The importance of solar insolation on the temperature variations for the past 110 kyr on the Chinese Loess Plateau

The late Quaternary temperature history on the Chinese Loess Plateau (CLP) is little known due to the absence of suitable paleothermometers. Here, we present a quantitative temperature record for the last 110 kyr from Lantian county, southern CLP, based on the distributions of bacterial tetraethers. Lantian loess temperature variations display significant correlation with the Northern Hemisphere insolation (35 degrees N) as well as the cave monsoon records at the precession band. We attribute such correlation to both the latent heat release from the East Asian summer monsoon and the direct influence of Sun&#39;s sensible heat over the semi-arid landscape. The long term cooling during the Marine Isotope Stage (MIS) 3 and the modest temperature drop of similar to 5 degrees C from 79 to 65 ka are best explained by interactions among multiple climate ford ngs including insolation, atmospheric CO2 concentration, global ice volume, and regional glacier activities. The cold and modestly wet climate during MIS 3 may have promoted maximum regional glacial advances, whereas the minimal temperature during local LGM reduced moisture transfer and led to glacial recession. The unusually early deglacial warming at similar to 22.5 ka highlights the importance of regional climate records to decipher the complex continental climate dynamics.</p

Major increase in winter and spring precipitation during the Little Ice Age in the westerly dominated northern Qinghai-Tibetan Plateau

There have been numerous attempts to use long-chain alkenones (LCAs) in saline lakes for paleotemperature and paleosalinity reconstructions. However, LCAs in saline lakes are often produced by multiple haptophyte species, which may confound data interpretations. Here we analyzed LCAs in a finely laminated, high sedimentation rate core from the hypersaline Lake Gahai in the northern Qinghai-Tibetan Plateau and compared our results with regional instrumental records. We find that LCA unsaturation ratios display a stepwise jump during the instrumental period, most likely originating from a sudden shift in the dominant alkenone producers. In contrast, the percentage of the C37:4 alkenone (%C37:4) displays strong correlations with spring and combined winter-spring precipitation amount (R2 = 0.83 and R2 = 0.81, respectively). We hypothesize that high winter-spring precipitation leads to greater freshening of lake surface water immediately after spring melting, promoting greater production of LCAs with relatively high %C37:4 values by the early blooming haptophyte species. Extending the instrumental calibration downcore allows us to quantitatively reconstruct regional spring and winter-spring precipitation for the past millennium. We find a major increase in winter-spring precipitation during the Little Ice Age (LIA), especially during the phases of solar minima. Our finding provides novel quantitative support to the previous studies suggesting relatively wet conditions during the LIA in the westerly dominated regions of central Asia and northern Tibetan Plateau and infers a substantial increase in regional winter-spring precipitation should the predicted grand solar minimum in the forthcoming decades become a reality.</p

Abundant C-4 plants on the Tibetan Plateau during the Lateglacial and early Holocene

Plants using the C-4 (Hatch-Slack) photosynthetic pathway are key for global food production and account for ca 25% of terrestrial primary productivity, mostly in relatively warm, dry regions. The discovery of modern naturally-occurring C-4 plant species at elevations up to 4500 m in Tibet and 3000 m in Africa and South America, however, suggests that C-4 plants are present in a wider range of environments than previously thought. Environmental conditions on the Tibetan Plateau, including high irradiance, rainfall focused in summer, and saline soils, can favor C-4 plants by offsetting the deleterious effects of low growing season temperature. We present evidence based on leaf wax carbon isotope ratios from Lake Qinghai that C-4 plants accounted for 50% of terrestrial primary productivity on the northeastern Tibetan Plateau throughout the Lateglacial and early Holocene. Despite cold conditions, C-4 plants flourished due to a combination of factors, including maximum summer insolation, pCO(2) ca 250 ppmv, and sufficient summer precipitation. The modern C-3 plant-dominated ecosystem around Lake Qinghai was established ca 6 thousand years ago as pCO(2) increased and summer temperature and precipitation decreased. C-4 plants were also intermittently abundant during the Last Glacial period; we propose that C-4 plants contributed a significant portion of local primary productivity by colonizing the exposed, saline Qinghai Lake bed during low stands. Our results contrast with state-of-the-art ecosystem models that simulate &lt;0.5% C-4 plant abundance on the Tibetan Plateau in modern and past environments. The past abundance of C-4 plants on the Tibetan Plateau suggests a wider temperature range for C-4 plants than can be inferred from modern distributions and model simulations, and provides paleoecological evidence to support recent findings that C-4 plant evolution and distribution was determined by a combination of climatic and environmental factors (temperature, irradiance, precipitation amount and seasonality, and soil salinity). Moreover, this finding highlights the exceptional sensitivity of high-elevation ecosystems to environmental change, and provides critical benchmarks for ecosystem model validation.</p

Heterodynes dominate precipitation isotopes in the East Asianmonsoon region, reflecting interaction of multiple climate factors

For the past decade, East Asian monsoon history has been interpreted in the context of an exceptionally well-dated, high-resolution composite record of speleothem oxygen isotopes (&delta;18Ocave) from the Yangtze River Valley. This record is characterized by a unique spectral response, with variance concentrated predominantly within the precession band and an enigmatic lack of variance at the eccentricity and obliquity bands. Here we examine the spectral characteristics of all existing &gt;250-kyr-long terrestrial water isotope records in Asia, including a new water isotope record using leaf wax hydrogen isotope ratios from the Chinese Loess Plateau. There exist profound differences in spectral characteristics among all orbital-scale Asian water isotope records. We demonstrate that these differences result from latitudinal gradients in the influence of the winter and summer monsoons, both of which impact climate and water isotopes throughout East Asia. Water isotope records therefore do not reflect precipitation during a single season or from a single circulation system. Rather, water isotope records in East Asia reflect the complex interplay of oceanic and continental moisture sources, operating at multiple Earth-orbital periods. These non-linear interactions are reflected in water isotope spectra by the presence of heterodynes. Although complex, we submit that water isotope records, when paired with rapidly developing isotope-enabled model simulations, will have the potential to elucidate mechanisms causing seasonal precipitation variability and moisture source variability in East Asia.</p

Temperature and leaf wax delta H-2 records demonstrate seasonal and regional controls on Asian monsoon proxies

Orbital-scale precipitation isotope records can elucidate climate forcing mechanisms and provide benchmarks for climate model validation. The ability to differentiate the influence of temperature, seasonality, and vapor transport history on precipitation isotope proxies is critical to both objectives. We present a 300 k.y. leaf wax hydrogen isotope record from the South China Sea with the effects of local condensation temperature removed (delta H-2(wax-T)). delta H-2(wax-T) reflects annually integrated precipitation delta H-2 in the Pearl River catchment of southeast China. Depleted delta H-2(wax-T) lags minimum precession (P-min) by 1.0 +/- 0.7 k.y., reflecting the influence of maximum summer insolation and minimum winter insolation, with a minor influence of global ice volume, which lags P-min by 3.3 +/- 0.4 k.y. In contrast, annually integrated cave delta O-18 minima in central China, 1000 km north of our site, lag P-min by 2.7 +/- 0.3 k.y., in phase with ice volume minima. This phase indicates that precipitation delta O-18 in central China is more strongly influenced by ice volume forcing at the precession band, with a lesser influence of Northern Hemisphere insolation. Our new delta H-2(wax-T) data demonstrate that precipitation isotopes in Asia have strong regional variability. Interpreting water isotope records within the context of regionally varying temperature, seasonality, and sensitivity to changing glacial boundary conditions is imperative to understanding Asian hydroclimatic change.</p

A climate threshold at the eastern edgeof the Tibetan plateau

Proxy records of summer monsoon moisture at Lake Qinghai on the northeastern Tibetan Plateau reveal a late Quaternary climate history that is subtly different from that of speleothems from southern and eastern China. Total organic carbon and authigenic carbonate in two independently analyzed and dated cores indicate (1) relative stability and aridity during the glacial interval, (2) small variations during the B&oslash;lling-Aller&oslash;d and the Younger Dryas intervals, (3) comparatively abrupt change at the late Pleistocene/Holocene transition, and (4) relatively high variability during a wet early Holocene. Taken together, the data suggest that a climate threshold exists for penetration of Asian monsoon rainfall onto the Tibetan Plateau, a threshold that was crossed at the beginning of the Holocene. Conceptually, the threshold simply may be related to the topographic barrier that the eastern margin of the Tibetan Plateau presents to the landward penetration of the monsoon, or it may be related to nonlinearities in the climate system itself, such as sudden shifts in the configuration of the Westerly jet stream. Different mechanisms for producing a threshold are not mutually exclusive and may have combined to affect the dynamics of the Asian monsoon. In any case, the threshold is related to the presence of the Tibetan Plateau, which has a profound influence on the Asia monsoon system.</p

Interplay between the Westerlies and Asian monsoon recorded in Lake Qinghai sediments since 32 ka

Two atmospheric circulation systems, the mid-latitude Westerlies and the Asian summer monsoon (ASM), play key roles in northern-hemisphere climatic changes. However, the variability of the Westerlies in Asia and their relationship to the ASM remain unclear. Here, we present the longest and highest-resolution drill core from Lake Qinghai on the northeastern Tibetan Plateau (TP), which uniquely records the variability of both the Westerlies and the ASM since 32 ka, reflecting the interplay of these two systems. These records document the anti-phase relationship of the Westerlies and the ASM for both glacial-interglacial and glacial millennial timescales. During the last glaciation, the influence of the Westerlies dominated; prominent dust-rich intervals, correlated with Heinrich events, reflect intensified Westerlies linked to northern high-latitude climate. During the Holocene, the dominant ASM circulation, punctuated by weak events, indicates linkages of the ASM to orbital forcing, North Atlantic abrupt events, and perhaps solar activity changes.</p