Lagoonal Mud Research Articles

Fringing reef growth in the Mid-Devonian: An example from the southern Rhenish Massif, Germany

The Givetian Balduinstein Reef developed upon mixed felsic lavas and volcaniclastic tuffs. The Givetian age of the reef is proven by conodonts (Polygnathus rhenanus/varcus Zone to the Polygnathus ansatus Zone) and the occurrence of the brachiopod Stringocephalus burtini. Remains of land plants and subaerially ejected volcanic bombs in the vicinity of the outcrops document that the volcano was an emerged island surrounded by a fringing reef in an unstable tectonic environment. Reef development is characterised by a complex interaction of volcanism, erosion and the rate and pattern of sea-level change, which resulted in a variety of facies settings and rock-types. Microfacies analysis and rock-types prove that the fringing reef was backed by a shallow lagoon. A lime mudstone/alkali rhyolitic breccia represents a lava flow, or more probably a dome collapse breccia, which flowed into the lagoonal mud and confirms a contemporaneous occurrence of volcanism and reef growth. Reefs flourished during several episodes of inactivity of the volcano, particularly in the upper part of the sections, suggesting that the reef development kept up with sea-level changes and/or local events. Lithofacies of the upper part of the youngest section point to a regressive phase. The distinctive depositional architecture and evolution of the Balduinstein Reef are interpreted as having been controlled mainly by regional tectonics and volcanism with contributions from eustasy.

Roman ‘Well-cisterns’, Navigational Routes, and Landscape Modifications in the Venice Lagoon and Northeastern Adriatic

Underwater investigations in the lagoon of Venice have revealed a partially preserved structure interpreted as a Roman ‘well-cistern’. It is indeed very similar to another, better-preserved building excavated in the same area. The analysis of the mortar, the evaluation of the depth of the structure base in relation to relative sea level, and the absence of foundation poles in the underlying soft lagoon mud support such interpretation. A comparison with other poorly-studied Roman buildings of the same type located along the North Adriatic littoral suggests that they were probably used to supply ships with fresh water. In the Middle Ages, similar structures were used in the islands of the lagoon for sustenance of its residents and became a characteristic of the small squares (campi) of Venice.

Occurrence mode of Holocene tsunami overwash controlled by the geomorphic development along the eastern Nankai Trough, central Japan

Re-analysis of the lithological, paleontological, and geochemical data of three lagoonal sediment cores reveal the tsunami overwash history from 8000 to 6500 cal BP along the northern Suruga Bay coast, facing the eastern Nankai Trough. The coring sites are located in a transcect 0.8, 1.5, and 1.7 km inland, respectively, from the coastline at the time the overwash events occurred. The washover sand beds are distinguished from the lagoonal muds by the presence of quartz and mica, which are abundant on the outer (marine) side of the lagoon, as well as marine shell fossils. The increased C/S ratio often associated with sand beds, reflects input of open seawater to the reductive lagoon, and supports this interpretation. A maximum of eight tsunami deposits were identified between ∼7800 and 6500 cal BP, many of which can be correlated between the three cores. The magnitude of tsunami overwash implied by the thickness and grain size of tsunami deposits, and increase of C/S ratio following their deposition is relatively large at ∼7500-7200 cal BP during the post-glacial sea level rise and decreases with time during the subsequent high sea level stand. This occurrence mode (timing and scale of formation) of tsunami washover is explained by temporal changes in coastal geomorphology, rather than differences in the magnitude of individual tsunamis. Around 7500-7200 cal BP, sea level rise equilibrated with or exceeded the growth of the bay mouth barrier, and tsunamis could easily cross the small barrier and enter the lagoon, leaving behind thick and coarse-grained deposits. As the barrier becomes larger during the subsequent sea-level highstand, only the highest-amplitude waves exceeded the height of the barrier. By ∼6500 years ago, the barrier was high enough to prevent tsunamis from entering the lagoon. Similarly, the occurrence mode of tsunami deposits reported along the coast of Suruga Bay and Enshu-nada over the past 6000 years can be explained in relation to the growth of coastal barriers.

Deciphering hydroclimate fluctuations of Nile watershed from Holocene sediment of Manzala lagoon: Bio-geochemical implications

A 19.5-m long Holocene sediment core (MZ-1) was retrieved from Manzala lagoon coast, the depocenter of the Nile Delta. AMS-dating shows the complete Holocene strata of MZ-1, mostly consisting of lagoon muds. The high-resolution dated geochemical elements (major and trace) and bio-markers (n-alkanes) of MZ-1 serve as effective hydroclimate proxies that oscillated during the Africa Humid Period (AHP). Our measurements show the highest value of diagnostic geochemical elements of provenance implications (e.g. Ti/Al, Zr/Al, Cr/Al) prior to 7.5 ka - Period I, indicating the peak period of the AHP, when the African monsoon was intense over the Nile watershed, mainly the East African Plateau (EAP). This is immediately followed by Period II ca. 7.5–4.5 ka, characterized by the highly variable hydroclimatic conditions when the Intertropical Convergence Zone (ITCZ) withdrew southwards. It is denoted by a sharp drop of many diagnostic elements, then succeeded by a gentle, but highly-fluctuated rise, especially the metallic elements Fe, Ti and Cr of basaltic origins of the Ethiopian Highlands. It ascribes to an instability of the hydroclimate in the Blue Nile watershed due to eastward shift of the Congo Air Boundary (CAB) during the AHP receding. In response, the biomarker (n-alkanes) showed remarkably high terrestrial derived primary productivity in Manzala lagoon during 7.5–4.5 ka, evidently reversal to the Early-Holocene marine-enhanced ecological setting. The high Nile floods triggered by unstable hydroclimate of the Nile watershed, particularly in the Ethiopian Highlands of Period II delivered much terrestrial organic matter to coast, making lagoon water less saline. Minor geo-elemental and biomarker fluctuations occurred after ca. 4.5 ka in response to the post-AHP termination in the upper Nile Basin.

Holocene evolution and signature of environmental change of the Burullus lagoon (Nile Delta) deciphered from a long sediment record

This study presents high-resolution multi-proxy analyses of a well-dated sediment core (BR-1, 19.20 m long) retrieved from the Nile Delta, aiming to reconstruct the Holocene evolution of the Burullus lagoon. In particular, we focus on the morphodynamical processes, sediment provenances and related hydro-climatic changes. The Holocene strata of the lagoon coast comprise Early-Holocene marine transgressive facies and the Mid-Late Holocene retrogressive deltaic facies. Although BR-1 shows no river-channel facies, two peaks of magnetic susceptibility (MS) at ca. 8.0–7.0 ka (also associated with a high sedimentation rate of 50 mm/yr) and 3.5–2.0 ka evoke phases of greater morphodynamic influence of palaeo-Nile branches on the lagoon's development. Both dated peat layers (ca. 13 m and 8 m below MSL) and lagoon muds intercalated in BR-1, together with previously-dated peat strata were benchmarked relatively to MSL for the reconstruction of relative Holocene sea level in the study area. Our results show considerable land lowering on the Burullus coast indicated by a sea-level elevation, ca. 4–1 m lower than the one reconstructed along adjacent coastlines where less land subsidence occurred. The lagoon system began to form after ca. 6.0 ka in response to deceleration in sea-level rise and, being a shallower water body, both macrofauna (Cerastoderma glaucum etc.) and ostracods (Cyprideis torosa) of brackish-water nature emerged. In addition, geochemical data from BR-1 provide insights into environmental change in association with basin-wide hydro-climate fluctuations. Concomitant peaks in Fe/Al, Ti/Al and Cr/Al ratios (mafic origin), together with higher Zr/Al and Hf/Al ratios (felsic origin) before ca. 8.0 ka suggest sediment provenances derived from the White and the Blue Nile during the main African Humid Period (AHP). Since then, Ti/Al kept slightly ascending core-upwards with two pulses at 7.8–7.3 ka and 3.5–2.0 ka, which was asynchronous to Zr/Al and Hf/Al. This corroborates more sediments of basaltic origin transported to the delta coast after the main AHP due to southerly shift in the position of the ITCZ. The two periods of Ti/Al pulse are consistent with the MS peaks in BR-1, suggesting that palaeo-Nile branches remained active during 8.0–7.0 ka and 3.0–2.0 ka nearby.

Sequence stratigraphy, sedimentary characteristics of barrier coastal sedimentary system of the Benxi Formation (Gaoqiao area, Ordos basin) and favorable reservoir distribution

Benxi Formation is an abundant natural gas reservoir in Gaoqiao area. Therefore, it is essential to analyze the sequence and sedimentary characteristics of the Benxi Formation to predict the distribution of favorable gas reservoirs . After the comprehensive use of the core, thin section, drilling, and logging data, the sequence framework, sedimentary microfacies, planar distribution and vertical microfacies associations of the Benxi formation were studied. The Benxi Formation in the Gaoqiao area is barrier coastal sedimentary system, which is divided into eight sedimentary microfacies, namely, tidal delta, tidal channel, sand flat, mixed flat, mud flat, peat flat, barrier bar, and lagoon mud microfacies. In addition, we concluded that i) From west to east, peat flat, mud flat, mixed flat, sand flat, lagoon mud, and barrier bar have been developed in the study area. ii) Four typical vertical sequences were identified: MA1, MA2, MA1, and MA4. The first three sequences developed extensively and completely in the study area, whereas the last one generally has one or more microfacies missing. iii) The sedimentary system of the Benxi Formation in the Gaoqiao area was primarily vulnerable to tectonism, sequence, and sea-level changes. The Second Member was the target gas-producing stratum of the Formation. Both barrier bar and sand flat microfacies exhibited an excellent gas display. However, the barrier bar microfacies were the most favorable natural gas distribution zones with giant and premium gas reserves.

Sequence Stratigraphy, Sedimentary Characteristics of Barrier Coastal Sedimentary System of the Benxi Formation (Gaoqiao Area, Ordos Basin) and Implications for Gas Exploration

Benxi Formation is an important abundant natural gas resource in Gaiqiao area. Therefore, to analyze the sequence and sedimentary characteristics of the Benxi Formation and to provide guidance for predicting the favorable gas reservoir distribution. In addition, after comprehensive use of core, thin section, drilling, and logging data, the sedimentary microfacies of Benxi formation were studied. The Benxi Formation in Gaoqiao area is concluded to be a barrier coastal sedimentary system, subdivided into eight sedimentary microfacies, namely, tidal delta, tidal channel, sand flat, mixed flat, mud flat, peat flat, barrier bar, and lagoon mud microfacies. Along with sequence framework and sedimentary microfacies characteristics, planar distribution characteristics of sedimentary microfacies and vertical sequence characteristics are also studied. It is concluded that i) From west to east, peat flat - mud flat - mixed flat - sand flat - lagoon mud - barrier bar have been developed in the study area ii) Four typical vertical sequences have been identified: MA1, MA2, MA1, MA4. The first three sequences have developed extensively and completely in the study area, while the last one generally has one or more microfacies missing; iii) The sedimentary system of the Benxi Formation in Gaoqiao area is mainly vulnerable to tectonism, sequence, and sea-level changes. The Second Member is the target gas-producing strata of the formation. Both barrier bar and sand flat microfacies have a good gas display. However, the barrier bar microfacies are the most favorable natural gas distribution zones with giant and premium gas reserves.

Late Quaternary landscape evolution of the buried incised valley of Concordia Sagittaria (Tagliamento River, NE Italy): A reconstruction of incision and transgression

While recent landscape changes can be usually easily read through accessible morphological features, relatively ancient processes can be difficult to detect as the associated morphologies and deposits are often buried below younger sediments. With the aim of understanding the evolution of the distal Venetian-Friulian Plain (NE Italy) after the Last Glacial Maximum (LGM), this work presents the analysis performed on a dataset of ca. 2300 mechanical and hand-made cores, which allowed a detailed reconstruction of the formation and evolution of an incised valley, now almost completely filled and with little to absent morphologic expression. Such valley, up to 1.2 km wide, with a depth of 20 m below the LGM alluvial plain and traced for a length of 25 km, is the result of the complex interplay between minor spring-fed rivers and the Tagliamento River. The detailed characterization of the infilling allowed to identify two main phases in the valley evolution. The first one is related to the activity of the paleo Tagliamento River and led to the deposition of a 10 m thick gravelly unit (ca. 19–9.5 ka cal BP). The second phase, which followed a disconnection of the Tagliamento, is linked to the Holocene marine transgression and led to the formation of a lagoon environment within the valley and to the deposition of a ca. 15 m thick unit of lagoon muds (ca. 8 ka cal BP - historic time). This latter unit lays on top of gyttja deposits, indicating for the first time in this area the presence of widespread lacustrine environments in the Early Holocene. This work presents an in-depth analysis on the evolution of a distal plain incised valley, from its formation to its final filling, providing at the same time the means to describe the development of the entire alluvial plain landscape, spanning from the end of the LGM to the middle Holocene. We present new data on the paleoenvironmental and morphologic evolution of the Venetian-Friulian Plain area as a consequence of the interplay between autogenic forces and sea-level rise. Our study allowed to understand the importance of both Alpine rivers and groundwater-fed streams in the formation of large incised valleys in the coastal sector of the whole Venetian-Friulian Plain. Finally, as during the transgressive phase the upstream sediment input in the valley was almost absent, this study provides insights into facies and architecture of a rare example of downstream-controlled filling of an incised valley.

Relict and contemporary influences on the postglacial geomorphology and evolution of a current swept shelf: The Eastern Cape Coast, South Africa

Few stratigraphic models of continental shelves incorporate the process of geostrophic current-sweeping, consequently its role in the stratigraphic record is often overlooked. We examine the narrow, current-swept Eastern Cape shelf of South Africa using a combination of geophysical techniques, seafloor sampling and video observations and interpret the role of current action on the transgressive stratigraphy of this steep subtropical shelf. During the Last Glacial Maximum, fluvial valleys incised the acoustic basement rocks. During the subsequent transgression, two distinct shorelines were formed and preserved at −105 m and − 60 m. Their development and preservation is linked to (i) high sediment supply from adjacent fluvial sources, (ii) early diagenesis and (iii) alternating sea-level stillstands and periods of rapid sea-level rise during melt water pulses 1A and 1B, respectively. The deeper shoreline formed in a sandy, wide coastal plain setting with limited bedrock influence, whereas the shallower shoreline comprised alternating rock headlands and embayments like the contemporary coast. Differences in antecedent topography and geology are responsible for the temporal variability in shoreline type; coastal squeeze is exacerbated with increasing bedrock control as the shoreline migrates landwardBetween the two shoreline complexes, in the mid-shelf, the transgressive stratigraphy records initial valley infill by progradation of coast-parallel sandy spits. These are capped by a stiff lagoonal mud deposited as ongoing sea-level rise overspilled the valley interfluves, onlapping the adjacent aeolianites. The uppermost stratigraphy comprises mounds of rhodoliths which interfinger with a sandy inner to middle shelf highstand wedge.After sea-level reached its present position ca 7.4 ka yr BP, the shelf became subject to reworking by the high-energy geostrophic Agulhas Current. This has had the following major effects on the shelf stratigraphy: 1. The topographic relief of the cemented palaeo-shorelines has been emphasised by removal of the post-transgressive cover; and 2. The shelf no longer acts as a depocenter; instead, the seabed consists of rhodoliths, gravel streamers, bedrock or gravel hash of the wave ravinement surface. Given the necessary antecedent conditions such as accommodation, sediment supply and favourable diagenetic climate, prominent shorelines can form and be preserved on the shelf. Strong current sweeping emphasises these morphological features on subtropical shelves.

Sedimentary characteristics based on sub-bottom profiling and the implications for mineralization of cobalt-rich ferromanganese crusts at Weijia Guyot, Western Pacific Ocean

Cobalt-rich ferromanganese crusts on seamounts have attracted much attention due to high economic potential of various metals. Studies showed that seamounts in the Western Pacific Ocean are rich in cobalt-rich crusts, and Weijia Guyot (Ita Mai Tai) is one of the most promising one. Cobalt-rich crusts were drilled from this seamount in our previous investigation. This study evaluates promising areas of cobalt-rich crusts on the whole guyot. Combined the sub-bottom profiles, topography, scientific ocean drilling data with related studies, this paper studied sedimentary characteristic and their implications for cobalt-rich crusts mineralization prospective areas on the summit of Weijia Guyot. Three types of stratum reflection characteristics were identified: pelagic deposits, oolitic limestone, and lagoonal mud. Reflection sequences in Chirp sub-bottom profiler records are well matched to stratigraphy obtained at Deep-Sea Drilling Project Sites 200 and 202. Cobalt-rich crusts promising mineralization areas were delineated based on the water depth, slope gradient and pelagic pinch-out line, with the area approximately 576.4 km2. This estimated number is 10% higher than the results from previous studies (approximately 436.6 km2). It provides great implication for exploration and mining-lease-block selection in the future.

Facies Analysis and Sequence Stratigraphy of Al-Kharrar Lagoon Coastal Sediments, Rabigh Area, Saudi Arabia: Impact of Sea-Level and Climate Changes on Coastal Evolution

This study employs the results of facies analyses and sequence stratigraphy of the coastal plain sediments of Al-Kharrar Lagoon, Rabigh, Saudi Arabia, to interpret the impact of Late Quaternary sea-level and climate changes on sedimentation, facies distribution and Red Sea coastal evolution. Facies analysis of the sediments that crop out in a recently excavated quarry section and those recovered from six shallow cores obtained from the tidal flat south Al-Kharrar Lagoon enabled identifying four facies associations characterizing fluvial, intertidal–supratidal, lagoonal and intertidal flat deposits. Facies architecture and sequence stratigraphic interpretations indicate two distinct depositional stages. The first stage included the deposition of marine-influenced gravel-dominated fluvial channel deposits interdigitating with or overlain by grey lagoonal mud and fine sands suggesting deposition during a period of progressive sea-level rise and enhanced precipitation. The second stage records deposition of prograding intertidal–supratidal deposits containing calcareous nodules and mottles and evaporite minerals at quarry section suggesting deposition during a sea-level fall under an arid climate. The results of this study are correlated with the Late Quaternary periods of humidity and aridity. It is found that the first stage corresponds to the postglacial early–middle Holocene sea-level rise and wet climate, whereas the second stage corresponds to the mid- to late Holocene isostatically controlled sea-level fall and arid climate.

Late Holocene erosion of the Canopic promontory (Nile Delta, Egypt)

The mouths of the Nile Delta are sensitive coastal areas, their geomorphology primarily being mediated by relative sea-level rise and sediment supply. To further document the Holocene evolution of the Nile's Canopic mouth, a core was taken from the southern shores of Abu Qir Bay, close to the ancient Canopic channel. Core bio-sedimentology and chronostratigraphy highlight four stages of marine incursion which are juxtaposed upon the general progradation trend of the Nile coast in this area. Compiled age-depth points from sediment cores taken in Abu Qir Bay underscore two phases of negative sediment budget at the Canopic mouth: (1) a first period, between 3.5 and 2kacal. yr BC, probably in relation to the well-documented mid-to-late Holocene decline in Nile flow; and (2) a second phase, after 0.5kacal. AD, linked to a decline in Canopic sediment supply to the coastal area, and concomitant with the development of the Rashid branch. The erosion and reworking of material flattened and lowered the promontory surface by up to 4m. The submersion of the Canopic promontory was completed by relative sea-level rise, primarily controlled by the compaction and liquefaction of unconsolidated lagoonal muds. The lowering of the Canopic mouth led to the submersion of two ancient estuarine-harbor cities, known as East-Canopus and Herakleion, whose remains lie 4–7m below present mean sea level. It is argued that the subsidence of the two cities cannot explain their abandonment during the late 7th–early 8th century AD, taking into account the regional occupation pattern during Antiquity. Rather, the longevity of the two cities, spanning more than 13 centuries, shows that adaptation to coastal risks including erosion, subsidence and high-energy events like storms or tsunamis, was the rule.

An Early Bronze Age pile-dwelling settlement of discovered in Alepu lagoon (municipality of Sozopol, department of Burgas), Bulgaria

A new pile-dwelling settlement has been discovered during coring investigations on the shores of the Alepu lagoon (municipality of Sozopol, department of Burgas), on the western Black Sea coast, in Bulgaria. A multi-disciplinary methodology was applied to analyze the archaeological dataset, composed of wood piles, abundant charcoals and wood fragments, seeds, fish and shell remains, a few small bone fragments, some lithic fragments and potsherds. The piles were trimmed from oak trees and sunk into lagoonal muds, and currently lie 5.8 to 6.8 m below mean sea level. It highlights a wooden building at the edge of Alepu palaeo-lagoon. Charcoal remains confirm the use of oak tree as a dominant timber resource, consistent with pollen data for this period. Palaeo-botanic remains highlight gathering activities and the consumption of wild grapes, raspberries and figs. The herbaceous assemblage evokes deforestation activities. Exploitation of coastal resources is well attested by the great density of fish remains, dominated by anchovy (61%), highlighting possible preservation of fish products. Five radiocarbon dates constrain the age of the site to between 3350 and 3000 cal. BC.The Alepu piles-dwelling settlement sheds new light on the very beginning of the Early Bronze Age in coastal Bulgaria. Adding fresh information to the local archaeological record, it completes the well-discussed issue of the protohistoric submerged settlements, revealing in turn the economic strategies of the societies at the end of the transitional period. Considerations about geomorphological settings of these sites underline the evolution of the regional settlement patterns, as well as the importance of lagoonal locations. Lagoon contexts not only offer abundant fish resources, as attested by our data from Alepu, but also good conditions for anchorage. These sites were later drowned, preserved and buried following the relative rise of the Black Sea level.

CORROSION MINERALOGY OF COMMON BIRD SHOT TYPES IN NORTH CAROLINA, USA, ENVIRONMENTS

This study compared corrosion of lead, steel, copper-coated lead, tungsten-steel, and bismuth varieties of shot in different sedimentary and aquatic environments in North Carolina. Sets of individual plastic vials containing all five types of shot were placed in the following environments: marine beach, marine dune/soil, barrier island lagoonal mud, saltwater, running (stream) freshwater above and below sediment surface, still (pond) freshwater above and below sediment surface, and forest soil and surface. After approximately 4.5 years exposure, samples were collected and analyzed with scanning electron microscopy/energy dispersive spectrometry (SEM/EDS) and powder X-ray diffraction (XRD). Samples that were placed in saltwater environments were the most severely corroded, while those in terrestrial or freshwater environments were much less corroded. Similarly, buried samples were generally more corroded than samples placed at or above the water table. Analyses reveal the production of Pb oxide, hydroxide, chloride, carbonate, sulfide, and sulfate compounds in lead and copper-coated lead shot. Steel and tungsten-steel shot exhibited a surface layer of Fe oxides/hydroxides. Bismuth shot and tungsten-steel shot showed little corrosion, limited to small amounts of oxide/chloride and oxide only in two of the sampled environments. In all shot types, 19 minerals and one non-mineral compound [Zn 4 (CO 3 )(OH) 6 •H 2 O] were identified as corrosion products.

Sedimentary architecture and depositional evolution of the Quaternary coastal plain of Maricá, Rio de Janeiro, Brazil

The coastal geomorphology of Marica (Rio de Janeiro state) is characterized by a large lagoon and by two sandy barriers that confine a series of small isolated chain-like lagoons. Data collected from ground-penetrating radar and boreholes from the central coastal plain of Marica provided information on the sedimentary architecture and evolution of this area in the Quaternary. Six lithological units were identified comprising three depositional sequences limited by erosional surfaces, related to barrier-lagoon systems that migrated onshore, offshore, and longshore, giving rise to a sedimentary deposit 25 m thick or more. The data reveal a retrograding barrier overlying a basal mud unit which rests in unconformity upon Precambrian basement, thus characterizing an important Pleistocene transgression. A second Pleistocene barrier of 45,000 cal years BP migrated over a lagoonal mud unit (48,000-45,000 cal years BP) reaching over the previous barrier. A progradational phase followed due to a fall of sea level. A long interval of erosion of the barrier created an unconformity that represents the Pleistocene-Holocene boundary. A beachrock in nearby Itaipuacu, 100 m offshore from the present-day beach, dated as 8,500 cal years BP marks the onset of Holocene sedimentation due to gradually rising sea level, which continued until at about 5,000 years ago. This promoted the retrogradation of the barrier-lagoon system. A brief episode of progradation is observed as a series of paleobeach scarps. Today's rising sea level is causing the retrogradation of the barrier.

Benthic communities and habitats in the near shore zone of the Curonian Spit (the south–eastern part of the Baltic Sea)

Benthic communities classified according to species diversity, abundance and composition of dominant complex were defined and mapped. Maps compiled represent the distribution of bottom sediment types, substrata, bathymetry and benthic communities in the pilot area. Combination of data on community distribution and several abiotic habitat features (grain size, substrate types, and photic conditions) allowed recognizing several benthic habitats, according to HELCOM habitat classification. New data on features of coastal benthic biotopes made evident the existence of unique seascape “ancient lagoon mud” in the study area and allowed recommending further establishment of new marine protected areas.

Identifying possible tsunami deposits on the Shizuoka Plain, Japan and their correlation with earthquake activity over the past 4000 years

Three washover sand beds, ranging from 15 to 34 cm in thickness, were recorded in a lagoonal mud sequence on the Shizuoka coast of central Japan, which faces the Suruga Trough. The sand beds were composed of well-sorted and well-rounded beach sand derived by a marine inundation. The basal erosion surface, mud clasts, and presence of both inverse and normal grading suggest that the sand beds formed as a result of high-energy deposition. The sand beds are multilayered, with fine alternations of sand sheets and mud drapes, which are consistent with deposition from a long-period wave train. Radiocarbon dates obtained from the three washover deposits are around ad 1000, 3565–3486 cal. yr BP, and 4000 cal. yr BP. Vertical changes in the diatom assemblages suggest a gradually increasing inflow of seawater up to the second sand bed, followed by a rapid change to freshwater conditions just above the bed. The gradual increase of seawater inflow begins again just above the second sand bed. We conclude that this series of sequential environmental changes indicates interseismic subsidence and coseismic uplift. The study area experienced around 1 m of uplift during the ad 1854 Ansei Tokai earthquake (Mw 8.4). In contrast, the area has subsided gradually ( c. 6 mm/yr) during the present interseismic period. The youngest sand bed may be correlated with the ad 1096 Eicho earthquake, which caused severe damage along the Shizuoka coast.

The Gyra washover fan in the Lefkada Lagoon, NW Greece—possible evidence of the 365 AD Crete earthquake and tsunami

Coastal geomorphological and geological archives store valuable information about the geodynamic evolution of coastal areas and the contributing geomorphodynamic processes. The coastal geomorphology of the Lefkada– Preveza coastal zone, NW Greece, holds evidence for the influence of both gradual, low-energy and episodic, high-energy coastal processes. Situated close to the Hellenic Arc and the Cefalonia transform fault, the area belongs to one of the seismically most active regions in the Mediterranean. Thus, tsunami events are assumed to have contributed to the coastal evolution in the study area according to tsunami catalogues and field evidence from previous investigations. The northern part of the Lefkada Lagoon, separated from the Ionian Sea by an extensive barrier beach system, is characterized by a number of fan-like washover structures, formed by different washover generations. In this paper, we present detailed geomorphological, sedimentological, geochemical and foraminiferal investigations on the most prominent fan structure, the Gyra fan, based on terrestrial and lagoonal sediment cores. The Gyra fan consists of a coarse-grained sequence of fully marine origin on top of lagoonal mud and a palaeosol, respectively. The sequence shows numerous characteristics of extreme wave deposits. Due to the dimension and morphology of the fan, the existence of up to four consistent and correlative stratigraphic subunits, their landward thinning and their comparable sedimentary characteristics, we argue that a tsunami event induced the formation of the Gyra fan, comprising at least four major inundation impulses. According to radiocarbon dates, this geomorphological structure was most likely formed by teletsunami effects triggered by the 365 AD earthquake off western Crete.

Flat-plate techniques for measuring reflectance of macro-algae (Ulva curvata)

We tested the consistency and accuracy of flat-plate spectral measurements (400–1000 nm) of the marine macrophyte Ulva curvata. With sequential addition of Ulva thallus layers, the reflectance progressively increased from 6% to 9% with six thalli in the visible (VIS) and from 5% to 19% with ten thalli in the near infrared (NIR). This progressive increase was simulated by a mathematical calculation based on an Ulva thallus diffuse reflectance weighted by a transmittance power series. Experimental and simulated reflectance differences that were particularly high in the NIR most likely resulted from residual water and layering structure unevenness in the experimental progression. High spectral overlap existed between fouled and non-fouled Ulva mats and the coexistent lagoon mud in the VIS, whereas in the NIR, spectral contrast was retained but substantially dampened by fouling.

Traces of Holocene tsunamis across the Sound of Lefkada, NW Greece

This paper gives evidence of multiple tsunami inundation of the Sound of Lefkada (NW Greece) since the mid-Holocene based on the analysis of sediment cores by means of geomorphological, sedimentological, geochemical, micromorphological and micropalaeontological methods. Layers of sand, gravel and shell debris, mostly unsorted, were found intersecting autochthonous lagoonal muds of the sheltered and quiescent inner-sound environment. They are further characterized by erosional unconformities at their base, rip-up clasts from the underlying sediments, fining upward sequences and an upward increase in sorting. The coarse grained high energy deposits include macro- and microfaunal remains typical of open-marine, partly even deep-water conditions which underlines their allochthonous character. Several distinct event layers indicate multiple tsunami passage across the sound. Earth resistivity measurements and vibracore transects revealed that the entire sound has been affected by catastrophic wave events. Thin sections of tsunami-influenced sediments found in lateral parts of the sound show a mixture of marine, lagoonal and terrigenous material. The inner Sound of Lefkada, well protected against storms, thus represents an excellent trap for tsunamigenic deposits. A preliminary local tsunami geochronology is based on 9 radiocarbon dates and diagnostic ceramic fragments. Several early tsunami impacts hit the sound between the 6th and 3rd millennium BC. Younger events seem to be consistent with tsunami landfalls that hit adjacent areas around 1000 cal BC, 395–247 cal BC as well as in Roman and medieval times. Vibracore data document an isthmus-like shallow-water environment which existed in the central sound and which was repeatedly inundated by tsunami wave action. Multiple tsunami passage eroded a natural channel which, we suggest, is the precursor of the famous waterway excavated by the Corinthians in the 7th century BC. Based on historical data, it is concluded that the navigable channel across the sound was repeatedly choked with sediments by tsunamigenic inundation during the following centuries. This study reveals a high tsunami risk for the Sound of Lefkada with at least one strong event in every 500–1000 years.