Distribution and composition of authigenic minerals in surface sediments of the western Gulf of Thailand

Generation, morphology, and distribution of authigenic minerals directly reflect sedimentary environment and material sources. Surface sediments were collected from the western Gulf of Thailand during 2011–2012, and 159 samples were analyzed to determine detrital minerals. Authigenic minerals, including siderite, pyrite, and glauconite, are abundant whereas secondary minerals, such as chlorite and limonite, are distributed widely in the study area. Siderite has a maximum content of 19.98 g/kg and appears in three types from nearshore to continental shelf, showing the process of forming-maturity-oxidation. In this process, the Mn O content in siderite decreases, but FeOand Mg O content increase. Colorless or transparent siderite pellets are fresh grains generated within a short time and widely distributed throughout the region; high content appears in coastal area where river inputs are discharged. Translucent cemented double pellets appearing light yellow to red are mature grains; high content is observed in the central shelf. Red-brown opaque granular pellets are oxidized grains,which are concentrated in the eastern gulf. Pyrite is mostly distributed in the central continental shelf with an approximately north–south strip. Pyrite are mainly observed in foraminifera shell and distributed in clayey silt sediments, which is similar to that in the Yangtze River mouth and the Yellow Sea. The pyrite in the gulf is deduced from genetic types associated with sulfate reduction and organic matter decomposition. Majority of glauconite are granular with few laminar. Glauconite is concentrated in the northern and southern parts within the boundary of 9.5° to 10.5°N and is affected by river input diffusion. The distribution of glauconite is closely correlated with that of chlorite and plagioclase, indicating that glauconite is possibly derived from altered products of chlorite and plagioclase. The KO content of glauconite is low or absent, indicating its short formation time.

安达曼海东南部海域表层沉积物稀土元素特征及其物源指示意义

对取自安达曼海东南部海域的95个表层沉积物样品进行了稀土元素（REE）地球化学研究,揭示了区内稀土元素分布特征及其指示的物质来源。结果表明研究区沉积物REE总量变化范围为31~228μg/g,平均值为117μg/g。沉积物REE球粒陨石配分模式呈现轻稀土元素相对富集而重稀土元素平坦,无明显Ce异常,呈现中等程度的Eu负异常等特征,表现出明显的陆源属性。REE的富集和分布受物质来源、沉积物类型、生物碳酸盐以及重矿物含量共同制约。根据沉积物REE特征参数将研究区分为4个物源区,各区沉积物稀土元素UCC标准化模式明显不同。物源判别分析显示,研究区西北部（Ⅰ区）沉积物主要来源于伊洛瓦底江陆源物质的输入;马来半岛西部浅海内陆架区（Ⅱ区）沉积物可能主要来自马来半岛沿岸水系输入的陆源物质以及现代近岸侵蚀;研究区北部（Ⅲ区）为残留沉积区,主要为全新世海侵形成的残留砂质沉积;研究区中部和南部大部分海域（Ⅳ区）为多源混合沉积区,主要为受印度季风驱动的西南季风流所搬运的伊洛瓦底江和马来半岛河流输入的陆源物质,该区沉积物受马来半岛入海物质影响更大。

Early divergence and differential population histories of the Indo-Pacific humpback dolphin in the Pacific and Indian Oceans

The currently recognized Indo-Pacific humpback dolphin occurs in estuaries and surrounding shallow waters from the South China Sea to the Asian coast of the Indian Ocean.However,a recent study suggested that the humpback dolphin from the Bay of Bengal may represent a distinct phylogenetic species.In this study,we sequenced 915-bp mtDNA segments from five geographic populations in both Chinese and Thai waters;together with previously published sequences,these data revealed that the ancestral Indo-Pacific humpback dolphin might have split during the transition from the Oligocene to Miocene(23.45 Mya,95%HPD:16.65–26.55 Mya),and then dispersed along the Pacific and Indian Ocean coasts of Asia.Genetic differentiation was detected between most of the examined populations,except for only a few pairwise populations in the northern South China Sea.Genetic differentiation/distance between the humpback dolphins from the northern and southern South China Sea met the sub-species threshold value proposed for marine mammals,whereas that between the humpback dolphins in the Pacific and the Indian Ocean was above the species threshold.Bayesian inference of historic gene flow indicated low but constant northward gene flow along the Indian Ocean coast;however,there was a recent abrupt increase in gene flow in the Pacific region,likely due to the shortening coastline at the low stand of sea level.Our results revealed that the current taxonomic classification of Indo-Pacific humpback dolphins may not reflect their phylogeography.

Novel insights into the spatial genetic patterns of the finless porpoise from East to Southeast Asia

Several cetacean species are endemically distributed throughout the Indo-Pacific.Due to disproportionate sampling and research efforts across the Indo-Pacific region,the spatial genetic structure of these species remains poorly understood.This has led to poor phylogeographic knowledge and ambiguous taxonomic classification of many Indo-Pacific cetacean species.Of these,the finless porpoise(genus Neophocaena)is a small cetacean obligatory to the inshore waters from East Asia to the Persian Gulf of the Indian Ocean.To date,two species are generally recognized:the narrow-ridged finless porpoise inhabiting temperate and subtropical waters,and wideridged finless porpoises inhabiting subtropical and tropical waters.Early research efforts focused on the temperate waters off the northern China.However,recent studies have proposed that the primary divergence within the genus may lie between the Indian Ocean and Pacific region,which remains to be tested with more samples from tropical and subtropical regions.Here,we examined the genetic relationship among the finless porpoises from the Gulf of Thailand to the Taiwan Strait using both mitochondrial and autosomal markers.Bayesian assignment analysis suggested a minimum of four genetic populations within the study areas,corresponding to the narrowridged finless porpoise from the Taiwan Strait(TWSn),and the three wide-ridged finless porpoise populations from the Taiwan Strait(TWSw),Pearl River Delta region(PRDw),and the Gulf of Thailand(Thaiw),respectively.The minimum spanning network of the mtDNA control region found shared haplotypes among finless porpoises in Chinese waters,but those from the Gulf of Thailand formed a unique matriline lineage.Consistently,the genetic differentiation or divergence within the South China Sea(Thaiw vs.PRDw)appears to be higher than that of most finless porpoise populations examined to date,and meets the threshold values of species or sub-species level proposed for the cetacean species.The Mantel test detected a strong correlation between the geographic and genetic matrices within the South China Sea(r>0.99,p<0.001),indicating that the divergence associated with isolation-by-distance(IBD)has been accumulating in recent history.Our results imply that the formation and maintenance of the spatial genetic pattern of the finless porpoise is more complex than previously thought.However,this cannot be addressed by the current taxonomic classification of the genus.

泰国湾西部表层沉积物重矿物分布特征：对物质来源和沉积环境的指示

对泰国湾西部159个表层沉积物进行了碎屑重矿物鉴定分析，其中57站重矿物颗粒数少于300粒，有效数据102站。结果表明，泰国湾西部表层沉积物中重矿物含量低，平均为1．628％，变化范围在0．016％～20．755％之间；共鉴定出重矿物29种，包括菱铁矿、褐铁矿、普通角闪石、自生黄铁矿、赤铁矿、绿帘石、磁铁矿、白钛石、锆石、紫苏辉石、石榴子石、黑云母、水黑云母、普通辉石、透闪石、白云母、钛铁矿、白云石、榍石、磷灰石、绢云母、蓝晶石、十字石、阳起石、电气石、金红石、棕闪石、独居石和透辉石，其中优势矿物为菱铁矿、普通角闪石和褐铁矿，平均含量（颗粒百分含量，下同）分别为41．6％，15．6％和13．4％，最高含量分别为99．7％，72％和94％；特征矿物为自生黄铁矿、磁铁矿、赤铁矿和锆石，平均含量分别为6．8％，4．1％，3．9％和0．9％，最高含量分别为97．3％，26．0％，91．7％和11．O％。利用数学聚类方法并结合矿物分布，在泰国湾西部划分出6个重矿物分区，基本以从北到南垂直岸线的东西向带状分布，矿物分区与物质来源和沉积环境具有较好的对应关系。重矿物来源主要为陆源和白生源：前者以普通角闪石、磁铁矿、赤铁矿、钛铁矿以及紫苏辉石等为代表性矿物，陆源物质输入较为明显的海区有3处，即曼谷湾的罗勇外部海区、春蓬和敦沙之间的近岸区域以及宋卡和北大年之间近岸区域，其中春蓬和敦沙之间的近岸区域表层沉积物中黑色金属矿物含量高，与附近有铁矿床分布相符合；后者以菱铁矿、自生黄铁矿以及菱铁矿风化形成的褐铁矿为代表性矿物，主要分布在陆架中南部、中东部和中北部。依据自生源矿物分布特征在陆架中部划分出不同的沉积环境，在陆架中南部和中北部为轻度还原环境，局部出现较强的还原环境，而在陆架中南部的轻度还原区域内出现氧化沉积环境。