With growing global interest in critical metals, large-scale operations are increasingly proposed by some for mining across deep-sea ecosystems, raising concerns about environmental impacts on and beyond the seafloor. Discharges of mining-derived sediments and effluent into the pelagic zone can spread contamination beyond benthic environments, particularly via diel vertically migrating species, affecting epipelagic and mesopelagic communities. This study investigates the effects of sediments from the Clarion-Clipperton zone (CCZ) on representative phytoplankton and zooplankton under laboratory conditions, focusing on nutrient availability, metal toxicity, and reproductive impacts. Sediment particles stimulated the growth of nitrogen- or metal-limited diatoms (Thalassiosira weissflogii, Phaeodactylum tricornutum, Skeletonema costatum), coccolithophores (Emiliania huxleyi), and cyanobacteria (Synechococcus sp.) by releasing nutrients (N, Co, Cu, Fe, Mn, Ni, and Zn). However, a reduced growth of the diatom T. weissflogii in metal-replete seawater and a limited response of cyanobacteria Synechococcus were observed, likely due to metal toxicity. The marine copepod Tigriopus californicus exhibited dose-dependent reductions in growth and reproduction to the CCZ sediment (2-50 mg L-1) and significant reductions in mating success, pregnancy rates, and offspring viability were also observed following exposure to the sediment from the North Pacific abyss. While the test species primarily inhabit surface waters, they were selected as established models to elucidate mechanistic responses to deep-sea sediment exposure. These findings provide one of the first assessments of ecological vulnerabilities to deep-sea mining waste discharges that are broadly relevant across pelagic ecosystems and could inform regulatory decisions by the International Seabed Authority or any individual nations seeking to mine the deep-sea beyond national jurisdiction.

The origin of dissolved organic carbon (DOC) in the ocean, which represents the largest active reservoir of exchangeable carbon on Earth, remains unknown due to the complexity of determining DOC exchange between marine sediments and seawater. Here, we emulate underlying processes of DOC dynamics using physics-based artificial intelligence (AI) to quantify DOC cycling and preservation in sediments at the global scale. We conclude that up to 11% of all particulate organic carbon deposited on the global seafloor returns to seawater as DOC following hydrolysis within the sediment, which is around half of the riverine DOC flux to the ocean. Around 50% of the total solid-phase organic carbon in the upper meter of sediments is formed through the sorption of DOC to minerals. Further, we show that the contribution of the abyssal plain to global DOC efflux and preservation, which is typically entirely overlooked in carbon cycle studies, is equivalent to half the contribution of the continental margins. This study also highlights the power of simpler AI algorithms compared to more complex models and extends the principle of parsimony in mathematical modeling to the context of physics-based AI.

Respiration is a key process in the organic carbon cycle of marine sediments, the understanding of which is key to future management decisions which aim to maximise sediment carbon storage. The microbial community is typically considered the dominant actor in overall sedimentary respiration, but knowledge is lacking about interactions with other components, particularly the macrofauna. The “inverted microbial loop” hypothesis suggests that macrofaunal activity stimulates the microbial respiration of organic carbon through the mixing of fresh organic carbon to depth, and subsequent priming (i.e. activation of refractory detritus by co-respiration with fresh detritus). We conducted experimental incubations to partition respiration amongst the microbial and macrofaunal components of the community and investigate interactions between them. We prepared sediment cores with native benthic communities, macrofauna only and microbial communities only. We added 13C labelled fresh organic matter to these cores and measured respiration over 7 d, quantifying both O2 consumption (reflecting remineralisation of all sedimentary organic C) and production of 13C dissolved inorganic C (DIC, reflecting remineralisation of labile organic C). Consumption of O2, which reflected remineralisation of ambient as well as added fresh organic C, showed greater rates when macrofaunal and microbial communities were present together than the sum of their separate rates. This provides direct experimental evidence that the inverted microbial loop mechanism stimulates mineralisation of less reactive, ambient organic C. Macrofaunal and microbial communities showed approximately equal contributions to the total community respiration, suggesting that faunal respiration should be more routinely included in carbon degradation modelling. The fate of the added fresh organic C in different treatments suggested competition for this resource between macrofauna and microbes, and some functional redundancy amongst different components of the benthic community. The enhanced understanding of sediment respiration generated by this study has implications for management of shelf seafloors to balance carbon storage with other human uses.

Abstract. Respiration is a key process in the organic carbon cycle of marine sediments. The microbial community is considered the dominant actor in the overall sedimentary respiration, but knowledge is lacking about interactions with other components, particularly the macrofauna. The ‘inverted microbial loop’ hypothesis suggests that macrofaunal activity stimulates the microbial respiration of organic carbon through the mixing of fresh organic carbon to depth, and subsequent priming (i.e. activation of refractory detritus by co-respiration with fresh detritus). We conducted experimental incubations to partition respiration amongst the microbial and macrofaunal components of the community and investigate interactions between them. We prepared sediment cores with native benthic communities, macrofauna only and microbial communities only. We added 13C labelled fresh organic matter to these cores and measured respiration over 7 days, quantifying both O2 consumption (reflecting remineralisation of all sedimentary organic C) and production of 13C dissolved inorganic C (DIC, reflecting remineralisation of labile organic C). Macrofaunal and microbial communities showed an approximately equal contribution to the total community respiration, while the fate of the added fresh organic C in different treatments suggested competition for this resource between macrofauna and microbes. Consumption of O2, which reflected remineralisation of ambient as well as added fresh organic C, showed greater rates when macrofaunal and microbial communities were present together than the sum of their separate rates. This provides direct experimental evidence that the inverted microbial loop mechanism stimulates mineralisation of less reactive, ambient organic C. The inverted microbial loop effect is likely to be enhanced following deposition of fresh organic C onto the seafloor, as occurs after a spring bloom.

Controls on organic carbon preservation in marine sediments remain controversial but crucial for understanding past and future climate dynamics. Here we develop a conceptual-mathematical model to determine the key processes for the preservation of organic carbon. The model considers the major processes involved in the breakdown of organic carbon, including dissolved organic carbon hydrolysis, mixing, remineralization, mineral sorption and molecular transformation. This allows redefining of burial efficiency as preservation efficiency, which considers both particulate organic carbon and mineral-phase organic carbon. We show that preservation efficiency is almost three times higher than the conventionally defined burial efficiency and reconciles predictions with global field data. Kinetic sorption and transformation are the dominant controls on organic carbon preservation. We conclude that a synergistic effect between kinetic sorption and molecular transformation (geopolymerization) creates a mineral shuttle in which mineral-phase organic carbon is protected from remineralization in the surface sediment and released at depth. The results explain why transformed organic carbon persists over long timescales and increases with depth.

Deep-seafloor organisms consume oxygen, which can be measured by in situ benthic chamber experiments. Here we report such experiments at the polymetallic nodule-covered abyssal seafloor in the Pacific Ocean in which oxygen increased over two days to more than three times the background concentration, which from ex situ incubations we attribute to the polymetallic nodules. Given high voltage potentials (up to 0.95 V) on nodule surfaces, we hypothesize that seawater electrolysis may contribute to this dark oxygen production. Oxygen is generated abiotically at the abyssal seafloor in the presence of polymetallic nodules, potentially by seawater electrolysis, according to in situ chamber and ex situ incubation experiments.

Natural Flood Management (NFM) aims to reduce flood hazard by working with nature and is gaining prominence worldwide. One particular NFM technique involves the use of channel-spanning woody dams that maintain a clearance height above baseflow. These dams function by increasing channel roughness during high flows and by forcing excessive water onto the floodplain. Whether these dams provide additional benefits to nature remains unclear. While there are many existing studies on natural in-stream wood structures, very few have documented the impact of NFM woody dams in particular. This study adopted a multidisciplinary approach and a Before-After Control-Impact (BACI) research design to assess whether NFM woody dams installed in a small upland catchment had driven changes in benthic macroinvertebrate assemblages and benthic metabolic activities through the geomorphic changes that they had created. Statistical results indicate that macroinvertebrate density, richness, and diversity did not show any difference between stream reaches with and without NFM woody dams. The metrics were generally not related to grain-size parameters and volumes of sediments eroded or deposited. However, individual genera such as Baetis and Rhithrogena became more dominant in the control reach towards the end of the study period, likely due to the higher flow velocities and coarser sediments there resulting from the lack of flow resistance in the absence of NFM woody dams. Rates of benthic respiration (but not rates of photosynthesis) were consistently significantly higher in woody dam reaches than in control reaches, likely due to the presence of patches of finer sediments in the former.

The balance between degradation and preservation of sedimentary organic carbon (OC) is important for global carbon and oxygen cycles 1 . The relative importance of different mechanisms and environmental conditions contributing to marine sedimentary OC preservation, however, remains unclear 2 – 8 . Simple organic molecules can be geopolymerized into recalcitrant forms by means of the Maillard reaction 5 , although reaction kinetics at marine sedimentary temperatures are thought to be slow 9 , 10 . More recent work in terrestrial systems suggests that the reaction can be catalysed by manganese minerals 11 – 13 , but the potential for the promotion of geopolymerized OC formation at marine sedimentary temperatures is uncertain. Here we present incubation experiments and find that iron and manganese ions and minerals abiotically catalyse the Maillard reaction by up to two orders of magnitude at temperatures relevant to continental margins where most preservation occurs 4 . Furthermore, the chemical signature of the reaction products closely resembles dissolved and total OC found in continental margin sediments globally. With the aid of a pore-water model 14 , we estimate that iron- and manganese-catalysed transformation of simple organic molecules into complex macromolecules might generate on the order of approximately 4.1 Tg C yr −1 for preservation in marine sediments. In the context of perhaps only about 63 Tg C yr −1 variation in sedimentary organic preservation over the past 300 million years 6 , we propose that variable iron and manganese inputs to the ocean could exert a substantial but hitherto unexplored impact on global OC preservation over geological time.

One popular Natural Flood Management (NFM) technique involves the construction of channel-spanning woody dams in low-order streams that maintain a clearance height above base flows. While extensive research has examined the geomorphic effects of natural wood accumulations, little has been documented of NFM woody dams, which are structurally distinct from natural accumulations and may produce different patterns of erosion and deposition. This consideration is crucial because changes in physical habitat characteristics have implications for flood management objectives as well as ecosystem structure and functioning. This study adopted a Before-After Control-Impact (BACI) design to assess the geomorphic effects of NFM woody dams in the upper River Cover catchment, United Kingdom. One baseline survey prior to and three monitoring surveys up to 2 years following dam construction were conducted. Structure-from-Motion (SfM) photogrammetry was employed to capture topographic change, supplemented by bathymetric surveys. Results highlight that where the dams remained secure in place, they promoted instream habitat diversity by creating underflow pools. Sediment storage was observed only where the dams had clearance heights <0.3 m from the stream bed. Additionally, the dams commonly led to bank erosion, likely enhanced by inherent bank instability in the study catchment as observed along the control reaches. However, volumes of sediments eroded and deposited were not statistically different between the control and woody dam reaches. Longer monitoring is required to determine whether these effects on channel morphology and habitat diversity will persist, amplify, or diminish over time, and to better understand the longevity of NFM woody dams.

Continental shelf sediments store large amounts of organic carbon. Protecting this carbon from release back into the marine system and managing the marine environment to maximize its rate of accumulation could both play a role in mitigating climate change. For these reasons, in the context of an expanding “Blue Carbon” concept, research interest in the quantity and vulnerability of carbon stored in continental shelf, slope, and deep ocean sediments is increasing. In these systems, carbon storage is physically distant from carbon sources, altered between source and sink, and disturbed by anthropogenic activities. The methodological approaches needed to obtain the evidence to assess shelf sea sediment carbon manageability and vulnerability within an evolving blue carbon framework cannot be transferred directly from those applied in coastal vegetated “traditional” blue carbon habitats. We present a toolbox of methods which can be applied in marine sediments to provide the evidence needed to establish where and when marine carbon in offshore sediments can contribute to climate mitigation, focusing on continental shelf sediments. These methods are discussed in the context of the marine carbon cycle and how they provide evidence on: (i) stock: how much carbon is there and how is it distributed? (ii) accumulation: how rapidly is carbon being added or removed? and (iii) anthropogenic pressures: is carbon stock and/or accumulation vulnerable to manageable human activities? Our toolbox provides a starting point to inform choice of techniques for future studies alongside consideration of their specific research questions and available resources. Where possible, a stepwise approach to analyses should be applied in which initial parameters are analysed to inform which samples, if any, will provide information of interest from more resource-intensive analyses. As studies increasingly address the knowledge gaps around continental shelf carbon stocks and accumulation – through both sampling and modelling – the management of this carbon with respect to human pressures will become the key question for understanding where it fits within the blue carbon framework and within the climate mitigation discourse.

The use of in-stream wood is one of the most commonly employed natural flood management (NFM) techniques. The effectiveness of NFM wood structures in reducing flood risks (i.e., their "primary" effect) has been relatively well documented. However, their additional or "secondary" effects on other natural processes have not been fully evaluated. These secondary effects can be inferred by reviewing previous studies that scrutinized natural wood accumulations or artificial wood structures constructed for purposes other than NFM. The degree of contact with base flows and the stream bed provides a broad classification of NFM wood structures. Having considered the similarities between NFM wood structures and other in-stream wood types, it is suggested that the following geomorphic effects are common to all types of NFM wood structures: pool formation; accumulation of clasts immediately upstream; buffering against stream bed coarsening; and bank erosion, causing channel widening and the formation of floodplain channels. These geomorphic changes contribute to stream bed heterogeneity, potentially creating new niches for aquatic organisms such as macroinvertebrates. Moreover, NFM wood structures may retain benthic organisms accidentally flushed away during flood events, serving as sources of colonists during phases of recovery. Geomorphic changes induced by NFM wood structures may also contribute to spatial variation in rates of biogeochemical processing. Accumulation of fine sediments in some areas may provide more surfaces for the attachment of organic matter and micro-organisms, hence increasing benthic metabolic rates. Stream bed scouring in other areas may lead to sediment instability, suppressing the growth of micro-organisms and benthic metabolic rates. This article is categorized under: Water and Life > Conservation, Management, and Awareness Science of Water > Water Extremes Engineering Water > Sustainable Engineering of Water

The coprecipitation of organic carbon with iron minerals is important for its preservation in soils and sediments, but the mechanisms for carbon-iron interactions and thus the controls on organic carbon cycling are far from understood. Here we coprecipitate carboxylic acids with iron (oxyhydr)oxide ferrihydrite and use near-edge X-ray absorption fine structure spectroscopy and wet chemical treatments to determine the relationship between sequestration mechanism and organic carbon stability against its release and chemical oxidative remineralisation. We show that organic carbon sequestration, stabilisation and persistence increase with an increasing number of carboxyl functional groups. We suggest that carboxyl-richness provides an important control on organic carbon preservation in the natural environment. Our work offers a mechanistic basis for understanding the stability and persistence of organic carbon in soils and sediments, which might be used to develop an overarching relationship between organic functional group-richness, mineral interactions and organic carbon preservation in the Earth system. Organic carbon sequestration, stabilisation and burial through its interaction with iron is enhanced by carboxyl-richness of the organic moiety, according to elemental and microstructure analysis of experimentally produced co-precipitates.

This study envisaged the likely impacts of future hydro-climatic changes on the susceptibility of coastal land to erosion through the development of raster-based geographical information system (GIS) model called land susceptibility to coastal erosion (LSCE). The model was applied to the coastal area of Bangladesh to assess future erosion susceptibility under four greenhouse gas (GHG) concentration trajectories: A1B, RCP2.6, RCP4.5 and RCP8.5. The results indicate considerable changes in future scenarios of coastal land susceptibility to erosion in the area compared to current baseline conditions. The current area of 276.33 km2 (0.61%) high and very high susceptible lands would be substantially increased to 1019.13 km2 (2.25% of land), 799.16 km2 (1.77%), 1181.38 km2 (2.61%) and 4040.71 km2 (8.96%) by 2080 under A1B, RCP2.6, RCP4.5 and RCP8.5 scenarios, respectively. Spatially, the western and eastern coastal zones would have low to moderate susceptibility to erosion, whereas the central coastal zone would have moderate to high/very high susceptibility to erosion. Seasonally, the model predicted the high erosion susceptibility during the monsoon seasons and very low erosion susceptibility during the winter seasons in the future. The model outputs were enhanced by integrating experts’ judgements through fuzzy cognitive mapping (FCM) approach. The LSCE model might be indispensable for coastal researchers in generating future scenarios of physical susceptibility to erosion for highly dynamic coastal areas around the world.