## 4 Discussion
It was found that muscovite is widely distributed with moderate contents ranging from 5.7% to 2.5%, while biotite contents were lower, with the highest value of only 1.5% in the silty fraction. In addition, [PERSON] et al. proposed that Yellow River materials were characterized by abundant mica minerals [25]. Biotite is one of the main minerals with high contents of up to 64.7% in underwater deltas. Muscovite is a secondary mineral and its content could reach approximately 5%. They also suggested that a high particle percentage ratio (value of 8.97) of biotite to muscovite could serve as a mineralogical indicator of Yellow River materials [25]. [PERSON] et al. also found that the biotite content in Yellow River sediments (up to 23.36%) was much higher than that in Yangtze River sediments (very low) [55]. It was further suggested that mica in the 0.063-0.125 mm fraction of Yellow River Estuary sediments was the main dominant heavy mineral because its particle percentage reached 39.2% [32]. They also found that the muscovite content (17.6%) was higher than that of biotite (16.3%) and hydrobiotite (5.3%) [32]. Figure 6: Comparison of the detrital mica contents (**a**) and ratios (**b**) in the very fine sand fraction of the sediments between the Yangtze River Estuary and ancient Yellow River Estuary. (**a**) shows the differences of mass percentage of mica (in blue) and particle percentages of biotite (in red), muscovite (in green), weathered mica (in purple) and sum of biotite and muscovite (in cyan). (**b**) shows the differences of particle percentage ratio of weathered mica to the sum of biotite and weathered mica (in blue), particle percentage ratio of the sum of biotite and weathered mica (in red), and particle percentage ratio of biotite to muscovite (in green). In river basins, geological conditions, climate types and hydrodynamic conditions notably impact minerals in sediments [7; 56], including detrital minerals. Thus, the causes of detrital mica differences in sediments between these two large river estuaries should be related to these factors (Figure 7). First, compared to the Yellow River drainage area, the exposed rocks in the Yangtze River drainage area are richer in mica. In the Yangtze River drainage area, granite, schist and gneiss are widely distributed in the middle-to-lower stream sections. This leads to a high content of detrital mica in Yangtze River sediments. However, in the Yellow River drainage area, loess widely covers the midstream area and is the main provenance of Yellow River detritus [35]. The mica content is approximately 18.4% in the \(>\)20 \(\upmu\)m fraction of loess paleosol, with a maximum value lower than 26% [57]. This indicates that the mica content in the very fine sand fraction is not overly high in Yellow River sediments. Hence, the difference in geological conditions between provenance areas is an important reason why the mass content of mica in the very fine sand fraction of Yangtze River Estuary sediments is significantly higher than that in the very fine sand fraction of ancient Yellow River Estuary sediments. Second, compared to the Yellow River, the climatic conditions in the Yangtze River drainage area are more favorable for weathering. These climatic conditions include abundant precipitation and a high temperature in most areas of the Yangtze River drainage area, except for the upstream area. However, the region is dry and cold, with a lower annual precipitation and poor hydrothermal conditions in the Yellow River drainage area. Research has demonstrated that the chemical weathering rate (61.58 t/ km\({}^{2}\cdot\)a) in the Yangtze River drainage area is significantly higher than that in the Yellow River drainage area (39.29 t/ km\({}^{2}\cdot\)a) [58]. Strong chemical weathering in the Yangtze River drainage area inevitably leads to the release of a large number of mica minerals in igneous and metamorphic rocks, such as granite, into the river.