Natural and human-induced Holocene paleoenvironmental changes on the Guadiana shelf (northern Gulf of Cadiz)

Abstract

Three contrasting sedimentary environments on the continental shelf off the Guadiana River (northern Gulf of Cadiz) were integrated in a chronological framework and analysed in terms of sedimentology and benthic foraminiferal assemblages to understand the Holocene paleoenvironmental evolution. The analysed environments differ in terms of their depositional regimes and benthic foraminiferal assemblages. However, a dominant fluvial origin of the sand fraction was observed in all three environments. Holocene sedimentary processes were mainly controlled by natural (sea level changes and climate variations) and human-induced processes (e.g. deforestation, agriculture) along four evolutionary stages. The three older stages were mainly influenced by natural processes, such as sea level variations and fluvial inputs, whereas the most recent stage reflects a combination of climatic-and human-induced processes. A deepening of sedimentary environments related to a period of rapid sea level rise, strongly influenced by river discharges occurred from c. 11,500 to c. 10,000 cal. yr BP. A reduction in sediment export to the shelf, as a result of the continuous and rapid sea level rise and enhanced estuary infilling reflects the second stage, from c. 10,000 to c. 5000 cal. yr BP. The beginning of the third stage, from c. 5000 to c. 1500-1000 cal. yr BP, is marked by a sea-level slowdown and the relatively stable climate and environmental conditions. The fourth stage, from c. 1500-1000 cal. yr BP to Recent times, reflects the intensification of human-induced processes and climatic variability in the Guadiana River basin. This stage also reflects modern depositional conditions, with the formation of a proximal prodeltaic wedge and a distal muddy body.

Exploring Offshore Sediment Evidence of the 1755 CE Tsunami (Faro, Portugal): Implications for the Study of Outer Shelf Tsunami Deposits

“…However, other possible older events show a very clear signature such as a sandy layer interbedded in silty normal marine background sediments with well supporting XRF-data and biomarker analysis [109]. The small signature of the 1755 CE tsunami could be explained by a change in sedimentation dynamics in general observed by PCA results including both cores (Figure 8c,d) (see also Figure 2), the mass normalized magnetic susceptibility variation (Figure 6) and other studies of the southern Portuguese shelf sedimentary records [110][111][112]. Several hypotheses are discussed for this change in sedimentation, i.e., of a decreasing trend in grain size and higher sedimentation rates: (1) the change of coastal morphology, such as the opening/closing of the Ria Formosa barrier island system [113], (2) the intensification of fine-grained loaded river discharge associated with the wetter climatic condition since the Little Ice Age [114], and (3) the increased settlement density induced land-use change, that probably resulted in an increased input of terrigenous material into the shelf [111].…”

Morphology of the last subaerial unconformity on a shelf: insights into transgressive ravinement and incised valley occurrence in the Gulf of Cádiz

“…This difference in water depths could be related to subsidence in the distal margin and/or the detachment of the depositional break from the lowstand shoreline, due to the influence of wave base levels. 14 C dates of deposits overlying the unconformity produce two consistent sets of ages: postglacial ages of transgressive coarse-grained deposits (Nelson et al 1999), and Holocene ages of highstand muddy deposits (Nelson et al 1999;Mendes et al 2012). Specifically, dates for the outer shelf post-unconformity sediment wedge showing gently dipping…”

“…The analysis of sedimentary facies indicates that large-scale flood events were significant processes leading to the formation of transgressive wedges (González et al 2004). Holocene sedimentation off the Guadiana River comprises poorly developed inner shelf facies and distal muddy belts (Nelson et al 1999;Mendes et al 2012). The proximal prodelta is composed of a mixture of coarse and fine sediments, whereas the distal muddy belt mostly comprises fine-grained sediments (Mendes et al 2012) that mostly prograde toward the southeast due to the influence of prevailing shelf current patterns (Nelson et al 1999).…”

Middle to late Holocene environmental evolution of the Pisa coastal plain (Tuscany, Italy) and early human settlements

“…(i) the complex interaction between human and natural forcing factors, such as climate/greenhouse gas concentrations and eustacy (IPCC, 2007;Kaplan et al, 2011;Ruddiman et al, 2011), (ii) the mechanisms of direct or indirect human influence on fluvial regimes and sediment fluxes (Syvitski et al, 2005;Wilkinson, 2005;Ericson et al, 2006;Gregory, 2006;Hooke, 2006;Knox, 2006;Syvitski and Saito, 2007;Hoffmann et al, 2010;Syvitski and Kettner, 2011), and (iii) the critical transition from natural to anthropogenic-dominated environments, as recorded within different depositional archives (Dinis et al, 2006;Carmona and Ruiz, 2011;Marinova et al, 2012;Mendes et al, 2012).…”