Zhangdong Jin

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.

Glacial-Interglacial Indian Summer Monsoon Dynamics

Indian summer monsoon changes during the Pleistocene were influenced by dynamic effects originating in both hemispheres. The modern Indian summer monsoon (ISM) is characterized by exceptionally strong interhemispheric transport, indicating the importance of both Northern and Southern Hemisphere processes driving monsoon variability. Here, we present a high-resolution continental record from southwestern China that demonstrates the importance of interhemispheric forcing in driving ISM variability at the glacial-interglacial time scale as well. Interglacial ISM maxima are dominated by an enhanced Indian low associated with global ice volume minima. In contrast, the glacial ISM reaches a minimum, and actually begins to increase, before global ice volume reaches a maximum. We attribute this early strengthening to an increased cross-equatorial pressure gradient derived from Southern Hemisphere high-latitude cooling. This mechanism explains much of the nonorbital scale variance in the Pleistocene ISM record.

Loss of carbon from the deep sea since the last glacial maximum

Moving Carbon During the last glacial maximum, approximately 23,000 years ago, both the atmosphere and the terrestrial biosphere contained much less carbon than in the immediately preindustrial era. The carbon must have been stored in the deep ocean, and the transfer of carbon to the air and land during deglaciation must have affected the carbonate chemistry and carbon isotopic composition of the sea. Yu et al. (p. 1084) estimated how deep-water carbonate concentrations changed over the course of the last deglaciation and combined their results with 13C/12C data to show that carbon released by the deep ocean between 17.5 and 14.5 thousand years ago mostly stayed in the atmosphere as CO2, while between 14 and 10 thousand years ago, a substantial fraction was absorbed by the terrestrial biosphere. Carbon loss from the ocean to the atmosphere and terrestrial biosphere occurred at different rates in the last deglaciation. Deep-ocean carbonate ion concentrations ([CO32–]) and carbon isotopic ratios (δ13C) place important constraints on past redistributions of carbon in the ocean-land-atmosphere system and hence provide clues to the causes of atmospheric CO2 concentration changes. However, existing deep-sea [CO32–] reconstructions conflict with one another, complicating paleoceanographic interpretations. Here, we present deep-sea [CO32–] for five cores from the three major oceans quantified using benthic foraminiferal boron/calcium ratios since the last glacial period. Combined benthic δ13C and [CO32–] results indicate that deep-sea-released CO2 during the early deglacial period (17.5 to 14.5 thousand years ago) was preferentially stored in the atmosphere, whereas during the late deglacial period (14 to 10 thousand years ago), besides contributing to the contemporary atmospheric CO2 rise, a substantial portion of CO2 released from oceans was absorbed by the terrestrial biosphere.

Seismic mountain building: Landslides associated with the 2008 Wenchuan earthquake in the context of a generalized model for earthquake volume balance

Here we assess earthquake volume balance and the growth of mountains in the context of a new landslide inventory for the Mw7.9 Wenchuan earthquake in central China. Coseismic landslides were mapped from high-resolution remote imagery using an automated algorithm and manual delineation, which allows us to distinguish clustered landslides that can bias landslide volume calculations. Employing a power-law landslide area-volume relation, we find that the volume of landslide-associated mass wasting (~2.8+0.9/-0.7 km3) is lower than previously estimated (~5.7-15.2 km3) and comparable to the volume of rock uplift (~2.6±1.2 km3) during the Wenchuan earthquake. If fluvial evacuation removes landslide debris within the earthquake cycle, then the volume addition from coseismic uplift will be effectively offset by landslide erosion. If all earthquakes in the region followed this volume budget pattern, the efficient counteraction of coseismic rock uplift raises a fundamental question about how earthquakes build mountainous topography. To provide a framework for addressing this question, we explore a group of scaling relations to assess earthquake volume balance. We predict coseismic uplift volumes for thrust-fault earthquakes based on geophysical models for coseismic surface deformation and relations between fault rupture parameters and moment magnitude, Mw. By coupling this scaling relation with landslide volume-Mw scaling, we obtain an earthquake volume balance relation in terms of moment magnitude Mw, which is consistent with the revised Wenchuan landslide volumes and observations from the 1999 Chi-Chi earthquake in Taiwan. Incorporating the Gutenburg-Richter frequency-Mw relation, we use this volume balance to derive an analytical expression for crustal thickening from coseismic deformation based on an index of seismic intensity over a defined area. This model yields reasonable rates of crustal thickening from coseismic deformation (e.g.~0.1-0.5 km Ma-1 in tectonically active convergent settings), and implies that moderate magnitude earthquakes (Mw≈6-8) are likely responsible for most of the coseismic contribution to rock uplift, because of their smaller landslide-associated volume reduction. Our first-order model does not consider a range of factors (e.g., lithology, climate conditions, epicentral depth and tectonic setting), nor does it account for viscoelastic or isostatic responses to erosion, and there remain important uncertainties on the scaling relationships used to quantify coseismic deformation. Nevertheless, our study provides a conceptual framework and invites more rigorous modeling of seismic mountain building.

Controls on fluvial evacuation of sediment from earthquake-triggered landslides

Large earthquakes in active mountain belts can trigger landslides, which mobilize large volumes of clastic sediment. Delivery of this material to river channels may result in aggradation and flooding, while sediment residing on hillslopes may increase the likelihood of subsequent landslides and debris flows. Despite recognition of these processes, the controls on the residence time of coseismic landslide sediment in river catchments remain poorly understood. Here we assess the residence time of fine-grained ( 5 mm day–1). Together with previous observations from the C.E. 1999 Chi-Chi earthquake in Taiwan, our results demonstrate the importance of landslide density and runoff intensity in setting the duration of earthquake-triggered landslide impacts on river systems.

Comparison of Elemental Carbon in Lake Sediments Measured by Three Different Methods and 150-Year Pollution History in Eastern China

Concentrations of elemental carbon (EC) were measured in a 150 yr sediment record collected from Lake Chaohu in Anhui Province, eastern China, using three different thermal analytical methods: IMPROVE_A thermal optical reflectance (TOR), STN_thermal optical transmittance (TOT), and chemothermal oxidation (CTO). Distribution patterns for EC concentrations are different among the three methods, most likely due to the operational definition of EC and different temperature treatments prescribed for each method. However, similar profiles were found for high-temperature EC fractions among different methods. Historical soot(TOR) (high-temperature EC fractions measured by the IMPROVE_A TOR method) from Lake Chaohu exhibited stable low concentrations prior to the late 1970s and a sharp increase thereafter, corresponding well with the rapid industrialization of China in the last three decades. This may suggest that high-temperature thermal protocols are suitable for differentiating between soot and other carbon fractions. A similar soot(TOR) record was also obtained from Lake Taihu (~200 km away), suggesting a regional source of soot. The ratio of char(TOR) (low-temperature EC fraction measured by the IMPROVE_A TOR method, after correction for pyrolysis) to soot(TOR) in Lake Chaohu shows an overall decreasing trend, consistent with gradual changes in fuel use from wood burning to increasing fossil fuel combustions. Average higher char(TOR)/soot(TOR) was observed in Lake Taihu than in Lake Chaohu in the past 150 years, consistent with the longer and more extensive industrialization around the Taihu region.

Elemental Carbon and Polycyclic Aromatic Compounds in a 150-Year Sediment Core from Lake Qinghai, Tibetan Plateau, China: Influence of Regional and Local Sources and Transport Pathways

Elemental carbon (EC) and polycyclic aromatic compounds (PACs) are potential proxies for the reconstruction of change in human activities and the origin of air masses in historic times. In this study, the historic deposition of char and soot (the two subtypes of EC) and PACs in a 150-year sediment core from different topographic subbasins of Lake Qinghai on the Qinghai Tibetan Plateau (QTP) were reconstructed. The objective was to explore how the variations in the concentrations of EC and PACs, in the ratios of char to soot and of oxygenated polycyclic aromatic hydrocarbons (OPAHs) to parent PAHs, and in the composition of the PAC mixtures reflect historical changes in climate and human activity and the origin of air masses arriving at the QTP. The deposition fluxes of soot in the different subbasins were similar, averaging 0.18 (range of 0.15-0.25) and 0.16 (0.13-0.23) g m(-2) year(-1), respectively, but they varied for char (averaging 0.11 and 0.22 g m(-2) year(-1), respectively), suggesting ubiquitous atmospheric deposition of soot and local river inputs of char. The different vertical distributions of the char/soot ratios in the different subbasins can be interpreted in terms of the different transport mechanisms of char and soot. An abrupt increase in soot concentrations since 1980 coincides with results from the QTP ice cores that were interpreted to be indicative of soot transport from South Asia. Similar concentration patterns of PAHs with soot and 9,10-anthraquinone/anthracene (9,10-AQ/ANT) ratios all >2.0 suggest regional PAC sources. Increasing PAH/soot ratios and decreasing 9,10-AQ/ANT ratios since the beginning of the 1970s indicate increasing local emissions. The historical trends of these diagnostic ratios indicate an increase in the fossil-fuel contribution since the beginning of the 1970s. The increase of perylene concentrations with increasing core depth and the ratio of perylene to its penta-aromatic isomers indicate that perylene originates mainly from in situ biogenic diagenesis. We demonstrate that the concentrations of EC, char, soot, and PACs in sediments can be used to reconstruct local, regional, and remote sources and transport pathways of pollutants to the QTP.

The effect of acidification on the determination of elemental carbon, char-, and soot-elemental carbon in soils and sediments

We studied the influence of acid pretreatment on the effective distinction between elemental carbon (EC) and organic carbon (OC), and between char-EC and soot-EC. Though widely employed in the pretreatment of soils and sediments for EC quantification, the use of HCl, HF, and HNO(3) could decrease soot thermal stability as acid remains, leading to an underestimation of soot-EC by thermal methods. We compared thermal optical reflectance (TOR) measurements of EC concentrations in char reference materials and in lacustrine and marine sediments following pretreatment with various acids. The results showed that pretreatment with 2M HCl, concentrated HNO(3), 7 M HNO(3), and 1 M HNO(3) did not result in EC oxidation. However, hot concentrated HNO(3) oxidized EC significantly, leading to lower concentrations of EC, char-EC and soot-EC. By comparing the removal of potentially interfering materials, which contain little fire-derived carbon, with different acid pretreatments, we recommend the HCl-HF-HCl and concentrated (not hot) HNO(3)-HF-HCl pretreatments for the determination of EC, char-EC, and soot-EC in soils and sediments using the TOR method.

Seasonal contributions of catchment weathering and eolian dust to river water chemistry, northeastern Tibetan Plateau: Chemical and Sr isotopic constraints

relative to winter. It is noticeable that both the lowest and the highest 87 Sr/ 86 Sr values of the Buha River waters occurred in the monsoon season, indicating a sensitive response of carbonate versus silicate weathering sources to hydrological forcing on a seasonal basis. A significant decrease in Na/cation, together with lower Sr isotope ratios, is consistent with a greater proportion of carbonate weathering relative to silicate weathering in the early monsoon season. High temperature and increased rainfall during the peak of the monsoon facilitate an increased proportion of ions derived from silicates, partly from groundwaters, to river water. In other seasons, elemental and 87 Sr/ 86 Sr ratios vary much less, indicating a constant ratio of silicate to carbonate weathering, consistent with limited variation in discharge. Our results highlight that in a semiarid region where climatic conditions vary seasonally, in addition to silicate and carbonate contributions, supply from eolian dust may also play a significant role in controlling seasonal variations in chemistry of river waters.

Boron isotope variations and its geochemical application in nature

The high geochemical reactivity of boron and the large relative mass difference between 10B and 11B lead to significant boron isotope fractionation in nature. So far the measured range of boron isotope composition (δ11B) varies between –70 and +75‰. The negative δ11B values are found in non-marine evaporite borate minerals and tourmalines, whereas positive δ11B values are common in salt lake brines and evaporated seawater. Since the 1980s, with improved measurement methods, applications of boron isotope analysis have increased rapidly. At present, boron isotopes are successfully applied to reconstruct ancient marine environments, to determine depositional environments and ore genesis, to trace groundwater pollution and seawater intrusion, and to study continental erosion. This paper summarises the methods for boron isotope analysis, the mechanisms of boron isotope fractionation and the distribution of boron isotopes in nature, reviews the achievements and the problems of boron isotopes in geochemical applications, and proposes research directions of boron isotopes in geochemical fields.