Background Sedimentation Rates Research Articles

Pyrite-lined shells as indicators of inefficient bioirrigation in the Holocene–Anthropocene stratigraphic record

Abstract. Although the depth of bioturbation can be estimated on the basis of ichnofabric, the timescale of sediment mixing (reworking) and irrigation (ventilation) by burrowers that affects carbonate preservation and biogeochemical cycles is difficult to estimate in the stratigraphic record. However, pyrite linings on the interior of shells can be a signature of slow and shallow irrigation. They indicate that shells of molluscs initially inhabiting oxic sediment pockets were immediately and permanently sequestered in reduced, iron-rich microenvironments within the mixed layer. Molluscan biomass-stimulated sulfate reduction and pyrite precipitation was confined to the location of decay under such conditions. A high abundance of pyrite-lined shells in the stratigraphic record can thus be diagnostic of limited exposure of organic tissues to O2 even when the seafloor is inhabited by abundant infauna disrupting and age-homogenizing sedimentary fabric as in the present-day northern Adriatic Sea. Here, we reconstruct this sequestration pathway characterized by slow irrigation (1) by assessing preservation and postmortem ages of pyrite-lined shells of the shallow-infaunal and hypoxia-tolerant bivalve Varicorbula gibba in sediment cores and (2) by evaluating whether an independently documented decline in the depth of mixing, driven by high frequency of seasonal hypoxia during the 20th century, affected the frequency of pyrite-lined shells in the stratigraphic record of the northern Adriatic Sea. First, at prodelta sites with a high sedimentation rate, linings of pyrite framboids form rapidly in the upper 5–10 cm as they already appear in the interiors of shells younger than 10 years and occur preferentially in well-preserved and articulated shells with periostracum. Second, increments deposited in the early 20th century contain < 20 % of shells lined with pyrite at the Po prodelta and 30 %–40 % at the Isonzo prodelta, whereas the late 20th century increments possess 50 %–80 % of shells lined with pyrite at both locations. At sites with slow sedimentation rate, the frequency of pyrite linings is low (< 10 %–20 %). Surface sediments remained well mixed by deposit and detritus feeders even in the late 20th century, thus maintaining the suboxic zone with dissolved iron. The upcore increase in the frequency of pyrite-lined shells thus indicates that the oxycline depth was reduced and bioirrigation rates declined during the 20th century. We hypothesize that the permanent preservation of pyrite linings within the shells of V. gibba in the subsurface stratigraphic record was enabled by slow recovery of infaunal communities from seasonal hypoxic events, leading to the dominance of surficial sediment modifiers with low irrigation potential. The presence of very young and well-preserved pyrite-lined valves in the uppermost zones of the mixed layer indicates that rapid obrution by episodic sediment deposition is not needed for preservation of pyrite linings when sediment irrigation is transient and background sedimentation rates are not low (here, exceeding ∼ 0.1 cm yr−1) and infaunal organisms die at their living position within the sediment. Abundance of well-preserved shells lined by pyrite exceeding ∼ 10 % per assemblage in apparently well-mixed sediments in the deep-time stratigraphic record can be an indicator of inefficient bioirrigation. Fine-grained prodelta sediments in the northern Adriatic Sea deposited since the mid-20th century, with high preservation potential of reduced microenvironments formed within a mixed layer, can represent taphonomic and early diagenetic analogues of deep-time skeletal assemblages with pyrite linings.

Holocene tropical reef accretion and lagoon sedimentation: A quantitative approach to the influence of sea‐level rise, climate and subsidence (Belize, Maldives, French Polynesia)

AbstractAccretion rates of Holocene tropical coral reefs in three areas in the Atlantic, Pacific and Indian Oceans have been quantified in 79 dated core sections in 34 reef cores from Belize, the Maldives and French Polynesia. Holocene vertical reef accretion rate averages 5.05 m/kyr and has decreased during the past 10 kyr. Accretion rates in branched and massive coral facies are statistically similar. Reef accretion rate is positively correlated with the rate of sea‐level rise, that is the degree of creation of accommodation space, and with climate as expressed in a Holocene sea surface temperature anomaly. Accommodation space is also created by subsidence, but at a rate one to two orders of magnitude lower than that created by glacio‐eustasy (0.04 to 0.16 m/kyr). Lagoonal background sedimentation in adjacent reef lagoons averages 0.89 m/kyr as measured in 72 dated core sections in 28 cores. Lagoonal carbonate sedimentation on top of underlying mangrove peat usually starts after a considerable hiatus of ca 3 kyr on average. The lagoonal background sedimentation rate increased during the Holocene, probably due to deepening. The differences between vertical reef accretion and lagoonal background sedimentation rates are a major factor in the production of the widely known saucer shapes typical of tropical reefs and carbonate platforms, that is the creation of unfilled accommodation space. Reef core recovery, used as a proxy for reef consolidation, and core depth exhibit a statistically negative correlation based on data from 326 core barrels. Recovery and marine cement abundance (average volume 8.6%) also decrease from windward to leeward core positions. These observations are presumably a result of both a decrease in the rate of sea‐level rise that is the increase in time available for submarine cementation during the Holocene and the amount of flushing of reef interstices by marine waters.

Pollution history from 256 BC to AD 2005 inferred from the accumulation of elements in a varve record of Lake Korttajärvi in Finland

In order to assess the importance of different pollution sources for the increase in element concentration and accumulation, historical changes in selected elements were studied in the annually laminated sediment of Lake Korttajarvi in Central Finland (62°20′N; 25°41′E). The sediment chronology based on varve counting (256 BC to AD 2005) provided a unique opportunity to explore and date signals of metal emissions, including the ancient metallurgical activities of the Roman Empire at the beginning of the Current Era. Records of this kind are mostly lacking in Finland and northernmost Europe. The stratigraphic sequence of element concentrations did not reflect any major changes in the lake, but changes in element accumulation rates provided distinct pollution signals caused by airborne fallout, catchment erosion, and to some extent municipal wastewater loading. The maximum bulk sedimentation recorded in the twentieth century was 11-fold and organic sedimentation 4-fold higher than the mean background sedimentation rate (256 BC to AD 1019). The increase in the accumulation rates of the majority of the elements, such as Cd, Sn, Pb, Si, Ni, B, Cu, Zn, Sr, Na, K, Sb, Ca, Cr, U and Mg, in descending order, was at least equal to that of bulk sedimentation or much greater, especially for Cd, Sn, and Pb. Changes in the accumulation of Co, Fe, Mn, Mo and As were small and mainly followed those of organic sedimentation. The earliest pollution signals were those of Pb recorded in AD 1055–1141. A weak signal of Pb pollution from the Roman Era was detected in metal concentrations, but this could not be confirmed by the accumulation rate data for Pb.

Giant earthquakes in South-Central Chile revealed by Holocene mass-wasting events in Lake Puyehue

Very high resolution reflection seismic profiling (3.5 kHz) revealed nine Holocene mass-wasting events in Lake Puyehue (South-Central Chile). These events are made up of numerous coeval mass-wasting deposits and some homogenites, which are radiocarbon-dated. The two youngest mass-wasting events could be attributed to the giant AD 1960 and AD 1575 Valdivia earthquakes. The most extensive event took place around 1660 cal yr BP. Evaluation of all possible slope failure processes led us to infer that giant earthquakes, roughly comparable to the AD 1960 Valdivia earthquake ( M w = 9.5), are the most likely trigger mechanisms of all mass-wasting events in Lake Puyehue. These occurred with a mean recurrence rate of 1000 yr although relatively aperiodically (ranging between 500 and 2000 yr). Quantitative comparison of mass-wasting events related to historically reported earthquakes (AD 1960 and AD 1575) showed significant differences although these earthquakes are assumed to have had a comparable strength. A lowered background sedimentation rate could be responsible for this variable earthquake recording, which highlights the importance of a thorough assessment of the depositional history before using lacustrine records for quantitative paleoseismic analysis.

In situ biodeposition rates of Pacific oysters (Crassostrea gigas) on a marine farm in Southern Tasmania (Australia)

In situ biodeposition rates of farmed Pacific oysters (Crassostrea gigas) were determined over 2–3 consecutive days in November 1995 and June 1996 as a means of assessing biodeposit loadings. The study was conducted on a 10 ha commercial intertidal oyster farm located in a marine lagoon in southern Tasmania. The farm had approximately 9.5 km of racks and at the time of study was being used to on-grow and condition oysters at a density of 360 oysters/m of racking. The hydrological characteristics of the lagoon were calculated using tide data recorded from gauges located within the lagoon.Average daily biodeposition rates differed between the two time periods with 180.5 g m−2 day−1 in November and 39.6 g m−2 day−1 in June, although background sedimentation rates were similar (7.28 and 8.00 g m−2 day−1, respectively). Biodeposit load from the lease at each sampling time was calculated based on the estimated percentage of baskets containing oysters. A daily average of 494 kg (dry wt) from 10 ha was calculated for the November period and 185 kg (dry wt) for June. Sediment samples collected showed low organic matter content (1.9–2.5%) and suggested that minimal biodeposits were accumulating under the racks, indicating that they were being transported and deposited elsewhere. Average residence time of water in the lagoon was approximately 1.5 days with an average exchange ratio of 0.53 and maximum of 0.97, indicating that at times there is near complete turnover of lagoon water within a tidal cycle. The average difference in total volume and low water volume was 1.38 million m3, with a mean tidal prism of approximately 73% of the total volume.Variation in biodeposition rates were attributed to the time of year and stocking density. Minimal enrichment of the sediment was found under the culture structures indicating that tidal water movement within the intertidal culture area is sufficient to disperse biodeposits. However, the impact of this material being transported and accumulating elsewhere in the lagoon remains unknown.

Origin and evolution of an Upper Jurassic complex of carbonate buildups from Zegarowe Rocks (Kraków–Wieluń Upland, Poland)

The Upper Jurassic complex of Zegarowe Rocks is situated on the Krakow–Wielun Upland in southern Poland. The complex is dominated by massive limestones representing carbonate buildups. The successive stages of carbonate buildup development include: colonisation, aggradational growth and progradation phases. In the colonisation phase, on top of loose peloidal-ooid sands micritic peloidal thrombolites developed. Peloidal and agglutinated thrombolites and stromatolites proliferated during the aggradational growth phase, whereas the progradation phase was characterised by shallowing and related development of agglutinated stromatolites with coprolites. The latter were the effect of periodical stabilisation of detrital sediments by microbial mats. The Zegarowe Rocks complex developed upon an elevation of the Late Jurassic stable northern shelf of the Tethys. This elevation was formed due to local decrease in subsidence rate, induced by the presence of a Palaeozoic granitoid intrusion in the shelf substratum. The carbonate buildups of the Zegarowe Rocks complex, initially developing as sediment-starved mounds upon fault-controlled intraplatform highs under strongly restricted background sedimentation rate, were replaced by agglutinated microbial reefs.

Laminated sediments preceding Heinrich event 1 in the Northern North Sea and Southern Norwegian Sea: Origin, processes and regional linkage

Rapidly deposited laminated sediments dated between 18 and 21 cal. ka BP have been observed at several core locations in the Northern North Sea and the Southern Norwegian Sea. Sedimentation rates range from 20 cm/ka up to 2000 cm/ka, compared to a normal background sedimentation rate in the area of 5–10 cm/ka. Dropstones, in situ paired bivalves and grain size distribution indicate a hemipelagic origin for the laminae in core MD99-2291, from the upper continental slope of Southern Norway. AMS radiocarbon dating and cyclic changes in the number of planktonic foraminifera and lithological parameters suggest that the laminations in the core are annual varves. The laminated sediments are interpreted to be suspension plume deposits and two phases of plume deposition have been dated to 21.5 and 18.6 cal. ka BP. Large fluxes of meltwater were generated during this period, as reflected in oxygen and carbon isotopes records from cores in the area. The meltwater and the laminated plume deposits are interpreted to be associated with the presence and disintegration of the Norwegian Channel Ice Stream. Observations from other regions demonstrate that laminated high rate deposition is very widespread on glaciated continental margins. Around 18.6 cal. ka BP increases in sediment fluxes also occurred on the Barra Fan, the North Sea Fan, the Lofoten contourite drift and the Svalbard margin, preceding increased sediment and meltwater fluxes on the Greenland margin and the Labrador Sea, associated with Heinrich event 1. Warm water inflow to the Norwegian Sea together with rising sea level probably triggered the destabilisation of the ice streams of the Fennoscandian Ice Sheet, while a similar response of the Laurentide ice sheet was delayed by 500–1000 yr.

Upper Jurassic thrombolite reservoir play, northeastern Gulf of Mexico

In the northeastern Gulf of Mexico, Upper Jurassic Smackover inner ramp, shallow-water thrombolite buildups developed on paleotopographic features in the eastern part of the Mississippi Interior Salt basin and in the Manila and Conecuh subbasins. These thrombolites attained a thickness of 58 m (190 ft) and were present in an area of as much as 6.2 km2 (2.4 mi2). Although these buildups have been exploration targets for some 30 yr, new field discoveries continue to be made in this region. Thrombolites were best developed on a hard substrate during a rise in sea level under initial zero to low background sedimentation rates in low-energy and eurytopic paleoenvironments. Extensive microbial growth occurred in response to available accommodation space. The demise of the thrombolites corresponded to changes in the paleoenvironmental conditions associated with an overall regression of the sea. The keys to drilling successful wildcat wells in the thrombolite reservoir play are to (1) use three-dimensional seismic reflection technology to find paleohighs and to determine whether potential thrombolite reservoir facies occur on the crest and/or flanks of these features and are above the oil-water contact; (2) use the characteristics of thrombolite bioherms and reefs as observed in outcrop to develop a three-dimensional geologic model to reconstruct the growth of thrombolite buildups on paleohighs for improved targeting of the preferred dendroidal and chaotic thrombolite reservoir facies; and (3) use the evaporative pumping mechanism instead of the seepage reflux or mixing zone models as a means for assessing potential dolomitization of the thrombolite boundstone.

Evaluating the Development of Upper Jurassic Reefs in the Smackover Formation, Eastern Gulf Coast, U.S.A. through Fuzzy Logic Computer Modeling

ABSTRACT A three-dimensional (3D) numerical stratigraphic model has been built to simulate the development of microbial reefs on a Jurassic carbonate platform using fuzzy logic modeling methods. The flexibility of the model allows for the examination of a large number of variables. The relative importance of local environmental conditions and global changes on the frequency of reef development are examined relative to the temporal and spatial constraints from Upper Jurassic (Oxfordian) Smackover reef datasets from two Alabama oil fields. The fuzzy model simulates the deposition of reefs and carbonate facies by integrating local and global variables. Local-scale factors include basement relief, sea-level change, climate, latitude, water energy, water depth, background sedimentation rate, and substrate conditions. Regional and global-scale changes include relative sea-level change, climate, and latitude. The model confirms preexisting theories that background sedimentation rate, water energy, substrate, relative sea-level change, and climate influenced the development of Jurassic reefs. Additionally, the model determines the degree of control each variable has on 'IF,' 'WHERE,' and 'WHEN' reefs develop. The variables that control 'IF' reefs develop are the global parameters of climate and amplitude of sea-level change. The model specifies that the variables that influence the lateral distribution of reefs (or 'WHERE' reefs occur) are substrate, water energy, and background sedimentation rate. The model also indicates a strong correlation between 'WHEN' reefs develop and the rate of sea-level rise. In all simulations, reefs initially develop in conjunction with the greatest rate of sea-level rise (i.e., the inflection point on the sea-level curve) and reach peak development before the time of deepest water.

Stratigraphic and structural expression of the lateral growth of thrust fault‐propagation folds: results and implications from kinematic modelling

AbstractIn order to better understand the development of thrust fault‐related folds, a 3D forward numerical model has been developed to investigate the effects that lateral slip distribution and propagation rate have on the fold geometry of pre‐ and syn‐tectonic strata. We consider a fault‐propagation fold in which the fault propagates upwards from a basal decollement and along‐strike normal to transport direction. Over a 1 Ma runtime, the fault reaches a maximum length of 10 km and accumulates a maximum displacement of 1 km. Deformation ahead of the propagating fault tip is modelled using trishear kinematics while backlimb deformation is modelled using kink‐band migration.The applicability of two different lateral slip distributions, namely linear‐taper and block‐taper, are firstly tested using a constant lateral propagation rate. A block‐taper slip distribution replicates the geometry of natural fold‐thrusts better and is then used to test the sensitivity of thrust‐fold morphology to varied propagation rates in a set of fault‐propagation folds that have identical final displacement to length (Dmax/Lmax) ratios.Two stratigraphic settings are considered: a model in which background sedimentation rates are high and no topography develops, and a model in which a topographic high develops above the growing fold and local erosion, transport and deposition occur. If the lateral propagation rate is rapid (or geologically instantaneous), the fault tips quickly become pinned as the fault reaches its maximum lateral extent (10 km), after which displacement accumulates. In both stratigraphic settings, this leads to strike‐parallel rotation of the syn‐tectonic strata near the fault tips; high sedimentation rates relative to rates of uplift result in along‐strike thinning over the structural high, while low sedimentation rates result in pinchout against it. In contrast, slower lateral propagation rates (i.e. up to one order of magnitude greater than slip rate) lead to the development of along‐strike growth triangles when sedimentation rates are high, whereas when sedimentation rates are low, offflap geometries result. Overall we find that the most rapid lateral propagation rates produce the most realistic geometries. In both settings, time‐equivalent units display both nongrowth and growth stratal geometries along‐strike and the transition from growth to nongrowth has the potential to delineate the time of fault/fold growth at a given location. This work highlights the importance of lateral fault‐propagation and fault tip pinning on fault and fold growth in three dimensions and the complex syn‐tectonic geometries that can result.

High-Frequency Explosive Volcanic Eruptions in a Late Jurassic Volcanic Arc: The Ameghino Formation, Antarctic Peninsula

ABSTRACT Ash layers in the Lower Tithonian Longing Member (Ameghino Formation) on the Antarctic Peninsula are derived from the fallout of tephra associated with explosive eruptions. These layers are preserved in a complete and continuous sedimentary record formed in a deep, anoxic marine environment, with high background sedimentation rates allowing rapid burial and preservation. Absence of bioturbation precluded the thin ash beds from mixing with enclosing sediment. The average interval between explosive eruptions in the volcanic arc of the Antarctic Peninsula during the Early Tithonian is estimated at between 700 and 1000 years, the highest frequency of explosive volcanic events ever defined in ancient marine rocks. This frequency is comparable to that of explosive eruptions in some presently active volcanic arcs. This indicates that deposition in some deep marine settings can produce records sufficiently complete to determine accurate frequencies of explosive eruptions in ancient magmatic arcs. The apparent lull in the volcanic activity in the Antarctic Peninsula during this time, inferred from dating of volcanic rocks from the arc itself, is not supported by the marine record in the adjacent basin and must be due to a bias of sampling.

Distribution of Ammonoids in the Lower Triassic Union Wash Formation (Eastern California): Evidence for Paleoceanographic Conditions during Recovery from the End-Permian Mass Extinction

Other| December 01, 2000 Distribution of Ammonoids in the Lower Triassic Union Wash Formation (Eastern California): Evidence for Paleoceanographic Conditions During Recovery from the End-Permian Mass Extinction ADAM D. WOODS; ADAM D. WOODS 1University of Southern California, Department of Earth Sciences, Los Angeles, CA 90089-0740 Search for other works by this author on: GSW Google Scholar DAVID J. BOTTJER DAVID J. BOTTJER 1University of Southern California, Department of Earth Sciences, Los Angeles, CA 90089-0740 Search for other works by this author on: GSW Google Scholar PALAIOS (2000) 15 (6): 535–545. https://doi.org/10.1669/0883-1351(2000)015<0535:DOAITL>2.0.CO;2 Article history accepted: 15 Aug 2000 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation ADAM D. WOODS, DAVID J. BOTTJER; Distribution of Ammonoids in the Lower Triassic Union Wash Formation (Eastern California): Evidence for Paleoceanographic Conditions During Recovery from the End-Permian Mass Extinction. PALAIOS 2000;; 15 (6): 535–545. doi: https://doi.org/10.1669/0883-1351(2000)015<0535:DOAITL>2.0.CO;2 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 SocietyPALAIOS Search Advanced Search Abstract The Lower Triassic (Smithian-Spathian) Union Wash Formation (east-central California), deposited along the western edge of Pangea, is generally barren of macrofossils with the exception of ammonoids. To better understand the paleoenvironmental conditions that existed in the region of Union Wash Formation deposition, data were gathered concerning the taphonomy, sedimentology, and distribution of these ammonoids. Results indicate Meekoceras bed deposition was due to a drop in background sedimentation rates, perhaps coupled with favorable environmental conditions. Disappearance of ammonoid shells in the stratigraphic interval above the Meekocerasbed appears to be the result of a shallow aragonite compensation depth (ACD); the return of ammonoids higher in the section is associated with the appearance of seafloor calcium carbonate cements that represent an increase in the degree of calcium carbonate supersaturation, related to the upwelling of anoxic, alkaline waters. The subsequent disappearance of seafloor cements suggests that seawater calcium carbonate chemistry returned to normal prior to deposition of the Parapopanocerasbed. The condensed nature of the Parapopanocerasbed, coupled with poor shell preservation, indicates deposition as a lag deposit. Bedding plane occurrences of ammonoids elsewhere in the sequence imply deposition as mass mortality events possibly related to anoxia (indicated by the depauperate and laminated nature of much of the unit) and/or hypercapnia (related to the growth of seafloor cements). The results of this study further attest to the unusual paleoceanographic conditions that were present off the western margin of Pangea during the Early Triassic recovery from the end-Permian mass extinction. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Distribution of Ammonoids in the Lower Triassic Union Wash Formation (Eastern California): Evidence for Paleoceanographic Conditions During Recovery from the End-Permian Mass Extinction

Abstract The Lower Triassic (Smithian-Spathian) Union Wash Formation (east-central California), deposited along the western edge of Pangea, is generally barren of macrofossils with the exception of ammonoids. To better understand the paleoenvironmental conditions that existed in the region of Union Wash Formation deposition, data were gathered concerning the taphonomy, sedimentology, and distribution of these ammonoids. Results indicate Meekoceras bed deposition was due to a drop in background sedimentation rates, perhaps coupled with favorable environmental conditions. Disappearance of ammonoid shells in the stratigraphic interval above the Meekocerasbed appears to be the result of a shallow aragonite compensation depth (ACD); the return of ammonoids higher in the section is associated with the appearance of seafloor calcium carbonate cements that represent an increase in the degree of calcium carbonate supersaturation, related to the upwelling of anoxic, alkaline waters. The subsequent disappearance of ...

Significance of a Halysitid-Heliolitid Mud-facies Autobiostrome from the Middle Silurian of Gotland, Sweden

Abstract A low-diversity halysitid-heliolitid autobiostrome from the Late Wenlock Halla Formation at Blahall 1, Gotland, indicates a brief period of particular environmental conditions. The reef is 0.3–0.5 m thick, internally without bedding structures, and had a sheet-like distribution on a low-relief soft-bottom, in marginal platform, graptolitiferous strata (height-width ratio is 1:900 in outcrop). The dominating coral species building this mud-facies biostrome were the tabulates Stelliporella cf. parvistella and Halysites laticatenatus. The rugose coral fauna is dominated by fixosessile and rhizosessile forms with well developed holdfasts, for example Dokophyllum elegantulum. Coral growth forms generally indicate high background sedimentation rates during the growth of the biostrome. Four successive stages of sea-floor development are discussed: (1) a pre-biostrome stage, (2) a pioneering community stage, (3) a climax-community stage, and (4) a post-biostrome stage. Stratigraphically, these stages sho...

Facies and palaeoecology of Upper Jurassic (Middle Oxfordian) coral reefs in England

This study documents the facies and fauna of Late Jurassic (Middle Oxfordian) coral reefs in England. Sedimentological and palaeoecological analysis of these reefs distinguishes three generic reef types: (1) small reef patches and thickets associated with siliciclastic deposits; (2) small reef patches and thickets associated with siliciclastic-free bioclastic grainstones and packstones; and (3) biostromal units associated with deep water facies. The depositional environments of these reef types are discussed. Two coral assemblages are identified: (1) the microsolenid assemblage; and (2) theThamnasteria, Isastraea, Fungiastraea andThecosmilia assemblage (Thamnasteria assemblage). TheThamnasteria assemblage developed in all shallow water environments in the study area, regardless of local environmental conditions. The fauna is very eurytopic,r-selected and can tolerate significant environmental fluctuations on short temporal scales (sub-seasonal). The main control on the development of the microsolenid assemblage was low light intensity, low background sedimentation rates and low hydrodynamic energy levels.

Upper Jurassic microsolenid biostromes of northern and central Europe: facies and depositional environment

Microsolenid biostromes are a common and important reef type in many Upper Jurassic reefal sections. Comparative study of these biostromes in western Europe allows the characterisation of this distinct biostrome type in terms of its taxonomic composition, palaeoecology and sedimentary regime. The biostromes are characterised by a coral community of relatively low generic diversity, being dominated by the Microsolenidae family of corals. The coral skeletal biovolume is generally high and is overwhelmingly dominated by platy ecomorphotypes. Sedimentologically these biostromes developed in low energy marl and limestone regimes which were subject to low sedimentation rates and varying degrees of siliciclastic influx. In contrast to the coral fauna the associated fauna shows a high generic diversity, in particular a rich assemblage of echinoids and bivalves. In addition the coral plates provide an ideal habitat for a well developed and strongly polarised encrusting and boring fauna. Top surfaces show limited post-mortem encrustation by serpulids and cementing bivalves, and are also bored by lithophagan bivalves. The undersides of the plates show a highly developed and diverse cryptic fauna of serpulids, thecidean brachiopods, bryozoans, calcareous sponges and sclerosponges. It is concluded that the main control on the development of these biostromes was low light intensities, though low energy level and low background sedimentation rates were also important. It is suggested that relatively high nutrient levels may have also played a significant role. The conditions for microsolenid biostrome development prevail in relatively deep-water environments, and hence these are the first coral-dominated reefal structures to have developed in deeper water shallowing upward sequences. The corals in these biostromes have adapted to these conditions by developing a strongly platy growth form, but also reducing their growth rates substantially. The microsolenids are particularly adapted to these environments in two ways: (1) They secreted an extremely porous calcium carbonate skeleton, which in environments where calcification rates are slow, such as deep water environments, is a distinct advantage; (2) In response to the limited potential of phototrophy in these environments the microsolenids have developed a pennular septal structure which is suggested to be a means of improving heterotrophic feeding. Trophic analysis of the reef fauna reveals that the nutrient limitation model developed for present-day coral reefs cannot be applied to these biostromes. It is demonstrated that the Microsolena and the present-day agariciid, Leptoseris show a number of significant morphological and ecological similarities.

Contaminant dispersal on the Palos Verdes continental margin: III. Processes controlling transport, accumulation and re-emergence of DDT-contaminated sediment particles

A large mass of DDT-contaminated sediment lies buried beneath a thin cover on the Palos Verdes shelf and slope off Los Angeles, California. Analyses, including several types of numerical simulation, have been applied to an extensive data set to evaluate the biological and physical processes controlling the fate of the strongly particle-reactive pollutants. Sequential measurements of the p,p′-DDE (an isomer of DDT) content in cores from monitoring locations indicate that contaminated sediment particles are being re-introduced into the marine environment from the buried historic deposits at a significant rate. There is considerable scatter in the data but the trends for ongoing release over the 8 years between 1983 and 1991 are statistically significant at the 90% level. A comprehensive mathematical model of shelf sediment dynamics (Resuspension Model) has been used to explore sediment erosion and deposition during major storms of the early 1980s and of 1988. This modeling shows that storms alone cannot explain the observed losses of contaminated sediment particles from the historical deposits of the Palos Verdes shelf. On the other hand, it also demonstrates that both severe and common storm events do re-entrain some of the bottom sediment at all water depths across the Palos Verdes shelf. A numerical model (Contaminant Release Model), which couples upward biodiffusion of DDT with storm removal, satisfactorily explains the observed losses. The infaunal activity that creates biodiffusion has been explored in two separate ways. In calibrating the Contaminant Release Model, the depth-dependent biodiffusion coefficient profile is estimated by comparing the predicted and measured depth concentration distributions. A separate analysis has been conducted using the measured vertical distribution of infaunal species (Stull et al. 1995; Swift et al., 1995). The agreement between these two methods is good. Both show that bioturbation extends to the level of the high contamination. Bioturbation is now playing a significant role in redistributing the buried historic contaminants. The Contaminant Release Model, in combination with evaluations of the total sedimentation rate and the natural background sedimentation rate, clearly shows that the flux of solids from the outfall enhances the rate of accumulation of natural sediment on the Palos Verdes shelf. A sharp reduction of the flux of solids, such as would accompany conversion from partial to full secondary treatment, will result in considerably prolonging the high rates with which DDT and other particle-reactive contaminants are re-entering the marine environment.

Contaminant dispersal on the Palos Verdes continental margin: III. Processes controlling transport, accumulation and re-emergence of DDT-contaminated sediment particles

A large mass of DDT-contaminated sediment lies buried beneath a thin cover on the Palos Verdes shelf and slope off Los Angeles, California. Analyses, including several types of numerical simulation, have been applied to an extensive data set to evaluate the biological and physical processes controlling the fate of the strongly particle-reactive pollutants. Sequential measurements of the p,p′-DDE (an isomer of DDT) content in cores from monitoring locations indicate that contaminated sediment particles are being re-introduced into the marine environment from the buried historic deposits at a significant rate. There is considerable scatter in the data but the trends for ongoing release over the 8 years between 1983 and 1991 are statistically significant at the 90% level. A comprehensive mathematical model of shelf sediment dynamics (Resuspension Model) has been used to explore sediment erosion and deposition during major storms of the early 1980s and of 1988. This modeling shows that storms alone cannot explain the observed losses of contaminated sediment particles from the historical deposits of the Palos Verdes shelf. On the other hand, it also demonstrates that both severe and common storm events do re-entrain some of the bottom sediment at all water depths across the Palos Verdes shelf. A numerical model (Contaminant Release Model), which couples upward biodiffusion of DDT with storm removal, satisfactorily explains the observed losses. The infaunal activity that creates biodiffusion has been explored in two separate ways. In calibrating the Contaminant Release Model, the depth-dependent biodiffusion coefficient profile is estimated by comparing the predicted and measured depth concentration distributions. A separate analysis has been conducted using the measured vertical distribution of infaunal species (Stull et al. 1995; Swift et al., 1995). The agreement between these two methods is good. Both show that bioturbation extends to the level of the high contamination. Bioturbation is now playing a significant role in redistributing the buried historic contaminants. The Contaminant Release Model, in combination with evaluations of the total sedimentation rate and the natural background sedimentation rate, clearly shows that the flux of solids from the outfall enhances the rate of accumulation of natural sediment on the Palos Verdes shelf. A sharp reduction of the flux of solids, such as would accompany conversion from partial to full secondary treatment, will result in considerably prolonging the high rates with which DDT and other particle-reactive contaminants are re-entering the marine environment.

Trace-fossil tiering in flysch sediments: examples from the Guipúzcoan flysch (Cretaceous-Paleogene), northern Spain

Distribution of trace fossils in flysch sediments is in great part conditioned by tiering; that is, the vertical partitioning of trace-producing organisms below the sediment-water interface. Ichnologic studies in the Guipúzcoan flysch (Upper Albian-Lower Eocene), northern Spain, show that tiering in flysch sediments is strongly controlled by sediment character, mode and rate of deposition, and degree of oxygenation of pore waters. Depending on the mode of deposition, trace-fossil tiering in flysch developed and was preserved in two radically different styles: (1) In event layers (chiefly turbidites), tiering is preserved without secondary distortion, owing to synchronous colonization and the lack of vertical ascent through time. Vertical tiering structure depends primarily on sediment composition, sequence and thicknesses of structural divisions, and oxygen profile. Variations in tiering are interpreted to reflect the particular oxygen tolerances, penetration potentials, and nutritional requirements of tracemakers; (2) In background sediments (hemipelagites and pelagites), tiering resulted from successive colonizations that were influenced by background sedimentation rates, sediment composition, and oxygen levels of pore waters. Tiering of actively or passively filled burrows is reflected by complex ichnofabrics and the superposition of different tiers by ascension. Tiering structure was frozen upon deposition of superjacent event layers. The original tiering of graphoglyptids (open burrows) may be reconstructed from their cross-cutting relationships on turbidite soles. These contrasting styles of tiering development are characteristic of flysch and other sediments composed of alternating event and background deposits.

Computer Simulation of Ddt Distribution in Palos Verdes Shelf Sediments

Prior to the imposition of effective source‐control measures in 1970, large quantities of DDT were discharged to the Los Angeles County municipal sewer system and, subsequently, to the Pacific Ocean. A fraction of this material accumulated among sediments of the Palos Verdes shelf. While the bulk of the DDT lies 10–40 cm below the sediment surface, its fate may be affected by future wastewater‐treatment decisions at the Joint Water Pollution Control Plant (JWPCP), Los Angeles County's 385‐mgd treatment plant. To assess the impact of impending JWPCP secondary treatment requirements on shelf sediment quality (processes that potentially affect the distribution of chemical tracers among those sediments, including background sedimentation rate), contributions from effluent‐related solids, sediment mixing, and diffusive transport were incorporated in mathematical models. Model projections of surface‐sediment quality at the most heavily contaminated sites are sensitive to projected effluent solids concentrations...