Represent Time Intervals Research Articles

A high-resolution Holocene record on the Southern Brazilian shelf: Paleoenvironmental implications

A high-resolution, multi-proxy record has been used to determine the environmental changes during the Holocene on the southern Brazilian shelf. Present oceanographic conditions reveal wind and freshwater input as the determinants of short-term productivity changes in the study area. Magnetic susceptibility and grain-size variations, together with proxies of productivity (organic carbon, carbon accumulation rate, Ba, Sr, and Ca content, Ba/Al, Ba/Ti, and Al/Ti ratios) were analyzed and compared with proxies of redox condition (V/Ti ratio), terrigenous input (Fe/Ca and Ti/Ca ratios), as well as other Element/Ti ratios, to evaluate the paleoceanographic and paleoclimatic changes over the period. The core covers a time interval of about 7650 years, with sedimentation rates varying from 0.025 to 0.250 cm a −1, which represent time intervals of between 8 and 80 a per sample. There is a clear change in the sedimentation rate at about 2800 B.P. All grain-size and elemental results indicate the occurrence of conspicuous changes between 5200 and 5000 cal. B.P., as well as between 3000 and 2800 cal. B.P. A comparison of the results with the palynological information available from the adjacent continental areas suggests that the sedimentary changes in this last interval may be correlated with the onset of modern climatic conditions in South America, and especially, with the onset of the Plata Plume Water, a water mass that carries cold, less saline waters towards the north. However, minor changes are observed at ca. 1500 B.P. and are correlated with an increase in the atmospheric humidity. Furthermore, a time-series analysis undertaken using several proxies indicated the occurrence of Sub-Milankovitch cycles, which may be compared with those reported worldwide.

Rapid responses of high‐mountain vegetation to early Holocene environmental changes in the Swiss Alps

Summary The Early Holocene sediment of a lake at tree line (Gouillé Rion, 2343 m a.s.l.) in the Swiss Central Alps was sampled for plant macrofossils. Thin (0.5 cm) slices, representing time intervals of c. 50 years each from 11 800 to 7800 cal. year bp, were analysed and the data compared with independent palaeoclimatic proxies to study vegetational responses to environmental change. Alpine plant communities (e.g. with Salix herbacea) were established at 11 600–11 500 cal. year bp, when oxygen‐isotope records showed that temperatures increased by c. 3–4 °C within decades. Larix decidua trees reached the site at c. 11 350 cal. year bp, probably in response to further warming by 1–2 °C. Forests dominated by L. decidua persisted until 9600 cal. year bp, when Pinus cembra became more important. The dominance of Larix decidua for two millennia is explained by dry summer conditions, and possibly low winter temperatures, which favoured it over the late‐successional Pinus cembra. Environmental conditions were a result of variations in the earth's orbit, leading to a maximum of summer and a minimum of winter solar radiation. Other heliophilous and drought‐adapted species, such as Dryas octopetala and Juniperus nana, could persist in the open L. decidua forests, but were out‐competed when the shade‐tolerant P. cembra expanded. The relative importance of Larix decidua decreased during periods of diminished solar radiation at 11 100, 10 100 and 9400 cal. year bp. Stable concentrations of L. decidua indicate that these percentage oscillations were caused by temporary increases of Pinus cembra, Dryas octopetala and Juniperus nana that can be explained by increases in moisture and/or decreases in summer temperature. The final collapse of Larix decidua at 8400 cal. year bp was possibly related to abrupt climatic cooling as a consequence of a large meltwater input to the North Atlantic. Similarly, the temporary exclusion of Pinus cembra from tree line at 10 600–10 200 cal. year bp may be related to slowing down of thermohaline circulation at 10 700–10 300 cal. year bp. Our results show that tree line vegetation was in dynamic equilibrium with climate, even during periods of extraordinarily rapid climatic change. They also imply that forecasted global warming may trigger rapid upslope movements of the tree line of up to 800 m within a few decades or centuries at most, probably inducing large‐scale displacements of plant species as well as irrecoverable biodiversity losses.

A formal model for the geologic time scale and global stratotype section and point, compatible with geospatial information transfer standards

The geologic time scale is a complex data structure composed of abstract elements that represent time intervals and instants and their relationships with specific concrete representations in the geologic record as well as the observations made of those concrete representations. The International Union of Geological Sciences’ International Commission on Stratigraphy guidelines recommends a very precise usage of the relationships between these components in order to establish a standard time scale for use in global correlations. However, this has been primarily described in text. Here, we present a formal representation of the model using the Unified Modeling Language (UML). The model builds on existing components from standardization of geospatial information systems. The use of a formal notation enforces precise definition of the relationships between the components. The UML platform also supports a direct mapping to an eXtensible Markup Language (XML)‐based file format, which may be used for the exchange of stratigraphic information using Web-service interfaces.

Repetitions and cycles in stratigraphy

Stratigraphic sections show repetitions of similar or identical conditions on all scales from millimeters to many hundreds of meters, representing time intervals from possibly seconds to many millions of years. Any such repetitions have been called cycles. To understand the reason why cycles occur, a time scale is essential. Beds can be with or without time information: the latter are known as event stratifications. The event, which triggers the formation of beds or any succession of sediments, could be a random effect, or it could be part of a mechanism in which cyclic behaviour is due to an oscillating physical system. Such systems are dynamic systems, which means that they involve the transport of masses over distances and in definite time intervals. Cycles generated by such systems can be used as time units. A special example is the earth solar system which can generate cycles from daily periods to orbital variations of thousands and millions of years. Orbital variations are known as Milankovitch cycles and they are the subject of cyclostratigraphy proper. The recognition of Milankovitch cycles or of any other cycle associated with oscillating systems, rests on the regularity or time periodicity of such patterns. The correct ways of analysing such cycles are the methods of time series analysis. Repetition patterns representing time intervals of millions and possibly many hundreds of millions of years, are known from sequence stratigraphy. We know as yet very little about the mechanisms which generate such long periods of repetition. It has been suggested that the earth's crust has reached a state of self organized criticality (Bak, P., 1996. How nature works. Springer, New York, 212 pp.), in which case sequence formation could be part of the complexity which undoubtedly is an attribute of the earth's crust and history.

Geomagnetic long-term secular variations in Italian Lower Cretaceous shallow-water carbonates

SUMMARY The remanent magnetic properties of an 88 m bore core are unrelated to either the dolomite content or the sedimentological textures and are considered to be carried primarily by biogenetic magnetite that was cemented in during very early diagenesis. Individual readings represent time intervals of c. 720±32 yr and, after 40 mT partial demagnetization, they provide an almost continuous record of averaged geomagnetic secular variations over a period of some 3.17 Myr. The magnitude of directional secular variation is twice that of the present day, despite being smoothed, and the secular variations appear to grade into polarity transitions, suggesting no diVerence in their mechanisms. The rates of change in direction between subjacent levels in the core have a log-normal distribution which extends smoothly beyond 90° and has a median value of 13°/700 yr, the same as for unsmoothed European secular variation during the last 2000 yr. The intensity of remanence, after 40 mT partial demagnetization, appears to provide a reasonable approximation to geomagnetic field intensity. This tends to be weaker when the direction is moving faster, reflecting averaging, but is unrelated to the distance of the vector from the mean direction; that is, it depends on the rate of change and not on the virtual pole latitude. The virtual poles, after correction for tectonic rotations about horizontal and vertical axes, have latitudes that form a strongly platykurtic Fisherian distribution, while their longitudes have a circular distribution on which are superimposed two Gaussian peaks, 180° apart. This bore core thus provides detailed information of smoothed geomagnetic secular variation in the Lower Cretaceous (127±3 Ma) which shows clear regularities in behaviour, some related to changes in the Earth’s orbital parameters.

Spectral magnitude representation for multi-band excitation speech coders

A method for encoding a speech signal into digital bits including the steps of dividing the speech signal into speech frames representing time intervals of the speech signal, determining voicing information for frequency bands of the speech frames, and determining spectral magnitudes representative of the magnitudes of the spectrum at determined frequencies across the frequency bands. The method further includes quantizing and encoding the spectral magnitudes and the voicing information. The steps of determining, quantizing and encoding the spectral magnitudes is done is such a manner that the spectral magnitudes independent of voicing information are available for later synthesizing.

The sequential occurrence of viral mutations in a liver transplant recipient re-infected with hepatitis B: hepatitis B immune globulin escape, famciclovir non-response, followed by lamivudine resistance resulting in graft loss

Background/Aims: The purpose of this study was to characterize the clinical, histological and virological events in an orthotopic liver transplant (OLT) recipient with recurrent hepatitis B infection who was initially managed with hepatitis B immune globulin (HBIg) and when viral recurrence occurred, with nucleoside analogue salvage therapy. The aims were to document the mutations occurring in the hepatitis B virus (HBV) polymerase gene as a consequence of HBIg escape, famciclovir non-response and subsequent lamivudine resistance. Methods: Throughout the follow-up of 796 days, the patient was seen at least 4-week intervals. Clinical, biochemical and virological data were registered according to protocol. HBV DNA was quantified throughout the treatment period. The viral polymerase gene was sequenced from serum samples collected at representative time intervals. Consecutive liver biopsies were scored according to the modified Knodell classification. Results: Clinically, the patient was in excellent condition until the development of acute hepatitis during the lamivudine therapy period, 765 days post-OLT. Until this terminal event, serum transaminase activity was only 1–2 times the upper limit of normal with serum bilirubin and prothrombin time within the normal range. Subsequent liver biopsies showed chronic active hepatitis with no signs of fibrosis. The post-mortem biopsy showed severe acute hepatitis B with massive necrosis. The HBV polymerase gene was sequenced during HBIg, famciclovir and lamivudine treatment. One mutation I533L was detected during HBIg treatment. No amino acid changes were selected during famciclovir treatment. Three amino acid changes were selected while the patient on lamivudine treatment, which include L5331, S559T and M550I. Conclusions: We have documented HBV recurrence in a liver transplant recipient with the emergence of a multidrug resistant HBV which caused graft loss. The primary resistance to famciclovir in spite of therapeutic penciclovir levels may be as a result of a combination of the mutations found in the polymerase region. After 300 days of lamivudine treatment, a drug-resistant population emerged which was associated with a greater than three log increase in HBV DNA and contributed to loss of graft function. This is the first report of such an adverse clinical outcome due to the emergence of a mutant virus as a consequence of immunoprophylactic and antiviral therapy in a liver transplant recipient.

Hepatitis B virus polymerase mutations during antiviral therapy in a patient following liver transplantation

Background/Aims: The purpose of this study was to investigate possible resistance mutations which arose in the polymerase gene of hepatitis B virus (HBV) in a patient with severe recurrent HBV infection following liver transplantation. The patient's management included antiviral chemotherapy for almost 4 years comprising ganciclovir, foscarnet and famciclovir. In the last 2.5 years of famciclovir treatment, an increase in serum HBV DNA levels and a reduced sensitivity of the virion-associated DNA polymerase to penciclovir triphosphate were observed. Methods: The viral polymerase gene and X gene were sequenced from serum samples collected at representative time intervals covering the entire treatment period. Results: No mutations were detected in the X gene. Three nucleotide mutations, each of which resulted in an altered amino acid sequence, were detected in the polymerase gene after 816 days of total antiviral therapy (370 days of famciclovir). Two of these mutations were detected by direct sequencing and the third was detected after cloning and was present in 10% of the clones. All three mutations occurred in “region B” of RNA-dependent DNA polymerases. The HBV polymerase has similarities to both RNA and DNA polymerases. These mutations in the HBV polymerase gene were located in a similar area to the penciclovir-induced mutations observed in the herpes simplex virus DNA polymerase gene. Conclusions: Three mutations within the HBV polymerase gene were detected which were associated with a reduced penciclovir sensitivity.

Equilibrium and response to climatic and tectonic forcing: A study of alluvial sequences in the Devonian Munster Basin, Ireland

Abstract The Upper Palaeozoic Munster Basin is characterised by thick (> 6 km), Late Devonian alluvial successions (the Old Red Sandstone) that are well exposed throughout southwest Ireland. These represent the deposits of large-scale, complex terminal fan systems which operated under a semi-arid climate. A coarse biostratigraphic subdivision of these sequences has been combined with established radiometric evidence for Devonian stage durations in order to estimate a sediment accumulation rate for this part of the Munster Basin (0.38–0.46 mm a −1 ). This rate has then been used to estimate the duration of cyclic variations in a range of sedimentary parameters that have been identified throughout both proximal and distal basin-fill sequences. ‘Spot’-data derived from variables, such as coset thickness, maximum bed thickness and sand-stone content, were used to generate time series which have been subjected to routine Fourier analysis. The application of the estimated sediment accumulation rate to the three most prominent cycle thicknesses (31–175 m) and the assessment of relative frequencies has been used to establish the approximate cycle periodicities, all of which appear to lie within the Milankovitch Band. It is suggested that these features represent the stratigraphic expression of climatic perturbations related to orbital forcing (particularly eccentricity variation). It is further suggested that the recognition of these cycles throughout the basin-fill may, in future, form the basis of high resolution relative timescale and allow the correlation of proximal and distal sections through the basin-fill at scales comparable to the high frequency cyclicities ( c. 40 m). The stratigraphic expression of these cycles and their interpretation as evidence of climatic rather than tectonic variation are considered to be a function of their relationship to the equilibrium time constant ( T eq ) of the Munster Basin. The establishment of T eq requires an assessment of diffusivity which is largely controlled by the rate of water supply to the depositional systems. The latter is assessed by palaeohydrological reconstructions of the scale and discharge characteristics of the major fluvial distributary systems within the Munster Basin. Two methods have been applied. Method I uses a series of established empirical relationships to estimate palaeohydrological variables and Method II uses the thickness of sets and cosets of cross-strata within alluvial sandbodies to assess discharge and channel depth. The resulting estimates of water flux allow the assessment of diffusivity which suggests a T eq for the Munster Basin of the order of 2 × 10 6 a. The Milankovitch Band (10 5 a) periodicities ( T ) of the cycles observed in the basin-fill are therefore considered to represent ‘rapid’ variations ( T≪T eq ) in water/sediment flux associated with relatively high frequency climatic perturbations. However, these cycles are super-imposed on kilometre-scale, sand-prone ‘wedges’ that dominate the basin-fill architecture. These are considered to record the basinward progradation and subsequent retreat of the terminal fan systems. The 1.5–2 km thicknesses of these units represent time intervals greater than T eq for the Munster Basin. These features are therefore considered to record ‘slow’ variations ( T>T eq ) in base level related to changes in tectonic subsidence rate and/or sediment flux.

Floodplains and paleosols in the Wyoming Eocene sequence: implications for the taphonomy and paleoecology of faunas

The Willwood Fm. of the southern Bighorn Basin of Wyoming, U.S.A., comprises 700 m of lower Eocene alluvial molasse, nearly all of which contains relict pedogenic features. These rocks are grouped into pedofacies–alluvial sediment prisms, thick with immature paleosols proximal to streams and thinner with mature paleosols distally. Pedofacies are bounded by either trunk-stream channel or crevasse-splay deposits, which represent time-stratigraphic markers. The floodplain widths of the Willwood rivers varied from 15 to 20+ km. Paleosols occur throughout the Willwood Formation and the most mature paleosols required about 60 Ka to form whereas the least mature, required 0.5 to 1.0 Ka. Paleosol thicknesses vary from about 0.3–8.0 m and are directly related to net sediment accumulation rate (NSAR) and profile maturity. Pedofacies also reflect NSAR controls; pedofacies are continuously superposed, 15–35-m-thick, and represent time intervals of 30–60 Ka.In the earliest Eocene, paleosol maturity rose sharply, and NSAR plummeted (Fort Union Fm./Willwood Fm. contact), after which maturity gradually declined (and NSAR rose) throughout the early Eocene. This decline was punctuated by two episodes of severe decline, each corresponding with major increases in NSAR, increased tectonism, and episodes of faunal turnover (“Biohorizons” A and B). Above the biohorizons, species earlier tied to particular paleosol maturities were replaced by closely related though more generalized species with no marked paleosol preferences. Time-stratigraphic reconstruction of the Willwood Fm. shows that “Biohorizons” B and C record the same faunal event; B the extinctions, and C the immigrations.The 1,300 Willwood fossil vertebrate localities, which are distributed throughout the entire formation, occur in the surface horizons of cumulative alluvial paleosols. All fossil accumulations in paleosols are attritional and formed during pedogenesis. The most complete remains occur in immature paleosols, whereas the most abundant remains are found in mature paleosols. Within the large-scale Willwood ecologic setting, studies of discrete (m's to tens of m's thick) stratigraphic intervals suggest that the paleontology and sedimentology of these intervals can be significantly influenced by lateral differences in paleosol hydromorphy (soil wetness) and maturity (lateral position of a fossil-bearing paleosol with respect to an ancient river channel). These smaller-scale controls on fossil occurrences are important for distinguishing between real and apparent changes in faunal compositions over time and emphasize the value of three-dimensional stratigraphic analysis for interpreting paleontologic events.Supported by National Geographic Society grant 3985-89.

Numerical ages of Cenozoic biostratigraphic datum levels: Results of South Atlantic Leg 73 drilling

Six sites were drilled in the South Atlantic during the Leg 73 cruise of the Deep Sea Drilling Project (DSDP). Hydraulic piston coring at five of the six sites obtained a nearly complete sequence of undisturbed Cenozoic samples. The magnetostratigraphy at those sites was investigated by close sampling representing time intervals of about 10 4 yr. Most of the Cenozoic nannofossil and many of the foraminiferal zonal boundaries were accurately determined and magnetostratigraphically calibrated at those five Leg 73 boreholes. Their numerical ages have been computed assuming a linear spreading rate and a radiometric age of 66.5 m.y. for the Cretaceous-Tertiary boundary. Alternative magnetostratigraphic ages were obtained with the adoption of a 63.5-m.y. age for the Cenozoic. Our data confirm previous determinations of the Pleistocene-Pliocene boundary at 1.8 (1.7) m.y. B.P. and of the Pliocene-Miocene boundary at 5.1 (5.0) m.y. B.P. The Miocene-Oligocene boundary is placed within chron C-6 C, with a magnetostratigraphic age of 23.8–24.0 (22.7–22.9) m.y. The Oligocene-Eocene age is also very precisely located within chron C-13 R, with a magnetostratigraphic age of 37.1–37.2 (35.5–35.6) m.y. The Eocene-Paleocene boundary should be located within an uncored interval of chron C-24, with a magnetostratigraphic age of 59.0 (55.4) ± 0.2 m.y. A general accord of the magnetostratigraphic ages from Leg 73 sites and the radiometric ages published in the literature for the various zonal boundaries validates the assumption of a linear sea-floor–spreading rate during the Cenozoic.

Mass Mortality and Its Environmental and Evolutionary Consequences

The latest Mesozoic and earliest Tertiary sediments at Deep Sea Drilling Project site 524 provide an amplified record of environmental and biostratographic changes at the end of Cretaceous. Closely spaced samples, representing time intervals as short as 10(2) or 10(3) years, were analyzed for their bulk carbonate and trace-metal compositions, and for oxygen and carbon isotopic compositions. The data indicate that at the end of Cretaceous, when a high proportion of the ocean's planktic organisms were eliminated, an associated reduction in productivity led to a partial transfer of dissolved carbon dioxide from the oceans to the atmosphere. This resulted in a large increase of the atmospheric carbon dioxide during the next 50,000 years, which is believed to have caused a temperature rise revealed by the oxygen-isotope data. The lowermost Tertiary sediments at site 524 include fossils with Cretaceous affinities, which may include both reworked individuals and some forms that survived for a while after the catastrophe. Our data indicate that many of the Cretaceous pelagic organisms became extinct over a period of a few tens of thousands of years, and do not contradict the scenario of cometary impact as a cause of mass mortality in the oceans, as suggested by an iridium anomaly at the Cretaceous-Tertiary boundary.

Time resolution in fluvial vertebrate assemblages

Calibrating levels of time resolution that are accessible in the fossil record is important in understanding what evolutionary phenomena can be profitably studied using fossils. A model for attritional bone assemblage formation in fluvial deposits, based on observations of taphonomic processes in modern environments, provides order-of-magnitude estimates for time intervals represented in single unit, ‘contemporaneous' vertebrate samples. In order to form units with adequate material for analysis of morphological variation or paleoecological associations, it appears that bones must be spatially concentrated or stratigraphically condensed by sedimentary processes or biological agencies. In many cases this means that significant periods of time will be represented by single unit assemblages. According to predictions from modern environments, carcasses contributed through normal attrition can accumulate in the soil to ‘fossiliferous' densities over time intervals of 102–104yrs. Attritional channel assemblages include bones from three sources: floodplain land surfaces, floodplain deposits, and the active channel, and represent time intervals on the order of 102–104yrs. Given additional limitations on the composition of the fossil sample imposed by circumstances of preservation, outcrop availability and collecting strategy, attritional fluvial assemblages probably can be resolved only to 103years even under the best conditions. Time intervals represented by fossils are not necessarily the same as those represented by sedimentary events in fluvial systems because bones can continue to accumulate and may be concentrated during times of erosion or non-deposition. Fluvial vertebrate assemblages of comparable taphonomic history can be used to document evolutionary changes over periods longer than their finest level of time resolution. While they may not be applicable to questions of punctuated or gradual transitions over shorter time scales, the longer-term patterns should have their own evolutionary significance.