164,602 research outputs found
Astronomical calibration of the late Oligocene through early Miocene Geomagnetic Polarity Time Scale
No full textAt Ocean Drilling Program (ODP) Site 1090 (subantarctic South Atlantic), benthic foraminiferal stable isotope data (from Cibicidoides and Oridorsalis) span the late Oligocene through early Miocene (~24–16 Ma) at a temporal resolution of ~5 ky. Over the same interval, a magnetic polarity stratigraphy can be unequivocally correlated to the geomagnetic polarity time scale (GPTS), thereby providing direct correlation of the isotope record to the GPTS. In an initial age model, we use the newly derived age of the Oligocene/Miocene (O/M) boundary of 23.0 Ma of Shackleton et al. [Geology 28 (2000) 447], revised to the new astronomical calculation (La2003) of Laskar et al. [Icarus (in press)] to recalculate the spline ages of Cande and Kent [J. Geophys. Res. 100 (1995) 6093]. We then tune the Site 1090 18O record to obliquity using La2003. In this manner, we are able to refine the ages of polarity chrons C7n through C5Cn.1n. The new age model is consistent, within one obliquity cycle, with previously tuned ages for polarity chrons C7n through C6Bn from Shackleton et al. [Geology 28 447–450 (2000)] when rescaled to La2003. The results from Site 1090 provide independent evidence for the revised age of the Oligocene/Miocene boundary of 23.0 Ma. For early Miocene polarity chrons C6AAr through C5Cn, our obliquity-scale age model is the first to allow a direct calibration to the GPTS. The new ages are generally within one obliquity cycle of those obtained by rescaling the Cande and Kent [J. Geophys. Res. 100 (1995) 6093] interpolation using the new age of the O/M boundary (23.0 Ma) and the same middle Miocene control point (14.8 Ma) used by Cande and Kent [J. Geophys. Res. 100 (1995) 6093]
Milankovitch Cyclicity and Stable Isotope Calibration in the Paleogene (invited)
No full textSignificant progress has been made over the last decade in the extension of astronomically calibrated geological time scales for the Neogene (Hilgen et al., 1999, Shackleton et al., 1999). The validity of these time scales has been supported by comparison of data from different parts of the world’s oceans, through the improvement of astronomical calculations, and independent dating methods and intercalibrations (Renne et al., 1994). While evidence of astronomical forcing has been found for intervals from most parts of the Cenozoic, extending astronomically calibrated time scales into the Paleogene faces some fundamental problems that require a different approach than the sophisticated “pattern matching” that worked so well for the Neogene. These challenges are related to uncertainties and limits of astronomical calculations, sparser data coverage, and a climate system that behaved quite differently to today’s “ice-house” setting. This contribution reviews some of the challenges that will have to tackled, presents a new set of astronomically calibrated benthic isotope data from the late Eocene, and suggests a new approach to synthesise astronomically calibrated durations for magnetic reversals that arise from floating time scales, which are so far more common in the Paleogene.Challenges and limits of astronomical calculations for time scale use:
One crucial challenge that is faced when tackling the astronomical calibration of the geological time scale is the fact that the Solar System is chaotic, limiting the age back to which one can compute astronomical solutions with confidence (Laskar, 1999). Thus, one has to constrain astronomical parameters from the rock record. Apart from tidal dissipation effects, which change the detailed interference of particularly obliquity and climatic precession cycles, a larger scale effect is more directly linked to the chaotic nature of the Solar System: amplitude variations of the obliquity and climatic precession cycles with periods of ~1.2 and ~2.4 million years can be affected by chaotic transitions in the planetary solutions. This contribution will review the effects these chaotic transitions can have on astronomical “target” curves, particularly during the Paleogene, and why astronomical time scale calibrations will have to take this into account.Astronomically calibrated stable benthic isotopes from the Eocene:
High-resolution lithological proxy measurements from ODP 1052 in the western Atlantic (Pälike et al., 2001) have provided duration estimates for magnetochrons from the late middle Eocene. This contribution presents high-resolution benthic stable isotope measurements from the same location. The astronomically calibrated isotope measurements co-vary with the lithological measurements and the astronomy in the obliquity frequency band, documenting the interaction of astronomy and climate during this transition from the Paleogene “green-house” world to the Oligocene “ice-house” world (Figure 1), and significant events that were not recognised in previous, lower resolution studies.Integration of floating time scales with magnetostratigraphy:
The geomagnetic polarity time scale (Cande & Kent, 1995) incorporates astronomically calibrated ages back to 5.23 Ma. Recent results have changed significantly the age of the Oligocene/ Miocene boundary (Shackleton et al. 1999, 2000). These changes, which we will show have now been corroborated by results from ODP 199, need to be incorporated into the astronomically calibrated polarity time scale. We present a new approach, using a combination of calibrated absolute ages, and constraints on sea-floor spreading rates obtained from astronomically calibrated magnetic reversals from floating time scales, to compute a consistent set of spline interpolated ages for sea-floor magnetic reversals. This approach allows us to incorporate durations of magnetic reversals that result from the floating time scales more common in the Paleogene so far. First results, constrained by results from ODP 1218 in the Oligocene, and ODP 1052 in the late middle Eocene, suggest that the Eocene/Oligocene boundary age could be slightly older than previously estimated. It is suggested that this approach might be a useful first step to integrate astronomically calibrated ages from the Cenozoic until a full coverage, with independent data from different ocean basins, becomes available
Shackleton, J H (James Harold), WX9252
No full textThis record was harvested from a previous catalogue system and will be withdrawn in 2025. Information in this record may be superseded or incomplete. Visit this record in UMA's new catalogue at: https://archives.library.unimelb.edu.au/nodes/view/416188Surname: SHACKLETON. Given Name(s) or Initials: J H (JAMES HAROLD). Military Service Number or Last Known Location: WX9252. Missing, Wounded and Prisoner of War Enquiry Card Index Number: 27026.238341
Item: [2016.0049.48449] "Shackleton, J H (James Harold), WX9252
Improved astronomically tuned timescales for the Late Neogene (abstract of paper presented at EUG X, Strasbourg, France, 28 Mar - 1 Apr 1999)
No full textRecent attempts to tune geological timescales to Milankovitch-type cycles rely on matching features in proxy records to astronomically calculated obliquity and precession cycles. They often use tuning targets such as the 65°N summer insolation curve. It has been shown (Laskar 1993) that the exact position of insolation peaks in time depends on the parameters chosen for dynamical ellipticity and tidal dissipation (the Earth model). Hence a better knowledge of these parameters is needed before precise timescales for the pre Pliocene can be developed using traditional methods (Lourens, 1996).High quality geological records that have been tuned to an astronomical target have been published from both the Pacific (especially ODP Leg 138; Shackleton et al., 1995) and the Atlantic (especially ODP Leg 154; Shackleton and Crowhurst, 1997) although two different astronomical solutions have been used for the two studies. We have re-examined these data sets to place them in a consistent time scale, and to improve signal-to-noise ratio by stacking different data sets. Our objective is to optimise the separation in the data of that variability that is independent of the chosen Earth model (the amplitude modulation of the precession and obliquity components, extracted by complex demodulation) from the components that do depend on the chosen Earth model (the mean frequencies for obliquity and climatic precession). This operation can be performed iteratively on older sequences to optimise the astronomical solution for longer time intervals
Emergence of the Shackleton Range from beneath the Antarctic Ice Sheet due to glacial erosion
Get PDFThis paper explores the long-term evolution of a subglacial fjord landscape in the Shackleton Range, Antarctica. We propose that prolonged ice-sheet erosion across a passive continental margin caused troughs to deepen and lower the surrounding ice-sheet surface, leaving adjacent mountains exposed. Geomorphological evidence suggests a change in the direction of regional ice flow accompanied emergence. Simple calculations suggest that isostatic compensation caused by the deepening of bounding ice-stream troughs lowered the ice-sheet surface relative to the mountains by ~ 800 m. Use of multiple cosmogenic isotopes on bedrock and erratics (26Al, 10Be, 21Ne) provides evidence that overriding of the massif and the deepening of the adjacent troughs occurred earlier than the Quaternary. Perhaps this occurred in the mid-Miocene, as elsewhere in East Antarctica in the McMurdo Dry Valleys and theLambert basin. The implication is that glacial erosion instigates feedback which can change ice-sheet thickness, extent and direction of flow. Indeed, as the sub-glacial troughs evolve over millions of years, they increase topographic relief and this changes the dynamics of the ice sheet
Astronomical calibration of the late Oligocene through early Miocene geomagnetic polarity time scale (abstract of paper presented at AGU Fall Meeting, San Francisco, 8-12 Dec 2003)
No full textAt Ocean Drilling Program Site 1090 (subantarctic South Atlantic) benthic foraminiferal stable isotope data (from Cibicidoides and Oridorsalis) span the late Oligocene through the early Miocene (24-16 Ma) at a temporal resolution of 5 kyr. In the same time interval, a magnetic polarity stratigraphy can be unequivocally correlated to the geomagnetic polarity timescale (GPTS), thereby providing direct correlation of the isotope record to the GPTS. In an initial age model we use the newly derived age of the Oligocene/Miocene boundary of 23.0 Ma (Shackleton et al., 2000) revised to the new astronomical calculation of Laskar (2001) to recalculate the spline ages of Cande and Kent (1995). We then tune the site 1090 oxygen isotope record to obliquity, assuming a 7.2 kyr phase lag, using the new astronomic solution of Laskar (2001). In this manner we are able to refine the ages of polarity chrons C7n through C5Cn.1n. The new age model is consistent, within one obliquity cycle, with previously tuned ages for polarity chrons C7n to C6Bn from Shackleton et al. (2000), rescaled to the new astronomical solution of Laskar (2001). For early Miocene polarity chrons C6AAr through C5Cn, our obliquity-scale age model is the first to allow a direct calibration to the GPTS. The new ages are also close to, within one obliquity cycle, to those obtained by rescaling the Cande and Kent (1995) interpolation using the new age of the O/M boundary (23.0 Ma), and the same middle Miocene control point (14.8 Ma) used by Cande and Kent (1992). Thus we have confidence in the orbitally tuned age model and the refined GPTS calibration for the late Oligocene through early Miocene
The effect of shot peening on notched low cycle fatigue
Get PDFThe improvement in low cycle fatigue life created by shot peening ferritic heat resistant steel was investigated in components of varying geometries based on those found in conventional power station steam turbine blades. It was found that the shape of the component did not affect the efficacy of the shot peening process, which was found to be beneficial even under the high stress amplitude three point bend loads applied. Furthermore, by varying the shot peening process parameters and considering fatigue life it has been shown that the three surface effects of shot peening; roughening, strain hardening and the generation of a compressive residual stress field must be included in remnant life models as physically separate entities. The compressive residual stress field during plane bending low cycle fatigue has been experimentally determined using X-ray diffraction at varying life fractions and found to be retained in a direction parallel to that of loading and to only relax to 80% of its original magnitude in a direction orthogonal to loading. This result, which contributes to the retention of fatigue life improvement in low cycle fatigue conditions, has been discussed in light of the specific stress distribution applied to the components. The ultimate aim of the research is to apply these results in a life assessment methodology which can be used to justify a reduction in the length of scheduled plant overhauls. This will result in significant cost savings for the generating utility
Caenota equustagna Shackleton & Webb, 2015, sp. nov.
No full textCaenota equustagna sp. nov. urn:lsid:zoobank.org:act:E 41 D 5326 -C 154 - 4 BF 2-8044 - 3764 A 58 AB 7 BE Figs 46–67 Diagnosis. This species is most nearly resembles C. cudonis. However, the male anterior antennal process, while relatively large, is smaller than the posterior antennal process; the male maxillary palpi have their segments directed anterad and each segment II anteriorly has a whorl of long, dark setae. The female head has 2 to 5 pairs of setose punctures on the head capsule medial and anterior to the scapes. The larval frontoclypeus has the lateral margins of the anterior section relatively straight, converging as they approach the anterior margin; the larval foretrochantins each have many setae along the anterodorsal margin. Description.Male. Length of each forewing 10–11.5 mm, n= 3. Head (Figs 46, 47) with anterior surface depressed, capsule short and wide; eyes positioned anteriorly on lateral margins; postocular setal warts present, wider dorsally, tapering ventrally; no other setal warts on head capsule. First flagellar antennal segment modified, its anterior surface somewhat concave with brush of short dark setae basally, produced anteriorly; anterior antennal process relatively large but smaller than posterior antennal process, broad, flattened, anterior margin with dense, thick, dark setae; posterior antennal process large, rounded in lateral view, with scattered dark setae, posterior and anterior margins with dense dark setae, inner surface concave, with fringes of setae on basal and dorsal margins, 2 clumps of long, dark setae in basal anterior and posterior corners, antennal scape protruding through anterior surface. Maxillary palpi each 5 -segmented; segment I broad, ventral surface with large pigmented area and densely scattered setae, lateral surface with narrow area of pigmentation elongated laterally in dorsal 1 / 2, inner surface densely covered with short yellow setae, anterodorsal corner with brush of long, dark setae; segment II broad, lateral surface with area of pigmentation near posterior margin elongate transversely, ventral surface with dense, long, dark setae, anteriorly with whorl of long, dark setae, inner surface with long pale setae along dorsal margin; segment III arising from dorsal 1 / 2 of anterior margin of segment II, short, bulbous, inner surface with pale setae dorsally, long dark setae on ventral surface; segment IV shorter than segment III, squat; segment V elongate, slightly shorter than segment II, with brush of long pale setae directed anterad. Labial palpi (broken in specimen) setose. Pronotum with 1 pair of broad setal warts. Forewings (Fig. 56) brown/grey with some white mottling; each with forks I and II sessile, fork III on pedestal, fork IV absent, crossvein r concave, crossvein cu (joining Cu 1 b to Cu 2) above arculus, nygmata in thyridial cell and fork II. Hind wings (Fig. 57) relatively broad; each with discoidal cell absent; forks I and II sessile, fork III on pedestal; nygma in fork II; crossvein m-cu (joining Cu 1 a to M) present; anal veins terminating in basal 1 / 8 th of wing, not at wing margin. Foreleg tarsal segments with short, dark setae dorsally; tibiae covered in dense, long, dark setae dorsally and laterally; femora with long, pale setae on dorsal and inner lateral surfaces; costae with inner margins covered in long pale setae; mid- and hind legs unmodified. Genitalia (Figs 49–52) having segment X in dorsal view with lateral margins concave in anterior 1 / 2, convex in posterior 1 / 2, tapering posteriorly, strongly incised in apical 1 / 3 rd, with scattered, strong, dark setae, dorsal surface with line of strong, dark setae from about 1 / 3 rd length to near apex; in lateral view broad, rounded apically, with small anteroventral protrusion subapically. Preanal appendages slender, apically rounded, about 1 / 2 length of segment X. Inferior appendages broad in basal 1 / 3 rd, each branching into slender ventral and broad dorsal processes; ventral process tapering to point, strongly curved mesad at about 1 / 3 rd from base, then strongly curved dorsolaterad at 2 / 3 rds from base; dorsal process extending beyond segment X, tapering to point, with slender, posteriorly directed process arising about 1 / 2 length on dorsal surface and tapering to point. Pair of large projections covered with fine, short, pale setae arising from phallocrypt, in dorsal and ventral views angled laterad, extending just beyond segment X; in lateral view lateral surface concave, with slender, sclerotised ridge extending from base of segment IX to less than 1 / 3 rd length of projection. Phallus (Fig. 52) in lateral view gradually curved ventrad, dorsal surface convex, lateral surfaces concave, lightly sclerotised along length except for membranous apex, phallotremal sclerite somewhat triangular, positioned dorsolaterally about 3 / 4 ths length. Female. Length of each forewing 11 –13.0 mm, n= 4. Head (Fig. 48) with anterior dorsal section between antennae slightly depressed; postocular area relatively large, eyes positioned anteriorly on lateral margins; postocular setal warts present, broader dorsally, tapering ventrally; posterior setal warts present, long; anterior setal warts present, teardrop-shaped, separated; 2 to 5 pairs of setose punctures on head capsule medial and anterior to scapes. Antennae slightly shorter than forewing length; antennal scapes relatively broad and long, each about as long as its pedicel and flagellar segment I combined. Maxillary palps each 5 -segmented, unmodified. Labial palps each 3 -segmented. Pronotum with 2 pairs of warts, mesal pair smaller than lateral. Wings (Figs 58, 59) with venation and colouring similar to males except that in hind wing fork II sessile and vein A 2 joining A 1 before 1 / 2 length of A 1. Abdominal segment VIII with single sclerotised fold on ventrolateral margin. Foreleg tibiae and tarsi with short, dark setae. Genitalia (Figs 53–55) with preanal appendages relatively slender, widening posteriorly, apically rounded; sclerotised keel present ventrally along midline. Pupa. Head (Fig. 60, 61) having frons with pair of rounded lobes along midline of head capsule; labrum with 3 setae on each distally projected lobe, each anterolateral margin with 4, long dark setae and 2 small pale setae on anterior margin; mandibles without small subapicomesal teeth; antennae broad basally, each with about 5 setae in line dorsally. Abdominal segment I with pair of friction pads dorsally; tergites III to VI each with 1 pair of anterior hookplates, tergite V with 1 pair of posterior hookplates, anterior hookplates (Fig. 63) with 2 hooks; posterior hookplates (Fig. 64) with 3 hooks; segment IX (Fig. 62) of male with pointed projections arising from ventrolateral margins, directed dorsolaterad; abdomen terminating in pair of tapering sclerotized processes curved dorsolaterad to acute apices, each with 4 basolateral setae, and around 16 subapical setae; ventral surface with three pairs of closely grouped setae near mid-line at about 1 / 2 length of segment, one pair of setae basad of posterior projections. Larva. Approximate length 9.5 –11.0 mm at maturity. Head (Fig. 65) having frontoclypeus with lateral margins of anterior section relatively straight, converging near anterior margin; setae 6 positioned at constriction; posterolateral corners at constriction relatively sharply angled. Pronotum (Fig. 66) having anterior margin with dark setae; dorsum with narrow transverse line of setae, 1 or 2 setae deep, about 1 / 3 rd distance from anterior margin. Foretrochantins (Fig. 67) each with row of long, dark setae along anterodorsal margin. Abdomen with gills either simple or branched; segment I without gills; segment II gills VL 1 and VL 2 either present or absent, VL 3 present, L 1 absent, L 2 present or absent, DL 1 either present or absent; segment III gills VL 1 present, L 1 present or absent, DL 1 present; segment IV without gills. Holotype. Male, [AUSTRALIA: New South Wales] Horse Swamp Ck at Horse Swamp campground, - 31 º 55 ' 35.68 ''S 151 º 23 ' 10.62 ''E, 24 Feb 2011, J. Mynott and M. Shackleton (AM MS 988). Paratypes. [AUSTRALIA: New South Wales] collected with holotype, 3 females (AM MS 773 –MS 775), 3 male pupae (AM MS 99, MS 789, MS 790), 3 larvae (AM MS 786 –MS 788. Collected at same site as holotype, 11 Nov 2010, J. Mynott and M. Shackleton, 1 male (AM MS 772); 24 Feb 2011, J. Mynott and M. Shackleton, 2 larvae (AM MS 981 and MS 982). New South Wales, Horse Swamp Ck, up stream of Horse Swamp, 31 ° 56 'S 151 ° 25 'E, 2 Oct 2008, J. Dean and D. Cartwright, 3 larvae (AM MS 307, MS 90, MS 96), 1 March 2011, J. Mynott and M. Shackleton, 1 female (AM MS 1063). Kunderang Brook on Racecourse Trail, Werrikimbe NP, 31 º 8 ' 8.77 ''S 152 º 17 ' 19.31 ''E, 28 Feb 2011, J. Mynott and M. Shackleton, 1 male (AM MS 1049). Etymology. From the Latin equus and stagna, combined as a pleural neuter noun in apposition to Caenota, meaning “the pools of the horse,” pertaining to the type locality.Published as part of Shackleton, M. E. & Webb, J. M., 2015, Revision of the genus Caenota Mosely (Trichoptera: Calocidae), with descriptions of 2 new species and the larva of C. nemorosa Neboiss, pp. 451-481 in Zootaxa 3972 (4) on pages 475-476, DOI: 10.11646/zootaxa.3972.4.1, http://zenodo.org/record/23363
Aspects of the curve complex and the mapping class group
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