# A preliminary study on the inorganic carbon sink function of mineral weathering during sediment transport in the Yangtze River mainstream

### CaO, MgO, calcite and dolomite contents of suspended sediments in the Yangtze River and its main tributaries

Figure 2 shows a declining tendency of the CaO + MgO and calcite + dolomite of suspended sediments in the mainstream of the Yangtze River from upstream to downstream. The total CaO + MgO contents along the Yangtze River were as follows: Tuotuo River, 16.33%; Yibin, 11.43%; Cuntan, 10.35%; Yichang (below the Three Gorges Dam), 6.17%; Wuhan Industrial Port, 6.61%; Datong, 4.80%; and Wusongkou, 4.60%. The total calcite + dolomite contents also decreased along the river as follows: Tuotuo River, 16.8%; Yibin, 9.1%; Cuntan, 6.2%; Yichang, 4.1%; Wuhan Industrial Port, 7.4%; Datong, 4.2%; and Wusongkou, 1.5%. The three stations closest to the estuary were dominated by dolomite and free of calcite.

Due to the dramatic terrain changes in the upper reaches of the Yangtze River, strong gravity erosion and physical weathering, the CaO + MgO and calcite + dolomite contents were high in the mainstream section of the Jinsha River above Yibin and decreased from the headwater to the estuary, which clearly illustrated the dissolution of calcium-magnesium silicate and carbonate minerals during the process of sediment transport in the river. The declining rates of CaO + MgO and calcite + dolomite contents in the upper reaches of the Yangtze River above Yichang were 0.12% / 100 km and 0.29% / 100 km, respectively, while the rates below Yichang were 0.09% / 100 km and 0.15 % / 100 km, respectively. The declining rate of calcite and dolomite contents was higher than that of CaO + MgO contents, which indicated that carbonate minerals were more likely to be dissolved than calcium-magnesium silicate minerals during sediment transport. Because the upstream reaches had greater slope gradients, faster flow velocities and consequently higher mineral dissolution rates, the CaO + MgO and calcite + dolomite contents of suspended sediments in the upper reaches had a higher declining rate than those in the middle and lower reaches.

The CaO + MgO and calcite + dolomite contents of the suspended sediment in the main tributaries of the Yangtze River were as follows: Minjiang River: 9.75% and 6.3%; Jialing River: 5.40% and 5.5%; Wujiang River: 10.87%; Xiangjiang River: 4.75%; Hanjiang River: 4.41%; and Ganjiang River: 2.87% (only CaO + MgO contents and no calcite + dolomite content data for the last four tributaries). Except for the Wujiang River, CaO + MgO contents and calcite + dolomite contents in the Minjiang River were higher than those in other tributaries and close to the Jinsha River (Yibin site) because the Minjiang River basin had similar environments for erosion and sediment transport to the Jinsha River Basin. The CaO + MgO contents of suspended sediments in the Wujiang River were quite different between July 2003 and July 2007, ie, 6.67% and 15.06%, respectively. The relatively high contents might be due to its widespread distribution of carbonate rock.

### Carbon sink capacity during suspended sediment transport in the Yangtze River mainstream

According to Eqs. (1) – (6), the calculated TCS capacity (C1) decreased gradually from the headwater to the estuary (Fig. 3) in the following order: Tuotuo River, 0.271 t / t; Cuntan, 0.151 t / t; Yichang, 0.117 t / t; Wuhan Industrial Port, 0.127 t / t; Datong, 0.092 t / t; and Wusongkou, 0.091 t / t (Table 2). As CaO + MgO contents decreased, so did the TCS capacities. This result verified that CO2 was consumed by the dissolution of Ca – Mg minerals during sediment transport from upstream to downstream and that a carbon sink function existed. The TCS capacities at Cuntan and Wusongkou were 0.151 t / t and 0.091 t / t, respectively. A total of 0.060 tons of CO2 per ton of suspended sediment was dissolved during transport from Cuntan to the sea.

The SCS capacities (C3) of silicate minerals in the sediment were in the range of 0.027–0.047 t / t and had little variation, except for the Tuotuo River (0.061 t / t). The NSCS capacity (C2) was consistent with the variation in the TCS capacity, and both had a gradual decreasing tendency from the headwater to the estuary (Fig. 3), as follows: Tuotuo River, 0.210 t / t; Cuntan, 0.104 t / t; Yichang, 0.078 t / t; Wuhan Industrial Port, 0.097 t / t; Datong, 0.065 t / t; and Wusongkou, 0.051 t / t. The silicate carbon sink capacity (SCS) was smaller and showed a smaller reduction than the NSCS along the Yangtze River, mainly due to the slower dissolution rate of silicate minerals or greater contribution of silicate rock clastics from the watersheds in the middle and lower reaches, which also limited CO2 consumption in comparison with carbonate minerals. Obviously, the decrease in TCS capacity from upstream to downstream was due to the intense dissolution of carbonate minerals in the Yangtze River.

An “ideal mainstream segment” refers to a segment where there was no sediment input from the tributaries or the amount of sediment supply from the tributaries was equal to the amount of sedimentation in the segment (meaning that the suspended sediment yields at the inlet and outlet were similar), and the calcium and magnesium mineral contents of suspended sediments in the tributaries and mainstream were similar. The carbon sinks via CO2 consumption by dissolution of calcium and magnesium minerals in the sediment transport process can be expressed as follows:

$${ text {Wt}} _ {{text text {h}} 1 – 2}} = { text {Ws}} _ {{text text {h}} 1 – 2}} times left ({{ text {C}} _ ​​{{text text {h}} 1}} {-} { text {C}} _ ​​{{text text {h}} 2}}} right)$$

(7)

where Wth1-2 is the consumed CO2 via the dissolution of calcium and magnesium minerals in the segment (h1–H2) (104 t / yr); WSh1-2 is the mean suspended sediment transported at the segment (h1–H2) (104 t / yr); Ch1 is the carbon sink capacity of the suspended sediment at the inlet of the segment (t / t); and Ch2 is the carbon sink capacity of the suspended sediment at the outlet of the segment (t / t).

The Yangtze River has a drainage area of ​​1785 × 106 km2in which the upper reaches above Yichang have an area of ​​1.05 × 106 km2. The average sediment yield from 1956 to 2000 was 5.01 × 108 t / yr at Yichang, while it was 4.33 × 108 t / yr at Datong, with a drainage area of ​​1,705 × 106 km2. In addition, it was 4.39 × 108 t / yr at Cuntan13 with a drainage area of ​​0.867 × 106 km2. Although there is a large drainage area of ​​the river segment between Cuntan and Datong (0.838 × 106 km2), the sediment yields at Cuntan and Datong were very similar, namely, 4.39 × 108 t / yr, and 4.33 × 108 t / yr, respectively, because sediment deposition in the channels of the segment offset the sediment supply from the tributaries. The contents of CaO and MgO in the suspended sediment in the tributaries (Hanjiang River, Ganjiang River and Xiangjiang River) in the middle and low reaches below the TGR dam were nearly the same as the values ​​in the mainstream of the Yangtze River (Hankou station )12. Thus, we regarded the mainstream segment between Cuntan and Datong as an ideal mainstream segment, with Cuntan being upstream of the TGR dam, which made the evaluation of the damming effects available. The CaO + MgO contents of suspended sediment samples for the three campaigns (July 2003, July 2005 and July 2007) and the calcite + dolomite contents of the sediment (July 2005) at the two sites (Table 2) were used to calculate differences in TCS, NSCS and SCS capacities for the segment. Taking the mean sediment yield of the two sites during the period from 1956 to 2000 for reference, the annual net TCS, NSCS and SCS between the two sites were 26.45 × 106 tons of CO217.51 ​​× 106 tons of CO2 and 8.94 × 106 tons of CO2, respectively. After 2001, due to hydropower exploration (especially the TGR project), ecological mitigation and soil conservation, the sediment yields at the two sites largely decreased and have stabilized since 2006. By comparison to the period before 2000, the sediment yields at Cuntan and Datong decreased by 72.4% and 71.6%, respectively, during the period 2006–201914. Due to the reduction in sediment yields, the annual net TCS, NSCS and SCS in the segment decreased by 18.52 × 106 tons of CO212.24 × 106 tons of CO2 and 6.28 × 106 tons of CO2respectively (Table 3).

### Carbon sink capacities of the global rivers and their implications

The amount of deposited sediments of the Three Gorges Reservoir (TGR) from June 2003 to December 2017 was 1.669 × 109 tons, and the average sedimentation rate was 114.5 × 106 t / yr15. According to the differences in the TCS, NSCS and SCS capacities of the suspended sediments between Cuntan and Datong, the losses of annual TCS, NSCS and SCS by sedimentation in the TGR were estimated to be approximately 6.756 × 106 tons of CO24,466 × 106 tons of CO2 and 2,290 × 106 tons of CO2× 106 tons of CO2 , respectively. The power generation of the Three Gorges Hydropower Station exceeded 100 × 109 kW / h in 2018, equivalent to saving 31.9 × 106 tons of standard coal and reducing 85.80 × 106 tons of CO2 emissions16. The reduction in inorganic carbon sinks from the sedimentation in the TGR was equivalent only to a limited amount (TCS for 7.9% and SCS for 2.7%) of the reduced CO2 emissions by the Three Gorges Hydropower Station. Moreover, the sediment deposited by the TGR could bury and store vast quantities of organic carbon6and particulate organic carbon (POC) contents in the TGR in recent reports17,18,19 varied from 0.26 to 9.2%. An average of 1.5% of POC in buried sediment was used to estimate annual buried organic carbon, and the relatively permanent sedimentation of organic carbon was equivalent to 6.30 × 106 tons of CO2 sequestration. The losses of annual TCS and SCS via silicate weathering by the TGR project could also offset 107.28% and 36.36% of its annual CO2 sequestration (6.30 × 106 tons) via permanent sedimentation, respectively.

From a global perspective, 4462 rivers with basin areas of more than 100 km2 showed that the current annual sediment flux of global rivers into the sea was 12.61 × 109 tons20. Taking the preimpoundment period TCS of 0.060 t / t for the stream segment between Cuntan and Wusongkou (the mouth of the Yangtze River) into consideration, the total inorganic carbon sink amount of 7.57 × 108 tons of CO2was derived from global rivers, which is equivalent to 71.6% of the total inorganic carbon sink of global rock weathering (1.056 × 109 tons of CO2), with weathered silicate being more than 26% of the total weathered rocks4,6,10. The enhanced silicate rock weathering (ERW) strategy proposed by Beerling et al.1 would create a higher annual SCS, reaching 2 × 109 tons of sequestered CO2. To achieve this goal, there is no doubt that the inorganic carbon sink amount contributed by the dissolution of calcium and magnesium minerals in the processes of river sediment transport accounted for a great portion of the carbon sink amount via global rock weathering. The collision and abrasion of river sediments combined with stirring and mixing could promote the dissolution of minerals during sediment transport processes (off-site weathering of the rock). Therefore, it was suggested that the dissolution rate of off-site rock weathering was higher than that of in situ weathering. In comparison to the periods with limited anthropogenic influences, the global sediment fluxes to the sea decreased by approximately 10%20and the corresponding total carbon sink loss in a year was estimated to be 0.757 × 109 tons of CO2 which was still less than the amount of CO2emission reduction contributed by hydropower exploration and the associated buried organic carbon per year.