Tidal marshes store large amounts of organic carbon in their soils. Field data quantifying soil organic carbon (SOC) stocks provide an important resource for researchers, natural resource managers, and policy-makers working towards the protection, restoration, and valuation of these ecosystems. We collated a global dataset of tidal marsh soil organic carbon (MarSOC) from 99 studies that includes location, soil depth, site name, dry bulk density, SOC, and/or soil organic matter (SOM). The MarSOC dataset includes 17,454 data points from 2,329 unique locations, and 29 countries. We generated a general transfer function for the conversion of SOM to SOC. Using this data we estimated a median (± median absolute deviation) value of 79.2 ± 38.1 Mg SOC ha −1 in the top 30 cm and 231 ± 134 Mg SOC ha −1 in the top 1 m of tidal marsh soils globally. This data can serve as a basis for future work, and may contribute to incorporation of tidal marsh ecosystems into climate change mitigation and adaptation strategies and policies.

Coastal observatories are key to improve the understanding of processes within the coastal area and their interactions with regional and global environmental changes. The land-sea transition zone is an essential area that allows research on unique scientific questions under anthropogenic and natural influences. Amid the Wadden Sea UNESCO world natural heritage site – the largest tidal flat region worldwide – the barrier island Spiekeroog is an excellent location for an observatory studying land-sea interactions. The integrated Spiekeroog Coastal Observatory (SCO) operated by the Institute for Chemistry and Biology of the Marine Environment (ICBM, University of Oldenburg) is dedicated to interdisciplinary marine and terrestrial ecosystem research. Its position within the tidal area and the multitude of research-field addressed establishes the SCO as a unique coastal observatory with the potential to identify patterns in long-term variability and simultaneously understanding short-term changes. The establishment of the Time-Series Station (TSS) Spiekeroog in a tidal channel west of Spiekeroog back in 2002 laid the foundation of the SCO. Since then, the observatory is expanding continuously and is now representing a valuable asset supporting education, industry, government, and environmental conservation efforts in the area. Summing up the infrastructure and technical components, the importance of the SCO is evident, and individual projects greatly benefit from the collaboration with the partners in and the elements of the SCO. Harmonizing the infrastructure and competences of contributing partners will be a next step to further consolidate the SCO. A challenge poses the maintenance of the SCO based on projects, which is focused on the addition of new facilities, not maintaining, refurbishing, or (if necessary) deconstructing existing infrastructure. Therefore, structural support and funding opportunities not linked to projects but aiming to sustain observational capacities are required.

The island Spiekeroog (Germany) at the southern North Sea coast exhibits an island tail (Ostplate) of which large parts formed after 1860 by gradual eastward deposition of marine sediments. This study aimed to (i) provide a first overview of soils on the Ostplate, (ii) verify whether soil formation stages in different landscape elements (dune chain, dune slacks, saltmarsh, pioneer zone) are linked to the island's gradual development and (iii) to assess the impact of tides and winds on soil formation. Standard physicochemical properties of dune -and dune slack topsoils and 19 pedons (i.a. pH, total organic carbon (TOC), total inorganic carbon (TIC), total nitrogen (TN), and mean weight diameter (MWD) of water stable aggregates) were analyzed. Soils of the dune slacks, the saltmarsh and the pioneer zone classified as Gleysols which showed no trend towards differences along the Ostplate. The dune chain exhibited Protic Arenosols that were initially acidified in lee positions (min. pH = 5.1). Carbon (C) and nitrogen (N) isotopes (delta C-13, delta N-15) signatures of saltmarsh sediments were used as a proxy for the tidal influence on soil formation. In addition, sulfur (S) isotope (delta S-34) signatures revealed insights on sulfur sources in the saltmarsh. The delta C-13 signatures of organic matter (OM) decreased and TOC/TN -and TOC/total sulfur (TS) ratios increased with increasing elevation and decreasing flooding frequency. At higher flooding frequency, delta N-15 resembled the dominant input of OM from the southern North Sea, at a lower interaction level, mixing of a wider range of sources was indicated. S isotopes indicated dissolved sulfate and reduced species as sulfur sources. It was concluded that an ongoing impact of wind-induced sand movement at the dunes, and flooding-induced sediment movement in the saltmarsh and the pioneer zone, repealed any potential disparities in soil formation stages, which might have existed along the Ostplate.

Low-lying barrier islands present ideal locations to investigate hydrogeochemical processes in sandy coastal aquifers, as they (i) host typical geographic features of the coast, including beaches, dunes, as well as salt marshes, (ii) are susceptible to the impacts of a global climate change, because of their low surface elevations, and (iii) are ubiquitous (>2,100) amongst the global coastlines. The aim of this study was to investigate the evolution of hydrogeochemical patterns in near-surface groundwater on developing barrier islands, using the Ostplate on Spiekeroog Island, Germany, as an example. The impact of inundation, season and vegetation on the hydrogeochemical processes were of particular interest. Six groundwater monitoring wells were installed for this purpose, covering main geographic features of the Ostplate, i.e., supratidal beaches, dunes and salt marshes. Groundwater samples were collected bimonthly from April 2016 to March 2017. Measured parameters included major ions, nutrients, dissolved organic carbon/matter (DOC/DOM), delta C-13 signatures of dissolved inorganic carbon, as well as delta S-34 signatures of dissolved sulfate and sulfide. We found major ion compositions to be a function of the annual flooding frequency, although cation concentrations were further impacted by calcite dissolution and cation exchange. The redox chemistry varied at the different sites and through seasons. Aerobic respiration was the dominant pathway for carbon oxidation at the dune locations, whereas organic matter mineralization in the salt marsh was also mediated by anaerobic processes. Regarding electron donors, the presence of labile DOM was linked to salinity, and terrestrial DOM signatures as well as gross inputs of organic carbon agreed with vegetation patterns. A physico-chemical interpretation of the pore water compositions revealed a potential for the authigenic formation of carbonate and phosphate minerals in the investigated sediments, triggered by the redox processes.

This study is concerned with the rate of soil formation in Arenosols of dunes on the barrier island Spiekeroog at the southern North Sea coast. It focuses on Fe dynamics in the soils in relation to slope aspect, as well as on the interaction between minerals and soil organic matter (SOM) in the course of soil formation. Ten quartz-rich (approximate to 95 wt%) Arenosols on slopes with different aspects (north/south), low in primary carbonates (approximate to 0.5 wt%) and total Fe (Fe-t,approximate to 0.15 wt%), were studied across a soil chronosequence with optically stimulated luminescence (OSL) ages between 43 +/- 7 and 279 +/- 26 years. During this period, pH (CaCl2) decreased from 7.1 in fresh sands to 3.1 in initially podzolized soils. SOMcontents and the optical density of oxalate extracts (ODOE) increasedwith soil age. Differences in Fe stocks, vertical distribution of Fed, Feo and Fep and horizontation of equally old soils indicated an influence of microclimate on pedogenesis determined by slope aspect. Initial podzolization features occurred after 179 +/- 17 years. Eluviation resulted in the formation of an E horizon free of amorphous and poorly crystalized Fe oxides (Feo-Fep), indicating that these oxides provided a source of Fe during podzolization. However, illuviation features were absent. Field findings, macroscopic examinations and carbon fractionations revealed, that SOM was exclusively present in free particulate form (free POM). This indicated, that prior to podzolization, mineral grains did not become enclosed by organic pigments during the formation of Ah horizons. (C) 2019 Elsevier B.V. All rights reserved.

Freshwater lenses below barrier islands are a precious resource for the local water supply and important for coastal ecosystems. The aim of this study was to investigate the hydrochemical evolution of a freshwater lens, using the barrier island Spiekeroog, Germany, as an example. For this purpose, groundwater samples were obtained during several campaigns, and hydrochemical data and C-13/C-12 isotope ratios of dissolved inorganic carbon were linked to apparent groundwater ages. Results show that apparent groundwater ages increase with depth and range between 4 and 51 years. All groundwater samples were close to equilibrium with respect to calcite and considerably enriched in calcium and bicarbonate, suggesting calcite dissolution in the unsaturated zone of the dune sediments. The estimated average rate of decalcification was similar to 13 mm/a, resulting in a decalcification depth of similar to 4.6 m for the oldest sediments of an approximate age of 350 years. Moreover, C-13/C-12 isotope ratios of dissolved inorganic carbon indicated secondary carbonate mineral reactions within the aquifer, such as recrystallization and closed-system calcite dissolution. Cation exchange was primarily observed in older groundwater, i.e. the deeper part of the aquifer, and the calculated time of complete freshening of the aquifer is similar to 600 years. Regarding redox reactants, dissolved oxygen and nitrate concentrations decreased rapidly in young groundwater, while dissolved manganese and iron were virtually absent in samples collected below the zones of oxygen and nitrate reduction. Dissolved sulfide species indicate sulfate reduction in older groundwater, and methanogenesis was detected locally. The absence of solute manganese and iron may be explained by the formation of minerals, such as iron sulfides, siderite, and rhodochrosite, that serve as possible sinks for redoxsensitive solutes. In conclusion, this study showed that hydrochemical data can be linked to groundwater ages to describe the hydrochemical evolution of a freshwater lens in a homogeneous sandy aquifer as a function of residence time.

Organic-poor, permeable quartz sands are often present at land-sea transition zones in coastal regions. Yet, the biogeochemical cycles of carbon, sulfur, and iron are not well studied here. The aim of this work was, therefore, to improve our understanding regarding the chemical processes in these prominent coastal sediments. A 10 m core was collected at a dune base of the barrier island Spiekeroog, Germany, for this purpose. Additionally, groundwater was sampled from a multi-level well for one year to record seasonal hydrochemical variations. Methods included the analyses of geochemical (total carbon, total inorganic carbon, reactive iron, total sulfur, reduced inorganic sulfur) and hydrochemical parameters (field parameters, major ions, DOC, and molecular compositions of DOM), as well as stable sulfur isotopes (δ34S-sulfate, -sulfide, -total reduced inorganic sulfur). Moreover, optically stimulated luminescence (OSL) dating was applied. Results show that the core sediments are very young (<500 a) and were rapidly deposited. They are characterized by remarkably low contents of organic carbon (<0.1% dw.), reactive iron (~10 mmol/kg), and iron sulfides (<3 mmol/kg). Groundwater salinities were low in the top core sediments and increased at depth during most times of the year. However, the sampling site is subject to (seasonally) varying salinities, which could be linked to the biogeochemical cycles. For instance, the infiltration of seawater-derived labile DOM during inundation events drives microbial respiration besides sedimentary organic matter. Oxygen and nitrate were the dominant electron acceptors for the decomposition of organic matter in near-surface groundwater, while sulfate reduction was constrained to the lower brackish sediments. Here, authigenic pyrite formation was inferred based on the detection of dissolved sulfide, intact pyrite framboids, and matching stable sulfur isotope signatures of dissolved and solid sulfides. We concluded that the extremely low organic carbon contents limit pyrite formation in the organic-poor, permeable quartz sands.

To get further insights into the development of landscape elements and soils of North Sea barrier islands; an old part in the West (formed between approx. 1650-1860) and a young part, largely unaffected by anthropogenic influences in the East (formed approx. 1960), of the island Spiekeroog were comparatively analyzed. Between the two parts beach soils (World Reference Base of Soil Resources (WRB), IWG WRB (2015): Eutric Huvic Tiadalic and Eutric Endosalic Arenosols; German Soil Classification System (GSCS), AD-Hoc-AG BODEN (2005): Nassstrand and Strand) did not differ, except for microbial mats, which were found only in the East. In the West, a dune complex comprising white, gray and brown dunes holds a soil chronosequence from unaltered sands (WRB: Eutric Protic Arenosol; GSCS: Lockersyrosem) to braunified and podzolized soils (With: Dystric Brunk Arenosol; GSCS: Braunerde). However, in the East, only a single white dune chain, with adjacent shallow dune mounds to the south, had formed. In both parts, dune slacks evinced remarkably rapid peat formation (WRB: Dystric Histic and Eutric Histic Gleysols; GSCS: Niedermoorgleye). Here, iron content of groundwaters determined the formation of redoximorphic features. Soils of the salt marshes (WRB: Eutric Fluvic and Eutric Gleysols; GSCS: Rohmarschen) showed the highest contents of soil organic carbon (SOC) in the West, where the thickest fine sediment layers were also found. Revealed shortcomings of the German soil classification system concerning the classification of 'Strand' and 'Rohmarsch' soils are discussed. It is suggested to exclude the obligatory carbonate (CaCO3) content in the definition of the Strand soil and to introduce an additional horizon prefix to indicate carbonate contents between > 0 and < 2%, in order to classify Rohmarsch soils, with primary carbonate contents < 2%. Finally, an outlook on the future landscape -and soil development is provided, which is expected to be most profound in the young East.