Cheng Zhao

Position of the Snake River watershed divide as an indicator of geodynamic processes in the greater Yellowstone region, western North America

Tectonic processes, fl exure due to crustal loading, and dynamic mantle fl ow each impart a unique imprint on topography and geomorphic responses over time scales of 104 to 10 6 yr. This paper explores the mobility of regional drainage divides as a key geomorphic metric that can distinguish between the various processes driving crustal deformation in the greater Yellowstone region of the northwestern United States. We propose a new analysis that quantifi es the differences between the location of the presentday drainage divide from divides synthetically generated from fi ltered topography to determine the relative impact of tectonic and dynamic mantle infl uences on landscape development. The greater Yellowstone region is an opportune location for this investigation because contrasting models have been proposed to explain the parabolic shape of elevated topography and active seismicity that outline the imprint of hypothesized hotspot activity. Drainage divides synthesized from topography fi ltered at 50, 100, and 150 km wavelengths within the greater Yellowstone region show that the locations of the actual and synthetic Snake River drainage divides are controlled by both dynamic and fl exural mechanisms in the eastern greater Yellowstone region, but by fl exural mechanisms only in the western greater Yellowstone region. The location of the actual divide deviates from its predicted position in the fi ltered topography where tectonic controls, such as active faults (e.g., Centennial and Teton faults), have uplifted large footwall blocks. Our results are consistent with the notion of a northeastward-propagating greater Yellow stone region topographic and seismic parabola, and suggest that Basin and Range extension follows from, rather than precedes, greater Yellowstone region dynamic topography. Furthermore, our analysis suggests that eastward migration of the Snake River drainage divide lags behind the continued northeastward propagation of high-standing topography associated with the Yellowstone geophysical anomaly by 1–2 m.y.

Biomarker-based reconstructions of Holocene lake-level changes at Lake Gahai on the northeastern Tibetan Plateau

Holocene hydrological changes in regions dominated by the westerlies significantly differ from those by the Asian summer monsoon. The high-elevated northeastern Tibetan Plateau, located in between, is likely influenced by the interactions of both circulation systems. Here, we attempt to use biomarkers, n-alkanes and alkenones, to reconstruct Holocene lake-level changes at Lake Gahai in the Qaidam Basin. We choose a sediment core drilled at the lake shore, where biomarkers would be sensitive to lake-level changes. The n-alkane records show high average chain length (ACL), high carbon preference index (CPI), and low proportion of aquatic macrophyte (Paq) values at 7–2 kyr (thousand calibrated years ago) with peaked values around 6 kyr, whereas low ACL, low CPI, and high Paq values occurred after 2 kyr and before 7 kyr. No alkenones were detected at 7–2 kyr, suggesting that lake level at this period was incapable of constantly reaching to the coring site. Therefore, combined results provide unambiguous evidence of relatively low lake level at 7–2 kyr, probably lowest at ~6 kyr. Holocene lake-level changes in this marginal region thus display a different pattern from either of the core regions dominated by the westerlies (anti-phase) and the Asian summer monsoon (out-of-phase). We suggest that in the arid marginal region, temperature-induced evaporation could significantly affect regional hydrological balance, resulting in the discrepancy with the Holocene long-term precipitation decreasing trend in Asian monsoon-dominated regions.

Hemlock (Tsuga canadensis) declines at 9800 and 5300 cal. yr BP caused by Holocene climatic shifts in northeastern North America

We present fossil pollen data from a sediment core at Lake Grinnell in northern New Jersey of the northeastern United States. The 12 500-yr chronology of the sediment record was controlled by seven calibrated AMS radiocarbon dates on terrestrial plant macrofossils. Similar to many pollen profiles in the region, our data show that vegetation changed from Picea- and Pinus-dominated woodland/forest at 12.5—11.4 ka (1 ka = 1000 cal. yr BP), through Pinus-dominated mixed forest at 11.4—9.3 ka, to Quercus-dominated forest after 9.3 ka. Some main tree taxa, including Tsuga, Ulmus and Acer, arrived and expanded during the Quercus expansion phase at 11.4—9.3 ka in the early Holocene, while other trees, including Fagus and Carya, established and expanded much later around 9 ka. Pollen data show that major forest shifts were in response to climatic change as independently inferred from oxygen-isotope records in the same core. These responses include major turnovers of tree species at the onset of the Holocene after the Younger Dryas, including the disappearance of Picea, declines of boreal taxa Betula and Alnus, and expansions of most other temperate trees. Our new records show two hemlock (Tsuga canadensis) decline events during the Holocene that were both likely caused by climatic change. The early-Holocene hemlock decline centering around 9.5 ka was marked by >10-fold decrease in hemlock populations, which was probably caused by summer high temperature as inferred from high oxygen isotope values. The mid-Holocene hemlock decline from 5.3 ka to 3.0 ka corresponds with a pronounced shift in oxygen isotopes at Grinnell and with a dry climate interval documented from other records in northeastern North America. Our results support the notion that a dry climate caused or triggered the mid-Holocene hemlock decline and that the Appalachian Forest has shown sensitive responses to climatic change in the Holocene.We present fossil pollen data from a sediment core at Lake Grinnell in northern New Jersey of the northeastern United States. The 12 500-yr chronology of the sediment record was controlled by seven...

Solar Imprints on Asian Inland Moisture Fluctuations over the Last Millennium

Solar irradiance changes are thought to play an important role in natural climate variability. How the hydrological conditions were affected by solar irradiance in westerly-controlled arid central Asia (ACA) on decadal/centennial timescales remains poorly understood because of the lack of high-quality records. Here, we integrate 1.2-year-resolution x-ray fluorescence (XRF) scanner-derived carbonate accumulation estimates with 6-year-resolution biomarker and magnetic records in a well-preserved shoreline core from Lake Manas, northwestern China, to reconstruct lake level fluctuations and potential solar imprints over the last millennium. Besides the generally confirmed cool-wet/warm-dry climate pattern in ACA, our data also consistently show frequent and substantial lake level fluctuations, resembling solar activity changes, especially during the ‘Little Ice Age’. Wavelet spectral analyses of our XRF data indicate strong 8- to 16-year, 64- to 128-year and 128- to 256-year cycles, coinciding with the ~11-year Schwabe cycle, ~70- to 100-year Gleissberg cycle, and the ~200-year Suess-de Vries cycle. We therefore suggest the existence of solar imprints on effective moisture fluctuations in ACA over the last millennium, and the potential occurrence of the Schwabe cycle even during the solar minima.

Onset of frequent dust storms in northern China at ~AD 1100.

Dust storms in northern China strongly affect the living and health of people there and the dusts could travel a full circle of the globe in a short time. Historically, more frequent dust storms occurred during cool periods, particularly the Little Ice Age (LIA), generally attributed to the strengthened Siberian High. However, limited by chronological uncertainties in proxy records, this mechanism may not fully reveal the causes of dust storm frequency changes. Here we present a late Holocene dust record from the Qaidam Basin, where hydrological changes were previously reconstructed, and examine dust records from northern China, including the ones from historical documents. The records, being broadly consistent, indicate the onset of frequent dust storms at ~AD 1100. Further, peaked dust storm events occurred at episodes of high total solar irradiance or warm-dry conditions in source regions, superimposed on the high background of frequent dust storms within the cool LIA period. We thus suggest that besides strong wind activities, the centennial-scale dust storm events over the last 1000 years appear to be linked to the increased availability of dust source. With the anticipated global warming and deteriorating vegetation coverage, frequent occurrence of dust storms in northern China would be expected to persist.

Millennial-scale hydroclimate variations in southwest China linked to tropical Indian Ocean since the Last Glacial Maximum

Indian summer monsoon (ISM) variations have been linked to the orbital-scale boreal summer insolation and millennial-scale North Atlantic climates. Recent studies show the critical role of Indian Ocean sea-surface temperatures (SSTs) in affecting deglacial millennial-scale monsoon oscillations. However, it is unclear whether SSTs can affect monsoon rainfall and terrestrial hydroclimate during the Holocene. Here we report multiproxy evidence of hydroclimate changes in southwest China since the Last Glacial Maximum. Similar to the often-documented gradual decrease in Holocene monsoon rainfall with superimposed millennial-scale variations, our records particularly show pronounced hydroclimate fluctuations including wet conditions at ∼5000-4000 yr ago, and perhaps over the past 1000 yr. We also find coherent variations between our records and sea-surface salinities in the eastern Indian Ocean, suggesting that terrestrial hydroclimate and resultant continental drainage have affected surface ocean conditions. These fluctuations are likely linked to changes in SSTs downstream of the monsoon source in the tropical western Indian Ocean, i.e., a warmer ocean and more monsoon rainfall. We conclude that the influence of both insolation and tropical SSTs on the ISM has persisted from the last deglaciation into Holocene.