Introduction

The rate of CO2 build-up in the atmosphere depends on the rate of fossil fuel combustion and the rate of CO2 uptake by the ocean and terrestrial biota. About half of the anthropogenic CO2 has been absorbed by land and ocean. Large rivers that connect the land and ocean may play an important role in the global carbon cycle1,2. On the one hand, river can transport a significant amount of dissolved and particulate carbon materials from the land to the ocean, which are subject to recycling and sedimentation in the estuaries, or further transportation to the marginal seas3,4. On the other hand, there may be high levels of nutrients in the river waters, which could enhance biological uptake of CO2 and subsequent carbon burial in the estuaries5,6.

The Yellow River, the second longest river in China following the Yangtze River, provides approximately 50% of the freshwater discharged into the Bohai Sea every year7. There were some studies on sedimentary organic carbon around the Yellow River Estuary (YRE), which were mainly conducted in the Yellow River Delta1,8,9 and in the shelf of the Bohai Sea10,11,12,13. Limited studies showed a large spatial variability (ranging from 0.7 to 7.7 g kg−1) in total organic carbon (TOC) in the YRE14, with the highest contribution (40–50%) of terrestrial organic carbon near the delta11. However, little is known about the TOC dynamics in the sediment for the transitional zone near the river mouth.

Limited studies of inorganic carbon dynamics have been conducted in the YRE. An earlier study showed that particulate inorganic carbon (1.8% ± 0.2%) was significantly higher than particulate organic carbon (0.5% ± 0.05%) in the water column of YRE15. A later analysis demonstrated that rate of CaCO3 precipitation was modestly higher than rate of biological production in the water columns of the estuary16. These findings suggest that there might be more inorganic carbon (TIC) than TOC accumulated in the sediment of the YRE. However, there is no evidence to support it because little is known on the magnitude and variability of TIC in the YRE. On the other hand, recent studies have showed that there was a large amount of carbonate in the soils of lower part of the Yellow River Basin, and higher level of carbonate was associated with high level of organic carbon17,18. One may expect a similar phenomenon in the sediment of the YRE.

As the world’s largest carrier of fluvial sediment, the Yellow River’s sediment load has continually decreased since the 1950s due to changes in water discharge and sediment concentration by anthropogenic changes19. On the other hand, climate change and human activities in the Yellow River basin have decreased fine sediment from the Loess Plateau and increased coarse sediment scouring from the lower river channel20. These changes may have profound impacts on the physical, biogeochemical and biological processes in the YRE. This study is the first to assess the dynamics of both TOC and TIC in the surface sediment of the YRE, focusing on the transitional zone near the river mouth21. The objective of this study is to test the hypothesis of more TIC than TOC accumulated in the sediment, and to explore the underlying mechanisms that regulate the variability of TOC and TIC in the YRE.

Results

Physical characteristics

The sampling sites covered most parts of the YRE, with water depth ranging from 1.5 m to 13.5 m (Fig. 1a). Dry bulk density (DBD) ranged from 0.74 to 1.55 g cm−3, with an average of 1.02 g cm−3 (Table 1). Generally, DBD was much higher in the shallow water area than in the deep water region, presenting high values mainly occurred in the south and north sides near the river mouth (Fig. 1b).

Figure 1
figure 1

Spatial distributions of (a) depth (m) and (b) dry bulk density (DBD, g cm−3) in surface sediments of the Yellow River Estuary. The maps were generated by ArcGIS 10.2 (http://www.esri.com/arcgis/about-arcgis).

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Table 1 Means, standard deviation (SD) and coefficients of variation (CV) of the main variables.
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Figure 2 showed the spatial distributions of the main granulometric variables of the surface sediment. In general, clay content was low, ranging from 1.4 to 10.8% (Table 1), with relatively higher values in the northern part than in the southern part. The highest clay content was found near the north side of the river mouth, and the lowest at the mouth section. Silt content was much high (69.4 ± 21.1%), exhibiting similar spatial distribution with clay. On the other hand, the highest content of sand was found at the mouth (Fig. 2c), where clay and silt contents were lowest (Fig. 2a,b). As expected, the spatial distribution of d(0.5) was similar to that of sand, displaying the highest values in the shallow river mouth section and lowest in the southern bay, indicating strong hydrodynamic effect in the former and weak in the latter.

Figure 2
figure 2

Distributions of (a) clay (%), (b) silt (%), (c) sand (%), (d) the median diameter (d(0.5), µm) in surface sediments of the Yellow River Estuary. The maps were generated by ArcGIS 10.2 (http://www.esri.com/arcgis/about-arcgis).

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Spatial distributions of TOC, TN, C:N and δ13Corg

Concentration of TOC was highly variable, with higher values (3.2–4.4 g kg−1) in the northernmost section of the estuary and the east deep water area (Fig. 3a). There was also a high value of TOC in the bay, south of the river mouth. On the other hand, lower TOC concentration (0.2–1.4 g kg−1) was observed in the south section. Similarly, TN value varied largely, from 0.06 to 0.68 g kg−1, with the lowest at the shallow water area near the river mouth and the highest in the north deep water section (Fig. 3b). Overall, the spatial distribution of TN was similar to that of TOC, both showing higher values in the north and east deeper water area.

Figure 3
figure 3

Spatial distributions of (a) TOC (g kg−1), (b) TN (g kg−1), (c) C:N, (d) δ13Corg (‰) in surface sediments of the Yellow River Estuary. The maps were generated by ArcGIS 10.2 (http://www.esri.com/arcgis/about-arcgis).

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The C:N ratio ranged from 2.1 to 10.1 (Fig. 3c). In general, C:N ratio was higher in the shallow water part relative to the deep water part. The highest C:N ratio (8–10) was found in the southern bay, and the lowest in the shallow water area near the river mouth (<4.5). Figure 3d showed a considerable spatial variability in the δ13Corg values with a range from −24.26‰ to −22.66‰. The δ13Corg value was more negative near the river mouth and its adjacent south bay, and less negative far away from the river mouth and the coast line.

Spatial distribution of TIC, δ13Ccarb and δ18Ocarb

There was a large spatial variation in TIC, as shown in Fig. 4a, ranging from 6.3 to 20.1 g kg−1, with higher concentration in the northern deep sea area (>17 g kg−1) away the mouth, and lower level in the south section (<13 g kg−1). Apparently, TIC also presented a high value in the north and east part. Overall, the spatial distribution of TIC was similar to that of TOC. The values of δ13Ccarb and δ18Ocarb ranged from −4.89‰ to −3.74‰ and −10.92‰ to −7.92‰, respectively (Table 1). Generally, the spatial distribution of δ13Ccarb exhibited more negative values in the north and east deep sea area, which was opposite to that of δ18Ocarb (Fig. 4b,c).

Figure 4
figure 4

Spatial distributions of (a) TIC (g kg−1), (b) δ13Ccarb (‰), and (c) δ18Ocarb (‰) in surface sediments of the Yellow River Estuary. The maps were generated by ArcGIS 10.2 (http://www.esri.com/arcgis/about-arcgis).

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Discussion

Sources for TOC in the Yellow River Estuary

It is well known that human activities such as industrial and agricultural development would cause an increase in riverine input of nutrients and organic materials, leading to enhancements in estuary productivity and TOC burial in the sediment22,23,24. There was evidence that δ13Corg was less negative in the central Bohai Sea (−21‰ to −22‰) than in the nearshore (~−27‰)11, indicating more negative δ13Corg in terrigenous OC. Provided that the δ13Corg values ranged from −24.26‰ to −22.66‰, organic carbon in surface sediment of the YRE might be mainly from marine sources.

Since C:N ratio is significantly smaller in marine particles than in terrestrial organic matters, one may use a two-end-member mixing model to quantify different sources of OC; such approach has been widely applied in studies of wetland and lake sediments25,26,27, and offshore and marine sediments28,29. Given that TOC:TN ratio was lower than 5.5 g:g at some sites in the YRE, it was reasonable to assume that there were terrestrial inputs of inorganic nitrogen. There was a significant corelation between TN and TOC (Fig. 5a), with an intercept of 0.0297 g N kg−1. Following Schubert and Calvert30, we calculated total organic nitrogen (TON) concentration of each sample by subtracting 0.0297 g N kg−1 (the intercept) from TN. As shown in Table 2, TOC:TON ratio was low (<7.1) in most sections, illustrating that TOC was mainly autochthonous in the surface sediment the YRE. On the other hand, mean TOC:TON ratio was 9.5 in the southern shallow bay; such high C:N ratio together with relatively more negative δ13Corg value (Table 2) suggested that there might be a large amount of allochthonous OC sources in this section.

Figure 5
figure 5

Relationship between (a) TOC and TN, (b) TOC and TIC in surface sediments of the Yellow River Estuary.

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Table 2 Means of the variables in different sections.
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To quantify the relative contributions of autochthonous and allochthonous OC in the surface sediments, we applied a two-end-member mixing model by using TOC:TON ratio, and assuming 6.6 mol:mol as the marine end-member. Using the average C:N ratio (10.8 g:g) from the soils collected near the river mouth (Table 1), we estimated that 75% of TOC was from soil OC source in the bay section, but only 12–28% in other sections of the YRE (Table 3). However, our approach could introduce bias or uncertainty due to the choice of end member value for soil C:N ratio. According to our recent study31, soil C:N ratio varied from 9.5 to 13.4 in the middle-lower parts of Yellow River Basin. If we chose 9.5 (or 13.4) as the soil C:N end member, the terrigenous contribution would be increased (or decreased) by 4–25%. Nevertheless, TOC in the surface sediment was primarily autochthonous in most parts of the YRE.

Table 3 Relative contributions (%) of marine and terrestrial sources using different soil C:N ratios as the end-member.
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TOC variability in the Yellow River Estuary

The magnitude and spatial distribution of TOC in estuarine sediment may reflect multiple and complex processes10,32. As shown in Fig. 2, the surface sediments were finer to the north than to the south. In general, coarser (finer) sediment particles usually indicated a stronger (weaker) water energy environment33,34. These analyses indicated that the relatively lower TOC values in the south section were attributable to higher kinetic energy level. On the other hand, a significantly positive relationship (r = 0.71, p < 0.01) between the δ13Corg value and water depth (Table 4) implied that the shallow sections in the YRE accumulated more terrigenous OC (with more negative δ13Corg values).

Table 4 Correlation coefficient (r) between various variables for the sediments.
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There is evidence that the magnitude and variability of OC is largely influenced by primary productivity, followed by sediment resuspension and riverine input in the Yellow-Bohai Sea35. In general, an increase of water productivity would cause enriched 13C in carbonate36,37. However, we found a significantly negative correlation (p < 0.01, Table 4) between TOC and δ13Ccarb in the YRE, indicating that higher levels of TOC (with more negative δ13Ccarb) were not a result of local biological production. Given that sediment resuspension played a large role in regulating the spatial-temporal variability of POC in the Yellow-Bohai Sea35,38, we inferred that the current system would cause re-distribution of POC thus TOC in the surface sediment. Therefore, more OC could deposit in the north and east deep water area (with lower kinetic energy levels) in the YRE.

Dynamics of TIC and underlying mechanisms

Concentration of TIC in the surface sediment of the YRE was relatively higher in the north section (16.2 g kg−1) than in the south section (12.8 g kg−1) (Table 2), which was consistent with TOC. As shown in Fig. 5b, there was a significantly positive correlation between TOC and TIC in the surface sediments in the YRE (r = 0.97, p < 0.01), implying a potential relationship between the two parameters. In general, OC production (i.e., uptake of CO2) can induce changes of chemical properties in the water column, which often leads to precipitation of carbonate36,37,39. Our analyses showed that the change ratio between TIC and TOC (i.e., the slope of 2.93 in Fig. 5b) in the surface sediment of the YRE was close to the ratio of 3.6 for IC:OC in particles in the water column by Gu, et al.15, indicating that the spatial variability of TIC might be driven by variability of POC.

While higher levels of TIC might be associated with higher levels of TOC, there was a big intercept (7.17 in Fig. 5b) for the TIC-TOC relationship in the surface sediment, suggesting that there were other processes of CaCO3 formation, which were not linked with biological production. If higher levels of TIC were a result of higher rates of biological production, one would expect an enrichment of 13C in carbonate; on the other hand, higher rate of respiration/decomposition would lead to depleted 13C in dissolved IC thus in carbonate36,37. The significantly negative relationship (p < 0.01) between δ13Ccarb and TIC in the YRE (Table 4) indicated that higher levels of TIC (with more negative δ13Ccarb) might result from high rates of decomposition of OC. Given that both TIC and TOC had a significantly negative correlation (p < 0.01, Table 4) with δ13Ccarb in the YRE, we speculated that there might be decomposition of TOC/POC associated with sediment resuspension, which would lead to an increase in dissolved IC thus promote carbonate precipitation and sedimentation.

Comparisons with other studies

There have been many studies of TOC but only a few studies of TIC from the estuarine sediments. Overall, TOC levels are lower in the surface sediments in most estuaries in China, relative to those in the South and Southeast Asia40,41, Europe42,43, North America and South America44,45. In general, sedimentary TOC concentration is relatively lower in large river estuaries (e.g., the Yangtze River Estuary46,http://www.esri.com/arcgis/about-arcgis).