A 1700-year n-alkanes hydrogen isotope record of moisture changes in sediments from Lake Sugan in the Qaidam Basin, northeastern Tibetan Plateau

We present a hydrogen isotopic record of long-chain n-alkanes in Lake Sugan to reconstruct regional moisture changes in the last 1700 years at a sampling resolution of 25 years. The D values decreased by over 30 parts per thousand from the Medieval Warm Period&#39; (MWP; c. ad 600-1500) to the Little Ice Age&#39; (LIA; c. ad 1500-1850), and indicated that the moisture pattern included a relatively humid climate before the MWP, becoming drier overall in the MWP, wetter in the LIA, and then tending towards drought in the post-industrial era. In the study region, Chenopodiceae shrubs were more abundant in dry climates than in wet. Meanwhile, D values of shrubs were more positive than those of grasses. Therefore, we suggest that the vegetation type (shrub or grass), which depends on moisture changes, is the controlling factor for D variations in Lake Sugan.</p

Salinity control on long-chain alkenone distributions in lake surface waters and sediments of the northern Qinghai-Tibetan Plateau, China

Long-chain alkenones in lacustrine settings are potentially excellent biomarkers for the reconstruction of past terrestrial environmental conditions, and have been found in many different types of lakes around the globe. A wider range of factors influence the occurrence and distribution of alkenones in lake sediments and waters when compared to marine systems. Lake environmental conditions, such as temperature (in particular) and salinity, are among the key factors controlling alkenone distributions in lacustrine settings. Here we investigated alkenone distribution patterns in lakes of the northern Qinghai-Tibetan Plateau, China, and their possible relationship with environmental conditions, by analyzing paired samples of suspended particulate matter in surface waters and surface sediments. Salinity of investigated lake waters ranges from almost 0 to similar to 100 g/L, while temperature variation among the lakes is minimal, effectively eliminating temperature effects on the alkenone distribution patterns observed here. We show that (1) alkenone concentrations vary substantially between the lakes, yet controlling mechanisms remain elusive; (2) C(37)/C(38) ratios are substantially lower in the lakes of the Qaidam Basin than in the Lake Qinghai region, probably indicating different alkenone producers in the two regions; and (3) large variations in %C(37:4) (the percentage of the C(37:4) alkenone), determined from both surface waters and sediments, are negatively correlated with salinity. We suggest that the %C(37:4) index could be used as a salinity indicator at least on a regional scale, with careful considerations of other potentially complicating factors. However, potential reasons for why salinity could significantly affect %C(37:4) values need further investigation.</p

Carbon isotope composition of long chain leaf wax n-alkanes in lakesediments: A dual indicator of paleoenvironment in the Qinghai-TibetPlateau

The carbon isotope composition (d13C values) of long chain n-alkanes in lake sediments has been considered a reliable means of tracking changes in the terrigenous contribution of plants with C3 and C4 photosynthetic pathways. A key premise is that long chain leaf wax components used for isotope analysis are derived primarily from terrigenous higher plants. The role of aquatic plants in affecting d13C values of long chain n-alkanes in lacustrine sediments may, however, have long been underestimated. In this study, we found that a large portion of long chain n-alkanes (C27 and C29) in nearshore sediments of the Lake Qinghai catchment was contributed by submerged aquatic plants, which displayed a relatively positive carbon isotope composition (e.g. 26.7&permil; to 15.7&permil; for C29) similar to that of terrestrial C4 plants. Thus, the use of d13C values of sedimentary C27 and C29 n-alkanes for tracing terrigenous vegetation composition may create a bias toward significant overestimation/underestimation of the proportion of terrestrial C4 plants. For sedimentary C31, however, the contribution from submerged plants was minor, so that the d13C values for C31 n-alkane in surface sediments were in accord with those of the modern terrestrial vegetation in the Lake Qinghai region. Moreover, we found that changes in the d13C values of sedimentary C27 and C29 n-alkanes were closely related to water depth variation. Downcore analysis further demonstrated the significant influence of endogenous lipids in lake sediments for the interpretation of terrestrial C4 vegetation and associated environment/climate reconstruction. In conclusion, our results suggest that the d13C values of sedimentary long chain n-alkanes (C27, C29 and C31) may carry different environmental signals. While the d13C values of C31 were a reliable proxy for C4/C3 terrestrial vegetation composition, the d13C values of C27 and C29 n-alkanes may have recorded lake ecological conditions and sources of organic carbon, which might be affected by lake water depth.</p

Carbon isotopic composition of isoprenoid tetraether in surface sediments of Lake Qinghai and surrounding soils

Isoprenoid GDGTs (iGDGTs), along with their constituent biphytanyl moieties, are biomarkers for archaea. In order to obtain more information on identifying the carbon source and potential carbon assimilation pathway of archaea in surface sediments of Lake Qinghai and the surrounding soils, the stable carbon isotopic composition of iGDGT-derived biphytanes as well as its relationship with delta C-13 values of total organic carbon (TOC) and dissolved inorganic carbon (DIC) was investigated. The delta C-13 values of iGDGT-derived biphytanes ranged from -42.7% to -20.3%. For example, the values for acyclic biphytanes (BP-0), the most abundant, varied from -25.0% to -22.1% in offshore sediments, -30.5% to -25.4% in surrounding soils and -42.7% to -32.0% in nearshore sediments. The wider variation than that of delta C-13(TOC) (-26.9% to -25.1%) in offshore sediments and that in surrounding soils indicated that delta C-13 values of BP-0 may be a sensitive indicator for examining the depositional environments between terrestrial and lacustrine systems. With average delta C-13 values of -23.6%, crenarchaeol-derived biphytanes (BP-cren) in offshore sediments were enriched in C-13 (ca. 3.6%) relative to TOC. In addition, the carbon isotopic fractionation between BP-cren and DIC was -21.3%, consistent with that reported for marine Thaumarchaeota. The enrichment in C-13 relative to TOC and the similar carbon isotopic fractionation indicated that Thaumarchaeota in offshore sediments of Qinghai Lake are autotrophic. With average delta C-13 values of -29.2%, BP-cren in surrounding soils was depleted by 0.9% to 12.8% in C-13 relative to DIC. The quite large scatter in carbon isotopic fractionation between BP-cren and DIC indicated that the biomass of soil Thaumarchaeota originated from inorganic and organic carbon sources, suggesting a mixotrophic lifestyle.</p

Late Miocene episodic lakes in the arid Tarim Basin, western China

The Tibetan Plateau uplift and Cenozoic global cooling are thought to induce enhanced aridification in the Asian interior. Although the onset of Asian desertification is proposed to have started in the earliest Miocene, prevailing desert environment in the Tarim Basin, currently providing much of the Asian eolian dust sources, is only a geologically recent phenomenon. Here we report episodic occurrences of lacustrine environments during the Late Miocene and investigate how the episodic lakes vanished in the basin. Our oxygen isotopic (delta O-18) record demonstrates that before the prevailing desert environment, episodic changes frequently alternating between lacustrine and fluvial-eolian environments can be linked to orbital variations. Wetter lacustrine phases generally corresponded to periods of high eccentricity and possibly high obliquity, and vice versa, suggesting a temperature control on the regional moisture level on orbital timescales. Boron isotopic (delta B-11) and delta O-18 records, together with other geochemical indicators, consistently show that the episodic lakes finally dried up at similar to 4.9 million years ago (Ma), permanently and irreversibly. Although the episodic occurrences of lakes appear to be linked to orbitally induced global climatic changes, the plateau (Tibetan, Pamir, and Tianshan) uplift was primarily responsible for the final vanishing of the episodic lakes in the Tarim Basin, occurring at a relatively warm, stable climate period.</p

Distribution of glycerol dialkyl glycerol tetraethers in surface sediments of Lake Qinghai and surrounding soil

Glycerol dialkyl glycerol tetraethers (GDGTs) are increasingly used as proxies for paleoclimate studies of marine and lacustrine environments. While GDGT-based proxies have been applied to a number of lake environments globally, little is known about the distribution of GDGTs on the Qinghai-Tibet Plateau. We have investigated the isoprenoid GDGTs (iGDGTs) and branched GDGTs (bGDGTs) in Lake Qinghai sediments and the surrounding surface soil in order to examine their potential use as paleoclimate proxies on the Qinghai-Tibet Plateau. The results show that (i) the values of the iGDGT/bGDGT ratio for surrounding soil were at the higher end among globally distributed soils and consequently BIT values (avg. 0.71) at the lower end, (ii) the TEX86 values decreased while the MBT and CBT values increased along an onshore soil-nearshore sediment-offshore sediment transect, (iii) the TEX86 values for the offshore sediments were almost identical and their inferred temperatures were close to mean summer surface water temperature and (iv) the bGDGT-inferred mean annual air temperature (MAAT) and pH for soil were consistent with measured MAAT and pH. However, the CBT-inferred pH for offshore sediments seemed inconsistent with the pH of lake water or sediment. Our results suggest that (i) the higher pH may be an important factor leading to the higher iGDGT/bGDGT values (and lower BIT values) in surrounding surface soil, (ii) both iGDGTs and bGDGTs may originate from terrestrial input and in situ production for this saline lake, especially for nearshore sediments. However, for offshore sediments, terrestrial iGDGT input seems minor, and TEX86 may be useful for paleoclimate studies of Lake Qinghai.</p

Assessing the ratio of archaeol to caldarchaeol as a salinity proxy in highland lakes on the northeastern Qinghai-Tibetan Plateau

The ratio of archaeol to caldarchaeol (the ACE index) has been proposed recently as an index for paleosalinity reconstruction and is based principally on archaeal core lipids (CLs) from coastal salt pans (Turich, C., Freeman, K.H., 2011. Archaeal lipids record paleosalinity in hypersaline systems. Organic Geochemistry 42, 1147-1157). We have examined possible relationships between salinity and ACE in both CLs and intact polar lipids (IPLs) from suspended particulate matter (SPM) and surface sediments of lakes and surrounding soils on the northeastern Qinghai-Tibetan Plateau. Our results showed that ACE values were positively correlated with salinity in all samples; however, CL ACE values were systematically higher than IPL ACE values, probably due to different degradation kinetics of intact polar (IP) archaeol and IP caldarchaeol. On the other hand, surface sediment ACE values from both CLs and IPLs were lower than SPM ACE values, probably due to enhanced production of caldarchaeol relative to archaeol in the sediment. Our results demonstrate that the ACE proxy reflects changes in salinity in diverse environments on the Qinghai-Tibetan Plateau, which is promising for paleosalinity reconstruction; however, caution should be used when applying the salinity proxy before we have a better understanding of degradation kinetics of archaeal IPLs and in situ production of caldarchaeol and archaeol in sediments.</p

Solar influenced late Holocene temperature changes on the northern Tibetan Plateau

Considerable efforts have been made to extend temperature records beyond the instrumental period through proxy reconstructions, in order to further understand the mechanisms of past climate variability. Yet, the global coverage of existing temperature records is still limited, especially for some key regions like the Tibetan Plateau and for earlier times including the Medieval Warm Period (MWP). Here we present decadally-resolved, alkenone-based, temperature records from two lakes on the northern Tibetan Plateau. Characterized by marked temperature variability, our records provide evidence that temperatures during the MWP were slightly higher than the modern period in this region. Further, our temperature reconstructions, within age uncertainty, can be well correlated with solar irradiance changes, suggesting a possible link between solar forcing and natural climate variability, at least on the northern Tibetan Plateau.</p

Water depth affecting thaumarchaeol production in Lake Qinghai, northeastern Qinghai-Tibetan plateau: Implications for paleo lake levels and paleoclimate

Archaeal glycerol dialkyl glycerol tetraethers (GDGTs) are increasingly popular and versatile tool for palaeolimnology studies, but their applications in paleohydrology are scarce, especially for thaumarchaeol which is specific for the newly proposed phylum Thaumarchaeota. After investigating our published GDGT data of Lake Qinghai, we found that both the concentration of thaumarchaeol and the relative abundance of thaumarchaeol to total archaeal GDGTs (%thaum) in core-top sediments increased significantly with increasing water depth (R=0.88 and 0.95, respectively), with lower concentrations of 5+/-5 ng/g in shallow areas (water depth&lt;5m) and higher concentrations of 121+/-65ng/g in deep areas (water depth&gt;10m). This is likely because that the producers of thaumarchaeol, Thaumarchaeota, prefer living in the relative deeper zone in lacustrine systems, where probably both competition of ammonium (the substrate) from other microbes and light intensity are low. Therefore, we proposed that thaumarchaeol was mainly produced in situ and changes in % thaum might reflect water-depth variations in this closed-basin lake. The application of % thaum as a water-depth indicator in a Holocene sediment sequence of core QH-2011 provided a high-resolution relative lake-level history of Lake Qinghai which resembles that inferred from the delta C-13(org) value obtained in the same core. This supports the use of %thaumas an indicator of lakewater depth in paleohydrology studies, especially for medium lakes. Moreover, the records of the two independent proxies in core QH-2011 confirmed a shallow Lake Qinghai in the early Holocene and a late-Holocene highstand, highlighting the importance of local temperature (and evaporation loss) in controlling effective moisture in the arid/semi-arid region.</p