Seasonal Mg isotopic variation in the middle Yellow River: Sources and fractionation

In order to better understand how stable metal isotope signals in large rivers can be used to constrain present and past weathering, the seasonal riverine Mg-Sr isotopic pattern in the middle Yellow River was systematically investigated based upon weekly collected samples for the whole year of 2013. The results demonstrate that Mg is mainly transported in the dissolved form (65%) in this river system and that 45% of the total dissolved Mg is transported during the monsoon seasons, with 2% exported over 4 days during a single storm event. Dissolved Mg in the middle Yellow River is dominantly derived from both silicate and carbonate (82-89%) in this semi-arid region, with limited evaporite contribution (similar to 7%). Lithological mixing is the first order control on riverine dissolved Mg and Sr isotopes, with a contribution from similar to 40% carbonate dissolution and similar to 60% from silicate dissolution in the dry seasons, and similar to 50% carbonate and similar to 50% silicate during the monsoon seasons according to delta 26Mg signals. Furthermore, a significant role of prior calcite precipitation (PCP) can be quantified, which fractionates Mg isotopes by about 0.17%o to 0.39%o positively depending on the choice of elemental and isotope partition of Mg in secondary carbonates. Clay formation following the PCP further fractionates riverine Mg isotopes to the negative side. An similar to 0.2%o decrease of riverine Mg isotopes is attributable to (1) a single storm event causing carbonate dissolution and (2) delayed delivery of depleted waters to rivers (similar to 3 months after the storm event) because of subsurface hydrological circulation. Annually, the weighted average riverine delta 26Mg (- 1.05%o) in the middle Yellow River is identical to the global average (-1.09%o). Despite the significant impact of lithology on the riverine dissolved Mg isotope signature, the mixing proportions of different Mg sources remain virtually constant, even when there are huge contrast of temperature, hydrology, and precipitation seasonally along the year, providing a basis for dissolved delta 26Mg response to climatic forcing on the continental scale. This means that significant changes in the sedimentary Mg isotope records would reflect extreme conditions in deep time.