Juzhi Hou

Geographic distance and pH drive bacterial distribution in alkaline lake sediments across Tibetan Plateau.

Continent-scale biogeography has been extensively studied in soils and marine systems, but little is known about biogeographical patterns in non-marine sediments. We used barcode pyrosequencing to quantify the effects of local geochemical properties and geographic distance for bacterial community structure and membership, using sediment samples from 15 lakes on the Tibetan Plateau (4–1670 km apart). Bacterial communities were surprisingly diverse, and distinct from soil communities. Four of 26 phyla detected were dominant: Proteobacteria, Bacteroidetes, Firmicutes and Actinobacteria, albeit 20.2% of sequences were unclassified at the phylum level. As previously observed in acidic soil, pH was the dominant factor influencing alkaline sediment community structure, phylotype richness and phylogenetic diversity. In contrast, archaeal communities were less affected by pH. More geographically distant sites had more dissimilar communities (r = 0.443, P = 0.030). Variance partitioning analysis showed that geographic distance (historical contingencies) contributed more to bacterial community variation (12.2%) than any other factor, although the environmental factors explained more variance when combined (28.9%). Together, our results show that pH is the best predictor of bacterial community structure in alkaline sediments, and confirm that both geographic distance and chemical factors govern bacterial biogeography in lake sediments.

Postglacial climate reconstruction based on compound‐specific D/H ratios of fatty acids from Blood Pond, New England

We determined hydrogen isotope ratios of individual fatty acids in a sediment core from Blood Pond, Massachusetts, USA, in order to reconstruct climate changes during the past 15 kyr. In addition to palmitic acid (C16n-acid), which has been shown to record lake water D/H ratios, our surface sediments and down core data indicate that behenic acid (C22n-acid), produced mainly by aquatic macrophytes, is also effective for capturing past environmental change. Calibration using surface sediments from two transects across eastern North America indicates that behenic acid records δD variation of lake water. Down core variations in δD values of behenic acid and pollen taxa are consistent with the known climate change history of New England. By evaluating the hypothesis that D/H fractionations of long chain even numbered fatty acids (C24-C32n-acids) relative to lake water provide independent estimates of relative humidity during the growing season, we find that differences between lake-level records and isotopically inferred humidity estimates may provide useful insight into seasonal aspects of the hydrologic cycle. Combined analyses of D/H of short and long chain fatty acids from lake sediment cores thus allow reconstructions of both past temperature and growing season relative humidity. Comparison of δD records from two lakes in New England provides critical information on regional climate variation and abrupt climate change, such as the 8.2 ka event.

Centennial‐scale compound‐specific hydrogen isotope record of Pleistocene–Holocene climate transition from southern New England

[1] Northeastern North America experienced major climate shifts during the Pleistocene–Holocene transition. However, there have been no high-resolution isotopic records of climate change from this region. Here, we present a centennial-scale record of climate change during the transition based on D/H ratios of behenic acid (C22 nacid) or dDBA from a sediment core in Blood Pond, Massachusetts. Surface calibrations from a transect of 19 lakes in eastern North America show that dDBA values track mean annual atmospheric temperature variations. The abrupt climate events observed in Blood Pond records show remarkable similarity with Greenland ice core d 18 O records during the Pleistocene. During the early Holocene, the northeastern North America dDBA record was more variable than Greenland, possibly due to the close proximity of the Laurentide ice sheet, and impact of freshwater outbursts as the ice sheet rapidly retreated. Citation: Hou, J., Y. Huang, W. W. Oswald, D. R. Foster, and B. Shuman (2007), Centennialscale compound-specific hydrogen isotope record of Pleistocene – Holocene climate transition from southern New England, Geophys. Res. Lett., 34, L19706, doi:10.1029/2007GL030303.

Radiocarbon dating of individual lignin phenols: A new approach for establishing chronology of late quaternary lake sediments

The reliability of chronology is a prerequisite for meaningful paleoclimate reconstructions from sedimentary archives. The conventional approach of radiocarbon dating bulk organic carbon in lake sediments is often hampered by the old carbon effect, i.e., the assimilation of ancient dissolved inorganic carbon (DIC) derived from carbonate bedrocks or other sources. Therefore, radiocarbon dating is ideally performed on organic compounds derived from land plants that use atmospheric CO(2) and rapidly delivered to sediments. We demonstrate that lignin phenols isolated from lake sediments using reversed phase high performance liquid chromatography (HPLC) can serve as effective (14)C dating materials for establishing chronology during the late Quaternary. We developed a procedure to purify lignin phenols, building upon a published method. By isolating lignin from standard wood reference substances, we show that our method yields pure lignin phenols and consistent ages as the consensus ages and that our procedure does not introduce radiocarbon contamination. We further demonstrate that lignin phenol ages are compatible with varve counted and macrofossil dated sediment horizons in Steel Lake and Fayetteville Green Lake. Applying the new method to lake sediment cores from Lake Qinghai demonstrates that lignin phenol ages in Lake Qinghai are consistently younger than bulk total organic carbon (TOC) ages which are contaminated by old carbon effect. We also show that the age offset between lignin and bulk organic carbon differs at different Lake Qinghai sedimentary horizons, suggesting a variable hard water effect at different times and that a uniform age correction throughout the core is inappropriate.

Large Holocene summer temperature oscillations and impact on the peopling of the northeastern Tibetan Plateau

Summer temperatures on the Tibetan Plateau (TP) significantly affect stability of glaciers that provide steady water resources to nearly half of the world population. However, lack of reliable, long-term proxy records greatly impedes understanding of regional temperature sensitivity to climate forcings. Here we present a 16ka long, alkenone-based summer temperature record from Lake Qinghai, northeastern TP that demonstrates major regional temperature response to changes in summer insolation and Atlantic Meridional Overturning Circulation during the Holocene and late glacial. Importantly, we find a period of sustained summer temperature decline (>4 degrees C) between 5 and 3.5ka, which coincides with expansion of Barents Sea ice coverage and is likely driven by intensification of the Westerlies. This unusually long and pronounced regional cooling event likely delayed permanent human settlements on the high-altitude regions (>3000m) of the TP by at least 500years.

Centennial-scale climate variability during the past 2000 years on the central Tibetan Plateau

It is currently suggested that climate change on the northeastern Tibetan Plateau (TP) was influenced alternately by the monsoon and the Westerlies. However, the mechanisms driving Holocene climate change on the TP remain unclear, since the extent of the influence of individual atmospheric circulation systems has not yet been clearly defined because of the shortage of high-quality paleoclimatic records. This is especially true in the central TP, where only a few ice core and paleolimnological records are available. Here, we present a decadal-resolution temperature record from Dagze Co in the central TP for the past 2000 years, based on the unsaturation index of long-chain alkenones, using an updated temperature calibration, and a record of precipitation isotopes from compound-specific isotope ratios of leaf waxes. The centennial-scale variation of the temperature and precipitation isotope records captures well-known climatic events over the past 1000 years, for example, the ‘Little Ice Age’, which was cooler and drier than the ‘Medieval Warm Period’. However, the relationship between temperature and the precipitation isotope records differed during the interval at 2000–1000 cal. yr BP compared to the past 1000 years, probably because of changes in precipitation seasonality and the additional influence of the Westerlies on the central TP. In addition, the temperature records exhibit a prominent 210-year cyclicity, suggesting a possible influence of solar radiation on temperature variability.

Salinity drives archaeal distribution patterns in high altitude lake sediments on the Tibetan Plateau

Archaeal communities and the factors regulating their diversity in high altitude lakes are poorly understood. Here, we provide the first high-throughput sequencing study of Archaea from Tibetan Plateau lake sediments. We analyzed twenty lake sediments from the worlds highest and largest plateau and found diverse archaeal assemblages that clustered into groups dominated by methanogenic Euryarchaeota, Crenarchaeota and Halobacteria/mixed euryarchaeal phylotypes. Statistical analysis inferred that salinity was the major driver of community composition, and that archaeal diversity increased with salinity. Sediments with the highest salinities were mostly dominated by Halobacteria. Crenarchaeota dominated at intermediate salinities, and methanogens were present in all lake sediments, albeit most abundant at low salinities. The distribution patterns of the three functional types of methanogens (hydrogenotrophic, acetotrophic and methylotrophic) were also related to changes in salinity. Our results show that salinity is a key factor controlling archaeal community diversity and composition in lake sediments on a spatial scale that spans nearly 2000 km on the Tibetan Plateau.

Abrupt cooling repeatedly punctuated early-Holocene climate in eastern North America

Climate proxy records and general circulation models suggest that Atlantic Meridional Overturning Circulation (AMOC) plays a key role for global climate changes. Paleoceanographic data document multiple episodes of prominent AMOC weakening during the early Holocene. However, proxy records at adjacent continents have not been demonstrated to fully capture the climate responses to multiple AMOC variation due to temporal resolution and/or the proxy sensitivity. Here we present decadal- to multidecadal-resolution hydrogen isotopic records of aquatic biomarkers from Blood Pond, Massachusetts during the early Holocene. Our data reveal a full series of prominent and abrupt cooling events centered on 10.6, 10.2, 9.5, 9.2, 8.8 and 8.4 ka. These abrupt climatic reversals coincide with key intervals of weakened AMOC, suggesting an apparent relationship between AMOC oscillations and the abrupt continental climate changes in northeastern North America. The noticeable connection implies that the AMOC variation did play an important role in the abrupt climate changes during the early Holocene. Our data also suggest that northeastern North America may experience significant climatic variations should the predicted major disturbance of AMOC occur in the coming century as a result of anthropogenic greenhouse gas emissions.

A 2540-year record of moisture variations derived from lacustrine sediment (Sasikul Lake) on the Pamir Plateau

Although the Pamir Plateau is an ideal place to investigate paleo-environmental changes in the westerlies-dominated high Central Asia, there are only few Holocene records from this region. We present a sub-centennially resolved lacustrine record of moisture variations from Sasikul Lake, central Pamir Plateau, based on geochemical, sedimentological, and mineralogical proxies. Our results show that generally dry conditions at Sasikul Lake during the past 2540 years were interrupted by a pronounced wet period between ad 1550 and 1900, corresponding to the ‘Little Ice Age’ (LIA). More negative values of carbonate δ18O, lower total inorganic carbon (TIC) and sand content during the LIA all indicate a relatively wet period with higher lake level. Higher TIC during the ‘Medieval Warm Period’ (MWP; ad 950–1200) reveals a lower lake level relative to the LIA. Low δ18O during this time is probably attributed to changes in the isotopic composition of input water and/or upstream moisture sources. The significant increase in detrital minerals and decrease in carbonate during the LIA provide further evidence for higher allochthonous input during the wet period at Sasikul Lake. The inferred moisture variations are consistent with existing records from regions of the northern Tibetan Plateau and Central Asia that are also influenced by the westerlies, but out-of-phase with those records from the Asian monsoon region, indicating that moisture variations at Sasikul Lake were mainly influenced by the strength and trajectories of the westerlies. The inferred water level at Sasikul Lake decreased significantly during the first half of the 20th century, and then increased in recent decades. This is consistent with the increase in lake area derived from satellite images and the monitoring data of large lake-level changes in Central Asia.

Molecular distributions and compound-specific stable carbon isotopic compositions of lipids in wintertime aerosols from Beijing.

Molecular distributions and stable carbon isotopic compositions (δ13C) of n-alkanes, fatty acids and n-alcohols were investigated in urban aerosols from Beijing, northern China to better understand the sources and long-range atmospheric transport of terrestrial organic matter during polluted and clear days in winter. n-Alkanes (C19–C36), fatty acids (C8–C32) and n-alcohols (C16–C32) detected in Beijing aerosols are characterized by the predominance of C23, C16 and C28, respectively. Carbon preference index (CPI) values of n-alkanes, the ratios of the sum of odd-numbered n-alkanes to the sum of even-numbered n-alkanes, are close to 1, indicating a heavy influence of fossil fuel combustion. Relatively higher ratios of C(18:0+16:0)/C(18:n+16:1) (fatty acids) on clear days than polluted days indicate that long-distance transport and/or photochemical aging are more significant during clear days. δ13C values of n-alkanes and low molecular weight fatty acids (C16:0, C18:0) ranged from –34.1 to −24.7% and −26.9 to −24.6%, respectively, which are generally heavier on polluted days than those on clear days. Such a wide range suggests that atmospheric lipids in Beijing aerosols originate from multiple sources and encounter complicated atmospheric processes during long-range transport in North China.