Temporal variability of flooding events of Guadiana River (Iberian Peninsula) during the middle to late Holocene: Imprints in the shallow-marine sediment record
Introduction
Holocene climatic fluctuations and associated changes in vegetation, ocean circulation and sedimentary environments exhibit a spatially variable pattern (Mayewski et al., 2004). A long-term cooling trend evidenced during the late Holocene in North Atlantic records (Marsicek et al., 2018) is associated to the southward shift of the Intertropical Convergence Zone that has promoted a progressive change of the North Atlantic Oscillation index (NAO) (Wanner et al., 2008). In this sense, the NAO has evolved from generally positive to variable or even negative values during the late Holocene (e.g. Trouet et al., 2009; Olsen et al., 2012). These changes have influenced long-term trends in atmospheric circulation and weather patterns, which in turn affected climatic variables such as regional patterns in temperature, rainfall and storm events (Toomey et al., 2013).
The influence of rainfall patterns on sediment production is very significant in fluvial drainage basins, with consequences for sediment transport to adjacent continental margins. Particularly, rainfall modifications affect the frequency and volume of river runoffs (Vallefuoco et al., 2012), associated flooding events (Sommerfield et al., 2002), and interfere with atmospheric processes of different intensities and periodicities, such as storms (Lesueur et al., 1996; Zhao et al., 2017). On the continental shelf, storms, major oceanographic reorganizations, or earthquakes may leave individual stratigraphic features in the geologic record (Gomez et al., 2007; Cosentino et al., 2017). The depositional signals of such interactions involve unconformities, gravity-driven mass flow, storm events, or drastic changes in sedimentation rates due to varying sediment supply. The latter process is ultimately recorded in the shelf depositional systems in the form of mud depocenters that can be exploited as high-resolution archives to reconstruct the climatic, oceanographic and geologic past (e.g. Walsh and Nittrouer, 2009; Gao and Collins, 2014; Bassetti et al., 2016; Penaud et al., 2020). These mud depocenters can extend over wide areas on a continental shelf (e.g. mudbelts), or can be locally confined by the preexisting physiography (e.g. mud entrapments; Hanebuth et al., 2015).
Depositional signals in shallow-water muddy depocenters may also be significantly altered by the onset of human occupation of the river basins, including the effect of diverse practices such as deforestation, forest firing, agriculture, mining and river damming (e.g. Chester and James, 1999; Boone and Worman, 2007; Gonzalez et al., 2007; Jabaloy-Sánchez et al., 2010; Chester, 2012; Mendes et al., 2015). These activities established an erosional regime in the drainage basins (Gomez et al., 2007) and led to a further imprint of flood events in the sedimentary shelf record (Sommerfield and Wheatcroft, 2007; Budillon et al., 2012).
The northern margin of the Gulf of Cadiz, located in southwest Iberian Peninsula, is a key area to understand spatial patterns of climatic variability between the Atlantic and Mediterranean. On the continental shelf off the Guadiana River (Fig. 1), late Holocene sedimentary formations can be considered as an outstanding paleoenvironmental archive of detailed temporal resolution (Mendes et al., 2010, Mendes et al., 2012a, Mendes et al., 2013; Rosa et al., 2011). The Guadiana River drains the southwestern Iberian Peninsula and therefore provides significant information about the transitional area between NW Atlantic and SW Mediterranean climates. Consequently, the goals of this study are: (1) to interpret changes in depositional environments in the area, which are directly influenced by the Guadiana River supplies; (2) to understand the influence of flooding events and other factors such as occurrence of storm events and recent anthropogenic actions on river supply patterns; (3) to compare the Guadiana record with those from other modern Iberian Atlantic depocenters to reveal latitudinal changes of the climatic variability during mid to late Holocene times.
Section snippets
Regional setting
The study area is located in the central part of the northern Gulf of Cadiz on the inner continental shelf (southwestern Iberian Peninsula) off the Guadiana River mouth (Fig. 1).
Material and methods
This study was based on a multi-proxy analysis of two sediment cores (GeoB19522-2 and GeoB19522-3), in order to reveal the recent infilling pattern of the main Guadiana River paleo-valley. The sediment cores were collected at site GeoB19522, at 17 m water depth (37°8′29.34” N, 7°16′46.8” W; Fig. 1), using an available high-resolution seismic profile that crossed the Guadiana River paleo-valley, during RV Poseidon cruise POS482 CADISED (Cadiz Shelf Sediment Depocentres), in March 2015 (Lantzsch
Overall architecture of the recent paleo-valley infilling phase
The seismo-acoustic profile collected across site GeoB19522 displays the Guadiana River paleo-valley and its sedimentary infill. The most recent valley erosional surface is related to the Last Glacial Maximum (LGM), whereas the most recent depositional unit is regarded as a mud entrapment (Fig. 1c). Core GeoB19522-3, located over the landward flank of the paleo-valley, penetrated almost the whole mud entrapment unit, approximately 4 m thick, and recorded two sub-parallel internal reflectors (
Depositional environments
During the last ca. 5800 cal yr BP two major depositional environments were identified, based on the combined analysis of various independent proxies. The lower Unit A extends from ca. 5800 to ca. 1300–1200 cal yr BP, whereas the upper Unit B extends from ca. 1300 to 1200 cal yr BP to the core top (Fig. 7).
Conclusions
Two major depositional environments have been distinguished in sediment core GeoB19522-3 recording the conditions since Holocene sea level highstand. This record was retrieved from a mud entrapment, which has formed in the drowned paleo-valley of the Guadiana River located on the adjacent shelf. Most of the middle to late Holocene (ca. 5800–1250 cal yr BP) times were characterized by low accumulation rates of coarse-grained sediment, high species richness, and high abundances of benthic
Funding
This study was made possible by the CADISED (Cadiz Shelf Sediment Depocentres) survey (MARUM research cruise POS482, Germany), and was also supported by projects UID/0350/2020 CIMA of the Portuguese Science Foundation (FCT) and CGL2011-30302-C02-02 and CTM2017-88237-P of the Spanish Ministry of Economy and Competitiveness. Isabel Mendes also thanks to FCT for Research Assistant contract DL57/2016/CP1361/CT0009.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank captain and crew of RV Poseidon for their professionalism during the research expedition CADISED. We acknowledge Margarida Ramirez (CIMA-UAlg) for technical support in grain size analyses, Rubens Cesar Lopes Figueira (LaQIMar) and Paulo Alves Ferreira (LaQIMar) for radionuclides (210Pb and 137Cs) results discussion. We also thank the reviewers Marc Hijma (Deltares) and an anonymous reviewer for their helpful and constructive comments.
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