The Holocene 1–8 © The Author(s) 2011 Reprints and permission: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0959683611423685 hol.sagepub.com Special Issue 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 palaeoen- vironments (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 (δ 13 C) values, between −32‰ and −21‰ (Deines, 1980; Meyers, 1994; Schid- lowski et al., 1983) than marine organic matter. Marine algae have C/N ratios <8 because of nitrogen enrichment (Bordovskiy, 1965) and δ 13 C values of −16‰ to −23‰ (Haines, 1976; Meyers, 1994). The δ 13 C content of marine particulate organic carbon (POC) is reported as ranging from −21‰ to −18‰ because marine phyto- plankton 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 δ 13 C values of −26‰ to −30‰ (Meyers, 1994; Schidlowski et al., 1983), while algae in C4-vegetation catchments can have relatively high δ 13 C values of ≥ –16‰ (O’Leary, 1988). In recent decades, C/N ratios and δ 13 C have been used success- fully 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 δ 13 C and C/N values (Sampei and Matsumoto, 2001), the direction of change in δ 13 C 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 δ 13 C and C/N ratios as sea level and sedimentary envi- ronmental indicators in an analysis of the Humber Estuary (UK) 423685HOL XX X 10.1177/09596 83611423685Zhan et al.The Holocene 1 East China Normal University, China 2 Shanghai Geological Survey, China Received 7 December 2010; revised manuscript accepted 28 June 2011 Corresponding author: Zhanghua Wang, State Key Laboratory for Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China. Email: zhwang@geo.ecnu.edu.cn Assessing C/N and δ 13 C as indicators of Holocene sea level and freshwater discharge changes in the subaqueous Yangtze delta, China Qing Zhan, 1 Zhanghua Wang, 1 Yan Xie, 1 Jianlei Xie 2 and Zhongfa He 2 Abstract To examine the applicability of C/N and organic carbon stable isotope (δ 13 C) 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 δ 13 C 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%), CaCO 3 (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 δ 13 C 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 δ 13 C 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 δ 13 C is more useful in reflecting freshwater discharge. Keywords organic carbon sources, postglacial sea level rise,Yangtze Estuary by guest on April 11, 2016 hol.sagepub.com Downloaded from