Extent Of Bioturbation Research Articles

Covariation between molybdenum and uranium isotopes in reducing marine sediments

The isotopic compositions of redox-sensitive elements such as uranium (U) and molybdenum (Mo) have been extensively used to track the evolution of redox conditions on Earth's surface. However, additional investigation of the behaviors of U and Mo in modern marine sediments—particularly in reducing sediments underlying oxic seawater—can bolster understanding and improve application of these systems as biogeochemical tracers. Here we present elemental and isotopic Mo and U data from reducing sediments from various sites in the Bering Sea where the extent of bioturbation is well characterized. Total Mo concentrations and total organic carbon contents (TOC) are positively correlated, as are authigenic Mo isotopic compositions and TOC, indicating the presence of authigenic Mo enrichments in these sediments. Similar to what has previously been observed in euxinic settings, we observe a positive correlation between authigenic U and Mo concentrations, and a negative correlation between the isotopic compositions of authigenic U and Mo (δ238Uauth and δ98Moauth). These patterns likely reflect close associations of U and Mo reduction to organic carbon degradation. Observed variation in δ98Moauth values could reflect variations in Mo removal efficiency or Mo isotopic fractionation between particles and seawater related to porewater H2S levels. Conversely, we interpret variation in δ238Uauth values in these Bering Sea samples to be related to varying extents of U reduction and removal, coupled with anaerobic carbon degradation. There is no significant correlation between the extent of bioturbation and Mo or U isotope values, suggesting that bioturbation would likely not have a significant influence on sedimentary Mo and U records. Further, the array of U and Mo isotopic data from these sediments is distinct from those observed in anoxic sediments below euxinic seawater—suggesting that a combination of U and Mo isotopic compositions could help to more accurately resolve the redox state of depositional environments from past intervals of Earth's history.

Evidence of changes in sedimentation rate and sediment fabric in a low-oxygen setting: Santa Monica Basin, CA

Abstract. The Southern California Bight is adjacent to one of the world's largest urban areas, Los Angeles. As a consequence, anthropogenic impacts could disrupt local marine ecosystems due to municipal and industrial waste discharge, pollution, flood control measures, and global warming. Santa Monica Basin (SMB), due to its unique setting in a low-oxygen and high-sedimentation environment, can provide an excellent sedimentary paleorecord of these anthropogenic changes. This study examined 10 sediment cores, collected from different parts of the SMB between spring and summer 2016, and compared them to existing cores in order to document changes in sedimentary dynamics during the last 250 years, with an emphasis on the last 40 years. The 210Pb-based mass accumulation rates (MARs) for the deepest and lowest oxygen-containing parts of the SMB basin (900–910 m) have been remarkably consistent during the past century, averaging 17.1±0.6 mg cm−2 yr−1. At slightly shallower sites (870–900 m), accumulation rates showed more variation but yield the same accumulation rate, 17.9±1.9 mg cm−2 yr−1. Excess 210Pb sedimentation rates were consistent with rates established using bomb test 137Cs profiles. We also examined 14C profiles from two cores collected in the deepest part of the SMB, where fine laminations are present up to about 450 yr BP. These data indicate that the MAR was slower prior to ∼1900 CE (rates obtained were 9 and 12 mg cm−2 yr−1). The δ13Corg profiles show a relatively constant value where laminations are present, suggesting that the change in sediment accumulation rate is not accompanied by a change in organic carbon sources to the basin. The increase in sedimentation rate towards the Recent occurs at about the time previous studies predicted an increase in siltation and the demise of a shelly shelf benthic fauna on the SMB shelf. X-radiographs show finely laminated sediments in the deepest part of the basin only, with centimeter-scale layering of sediments or no layering whatsoever in shallower parts of the SMB basin. The absence of finely laminated sediments in cores MUC 10 (893 m) and MUC 3 (777 m) suggests that the rate at which anoxia is spreading has not increased appreciably since cores were last analyzed in the 1980s. Based on core top data collected during the past half century, sedimentary dynamics within SMB have changed minimally during the last 40 years. Specifically, mass accumulation rates, laminated sediment fabric, extent of bioturbation and % Corg have not changed. The only parameter that appeared to have changed in the last 450 years was the MAR, with an apparent > 50 % increase occurring between ∼1850 CE and the early 1900s. The post-1900 CE constancy of sedimentation through a period of massive urbanization in Los Angeles is surprising.

Phanerozoic shallow marine sole marks and substrate evolution

Research Article| August 01, 2018 Phanerozoic shallow marine sole marks and substrate evolution Lidya G. Tarhan Lidya G. Tarhan * 1Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA *E-mail: lidya.tarhan@yale.edu Search for other works by this author on: GSW Google Scholar Geology (2018) 46 (9): 755–758. https://doi.org/10.1130/G45055.1 Article history received: 19 Apr 2018 rev-recd: 29 Jun 2018 accepted: 04 Jul 2018 first online: 01 Aug 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation Lidya G. Tarhan; Phanerozoic shallow marine sole marks and substrate evolution. Geology 2018;; 46 (9): 755–758. doi: https://doi.org/10.1130/G45055.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyGeology Search Advanced Search Abstract The presence of erosive sole structures such as tool and flute marks in the sedimentary record can be used to interpret flow dynamics of ancient storm-dominated settings, as well as seafloor sediment properties. The formation and preservation of these structures require a hydroplastic and cohesive substrate. Therefore, the temporal distribution of these structures can be employed to track secular changes in seafloor rheology, which can significantly alter early diagenetic processes and thus shape global biogeochemical cycling. Herein I present a global compilation of Phanerozoic shallow marine successions bearing erosive sole structures, derived from assessment of the sedimentological, tectonic, and paleontological literature. The frequency of sole mark preservation in shallow marine environments has dramatically declined through the Phanerozoic. This first-order trend suggests that the rheological nature of the seafloor has experienced major secular transformations over the past 541 m.y. This record provides evidence for a marked decrease in sediment cohesion and increase in sediment porosity of the uppermost seafloor sediment column through time. I attribute this change in seafloor sedimentary properties to increases in the extent of bioturbation through the Phanerozoic, because the activity of sediment-mixing infauna dramatically increases the fluid content of the shallow sediment pile, decreasing the hydroplasticity necessary for sole mark formation and preservation. The gradual decline in the frequency of abiogenic sole marks in shallow marine successions through the Phanerozoic indicates that the development of bioturbation has been a similarly protracted process. You do not currently have access to this article.

Comparison of Large and Ultra-Small Δ14C Measurements in Core Top Benthic Foraminifera from the Okhotsk Sea

The radiocarbon activity of benthic foraminifera was investigated in surface sediments from a high deposition rate location at a depth of 1000 m in the Okhotsk Sea. Sediments were preserved and stained with Rose Bengal to identify foraminifera that contain cytoplasm. The benthic fauna at this site is dominated by large specimens of Uvigerina peregrina, and bulk samples (∼150 individuals) of stained and unstained specimens were dated. The stained sample was about 240 14C yr younger than the unstained, and the presence of bomb 14C is inferred by comparison to water column data in the nearby open North Pacific. Using new methods, multiple measurements were also made on samples of three stained and unstained individuals (as small as 7 μg C). Results are consistent with those from the bulk samples. This suggests that similar ultra-small measurements could be made at other locations to reveal the age distribution of individuals in a sediment sample in order to assess the extent of bioturbation and the presence of bomb 14C contamination.

Burgess shale−type biotas were not entirely burrowed away

Research Article| March 01, 2012 Burgess shale−type biotas were not entirely burrowed away Robert R. Gaines; Robert R. Gaines 1Geology Department, Pomona College, 185 E. Sixth Street, Claremont, California 91711, USA Search for other works by this author on: GSW Google Scholar Mary L. Droser; Mary L. Droser 2Department of Earth Sciences, University of California−Riverside, Riverside, California 92521, USA Search for other works by this author on: GSW Google Scholar Patrick J. Orr; Patrick J. Orr 3UCD School of Geological Sciences, University College Dublin, Dublin 4, Ireland Search for other works by this author on: GSW Google Scholar Daniel Garson; Daniel Garson 2Department of Earth Sciences, University of California−Riverside, Riverside, California 92521, USA Search for other works by this author on: GSW Google Scholar Emma Hammarlund; Emma Hammarlund 4Nordic Center for Earth Evolution, DK-5230 Odense M, Denmark5Department of Palaeozoology, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden Search for other works by this author on: GSW Google Scholar Changshi Qi; Changshi Qi 6Yunnan Key Laboratory for Palaeobiology, Yunnan University, Kunming 650091, China Search for other works by this author on: GSW Google Scholar Donald E. Canfield Donald E. Canfield 4Nordic Center for Earth Evolution, DK-5230 Odense M, Denmark Search for other works by this author on: GSW Google Scholar Author and Article Information Robert R. Gaines 1Geology Department, Pomona College, 185 E. Sixth Street, Claremont, California 91711, USA Mary L. Droser 2Department of Earth Sciences, University of California−Riverside, Riverside, California 92521, USA Patrick J. Orr 3UCD School of Geological Sciences, University College Dublin, Dublin 4, Ireland Daniel Garson 2Department of Earth Sciences, University of California−Riverside, Riverside, California 92521, USA Emma Hammarlund 4Nordic Center for Earth Evolution, DK-5230 Odense M, Denmark5Department of Palaeozoology, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden Changshi Qi 6Yunnan Key Laboratory for Palaeobiology, Yunnan University, Kunming 650091, China Donald E. Canfield 4Nordic Center for Earth Evolution, DK-5230 Odense M, Denmark Publisher: Geological Society of America Received: 14 Jun 2011 Revision Received: 24 Oct 2011 Accepted: 27 Oct 2011 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2012 Geological Society of America Geology (2012) 40 (3): 283–286. https://doi.org/10.1130/G32555.1 Article history Received: 14 Jun 2011 Revision Received: 24 Oct 2011 Accepted: 27 Oct 2011 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Robert R. Gaines, Mary L. Droser, Patrick J. Orr, Daniel Garson, Emma Hammarlund, Changshi Qi, Donald E. Canfield; Burgess shale−type biotas were not entirely burrowed away. Geology 2012;; 40 (3): 283–286. doi: https://doi.org/10.1130/G32555.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Burgess Shale−type biotas occur globally in the Cambrian record and offer unparalleled insight into the Cambrian explosion, the initial Phanerozoic radiation of the Metazoa. Deposits bearing exceptionally preserved soft-bodied fossils are unusually common in Cambrian strata; more than 40 are now known. The well-documented decline of soft-bodied preservation following the Middle Cambrian represents the closure of a taphonomic window that was only intermittently open in marine environments thereafter. The prevailing hypothesis for this secular shift in taphonomic conditions of outer shelf environments is that soft-bodied biotas were literally burrowed away from the fossil record by increasing infaunal activity in muddy substrate environments; this would have affected geochemical gradients and increased the efficiency of organic matter recycling in sediments. New and recently published data, however, suggest a more complex scenario. Ichnologic and microstratigraphic data from Burgess Shale−type deposits indicate that (1) bioturbation exerts a limiting effect on soft-bodied preservation; (2) the observed increase in the depth and extent of bioturbation following the Middle Cambrian would have restricted preservation of Burgess Shale−type biotas in a number of settings; but (3) increasing depth and extent of bioturbation would not have affected preservation in many other settings, including the most richly fossiliferous portions of the Chengjiang (China) deposit and the Greater Phyllopod Bed of the Burgess Shale (Canada). Therefore, increasing bioturbation cannot account for the apparent loss of this pathway from the fossil record, and requires that other circumstances, including, but not limited to, widespread benthic anoxia, facilitated widespread exceptional preservation in the Cambrian. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Temporal Distribution of Diagnostic Biofabrics in the Lower and Middle Ordovician in North China: Clues to the Geobiology of the Great Ordovician Biodiversification Event

Abstract:The temporal distribution of the diagnostic biofabrics in the Lower and Middle Ordovician in North China distinctly illustrates that the sedimentary systems on the paleoplate have been changed markedly as consequences of the Great Ordovician Biodiversification Event (GOBE). The pre‐GOBE sedimentary systems deposited in Tremadoc display widespread microbialite and flat‐pebble conglomerates, and a less extent of bioturbation. Through the transitional period of early Floian, the sedimentary systems in the rest of the Early and Mid‐ Ordovician change to GOBE type and are characterized by intensive bioturbation and vanishing flat‐pebble conglomerates and subtidal microbial sediments. The irreversible changes in sedimentary systems in North China are linked to the GOBE, which conduced the increase in infaunal tiering, the expansion of infaunal ecospace, and the appearance of new burrowers related to the development of the Paleozoic Evolutionary Fauna during the Ordovician biodiversification. Thus, changes in sedimentary systems during the pivotal period of the GOBE were consequences of a steep diversification of benthic faunas rather than the GOBE's environmental background.

Benthic biological and biogeochemical patterns and processes across an oxygen minimum zone (Pakistan margin, NE Arabian Sea)

Abstract Oxygen minimum zones (OMZs) impinging on continental margins present sharp gradients ideal for testing environmental factors thought to influence C cycling and other benthic processes, and for identifying the roles that biota play in these processes. Here we introduce the objectives and initial results of a multinational research program designed to address the influences of water depth, the OMZ (∼150–1300 m), and organic matter (OM) availability on benthic communities and processes across the Pakistan Margin of the Arabian Sea. Hydrologic, sediment, and faunal characterizations were combined with in-situ and shipboard experiments to quantify and compare biogeochemical processes and fluxes, OM burial efficiency, and the contributions of benthic communities, across the OMZ. In this introductory paper, we briefly review previous related work in the Arabian Sea, building the rationale for integrative biogeochemical and ecological process studies. This is followed by a summary of individual volume contributions and a brief synthesis of results. Five primary stations were studied, at 140, 300, 940, 1200 and 1850 m water depth, with sampling in March–May (intermonsoon) and August–October (late-to-postmonsoon) 2003. Taken together, the contributed papers demonstrate distinct cross-margin gradients, not only in oxygenation and sediment OM content, but in benthic community structure and function, including microbial processes, the extent of bioturbation, and faunal roles in C cycling. Hydrographic studies demonstrated changes in the intensity and extent of the OMZ during the SW monsoon, with a shoaling of the upper OMZ boundary that engulfed the previously oxygenated 140-m site. Oxygen profiling and microbial process rate determinations demonstrated dramatic differences in oxygen penetration and consumption across the margin, and in the relative importance of anaerobic processes, but surprisingly little seasonal change. A broad maximum in sediment OM content occurred on the upper slope, roughly coincident with the OMZ; but the otherwise poor correlation with bottom-water oxygen concentrations indicated that other factors are important in determining sediment OM distributions. Downcore profiles generally showed little clear evidence of in-situ OM alteration, and there was little sign of OM enrichment resulting from the southwest monsoon in sediments collected in the late-to-postmonsoon sampling. This is interpreted to be due to rapid cycling of labile OM. Organic geochemical studies confirmed that sediment OM is overwhelmingly of marine origin across the margin, but also that it is heavily altered, with only small changes in degradation state across the OMZ. More negative stable C isotopic compositions in surficial sediments at hypoxic sites within the OMZ core are attributed to a chemosynthetic bacterial imprint. Dramatic changes in benthic community structure occurred across the lower OMZ transition, apparently related to OM availability and quality as well as to DO concentrations. High-resolution sampling, biomarkers and isotope tracer studies revealed that oxygen availability appears to exert threshold-type controls on benthic community structure and early faunal C processing. Biomarker studies also provided evidence of faunal influence on sediment OM composition. Together, the results offer strong evidence that benthic fauna at sites across the margin play important roles in the early cycling of sediment OM through differential feeding and bioturbation activities.

Global patterns of bioturbation intensity and mixed depth of marine soft sediments

AB Aquatic Biology Contact the journal Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AB 2:207-218 (2008) - DOI: https://doi.org/10.3354/ab00052 Theme Section: Bioturbation in aquatic environments: linking past and present Global patterns of bioturbation intensity and mixed depth of marine soft sediments L. R. Teal1,2,*, M. T. Bulling1,3, E. R. Parker2, M. Solan1 1Oceanlab, University of Aberdeen, Main Street, Newburgh, Aberdeenshire AB41 6AA, UK 2CEFAS, Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK 3University of York, Environmental Department, Heslington, York YO10 5DD, UK *Email: l.teal@abdn.ac.uk ABSTRACT: The importance of bioturbation in mediating biogeochemical processes in the upper centimetres of oceanic sediments provides a compelling reason for wanting to quantify in situ rates of bioturbation. Whilst several approaches can be used for estimating the rate and extent of bioturbation, most often it is characterized by calculating an intensity coefficient (Db) and/or a mixed layer depth (L). Using measures of Db (n = 447) and L (n = 784) collated largely from peer-reviewed literature, we have assembled a global database and examined patterns of both L and Db. At the broadest level, this database reveals that there are considerable gaps in our knowledge of bioturbation for all major oceans other than the North Atlantic, and almost universally for the deep ocean. Similarly, there is an appreciable bias towards observations in the Northern Hemisphere, particularly along the coastal regions of North America and Europe. For the assembled dataset, we find large discrepancies in estimations of L and Db that reflect differences in boundary conditions and reaction properties of the methods used. Tracers with longer half-lives tend to give lower Db estimates and deeper mixing depths than tracers with shorter half-lives. Estimates of L based on sediment profile imaging are significantly lower than estimates based on tracer methods. Estimations of L, but not Db, differ between biogeographical realms at the global level and, at least for the Temperate Northern Atlantic realm, also at the regional level. There are significant effects of season irrespective of location, with higher activities (Db) observed during summer and deeper mixing depths (L) observed during autumn. Our evaluation demonstrates that we have reasonable estimates of bioturbation for only a limited set of conditions and regions of the world. For these data, and based on a conservative global mean (±SD) L of 5.75 ± 5.67 cm (n = 791), we calculate the global volume of bioturbated sediment to be >20700 km3. Whilst it is clear that the role of benthic invertebrates in mediating global ecosystem processes is substantial, the level of uncertainty at the regional level is unacceptably high for much of the globe. KEY WORDS: Bioturbation · Sediment mixed depth · Bioturbation coefficient · Global analysis · Tracer · Sediment profile imaging Full text in pdf format PreviousNextCite this article as: Teal LR, Bulling MT, Parker ER, Solan M (2008) Global patterns of bioturbation intensity and mixed depth of marine soft sediments. Aquat Biol 2:207-218. https://doi.org/10.3354/ab00052Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AB Vol. 2, No. 3. Online publication date: June 19, 2008 Print ISSN: 1864-7782; Online ISSN: 1864-7790 Copyright © 2008 Inter-Research.

The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy

Diagenesis exerts a strong control on the quality and heterogeneity of most clastic reservoirs. Variations in the distribution of diagenetic alterations usually accentuate the variations in depositional porosity and permeability. Linking the types and distribution of diagenetic processes to the depositional facies and sequence-stratigraphic framework of clastic successions provides a powerful tool to predict the distribution of diagenetic alterations controlling quality and heterogeneity. The heterogeneity patterns of sandstone reservoirs, which determine the volumes, flow rates, and recovery of hydrocarbons, are controlled by geometry and internal structures of sand bodies, grain size, sorting, degree of bioturbation, provenance, and by the types, volumes, and distribution of diagenetic alterations. Variations in the pathways of diagenetic evolution are linked to (1) depositional facies, hence pore-water chemistry, depositional porosity and permeability, types and amounts of intrabasinal grains, and extent of bioturbation; (2) detrital sand composition; (3) rate of deposition (controlling residence time of sediments at specific near-surface, geochemical conditions); and (4) burial thermal history of the basin. The amounts and types of intrabasinal grains are also controlled by changes in the relative sea level and, therefore, can be predicted in the context of sequence stratigraphy, particularly in paralic and shallow marine environments. Changes in the relative sea level exert significant control on the types and extent of near-surface shallow burial diagenetic alterations, which in turn influence the pathways of burial diagenetic and reservoir quality evolution of clastic reservoirs. Carbonate cementation is more extensive in transgressive systems tract (TST) sandstones, particularly below parasequence boundaries, transgressive surface , and maximum flooding surface because of the abundance of carbonate bioclasts and organic matter, bioturbation, and prolonged residence time of the sediments at and immediately below the sea floor caused by low sedimentation rates, which also enhance the formation of glaucony. Eogenetic grain-coating berthierine, odinite, and smectite, formed mostly in TST and early highstand systems tract deltaic and estuarine sandstones, are transformed into ferrous chlorite during mesodiagenesis, helping preserve reservoir quality through the inhibition of quartz cementation. The infiltration of grain-coating smectitic clays is more extensive in braided than in meandering fluvial sandstones, forming flow barriers in braided amalgamated reservoirs, and may either help preserve porosity during burial because of quartz overgrowth inhibition or reduce it by enhancing intergranular pressure dissolution. Diagenetic modifications along sequence boundaries are characterized by considerable dissolution and kaolinization of feldspars, micas, and mud intraclasts under wet and warm climates, whereas a semiarid climate may lead to the formation of calcrete dolocrete cemented layers. Turbidite sandstones are typically cemented by carbonate along the contacts with interbedded mudrocks or carbonate mudstones and marls, as well as along layers of concentration of carbonate bioclasts and intraclasts. Commonly, hybrid carbonate turbidite arenites are pervasively cemented. Proximal, massive turbidites normally show only scattered spherical or ovoid carbonate concretions. Improved geologic models based on the connections among diagenesis, depositional facies, and sequence-stratigraphic surfaces and intervals may not only contribute to optimized production through the design of appropriate simulation models for improved or enhanced oil recovery strategies, as well as for CO2 geologic sequestration, but also support more effective hydrocarbon exploration through reservoir quality prediction.

Interacting effects of Hydrobia ulvae bioturbation and microphytobenthos on the erodibility of mudflat sediments

Microphytobenthos-macrofauna sediment interactions and their effects on sediment erodability were examined in laboratory experiments. Sediment beds were manipulated in a tidal mesocosm to produce diatom mats in exponential or in stationary phases of development after 6, 8 or 11 d of culture. These sediment beds were used in flume experiments to investigate the influence of bioturbation by the gastropod Hydrobia ulvae on both sediment and pigment resuspension as a function of the physiological state of the microphytobenthic mats. In most experiments, only a surface layer was resuspended. A model was used to analyse in detail the contribution of each variable to this surface-layer erosion. Bioturbation was the major factor controlling resuspension, and its extent was influenced by sediment density and the growth stage of the microphytobenthos. The amount and extent of bioturbation is assumed to be influenced by sediment density and chlorophyll a content. Snail bioturbation can, in turn, influence the amount of microalgal resuspension. The quantity of pigment resuspended due to bioturbation increased by a factor of 15 when the diatom mats were in exponential growth stages. However, as the age of the mat increased, the influence of bioturbation on pigment resuspension declined. When the mats became senescent, Type I erosion occurred with erosion rates high enough to obscure any effects of bioturbation. To summarise, we assume that there are 2 causes of microphytobenthos resuspention, depending on the physiological state of the mat: (1) in the exponential phase, bioturbation substantially affects the resuspension of pigments which are present in the surface layer (the biogenic fluff layer) and (2) in the senescent phase, the increase in bed roughness and water content renders the mat fragile, leading to bed erosion.

Sedimentology and depositional history of the Claret Formation (‘carse clay’) at Bothkennar, near Grangemouth

Synopsis The estuarine sediments of Flandrian age which comprise the Claret Formation in the Bothkennar area can be divided into bedded, mottled and laminated facies on the basis of primary sedimentary structures, frequency of silty laminae and the nature and extent of bioturbation. The mottled facies can be further subdivided by the style of bioturbation. The facies are very similar in overall particle size distribution and mineralogy. Biogenic traces occur in all the facies and have been tentatively matched to organisms present in the modern infauna Equation ()of the Firth of Forth. The succession of facies has been controlled by the fall of relative sea-level since the Flandrian maximum and records an emergent sequence from sub-tidal (probable water depth around 20 m) to inter-tidal. In the Bothkennar area four local subdivisions can be recognized in the Claret Formation, which reflect changes in sedimentary environment consequent on the reduction in water depth.

Modeling Variability in 210Pb and Sediment Fluxes Near the Whites Point Outfalls, Palos Verdes Shelf, California

Many researchers have had difficulty interpreting sediment data collected from the Palos Verdes Shelf, southern California. Factors that have been difficult to reconcile include the distribution of 210Pb and metals, the depth and extent of bioturbation, and the rate of sedimentation. This paper presents a simple model that includes these elements and simulates the flux of 210Pb, sediment, and metals to the sea floor near the Whites Point wastewater outfalls. The model uses known particle and metals emission rates from the outfalls and 210Pb fluxes to the sediments that vary in proportion to the flux of sediment mass to the sea floor. Model-predicted metals and 210Pb concentration profiles in the sediments agree well with data from cores collected at three locations on the Palos Verdes Shelf between 1972 and 1997. The implication of the model results is that 210Pb fluxes to the sediments in this area have varied greatly over the past 60 years. The model suggests that subsurface 210Pb maxima and uniform 210Pb concentrations to depths within the sediments of roughly 30 cm have resulted from time-variable 210Pb fluxes to the sediments and relatively shallow bioturbation and that natural sedimentation rates are relatively high (roughly 500−1000 mg cm-2 yr-1, or about 0.7−1.3 cm yr-1 of unconsolidated sediment).

Trace fossil analysis of lacustrine facies and basins

Abstract Two ichnofacies are typical of lacustrine depositional systems. The Scoyenia ichnofacies characterizes transitional terrestrial/nonmarine aquatic substrates, periodically inundated or desiccated, and therefore is commonly present in lake margin facies. The Mermia ichnofacies is associated with well oxygenated, permanent subaqueous, fine-grained substrates of hydrologically open, perennial lakes. Bathymetric zonations within the Mermia ichnofacies are complicated by the wide variability of lacustrine systems. Detected proximal–distal trends are useful within particular lake basins, but commonly difficult to extrapolate to other lakes. Other potential ichnofacies include the typically marine Skolithos ichnofacies for high-energy zones of lakes and substrate-controlled, still unnamed ichnofacies, associated to lake margin deposits. Trace fossils are useful for sedimentologic analysis of event beds. Lacustrine turbidites are characterized by low-diversity suites, reflecting colonization by opportunistic organisms after the turbidite event. Underflow current beds record animal activity contemporaneous with nearly continuous sedimentation. Ichnologic studies may also help to distinguish between marine and lacustrine turbidites. Deep-marine turbidites host the Nereites ichnofacies that consists of high diversity of ornate grazing traces and graphoglyptids, recording highly specialized feeding strategies developed to solve the problem of the scarcity of food in the deep sea. Deep lacustrine environments contain the Mermia ichnofacies, which is dominated by unspecialized grazing and feeding traces probably related to the abundance and accessibility of food in lacustrine systems. The lower diversity of lacustrine ichnofaunas in comparison with deep-sea assemblages more likely reflects lower species diversity as a consequence of less stable conditions. Increase of depth and extent of bioturbation through geologic time produced a clear signature in the ichnofabric record of lacustrine facies. Paleozoic lacustrine ichnofaunas are typically dominated by surface trails with little associated bioturbation. During the Mesozoic, bioturbation depth was higher in lake margin facies than in fully lacustrine deposits. While significant degrees of bioturbation were attained in lake margin facies during the Triassic, major biogenic disruption of primary bedding in subaqueous lacustrine deposits did not occur until the Cretaceous.

Trends in depth and extent of bioturbation in Cambrian carbonate marine environments, western United States

Cambrian carbonate strata of the Great Basin deposited below fairweather wave base but above mean storm wave base (inner shelf), and below mean storm wave base but above maximum storm wave base (middle shelf) were examined for extent and depth of bioturbation. Data indicate that initial extensive colonization of marine infaunal habitats occurred between Tommotian and Atdabanian time. Throughout the remaining 65 m.y. of Cambrian time bioturbation increased, although depth of bioturbation remained relatively constant at less than 6 cm. During this time, the amount of bioturbation in the inner shelf was greater than that in the middle shelf.

An Integrated Geochemical and Paleoecological Approach to Petroleum Source Rock Evaluation, Lower Niobrara Formation (Cretaceous), Lyons, Colorado

A detailed study of paleoecological, geochemical, and stable isotopic properties of the lower Niobrara Formation (upper Turonian to lower Coniacian) was undertaken in order to evaluate petroleum source rock potential and to gain an understanding of the processes affecting variation in organic carbon content. The highest organic carbon contents In the lower Niobrara Formation occur in the lower shale unit of the Smoky Hill Shale Membel Trends in extent of bioturbation, organic carbon contents, and oxygen isotopic ratios of carbonates suggest that paleoclimatic factors influenced bottom water environments during deposition of this unit. A shift toward a more negative oxygen isotopic ratio in the lower shale unit is interpreted to be a result of decreased surface water salinity due to increased fresh water input and possibly to climatic warming. Resultant stratification of the water column limited benthic oxygenation thereby limiting benthic activity, enhancing the preservation of marine organic matter, and increasing source rock potential for petroleum. Data from underlying and overlying units in the lower Niobrara Formation suggest more normal marine conditions with well-oxygenated bottom waters, normal levels of bioturbation, and relatively low organic carbon contents. Pyrolysis data are interpreted to reflect a principally marine source of organic matter with substantial alteration due to bioturbation and thermal evolution. Elevated thermal maturity of the section at Lyons is inferred to be a local feature caused by local heating associated with fluid movement along fault zones or with emplacement of Tertiary sills.

Influence of Paleoenvironmental Factors on Preservation of Organic Matter in Middle Cretaceous Greenhorn Formation, Pueblo, Colorado

Sedimentological and geochemical studies of the Upper Cretaceous Hartland Shale and overlying Bridge Creek Limestone Members of the Greenhorn Formation reveal close associations between abundance of current-induced sedimentary structures, extent of bioturbation, and type of preserved organic matter. Abundant hydrogen-rich organic matter (4-5 wt. %) and low sulfur to organic carbon ratios are characteristic of the laminated facies, which lack current-induced sedimentary structures. Sparse hydrogen-poor organic matter (0.1-0.5 wt. %) and relatively high sulfur to organic carbon ratios are characteristic of the bioturbated facies, which contain numerous sedimentary structures indicative of currents. The concentration of oxygen in the benthic environment and the degree of bio urbation of the sediment apparently were determined principally by the rate that oxygen was supplied advectively (by currents) rather than by the rate of oxygen consumption (by decomposition). Thus, paleoclimatic and paleo-oceanographic factors that influenced mixing and current strength in the water column profoundly affected the amount and type of organic matter preserved in these units. It is proposed that episodes of high river discharge lead to density stratification of the water column and in turn to quiescent oxygen-depleted bottom water and high preservation of organic matter, thus accounting for the intermittent widespread deposition of organic-rich strata during maximum transgression and depth of the Greenhorn sea. The lateral association of voluminous deltaic deposits with dee -water black shales in other elongate or restricted marine basins may have causes similar to those envisioned for the Hartland Shale and Bridge Creek Limestone Members.