Geochemistry of organic matter and elements of black shale during weathering in Northern Guizhou,Southwestern China: Their mobilization and inter-connection

Different from previous studies on effect of weathering upon geochemical variation along a single weathered profile, this paper provides a new methodology validated by comparing a weathered outcrop samples and their stratigraphic counterpart un-weathered core samples in a nearby shallow borehole. This outcrop and borehole penetrated the Ordovician-Silurian Wufeng–Longmaxi shales, located in the same anticline structure in the northern part of Guizhou Province, Southern China. The mineral composition, major, trace and rare earth elements (REEs) composition and Rock-Eval parameters of outcrop and core samples were analyzed and compared. Organic matter (OM) was observed in the microscope and extracted for elements analysis. The results show that short-term weathering still has significant influence on OM, mineral and elemental composition of black shales. The elements composition shows the outcrop profile was moderately weathered. The REEs compositions do not alter much during weathering process and the REEs composition and their relative ratios still are valid for rock origin determination. The OM, mainly composed by graptolite and bitumen, even entering the highly-over thermal maturity, is still sensitive to the weathering with a systematic loss 30–50% of TOC along the outcrop profile, which suggests that the OM consumption is predominantly controlled by weathering duration and the distance from the weathering surface. In turn, OM has significant influence on the trace elements transportation behavior during weathering. Some trace elements associated with the OM such as V, Cr, Th, U, Ni and Co, change significantly in their absolute concentration during weathering, but their relative ratios do not necessarily change too much and might be still reliable proxies for paleo-environmental determination. The mobility of shale minerals during weathering is in the following order: plagioclase?>?potassium feldspar and dolomite >pyrite and OM. Short-term weathering can also result in considerable transportation of elements and significant variation of minerals content in black shale, which may pose potentially high environmental and engineering risk in the regions rich in black shale.     

有机质热演化过程中地层压力的作用与影响

压力对有机质演化的影响长期以来争论不断,共有3种不同意见：压力对有机质演化没有影响,压力促进有机质的演化,压力抑制有机质的演化。通过对前人所做实验的实验设备、实验样品及实验结果的分析对比发现：不同实验结果主要是由于不同实验条件引起的,特别是实验系统和加压方式的不同,对实验结果影响最大,所代表的地质意义也不同。综合分析结果表明,当压力的增加表现为沉积有机质受到的有效应力增加时可以促进有机质生烃;当压力的增加表现为孔隙流体压力增加时,有机质生烃过程将受到抑制;另外,这两种情况都可能使干酪根的镜质组反射率降低,易使人得出压力增加抑制有机质演化的结论。进一步对压力影响有机质演化的成因机制进行了分析,发现在有机质演化的不同阶段因异常压力的增压机制不同,压力在有机质演化过程中所表现出来的作用也有差异。     

近200年来黑河下游天鹅湖湖泊沉积记录的环境变迁

根据2002-2004年洞庭湖水质监测数据,参照GB3838-2002中Ⅲ类水质标准,选用内梅罗水污染指数法和黄浦江污染指数对洞庭湖水质现状进行评价,结果表明:(1)洞庭湖水体的主要污染指标是总磷,总氮和粪大肠菌群;(2)黄浦江污染指数平均值为0．27,所以洞庭湖12个断面水质无黑臭现象发生;(3)枯水期西洞庭湖和南洞庭湖水质污染最严重,平水期西洞庭湖水质污染最严重,洞庭湖丰水期的污染程度小于平水期;(4)洞庭湖的大部分水体的水质主要处于轻度污染的状态,局部水体的水质在枯水期达到重污染的状态．     

柴达木盆地第三系咸水湖相生油岩古沉积环境地球化学特征

在系统分析柴达木盆地西部第三系咸水湖相生油岩样品的微量元素、有机物、粘土矿物组成的基础上,揭示了研究区生油岩的无机元素和有机质组成特征;并结合沉积环境的无机与有机地球化学指标,重建了生油岩沉积时的古沉积环境。生油岩的B、Cl-及伽玛蜡烷含量表明,沉积时的水介质条件是半咸水—咸水环境,并伴随湖盆沉积中心北迁过程各层位地层的盐度发生规律性时空]化;生油岩的Fe2 /Fe3 、S2-、Pr/Ph比值、藿烷碳数分布模式及黄铁矿含量反映其沉积时的沉积环境为强还原性;粘土矿物组合特征及含量揭示这些生油岩的沉积相带介于中心咸湖相和边缘咸湖相之间。     

南极半岛邻近海域夏季POC分布特征及其影响因素

海水中的颗粒有机碳(POC)与生物的生命过程、初级生产力关系密切,是海洋食物链中重要的物质基础和能量来源,因此POC的分布特征可以有效反映其生物地球化学环境。利用中国第33次南极考察期间(2016年12月至2017年1月)在南极半岛邻近海域采集的海水颗粒物样品,研究POC的空间分布特征及其影响因素。结果表明,斯科舍海0—200 m的POC浓度范围为7.44—193.52μg·L~(-1),平均浓度为(48.84±35.09)μg·L~(-1);南斯科舍海岭0—200 m的POC浓度范围为9.13—62.17μg·L~(-1),平均浓度为(29.76±14.12)μg·L~(-1);鲍威尔海盆0—200 m的POC浓度范围为5.87—270.72μg·L~(-1),平均浓度为(48.57±38.92)μg·L~(-1)。表层POC高值出现在斯科舍海区和鲍威尔海盆区,而低值出现在海岭区,与叶绿素a(Chla)的变化趋势一致,与营养盐的变化趋势相反。垂向分布上,各个区域POC平均浓度随深度的增加而减少,鲍威尔海盆和斯科舍海POC最高值都出现在25 m层。分析结果表明光合浮游植物是研究海域POC的主要来源, POC的主要影响因素为温度、水团混合以及海冰环境。斯科舍海与鲍威尔海盆整体非生命POC占比高,可能是由于高磷虾生物量、海冰碎屑以及陆源输入的干扰;南斯科舍海岭整体非生命POC占比低。     

Organic matter in deepwater sediments of the Northern Gulf of Mexico and its relationship to the distribution of benthic organisms

Sediment samples were selected from 28 sites across the deep (212–3527 m) northern Gulf of Mexico (GOM) as part of the Deep Gulf of Mexico Benthos (DGoMB) program, and analyzed for geochemical parameters related to organic carbon (OC) distribution and characteristics. The results of this study indicate that the OC content of sediments in the deep northern GOM is controlled by several factors; including water depth, overlying water productivity, sediment carbonate content, sediment oxygen exposure time, OC sources, and regional influences. The best correlation between sediment OC content, on a CaCO₃-free basis, and other parameters examined was an inverse correlation of OC with water depth. The OC/SA ratio had a wide range of values and, along with variable sources of sedimentary OC, indicated that the organism-available concentration of metabolizable organic matter may not be simply related to sedimentary OC content. This was perhaps reflected in the observation that benthic macrofaunal and meiofaunal biomass abundances were well correlated with sedimentary OC, but the abundance of bacteria in sediments was not.     

Macrofaunal density and biomass in the Campeche Canyon, Southwestern Gulf of Mexico

The composition, density and community structure of the benthic macrofauna were investigated in sediments of the Campeche Canyon in the SW Gulf of Mexico. Total macrofaunal density ranged from 9466±2736 ind m⁻² at the continental shelf station to 1550±195 ind m⁻² in the canyon. Density values significantly diminished with distance from the coast and depth; only a few stations in the center of the canyon displayed larger density values (E-37 with 4666±1530 ind m⁻², E-36 with 5791±642 ind m⁻² and E-26 with 6925±2258 ind m⁻²). Densities were positively correlated to organic nitrogen in the sediment (r=0.82) and coarse silt (r=0.43), and negatively with depth (r=−0.74) and distance from the coast (r=−0.68). At all stations, the polychaete worms contributed most to the multi-species community structure. The nematodes and Foraminifera displayed their highest densities in the center of the canyon. The biomass values declined significantly with depth. We conclude that the macrofauna density and biomass changed in response to organic matter contents in the sediment, both with distance from the coast and with depth.     

Methylmercury production in sediments of Chesapeake Bay and the mid-Atlantic continental margin

Methylmercury (MeHg) concentration and production rates were studied in bottom sediments along the mainstem of Chesapeake Bay and on the adjoining continental shelf and slope. Our objectives were to 1) observe spatial and temporal changes in total mercury (HgT) and MeHg concentrations in the mid-Atlantic coastal region, 2) investigate biogeochemical factors that affect MeHg production, and 3) examine the potential of these sediments as sources of MeHg to coastal and open waters. Estuarine, shelf and slope sediments contained on average 0.5 to 1.5% Hg as MeHg (% MeHg), which increased significantly with salinity across our study site, with weak seasonal trends. Methylation rate constants (k_meth), estimated using enriched stable mercury isotope spikes to intact cores, showed a similar, but weaker, salinity trend, but strong seasonality, and was highly correlated with % MeHg. Together, these patterns suggest that some fraction of MeHg is preserved thru seasons, as found by others [Orihel, D.M., Paterson, M.J., Blanchfield, P.J., Bodaly, R.A., Gilmour, C.C., Hintelmann, H., 2008. Temporal changes in the distribution, methylation, and bioaccumulation of newly deposited mercury in an aquatic ecosystem. Environmental Pollution 154, 77] Similar to other ecosystems, methylation was most favored in sediment depth horizons where sulfate was available, but sulfide concentrations were low (between 0.1 and 10 μM). MeHg production was maximal at the sediment surface in the organic sediments of the upper and mid Bay where oxygen penetration was small, but was found at increasingly deeper depths, and across a wider vertical range, as salinity increased, where oxygen penetration was deeper. Vertical trends in MeHg production mirrored the deeper, vertically expanded redox boundary layers in these offshore sediments. The organic content of the sediments had a strong impact on the sediment:water partitioning of Hg, and therefore, on methylation rates. However, the HgT distribution coefficient (K_D) normalized to organic matter varied by more than an order of magnitude across the study area, suggesting an important role of organic matter quality in Hg sequestration. We hypothesize that the lower sulfur content organic matter of shelf and slope sediments has a lower binding capacity for Hg resulting in higher MeHg production, relative to sediments in the estuary. Substantially higher MeHg concentrations in pore water relative to the water column indicate all sites are sources of MeHg to the water column throughout the seasons studied. Calculated diffusional fluxes for MeHg averaged  1 pmol m^− 2 day^− 1. It is likely that the total MeHg flux in sediments of the lower Bay and continental margin are significantly higher than their estimated diffusive fluxes due to enhanced MeHg mobilization by biological and/or physical processes. Our flux estimates across the full salinity gradient of Chesapeake Bay and its adjacent slope and shelf strongly suggest that the flux from coastal sediments is of the same order as other sources and contributes substantially to the coastal MeHg budget.     

Trace element and molecular markers of organic carbon dynamics along a shelf–basin continuum in sediments of the western Arctic Ocean

Molecular organic biomarkers together with trace element composition were investigated in sediments east of Barrow Canyon in the western Arctic Ocean to determine sources and recycling of organic carbon in a continuum from the shelf to the basin. Algal biomarkers (polyunsaturated and short-chain saturated fatty acids, 24-methylcholesta-5,24(28)-dien-3β-ol, dinosterol) highlight the substantial contribution of organic matter from water column and sea-ice primary productivity in shelf environments, while redox markers such as acid volatile sulfide (AVS), Mn, and Re indicate intense metabolism of this material leading to sediment anoxia. Shelf sediments also receive considerable inputs from terrestrial organic carbon, with biomarker composition suggesting the presence of multiple pools of terrestrial organic matter segregated by age/lability or hydrodynamic sorting. Sedimentary metabolism was not as intense in slope sediments as on the shelf; however, sufficient labile organic matter is present to create suboxic and anoxic conditions, at least intermittently, as organic matter is focused towards the slope. Basin sediments also showed evidence for episodic delivery of labile organic carbon inputs despite the strong physical controls of water depth and sea-ice cover. Principal components analysis of the lipid biomarker data was used to estimate fractions of preserved recalcitrant (of terrestrial origin) and labile (of marine origin) organic matter in the sediments, with ranges of 12–79%, 14–45%, and 37–66% found for the shelf, slope, and basin cores, respectively. On average, the relative preserved terrestrial organic matter in basin sediments was 56%, suggesting exchange of organic carbon between nearshore and basin environments in the western Arctic.