Geochemical and isotopic evidence for upward flow of saline fluid to the Mississippi River Valley alluvial aquifer,southeastern Arkansas,USA

Hydrogeology Journal - Groundwater from the Quaternary Mississippi River Valley Alluvial (MRVA) aquifer in southeastern Arkansas (SE AR), USA, has higher salinity compared to other MRVA...     

Origin and extent of fresh groundwater, salty paleowaters and recent saltwater intrusions in Red River flood plain aquifers, Vietnam

A model has been established on the origin and extent of fresh groundwater, salty paleowaters and saltwater from recent seawater intrusions in the Red River flood plain in Vietnam. This was done with geological observations, geophysical borehole logging and transient electromagnetic methods. Salt paleowater is present up to 50–75?km from the coastline, with occurrence controlled by the Holocene transgression. A density-driven leaching of salty porewater has occurred from high-permeability Holocene sediments into underlying Pleistocene deposits, whereas diffusion has dominated in low-permeability layers. In the Pleistocene aquifer, the highest content of dissolved solids is found below two intrinsic valleys with Holocene marine sediments and along the coastline. Recent intrusion of saltwater from the South China Sea is observed in shallow groundwater 35?km inland, probably a result of transport of salty water inland in rivers or leaching of paleowaters from very young near-coast marine sediments. The observed inverted salinity profile, with high saline water overlying fresher groundwater, has been formed due to the global eustatic sea-level changes during the last 8,000–9,000?years. The proposed model may therefore be applicable to other coastal aquifers, with a proper incorporation of the local geological environments.     

A 10Be‐based reconstruction of the last deglaciation in southern Sweden

We present 23 cosmogenic surface exposure ages from 10 localities in southern Sweden. The new ¹⁰Be ages allow a direct correlation between the east and west coasts of southern Sweden, based on the same dating technique, and provide new information about the deglaciation of the Fennoscandian Ice Sheet in the circum‐Baltic area. In western Skåne, southernmost Sweden, a single cosmogenic surface exposure sample gave an age of 16.8±1.0 ka, whereas two samples from the central part of Skåne gave ages of 17.0±0.9 and 14.1±0.8 ka. Further northeast, in southern Småland, two localities gave ages ranging from 15.2±0.8 to 16.9±0.9 ka (n=5) indicating a somewhat earlier deglaciation of the area than has previously been suggested. Our third locality, in S Småland, gave ages ranging from 10.2±0.5 to 18.4±1.6 ka (n=3), which are probably not representative of the timing of deglaciation. In central Småland one locality was dated to 14.5±0.8 ka (n=3), whereas our northernmost locality, situated in northern Småland, was dated to 13.8±0.8 ka (n=3). Samples from the island of Gotland suggest deglaciation before 13 ka ago. We combined the new ¹⁰Be ages with previously published deglaciation ages to constrain the deglaciation chronology of southern Sweden. The combined deglaciation chronology suggests a rather steady deglaciation in southern Sweden starting at c. 17.9 cal. ka BP in NW Skåne and reaching northern Småland, ～200 km further north, c. 13.8 ka ago. Overall the new deglaciation ages agree reasonably well with existing deglaciation chronologies, but suggest a somewhat earlier deglaciation in Småland.     

Harpacticoid copepod response to epiphyte load variations in Posidonia oceanica (L.) Delile meadows

We conducted a field experiment to assess the response of phytal harpacticoids to nutrient‐driven increases of epiphyte load in Posidonia oceanica meadows. First, we evaluated differences in species richness, diversity and assemblage structure of phytal harpacticoids in P. oceanica meadows with differing epiphyte loads. Secondly, we conducted a field experiment where epiphyte load was increased through an in situ addition of nutrients to the water column and evaluated the responses of the harpacticoid assemblages. We predicted that there would be changes in the harpacticoid assemblages as a result of nutrient‐driven increases of epiphyte load, and that these changes would be of a larger magnitude in meadows of low epiphyte load. Our results show that the harpacticoid fauna (>500 μm) present in P. oceanica meadows in the Bay of Palma comprised taxa which are considered phytal and other less abundant ones previously described as sediment dwellers or commensal on other invertebrate species. Nutrient addition had an overall significant effect on epiphyte biomass and on harpacticoid abundance, diversity and assemblage structure, possibly as a response to the increased resources and habitat complexity provided by epiphytes. The abundance of dominant species at each location was favoured by nutrient addition and in some cases correlated with epiphytic biomass, although never strongly. This may indicate that structural complexity or diversity of the epiphytic cover might be more important than the actual epiphytic biomass for the harpacticoid species investigated. More species‐specific studies are necessary to ascertain this and clarify the relationships between harpacticoids and epiphytes in seagrass meadows. To our knowledge, this is the first account of harpacticoid species associated with P. oceanica leaves and the epiphytic community they harbour in the Mediterranean Sea.     

Impact melt rocks from the Late Paleocene Hiawatha impact structure,northwest Greenland

Impact melt rocks formed during hypervelocity impact events are ideal for studying impact structures. Here, we describe impact melt rock samples collected proximal to the 31 km wide 58 Ma Hiawatha impact structure, northwest Greenland, which is completely covered by the Greenland Ice Sheet. The melt rocks contain diagnostic shock indicators (e.g., planar deformation features [PDF] in quartz and shocked zircon) and form three groups based on melt textures and chemistry: (i) hypocrystalline, (ii) glassy, and (iii) carbonate-based melt rocks. The exposed foreland directly in front of the structure consists of metasedimentary successions and igneous plutons; however, the carbonate-based impactites indicate a mixed target sequence with a significant carbonate-rich component. Well-preserved organic material in some melt rocks indicates that North Greenland at the time of impact was host to abundant organic material, likely a dense high-latitude temperate forest. Geochemical signatures of platinum-group elements in selected samples indicate an extraterrestrial component and support previous identification of a highly fractionated iron impactor in glaciofluvial sand. Our results illustrate the possibility to study impact structures hidden beneath a thick ice sheet based on transported samples and this opens a new avenue for identifying other potential impact craters in Greenland and Antarctica.