赣南版石-蔡坊火山岩年代学、地球化学和锆石Hf同位素特征及其地质意义

赣南版石和蔡坊盆地流纹质火山岩LA-ICP-MS锆石U-Pb定年结果分别为129.2±2.3 Ma和138.0±2.4 Ma。综合报道的同时期火山岩年龄,可以确定武夷山西缘发育的早白垩世火山岩形成时限大约在145~130 Ma。版石和蔡坊火山岩SiO_270%,属于高硅流纹质岩石,具有较低的Fe_2O_3~t/MgO比值(平均值10),富集大离子亲石元素和轻稀土元素,亏损中-重稀土元素,具显著的Eu负异常(δEu=0.06~0.20),类似于湿冷氧化性流纹岩,可能由来自交代岩石圈地幔的富钾岩浆结晶分异形成。湿冷氧化性的版石和蔡坊流纹质火山岩在武夷山西缘出现,并没有扩展到华南内陆,很可能表明古太平洋俯冲对华南地幔的影响范围主要位于武夷山西缘及其以东地区。     

自举法对我国进口铁矿二氧化硅含量的代表值估计

对我国近两年490批次、23个进口国的进口铁矿中SiO2含量进行了总体统计分析，在数据统计分布特征研究基础上，使用内核密度估计对进口铁矿SiO2含量进行数据多态性分析，使用自举法对原始数据样本值重复取样以获得稳健的SiO2含量代表值估计及标准偏差，并证明以自举法重新取样样本分布的均值与标准偏差作为有限单次样本代表值是合理、有效的。     

长江与黄河入海活性硅输送规律及变化趋势

根据长江与黄河各一个完整水文年的调查数据,并结合历史资料分析了我国这两条大型河流活性硅((RSi,RSi=溶解硅(DSi)+生物硅(BSi))的入海通量及长时间序列的变化规律与影响因素.结果表明,长江与黄河RSi的组成存在显著的差异,二者水体中BSi/RSi的平均比值分别为0.22和0.49;黄河DSi的年平均浓度为长江的74%,而BSi年平均浓度却是长江的3倍.黄河水体中相对较高的BSi浓度反映了黄河流域水体浑浊度与土壤侵蚀程度较高,源自黄土高原高的泥沙输送量是导致黄河水体中BSi浓度较长江高的主要原因.长江与黄河下游RSi通量在丰水期、平水期与枯水期的比值分别为5.3∶3.1∶1.6与3.8∶3.4∶2.8,长江半数以上的RSi入海通量是在丰水期输出的,而黄河在3个时期的差异不明显.相比于径流的变化,1958-2014年间长江DSi通量变化主要是由DSi浓度的变化引起的,流域气候变化(如温度变化)是其浓度及其通量年代际变化的重要原因;而黄河1985-2001年间DSi通量下降是由于径流量与DSi浓度降低的双重原因引起的.气候变化,特别是温度的变化会对流域硅的风化速率与硅的产出产生重要影响,但其具体的影响有待进一步揭示.     

湖光岩玛珥湖沉积物硅藻硅同位素环境示踪

硅是硅藻生长所必需的养分,硅藻在生长过程中从水体吸收主要以硅酸(H_4SiO_4)形式存在的溶解硅.硅藻δ~(30)Si_(distom)可反映硅藻吸收硅酸比例的变化,进一步可以反映湖区气候与环境变化.为探索湖泊沉积物硅藻δ~(30)Si_(distom)在指示湖泊古环境方面的潜力,我们测定了湖光岩玛珥湖沉积物硅藻硅同位素变化.研究结果表明,沉积物δ~(30)Si_(distom)变化与生物硅和有机碳含量变化呈显著相似性,可作为一种有效的古环境代用指标.同时,与已有温度记录的对比分析表明,两者具有很好一致性.较高的如δ~(30)Si_(distom)指示了温度较高的气候条件下硅藻对硅酸的利用率较高;当δ~(30)Si_(distom)较低时,对应着在温度较低的气候条件下硅藻对硅酸的利用率较低.目前,热带陆地区域古气候重建的方法中可用的温度代用指标很少,此项研究证明了δ~(30)Si_(distom)是一种很有潜力的古环境代用指标,有望在低纬度地区古温度重建中发挥重要作用,值得进一步深入研究.     

Estimation of biogenic silica contents in marine sediments using seismic and well log data: Sediment Drift 7, Antarctica

Petrophysical properties (wet bulk density, porosity, P-wave velocity) are used to predict biogenic silica contents along a seismic reflection profile that ties two well sites, 1095 and 1096, drilled by Ocean Drilling Program (ODP) Leg 178 on sediment drifts on the Pacific continental margin of the Antarctic Peninsula. The biogenic silica contents along the seismic reflection profile were estimated on the basis of three hypotheses about petrophysical properties distributions in the two boreholes and statistical relationships between biogenic silica and other petrophysical properties, which were established on various sediment layers within the boreholes. Our study demonstrates the possibility to reliably predict the distribution of biogenic silica in the sub-seabed sediments if seismic data processed with amplitude preservation are used and statistical relations are considered. We conclude that the statistical extrapolation of biogenic silica content along seismic reflection profiles tied to borehole data is an efficient tool to quantify the amounts of silica undergoing crystalline transformation, which may have strong implications for submarine slope destabilisation.     

Influence of Asian monsoon and ENSO events on particle fluxes in the western subtropical Pacific

A time-series sediment trap was deployed from October 2007 to May 2011 in the western subtropical Pacific with the aim of understanding the seasonal and inter-annual variability on particle flux in response to El Niño-Southern Oscillation (ENSO) events. Total mass fluxes varied from 3.04 mg m⁻² day⁻¹ to 31.1 mg m⁻² day⁻¹, with high fluxes during February–April and low fluxes during other months. This seasonal variation was also characterized by a distinct change in the CaCO₃ flux between the two periods. The marked increase in particle flux during February–April may be attributed to enhanced biological productivity in surface waters caused by strong wind-driven mixing in response to the western North Pacific monsoon system. The 2009/10 strong El Niño was accompanied by a significant reduction in particle flux, whereas the La Niña had no recognizable effect on particle flux in the subtropical Pacific. In particular, in the mature phase of the 2009/10 strong El Niño, the fluxes of organic carbon and biogenic silica decreased by 70–80% compared with those during the normal period, implying that the El Niño acted to suppress biological productivity in surface waters. The suppression of biological productivity during the 2009/10 strong El Niño is attributed to the decrease in precipitation due to the shift in the western Pacific warm pool. This finding is opposite that of other studies of the western equatorial Pacific, where El Niño events were observed to result in an increase in biological productivity and particle flux. The difference in particle flux between the western equatorial and subtropical Pacific is attributed to the regional differences in oceanic and atmospheric circulation systems generated by the strong El Niño.     

Annual lamination and its sedimentary implications in the Yangtze River delta inferred from High-resolution biogenic silica and sensitive grain-size records

Annual sedimentary layers in core A from the Yangtze River (Changjiang) subaqueous delta were identified based on high-resolution biogenic silica (BSi) and grain-size records. The contents of BSi, the sediment grain-size and the sensitive grain-size fractions of <13.14 μm and 13.14-250 μm illustrated steady coincident fluctuations with the depth profile of core A. These fluctuations were inferred to reflect seasonal changes in physical and biological processes. For example, variations in the content of BSi likely represent changes in primary production in flood versus dry seasons. It in flood seasons was several times higher than that in dry seasons. The amounts of <13.14 and 13.14-250 μm fractions were complimentary to each other and co-varied with the BSi amount. Therefore, the BSi, grain-size and sensitive fractions can be used as proxies for the annual sedimentation. From 1992 to 2005, the estimated sedimentary rate based on the annual layer varied from 1.0 to 3.5  cm/a (mean of 2.07  cm/a), and these values varies with the annual sediment discharge from the Yangtze River to the sea. The significant seasonal variations in the water discharge, sediment load, ocean hydrodynamics and primary production, which are common in the area affected by the East Asian monsoon, are likely responsible for the formation of the annual sedimentary lamination.     

Chert spheroids of the Monterey Formation,California (USA): early‐diagenetic structures of bedded siliceous deposits

Chert spheroids are distinctive, early‐diagenetic features that occur in bedded siliceous deposits spanning the Phanerozoic. These features are distinct in structure and genesis from similar, concentrically banded ‘wood‐grain’ or ‘onion‐skin’ chert nodules from carbonate successions. In the Miocene Monterey Formation of California (USA), chert spheroids are irregular, concentrically banded nodules, which formed by a unique version of brittle differential compaction that results from the contrasting physical properties of chert and diatomite. During shortening, there is brittle fracture of diatomite around, and horizontally away from, the convex surface of strain‐resistant chert nodules. Unlike most older siliceous deposits, the Monterey Formation still preserves all stages of silica diagenesis, thus retaining textural, mineralogical and geochemical features key to unravelling the origin of chert spheroids and other enigmatic chert structures. Chert spheroids found in opal‐A diatomite form individual nodules composed of alternating bands of impure opal‐CT chert and pure opal‐CT or chalcedony. With increased burial diagenesis, surrounding diatomite transforms to bedded porcelanite or chert, and spheroids no longer form discrete nodules, yet still display characteristic concentric bands of pure and impure microcrystalline quartz and chalcedony. Petrographic observations show that the purer silica bands are composed of void‐filling cement that precipitated in curved dilational fractures, and do not reflect geochemical growth‐banding in the manner of Liesegang phenomena invoked to explain concentrically banded chert nodules in limestone. Chertification of bedded siliceous sediment can occur more shallowly (< 100 m) and rapidly (< 1 Myr) than the bulk silica phase transitions forming porcelanite or siliceous shale in the Monterey Formation and other hemipelagic/pelagic siliceous deposits. Early diagenesis is indicated by physical properties, deformational style and oxygen‐isotopic composition of chert spheroids. Early‐formed cherts formed by pore‐filling impregnation of the purest primary diatomaceous beds, along permeable fractures and in calcareous–siliceous strata.     

Structure of a silica diagenetic transformation zone: the Gjallar Ridge,offshore Norway

Three‐dimensional seismic data from the Gjallar Ridge were used together with X‐ray diffraction data, scientific boreholes and wireline logs to analyse the seismic structure of a silica diagenetic transformation zone. The following features were identified: (i) an interval some 150 to 300 m thick that contains anomalously high‐amplitude reflections; (ii) a strong reflection event at the top of this interval which cross‐cuts stratigraphy, interpreted as the transformation boundary between opal‐A‐rich and opal‐CT‐rich sediment; (iii) amplitude variations on stratigraphic reflections within the interval, attributed to variations in the proportions of opal‐A and opal‐CT; and (iv) a second, deeper, cross‐cutting reflection event within the interval, interpreted as the transformation boundary between opal‐CT‐rich and quartz‐rich sediment. The base of the interval containing the anomalously high‐amplitude reflections is interpreted as a stratigraphic reflection demarcating the base of the silica‐rich strata. On a stratigraphic reflection within the interval of high amplitudes, roughly circular regions of anomalously high amplitude with diameters of 0·8 to 2·5 km are separated by lower amplitude regions. This pattern is similar to the cells previously identified at the opal‐A to opal‐CT transformation boundary and probably results from more complete transformation of opal‐A to opal‐CT. All of these observations provide the first recognition from seismic data that silica diagenetic transformations are not always narrow boundaries represented by single cross‐cutting seismic reflection events, as implied previously, but can be heterogeneous and hundreds of metres in thickness, as observed at outcrop.