Sources of magnetic susceptibility in a slate     

Graham Borradaile  John Mothersill  Don Tarling  Craig Alford 《Earth and Planetary Science Letters》1986,76(3-4)

Magnetic susceptibility and its anisotropy in the Borrowdale Volcanic slates at Kentmere in the English Lake District are attributed largely to preferred orientation of a paramagnetic chlorite of diabantite-ripidolite composition. In units of 10⁻⁶ cgs/g, the principal susceptibilities for the slates are 9.61; 9.42; 8.69 and for the chlorite grains the minimum anisotropy is represented by principal susceptibilities of 11.57; 11.22 and 9.15. Because the magnetic susceptibility is carried by a tightly packed, matrix-forming mineral that has recrystallised during the deformation it is not possible to imagine simple grain rotation as being responsible for the anisotropy of susceptibility.  相似文献   

Dating of paleomagnetic logs by tephra layers,Mara Lake,Canada     

John S. Mothersill 《Studia Geophysica et Geodaetica》1991,35(4):302-313

Summary The paleodeclination and paleoinclination logs compiled from the cores taken from Mara Lake show consistent, well defined oscillations. It was hoped that the Mt. St. Helens and Mt. Mazama tephra layers encountered in the cores would provide accurate, absolute dating control of the cores as well for the paleodeclination and paleoinclination logs. Unfortunately the dates of 3300 BP for the Mt. St. Helens tephra and 6845 ± 50 BP for the Mt. Mazama tephra are incompatible with a constant rate of sedimentation. It would appear that the Mt. Mazama date is valid and a redetermination of the Mt. St. Helens date based on the 6845 ± 50 BP date for the Mt. Mazama tephra and a constant rate of sedimentation from Mazama time to the present gives a date of 4100 BP for the Mt. St. Helens tephra. This dating control has been used to construct paleodeclination and paleoinclination logs versus time which correlate well with paleomagnetic logs from Fish Lake, Oregon and Shawnigan Lake on Vancouver Island.Presented at 2nd conference on New Trends in Geomagnetism, Castle Bechyn, Czechoslovakia, September 24–29, 1990.  相似文献   

Hepatic biomarkers of sediment-associated pollution in juvenile turbot, Scophthalmus maximus L     

Hartl MG  Kilemade M  Sheehan D  Mothersill C  O'Halloran J  O'Brien NM  van Pelt FN 《Marine environmental research》2007,64(2):191-208

Hatchery-reared turbot (Scophthalmus maximus L.) were exposed for 3 weeks, under laboratory conditions, to sediment collected from polluted sites in Cork Harbour and a reference site at Ballymacoda, Co. Cork, Ireland. The potential of surficial sediment for inducing hepatic biomarkers was assessed at two levels of biological organisation: expression of cytochrome P450 [Western blotting analysis and 7-ethoxy-resorufin O-dealkylase (EROD), 7-benzoxy resorufin O-dealkylase (BROD), 7-methoxy resorufin O-dealkylase (MROD), 7-pentoxy-resorufin O-dealkylase (PROD) activities] and DNA integrity (Comet assay). Positive controls were generated, either by exposing turbot to cadmium chloride-spiked seawater (Comet assay) or to beta-naphthaflavone by intraperitoneal injection (cytochrome P450 induction). The induction of cytochrome P450 activity (EROD, MROD and PROD) in animals following a 7-day exposure to contaminated sediments was significantly higher than those exposed to reference site sediment and remained elevated thereafter; BROD was not induced. DNA single-strand breaks were also significantly higher following exposure to contaminated sediments throughout the experiment. Although no direct correlation between induction of alkoxyresorufin O-dealkylase activities and a particular chemical class was established, the induction of MROD and PROD activities in fish exposed to sediments containing complex contaminant mixtures, appeared to be more sensitive than conventional EROD activity assays. We conclude from the present laboratory study that S. maximus is a suitable sentinel species for the assessment of moderately contaminated sediments and therefore allows for the further development of this model for future, ecologically relevant, field studies.  相似文献   

Coaxial deformed and magnetic fabrics without simply correlated magnitudes of principal values     

Graham J. Borradaile  John S. Mothersill 《Physics of the Earth and Planetary Interiors》1984,35(4):294-300

Results of a detailed strain analysis of accretionary lapilli in the Borrowdale Volcanic sequence of England indicate a bedding-memory component to the rock's fabric. A simple, new strain analysis method uses variation in thickness of lapilli rims and appears not to be influenced by the bedding fabric. The orientations of the principal magnetic susceptibilities show a strong correlation with the principal strain orientations, as in other studies, but we can detect no correlation between strain magnitudes and principal susceptibility magnitudes in the same small specimens.  相似文献   

An assessment of the pollutant status of surficial sediment in Cork Harbour in the South East of Ireland with particular reference to polycyclic aromatic hydrocarbons     

Kilemade M  Hartl MG  Sheehan D  Mothersill C  van Pelt FN  O'Brien NM  O'Halloran J 《Marine pollution bulletin》2004,49(11-12):1084-1096

Surface sediment from three polluted sites within Cork Harbour, Ireland, and from a relatively clean reference site were collected and analysed for polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs), brominated flame retardants (BFRs), organotins (OTs), and heavy metals. PAHs were determined to be the most abundant class of contaminant. Concentrations of the sum (Sigma) of the 21 PAHs measured from the Harbour sites (2877.70 ng g(-1), 1000.7 ng g(-1) and 924.40 ng g(-1) dry weight respectively) were significantly higher than that of the sediment from the reference site (528.30 ng g(-1) dry weight). An inner harbour site, Douglas being the more contaminated of the three harbour sites. A similar pattern was observed with the other contaminants however, these compounds, with the exception of the heavy metals, all tended to be detected at concentrations on or below detection limits.  相似文献   

Implications of seasonal priming and reproductive activity on the interpretation of Comet assay data derived from the clam, Tapes semidecussatus Reeves 1864, exposed to contaminated sediments     

Hartl MG  Coughlan BM  Sheehan D  Mothersill C  van Pelt FN  O'Reilly SJ  Heffron JJ  O'Halloran J  O'Brien NM 《Marine environmental research》2004,57(4):295-310

We explore the use of the clam Tapes semidecussatus Reeves 1864 as an indicator for the presence of potentially genotoxic substances in estuarine sediments. The limitations associated with the interpretation of Comet assay data (expressed as % DNA in tail) in terms of clam reproductive state, size (age) and thermal exposure history following laboratory acclimation are discussed. Hatchery-reared clams, subjected to ambient temperature fluctuations during growth, were exposed in vivo under laboratory conditions for three weeks to sediment samples collected from a polluted site and a "clean" reference site. The DNA damage observed in haemocytes, gill and digestive gland cells was significantly higher in animals exposed to contaminated sediment compared to those exposed to sediment from the reference site. The extent of DNA damage recorded was not correlated with size (age). Spawning was not observed during the experiment. Nevertheless, clams with well-developed gonads showed a statistically higher degree of DNA damage in gill and digestive gland cells- but not haemocytes, demonstrating an increased sensitivity to potential genotoxic compounds, possibly caused by impaired DNA repair capacity due to reproductive activity. Furthermore, the degree of DNA damage in clams exposed to contaminated sediments was higher in autumn and winter compared to spring and summer, suggesting an effect of seasonal priming.  相似文献   

The timing of regional Lateglacial events and post-glacial sedimentation rates from Lake Superior     

Andy Breckenridge  Thomas C. Johnson  Suzanne Beske-Diehl  John S. Mothersill 《Quaternary Science Reviews》2004,23(23-24):2355-2367

We analyze both new and previously published paleomagnetic records of secular variation (PSV) from Lake Superior sediment cores and compare these records to correlated rhythmite (varve) thickness records to determine post-glacial sedimentation rates and to reassess the termination of glaciolacustrine varves in the basin. The results suggest that offshore sedimentation rates have exhibited considerable spatial variation over the past 8000 years, particularly during the mid-Holocene. We attribute offshore, mid-Holocene sedimentation changes to alterations in whole basin circulation, perhaps precipitated by a greater dominance of the Gulf of Mexico air mass during the summer season. Nearshore bays are characterized by high sedimentation rates for at least 1000 years after varve cessation and during a period between around 4500 and 2000 cal. BP. After 2000 cal. BP, sedimentation rates subsided to earlier rates. The increases between 4500 and 2000 cal. BP are probably due to lake level fall after the Nipissing II highstand.The older glaciolacustrine varve thickness records suggest that the influx of glacially derived sediment ended abruptly everywhere in the lake, except near the Lake Nipigon inlets. Multiple sediment cores reveal 36 anomalously thick varves, previously ascribed to the formation of the Nakina moraine, which were deposited just prior to varve cessation in the open lake. The PSV records support the observation that the cessation of these thick varves is a temporally correlative event, occurring at 9035±170 cal. BP (calibrated years before 1950, ca 7950–8250 ¹⁴C BP). This date would correlate to the eastern diversion of Lake Agassiz and glacial meltwater into Lake Ojibway.  相似文献   

Paleomagnetic results from lakes Victoria and albert,Uganda     

J. S. Mothersill 《Studia Geophysica et Geodaetica》1996,40(1):25-35

Summary Unfortunately most of the cores taken from Salisbury Channel, Lake Victoria which contained discrete pieces of wood suitable for¹⁴C dating were organic-rich and therefore showed a considerable scattering of the remanent magnetic direction plots resulting in poorly defined oscillations. However, the cores from Pilkington Bay as well as core C5 from Salisbury Channel, Lake Victoria and the cores from Lake Albert show well-defined paleodeclination and paleoinclination logs. Several cores from Pilkington Bay and Lake Albert with some¹⁴C control can be used to verify the isochronous correlation of the paleodeclination and paleoinclination oscillations. Core C5, from Salisbury Channel, Lake Victoria, which did not contain any discrete pieces of plant or shell material has been dated by correlation with other cores which had¹⁴C control. This core which shows the best-defined paleodeclination and paleoinclination logs of this study would appear to have bottomed in sediments in excess of 6000 years BP age and with the work of Maley et al. (1990) from the Cameroons provides a complete record of the Holocene for Central Africa.  相似文献   

Utilization of magnetic susceptibility and intensity of remanent magnetisation for chronostratigraphic correlation of lake sediments     

John S. Mothersill 《Studia Geophysica et Geodaetica》1991,35(4):314-324

Summary Magnetic susceptibility (k) and intensity of remanent magnetisation (J) logs can be used for chronostratigraphic correlation of the late Holocene stratigraphic sequence of the southern basin of Mara Lake. Q-ratio (J/k) logs also show a good correlation from core to core within the lake basin.Chronostratigraphic correlations of cores from Mara, Shawnigan and Christina Lakes were carried out on the basis of paleodeclination and paleoinclination log information and substantiated by the relative position of the Mt. Mazama tephra in the Mara and Shawnigan Lake stratigraphic sequences. This information was used as dating control to construct and compare magnetic susceptibility, intensity of remanent magnetisation and the Q ratio record for each of the three lakes. None of these three parameters would appear to be useful for correlation from lake basin to lake basin. Therefore, on the basis of this study magnetic susceptibility, intensity of remanent magnetisation and Q ratio logs can be used for intra-lake basin correlation but not for lake to lake correlation.Presented at 2nd conference on New Trends in Geomagnetism, Castle of Bechyn, Czechoslovakia, September 24–29, 1990.  相似文献   

Paleomagnetic Logs: A Tool for Dating Lake Erie's Past     

Mothersill  J.S.  Schurer  P.J. 《Studia Geophysica et Geodaetica》2003,47(2):289-300

An examination of the lithology, paleomagnetic and Mossbauer Effect Spectroscopy (MES) log data and ¹⁴C determinations of cores taken from three small basins located in western Lake Erie, in conjunction with data obtained from earlier studies, has been used to re-evaluate the postglacial history of the area. After the re-advance of the Laurentide Ice Sheet into the Huron basin and the eastern part of the Erie basin during the Port Huron stadial about 13,000 yrs B.P., lacustrine clay was deposited across much of the western Erie basin in a lake ponded against the glacial ice front to the east. However, by 12,000 yrs B.P. as the ice front retreated, the waters from the Huron basin bypassed Lake Erie, first by the Kirkfield outlet to Lake Ontario, and later by the Fossmill and North Bay outlets to the Ottawa River. This resulted in the draining of water from most of the western Erie basin. Extensive swamps choked with plants formed in the small basin areas, and the surrounding low-lying former lake bottom was subjected to subaerial weathering. It would appear that by about 9,500 yrs B.P. there was an increase in surface run off into these basins and the swamps evolved into shallow, relatively warm-water lakes in which calcareous-rich clay was deposited. Isostatic rebound of the northeastern outlets of the Huron basin led to a gradual tilting of the basin and a progressive migration of the southern shoreline of Lake Huron (Lake Stanley phase) to the southwest. By about 5000 yrs B.P. the water level in the southern Huron basin had been raised sufficiently to re-open the Port Huron outlet into the Erie basin. This resulted in a substantial rise in the lake level in the Erie basin, terminated the deposition of calcareous-rich clay in the small basins, and led to the deposition of normal lacustrine sediments in the modern phase of Lake Erie.  相似文献