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Abstract: The Mejerda River is the one of the major rivers in Tunisia and several studies dealt with the importance of this river in terms of ecology, geochemistry and sociology. However, the sedimentary contributions to the coastal zone from the Mejerda River were poorly observed, and we did not find any continuous monitoring. The main objective of this work is to assess the impact of reduction on sediment transport from the upper basins to the coastal area, after the construction of large dams which may have strong and long-lasting effects on coastal geomorphology and ecosystems. In this paper, a paleo-hydrological approach was applied through the study of sediment cores sampled in the low valley meander on alluvial terrace. We attempt to emphasize, through this approach, the reduction in sediment loads to the sea (Gulf of Tunis) due to the construction of dams on the Mejerda River and to changes in climate conditions. Three cores (C1, C3, C5) were sampled in the lower part of the Mejerda near the hydrometric station of Jdaida with different depths (0.9 m, 1.2 m and 3.0 m) and in different dates (2014–2015). Visible successions of sedimentary layers corresponding to the deposits of successive flood events were identified and the history of the sedimentary contributions to the Mejerda was reconstructed. The thickest layers of sedimentary deposits were related to the exceptional events. They are mainly concentrated on the lower part of the core and are predominantly composed of sands. Since the construction of the Sidi Salem dam in 1981, all of the cores C1, C3 and specially C5 presented mostly a succession of small layers of fine material (silt or clay) without any sand deposits in the downstream River bed. In C5, 18 individual slackwater flood units were identified. A strong decrease in the accumulation rate of sediment was observed based on the length of the flood units and the number of years between flood events. A lower sedimentation rate is observed between 1982 and 2015 of about 2.3 cm/year, while it was much more important between 1963 and 1981 of about 4.75 cm/year, and around 10 cm/year between 1950 and 1962. The statistical study of precipitation indicated three climatic successive periods between 1951 and 2009 characterized as wet, dry and intermediate. Thus, the sediment rate was higher before 1974 reaching about 8.47 cm/year for the wet condition, of about 4 cm/year for the dry period between 1974 and 1989, and about 1.6 cm/year in the intermediate period afterward to 2009 that also witnessed the construction of the major dams. [ABSTRACT FROM AUTHOR]

The present study aims at reconstructing mid-to late Holocene paleoenvironmental evolution of the Tahaddart lower estuary (NW of Morocco) and to find possible sedimentary evidence of past marine submersion events (storm surges and tsunamis). This is achieved through the study of four sedimentary cores and by using a multiproxy approach that combines sedimentological, geochemical and foraminiferal analyses. Radioisotope measurements of 210Pb ex , 137Cs and 14C were used to provide a chronological framework of the studied cores. The results show that the Tahaddart lower estuary has evolved from a bay-like environment characterized by very shelly sandy facies (i.e., marine phase) between ∼6800 and ∼625 cal BP, to an estuarine system after ∼625 cal BP, with a dominance of fine-grained sediments (i.e., estuarine phase). We further identified several sedimentary deposits that we interpret as possible marine submersion events. Two events were identified in the marine phase at ∼3800 cal BP and ∼2700 cal BP, in coherence with previously documented marine high-energy deposits located on the Atlantic Iberian coast. In the estuarine phase of the cores, two other events were detected and dated at ∼625 cal BP and ∼200 cal BP. The latter coincides with the 1755 CE Lisbon tsunami (195 cal BP). • Four sediment cores are extracted from the Tahaddart lower estuary. • The studied area was a bay-like environment between ∼6800 and ∼625 cal BP. • Four marine submersion events are identified, including the 1755 CE Lisbon tsunami. [ABSTRACT FROM AUTHOR]

Abstract: Trace metals (Cd, U, Co, Ni, Cu, Ba, Fe, Mn), total organic carbon (TOC) and C and N stable isotope signatures (δ13C and δ15N) were determined in short sediments cores from the inner and outer shelf off Concepción, Chile (∼36°S). The objectives were to establish the effect of environmental conditions on trace metal distributions at two shelf sites, one affected by seasonal oxygenation and the other by permanent low oxygen conditions due to the presence of the oxygen minimum zone (OMZ). We evaluate trace metals as proxies of past changes in primary productivity and the bottom water oxygen regime. Concentrations of pore water sulfides and were also measured as indicators of the main diagenetic pathways at each site. Our results for the inner shelf (seasonal suboxia) suggest that the oxidative state of the sediments responds to seasonal pulses of organic matter and that seasonal oxygenation develops during high and low primary productivity in the water column. Here, positive fluxes (to the water column) estimated from pore water concentrations of several elements were observed (Ba, Co, Ni, Fe and Mn). The less reduced environment at this site produces authigenic enrichment of Cu associated with the formation of oxides in the oxic surface sediment layer, and the reduction of U within deeper sediment sections occur consistently with negative estimated pore water fluxes. In the outer shelf sediments (permanent suboxia, OMZ site), negative fluxes (to the sediment) were estimated for all elements, but these sediments showed authigenic enrichments only for Cd, Cu and U. The short oxygenation period during the winter season did not affect the accumulation of these metals on the shelf. The distribution of Cu, Cd and U have been preserved within the sediments and the authigenic accumulation rates estimated showed a decrease from the deep sections of the core to the surface sediments. This could be explained by a gradual decrease in the strength of the OMZ in the last 100 years, in combination with periods of strong oxygenations during El Niño events. [Copyright &y& Elsevier]

Abstract: This study analyzes the records of several trace metals sensitive to redox conditions in continental shelf sediments off Concepción, Chile (36°S). The continental margin off Concepción (36°S; 73°W) lies beneath an important upwelling center characterized by high primary production rates and, consequently, high fluxes of organic matter. In spring and summer, this material settles to the seafloor where it decays, producing periods of very low oxygen content in bottom waters (<1mLL−1). In addition, an oxygen minimum zone develops at ∼100–400m water depth, where dissolved oxygen levels are <0.5mLL−1. This situation changes during strong El Niño events, when dissolved oxygen at the bottom increases drastically (>1mLL−1). The goals of this study were to determine the input of trace metals to the sediment and to decipher how this information can be used to reveal variations in primary productivity or bottom oxygenation. Gravity cores collected at two stations – VG06-2 over the mid-shelf station (88m water depth, upper boundary of the oxygen minimum zone) and VG06-3 over the outer shelf (120m water depth, within the oxygen minimum zone) – were sampled for high resolution profiles (1cm) of trace metals, biogenic opal, stable isotopes, and total organic carbon. The results suggest that the variability in the trace metal distribution on the continental shelf off Concepción is determined by redox conditions and the organic carbon flux to the bottom. Some sections of the sediment cores from the outer shelf showed appreciable authigenic enrichment of U, Cd, and Mo (EF: 5–10, 2–5, and 10–16 respectively) along with heavier values of δ15N, suggesting periods of suboxic conditions. During these periods, fluxes of organic material to the bottom were higher, as indicated by elevated TOC and opal contents. Alternating periods of higher and lower trace metal contents were not observed mid-shelf as they were on the outer shelf. Rather, the mid-shelf samples showed authigenic enrichment of U, Cd, and Mo (EF: 1–6, 4–5, and 10–20, respectively) throughout the core except in a 10-cm-thick gray layer. In general, authigenic enrichment of U, Mo, and Cd occurred at both sites, coincident with olive green layers in the cores. These layers were associated with periods of elevated primary productivity and suboxic conditions. Such periods did not seem to last as long as the oxygenated periods, which had higher inputs of refractive detrital material, coincident with the occurrence of distinct gray sediment layers. [Copyright &y& Elsevier]

Fire is a potential significant driver of soil erosion in the Mediterranean area, as it results in the partial removal of vegetation and the alteration of organic matter, affecting soil structure and stability. The erosion of particle sizes is influenced by the intensity and duration of rainfall, regulated by disturbance regimes and vegetation cover. We hypothesize that, during the Holocene, high fire frequency and intensity under precipitation control may have contributed to soil erosion, while plant cover and composition could have mitigated it. This plant cover, in turn, might have been influenced by biomass burning. To test this hypothesis, we conducted a detailed analysis of sediments spanning the last 11,500 years from a small mountain lake, Corsica, situated in the black pine forest belt. The high temporal resolution analysis (∼10 years per sample), used granulometry and loss-on-ignition as proxies for erosion and ecosystem productivity in both the lake and watershed, and fire and plant macroremains as fire and tree cover proxies, respectively. The correlation between particle size and the contents of organic or mineral matter with mean fire return intervals (FRI) revealed significant patterns. Long fire intervals were associated with more fine and coarse sands, whereas short mean FRI positively correlated with clay and, fine and coarse silt, along with higher total mineral and organic matter contents. These findings suggest that fires were more frequent when rain duration was sustained but runoff intensity was lower. Conversely, wildfires were less frequent during intense runoff periods (wetter climate). Unlike fire frequency, fire severity did not correlate with erosion, and tree cover and plant richness had minimal to no effect. The 8.2 kyr event was characterized by runoff transporting primarily coarse sands, i.e. a dry period with very intense rains. This suggests that the system is primarily top-down controlled by climate. Multimillennial erosion trends are influenced by fire frequency and precipitation regimes, whereas vegetation does not seem to have a mitigating effect on this process. [Display omitted] • Over the Holocene, fire frequency was well correlated with clay and silt. • Periods with more coarse sands and low fire frequency were related to wetter climate. • Vegetation and fire severity had no significant effect on soil erosion. • Early Holocene experienced intense rainfall and runoff, contrary to the Late Holocene. • 8.2 kyr event share precipitation and recovery time pattern with the Younger Dryas. [ABSTRACT FROM AUTHOR]

The reconstruction of paleofloods in the late Holocene has become an important research highlight in the context of the present climate change. The Tunisian Mediterranean coast is a region that is prone to experiencing severe weather. The flood events have a significant impact on the infrastructure and populations residing in coastal lowlands. Studies on lagoon sediments have recently revealed important information about the past extreme hydrological events as well as their relationship with past climate changes. This study aims to reconstruct the historical extreme flood-event record using sediment cores taken from the El Bibane coastal lagoon, southeast of Tunisia. Three sediment cores BL12–5, BL12–7 and BL12–10 were studied. Through a combination of analytical techniques such as the chronological, geochemical and sedimentological analyses, past flash floods were identified. Results display that the sedimentological signature of the paleoflood levels recognized within the lagoonal sequence is characterized by high values of the elemental ratios (Ti/Ca and Fe/Ca) and high content of fine material such as clay and silt. These floods are thought to have occurred as a result of intense precipitation that increased the terrigenous input to the El Bibane lagoon by the Fessi River. Chronologies of 210Pb, 137Cs, and 14C have been used to calculate the age of these floods. The records offer a good temporal correlation with historical floods recorded in the central Mediterranean region. In the southeast of Tunisia, periods with more frequent floods coincide with periods generally under alternation of wetter and drier conditions and minima of solar activity. The paleoflood record of the El Bibane lagoon can help researchers better understand the physical mechanisms responsible for changes in the intensity and/or frequency of extreme events in the south of Tunisia. Our research proves that the proxies utilized here have a lot of potential for reconstructing past floods, which is important knowledge for future flood prediction and mitigation in southeast Tunisia. • Reconstruct the past extreme flood events in the El Bibane Lagoon (southeast of Tunisia). • Our multiproxy approach registers reliably flood events during the upper Holocene. • The combination of chronological, geochemical and granulometric analysis have permitted us to characterize the sedimentological signature of paleofloods levels. • A good correlation is found between the flood events recorded in the El Bibane lagoon and those obtained in the sebkha el Mhabeul (Southern Tunisia). • The periods with more frequent floods events coincide with periods of minima solar activity and with the last two Bond events. [ABSTRACT FROM AUTHOR]

Big storm events represent a major risk for populations and infrastructures settled on coastal lowlands. In the Western Mediterranean, where human societies colonized and occupied the coastal areas since the Ancient times, the variability of storm activity for the past three millennia was investigated with a multi-proxy sedimentological and geochemical study from a lagoonal sequence. Mappings of the geochemistry and magnetic susceptibility of detrital sources in the watershed of the lagoon and from the coastal barriers were undertaken in order to track the terrestrial or coastal/marine origin of sediments deposited into the lagoon. The multi-proxy analysis shows that coarser material, low magnetic susceptibility, and high strontium content characterize the sedimentological signature of the paleostorm levels identified in the lagoonal sequence. A comparison with North Atlantic and Western Mediterranean paleoclimate proxies shows that the phases of high storm activity occurred during cold periods, suggesting a climatically-controlled mechanism for the occurrence of these storm periods. Besides, an in-phase storm activity pattern is found between the Western Mediterranean and Northern Europe. Spectral analyses performed on the Sr content revealed a new 270-year solar-driven pattern of storm cyclicity. For the last 3000 years, this 270-year cycle defines a succession of ten major storm periods (SP) with a mean duration of 96 ± 54 yr. Periods of higher storm activity are recorded from >680 to 560 cal yr BC (SP10, end of the Iron Age Cold Period), from 140 to 820 cal yr AD (SP7 to SP5) with a climax of storminess between 400 and 800 cal yr AD (Dark Ages Cold Period), and from 1230 to >1800 cal yr AD (SP3 to SP1, Little Ice Age). Periods of low storm activity occurred from 560 cal yr BC to 140 cal yr AD (SP9 and SP8, Roman Warm Period) and from 820 to 1230 cal yr AD (SP4, Medieval Warm Period). [ABSTRACT FROM AUTHOR]

Recent reductions of sediment transport are observed in Bouregreg River and estuary close to the city of Rabat in Morocco. The cause of this evolution in sediment transport which may be due to human activities (dam, pollution), climate variability and natural events has been investigated from a paleo-hydrological approach. Sedimentological and geochemical analyses of slackwater flood deposits are used to assess the impact of a dam installed in 1974 and the climate change on the evolution of sediment transport during about 60 years. Higher accumulation rate of about 3,64 cm/year is observed between 1950 and 1978, whereas it was much lower about 0,41 cm/year between 1990 and 2017. This strong decrease in sediment rate appears much more due to the impact of dam construction rather than to climate change, however climate change in an increasing concern for the future. • The paleo-hydrology is an excellent approach to estimate the impact of dam and climate change on fluvial sediment transport. • One potential climatic driver of river discharge in Morocco is the North Atlantic Oscillation (NAO). • Recent reduction of sediment transport is observed in Bouregreg River in Morocco, after the construction of dam since 1974. • Strong decrease in sediment rate appears much more due to dam effect than to climate change. [ABSTRACT FROM AUTHOR]

This study presents the geochemical composition of superficial sediment under oxic and suboxic bottom water conditions along the Chilean continental margin (SE Pacific), where evidence for benthic chemosynthetic activity associated with diffuse seeping of chemically reduced fluids has been reported. The exploration was carried out at: (1) the Chilean Triple Junction (CTJ), at a water depth of ∼2900 m, with the additional indication of hydrothermal activity near a methane-rich cold-seep area (46°S) (German et al., 2010); and (2) the El Quisco methane seep site (EQSS), at ∼340 m water depth (33°S) (Melo et al., 2007; Krylova et al., 2014). While the deeper CTJ is located within an oxic environment (dissolved oxygen in the bottom waters: 164 μM), the shallower EQSS lies within a suboxic environment (dissolved oxygen in bottom water: 23 μM), located within the lower limit of the SE Pacific oxygen minimum zone (OMZ). Pore water from short cores was analyzed for dissolved major, minor, and trace elements (Cl, Na, Mg, K, Ca, Sr, Si, B, P, Ba, Pb, Mn, Fe, Cd, U, and Mo), δ 13 DIC, sulfide, sulfate, and methane. The solid sediment fraction was likewise analyzed for total organic carbon (TOC), metals, and redox potential. Elevated sediment temperatures were found in superficial sediments (5–13 °C) at the CTJ site, which could be due to warm fluids associated with the proximity of the ridge, where hydrothermal vents may occur. Reduced fluids were also present here, indicated by higher Mn fluxes toward the water column even in oxidized sediments (RPD > 8 cm), which contrasted with the lower fluxes in reduced sediments of the EQSS site (RPD ∼ 2 cm). 13 C-depleted DIC, anomalously low pore water Cl (∼15 ppb), and low concentrations of other major elements may be the result of dilution by fluid seeping and precipitation of major elements, producing authigenic enrichment (Ca, Mg, Sr). The fluid could also: (a) be diluted by pure water produced during methane hydrate dissociation, as observed in other cold-seep areas; and (b) correspond to clay mineral dehydration, as reported in plate subduction systems. The reducing conditions established at the CTJ conduct the Cd enrichment at a similar magnitude of that seen at the shallower suboxic site (EQSS). Evidence of chimney or vent fauna was not observed. At the EQSS, higher TOC and total sulfide contents were consistent with enhanced deposition of organic matter and reducing conditions developed in the OMZ, favoring the authigenic enrichment of Cd, U and Pb. The geochemical evidence, based only on methane concentrations and δ 13 DIC, is insufficient to establish the presence of methane seeps, as previously reported. [ABSTRACT FROM AUTHOR]

• New sedimentary and historical records of river flooding in the western Mediterranean. • A long period of increased flooding occurred during the Medieval Climate Anomaly. • Increased flooding associated with higher air and sea surface temperature. • Higher flood frequencies related to the positive phase of the East Atlantic pattern. • Intensification of land use led to increased flooding during the Middle Ages. River flooding triggered by extreme rainfall events is a major natural hazard that frequently causes fatalities and damages in southeastern France. A 2000-year record of flooding from the Hérault River basin was reconstructed using magnetic susceptibility data from lagoon deposits, supported by documentary databases of floods in coastal river basins from Languedoc over the last centuries. Flooding was particularly important between 800 and 1350 cal CE, i.e. during the warm climatic conditions of the Medieval Climate Anomaly. Moreover, several periods of increased flooding occurred in the 2nd, 6th-7th, 16th, 18th and 20th centuries. Increased flooding was significantly correlated with higher air temperature in the northwestern Mediterranean and with higher sea surface temperature in the Gulf of Lions. The analysis of relationships between river flooding and atmospheric conditions shows that autumn flood events in Languedoc were generally more frequent during the positive phase of the East Atlantic pattern, which is associated with southerly to southeasterly winds in the Gulf of Lions. Besides, increased flooding could also have been controlled by forest decline and human activities after 500 cal CE. Between 1400 and 1800 cal CE, palaeobotanical data show an intensification of land use characterized by an expansion of grasslands and cultivated areas in the coastal lowlands east of the Hérault River. Forest clearings and the development of agriculture seem to have led to a relative increase in river flooding, especially in the late Middle Ages and early modern period. [ABSTRACT FROM AUTHOR]