Assessing C/N and δ 13 C as indicators of Holocene sea level and freshwater discharge changes in the subaqueous Yangtze delta,China

Abstract

To examine the applicability of C/N and organic carbon stable isotope (δ¹³C) in studies of the Holocene sea level and freshwater discharge in the large river mouth of Yangtze, we observed the distribution of carbon, nitrogen and δ¹³C in a late-Quaternary core (ZK9) collected from the present subaqueous delta. We also collected published data of the two proxies for the suspended particulate matter (SPM) and surficial sediments from the lower Yangtze River to the adjacent East China Sea. The results show that the estuarine front is an important boundary for terrestrial and marine contribution of the organic component in the modern sedimentary environment. In the core ZK9, sediments deposited during c. 13–9 cal. ka BP are characterized by high values of TOC (0.54–1.16%), CaCO₃ (0.35% on average), and C/N (>12), which reflect an inner tidal estuarine environment dominated by C3 terrestrial organic carbon input. During c. 9–0.7 cal. ka BP, both TOC content (0.57% on average) and C/N ratio (<10) decrease remarkably while TN increases, indicating a lower estuarine or shallow marine environment. An abrupt sea level rise from c. 9 cal. ka BP resulted in a deeper water environment and reduced terrestrial input at the core location. The low δ¹³C values (−24.23‰ on average) before c. 6 cal. ka BP reflect a dominantly terrestrial source of organic matter associated with increased freshwater discharge into the estuary during that time. The sediments since c. 6 cal. ka BP are characterized by increasing δ¹³C up to −24.1 to −23.39‰, reflecting more contribution from marine algae as freshwater discharge fell. We suggest that in the Yangtze River mouth the C/N ratio indicates an abrupt sea level rise at c. 9 cal. ka BP, while δ¹³C is more useful in reflecting freshwater discharge.

Keywords

organic carbon sources postglacial sea level rise Yangtze Estuary

Introduction

Microfossils are used traditionally to reconstruct past sea levels and palaeoenvironmental change in coastal areas (Horton, 1999; Innes et al., 1996; Wang et al., 2010; Zong and Horton, 1998). In some cases microfossils are spatially restricted and sparse in the coastal zone as they are susceptible to chemical and mechanical damage. The absence of identifiable microfossils in sediments seriously hinders studies of Holocene relative sea levels (RSL) and palaeoenvironments (Gonzalez et al., 2000; Wilson et al., 2005a). In these cases other indictors of RSL need to be assessed.

Coastal sediments receive organic material from both terrestial and marine sources (Lamb et al., 2006). Previous studies have shown that because of the predominant contribution of C3 plant detritus, terrestrial organic matter has significantly higher C/N ratios (>12, Prahl et al., 1980) and lower organic carbon isotope (δ¹³C) values, between −32‰ and −21‰ (Deines, 1980; Meyers, 1994; Schidlowski et al., 1983) than marine organic matter. Marine algae have C/N ratios <8 because of nitrogen enrichment (Bordovskiy, 1965) and δ¹³C values of −16‰ to −23‰ (Haines, 1976; Meyers, 1994). The δ¹³C content of marine particulate organic carbon (POC) is reported as ranging from −21‰ to −18‰ because marine phytoplankton is the dominant source (Middelburg and Nieuwenhuize, 1998; Peters et al., 1978; Wada et al., 1987; Yamaguchi et al., 2003). Freshwater algae in C3-dominated environments tend to have lower δ¹³C values of −26‰ to −30‰ (Meyers, 1994; Schidlowski et al., 1983), while algae in C4-vegetation catchments can have relatively high δ¹³C values of ≥ –16‰ (O’Leary, 1988).

In recent decades, C/N ratios and δ¹³C have been used successfully to distinguish the provenance of organic material in coastal and marine sediments and then applied to reveal paleoenvironmental and sea level changes (Lamb et al., 2007; Mariotti et al., 1991; Stuiver et al., 1995; Wilson et al., 2005b; Wurster et al., 2010; Yang et al., 2011; Yu et al., 2010; Zong et al., 2010). Although organic matter decomposition has been shown to change sediment δ¹³C and C/N values (Sampei and Matsumoto, 2001), the direction of change in δ¹³C and C/N, rather than their absolute values, is the key for interpreting changes in relative sea level and such directional changes are commonly preserved (Lamb et al., 2006). Lamb et al. (2007) used δ¹³C and C/N ratios as sea level and sedimentary environmental indicators in an analysis of the Humber Estuary (UK) and found an estuary-wide expansion of marine conditions from c. 3.3 cal. ka BP, followed by a contraction after c. 2.7 cal. ka BP. Wilson et al. (2005b) analyzed part of an early-Holocene sediment core from the Mersey Estuary and demonstrated that organic δ¹³C and C/N analysis is likely to provide continuous records of RSL and sedimentary environmental change. δ¹³C is also a good indicator of the freshwater discharge that drives the land–sea interaction and thus the isotopic composition of organic carbon in the estuarine region (Fernandes et al., 2009; Morrill et al., 2003; Xiao et al., 2002, 2006; Zong et al., 2006). For instance, Zong et al. (2006) found a significant increase in freshwater flux resulting from the enhanced summer monsoon regime in the Pearl River delta from 8500 cal. yr BP by analyzing the organic carbon (δ¹³C and C/N) and diatoms in sediments.

The Yangtze River mouth is characterized by huge discharges of freshwater (924 × 10⁹ m³/yr) and suspended sediment (486 mt/yr; Chen et al., 1985), which means large amounts of terrestrial organic carbon in the coastal sediments. Recent work by Yang et al. (2011) based on a Holocene core from an estuarine island suggested that the response of organic matter to monsoon variability is weak in the Yangtze estuary possibly because of a freshwater-dominant environment of the core location and the big change of sedimentary facies throughout the core. Therefore, variables that influence whether the organic material supply is from a terrestrial or a marine source include sea level change, freshwater flux, and coastal/delta progradation in the Yangtze River mouth. We suggest it is important to assess δ¹³C and C/N ratios as paleoenvironmental indicators in the Holocene sediments in such a large river mouth as the Yangtze.

There have been many studies focusing on the provenance of surficial and suspended sediments and dissolved nutrients based on analysis of δ¹³C and C/N ratios in the Yangtze River mouth (Liu et al., 2006; Wu et al., 2003, 2007; Zhang et al., 2007; Zhou et al., 2006; Zhu et al., 2011). These previous works have shown a progressive change of δ¹³C and C/N ratios from the freshwater river to the shallow marine environment. For example, Wu et al. (2007) revealed the Yangtze riverine particulate organic matter is mostly derived from soil which is dominated by C3 plants; the contribution from phytoplankton is minor and difficult to trace. Zhou et al. (2006) revealed that δ¹³C is c. −26‰ to −24‰ for the high marsh sediment, −24.5‰ to −22.5‰ for the low marsh and −24‰ to −22‰ for the bare tidal flat; C/N is generally higher than 10 for the marsh sediments while it is around 7 for the bare tidal flat. Their results also indicated that riverine organic carbon contribution accounts for 44.4–76.4% of the tidal flat sediments and decomposition of the organic matter occurs mainly on the surface of the high marsh because of the lower sedimentation rate. Zhu et al. (2011) demonstrated C/N ratios being 12.2±1.8 in the lower Yangtze River sediments, 9.2±3.0 in the Yangtze estuary, 8.7±1.7 in the longshore shelf, and 7.6±1.9 in the open shelf of the East China Sea. The above results imply the possible applicability of δ¹³C and C/N ratios in Holocene environmental evolution of the Yangtze River mouth controlled by the sea level and climate changes, if end members of the proxies for different sedimentary environment can be figured out. The core location is also important, i.e. at the relative fringe of the Holocene depocenter so that the sedimentation is sensitive to the change in sea level or freshwater discharge in addition to the delta progradation.

In this paper, we first collect all data published by previous workers of C/N ratios and δ¹³C of the surficial and suspended sediments from the lower Yangtze River channel to the inner shelf of the East China Sea (Figure 1; Table 1). Then we analyze the Holocene sediments from a core (ZK9) obtained from the prodelta of the present subaqueous Yangtze delta (Wang et al., 2010). We aim to examine the applicability of C/N and δ¹³C as indicators of fluctuations of relative sea level and freshwater discharge in the large river mouth of Yangtze during the Holocene.

Figure 1.

Map of the study area showing the location of core ZK9 and the turbidity maximum, estuarine front, and plume front of the present-day subaqueous delta. The water depth at core ZK9 is 12.5 m. Approximate locations of surficial and suspended samples collected in previous studies are also indicated. Also indicated are three representative sedimentary environments: riverine, river mouth, and shallow marine. XLJ, Xu-Liu-Jing, where the apex of present subdelta lobe occurs

Table 1.

δ¹³C and C/N values of three representative modern sedimentary environments from lower Yangtze River to the East China Sea collected from previous publications. Data sources and material measured are also indicated

Sedimentary environment	C/N	δ¹³C (‰)	Number of sample	Material measured	Data sources
Riverine	17.5±2.5	—	—	SPM	Milliman et al. (1984)
	—	−25.5±0.9	—	SPM	Wu et al. (2000)
	13.4±2.6	−28.7±1.0	7	SPM	Zhang et al. (2002)
	12.0±5.1	−26.2±3.0	6	SPM	Zhang et al. (2007)
	—	−27.6±3.1	—	SPM	Cai and Han (1998)
	12.2±4.6	−26.0±0.8	7	SPM	Wu et al. (2007)
	18.5±11.0	−26.6±2.3	5	Soil	Wu et al. (2007)
	18.9±12.8	−24.4±0.5	12	Sediments	Tang et al. (2006)
	12.2±1.8	—	12	Sediments	Zhu et al. (2011)
River mouth
upper	15.0±5.0	−25.0±1.0	—	SPM	Tan et al. (1991)
	12.3±0.1	−23.1±1.5	20	SPM	Zhang et al. (2007)
	10.0±2.0	−26.8±3.0	10	Sediments	Liu et al. (2006)
middle	14.4±0.9	−21.8±0.6	19	SPM	Zhang et al. (2007)
	17.5±6.5	−24.3±1.7	17	SPM	Cai et al. (1992)
	10.5±4.5	−20.5±1.5	—	SPM	Tan et al. (1991)
lower	6.4±1.4	—	2	SPM	Zhang et al. (2007)
	8.7±2.5	−23.1±3.3	6	SPM	Cai et al. (1992)
	7.6±0.2	−22.1±0.3	3	Sediments	Kao et al. (2003)
Shallow marine	7.5±2.5	—	—	SPM	Milliman et al. (1984)
	—	−20.2±1.4	—	SPM	Shi (1993)
	5.5±1.5	−20.0±1.0	—	SPM	Wu et al. (2003)
	—	−21.9±1.0	18	Sediments	Cai et al. (1992)
	6.9±0.5	−20.1±0.6	16	Sediments	Kao et al. (2003)
	3.8±0.3	−22.7±0.3	4	Sediments	Tang et al. (2006)
	7.6±1.9	—	64	Sediments	Zhu et al. (2011)

SPM, suspended particulate matter.

Materials and methods

Surficial and suspended sediment data

We defined the lower Yangtze River being upstream from the site of Xu-Liu-Jing (XLJ) where distributaries start and freshwater predominates even during peak spring high tide (Figure 1). The river mouth, starting from XLJ, is characterized by an estuarine front and a plume front. We further divided the river mouth into upper, middle, and lower parts. The upper part consists of the distributaries; the middle part is between the mouth and the estuarine front, where a turbidity maximum occurs; the lower part is between the estuarine front and the plume front. Estuarine water of high SSC (suspended sediment concentration; highest exceeding 2 kg/m³) and low salinity (<5‰) occurs within the middle part. Plume water floats over the salt wedge in the lower part, where SSC decreases to <0.6 kg/m³. The shallow marine zone is defined as seaward of the summer plume front where salt water occurs during the whole year (Chen et al., 1999).

We collected from previous publications (Figure 1; Table 1) C/N and δ¹³C values of surficial sediments, soil, and suspended particulate matter (SPM) from three representative sedimentary environments including the lower Yangtze River, Yangtze River mouth, and the adjacent shallow marine areas of the East China Sea (references are given in Table 1). We used these data to determine the distribution and end members of organic elements in the modern environment from land to sea (Figure 2) and thus help discriminate the organic matter sources for the Holocene core sediments.

Figure 2.

δ¹³C and C/N values of three representative modern sedimentary environments from the lower Yangtze River to the East China Sea: riverine, river mouth, and shallow marine (data source indicated in Table 1). The river mouth is further subdivided into upper, middle, and lower part

Sediment core and the chronostratigraphy

The 50 m long late-Quaternary core (ZK9) was obtained in 2007 at a water depth 12.5 m in the subaqueous Yangtze delta (122°23′E, 30°43′N; Figure 1), where plume water dominates at present and the average and maximum tidal range is around 2.7 m and 4.6 m, respectively. Core information has been published in Wang et al. (2010), including chronostratigraphy and changes in sedimentary environment and climate, based on AMS ¹⁴C dating, lithology, pollen, spore and dinoflagellate cyst identification. Here we summarise the lithology and sedimentary environmental interpretation in order of both age and depth (Figure 3).

Figure 3.

Vertical distribution of TN, TC, TOC and CaCO₃ concentration, TOC/TN ratio, and organic carbon isotopes (δ¹³C) in the core sediments. Calibrated age, grain size and interpretation of sedimentary facies are also indicated. Md, median diameter of grain size

Last glaciation (core depth 43–50 m)

Fluvial in-channel deposits of gray sand interbedded with yellowish gray mud occurs at the bottom of the core, overlain by a layer of ~2.5 m thick hard gray homogeneous mud that was formed during the last glacial maximum (LGM). No marine fossils were found and fossil pollen and spores are rare.

Postglacial–~10.0 cal. ka BP (core depth 30–43 m)

Dark gray mud is interbedded thinly with sand. The sand interbeds change to thin or thick laminations upward. Shell and plant fragments are present locally. Marine fossils appear with low abundance. Pollen and spore concentrations increase significantly and this is interpreted as an intertidal to subtidal environment.

~10.0–8.4 cal. ka BP (core depth 16.4–30 m)

Sand-mud couplets dominate, with an increase in mud thickness downcore during this time period. Both marine fossil abundance and pollen and spore concentrations increase substantially. This is interpreted as an estuary front environment.

~8.4–5.9 cal. ka BP (core depth 14.5–16.4 m)

This period is characterized by a very low sedimentation rate in a sequence of highly laminated mud and mixed deposition of sand-mud-shell fragments. It is interpreted as a transitional environment from estuarine to deltaic conditions and sediment starvation due to the landward shift of the depocenter (Wang et al., 2010).

~5.9 cal. ka BP to present (core depth 0–14.5 m)

Homogenous mud dominates the deltaic deposition after 5.9 cal. ka BP at the core location. Silty laminations appear at ~2.0 cal. ka BP and clearly increase during the last ~1.0 ka. Both marine microfossils, and pollen and spores are abundant. This environment is interpreted as a nearshore shelf overlain by the prodelta.

As the location where the core was collected lies outside the maximum turbidity zone and the estuarine front (Figure 1) and the dominant lithology is homogeneous mud of deltaic origin and sand-mud couplets of estuarine formation, we suggest that it is located at the periphery of the Yangtze River mouth depocenter although it is in relatively shallow water. Liu et al. (2010) described thicker deltaic deposition (~20 m) and an earlier initial age of ~7.3 cal. ka BP from a core northeast of ZK9, further evidence the relatively marginal position of ZK9. Therefore, we believe the core location of ZK9 is a reasonable place for studying sea level and Yangtze freshwater discharge changes by the organic elemental proxies.

Organic elemental analyses

Ninety-five (95) samples, each 5–10 cm thick, were taken at ~50 cm intervals along the core and were stored at −4°C. All samples for organic element analysis were dried in an oven at 40°C and ground to powder. Two aliquots were prepared for each sample: (1) about 20 mg of powder was wrapped in silver paper to measure total carbon (TC) and total nitrogen (TN) using an EA1110 Elemental Analyzer (Carlo Erba, Italy); (2) about 0.5 g of powder was mixed with in 0.1 M HCl for 24 h to remove the carbonate and then washed with deionized water thoroughly until the pH was neutral. The neutral specimen was dried at 60°C in an oven and then used for measurements of TOC by EA1110 Elemental Analyzer and δ¹³C by MAT-251 Mass Spectrometer. Eight replicate samples, distributed evenly down the core, were analyzed to examine the error of measurement. Carbonate content was calculated by subtracting the TOC content from TC.

Results

Organic proxies of surficial and suspended sediments from the lower Yangtze River to the East China Sea

The surficial and suspended sediments from the lower Yangtze River have clearly different ranges of C/N and δ¹³C to those from the continental shelf of the East China Sea (Table 1, Figure 2). C/N is higher, 12.0–18.9 on average, and δ¹³C is noticeably lower (−28.7 to −24.4‰ on average) for the riverine sediments of the lower Yangtze. There is no clear difference for the values of SPM, soil, and surficial sediments in the riverine environment. In contrast, in the shallow marine environment, SPM and surficial sediments have much lower C/N and higher δ¹³C values, being 3.8–7.6 and −22.7 to −20.0‰ on average, respectively. The average value of C/N is 10.0–15.0 for the SPM and surficial sediments in the upper river mouth, 10.5–17.5 for the SPM in middle river mouth, and 6.4–8.7 for the SPM and surficial sediments in lower river mouth. Average values of δ¹³C are −26.8 to −23.1‰, −24.3 to −20.5‰, and −23.1 to −22.1‰ for SPM and surficial sediments of the upper, middle and lower river mouth, respectively.

Distribution of organic carbon in the late-Quaternary core sediments of ZK9

Replicate samples were measured to assess the machine precision and results show that the standard deviation of TC, TOC and TN is <5%, and that of δ¹³C is <7%, which indicates quality of measurements. Results show that contents of TC, TOC, and CaCO₃ and values of C/N fluctuate consistently while TN has a reverse pattern (Figure 3). δ¹³C changes clearly along the core sections. According to changes of all proxies, we divided the core sediments into six units (I–VI) in ascending order, two for sediments deposited during the last glaciation (core depth 43–50 m) and four during the postglacial period (above 43 m; Figure 3).

Unit I: last glaciation (47.1–50 m)

Low content of TN (0.044%), but highest TC (>1.3%), TOC (>1.2%) and CaCO₃ values (0.98–1.21%) characterize the fluvial in-channel sediments of this unit (Wang et al., 2010). The C/N ratios (20.6–43.5) are also the highest throughout the core. Values of δ¹³C are low but increase significantly from bottom (−25.6‰) to top (−24.2‰).

Unit II: last glacial maximum (43–47.1 m)

TN values increase slightly to ~0.052%, while TC, TOC and C/N decrease to the lowest of the core. CaCO₃ is close to zero. δ¹³C increases to −19.7 to −23.12‰, which is the highest in the whole core.

Unit III: c. 13–9 cal. ka BP (27–43 m)

The TN is around 0.062%. TOC content fluctuates in the range 0.54–1.16%, with an average value of 0.93%. The C/N ratio generally exceeds 12 and the average is 15.44. CaCO₃ values increase to a high range (0.06–0.79%) with an average of 0.35%. δ¹³C values decrease clearly and fluctuate between −25.16‰ and −23.82‰, with an average of −24.21‰.

Unit IV: c. 9–6 cal. ka BP (14.5–27 m)

TN increases upward and has an average value of 0.083%. The average values of TOC and CaCO₃ both decrease sharply to 0.52% and 0.04%, respectively. The C/N ratio decreases also to a low range of 4.47–9.44 (average 6.45). δ¹³C values are from a low of −25.66‰, and the average is −24.25‰.

Unit V: c. 6–0.7 cal. ka BP (2.1–14.5 m)

Values of TN (0.07–0.11%), TOC (0.48–0.71%), and CaCO₃ (0–0.05%) are similar to the underlying section III. C/N ratio is low at 5.85–7.79. δ¹³C increases to −23.96 to −23.39‰ with an average of −23.64‰.

Unit VI: c. 0.7 cal. ka BP to present (0–2.1 m)

Values of TOC, C/N and CaCO₃ clearly increase again and fluctuate over a wide range. The highest content of TOC reaches 1.29% and highest C/N ratio is 16.42. TN decreases slightly. δ¹³C value falls to −24.1 to −23.81‰ with an average of −23.94‰.

Discussion

End-members for the organic matter and barrier effect of the estuarine front

We can first determine two end-member conditions for the organic material: the Yangtze riverine and East China Sea, according to the distributions of organic proxies in the modern environment (Figure 2). The riverine environment is characterized by higher C/N ratios of >10 and lower δ¹³C value of <−24‰, and the shallow marine has lower C/N ratios of <9 and higher δ¹³C value of >−23‰. There are some riverine samples with C/N ratios <10, and δ¹³C <−24‰ (Figure 2), possibly indicating freshwater algae as the source. Previous studies demonstrate another end-member of material of deltaic source, which is characterized by higher δ¹³C value of ~22.1±1.5‰ because of the introduction of a C4 plant, Spartina, since the 1990s (Li et al., 2009; Zhou, 2005). This might explain the relatively higher value of δ¹³C of the modern estuarine samples, which have no analogy in the Holocene sediments.

The organic proxies of SPM and surficial sediments in the upper river mouth are close to those of the lower Yangtze River, while organic proxies of the lower river mouth are closer to those of shallow marine area (Figure 2). Organic proxies of SPM in the middle river mouth, lying between the riverine and shallow marine, show higher C/N and higher δ¹³C values, reflecting the contribution from a modern deltaic source (Zhou, 2005). We suggest that the significant change of C/N ratios from mostly >10 in the middle river mouth, to <10 in the lower river mouth reflects the barrier effect of the estuarine front, which traps most terrestrial suspended particulates within the estuarine water (Chen et al., 1999). This means that terrestrial contributions of organic matter occur mostly within the estuarine front, while shallow marine contributions prevail seaward of it.

Organic matter sources of the late-Quaternary sediments in ZK9

The C/N and δ¹³C of the postglacial sediments (units III–VI) are similar to those of the present Yangtze River mouth surficial and suspended sediments, while the underlying sediment of Late Pleistocene age (units I–II) is more similar to the riverine and marine sediments (Figure 4). The values of C/N and δ¹³C of unit I fall totally within the range of C3 terrestrial plants (Figure 5), indicating a terrestrial organic source which is in agreement with the interpretation of the fluvial sedimentary environment (Figure 3; Wang et al., 2010). Most of the δ¹³C and C/N compositions of unit II locate in the intersecting region of marine algae, marine POC and bacteria (Figure 5). Our previous work shows no marine fossils are found in this unit and it is interpreted as paleosol formed during the LGM (Wang et al., 2010). Magnetic measurement of this unit demonstrates significantly high values of frequency magnetic susceptibility (data not shown) which is evidence of strong pedogenesis. Chen et al. (2008) also reported the pedogenesis in this unit of hard mud in the Yangtze delta. We therefore suggest bacterial decomposition of organic matter explains the features of the organic proxies in unit II (Rice and Hanson, 1984).

Figure 4.

Distribution of δ¹³C and C/N of the core sediments in the correlation plots of SPM and surficial sediments from the lower Yangtze to the East China Sea

Figure 5.

Distribution of δ¹³C and C/N of the core sediments in the correlation plots of different organic sources (modified after Lamb et al., 2006)

The sediments of unit III formed during c. 13–9 cal. ka BP have δ¹³C and C/N values similar to marine dissolved organic carbon (DOC; Figure 5), also overlapping significantly with those of C3 plants. This is because marine DOC is principally derived from a mixture of terrigenous organic matter and phytoplankton in both fluvial and marine environments (Rashid, 1985). Compared with the SPM and surficial sediments from modern riverine to shallow marine facies, the δ¹³C versus C/N correlation of this unit is typically of middle and upper river mouth nature, and much closer to riverine rather than shallow marine (Figure 4). Terrigenous organic matter is suggested as the major source in this unit of 13–9 cal. ka BP.

A sharp reduction of both TC and TOC with a clear decrease in the C/N ratio is a feature of the sediments of unit IV deposited during c. 9–6 cal. ka BP (Figure 3). The δ¹³C versus C/N correlation falls mostly within the range of the lower river mouth sediments, further suggesting that the organic composition is closer to shallow marine surficial sediments rather than riverine (Figure 4). The samples straddle the boundary between marine algae/POC and freshwater algae/POC. The C/N ratio suggests material dominated by algae, while the δ¹³C values suggest a mixture of marine and freshwater sources (Figure 5). Therefore we suggest that terrigenous organic input to the core location decreased abruptly at the boundary between unit III and unit IV (c. 9 cal. ka BP).

Sediments of unit V deposited during c. 6–0.7 cal. ka BP have higher δ¹³C (Figure 3). Their δ¹³C versus C/N correlation moves slightly closer to the surficial sediments of shallow marine (Figure 4) or marine algae and POC (Figure 5). We suggest more marine contribution of organic matter during the time period 6–0.7 cal. ka BP.

In the top unit of the recent 0.7 ka, δ¹³C and C/N compositions of three samples fall within the bottom region of marine POC or algae while the other two within the ranges of C3 and marine DOC (Figure 5). We suggest more terrestrial organic input resulting from the strong human activities in the drainage basin during the last 700 years (Wang et al., 2011).

Abrupt sea level rise from c. 9 cal. ka BP

The significant reduction of TOC content and C/N ratio from unit III to IV (Figures 3–5) reveals increased marine contribution of organic carbon since c. 9.0 cal. ka BP. The similar change in organic carbon composition from middle river mouth to the lower one of the modern Yangtze is analogous to the above change in the sediment core (Figure 2). This change in core could be explained by an abrupt sea level rise and associated shoreline retreat, or the significant decline in freshwater flux which led to the reduction in delivery of organic material of terrestrial origin to the core site. However, our previous study of pollen and spores from ZK9 revealed a cool/dry event that started at c. 9.4 cal. ka BP (Wang et al., 2010). Dykoski et al. (2005) revealed the high Asia monsoon intensity was punctuated by centennial-scale events at 9.2 ka, 8.3 ka, and 8.1 ka during the early mid-Holocene. We thus argue that the mechanism causing this shift is an abrupt change of sedimentary environment at the core location because of landward retreat of the Yangtze estuary caused by an abrupt sea level rise from c. 9 cal. ka BP. The sharp decline of CaCO₃ also supports the reduction of terrestrial material input to the core site because of shoreline retreat (Figure 3). Hori and Saito (2007) suggested a sea level jump at c. 9–8.5 cal. ka BP based on the large decreases in sediment accumulation rates in the incised valley of the Yangtze, Songhong, and Kiso River mouths. Tamura et al. (2009) reported c. 5 m sea level rise in the Mekong delta at 8.5–8.4 cal. ka BP. Bird et al. (2010) also reported rapid sea level rise at a rate of 1.8 m/100 yr during 8.9–8.1 cal. ka BP based on their work from Singapore.

Hori et al. (2002) showed that the Yangtze estuary retreated and the depocenter shifted landward abruptly at c. 8–9 cal. ka BP as result of rapid sea level rise. Therefore, terrestrial input to the present core site reduced significantly, as also indicated by the sharp decline of TC and TOC contents in the present study. Sediment at core depth 27–16.1 m, which was deposited during c. 9–8.4 cal. ka BP (Wang et al., 2010) was accumulated mainly by marine dynamics including tidal currents rather than fluvial discharge. Sediment starvation then occurred at the core site since c. 8.4 cal. ka BP because of the sustained sea level rise. Deposition recovered until 6 cal. ka BP when the sea level was stable and delta progradation prevailed.

Decline of the Yangtze freshwater discharge since c. 6 cal. ka BP

The average value of δ¹³C increases slightly from −24.23‰ during c. 13–6 cal. ka BP to −23.64‰ during c. 6–0.7 cal. ka BP (Figure 3). We suggest it reflects strong summer monsoon precipitation and associated large freshwater flux from the Yangtze River before 6 cal. ka BP, followed by a decline. Although marine processes dominated during the rapid sea level rise period of c. 9–8.4 cal. ka BP, the lowest values of δ¹³C suggest a major contribution of C3 plants to the organic carbon, which reflects strong freshwater discharge due to the strengthened summer monsoon (Dykoski et al., 2005; Shao et al., 2006). Sea level tends to be stable since c. 6.5 cal. ka BP (Bird et al., 2010) and the delta has prograded significantly since c. 6 cal. ka BP (Hori et al., 2002), which would lead to the core site becoming more proximal to riverine source and an increase in freshwater-sourced organic material if no other changes were taking place. The fact that the trend is the opposite supports the assertion that there is a reduction in freshwater flux linked to reduced precipitation. Previous studies using oxygen isotopes in stalagmites in the upper and middle Yangtze drainage basin revealed that monsoon precipitation has declined significantly since c. 6 cal. ka BP (Dykoski et al., 2005; Shao et al., 2006). Zong et al. (2006) analysed the diatom and organic carbon data from a sediment core in the Pearl estuary and suggested the same reduction in monsoon precipitation after c. 6 cal ka BP.

Conclusions

In the mouth of the Yangtze river, proxies of C/N and δ¹³C are indicative of changes in the sedimentary environment associated with changes in sea level and freshwater discharge. Terrestrial organic matter dominates both in SPM and surficial sediments in the upper and middle part of the modern Yangtze River mouth as trapped by the estuarine front. Organic matter is mainly contributed by marine sources in lower river mouth and the East China Sea. In the Holocene sediment core ZK9, before c. 9.0 cal. ka BP the organic proxies indicate this site was in the upper and middle river mouth. There is a rapid change at that time to lower river and shallow marine conditions, indicating an abrupt sea level rise. The continuous higher values of δ¹³C since 6.0 cal. ka BP reflect the decline of Yangtze freshwater discharge in a situation of stable sea level and delta progradation.

Footnotes

Acknowledgements

The authors are grateful to Dr Brian Finlayson who kindly helped improve the language.

The study is supported by grants from the Ministry of Science and Technology of China (Grant No. SKLEC-2009KYYW02; SKLEC-2010RCDW05), Natural Science Foundation of China (Grant No. 41176070), and the Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Ministry of Land and Resources (Grant No. MRE201001).

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