Huanye Wang

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

Significance The relative roles of the two plausible causes, Cenozoic global cooling and Tibetan Plateau uplift, for the Asian interior aridification/desertification are often difficult to disentangle. High-quality terrestrial records from the Tarim Basin document how enhanced aridification occurred over the last 7 my, from episodic lacustrine environments to the currently prevailing desert environments. During the Late Miocene–Early Pliocene period, episodic occurrences of great lakes and thus wetter climate closely corresponded to periods of high eccentricity and possibly high obliquity, suggesting that the uplifted plateau by then did not effectively block the moisture availability at optimum climatic conditions. The finally irreversible drying up of great lakes at ∼4.9 Ma could be attributed to the plateau uplift. 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 (δ18O) 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 (δ11B) and δ18O records, together with other geochemical indicators, consistently show that the episodic lakes finally dried up at ∼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.

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 δD values decreased by over 30‰ from the ‘Medieval Warm Period’ (MWP; c. ad 600–1500) to the ‘Little Ice Age’ (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, δD 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 δD variations in Lake Sugan.

Influence of aquatic plants on the hydrogen isotope composition of sedimentary long-chain n-alkanes in the Lake Qinghai region, Qinghai-Tibet Plateau

The hydrogen isotopic composition (δD) of leaf wax long-chain n-alkanes (C27, C29, and C31) from lacustrine sediments has been widely applied to reconstruct terrestrial paleoclimatic and paleohydrological changes. However, few studies have addressed whether the aquatic-derived n-alkanes can affect the δD values of lake sedimentary long-chain n-alkanes, which are usually regarded as a recorder of the terrestrial hydrological signals. Here we systematically investigated δD values of long-chain n-alkanes from modern aquatic plants, both near-shore and off-shore surface sediments, surrounding terrestrial plant litters, as well as river water and lake water in Lake Qinghai and its satellite lakes on the northeastern Qinghai-Tibet Plateau. Our data showed that (i) δD values of long-chain n-alkanes from aquatic plants varied from -184‰ to -132‰ for n-C27, from -183‰ to -138‰ for n-C29, and from -189‰ to -130‰ for n-C31, respectively, with no significant differences among the three n-alkanes homologues; (ii) δD values of long-chain n-alkanes from aquatic plants were generally more positive than those from surrounding terrestrial plants, possibly because that they recorded the D-enrichment of lake water in this semi-arid region; (iii) δD values of long-chain n-alkanes from surface sediments showed significant differences among the three n-alkanes homologues, due to the larger aquatic input of n-C27 to the sedimentary lipid pool than that of n-C31, and (iv) n-C27δD values of near-shore aquatic plants and near-shore sediments are more negative than those from off-shore as a result of lower δD values of near-shore lake water. Our findings indicate that in this region (i) the offset between sedimentary n-C27 and n-C31δD values (ΔδDC27-C31) could potentially be used to evaluate if sedimentary long-chain n-alkanes are derived from a single source; (ii) while δD values of n-C27 may be influenced by lake water hydrological changes, sedimentary n-C31 is derived predominantly from terrestrial plants and thus its δD can serve as a relatively reliable indicator for terrestrial paleoclimatic and paleohydrological reconstructions.

amoA-encoding archaea and thaumarchaeol in the lakes on the northeastern Qinghai-Tibetan Plateau, China

All known ammonia-oxidizing archaea (AOA) belong to the phylum Thaumarchaeota within the domain Archaea. AOA possess the diagnostic amoA gene (encoding the alpha subunit of ammonia monooxygenase) and produce lipid biomarker thaumarchaeol. Although the abundance and diversity of amoA gene-encoding archaea (AEA) in freshwater lakes have been well-studied, little is known about AEA ecology in saline/hypersaline lakes. In this study, the distribution of the archaeal amoA gene and thaumarchaeol were investigated in nine Qinghai–Tibetan lakes with a salinity range from freshwater to salt-saturation (salinity: 325 g L-1). The results showed that the archaeal amoA gene was present in hypersaline lakes with salinity up to 160 g L-1. The archaeal amoA gene diversity in Tibetan lakes was different from those in other lakes worldwide, suggesting Tibetan lakes (high elevation, strong ultraviolet, and dry climate) may host a unique AEA population of different evolutionary origin from those in other lakes. Thaumarchaeol was present in all of the studied hypersaline lakes, even in those where no AEA amoA gene was observed. Future research is needed to determine the ecological function of AEA and possible sources of thaumarchaeol in the Qinghai–Tibetan hypersaline lakes.

Impacts of temperature and pH on the distribution of archaeal lipids in Yunnan hot springs, China

In culture experiments and many low temperature environments, the distribution of isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) commonly shows a strong correlation with temperature; however, this is often not the case in hot springs. We studied 26 hot springs in Yunnan, China, in order to determine whether temperature or other factors control the distribution of GDGTs in these environments. The hot springs ranged in temperature from 39.0 to 94.0°C, and in pH from 2.35 to 9.11. Water chemistry including nitrogen-, sulfur-, and iron species was also determined. Lipids from the samples were analyzed using liquid chromatography–mass spectrometry (LC–MS). Distributions of GDGTs in these hot springs were examined using cluster analysis, which resulted in two major groups. Group 1 was characterized by the lack of dominance of any individual GDGTs, while Group 2 was defined by the dominance of GDGT-0 or thaumarchaeol. Temperature was the main control on GDGT distribution in Group 1, whereas pH played an important role in the distribution of GDGTs in Group 2. However, no correlations were found between the distribution of GDGTs and any of the nitrogen-, sulfur-, or iron species. Results of this study indicate the dominance of temperature or pH control on archaeal lipid distribution, which can be better evaluated in the context of lipid classification.

A 12-kyr record of microbial branched and isoprenoid tetraether index in Lake Qinghai, northeastern Qinghai-Tibet Plateau: Implications for paleoclimate reconstruction

Branched and Isoprenoid Tetraether (BIT) index was considered as a proxy for terrestrial organic matter input in lake sediments, based on the assumption that branched glycerol dialkyl glycerol tetraethers (bGDGTs) are mainly derived from terrestrial soils. However, mounting evidences have showed that the in situ production of bGDGTs is widespread in lakes, challenging BIT as a reliable terrestrial input proxy. Recently, BIT has been proven to be a reliable proxy for paleohydrology in a small crater lake (Lake Challa) in accordance with a different mechanism. However, the response of BIT to paleohydrology variation may differ for different lakes. In this study, we investigate the variations in the BIT index and the concentrations of its related GDGTs in a 12-ka sediment core from Lake Qinghai, in combination with our previous results for surface sediments. We find that variations in BIT strongly depend on the concentration of crenarchaeol in both surface and ancient sediments of this lake, whereas bGDGT concentration varies much less remarkably. Considering that crenarchaeol production is positively correlated with water depth in Lake Qinghai, water depth may exert negative control on the BIT index in this lake. This case is inconsistent with the positive relationship between BIT and lake levels or rainfall intensity reported for Lake Challa, suggesting that the response of BIT to local paleohydrology is site specific in lacustrine systems. Hence, the application of sedimentary BIT as a paleohydrological proxy in a specific lake requires caution before confirming the environmental controls on BIT in that lake.

Transient temperature asymmetry between hemispheres in the Palaeogene Atlantic Ocean

During the Late Palaeogene between ~40 and 23 million years ago (Ma), Earth transitioned from a warm non-glaciated climate state and developed large dynamic ice sheets on Antarctica. This transition is largely inferred from the deep-sea oxygen isotope record because records from independent temperature proxies are sparse. Here we present a 25-million-year-long alkenone-based record of surface temperature change from the North Atlantic Ocean. Our long temperature record documents peak warmth (~29 °C) during the middle Eocene, a slow overall decline to the Eocene/Oligocene transition (EOT, ~34 Ma) and high-amplitude variability (between ~28 and 24 °C) during the Oligo–Miocene. The overall structure of the record is similar to that of the deep-sea record, but a distinct anomaly is also evident. We find no evidence of surface cooling in the North Atlantic directly coinciding with the EOT when Antarctica first became cold enough to sustain large ice sheets and subantarctic waters cooled substantially. Surface ocean cooling during the EOT was therefore strongly asymmetric between hemispheres. This transient thermal decoupling of the North Atlantic Ocean from the southern high latitudes suggests that Antarctic glaciation triggered changes in ocean circulation-driven heat transport and influenced the far-field climate response.Northern and Southern hemisphere temperatures were decoupled during the Eocene/Oligocene transition, suggests a sea surface temperature record from the North Atlantic.

A rapid lake-shallowing event terminated preservation of the Miocene Clarkia Fossil Konservat-Lagerstätte (Idaho, USA)

The world-renowned middle Miocene Clarkia lacustrine deposits (15.4–16.0 Ma) in northern Idaho, United States, known as Fossil Lagerstatten , yield extraordinary fossils that preserve in situ ancient biomolecules and organic biomarkers. The sudden formation of the Clarkia Lake basin by means of the Columbia River Basalt damming the proto–St. Maries River is well documented, but less is known about the tempo and mode of the lake environmental succession which impacted on the preservation of these Fossil Lagerstatten . Here, we present evidence for a previously unrecognized, geologically instantaneous drop in the Clarkia Lake water level, using tetraether-based water-depth proxies from a continuous sedimentary sequence at the classic P-33 site. Terrestrial hydrological conditions inferred from compound-specific hydrogen isotope compositions (δD) and tetraether-derived temperature estimates from the same sequence show that the rapid shallowing by >10 m was independent of regional climatic changes. We hypothesize that a volcanic-related geological event was primarily responsible for the rapid reduction of Clarkia Lake water depth—an event that played a decisive role in switching depositional conditions for Clarkia Fossil Lagerstatten from a conservation deposit to a concentration deposit.