Description: Although the north-western coast of Western Australia is highly vulnerable to tropical cyclones and tsunamis, little is known about the geological imprint of historic and prehistoric extreme wave events in this particular area. Despite a number of site-specific difficulties such as post-depositional changes and the preservation potential of event deposits, both tropical cyclones and tsunamis may be inferred from the geomorphology and the stratigraphy of beach ridge sequences, washover fans, and coastal lagoons or marshes. A further challenge is the differentiation between tsunami and storm deposits in the geological record, particularly where modern deposits and/or historical reports on the event are not available. This study presents a high resolution sedimentary record of washover events from the Ashburton River delta (Western Australia) spanning approximately the last 150 years. A detailed characterization of event deposits is provided, and a robust chronostratigraphy for the investigated washover sequence is established based on multi-proxy sediment analyses and optically stimulated luminescence dating. Combining sedimentological, geochemical and high-resolution optically stimulated luminescence data, event layers are assigned to known historical events and tropical cyclone deposits are separated from tsunami deposits. For the first time, the 1883 Krakatoa and 1977 Sumba tsunamis are inferred from sedimentary records of northwest Australia. It is demonstrated that optically stimulated luminescence applied in coastal sedimentary archives with favourable luminescence characteristics can provide accurate chronostratigraphies even on a decadal time scale. The results contribute to the data pool of tropical cyclone and tsunami deposits in Holocene stratigraphies; however, they also demonstrate how short-lived sediment archives may be in dynamic sedimentary environments. This article is protected by copyright. All rights reserved.

Description: The Arctic Ocean plays a key role in regulating the global climate, while being highly sensitive to climate change. Temperature in the Arctic increases faster than the global average, causing a loss of multiyear sea-ice and affecting marine ecosystem structure and functioning. As a result, Arctic primary production and biogeochemical cycling are changing. Here, we investigated inter-annual changes in the concentrations of particulate and dissolved organic carbon (POC, DOC) together with biological drivers, such as phyto- and bacterioplankton abundance in the Fram Strait, the Atlantic gateway to the Central Arctic Ocean. Data have been collected in summer at the Long-Term Ecological Research observatory HAUSGARTEN during eight cruises from 2009 to 2017. Our results suggest that the dynamic physical system of the Fram Strait induces strong heterogeneity of the ecosystem that displays considerable intra-seasonal as well as inter-annual variability. Over the observational period, DOC concentrations were significantly negatively related to temperature and salinity, suggesting that outflow of Central Arctic waters carrying a high DOC load is the main control of DOC concentration in this region. POC concentration was not linked to temperature or salinity but tightly related to phytoplankton biomass as estimated from chlorophyll-a concentrations (Chl-a). For the years 2009–2017, no temporal trends in the depth-integrated (0–100 m) amounts of DOC and Chl-a were observed. In contrast, depth-integrated (0–100 m) amounts of POC, as well as the ratio [POC]:[TOC], decreased significantly over time. This suggests a higher partitioning of organic carbon into the dissolved phase. Potential causes and consequences of the observed changes in organic carbon stocks for food-web structure and CO2 sequestration are discussed.

Description: Numerical simulations of ocean biogeochemical cycles need to adequately represent particle sinking velocities (SV). For decades, Stokes' Law estimating particle SV from density and size has been widely used. But while Stokes' Law holds for small, smooth, and rigid spheres settling at low Reynolds number, it fails when applied to marine aggregates complex in shape, structure, and composition. Minerals and zooplankton can alter phytoplankton aggregates in ways that change their SV, potentially improving the applicability of Stokes' models. Using rolling cylinders, we experimentally produced diatom aggregates in the presence and absence of minerals and/or microzooplankton. Minerals and to a lesser extent microzooplankton decreased aggregate size and roughness and increased their sphericity and compactness. Stokes' Law parameterized with a fractal porosity modeled adequately size‐SV relationships for mineral‐loaded aggregates. Phytoplankton‐only aggregates and those exposed to microzooplankton followed the general Navier‐Stokes drag equation suggesting an indiscernible effect of microzooplankton and a drag coefficient too complex to be calculated with a Stokes' assumption. We compared our results with a larger data set of ballasted and nonballasted marine aggregates. This confirmed that the size‐SV relationships for ballasted aggregates can be simulated by Stokes' models with an adequate fractal porosity parameterization. Given the importance of mineral ballasting in the ocean, our findings could ease biogeochemical model parameterization for a significant pool of particles in the ocean and especially in the mesopelagic zone where the particulate organic matter : mineral ratio decreases. Our results also reinforce the importance of accounting for porosity as a decisive predictor of marine aggregate SV.

Description: Carbohydrates represent an important fraction of labile and semi-labile marine organic matter that is mainly comprised of exopolymeric substances derived from phytoplankton exudation and decay. This study investigates the composition of total combined carbohydrates (tCCHO; 〉 1 kDa) and the community development of free-living (0.2 - 3 μm) and particle-associated (3 - 10 μm) bacterioplankton during a spring phytoplankton bloom in the southern North Sea. Furthermore, rates were determined for the extracellular enzymatic hydrolysis that catalyzes the initial step in bacterial organic matter remineralization. Concentrations of tCCHO greatly increased during bloom development, while the composition showed only minor changes over time. The combined concentration of glucose, galactose, fucose, rhamnose, galactosamine, glucosamine and glucuronic acid in tCCHO was a significant factor shaping the community composition of the particle-associated bacteria. The richness of particle-associated bacteria greatly increased in the post‐bloom phase. At the same time, the increase in extracellular β‐glucosidase activity was sufficient to explain the observed decrease in tCCHO, indicating the efficient utilization of carbohydrates by the bacterioplankton community during the post-bloom phase. Our results suggest that carbohydrate concentration and composition are important factors in the multifactorial environmental control of bacterioplankton succession and the enzymatic hydrolysis of organic matter during phytoplankton blooms.

Description: In recent decades, the Eurasian Basin of the Arctic Ocean has undergone remarkable variations as part of the large-scale environmental changes facing the planet. The Fram Strait connects the Arctic Ocean to the North Atlantic, and provides the main gateway for water exchange between the Arctic and the global oceans. Two major current systems are present in Fram Strait: the West Spitsbergen Current (WSC) carries Atlantic water northwards, and the East Greenland Current (EGC) brings cold Arctic waters and ice southwards (Fig 1 and 2). The proximity of these two distinct current systems creates a valuable opportunity for studying differences in microbial community composition across strong gradients of temperature and ice cover. Here we present a first preliminary investigation of both free-living and particle-associated pelagic bacterial communities in the upper water column across a longitudinal transect of the entire Fram Strait, conducted during RV Polarstern expedition PS85 (ARK-XXVIII/2) in June 2014.

Description: Transparent exopolymer particles (TEP) are a class of marine gel particles and important links between surface ocean biology and atmospheric processes. Derived from marine microorganisms, these particles can facilitate the biological pumping of carbon dioxide to the deep sea, or act as cloud condensation and ice nucleation particles in the atmosphere. Yet, environmental controls on TEP abundance in the ocean are poorly known. Here, we investigated some of these controls during the first multiyear time-series on TEP abundance for the Fram Strait, the Atlantic gateway to the Central Arctic Ocean. Data collected at the Long-Term Ecological Research observatory HAUSGARTEN during 2009 to 2014 indicate a strong biological control with highest abundance co-occurring with the prymnesiophyte Phaeocystis pouchetii. Higher occurrence of P. pouchetii in the Arctic Ocean has previously been related to northward advection of warmer Atlantic waters, which is expected to increase in the future. Our study highlights the role of plankton key species in driving climate relevant processes; thus, changes in plankton distribution need to be accounted for when estimating the ocean's biogeochemical response to global change.

Description: Climate models project that the Arctic Ocean may experience ice-free summers by the second half of this century. This may have severe repercussions on phytoplankton bloom dynamics and the associated cycling of carbon in surface waters. We currently lack baseline knowledge of the seasonal dynamics of Arctic microbial communities, which is needed in order to better estimate the effects of such changes on ecosystem functioning. Here we present a comparative study of polar summer microbial communities in the ice-free (eastern) and ice-covered (western) hydrographic regimes at the LTER HAUSGARTEN in Fram Strait, the main gateway between the Arctic and North Atlantic Oceans. Based on measured and modeled biogeochemical parameters, we tentatively identified two different ecosystem states (i.e., different phytoplankton bloom stages) in the distinct regions. Using Illumina tag-sequencing, we determined the community composition of both free-living and particle-associated bacteria as well as microbial eukaryotes in the photic layer. Despite substantial horizontal mixing by eddies in Fram Strait, pelagic microbial communities showed distinct differences between the two regimes, with a proposed early spring (pre-bloom) community in the ice-covered western regime (with higher representation of SAR11, SAR202, SAR406 and eukaryotic MALVs) and a community indicative of late summer conditions (post-bloom) in the ice-free eastern regime (with higher representation of Flavobacteria, Gammaproteobacteria and eukaryotic heterotrophs). Co-occurrence networks revealed specific taxon-taxon associations between bacterial and eukaryotic taxa in the two regions. Our results suggest that the predicted changes in sea ice cover and phytoplankton bloom dynamics will have a strong impact on bacterial community dynamics and potentially on biogeochemical cycles in this region.