The distribution of oxygen in the ocean is controlled by physical, biogeochemical and biological processes. Both the supply and consumption of oxygen are sensitive to climate change in ways that are not fully understood. Recent observations suggest that the oxygen content of the ocean is declining (ocean deoxygenation) and the oxygen minimum zones and costal hypoxia sites are expanding with tremendous effects on the ocean’s ecosystems and living organisms. Following the EUR-OCEANS conference on “Ocean Deoxygenation” in Toulouse, France, in 2011, the 46th International Liège Colloquium on “Low Oxygen Environments in Marine, Fresh and Estuarine Waters” in 2014 and the discussion meeting of the Royal Society London on “Ocean Ventilation and Deoxygenation in a Warming World” in 2016, the purpose of the international conference on “Ocean Deoxygenation: Drivers & Consequences – Past|Present|Future” that took place in Kiel, 3–7 Sept 2018 organised by the Collaborative Research Centre 754 “Climate-Biogeochemical Interactions in the Tropical Ocean” was to − focus on the past, present and future state of oxygen in the ocean on global, regional and local scales − analyse mechanisms and feedbacks critical to identify natural and anthropogenic causes of oxygen variability − determine impacts on biogeochemical cycles and ecosystems.

This special issue is open to all submissions on ocean deoxygenation and is not limited to conference participants.

Purpose of special issue: The Collaborative Research Centre 754 “Climate-Biogeochemical Interactions in the Tropical Ocean” has been investigating the newly recognised threat of ocean deoxygenation since 2008. Before the project's end in 2019, the International Conference on Ocean Deoxygenation was organised in Kiel, Germany in September 2018 with over 300 participants (https://conference.sfb754.de/event/1/). Following the conference the proposed inter-journal special issue aims at presenting the recent research advances and highlights in the field of ocean deoxygenation. The special issue shall not be limited to conference participants but be open to all submissions.

Tropical ecosystems are epicentres of biodiversity and play key roles in global biogeochemical cycles, for example in the sequestration of terrestrial and coastal carbon. Tropical coasts are characterised by large river systems delivering huge amounts of material into the coastal ocean, with fluvial inputs of nutrients, carbon, and sediment from tropical rivers playing a globally significant role. The majority of the population in tropical regions lives along these rivers and coasts and depends economically on aquatic resources. Yet rapid development in many tropical regions, such as Borneo, is causing large-scale impacts on riverine and coastal ecosystems, the biogeochemical consequences of which remain poorly known. Interestingly, it has been suggested that coastal systems in tropical regions react differently to anthropogenic stresses compared to coasts in temperate and subtropical regions. However, our understanding of tropical rivers, estuaries, and coasts is still too basic to predict the impacts of increasing anthropogenic pressures. An improved understanding of these ecosystems and their biogeochemistry is therefore urgently needed. This special issue calls for work that examines biogeochemical processes of tropical rivers, estuaries, and coasts as well as their response to environmental change and the interplay of natural and anthropogenic control factors.

After more than 20 years, mining of mineral resources, such as polymetallic nodules and massive sulfides, in the deep sea is back on the agenda of many countries and companies in their search for high-tech raw metals. In this course, the past 10 years have seen an immense increase of exploration licences issued by the International Seabed Authority (ISA), which organizes and controls activities at the ocean floor beyond national jurisdiction, generally termed the "area". The seven pilot investors are renewing their exploration licences for another five years, after which they will need to apply for an exploitation licence (or lose their contracted claim). Thus, ISA is currently developing the regulatory framework for the exploitation of seafloor mineral resources, which includes legislation to "ensure effective protection of the marine environment from harmful effects" according to UNCLOS article 145. In order to support decision-making for ISA's regulations, several benthic impact experiments were conducted more than 20 years ago. However, the impacts have only been studied over the following 5–7 years. Until now, no information has been available for the expected longer-term impacts induced by mining operations, i.e. in case of nodule mining the removal of the surface sediments and dispersal of the suspended particle plume and subsequent seafloor blanketing. Therefore, in 2015 the European project MiningImpact, funded through the Joint Programming Initiative Healthy Seas and Oceans (JPIO), revisited several impact scars at the seafloor in the nodule exploration licence areas of the Clarion–Clipperton Fracture Zone (CCZ) as well as the benthic impact experiment DISCOL (DIsturbance and reCOLonization) in the Peru Basin. Our scientific study provides the first results of environmental consequences on the deep-sea ecosystem that prevail for up to 40 years after a disturbance was created. Also, the impacted areas vary in size from single 1–2 nm long dredge or sled tracks to the 78 criss-crossing, 4 m wide plough harrow marks in the 11 km² large DISCOL experimental area. This special issue presents the scientific results of the biological, biogeochemical, geological, and oceanographic investigations carried out in the MiningImpact project (consisting of 25 institutes from 11 European countries) that is used to provide recommendations for the exploitation mining code of ISA (e.g. improved methods for the management and monitoring of mining operations) and to improve our understanding of the ecological functions of the deep seafloor.

Paleoclimate data provide unique insights into climate dynamics across a range of timescales. Importantly, paleoclimate data are the only means of evaluating and constraining climate models under boundary conditions different from today and the recent past. However, most paleoclimate data are presently archived in a fragmented and non-standardized way, necessitating synthesis efforts in order to allow meaningful analysis of spatio-temporal climate dynamics and data-model comparison. Moreover, paleoclimate data are inherently uncertain since they are based on indirect evidence (proxies) and associated with chronological error, requiring rigorous uncertainty analysis to separate signal from noise and to make full use of paleoclimate data syntheses.

This special issue provides a platform to present paleoclimate synthesis products, to review the current state of proxy uncertainty analysis, as well as to present new developments. The issue is organized within the paleoclimate data synthesis working group of the PALMOD (http://www.palmod.de/ ) project, which focuses on the past 130,000 years. However, this is an open submission issue and we explicitly invite contributions from across the paleoclimate community describing synthesis methods and results from any kind of archive and/or parameter. We welcome contributions presenting paleoclimate synthesis products and their analysis across timescales, with regional or global focus and both time slice and transient approaches as well as conceptual contributions to proxy data uncertainty analysis (theoretical, empirical, Bayesian).

NETCARE (Network on Aerosols and Climate: Addressing Key Uncertainties in Remote Canadian Environments) is a large research network focusing on aerosol–cloud–climate interactions. While Canadian-based, it operates with many international collaborations. It is comprised of scientists working in both atmospheric science and marine biogeochemistry, with particular attention given to a suite of intensive field measurements (with both atmospheric and oceanic components) and model evaluation and development. There are three major research directions within the network: 1. Carbonaceous Aerosol, 2. Arctic Clouds, and 3. Ocean–Atmosphere Interactions. A large amount of the research has an Arctic focus, it being a region especially susceptible to anthropogenic input and experiencing a large degree of biogeochemical change. The website for the network is http://www.netcare-project.ca/ . On the website, there is more information on research activities, field campaign details, modeling activities, data products, and personnel.