As the planet warms, the winter season becomes shorter and warmer. The consequences of these changes are particularly problematic in Alpine regions. These areas are deprived of the time and temperature necessary to accumulate an adequate snowpack to supply the valleys with fresh water throughout spring and early summer. Understanding how the snowpack has changed is critical, but snow is fleeting and the research community lacked a proxy to record and reconstruct seasonal river flow.
Today, a team of Japanese researchers are using the geochemical signature obtained from the shells of freshwater bivalves to find this essential information. The results of their study, published in Paleogeography, Paleoclimatology, Paleoecology, produced a nearly 7-decade seasonal record of river flow in northern Japan.
“The indirect and / or geological data that could reconstruct the environmental conditions of the river in the past are very limited,” said Tsuyoshi Watanabe, lecturer at Hokkaido University and first author of the study. “I was surprised that [the shells of] these long lives [mussels] could capture very detailed information about river environments [at a] daily scale.
For more than 4 decades, geochemical records obtained from the shells of bivalves, microplankton and corals have proven to be effective indicators in reconstructing past environmental conditions, including temperature, salinity and dissolved oxygen. In this study, Watanabe and his team focused on the freshwater mussel species Margaritifera, which has a pronounced lifespan of up to 200 years. Previous studies have shown that annual growth patterns for this species reflect temperature and precipitation during the summer months. Watanabe and his team set out to determine how the timing and chemistry of the growth bands on the shell material correlated with the large amount of environmental data for the region around the Shiribetsu River, where mussel colonies are common.
In mid to late spring, the snow along Mount Fure in Hokkaido melts and flows through various streams that merge and empty into the Shiribetsu, which eventually empties into the Sea of Japan. Snowmelt contributes almost half the volume of the river, causing a seasonal change in the volume, temperature and chemistry of the water. Shiribetsu has been monitored for water temperature, pH, turbidity, chemical oxygen demand, conductivity, and dissolved oxygen content since 2001 and for water volume since 1965.
Like the trees, Margaritifera grows by depositing material along defined lines. Watanabe and his team have established a timeline for the species M. laevis by counting the growth pattern over 12 shells, yielding a history of 67 years. They applied laser ablation by induction coupled plasma mass spectrometry along the growth line to obtain the concentration of three isotopes in the shell material: magnesium-24, manganese-55 and barium-138. Each element has been normalized to calcium-43 and represented by a molar ratio. Each molar ratio was regressed based on long-term environmental factors of the river, as well as precipitation and maximum snow depth.
Barium reflects the flow of the river
The researchers found that river flow had the greatest influence on the growth lines of the shells. Additionally, the highest and most pronounced peaks occurred in the barium data in early spring. While the environmental source of the barium remains uncertain, the peak roughly matches the spring meltwater pulses.
The team also found that the magnesium concentration is low in the spring and increases as the summer progresses. They were unable to draw any conclusions from this signal due to the complexity of the patterns for magnesium, as manganese concentration was closely related (negatively) to the seasonal recording of dissolved oxygen, suggesting that it reflects redox conditions.
“Great potential for this type of proxy”
“There is great potential here for this type of proxy,” said Paul Butler, senior geography researcher at the University of Exeter who did not participate in the study. “But if you want to look at the changes that took place before modern warming started, you have to date the dead seashells in the timeline, and for that you have to get material from the sediments of the river bed or use dated specimens. museums. Even if you can find them, this technique requires the use of multiple samples that overlap in time to distinguish a significant common signal.
Watanabe and his team made some initial comparisons between mussel data and several climate records that influence northern Japan, including the North Pacific Index and the Pacific Decadal Oscillation.
The comparison “indicated that the volume of the river could change on a decadal scale with the winter monsoon and the Pacific Decadal Oscillation,” Watanabe said. “If we could apply this method to long-lived and / or fossil samples to compare the recent and the past, perhaps we could assess how the freshwater supply will be influenced by recent global warming and / or future in this region. “
—Stacy Kish (@StacyWKish), science writer