Contrasting lipid biomarkers in mountain rivers in the Nepal Himalayas:Organic matter characteristics and contribution to the fluvial carbon pool

The Nepal Himalayas is the source of many glacial and spring-fed river systems crisscrossing the moun-tainous terrain.There is an increasing recognition of small mountain rivers(SMRs)to have a significant combined export of dissolved and particulate organic carbon to the global carbon flux.We analyzed flu-vial sediments from two SMRs and compared the results with two large mountain rivers(LMRs)in Nepal.We investigated the organic matter(OM),its compositional variability,and seasonal export using a suite of lipid biomarkers,namely n-alkanes,n-alkanoic acids,n-alkanols,and sterols.The SMRs indicated a similarity in lipid distribution and were affected by a strong seasonal variability.The LMRs showed a dis-tinct contrast in the distribution of lipids in suspended sediments.Bedload sediments in SMRs were derived from diverse sources with weak terrigenous dominance all-year-round compared to the sus-pended load.Functional lipids(n-alkanoic acids and n-alkanols)were the major constituents in SMR sed-iments,indicating better preservation.In contrast,n-alkane concentration dominated over other fractions in suspended sediments retrieved from LMRs.The biomarker trends differentiate SMRs from LMRs with lower transformed/degraded OM in SMRs.A common observation was the strong presence of even carbon compounds in short-chain n-alkanes in SMR bedload sediments and their predominance in suspended sediments in LMRs.Such an unusual trend is attributed to specific biomarker sources from the catchment and ongoing processes in fluvial systems.Topsoil colonized by fungal species under moist acidic condi-tions and autochthonous bacteria contributes to the organic matter pool in shallow SMRs.In LMRs,the contribution from thermally mature sedimentary hydrocarbons and the diagenetic reduction of n-alkanoic acids to n-alkanes are additional contributors to the allochthonous carbon pool.The differences in lipid concentrations,their distribution,seasonality,and the size of rivers suggest differential preserva-tion/degradation of the organic matter pool and their importance in contributing to the carbon budget.     

鸭绿江河口西岸潮间带柱状沉积物中重金属的分布特征及其对流域变化的响应

2010年和2011年在鸭绿江西水道和西岸潮间带共采集4根柱状样,通过对2mPb测年、粒度、总有机碳、重金属元素等多指标综合分析,探讨了：（1）鸭绿江河口西水道和西岸潮间带柱状沉积物中重金属的垂向分布及其来源：（2）重金属分布的粒度控制作用：（3）不同时期的粒度和重金属分布变化及其对流域变化的响应。结果表明：f1）Cu和zn可能来源于有机质降解的内源释放：Cr和Ni表征了岩石风化剥蚀形成陆源碎屑的自然来源：Cd和Pb反映了人类活动的影响。（2）除西岸潮间带的Cd和Pb含量可能部分受来源影响外,研究区的粒度效应是控制鸭绿江地区重金属含量分布的最主要因素。（3）粒度变化与流域演变密切相关,重金属含量对流域变化和人类活动响应明显,大致以1940年、1970年、1995年为界分为四个沉积阶段：1940年以前,自然演变对鸭绿江河口西岸潮间带的重金属分布控制明显,而1940年来至今,人类活动的控制作用日益凸显。     

U-series disequilibria in suspended river sediments and implication for sediment transfer time in alluvial plains: The case of the Himalayan rivers

²³⁸U-²³⁴U-²³⁰Th radioactive disequilibria were analyzed in suspended sediments (collected at different depths) from the Ganges River and one of its main tributaries: the Narayani-Gandak River. Results associated with bedload sediment data suggest that uranium-series (U-series) disequilibria in river sediments of the Ganges basin vary with grain size and sampling location. The range of observed U-series disequilibria is explained by a mixing model between a coarse-grained sediment end-member, represented by bedload and bank sediments, and a fine-grained end-member that both originate from Himalaya but undergo different transfer histories within the plain. The coarse-grained sediment end-member transits slowly (i.e. >several 100’s ky) in the plain whereas the fine-grained sediment end-member is transferred much faster (<20-25 ky), as indicated by the absence of significant variations in Th isotope composition of the fine-grained sediment end-members. These results show that U-series isotopes can be used to quantify the various transfer times of river sediments of different sizes and infer that there can be an order of magnitude of difference, or more, between the transfer time of suspended and bedload sediments. This underlines that a good knowledge of the proportion of suspended vs. bedload sediments transported in the river is required to accurately assess how fast erosion products are transferred in a catchment and how fast a catchment is likely to respond to external forcing factors.     

A multi‐proxy study of Holocene lake development,lake settlement and vegetation history in central Ireland

Stratigraphical investigations, geomorphological mapping, and diatom, plant macrofossil and pollen analyses were undertaken in and around two lakes in central Ireland to establish correlations between changes in lake conditions and catchment vegetation throughout the Holocene. Similar investigations of an adjacent mire reveal early Holocene changes in lake level and area. The palaeoecological data show high correlations related to variations in lake depth and area, catchment vegetation type, organic inputs and trophic status. Catchment‐scale deforestation is gradual and occurs through the Bronze and the Iron Ages, and the construction of a crannog in the early Medieval period (seventh century AD ) appears to be associated with a widespread increase in deforestation and mixed agriculture in the catchment. Both pollen and plant macrofossils suggest that one of the crannogs was used for crop storage in addition to domestic and any other activities. In the early to middle Holocene similarities in the proxy‐data appear to be climatically driven through changing lake levels and areal extent whereas the later Holocene record is clearly dominated by anthropogenic changes within the catchment and the construction of crannogs in the lakes. The advantages of combining multi‐proxy indicators of lake hydroecology with the vegetation record are illustrated. Copyright © 2005 John Wiley & Sons, Ltd.     

A trace-element geochemical model for imperfect fractional crystallization associated with the development of crystal zoning

Although many petrological studies of volcanic rocks have suggested that crystallization proceeds within magma bodies, highly compatible trace elements do not display the marked variations and extreme depletions predicted to result from perfect fractional crystallization. Imperfect crystal-liquid separation is a key process in explaining this paradox. The presence of suspended crystals greatly affects variations in highly compatible elements, and has been quantitatively modeled by assuming perfect equilibrium between the suspended crystals and the liquid (equilibrium crystallization and imperfect separation; ECIS); however, volcanic rocks generally contain zoned phenocrysts that reflect the absence of solid-state equilibration. The present study develops a mass-balance model for zoned crystallization and imperfect separation (ZCIS). The ZCIS process is more efficient than the conventional ECIS process in generating depleted compatible elements. These two end-member models are able to explain the compositional range of igneous rocks that experienced imperfect fractional crystallization under natural conditions. The predicted compositional regions in bivariate trace-element diagrams successfully account for the sizes and shapes of the regions defined by whole-rock and melt-inclusion data from the Bishop Tuff, CA, USA.     

Carbonate and silicate weathering in two presently glaciated, crystalline catchments in the Swiss Alps

We present a weathering mass balance of the presently glaciated Rhône and Oberaar catchments, located within the crystalline Aar massif (central Switzerland). Annual chemical and physical weathering fluxes are calculated from the monthly weighted means of meltwater samples taken from July, 1999 to May, 2001 and are corrected for precipitation inputs. The meltwater composition issuing from the Oberaar and Rhône catchments is dominated by calcium, which represents 81% and 55% of the total cation flux respectively (i.e. 555 and 82-96 keq km⁻² yr⁻¹). The six to seven times higher Ca²⁺ denudation flux from the Oberaar catchment is attributed to the presence of a strongly foliated gneissic zone. The gneissic zone has an elevated calcite content (as reflected by the 4.6 times higher calcite content of the suspended sediments from Oberaar compared to Rhône) and a higher mechanical erosion rate (resulting in a higher flux of suspended sediment). The mean flux of suspended calcite of the Oberaar meltwaters during the ablation period is 7 times greater than that of the Rhône meltwaters. Taking the suspended calcite as a proxy for the total (including sub-glacial sediments) weathering calcite surface area, it appears that the available surface area is an important factor in controlling weathering rates. However, we also observe an increased supply of protons for carbonate dissolution in the Oberaar catchment, where the sulphate denudation flux is six times greater. Carbonic acid is the second important source of protons, and we calculate that three times as much atmospheric CO₂ is drawn down (short term) in the Oberaar catchment. Silica fluxes from the two catchments are comparable with each other, but are 100 kmol km² yr⁻¹ lower than fluxes from physically comparable, non-glaciated basins.     

Compositional and thermal convection in magma chambers

Magma chambers cool and crystallize at a rate determined by the heat flux from the chamber. The heat is lost predominantly through the roof, whereas crystallization takes place mainly at the floor. Both processes provide destabilizing buoyancy fluxes which drive highly unsteady, chaotic convection in the magma. Even at the lowest cooling rates the thermal Rayleigh number Ra is found to be extremely large for both mafic and granitic magmas. Since the compositional and thermal buoyancy fluxes are directly related it can be shown that the compositional Rayleigh number Rs (and therefore a total Rayleigh number) is very much greater than Ra. In the case of basaltic melt crystallizing olivine Rs is up to 10⁶ times greater than Ra. However compositional and thermal buoyancy fluxes are roughly equal. Therefore thermal and compositional density gradients contribute equally to convection velocities in the interior of the magma. Effects of thermal buoyancy generated by latent heat release at the floor are included.The latent heat boundary layer at the floor of a basaltic chamber is shown to be of the order of 1 m thick with very low thermal gradients whereas the compositional boundary layer is about 1 cm thick with large compositional gradients. As a consequence, the variation in the degree of supercooling in front of the crystal-liquid interface is dominated by compositional effects. The habit and composition of the growing crystals is also controlled by the nature of the compositional boundary layer. Elongate crystals are predicted to form when the thickness of the compositional boundary layer is small compared with the crystal size (as in laboratory experiments with aqueous solutions). In contrast, equant crystals form when the boundary layer is thicker than the crystals (as in most magma chambers). Instability of the boundary layer in the latter case gives rise to zoning within crystals. Diffusion of compatible trace elements through the boundary layer can also explain an inverse correlation, observed in layered intrusions, between Ni concentration in olivine and the proportion of Ni-bearing phases in the crystallizing assemblage.     

黑河上游高寒山区降雨-径流形成过程的同位素示踪

为揭示中、低纬度高寒山区降雨-径流的形成过程,指导水资源的合理开发利用,选择黑河上游红泥沟小流域为研究区,基于河道径流量及雨水和河水稳定同位素的观测数据,构建二元混合模型,计算了2013年7月23日及8月21日两次典型降雨-径流事件中事件水（降雨）和事件前水（流域前期储水）对河道径流的贡献及其动态变化.结果显示:两次降雨事件中事件前水的贡献比例分别为68.69%和54.46%;事件前水的贡献比例在涨水阶段减小,在退水阶段增大.结合河水电导率的观测结果,进一步分析了降雨-径流的形成过程:河道径流的形成主要受饱和区蓄满产流、河岸带地下径流和山坡地下径流3种产流机制控制;事件水主要源于蓄满产流,事件前水主要源于河岸带和山坡地下径流;事件初期和末期以河岸带地下水补给为主,涨水阶段后期和退水阶段前期转为以蓄满产流和山坡地下水补给为主,洪峰期间蓄满产流的贡献达到最大.两次事件的对比表明,事件前的湿度条件和降雨强度对降雨-径流的形成过程有着重要影响:前期越湿润,流域储水能力越弱,导水能力越强,事件水的贡献越大,河道径流对降雨的响应越迅速;降雨强度越大,蓄满产流及其中的事件水比例越高,河道径流中事件水的比例也越高.      

悬浮泥沙对藻类生长影响的实测与分析

通过对长江下游镇扬段的牛轭湖—内江的泥沙、水质、光强、藻类等进行现场监测和数据分析,探讨了内江悬浮泥沙对水下光强的衰减规律以及对藻类生长的影响。结果表明:内江水体中存在80%以上的粒径小于75μm的微细悬浮颗粒,其中不易沉降的粘土颗粒占到20%～50%左右,枯水期平均含量为40 mg/L,丰水期平均含量为70 mg/L,而藻细胞生物总量基本都在100万个/L以内,其中绿藻、硅藻为优势种。数据分析表明:泥沙对水下光强的衰减遵循指数函数衰减规律,光衰减系数与泥沙含量呈较好的线性相关性,内江的优势种绿藻、硅藻的生物量与泥沙含量呈显著的指数相关。认为,泥沙通过衰减光强间接地抑制藻类繁殖,这一抑制作用可以由泥沙含量和藻细胞密度之间的关系进行定量。     

The partitioning of the total sediment load of a river into suspended load and bedload: a review of empirical data

The partitioning of the total sediment load of a river into suspended load and bedload is an important problem in fluvial geomorphology, sedimentation engineering and sedimentology. Bedload transport rates are notoriously hard to measure and, at many sites, only suspended load data are available. Often the bedload fraction is estimated with ‘rule of thumb’ methods such as Maddock’s Table, which are inadequately field‐tested. Here, the partitioning of sediment load for the Pitzbach is discussed, an Austrian mountain stream for which high temporal resolution data on both bedload and suspended load are available. The available data show large scatter on all scales. The fraction of the total load transported in suspension may vary between zero and one at the Pitzbach, while its average decreases with rising discharge (i.e. bedload transport is more important during floods). Existing data on short‐term and long‐term partitioning is reviewed and an empirical equation to estimate bedload transport rates from measured suspended load transport rates is suggested. The partitioning averaged over a flood can vary strongly from event to event. Similar variations may occur in the year‐to‐year averages. Using published simultaneous short‐term field measurements of bedload and suspended load transport rates, Maddock’s Table is reviewed and updated. Long‐term average partitioning could be a function of the catchment geology, the fraction of the catchment covered by glaciers and the extent of forest, but the available data are insufficient to draw final conclusions. At a given drainage area, scatter is large, but the data show a minimal fraction of sediment transported in suspended load, which increases with increasing drainage area and with decreasing rock strength for gravel‐bed rivers, whereby in large catchments the bedload fraction is insignificant at ca 1%. For sand‐bed rivers, the bedload fraction may be substantial (30% to 50%) even for large catchments. However, available data are scarce and of varying quality. Long‐term partitioning varies widely among catchments and the available data are currently not sufficient to discriminate control parameters effectively.     

Investigating hydrological drivers of nitrate export dynamics in two agricultural catchments in Germany using high-frequency data series

In this study near-continuous time series of nitrate, electrical conductivity, and discharge were used to identify the dominating hydrological mechanisms that control nitrate export dynamics in two agricultural catchments. The main goal was to assess relationships between contrasting event based as well as long-term nitrate transport behaviour and catchment hydrology. Data records were obtained from online probes that allow field based high-frequency analyses over long time periods. The catchments of the Ammer River (southwestern Germany) and the Weida River (eastern Germany) are similar with respect to size (~100 km²), morphology, and climate and are dominated by agricultural use. Main differences are the stronger urbanization and the occurrence of karstic rocks in the Ammer catchment. Nitrate concentrations are high in water of both streams and range mostly between 20 and 50 mg l^?1. Nitrate export in the Ammer catchment is dominated by baseflow and a minor second, diluting runoff component generated in urbanized areas. In contrast, nitrate dynamics of the Weida catchment is governed by the interplay of at least three runoff components, while the largest amount of nitrate is mobilized intermittently by a delayed fast component generated in the catchment’s soils during wet conditions. These interpretations, derived with one online probe at the outlet of each catchment, are well in line with the former modeling results. This study shows that high-resolution data obtained by online techniques offers a large potential to improve the conceptualization of dominating flow and transport processes at catchment scales at relatively low costs and effort.     

含沙量和悬沙粒径变化对长江宜昌-汉口河段年冲淤量的影响

依据三峡水库修建以前的资料,运用数理统计方法对含沙量和悬沙粒径变化对长江宜昌-汉口河段年冲淤量的影响进行研究,以期为三峡水库修建以后库下游河道冲淤特性的预测提供参考。建立了1980-1997年间宜昌-汉口#河段年冲淤量与宜昌站年均含沙量C宜昌之间的回归方程,据此估算出使宜昌-汉口#河段处于不冲不淤状态的宜昌站临界年均含沙量为0.734 kg/m3。以宜昌-汉口冲淤量作为因变量,以宜昌站的含沙量、悬沙中径D50、最大流量和三口分流比作为影响变量,建立了多元回归方程。基于1980-1997年资料的方程表明,宜昌站含沙量越高,悬沙中径越粗,宜昌站洪水流量越大,宜昌-汉口河段年淤积量越大;三口分流比越小,宜昌-汉口河段年淤积量越大。     

Lead contamination of fluvial sediments in an eroding blanket peat catchment

Over the last few years there has been growing concern over the mobilisation of anthropogenically derived, atmospherically deposited Pb from upland blanket peat soils to receiving surface waters. The near-surface layer of blanket peat soils of the Peak District, southern Pennines, UK, is severely contaminated with high concentrations of Pb. Erosion of peat soils in this upland area may be releasing large quantities of previously deposited Pb into the fluvial system. Samples of fluvial sediments (suspended, floodplain, streamside fan, trash-line and channel bed) were collected from a severely eroding blanket peat catchment in the Peak District in order to investigate Pb contamination of fluvial sediments, to determine the mechanism for fluvial Pb transport and to determine if erosion of contaminated peat soils in the catchment is releasing Pb into the fluvial system. Concentrations of Pb associated with fluvial sediments are considerably higher than those in the catchment geology, but not as high as those in peat soils in the catchment. Intra- and inter-storm variability in the Pb content of suspended sediments can be explained by differences in organic matter content of these sediments and differences in erosion processes operating within the catchment. High Pb concentrations are associated with suspended sediments that have a high organic matter content. The results of this study suggest that organic matter is the principle vector for sediment-associated Pb in the fluvial system. Erosion of contaminated peat soils in the Peak District is releasing Pb into the fluvial system. The extent to which this is a problem in other peatland environments is an area requiring further research.     

The Correlation of Inorganic C,O Isotopic Values for Lake Chenghai Sediments and Its Environmental Implications

As one of the lakes on the Yunnan-Guizhou plateau, Lake Chenghai, which is a typical closed lake with the precipitation accounting for one-third or more of the annual water input, has a high total salinity (almost like a saline lake). The inorganic C, O isotopic composition of lake sediments bears much sensitive information about environmental change in the catchment, while their correlations revealed the hydrological conditions under which the lake was closed. Their compositional variations are controlled by temperature, precipitation, photosynthesis, dissolving equilibrium of the carbonate system and hydrological condition. According to our research on inorganic C, O isotopic composition of Lake Chenghai sediments, we investigated the environmental change of this catchment several decades ago. The results showed that Lake Chenghai has kept good hydrological closing conditions in the past several decades, as indicated by the good correlation of inorganic C, O isotopic composition of sediments; and that the environmental change in this catchment shows a tendency of periodical evolution on a 10⁻¹¹-years scale, although the signal noise is relatively high at the bottom of the sediment core. And we also can extend C, O isotopes, a sensitive environmental indicator, to nearly saline lake environments with a high degree of mineralization.     

辽宁赛马碱性岩体层硅铈钛矿化学成分变化及其对碱性岩浆演化的指示意义

辽宁赛马岩体是我国典型的产铀碱性杂岩体,但其稀土矿化机制尚不明确.通过光学显微镜、扫描电镜和电子探针分析,得知该岩体从霞石正长岩经霓霞正长伟晶岩至晚期异霞正长岩,代表性稀土矿物层硅铈钛矿[Na₂Ca₄REETi（Si₂O₇）₂OF_3]不断富集,Nb、Zr和REE（特别是HREE）等高场强元素含量不断升高,部分颗粒具Zr、REE等元素成分环带,以上成分变化与稀土等不相容元素性质、碱性岩浆成分和岩浆结晶分异过程密切相关.此外,层硅铈钛矿经历了一系列的热液蚀变,蚀变部分Ti、Ca、Sr、Na含量增加而Zr、REE含量降低,最终形成由残余层硅铈钛矿+方解石+萤石+铈硅磷灰石组成的假晶,可能与富碱质、F和CO₂的自交代流体作用有关.该研究揭示了碱性岩浆演化过程中,层硅铈钛矿成分变化及热液蚀变组合对指示岩浆结晶分异程度、探究稀土元素分馏及其热液活动性的具有重要意义.      

过去约8000年来程海摇蚊组合记录的气候与环境变化

季风气候是影响地区古环境变化的主要因素。文章通过程海长556cm岩芯的沉积摇蚊亚化石、粒度、TOC、TN和TOC/TN记录,探讨了季风气候影响下程海地区约7660cal.a B.P.以来的降水变化情况。结果表明,沉积物中摇蚊属种以Procladius Skuse、Chironomus plumosus、Tanytarsus mendax和Microchironomus tener为主。约7660~2800cal.a B.P.时期流域受印度季风影响强烈,气候暖湿、降水量大,湖泊生产力水平较高;约2800cal.a B.P.以来程海流域降水量逐渐减少,湖泊生产力水平降低。Procladius Skuse与T.mendax的组合关系反映了程海在约6850~6500cal.a B.P.、约5500~5000cal.a B.P.、约4150~3700cal.a B.P.、约2800~2300cal.a B.P.和约1400~1100cal.a B.P.这5个时期由于印度季风减弱,发生了短暂性的干旱事件,研究表明季风因素驱动下的程海摇蚊亚化石组合主要受流域降水侵蚀因素的影响。

     

波流共同作用下潮滩剖面沉积物和地貌分异规律——以长江口崇明东滩为例

通过对崇明东滩两个海滩剖面、表层沉积物和悬沙粒度以及同步水沙资料的分析,探讨波流共同作用下表层沉积物和地貌的分异规律。受波流共同作用的影响,表层沉积物中值粒径由破波带向两侧逐渐变细,分选由破波带向两侧逐渐变差,偏度由极正偏变为正偏,峭度由很窄尖变为宽平和中等峭度。由破波带向岸方向,流速逐渐减小,含沙量逐渐增加。悬沙和表层沉积物粒度特征的对比分析表明,潮间带上部的悬沙主要来源于破波带泥沙的再悬浮。破波带内泥沙以“波浪掀沙”引起的分选运移为主,而破波带两侧的泥沙以潮流对破波带水体的“平流输移”为主。以潮汐水位和高精度海滩剖面数据对崇明东滩微地貌类型按高程进行了新的划分。     

Impact of Ditching in a Small Forested Catchment on Concentrations of Suspended Material, Organic Carbon, Hydrogen Ions and Metals in Stream Water

Stream-water samples were collected during a 4-year-period in twosmall streams, one whose catchment was ditched for forestry halfway through the sampling period and another nearby reference stream whose catchment was not ditched during this period. The main aim was to study the impact of forest ditching on stream-water quality. Whereas the artificial drainage did not change the hydrograph pattern, it had a large effect on stream hydrochemistry: the concentrations of suspended material, Mn, Ca, Mg and Al increased, theconcentrations of total organic carbon decreased, and pH increased by approximately one unit, from an average of 4.4 to 5.4. The increase in suspended material, Mn and Al concentrations is explainedby the physical mobilisation of mineral particulates/colloids from mineral soils (till) exposed on the ditch slopes beneath the peat layer, while the increase in Ca and Mg loads is explained by the release of Ca²⁺ and Mg²⁺ in exchange reactions in this same soil layer. The increase in pH and decrease in TOC concentrations after ditching are related to changes in hydrological flow paths in the catchments, and most likely to immobilisation of both hydrogen ions and humic substances in the near neutral till horizon exposed beneath the peat layer. Only the aquatic abundance of Fewas not significantly affected by the ditching.     

Variability in bedform morphology and kinematics with transport stage

Bedform geometry is widely recognized to be a function of transport stage. Bedform aspect ratio (height/length) increases with transport stage, reaches a maximum, then decreases as bedforms washout to a plane bed. Bedform migration rates are also linked to bedform geometry, in so far as smaller bedforms in coarser sediment tend to migrate faster than larger bedforms in finer sediment. However, how bedform morphology (height, length and shape) and kinematics (translation and deformation) change with transport stage and suspension have not been examined. A series of experiments is presented where initial flow depth and grain size were held constant and the transport stage was varied to produce bedload dominated, mixed‐load dominated and suspended‐load dominated conditions. The results show that the commonly observed pattern in bedform aspect ratio occurs because bedform height increases then decreases with transport stage, against a continuously increasing bedform length. Bedform size variability increased with transport stage, leading to less uniform bedform fields at higher transport stage. Total translation‐related and deformation‐related sediment fluxes all increased with transport stage. However, the relative contribution to the total flux changed. At the bedload dominated stage, translation‐related and deformation‐related flux contributed equally to the total flux. As the transport stage increased, the fraction of the total load contributed by translation increased and the fraction contributed by deformation declined because the bedforms got bigger and moved faster. At the suspended‐load dominated transport stage, the deformation flux increased and the translation flux decreased as a fraction of the total load, approaching one and zero, respectively, as bedforms washed out to a plane bed.     

Steroid geochemistry in the oxygen minimum zone of the eastern tropical North Pacific Ocean

Particulate matter was collected in the eastern tropical North Pacific Ocean, an area characterized by a shallow and intense oxygen minimum zone, in order to investigate the geochemistry of particulate organic matter in the ocean. Sinking, large particles and suspended, small particles were analyzed for sterols, 3-ketosteroids, 3-methoxy-steroids and sterenes. Vertical fluxes of steroid classes in sinking particles and concentrations of steroids in suspended particles decreased rapidly below the euphotic zone consistent with upper ocean sources and deep water decomposition. Significant compositional changes were observed with increasing depth and as a function of particle size. Sinking particles were enriched in C₂₇-Sterols but deficient in C₂₈ and C₂₉-sterols compared to the suspended particles. Suspended particles, especially in the oxygen minimum zone, were enriched in steroidal ketones and sterenes compared to sinking particles. Steroid distributions suggest that the oxygen minimum zone is the site of active “diagenetic” transformations, most likely microbially-mediated, of stenols to steroidal ketones, stanols and sterenes. These transformations occur preferentially in suspended particles relative to sinking particles.