The Arctic environment is changing substantially and rapidly owing to global warming. Sea ice, ice sheet, glacier, and frozen soil in the Arctic are melting and retreating year by year. The changes in the Arctic are thought to accelerate the global warming rate (positive feedback effect on global warming), and affect ecological changes. We must interpret these signs as the last warning to our sustainable human life. Geochemical society should show evidences and future prospect of changes of the Arctic environment. The aim of this special issue “Geochemistry of the Arctic” is to broaden the history and current trends of the Arctic geochemical research to the future scientists, members of the Geochemical Society of Japan, and other scientific societies.

Due to global warming, the Arctic has been changing drastically and rapidly. The changes in the Arctic cryosphere affect not only the Arctic climate and environment but also the global climate system. There is an urgent need to improve the projections of future Arctic climate and environment, including mass loss of the Greenland ice sheet, which affects the global sea level, ocean circulation and global climate. To achieve these goals, we need to advance ice sheet and climate modeling. Long-term records of the past Arctic warmings and their impacts, and the understanding of the mechanisms are necessary. Arctic ice cores have been providing us with valuable information on different time-scales from decadal to orbital time-scales. For example, deep ice cores from Greenland have revealed abrupt warming events in the glacial and deglacial periods and their links to global environmental changes. Multiple ice cores from the Arctic have been used to reconstruct the elevations of the past Greenland ice sheet. Shallow ice cores from circum-Arctic ice caps and Greenland have shown anthropogenic increases of acids, toxic metals etc. after the industrial revolution. This paper briefly reviews the history of ice core studies in the Arctic and discusses future prospects.

In the Arctic region, the glacier and the sea ice are melting rapidly. This melting process leads to the drastic changes of the surface ocean environments by supplying fresh water and biogeochemical components. For example, the glacier melting provides minerals and nutrients from the land to the ocean, and these processes were depending on the type of the glacier between land- and marine-terminating glaciers. In addition, sea ice melting dilutes the chemical components (e.g., CO₂) in the surface water, leading the decrease of the CO₂ concentration with respect to the atmosphere, and increases the absorption of the atmospheric CO₂. In this review, we assess the effects of glacier and sea ice melting on the biogeochemical cycles of the surface water (e.g., nutrients, iron, CO₂, and CH₄) in the Arctic Ocean based on the previous and our studies. We also discuss future biogeochemical cycles in the Arctic Ocean.

Since 1980’s many of atmospheric chemists observed the arctic surface atmosphere, and they concluded that ozone depletion in the arctic air in polar sunrise is caused by catalytic reaction of Br atom originated from the ocean. However, production processes of Br atom are not well known yet. Photolytic reaction at the surfaces of sea-salt and sea-ice is believed to be the main source of Br atom. Photolysis of organic bromine compounds such as bromoform (CHBr₃) produced by macro algae and micro algae should contribute to the Br atom supply. Some studies proposed that CHBr₃ is produced by abiotic reaction at the interface between sea-ice and snow on the sea ice. This review summarizes studies of ozone depleting Br compounds and introduce some recent studies concerning organic bromine compounds.

Also, an organic sulfur gas, dimethyl sulfide (DMS) plays potential roles in atmospheric chemistry and Earth’s climate. DMS is produced from the decomposition of dimethylsulfoniopropionate (DMSP), which is formed within marine algae cells. Recent climate changes, especially in the Arctic Ocean, induce transition of marine biota and its physiology, implying the production/consumption of the marine biogenic compounds will change following the climate change. Here, we also review sulfur cycle in the Arctic Ocean including sea ice zone.

Arctic Ocean is considered especially vulnerable to ocean acidification. Here we summarize current understandings of reasons of the vulnerability, state and future perspective of the ocean acidification in the Arctic Ocean, with an emphasis on regional variability. In general, Arctic Ocean has low calcium carbonate saturation state (Ω) and low pH buffer capacity to added carbon dioxide, because of its low temperature and dilution by various freshwater sources. In coastal shelf area, local physical and biogeochemical processes, such as river discharge and high biological activity, characterize Ω and pH in each region. Recent climate change is affecting each of these processes, to complicate ocean acidification in the Arctic Ocean. Despite the increasing attention, long-term observations are still insufficient for most part of the Arctic Ocean. Understanding and monitoring of Arctic Ocean would provide much knowledge about biogeochemical consequences of ocean acidification and concurrent climate change, in order to better predict future of our ocean.

This review demonstrates carbon cycle and methane dynamics in the Arctic ecosystem, which plays an important role in carbon dioxide and methane emissions because of large stocks of soil organic matter, rapid Arctic warming, and permafrost thaw. Exchange of carbon dioxide between terrestrial ecosystem and the atmosphere is determined by the balance between ecosystem respiration and primary production. Ecosystem respiration can be enhanced not only by soil temperature increase during the summer but also that during the autumn and the winter. Vegetation change is a highly uncertain factor on primary production. Some observations reported increase of shrubs which are important species in Arctic carbon cycle. Tree line is predicted to shift northward under Arcitic warming, and observational studies suggest that the shift depends on seed dispersal or microtopography and hydrology of permafrost ecosystem. Effects of climate change such as snow increase on vegetation have been also investigated by in situ manipulation experiments.

Effect of permafrost thaw on methane emission has been assessed by observing methane flux across sites with different permafrost conditions. Satellite imagery can be used to evaluate changes in wetland area by permafrost thaw and to estimate methane emission changes on local or regional scales. Production, oxidation, and transport processes of methane, which are necessary for understanding the relations between methane emission and environmental factors, can be assessed by carbon and hydrogen isotopes of methane.