Studies focused on field measurements have been conducted to investigate how leaf wax n-alkane δ13C values vary among photosynthetic pathways (C3 vs. C4) or plant types (dicots, monocots, ...gymnosperms, magnoliids), and how n-alkane δ13C values respond to climate factors (precipitation, temperature). However, a systematic global analysis of plant n-alkane δ13C values with respect to major plant groups is still lacking, and the corresponding mechanism remains unknown. In this study, we compiled globally published n-alkanes δ13C data in modern plants. We find significantly negative correlations between δ13Cwt values and mean annual precipitation (MAP) for C3 plants (R2 = 0.30; p < .05), dicots (R2 = 0.28; p < .05) and magnoliids (R2 = 0.55; p < .05), but non-significant correlation between δ13Cwt values and MAP for C4 plants (R2 = 0.08; p = .12) or monocots (R2 = 0.01; p = .89). However, there are significantly positive correlations between δ13Cwt values and mean annual temperature (MAT) in C4 plants (R2 = 0.33; p < .05) and monocots (R2 = 0.35; p < .05), instead of weak correlations in C3 plants (R2 = 0.09; p < .05), dicots (R2 = 0.03; p < .05) and gymnosperms (R2 = 0.06; p = .06). These results indicate that n-alkane δ13C values in C3 dicots have more potential for paleoprecipitation reconstruction, while those in C4 monocots are more likely to indicate paleotemperature changes. Our study supports use of leaf wax n-alkane δ13C values to gain information about paleoprecipitation and paleotemperature, but only in an appropriate paleoecological context
The Anthropocene as a potential new unit of the International Chronostratigraphic Chart (which serves as the basis of the Geological Time Scale) is assessed in terms of the stratigraphic markers and ...approximate boundary levels available to define the base of the unit. The task of assessing and selecting potential Global Boundary Stratotype Section and Point (GSSP) candidate sections, a required part of the process in seeking formalisation of the term, is now being actively pursued. Here, we review the suitability of different stratified palaeoenvironmental settings and facies as potential hosts for a candidate GSSP and auxiliary sections, and the relevant stratigraphical markers for correlation. Published examples are evaluated for their strengths and weaknesses in this respect. A marked upturn in abundance of radioisotopes of 239Pu or 14C, approximately in 1952 and 1954CE respectively, broadly coincident with a downturn in δ13C values, is applicable across most environments. Principal palaeoenvironments examined include: settings associated with accumulations of anthropogenic material, marine anoxic basins, coral reefs, estuaries and deltas, lakes at various latitudes, peat bogs, snow/ice layers, speleothems and trees. Together, many of these geographically diverse palaeoenvironments offer annual/subannual laminae that can be counted and independently dated radiometrically (e.g. by 210Pb). Examples of possible sections offer the possibility of correlation with annual/seasonal resolution. From among such examples, a small number of potentially representative sites require the acquisition of more systematic and comprehensive datasets, with correlation established between sections, to allow selection of a candidate GSSP and auxiliary stratotypes. The assessments in this paper will help find the optimal locations for these sections.
Numerous field studies have shown that leaf wax
n
-alkane
δ
D values (
δ
D
n
-alkane
) can be used as a proxy for paleoelevation reconstruction. However, the lack of systematic global analysis of
δ
D
...n
-alkane
values with respect to altitude limits the reliability of
δ
D
n
-alkane
values for paleoaltimetry at the global scale. In this study, we analyzed the variation of
δ
D
n
-alkane
values with altitude in terrestrial higher plants based upon our compiled global
δ
D
n
-alkane
database. The general decrease in
δ
D
n
-alkane
values with altitude corresponds to the variation in precipitation
δ
D (
δ
D
prec
) with altitude, suggesting that the
δ
D
n
-alkane
values effectively reflect
δ
D
prec
values. When eliminating the effect of latitude at the global scale, the global
δ
D
n
-alkane
-altitude lapse rate for all species was −1.01±0.10‰ (100 m)
−1
, while these lapse rates were −1.03±0.13‰ and −0.47±0.13‰ (100 m)
−1
for dicots and monocots, respectively. Our study reveals that it is reliable to use
δ
D
n
-alkane
values to gain information about paleoelevation at the global scale, and the global
δ
D
n
-alkane
-altitude lapse rates provide an alternative to reconstruct paleoelevation at some
δ
D
n
-alkane
-uninvestigated areas.
Severe haze in northern China An, Zhisheng; Huang, Ru-Jin; Zhang, Renyi ...
Proceedings of the National Academy of Sciences - PNAS,
04/2019, Letnik:
116, Številka:
18
Journal Article
Recenzirano
Odprti dostop
Regional severe haze represents an enormous environmental problem in China, influencing air quality, human health, ecosystem, weather, and climate. These extremes are characterized by exceedingly ...high concentrations of fine particulate matter (smaller than 2.5 μm, or PM2.5) and occur with extensive temporal (on a daily, weekly, to monthly timescale) and spatial (over a million square kilometers) coverage. Although significant advances have been made in field measurements, model simulations, and laboratory experiments for fine PM over recent years, the causes for severe haze formation have not yet to be systematically/comprehensively evaluated. This review provides a synthetic synopsis of recent advances in understanding the fundamental mechanisms of severe haze formation in northern China, focusing on emission sources, chemical formation and transformation, and meteorological and climatic conditions. In particular, we highlight the synergetic effects from the interactions between anthropogenic emissions and atmospheric processes. Current challenges and future research directions to improve the understanding of severe haze pollution as well as plausible regulatory implications on a scientific basis are also discussed.
The mid-Pleistocene transition (MPT) is widely recognized as a shift in paleoclimatic periodicity from 41- to 100-kyr cycles, which largely reflects integrated changes in global ice volume, sea ...level, and ocean temperature from the marine realm. However, much less is known about monsoon-induced terrestrial vegetation change across the MPT. Here, on the basis of a 1.7-million-year δ
C record of loess carbonates from the Chinese Loess Plateau, we document a unique MPT reflecting terrestrial vegetation changes from a dominant 23-kyr periodicity before 1.2 Ma to combined 100, 41, and 23-kyr cycles after 0.7 Ma, very different from the conventional MPT characteristics. Model simulations further reveal that the MPT transition likely reflects decreased sensitivity of monsoonal hydroclimate to insolation forcing as the Northern Hemisphere became increasingly glaciated through the MPT. Our proxy-model comparison suggests varied responses of temperature and precipitation to astronomical forcing under different ice/CO
boundary conditions, which greatly improves our understanding of monsoon variability and dynamics from the natural past to the anthropogenic future.
The complicity of long-term land surface temperature (LST) changes has been under investigated and less understood, hindering our understanding of the history and mechanism of terrestrial climate ...change. Here, we report the longest (800 thousand years) LSTs based on distributions of soil fossil bacterial glycerol dialkyl glycerol tetraethers preserved in well-dated loess-paleosol sequences at the center of the Chinese Loess Plateau. We have found a previously-unrecognized increasing early and prolonged warming pattern toward the northwestern plateau at the onset of the past seven deglaciations, corresponding to the decrease in vegetation coverage, suggesting underlying surface vegetation or lack of has played an important role in regulating LSTs, superimposed on the fundamental global glacial-interglacial changes. Our results support that LSTs in semi-humid and semi-arid regions with little vegetation will be more sensitive to the anticipated global temperature rise, while improving vegetation coverage would reduce LSTs and thus ecological impacts.
•Understanding provenance fluctuations of Asian aeolian deposits requires discrimination of the primary and secondary sources.•Integrating different tracers distinguishes three primary and ten ...secondary sources of Asian aeolian deposits.•Combination of mineral and isotopic tracers can decipher the source-to-sink fluctuations since the late Oligocene.•Tectonic and climatic factors had major impacts on Asian dust cycling at tectonic and glacial-interglacial timescales.
The source-to-sink fluctuations of Asian aeolian deposits have been investigated over the past two decades using multiple mineralogical and geochemical tracers. These source tracers have substantially improved our understanding of the processes of production, transport and deposition of Asian dust cycling from the Asian inland basins to the North Pacific Ocean. However, due to differences in the sensitivity and interpretation of these various tracers, there is controversy about which regions were the dominant dust sources and whether the dust provenances were stable at tectonic and glacial-interglacial timescales. Here we comprehensively review the source-to-sink system of Asian aeolian deposits on the Chinese Loess Plateau since the late Oligocene. By assessing the utility of recently employed provenance tracers, we conclude that integrating different tracers can effectively distinguish major primary and secondary dust sources. The combination of Sr-Nd isotopes, quartz ESR-CI-δ18O data, and zircon age spectra can decipher the source-to-sink fluctuations of Asian aeolian deposits at tectonic and glacial-interglacial timescales. Comparison of these sensitive source tracers with tectonic and climatic evidence indicates that the phased growth of the northern Tibetan Plateau played a key role in Asian dust production and transport during the late Oligocene to Miocene, while the onset and expansion of the Northern Hemisphere ice-sheets had a substantial impact on the source-to-sink fluctuations since the late Pliocene.
A speleothem δ ¹⁸O record from Xiaobailong cave in southwest China characterizes changes in summer monsoon precipitation in Northeastern India, the Himalayan foothills, Bangladesh, and northern ...Indochina over the last 252 kyr. This record is dominated by 23-kyr precessional cycles punctuated by prominent millennial-scale oscillations that are synchronous with Heinrich events in the North Atlantic. It also shows clear glacial–interglacial variations that are consistent with marine and other terrestrial proxies but are different from the cave records in East China. Corroborated by isotope-enabled global circulation modeling, we hypothesize that this disparity reflects differing changes in atmospheric circulation and moisture trajectories associated with climate forcing as well as with associated topographic changes during glacial periods, in particular redistribution of air mass above the growing ice sheets and the exposure of the “land bridge” in the Maritime continents in the western equatorial Pacific.
Significance This paper presents a new long speleothem δ ¹⁸O time series from Xiaobailong cave in southwest China that characterizes changes in a major branch of Indian summer monsoon precipitation over the last 252 kyrs. This record shows not only 23-kyr precessional cycles punctuated by prominent millennial-scale weak monsoon events synchronous with Heinrich events in the North Atlantic, but also clear glacial–interglacial variations that are consistent with marine records but different from the cave records in East China. The speleothem records of Xiaobailong and other caves in East China show that the relationship between the Indian and the East Asian summer monsoon precipitation is not invariant, but rather varies on different timescales depending on the nature and magnitude of the climate forcing.
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