithophore recordPublication . González-Lanchas, Alba; Flores, José-Abel; Sierro, Francisco J.; Bárcena, María Ángeles; Rigual-Hernández, Andrés S.; Oliveira, Dulce; Azibeiro, Lucía A.; Marino, Maria; Maiorano, Patrizia; Cortina, Aleix; Cacho, Isabel; Grimalt, Joan O.A high-resolution study of the MIS 12/MIS 11 transition and the MIS 11 (430-376 kyr) coccolithophore assemblages at Ocean Drilling Program Site 977 was conducted to reconstruct the palaeoceanographic and climatic changes in the Alboran Sea from the variability in surface water conditions. The nannofossil record was integrated with the planktonic oxygen and carbon stable isotopes, as well as the U-37(k') Sea Surface Temperature (SST) at the studied site during the investigated interval. The coccolithophore primary productivity, reconstructed from the PPP (primary productivity proxy = absolute values of Gephyrocapsa caribbeanica + small Gephyrocapsa group) revealed pronounced fluctuations, that were strongly associated with variations in the intensity of the regional Alboran Upwelling System. The comparison of the nannoplankton record with opal phytolith content for the studied site and the already available pollen record at the nearby Integrated Ocean Drilling Program Site U1385, suggests an association of the upwelling dynamics with the variability of the North Atlantic Oscillation-like (NAO-like) phase. High PPP during positive (+) NAO-like phases is the result of intensified upwelling, owing to the complete development of the surface hydrological structures at the Alboran Sea. This scenario was identified during the MIS 12/MIS 11 transition (428-422 kyr), the late MIS 11c (405-397 kyr), and MIS11 b to MIS 11a (397-376 kyr). Two short-term minima in the PPP and SST were observed during MIS 11 b and were coeval with the North Atlantic Heinrich-type (Ht) events Ht3 (similar to 390 kyr) and Ht2 (similar to 384 kyr). Increased abundance of the subpolar Coccolithus pelagicus subsp. pelagicus and Gephyrocapsa muellerae was consistent with the inflow of cold surface waters into the Mediterranean Sea during the Ht events. Lowered PPP during negative (-) NAO-like phases is the result of moderate upwelling by the incomplete development of surface hydrological structures at the Alboran Sea. This scenario is expressed during the early MIS 11c (422-405 kyr). Overall, the results of our study provide evidence of the important role of atmospheric circulation patterns in the North Atlantic region for controlling phytoplankton primary production and oceanographic circulation dynamics in the Western Mediterranean during MIS 11.
- Ocean productivity in the Gulf of Cadiz over the last 50 kyrPublication . Penaud, A.; Eynaud, F.; Etourneau, J.; Bonnin, J.; Vernal, A.; Zaragosi, S.; Kim, J.-H.; Oliveira, Dulce; Waelbroeck, C.Reconstructions of ocean primary productivity (PP) help to explain past and present biogeochemical cycles and climate changes in the oceans. We document PP variations over the last 50 kyr in a currently oligotrophic subtropical region, the Gulf of Cadiz. Data combine refined results from previous investigations on dinocyst assemblages, alkenones, and stable isotopes ( 18O, 13C) in planktonic (Globigerina bulloides) and endobenthic (Uvigerina mediterranea) foraminifera from cores MD04-2805 CQ and MD99-2339, with new isotopic measurements on epibenthic (Cibicides pachyderma–Cibicidoides wuellerstorfi) foraminifera and dinocyst-based estimates of PP using the new n = 1,968 modern database. We constrain PP variations and export production by integrating qualitative information from bio-indicators with dinocyst-based quantitative reconstructions such as PP and seasonal sea-surface temperature and information about remineralization from the benthic 13C (difference between epi- and endo-benthic foraminiferal 13C signatures). This study also includes new information on alkenone-based SST and total organic carbon which provides insights into the relationship between past regional hydrological activity and PP regime change. We show that PP, carbon export, and remineralization were generally high in the NE subtropical Atlantic Ocean during the last glacial period and that the Last Glacial Maximum (LGM) had lower 13C than the Heinrich Stadials with sustained high PP, likely allowing enhanced carbon sequestration. We link these PP periods to the dynamics of upwelling, active almost year-round during stadials, but restricted to spring-summer during interstadials and LGM, like today. During interstadials, nutrient advection through freshwater inputs during autumn–winter needs also to be considered to fully understand PP regimes.
- Pollen from the deep-sea: A breakthrough in the mystery of the ice agesPublication . Goni, Maria F. Sanchez; Desprat, Stephanie; Fletcher, William J.; Morales-Molino, Cesar; Naughton, Filipa; Oliveira, Dulce; Urrego, Dunia H.; Zorzi, CoraliePollen from deep-sea sedimentary sequences provides an integrated regional reconstruction of vegetation and climate (temperature, precipitation, and seasonality) on the adjacent continent. More importantly, the direct correlation of pollen, marine and ice indicators allows comparison of the atmospheric climatic changes that have affected the continent with the response of the Earth's other reservoirs, i.e., the oceans and cryosphere, without any chronological uncertainty. The study of long continuous pollen records from the European margin has revealed a changing and complex interplay between European climate, North Atlantic sea surface temperatures (SSTs), ice growth and decay, and high-and low-latitude forcing at orbital and millennial timescales. These records have shown that the amplitude of the last five terrestrial interglacials was similar above 40 degrees N, while below 40 degrees N their magnitude differed due to precession-modulated changes in seasonality and, particularly, winter precipitation. These records also showed that vegetation response was in dynamic equilibrium with rapid climate changes such as the Dangaard-Oeschger (D-O) cycles and Heinrich events, similar in magnitude and velocity to the ongoing global warming. However, the magnitude of the millennial-scale warming events of the last glacial period was regionally-specific. Precession seems to have imprinted regions below 40 degrees N while obliquity, which controls average annual temperature, probably mediated the impact of D-O warming events above 40 degrees N. A decoupling between high-and low-latitude climate was also observed within last glacial warm (Greenland interstadials) and cold phases (Greenland stadials). The synchronous response of western European vegetation/climate and eastern North Atlantic SSTs to D-O cycles was not a pervasive feature throughout the Quaternary. During periods of ice growth such as MIS 5a/4, MIS 11c/b and MIS 19c/b, repeated millennial-scale cold-air/warm-sea decoupling events occurred on the European margin superimposed to a long-term air-sea decoupling trend. Strong air-sea thermal contrasts promoted the production of water vapor that was then transported northward by the westerlies and fed ice sheets. This interaction between long-term and shorter timescale climatic variability may have amplified insolation decreases and thus explain the Ice Ages. This hypothesis should be tested by the integration of stochastic processes in Earth models of intermediate complexity.