## 4 Discussion  
Differences in Detrital Mica Contents in Sediments between the Yangtze River Estuary and Ancient Yellow River Estuary and the Causes of these Differences  
Detrital minerals in sediments differ with the original rock, weathering degree, tectonic activity, climate change and other supergene environmental conditions [2; 7], and micas are no exception. Their contents in large river sediments differ with watershed environments [7]. As shown in Figure 6, the contents and ratios of detrital mica in sediments between the Yangtze River Estuary and ancient Yellow River Estuary significantly differed. Specifically, the mass percentage of mica, particle percentage of weathered mica, particle percentage of muscovite, and particle percentage ratio of weathered mica to the sum of biotite and weathered mica in the very fine sand fraction of Yangtze River Estuary sediments were 2.47, 1.89, 1.15 and 2.06 times higher, respectively, than those in the very fine sand fraction of ancient Yellow River Estuary sediments. However, the particle percentage of biotite, particle percentage ratio of biotite to muscovite, and particle percentage ratio of the sum of biotite and weathered mica to muscovite in the very fine sand fraction of the ancient Yellow River Estuary sediments was 3.33, 4.05 and 1.28 times higher, respectively, than those in the very fine sand fraction of Yangtze River Estuary sediments. All of these results indicate that the mass percentage of mica and the particle percentage of weathered mica in the very fine sand fraction of the sediments in the Yangtze River Estuary were significantly higher than those in the very fine sand fraction of the sediments in the ancient Yellow River Estuary. The weathering degree of mica (the particle percentage ratio of weathered mica to the sum of biotite and weathered mica) was much higher than that in the ancient Yellow River Estuary. All of these results also suggested that the particle percentage of biotite and the particle percentage ratio of biotite to muscovite in the very fine sand fraction of the sediments in the ancient Yellow River Estuary were significantly higher than those in the very fine sand fraction of the sediments in the Yangtze River Estuary, although the weathering degree of mica was very low. Moreover, the particle percentage ratio of easily weathered mica (sum  
Figure 5: Mass percentage of mica and particle percentages of biotite, weathered mica and muscovite in the very fine sand fraction of Subei littoral plain area sediments (mass percentage of mica = 100 * total mica mass/total mineral mass. Particle percentage of biotite or weathered mica or muscovite = 100 * particle number of biotite or weathered mica or muscovite/total particle number of mica). of biotite and weathered mica) to difficult-to-weather mica (muscovite) in the very fine sand fraction of the sediments in the ancient Yellow River Estuary was also higher than that in the very fine sand fraction of the sediments in the Yangtze River Estuary. Researchers have previously reported similar characteristics, such as high mica and muscovite contents in the very fine sand fraction of Yangtze River Estuary sediments, a high biotite content and high particle percentage ratio of biotite to muscovite in ancient Yellow River sediments. For example, [PERSON] found that the content of schistoe mica minerals in the 0.063-0.125 mm fraction of sediments in the Yangtze River Estuary was very high, with a maximum value of up to 60% [26]. This value was thought to represent a basic feature of the heavy mineral assemblages in the very fine-grained (0.063-0.125 mm) sediments of the Yangtze River Estuary [26]. [PERSON] and [PERSON] also proposed that mica is one of the dominant minerals in Yangtze River Estuary sediments [33]. The highest mica content reached 32.4%, with a content of 17.1% in the silt fraction. [PERSON] et al. noted that muscovite is one of the characteristic minerals of underwater delta sediments in the Yangtze River [54].