Diversity and Biogeography of Bathyal and Abyssal Seafloor Bacteria and Archaea Along a Mediterranean—Atlantic Gradient

Seafloor sediments cover the majority of planet Earth and microorganisms inhabiting these environments play a central role in marine biogeochemical cycles. Yet, description of the biogeography and distribution of sedimentary microbial life is still too sparse to evaluate the relative contribution of processes driving this distribution, such as the levels of drift, connectivity, and specialization. To address this question, we analyzed 210 archaeal and bacterial metabarcoding libraries from a standardized and horizon-resolved collection of sediment samples from 18 stations along a longitudinal gradient from the eastern Mediterranean to the western Atlantic. Overall, we found that biogeographic patterns depended on the scale considered: while at local scale the selective influence of contemporary environmental conditions appeared strongest, the heritage of historic processes through dispersal limitation and drift became more apparent at regional scale, and ended up superseding contemporary influences at inter-regional scale. When looking at environmental factors, the structure of microbial communities was correlated primarily with water depth, with a clear transition between 800 and 1,200 meters below sea level. Oceanic basin, water temperature, and sediment depth were other important explanatory parameters of community structure. Finally, we propose increasing dispersal limitation and ecological drift with sediment depth as a probable factor for the enhanced divergence of deeper horizons communities.

Development of a Radiation Survey Device for a Multipurpose Unmanned Surface Vehicle

Large quantities of volatile radionuclides were released into the atmosphere after the Fukushima Daiichi Nuclear Power Station (FDNPS) accident in March 2011. An inventory of these radionuclides in seafloor sediments is important for evaluating their environmental behavior and effects on aquatic organisms. We developed a dedicated radiation survey device that can be installed on a multipurpose unmanned surface vehicle called the Intelligent Boat for Oceanological Survey from Hama-dori (i-BoSH). Both the i-BoSH and dedicated radiation survey device were developed by a local consortium to survey places of heavy damage from the accident. The radiation survey device can measure radioactivity in-situ and collect eight sediment samples. The device also contains sensors to measure the relative distance between the i-BoSH and the seafloor as well as other fundamental seawater information such as temperature. A comprehensive test that included in-situ radiation measurements in sediment and collection of sediment core samples was conducted offshore from a radiocesium-contaminated fishing port in Fukushima. Consequently, the effectiveness of this device was confirmed via a successful operation at 12 sampling points with two cruises from the fishing port. Good correlations were observed between the laboratory and field measurement results of the sampled sediment cores and the in-situ radiation survey, respectively. Successful application of this device is expected to go beyond environmental surveys around the FDNPS to more general radiation monitoring around nuclear facilities and longer post-accident periods.

Ensemble Mapping and Change Analysis of the Seafloor Sediment Distribution in the Sylt Outer Reef, German North Sea from 2016 to 2018

Recent studies on seafloor mapping have presented different modelling methods for the automatic classification of seafloor sediments. However, most of these studies have applied these models to seafloor data with appropriate numbers of ground-truth samples and without consideration of the imbalances in the ground-truth datasets. In this study, we aim to address these issues by conducting class-specific predictions using ensemble modelling to map seafloor sediment distributions with minimal ground-truth data combined with hydroacoustic datasets. The resulting class-specific maps were then assembled into a sediment classification map, in which the most probable class was assigned to the appropriate location. Our approach was able to predict sediment classes without bias to the class with more ground-truth data and produced reliable seafloor sediment distributions maps that can be used for seafloor monitoring. The methods presented can also be used for other underwater exploration studies with minimal ground-truth data. Sediment shifts of a heterogenous seafloor in the Sylt Outer Reef, German North Sea were also assessed to understand the sediment dynamics in the marine conservation area during two different short timescales: 2016–2018 (17 months) and 2018–2019 (4 months). The analyses of the sediment shifts showed that the western area of the Sylt Outer Reef experienced sediment fluctuations but the morphology of the bedform features was relatively stable. The results provided information on the seafloor dynamics, which can assist in the management of the marine conservation area.

PLATE TECTONICS IN PORTUGUESE AND SPANISH SCIENCE TEXTBOOKS: FROM THE 1960s TO THE 1980s

Plate tectonics caused a revolution within earth sciences which then was transposed into science textbooks. The main objective of this paper is to explore how plate tectonics influenced Portuguese and Spanish science textbooks published from the 1960s through the 1980s. For this purpose, a qualitative method based on the concept of didactic transposition is used. The didactic transposition of seafloor spreading evidence such as ridges, rifts and trenches, transform faults, seafloor sediments, the age of seafloor basaltic rocks, the magnetic anomalies on the seafloor, the Benioff zones and the subduction process, and also the didactic transposition of the formation of mountains ranges and island arcs, convection currents, plate tectonics concepts, boundaries and motion, and plate tectonics acceptance are studied in a comprehensive sample of science textbooks. The analysis of textbooks shows that the didactic transposition of seafloor spreading, and plate tectonics started mainly in 1970s Portuguese and Spanish textbooks and had a strong development in 1980s textbooks. No major differences were found between the approaches to plate tectonics in similar age Portuguese and Spanish textbooks. At the beginning of the 1970s, textbooks presented partial evidence for seafloor spreading, such as magnetic anomalies and the characteristics of ridges, rifts and trenches. They also addressed convection currents but only those that were related to geosynclines. In the mid 1970s and in the 1980s, textbooks presented more comprehensive evidence of seafloor spreading, by adding didactical transpositions of transform faults, seafloor sediments and the age of seafloor rocks. They also presented in more detail topics such as magnetic anomalies, the Benioff zones, orogenic processes and the tectonic significance of ridges, rifts and trenches. Plate tectonic theory was presented in major textbooks as widely accepted, and discussions about speculative facts or processes were rare.

Ensemble Mapping and Change Analysis of the Seafloor Sediment Distribution in the Sylt Outer Reef, German North Sea from 2016-2018

Recent studies on seafloor mapping have presented different modelling methods for the automatic classification of seafloor sediments. However, most of these studies have applied these models to seafloor data with appropriate number of ground-truth samples, which raises the question whether these methods are applicable to studies with smaller numbers of ground-truth data. In this study, we aim to address this issue by conducting sediment class-specific predictions using ensemble modelling to map areas with limited or without ground-truth data and combined with hydro-acoustic datasets. The resulting class-specific maps were then assembled into one map, where the most probable class was assigned to the appropriate location. Our approach was able to predict sediment classes without bias to the class with more ground-truth data and produced reliable seafloor sediment distributions maps that can be used for seafloor monitoring. Sediment shifts of a heterogenous seafloor in the Sylt Outer Reef, German North Sea were also assessed to understand the sediment dynamics in the area. The analyses of sediment shifts showed that the western area of the Sylt Outer Reef is highly active, and the results of the analyses assisted in providing recommendations on future seafloor monitoring activities.

Mixed-Convective Processes Within Seafloor Sediments Arising From Fresh Groundwater Discharge

The dependence of near-shore ecosystems on the freshwater component of submarine groundwater discharge (SFGD) is well recognized. Previous studies of SFGD have typically assumed that SFGD occurs through aquitards that are in direct contact with seawater. These studies provide no guidance on the distribution of freshwater discharge to the seafloor where SFGD occurs through sandy sediments, even though in most situations, seabed sediments are permeable. We find that SFGD may occur in unconfined, seafloor sediments as density-driven flow in the form of fingers, or otherwise, diffusive freshwater discharge is also possible. Unstable, buoyancy-driven flow within seabed sediments follows similar patterns (except inverted) to the downward free convection of unstable (dense over less-dense groundwater) situations. Consequently, the same theoretical controlling factors as those developed for downward mixed-convective flow are expected to apply. Although, there are important differences, in particular the boundary conditions, between subsea freshwater-seawater interactions and previous mixed-convective problems. Simplified numerical experiments in SEAWAT indicate that the behavior of fresh buoyant plumes depends on the aquifer lower boundary, which in turn controls the rate and pattern of SFGD to the seafloor. This article provides an important initial step in the understanding of SFGD behavior in regions of sandy seafloor sediments and analyses for the first time the mixed-convective processes that occur when freshwater rises into an otherwise saline groundwater body.