Excursion Interval Research Articles

Chemostratigraphy in the Swedish Upper Ordovician: Regional significance of the Hirnantian δ13C excursion (HICE) in the Boda Limestone of the Siljan region

Samples from the Boda Limestone and immediately overlying strata at Osmundsberget in the Siljan region have produced an excellent Hirnantian isotope excursion (HICE) curve, in which the excursion interval is about 22 m thick and the HICE reaches maximum δ13C values of between +5‰ and +6‰. Both the HA and HB stratigraphic unconformities are recognized in the study succession as well as in the coeval stratigraphic interval in the Loka Formation in Västergötland and at Meifod in central Wales. A comparison between these and some other shallow-water successions in North America and northern Europe shows striking similarities in the δ13C curve, the lithology, and the stratigraphic sequence development. This is taken as support for the idea that glacio-eustacy exercised a major control on the depositional pattern in shallow-water successions during Hirnantian time. This applies also to the Osmundsberget succession where the formation of carbonate mounds was stopped by the HA regression that was caused by a major glaciation in Gondwana.

Mode and tempo of the Paleocene-Eocene thermal maximum in an expanded section from the Venetian pre-Alps

Mode and tempo of an expanded Paleocene Eocene Thermal Maximum section in the Venetian Pre-Alps

Occupation time theorems for a class of one-dimensional diffusion processes

The long time asymptotic behavior of the occupation times on a half line is studied for a class of one-dimensional diffusion processes whose excursion intervals have very heavy tail probability

Additive functionals and excursions of Kuznetsov processes

Let B be a continuous additive functional for a standard process and let be a stationary Kuznetsov process with the same semigroup of transition. In this paper, we give the excursion laws of conditioned on the strict past and future without duality hypothesis. We study excursions of a general regenerative system and of a regenerative system consisting of the closure of the set of times the regular points of B are visited. In both cases, those conditioned excursion laws depend only on two points and Xd, where ]g, d[ is an excursion interval of the regenerative set M. We use the ‐predictable exit system to bring together the isolated points of M and its perfect part and replace the classical optional exit system. This has been a subject in literature before (e.g., Kaspi (1988)) under the classical duality hypothesis. We define an “additive functional” for with B, we generalize the laws cited before to , and we express laws of pairs of excursions.

NEW EARLY EOCENE MAMMALIAN FOSSILS FROM THE HENGYANG BASIN, HUNAN CHINA

New fossil mammal materials found in the upper Lingcha Formation of the Hengyang Basin, Hunan, China, include a complete skull of Hapalodectes hetangensis and a taxon new to the fauna, Dissacus zengi, n. sp. H. hetangensis is the morphologically least derived species of the genus and currently is the earliest known record of the genus. D. zengi, n. sp. is the second Asian early Eocene record of the genus and represents the last occurrence of the genus in Asia. The Lingcha Formation includes two fossiliferous intervals. The upper contains about 12 species belonging to 7 mammalian orders, and the lower has yielded one mammalian species and one reptilian species. Biostratigraphic, chemostratigraphic, and magnetostratographic study in the Lingcha Formation indicates that the transient carbon isotope excursion that marks the Paleocene–Eocene boundary as currently advocated is present in the upper Lingcha Formation. The fauna from the upper Lingcha Formation occurs within the excursion interval, and is of earliest Eocene age. It is correlative with the Wasatchian-0 faunal zone in the Big Horn Basin, Wyoming, North America. The lower Lingcha Formation is of Paleocene age.

Evidence for the Kamikatsura and Santa Rosa excursions recorded in eolian deposits from the southern Chinese Loess Plateau

A detailed paleomagnetic investigation of a loess‐paleosol sequence in Baoji, southern Chinese Loess Plateau, reveals two distinct geomagnetic excursions accompanied by inferred relative paleointensity minima in the late Matuyama Chron. Rock magnetic experiments demonstrate that the sediments of the excursion intervals, in loess unit L9, have the same magnetic properties as those from surrounding normal and reversed polarity intervals. Assuming constant sedimentation rate between the Jaramillo‐Matuyama and Matuyama‐Brunhes magnetic polarity boundaries, the midpoint ages and durations for these two excursions are estimated at 0.89 Ma and 16 kyr, and 0.92 Ma and 7 kyr, respectively. In addition, these two events are separated by an interval with fully reversed polarity that had a duration of 16 kyr. These estimates are supported by paleoclimatologically inferred age constraints. The two excursions are probably correlated with the Kamikatsura and Santa Rosa excursions, respectively.

High-resolution records of methane hydrate dissociation: ODP Site 893, Santa Barbara Basin

Sediment records from ODP Site 893 (Santa Barbara Basin) have documented brief, negative carbon isotopic excursions in the late Quaternary in planktonic and benthic foraminifera. These excursions were interpreted as evidence of major releases of methane from the marine hydrate reservoir, affecting the entire water column and likely reaching the atmosphere. These carbon isotopic excursions are further recorded in the δ 13C values and concentration of lipid biomarkers of marine methanotrophs in the basin. Three previously described carbon isotopic excursions are re-examined during this study, at a resolution of every 3–7 years. The original isotopic excursions were sampled at a resolution of once every 50–70 years. Examination using light microscopy and environmental scanning electron microscopy (ESEM) indicates that specimens in this study are in excellent condition, with no apparent evidence of postdepositional diagenesis or alteration of the foraminiferal shells. Alternative hypotheses to account for these conspicuous carbon isotopic excursions include postdepositional carbonate diagenesis of the excursion intervals, and changes in surface ocean productivity and carbon export to the sediments. In this study, we further test the hypothesis that these excursions record the primary signal of methane in the water column by intensive analyses of the previously described carbon isotopic excursions. All four carbon isotopic excursions studied are associated with interstadial warming in the basin, time periods that are associated with shifts in the benthic foraminiferal assemblages toward low oxygen fauna. At high resolution, δ 13C excursions appear very brief (5–10 years duration), and within this time, planktonic and benthic species record a significant shift in δ 13C composition (planktonics to −3‰ and benthics to −6‰). Excursion events are synchronous in benthic and planktonic species within the resolution of these records. In one interval studied (∼37.5 ka), the bulk organic carbon δ 13C records a 1‰ negative shift, in concert with the carbon isotopic excursion in benthic and planktonic foraminifera, consistent with a carbon isotopic shift of photosynthetically derived marine organic carbon. We suggest that alternate explanations invoking changes in productivity and/or postdepositional alteration of these intervals cannot account for the entire magnitude of the carbon isotopic excursion nor the diversity of evidence presented here. Thus, methane hydrate destabilization remains a compelling hypothesis to explain the conspicuous δ 13C excursions observed in ODP Site 893 and similar late Quaternary excursions described elsewhere.

Eccentric Behaviors of the Brownian Sheet Along Lines

Distinct excursion intervals of a Brownian motion (that correspond to a fixed level) have no common endpoints. What is the situation for distinct excursion sets of a Brownian sheet? These sets are termed Brownian bubbles in the literature, and this paper examines how bubbles from fixed or random levels come into contact with each other, by examining whether or not the Brownian sheet restricted to a specific type of curve can have a point of increase. At random levels, we show that points of increase can occur along horizontal lines, while at fixed levels, such a point of increase can occur at the corner of a broken line segment with a right-angle. In addition, the Hausdorff dimension of the set of points with this last property is shown to be 1/2 a.s.

Floral response to rapid warming in the earliest Eocene and implications for concurrent faunal change

During the first 10–20 Kyr of the Eocene temperatures warmed by 4–8°C in middle and high latitudes, then cooled again over the succeeding ∼200 Kyr. Major changes in the composition of marine and terrestrial faunas, including one of the largest mammalian turnover events of the Cenozoic, occurred during this temperature excursion. To better understand the effects of rapid climatic change on continental biotas, we studied 60 fossil pollen samples collected from 900 m of section spanning approximately three million years of the late Paleocene and early Eocene; the samples come from the Fort Union Formation and Willwood Formation in the Bighorn Basin of northwestern Wyoming, paleolatitude approximately 47°N. There are 40 samples from the 500 m of rock deposited during the one million year interval centered on the Paleocene/Eocene boundary, although pollen was not preserved well in rocks representing the short warm interval at the base of the Eocene.Overall, the palynoflora shows moderate change in composition and diversity. Two pollen taxa clearly expanded their ranges to include North America in the first 400 Kyr of the Eocene,Platycarya(Juglandaceae), andIntratriporopollenites instructus(cf.Tilia), but they account for less than 5% of pollen grains in the early Eocene. There are no last appearances of common taxa associated with the Paleocene/Eocene boundary. The most noticeable palynological changes are the decrease in abundance ofCaryapollenitesspp. andPolyatriopollenites vermontensis(Juglandaceae), and the increase in abundance of Taxodiaceae (bald cypress family), Ulmaceae (elm family), and Betulaceae (birch family), particularlyAlnipollenitesspp. (alder). There are 22% more species in the Eocene samples than in the Paleocene samples; mean richness of Eocene samples is 17% higher than the mean of Paleocene samples. The mean evenness of Eocene samples is higher than that of Paleocene samples, but the difference is not significant.The modest level of floral change during the late Paleocene and early Eocene contrasts with the major taxonomic turnover and ecological rearrangement of North American mammalian faunas observed at the same time. Faunal change probably resulted from intercontinental range expansion across Arctic land bridges that became habitable as a result of high-latitude warming, so it is surprising that climatically sensitive plants did not also experience a major episode of interchange. The absence of fossil plants from the temperature excursion interval itself could prevent us from recognizing a transient shift in floral composition, but it is clear that the flora did not undergo a major and permanent restructuring like that seen in the mammals. The contrast between the moderate floral response to warming and the strong faunal response is consistent with the idea that interactions between immigrant and native taxa, rather than climate directly, were the primary cause of terrestrial biotic change across the Paleocene/Eocene boundary.

Evolutionary consequences of the latest Paleocene thermal maximum for tropical planktonic foraminifera

Study of planktonic foraminiferal assemblages preserved in the central, equatorial Pacific (ODP Site 865) reveal that genera which inhabited the near-surface, mixed layer diversified during the latest Paleocene thermal maximum (LPTM). This transient diversification gave rise to a suite of short-lived (50 to several 100 ka), distinctive morphotypes ( Morozovella allisonensis sp. nov., M. africana, and Acarinina sibaiyaensis) that are restricted to the LPTM interval. Parallel morphometric and stable isotopic data have been recorded from individual shells of members of the M. velascoensis/ M. allisonensis and A. soldadoensis/ A. sibaiyaensis lineages. The single-specimen isotope data confirm that M. allisonensis and A. sibaiyaensis are indeed stratigraphically restricted to the LPTM carbon isotope excursion; all specimens recorded anomalously low δ 13C values. A stratigraphic succession of single-specimen isotope data from within the δ 13C excursion interval was used to reconstruct the complex population dynamics which mediated the rapid (<10 ka) evolution of the LPTM taxa. These data reveal that local populations of ancestral M. velascoensis temporarily collapsed with the descendant M. allisonensis suddenly originating from a peripherally isolated population. This pattern is most consistent with peripatric speciation. Overall, the evolutionary transition from Acarinina soldadoensis to A. sibaiyaensis is most consistent with sympatric speciation, although certain elements of this transition are suggestive of a parapatric mechanism. We postulate that the rapid evolution of the LPTM taxa A. sibaiyaensis and M. allisonensis was fostered by a deepening of the nutrient-depleted, mixed layer. Alternatively, the LPTM morphotypes M. allisonensis and A. sibaiyaensis may have been extreme ecophenotypic variants arrayed along an intensified ecological gradient. Support for this latter interpretation is derived from the morphometric data which clearly show the morphological intergradation of the LPTM taxa with their respective ancestors. Thus, the morphologic diversification seen during the LPTM may represent failed speciations. Both scenarios invoke a temporary steepening of clinal gradients in response to intensified oligotrophy. The strong size-dependencies displayed by the δ 18O and δ 13C signatures of M. allisonensis and A. sibaiyaensis suggest that photosymbiosis facilitated the evolutionary success of the morozovellids and acarininids during the LPTM.

Extreme Lengths in Brownian and Bessel Excursions

VI ( t) >~ V2(t) 0 ? . ??Vn(t) 3 . , (1.1) the ordered excursion lengths of R over (0, t), the last zero-free interval (A-(t), t) being considered as an (incomplete) excursion interval. Discussions on excursion intervals excluding (A-(t), t) are postponed to Section 5. A remarkable development (Pitman and Yor 1992; 1997) in the study of ordered excursion lengths is that for any fixed positive numbers r, s and t and any integer k > 1,

Evaluation of human postprandial antral motor function using ultrasound

We used high-resolution ultrasound to characterize postprandial antral excursion characteristics in 15 healthy volunteers. Antral circumference was repeatedly measured in the sagittal plane anterior to the abdominal aorta, at 15-min intervals following ingestion of a standard meal. At each recording period, the maximal and minimal antral circumference was noted. In seven volunteers, antral circumference was studied continuously for 5 min at each 15-min recording period to assess the frequency and regularity of antral excursions. Antral circumference increased immediately after meal ingestion from preprandial values of 64 +/- 2.6 (means +/- SE) to 138 +/- 3.1 mm and then gradually decreased with meal emptying to 72 +/- 2.6 mm at 90 min. The continuous recordings of antral images revealed that antral circumference transiently decreased at regular intervals; the interval between successive excursions was 20.1 +/- 0.2 s. There was no change in the excursion interval with emptying. The excursion amplitude decreased from 33 +/- 3.9 mm immediately post-meal ingestion to 13 +/- 1.5 mm at 90 min (r2 = 0.62 +/- 0.07). The ratio between the excursion amplitude and circumference of the relaxed antrum changed only slightly with time (slope = 0.0009 +/- 0.00003). Comparison of repeat studies in 10 volunteers showed a low intra-individual variation for postprandial antral circumference and excursion amplitude.