Browsing by Author "Rodrigues, T."
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- Climate variability across the last deglaciation in NW Iberia and its marginPublication . Naughton, F.; Goni, Maria F. Sanchez; Rodrigues, T.; Salgueiro, Emilia; Costas, Susana; Desprat, S.; Duprat, J.; Michel, E.; Rossignol, L.; Zaragosi, S.; Voelker, Antje; Abrantes, FatimaThe direct comparison between marine and terrestrial data from the NW Iberian margin, core MD03-2697, allows us to accurately evaluate, without chronological ambiguity, the vegetation response to North Atlantic climate events across the last deglaciation. Comparison of MD03-2697 data with other marine and terrestrial records from a vast area stretching from the Azores to western (W) France, Iberia and its margin, the W Mediterranean and NW Africa reveals the importance of enhanced winter North Atlantic westerlies episodes in driving a heterogeneous regional climatic signal during particular events of the last deglaciation. Heinrich Stadial 1 (HS1)/Oldest Dryas is a complex event marked by three synchronous main phases (a: extremely cold/relatively wet; b: cool/dry; c: relatively warmer/increasing moisture availability) in regions directly influenced by the North Atlantic while it is characterized by a single phase (cold and dry) in most inland and high altitude areas. Changes in the strength and position of North Atlantic westerlies could explain the variability in moisture during HS1 from W Pyrenees to W Mediterranean. The Bolling-Allerod (B-A) event is marked by a synchronous progressive increase of ocean and atmospheric temperatures and precipitation from the Bolling to the Allerod in W Iberia and W Pyrenees contrasting with the Greenland temperature pattern. Mid-to high latitudes thermal contrast and the gradual strengthening of the Atlantic Meridional Overturning Circulation (AMOC) triggered the continuing enhancement of westerlies, and moisture, along this period.Finally, the Younger Dryas (YD) Stadial is characterized by a land-ocean synchronous returning cold conditions (but not so cold as HS1) and, although dry conditions persisted during this period, extreme winter precipitation occurrences might have affected the most coastal areas of north and central W Iberia and Pyrenees. In contrast, dry conditions persisted during the YD in the western Mediterranean regions. The westerlies migrated further north following the displacement of the Polar Front during the deglaciation precluding the input of moisture in the W Mediterranean region, which persisted very dry during the YD Stadial. (C) 2016 Published by Elsevier Ltd.
- Identifying imprints of externally derived dust and halogens in the sedimentary record of an Iberian alpine lake for the past ∼13,500 years – Lake Peixão, Serra da Estrela (Central Portugal)Publication . Moreno, J.; Ramos, A. M.; Raposeiro, P. M.; Santos, R. N.; Rodrigues, T.; Naughton, F.; Moreno, F.; Trigo, R. M.; Ibañez-Insa, J.; Ludwig, P.; Shi, X.; Hernández, A.Iberian lacustrine sediments are a valuable archive to document environmental changes since the last glacial termination, seen as key for anticipating future climate/environmental changes and their far-reaching implica -tions for generations to come. Herein, multi-proxy-based indicators of a mountain lake record from Serra da Estrela were used to reconstruct atmospheric (in)fluxes and associated climatic/environmental changes over the last similar to 13.5 ka. Depositions of long-range transported dust (likely from the Sahara) and halogens (primarily derived from seawater) were higher for the pre-Holocene, particularly in the late Bolling-Allerod-Younger Dryas period, compared to the Holocene. This synchronous increase could be related to a recognized dust-laden at-mosphere, along with the combined effect of (i) an earlier proposed effective transport of Sahara dust for higher latitudes during cold periods and (ii) the progressive Polar Front expansion southwards, with the amplification of halogen activation reactions in lower latitudes due to greater closeness to snow/sea ice (halide-laden) surfaces. Additionally, the orographic blocking of Serra da Estrela may have played a critical role in increasing precipi-tation of Atlantic origin at higher altitudes, with the presence of snow prompting physical and chemical processes involving halogen species. In the Late Holocene, the dust proxy records highlighted two periods of enhanced input to Lake Peixao, the first (similar to 3.5-2.7 ka BP) after the end of the last African Humid Period and the second, from the 19th century onwards, agreeing with the advent of commercial agriculture, and human contribution to land degradation and dust emission in the Sahara/Sahel region. The oceanic imprints throughout the Holocene matched well with North Atlantic rapid climatic changes that, in turn, coincided with ice-rafted debris or Bond events and other records of increased storminess for the European coasts. Positive parallel peaks in halogens were found in recent times, probably connected to fire extinction by halogenated alkanes and roadway de-icing.
- Influence of environmental variability and Emiliania huxleyi ecotypes on alkenone-derived temperature reconstructions in the subantarctic Southern OceanPublication . Rigual-Hernández, A.S.; Sierro, F.J.; Flores, J.A.; Trull, T.W.; Rodrigues, T.; Martrat, B.; Sikes, E.L.; Nodder, S.D.; Eriksen, R.S.; Davies, D.; Bravo, N.; Sánchez-Santos, J.M.; Abrantes, F.Long-chain unsaturated alkenones produced by haptophyte algae are widely used as paleotemperature indicators. The unsaturation relationship to temperature is linear at mid-latitudes, however, non-linear responses detected in subpolar regions of both hemispheres have suggested complicating factors in these environments. To assess the influence of biotic and abiotic factors in alkenone production and preservation in the Subantarctic Zone, alkenone fluxes were quantified in three vertically-moored sediment traps deployed at the SOTS observatory (140°E, 47°S) during a year. Alkenone fluxes were compared with coccolithophore assemblages, satellite measurements and surface-water properties obtained by sensors at SOTS. Alkenone-based temperature reconstructions generally mirrored the seasonal variations of SSTs, except for late winter when significant deviations were observed (3–10 °C). Annual flux-weighted averages in the 3800 m trap returned alkenone-derived temperatures ~1.5 °C warmer than those derived from the 1000 m trap, a distortion attributed to surface production and signal preservation during its transit through the water column. Notably, changes in the relative abundance of E. huxleyi var. huxleyi were positively correlated with temperature deviations between the alkenone-derived temperatures and in situ SSTs (r = 0.6 and 0.7 at 1000 and 2000 m, respectively), while E. huxleyi var. aurorae, displayed an opposite trend. Our results suggest that E. huxleyi var. aurorae produces a higher proportion of C37:3 relative to C37:2 compared to its counterparts. Therefore, the dom inance of var. aurorae south of the Subtropical Front could be at least partially responsible for the less accurate alkenone-based SST reconstructions in the Southern Ocean using global calibrations. However, the observed correla tions were largely influenced by the samples collected during winter, a period characterized by low particle fluxes and slow sinking rates. Thus, it is likely that other factors such as selective degradation of the most unsaturated alkenones could also account for the deviations of the alkenone paleothermometer.
- Variations in mid-latitude North Atlantic surface water properties during the mid-Brunhes (MIS 9-14) and their implications for the thermohaline circulationPublication . Voelker, Antje; Rodrigues, T.; Billups, K.; Oppo, D.; McManus, J.; Stein, R.; Hefter, J.; Grimalt, J.O.Stable isotope and ice-rafted debris records from three core sites in the mid-latitude North Atlantic (IODP Site U1313, MD01-2446, MD03-2699) are combined with records of ODP Sites 1056/1058 and 980 to reconstruct hydrographic conditions during the middle Pleistocene spanning Marine Isotope Stages (MIS) 9-14 (300-540 ka). Core MD03-2699 is the first high-resolution mid-Brunhes record from the North Atlantic's eastern boundary upwelling system covering the complete MIS 11c interval and MIS 13. The array of sites reflect western and eastern basin boundary current as well as north to south transect sampling of subpolar and transitional water masses and allow the reconstruction of transport pathways in the upper limb of the North Atlantic's circulation. Hydrographic conditions in the surface and deep ocean during peak interglacial MIS 9 and 11 were similar among all the sites with relative stable conditions and confirm prolonged warmth during MIS 11c also for the mid-latitudes. Sea surface temperature (SST) reconstructions further reveal that in the mid-latitude North Atlantic MIS 11c is associated with two plateaus, the younger one of which is slightly warmer. Enhanced subsurface northward heat transport in the eastern boundary current system, especially during early MIS 11c, is denoted by the presence of tropical planktic foraminifer species and raises the question how strongly it impacted the Portuguese upwelling system. Deep water ventilation at the onset of MIS 11c significantly preceded surface water ventilation. Although MIS 13 was generally colder and more variable than the younger interglacials the surface water circulation scheme was the same. The greatest differences between the sites existed during the glacial inceptions and glacials. Then a north - south trending hydrographic front separated the nearshore and offshore waters off Portugal. While offshore waters originated from the North Atlantic Current as indicated by the similarities between the records of IODP Site U1313, ODP Site 980 and MD01-2446, nearshore waters as recorded in core MD03-2699 derived from the Azores Current and thus the subtropical gyre. Except for MIS 12, Azores Current influence seems to be related to eastern boundary system dynamics and not to changes in the Atlantic overturning circulation.