Science China Earth Sciences

Carbon isotope, sulfur isotope, and trace element (including Rare Earth Elements, REE) analyses were conducted on the carbonates of the Dengying Formation at Lianghekou section in southern Shaanxi to reconstruct the terminal Ediacaran shallow-water environment on the northwestern margin of the Yangtze Platform. At Lianghekou section, samples in the middle 50-m of the Beiwan Member show characteristics of low ΣREE concentrations, no MREE-enriched REE distribution patterns, high Ce/Ce* values close to 1, and enriched redox-sensitive elements, whereas samples in the lower 30-m and upper 10-m show opposite characteristics of high ΣREE concentrations, MREE-enriched REE distribution patterns, low Ce/Ce* values around 0.6, and no redox-sensitive elements enriched, indicating that oxygenation did occur in the shallow water on the northwestern margin of the Yangtze Platform and redox conditions of the shallow water fluctuated from relatively oxygenated to anoxic and then back to oxygenated again. We propose that the anoxia appeared in middle of the Beiwan time may associate with the anoxic upwelled water. On one hand, abundant nutrients were brought in by this upwelling event, which stimulated the photosynthetic carbon fixation and increased the organic carbon burial under this anoxic condition, causing a peak of 3.6‰ in γ 13 C. On the other hand, because the anoxic upwelled water replaced the oxic shallow water, together with the increasing organic matter in the water column, bacterial sulfate reduction was enhanced and therefore quickly reduced the sulfate concentration, which eventually caused γ 34 S increasing to 50‰. However, as the upwelling gradually disappeared, γ 13 C and γ 34 SCAS values decreased as well in the late Beiwan time, indicating the shallow water went back to suboxic or oxic again.

Stable isotope paleoaltimetry has provided unprecedented insights into the topographic histories of many of the world’s highest mountain ranges. However, on the Tibetan Plateau (TP), stable isotopes from paleosols generally yield much higher paleoaltitudes than those based on fossils. It is therefore essential when attempting to interpret accurately this region’s paleoaltitudes that the empirical calibrations of local stable isotopes and the relations between them are established. Additionally, it is vital that careful estimations be made when estimate how different isotopes sourced from different areas may have been influenced by different controls. We present here 29 hydrogen isotopic values for leaf wax-derived n-alkanes (i.e., δDwax values, and abundance-weighted average δD values of C29 and C31) in surface soils, as well as the δD values of soil water (δDsw) samples (totaling 22) from Mount Longmen (LM), on the eastern TP (altitude ~0.8–4.0 km above sea level (asl), a region climatically affected by the East Asian Monsoon (EAM). We compared our results with published data from Mount Gongga (GG). In addition, 47 river water samples, 55 spring water samples, and the daily and monthly summer precipitation records (from May to October, 2015) from two precipitation observation stations were collected along the GG transect for δD analysis. LM soil δDwax values showed regional differences and responded strongly to altitude, varying from‒160‰ to‒219‰, with an altitudinal lapse rate (ALR) of‒18‰ km‒1 (R 2=0.83; p<0.0001; n=29). These δDwax values appeared more enriched than those from the GG transect by ~40‰. We found that both the climate and moisture sources led to the differences observed in soil δDwax values between the LM and GG transects. We found that, as a general rule, ε wax/rw, ε wax/p and ε wax/sw values (i.e., the isotopic fractionation of δDwax corresponding to δDrw, δDp and δDsw) increased with increasing altitude along both the LM and GG transects (up to 34‰and 50‰, respectively). Basing its research on a comparative study of δDwax, δDp, δDrw(δDspringw) and δDsw, this paper discusses the effects of moisture recycling, glacier-fed meltwater, relative humidity (RH), evapotranspiration (ET), vegetation cover, latitude, topography and/or other factors on ε wax/p values. Clearly, if ε wax-p values at higher altitudes are calculated using smaller ε wax-p values from lower altitudes, the calculated paleowaterδDp values are going to be more depleted than the actual δD values, and any paleoaltitude would therefore be overestimated.

Boron isotopic compositions in marine planktonic foraminifers can record changes in seawater pH and hence provide a new tool to reconstruct the changes of paleo-atmospheric PCO2. Here a comparative study was done on boron isotopes and trace element abundances of planktonic foraminifers, Globigerinoides sacculifer and Globigerinoides ruber from three tropical ODP sites, Site 806B, Site 664C and Site 999A. It is demonstrated here that G. ruber is a better species to be used for downcore boron isotope analyses on glacial-interglacial scales, particularly since G. sacculifer is prone to dissolution causing unexpectedly light boron isotopes during glacial times. It has been shown that the trace elements composition of foraminifers (i.e., Mg/Ca, U/Ca, B/Ca, and Li/Ca) can reflect the variations of seawater temperature and/or [CO3 2−]. However, all of these systems are highly species de- pendent and affected by other vital effects, and it is therefore important to perform necessary calibrations when applying them to the downcore analyses.

The Early Cambrian Chengjiang Biota of China has preserved many species of vermiform animals in exquisite detail, which provide a unique window into the origin and early evolution of different vermiform phyla. This article reviews and discusses the forty-five Chengjiang vermiform animals discovered so far, and specifically comments on the phylogeny, modes of life, evolutionary significance as well as existing debates concerning the Chengjiang priapulid and lobopodian phyla. This article analyzes existing problems with Chengjiang vermiform morphological research and highlights various factors that affect the final preserved fossil morphology, including taphonomy and shape change caused by vermiform movement, as well as proposing that Anningvermis multispinosa and Corynetis brevis are synonymous. Chengjiang vermiform animals displayed a great diversity of morphologies and ecological roles, which is significant for understanding the origin and evolution of early metazoans; the potential for future research is enormous but needs to be broadened by interdisciplinary work involving taphonomy, geochemistry, cladistics, and genetics.

Large perturbations in chlorophyll (Chl) are observed to coexist at interannual and tropical instability wave (TIW) scales in the tropical Pacific; at present, their combined effects on El Niño-Southern Oscillation (ENSO) through ocean biology-induced heating (OBH) feedbacks are not understood well. Here, a hybrid coupled model (HCM) for the atmosphere and ocean physics-biogeochemistry (AOPB) in the tropical Pacific is adopted to quantify how ENSO can be modulated by Chl perturbations at interannual and TIW scales, individually or collectively, respectively. The HCM-based sensitivity experiments demonstrate a counteracting effect on ENSO: the bio-climate feedback due to large-scale Chl interannual variability acts to damp ENSO through its impact on upper-ocean stratification and vertical mixing, whereas that due to TIW-scale Chl perturbations tends to amplify ENSO. Because ENSO simulations are sensitively dependent on the ways Chl effects are represented at these different scales, it is necessary to adequately take into account these related differential Chl effects in climate modeling. A bias source for ENSO simulations is illustrated that is related with the Chl effects in the tropical Pacific, adding in a new insight into interactions between the climate system and ocean ecosystem on different scales in the region. These results reveal a level of complexity of ENSO modulations resulting from Chl effects at interannual and TIW scales, which are associated with ocean biogeochemical processes and their interactions with physical processes in the tropical Pacific.

We studied the uptake of ammonium, nitrate, and a variety of amino acids by alpine plant species in the Kobresia humilis alpine meadow ecosystem in situ. We examined the extent of niche separation in uptake of N source by different plant species in alpine communities, and investigated the contribution of symbiotically fixed N to the total N in alpine meadow. The results are (1) δ 15N natural abundance values of 13 plant species lie between −2.680‰ and 5.169‰, and the scope is 7.849‰. (2) Leguminous plants, such as Trigonella ruthenica, Gueldenstaedtia diversiffolia, and Oxytyopis ochrocephala, and non-leguminous plant Gentiana straminea uptake low amounts of 15N labeled ammonium, nitrate, glycine or aspartate in soil. (3) As far as the plant uptake of organic N is concerned, Kobresia humilis, Poa pratensis, and Gentiuna spathutifolta can effectively uptake organic nitrogen, and about 37%–40% of the nitrogen of these species comes from soil organic nitrogen sources (such as glycine and aspartate). Stipa aliena can effectively uptake nitrate, and 60% of its nitrogen comes from soil nitrate. Potentilla anserina, Poa pratensis, and Thalictrum alpinum can effectively absorb ammonium in comparason to other plant species in the meadow, and about 25%–27% of the nitrogen in these plants comes from soil ammonium. (4) The contribution of leguminous fixed N to total N is 7.48%–9.26% in Kobresia humilis alpine meadow. (5) These data show many plant species of alpine meadow may effectively utilize dissolved organic nitrogen such as amino acids, and these plants have diverse ways to uptake soil nitrogen in alpine meadows. Based on the results we can partly explain why there are abundant biodiversities and how plants at alpine habitat utilize the limited soil N sources.

In this study, to further promote the application of the stable carbon isotope natural abundance (SCINA) method to the study of CH4 in paddy fields in China, field experiments were carried out to investigate carbon isotope fractionation during CH4 transport in both rice- and non-rice-growing seasons. More importantly, two new methods for the measurement of the CH4 transport fractionation factor (ɛ transport) in paddy fields were introduced. The results indicated that the closed chamber + syringe method was much better for the determination of ɛ transport during the non-rice-growing season. Presently, ɛ transport was calculated using the δ 13C value of the CH4 emitted from a rice field minus that of the CH4 in the floodwater (−6.7‰ to −3.0‰). In addition, there were three methods available for estimating ɛ transport during the rice-growing season: deduction of the δ 13C value of the CH4 in the floodwater from that of the CH4 emitted from the field (−16.6‰ to −15.2‰); deduction of the δ 13C value of the CH4 in the soil pore water from that of the CH4 emitted from the field (−13.2‰ to −1.1‰); and deduction of the δ 13C value of the CH4 in the aerenchyma of plants from that of the CH4 emitted from plants (−16.3‰ to −10.9‰). Unfortunately, the first two methods showed relatively large uncertainties. Only the last one, the dividing + cutting method, was not only scientific and reliable but also provided accurate measurements.

Although the involvement of hydrous fluids has been widely invoked in formation of podiform chromitites in ophiolites, there is lack of natural evidence to signify the role and mechanism of fluids. In this study, a new model for the genesis of podiform chromitite is proposed on basis of revisits of comprehensive petrological, mineralogical and geochemical results of the well-preserved Kızıldağ ophiolite and the well-characterized Luobusa chromite deposit. In this model, ascending magmas intruding oceanic lithospheric mantle would presumably form a series of small magma chambers continuously connected by conduits. Tiny chromite nuclei would collect fluids dispersed in such magmas to form nascent droplets. They tend to float upward in the magma chamber and would be easily transported upward by flowing magmas. Chromite-rich droplets would be enlarged via coalescence of dispersed droplets during mingling and circulation in the magma chamber and/or transport in magma conduits. Crystallization of the chromite-rich liquid droplets would proceed from the margin of the droplet inward, leaving liquid entrapped within grains as precursor of mineral inclusions. With preferential upward transportation, immiscible chromite-rich liquids would coalesce to a large pool in a magma chamber. Large volumes of chromite would crystallize in situ, forming podiform chromitite and resulting in fluid enrichment in the chamber. The fluids would penetrate and compositionally modify ambient dunite and harzburgite, leading to significant fractionations of elemental and isotopic compositions between melts and fluids from which dunite and chromitite respectively formed. Therefore, fluid immiscibility during basaltic magma ascent plays a vital role in chromitite formation.