Constant Sedimentation Rate Research Articles

Sequence of events during the last deglaciation in Southern Ocean sediments and Antarctic ice cores

The last glacial to interglacial transition was studied using down core records of stable isotopes in diatoms and foraminifera as well as surface water temperature, sea ice extent, and ice‐rafted debris (IRD) concentrations from a piston core retrieved from the Atlantic sector of the Southern Ocean. Sea ice is the first variable to change during the last deglaciation, followed by nutrient proxies and sea surface temperature. This sequence of events is independent of the age model adopted for the core. The comparison of the marine records to Antarctic ice CO2 variation depends on the age model as 14C determinations cannot be obtained for the time interval of 29.5–14.5 ka. Assuming a constant sedimentation rate for this interval, our data suggest that sea ice and nutrient changes at about 19 ka B.P. lead the increase in atmospheric pCO2 by approximately 2000 years. Our diatom‐based sea ice record is in phase with the sodium record of the Vostok ice core, which is related to sea ice cover and similarly leads the increase in atmospheric CO2. If gas exchange played a major role in determining glacial to interglacial CO2 variations, then a delay mechanism of a few thousand years is needed to explain the observed sequence of events. Otherwise, the main cause of atmospheric pCO2 change must be sought elsewhere, rather than in the Southern Ocean.

AMS- 14C chronology of 40.0 cal ka BP continuous deposits from a crater lake (Lake Massoko, Tanzania) : Modern water balance and environmental implications

Lake Massoko, Tanzania (8°20′S, 33°45′E; 870 m a.s.l.) is a freshwater maar lake belonging to the Rungwe volcanic area, 15 km northwest of the Livingstone Mountains and Lake Malawi. The rainfall regime is driven by the yearly oscillation of the Intertropical Convergence Zone, which determines a wet summer and a dry and windy winter. The presence of Lake Malawi to the south, and high ranges to the north result in a higher annual precipitation than that observed at a regional scale. Air masses overloaded with humidity bypassing Lake Malawi are submitted to ascending currents, producing rainfall up to 2400 mm yr −1 in the Lake Massoko area. Important exchanges between the lake and groundwater reservoirs have been demonstrated from hydrological and isotopic balances of Lake Massoko on a regional scale. These exchanges are superimposed on groundwater circulation. The hydrological setting of Lake Massoko is also influenced by the tectonics of the region dominated by the northwest/southeast tilt of tectonic blocks. The sedimentary sequences of Lake Massoko cored in 1995 and 1996 under the EEC-Project RUKWA [ RUKWA Project – Final Report, 1997] likely register the palaeoenvironmental changes on the catchment. The sediment record is nearly continuous due to the near-constant high water level of the lake. In contrast with many other lacustrine sequences, in which reduced deposition rates, discrepancies and/or gaps characterise arid periods such as the Last Glacial Maximum and the Younger Dryas event, all the AMS- 14C dates on authigenic organic samples fit with the stratigraphy, and the palaoenvironmental record of Lake Massoko exhibits a constant mean sedimentation rate of 0.628 mm yr −1 for the last ca. 40 cal ka BP.

Constructing a stacked benthic δ18O record

Composite stacks were constructed by superimposing 6 to 13 benthic foraminiferal δ 1 8 O records covering the period 0-850 ka. An initial timescale for each core was established using radioisotopic age control points and assuming constant sedimentation rates between these points. The average of these records is our 13-core untuned stack. Next, we matched the 41 kyr component of each record individually to variations in Earth's obliquity. Four of the 13 records produced timescales that were inconsistent with one or more of the known radioisotopic ages. The nine remaining cores were averaged to create a stack. Six of the minimally tuned cores were assembled into a tropical stack. For each stack we estimated the uncertainty envelope from the standard deviation of the constituents. Spectral analysis of the three stacks indicates that benthic δ 1 8 O is dominated by a 100 kyr oscillation that has a narrow spectral peak. The contribution of precession to the total variance is small when compared to prior results from planktic stacks.

Sedimentation rates and pollution history of a dried lake: Al-Oteibeh Lake

Sediment accumulation rates as well as the distribution of selected elements in a dried Syrian lake (Al-Oteibeh Lake), near Damascus City, are reported. Five core samples from different locations of the lake were collected, and four major elements (Fe, K, Mg and Na) and six trace metals (Co, Ni, Cr, Pb, Zn, U and Cu) were analyzed. Sedimentation rates were determined applying the 210Pb dating method and found to vary between 0.100 and 0.793 cm year −1. The results showed that the constant flux constant sedimentation rate (CF: CS) simple dating model is applicable for dating recent dried sediment and recording the past historical pollution of the last 100 years. However, the method was found to be only applicable for dating trace and major elements, which cannot be leached to deeper layers by rainwater. In addition, the obtained records can be used to verify the date of water level declining.

U–Th dating of marine isotope stage 7 in Bahamas slope sediments

In order to understand the driving forces for Pleistocene climate change more fully we need to compare the timing of climate events with their possible forcing. In contrast to the last interglacial (marine isotope stage (MIS) 5) the timing of the penultimate interglacial (MIS 7) is poorly constrained. This study constrains its timing and structure by precise U–Th dating of high-resolution δ 18O records from aragonite-rich Bahamian slope sediments of ODP Leg 166 (Sites 1008 and 1009). The major glacial–interglacial cycles in δ 18O are distinct within these cores and some MIS 7 substages can be identified. These sediments are well suited for U–Th dating because they have uranium concentrations of up to 12 ppm and very low initial 230Th contributions with most samples showing 230Th/ 232Th activity ratio of >75. U and Th concentrations and isotope ratios were measured by thermal ionisation mass spectrometry and multiple collector inductively coupled plasma mass spectrometry, with the latter providing dramatically better precision. Twenty-nine of the 41 samples measured have a δ 234U value close to modern seawater suggesting that they have experienced little diagenesis. Ages from 27 of the 41 samples were deemed reliable on the basis of both their U and their Th isotope ratios. Ages generally increase with depth, although we see a repeated section of stratigraphy in one core. Extrapolation of constant sedimentation rate through each substage suggests that the peak of MIS 7e lasted from ∼237 to 228 ka and that 7c began at 215 ka. This timing is consistent with existing low precision radiometric dates from speleothem deposits. The beginning of both these substages appears to be slightly later than in orbitally tuned timescales. The end of MIS 7 is complex, but also appears to be somewhat later than is suggested by orbitally tuned timescales, although this event is not particularly well defined in these cores.

Gas generation and overpressure: Effects on seismic attributes

Drilling of deep gas resources is hampered by high risk associated with unexpected overpressure zones. Knowledge of pore pressure using seismic data, as for instance from seismic‐while‐drilling techniques, will help producers plan the drilling process in real time to control potentially dangerous abnormal pressures. We assume a simple basin‐evolution model with a constant sedimentation rate and a constant geothermal gradient. Oil/gas conversion starts at a given depth in a reservoir volume sealed with faults whose permeability is sufficiently low so that the increase in pressure caused by gas generation greatly exceeds the dissipation of pressure by flow. Assuming a first‐order kinetic reaction, with a reaction rate satisfying the Arrhenius equation, the oil/gas conversion fraction is calculated. Balancing mass and volume fractions in the pore space yields the excess pore pressure and the fluid saturations. This excess pore pressure determines the effective pressure, which in turn determines the skeleton bulk moduli. If the generated gas goes into solution in the oil, this effect does not greatly change the depth and oil/gas conversion fraction for which the hydrostatic pressure approaches the lithostatic pressure. The seismic velocities versus pore pressure and differential pressure are computed by using a model for wave propagation in a porous medium saturated with oil and gas. Moreover, the velocities and attenuation factors versus frequency are obtained by including rock‐frame/fluid viscoelastic effects to match ultrasonic experimental velocities. For the basin‐evolution model used here, pore pressure is seismically visible when the effective pressure is less than about 15 MPa and the oil/gas conversion is about 2.5% percent.

New 40Ar/39Ar age of the Bishop Tuff from multiple sites and sediment rate calibration for the Matuyama‐Brunhes boundary

Precise dating of sanidine from proximal ash flow Bishop Tuff and air fall Bishop pumice and ash, California, can be used to derive an absolute age of the Matuyama Reversed‐Brunhes Normal (M‐B) paleomagnetic transition, identified stratigraphically close beneath the Bishop Tuff and ash at many sites in the western United States. An average age of 758.9±1.8 ka, standard error of the mean (SEM), was obtained for individual sanidine crystals or groups of several crystals, determined from ∼70 individual analyses of sanidine separates from 11 sample groups obtained at five localities. The basal air fall pumice (757.7±1.8 ka) and overlying ash flow tuff (762.2±4.7 ka) from near the source yield essentially the same dates within errors of analysis, suggesting that the two units were emplaced close in time. A date on distal Bishop air fall ash bed at Priant, California, ∼100 km to the west of the source area, is younger, 750.1±4.3 ka, but not significantly different within analytical error (±1 standard deviation). Previous dates of the Bishop Tuff, obtained by others using conventional K‐Ar and the fission track method on zircons, ranged from ∼650 ka to ∼1.0 Ma. The most recent, generally accepted date by the K‐Ar method on sanidine was 738±3 ka. We infer, as others before, that many K‐Ar dates on sanidine feldspar are too young owing to incomplete degassing of radiogenic Ar during fusion in the K‐Ar technique and that many older K‐Ar dates are too old owing to detrital or xenocrystic contamination in the larger samples that are necessary for the technique. The new dates are similar to recent 40Ar/39Ar ages of the Bishop Tuff determined on individual samples by others but are derived from a larger proximal sample population and from multiple analysis of each sample. The results provide a definitive and precise age calibration of this widespread chronostratigraphic marker in the western United States and northeastern Pacific Ocean. We calculated the age of the M‐B transition at five sites, assuming constant sedimentation rates, the age of the Bishop ash bed and one or more well‐dated chronostratigraphic horizons above and below the Bishop Tuff ash bed and M‐B transition, and stratigraphic separations between these datum levels. The age of the M‐B transition is 774.2±2.8 ka, based on the average of eight such calculations, close to other recent determinations, and similar to that determined from the astronomically tuned polarity timescale. Our approach provides an alternative and surprisingly precise method for determining the age of the M‐B and other chronostratigraphic levels. The above dates, calculated using U.S. Geological Survey values of 27.92 Ma for the Taylor Creek (TC) sanidine can be recalculated to other widely used values for these monitors. For example, using recently published values of 28.34 Ma (TC) and 523.1 Ma (McLure Mountain hornblende, MMhb‐1), the resulting ages are ∼774 ka for the Bishop Tuff and ash bed and ∼789 ka for the M‐B transition.

Studies of the Influence of Graded Storm Layers on 210Pb and Heavy Metals Profiles in Great Barrier Reef Sediments

Intense storm activity can resuspend surficial sediments in coastal regions resulting in the formation of graded layers in the sediment. This occurs in the Great Barrier Reef area of northeastern Australia where tropical cyclones are common. The vertical redistribution of 210Pb and heavy metals in these graded sediment layers has been examined mathematically using a model based on Stokes's Law of settling and experimentally using laboratory simulations. The vertical profiles of 210Pb and heavy metal concentrations show an increase with the increasing proportion of fine particles toward the sediment−water interface. The calculated and experimental results mimic 210Pb-depth profiles produced where there has been a constant rate of sedimentation. The use of 210Pb as a geochromometer in sediments including unidentified graded storm layers may yield incorrect sedimentation rates and false geochronologies. The redistribution of heavy metals is similar and may incorrectly suggest recent increases in heavy metal inputs. This misinterpretation is significant where the results are used in making environmental management decisions.

Geomagnetic Reversals

In their report “Lake Baikal record of continental climate response to orbital insolation during the past 5 million years” (7 Nov. 1997, p. 1114), D. F. Williams et al. present a sedimentary record of biogenic silica from Lake Baikal in Siberia. Magnetic polarity reversal data provided age control points that were used to arrive at a constant and continuous sedimentation rate of 4 centimeters per thousand years. In figure 1A of the report (p. 1115), a reference geomagnetic polarity time scale shows a number of short periods of normal polarity that do not appear in the references (1) listed in that figure. These periods appear to be based on the magnetic inclination record derived from the Lake Baikal samples, but are not shown in another publication related to the Baikal Drilling Project (2). “Events” at about 0.85, 1.2, and 2.4 million years ago appear to be the Kamikatsura, Cobb Mountain, and “X” subchrons, respectively (3). Normal events at about 1.55 and 2.03 million years ago do not appear in reference geomagnetic polarity time scales. The latter may argue for a split Reunion Event (4), although only a single event has been dated at 2.14 million years ago in the “type locality” (5).

Simultaneous thermoluminescence and optically stimulated luminescence dating of late Pleistocene sediments from Lake Baikal

When the thermoluminescence (TL) method is applied to dating of partially bleached sediments the need to correct for the residual TL signal left in the mineral crystals at time of deposition is a complicated problem. In dating sediments from Lake Baikal we have attempted a combination of TL and optical stimulated luminescence (OSL). The method was applied to three samples from the uppermost 50m of the sediment cored within the Baikal drilling project in 1993. The results indicate a fairly constant sedimentation rate back to 250 ka BP. This contradicts with results from radiocarbon dating of the same core but agrees reasonably well with preliminary paleomagnetic data and disequilibrium U–Th age determinations.

Large sedimentation rate in the Bengal delta: Magnetostratigraphic dating of Cenozoic sediments from northeastern Bangladesh

Research Article| June 01, 1998 Large sedimentation rate in the Bengal delta: Magnetostratigraphic dating of Cenozoic sediments from northeastern Bangladesh H.-U. Worm; H.-U. Worm 1Institut für Geophysik, Universität Göttingen, Herzberger Landstrasse 180, 37075 Göttingen, Germany Search for other works by this author on: GSW Google Scholar A. M. M. Ahmed; A. M. M. Ahmed 2Bangladesh Petroleum Exploration Company (BAPEX), Dhaka, Bangladesh Search for other works by this author on: GSW Google Scholar N. U. Ahmed; N. U. Ahmed 2Bangladesh Petroleum Exploration Company (BAPEX), Dhaka, Bangladesh Search for other works by this author on: GSW Google Scholar H. O. Islam; H. O. Islam 2Bangladesh Petroleum Exploration Company (BAPEX), Dhaka, Bangladesh Search for other works by this author on: GSW Google Scholar M. M. Huq; M. M. Huq 2Bangladesh Petroleum Exploration Company (BAPEX), Dhaka, Bangladesh Search for other works by this author on: GSW Google Scholar U. Hambach; U. Hambach 3Geologisches Institut, Universität zu Köln, Germany Search for other works by this author on: GSW Google Scholar J. Lietz J. Lietz 4Federal Institute for Geosciences & Natural Resources (BGR), Hannover, Germany Search for other works by this author on: GSW Google Scholar Geology (1998) 26 (6): 487–490. https://doi.org/10.1130/0091-7613(1998)026<0487:LSRITB>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation H.-U. Worm, A. M. M. Ahmed, N. U. Ahmed, H. O. Islam, M. M. Huq, U. Hambach, J. Lietz; Large sedimentation rate in the Bengal delta: Magnetostratigraphic dating of Cenozoic sediments from northeastern Bangladesh. Geology 1998;; 26 (6): 487–490. doi: https://doi.org/10.1130/0091-7613(1998)026<0487:LSRITB>2.3.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 SocietyGeology Search Advanced Search Abstract A 4.3-km-thick section of clastic sediments of the Surma basin, northeastern Bangladesh, that is exposed along the Shari River was sampled for a magnetostratigraphic study. Miocene through Pleistocene ages have been discussed previously; however, there has been no firm dating through biostratigraphy, or radiometric or other methods. Primary paleomagnetic remanent magnetizations of normal or reversed polarity were determined for most of the 300 collected samples. On the basis of assumptions of (1) constant sedimentation rates, on average, throughout the deposition of the sedimentary sequence, (2) no major gaps in sedimentation, and (3) a fairly young (< 2 Ma) age for the topmost Dupi Tila deposits, we find good agreement between the measured profile and the geomagnetic polarity time scale if the entire sedimentary sequence is between 4.9 and 1.4 Ma in age. The Upper Marine Shales, an important seismic marker horizon, are dated as 3.5 Ma and are therefore ∼1.5 m.y. younger than previously assigned by biostratigraphic results. The Upper Marine Shales presumably correspond to the last pre-Pleistocene sea-level highstand. The dating of rock units below the Upper Marine Shales is tentative because of a large gap in exposure. The inferred sedimentation rate—and thus the rate of delta subsidence—of 1.2 m/k.y. is much larger than previously assumed; it appears to be one of the highest sedimentation rates in Earth history that was sustained for millions of years. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The timing of major climate terminations

A simple, untuned “constant sedimentation rate” timescale developed using three radiometric age constraints and eleven δ18O records longer than 0.8 Myr provides strong support for the validity of the SPECMAP timescale of the late Quaternary [Imbrie et al., 1984]. In particular, the present study independently confirms the link between major deglaciations (terminations) and increases in northern hemisphere summer radiation at high latitudes and shows that this correlation is not an artifact of orbital tuning. In addition, the excess ice characteristic of late Quaternary “100‐kyr” climate cycles typically accumulates when July insolation at 65°N has been unusually low for more than a full precessional cycle, or &gt;21 kyr, and once established does not last beyond the next increase in summer insolation. Thus, the timing of the growth and decay of large 100‐kyr ice sheets, as depicted in the deep sea δ18O record, is strongly (and semipredictably) influenced by eccentricity through its modulation of the orbital precession component of northern hemisphere summer insolation.

Simultaneous presence of orbital inclination and eccentricity in proxy climate records from Ocean Drilling Program Site 806

Ocean Drilling Program Site 806 in the western Pacific shows evidence of a remarkably constant average sedimentation rate. This feature allows us to analyze ancient climate proxies without the need for “orbital tuning,” a standard procedure in prior work, but one that can lead to biased results. Spectral analysis of stable oxygen isotope ratios at this site, a proxy for global ice volume, shows a single narrow peak with a period ≈ 100 k.y., a result that supports our model which links glacial cycles to variations in the inclination of the Earth's orbit. In contrast, spectral analysis of the coarse component fraction of the sediment (primarily foraminifera) shows a structure characteristic of standard Milankovitch theory, with a triplet of peaks with periods near those expected from the Earth's eccentricity: 95, 125, and 400 k.y. Bispectral analysis confirms these linkages but suggests that orbital inclination also plays some role in the coarse fraction. From the clear presence of both signals in different proxies at the same site, we conclude that although eccentricity affected the local climate, it is orbital inclination that drove the variations in the global ice volume for the past million years.

Magnetostratigraphy of the lower member of the hadar formation (Ethiopia): Evidence for a short normal event in the mammoth subchron

Rock magnetism and magnetostratigraphy of the lower part of the Hadar Formation (Afar, Ethiopia) is presented after analysis of multiple new collection of samples from over 84 horizons. The Hadar Formation is composed of lacustrine, lake margin, fluvial and flood plain sediments and known for important Pliocene vertebrate faunas including Australopithecus afarensis. Hysteresis measurements, thermomagnetic analysis, growth and decay of isothermal remanent magnetisation are used to unravel the complex magnetic mineralogy of the different representative lithologies. Ferrimagnetic minerals of magnetite or titanomagnetite in composition, in the stable pseudo-single domain (PSD) size range are found to be the main carriers of the remanence. In most sites the characteristic remanence was isolated using stepwise thermal demagnetisation. The overall mean direction for about 72 horizons (434 samples) is D=358·6°, I=7° (k=17·9, α95=4°) implying some 14° of inclination shallowing, related to sediment compaction due to the very rapid sedimentation history of the site. Five successive polarity zones (N1-R1-N2-R2-N3) are identified and correlation with the lower Gauss chron of the astronomically calibrated geomagnetic polarity time scale (GPTS) is proposed using the existing40Ar/39Ar ages. This implies the existence of a short normal polarity event (N2), identified on six different sites, within the reversed Mammoth subchron, called the Kada-Hadar event. The age calculated for the Kada-Hadar event, using linear interpolation of the dated horizons, assuming a constant rate of sedimentation is 3.246 Ma and its duration is about 8 kyr.

Barite fronts in continental margin sediments: a new look at barium remobilization in the zone of sulfate reduction and formation of heavy barites in diagenetic fronts

Micro-crystalline barites recovered by deep-sea drilling from Site 684 on the Peru margin and Site 799 in the Japan Sea are highly enriched in the heavy sulfur isotope relative to seawater (δ 34S up to + 84%). This isotopic composition is consistent with remobilization of biogenic barite triggered by sulfate reduction, and subsequent reprecipitation as a diagenetic barite front. The high levels of barium sulfate in these deposits (10–50%) cannot be explained by a diffusive transport model in sediments experiencing a constant rate of sedimentation. When sedimentation rates change radically, the barite front will remain at a given depth interval leading to large accumulations of barium sulfate. Such conditions may have generated the barite deposits at Site 799. At Site 684, on the other hand, there is evidence that the barite deposits are a result of the tectonically-driven advection of sulfate-bearing fluids through the sediment column.

A geomagnetic excursion during the late Matuyama Chron, the Osaka Group, southwest Japan

Duplicate records of a geomagnetic excursion during the late Matuyama Chron were obtained from sediments gathered at the Takatsuki and Izumi sites in southwest Japan. The excursion is dominated by westerly declinations with shallow inclinations, and was found just above the volcanic ash known as the Azuki Tuff, in a marine clay layer of the Osaka Group. The excursion occurred at about 0.85 Ma, a recently revised age for the Azuki Tuff. By extrapolating constant sedimentation rates for the Holocene marine clay dated by the radiocarbon method, the duration of the excursion is estimated at about 240 yr, with a range of 200–500 yr if we consider possible fluctuations in sedimentation rates. The Kamikatsura excursion previously reported around this time from the Osaka Group data is estimated to be a few tens of thousands of years older than the excursion reported in this study, based on correlations of changes in sea level recorded in the sediments of the Osaka Group and the oxygen isotope record of deep-sea cores. These two excursions in Japan, together with short normal events reported in the Southern Ocean, North America and China, suggest that during the period from 0.87-0.84 Ma the geomagnetic field may have been unstable, and excursions or short reversals may have occurred regionally and sporadically over the earth.

Age and Survivorship of Diapausing Eggs in a Sediment Egg Bank

We determined the densities of diapausing eggs of the copepod Diaptomus Sanguieneus in sediments from two small freshwater lakes in Rhode Island. Sediment cores, sliced at 1—cm intervals, showed that egg densities ranged between 4 x 104 and 8 x 104 eggs/m2 near the sediment surface and declined to very low values at depths of 10—15 cm in both lakes, although eggs were found as deep as 30 cm in the sediment of one lake. Between 10 and 50% of these eggs hatched in short—term laboratory experiments, and actual egg viability is probably higher. 210Pb—dating revealed relatively constant sedimentation rates in both lakes, and we use this information to estimate egg ages. In one lake, the mean diapausing—egg age is 70.4 yr (median age = 45.9 yr) and the maximum age of eggs we hatched was 332 yr. In the other lake, the mean egg age is 48.9 yr (median age = 35.9 yr) and the maximum age of eggs we hatched was 112 yr. We calculated egg mortality rates by regressing 1n(egg density) on the age of the sediment from which the eggs were taken to obtain estimates of 1.1 and 1.5% mortality/yr for the two lakes. Diapausing eggs of zooplankton represent a long—lived life history stage of an otherwise short—lived organism. They provide generation overlap that can have substantial significance for both ecological and evolutionary dynamics.

Current and historical inputs of mercury to high-latitude lakes in Canada and to Hudson Bay

Sediment cores collected from several lakes in northern Canada have been analyzed for mercury and several other chemical contaminants. Sites ranged from the Experimental Lakes Area of northwestern Ontario, north to Cornwallis Island, and west to the southern Yukon. Cores were sliced at sites of collection and individual slices were freeze dried and analyzed for Pb-210 and Cs-137 to estimate average time intervals of deposition. The earliest date estimated by Pb-210 was about 1850, and mercury concentrations in some lakes were clearly increasing before then, assuming no vertical movements of mercury within the sediments. Extrapolation of dates downward to deeper slices, assuming a constant sedimentation rate, indicated that in some lakes mercury inputs increased slowly even in the 1500's, more rapidly after 1750, and more rapidly yet over the current century. These increases are interpreted as increased fluxes of mercury to the lakes as a result of long-range transport of atmospheric mercury, since there are no local industrial sources of mercury. Slices taken near the bottom of a core are taken to estimate the geological component while elevations in excess of that in surface slices are taken to represent contamination from fallout. This partitioning suggests that sediments in the eastern Northwest Territories are dominated by pollution, while those from the western Northwest Territories are influenced more by their geological settings. Two cores from Hudson Bay suggest that mercury is increasing there too, but has not yet exceeded geological sources. Mercury shows little or no tendency to decline in the most recent slices; indicating that inputs of mercury remain at or near their historical maxima. Given relatively high and continuing inputs of mercury to northern lakes it seems likely that some portion of that mercury may find its way into the food chain, hence the long-term prospect is for increasing levels of mercury in northern fish.

Fourier stratigraphy: Spectral gain adjustment of orbital ice mass models as an aid in dating Late Neogene deep‐sea sediments

Orbital tuning has become an important means of dating deep‐sea sediments of Pliocene‐Pleistocene age. One approach is to generate a template simulating ice mass history based on Milankovitch forcing for comparison with a given δ18O record. To obtain a template that closely mimics a given δ18O record, we propose the following steps: (1) generate a preliminary template based on a simple ice mass model, (2) find the Fourier matrix for this model, (3) find the Fourier matrix for the δ18O record to be tuned (if necessary, adjust preliminary sedimentation rate so that maxima appear at the correct spectral lines), (4) import the spectral power distribution from the matrix of the record into the matrix of the template (while preserving phase of template), and (5) generate the new template from the gain‐adjusted template matrix. The record should now show optimum coherency with the template over the appropriate interval. The efficiency of this method (“Fourier matrix conjugation”) in producing hybrid templates is illustrated using four examples: (1) testing the stacked record of Prell et al. (1986) for constant sedimentation rate, (2) testing the date of 790 ka of the Brunhes/Matuyama boundary in Ontong Java Plateau Ocean Drilling Program Hole 805C (Berger et al., 1993a), (3) testing the age scale for a core from the Norwegian Sea containing meltwater events (Jansen, 1989), and (4) checking the age model of Ocean Drilling Program Site 677 (Shackleton et al., 1990) for internal consistency. The template shows just where the given age models have optimum coherency with the selected orbital model and where in the record discrepancies arise.

Geometric and numerical model of progressive limb rotation in detachment folds

A simple, area-balanced, geometric model of progressive limb rotation in detachment folds allows the quantification of displacement, uplift, and rotation and the assessment of their effect upon growth strata geometries. Translation of this geometric model into a velocity model of deformation allows the modeling of associated growth strata resulting from background sedimentation and local erosion, transport, and deposition. We show that with constant displacement and sedimentation rate, this model predicts a progressive unconformity, indicating the onlap and eventual overlap of the fold limb caused by a decreasing uplift rate. This decrease is a consequence of the kinematics of progressive limb rotation. Increases in displacement rate, with sedimentation rate kept constant, can lead to offlap and angular unconformities, although subsequent onlap does not necessarily reflect a return to a lower displacement rate. Modeled growth strata geometries exhibit many of the features reported from the literature.