Holocene Coastal Deposits Research Articles

Hydrogeochemical processes as a conditioner of freshwater lenses associated with coastal deposits

AbstractThe aim of this work was to study the processes and factors conditioning the hydrochemistry of freshwater lenses associated with Holocene coastal deposits in an area of the Argentinean Atlantic coast. For this purpose, geomorphological units were mapped and a shallow groundwater monitoring network was established, including pre‐existing boreholes and shallow exploration wells. During the construction of the shallow exploration wells, sediment samples were extracted to determine the texture and mineralogical characteristics. Four samplings were carried out to measure in situ physicochemical parameters and to determine majority ions in the water. The chemical data were analysed considering the associated geomorphology, lithology, water balance and hydrodynamics. The geomorphological and lithological characteristics are the combined factors that favour the preferential rainwater recharge processes. Geoforms with greater topography and extension (continental and coastal dunes) tend to store freshwater lenses, while those with lower topography and little areal extension (sand sheets and beach ridges) have very thin lenses and tend to become salinized. Climate and local water balance are factor that determines that groundwater recharge occurs preferentially in winter. This causes increases in groundwater levels and a decrease in the groundwater EC. On the other hand, mineralogy is a factor that conditions the geochemical processes associated with the water–sediment interaction. Within the main geochemical processes, the dissolution and/or mineral weathering, the dissolution of CO2(g) and ion exchange determine the presence of different hydrochemical facies, where Ca‐HCO3 facies dominates in elevated geoforms, while Na‐Cl in lower ones. The results obtained in this work have provided an understanding of the factors and processes that condition groundwater chemistry in coastal deposits, being of vital importance for the management of water resources.

Middle Pleistocene to Holocene estuarine and aeolian deposits at L’Amélie (Médoc peninsula, Southwest France).

The chronostratigraphy of Pleistocene and Holocene coastal deposits in the Médoc Peninsula (southwest France) was revised during preventive archaeology work carried out prior to the restoration of the riprap protecting L'Amélie seaside resort. This study revealed that the Pleistocene deposits were more complex than initially expected. At the base, they comprise subtidal to supratidal estuarine units (“Argiles du Gurp”: laminated silts, green clay, sandy peat with tree stumps) related to the high sea level of Marine Isotopic Stage (MIS) 11 (Holsteinian). These deposits were incised by a fluvial/deltaic channel at the end of the interglacial, then covered by aeolian sands dating back to MIS 10. The whole area is covered unconformably by gravelly sand colluvium. Holocene aeolian deposits fill a small valley cut into the Pleistocene deposits and indicate the arrival of the coastal dune massif in the area during the late Antiquity. An overview of the results obtained in Médoc shows successive bodies of estuarine deposits indicating a gradual northward migration of the Gironde estuary. Only a few high marine levels are recorded (MIS 1, MIS 9, MIS 11, Gelasian, Piacenzian), presumably both because the sea level reached was similar to or higher than the Holocene level and because the deposits were preserved from erosion during the periods of valley incision.

Morpho-Sedimentary Constraints in the Groundwater Dynamics of Low-Lying Coastal Area: The Southern Margin of the Venice Lagoon, Italy

Complex freshwater–saltwater exchanges characterize most Holocene groundwater aquifers in low-lying coastal plains around the world, particularly in mechanically drained territories. This is due to the combination of several factors that control groundwater dynamics, including the high variability of the Holocene coastal deposits that host the shallow aquifers and the water management practices. The relationships between the stratal architecture of sedimentary deposits and the vertical changes in the salinity of the phreatic aquifer are poorly studied although they represent an issue of primary importance for a sustainable use of water resources and for agriculture. This research work is focused on the influence of sedimentary constraints, i.e., stratigraphic discontinuities and related changes in permeability in shaping salinity stratification into the unconfined aquifer at the southern margin of the Venice lagoon (Italy). Nine sites have been investigated by collecting sediment cores for facies analysis and monitoring water electrical conductivity in piezometric wells. The results show that buried channelized sandy deposits can enhance salinity mitigation of the phreatic aquifer in conjunction with precipitations and sufficient freshwater supply from nearby rivers and irrigation channels. Our analyses also reveal that the differences in stratigraphic architecture of the upper 10 m of the subsoil determine different fresh–saltwater dynamics of the phreatic aquifer. In particular, three possible behaviors can occur: (i) where the subsurface is characterized by the presence of a thick, up to 5 m, paleochannel, a freshwater lens is always present in the most surficial part of the phreatic aquifer; (ii) where the subsurface is composed by fine-grained sediments of marsh and lagoon paleo-environment, the phreatic aquifer tends to be salt-contaminated over its entire thickness; (iii) where the subsurface contains thin, up to 2–3 m, paleochannel deposits, the fresh–saltwater dynamics of the most surficial part of the phreatic aquifer varies more during the year, as a result of seasonal precipitation trend. The provided characterization of saltwater dynamics represents the basis for planning mitigation measures to improve the farmland productivity of the Venetian coastal plains.

Cenozoic coastal carbonate deposits of Qatar: Evidence for dolomite preservation bias in highly‐arid systems

AbstractIn the ancient rock record, early replacement of metastable marine calcium carbonate deposits by dolomite has long been associated with evidence of arid depositional environments. Such associations led to the development of the seepage reflux dolomitization model, whereby magnesium‐rich marine waters concentrated by evaporation descend into underlying sediments, replacing primary aragonite and calcite deposits with dolomite through rock–water interaction. In the modern arid coastal systems of Qatar, where marine waters concentrate to halite saturation, both aqueous geochemical measurements and mineralogical investigations show that replacement of calcium carbonate deposits by dolomite is not occurring to any consequential degree. At best dolomite formation is minor and localized. Instead, modern and mid‐Holocene coastal deposits retain their original mineralogy, showing little evidence of carbonate precipitation reactions with the notable exception of beachrock formation. Pleistocene coastal deposits are mostly absent in comparison with both Qatar Holocene coastal deposits, and Pleistocene coastal deposits from more humid settings. The lack of onshore Pleistocene‐aged carbonate outcrops in Qatar, as well as regionally, is interpreted to reflect coastal sediment denudation during emergence due to: (i) the absence of coastal marine lithification; and (ii) the absence of meteoric cementation and root stabilization in the highly‐arid realm. In contrast, marine Palaeogene and Miocene carbonate deposits are preserved in outcrop in Qatar, having suffered marine lithification by dolomite, thus promoting retention of carbonate strata through subsequent lowstands. These older deposits formed during times of ocean acidification, which, based on natural system and laboratory investigations, is interpreted to promote metastable carbonate dissolution and dolomite formation. The highly‐arid Holocene and Pleistocene coastal systems of Qatar represent limestone factories, but these deposits are not being retained to the rock record. Based on these observations, the association between the occurrence of dolomite and arid depositional settings may be to some degree a preservation bias.

Coastal peat-beds and peatlands of the southern North Sea: their past, present and future.

Peat layers are well represented in the Holocene coastal deposits of the southern North Sea and provide evidence as to the extent and nature of the fens and bogs that occupied the region in the mid and late Holocene. While natural processes contributed to their demise, without human interference extensive areas of peatland would remain. We review the characteristics of the vegetation of these peatlands along with the processes that influenced their development. Spatial and temporal trends are explored through the use of palaeogeographic maps from three areas: the East Anglian Fenland, the Romney Marsh area and the Netherlands. The palaeoecological evidence indicates that eutrophic vegetation promoted by rising relative sea level (RSL) dominated in the mid Holocene, with a trend towards the development of oligotrophic and ombrotrophic vegetation in the late Holocene as the rate of RSL rise declined. Nevertheless, areas of eutrophic vegetation appear capable of long-term stability with areas of fen woodland and herbaceous fen persisting at some locations for several thousand years in the mid and late Holocene. Areas of active peat growth in the region are now largely confined to small remnants within agricultural settings. To retain their characteristic biodiversity these remnants have been managed using traditional practices, although their small size and fragmented distribution limits their biodiversity value. Biodiversity concerns and the ecosystem services peatlands provide, notably carbon sequestration and flood attenuation, underlie recent restoration projects. These efforts are likely to receive additional impetus as a consequence of rising water levels, given projected rates of RSL rise. Future large-scale restoration can be informed by a greater understanding of the processes that formed and sustained coastal peatlands in the past. We identify advances in palaeoenvironmental research that could enhance restoration efforts and help maximise the ecosystem services delivered through such projects.

Holocene palaeogeographical reconstruction and relative sea-level changes in the southeastern part of the island of Samos (Greece)

In this paper, we focus on the southeastern coastal zone of the island of Samos (east-central Aegean Sea), in order to reconstruct the evolution of coastal landscapes and the relative sea-level changes during the late Holocene. We use geomorphological mapping, sedimentological and palaeontological proxies of late Holocene coastal deposits from two lagoon environments. We further compare our results with previously published sea-level data and we show that the southeastern part of Samos was characterized by a subsidence trend at least during the late Holocene, with RSL rise rates of ∼0.8 mm/yr. Our study additionally highlights that local-scale tectonics is responsible for the evolution of the coastal zone of Samos.

Late Holocene palaeogeographical evolution of Paroikia Bay (Paros Island, Greece)

Although there is rich evidence for human occupation of Paros’ coastline, there is a dearth of data with regards to the evolution of the island's seaboard palaeoenvironments. In this paper, we use sedimentological and palaeontological proxies of late Holocene coastal deposits from lagoonal environment to reconstruct the evolution of coastal landscapes in Paroikia Bay (Paros Island, Greece). A semi-enclosed lagoon existed in the northeastern part of Paroikia from at least 2915–2551 BC, which was gradually infilled after around 780–436 BC. Although it was not possible to chronologically constrain the timing of the infill, it is most likely relatively young, indicating anthropogenic effects. A correlation of our chronostratigraphic data with archaeological remains and tidal notches in the study area suggests that the subsidence observed on Paros Island is linked to long-term subsidence in combination with vertical seismic displacements.

Chronology and late-Holocene evolution of Caleta de los Loros, NE Patagonia, Argentina

Geomorphological and sedimentary records of Holocene coastal deposits may serve as archives for the local reconstruction of trends in coastal evolution and of the key forcing parameters controlling long-term change. We here present new observations on the sedimentology, chronology, and surface properties of a coupled beach ridge and coastal lagoon system located on the northern shore of San Matías Gulf, NE Patagonia, Argentina. The study is based on remotely sensed data, sediment cores, and a large number of samples dated using optically stimulated luminescence (OSL). The field site is located in a topographical depression within a cliffed shoreline composed of friable sand and gravel stones. The oldest preserved lagoonal deposits formed in the protected inner part of the system c. 2300 years ago. An up to 4-km-wide strandplain prograded rapidly between c. 1000 and 500 years in the more exposed western part of the system. Lagoonal deposition occurred primarily during the last 500 years. The chronology and spatial arrangement of landforms appear to result from a switch-over in sediment delivery probably caused by local implications of major shifts in climate regime. Even though we were not able to identify or benchmark the precise triggers of geomorphological change at Caleta de los Loros, our study presents an example of the potential importance of environmental changes on the rapid and non-linear development of coastal sedimentary systems.

Sedimentary characteristics of relict deposits on the western South Yellow Sea

Integrated studies of vertical sedimentary sequences, grain sizes, and benthic foraminifera and ostracoda, in combination with AMS 14C dating, and 210Pb and 137Cs analysis were carried out in three vibracores taken from the area of relict deposits on the western South Yellow Sea. The relict sands, which are about 0.4 m thick, overlie on the Early Holocene coastal marsh or tidal flat deposits with an evident erosional interface in between. The middle and upper parts or sometimes the whole of the relict sands have been reworked under the modern dynamic environment. The sedimentation rate varies between 0.20–0.30 cm year−1. The relict sands show a bimodal grain-size distribution pattern in frequency curves, with a sharp peak in the coarse fraction between 3Φ and 4Φ and a secondary peak in the fine fraction of about 7Φ. Of the benthic foraminiferal and ostracod assemblages, the reworked relict sands are characterized by the mixing of the nearshore euryhaline shallow-water species and deeper water species. The erosional interface at the bottom of the relict sands is considered as a regional ravinement surface formed during the transgression in the Early Holocene due to shoreface retreating landwards. The relict sands were accumulated on the ravinement surface during the transgression in the deglaciation period as lag deposits after winnowing and reworking by marine dynamic processes. And the secondary peak of fine fraction in the frequency curve for the relict sands suggests the input of fine-grained sediments during the reworking process. As the conclusion, the relict sands in the study area are interpreted as a type of reworked relict sediments.

Palaeoenvironmental evolution and sea-level changes in the coastal area of NE Lemnos Island (Greece) during the Holocene

The study area is located on the northeastern coast of Lemnos Island (North Aegean Sea, 40°2′30″N, 25°00′00″E). It covers the archaeological settlement of Hephestia located in the Purnia Gulf and the coastal area of Alyki Lagoon. Reconstruction of the palaeogeography used palaeoenvironment data delivered from foraminifera and sea-level changes, providing archeological data about the presence of a potential harbour in the area.Geomorphological mapping as well as sedimentological and micropalaeontological studies of the Holocene coastal deposits were conducted. Four boreholes were drilled from the Alyki Lagoon, reaching 2.5 m to 11 m at the deepest and two from Hephestia area, reaching a depth of 6 m. Twenty-four samples were dated using the AMS radiocarbon method, providing a chronology of the sedimentary units.The calculated age from the boreholes provides dates from 7050 cal BP to 990 cal BP There was a constant sea-level rise during the last 7000 cal BP in the study area. Results from the model of Lambeck sea level curve and sea level index point from Hephestia and Alyki show a vertical difference of 0.5–1.5 m below sea level. The foraminiferal fauna analysis shows the development of shallow marine conditions with fresh water influx during 7000–4000 cal BP and a temporary lagoon environment with fluctuations to shallow bay environment from 4000 to 990 cal BP.

Evolution of an incised valley coastal plain estuary under low sediment supply: a ‘give‐up’ estuary

AbstractThe literature on incised river valley sedimentology is dominated by studies of sediment‐rich systems in which the valley has been filled during and/or shortly after drowning. In contrast, the Holocene evolution of the Kosi Lagoon, South Africa (an incised coastal plain river valley) took place under very low sedimentation rates which have produced a distinctive stratigraphy and contemporary sedimentary environments. The findings are based on a synthesis of the results of studies of seismic stratigraphy, sediment distribution, morphodynamics and geomorphology. Barrier migration was prevented by a high pre‐Holocene dune barrier against which Holocene coastal deposits accumulated in an aggradational sequence. Holocene evolution of the back barrier involved: (i) drowning of the incised valley; (ii) wave‐induced modification of the back‐barrier shoreline leading to segmentation during the highstand; and (iii) marine sedimentation adjacent to the tidal inlet. Segmentation has divided the estuary into a series of geochemically and sedimentologically distinctive basins connected by channels in the estuarine barriers. The seismic stratigraphy of the back barrier essentially lacks a transgressive systems tract, shoreline modification and deposition having been accomplished during the highstand. The lack of historical geomorphological change suggests that the system has achieved morphological equilibrium with ambient energy conditions and low sediment supply. This study presents a classification for estuarine incised valley fills based on the balance between sea‐level rise and sedimentation in which Kosi represents a ‘give‐up’ estuary where much of the relict incised channel form is drowned and preserved. It exhibits a fundamentally different set of evolutionary processes and stratigraphic sequences to those of the better known incised valley systems in which sedimentation either keeps pace with sea‐level (‘keep‐up’ estuaries) or occurs after initial drowning (‘catch‐up’ estuaries).

Geochemistry of salt marsh sediments deposited during simulated sea-level rise and consequences for recent and Holocene coastal development of NW Germany

De-embankment in the salt marshes of the island of Langeoog was carried out in 2004, thereby inducing an artificial transgression within an area of 2.2 km2. Material from three suspended matter traps (SMTs) located along a N–S transect was collected monthly between January 2006 and February 2007. Besides geochemical (major and trace elements) and grain-size analyses, the duration and height of water cover were continuously measured by pressure gauges during the sampling period at two sites, thus revealing inundation frequency (max. 280 year−1) and level (max. 2.4 m). Generally, the silt-dominated SMT material exhibits a geochemical composition similar to that of suspended particulate matter from the adjacent Wadden Sea. However, distinctly increasing enrichments of TOC, P, Mn and Mo from the shoreline towards the higher salt marsh clearly indicate fractionation processes during material transport. Geochemical comparison with older Holocene coastal deposits reveals a mixture of brackish and tidal flat sediments, thus reflecting an early stage of sea-level rise and the development from a terrestrial towards a marine-dominated system. Sedimentation rates are higher than the local sea-level rise, as revealed by vertical salt marsh growth. Storm surges deliver the highest amounts of sediment and play an important role in salt marsh accumulation within the study area. Average accumulation rates of TOC (780 t year−1), P (54 t year−1) and Mn (5.2 t year−1) in the de-embanked area suggest that the former sand-dominated sediments currently receive significant amounts of reactive organic-rich material, thus fostering biogeochemical cycling.

Mid Holocene lake level and shoreline behavior during the Nipissing phase of the upper Great Lakes at Alpena, Michigan, USA

The Nipissing phase was the last pre-modern high-water stage of the upper Great Lakes. Represented as either a one- or two-peak highstand, the Nipissing occurred following a long-term lake-level rise. This transgression was primarily an erosional event with only the final stage of the transgression preserved as barriers, spits, and strandplains of beach ridges. South of Alpena, Michigan, mid to late Holocene coastal deposits occur as a strandplain between Devils Lake and Lake Huron. The landward part of this strandplain is a higher elevation platform that formed during the final stage of lake-level rise to the Nipissing peak. The pre-Nipissing shoreline transgressed over Devils Lake lagoonal deposits from 6.4 to 6.1 ka. The first beach ridge formed ~ 6 ka, and then the shoreline advanced toward Lake Huron, producing beach ridges about every 70 years. This depositional regression produced a slightly thickening wedge of sediment during a lake-level rise that formed 20 beach ridges. The rise ended at 4.5 ka at the Nipissing peak. This peak was short-lived, as lake level fell > 4 m during the following 500 years. During this lake-level rise and subsequent fall, the shoreline underwent several forms of shoreline behavior, including erosional transgression, aggradation, depositional transgression, depositional regression, and forced regression. Other upper Great Lakes Nipissing platforms indicate that the lake-level change observed at Alpena of a rapid pre-Nipissing lake-level rise followed by a slower rise to the Nipissing peak, and a post-Nipissing rapid lake-level fall is representative of mid Holocene lake level in the upper Great Lakes.

Paleoenvironmental changes since 3000 BC in the coastal marsh of Vravron (Attica, SE Greece)

The coastal area of the Vravron Bay, in the vicinity of the homonym archaeological site, is a marshy plain located on the eastern part of the Attica Peninsula (eastern Greece). In order to provide evidence for palaeoenvironmental changes and landscape evolution of the area, detailed micropaleontological, palynological and sedimentological analyses have been conducted at the underlying Late Holocene coastal deposits. The recovered sediments (lithostratigraphic Units A–D) represent a continuous record of the environmental history of the area since the Early Bronze Age, covering all subsequent historical periods. The base of the recovered deposits is older than the 3rd millennium BC. Until late medieval times, the studied sequence contains plenty evidence for the existence of the natural port reported from the Mycenaean (lithostratigraphic Unit A), that infilled after repeated flooding events during the Classic period (lithostratigraphic Unit B) and gradually developed into a shallower marine environment (lithostratigraphic Unit C). In the uppermost part of the sequence (lithostratigraphic unit D), a transition towards brackish environment with intense fresh water input is featured around 1540 AD. Signs of human agricultural activities in Vravron appear since 3000 BC (Early Bronze Age) and are linked to the history of inhabitation in the area.

Paleoenvironmental evolution and prehistoric human environment, in the embayment of Palamari (Skyros Island, Greece) during Middle-Late Holocene

Palamari Bay is located on the northeastern coast of Skyros Island (Sporades Islands, Aegean Sea). At the northern edge of the bay a fortified prehistoric settlement is found, dated between 2800 and 1700 BC (Early Bronze Age II–Middle Bronze Age I). Detailed geomorphological mapping of the coastal alluvial plain and paleontological, micropaleontological, palynological, sedimentological and micromorphological studies of the Holocene coastal deposits have been conducted in order to reconstruct the palaeoenvironment and the landscape evolution of the broader area of Palamari Bay. Three main sedimentary units were recognized (A, B and C, from oldest to youngest). The lowermost sedimentary unit A, deposited between before 7500 and 3500 cal BP, consists of sediment deposited from high to moderate energy fresh water flows with some suspended load fallout in established water bodies. The microfauna indicates a shallow fresh water environment. However, a tendency to oligohaline conditions was established gradually. During the same period, the Palamari area was characterized by open mixed deciduous forests that gradually retreated as a possible consequence of the intensification of anthropic activity, associated with the settlement of Palamari. Indications of cultivating and grazing activities in the vicinity of the lagoon were identified, pointing to a strong human presence since the Neolithic. Between about 6000 and 3500 cal BP, the embayment was a lagoon southeasterly connected to the sea, therefore sheltered and protected from northeastern winds. The overlying unit B (ca. 3500–800 cal BP) is characterized by the dominance of brackish water microfauna, indicating a brackish stagnant shallow water depositional environment, which was periodically supplied with fresh water from the surrounding springs. As the result of the continuous sea-level rise during the Late Holocene, part of the northern headland was submerged. The decline of the Palamari settlement at the time of the establishment of Unit B might be related to the observed changes that rendered the embayment a restricted body of water. The uppermost sedimentary unit C corresponds to a backshore environment dominated by aeolian activity modified by fluvial processes.

The Streif classification system: a tribute to an alternative system for organising and mapping Holocene coastal deposits

The classification system for organising and mapping Holocene coastal deposits as introduced by Streif is highlighted with respect to mapping requirements. Apart from the requirements such as a user-friendly legend and map, the linkage of the classification system with the history of deposition is demonstrated for the tide-dominated lowlands of the southern North Sea and English Channel. The linkage becomes obvious when considering the effect of the changes in the rate of relative sea-level rise. The difficulties that surrounded the lithostratigraphy of Holocene coastal deposits and the principles of the classification system which eventually made an end to the well-known and long-standing debate about the lithostratigraphy are briefly recalled.

A geophysical profile at the foot of the Dutch coastal dunes near the former outlet of the ‘Old Rhine’

AbstractThe apparent electrical conductivity of the subsurface along the Dutch coast between ‘Wassenaarse Slag’ (beach post 92.250) and north of Noordwijk aan Zee (beach post 77.750) reflects the spatial distribution of Holocene coastal deposits. Using a Geonics EM-34 a threefold division based on the mean electrical conductivity could be made in the outlet area of the River Old Rhine. 1) A section showing a sequence of sandy deposits that resembles the Schoorl Member of the Naaldwijk Formation on top of the Zandvoort Member of the Naaldwijk Formation. 2) A section underlain by a shallow clay/peat layer of varying thickness belonging to the Naaldwijk Formation (Walcheren Member) / Nieuwkoop Formation (Hollandveen Member), intercalated within the Schoorl Member and Zandvoort Member of the Naaldwijk Formation. South of the Old Rhine locally Wormer Member deposits of the Naaldwijk Formation underlie these clay/peat layers. 3) A section characterized by channel fill deposits of the former Rhine reaching to -20 m below Normaal Amsterdam Peil (N.A.P.), and belonging to deposits of the Wormer Member of the Naaldwijk Formation, and to two facies of the Naaldwijk Formation (Walcheren Member). The geophysical transect was calibrated using existing nearby drill hole data, so that a continuous snapshot of the subsurface was gained. Therefore this research also contributes to the recent genesis of this part of the Dutch coast.

Sequential growth of early diagenetic freshwater siderites in the Holocene coastal deposits, Korea

Spherulitic siderite grains ranging from 100 to 250 μm in diameter occur abundantly in a sediment layer, which is almost 8600 years old, in Namyang Bay, on the west coast of Korea. These large siderite grains consist predominantly of FeCO 3 (average 65%) and MnCO 3 (average 22%) and have low Mg/Ca and Mg/Fe ratios. Compositional scatter plots, together with the high Mn and low Mg contents, indicate that meteoric pore water was mainly involved in the siderite precipitation. Examination of the siderite grains with an electron microprobe reveals a systematic, micron-scale, center-to-margin compositional variation within individual grains. Three zones are recognized on the basis of the compositional variations: Zone I (center) in which Mn decreases and Fe increases, Zone II (intermediate) in which Mn increases and Fe decreases, and Zone III (outer) in which both Mn and Fe decrease and Mg increases. This zoning is interpreted as a result of depth-related early diagenetic reactions with different Fe, Mn, and Mg concentrations in the sediment pore waters. Meanwhile, compositional mapping of spherulitic grains implies that a siderite grain consists of numerous tiny siderite precipitates. Such dust-like tiny siderite precipitates appear to be adhered to the large spherulitic siderite grain. In some cases, large siderite grains are adjoined together and occurred as a siderite aggregate. We suggest that a spherulitic siderite grain is growing sequentially by adhering of numerous tiny siderite precipitates formed in each diagenetic zone onto the siderite grain like the formation of a snowball.

Distribution and palaeoecological indicator value of diatoms in Holocene coastal deposits of the southern North Sea (Lower Saxony, Germany)

Abstract. Die Entstehung der niedersächsischen Küstenlandschaften ist während der letzten 8000 Jahre eng mit Meeresspiegelfluktuationen verbunden, die Phasen von Meeresspiegelabsenkungen und -anstiegen ebenso umfassen wie Stillstandsphasen. Transgressive Abschnitte zeichnen sich vor allem durch sandige und tonig-schluffige Ablagerungen aus, während Regressionsphasen zum Teil durch Torf gekennzeichnet sind, der sich in Phasen verlangsamten Meeresspiegelanstieges seewärts auf marine Ablagerungen ausbreiten konnte. Auch auf den Ostfriesischen Inseln ergeben sich Möglichkeiten paläoökologischer Untersuchungen, die ungefähr bis in die Zeitenwende zurückreichen. Hier können vor allem ältere Salzwiesenhorizonte und Wattschichten genutzt werden, die heute infolge von Strandverschiebungen am seeseitigen Strand einiger Inseln erosiv freigelegt werden. Diese Horizonte bilden natürliche Pegelmarken, an denen zurückliegende Meeresspiegelstände rekonstruiert werden können. In diesem Artikel werden diatomologische Ergebnisse vorgestellt, die an Bohrungen und Profilen der festländischen Küste und Ostfriesischen Inseln erarbeitet wurden. Die Diatomeen-Vergesellschaftungen klastischer Sedimente, die während Transgressions-phasen abgesetzt wurden, sind generell durch pelagische und litorale marine Arten charakterisiert. Im Kontakt zu den Ablagerungen des Basaltorfes treten verstärkt oligo- bis mesohalobe Arten hinzu. Dies legt den Schluss nahe, dass sich dieser erste transgressive Kontakt nicht erosiv, sondern fließend vollzogen hat. Die Wattablagerungen sind vornehmlich durch eine Artenkombination poly- bis mesohalober Arten gekennzeichnet, während lagunäre Sedimente häufiger litorale brackische und schwach verkieselte Arten aufweisen. Dies deutet auf einen geringeren Salzgehalt und ruhige Ablagerungsbedingungen hin. Die eingeschalteten Niedermoortorfe mit Vhragmites zeigen einen Anstieg litoraler oligohalober Arten und bezeugen somit den Rückgang der Salinität und mithin eine regressive Phase innerhalb der Meeresspiegel-Entwicklung. Salzwiesenablagerungen sowohl auf den Inseln als auch im Kontakt zum pleistozänen Hinterland werden durch aerophile litorale Spezies charakterisiert, Überflutungsereignisse deuten sich durch stärkere Nachweise pelagi-scher mariner Diatomeen an. Sowohl Ablagerungen feuchter Dünentäler, die im Untersuchungsgebiet nur auf den Ostfriesischen Inseln auftreten, als auch die am höchsten gelegenen Bereiche der Salzwiese beherbergen eine Vielzahl oligohalober Spezies und unterstreichen auf diese Weise den zurückgehenden marinen Einfluss.

Spherulitic siderites in the Holocene coastal deposits of Korea (eastern Yellow Sea): elemental and isotopic composition and depositional environment

Siderite concretions are recovered from both freshwater (Unit II) and tidal (Unit I) deposits in the Holocene macrotidal flat of Kyunggi Bay, west coast of Korea. Siderites from both units show a spherulitic and well-rounded texture and well-developed equant rhombs on the external surfaces. The size of the concretions in Unit II (50–150 μm in diameter) is normally larger than that in Unit I (10–20 μm). The siderites in Unit II are chemically pure and highly enriched in Fe, whereas those of Unit I are relatively impure and distinctly zoned, demarcating high Mn (up to 69.1 mol%) in the core and high Fe (up to 75.9 mol%) in the margin. Within siderites of Unit I, substitution for Fe by Ca and Mg reaches up to 20.3 mol% (mean 15.1 mol%) and 16.1 mol% (mean 12.6 mol%), respectively. Bulk δ 18O values (−0.7 to +1.3‰ PeeDee Belemnite (PDB)) for siderites in Unit I are relatively higher than those (−7.1 to +0.5‰ PDB) in Unit II. Regardless of units, siderite δ 13C values (−21.3 to −11.2‰ PDB) are persistently low, indicating microbially-mediated precipitation at shallow burial depth. Extensive Mn, Ca and Mg substitution coupled with enrichment of 18O in the siderites of Unit I seems to reflect marine influence during the Holocene transgression. The occurrence of early diagenetic siderites provides supporting evidence on the depositional environment (marine vs. non-marine) of the Holocene muddy coastal deposits of Korea (eastern Yellow Sea).