Non-conservative Behavior of Dissolved Organic Matter and Trace Metals (Mn, Fe, Ba) Driven by Porewater Exchange in a Subtropical Mangrove-Estuary

Estuaries play a key role in controlling the land-ocean fluxes of dissolved organic matter (DOM), nutrients and trace metals. Here, we study how mangrove-fringed areas affect the molecular DOM and trace metal composition in a subtropical estuary. We combined molecular analysis of solid-phase extractable (SPE) DOM using ultrahigh-resolution mass spectrometry with organic and inorganic bulk parameter analyses in surface and porewater along the estuarine gradient of a mangrove-fringed estuary in Australia (Coffs Creek). Statistical analysis and mixing models demonstrate that the fluvial and mangrove-porewater derived DOM and inorganic chemical species were altered and/or removed by the estuarine filter before reaching the coastal ocean. The mangrove-fringed central estuary was a net source for dissolved Mn and Ba as well as total dissolved nitrogen (TDN) and dissolved organic carbon (DOC) to the tidal creek, likely due to the exchange of mangrove-porewater strongly enriched in these constituents. Dissolved Fe was removed from the water column, probably during the tidally driven circulation of creek water through the sulfidic mangrove sediments. In the mangrove-porewater dominated tidal creek, sulfur- and nitrogen-containing as well as aromatic DOM compounds were relatively enriched, whereas phosphorous-containing DOM was relatively depleted compared to non-mangrove fringed areas. In areas with intense mixing of estuarine and marine water masses we observed a strong decrease of these DOM compounds relative to values expected from conservative mixing, suggesting their removal by photodegradation and co-precipitation with particles such as Mn(hydr)oxides and/or as organometallic complexes, leading to more aliphatic DOM signatures at the creek-mouth. Tidally driven porewater exchange and surface water runoff from the mangroves had a stronger effect on the biogeochemical cycling in the estuary than the fluvial input during a dry compared to a wet season. Our study confirms that mangroves can significantly contribute to biogeochemical budgets of (sub)tropical estuaries

DSi as a tracer for submarine groundwater discharge

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Oehler, T., Tamborski, J., Rahman, S., Moosdorf, N., Ahrens, J., Mori, C., Neuholz, R., Schnetger, B., & Beck, M. DSi as a tracer for submarine groundwater discharge. Frontiers in Marine Science, 6, (2019): 563, doi:10.3389/fmars.2019.00563.Submarine groundwater discharge (SGD) is an important source of nutrients and metals to the coastal ocean, affects coastal ecosystems, and is gaining recognition as a relevant water resource. SGD is usually quantified using geochemical tracers such as radon or radium. However, a few studies have also used dissolved silicon (DSi) as a tracer for SGD, as DSi is usually enriched in groundwater when compared to surface waters. In this study, we discuss the potential of DSi as a tracer in SGD studies based on a literature review and two case studies from contrasting environments. In the first case study, DSi is used to calculate SGD fluxes in a tropical volcanic-carbonate karstic region (southern Java, Indonesia), where SGD is dominated by terrestrial groundwater discharge. The second case study discusses DSi as a tracer for marine SGD (i.e., recirculated seawater) in the tidal flat area of Spiekeroog (southern North Sea), where SGD is dominantly driven by tidal pumping through beach sands. Our results indicate that DSi is a useful tracer for SGD in various lithologies (e.g., karstic, volcanic, complex) to quantify terrestrial and marine SGD fluxes. DSi can also be used to trace groundwater transport processes in the sediment and the coastal aquifer. Care has to be taken that all sources and sinks of DSi are known and can be quantified or neglected. One major limitation is that DSi is used by siliceous phytoplankton and therefore limits its applicability to times of the year when primary production of siliceous phytoplankton is low. In general, DSi is a powerful tracer for SGD in many environments. We recommend that DSi should be used to complement other conventionally used tracers, such as radon or radium, to help account for their own shortcomings.TO, NM, and the presented case study 1 were funded through the BMBF junior research group SGD-NUT (grant #01LN1307A). Open access publication fees are paid by Leibniz-Centre for Tropical Marine Research internal funds. The presented case study 2 was financially supported by the DFG Research Group “BioGeoChemsitry of Tidal Flats”, the Ph.D. Research Training Group “The ecology of molecules” funded by the Ministry for Science and Culture of Lower Saxony, and the Institute for Chemistry and Biology of the Marine Environment, University of Oldenburg

Impact of the Major Baltic Inflow in 2014 on Manganese Cycling in the Gotland Deep (Baltic Sea)

The deep basins of the Baltic Sea, including the Gotland and Landsort Deeps, are well-known for the exceptional occurrence of sedimentary Mn carbonate. Although the details of the mechanisms of Mn carbonate formation are still under debate, a close relationship with episodic major Baltic inflows (MBIs) is generally assumed, at least for the Gotland Basin. However, the few studies on Mn cycling during MBIs suffer from a limited temporal resolution. Here we report on Mn dynamics in the water column and sediments of the Gotland Deep following an MBI that entered the Baltic Sea in December 2014. Water column profiles of dissolved Mn were obtained at a monthly to bi-monthly resolution between February 2015 and March 2017 and revealed an impact of the MBI on the Gotland Deep bottom waters beginning in March 2015. Water column profiles and budget estimates provided evidence for remarkable losses of dissolved Mn associated with the enhanced deposition of Mn oxide particles, as documented in sediment trap samples and surface sediments. In July 2015, subsequent to the nearly full oxygenation of the water column, clear signals of the re-establishment of bottom water anoxia appeared, interrupted by a second inflow pulse around February 2016. However, dissolved Mn concentrations of up to 40 μM in the bottom waters in June 2016 again indicated a pronounced reduction of Mn oxide and the escape of dissolved Mn back into the open water column. The absence of substantial amounts of Mn carbonate in the surface sediments at the end of the observation period suggested that the duration of bottom water oxygenation plays an important role in the formation of this mineral. Data from both an instrumental time series and a dated sediment core from the Gotland Deep supported this conclusion. Enhanced Mn carbonate formation occurred especially between the 1960s and mid-1970s, when several MBIs caused a long-lasting oxygenation of the water column. By contrast, Mn carbonate layers were much less pronounced or even missing after single MBIs in 1993, 2003, and 2014, each of which provided a comparatively short-term supply of O2 to the deeper water column

Historical anthropogenic heavy metal input to the south-eastern North Sea

The Helgoland Mud Area (HMA) in the German Bight, covering an area of approximately 500 km2, is one of a few depocentres for finer sediments in the North Sea. Radiocarbon and 210Pb analyses revealed continuous sedimentation over the last several centuries. Zinc (Zn) and lead (Pb) contents in the sediments show a distinct increase towards the youngest most sediments with the thickness of the heavy metal enriched sediments ranging from 15 to 103 cm. Stratigraphic data indicate that the onset of heavy metal enrichment is diachronous progressing north-westward over the depocentre, paralleled by a decrease in the thickness of the enriched layer. Beginning already during medieval times, the enhanced input of Zn and Pb seemingly is related to silver and zinc mining in the Harz Mountains and the Erzgebirge, well-known mining areas since the Bronze Age. Both regions are directly connected to the HMA by the Elbe and Weser rivers. Zn and Pb enrichment began in the south-eastern HMA and progressed subsequently with an average of 10 m per year north-westward, most likely triggered by variations in river discharge and by the hydrodynamic setting. Quantitative assessments of the Zn and Pb content in the sediments suggest that since the onset of enhanced Zn and Pb deposition, the anthropic Zn and Pb input in the HMA amounts to ~ 12,000 t and ~ 4000 t, respectively

Sulfurization of dissolved organic matter in the anoxic water column of the Black Sea

Today’s oceans store as much dissolved organic carbon (DOC) in the water column as there is CO2 in the atmosphere, and as such dissolved organic matter (DOM) is an important component of the global carbon cycle. It was shown that in anoxic marine sediments, reduced sulfur species (e.g., H2S) abiotically react with organic matter, contributing to carbon preservation. It is not known whether such processes also contribute to preserving DOM in ocean waters. Here, we show DOM sulfurization within the sulfidic waters of the Black Sea, by combining elemental, isotopic, and molecular analyses. Dissolved organic sulfur (DOS) is formed largely in the water column and not derived from sediments or allochthonous nonmarine sources. Our findings suggest that during large-scale anoxic events, DOM may accumulate through abiotic reactions with reduced sulfur species, having long-lasting effects on global climate by enhancing organic carbon sequestration

Microbial hitchhikers on intercontinental dust: high-throughput sequencing to catalogue microbes in small sand samples

Microbiological studies on the intercontinental transport of dust are confounded by the difficulty of obtaining sufficient material for analysis. Axenic samples of dust collected at high altitudes or historic specimens in museums are often so small and precious that the material can only be sacrificed when positive results are assured. With this in mind, we evaluated current methods and developed new ones in an attempt to catalogue all microbes present in small dust or sand samples. The methods used included classical microbiological approaches in which sand extracts were plated out on a variety of different media, polymerase chain reaction (PCR)-based amplification of 16S/18S rRNA sequences followed by construction of clone libraries, PCR amplification of 16S rRNA sequences followed by high-throughput sequencing (HtS) of the products and direct HtS of DNA extracted from the sand. A representative sand sample collected at Bahaï Wadi in the desert of the Republic of Chad was used. HtS with or without amplification showed the most promise and can be performed on ≤100ng DNA. Since living microbes are often required, current best practices would involve geochemical and microscopic characterisation of the sample, followed by DNA isolation and direct HtS. Once the microbial content of the sample has been deciphered, growth conditions (including media) can be tailored to isolate the micro-organisms of interes

Trace Metal Dynamics in Shallow Hydrothermal Plumes at the Kermadec Arc

Hydrothermal vents are a source of many trace metals to the oceans. Compared to mid-ocean ridges, hydrothermal vent systems at arcs occur in shallower water depth and are much more diverse in fluid composition, resulting in highly variable water column trace metal concentrations. However, only few studies have focused on trace metal dynamics in hydrothermal plumes at volcanic arcs. During R/V Sonne cruise SO253 in 2016/2017, hydrothermal plumes from two hydrothermally active submarine volcanoes along the Kermadec arc in the Southwest Pacific Ocean were sampled: (1) Macauley, a magmatic dominated vent site located in water depths between 300 and 680 m, and (2) Brothers, located between 1,200 and 1,600 m water depth, where hydrothermalism influenced by water rock interactions and magmatically influenced vent sites occur near each other. Surface currents estimated from satellite-altimeter derived currents and direct measurements at the sites using lowered acoustic Doppler current profilers indicate the oceanic regime is dominated by mesoscale eddies. At both volcanoes, results indicated strong plumes of dissolved trace metals, notably Mn, Fe, Co, Ni, Cu, Zn, Cd, La, and Pb, some of which are essential micronutrients. Dissolved metal concentrations commonly decreased with distance from the vents, as to be expected, however, certain element/Fe ratios increased, suggesting a higher solubility of these elements and/or their stronger stabilization (e.g., for Zn compared to Fe). Our data indicate that at the magmatically influenced Macauley and Brothers cone sites, the transport of trace metals is strongly controlled by sulfide nanoparticles, while at the Brothers NW caldera wall site iron oxyhydroxides seem to dominate the trace metal transport over sulfides. Solution stabilization of trace metals by organic complexation appears to compete with particle adsorption processes. As well as extending the generally sparse data set for hydrothermal plumes at volcanic arc systems, our study presents the first data on several dissolved trace metals in the Macauley system, and extends the existing plume dataset of Brothers volcano. Our data further indicate that chemical signatures and processes at arc volcanoes are highly diverse, even on small scales