Fossil Stomatocysts in Upper Cretaceous Sedimentary Pyrite from Central Mexico

In this paper six fossil forms of Chrysophycean stomatocysts are described. The material was collected from sedimentary pyrite embedded in well-laminated organic-rich limestone of the Upper Cretaceous Agua Nueva Formation at the locality of Xilitla, Central Mexico. The stomatocysts are represented by two spherical specimens with smooth surfaces lacking of ornamentation, three ovoid forms with rugose textures, one of them exposing presence of pore without collar, and one spherical specimen showing rugose texture and the presence of two short and rounded projections. The specimens here described showed affinity with some stomatocysts morphotypes described for brackish and fresh water, but not for known marine specimens. The presence of these microfossils in the Agua Nueva Formation represents the first formal description of fossil stomatocysts in Upper Cretaceous sedimentary pyrite in Mexico. The occurrence of both micro (planktonic foraminifera, calcispheres, radiolarians) and macrobiota (ammonites, inoceramid bivalves and fishes) and the presence of the specimens in sedimentary pyrite suggest that the stomatocysts were preserved under oxygen-deficiency conditions in a low energy environment. This event could have occurred in open marine waters in the Tampico-Misantla basin (Central Mexico) during the late Cenomanian throughout the early Turonian.

Natural and Anthropogenic Influence in Water Quality:The Case of Linares City,NE Mexico

The aim of this research paper was to identify whether the water quality had been influenced either by the natural environment or by anthropogenic activities or both in the municipality of Linares, the second largest city of the State of Nuevo Leon, NE Mexico. The superficial water (Pablillo River) and the groundwater quality (from a fractured and a porous aquifers hydraulically interconnected) were determined by comparing their chemical composition with maximum permissible limits for water consumption and irrigation use. A hydrogeochemical modeling was performed to identify the distribution of aqueous species responsible for the presence of some dissolved or precipitated mineral species, as well as an identification of geochemical factors responsible of superficial and groundwater quality. A canonical correspondence analysis was allowed to determine if the natural environment and/or anthropogenic activities were responsible for water quality. The parameters analysed in both aquifers, as well as in the Pablillo River, were total solids, suspended solids, nitrate, and chloride;barium and mercury were present in both aquifers. As a natural influence, the predominant mineral species are as following: under-saturated anhydrite (porous aquifer), over-saturated aragonite, calcite, dolomite, and gypsum (both aquifers and Pablillo River), barite and whiterite (only in fractured aquifer). The geochemical factors responsible for natural contamination were rock dominance (fractured aquifer), and evaporation dominance (porous aquifer, Pablillo River). On the other hand, anthropogenic activities such as changes in soil use and the presence of point (old municipal landfill, pig farms, barite deposit), and diffuse (agricultural areas, septic tanks and latrines) pollution sources had influence in the presence of contaminants such as total and fecal coliforms, nitrates, chlorides, mercury and barium. Several control and remediation strategies should be taken into account to prevent this pollution in the future.

Textural and geochemical characteristics of late Pleistocene to Holocene fine-grained deep-sea sediment cores(GM6 and GM7),recovered from southwestern Gulf of Mexico

Texture, mineralogy, geochemistry, and 14C ages of two deep-sea sediment cores （GM6 and GM7） recovered in the southwestern Gulf of Mexico were investigated to infer their provenance and depositional condition. The sediments are enriched in fine-grained silt and clay and poor in sand content. Mineralogically, the sediments consist of quartz, calcite, smectite, and kaolinite. Based on the ]4C data, the age of the GM6 and GM7 sediment cores were calculated as 23,615 cal yrs. BP and 19,007 cal yrs. BP, respectively. The weathering indices such as chemical index of alteration （CIA）, chemical index of weathering （CIW）, and plagioclase index of alteration （PIA） in GM6 （56-69, 60-74, and 6]-73, respectively） and GM7 （54-69, 57-76, and 55-74, respectively） cores revealed a moderate-to-high intensity of weathering. The rare earth element （REE） patterns suggested that the sediments were derived mostly by the weathering of intermediate rocks, exposed along the coastal regions of the Gulf of Mexico. The trace elemental ratios like V/Cr （〈2）, Ni/Co （〈2）, and Cu/Zn （〈1）, authigenic uranium content （〈1）, and Ce anomaly （Ce/Ce＊ ≤1） suggested that the sediments were deposited under an oxic condition, which was also revealed by the benthic foraminiferal assemblages throughout the GM6 and GM7 sediment cores.