Salinity reflux and dolomitization of southern Australian slope sediments: the importance of low carbonate saturation levels

Anomalously saline waters in Ocean Drilling Program Holes 1127, 1129, 1130, 1131 and 1132, which penetrate southern Australian slope sediments, and isotopic analyses of large benthic foraminifera from southern Australian continental shelf sediments, indicate that Pleistocene–Holocene meso‐haline salinity reflux is occurring along the southern Australian margin. Ongoing dolomite formation is observed in slope sediments associated with marine waters commonly exceeding 50‰ salinity. A well‐flushed zone at the top of all holes contains pore waters with normal marine trace element contents, alkalinities and pH values. Dolomite precipitation occurs directly below the well‐flushed zone in two phases. Phase 1 is a nucleation stage associated with waters of relatively low pH (ca 7) caused by oxidation of H₂S diffusing upward from below. This dolomite precipitates in sediments < 80 m below the sea floor and has δ¹³C values consistent with having formed from normal sea water (? 1‰ to + 1‰ Vienna Pee Dee Belemnite). The Sr content of Phase 1 dolomite indicates that precipitation can occur prior to substantial metastable carbonate dissolution (< 300 ppm in Holes 1129 and 1127). Dolomite nucleation is interpreted to occur because the system is undersaturated with respect to the less stable minerals aragonite and Mg‐calcite, which form more readily in normal ocean water. Phase 2 is a growth stage associated with the dissolution of metastable carbonate in the acidified sea water. Analysis of large dolomite rhombs demonstrates that at depths > 80 m below the sea floor, Phase 2 dolomite grows on dolomite cores precipitated during Phase 1. Phase 2 dolomite has δ¹³C values similar to those of the surrounding bulk carbonate and high Sr values relative to Phase 1 dolomite, consistent with having formed in waters affected by aragonite and calcite dissolution. The nucleation stage in this model (Phase 1) challenges the more commonly accepted paradigm that inhibition of dolomitization by sea water is overcome by effectively increasing the saturation state of dolomite in sea water.     

裂缝对和田河气田石炭系生屑灰岩段储层的控制作用

和田河气田是塔里木盆地已探明的大型碳酸盐岩气田。石炭系生屑灰岩段是和田河气田产气的主要产层,其储量占总储量的53% ,生屑灰岩段储层经历了同生期岩溶作用、埋藏期岩溶作用。埋藏溶蚀作用产生的溶蚀孔隙是该储层的主要储集空间,占储层总孔隙度的47%～ 61%。H2 S可能是生屑灰岩段埋藏溶蚀作用的一种重要营力,埋藏溶蚀作用沿裂缝进行。裂缝以构造缝为主,裂缝不但是储层渗透率的主要贡献者,而且控制孔隙在垂向及平面上的分布: 在纵向上,裂缝发育段也是基质孔隙发育段;而平面上,构造高部位等裂缝发育处孔隙度也相对较大,而在构造鞍部裂缝不发育区,孔隙度也低。     

Infrared estimates of aliphatic kerogen carbon in sedimentary rocks

An infrared routine has been developed to estimate the aliphatic portion of kerogen carbon in sedimentary rocks. The procedure does not require isolation of the organic matter and is based on a computer-assisted determination of global band areas in the region of the aliphatic carbon-hydrogen stretching vibrations around 2900cm⁻¹. From these integrated absorptions the amount of aliphatic carbon C_al (mg of aliphatic carbon per gram of solvent-extracted rock) is calculated by means of a calibration with model rocks. Carbonate overtones which interfere in the case of limestones are eliminated by comparison to a CaCO₃ standard.The method has been applied to rocks containing kerogens of different types and maturities at TOC levels of 0.5 to 12%. The aliphatic carbon concentrations range from 0.5 to 60mg·g⁻¹ and correlate reasonably well with the residual genetic potentials of the rocks as measured by S₂ values from Rock-Eval pyrolysis. The ratio S₂/C_al is found to decrease with burial depth reflecting a maturity enhanced conversion of aliphatic carbon to fixed aromatic carbon under Rock-Eval conditions.     

埋藏环境下碳酸盐岩建设性成岩作用类型、特征及其意义

本文通过对南方中扬子地区和北方鄂尔多斯盆地西缘的寒武系、奥陶系埋藏成岩作用的详细研究,认为:（1）埋藏破裂、裂隙作用虽然能使岩石孔隙度有所增加。但更重要的是连通原已存在的孔隙。从而提高岩石的孔隙度和渗透率:（2）埋藏白云化作用形成的晶间孔、缝、晶内孔、晶间溶孔等是油气储集的一种最有利的空间;（3）与构造作用相关的埋藏溶解作用和深部埋藏作用能形成有意义的油气储集场所。上述3冲作用相互叠加时。岩石的孔隙度和渗透率将急剧提高。本文提出鄂尔多斯北缘和东缘奥陶系是油气勘探的战略远景区。同时首次提出了识别埋藏白云石化作用和埋藏溶解作用的标志和际准。     

Physics of Atoll Groundwater Flow

Large-scale steady-state groundwater flow in atoll carbonate platforms results from temperature and salinity-induced density gradients. Atolls are built on top of a basaltic substrate that provides geothermal heating from beneath. Moreover, they are immersed in the tropical ocean where temperature decreases rapidly with depth. Groundwater circulation in these platforms has long been associated with the geothermal heat flux because it is capable of generating inward and upward flow of oceanic origin water by buoyancy effects. This study shows that hydraulic circulation occurs even in the absence of a geothermal flux because the combination of the cold subsurface ocean waters with the warm surface conditions is sufficient to maintain a convection cell within the carbonate platform. Using a one-dimensional analytical model, validated by more sophisticated two-dimensional simulations, we can investigate the interaction between these two driving forces. The flow rate inside the platform is, in fact, a function of the ratio of the geothermal flux to the temperature gradient in the ocean. It increases with the geothermal flux but decreases with the oceanic temperature gradient. This one-dimensional model also shows that taking salinity effects on density into account increases the flow rates transiting through the platform by a third.     

Mg-Sr isotopes of low-δ26Mg basalts tracing recycled carbonate species: Implication for the initial melting depth of the carbonated mantle in Eastern China

ABSTRACT

Recent studies show that crustal carbonates recycled into the mantle can be traced using Mg isotopes of basalts. However, the species of recycled carbonates are poorly constrained. Carbonates have lower δ²⁶Mg values and higher ⁸⁷Sr/⁸⁶Sr ratios relative to the mantle, but different carbonate species display different mixing curves with the mantle in the Mg-Sr isotopic diagram because of differences in their Sr and Mg contents. Thus a combined study of Mg-Sr isotopes can constrain the species of deeply recycled carbonates. Here, we present newly determined ⁸⁷Sr/⁸⁶Sr ratios of the <110 Ma basalts from Eastern China, and together with published Mg isotopic data we evaluate the species of recycled carbonates in the mantle and discuss their implication. The <110 Ma basalts display low δ²⁶Mg values of ?0.60 to ?0.30‰ and relatively low initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70328 to 0.70537, suggesting that their mantle source was hybridized by recycled carbonates with a light Mg isotopic composition which had more significant effects on Mg than Sr isotope ratios. Mg-Sr isotopic data indicate that the recycled carbonates consist of magnesite and aragonite, but the possibility of calcite and dolomite cannot be eliminated. Based on the carbonated peridotite solidus, the equilibrium line between dolomite and magnesite + aragonite, as well as the mantle adiabat, the initial melting depth of the carbonated mantle, the source region of the studied basalts, was constrained at ~300–360 km. Thus, the subducted depth of the west Pacific slab underlying the carbonated mantle and supplying recycled carbonates should be greater than ~300–360 km, consistent with the seismic tomography result that the west Pacific slab now stagnates in the mantle transition zone.     

洋岛,海山碳酸盐岩的沉积特征及其古地理意义

高出水面的洋岛和潜伏水下的海山普遍存在于现代地球表面的各大洋中，地质历史中存在的古洋岛和古海山也逐渐为人们所认识。笔者从地形特征，沉积学特征，成岩作用和生物学特征几个方面对洋岛，海山进行了分析，洋岛，海山通常具有洋岛型火山岩基底和碳酸盐盖层的双层式地层结构，具有低分异度，探讨了古洋岛，古海山的鉴别对于再造古海洋，古地理格局的现实意义。     

High-Resolution Records of the Holocene Paleoenvironmental Variation Reflected by Carbonate and Its Isotopic Compositions in Bosten Lake and Response to Glacial Activities

Abstract: The Early Holocene paleoclimate in Bosten Lake on the northern margin of the Tarim Basin, southern Xinjiang, is reconstructed through an analysis of a 953 cm long core (BSTC2000) taken from Bosten Lake. Multiple proxies of this core, including the mineral components of carbonate, carbonate content, stable isotopic compositions of carbonate, Ca/Sr, TOC and C/N and C/S of organic matter, are used to reconstruct the climatic change since 8500 a B.P. The chronology model is made by nine AMS 14C ages of leaves, seeds and organic matter contained in two parallel cores. The climate was cold and wet during 8500 to 8100 a B.P. Temperature increased from 8100 to 6400 a B.P., the climate was warm and humid, and the lake expanded. The lake level was highest during this stage. Then from 6400 to 5100 a B.P., the climate became cold and the lake level decreased slightly. During the late mid-Holocene, the climate was hot and dry from 5100 to 3100 a B.P., but there was a short cold period during 4400 to 3800 a B.P. At this temporal interval, a mass of ice and snow melting water supplied the lake at the early time and made the lake level rise. The second highest lake level stage occurred during 5200 to 3800 a B.P. The climate was cool and wet during 3100 to 2200 a B.P., when the lake expanded with decreasing evaporation. The lake had the last short-term high level during 3100 to 2800 a B.P. After this short high lake level period, the lake shrank because of the long-term lower temperature and reduced water supply. From 2200 to 1200 a B.P., the climate was hot and dry, and the lake shrank greatly. Although the temperature decreased somewhat from 1200 a B.P. to the present, the climate was warm and dry. The lake level began to rise a little again, but it did not reach the river bed altitude of the Konqi River, an outflow river of the Bosten Lake.     

Long-distance correlation between tectonic-controlled, isolated carbonate platforms by cyclostratigraphy and sequence stratigraphy in the Devonian of South China

During the early Middle Devonian in South China, an extensive carbonate platform was broken up through extension to create a complex pattern of platforms, and interplatform basins. In Givetian and Frasnian carbonate successions, five depositional facies, including peritidal, restricted shallow subtidal, semi‐restricted subtidal, intermediate subtidal and deep subtidal facies, and 18 lithofacies units are recognized from measured sections on three isolated platforms. These deposits are arranged into metre‐scale, upward‐shallowing peritidal and subtidal cycles. Nine third‐order sequences are identified from changes in cycle stacking patterns, vertical facies changes and the stratigraphic distribution of subaerial exposure indicators. These sequences mostly consist of a lower transgressive part and an upper regressive part. Transgressive packages are dominated by thicker‐than‐average subtidal cycles, and regressive packages by thinner‐than‐average peritidal cycles. Sequence boundaries are transitional zones composed of stacked, high‐frequency, thinner‐than‐average cycles with upward‐increasing intensity of subaerial exposure, rather than individual, laterally traceable surfaces. These sequences can be further grouped into catch‐up and keep‐up sequence sets from the long‐term (second‐order) changes in accommodation and vertical facies changes. Catch‐up sequences are characterized by relatively thick cycle packages with a high percentage of intermediate to shallow subtidal facies, and even deep subtidal facies locally within some individual sequences, recording long‐term accommodation gain. Keep‐up sequences are characterized by relatively thin cycle packages with a high percentage of peritidal facies within sequences, recording long‐term accommodation loss. Correlation of long‐term accommodation changes expressed by Fischer plots reveals that during the late Givetian to early Frasnian increased accommodation loss on platforms coincided with increased accommodation gain in interplatform basins. This suggests that movement on faults resulted in the relative uplift of platforms and subsidence of interplatform basins. In the early Frasnian, extensive siliceous deposits in most interplatform basins and megabreccias at basin margins correspond to exposure disconformities on platforms.     

云贵高原湖泊沉积物碳酸盐和有机质的同位素组成及环境信息记录

云贵高原湖泊沉积物碳酸盐和有机质的同位素组成及环境信息记录＠肖保华￥中国科学院地球化学研究所高原湖泊,碳酸盐,有机质,碳同位素,地球化学记录,环境气候信息云贵高原湖泊沉积物碳酸盐和有机质的同位素组成及环境信息记录肖保华（中国科学院地球化学研究所,贵阳５５０...