Major increase in winter and spring precipitation during the Little Ice Age in the westerly dominated northern Qinghai-Tibetan Plateau
Introduction
Long-chain alkenones (LCAs) are important lipid biomarkers exclusively produced by haptophyte algae within the order Isochrysidales (e.g., Marlowe et al., 1984; Volkman et al., 1980, 1995; Müller et al., 1998; Versteegh et al., 2001). In the global ocean, the alkenone unsaturation indices ( and ) are well-established proxies for past sea surface temperature reconstructions (e.g., Brassell et al., 1986; Prahl and Wakeham, 1987; Müller et al., 1998). The wide spread application of or indices in ocean settings have led many lacustrine studies to simply inherit similar approaches to reconstruct past temperatures (e.g., Liu et al., 2006; He et al., 2013; Song et al., 2015; Randlett et al., 2014; Hou et al., 2016; Zhao et al., 2017). However, unlike the relatively uniform LCA producers in the open ocean, recent studies clearly indicate the presence of more than one LCA-producing haptophyte species in saline lakes (e.g., Coolen et al., 2004; Theroux et al., 2010; Randlett et al., 2014; Longo et al., 2016; Zhao et al., 2017). For example, sudden shifts in LCA unsaturation ratios have been found in sediment cores from saline lakes and attributed to temporal shifts in haptophyte species (Theroux et al., 2010; Toney et al., 2012; Randlett et al., 2014; Zhao et al., 2017). In contrast to the complications in natural systems, pure culture studies demonstrate that individual lacustrine and brackish-water haptophyte species display excellent linear regressions between (or ) and temperatures, although the traditional index used for ocean settings is less correlated to temperature (Zheng et al., 2016). Therefore, the loss of a simple temperature relationship in sediment cores of saline lakes most likely originates from mixing or alternating dominance of two or more haptophyte species at different times in the lakes (Theroux et al., 2010; Toney et al., 2012; Randlett et al., 2014; Zhao et al., 2017). There could also be seasonal variations in productivity among different haptophyte species that experience different growth temperatures. When alkenones from multiple species are combined in the sediments, the temperature resulting from would represent a weighted average temperature and such weighting may have changed temporally in the past, complicating paleoclimate interpretations.
In addition to paleotemperature implications, many studies have also identified an apparent relationship between %C37:4 (i.e., percentage of tetra-unsaturated relative to the total di-, tri- and tetra-unsaturated C37 alkenones) and sea surface salinity, with higher %C37:4 values corresponding to fresher conditions (e.g., Rosell-Melé, 1998; Sicre et al., 2002; Harada et al., 2003). The underlying causes for such relationship are, however, debated (e.g., Sikes and Sicre, 2002; Bendle et al., 2005; Kaiser et al., 2017). Cultures of individual lacustrine (or ocean) haptophyte species do not yield corresponding %C37:4 changes as salinity varies (Chivall et al., 2014; M'boule et al., 2014). The apparent correlation between %C37:4 and salinity most likely originates from production of alkenones with different %C37:4 by different haptophyte species, such as in the Baltic Sea (Kaiser et al., 2017). Salinity reconstructions from saline lakes have so far simply inherited the same rationale from ocean settings (e.g., Liu et al., 2006; Liu et al., 2008, 2011; He et al., 2013; Song et al., 2015). Most likely, there could also be different haptophyte species producing different amounts of %C37:4 at different salinity, and when salinity decreases, species producing greater amounts of C37:4 alkenone grow more successfully than the one producing less C37:4 alkenone, resulting in an apparent correlation between %C37:4 and salinity. Such apparent correlation has been identified in Lake Sugan by comparing with instrumental records of precipitation, with higher annual precipitation leads to lower %C37:4 in sediments (He et al., 2013). In Lake Van (Huguet et al., 2011; Randlett et al., 2014), increased precipitation lowers surface salinity and leads to increase %C37:4 in sediment trap samples.
Multiple proxy reconstructions from northern Qinghai-Tibetan plateau have shown contrasting precipitation changes over the late Holocene (e.g., Thompson et al., 1995; Yao et al., 1996; Chen et al., 2006, 2010; Liu et al., 2006; Liu et al., 2009; Zhao et al., 2009; He et al., 2013), divided by the summer monsoon dominated southeastern region and westerly dominated northwestern region (Fig. 1). In the northeastern Qinghai-Tibetan plateau, the division line is primarily defined topographically (mountain ridges), since summer monsoon moisture rarely crosses the Kunlun Mountains into the Qaidam basin (Fig. 1b). Unlike the relative dry “Little Ice Age” (LIA: AD 1400–1850; He et al., 2013) in the monsoon dominated northeastern region of the Qinghai-Tibetan plateau (e.g., Zhang et al., 2003; Shao et al., 2005; Liu et al., 2006; Yang et al., 2014; Xu et al., 2016), the westerly-dominated central Asia was relatively wet during the LIA (e.g., Thompson et al., 1995; Yao et al., 1996; Ma and Edmunds, 2006; Sorrel et al., 2006; Austin et al., 2007; Chen et al., 2006, 2010; He et al., 2013; Aichner et al., 2015; Chen et al., 2015). However, there has been no quantitative reconstruction of precipitation changes over this time interval in the westerly-dominated northern Qinghai-Tibetan Plateau (e.g., Qaidam Basin). Importantly, the seasonality of the LIA precipitation increase is unclear. In this arid region, winter/spring precipitation is particularly important for agriculture and natural ecosystems, as plants germination relies on sufficient winter snow (producing melt water pools on the landscape) and spring precipitation (Ehleringer et al., 1991). In addition, satellite-derived Normalized Difference Vegetation Index (NDVI) data suggested that desert vegetation is highly sensitive to winter and spring precipitation in northwestern China, but much less so to summer/autumn precipitation (Zhao et al., 2011).
Here we analyzed LCAs in a high sedimentation rate short core from the hypersaline Lake Gahai in the northern Qinghai-Tibetan Plateau (Fig. 1a; Fig. S1). Previous studies published the robust chronologies of short sediment cores based on 137Cs and 210Pb dating in Lake Gahai (Zhao et al., 2008; Li et al., 2012). The sedimentation rate (up to ∼0.5 cm/year) of Lake Gahai (Zhao et al., 2008) is exceptionally high, allowing direct comparison with the instrumental data collected at the Delingha meteorological station located in the basin. In this study, we take advantage of the high sedimentation rate of Lake Gahai to perform an instrumental calibration of alkenone distributions. We identify a strong relationship between %C37:4 and combined winter-spring precipitation. Based on this instrumental calibrated transfer function, we quantitatively convert a published high resolution %C37:4 record (He et al., 2013) to spring and winter-spring precipitation changes spanning the past millennium. Our ultimate objective is to unravel underlying mechanisms controlling the regional precipitation patterns and develop possible scenarios of future precipitation trends.
Section snippets
Study site
The northern Qinghai-Tibetan Plateau is a climatologically important region as it involves complex interactions between the mid-latitude westerly and the subtropical Asian monsoon circulations (Chen et al., 2010; Yang et al., 2011). The Qaidam Basin in the northern Qinghai–Tibetan Plateau is one of the largest hyper-arid basins in the northern hemisphere, with an area of 120,000 km2. Lake Gahai (37.12°N, 97.55°E, 2, 848 m above sea level) is a topographically closed-basin lake, situated in the
The impact of alkenone-producing haptophyte species on the C37 LCA temperature proxies
LCAs were quantified in 39 of 40 sediment samples in the core (LCA concentration was too low in 3.5–4 cm depth for accurate measurements). C37 and C38 concentrations ranged from 1.1 to 233.3 μg/gdw (micrograms of alkenones per gram of dry sediment) with an average of 23.5 μg/gdw (Fig. 3; Table S3). The absence of the C37:3 alkenone isomer and C38 methyl alkenones is typical of LCA distribution in Group II haptophyte species (Fig. S2; Theroux et al., 2010; Longo et al., 2016; Zheng et al., 2016,
Conclusions
We demonstrate that C37 alkenone unsaturation ratios (regardless of which indices , or are used) can't be simply translated into past temperature changes in Lake Gahai, due to major changes in alkenone-producing haptophyte species. Results from our comparison with instrumental temperature records suggest a temporally alternating dominance of two different alkenone-producing haptophyte species, one blooming during the early spring (May) and the other blooming during the middle
Acknowledgments
This research was funded by the National Natural Science Foundation of China (Grant No. 41573113 to Y. Huang; Grant No. 41602193 to J. Zhao and Grant No. 41702187 to Y. Yao), project from the State Key Laboratory of Loess and Quaternary Geology in the Institute of Earth Environment of the Chinese Academy of Sciences (SKLLQGPY1704) and U.S. National Science Foundation grants ATM-0902805, EAR-1122749, PLR-1503846, EAR-1502455; EAR-1762431 to Y. Huang. We are grateful to two anonymous reviewers
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