Chloride retention in forest soil by microbial uptake and by natural chlorination of organic matter

Inorganic chlorine (i.e. chloride; Cl_in) is generally considered inert in soil and is often used as a tracer of soil and ground water movements. However, recent studies indicate that substantial retention or release of Cl_in can occur in soil, but the rates and processes responsible under different environmental conditions are largely unknown. We performed ³⁶Cl tracer experiments which indicated that short-term microbial uptake and release of Cl_in, in combination with more long-term natural formation of chlorinated organic matter (Cl_org), caused Cl_in imbalances in coniferous forest soil. Extensive microbial uptake and release of Cl_in occurred over short time scales, and were probably associated with changes in environmental conditions. Up to 24% of the initially available Cl_in within pore water was retained by microbial uptake within a week in our experiments, but most of this Cl_in was released to the pore water again within a month, probably associated with decreasing microbial populations. The natural formation of Cl_org resulted in a net immobilization of 4% of the initial pore water Cl_in over four months. If this rate is representative for the area where soil was collected, Cl_org formation would correspond to a conversion of 25% of the yearly wet deposition of Cl_in. The study illustrates the potential of two Cl_in retaining processes in addition to those previously addressed elsewhere (e.g. uptake of chloride by vegetation). Hence, several processes operating at different time scales and with different regulation mechanisms can cause Cl_in imbalances in soil. Altogether, the results of the present study (1) provide evidence that Cl_in cannot be assumed to be inert in soil, (2) show that microbial exchange can regulate pore water Cl_in concentrations and (3) confirm the controversial idea of substantial natural chlorination of soil organic matter.     

四川黄龙沟景区钙华的起源和形成机理研究

对四川黄龙沟钙华景区的水化学测试发现，形成黄龙沟钙华的泉水具有很高的Ca^2 和HC03^-离子浓度，相应地，泉水的C02分压显著高于大气和土壤生物成因所能产生的C02分压。结合泉水出露的地质条件和泉口C02气体碳稳定同位素组成(δ^13C＝—6．8‰)的分析，进一步发现，高C02分压与深部成因的C02有关。可见，黄龙沟钙华属于热成因类钙华，而非原来普遍认为的“是气候岩溶作用的产物”。此外，黄龙沟钙华的大量出现与水中方解石的迅速沉积、Ca^2 和HC03^-浓度的大量降低有关。随着地下水自泉口出露，由于水的C02分压远高于大气，水中C02大量释放于大气，结果水的pH值迅速升高，方解石饱和指数由泉口的负值很快转变为高的正值，为方解石的沉积奠定了必要的物理化学基础。放置于水中的大理岩石片观测表明，流速较快的边石坝处的方解石沉积速率是其附近水池内的2—5倍，这清楚地显示了水动力条件对沉积速率的控制。进一步根据DBL理论模型分析发现，水动力条件对方解石沉积速率的控制在于其对固液界面间扩散边界层(DBL)厚度的影响，流速愈快，DBL厚度愈薄。且DBL厚度最终制约着沉积表面的化学组成浓度，即厚度愈小，表面H^ 浓度愈低(或pH愈高)、方解石饱和指数愈高，进而方解石沉积愈快。     

Impacts of Climate Change on Growth Period and Planting Boundaries of Spring Wheat in China under RCP4.5 Scenario

This article contributes to research on how climate change will impact crops in China by moving from ex-post empirical analysis to forecasting. We construct a multiple regression model, using agricultural observations and meteorological simulations by GCMs, to simulate the possible planting boundaries and suitable planting regions of spring wheat under RCP4.5 scenario for the base period 2040s and 2070s. We find that the south boundary of possible planting region for spring wheat spreads along the belt: south Shandong-north Jiangsu-north Anhui-central Henan-north Hubei-southeast Sichuan-north Yunnan provinces, and will likely move northward under RCP4.5 scenario in 2040s and 2070s, resulting in the decrease of possible planting area in China. Moreover, the sowing and harvest date of spring wheat in the base period shows a gradually delayed phenomenon from the belt: south Xinjiang - Gansu, to the Tibet Plateau. As a result, the growth period of spring wheat in China will shorten because of the impacts of climate change. These results imply that a variety of adaptations measures should be set up in response to changing climatic conditions, including developing the planting base for spring wheat, restricting the planting area of spring wheat in sub-suitable areas at risk while expanding the planting area of optimal crops.     

Relation between neighbouring grains in the upper part of the NorthGRIP ice core — Implications for rotation recrystallization

We propose a new method to investigate the relationships between neighbouring crystals and apply it to the textures measured along the upper 900 m of the NorthGRIP ice core. This method shows unexpected correlations between neighbours in the so-called normal grain growth regime, questioning the classical view on the onset of rotation recrystallization in ice-sheets. Moreover, the fractionation rate associated to the rotation recrystallization appears constant through time. Finally, grains with low-angle boundaries do not present a special distribution pattern of their c-axes. This suggests that rotation recrystallization is an isotropic process not affected by the direction of the macroscopic strain.     

‘Modelling the Arctic Boundary Layer: An Evaluation of Six Arcmip Regional-Scale Models using Data from the Sheba Project’

A primary climate change signal in the central Arctic is the melting of sea ice. This is dependent on the interplay between the atmosphere and the sea ice, which is critically dependent on the exchange of momentum, heat and moisture at the surface. In assessing the realism of climate change scenarios it is vital to know the quality by which these exchanges are modelled in climate simulations. Six state-of-the-art regional-climate models are run for one year in the western Arctic, on a common domain that encompasses the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment ice-drift track. Surface variables, surface fluxes and the vertical structure of the lower troposphere are evaluated using data from the SHEBA experiment. All the models are driven by the same lateral boundary conditions, sea-ice fraction and sea and sea-ice surface temperatures. Surface pressure, near-surface air temperature, specific humidity and wind speed agree well with observations, with a falling degree of accuracy in that order. Wind speeds have systematic biases in some models, by as much as a few metres per second. The surface radiation fluxes are also surprisingly accurate, given the complexity of the problem. The turbulent momentum flux is acceptable, on average, in most models, but the turbulent heat fluxes are, however, mostly unreliable. Their correlation with observed fluxes is, in principle, insignificant, and they accumulate over a year to values an order of magnitude larger than observed. Typical instantaneous errors are easily of the same order of magnitude as the observed net atmospheric heat flux. In the light of the sensitivity of the atmosphere–ice interaction to errors in these fluxes, the ice-melt in climate change scenarios must be viewed with considerable caution.     

The Greenland Ice Core Chronology 2005, 15–42 ka. Part 2: comparison to other records

A new Greenland Ice Core Chronology (GICC05) based on multi-parameter counting of annual layers has been obtained for the last 42 ka. Here we compare the glacial part of the new time scale, which is based entirely on records from the NorthGRIP ice core, to existing time scales and reference horizons covering the same period. These include the GRIP and NorthGRIP modelled time scales, the Meese-Sowers GISP2 counted time scale, the Shackleton–Fairbanks GRIP time scale (SFCP04) based on ¹⁴C calibration of a marine core, the Hulu Cave record, three volcanic reference horizons, and the Laschamp geomagnetic excursion event occurring around Greenland Interstadial 10. GICC05 is generally in good long-term agreement with the existing Greenland ice core chronologies and with the Hulu Cave record, but on shorter time scales there are significant discrepancies. Around the Last Glacial Maximum there is a more than 1 ka age difference between GICC05 and SFCP04 and a more than 0.5 ka discrepancy in the same direction between GICC05 and the age of a recently identified tephra layer in the NorthGRIP ice core. Both SFCP04 and the tephra age are based on ¹⁴C-dated marine cores and fixed marine reservoir ages. For the Laschamp event, GICC05 agrees with a recent independent dating within the uncertainties.     

Reading the climate record of the martian polar layered deposits

The martian polar regions have layered deposits of ice and dust. The stratigraphy of these deposits is exposed within scarps and trough walls and is thought to have formed due to climate variations in the past. Insolation has varied significantly over time and caused dramatic changes in climate, but it has remained unclear whether insolation variations could be linked to the stratigraphic record. We present a model of layer formation based on physical processes that expresses polar deposition rates of ice and dust in terms of insolation. In this model, layer formation is controlled by the insolation record, and dust-rich layers form by two mechanisms: (1) increased summer sublimation during high obliquity, and (2) variations in the polar deposition of dust modulated by obliquity variations. The model is simple, yet physically plausible, and allows for investigations of the climate control of the polar layered deposits (PLD). We compare the model to a stratigraphic column obtained from the north polar layered deposits (NPLD) (Fishbaugh, K.E., Hvidberg, C.S., Byrne, S., Russel, P.S., Herkenhoff, K.E., Winstrup, M., Kirk, R. [2010a]. Geophys. Res. Lett., 37, L07201) and show that the model can be tuned to reproduce complex layer sequences. The comparison with observations cannot uniquely constrain the PLD chronology, and it is limited by our interpretation of the observed stratigraphic column as a proxy for NPLD composition. We identified, however, a set of parameters that provides a chronology of the NPLD tied to the insolation record and consistently explains layer formation in accordance with observations of NPLD stratigraphy. This model dates the top 500 m of the NPLD back to ～1 million years with an average net deposition rate of ice and dust of 0.55 mm a^?1. The model stratigraphy contains a quasi-periodic ～30 m cycle, similar to a previously suggested cycle in brightness profiles from the NPLD (Laskar, J., Levrard, B., Mustard, F. [2002]. Nature, 419, 375–377; Milkovich, S., Head, J.W. [2005]. J. Geophys. Res. 110), but here related to half of the obliquity cycles of 120 and 99 kyr and resulting from a combination of the two layer formation mechanisms. Further investigations of the non-linear insolation control of PLD formation should consider data from other geographical locations and include radar data and other stratigraphic datasets that can constrain the composition and stratigraphy of the NPLD layers.