Murray K. Gingras

The nature of Miocene Amazonian epicontinental embayment: High-frequency shifts of the low-gradient coastline

A sedimentological and ichnological data set that covers the Lower-Upper Miocene sedimentary series of western Amazonian foreland basin indicates that widespread, restricted marine ingressions shaped western Amazonia throughout the Miocene. The late Lower–early Upper Miocene sedimentary series (Pebas Formation) consists of stacked, 3- to 10-m-thick, tidally influenced, brackish to freshwater, bay-margin sequences. The overlying Upper Miocene (“post-Pebas”) strata bear tidally influenced, low-salinity, channel deposits that are interbedded with continental deposits. The data suggest that several tens of high-frequency ingressions reached the basin during the Miocene. The ingressions were shallow and restricted, and were interspersed with rapid progradation. Along with the prograding shorelines, the continental environments—swamps, lagoons, floodplains and forests—constrained the extent of the marginal marine embayment. Consequently, the Miocene marginal marine and continental strata are closely interbedded throughout the basin. These results refine the recent depositional models for Miocene Amazonia, and challenge the theory that marine ingressions shaped the area only during one brief time interval (late Middle-early Late Miocene) during the epoch. Much of recent literature has documented fossils of mangrove pollen, brackish-euryhaline fish and brackish-water ostracods, brackish-water trace fossil assemblages, and tidal deposits from various Miocene stratigraphic levels. Commonly, these data sets are collected from the same outcrops as those for which data sets imply freshwater conditions. We propose that these seemingly contrasting data sets can be unified, if the repetitive nature of the ingressions is considered, and all the paleoenvironmental data are presented in a detailed lithological and stratigraphical context.

SIGNIFICANCE OF ATLANTIC STURGEON FEEDING EXCAVATIONS, MARY'S POINT, BAY OF FUNDY, NEW BRUNSWICK, CANADA

Abstract In this study we report on the occurrence and potential significance of Atlantic sturgeon (Acipenser oxyrhynchus) feeding traces observed in the Bay of Fundy in great abundance on the intertidal mud flats of Marys Point, New Brunswick, Canada. The traces comprise a crescent-shaped impression and a plug-shaped excavation and are considered to be a modern analogue for the trace fossil Piscichnus. Local areas exhibit relatively great numbers of the feeding structure: the sediment in these zones contains bivalves (primarily Macoma balthica), worms (generally nereid polychaetes), and amphipods (Corophium volutator). Analysis of the feeding-trace distribution and orientation shows that activity is greatest within 500 m of mean high water and coincides with the highest population densities of amphipods (up to 30,000 individuals per m2). Where sturgeon feeding is most intense, voluminous quantities of clay and silt are redistributed. Within the study area, as much as 1,220 m3 of intertidal sediment is resuspended during the 6 summer weeks that mark peak sturgeon activity. The reworked sediment contributes to the extensive soupy substrate, which accumulates from suspension deposition of silts and minor amounts of clay during slack tide. Subsequent to their excavation, feeding depressions trap sediment. Thus, feeding by the Atlantic sturgeon locally represents an important erosional-depositional agent in the intertidal mud flat zone within Marys Point.

Wave-generated tidal bundles as an indicator of wave-dominated tidal flats

We appreciate the Comment by [Mazumder (2008)][1] on our recent paper ([Yang et al., 2008][2]), and hope that he will be a part of continuing discussion and study on the subject. Mazumders main point is that the ratio between wave components and tidal currents plays an important role in the

WINTERING CHIRONOMIDS MINE OXYGEN

Abstract Seasonally sampled cores of burrowed sediment containing chironomid larvae were collected from Cooking Lake, Alberta, and analyzed to (1) assess and establish the typical burrowing behavior and burrow architecture of chironomid larvae; (2) record micrometer-scale geochemical profiles of O2, H2S, and pH in the uppermost sedimentary layers throughout a seasonal cycle; and (3) link changing geochemical conditions to changing burrowing behaviors. We observed that the larvae lived in soft, water-saturated sediment, maintained by open burrows accreted by the animals mucous. Chironomid-larvae burrows were small and Y-shaped (e.g., Polykladichnus-like) or Y-shaped with basal branches (Thalassinoides-like) and were 20 cm deep. The larvae moved up and down from the oxygenated zone (“sounding” behavior) to exploit food in suboxic and anoxic sediment. Geochemical analyses showed that H2S was present in the pore waters to within 1.5 mm of the sediment-water interface during the summer, when lake-bottom algae and cyanobacteria generated sufficient O2 to drive the oxic-anoxic redoxcline into the sediment. In the winter, the H2S front extended upward into the water column owing to the cessation of algal and cyanobacterial activity. The prevalence of H2S results from a combination of high-dissolved-sulfate concentrations in the lake and the abundance of microbial biomass that fuels an active subsurface population of sulfate-reducing bacteria. Interestingly, burrowing behavior was not linked to seasonal changes in the sediment chemistry. This is in part due to the ability of chironomid larvae to exploit oxygen islands in the sediment: in the winter, the chironomid larvae harvest their oxygen from the uppermost photosynthetic layer in otherwise O2-impoverished sediments.

LINKING GEOMICROBIOLOGY WITH ICHNOLOGY IN MARINE SEDIMENTS

In the last few years, the wealth of new information on microbial diversity, metabolic capabilities, and environmental constraints has led to significant insights into the ways in which microbes interact with their local environments. Indeed, microbes are integral to mineral dissolution, sorption and precipitation reactions, aqueous redox processes, and, ultimately, global elemental cycles. In this regard, they have helped shape our planet over the last 4 billion years and made it habitable for higher forms of life. The role of microbial communities in driving the major sedimentary diagenetic reactions is one aspect of geomicrobiology that has received significant attention. Through various chemoheterotrophic pathways, microorganisms are ultimately responsible for the conversion of organic carbon to CO more labile materials are degraded in the shallow subsurface on time scales of days to years; more refractory materials are broken down deeper in the sediment on time scales of hundreds to thousands of years, while the most resistant materials, precursors to fossil fuels, are transformed only on time scales of millions of years. Pore-water and mineralogical changes during diagenesis are also directly related to the bacterial reduction of aqueous species (O 2 ,N O3 ,S O4 2 ,C O 2) or metal oxyhydroxides in the sediment. The terminal electron-accepting process that occurs at any given depth depends on which oxidants are available and, in situations in which multiple electron acceptors are present, as in the uppermost sediment layers, on the free energy yield of the specific reaction. The decomposition of freshly deposited organic material thus proceeds in a continuous sequence of redox reactions, with the most electropositive oxidants being consumed at or near the surface and progressively poorer oxidants being consumed at depth, until the labile organic fraction is