The role of airborne volcanic ash for the surface ocean biogeochemical iron-cycle: a review

Iron is a key micronutrient for phytoplankton growth in the surface ocean. Yet the significance of volcanism for the marine biogeochemical iron-cycle is poorly constrained. Recent studies, however, suggest that offshore deposition of airborne ash from volcanic eruptions is a way to inject significant amounts of bio-available iron into the surface ocean. Volcanic ash may be transported up to several tens of kilometers high into the atmosphere during large-scale eruptions and fine ash may stay aloft for days to weeks, thereby reaching even the remotest and most iron-starved oceanic regions. Scientific ocean drilling demonstrates that volcanic ash layers and dispersed ash particles are frequently found in marine sediments and that therefore volcanic ash deposition and iron-injection into the oceans took place throughout much of the Earth's history. Natural evidence and the data now available from geochemical and biological experiments and satellite techniques suggest that volcanic ash is a so far underestimated source for iron in the surface ocean, possibly of similar importance as aeolian dust. Here we summarise the development of and the knowledge in this fairly young research field. The paper covers a wide range of chemical and biological issues and we make recommendations for future directions in these areas. The review paper may thus be helpful to improve our understanding of the role of volcanic ash for the marine biogeochemical iron-cycle, marine primary productivity and the ocean-atmosphere exchange of CO2 and other gases relevant for climate in the Earth's history

Influence of trace metal release from volcanic ash on growth of Thalassiosira pseudonana and Emiliania huxleyi

Recent studies demonstrate that volcanic ash has the potential to increase phytoplankton biomass in the open ocean. However, besides fertilizing trace metals such as Fe, volcanic ash contains a variety of potentially toxic metals such as Cd, Cu, Pb, and Zn. Especially in coastal regions closer to the volcanic eruption, where ash depositions can be very high, toxic effects are possible. Here we present the first results of laboratory experiments, showing that trace metal release from different volcanic materials can have both fertilizing and toxic effects on marine phytoplankton in natural coastal seawater. The diatom Thalassiosira pseudonana generally showed higher growth rates in seawater that was in short contact with volcanic ash compared to the controls without ash addition. In contrast to that, the addition of volcanic ash had either no effect or significantly decreased the growth rate of the coccolithophoride Emiliania huxleyi. It was not possible to attribute the effects to single trace metals, however, our results suggest that Mn plays an important role in regulating the antagonistic and synergistic effects of the different trace metals. This study shows that volcanic ash can lead to changes in the phytoplankton species composition in the high fall-out area of the surface ocean. Highlights: ► We tested the effect of volcanic ash on growth of T. pseudonana and E. huxleyi ► Volcanic ash increased growth of T. pseudonana but not of E. huxleyi ► Mn seems important to regulate the effects of different trace metals from the ash ► Volcanic eruptions have the potential to change phytoplankton community structure

Fertilising the surface ocean – the role of volcanoes

The oceans are by far the largest global reservoir of carbon that is available on climate relevant timescales (< 1000 yrs). A fraction of this oceanic carbon pool, comparable in magnitude with the CO2 inventory of today’s atmosphere, is transformed via biological assimilation of inorganic carbon into dissolved or particulate organic material within the sun-lit surface ocean. Subsequently this material can be respired, returning to the ocean as CO2, or it can sink to the sediments and this forms the basis of the ‘biological pump’. The efficiency of this pump is limited by the availability of nutrients, which are essential prerequisites for the growth of phytoplankton. We now know that vast areas of the surface ocean have extremely low nutrient concentrations limiting productivity. For instance, in the subtropical oceanic gyres, which comprise more than 40% of the Earth’s surface, the macronutrients nitrate, nitrite, ammonia and phosphate are depleted to trace levels which limit phytoplankton abundance so strongly such that the term “oceanic desert” was coined for these regions

Carbonate recrystallisation and organic matter maturation in heat-affected sediments from the Shaban Deep, Red Sea

Parasound profiles across the Shaban Deep in the Red Sea indicate turbiditic transport of surface sediments from the topographic hight (basalt ridge) into the interior of the deep. This is supported by petrographical and (isotope-) geochemical evidence in the East Basin of the Shaban Deep where the presence of variable mixtures of authochtonous and allochthonous sediment compounds had been found. The uppermost 170 cm of both sediment cores 17008-1 and 17009-3 reveal “normal” stable oxygen isotope values for the planktonic foraminifera G. ruber near -1 ‰ which is indicative for carbonate formation in Red Sea surface water around 27°C. However, below 182 cm in core 17008-1 highly variable δ 18O values for G. ruber between 0.26 and -10.68 ‰ occur which are not the result of temperature-controlled oxygen isotope fractionation between foraminiferal carbonate and Red Sea surface water. The lowest δ18O values of -10.68 ‰ measured for highly-altered foraminifera shells suggests carbonate precipitation higher than 90°C. Organic petrographical observations show a great diversity of marine-derived macerals and terrigenous organic particles. Based on petrographical investigations sediment core 17008-1 can be subdivided in intervals predominantly of authochtonous character (i.e. 1, 3, 5 corresponding to core depths 0-170 cm, 370-415 cm, 69-136 cm), and allochthonous/thermally altered character (e.g. 2, 4 corresponding to core depths 189-353 cm, 515-671 cm). Allochthonous/thermally altered material displays a wide to an extremely wide range of maturities (0.38-1.42 % Rr) and also natural coke particles were found. Similarily, the organic geochemical and pyrolysis data indicate the predominance of well-preserved, immature algal and bacterial remains with a minor contribution of land plant material. Sediments below 170 cm (core 17008-1) contain contributions of re-sedimented pre-heated material most likely from the area of the basaltic ridge. This is documented by individual coke particles reduced hydrogen indices and elevated Tmax values up to 440°C. An “oil-type” contribution (evidenced by mature biomarkers, hopene/hopane ratios, elevated background fluorescence, n-alkane distribution) is also present in the sediments which most likely originated at greater depth and impregnated the surface sediments. The heat source responsible for recrystallisation of foraminiferal carbonate and maturation of organic particles in Shaban Deep sediments most likely is attributed to modern basalt extrusions which now separate the Shaban Deep subbasins

Multidisciplinary investigation on cold seeps with vigorous gas emissions in the Sea of Marmara (MarsiteCruise): Strategy for site detection and sampling and first scientific outcome

MarsiteCruise was undertaken in October/November 2014 in the Sea of Marmara to gain detailed insight into the fate of fluids migrating within the sedimentary column and partially released into the water column. The overall objective of the project was to achieve a more global understanding of cold-seep dynamics in the context of a major active strike-slip fault. Five remotely operated vehicle (ROV) dives were performed at selected areas along the North Anatolian Fault and inherited faults. To efficiently detect, select and sample the gas seeps, we applied an original procedure. It combines sequentially (1) the acquisition of ship-borne multibeam acoustic data from the water column prior to each dive to detect gas emission sites and to design the tracks of the ROV dives, (2) in situ and real-time Raman spectroscopy analysis of the gas stream, and (3) onboard determination of molecular and isotopic compositions of the collected gas bubbles. The in situ Raman spectroscopy was used as a decision-making tool to evaluate the need for continuing with the sampling of gases from the discovered seep, or to move to another one. Push cores were gathered to study buried carbonates and pore waters at the surficial sediment, while CTD-Rosette allowed collecting samples to measure dissolved-methane concentration within the water column followed by a comparison with measurements from samples collected with the submersible Nautile during the Marnaut cruise in 2007. Overall, the visited sites were characterized by a wide diversity of seeps. CO2- and oil-rich seeps were found at the westernmost part of the sea in the Tekirdag Basin, while amphipods, anemones and coral populated the sites visited at the easternmost part in the Cinarcik Basin. Methane-derived authigenic carbonates and bacterial mats were widespread on the seafloor at all sites with variable size and distributions. The measured methane concentrations in the water column were up to 377 μmol, and the dissolved pore-water profiles indicated the occurrence of sulfate depleting processes accompanied with carbonate precipitation. The pore-water profiles display evidence of biogeochemical transformations leading to the fast depletion of seawater sulfate within the first 25-cm depth of the sediment. These results show that the North Anatolian Fault and inherited faults are important migration paths for fluids for which a significant part is discharged into the water column, contributing to the increase of methane concentration at the bottom seawater and favoring the development of specific ecosystems

Impacts of airborne volcanic ash on the surface ocean biogeochemistry and marine ecosystems

The availability of nutrients in the surface ocean affects the marine primary productivity (MPP), and the atmosphere represents an important source of nutrients to the euphotic zone of the in the global ocean. Volcanic eruptions, can through atmospheric transport, deposit volcanic ash into the ocean and partial dissolution of ash particles in seawater can provide biologically relevant elements for the ocean. Despite the high input of volcanic ash into the ocean, the role of volcanic eruptions in the surface ocean biogeochemistry was largely overlooked. The primary goal of this study is to evaluate the possible impact of explosive volcanic eruptions on the surface ocean nutrient and its influence on MPP in different oceanic regions (in high- and low-nutrient, low production areas), and influence on the oceanic food-web. By using new clean-laboratory-based geochemical nutrient release data from volcanic ash, and estimates of the marine flux of volcanic ash over long-term periods and/or during single major eruptions, the study provides a new constraints on the importance of volcanic ash deposition for marine nutrient-cycles compared to other major sources such as desert dust. It is also indicated that increased phytoplankton abundance in the surface ocean in large-scale ash fall-out regions can impact the marine food-web (e.g., the increased abundance of zooplankton and salmon fish). Shifts in the phytoplankton community upon addition of volcanic ash can influence the export of biogenic material into the deep ocean, and may eventually affect the carbon cycle.Die Verfügbarkeit von Nährstoffen im Oberflächenozean beeinflusst die marine Primärproduktion (MPP). Die Atmosphäre stellt eine wichtige Nährstoff-Quelle für die euphotische Zone dar. Vulkanausbrüche können durch atmosphärischen Transport Vulkanasche in den Oberflächenozean eintragen, wobei die partielle Auflösung von Vulkanaschepartikeln in Berührung mit Meerwasser biologisch relevante Elemente freisetzen können. Trotz des bekanntermaßen hohen Eintrags von Vulkanasche war die Bedeutung von vulkanischen Eruptionen für die Biogeochemie des Oberflächenozeans bislang weitgehend übersehen worden. Das primäre Ziel der vorliegenden Dissertation ist es, den möglichen Impakt explosiver Vulkanausbrüche auf das Nährstoffbudget des Oberflächenozeans sowie den Einfluss auf die MPP in unterschiedlichen Ozeanregionen (in sowohl Hoch- als auch Niedrignährstoffgebieten mit niedriger MPP) zu evaluieren. Weiterhin sollte der mögliche Einfluss auf das marine Nahrungsnetz mit in die Betrachtungen einbezogen werden. Dies wurde erreicht durch die Generierung neuer Daten zur Freisetzung von Nährstoffen von Vulkanaschen in Reinraumlaboratorien, Abschätzungen zum Fluss vulkanischer Asche in den regionalen und globalen Ozean sowohl im Zusammenhang mit einzelnen größeren Ausbrüchen als auch über geologische Zeiträume für Ozeanbecken. Die Untersuchungen führen hierdurch und durch den Vergleich mit Wüstenstaub als andere wichtige Nährstoffquelle zu neuen Einsichten und einem besseren Verständnis der Bedeutung von Vulkanasche für marine Nährstoffkreisläufe. Es gibt Hinweise, dass Vulkanausbrüche durch die Anregung der MPP innerhalb großskalierter Aschefallregionen sogar Einfluss auf das marine Nahrungsnetz nehmen können (z.B. der Erhöhung der Häufigkeit von Zooplankton und Lachs). Weiterhin kann der Eintrag vulkanischer Asche in den Oberflächenozean zu Änderungen der Phytoplanktonvergesellschaftung innerhalb der Aschefallregion führen, was über den Einfluss auf den Export von biogenem Material in den tiefen Ozean den Kohlenstoffkreislauf berühren kann