Globally, rivers deliver significant quantities of nitrogen (N) and phosphorus (P) to the coastal ocean each year. Currently, there are no viable estimates of how much of this N and P escapes biogeochemical processing on the shelf to be exported to the open ocean; most models of N and P cycling assume that either all or none of the riverine nutrients reach the open ocean. We address this problem by using a simple mechanistic model of how a low-salinity plume behaves outside an estuary mouth. The model results in a global map of riverine water residence times on the shelf, typically a few weeks at low latitudes and up to a year at higher latitudes, which agrees well with observations. We combine the map of plume residence times on the shelf with empirical relationships that link residence time to the proportions of dissolved inorganic N (DIN) and P (DIP) exported and use a database of riverine nutrient loads to estimate the global distribution of riverine DIN and DIP supplied to the open ocean. We estimate that 75% of DIN and 80% of DIP reaches the open ocean. Ignoring processing within estuaries yields annual totals of 17 Tg DIN and 1.2 Tg DIP reaching the open ocean. For DIN this supply is about 50% of that supplied via atmospheric deposition, with significant east-west contrasts across the main ocean basins. The main sources of uncertainty are exchange rates across the shelf break and the empirical relationships between nutrient processing and plume residence time.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2016GB005483

What proportion of riverine nutrients reaches the open ocean

An area of about 40000 km2 of desalinated upper layer waters with a salinity of less than 25 psu was found during cruise 54 of the R/V Akademik Mstislav Keldysh in the southwestern part of the Kara Sea (September 2007). Close to the boundary of this region located near the eastern coast of Novaya Zemlya, the salinity was less than 16 psu. The thickness of the desalinated layer was about 10 m. The results of the chemical analysis revealed that the observed desalination of the sea water was produced, first of all, by the Yenisei River, while the contribution of the Ob River’s waters was secondary. However, the most desalinated region near the eastern coast of Novaya Zemlya was separated from the Ob-Yenisei estuary and corresponded to a quasi-isolated lens. It is likely that the formation of this lens, as well as the major part of the desalinated upper layer waters, occurred in June when the flood of the Yenisei was maximal, while the further drift of the desalinated waters to the west of the Ob-Yenisei estuary was forced by the prevailing northern wind. The additional desalination (by 2–3 psu and even more) of the upper layer waters near the eastern coast of Novaya Zemlya might be related to the melting of the Novozemelskiy ice massif. The regularities of the temporal evolution of the upper desalinated layer, as well as the influence of this layer on the hydrological structure and dynamics of the southwestern Kara Sea, are discussed.

https://link.springer.com/content/pdf/10.1134/S0001437010050036.pdf

The upper desalinated layer in the Kara Sea

The results of observations of submesoscale eddies (with a diameter of 2–8 km) on the narrow Black Sea shelf are presented. These observations were carried out in the Gelendzhik region in the autumn seasons of 2007–2008 using traditional and new methods of hydrophysical investigations. The mechanisms of generation of such eddies are discussed.

Submesoscale Eddies at the Caucasus Black Sea Shelf and the Mechanisms of Their Generation

Long-term variations of the Black Sea dynamics and their impact on the marine ecosystem

The Holocene marine transgression starting at ~19 ka flooded the Arctic shelves driving extensive thawing of terrestrial permafrost. It thereby promoted methanogenesis within sediments, the dissociation of gas hydrates, and the release of formerly trapped gas, with the accumulation in pressure of released methane eventually triggering blowouts through weakened zones in the overlying and thinned permafrost. Here we present a range of geophysical and chemical scenarios for the formation of pingo-like formations (PLFs) leading to potential blowouts. Specifically, we report on methane anomalies from the South Kara Sea shelf focusing on two PLFs imaged from high-resolution seismic records. A variety of geochemical methods are applied to study concentrations and types of gas, its character, and genesis. PLF 1 demonstrates ubiquitously low-methane concentrations (14.2–55.3 ppm) that are likely due to partly unfrozen sediments with an ice-saturated internal core reaching close to the seafloor. In contrast, PLF 2 reveals anomalously high-methane concentrations of >120,000 ppm where frozen sediments are completely absent. The methane in all recovered samples is of microbial and not of thermogenic origin from deep hydrocarbon sources. However, the relatively low organic matter content (0.52–1.69%) of seafloor sediments restricts extensive in situ methane production. As a consequence, we hypothesize that the high-methane concentrations at PLF 2 are due to microbial methane production and migration from a deeper source.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015JF003467

Methane release from pingo-like features across the South Kara Sea shelf, an area of thawing offshore permafrost

During cruise 54 of the R/V Akademik Mstislav Keldysh to the southwestern Kara Sea (September 6 to October 7, 2007), a large amount of hydrophysical data with unique spatial resolution was obtained on the basis of measurements using different instruments. The analysis of the data gave us the possibility to study the dynamics and hydrological structure of the southwestern Kara Sea basin. The main elements of the general circulation are the following: the Yamal Current, the Eastern Novaya Zemlya Current, and the St. Anna Trough Current. All these currents are topographically controlled; they flow over the bottom slopes along the isobaths. The Yamal Current begins at the Kara Gates Strait and turns to the east as part of the cyclonic circulation. Then, it turns to the north and propagates along the Yamal coast over the 100-m isobath. The Eastern Novaya Zemlya Current (its core is located over the eastern slope of the Novaya Zemlya Trough) flows to the northeast. Near the northern edge of Novaya Zemlya, it encounters the St. Anna Trough Current, separates from the coast, and flows practically to the east merging with the continuation of the Yamal Current. A strong frontal zone is formed in the region where the two currents merge above the threshold that separates the St. Anna Trough from the Novaya Zemlya Trough and divides the warm and saline Arctic waters from the cooler and fresher waters of the southwestern part of the Kara Sea. This threshold, whose depth does not exceed 100–150 m, is a barrier that prevents the spreading of the Barents Sea and Arctic waters to the southwestern part of the Kara Sea basin through the St. Anna Trough.

Circulation in the southwestern part of the Kara Sea in September 2007

The contemporary state and current changes in the Arctic Basic, both climatic and anthropogenic, are in many respects determined by the processes occurring on the shelves of marginal Arctic seas, in regions with the largest impact of continental runoff on the Arctic sea environment and biota. The Siberian sea shelves are regions with vast volumes of river runoff into the Arctic, ~2000 km3/year, as well as its transformation. The processes occurring in the mouth areas of Siberian rivers and on the shelf of epicontinental Arctic seas affect the Central Arctic Basin via a system of cross-shelf and cross-slope transport. They play the most important role in the formation of hydrophysical and hydrodynamic properties of water masses, in the regulation of hydrochemical and biogeochemical regimes, in the creation of biological production, and in the regulation of matter f luxes, including anthropogenic pollution, in the contemporary Arctic ecosystem as a whole. A comprehensive assay of these processes and the factors governing them is the basis for understanding the Arctic as a system and its modern evolution under the impact of climate change and anthropogenic factors, as well as predicting and evaluating the reversibility of modern changes taking place in the Arctic. To study Arctic ecosystems, the Shirshov Institute of Oceanology, Russian Academy of Sciences (RAS), organized an expedition to the Kara, Laptev, and East Siberian seas: cruise 69 of the R/V Akademik Mstislav Keldysh. The expedition set off from the port of Arkhangelsk on August 22, 2017, and lasted 45 days. The leader of the expedition was the deputy director of the institute, RAS Corresponding Member M.V. Flint. The vessel was commanded by Captain Yu.N. Gorbach. Figure 1 shows the route of the expedition, which covered 8108 NM. The expedition continued comprehensive research of Russian Arctic ecosystems conducted by the institute since 2007 [1–3] (Fig. 1). Seventy-five people made up the expedition’s scientific team. In addition to employees of the Shirshov Institute of Oceanology and its Atlantic, Southern, and Northwestern branches, the expedition included scientists from the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, RAS; the Institute of General Genetics, RAS; the Institute for Biology of Inland Waters, RAS; the Institute of Geochemistry and Analytical Chemistry, RAS; the Marine Biology Research Institute, RAS; the Scientific Research Center Kurchatov Institute; Moscow State University; and Moscow Physicotechnical University. The research made it possible to obtain the characteristic of spatiotemporal variability of the thermocline structure of waters in the areas where large Siberian rivers impact the Arctic shelf in the Kara, Laptev, and East Siberian seas. Continuous observations of the variability of the thermohaline characteristics of the surface layer of Siberian seas from the Kara Strait to the central part of the East Siberian Sea were conducted, which showed freshening almost everywhere up to <28 PSU. Hydrophysical observations revealed separation of the East Siberian Sea into two regions: the western more freshened region under the influence of Laptev Sea waters and the eastern part influenced by the Chukchi Sea and the Central Arctic Basin. A unique feature of the eastern part of the East Siberian Sea was the existence of an extended (around 150 km in the meridional direction) region of water nearly homogeneous in density, where vertical variations in salinity from the surface to the bottom were only from 28.1 to 28.2 PSU. Clearly, in this area, seasonal cooling causes vertical convection that encompasses the entire water column. Observations in the mouth of the Khatanga River and the adjacent shelf revealed the presence of an estuarine front with a longitudinal extent of around 30–40 km. Study of the optical properties of water in the East Siberian Sea has made it possible for the first time to estimate the latitudinal extent of the anomalously turbid near-bottom layer with a Secchi depth from 27 to

Ecosystems of the Siberian Arctic Seas-2017 (Cruise 69 of the R/V Akademik Mstislav Keldysh)

Results of the studies of the spatiotemporal variability in the hydrophysical and hydrodynamical structure of the shelf area and its dependence on the water dynamics of the deep-water zone of the Black Sea off Gelendzhik are presented and discussed. The field materials were collected in early October of 2006 with the use of traditional and new measurement techniques. A combined analysis of on-line satellite oceanological information, data of onboard surveys, and data time series of the vertical profiling of the hydrophysical characteristics at a moored buoy station was performed.

Studies of the hydrophysical processes over the shelf and upper part of the continental slope of the Black Sea with the use of traditional and new observation techniques

The ecosystems of the maritime Arctic have undergone pronounced restructuring over the past several decades, which is largely caused by global and regional climate changes, as well as by various anthropogenic impact. Many Arctic regions have accumulated high ecological risks. Geopolitical, economic, and ecological considerations determine the need to know which changes in the Arctic natural complexes should be expected in the near future. All of the above generates strong interest in studying Arctic ecosystems. The current state of both climatic and anthropogenic changes in the Arctic Basin largely depend on the processes that take place on the shelves of epicontinental Arctic seas. The shelves are zones of interaction between the Arctic Basin and continents, regions from where huge volumes (2300–2500 km3 per year) of continental runoff f low into the Arctic and where the runoff undergoes transformation. Over the Arctic Shelf waters form which lately enter the Central Arctic Basin. The process determines the transfer of nutrients, suspended and dissolved matter, and allochthonous materials, including pollutants brought to the shelf by river runoff. Russia, the largest Arctic state, whose jurisdiction embraces more than 70% of the Arctic Shelf, is destined to play a significant role in Arctic studies. Moreover, this has been established by the Strategy for Developing the Arctic Zone of the Russian Federation and Ensuring National Security until 2020, signed by the President of the Russian Federation on February 20, 2013.

S. G. Poyarkov

Papers

The upper desalinated layer in the Kara Sea

Circulation in the southwestern part of the Kara Sea in September 2007

Ecosystems of the Siberian Arctic Seas-2017 (Cruise 69 of the R/V Akademik Mstislav Keldysh)

Studies of the hydrophysical processes over the shelf and upper part of the continental slope of the Black Sea with the use of traditional and new observation techniques

Ecosystems of the Russian Arctic-2015 (63rd Cruise of the research vessel Akademik Mstislav Keldysh)