Controls on the sources and cycling of dissolved inorganic carbon in the Changjiang and Huanghe River estuaries, China: ¹⁴C and ¹³C studies

Abstract

The Changjiang (Yangtze River) and Huanghe (Yellow River) are the two largest rivers in China, and they transport large amounts of terrestrial carbon to the coastal waters of the East China Sea and the Bohai Sea. The sources and cycling of riverine carbon in these two large river estuaries, however, have not been well studied. In this article, we present the results of dual isotope (D 14 C and d 13 C) measurements of dissolved inorganic carbon (DIC) collected in the low reaches of the Changjiang and Huanghe and their estuaries during two cruises in 2014. Our results indicate that both the Changjiang and Huanghe carry very high concentrations of DIC ranging from 1384 lmol kg 21 to 1732 lmol kg 21 and 2711 lmol kg 21 to 4120 lmol kg 21 , respectively, and DIC levels varied with flow rates during high and low discharge periods. The cycling of DIC exhibited conservative behavior in both the Changjiang and Huanghe estuaries, suggesting DIC levels were controlled mainly by physical mixing processes.D 14 C-DIC values indicate that the Changjiang and Huanghe transport aged DIC (1060-1380 yr old). Both D 14 C-DIC and d 13 C-DIC values also showed conservative mixing in the two estuaries. Using a dual carbon isotopic model, we calculated that atmospheric CO 2 consumed mainly by silicate weathering was a major source, contributing 65.2 6 9.0% and 73.4 6 3.0% of DIC in the Changjiang and Huanghe, and 96.9-97.7% (by air-sea exchange) of DIC in the coastal waters of the East China Sea (ECS) and Bohai Sea, respectively. Our results indicate that carbonate dissolution was an important (12.3-17.4%) but not major process controlling the high DIC levels in both rivers, as suggested previously. Compared with the large Amazon River, respiration of riverine organic matter (OM) played a less important role, contributing only 15.4-17.2% of DIC in the two Asian rivers. Flux calculations indicate that the Changjiang and Huanghe discharged 1.46 3 10 13 g and 6.28 3 10 11 g DIC into the ECS and Bohai Sea in 2014, which were 9 and 17 times higher than the DOC fluxes in the two rivers. These large fluxes of riverine DIC, especially of aged DIC, could have significant impacts on primary production and carbon cycling in the ECS and Bohai Sea.

Temporal variability in composition and fluxes of Yellow River particulate organic matter

“…4a). This trend is consistent with higher concentrations and a more 13 C-depleted DIC signature in autumn as a consequence of OM decomposition and also terrestrially-derived groundwater DIC inputs (Hotchkiss et al 2015), while more 13 C-enriched DIC signals stem from dissolution of carbonate and siliciclastic rocks during weathering reactions in spring and winter (Wang et al 2016). The pattern of seasonal variability in d 13 C of short-chain n-FA is similar to that of the n-C 17 alkane (d 13 C 17alkane ), albeit lower in amplitude (Fig.…”

“…a). Although there are only two data point available for Δ 14 C DIC in the Yellow River (Wang et al ), higher Δ 14 C 16 + 18FA values (−2‰) and Δ 14 C DIC values (−125‰) were observed in October, while lower Δ 14 C 16 + 18FA values (−50‰) and Δ 14 C DIC values (−164‰) were evident in Spring (April). These parallel time‐series variations suggest that aquatic production is likely a source of short‐chain n ‐FAs, and hence corresponding carbon isotopic compositions of short‐chain n ‐FAs echoes that of aquatic biomass produced via photosynthetic DIC fixation.…”

Carbon Cycling in the World's Deepest Blue Hole

“…Since air-sea exchange likely only affects water to a depth of 80 m, DO declines after this depth because of limited inputs from oxygen-rich surface waters, and then H 2 S, CH 4 , and other dissolved gases start to accumulate, forming the chemocline (Figure 2). With the decrease of DO, DIC concentrations increase and the DIC δ 13 C and Δ 14 C values decrease (Figures 2 and 3), which are significantly different from open ocean waters and river waters (p < 0.01) ( Figures 5 and S7) Wang et al, 2012Wang et al, , 2016, but similar values have been observed in other marine cave systems (McGee et al, 2010), further suggesting a minor contribution from the atmosphere. Carbon isotopic disequilibrium between deep water in the YBH and atmospheric CO 2 or marine bicarbonate values are likely due primarily to the 13 C-depleted carbon produced by bacterial respiration (McGee et al, 2010).…”

“…The profiles of DIC concentration and isotopic compositions in the YBH indicate that the DIC in surface and deep waters are controlled by different mechanisms. The concentration and isotopic compositions of DIC in surface waters of the YBH (from 0 to 80 m) are relatively constant (Figure 3b) and very close to open ocean and estuarine values ( Figure 5), which are mainly controlled by air-sea exchange, suggesting adequate mixing of the surface waters Wang et al, 2012Wang et al, , 2016. Since air-sea exchange likely only affects water to a depth of 80 m, DO declines after this depth because of limited inputs from oxygen-rich surface waters, and then H 2 S, CH 4 , and other dissolved gases start to accumulate, forming the chemocline (Figure 2).…”

Influence of Hydrodynamic Processes on the Fate of Sedimentary Organic Matter on Continental Margins

“…Bubble sizes correspond to approximate water depth. The dashed boxes indicate approximate ranges of Δ 14 C OC and δ 13 C OC values of marine OM (Blair & Aller, ; Wang et al, ) and of Yellow and Yangtze River suspended particle organic matter (SPM; Marwick et al, ; Tao et al, ; Wang et al, ). The data from Wu et al () and Kao et al () (solid symbols) are plotted without water depth information in the lower panel.…”