A 90° spin on Rodinia: possible causal links between the Neoproterozoic supercontinent, superplume, true polar wander and low-latitude glaciation

We report here new geochronological and paleomagnetic data from the 802±10 Ma Xiaofeng dykes in South China. Together with existing data, these results suggest that Rodinia probably spread from the equator to the polar region at ca. 800 Ma, followed by a rapid ca. 90° rotation around an axis near Greenland that brought the entire supercontinent to a low-latitude position by ca. 750 Ma. We propose that it was the initiation of a mantle superplume under the polar end of Rodinia that triggered an episode of true polar wander (TPW) which brought the entire supercontinent into equatorial latitudes. An unusually extensive emerged land area at the equator increased both atmospheric CO₂ drawdown and global albedo, which, along with waning plume volcanism led directly to the low-latitude Sturtian glaciation at ca. 750–720 Ma.     

A major change in magma sources in late Mesozoic active margin of the circum-Sea of Japan domain: Geochemical constraints from late Paleozoic to Paleogene mafic dykes in the Sergeevka belt,southern Primorye,Russia

More than 30 mafic dykes crop out in the Sergeevka belt in the coastal South Primorye, Far East Russia, of which geologic settings have been unclear for years. This study conducted major- and trace elements characterization, Sr–Nd isotope analyses, and Ar–Ar amphibole and U–Pb zircon datings for these rocks in order to identify their origin. The results demonstrated that all dykes are characterized by high Ba/Yb and low Nb/Y, Zr/Y, and Th/Yb ratios, which suggest their origin from arc melts derived from thin wedge mantle and shallow-dipping slab. These dykes are clearly separated into two distinct age/geochemistry suites; that is, the Paleogene and Early Cretaceous one with dolerites/basalts and adakitic rocks, and the Permian–Triassic one with high-Mg and high-Al gabbro-dolerite varieties. Their geochemistry suggests that the older suite was sourced from a primitive depleted MORB mantle (DMM)-type mantle, whereas the younger suite from an enriched mantle II (EM2)-type mantle domain. The transition in source type from DMM to EM2 occurred during the Jurassic-earliest Cretaceous time, probably by a strong influence of a mantle plume onto the long-continuing subduction-related magmatism. The plume influence reached the maximum when the unique meimechite-picrite complex formed in the region.     

基于生态系统恢复力的干旱区植被生态需水阈值计算方法与应用

本文以石羊河流域中下游为研究区,采用考虑生态系统恢复力(latitude of ecosystem resilience, LER)的月尺度生态需水评估方法计算1982-2020年植被月适宜生态需水量、最小生态需水量以及相应的生态缺水量,并与土壤水分特征值法(characteristic value of soil water, CVSW)进行比较,分析不同类型植被生长期的水分盈亏关系。结果表明:LER法和CVSW法计算结果相近,但LER法具有更大的生态需水阈值区间;天然降水基本可以满足植被的基本生存,但无法满足正常生长需求;LER法的适宜需水条件下,各植被生长期总体处于缺水状态,缺水严重程度排序为灌木林＞其他林地＞疏林地＞高覆盖度草地＞中覆盖度草地＞有林地,全部植被生长期总适宜生态需水量为3.7×10⁸ m³,亏缺水量为1.2×10⁸ m³,同一植被亏缺水量基本符合春秋多、夏季少的规律;最小需水条件下,只有其他林地存在生长期缺水情况,全部植被生长期总最小生态需水量为0.8×10⁸ m³;在缺乏土壤水分数据的干旱地区,LER法具有良好的适用性。研究结果可为石羊河流域水资源高效利用和干旱区生态系统的恢复与重建提供理论参考。     

Statistical analysis of flash flood events for designing water harvesting systems in an extremely arid environment

A water harvesting system for research purposes has been established in the Lisan Peninsula of the Dead Sea in the middle of the Jordan Rift Valley, where no authorized guideline is available for designing water harvesting systems. Rainfall and runoff, which occurred as flash floods, were observed at the downstream end of a gorge with a 1.12 km² barren catchment area from October 2014 through July 2019. Due to the extremely arid environment, runoff from the catchment is ephemeral, and the flash flood events can be clearly distinguishable from each other. Thirteen flash flood events with a total runoff volume of more than 100 m³ were successfully recorded during the five rainy seasons. Pearson and Spearman correlations between duration, total rainfall depths at two points, total runoff volume, maximum runoff discharge, bulk runoff coefficient, total variation in runoff discharge and maximum variation in runoff discharge of each flash flood event were examined, revealing no straightforward relationship between rainfall and runoff. The performance of the conventional SCS runoff curve number method was also deficient in reproducing any rainfall–runoff relationship. Therefore, probability distribution fitting was performed for each random variable, focusing on the lognormal distribution with three parameters and the generalized extreme value distribution. The maximum goodness-of-fit estimation turns out to be a more rational and efficient method in obtaining the parameter values of those probability distributions rather than the standard maximum likelihood estimation, which has known disadvantages. Results support the design of the water harvesting system and provide quantitative information for designing and operating similar systems in the future.     

基于长时间序列(1975—2020年)生态耗水的岱海动态生态需水分析

生态需水是湖泊生态系统的重要指标,维持着湖泊生态系统的良性循环.以内蒙古中部半干旱湖泊岱海为研究对象,对湖泊动态生态需水进行分析.本研究在遥感和气象数据的基础上,获得1975-2020年长时间序列高精度水文要素数据,分析岱海水文要素时空演变规律;通过天然生态水深分析法、水深经验频率分析法和湖泊形态分析法分析岱海的水深随面积变化的关键水深;构建基于生态耗水规律的湖泊生态需水模型,计算自然状态下岱海生态需水动态变化范围.研究结果如下:岱海地区6-9月为丰水期,10月至次年5月为枯水期;45 a以来岱海水面面积呈显著下降趋势,近年来下降速率减缓;枯水期岱海适宜生态水深为8.72~9.92 m,丰水期为9.40~10.69 m,适宜生态需水量为5.62亿~7.71亿m³,适宜湖面面积为70.92~84.77 km².本文构建了长时间序列气候水文数据库,确定岱海动态生态需水范围可以实现对湖泊生态健康的实时监测,为相关规划与管理提供科学依据及可操作性指导,从而为岱海湖泊治理提供理论参考.     

Bundelkhand mafic dykes, Central Indian Shield: Implications for the role of sediment subduction in Proterozoic crustal evolution

Abstract The Archean to Paleo–Proterozoic Bundelkhand massif basement of the central Indian shield has been dissected by numerous mafic dykes of Proterozoic age. These dykes are low‐Ti tholeiites, ranging in composition from subalkaline basalt through basaltic‐andesite to dacite. They are enriched in light rare earth elements (LREE), large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE: Nb, P and Ti). Negative Sr anomaly is conspicuous. Nb/La ratios of the dykes are much lower compared with the primitive mantle, not much different from the average crustal values, but quite similar to those of continental and subduction related basaltic rocks. Bulk contamination of the mantle derived magma by crustal material is inadequate to explain the observed geochemical characteristics; instead contamination of the mantle/lithospheric source(s) via subduction of sediment is a better proposition. Thus, in addition to generating juvenile crust along the former island arcs, subduction processes appear to have influence on the development of enriched mantle/lithospheric source(s). The Bundelkhand massif basement is inferred to represent subduction related juvenile crust, that experienced lithospheric extension and rifting possibly in response to mantle plume activities. The latter probably supplied the required heat, material (fluids) and extensional environment to trigger melting in the refractory lithospheric source(s) and emplacement of the mafic dykes. Proterozoic mafic magmatic rocks from Bundelkhand, Aravalli, Singhbhum and Bastar regions of the Indian shield and those from the Garhwal region of the Lesser Himalaya display remarkably similar enriched incompatible trace elements characteristics, although limited chemical variations are observed in all these rocks. This may indicate the existence of a large magmatic province, different parts of which might have experienced similar petrogenetic processes and were probably derived from mantle/lithospheric source(s) with similar trace element characteristics. The minor, less enriched to depleted components of the Jharol Group of the Aravalli terrane and those from the Singhbhum terrane may represent protracted phases of rifting, that probably caused thinning and mobilization of the lithosphere, facilitating the eruption/emplacement of the asthenospheric melts (with N‐ to T‐types mid‐oceanic ridge basalts signatures) and deposition of deep water facies sediments in the younger developing oceanic basins. In contrast, Bundelkhand region did not experience such protracted rifting, although dyke swarms were emplaced and shallow water Bijawar Group and Vindhyan Supergroup sediments were deposited in continental rift basins. All these discrete Proterozoic terranes appear to have experienced similar petrogenetic processes, tectonomagmatic and possibly temporal evolution involving subduction processes, influencing the lithospheric source characteristics, followed by probably mantle plume induced ensialic rifting through to the development of oceanic basins in the Indian shield regions and their extension in the Lesser Himalaya.     

The age and tectonic environment of the rhyolitic rocks on the western side of Wuyi Mountain,South China

During the geological survey of the metamorphic rocks in Xingning-Wuhua region on the western side of Wuyi Mountain, South China, we discovered the Neoproterozoic rhyolite and rhyolitic greywacke for the first time that outcrop in the Proterozoic metamorphic rocks near Jingnan Town of Xingning County, eastern Guangdong Province. A systematic research on petrology, geochemistry and geochronology of rhyolitic rocks was conducted to understand their tectonic setting and formation age. The Jingnan rhyolite is interbedded with a coeval greywacke, with a total thickness of 60 m; both rhyolite and greywacke display a similar folding and metamorphic pattern. Meta-rhyolite consists of groundmass and phenocrystals including sanidine, orthoclase, and quartz with distinct undulose extinction; the groundmass has been recrystallized into fine-grain feldspar, quartz and sericite aggregation. Meta-greywacke is composed of crystallinoclastic grains (sanidine, orthoclase, quartz and oligoclase) and clay groundmass. Zircon grains used for the SHRIMP U-Pb analysis are light brown-colored and euhedral or subeuhedral. Dating data suggest two age groups; eight grains of magmatype zircon with an idiomorphic form yield an age of 972±8 Ma, and the other seven weakly corroded grains of zircon with euhedral to subeuhedral shape construct an average age of 1097±11 Ma, which were captured from older rocks by an uplifting magma, implying that a late Mesoproterozoic basement exists in the Nanling region. In addition, one Paleoproterozoic age, 2035±11 Ma, is obtained from a rounded detrital zircon, indicating that a Paleoproterozoic thermal event took place in the South China. Geochemically, the Jingnan rhyolitic rocks are characterized by high K2O content, intermediate Al2O3 content, with the ACNK value 0.98―1.11, and belonging to high-K alkaline series. They are rich in ΣREE, Rb, Th and Ce, depleted in Ba, Sr, Eu, Ti, P and Nb-Ta, and with moderate negative Eu and Sr anomalies. These features indicate that the Jingnan volcanic rocks have an affinity of continental arc that is similar to those of acid volcanic rocks in the SE-China Coastal Region, in other words, a Neoproterozoic tectonomagma event might have taken place in the western Wuyi region, leading to an eruption of high-K calc-alkaline granitic magma.     

New SHRIMP U-Pb age from the Wuqiangxi Formation of Banxi Group: Implications for rifting and stratigraphic erosion associated with the early Cryogenian (Sturtian) glaciation in South China

In South China, the Wuqiangxi Formation of the Banxi Group and its equivalents underlie the early Cryogenian (Sturtian) glacial deposits but their thickness varies from <200 m to >2000 m. In the Guzhang section of western Hunan, the Wuqiangxi Formation is only 152 m thick, and an ash bed 58 m below the glacial diamictite yielded a SHRIMP U-Pb age of 809.3±8.4 Ma. In contrast, 90 km south of the Guzhang section towards the basin in Zhijiang area where the Wuqiangxi Formation is ~2200 m thick, an age of 725±10 Ma has been reported from the top of this unit, 300 m below the glacial diamictite. These ages provide new evidence for the regional stratigraphic correlation across the Nanhua basin, and suggest unusually large (>2 km) stratigraphic erosion potentially associated with the Sturtian glaciation in South China. The magnitude of erosion may imply significant uplifting and tectonotopography at the onset of the Sturtian glaciation.     

Quantitative analysis of the driving factors for groundwater resource changes in arid irrigated areas

How to quantify the impact of climate change and human activities on groundwater is not only a hot topic of current research but also a key point of water resource management in arid irrigated areas. Therefore, this paper analyzes the changes in the trends of land use, climate, and groundwater extraction in the Yanqi Basin in recent years and uses the distributed hydrological model MIKE-SHE to quantitatively analyze the impacts of these three factors on groundwater resources. The results show that: 1. The Nash coefficients of the simulated and observed groundwater levels during the verification period are 0.84, 0.79 and 0.76; the correlation coefficient between the simulated and observed soil moisture is 0.86. Although there are some uncertainties in the simulation, the results prove that the model can be used to simulate arid irrigated areas. 2. The effects of these three factors on groundwater levels are 5, 12.5 and 82.5%, respectively, and have caused the regional average groundwater level to decrease by a maximum of 0.07, 0.23 and 1.79 m, respectively. The effects of these three factors on the interactions between surface water and groundwater were 7.04, 3.63 and 89.33%. Groundwater extraction has become the main influencing factor of regional groundwater resources changes due to its more direct influence. 3. The influence of groundwater extraction has a strong spatial distribution characteristic and 10% of the study area has been greatly impacted by the groundwater extraction. Base on the above results, integrating multidisciplinary knowledge to establish the relationship between ecological environment and groundwater changes can provide strategies for the sustainable development of groundwater.     

华南沿海地区地壳厚度与泊松比研究

华南沿海地区地壳厚度和泊松比分布是了解该地区板块运动和地球内部物质结构的重要依据.本文利用广东台网和福建台网共75个固定地震台记录到的2001—2008年远震波形的接收函数,通过H-κ叠加法获得了74个台站下方平均地壳厚度和泊松比值的有效估计,结合该区断裂分布、地震活动和地形地貌特征,得出以下结论:(1)该区地壳厚度范围在26~32 km之间,由陆向洋、自北向南减薄.泊松比0.23~0.28,由陆向洋增加,地壳厚度与泊松比变化分别和地表地形呈正相关和负相关;(2)地壳厚度和泊松比呈块状和带状分布,对应该区的断裂活动,其过渡带变化地区与地震分布一致,震群区和地幔上隆区对应泊松比的急剧增加;(3)该区主要以中、酸性地壳物质为主,地壳厚度与泊松比在近海区的构造伸展作用和褶皱区的逆冲推覆作用影响下,分别呈正、负消长关系,揭示该区不同时代和不同区域构造演化模式的差异.     

Spectral study of the Bouguer anomaly map of a rift valley and adjacent areas in Central India

The spectral study of the Bouguer anomally map of Central India suggests an uplifted crust-mantle interface under the Mahandi graben. This study has delineated three subsurface levels of anomalous masses at the respective depths of 23 km, 8 km, and 2 km apparently representing the Moho, an intermediate discontinuity in the sialic part of the crust and the basement, respectively. Model study of the Bouguer anomaly along a profile suggests a typical continental graben type subsurface structure with a low density depression in the sialic part of the crust between 8 and 18 km supported by an elevated upper mantle of intermediate density (3.4 g/cm³) varying in depth from 25 km to 55 km. The depths of the inferred interfaces in case of Bundelkhand granite are 32 km, 11 km, and 1.5 km, which might represent the Moho, the base of intruded granite massif, and some shallow compositional variation. Similar studies in case of Vindhyan basin have brought out three discontinuities at the respective depths of 16 km, 6–4.5 km, and 2.4 km. The first horizon at the depth of 16 km probably represents the interface between the granitic and the basaltic part of the crust. The 6–4.5 km is the depth of the basement, with the 2.4 km interface separating Bijawar rocks from Vindhyans wherever they are present. A generalized inversion of a profile across a positive belt of Bouguer anomaly representing the subsurface Bijawar rocks support the above result.     

Impact analysis of government investment on water projects in the arid Gansu Province of China

In this paper, we introduced three-nested Constant Elasticity of Substitution (CES) production function into a static Computable General Equilibrium (CGE) Model. Through four levels of factor productivity, we constructed three nested production function of land use productivity in the conceptual modeling frameworks. The first level of factor productivity is generated by the basic value-added land. On the second level, factor productivity in each sector is generated by human activities that presents human intervention to the first level of factor productivity. On the third level of factor productivity, water allocation reshapes the non-linear structure of transaction among first and second levels. From the perspective of resource utilization, we examined the economic efficiency of water allocation. The scenario-based empirical analysis results show that the three-nested CES production function within CGE model is well-behaved to present the economy system of the case study area. Firstly, water scarcity harmed economic production. Government investment on water projects in Gansu thereby had impacts on economic outcomes. Secondly, huge governmental financing on water projects bring depreciation of present value of social welfare. Moreover, water use for environment adaptation pressures on water supply. The theoretical water price can be sharply increased due to the increasing costs of factor inputs. Thirdly, water use efficiency can be improved by water projects, typically can be benefited from the expansion of water-saving irrigation areas even in those expanding dry area in Gansu. Therefore, increasing governmental financing on water projects can depreciate present value of social welfare but benefit economic efficiency for future generation.     

华南地区地磁场长期变化特征

使用华南地区5个国家基准地磁台2008—2018年地磁观测数据,通过对磁偏角D、磁倾角I、总强度F、北向分量X、东向分量Y、水平分量H、垂直分量Z月均值及年变率的分析,研究该区地磁场长期变化特征。结果表明,华南地区5个地磁台近10年来七要素月均值及年变率均呈缓慢变化趋势,且变化量相当,反映了该区域地磁场长期变化的特征。     

Numerical groundwater modelling as an effective tool for management of water resources in arid areas

Abstract

Groundwater, possibly of fossil origin, is used for water supply in some arid regions where the replenishment of groundwater by precipitation is low. Numerical modelling is a helpful tool in the assessment of groundwater resources and analysis of future exploitation scenarios. To quantify the groundwater resources of the East Owienat area in the southwest of the Western Desert, Egypt, the present study assesses the groundwater resources management of the Nubian aquifer. Groundwater withdrawals have increased in this area, resulting in a disturbance of the aquifer’s natural equilibrium, and the large-scale and ongoing depletion of this critical water reserve. Negative impacts, such as a decline in water levels and increase in salinity, have been experienced. The methodology includes application of numerical groundwater modelling in steady and transient states under different measured and abstraction scenarios. The numerical simulation model developed was applied to assess the responses of the Nubian aquifer water level under different pumping scenarios during the next 30 years. Groundwater management scenarios are evaluated to find an optimal management solution to satisfy future needs. Based on analysis of three different development schemes that were formulated to predict the future response of the aquifer under long-term water stress, a gradual increase in groundwater pumping to 150% of present levels should be adopted for protection and better management of the aquifer. Similar techniques could be used to improve groundwater management in other parts of the country, as well as other similar arid regions.

Editor D. Koutsoyiannis; Associate editor X. Chen     

Simulation of site-scale water fluxes in desert and natural oasis ecosystems of the arid region in Northwest China

The study of water fluxes is important to better understand hydrological cycles in arid regions. Data-driven machine learning models have been recently applied to water flux simulation. Previous studies have built site-scale simulation models of water fluxes for individual sites separately, requiring a large amount of data from each site and significant computation time. For arid areas, there is no consensus as to the optimal model and variable selection method to simulate water fluxes. Using data from seven flux observation sites in the arid region of Northwest China, this study compared the performance of random forest (RF), support vector machine (SVM), back propagation neural network (BPNN), and multiple linear regression (MLR) models in simulating water fluxes. Additionally, the study investigated inter-annual and seasonal variation in water fluxes and the dominant drivers of this variation at different sites. A universal simulation model for water flux was built using the RF approach and key variables as determined by MLR, incorporating data from all sites. Model performance of the SVM algorithm (R² = 0.25–0.90) was slightly worse than that of the RF algorithm (R² = 0.41–0.91); the BPNN algorithm performed poorly in most cases (R² = 0.15–0.88). Similarly, the MLR results were limited and unreliable (R² = 0.00–0.66). Using the universal RF model, annual water fluxes were found to be much higher than the precipitation received at each site, and natural oases showed higher fluxes than desert ecosystems. Water fluxes were highest during the growing season (May–September) and lowest during the non-growing season (October–April). Furthermore, the dominant drivers of water flux variation were various among different sites, but the normalized difference vegetation index (NDVI), soil moisture and soil temperature were important at most sites. This study provides useful insights for simulating water fluxes in desert and oasis ecosystems, understanding patterns of variation and the underlying mechanisms. Besides, these results can make a contribution as the decision-making basis to the water management in desert and oasis ecosystems.     

西沙群岛西北深水海域的内潮特征分析

基于海南岛至西沙群岛之间深水海域一长达5年的锚定潜标测流资料,采用谱分析、调和分析和动力模态分解等方法主要分析了局部海域内潮的基本特征.结果表明:研究海域的正压和斜压潮均以全日振荡为主,500~900 m的海洋中层全日等密度线垂向振幅可达40 m;全日内潮主要沿垂直于陆坡方向传播,与天文潮锁相的全日内潮可占全日内潮总能量的41%;海洋上层,O1内潮垂向平均振幅与局地海面高度呈显著正相关关系,K1内潮则表现为夏、冬季增强的半年循环特征;超过70%的O1分潮能量集中于第一、二斜压模态上,K1分潮在第三斜压模态上亦有相当能量.     

Climate change and runoff response based on isotope analysis in an arid mountain watershed of the western Kunlun Mountains

Climate change and runoff response were assessed for the Tizinafu River basin in the western Kunlun Mountains, China, based on isotope analysis. We examined climate change in the past 50 years using meteorological data from 1957 to 2010. Results of the Mann-Kendall non-parametric technique test indicated that temperature in the entire basin and precipitation in the mountains exhibited significant increasing trends. Climate change also led to significant increasing trends in autumn and winter runoff but not in spring runoff. By using 122 isotope samples, we investigated the variations of isotopes in different water sources and analysed the contributions of different water sources based on isotope hydrograph separation. The results show that meltwater, groundwater and rainfall contribute 17%, 40% and 43% of the annual streamflow, respectively. Isotope analysis was also used to explain the difference in seasonal runoff responses to climate change. As the Tizinafu is a precipitation-dependent river, future climate change in precipitation is a major concern for water resource management.

EDITOR A. Castellarin; ASSOCIATE EDITOR S. Huang     

Statistical interpretation of the daily variation of urban water consumption in Beijing,China

Abstract

The efficient planning and design of water networks, as well as the management and strategies of existing water supply systems, require accurate short-term water demand forecasts. In this study, a statistical model for the estimation of daily urban water consumption was developed. The model was applied to analyse and forecast the daily water consumption in the main district of Beijing, China, from 2006 to 2010. The model exhibited good performance with a coefficient of determination, R², greater than 0.9 in both the calibration and validation periods. The results indicate that: (a) the 7-day moving average temperature is an efficient variable that can be used to depict water-use variation in a year; (b) a daily maximum temperature of 31.1°C and the occurrence of precipitation are two thresholds of water-use behaviour; and (c) the current day’s water consumption has a strong correlation with the consumption of one, two and seven days ago.

Editor Z.W. Kundzewicz; Associate editor D. Yang

Citation Zhang, D.W., et al., 2013. Statistical interpretation of the daily variation of urban water consumption in Beijing, China. Hydrological Sciences Journal, 59 (1), 181–192.     

Discovery of native aluminum and its possible origin from prospective gas hydrate areas in the South China Sea

Aluminum is an active amphoteric metal element, but native aluminum (Al°) can be formed and preserved in special condi- tions so that its occurrence has unique geological significance. In this paper, the coarse fractions ( 63 μm) of 269 samples of surface sediments and of 165 sediment samples in four cores from the South China Sea (SCS) were picked out and analyzed. Al° particles were found in only five surface samples and three samples in two sedimentary cores. The particles are grayish or silver white in color with a strongly metallic luster and high plasticity, and they occur as irregular plats, elongated shapes and spherules with botryoidal structures. The identified Al° particles belong to cubical system, Fm3m space group with a varying from 4.028×10?1 nm to 4.059×10?1 nm. Their chemical compositions consist mainly of Al (95.07%–99.84%) and minor amounts of Si, Fe, Ti, S, Zn, Mg, Ca and trace amounts of K, Na, Cu, Co, P and Mn. Our results show that the identified Al° is authigenic, formed in situ in the prospective areas of gas hydrates, and that it is an unreported type of Al° occurrence. A possi- ble mechanism of Al° formation is proposed: under the strongly reducing micro environments at cold seeps, dissolved Al3+ in sediments is reduced to its metallic state by the strongly reducing agent of H2 through microbial processes and enzyme action. As a result, the Al° particles from the prospective gas hydrate areas in the SCS are the special products, representing the first report on Al° from the sediments at cold seeps. The proposed mechanism provides a theoretical basis for further studies on the special biogeochemical processes at cold seeps.