As the planet warms, the winter season is growing shorter and warmer. The consequences of these changes are particularly problematic in alpine regions. These areas are being robbed of the time and temperature to accumulate an adequate snowpack to supply the valleys below with fresh water throughout the spring and early summer. Understanding how the snowpack has changed is critical, but snow is ephemeral, and the research community has lacked a proxy to record and reconstruct seasonal river flow.
Now a Japanese team of researchers is using the geochemical signature obtained from the shells of freshwater bivalves to find this critical piece of information. The results of their study, published in Palaeogeography, Palaeoclimatology, Palaeoecology, produced an almost 7-decade seasonal record of river flow in northern Japan.
“The proxy and/or geological records that could reconstruct the river environmental condition in the past are very limited,” said Tsuyoshi Watanabe, a lecturer at Hokkaido University and first author on the study. “I was surprised that [the shells of] these long-living [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 proxies to reconstruct 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 life span of up to 200 years. Previous studies have found that the annual growth patterns of this species reflect temperature and precipitation during the summer months. Watanabe and his team set out to determine how the timing and chemistry of growth bands on the shell material correlated to the vast amount of environmental data for the region around the Shiribetsu River, where colonies of the mussel are common.
In middle to late spring, snow along Hokkaido’s Mount Fure melts and flows through various streams that merge and feed into the Shiribetsu, which eventually flows into the Sea of Japan. Snowmelt contributes almost half of the volume of the river, driving a seasonal change in water volume, temperature, and chemistry. The 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 trees, Margaritifera grows by depositing material along defined lines. Watanabe and his team established a chronology for the species M. laevis by counting the growth pattern on 12 shells, producing a 67-year history. They applied laser ablation inductively 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 was normalized to calcium-43 and represented as a molar ratio. Each molar ratio was regressed against the long-term river environmental factors, as well as precipitation and maximum snow depth.
Barium Reflects River Flux
Researchers found river discharge has the greatest influence on shell growth lines. In addition, the highest and most pronounced spikes occurred in the barium data in early spring. Although the environmental source of the barium remains unclear, the peak roughly corresponds to pulses in spring meltwater.
The team also found that the concentration of magnesium is low in spring and increases as the summer progresses. They were unable to draw any conclusions from this signal because of the complexity of patterns for magnesium, whereas the manganese concentration was closely tied (negatively) to the seasonal dissolved oxygen record, suggesting it reflects redox conditions.
“Great Potential for This Kind of Proxy”
“There is great potential here for this kind of proxy,” said Paul Butler, a senior research fellow in geography at the University of Exeter who was not involved in the study. “But if you want to look at changes that took place before the onset of modern warming, you’d need to cross-date dead shells into the chronology, and for that you’d need to obtain material from riverbed sediments or use dated specimens from museums. Even if you can find those, this technique requires the use of multiple specimens that overlap in time to distinguish a significant common signal.”
Watanabe and his team did some initial comparisons between the mussel data and several climate records that influence northern Japan, including the North Pacific Index and the Pacific Decadal Oscillation.
The comparison “indicated that river volume could change in decadal scale with the winter monsoon and Pacific Decadal Oscillation,” said Watanabe. “If we could apply this method to long-lived and/or fossil samples to compare between the recent and past, we may be able to evaluate how the freshwater supply will be influenced by recent and/or future global warming in this region.”
—Stacy Kish (@StacyWKish), Science Writer