Description: Riverine nutrient export is an important process in marine coastal biogeochemistry and also impacts global marine biology. The nitrogen cycle is a key player here. Internal feedbacks regulate not only nitrogen distribution, but also primary production and thereby oxygen concentrations. Phosphorus is another essential nutrient and interacts with the nitrogen cycle via different feedback mechanisms. After a previous study of the marine nitrogen cycle response to riverine nitrogen supply, we here additionally include phosphorus from river export with different phosphorus burial scenarios and study the impact of phosphorus alone and in combination with nitrogen in a global 3-D ocean biogeochemistry model. Again, we analyse the effects on near coastal and open ocean biogeochemistry. We find that the addition of bio-available riverine phosphorus alone or together with nitrogen affects marine biology on millennial timescales more than riverine nitrogen alone. Biogeochemical feedbacks in the marine nitrogen cycle are strongly influenced by the additional phosphorus. Where bio-available phosphorus is increased by river input, nitrogen concentrations increase as well, except for regions with high denitrification rates. High phosphorus burial rates decrease biological production significantly. Globally, riverine phosphorus leads to elevated primary production rates in the coastal and open oceans.

Description: Spatial predictions of total organic carbon (TOC) concentrations and stocks are crucial for understanding marine sediments’ role as a significant carbon sink in the global carbon cycle. In this study, we present a geospatial prediction of TOC concentrations and stocks at a 5 x 5 arc minute grid scale, using a deep learning model — a novel machine learning approach based on a new compilation of over 22,000 global TOC measurements and a new set of predictors, such as seafloor lithologies, grain size distribution, and an alpha-chlorophyll satellite data. In our study, we compared the predictions and discuss the limitations from various machine learning methods. Our findings reveal that the neural network approach outperforms methods such as k Nearest Neighbors and random forests, which tend to overfit to the training data, especially in highly heterogeneous and complex geological settings. We provide estimates of mean TOC concentrations and total carbon stock in both continental shelves and deep sea settings across various marine regions and oceans. Our model suggests that the upper 10 cm of oceanic sediments harbors approximately 171 Pg of TOC stock and has a mean TOC concentration of 0.68 %. Furthermore, we introduce a standardized methodology for quantifying predictive uncertainty using Monte Carlo dropout and present a map of information gain, that measures the expected increase in model knowledge achieved through in-situ sampling at specific locations which is pivotal for sampling strategy planning.

Description: Submarine landslides pose a hazard to coastal communities due to the tsunamis they can generate, and can damage critical seafloor infrastructure, such as the network of cables that underpin global data transfer and communications. These mass movements can be orders of magnitude larger than their onshore equivalents and are found on all of the world’s continental margins; from coastal zones to hadal trenches. Despite their prevalence, and importance to society, offshore monitoring studies have been limited by the largely unpredictable occurrence of submarine landslide and the need to cover large regions of extensive continental margins. Recent subsea monitoring has provided new insights into the preconditioning and run-out of submarine landslides using active geophysical techniques, but these tools only measure a very small spatial footprint, and are power and memory intensive, thus limiting long duration monitoring campaigns. Most landslide events therefore remain entirely unrecorded. Here we first show how passive acoustic and seismologic techniques can record acoustic emissions and ground motions created by terrestrial landslides. We then show how this terrestrial-focused research has catalysed advances in the detection and characterisation of submarine landslides, using both onshore and offshore networks of broadband seismometers, hydrophones and geophones. We then discuss some of the new insights into submarine landslide preconditioning, timing, location, velocity and their down-slope evolution that is arising from these advances. We finally outline some of the outstanding challenges, in particular emphasising the need for calibration of seismic and acoustic signals generated by submarine landslides and their run-out. Once confidence can be enhanced in submarine landslide signal detection and interpretation, passive seismic and acoustic sensing has strong potential to enable more complete hazard catalogues to be built, and opens the door to emerging techniques (such as fibre-optic sensing), to fill key, but outstanding, knowledge gaps concerning these important underwater phenomena.

Description: Identification of seismically active fault zones and the definition of sufficiently large respect distances from these faults which enable avoiding the damaged rock zone surrounding the ruptured ground commonly are amongst the first steps to take in the geoscientific evaluation of sites suitable for nuclear waste disposal. In this work we present a GIS-based approach, using the earthquake-epicentre locations from the instrumental earthquake record of South-Korea to identify potentially active fault zones in the country, and compare different strategies for fault zone buffer creation as originally developed for site search in the high seismicity country Japan, and the low-to-moderate seismicity countries Germany and Sweden. In order to characterize the hazard potential of the Korean fault zones, we moreover conducted slip tendency analysis, here for the first time covering the fault zones of the entire Korean Peninsula. For our analyses we used the geo-spatial information from a new version of the Geological map of South-Korea, containing the outlines of 11 rock units, which we simplified to distinguish between 4 different rock types (granites, metamorphic rocks, sedimentary rocks and igneous rocks) and the surface traces of 1,528 fault zones and 6,654 lineaments identified through years of field work and data processing, a rich geo-dataset which we will publish along with this manuscript. Our approach for identification of active fault zones was developed without prior knowledge of already known seismically active fault zones, and as a proof of concept the results later were compared to a map containing already identified active fault zones. The comparison revealed that our approach identified 16 of the 21 known seismically active faults and added 472 previously unknown potentially active faults. The 5 seismically active fault zones which were not identified by our approach are located in the NE- and SW-sectors of the Korean Peninsula, which haven’t seen much recent seismic activity, and thus are not sufficiently well covered by the seismic record. The strike directions of fault zones identified as active are in good agreement with the orientation of the current stress field of the peninsula and slip tendency analysis provided first insights into subsurface geometry such as the dip angles of both active and inactive fault zones. The results of our work are of major importance for the early-stage seismic hazard assessment that has to be conducted in support of the nuclear waste disposal siting in South-Korea. Moreover, the GIS-based methods for identification of active fault zones and buffering of respect areas around fault zone traces presented here, are applicable also elsewhere.

Description: Nitrogen fixers, or diazotrophs, play a key role in the carbon and nitrogen cycle of the world oceans, but the controlling mechanisms are not comprehensively understood yet. The present study compares two paradigms on the ecological niche of diazotrophs in an Earth System Model (ESM). In our standard model configuration, which is representative for most of the state-of-the-art pelagic ecosystem models, diazotrophs take advantage of zooplankton featuring a lower food preference for diazotrophs than for ordinary phytoplankton. We compare this paradigm with the idea that diazotrophs are more competitive under oligotrophic conditions, characterized by low (dissolved, particulate, organic and inorganic) phosphorous availability. Both paradigms are supported by observational evidence and lead to a similar good agreement to the most recent and advanced observation-based nitrogen fixation estimate in our ESM framework. Further, we illustrate that the similarity between the two paradigms breaks in a RCP 8.5 anthropogenic emission scenario. We conclude that a more advanced understanding of the ecological niche of diazotrophs is mandatory for assessing the cycling of essential nutrients, especially under changing environmental conditions. Our results call for more in-situ measurements of cyanobacteria biomass if major controls of nitrogen fixation in the oceans are to be dissected.

Description: Bottom trawling represents the most widespread anthropogenic physical disturbance to shelf sea sediments. While trawling-induced mortality in benthic fauna has been extensively investigated, its impacts on ecosystem functioning and carbon cycling at regional scales remain unclear. Using the North Sea as an example, we address these issues by synthesizing a high-resolution dataset of bottom trawling impact on sediments, feeding this dataset into a 3-dimensional physical–biogeochemical model to estimate trawling-induced changes in biomass, bioturbation and sedimentary organic carbon, and assessing model results with field samples. Results suggest a trawling-induced net reduction in macrobenthic biomass by 10-27%. Trawling-induced resuspension and reduction of bioturbation jointly and accumulatively reduce the regional sedimentary organic carbon sequestration capacity by 21-67%, equivalent to 0.58-1.84 Mt CO2 yr-1. Our study emphasizes the need for proper management of trawling on muddy seabeds, if the natural capacity of shelf seas for carbon sequestration should be conserved and restored.