The formation and stabilization of soil organic carbon (SOC) are mediated by the activity of arbuscular mycorrhizal (AM) fungi. Understanding the distribution of AM fungi across tidal flat gradients in coastal salt marshes and their community responses to soil carbon sequestration is critical for predicting and managing blue carbon ecosystems under a warming climate. We combined high-throughput DNA sequencing with biomarker analysis to examine shifts in AM fungal communities along a seaward-to-landward tidal gradient and to evaluate their potential functions in SOC formation within a coastal salt marsh. Both AM fungal alpha diversity and network complexity followed a unimodal pattern along this gradient, showing positive correlations with soil available nitrogen, salinity, and available phosphorus, but a negative correlation with soil pH. Stochastic processes remained predominant in AM fungal community assembly across the tidal gradient, although the relative contribution of deterministic processes gradually increased from seaward to landward habitats. Fungal necromass carbon exhibited an initial rise, a subsequent decline, and a final increase along the tidal flat gradient. Changes in AM fungal community composition associated with soil physical properties were closely linked to variations in fungal necromass carbon and SOC accumulation in salt marsh ecosystems. Key fungal genera, including Archaeospora, Glomus, and Paraglomus, were identified as potenial contributors to the accumulation of fungal necromass carbon and SOC. This study suggests a role of AM fungi in coastal salt marsh carbon sequestration and calls for further investigation into the ecological functions of AM fungi in wetland ecosystems.

Marginal seas are important repositories of both terrestrial and marine organic carbon (OC), accounting for over 90% of the global marine sedimentary OC burial flux. The northeastern South China Sea (NESCS), as the largest semi-enclosed marginal sea in the western Pacific, provides an exceptional natural laboratory to investigate millennial-scale OC dynamics. This study analyzes total organic carbon (TOC), stable carbon isotopes (delta C-13), radiocarbon isotopes (C-14), and lignin phenols in Core S04 from the Taiwan Shoal to examine OC burial variations and their environmental controls since similar to 36.5 ka. The results show that deep-water OC burial in the NESCS is not simply controlled by variations in organic matter inputs. Instead, it reflects the combined influence of sea-level-controlled source-sink configuration, monsoon-driven sedimentary dynamics, and pre-depositional reworking and mineral protection processes. During MIS 3, relatively high TOC contents indicate OC burial under a stable source-sink configuration, dominated by marine-derived inputs with a secondary contribution from terrestrial biospheric carbon. During MIS 2, extremely low sea level produced two contrasting regimes. The early to middle glacial period was characterized by high-energy cross-shelf transport and intense particle sorting, which suppressed OC loading efficiency. In contrast, the subsequent deglacial period saw enhanced retention of fine particles and intensified sediment reworking, leading to longer OC residence times and older apparent radiocarbon ages. During the Holocene, strengthened East Asian Summer Monsoon conditions increased terrestrial material supply, but high sea level combined with persistent bottom-current activity maintained a low-loading stable state, in which OC preservation became weakly responsive to source variations and increasingly controlled by transport and reworking processes. Overall, OC burial in the deep basin of the NESCS reflects the coupled effects of climatic forcing and sedimentary transport-preservation processes. These findings provide new process-based constraints on the response of deep-water carbon sinks in continental margin systems to glacial-interglacial climate variability.

Based on sedimentological and organic carbon and nitrogen geochemical analyses of four sediment cores from the Paleo-Yangtze Grand Delta, this study reconstructs sedimentary environmental evolution and organic matter (OM) significance since Marine Isotope Stage 6 (MIS 6). During glacial periods, MIS 6 was characterized by fluvial facies deposition, with organic carbon preservation primarily controlled by sediment grain size. The consistently positive δ13Corg values observed in MIS 2 sediments are attributed to an increased contribution from C4 plants. During interglacials, grain size remained key, although its influence was modulated by other factors. Despite the most extensive transgression in MIS 5, strong terrestrial input partially masked the marine signal. During MIS 3, a shelf–delta front–tidal flat sedimentary system developed, with sedimentary facies differentiation and grain size characteristics jointly regulating the organic carbon preservation pattern. By MIS 1, rising sea level further enhanced marine influence, leading to the formation of a shelf–tidal sand ridge–tidal flat depositional system. This study demonstrates that integrating organic carbon–nitrogen geochemistry with sedimentary facies analysis effectively traces OM sources and elucidates the glacial–interglacial environmental evolution. The findings advance our understanding of OM burial mechanisms under varying climate and sea-level conditions and provide insights into sedimentary and carbon–cycle responses of marginal seas to global change.

Abstract The interaction between organic carbon (OC) and reactive iron (FeR) plays a critical role in suppressing OC mineralization, with the resulting FeR-bound OC (FeR-OC) constituting an important carbon sink in marine sediments. Traditional views suggest that under anoxic conditions, associations between FeR and OC not only enhance the OC accumulation and persistence but also inhibit the reductive dissolution of FeR. However, this framework leaves unresolved uncertainties regarding how FeR and OC mutually promote their costabilization under anoxic conditions. In this study, sequential chemical extraction was employed to distinguish organically complexed iron-bound OC (Fep-OC) from iron-oxide-bound OC (Feox-OC). Our results reveal that Fep represents a previously underrecognized iron-associated OC pool that contributes to the OC preservation in marine sediments. Under anoxic conditions, Fep promotes OC retention and offsets the decline in Feox-OC. Despite its lower abundance, Fep accounts for the stabilization of an average of 20.59% of the sedimentary OC. Fourier transform infrared spectroscopy (FTIR) analyses further indicate that Feox-OC is relatively enriched in aromatic and alcohol/phenol functional groups compared with bulk OC, whereas Fep-OC exhibits no distinct enrichment of specific functional groups. Collectively, this study provides field-based evidence for the redistribution of OC among different FeR-bound fractions and highlights the potentially important contribution of Fep to OC preservation under anoxic conditions.

Understanding past changes in oceanic circulation and the corresponding heat, salt delivery variations are essential for assessing the climatic roles of ocean dynamic processes since the last glacial period. Unravelling salinity budget variation in the North Pacific and its controls is important to better understand the North Pacific Intermediate/Deep Water formation and associated climate impacts. The Tsushima Warm Current (TWC), the northernmost Kuroshio branch entering the semi-closed Japan Sea through the shallow strait, is regarded as a fundamental component for oceanographic changes in the Japan Sea. To obtain a comprehensive history of the Japan Sea salinity budget, this study reconstructed the evolution history of the TWC inflow by compiling paleohydrographic records in the Japan Sea. Following a persistent but weakened TWC inflow during Marine Isotope Stage 3, radiolarian assemblage data revealed that the TWC taxa disappeared since -30 thousand years ago (ka). The synchronous onset of the low salinity anomaly event was in response to the cutoff of saline TWC inflow due to the rapid fall in global sea level at 30 ka. Extreme restriction of seawater exchange caused a persistent freshening of the glacial Japan Sea and formed a low-salinity water mass in the upper ocean. The compiled paleodata confirmed that the Japan Sea accumulated excess freshwater during the glacial sea-level lowstands and the low-salinity pool extended downward to -900 m depths. Coinciding with the peak of the low salinity anomaly event, re-emerging TWC inflow after 19 ka reflected the reconnection of the open ocean to the Japan Sea. The persistent TWC inflow mainly drove the reduction in magnitude of the deep low-salinity pool during the last deglaciation. As a large and isolated freshwater sink for the glacial North Pacific, the deep low-salinity pool evolution could potentially have strong impacts on the North Pacific salinity budget and subsequent large-scale circulation.

The grain size characteristics and aging of beach sediments from six islands of the Yongle Atoll, South China Sea, were measured to examine the taphonomic degradation of coral clasts in sediment reservoirs of reef environ-ments. Seventy-one beach sediment samples were collected from 22 transects on 6 islands for sediment char-acteristic analyses, and 16 bulk sand samples and 28 coral samples were selected for 230Th dating. The mean grain size and ages of coral clasts showed a significant negative correlation (F25 =-0.721, p = 0.01). A grain size age model was established to explain the taphonomic degradation mechanism of coral clasts over time, and an empirical relationship between the mean grain size and 230Th ages was obtained from the Yongle Atoll. The mismatch of the taphonomic clock by grain size data was analyzed, and it was mainly caused by the influence of sorting processes and the intrinsic morphology of coral skeletons that occurred at the early stage of the break-down process. The application of the model to a nearby atoll island showed that for coral sediments with ho-mogeneous taphonomic conditions, the predicted age was well matched with the measured age. This work presents a general framework for understanding coral clast degradation trajectories with time in taphonomic environments, and it provides a feasible method to predict the coral age independent of other dating methods.

为探究东海扬子浅滩晚更新世沉积物的来源,对该区域的YZ05钻孔MIS6阶段沉积物中的石英流体包裹体进行了岩相学分析,并与长江干流表层沉积物中的流体包裹体进行对比研究.结果显示,YZ05钻孔晚更新世沉积物中单位体积(0.001 mm3)石英颗粒的包裹体数量在17～47个之间,个体大小多为2～5 μm,以孤立分布为主.其中次生包裹体数量较多,约占包裹体总数的88％～96％,原生包裹体以椭圆或四边形居多,室温下多为富液相,气液比大多为10％～30％.研究区及现代长江干流表层沉积物中的流体包裹体形态较为一致.此外,研究还发现了颈缩形态的包裹体及气液比大于80％的富气相原生包裹体,此类包裹体在长江上游石鼓地区较为常见.因此,可以推测晚更新世时(至少在47 ka BP),长江上游物质已经到达扬子浅滩西南海域.

The numerous foraminifera in sediments near the equatorial western Indian Ocean act as historiographers that record abundant information about the marine environment. However, few studies have focused on the foraminiferal distribution in surface sediments within this area and the associated environmental and productivity implications. In this study, planktonic and benthic foraminifera were obtained from 28 surface sediments. The planktonic foraminifera were found to be mainly warm-water species; their distribution was mainly influenced by depth-controlled dissolution rate of carbonate, temperature, and salinity. Combined vertical distribution of foraminifera, fragment rate, resistant species rate, and Goborotalia menardii fragmentation index, the carbonate lysocline depth of the equatorial western Indian Ocean is similar to 3900 m. The benthic foraminifera were mostly oxygen-rich and oligotrophic species. Their distribution and organic carbon, the oxygen content, and the water mass related. Four assemblages were identified: Assemblage 1, Cibicidoides wuellerstorfi, Oridorsalis umbonatus [indicative of Antarctic Bottom Water (AABW)]; Assemblage 2, Pullenia bulloides, Oridorsalis umbonatus (indicative of an oxygen-rich environment with intermediate to low productivity); Assemblage 3, Epistominella exigua, Turrilina cf. brevispira (indicating a pulsed organic matter input and seasonal fluctuations in productivity); Assemblage 4, Globocassidulina subglobosa, Astrononion novozealandicum (suggestive of a low-productivity environment). The overall productivity is low near the equatorial western Indian Ocean. The foraminifera species, TOC, and biogenic barium data show that the western side of the Central Indian Ridge is relatively more productive than the eastern region. These results can be used as a basis for reconstructing the distribution of paleoAABW and productivity within the Indian Ocean.

This study evaluated the structure, mineralogy, and chemical composition of ferromanganese micronodules in gravity core GC01 obtained from the eastern southwest Indian ridge using micro-Raman spectroscopy, WITec Alpha 300R confocal Raman spectroscopy, scanning electron microscopy with energy-dispersive X-ray spec-troscopy, TESCAN integrated mineral analysis and electron microprobe analysis. The results revealed vernadite as the major crystalline phase in the ferromanganese micronodules. In addition, the micronodules contained small amounts of Fe oxyhydroxides and silicate minerals. All micronodules had low Mn/Fe ratios (similar to 1), and their internal structures included concentric, clastic, dendritic, and foraminifera-like structures, among which the concentric and clastic structures were predominant. These characteristics suggest that micronodules are formed in an oxidizing environment via hydrogenetic processes. Volcanic materials and biological debris may act as "seeds " for micronodules, among which volcanic materials may serve as material sources. During water-rock interaction, volcanic clastics and biological debris react with seawater, and some cations (such as Ca2+, Na+, Mg2+) are leached out; Fe2+ and Mn2+ remain and generate Fe oxides or oxyhydroxides and Mn oxides which forms ferromanganese micronodules. Conclusively, the formation of ferromanganese micronodules in this study was controlled by a coupled dissolution-reprecipitation mechanism.

Tectonic-related hydrothermal fields constitute about half of the hydrothermal activities on slow-ultraslow spreading ridges, and may form sulfides large in size and rich in precious metals. Ore-forming fluids are critical for element migration progressions, thus are key parameter to reveal their genesis and sulfide mineralization processes. However, previous studies of this kind of hydrothermal activities primarily focused on vent fluids which have transient effects or based on surface samples, while the ore-forming fluid evolution and related subseafloor mineralization process are still poorly constrained. In this study, we analyzed the characteristics of fluid inclusions in the stockwork mineralization of the East Longjing-2 hydrothermal field (ELHF-2) on the Southwest Indian Ridge. The results showed that the fluids evolved gradually from medium-high temperatures (260-315 degrees C, mean of 284 degrees C) and low salinities (0.70-3.70 wt.% NaCl eq.) in the disseminated pyrite mineralization stage (Stage I) to medium temperatures (239-261 degrees C, mean of 249 degrees C) and low salinities (1.05-3.85 wt.% NaCl eq.) in the chalcopyrite-pyrite-quartz veinlet mineralization stage (Stage II). Laser Raman specular analysis indicated the fluid inclusions were mainly composed of H2O. The low salinity (lowest 0.7 wt.% NaCl eq.) and various vapor volume (0.05-0.5) observed in these fluid inclusions suggested the ore-forming fluids were probably mixtures of the low-salinity vapor phase formed by phase separation with large amounts of seawater. The precipitation of the ore-forming elements was likely induced by temperature decreasing. We propose that hydrothermal circulation in ELHF-2 was a result of coupled high-permeability detachment fault and shallow gabbro intrusion. Our results suggest that high temperature hydrothermal activities could be developed on distal axis area on ultraslow spreading ridges.

The formation of intermediate and deep water plays a key role in regulating climate changes at a variety of time scales through the heat redistribution and carbon cycling. The Japan Sea has unique water-mass characteristics in the North Pacific with its own deep-water formation within the Sea itself called Japan Sea Proper Water (JSPW). Latitudinal ventilation changes in the Japan Sea were reconstructed using radiolarian assemblage from three sediment cores, extending from the southwestern, central to northwestern Japan Sea. Here, we present downcore faunal records spanning the last 25 ka as well as other existing ventilation records in the Japan Sea, and provide reliable evidence to evaluate the potential controlling mechanism that lead to onset and interruption of JSPW ventilation. Taking all together, we argue that radiolarian assemblage records have revealed a distinct basin-scale transition in deep-water conditions from anoxic to oxic during the deglaciation related to changing surface hydrography. However, it should be recognized that there is significant potential for bias in the timing of the ventilation changes among regions. Deep ventilation in the central Japan Sea has been in an interglacial mode during the Bølling/Allerød presumably related to northward volume transport of the Tsushima Warm Current. Moreover, the decrease of JSPW Assemblage at the B/A in southwestern Japan Sea was attributed to higher export productivity, facilitating suboxic deepwater condition through enhanced consumption of oxygen, which was probably caused by coastal upwelling. In contrast, the weakening ventilation of the northwestern Japan Sea during the B/A and YD periods was probably caused by the blocking effect of the sea ice. Note: This study was supported by the National Natural Science Foundation of China (Grant No. 41420104005, U1606401) and National Program on Global Change and Air-Sea Interaction (GASI-GEOGE-04).

南沙群岛的珊瑚礁以环礁形式存在,拥有各具特色的地貌形态.九章环礁中的牛轭礁和西门礁是两座发育有新生沙洲的环礁,代表灰沙岛形成前一个重要的过渡性地貌演化阶段.这两处新生沙洲表层沉积物样品的粒度分析表明:沉积物主要组分为珊瑚碎屑,长轴中值粒径在14～45mm之间,属于砾石级别;样品分选系数在4.5～31之间,分选性很差;球度值为0.52～0.68,球度差.粒度特征值中,中值粒径具有随着到水边线距离的增大而升高的趋势,而分选性和球度与距水边线距离关系不大.整体上中值粒径与分选系数呈正相关关系,而与球度呈负相关关系.历史文献记载和卫星图像表明,牛轭礁和西门礁上的新生沙洲都是近年来才形成的,与风暴过程及冬季风有关.牛轭礁上沙洲向北迁移,同时其长轴向东南方向延伸;西门礁上沙洲整体向东南方向迁移,并形成回弯形沙嘴形态.新生沙洲的出现与珊瑚碎屑物质供给变化或风暴浪作用变化的关系还需进一步探讨.

The characteristics of quartz-hosted fluid inclusions in fluvial sediments from five locations in the upstream, midstream, and estuary of the Changjiang River, China, are analyzed. The sources of sediments are discussed concerning their differences in the shape, size, number, gas percentage and genetic type of quartz-hosted fluid inclusions. From upstream to downstream, the characteristics of quartz-hosted fluid inclusions in sediments are different. The fluid inclusion types in the samples from upstream to estuary are gradually enriched. The sediment influx from the tributaries of the Changjiang River makes new types of quartz-hosted fluid inclusions in the downstream and estuary. In terms of the number and size, most quartz-hosted fluid inclusions are concentrated in the range of 2–4 µm in diameters and 10–150 in number per 10−3 mm3. The number and size ranges of the fluid inclusions from different positions are also different. The fluid inclusions in the sample collected from the Shigu, upstream of the Changjiang River, are 2–18 µm in size, with the number of 2–166 per 10−3 mm3. Among the samples collected from Yibin, Yichang and Wuhan, the sizes of fluid inclusions are 2–15, 2–10, 2–12 µm, with the number of 1–270, 2–220, and 1–308 per 10−3 mm3, respectively. The proportion of primary fluid inclusions in the sample of the upstream (14%) is higher than that of the midstream (6%–8%) and the estuary (5%), suggesting that different types of source rocks have been input into the river by the tributaries. The characteristics of quartz-hosted fluid inclusions in the fluvial sediments could offer a new perspective for exploration of the source of sediments.

本文对泰国湾西部T93柱状沉积物样品进行了常微量元素测试,探讨了沉积物物源和沉积环境变迁.T93柱状样全长381 cm,底层年代约14640 cal.a B.P.,间隔10 cm取样测试,样品平均年代间隔375 a.测试结果表明,末次冰消期以来泰国湾沉积物来源主要经历了4个阶段的变迁:1)14640～11700 cal.a B.P.期间,该阶段为低海平面时期的陆相沉积,沉积物为粘土质粉砂,Na、K、Ca等活跃的碱金属和碱土金属元素含量较低,Ti、Zr、Ba等含量高,表现出强烈的化学风化特征;δEu平均为0.60,(La/Yb)PAAS平均0.92,物质来源以马来半岛为主,但分异程度高,成分较复杂.2)11700～8000 cal.a B.P.期间,该阶段为海平面快速上升期的滨海沉积,沉积物主要为粘土质粉砂,粉砂组分含量开始增加,Mg和Mn元素含量大幅升高,Fe、Ti含量减少;δEu平均为0.58,(La/Yb)PAAS平均0.99,物质来源主要为马来半岛.3)8000～4000 cal.a B.P.期间,该阶段为高海平面时期的陆架沉积,沉积物包括粉砂和粘土质粉砂,Mg、Na、Ca含量明显增大,化学风化程度减弱;δEu平均为0.60,(La/Yb)PAAS平均1.08,沉积物主要来自中南半岛.4)4000 cal.a B.P.至今,该阶段为海平面稍有下降的稳定陆架沉积,沉积物主要为粉砂,沉积物元素总体稳定,现代沉积体系基本形成;δEu平均为0.62,(La/Yb)PAAS平均0.97,沉积物以马来半岛物质为主,中南半岛物质为辅,物源与现代一致.

通过对中印度洋海盆GC18孔沉积物元素地球化学特征和微体化石组合进行分析,恢复了该区域碳酸盐补偿深度(Carbonate Compensation Depth,CCD)的演变及环境演化历史.结果表明,GC18孔从上到下可分为五段.第Ⅰ段(0～30 cm) CaO含量和总无机碳(Total Inorganic Carbon,TIC)从表层向下呈显著下降趋势,SiO2呈显著上升趋势,微体化石主要是有孔虫碎屑,还含有少量钙质超微化石Calcidiscus leptoporus(弱脐钙盘藻)和Ceratolithus cristatus(具冠毛角石藻),沉积时代为第四纪.第Ⅲ段(150～280 cm) CaO和TIC含量少于第Ⅰ段,两者从上到下经历了上升-稳定-下降的过程,SiO2变化趋势相反,含钙质超微化石,主要为Discoaster deflandrei(德佛兰盘星石藻),沉积时代为中中新世晚期或更早.第Ⅴ段(300～415 cm)沉积物组分波动相对较大,CaO和TIC平均含量达到最高,SiO2含量较低,钙质超微化石非常丰富,以Umbilicosphaera Jafari(贾法脐球藻),Umbilicosphaera rotula(轮形脐球藻)和Discoaster deflandrei为主,沉积时代为中中新世-早中新世.第Ⅱ段(30～150 cm)和第Ⅳ段(280～300 cm)相似,均呈现低CaO和高SiO2特征且相对稳定,TIC含量很低,未发现微体化石,主要是深海黏土.第Ⅰ,Ⅲ和Ⅴ段CaO含量较高,钙质超微化石均有保存,反映当时沉积均处于CCD以上.第Ⅱ,Ⅳ段CaO含量低,SiO2含量高,钙质超微化石没有保存,反映当时沉积处于CCD以下从第Ⅴ段到第Ⅱ段(中中新世-晚中新世),CCD整体逐渐变浅,这与全球CCD在此期间的变化特征一致.中印度洋海盆CCD自中新世以来经历了深-浅-深-浅-深的变化历史.

Assessing modern radiolarian distribution in the marginal seas is essential for understanding its relationship to hydrographic and environmental variables, upon which subsequent interpretation of past radiolarian records can be made reliably. Here, we combine published radiolarian assemblages in 69 surface sediments from the Japan Sea with those obtained from 15 new surface sediments in this study to determine the modern distribution of radiolarians. The Tsushima Warm Current (TWC) taxa are only found in the south of the Subpolar Front. The results of redundancy analysis confirm that the temperature at 10 m depth (temperature(10m)) is the main factor controlling the spatial variability of TWC taxa. The water depth plays a key role in affecting the abundances of Japan Sea Proper Water (JSPW) Assemblage including Larcopyle buetschlii Dreyer (adult), Cycladophora davisiana Ehrenberg, and Actinomma group. The JSPW Assemblage can be used as a reliable indicator of ventilation changes in the Japan Sea. Based on the newly developed TWC taxa and JSPW Assemblage, we further reconstructed the water column structure in the central Japan Sea over the past 25 ka. There was weak ventilation in the central Japan Sea even during the Last Glacial Maximum (LGM), although a distinct decrease in the TWC inflow resulted in surface-water freshening and water column stratification. After the LGM, the TWC taxa together with JSPW Assemblage resumed in the central Japan Sea at the 19 ka meltwater pulse (MWP). The MWP-1A was clearly marked by an enhanced intrusion of the TWC. Synchronously, JSPW was ventilated deeply down after similar to 14.6 ka attaining to the similar level of the Holocene as a result of changing sea surface conditions. Along with the continuing sea-level rise, the matured TWC entered the Japan Sea at 7 ka and since then the modern hydrographic condition has been established. We suggest that the changes in the sea level and TWC strength predominantly determined the surface hydrography and deep ventilation in the central Japan Sea since 25 ka, and the TWC might have notably affected the Japan Sea since the Bolling-Allerod.

为研究人类活动及鸭绿江陆源输入对河口海岸及陆架泥质区沉积环境的影响,本文通过对2017年于辽东半岛泥质区东部采集的Q02柱与西部Q04柱进行210 Pb定年、粒度、总有机碳、总有机氮、δ13C同位素等指标综合分析,利用Meyers研究模型结合C/N值及δ13C同位素含量分析不同物源有机质贡献率.结果 表明:(1)两根柱样TOC与TN含量近百年来逐渐升高,C/N值在9～11之间为海陆混合来源.沉积物主要以粉砂及粘土为主,粉砂含量最高,粘土次之,砂含量较少.(2)有机碳来源占比均呈陆源降低海源升高的特点,分别反映了泥质区东部与西部不同主导因素对海域环境的影响.东部沉积环境主要受流域环境变化与海域共同作用,垦荒、自然灾害、日俄战争等事件造成了植被覆盖降低,水库建设减少了陆源有机碳输入;西部沉积环境主要受沿岸人类活动、水体富营养化的影响,“弛禁”政策导致植被覆盖被破坏,港口修建、水产养殖等活动造成水体营养物质增多,引起海洋环境恶化.

对于开阔大洋海山结壳的矿物组成和地球化学特征研究已经比较深入,但对边缘海海山铁锰结壳的研究却较少.以在南海东部管事海山上采集的两块铁锰结壳为研究材料,对其进行了X射线衍射、扫描电镜及电子探针微区的分析,结果显示结壳矿物以水羟锰矿为主,含有少量针铁矿,碎屑矿物主要是石英和长石.结壳的显微结构呈现条带状分布,有10～15μm长的空隙和0.5μm直径大小的孔洞.相对于大洋结壳,结壳的Mn含量较高,与马尼拉海沟附近火山喷发物质有关,但Cu、Co和Ni含量更低,可能与边缘海沉积速率高或有机络合物吸附有关.矿物组成及元素比值表明结壳主要是水成的,但未发生明显磷酸盐化作用.元素比值和相关性表明,Mn主要来源于南海扩张停止后火山喷发、玄武岩风化形成溶解Mn;Fe与Si元素均来自火山碎屑、生源物质和陆源物质,还受到管状蠕虫吸附的影响.南海管事海山结壳是在富氧环境中上升流作用下选择性吸附水体中溶解化学元素缓慢沉积而成.结壳中元素含量随时间变化趋势反映了南海的古环境变化.

Low-lying reef islands constructed from unconsolidated sediments are of growing interest due to concerns over the future stability of island landforms in response to climatic change and sea-level rise. Sediment characteristics, combined with the constituents and physical processes, are fundamental to understand the links between sediment production and their supply to an island's beach reservoir. The grain size, constituents, and Th-230 age of beach sediments from five islands at Yongle Atoll in the South China Sea were analyzed. Data reveal that coral grains are the dominant constituents of the beach sediments, followed by mollusks, coralline algae, foraminifera, Halimeda, and bryozoans. The mean grain sizes of the beach sediment were within a range of 226-4215 mu m, and the coarsest sands were located near the passages in the atoll rim. Transportation by tropical cyclone generated waves and winnowing by tides are potentially the main factors that influenced the grain-size characteristics. Calibrated Th-230 age dates for bulk sand samples date the beach sediment between 424 yr BP and 5953 yr BP. Dating of coral sand samples from the beach sediment provides ages of 686-5780 yr BP; the coral sands from a vertical section (1.2 m depth) located on the beach at Jinqing Island were dated at 1154-3824 yr BP. The age reversal on the vertical section, with similar ages of beach sediment on any single island and different age ranges on different islands in an atoll, indicates that the coral sediment supply processes occurred within a relatively close time period and that different islands had different sediment supply rates. The temporal link between the coral-dominated sediment production and the beach reservoir provides an important insight into understanding the sediments supply mechanism of atoll islands in the northwestern Pacific Ocean.