A self-consistent top–down model for differentiation in bimodal suites: application to the Sonju Lake Intrusion–Finland granite system (MN)

The Sonju Lake Intrusion (SLI) is a 1200-meter thick layered mafic intrusion that directly underlies an equally large silicic pluton, the Finland granophyre (FG) within the Beaver Bay Complex of the Mid-Continent Rift (MN, USA). The SLI, with a simple mineralogical and compositional stratigraphy, provides an excellent case study for examining the changes in iron isotope ratios (δ⁵⁶Fe). Here new Fe isotope data along with ⁸⁷Sr/⁸⁶Sr for a set of stratigraphically controlled samples from the SLI and FG are presented. The Fe isotope data show systematic changes within two differentiation sequences found in the lowermost FG as well as the upper portion of the SLI. Specifically, δ⁵⁶Fe is observed to start at low values and increase to heavy values going stratigraphically up through each differentiation sequence. Within the middle portion of the SLI, δ⁵⁶Fe varies between 0 and 0.1. Two samples from the SLI bottom are isotopically lighter than the middle SLI. The origin of the Fe isotope variations is discussed in terms of recently proposed explanations. A quantitative model shows that the observed spatial variation is consistent with the prediction of a temperature gradient model. Using present constraints on equilibrium phase partitioning, the iron isotope variations do not appear consistent with production by fractional crystallization. Based on these observations, a top–down sill emplacement process coupled with in situ differentiation remains a viable alternative model for forming this layered intrusion.     

Climate change in southern Illinois, USA, based on the age and δC of organic matter in cave sediments

Matrix-supported diamicton and uniform to laminated, silty, fine-grained sediment deposited from about 42,500 to 27,600 cal yr B.P. under slackwater conditions nearly filled two caves in southwestern Illinois. At some point, most of the sediment was flushed from the caves and from about 22,700 to 4000 cal yr B.P., floods deposited a drape of sandy and silty sediment on remnant slackwater successions, cobbly alluvium, and bedrock (especially from 7700 to 4000 cal yr B.P.). Clay mineral analyses of the slackwater cave sediment reveal a provenance of chiefly Petersburg Silt, a smectite- and illite-rich proglacial lacustrine unit present in the overlying Illinois Episode glacial succession. Today, remnants of the ancient subterranean slackwater deposits nearly fill several secondary passages and, in at least two locations, cover a cobble-mantled strath terrace 1.3 to 1.5 m above active stream channels. Slumping and sinkhole formation appear to have been important mechanisms for deposition of the ancient subterranean deposits. Slumping of these surficial deposits and associated vegetation can occur along the flanks of sinkholes (in addition to sinkhole formation) and enter caves; however, the finer organics, some of them comminuted during transport into the caves, become part of the cave alluvium. This finer organic fraction is the modern analog of the humified organic matter disseminated in slackwater sediment dated in this investigation by radiocarbon methods. Twenty-four ¹⁴C ages on humified organic matter provide chronologic control. The δ¹³C values of the organic matter reflect the proportion of C₄-type to C₃-type vegetation growing in and around swallets and sinkholes at the time of redeposition. Drought-tolerant C₄-type vegetation was more prevalent relative to C₃-type vegetation from 42,500 to 31,200 cal yr B.P. compared to conditions from 28,800 cal yr B.P. to the present. The δ¹³C values are consistent with the results from other investigations of speleothems and organic matter from loessial paleosols.     

黄土丘陵沟壑区典型小流域土地持续利用案例研究

本文选择我国水土流失最为严重的黄土丘陵沟壑区作为研究地区,通过野外调查、土地利用填图,结合区域国民经济发展规划提出了4种基本土地利用规划方案附加2种保护性耕作措施;开展了生态适宜性、经济可行性和社会可接受性评价。提出:在得到大量外部经济支持的前提下,应积极实行15°以上的坡耕地全部退耕还林还草的方案,其中在中等坡度的地区(15°-25°)应发展果园和经济林;若缺少外部强有力的经济支持,该区土地利用调整应逐步开展,短期内(约0-5年),建议坡度大于25°的坡耕地逐步退耕还林还草,其中,坡度大于25°,地形条件较好的地区应种植果园和经济林;中期(约5-10年),坡度大于20°的黄土地区应逐步退耕还林还草,其中坡度介于20°-25°之间的地区应转变为果园和经济林;10年之后,建议大于15°的坡耕地全部转变为其他用途,其中坡度介于15°-25°的黄土地区应转变为果园和经济林,坡度大于25°的地区转变为林地/灌丛和草地     

Climatic and limnological changes at Lake Karakul (Tajikistan) during the last ~29 cal ka

We present results of analyses on a sediment core from Lake Karakul, located in the eastern Pamir Mountains, Tajikistan. The core spans the last ~29 cal ka. We investigated and assessed processes internal and external to the lake to infer changes in past moisture availability. Among the variables used to infer lake-external processes, high values of grain-size end-member (EM) 3 (wide grain-size distribution that reflects fluvial input) and high Sr/Rb and Zr/Rb ratios (coinciding with coarse grain sizes), are indicative of moister conditions. High values in EM1, EM2 (peaks of small grain sizes that reflect long-distance dust transport or fine, glacially derived clastic input) and TiO₂ (terrigenous input) are thought to reflect greater influence of dry air masses, most likely of Westerly origin. High input of dust from distant sources, beginning before the Last Glacial Maximum (LGM) and continuing to the late glacial, reflects the influence of dry Westerlies, whereas peaks in fluvial input suggest increased moisture availability. The early to early-middle Holocene is characterised by coarse mean grain sizes, indicating constant, high fluvial input and moister conditions in the region. A steady increase in terrigenous dust and a decrease in fluvial input from 6.6 cal ka BP onwards points to the Westerlies as the predominant atmospheric circulation through to present, and marks a return to drier and even arid conditions in the area. Proxies for productivity (TOC, TOC/TN, TOC _Br ), redox potential (Fe/Mn) and changes in the endogenic carbonate precipitation (TIC, δ¹⁸O _Carb ) indicate changes within the lake. Low productivity characterised the lake from the late Pleistocene until 6.6 cal ka BP, and increased rapidly afterwards. Lake level remained low until the LGM, but water depth increased to a maximum during the late glacial and remained high into the early Holocene. Subsequently, the water level decreased to its present stage. Today the lake system is mainly climatically controlled, but the depositional regime is also driven by internal limnogeological processes.     

Influence of aspect and slope gradient on hydraulic conductivity measured by tension infiltrometer

A tension infiltrometer technique was used to characterize differences in hydraulic conductivity (K) in two rain‐fed hillsides (north‐facing and south‐facing) in central Chile. For the north‐facing locations, smaller values of K (at a range of supply water pressure heads ψ) compared with south‐facing locations were found, with accentuated differences close to saturation (zero pressure head). The differences were attributed to differences in texture and organic matter contents observed for the two sites. Furthermore, K(ψ) had a tendency to increase with increasing slope gradient. This tendency was to an extent explained by the deviation from requirements of measurements on level ground. The differences found in K(ψ) between different slope gradients were explained by the differences in the vertical and lateral hydraulic conductivity and by the occurrence of surface sealing in low slope plots. Copyright © 2000 John Wiley & Sons, Ltd.     

Magnesium isotopic composition of igneous rock standards measured by MC-ICP-MS

To provide inter-lab comparison for high-precision Mg isotope analysis, Mg isotope compositions (expressed as δ²⁶Mg relative to DSM-3) for commercially accessible peridotite, basalt, andesite, and granite geo-standards have been measured by multi-collector inductively coupled mass-spectrometry (Nu-Plasma) using sample-standard bracketing method. There is a large tolerance of matrix cations during the measurement of Mg isotopes, as intensity ratios of ²³Na/²⁴Mg and ²⁷Al/²⁴Mg of about 20% only change the δ²⁶Mg by less than 0.1‰, and low ⁵⁵Mn/²⁴Mg (<0.1) and ⁵⁸Ni/²⁴Mg (<0.01) do not cause significant mass bias either. Concentration match between samples and standards within 90% is adequate to obtain accurate isotope analysis, which also mitigates the isobaric interference of ¹²C¹⁴N⁺ on ²⁶Mg. Organic matrix from chemical purification can cause significant analytical errors when the mass of Mg processed is small. The long-term reproducibility of δ²⁶Mg_DSM-3 for samples with relatively higher MgO content is about 0.11‰ (2SD), and granites with lower MgO content is about 0.2‰ (2SD). Although the standards in this study have wide ranges of major element compositions with SiO₂ from 40 to 70 wt.% and MgO from 0.75 to 49.6 wt.%, they exhibit a variation of Mg isotopic compositions with δ²⁶Mg from −0.07 to −0.40‰. δ²⁶Mg do not correlate with SiO₂ or MgO contents, suggesting homogenous Mg isotope compositions in igneous rocks at the level of current precision, relative to low temperature samples including sediments and riverine and sea waters. Our data do not support a non-chondritic Mg isotope composition of the Earth.