Percorrer por autor "Alberto, Ana"
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- Early Pleistocene (1.94–1.46 Ma) records for the upper mediterranean outflow Water branch reveal low and high latitude climate influencesPublication . Voelker, Antje; Ducassou, Emanuelle; Balestra, Barbara; Flores, Jose Abel; Acton, Gary D.; Richter, Carl; Xuan, Chuang; Lofi, Johanna; Alberto, Ana; Kuhnert, Henning; Zarikian, Carlos A. AlvarezThe Mediterranean Outflow Water (MOW), modified by paleoceanographic conditions and tectonic processes, played a significant role in the formation of sediments drifts along the Iberian Margin. Using sediment samples from IODP Hole U1387C, we explore the Early Pleistocene history of the upper MOW core above the central Faro Drift in the Gulf of Cadiz. The time series of benthic foraminifer stable isotope and grain size related data have a rigorous stratigraphic framework consisting of nannofossil biostratigraphy and paleomagnetic and delta 18O stratigraphy. The paleoenvironmental records are supplemented by natural gamma ray downhole logging data. Above the hiatus associated with the youngest dolostone, sandy to muddy contourite sedimentation started at 1.946 Ma, i.e., within Marine Isotope Stage (MIS) 74, at IODP Site U1387, slightly younger than at IODP Site U1389. Formation of contourite layers, reflected in the sortable silt and sand percentage records, strongly reacted to precession forcing, including semi- and quarter-precession cycles. The majority of the contourite beds developed during stadial (colder) climate periods, like previous observations from the Early to Late Pleistocene. Formation of contourite layers within MIS 53, MIS 55 and MIS 65, however, appear to be linked to the prevailing atmospheric conditions over North Africa. Periods of poor ventilation in the upper MOW were linked to insolation maxima and reduced ventilation in the Mediterranean Sea. Here, MIS 51 presents a peculiar case as poor ventilation reached from the surface to the lower North Atlantic Deep Water range, reflecting unique interglacial conditions that merit future exploration.
- Mediterranean Outflow and surface water variability off southern Portugal during the early Pleistocene: A snapshot at Marine Isotope Stages 29 to 34 (1020-1135 ka)Publication . Voelker, Antje H. L.; Salgueiro, Emilia; Rodrigues, Teresa; Jiménez-Espejo, Francisco J.; Bahr, Andre; Alberto, Ana; Loureiro, Isabel; Padilha, Maria; Rebotim, Andreia; Roehl, UrsulaCentennial-to-millennial scale records from IODP Site U1387, drilled during IODP Expedition 339 into the Faro Drift at 558 m water depth, now allow evaluating the climatic history of the upper core of the Mediterranean Outflow (MOW) and of the surface waters in the northern Gulf of Cadiz during the early Pleistocene. This study focuses on the period from Marine Isotope Stages (MIS) 29 to 34, i.e. the interval surrounding extreme interglacial MIS 31. Conditions in the upper MOW reflect obliquity, precession and millennial-scale variations. The benthic delta O-18 signal follows obliquity with the exception of an additional, smaller delta O-18 peak that marks the MIS 32/31 transition. Insolation maxima (precession minima) led to poor ventilation and a sluggish upper MOW core, whereas insolation minima were associated with enhanced ventilation and often also increased bottom current velocity. Millennial-scale periods of colder sea-surface temperatures (SST) were associated with short-term maxima in flow velocity and better ventilation, reminiscent of conditions known from MIS 3.A prominent contourite layer, coinciding with insolation cycle 100, was formed during MIS 31 and represents one of the few contourites developing within an interglacial period. MIS 31 surface water conditions were characterized by an extended period (1065-1091 ka) of warm SST, but SST were not much warmer than during MIS 33. Interglacial to glacial transitions experienced 2 to 3 stadial/interstadial cycles, just like their mid-to-late Pleistocene counterparts. Glacial MIS 30 and 32 recorded periods of extremely cold (<12 degrees C) SST that in their climatic impact were comparable with the Heinrich events of the mid and late Pleistocene. Glacial MIS 34, on the other hand, was a relative warm glacial period off southern Portugal. Overall, surface water and MOW conditions at Site U1387 show a strong congruence with Mediterranean climate, whereas millennial-scale variations are closely linked to North Atlantic circulation changes. (C) 2015 Elsevier B.V. All rights reserved.
- Multiproxy characterization of high energy layers in the nner continental shelf of Quarteira (Southern Portugal) - Preliminary resultsPublication . Santos, Jacqueline; Drago, Teresa; Moura, Delminda; Magalhães, Vitor; Roque, Cristina; Silva, Pedro F.; Rodrigues, Ana Isabel; Terrinha, Pedro; Anxo, Mena; Francés, Guillermo; Lopes, Ana; Alberto, Ana; Baptista, Maria AnaThe occurrence of sedimentary layers indicating high energy events is common in the continental shelves sedimentary record. Their presence has been usually related either to tsunami waves or storm waves. In both cases, the identification of the transport mechanism requires a variety of proxies. This work presents the preliminary results of the analysis of a gravity core (MW-107) collected at ca. 57 m water depth in the inner continental shelf of Quarteira (Algarve) and aims to identify potential high energy events.
- Neoichnology and sediment distribution in tidal flats of the mud‐dominated mesotidal mira river estuary (Portugal)Publication . Shchepetkina, Alina; Drago, Teresa; Santos, Jacqueline; Alberto, Ana; Fatela, Francisco; Freitas, Maria da ConceiçãoInvertebrate burrow morphologies and distributions are presented for the tidal flats of the middle and lower reaches of the tide-influenced (mesotidal), mud-dominated Mira River estuary (SW Portugal) as an analogue for interpreting the ichnology of palaeoestuarine successions. Burrow distributions are revealed using field observations of biogenic and physical sedimentary structures, trace makers, collected grab samples, push cores and lab measurements of grain size, organic matter and calcium carbonate content and linked to physico-chemical stresses. Estuarine tidal-flat surface open burrow distribution and bioturbation intensity are analysed in plan view using free scientific image analysis software (ImageJ) to obtain burrow density, total burrow area and minimum and maximum burrow diameters. The total tidal flat area occupied by organisms ranges from 0.8% to 4.1% in the middle estuary and is 1.4% in the lower estuary. These equate to a bioturbation index of one to two (BI 1–2). However, computed tomography (CT) of push cores (cross-sectional view) taken on the same tidal flats shows sparse to complete bioturbation intensity (BI 1–6), revealing that tidal flat sediments tend to be more biogenically reworked cumulatively over time with overprinting of subsequent tiers and/or recolonized. CT scanning also allows the identification of more burrow types, demonstrates infaunal tiering and provides insights into trace-fossil preservation potential. This research shows that the trace communities are heterogeneous and change spatially and temporally along the Mira River estuary, reflecting a physico-chemical gradient and seasonality. This trend signals changes in sediment composition, substrate type and consistency (sediment compaction and subaerial exposure), salinity, oxygenation, temperature, pH and interplay of tidal versus hydraulic energy.
- Offshore 1755 CE Lisbon Tsunami deposit in the southern portuguese continental shelfPublication . Kümmerer, Vincent; Drago, Teresa; Veiga-Pires, Cristina; Silva, Pedro; Lopes, Ana; Magalhães, Vitor; Roque, Cristina; Rodrigues, Ana Isabel; Terrinha, Pedro; Mena, Anxo; Francés, Guillermo; Kopf, Achim; Völker, David; Salgueiro, Emilia; Alberto, Ana; C. Lopes, Cristina; Costa, Pedro; Baptista, Maria AnaThe importance of tsunami hazard assessment is only possible if a complete dataset of events is available, allowing the determination of the recurrence intervals of the tsunamis adapted to local and regional conditions. One possible way to know these intervals is to study the offshore sedimentary record, looking for sediment remobilised and transported by the incoming tsunami waves and generated backwash currents. Even if these deposits are not of easy access (and not so well studied), the tsunami depositional signature has potential to be better preserved than those located onshore.