bifurcation structure and noise assisted transitions in the pleistocene glacial cycles 1 the glacial cycles are attributed to the climatic response of the orbital changes in the irradiance to the earth these changes in the forcing are too small to explain the observed climate variations as simple linear responses nonlinear amplifications of the orbital forcing are necessary to account for the glacial cycles here an empirical model of the nonlinear response is presented from the model it is possible to assess the role of stochastic noise in comparison to the deterministic orbital forcing of the ice ages the model is based on the bifurcation structure derived from the climate history it indicates the dynamical origin of the mid pleistocene transition from the 41 ka world to the 100 ka world the dominant forcing in the latter is still the 41 ka obliquity cycle but the bifurcation structure of the climate system is changed the model suggests that transitions between glacial and interglacial climate are assisted by internal stochastic noise in the period prior to the last five glacial cycles while the last five cycles are deterministic responses to the orbital forcing

the mid quaternary climatic transition as the free response of a three variable dynamical model abstract a simplified version of a previously described dynamical model governing global ice mass atmospheric carbon dioxide and mean ocean temperature that may also be a proxy for some other co2 controlling oceanic variable e g nutrient supply is shown to possess solutions in a realistic parameter range that can replicate the main features of the climatic variations implied by the full two million years quarternary 18o record these variations include a major transition between a low ice 18o low variance mode before roughly 900 kyr bp to a high ice near 100 kyr period variance mode after that time to the present the model contains only three free parameters in this simplified form no external earth orbital forcing is prescribed i e the model represents only internal dynamics from the previous studies it seems clear that additional variance representing such features as the rapid deglaciation and the phase of the major quaternary oscillation can be largely explained with no more

major features and forcing of high latitude northern hemisphere atmospheric circulation using a 110 000 year long glaciochemical series the greenland ice sheet project 2 glaciochemical series sodium potassium ammonium calcium magnesium sulfate nitrate and chloride provides a unique view of the chemistry of the atmosphere and the history of atmospheric circulation over both the high latitudes and mid low latitudes of the northern hemisphere interpretation of this record reveals a diverse array of environmental signatures that include the documentation of anthropogenically derived pollutants volcanic and biomass burning events storminess over marine surfaces continental aridity and biogenic source strength plus information related to the controls on both high and low frequency climate events of the last 110 000 years climate forcings investigated include changes in insolation of the order of the major orbital cycles that control the long term behavior of atmospheric circulation patterns through changes in ice volume sea level events such as the heinrich events massive discharges of icebergs first identified in the marine record that are found to operate on a 6100 year cycle due largely to the lagged response of ice sheets to changes in insolation and consequent glacier dynamics and rapid climate change events massive reorganizations of atmospheric circulation that are demonstrated to operate on 1450 year cycles changes in insolation and associated positive feedbacks related to ice sheets may assist in explaining favorable time periods and controls on the amplitude of massive rapid climate change events explanation for the exact timing and global synchroneity of these events is however more complicated preliminary evidence points to possible solar variability climate associations for these events and perhaps others that are embedded in our ice core derived atmospheric circulation records

case study of a volcanic geothermal system mount pelee martinique abstract mount pelee is one of the active volcanoes of the lesser antilles arc with more than 20 eruptions during the last 5000 years despite this abundant potential heat flow there are few surface manifestations indicative of the existence of a large high temperature hydrothermal system a few na hco 3 so 4 type hot springs with emergence temperatures not exceeding 65 c occur on the south and west flanks of the volcano two small hydrothermal eruptions occurred in 1792 and 1851 on the southwestern flank on the site of now extinct sulfur springs the heat source provided by the mount pelee magma chamber should occur according to petrologic data at shallow depth 5 8 km beneath the summit region we assume abundant groundwater recharge to take place within the central calderas where faults and magmatic conduits provide vertical channelways for deep infiltration of cold meteoric water this would be expected to depress a presumed nacl hydrothermal system located deep in the core of the volcano upwelling of high temperature fluids and gases is thought to be found at the periphery of the central calderas along the arcuate caldera related faults and or along regional faults and any permeable stratigraphic levels connected to the central caldera structure steam heating of shallow meteoric waters by a degassing nacl system could produce a na hco 3 so 4 type system which would develop preferentially on the periphery of the central caldera structure or even outside it in an adjacent zone of fracture permeability caldera or regional faults where cold meteoric flows are less abundant this na hco 3 so 4 system is assumed to be located beneath the south flank of the volcano and the major upflow zone seems to be the southern limit of the intermediate caldera as evidenced by the location of hot springs and hydrothermal eruptions the scarcity of thermal manifestations on mount pelee is not surprising in view of its groundwater hydrogeology the na hco 3 so 4 fluids which ascend will be masked wherever a highly permeable subhorizontal aquifer occurs near the surface and they commonly mix with cold waters before discharging

an archaeopedological and phytolitarian study of the dark earth on the site of rue de dinant brussels belgium during the excavations of the site of rue de dinant in the historic centre of brussels belgium remains of the first city wall dating from the 13th century have been discovered intra muros thick dark sediment units predating this wall are observed these dark units are among the oldest occupation traces so far encountered in this higher eastern part of the historic centre of brussels in order to characterise them and to understand the human activities and natural processes behind their formation an interdisciplinary research integrating archaeopedology and phytolith study has been undertaken this approach permitted us to identify a range of natural and cultural processes leading to the formation of these dark layers and to characterise different activities remains of a hearth old plough land cropfields and grassland have been identified all activities referring to a rural landscape organisation later on the area changes into an enormous silt extraction area whereby dark earth is thrown into the dug pits finally the construction of the first city wall marks the area as becoming part of the city the study of dark earth shows to be a potentially valuable tool to shed light on the development and spatial organisation of pre urban brussels a historically very poorly documented period

influence of late cenozoic mountain building on ocean geochemical cycles in a steady state ocean input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes for the elements strontium calcium and carbon rivers provide the primary input whereas marine biogenic sedimentation dominates removal dissolved fluxes in rivers are related to rates of continental weathering which in turn are strongly dependent on rates of uplift the largest dissolved fluxes today arise in the himalayan and andean mountain ranges and the tibetan plateau during the past 5 m y uplift rates in these areas have increased significantly this suggests that weathering rates and river fluxes may have increased also the oceanic records of carbonate sedimentation level of the calcite compensation depth and delta sup 13 c and delta sup 87 sr in biogenic sediments are consistent with a global increase in river fluxes since the late miocene the cooling of global climate over the past few million years may be linked to a decrease in atmospheric co sub 2 driven by enhanced continental weathering in these tectonically active regions

obliquity and precession as pacemakers of pleistocene deglaciations abstract the milankovitch theory states that the orbital eccentricity precession and obliquity of the earth influence our climate by modulating the summer insolation at high latitudes in the northern hemisphere despite considerable success of this theory in explaining climate change over the pleistocene epoch 2 6 0 01 myr ago it is inconclusive with regard to which combination of orbital elements paced the 100 kyr glacial interglacial cycles over the late pleistocene here we explore the role of the orbital elements in pacing the pleistocene deglaciations by modeling ice volume variations in a bayesian approach when comparing models this approach takes into account the uncertainties in the data as well as the different degrees of model complexity we find that the earth s obliquity axial tilt plays a dominant role in pacing the glacial cycles over the whole pleistocene while precession only becomes important in pacing major deglaciations after the transition of the dominant period from 41 kyr to 100 kyr the mid pleistocene transition we also find that geomagnetic field and orbital inclination variations are unlikely to have paced the pleistocene deglaciations we estimate that the mid pleistocene transition took place over a 220 kyr interval centered on a time 715 kyr ago although the data permit a range of 600 1000 kyr this transition occurring within just two 100 kyr cycles indicates a relatively rapid change in the climate response to insolation

the evolution of pleistocene climate a time series approach abstract in the later part of the pleistocene variations in global ice volume have been dominated by an approximate 100 000 year cycle analysis of 2 myr long oxygen isotope record from an equatorial pacific core indicates that this is true only for the last 900 000 years prior to this time the amplitude of the 100 000 year cycle is much reduced as is the variance of all oscillations with periods greater than 60 000 years based on results of time series analysis of this 2 myr long record the pleistocene glacial cycles can be divided into three sections 1 the late pleistocene 0 900 kyr b p where the variations in the isotope record are dominated by the 100 000 year cycle 2 the middle pleistocene 900 1450 kyr b p in which low frequency components are not as important as in the later period of the pleistocene and 3 the early pleistocene late pliocene 1450 2000 kyr b p where general reductions of importance at all frequencies is seen as compared to the later intervals recent modeling efforts which describe variations in global ice volume show that the dominant low frequency component observed in the late pleistocene can result from different time constants for the rate of glacial growth and decay in response to variations in the earth s orbital parameters it is hypothesized that during the early pleistocene the rate of growth and decay of glaciers were more similar and that continental erosion by successive glacial advances lowered the land surface in areas of ice cap formation to below sea level when the ice caps became marine based more rapid decay of the ice became possible

relict soils and early arable land management in lofoten norway relict arable soils dating fromc ad700 provide an opportunity to identify early arable land management practices in the agriculturally marginal landscapes of the lofoten archipelago northern norway synthesis of field survey and soil thin section micromorphology supported by image analysis suggests that there was deliberate management of erodible sandy soils in sloping locations to create small areas of cultivation terrace with a range of materials used as soil amendments to stabilize the accumulated soil and enhance fertility small areas of sandy soils in more gently sloping locations were also cultivated again with a range of materials used as amendments and which contributed to a significant increase in soil thickness the last phase of cultivation commenced during the late 1800s and involved the reclamation of wetter peaty soils by spade these patterns of arable land management are repeated in different parts of lofoten and indicate that despite the climatic and economic marginality of arable activity in lofoten land management practices were developed and applied to permit barley production from small areas

a low order dynamical model of global climatic variability over the full pleistocene a previously formulated dynamical model of the late pleistocene ice ages based on the hypothesis that the global co sub 2 system can provide the instability to drive a natural oscillation involving feedbacks between the cryosphere atmosphere and ocean is extended to include 1 additive earth orbital forcing summer insolation changes at 65 degree n and 2 tectonic forcing in the form of a postulated variation in the multiplicative parameters rate constants of the model system the structural e g bifurcation properties of the model are examined in detail to reveal the regions of parameter space wherein the geologically inferred features of the full pleistocene can be simulated including the observed chronology the phase relationships between ice co sub 2 and north atlantic deep water formation and the mid pleistocene transition

the origin of the 100 kiloyear ice sheet cycle in the pleistocene the response of a simple ice sheet model forced by a periodic change in the climate point exhibits a bifurcation when certain dimensionless parameters undergo realistic changes it is postulated that this bifurcation may be responsible for the initiation of large glacial cycles about 700 000 years ago following smaller higher frequency ice sheet advances and retreats before that time

early to mid pleistocene ostracod δ18o and δ13c in the central tibetan plateau implication for indian monsoon change as one of the pioneering projects of the chinese environmental scientific drilling cesd program a 206 5 m long sediment core ce was retrieved from co ngoin co lake basin in the central tibetan plateau limnic records of the abundance of ostracod shells and their oxygen and carbon isotope measurements together with the geochemical and the sedimentological investigations were carried out on the interval between 143 and 52 m core depths corresponding to ca 2 01 0 84 myr distinct oscillations of the abundance and the chemical compositions of ostracod shells have yielded the original information about variations in lakewater chemistry in association with the change in the indian monsoon over the period of 2 01 to 0 84 myr the climate conditions during the early pleistocene were characterized by frequent oscillations and the ostracod records had expressed its synchronous response to these oscillations a major climate change occurred in the co ngoin area at about 0 93 myr a pronounced fluctuation in ostracod delta 18o and delta 13c during the mid pleistocene transition mpt is possibly associated with an increase in continental ice sheets the compositional and the sedimentological characters of the immediately overlying sediments imply a shallow aquatic environment and also a harsh salinity condition beyond the tolerance limits for ostracod survival resulting in the disappearance of the qinghaicypris crassa huang 1979 around 0 84 myr

carbon cycle instability as a cause of the late pleistocene ice age oscillations modeling the asymmetric response a dynamical model of the pleistocene ice ages incorporating many of the qualitative ideas advanced recently regarding the possible role of ocean circulation chemistry temperature and productivity in regulating long term atmospheric carbon dioxide variations has been constructed this model involves one additional term and free parameter beyond that included in a previous model b saltzman and a sutera 1987 providing the capacity for an asymmetic for example saw toothed response it is shown that many of the main features exhibited by the 18o derived ice record and the vostok core 13c derived carbon dioxide record in the late pleistocene can be deduced as a free oscillatory solution of the model including a rapid deglaciation during which a spike of high co2 and a rapid surge in north atlantic deep water production occurs it is expected that the addition of reasonable levels of external for example earth orbital forcing will enable the model to account for a significant amount of the remaining observed variance and covariance of the slow response climatic variables over the full pleistocene including the mid pleistocene transition

late holocene land use at orstad jaeren southwestern norway evidence from pollen analysis and soil micromorphology abstract pollen analysis was combined with radiocarbon dating physical chemical and biological soil analyses and soil micromorphology to investigate the prehistoric land use at orstad in jaeren southwestern norway orstad is an early bronze age clearance cairn field in a cultural landscape with traces of land use back to the neolithic samples were mainly collected from an 84 m long excavated trench with mainly podzolic soils although pollen preservation was poor it was possible to differentiate five phases in the local vegetation and land use history human impact on the site could be traces back to about 4400 4000 bp uncal 3045 2600 cal bc wheat and barley have been cultivated at orstad during a period from ca 3600 bp uncal 1945 1750 cal bc until the older iron age when heather vegetation began to spread on the site soil micromorphology revealed different soil management practices for this time period the soils seem to have been cleared by fire prior to cultivation until ca 3200 bp uncal 1610 1455 cal bc then new cultivation fields were laid out on higher levels and seem to have been improved by adding of organic materials mainly turves taken from podzolic top soils and peats from nearby localities

the late quaternary glaciations as the response of a three component feedback system to earth orbital forcing abstract a climatic feedback system previously described consisting of three prognostic nonlinear equations governing the mass of ice sheets the mass of marine and continental marginal ice and the mean ocean temperature is forced by a representation of the effects of external earth orbital variations with reasonable amplitudes for the eccentricity obliquity and precession forcing the free oscillatory solutions of major period near 100 kyr can be modified in a way that substantially agrees with the 18o derived observations of ice mass evolution in particular a proper structure variance spectrum and phase lock of the major variations are obtained over the last 400 kyr an analysis of the sensitivity of these results to variations in the model parameters and to random perturbations shows that the solution is robust for small changes in all but a few of the equation coefficients concomitant variability in the marine ice mass ocean temperature and net radiation at the top of the atmospher

vostok ice core provides 160 000 year record of atmospheric co2 direct evidence of past atmospheric co2 changes has been extended to the past 160 000 years from the vostok ice core these changes are most notably an inherent phenomenon of change between glacial and interglacial periods besides this major 100 000 year cycle the co2 record seems to exhibit a cyclic change with a period of some 21 000 years

the use of image analysis in the micromorphological study of old cultivated soils an evaluation based on soils from the island of papa stour shetland this paper presents data from the first part of a major project designed to investigate the micromorphology of cultivated soils in particular the initial phase of the project is concerned with an investigation into methods for the identification and quantification of cultivation features in soil thin sections using image analysis techniques in 1993 a range of soil samples of known cultivation contexts were taken from an abandoned farm at olligarth which is located on the small island of papa stour to the immediate west of mainland shetland cultivation on this farm was carried out using a delling spade until the late 1940s samples from four contrasting sites at olligarth site 6 a planticrue site 7 the kale yard site 8 small fields and site 9 heathland were subsequently analysed for total phosphate organic content expressed as loss on ignition image analysis and traditional soil micromorphology were undertaken on samples from these sites results from the image analysis traditional micromorphological analysis and chemical analysis for each site were compared overall it was found that image analysis can contribute significantly to micromorphological description and interpretation of old cultivated soils however image analysis should not be seen as providing a replacement for the standard description of thin sections instead the use of image analysis in the study of ancient cultivated soils should not be undertaken in isolation other forms of analyses phosphate pollen organic matter traditional micromorphology should be undertaken in combination with image analysis in order to investigate the formation of cultivated soils 1996 academic press limited

linear and nonlinear couplings between orbital forcing and the marine δ18o record during the late neocene previous investigations of the response of plio pleistocene climatic records to long term orbitally induced changes in radiation have considered a linear response of climate while the second order statistics of power spectra and cross spectra provide necessary information on linear processes insight into the nonlinear characteristics of pliocene and pleistocene climate is not provided by these statistical quantities second order statistics do not contain the phase information necessary to investigate nonlinear phase coupled processes such information is provided by higher order statistical quantities in particular bispectral analysis indicates that nonlinear couplings are present in the climatic radiative forcing at the milankovitch frequencies through a linear transfer this forcing produces similar nonlinear couplings in deep sea sedimentary oxygen isotope records odp site 677 and dsdp site 607 from 1 0 to 0 ma during the late neogene this analysis suggests that during the late pleistocene the dominance of the 100 000 year cycle in the climate record is consistent with a linear resonant response to eccentricity forcing in the period from 2 6 to 1 0 ma a change in the nature of the climatic response to orbital forcing is indicated as phase couplings present in the isotopic time series are not similar to the phase couplings present in the insolation forcing third order moments skewness and asymmetry are used to quantify the shape of the climatic response from 2 6 ma to present an increase in the asymmetry sawtoothness of the oxygen isotopic records is accompanied by a corresponding decrease in the skewness peakedness of the records this indicates an evolution in the nature of the phase coupling within the climate system these results may provide important constraints useful in development of models of paleoclimate

exploring the structure of the mid pleistocene revolution with advanced methods of time series analysis the mid pleistocene climate transition is a complex global change leading to late pleistocene ice ages with increased mean ice volume and dominant 100 ka cycle a thorough understanding of this transition demands quantitative investigations in the time and frequency domains and in the stochastic domain three methods of time series analysis are presented which have been adapted for this purpose they are tested by means of predefined artificial time series and applied to benthic oxygen isotope d18o records which serve as ice volume indicator results are as follows a the time dependent mean shows an increase of 0 35 vs pdb from 942 to 892 ka b evolutionary spectral analysis reveals an abrupt in crease of 100 ka cycle amplitude at approximately 650 ka c probability density function exhibits a bifur cation behavior at approximately 725 ka these find ings point to a multiple transition from a more linear climate system to a strong nonlinear system the signif icant lead of the transition in mean in relation to the 100 ka amplitude change and bifurcation is left open for explanation

relict properties of anthropogenic deep top soils as indicators of infield management in marwick west mainland orkney relict properties of anthropogenic deep top soils formed between the late 1200s and late 1800s are used to identify infield management practices in the coastal area of marwick west mainland orkney properties examined include particle size distribution total phosphate 13c values and micromorphological features interpretation of infield management practices from these properties is based upon their known characteristics through comparisons with known materials and by comparisons with other areas of the marwick landscape the observed properties suggest that manuring practice involved the application of grassy turf material that came exclusively from the hill land together with composted animal manure and a minor seaweed component materials from other possible sources were not utilized and differences in relict soil properties within and between the deep top soils in marwick suggest that varying proportions of turf and manure were applied across the area of land that developed as deep top soil cultivation practices associated with the deep top soils were sufficiently intense to contribute to the downslope and down profile movement of fine material the implications of the interpretations are discussed in relation to the wider organization of the cultural landscape in orkney more general conclusions are drawn on the implications for soils based studies of infield systems