Research on the Metal Corrosion Process in the Sea Mud/Seawater/Atmosphere Interface Zone

Corrosion in the interface zone is a complicated local corrosion phenomenon. The conventional single-electrode method finds it difficult to obtain the kinetic information of corrosion occurrence and development process. In this paper, metal corrosion was studied by Wire Beam Electrode (WBE) technology on the interfaces of sea mud/seawater and seawater/atmosphere. The study found that the metal corrosion in the interface is a process of coupling a dual corrosion cell into a single corrosion cell. Initially, a corrosion cell is formed with the seawater/atmosphere interface acting as the cathode and the upper part of the metal in the seawater area as the anode. This is due to the oxygen concentration cell caused by the waterline effect. The cathode area is always enriched near the seawater/atmosphere interface. The lower part of the metal in the seawater area and the metal in the sea mud area are the anode and the cathode, respectively, of another corrosion cell. Along with the immersion time, the anodic area of the first corrosion cell gradually extends to the lower part of the metal in the seawater zone and finally the sea mud zone, resulting in the disappearance of the second corrosion cell. In the single corrosion cell stage, the seawater/atmosphere interface is the cathode area; the seawater area and the sea mud area are the anode areas, and the electrode adjacent to the cathode area becomes the anode area with the largest current density. During the whole experiment, the sea mud zone is a process of polarity transition from the cathode zone to the anode zone, and finally forms the anode zone of the whole electrode together with the anode zone in the sea zone.

Subgroup level differences of physiological activities in marine Lokiarchaeota

Asgard is a recently discovered archaeal superphylum, closely linked to the emergence of eukaryotes. Among Asgard archaea, Lokiarchaeota are abundant in marine sediments, but their in situ activities are largely unknown except for Candidatus ‘Prometheoarchaeum syntrophicum’. Here, we tracked the activity of Lokiarchaeota in incubations with Helgoland mud area sediments (North Sea) by stable isotope probing (SIP) with organic polymers, 13C-labelled inorganic carbon, fermentation intermediates and proteins. Within the active archaea, we detected members of the Lokiarchaeota class Loki-3, which appeared to mixotrophically participate in the degradation of lignin and humic acids while assimilating CO2, or heterotrophically used lactate. In contrast, members of the Lokiarchaeota class Loki-2 utilized protein and inorganic carbon, and degraded bacterial biomass formed in incubations. Metagenomic analysis revealed pathways for lactate degradation, and involvement in aromatic compound degradation in Loki-3, while the less globally distributed Loki-2 instead rely on protein degradation. We conclude that Lokiarchaeotal subgroups vary in their metabolic capabilities despite overlaps in their genomic equipment, and suggest that these subgroups occupy different ecologic niches in marine sediments.

Stable iron isotope signals as indicators for iron reduction pathways in deep methanic sediments

<p>A number of studies have shown that iron reduction in marine sediments is not confined to sulfate- or sulfide-containing depths but may also affect deep methanic intervals. In particular dynamic depositional settings often show the release of dissolved iron below the sulphate-methane transition (SMT). The specific process behind this deep iron release is not well understood. It has been suggested that anaerobic oxidation of methane (AOM) mediated by Fe oxide reduction plays an important role. So there might be a close, so far unaccounted link between the Fe and C cycles in deep marine sediments.</p><p>Here we present a compilation of inorganic geochemical data including δ<sup>56</sup>Fe values of pore water and reactive Fe fractions for sediments of the Helgoland mud area (North Sea) for which a coupling between deep iron reduction and AOM has been proposed [1]. The sediments show a shallow SMT and increasing dissolved Fe concentrations of up to 400 µM further below. High sedimentation rates led to a fast burial and preservation of reactive Fe (oxyhydr)oxides, enabling deep iron reduction as we observe it today.</p><p>Isotopic fractionation of Fe has been demonstrated for DIR in culture experiments and in shallow marine sediments. Such studies build upon the principle that microbes preferentially utilize light Fe isotopes (<sup>54</sup>Fe) causing a fractionation between solid ferric and dissolved ferrous iron. For alternative biotic Fe reduction pathways in methanic environments, there are practically no data. We hypothesized that any microbially mediated iron reduction process would result in a similar preferential release of <sup>54</sup>Fe and, thus, shift pore water δ<sup>56</sup>Fe towards negative values. Furthermore we hypothesized that the microbial utilization of a specific Fe (oxyhydr)oxide pool would result in a relative enrichment of <sup>56</sup>Fe in the residual ferric substrate.</p><p>Close to the sediment-water interface pore water δ<sup>56</sup>Fe in the mud area is generally negative and shows a downward trend towards positive values as it can be expected for in-situ dissimilatory iron reduction (DIR) [2]. The Fe isotope signal close to the sulfidic interval is ~1‰ heavier than above and below as Fe sulfide precipitation preferentially removes <sup>54</sup>Fe from pore water. A pronounced downward shift of pore-water δ<sup>56</sup>Fe to more negative values within the methanic zone is a clear indication for microbial Fe reduction coupled to organic matter degradation. However, this shift does not coincide with the main interval of Fe release for which potential for Fe-AOM had been demonstrated [1]. In this deeper interval, the released Fe has an isotopic composition that matches that of the ferric substrates. We conclude that either 1) Fe-AOM plays a subordinate role for Fe release at depth or 2) does not go along with significant Fe isotope fractionation, which might be explained by different ways of electron transfer between microbe and the iron oxide compared to DIR.</p><p>[1] Aromokeye, D. et al., 2019. Frontiers in Microbiology, doi: 10.3389/fmicb.2019.03041.</p><p>[2] Henkel, S. et al., 2016. Chemical Geology 421: 93-102.</p>

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.

Phytoplankton productivity and community structure variations over the last 160 years in the East China Sea coast in response to natural and human-induced environmental changes

Eutrophication has caused drastic changes to the marine ecosystem of the East China Sea during the past decades. However, there is relatively sparse evidence of historical changes, as well as the explicit effects of climatic changes and anthropogenic activities on the primary productivity of marine coastal ecosystems. In this study, surface and core sediments from the Zhejiang-Fujian coastal mud area, East China Sea coast, were analyzed using the bulk and molecular biomarkers. The results showed that ecosystem changes were characterized by increased phytoplankton productivity and a fluctuant transition from blooms mostly dominated by diatoms to red tide events dominated by dinoflagellates. Variations from the early 1850s to the early 2010s can be divided into a nature-dominated period (the early 1850s–1960s) and a human-impacted period (1960s–the early 2010s). Particularly, natural forcing such as heavy floods (e.g. 1998, 1954, and 1931) in the whole of the Yangtze River catchment, variations in the intensity of East Asia Monsoon, and strengthened or weakened Kuroshio intrusion/positive or negative Pacific Decadal Oscillation phase in the coastal mud area have substantially affected the phytoplankton productivity and community structure during the nature-dominated period. In contrast, changes in nutrient supply and compositions were more apparent during the human-impacted period, which could have been because of increased fertilizer usage, discharges of industrial wastewater and domestic sewage, and large-scale human projects (e.g. Danjiangkou Reservoir and Three Gorges Dam) in the Yangtze River drainage area, leading to significant phytoplankton productivity and community structure variations in the coastal mud area system of East China Sea.

CO2conversion to methane and biomass in obligate methylotrophic methanogens in marine sediments

Methyl substrates are important compounds for methanogenesis in marine sediments but diversity and carbon utilization by methylotrophic methanogenic archaea have not been clarified. Here, we demonstrate that RNA-stable isotope probing (SIP) requires13C-labeled bicarbonate as co-substrate for identification of methylotrophic methanogens in sediment samples of the Helgoland mud area, North Sea. Using lipid-SIP, we found that methylotrophic methanogens incorporate 60 to 86% of dissolved inorganic carbon (DIC) into lipids, and thus considerably more than what can be predicted from known metabolic pathways (∼40% contribution). In slurry experiments amended with the marine methylotrophMethanococcoides methylutens, up to 12% of methane was produced from CO2, indicating that CO2-dependent methanogenesis is an alternative methanogenic pathway and suggesting that obligate methylotrophic methanogens grow in fact mixotrophically on methyl compounds and DIC. Thus, the observed high DIC incorporation into lipds is likely linked to CO2-dependent methanogenesis, which was triggered when methane production rates were low. Since methylotrophic methanogenesis rates are much lower in marine sediments than under optimal conditions in pure culture, CO2conversion to methane is an important but previously overlooked methanogenic process in sediments for methylotrophic methanogens.

Late-Holocene high-frequency East Asia Winter Monsoon variability inferred from the environmentally sensitive grain size component in the distal shelf mud area, East China Sea

The B2 (B2G) and I4 sediment cores recovered from the centre of the distal mud area of the East China Sea (ECS) were analysed for grain size distribution. Proxies for environmentally sensitive grain size components (ESGSC) retrieved from the composite B2 core, namely, variations in the volumetric content and mean grain size of specific grain size fractions, reveal a detailed history of the East Asia Winter Monsoon (EAWM) including centennial to decadal-scale variations spanning the last 2300 calendar years before present (cal. yr BP). The results indicate that EAWM variations are consistent with temperature changes in eastern China (as inferred from historical documents). Additionally, the sea surface temperature (SST) in the Southern Okinawa Trough, the δ18O of stalagmite from the Sanbao cave and the drift ice indices from the North Atlantic, along with strong or weak EAWMs, corresponding to low or high temperatures, respectively. Four periods of EAWM variations were identified, namely, a weak EAWM stage from 2300 to 2050 cal. yr BP; a comparatively enhanced EAWM between 2050 and 1700 cal. yr BP; a return to a weak EAWM from 1700 to 700 cal. yr BP, including the Roman Warm Period (RWP), the Sui–Tang Dynasty Warm Period (STWP) and the ‘Medieval Warm Period’ (MWP) and a strongly developed EAWM between 700 and 100 cal. yr BP, corresponding to a ‘Little Ice Age’. An important abrupt warm to cold climate change event occurred around 678 cal. yr BP. During this period, the climate change was likely related to global scale changes in atmospheric circulation. Spectral analyses of the ESGSC proxies show high-frequency cycles and a close solar–monsoon connection to the EAWM, suggesting that one of the primary controls for centennial to decadal-scale change in EAWM intensity was the variation in solar radiation during that time.