Recent advances in clean technique and highly sensitive analytical technique made it possible to analyze trace elements and their isotopes (TEIs) in open ocean waters. Now, it is realized that many trace metals are important for marine primary production, and some trace element isotopes are useful as tracers for marine biogeochemical cycles. To accumulate high-quality data of TEIs in the ocean at global scale, International Study of the Marine Biogeochemical Cycles of Trace Elements and Their Isotopes (GEOTRACES) started in 2000s. Three of intermediate data products have been already published, and the accomplishments of GEOTRACES are highly evaluated now. In this special issue, Japanese Geotracers reviewed these recent progresses in marine geochemistry of TEIs and indicated remaining problems in this area. We hope this special issue will deepen our present understanding of marine biogeochemical cycles of TEIs.

Recently, the biogeochemical cycles of trace elements and their isotopes (TEIs) in the ocean have been investigated intensively. The main driver of this rapid improvement is GEOTRACES project (An international study of the marine biogeochemical cycles of trace elements and their isotopes). The development and implementation of clean technique and advances in analytical chemistry made it possible to reveal TEIs distributions precisely and give us new insights about the processes controlling TEIs in the ocean. In this review, we give an overview of recent findings in GEOTRACES by focusing on three main themes, (1) fluxes and processes at ocean interfaces, (2) internal cycling of TEIs and (3) development of proxies for past change. We also introduce the recent achievements by GEOTRACES-Japan in which Japanese marine geochemistry community has been actively involved. The project is still on the halfway, but the findings have certainly deepened our understanding of marine biogeochemical cycles of TEIs.

In GEOTRACES, a great deal of effort is put into compiling trace elements and isotopes data observed in all ocean basin by researchers all over the world to create a single global data set. GEOTRACES does not wait for the final data product at the end of the program, but produces intermediate data products (IDP) as the program progresses over a period of time. This review paper introduces the contents of the latest IDP published in 2021 and provides an overview of the activities that have been undertaken to build a global dataset for the GEOTRACES project, including data collection, data quality control, and the development of terms of reference for data use.

Radiocarbon (¹⁴C) as a tracer for (1) the turnover time of the deep ocean and (2) CO₂ gas exchange between the atmosphere and surface ocean is reviewed. Just after the discovery of radiocarbon in the late 1940s, oceanographers realized that the distribution of radiocarbon in the ocean could reveal the turnover time of the deep ocean, which was debated for a long time. GEOSECS (Geochemical Ocean Section Study) in the 1970s achieved success in more than two thousand measurements of radiocarbon around the world ocean. And the radiocarbon data showed that the turnover time of the deep ocean was estimated to be on centennial timescales (500–1000 years). The radiocarbon from GEOSECS also revealed the global mean of the CO₂ exchange at the sea surface. The mean exchange rates estimated from the natural- and bomb-produced radiocarbon agreed well. However, the exchange rate derived from the GEOSECS radiocarbon data was found to be overestimated by about 25% by more radiocarbon data from WOCE (World Ocean Circulation Experiment) in the 1990s. As a result, the empirical equation for the relationship between the gas exchange rate, which depends on the radiocarbon budget in the ocean, and wind speed has been revised downward too.

Neodymium (Nd) isotopic compositions of ferromanganese (Fe–Mn) crusts, bulk sediment leachates, foraminiferal Fe–Mn hydroxides, biogenic apatite, and deep-sea corals have been used as a proxy of Nd isotopes in bottom seawater to reconstruct continental weathering and ocean circulation and mixing changes over various time scales. Proper understanding and appropriate use of this proxy are the basis for more detailed and accurate reconstructions of paleoceanographic variability recorded in marine sediments. In this review, I summarize current knowledge on the behavior of Nd isotopes in the modern ocean and the characteristics of bottom-water Nd isotope proxies to further advance the study of past ocean circulation. In addition, several noteworthy Nd isotope studies are presented to provide basic knowledge to researchers who will work on paleoceanography using Nd isotopes.

Isotope ratios of trace metals in the ocean have been used as tracers to understand the sources of trace metals and their biogeochemical processes. This paper focuses on iron (Fe), nickel (Ni), copper (Cu), zinc (Zn), and cadmium (Cd), which are essential or toxic to marine organisms, and reviews the isotopic distribution of these metals in the ocean and the factors controlling their distribution. The distribution of Fe isotope ratios (δ^56/54Fe) varies greatly among ocean regions. This is explained by the supply of Fe from different sources (atmospheric aerosols, sediments, and hydrothermal vent). The isotope ratios of Ni and Cd in the ocean (δ^60/58Ni and δ^114/110Cd) are globally constant in the deep water below 1000 m, and increase with decreasing their concentrations in the surface water above 1000 m, suggesting preferential uptake of lighter isotopes by phytoplankton. The Cu isotope ratios (δ^65/63Cu) in the ocean show large variations in both the surface and deep waters, which are accounted for phytoplankton uptake and atmospheric aerosol deposition in the surface water and adsorption on particles in the deep water. The isotope ratios of Zn (δ^66/64Zn) in the ocean are almost constant in the deep water, and shift positively and negatively in the surface water. This variation is explained by phytoplankton uptake, adsorption on biogenic particles, and input of anthropogenic substances.

Dry and wet depositions of aerosol iron (Fe) is one of the dominant sources of dissolved Fe in surface seawater. Although much effort has been paid to understanding the factors controlling fractional Fe solubility in aerosol particles, their uncertainties still remain. This paper summarizes the factors controlling the fractional Fe solubility in aerosol particles (e.g., emission sources and atmospheric processes) based on the observational results of size-fractionated aerosol particles collected in the marine atmosphere. In particular, we mainly focus on two topics: (1) source apportionment of Fe using the Fe isotope ratio and (2) chemical alteration processes of Fe-bearing particles in the ambient atmosphere. Finally, future perspectives and strategies for field observation of trace metals in aerosol particles will be discussed.

This review summarized the current status of trace metals research in the Cryosphere and identified subjects of research that need to be addressed. This paper outlined the characteristics and origin of bioactive trace metals (Bioact TMs) in the ice sheet, glaciers, and sea ice with focusing on the Antarctic region. The melting of glaciers and sea ice has a non-negligible impact on the distribution of Bioact TMs concentrations in seawater. Uncertainties in Bioact TMs fluxes associated with these ice melting remain large. Studies are needed to quantify Bioact TMs fluxes via meltwater accurately, including clarification of the removal mechanisms of Bioact TMs when the freshwater mixes into seawater. This paper also outlined that the melting of glaciers and sea ice affects primary production in the euphotic zone by enhancing upper-ocean stratification. Yet, the impact of ice-derived Bioact TMs on primary production is not well understood. Studies are needed to clarify the bioavailability of Bioact TMs supplied by meltwater, especially for the particulate form. The Antarctic ice sheet and glaciers are currently losing mass due to the submarine melting and calving of ice shelves, as well as the ocean warming. Although no statistically significant change in the sea ice extent in the Antarctic region to date, it will begin to decline in the next hundred years, given the current status of the ocean warming. Research will become important to investigate how melting of ice affects the biogeochemical cycles of Bioact TMs and how it changes the marine ecosystem.

Platinum group elements have low abundance on the upper crust and are recognized as precious metals. Since their concentrations in the natural water samples are very low, solid analytical methods have not been established yet for seawater. Recent improvements in analytical techniques gradually made it possible to clarify their distributions and behaviors in the aquatic environments. Here, we will introduce the recent progresses in the analytical methods and compile the recent studies on platinum, palladium, and rhodium in estuaries and open ocean.

This review summarizes recent advances in studies of the distribution of total and speciated mercury (Hg) in the ocean including the Pacific, Atlantic and polar regions. In addition, clean sampling technologies and the analytical methods for trace Hg-species are also presented. The distribution of the Total Hg in the Pacific and Atlantic Oceans represents at “Scavenge-type” in subsurface layers and “Nutrient-type” in mesopelagic zone, whereas the distributions of Total Hg in the polar oceans are unique with higher concentrations near the surface due to the inclusion of melting sea-ice and brine. Mono-Methyl Hg (MMHg) and Methylated Hg (MeHg) represent at “Nutrient-type” distribution. They are correlated with the apparent oxygen utilization in the Pacific and the Southern Oceans, suggesting the Hg methylation is related to organic carbon remineralization by microbial respiration. However, this correlation is not significantly observed in the North Atlantic Ocean and the Arctic. In addition, mechanisms of MMHg and Di-Methyl Hg (DMHg) production in water column are still under discussion. A further understanding of the distribution of speciated Hg and their interactions in oceanic environments are required for establishing a global model which are able to predict the future Hg biogeochemical cycle including the influences by climate change.

Mercury is known as one of the highly toxic heavy metals and presents at low levels in the ocean. Especially methylmercury efficiently accumulates in fish, which leads to human and environmental health risks. Thus, information about the distribution and transformations of mercury and methylmercury in the ocean is important for understanding the accumulation process of mercury in marine biota. In marine environments, microorganisms (bacteria and archaea in this study) are involved in mercury transformations, i.e., methylation, demethylation, and reduction. Genomic and metagenomic studies have revealed the key genes for mercury methylation (hgcAB),demethylation (merB),and reduction (merA).The distributions of these genes in several marine environments have been reported. In this review, we introduce the current understanding of the distribution of total mercury and methylmercury in the ocean, the phylogeny and diversity of the genes involved in the mercury transformations, and highlight the distribution of these genes in the marine environments.

Vitamin B₁₂ (cobalamin) is a micronutrient involved in various metabolic processes in the ocean, such as the citric acid cycle and DNA synthesis. In oceanography, the distribution and function of vitamin B₁₂ have been investigated mainly in coastal areas since the 1950s, but the concentration level is extremely low at the 10-¹⁴–10-¹⁰ molar level, so, there are many unclear points about its oceanic cycle compared to major nutrients and trace metal elements. On the other hand, with the recent development of chemical analysis and gene analysis techniques, its importance is gradually reconfirmed. This paper outlines the findings obtained so far regarding the dynamics of vitamin B₁₂ in the ocean and extracts future issues for research on marine vitamin B₁₂.

Iron is an important micronutrient that controls primary production in the ocean. The iron cycle has been incorporated into many numerical models that predict future global warming as an important component. Accumulation of the GEOTRACES data regarding iron and its isotope has revealed various overlooked processes that are playing important roles on the marine iron cycle. In this paper, we describe the iron supply processes from the outside to the ocean and the cycle inside the ocean, which have recently been highlighted, and how they are incorporated into numerical models. We summarize the issues remaining and future perspectives for the development of marine iron cycle models.

Over the past 800,000 years, atmospheric partial pressure of carbon dioxide (pCO₂) during glacial periods was lower by about 90 ppmv than during interglacial periods. Changes in the ocean carbon cycle are recognized as the primary driver of the glacial-interglacial changes in atmospheric pCO₂. The changes in the ocean carbon cycle would result from multiple processes, including changes in CO₂ solubility, the redistribution of carbon by ocean biological pumps and ocean circulation, and carbonate sedimentation processes although the quantitative contribution of individual oceanic processes remains controversial. This paper reviews previous modeling studies of the glacial ocean carbon cycle, with a particular focus on environmental changes in the Southern Ocean. During glacial periods, geological records have shown that there were physical changes in sea ice, stratification, and ocean circulation, as well as biogeochemical changes in the ecosystem in the Southern Ocean. As the Southern Ocean is a region where deep waters ventilate to the surface ocean, its environmental changes affect the carbon cycle of the entire ocean and atmospheric pCO₂. By presenting a comprehensive study of these changes in comparison with the constraints of geological records, we discuss current challenges and prospects for understanding the mechanisms of past changes in the ocean carbon cycle.