Oxygen isotope ratio and other chemical tracers are used to distinguish water mass modification processes in the Sea of Okhotsk and the Arctic Ocean. Distributions of tracers show that formation of the dense water during sea ice formation is a key process in both regions. This process freshens, cools, and ventilates water below the surface. In the Sea of Okhotsk, however, diapycnal mixing in the Kuril Straits is found to contribute more than the dense water formation to form the Okhotsk Sea Intermediate Water. This process will also affect the properties of North Pacific Intermediate Water, which receives anthropogenic CO₂ and freshwater by mixing with Okhotsk Sea Intermediate Water. In the Arctic Ocean, formation of dense water and distribution of fresh water determine the salinity distribution in the upper part of the ocean. A newly investigated tracer, alkalinity, reveals that Siberian river runoff and meteoric water from the Bering Sea are the main freshwater sources, even in the Canada Basin. Historical alkalinity and oxygen isotope ratio data from the past 70 years are combined to draw distributions of sea ice meltwater/brine and other freshwaters for the entire Arctic Ocean. Accumulation of freshwater with brine in the Canada Basin indicates that this is the region where massive amount of freshwater enters, and a large portion of freshwater is converted into sea ice.