The Arctic Ocean has experienced rapid warming and sea ice loss in recent decades, becoming the first open-ocean basin to experience widespread aragonite undersaturation [saturation state of aragonite (Ω _arag ) < 1]. However, its trend toward long-term ocean acidification and the underlying mechanisms remain undocumented. Here, we report rapid acidification there, with rates three to four times higher than in other ocean basins, and attribute it to changing sea ice coverage on a decadal time scale. Sea ice melt exposes seawater to the atmosphere and promotes rapid uptake of atmospheric carbon dioxide, lowering its alkalinity and buffer capacity and thus leading to sharp declines in pH and Ω _arag . We predict a further decrease in pH, particularly at higher latitudes where sea ice retreat is active, whereas Arctic warming may counteract decreases in Ω _arag in the future.

Time series data from a sediment trap at approximately 1,000‐m depth in the northwestern South China Sea, July 2012 to April 2013, were used in combination with remote sensing and shipboard physical and biogeochemical data to quantify the effects of multiscale physical processes on sinking particle fluxes and the function of the local biological carbon pump. Total mass flux and the fluxes of its primary components—particulate organic carbon, calcium carbonate, opal, and lithogenic matter—exhibited similar temporal trends, with three conspicuous high‐flux events. These events occurred in the summer, autumn, and winter, attributable to upwelling, aerosol deposition, and the northeast monsoon, respectively. Together, these events accounted for only 40% of the total time but approximately 80% of the 10‐month total mass flux (21% during the upwelling event, 23% during the aerosol event, and 36% during the northeast monsoon event). Each event contributed its own unique biogeochemical overprint. For example, Calcareous phytoplankton benefited from both the upwelling and aerosol deposition events, but siliceous phytoplankton benefited more than calcifiers during the upwelling conditions. The biogenic sinking particles were primarily marine authigenic, while the lithogenic matter was mainly resuspended sediment from the neighboring slope. The high flux of lithogenic matter was due to scavenging by sinking organic matter and the ballasting effect, especially during the high‐flux periods. This study illustrates that episodic intraseasonal physical processes are as important as the most common major seasonal event—the northeast monsoon—in modulating the strength of the South China Sea biological carbon pump.

The international and interdisciplinary sea-ice drift expedition “The Multidisciplinary drifting Observatory for the Study of Arctic Climate” (MOSAiC) was conducted from October 2019 to September 2020. The aim of MOSAiC was to study the interconnected physical, chemical, and biological characteristics and processes from the atmosphere to the deep sea of the central Arctic system. The ecosystem team addressed current knowledge gaps and explored unknown biological properties over a complete seasonal cycle focusing on three major research areas: biodiversity, biogeochemical cycles, and linkages to the environment. In addition to the measurements of core properties along a complete seasonal cycle, dedicated projects covered specific processes and habitats, or organisms on higher taxonomic or temporal resolution in specific time windows. A wide range of sampling instruments and approaches, including sea-ice coring, lead sampling with pumps, rosette-based water sampling, plankton nets, remotely operated vehicles, and acoustic buoys, was applied to address the science objectives. Further, a broad range of process-related measurements to address, for example, productivity patterns, seasonal migrations, and diversity shifts, were made both in situ and onboard RV Polarstern. This article provides a detailed overview of the sampling approaches used to address the three main science objectives. It highlights the core sampling program and provides examples of habitat- or process-specific sampling. The initial results presented include high biological activities in wintertime and the discovery of biological hotspots in underexplored habitats. The unique interconnectivity of the coordinated sampling efforts also revealed insights into cross-disciplinary interactions like the impact of biota on Arctic cloud formation. This overview further presents both lessons learned from conducting such a demanding field campaign and an outlook on spin-off projects to be conducted over the next years.

Seasonal variations in the transports of total alkalinity (TAlk) and dissolved inorganic carbon (DIC) from the Lower Changjiang (Yangtze) River/Estuary to the East China Sea were investigated based on a series of field surveys in 2015–2017, including monthly samplings at Datong Station and seasonal mapping cruises in the Changjiang Estuary and the adjacent northwestern East China Sea. In comparison with historical data sets, the Changjiang TAlk flux varied around a nearly stable average over the past 55 years. This is much different from some American rivers, where TAlk export fluxes increased for a century long. To assess effects of riverine carbonate inputs on coastal carbonate chemistry, we compared several cases showing freshwater‐dilution‐induced decline in coastal aragonite saturation state (Ω_arag), including rainwater dilution and riverine water dilution. Without riverine carbonate inputs, the effect of a unit of salinity decrease (due to rainwater dilution) on Ω_arag was expected to be counteracted by a DIC removal of 10 μmol/kg relative to the baseline value along relevant conservative mixing line, when coastal Ω_arag was close to a critical value of 1.5. Considering terrestrial carbonate inputs from Changjiang, however, the freshwater‐dilution‐induced coastal Ω_arag suppression decreased by 12%. Our data also showed that more than 10% of wet‐season DIC flux discharged from the Changjiang Estuary was sequestered by biological activities in nearshore areas, while the TAlk flux was rarely affected. This biological alteration effectively transformed the terrestrial carbonate system from a feature of DIC:TAlk >1.0 to the usual seawater feature of DIC:TAlk <0.9.