Late Quaternary environmental change of Yellow River Basin: An organic geochemical record in Bohai Sea (North China)
Introduction
As the cradle of Chinese civilization, the Yellow River Basin (YRB) has received considerable attention from the scientific community to understand Holocene climate change and its impact on local ecosystems and society (An et al., 1991, Pang and Huang, 2006, Huang et al., 2009). A consensus has been reached that the mid-Holocene climate in China (ca 8.5–3.0 ka BP) was warmer and wetter than present as a response to Asian Monsoon variability (Shi et al., 1992). This Holocene Optimum is consistent with paleo-records in many other places of the world (Steig, 1999). Archaeological and historical references revealed that Chinese Loess Plateau (CLP), which is located in upper and mid-reaches of YRB and currently dominated by steppe and grassland, was occupied by a forest dominated ecosystem during the mid-Holocene (Shi, 1991). This conclusion was supported by pollen data extracted from western and southern parts of the CLP where forest extensively developed in valley terraces and mountain areas during the mid-Holocene (Feng et al., 2006, Zou et al., 2009, Shang and Li, 2010). However, the studies for Yuan lands (the plain area) of the CLP suggested herb and shrub plants as the dominant vegetation throughout the Holocene (Liu et al., 1996, Liu et al., 2005, Xie et al., 2002). Such controversial issues may be a result of climatic and topographic complexity of the CLP (Lu et al., 2003). The reconstruction of vegetation history in the YRB was primarily based on continental archives such as loess-paleosol sequences and lake sediments, which likely record local rather than regional environmental signals. In contrast, marine sediments close to the Yellow River outflow (e.g., Bohai Sea) are strongly controlled by fluvial materials and thus contain catchment-integrated environmental signals from upper to lower reaches of the Yellow River.
One challenge of extracting continental signals in the coastal sediment is the high heterogeneity (e.g., mixing of marine and continental materials). Organic matter biomarkers, natural compounds biosynthesized by certain organisms, are powerful paleoclimate and paleoenvironmental proxies (Eglinton and Eglinton, 2008) and particularly useful when applied to complex systems such as the coasts because of their utility in distinguishing organic matter sources (Xu et al., 2007). To the best of our knowledge, a long term biomarker record for Bohai Sea (>10 ky) has not been reported. In this study we analyzed organic matter biomarkers, bulk geochemical characteristics and grain size distributions in a 21 m sediment core from the Bohai Sea, North China, spanning the past 21 ka (based on 14C dating). Three classes of biomarkers, namely lignins, straight chain aliphatic hydrocarbons (n-alkanes) and glycerol dialkyl glycerol tetraethers (GDGTs), were discussed. Lignins are exclusively present in vascular plants, the second most abundant natural organic compound on Earth after cellulose and highly resistant to degradation, making them an ideal tracer of terrigenous organic matter (TOM) (Xu et al., 2009). Long chain n-alkanes (⩾C25) with a strong odd/even carbon predominance (OEP) are predominantly derived from higher plant leaf waxes (Eglinton and Hamilton, 1967), while short chain n-alkanes (<C20) without OEP are predominantly produced by aquatic microbes and/or algae (Meyers, 1997). GDGTs are membrane lipids in Archaea and anaerobic soil bacteria and the branched and isoprenoid tetraether (BIT) index indicates relative importance of soil organic matter versus autochthonous marine or lacustrine organic matter (Hopmans et al., 2004, Weijers et al., 2006). Three questions were mainly addressed in this study: (1) what was the major source of sedimentary organic matter in the Bohai Sea during the Holocene; (2) did the mid-Holocene climate optimum aid in the development of vegetation cover and soil in the YRB; and (3) did woody plants significantly expand in the YRB during the mid-Holocene as a response to climate changes?
Section snippets
Study area and sampling
Bohai Sea is a shallow sea in North China with the surface area of 77,000 km2 and an average water depth of 18 m. It is enclosed by Liaodong and Shandong peninsulas, and connected to the northern Yellow Sea by the Bohai Strait. The Holocene seawater transgression reached the Bohai Sea in early Holocene (Wang and VanStrydonck, 1997). The Yellow River, despite frequently shifting its course, drained into the Bohai Sea for most time of the Holocene (Saito et al., 2001). As the world’s second largest
14C dating and age model
Radiocarbon (14C) dating was undertaken at Peking University Accelerator Mass Spectrometry (AMS) lab. The materials for 14C dating are intact bivalve shells and peat organic matter. The age model was established based on 11 14C determinations. The 14C age was calibrated using the CALIB 5.0 program with two standard deviation uncertainty (2σ) (Stuiver et al., 2005). The marine reservoir effect is 141 yr (Marine Reservoir Correction Database). All ages reported have been calibrated into a calendar
Chronology
The AMS 14C dating for 11 samples is shown in Table 1. An age model is based on linear interpolation between the relevant age points. Since the peat samples at 14.1 and 14.2 m are different by only 10 cm, they are combined as one age point. The sample at 23 m depth was not included in age model because its 14C age is anomalously older than the samples from 23.4 m and 28.9 m depth. In addition we set the surface sediment to the year 2009. Our age model shows the 2.0–21 m section of the TP 23 covers
Source of sedimentary organic matter in Bohai Sea sediments
Each year rivers transport approximately 0.4 × 1015 g organic carbon from continents to the ocean (Hedges et al., 1997), representing one of the most important pathways for organic carbon exchange between land and ocean. Tracing the fate of TOM in oceans is indispensable to understand global biogeochemical cycle. Carbon stable isotope composition has been widely used to determine organic carbon inputs to marine sediments given the fact that C3 continental plants have δ13Corg values of −28‰ to −25‰
Conclusions
The distributions of molecular biomarkers, bulk geochemical property and grain size distribution in Bohai Sea sediments close to the Yellow River outflow record variability in source of organic matter inputs and paleoenvironmental changes since the latest Pleistocene. Four conclusions can be drawn based on this study:
- 1.
All geochemical proxies presented remarkable differences between the late glacial period and the Holocene, reflecting a fluvial plain and marine environment, respectively, due to
Acknowledgements
This project is financially supported by NSFC (41006042), State Education Ministry (SRF for ROCS, SEM) and 100 Talent Project of Peking University. Dr. Guoan Wang is thanked for stable carbon isotope analyses Dr. Rudolf Jaffe and two anonymous reviewers are thanked for constructive comments.
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