Unlike the black currant (a small, sour berry often made into jellies and jams), the Black Current is a strong, northeast flowing ocean current between the Philippines and Japan. Also called the Kuroshio Current, this massive flux of heat, salt, and moisture from low latitudes up the western edge of the Pacific Ocean plays a pivotal role in the weather and climate of this region.
Today, the Black Current flows through a waterway called the Okinawa Trough to a number of marginal seas, such as the East China Sea and the Yellow Sea. However, some studies suggest this may not have always been the case. For example, dips in sea level and a theorized land bridge between Taiwan and Japan’s Ryukyu Islands during the Last Glacial Maximum (when ice sheets were most recently at their greatest extension, around 30,000 years ago) could have allowed the current to bypass the trough, and these seas, completely. Meanwhile, other recent studies say such a shift is unlikely to have happened.
To further decode the Black Current’s murky past, Lim et al. examined samples of mercury from cores of sediment extracted from several active hydrothermal vents in the seafloor of the Okinawa Trough. Each core is a time capsule from the past 20,000 years; the mercury encased within each one was deposited by natural sources, such as volcanic and geothermal activity, whereas modern mercury levels are influenced by human activity, such as fossil fuel combustion.
By comparing the levels of mercury in these cores to modern levels, the researchers were able to probe past climate and geological changes in the region.
First, they found anomalously high levels of mercury in the sediment deposits from the Holocene epoch, which encompasses the past 12,000 years. Of particular interest was a sharp spike around 10,000 years ago. Because most of the mercury in the samples probably came straight from the hydrothermal vents on the ocean floor, the researchers think this spike must have been caused by the Black Current passing through the area, triggering deepwater circulation.
Furthermore, they noticed a big dip in the mercury levels around 3,000–5,000 years ago, which aligns with a known cold period associated with the regrowth of glaciers during the late Holocene. This suggests the Black Current was temporarily interrupted by the cold period, weakening or halting deepwater circulation.
Overall, the researchers found substantial evidence to support the idea that the Black Current experienced changes in its intensity and route during past glacial and interglacial periods. Not only did the study show hydrothermal mercury to be an effective tool for reconstructing paleoenvironmental conditions, but it shed light on the prehistory of a major ocean current underlying East Asian climate and many aquatic ecosystems. (Paleoceanography, https://doi.org/10.1002/2017PA003116, 2017)
—Sarah Witman, Freelance Writer