Sedimentary records of bulk organic matter and lipid biomarkers in the Bering Sea: A centennial perspective of sea-ice variability and phytoplankton community
Introduction
The sub-Arctic Bering Sea, a transition region between the Pacific and Arctic Oceans, is a key area characterized by high productive and sensitive ecosystem dynamics (Grebmeier et al., 2006; Sigler et al., 2010). Due to its specific geographical location, seasonal sea-ice occurrence, high primary production and abundant biological resources, the Bering Sea plays an important role in the global carbon biogeochemical cycle with significant interaction and response to the recent climate change (e.g., Springer et al., 1996; Chen et al., 2002). Especially, the seasonal sea-ice coverage of the Bering Sea is a major driver of its ecology, which makes this ecosystem particularly sensitive to climate change (Guo et al., 2004; Sigler et al., 2010).
Over the past few decades, sea surface temperature (SST) has increased by about 3 °C in the Bering Sea (Stabeno et al., 2007, Stabeno et al., 2010), and the sea-ice cover has decreased in both concentration and duration (Stroeve et al., 2007, Stroeve et al., 2012). These recent changes have strongly impacted the ecosystem of the Bering Sea since the 1970s; rapid changes in the abundance of phytoplankton, zooplankton and fish have also been observed (Hunt et al., 2002). Recent changes in environmental condition, especially in the late 1970s, were closely associated with the well-documented 1976–1977 climate regime shift of the North Pacific, resulting in warming of the Bering Sea (Hare and Mantua, 2000; Bond et al., 2003). Numerous studies of the evolution of the Bering Sea ecosystem have focused on sea-ice loss connected with this regime shift, even its recognition is complicated by the presence of high-frequency, interannual variations in both physical and biological time series (e.g., Stabeno and Overland, 2001; Hunt et al., 2002; Wooster and Zhang, 2004; Overland and Stabeno, 2004; Stabeno et al., 2007). A dramatic increase in alkenones content around 2000 may have reflected changing hydrologic condition (e.g. stratification) in the northern Bering shelf (Harada et al., 2012); within the same period, annual sea-ice persistence started to increase dramatically in the southern Bering Sea (Frey et al., 2015), reflecting greater spatial heterogeneity in sea-ice variability (Brown and Arrigo, 2012; Frey et al., 2018). In spite of recent interannual variability especially towards the southern Bering Sea (Brown and Arrigo, 2012), considerable evidences has demonstrated that the long-term reduction in sea-ice cover has caused concomitant ecosystem change in the Bering Sea (e.g., Hunt et al., 2002; Grebmeier et al., 2006). Therefore, the Bering Sea is thought to have experienced significant environmental changes under climate warming within the Pacific Arctic region (e.g., Grebmeier et al., 2006; Stroeve et al., 2007; Giesbrecht et al., 2019; Cornwall, 2019).
The sea-ice cover is a critical component of the Bering Sea ecosystem; its seasonal variability is extremely sensitive to changes in both weather and climate (e.g., Overland, 1981; Brown and Arrigo, 2012). The duration and extent of sea ice, sea water temperature and water mass structures are critical controls of the ecosystem dynamics in the Pacific Arctic (Grebmeier et al., 2006). Ecosystem changes in the Bering Sea during recent decades have commonly been attributed to earlier sea-ice retreat and regional atmospheric–oceanic forcing (e.g. the Aleutian Low and sea-water warming) (Grebmeier et al., 2006). Long-term shifts towards earlier algal growth have also been observed, which indicates an ongoing ecosystem shift in this high-latitude marine ecosystem (Fietzke et al., 2015).
Climate-driven regional regime shifts have been reported in the northern Pacific, with potential linkages to environmental changes at interannual or seasonal timescales (Stabeno et al., 1999). However, few studies have coupled a long-term regional climate change with corresponding primary productivity and community structure and their responses, especially in the western Bering Sea. The lack of in situ observations over decadal timescales makes it impossible to identify long-term changes in primary production in this highly productive but sensitive Pacific Arctic region (Hill et al., 2018). Accordingly, analyses of decadal sediment records using sea-ice indicators and phytoplankton-based biomarkers may facilitate the evaluation of the change towards the evolution and/or shifting of phytoplankton community and their relationship to the sea-ice variation.
In recent decades, the organic geochemical biomarker IP25, a highly branched, monounsatuarated C25 isoprenoid (HBI) alkene derived from sea ice diatoms, has been developed as sea ice proxy (Belt et al., 2007). IP25 as well as the so-called PIP25 (Phytoplankton-IP25) have subsequently been used to reconstruct recent and past sea-ice conditions in Arctic and sub-Arctic regions (e.g., Müller et al., 2009, Müller et al., 2011; Fahl and Stein, 2012; Méheust et al., 2013; Xiao et al., 2013, Xiao et al., 2015; Belt et al., 2015; Stein et al., 2016; Ruan et al., 2017; Bai et al., 2019; for reviews see Stein et al., 2012; Belt and Müller, 2013; Belt, 2018). In the Chukchi/Bering Sea area, i.e., a region close to our study area, IP25 and phytoplankton biomarker records reflecting the Last Glacial and Holocene history of sea-ice and primary productivity are published by Méheust et al., 2015, Méheust et al., 2018, Polyak et al. (2016) and Stein et al. (2017).
In the present study, we analyzed these biomarkers in two sediment cores well-dated in 210Pb from the western Bering Sea to determine a long-term phytoplankton community evolution and biomarker-related sea-ice occurrence over the past century. The main objectives were to examine long-term sea-ice conditions and phytoplankton evolution, and to identify the relationships between the sea-ice conditions and phytoplankton productivity as well as its spatial variability in the context of the recent climate regime shifts in the Bering Sea.
Section snippets
Regional setting
The Bering Sea, which is characterized by seasonal variation in sea-ice coverage, is the gateway for the imports from the Pacific Ocean to the Arctic Ocean (Jones et al., 2003). The physical oceanography of the Bering Sea is influenced by tides, winds, topography, flows through passages and the annual formation, drift and melting of sea ice (Schumacher et al., 2003). In summer, three major water masses flow northward over the northern Bering shelf (Fig. 1). On this shallow shelf, the Alaska
Sampling and sediment core dating
Two sediment cores (BL16 and LV63–23) were recovered from the Bering Sea during the 2012 Chinese Arctic Research Expedition (CHINARE-2012) and the 2013 China-Russia Joint Expedition, respectively, using a multicorer (Fig. 1). The two cores were subsampled at 1 cm intervals and then stored at −20 °C until further processing.
Sedimentation rates and the chronology framework were determined for each core by analyzing 210Pb and 137Cs activity in selected horizons (Fig. 2). Age model for each of the
210Pb and 137Cs profiles and sediment chronology of the two cores
Profiles of excess 210Pb (210Pbex) in the two cores are shown in Fig. 2 and linear sediment rates were calculated based on the constant initial concentration (CIC) model according to the exponential decreased trend of 210Pbex throughout the core (Fig. 2). The log-linear 210Pbex profiles coupled with steady sediment composition throughout the two cores indicated limited bioturbation or sediment disturbance, although a few vertically homogeneous trends were visible in the upper horizons (e.g.,
Source identification of sedimentary organic matter (SOM)
The low TOC/TN ratios observed in core BL16 (5.86 ± 0.49, Table S2) from the northern shelf may be partly related to the presence of soil-derived bound ammonium due to adsorption by clays, causing (too) low ratios (Schubert and Calvert, 2001; Stein and Macdonald, 2004). This finding is also in line with the previous reports of OM sources along the high-latitude coastal margins of the southern Chukchi Sea, which showed more refractory OM near shore versus more labile OM at offshore and/or in the
Conclusions
Sedimentary bulk organic matter and biomarker records indicated decadal to centennial timescales of sea-ice variability across the Bering Sea and corresponding phytoplankton community changes in this sensitive Pacific Arctic region. A trend of δ13CTOC depletion since the late 1970s observed in the core sampled from the northern Bering shelf revealed increased terrigenous input, which could be ascribed to warming scenario during recent decades. Compared to the northern shelf, where OM
Declaration of Competing Interest
None.
Acknowledgments
We wish to thank the participants of R/V Xuelong and R/V Akademik M.A. Lavrentyev for sampling assistance during the cruise. We are most grateful to members of the AWI laboratory (Lester Lembke-Jene and Nicoletta Ruggieri) and the Kochi University (Minoru Ikehara) for help with sample processing and organic chemical analysis. This work was financially supported by Qingdao National Laboratory for Marine Science and Technology (2016ASKJ13, 2018SDKJ0104-3), the National Natural Science Foundation
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