Determination of experimentally enriched15N in nitrate nitrogen based on an improved method of azo dye formation

A method has been developed for determination of¹⁵N isotope ratio in nitrate nitrogen, which is a major analytical step in tracer experiments for studies of nitrate metabolism in the marine environment. The method is based on diazotization of nitrite with sulfanilic acid following reduction of nitrate to nitrite by a cadmium-copper column. The diazonium compound is then subject to the azo coupling reaction with 2-naphthol, and the azo dye formed is extracted by a solid phase extraction column. The dye eluted from the column is collected, and total nitrogen and¹⁵N content of the dye are determined by mass spectrometry. Sulfanilic acid can also remove preexisting nitrite by heating the sample under acidic conditions before passing through the cadmium-copper reduction column. The average recovery of nitrate nitrogen was 86%. A procedure for reducing the background nitrogen that derives from the analytical operations has been developed; background nitrogen was limited to about 0.25 μg-atomN. The variation in the background nitrogen levels reflects the range of error in¹⁵N determination of nitrate nitrogen by this method. Application of the present method to a¹⁵NO₃ ⁻ isotope dilution experiment for determination of nitrification rate in sea water is demonstrated.     

海洋沉积物不同相态中Sr、Nd同位素提取方法研究

海洋沉积物中Fe-Mn氧化物相和残渣态的Sr、Nd同位素组成能够敏感地指示洋流循环及物质来源,但实验室对沉积物中上述相态的Sr和Nd同位素的提取易产生过量或提取不完全,进而影响同位素测定结果的准确性,因此制定有效的提取流程显得非常重要。本文采用不同浓度盐酸羟胺(Hydroxylamine Hydrochloride,HH)与醋酸(Acetic Acid,HAc)混合溶液对中印度洋海盆深海沸石黏土、北极半深海沉积物以及安达曼海近海沉积物的Fe-Mn氧化物相进行提取,残渣态用HNO3-HF高压密闭消解法溶融,测定了各相态的主微量元素含量及Sr、Nd同位素组成,通过分析不同实验条件下得到的Fe-Mn氧化物相与残渣态的稀土元素(REE)配分模式、Al/Nd含量比值及Sr、Nd同位素组成,建立了3种不同成因类型海洋沉积物不同相态的化学提取方法。提取深海沸石黏土中Fe-Mn氧化物相的理想试剂条件为0.25 mol/L HH和15%HAc,北极半深海沉积物和安达曼海近海沉积物的试剂条件为0.5 mol/L HH和15%HAc。该方法可以准确获得沉积物中Fe-Mn氧化物相与残渣态的Sr、Nd同位素组成信息,为古海洋学的研究提供方法支持。     

生物固氮速率测定中15N2同位素示踪剂的引入

生物固氮作用是一个重要的海洋新氮来源,在海洋生物地球化学循环中扮演着不可替代的角色。基于稳定同位素（15N₂）示踪吸收法,是目前直接测定海洋生物固氮速率最有效的手段。其中,高效、洁净地将15N₂引入海水培养体系,并准确定量培养体系底物的同位素示踪水平,是同位素示踪吸收法准确获取固氮速率的关键。本研究针对15N₂同位素示踪剂引入这一关键环节进行了探讨,确认改进气泡法是将15N₂引入海水培养体系的首选操作。在对培养体系造成的较小扰动的情况下,可将培养体系氮气底物的15N原子丰度提升至10%以上,相对于另一种导入同位素示踪剂的手段——预溶解海水法,改进气泡法将培养瓶中氮气底物的15N原子丰度提升了近200%。此外,改进气泡法还具有最小化痕量金属沾污、操作简便等优点。将改进气泡法结合与稳定同位素比值质谱测定结合,是准确测定水体生物固氮速率的推荐方法。     

Precipitation of authigenic uranium in suboxic continental margin sediments from the Okinawa Trough

Concentrations of U and Th isotopes in Okinawa Trough and East China Sea sediment cores were determined by isotope dilution inductively coupled plasma-mass spectrometry (ID-ICP-MS) to investigate the behavior of redox sensitive uranium in suboxic hemipelagic sediments and determine their significance in oceanic uranium balance. ²³⁸U concentrations and ²³⁸U/²³²Th activity ratios in the East China Sea sediments showed no remarkable variation with depth. However, ²³⁸U and ²³⁸U/²³²Th ratios in the Okinawa Trough sediments were low in the surface oxidizing layer but increased where the suboxic condition was encountered. The distribution profiles of ²³⁰Th and ²³²Th concentrations were relatively constant with depth in both the Okinawa Trough and East China Sea sediment cores. These results suggested that there has been post-depositional precipitation of authigenic uranium within the suboxic Okinawa Trough sediment column. The post-depositional precipitation rates of authigenic uranium were estimated to be 47 ± 5 to >62 ± 8 ng cm⁻² yr⁻¹; these rates were comparable to those previously reported for several anoxic sediments. A mechanism controlling precipitation of uranium may be the downward diffusion of uranium U(VI), reduction to U(IV) and finally precipitation onto the solid phase. The accumulation rate of uranium for the Okinawa Trough sediments was approximately eight times higher than the world average rate reported for suboxic sediments. This removal of uranium in the oceanic budget increases the importance of the suboxic sediment sink.     

Stable isotope composition of modern bryozoan skeletal carbonate from the Otago Shelf,New Zealand

The oxygen (δ¹⁸O) and carbon (δ¹³C) isotope ratios of 10 species of living Bryozoa collected from the Otago Shelf, New Zealand were analysed to assess the extent to which isotopic equilibrium (relative to inorganic equilibrium isotope fractionation) is attained during the precipitation of skeletal calcium carbonate. The data reveal that whereas eight species of Bryozoa synthesise skeletal carbonate in apparent oxygen isotope equilibrium with respect to environmental conditions, two species (Celleporina grandis and Hippomonavella flexuosa) yield δ¹⁸O_calcite values which indicate significant disequilibrium oxygen isotope fractionation during calcification. Sufficient data are available from one species (C. grandis) to demonstrate that disequilibrium is probably related to kinetic factors associated with diffusion‐controlled transport of HCO₃‐ to the site of calcite precipitation. Carbon isotope signatures indicate significant departures from inorganic isotope equilibrium in all but one bryozoan species (Hippomenella vellicata). Although greater uncertainties are associated with estimates of the isotopic composition of total dissolved inorganic carbon (δ¹³C_SDIC), the data suggest that two factors—kinetic fractionation and incorporation of respiratory CO₂—are important in controlling carbon isotope disequilibrium. Where bryozoan species exhibit evidence for disequilibrium in both oxygen and carbon isotope systems (C. grandis, H. flexuosa), it is likely that kinetic factors are primarily responsible for observed departures from carbon isotope equilibrium. In contrast, the probable explanation for those species which display evidence for carbon isotope disequilibrium only, is that skeletal carbonate is precipitated from a DIC pool modified by the incorporation of respiratory CO₂. Differences between the carbon isotope composition of skeletal elements from the same species and co‐existing species living in the same community suggests that significant variations may occur in the extent to which marine DIC and respiratory CO₂ are utilised during calcification. Additional studies of carbon pathways associated with calcification are required to assess the relative effects of kinetic, metabolic, and environmental factors on the carbon isotopic composition of bryozoan skeletal carbonate.     

Characterizing lipid biomarkers in methanotrophic communities of gas hydrate-bearing sediments in the Sea of Okhotsk

We studied specific lipid biomarkers of archaea and bacteria, that are associated with the anaerobic oxidation of methane (AOM) in a cold seep environment as well as the origin of sedimentary organic matter on the continental slope off NE Sakhalin in the Sea of Okhotsk. The organic geochemical parameters demonstrated that most of the sedimentary organic matter containing hydrate layers could be derived from marine phytoplankton and bacteria, except for a station (LV39-29H) which was remarkably affected by terrestrial vascular plant. Specific methanotrophic archaea biomarkers was vertically detected in hydrate-bearing cores (LV39-40H), coinciding with the negative excursion of the δ¹³C_org at core depths of 90–100 cm below the seafloor. These results suggest that methane provided from gas hydrates are already available substrates for microbes thriving in this sediment depth. In addition, the stable isotope mass balance method revealed that approximately 2.77–3.41% of the total organic carbon (or 0.036–0.044% dry weight sediment) was generated by the activity of the AOM consortium in the corresponding depth of core LV39-40H. On the other hand, the heavier δ¹³C values of archaeol in the gas hydrate stability zone may allow ongoing methanogenesis in deeper sediment depth.     

Characterizing lipid biomarkers in methanotrophic communities of gas hydrate-bearing sediments in the Sea of Okhotsk

We studied specific lipid biomarkers of archaea and bacteria, that are associated with the anaerobic oxidation of methane (AOM) in a cold seep environment as well as the origin of sedimentary organic matter on the continental slope off NE Sakhalin in the Sea of Okhotsk. The organic geochemical parameters demonstrated that most of the sedimentary organic matter containing hydrate layers could be derived from marine phytoplankton and bacteria, except for a station (LV39-29H) which was remarkably affected by terrestrial vascular plant. Specific methanotrophic archaea biomarkers was vertically detected in hydrate-bearing cores (LV39-40H), coinciding with the negative excursion of the δ¹³C_org at core depths of 90–100 cm below the seafloor. These results suggest that methane provided from gas hydrates are already available substrates for microbes thriving in this sediment depth. In addition, the stable isotope mass balance method revealed that approximately 2.77–3.41% of the total organic carbon (or 0.036–0.044% dry weight sediment) was generated by the activity of the AOM consortium in the corresponding depth of core LV39-40H. On the other hand, the heavier δ¹³C values of archaeol in the gas hydrate stability zone may allow ongoing methanogenesis in deeper sediment depth.     

Origin and fate of particulate organic matter in the southern Beaufort Sea – Amundsen Gulf region,Canadian Arctic

To establish the relative importance of terrigenous and marine organic matter in the southern Beaufort Sea, we measured the concentrations and the stable isotopic compositions of organic carbon and total nitrogen in sediments and in settling particles intercepted by sediment traps. The organic carbon content of surface sediment in the Chukchi and southern Beaufort Seas ranged from 0.6 to 1.6% dry wt., without a clear geographical pattern. The C_ORG:N_TOT ratio ranged from 7.0 to 10.4 and did not vary significantly downcore at any one station. Values of δ¹³C_ORG and δ¹⁵N_TOT in the sediment samples were strongly correlated, with the highest values, indicative of a more marine contribution, in the Amundsen Gulf. In contrast, the organic matter content, elemental (C_ORG:N_TOT ratio) and isotopic (δ¹³C_ORG and δ¹⁵N_TOT) composition of the settling particles was different from and much more variable than in the bottom sediments. The isotopic signature of organic matter in the Beaufort Sea is well constrained by three distinct end-members: a labile marine component produced in situ by planktonic organisms, a refractory marine component, the end product of respiration and diagenesis, and a refractory terrigenous component. A three-component mixing model explains the scatter observed in the stable isotope signatures of the sediment trap samples and accommodates an apparent two-component mixing model of the organic matter in sediments. The suspended matter in the water column contains organic matter varying from essentially labile and marine to mostly refractory and terrigenous. As it settles through the water column, the labile marine organic matter is degraded, and its original stable isotope signature changes towards the signature of the marine refractory component. This process continues in the bottom sediment with the result that the sedimentary organic matter becomes dominated by the refractory terrigenous and marine components.     

Distribution and inventories of 90Sr, 137Cs, 241Am and Pu isotopes in sediments of the Northwest Pacific Ocean

Bottom sediments collected in the Northwest (NW) Pacific Ocean in 1997 were analysed for ⁹⁰Sr, ¹³⁷Cs, ^239,240Pu and ²⁴¹Am contents to determine their distribution patterns, inventories and sources. Enhanced inventories of ^239,240Pu and ²⁴¹Am were observed in the latitudinal belts of 10–20°N and 30–40°N, which correspond to major areas of local (tropospheric) and global (stratospheric) fallout (with a contribution from local fallout), respectively. The sediment inventory of ^239,240Pu near the Bikini Atoll exceeded its overlying water inventory, however, in the mid-latitudes, more than 70% of ^239,240Pu still remains in the water column. ²⁴¹Am inventories in sediments exceeded that of the water column for the entire NW Pacific Ocean. Higher ¹³⁷Cs and ⁹⁰Sr sediment inventories in the latitudinal belt of 30–40°N are due to global fallout, and they account for about 10% and less than 5% of the water column inventories, respectively. The observed activity ratios of ¹³⁷Cs/⁹⁰Sr, ²³⁸Pu/^239,240Pu and ²⁴¹Am/^239,240Pu in sediment were at some stations higher than the global fallout ratios due to contributions from local fallout and due to specific processes in the water column. Two end-member mixing model based on the ²⁴⁰Pu/²³⁹Pu atom ratios observed in global and local fallout yielded ∼60% contribution of the local fallout in the bottom sediments near the Bikini Atoll. The upward decrease in the ²⁴⁰Pu/²³⁹Pu atom ratios in the sediment column indicates a decrease in the contribution of local fallout to the Pu inventory with time. ²⁴¹Am and ²⁴¹Pu dating of sediment layers was utilized to explain a hiatus in sediment accumulation in the deep seafloor.     

A review of present techniques and methodological advances in analyzing Th in aquatic systems

The short-lived thorium isotope ²³⁴Th (half-life 24.1 days) has been used as a tracer for a variety of transport processes in aquatic systems. Its use as a tracer of oceanic export via sinking particles has stimulated a rapidly increasing number of studies that require analyses of ²³⁴Th in both marine and freshwater systems. The original ²³⁴Th method is labor intensive. Thus, there has been a quest for simpler techniques that require smaller sample volumes. Here, we review current methodologies in the collection and analysis of ²³⁴Th from the water column, discuss their individual strengths and weaknesses, and provide an outlook on possible further improvements and future challenges. Also included in this review are recommendations on calibration procedures and the production of standard reference materials as well as a flow chart designed to help researchers find the most appropriate ²³⁴Th analytical technique for a specific aquatic regime and known sampling constraints.     

Interactions of marine plankton with transuranic elements. II. Influence of dissolved organic compounds on americium and plutonium accumulation in a diatom

To assess the significance of naturally occurring dissolved organic matter (DOM) on complexation of transuranic elements in seawater, a series of bioassay experiments was conducted in which the effect of DOM on the accumulation of ²⁴¹ Am, ²³⁷Pu (III–IV), and ²³⁷Pu (V–VI) by the marine diatom Thalassiosira pseudonana was measured. EDTA at 0.3μM complexed both metals substantially, resulting in reduced radio-isotope uptake by the diatom; the greatest effect was on Pu (III–IV). In contrast, there was no apparent complexation of either element by equimolar concentrations of marine fulvic (MFA) or humic acids (MHA), naturally occurring photooxidizable DOM (uncharacterized), or diatom exudates, as none of these materials reduced isotope uptake; on the contrary, there were indications that some of this DOM enhanced transuranic bioaccumulation in the diatom slightly. Subsequent experiments showed this enhancement was probably due to complexation of transition metals by the DOM, leading to fewer ambient ions ‘competing’ for binding sites on the cells; ²⁴¹ Am uptake rates were negatively correlated (r =? 0.846, P < .01) with Σ ASV-labile Cu + Zn + Cd + Pb. These experiments suggest that naturally occurring DOM may not appreciably complex Am or Pu or greatly affect their bioavailability in the sea.     

Bulk organic δC and C/N as indicators for sediment sources in the Pearl River delta and estuary,southern China

Preservation of organic matter in estuarine and coastal areas is an important process in the global carbon cycle. This paper presents bulk δ¹³C and C/N of organic matter from source to sink in the Pearl River catchment, delta and estuary, and discusses the applicability of δ¹³C and C/N as indicators for sources of organic matter in deltaic and estuarine sediments. In addition to the 91 surface sediment samples, other materials collected in this study cover the main sources of organic material to estuarine sediment. These are: terrestrial organic matter (TOM), including plants and soil samples from the catchment; estuarine and marine suspended particulate organic carbon (POC) from both summer and winter. Results show that the average δ¹³C of estuarine surface sediment increases from −25.0 ± 1.3‰ in the freshwater environment to −21.0 ± 0.2‰ in the marine environment, with C/N decreasing from 15.2 ± 3.3 to 6.8 ± 0.2. In the source areas, C₃ plants have lower δ¹³C than C₄ plants (−29.0 ± 1.8‰ and −13.1 ± 0.5‰ respectively). δ¹³C increases from −28.3 ± 0.8‰ in the forest soil to around −24.1‰ in both riverbank soil and mangrove soil due to increasing proportion of C₄ grasses. The δ¹³C_POC increases from −27.6 ± 0.8‰ in the freshwater areas to −22.4 ± 0.5‰ in the marine-brackish-water areas in winter, and ranges between −24.0‰ in freshwater areas and −25.4‰ in brackish-water areas in summer. Comparison of the δ¹³C and C/N between the sources and sink indicates a weakening TOM and freshwater POC input in the surface sedimentary organic matter seawards, and a strengthening contribution from the marine organic matter. Thus we suggest that bulk organic δ¹³C and C/N analysis can be used to indicate sources of sedimentary organic matter in estuarine environments. Organic carbon in surface sediments derived from anthropogenic sources such as human waste and organic pollutants from industrial and agricultural activities accounts for less than 10% of the total organic carbon (TOC). Although results also indicate elevated δ¹³C of sedimentary organic matter due to some agricultural products such as sugarcane, C₃ plants are still the dominant vegetation type in this area, and the bulk organic δ¹³C and C/N is still an effective indicator for sources of organic matter in estuarine sediments.     

天然气水合物赋存区甲烷渗漏活动的地球化学响应特征

综述了天然气水合物赋存区甲烷渗漏活动的地球化学响应指标的研究进展,分析了应用单一指标识别甲烷渗漏活动各自所存在的问题,包括浅表层沉积物孔隙水中CH_4、SO_4~(2–)、Cl~–等离子浓度随深度的变化;浅层沉积物全岩W_(TOC)(W表示质量分数,TOC表示总有机碳)和W_(TS)(TS表示总硫)之间的相关性及比值;自生碳酸盐岩δ~(13)C和δ~(18)O;自生矿物重晶石、黄铁矿、自生石膏的δ~(34)S;有孔虫壳体和生物标志化合物的δ~(13)C等。结果表明孔隙水中的CH_4、SO4_~(2–)浓度及溶解无机碳的碳同位素组成可以用来识别目前正在发生的甲烷渗漏活动;而沉积物中的WTS、自生矿物的δ~(34)S、钡含量及其异常峰值和生物标志化合物的δ~(13)C等指标的联合使用可以更真实准确地反映地质历史时期天然气水合物赋存区的甲烷渗漏活动。因此,在实际研究过程中,可将孔隙水和沉积物两种介质的多种指标相结合。随着非传统稳定同位素(Fe、Ca、Mg等)和沉积物氧化还原敏感元素(Mo、V、U等)等研究的发展,甲烷渗漏活动地球化学响应指标的研究也将得到拓展,而多种地球化学指标的联合使用将为天然气水合物勘探及其形成分解过程识别研究提供重要的科学依据。     

基于双还原体系与膜进样质谱快速测定15N加富水样中15NO3-的方法

海洋中的氮循环是海洋生物地球化学研究的热点领域之一,而硝化过程是氮循环的关键一环,准确获取硝化速率对于丰富海洋氮循环的认识至关重要。¹⁵N标记同位素技术是目前国际上最为广泛使用的硝化速率测定方法,该方法的核心在于准确测定¹⁵N加富样品产生的¹⁵NO₂^-和¹⁵NO₃^-的含量,但目前的方法普遍存在测试时间较长、测试成本较高、所需样品体积较大或者检测限较高等问题。研究以低成本的膜进样质谱作为¹⁵N加富样品测试设备,建立了基于镉柱与氨基磺酸双还原体系测定¹⁵N加富样品中¹⁵NO₃^-含量的方法。经条件优化实验确定的具体方法:采用1 mol/L HCl配制15 mmol/L的氨基磺酸（SA）作为反应试剂除去样品原有的NO₂^-,然后利用镉柱将¹⁵NO     

Sedimentation and mixing rates of radionuclides in Barents Sea sediments off Novaya Zemlya

Radionuclide measurements have been conducted on sediment cores collected in 1992 in the south-eastern region of the Barents Sea, known as the Pechora Sea. Cesium-137 and ^239,24OPu activities in surface sediments are generally less than 30 Bq/kg, with the highest levels being measured in sediments off the southwestern coastline of the island of Novaya Zemlya. High correlations between both ¹³⁷Cs and ^239,24OPu and the concentration of fine (< 63 μm) particles in surface sediments indicate that much of the variance in radionuclide concentrations throughout the Pechora Sea can be explained by particle size fractionation. However, elevated activities of ¹³⁷Cs (138 Bq/kg), ⁶⁰Co (92 Bq/kg), ²⁴¹Am (433 Bq/kg), and especially ^239,24OPu (8.47 × 103 Bq/kg) were measured in one surface sediment sample from the fjord of Chernaya Bay on the southern coast of Novaya Zemlya. The source of radioactive contamination is two underwater nuclear tests conducted in Chernaya Bay in 1955 and 1957.The ²³⁸Pu/^239,240Vu activity ratio of 0.0245 in Chernaya Bay is equivalent to values measured in global fallout. The ²⁴⁰Pu/²³⁹Pu atom ratio (0.0304), measured by mass spectrometry, is much lower than values (0.18) typical of global fallout, but is consistent with ratios measured for fallout from the early (1951–1955) series of weapons tests at the Nevada Test Site. The timing of the Chernaya Bay source term, estimated from the ²⁴¹Am/²⁴¹Pu ratio, is consistent with the timing of the 1955 and 1957 underwater nuclear tests. Relatively low initial yields of ²⁴¹Pu (²⁴¹Pu/²³⁹Pu atom RATIO = 0.00 123) in these tests have resulted in relatively low ²⁴¹Am/^239,240Pu activity ratios (0.05) in recent sediments in Chernaya Bay.Radionuclide tracer profiles in cores from the Pechora Sea can be simulated using a two-layer biodiffusion model with rapid, near-homogeneous mixing in the surface mixed layer and reduced mixing in the deep layer. Lead-210 profiles are consistent with a wide range of sedimentation and mixing rates in the deep sediment layer. However, the ¹³7Cs and ^239,240Pu results further constrain the model parameters and indicate that the downward transport of radionuclides in the sediments is governed primarily by sediment mixing, with sediment burial playing a secondary role.     

Geochemical investigation of oils from Cretaceous to Eocene sedimentary sequences of the Abadan Plain,Southwest Iran

Oil samples from Lower Cretaceous to Eocene reservoirs in southwest Iran were analyzed using gas chromatography–mass spectrometry and gas chromatography–isotope ratio mass spectrometry for genetic classification of oil families and determining their maturity. The Studied oil samples are non-biodegraded and their gravity range from 18.3 to 37° API. The slight even/odd n-alkane predominance, coupled with low Pr/Ph values, suggests their likely source rocks with a predominance of algal organic matter, type IIS kerogen deposited under strongly reducing marine environments. The biomarker distribution of investigated oils is characterized by high concentration of both C₂₉ and C₃₀ hopanes and ratios of C₂₉/C₃₀H are generally greater than unity. There is a marked predominance of C₂₉ regular sterane over C₂₇ and C₂₈ homologs in our studied oils. High sterane/hopane values and cross plot of the δ¹³C sat versus δ¹³C aro show contribution of marine organic matter. Medium value of gammacerane index and other salinity indices show water density stratification and high salinity conditions of the environment of deposition. It can be concluded that the studied reservoirs, due to their variable maturity have different API gravity and contain two oil families (types A and B) with latter being deeper and comprising more mature oils.     

Effect of solid to solution ratio on biogenic silica determination in coastal sediments

The method of DeMaster (1981) for measuring biogenic silica (BSi) in marine sediments was modified. We found a considerable effect of the solid to solution (S/S) ratio on BSi determination in coastal sediments. The BSi contents determined by DeMaster’s method were overestimated due to the contribution of clay mineral-derived extractable Si at a low S/S ratio (<1 g l⁻¹), and incomplete recovery of BSi was observed due to incomplete extraction when using a high S/S ratio (>2 g l⁻¹). For BSi analysis of coastal sediment samples, the BSi content of representative samples must be measured using various S/S ratios to determine the appropriate S/S ratio in order to minimize the contribution of extractable Si derived from clay minerals and to obtain sufficiently efficient extraction with 1% Na₂CO₃ before actual sample measurement. One way to determine the appropriate S/S ratio is to use artificial sediment which has a theoretical Si and clay mineral content similar to the sediment sample composition. Using artificial sediment, the S/S ratio of 2 g l⁻¹ seems appropriate for measuring the BSi content of the coastal sediments investigated in this study, because 99.6% recovery can be achieved.     

Natural abundances of 15N as a source indicator for near-shore marine sedimentary and dissolved nitrogen

Because organic matter originating in the euphotic zone of the ocean may have a distinctive nitrogen isotope composition (¹⁵N/¹⁴N), as compared to organic matter originating in terrestrial soils, it may be used to evaluate the relative nitrogen contribution to marine and estuarine sediment. The nitrogen isotope ratios of 42 sediment samples of total nitrogen and 38 dissolved pore-water ammonium samples from Santa Barbara Basin sediment cores were measured. The range of δ¹⁵N values for total nitrogen was $\text{+2.89 – +9.4‰}$ with a mean of $\text{+6.8‰}$ and for pore water ammonium, $\text{+8.2 – +12.4‰}$ with a mean of 10.2‰.The results suggest that the dissolved ammonium in the pore water is produced from bacterial degradation of marine organic matter. The range of δ¹⁵N values for total nitrogen in the sediment is interpreted as resulting from an admixture of nitrogen derived from marine ($\text{+10‰}$) and terrestrial (+2‰ marines. The marine component of this mixture, composed principally of calcium carbonate with smaller amounts of opal and organic matter, contains ～ 1.0% nitrogen. The terrestrial component, which comprises over 80% of the sediment, contains ～ 0.1% organically bound nitrogen and accounts for > 25% of the total nitrogen in Santa Barbara Basin sediment.     

Measuring low concentrations of Th in water and sediment

²³⁴Th/²³⁸U disequilibria have been used extensively in studies of particle dynamics and the fate and transport of particle-reactive matter in marine environments. Similar work in low salinity, estuarine, and freshwater systems has not occurred primarily because the lower concentrations of both parent and daughter nuclides that are typical of these systems often render established methods for the analysis of ²³⁴Th inadequate. The application of this radionuclide tracer technique to these systems, however, has great potential. To this end, we present a method for measuring low activities of ²³⁴Th in relatively small samples (<200 l) using low background gas-flow proportional counters, a ²²⁹Th yield monitor, and empirical corrections for the interferences from real and apparent betas that are emitted by other thorium isotopes and their progeny. For samples with low ²³⁴Th/²²⁸Th activity ratios, we improve upon current beta counting methodologies that rely on immediate sample counting, weak beta absorption, or multiple beta counts so that, using the analytical approach outlined here, it should be possible to measure ²³⁴Th activities (i) as low as 1.5 dpm/total sample, (ii) up to 2 weeks after radiochemical purification of thorium, and (iii) with only one sample count for alpha and beta activity.     

Measurement of 129I in ferromanganese crust with AMS

In the present study,the analytical method for 129iodine(129I) in ferromanganese crusts is developed and129iodine/127iodine(129I/127I) ratio in ferromanganese crusts is measured by the accelerator mass spectrometry(AMS).The developed method is applied to analyze 129I/127 I ratio in two ferromanganese crusts MP5D44 and CXD08-1 collected from the Mid-Pacific Ocean.The results show that 129I/127 I ratio in MP5D44 and CXD08-1crusts varies from 7×10–14 to 1.27×10–12,with the lowest value falling on the detection limit level of AMS reported by previous literatures.For the depth distribution of 129I/127 I,it is found that both MP5D44 and CXD08-1 crusts have two growth generations,and the 129I/127 I profiles in two generations all displayed an approximate exponential decay.According to the 129I/127 I ratio,the generate age of bottom layer of MP5D44 and CXD08-1 was estimated to be 54.77 and 69.69 Ma,respectively.