Climatic control on river chemistry in Yangtze River Basin: Cooling forced Cenozoic oceanic Sr/Ca and Mg/Ca evolution?

Abstract:

Rivers act as a primary source of oceanic solute, therefore variations of riverine chemistry may cause variations in ocean chemistry. In order to investigate short-term climatic control on river chemistry, riverine samples from Yangtze River were collected in Nanjing twice a month during May, 2010 to April, 2011. Major ions (Cl, SO₄, NO₃, Ca, Mg , K, Na, Sr ), ⁸⁷Sr/⁸⁶Sr ratios and magnesium isotopic composition in the dissolved load were determined. Results show that in the monsoon season, ⁸⁷Sr/⁸⁶Sr, Sr/Ca and Mg/Ca ratios were lower than those ratios during the dry season, implying that carbonate weathering is relatively intense during high discharge periods. But variations in Sr/Ca and Mg/Ca ratios are more significant than the changes in ⁸⁷Sr/⁸⁶Sr and δ26/24Mg ratios (Fig. 1), and are both negatively correlated with discharge (Fig. 2). This can’t be explained by changes in mixing portions of silicate and carbonate weathering products, but by that more secondary carbonate (low Sr/Ca and Mg/Ca ratios) precipitates when discharge is lower. Because Mg is hardly incorporated into secondary carbonates, this process exerts little control on riverine δ26/24Mg ratio.

If the riverine chemistry change during the late Cenozoic cooling is similar to the regime above, the riverine Sr/Ca and Mg/Ca would been elevated, causing large increase in oceanic Sr/Ca and Mg/Ca ratios but stability in oceanic δ26/24Mg ratio.