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The role of climate, marine influence and sedimentation rates in late-Holocene estuarine evolution (SW Portugal)

Publication . Costa, Ana Maria; Freitas, Maria da Conceicao; Leira, Manel; Costas, S.; Costa, Pedro J. M.; Andrade, Cesar; Bao, Roberto; Duarte, Joao; Rodrigues, Aurora; Cachao, Mario; Araujo, Ana Cristina; Diniz, Mariana; Arias, Pablo

Estuaries are sensitive to changes in global to regional sea level, to climate-driven variation in rainfall and to fluvial discharge. In this study, we use source and environmentally sensitive proxies together with radiocarbon dating to examine a 7-m-thick sedimentary record from the Sado estuary accumulated throughout the last 3.6 kyr. The lithofacies, geochemistry and diatom assemblages in the sediments accumulated between 3570 and 3240 cal. BP indicate a mixture between terrestrial and marine sources. The relative contribution of each source varied through time as sedimentation progressed in a low intertidal to high subtidal and low-energy accreting tidal flat. The sedimentation proceeded under a general pattern of drier and higher aridity conditions, punctuated by century-long changes of the rainfall regime that mirror an increase in storminess that affected SW Portugal and Europe. The sediment sequence contains evidence of two periods characterized by downstream displacement of the estuarine/freshwater transitional boundary, dated to 3570-3400 cal. BP and 3300-3240 cal. BP. These are intercalated by one episode where marine influence shifted upstream. All sedimentation episodes developed under high terrestrial sediment delivery to this transitional region, leading to exceptionally high sedimentation rates, independently of the relative expression of terrestrial/marine influences in sediment facies. Our data show that these disturbances are mainly climate-driven and related to variations in rainfall and only secondarily with regional sea-level oscillations. From 3240 cal. BP onwards, an abrupt change in sediment facies is noted, in which the silting estuarine bottom reaches mean sea level and continued accreting until present under prevailing freshwater conditions, the tidal flat changing to an alluvial plain. The environmental modification is accompanied by a pronounced change in sedimentation rate that decreased by two orders of magnitude, reflecting the loss of accommodation space rather than the influence of climate or regional sea-level drivers.

2019-04Journal article

Open Access

Impact of sea-level rise on earthquake and landslide triggering offshore the Alentejo margin (SW Iberia)

Publication . C. Neves, Maria; Roque, C.; Luttrell, K. M.; Vazquez, J. T.; Alonso, B.

Earthquakes and submarine landslides are recurrent and widespread manifestations of fault activity offshore SW Iberia. The present work tests the effects of sea-level rise on offshore fault systems using Coulomb stress change calculations across the Alentejo margin. Large-scale faults capable of generating large earthquakes and tsunamis in the region, especially NE-SW trending thrusts and WNW-ESE trending dextral strike-slip faults imaged at basement depths, are either blocked or unaffected by flexural effects related to sea-level changes. Large-magnitude earthquakes occurring along these structures may, therefore, be less frequent during periods of sea-level rise. In contrast, sea-level rise promotes shallow fault ruptures within the sedimentary sequence along the continental slope and upper rise within distances of <100 km from the coast. The results suggest that the occurrence of continental slope failures may either increase (if triggered by shallow fault ruptures) or decrease (if triggered by deep fault ruptures) as a result of sea-level rise. Moreover, observations of slope failures affecting the area of the Sines contourite drift highlight the role of sediment properties as preconditioning factors in this region.

2016-12Journal article

Open Access

The response of piezometric levels in Portugal to NAO, EA, and SCAND climate patterns

Publication . Neves, Maria; Jerez, Sonia; Trigo, Ricardo M.

Under the increasing risk of water scarcity, aquifer management strategies can take advantage of a deeper knowledge about the natural long-term fluctuations driven by climate patterns. This study examines the links between major large-scale atmospheric circulation modes and inter-annual to decadal oscillations in groundwater levels in Portugal. Precipitation and piezometric records (1987-2016) from two aquifer systems, Leirosa-Monte Real in the north and Querenca-Silves in the south, are analyzed using wavelet transform methods and singular spectral analysis. Wavelet coherences computed between hydrologic time series and the North Atlantic Oscillation (NAO), East Atlantic (EA) and Scandinavia (SCAND) climate patterns show non-stationary relationships that are nonetheless consistent in distinct period bands. The strongest covariability occurs in the 6-10 years band for NAO, in the 2-4 years band for EA (especially after 1999) and in the 4-6 years band for SCAND (mainly after 2005). NAO is the mode playing the most relevant role with a stronger influence in the south (60% on average) than in the north (40% on average). The relatively higher frequency ( < 5 year period) contributions of EA and SCAND are difficult to set apart but their joint impact accounts for approximately 20% and 40% of the total variance of groundwater levels in the south and north of the country, respectively. Wavelet coherence patterns also expose transitive couplings between NAO, EA and SCAND. Often, coupled phases between climate modes mark abrupt transitions in synchronization patterns and shifts in the time-frequency domain. Extremes in groundwater storage coincide with anti-phase NAO and EA combinations: maximum piezometric levels occur during NAO-EA + (coincidentally also SCAND + ) phases while minimum levels occur during NAO + EA - phases.

2019-01Journal article

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