inferring domain state from magnetic hysteresis in high coercivity dolerites bearing magnetite with ilmenite lamellae abstract hysteresis loops were measured for 16 dolerite samples from separate precambrian dykes whose remanence is probably primary remanence is carried by grains of magnetite subdivided into fine particles usually by ilmenite lamellae as seems common in mafic igneous rocks of high coercivity coercive force h c ranges from 9 to 40 10 3 a m 110 500 oe the ratio j rs j s of saturation remanence to saturation magnetization ranges from 0 10 to 0 45 j rs j s varies in approximate proportion to h c with a constant of proportionality appropriate for pseudo single domain magnetite with one or two domain walls per particle a cooling cycle to 77k causes large 40 on average demagnetization of j rs in most samples supporting the dominance of pseudo single domain magnetite most samples also show a large 29 on average decrease in h c on cooling to 140k suggesting magnetostrictive control of h c perhaps through internal stresses opposing domain wall motion only the three dolerites with h c greater than 30 10 3 a m 380 oe show little change in h c on cooling suggesting dominance by single domain particles with shape anisotropy in magnetite grains subdivided into single domain particles by ilmenite lamellae magnetic interaction may lower j rs j s from the expected 0 5 some theories 1 2 suggests that this is due to a self demagnetizing field from magnetic poles on the surface of each grain of magnetite subdivided by ilmenite and should be corrected for another theory 3 suggests that this correction is not necessary because the self demagnetizing field is already approximately cancelled due to magnetic poles inside each subdivided grain at the ends of magnetite particles the latter suggestion is favoured by our three highest h c dolerites whose j rs j s averages 0 42 0 04 before and 0 66 0 06 after correction for the self demagnetizing field from poles on the surface of magnetite ilmenite grains applying this correction to our other dolerites usually raises j rs j s significantly above 0 5 which is unreasonable for pseudo single domain magnetite hence correcting for self demagnetizing fields from poles on the surface of magnetite ilmenite grains in rocks is not recommended whatever the domain state

sourcing geochemically identical obsidian multiscalar magnetic variations in the gutansar volcanic complex and implications for palaeolithic research in armenia abstract in obsidian rich armenia one of the most archaeologically significant obsidian resources is the gutansar volcanic complex gvc numerous palaeolithic sites across armenia consist of little more than deposits of obsidian tools and debris and within the hrazdan gorge the vast majority of artifacts is crafted from gvc obsidian for example about 93 of the lithic assemblages from the lower palaeolithic site of nor geghi 1 and the middle palaeolithic site of lusakert cave 1 are gvc obsidian although erupted from multiple volcanic centers gvc obsidians are geochemically identical and cover an area more than 70 km2 for studies seeking to reconstruct movements of palaeolithic hunter gatherers throughout the local landscape the expansive gvc is problematic geochemically identical obsidians dominate the foraging territories of sites within and near the complex here we explore the potential of obsidian magnetic properties to reveal hitherto unobservable patterns in gvc obsidian utilization with the ultimate goal of elucidating palaeolithic behavioral patterns our results show that magnetic properties vary over multiple scales within the gvc allowing obsidian sourcing to become multiscalar these measurements are rapid inexpensive and entirely nondestructive for small artifacts measuring a few basic magnetic properties enables us to distinguish geochemically identical obsidians from three eruptive centers and potentially to attribute artifacts to specific quarrying sites recognize changes in provisioning strategy and estimate a minimum number of cores represented by a lithic scatter

calibration of magnetic granulometric trends in oceanic basalts abstract the validity of magnetic granulometric estimates relies heavily on the ability to distinguish ultrafine particles from coarser grains for example populations with dominantly superparamagnetic sp or multidomain md grains both are characterized by low remanence and coercivity and distinguishing these endmembers may provide valuable clues to the origin of magnetization in the intervening stable single domain sd size range the natural grain size variations associated with variable cooling rates in submarine lavas provide a rare opportunity for examining progressive changes in average magnetic grain size from sp sd mixtures in submarine basaltic glass to sd md mixtures in flow interiors based on microanalysis and rock magnetic measurements on pillow basalt samples dredged from the flanks of the mid atlantic ridge ages

magnetic hysteresis as a function of low temperature for deep sea basalts containing large titanomagnetite grains inference of domain state and controls on coercivity hysteresis loops to 1200 oe 95 ka m 1 are measured between 295 and 105 k for two deep sea basalts dsdp legs 34 and 37 containing large 200 m unexsolved titanomagnetite grains the curie points electron microprobe analyses and saturation magnetizations of the magnetic grains are the same as for synthetic titanomagnetite xfe2tio4 1 x fe3o4 with x 0 6 as temperature is lowered from 295 to 190 k coercive force hc slowly rises from 40 to 95 oe 3 2 to 7 6 ka m 1 approximately in proportion to the rise in the magnetostriction constant presumably hc is controlled by through internal stresses impeding domain wall motion as expected of multidomain grains the ratio of saturation remanence to saturation magnetization in 1200 oe 95 ka m 1 cycles jr js rises approximately in proportion to hc with a constant of proportionality consistent with titanomagnetite x 0 6 as temperature is lowered from 190 to 120 k hc rises rapidly to 400 oe 32 ka m 1 as a roughly linear function of the

carbon isotopic ratios of wetland and terrace soil sequences in the maya lowlands of belize and guatemala abstract this article provides new data and synthesizes earlier findings on the carbon isotope ratios of the humin part of soil organic matter from a range of sites in the central maya lowlands changes down the soil profile in carbon isotope ratios can provide an important line of evidence for vegetation change and erosion over time especially in well dated aggrading profiles research thus far has provided substantial evidence for significant inputs from c4 vegetation in buried layers from the ancient maya periods in depositional soils but equivocal evidence from sloping soils we present new findings from soil profiles through ancient maya wetland fields upland karst wetlands ancient maya aguadas reservoirs and ancient maya terraces most of the profiles exhibited 13c enrichment greater than the 2 5 3 typical from bacterial fractionation seven of nine ancient maya wetland profiles showed 13c enrichment ranging from 4 25 to 8 56 in ancient maya dated sediments that also contained phytolith and pollen evidence of grass c4 species dominance upland karst sinks and ancient reservoirs produced more modest results for 13c enrichment these seasonal wetland profiles exhibited 13c enrichment ranging from 1 to 7 3 from the surface to ancient maya period sediments agricultural terraces produced mixed results with two terraces having substantial 13c enrichment of 5 34 and 5 66 and two producing only equivocal results of 1 88 and 3 03 from modern topsoils to maya classic period buried soils altogether these findings indicate that c4 plants made up c 25 of the vegetation at our sites in the maya classic period and only a few percent today these findings advance the small corpus of studies from ancient terraces karst sinks and ancient wetland fields by demonstrating substantial 13c and thus c4 plant enrichment in soil profile sections dated to ancient maya times these studies are also providing a new line of evidence about local and regional soil and ecological change in this region of widespread environmental change in the late holocene

magnetic properties and opaque mineralogy of drilled submarine intrusive rocks summary measurements of natural remanent magnetization nrm curie temperature ferrimagnetic and paramagnetic susceptibility saturation induced and remanent magnetizations coercive forces alternating field properties and viscous magnetization are reported for 50 submarine intrusive rocks drilled during legs 30 37 and 45 of dsdp the collection includes doleritic sill rocks fresh and serpentinized cumulate gabbros and serpentinized cumulate peridotites and serpentinized lherzolites believed to have originated in layers 2b 3b and 4 respectively magnetite with a curie temperature between 520 and 580 c is the principal magnetic mineral in all samples there is no indication of maghemitization or of metamorphism to greenschist facies or above the magnetite in the doleritic and cumulate gabbros is a product of deuteric alteration of titanomagnetite and pyroxene the stable nrm is a primary trm the magnetite in the serpentinized rocks is a secondary product of serpentinization the stable nrm is a crm in most rock types the magnetite is of single domain or pseudo single domain size and soft components of nrm are small the magnetite grain size in some of the doleritic gabbros is much coarser these rocks acquire large viscous magnetizations which however are readily removed by alternating field cleaning the cleaned nrms of legs 30 and 45 rocks have approximately dipole inclinations but the nrms of most of the mutually intruded leg 37 units have been dispersed by tectonic rotations the doleritic gabbros and the serpentinized peridotites have stable directionally coherent nrms 10 3emu cm 3 in intensity their counterparts in layers 2b and 3b 4 are likely contributors to oceanic magnetic anomalies layer 3b cumulate gabbros and tectonic serpentinites in the middle and upper crust are less likely anomaly sources

subsolidus evolution and alteration of titanomagnetite in ocean ridge basalts from deep sea drilling project ocean drilling program hole 504b9 leg 83 implications for the timing of magnetization magnetic minerals in six samples of oceanic basalts of the transition zone and upper sheeted dikes from deep sea drilling project ocean drilling program dsdp odp hole 504b leg 83 were studied by methods of rock magnetism and transmission electron microscopy tem tem observations showed that the magnetic mineral in these basalts is end member magnetite tmo of extremely fine grain size 30 100 nm primarily in the range of pseudosingle domain magnetite consistent with the rock magnetic properties including hysteresis parameters curie temperature and low temperature measurements verwey transition magnetite formed by two different processes 1 oxidation exsolution true exsolution and hydrothermal alteration and 2 oxidation exsolution a second stage of oxidation exsolution and hydrothermal alteration the primary titanomagnetite tm60 70 that crystallized from the melt thus evolved to end member magnetite coexisting with titanite sphene kassite ulvospinel tm 87 and ilmenite on a submicroscopic scale on the basis of the formation mechanisms of the magnetic carrier the primary titanomagnetite tm 60 with curie temperature of 180 c did not acquire thermoremanent magnetization trm in these basalts instead the ti bearing magnetite tm 10 20 that formed as oxidized or exsolved lamellae acquired its first thermal chemical remanent magnetization crm at 500 400 c during subsolidus cooling upon the onset of hydrothermal alteration the recrystallized end member magnetite acquired a second crm the natural remanent magnetization of the basalts from the transition zone and upper sheeted dikes is therefore characteristic of crms that were acquired when titanomagnetite altered in part to magnetite during subsolidus cooling and hydrothermal alteration close to the ridge axis

magnetic properties of a soil from southeastern brazil in the presence of magnetic biomineralization by social insects abstract it is known that the migratory ant pachycondyla marginata and its prey the termite neocapritermes opacus synthesize magnetic minerals which may contribute to the magnetic characteristics of soil a comparative study of the magnetic properties of the minerals found in those species as well as their nests and surrounding soil aimed at investigating the contribution of the biogenic minerals to the magnetization of the soil partially oxidized magnetite t v 120 k was found in the pachycondyla marginata as shown by the verwey transition in the zfc fc and rtsirm curves low temperature remanence measurements of the neocapritermes opacus indicated the presence of maghemite and or titanomaghemite while partially oxidized pseudo single domain magnetites and or titanomaghemite were recognized in the soil and insect nests based on the similarity in the magnetic characteristic of the samples it is concluded that there is significant influence of biogenic mineralization upon the pedogenic contribution

stability and instability on maya lowlands tropical hillslope soils abstract substantial lake core and other evidence shows accelerated soil erosion occurred in the maya lowlands of central america over ancient maya history from 3000 to 1000 years ago but we have little evidence of the wider network of the sources and sinks of that eroded sediment cascade this study begins to solve the mystery of missing soil with new research and a synthesis of existing studies of tropical forest soils along slopes in nw belize the research aim is to understand soil formation long term human impacts on slopes and slope stability over time and explore ecological implications we studied soils on seven slopes in tropical forest areas that have experienced intensive ancient human impacts and those with little ancient impacts all of our soil catenas except for one deforested from old growth two years before contain evidence for about 1000 years of stable tropical forest cover since maya abandonment we characterized the physical chemical and taxonomic characteristics of soils at crest shoulder backslopes footslopes and depression locations analyzing typical soil parameters chemical elements and carbon isotopes 13 c in dated and undated sequences four footslopes or depressions in areas of high ancient occupation preserved evidence of buried clay textured soils covered by coarser sediment dating from the maya classic period three footslopes from areas with scant evidence of ancient occupation had little discernable deposition these findings add to a growing corpus of soil toposequences with similar facies changes in footslopes and depressions that date to the maya period using major elemental concentrations across a range of catenas we derived a measure ca mg al fe mn of the relative contributions of autochthonous and allochthonous materials and the relative age of soil catenas we found very low ratios in clearly older buried soils in footslopes and depressions and on slopes that had not undergone ancient maya erosion we found high ca mg al fe mn values on slopes with several lines of evidence that suggest relative youth soils possibly formed since maya abandonment carbon isotopes 13 c also provide some evidence of past vegetation change on slopes we found strong evidence for maize or other alien c 4 species in an ancient terrace soil and additional evidence in buried footslopes but only evidence for c 3 species like tropical trees on the backslopes and other crest shoulders the fact that steep slopes preserved no evidence of c 4 species inputs may mean that the ancient maya maintained forests here alternatively ancient maya land uses eroded slopes with the 13 c signatures detected today being the result of more recent soil development under forest over the last millennium additional evidence that these soils are recent in age includes elevated ca mg al fe mn values skeletal soil profiles and low soil magnetic susceptibility besides the evidence for truncating backslopes and aggrading footslopes the ancient maya built agricultural terraces that accumulated soils and altered drainage all these ancient maya slope alterations would have influenced modern tree distributions because many tree species in the modern forest show strong preferences for different soil types and topographic situations that the ancient maya changed

volcanism at 1 45 ma within the yellowstone volcanic field united states abstract rhyolitic volcanism in the yellowstone volcanic field has spanned over two million years and consisted of both explosive caldera forming eruptions and smaller effusive flows and domes effusive eruptions have been documented preceding and following caldera forming eruptions however the temporal and petrogenetic relationships of these magmas to the caldera forming eruptions are relatively unknown here we present new 40ar 39ar dates for four small volume eruptions located on the western rim of the second cycle caldera the source of the 1 300 0 001 ma mesa falls tuff we supplement our new eruption ages with whole rock major and trace element chemistry pb isotopic ratios of feldspar and paleomagnetic and rock magnetic analyses eruption ages for the effusive green canyon flow 1 299 0 002 ma and moonshine mountain dome 1 302 0 003 ma are in close temporal proximity to the eruption age of the mesa falls tuff in contrast our results indicate a period of volcanism at ca 1 45 ma within the yellowstone volcanic field including the eruption of the bishop mountain flow 1 458 0 002 ma and tuff of lyle spring 1 450 0 003 ma these high silica rhyolites are chemically and isotopically distinct from the mesa falls tuff and related 1 3 ma effusive eruptions the 40ar 39ar data from the tuff of lyle spring demonstrate significant antecrystic inheritance prevalent within the upper welded ash flow tuff matrix and minimal within individual pumice antecrysts are up to 20 kyr older than the eruption with subpopulations of grains occurring every few thousand years we interpret these results as an indicator for the timing of magmatic pulses into a growing magmatic system that would ultimately erupt the tuff of lyle spring and which we more broadly interpret as the tempo of crustal accumulation associated with bimodal magmatism we propose a system whereby chemically isotopically and temporally distinct isolated small volume magma batches are periodically generated and erupted in a low magmatic flux state which is punctuated by larger volume caldera forming eruptions

high paleointensities of the geomagnetic field from thermomagnetic studies on rift valley pillow basalts from the mid atlantic ridge nineteen pillow basalts dredged within the rift valley of the mid atlantic ridge at 36 8on were studied by the thellier stepwise heating method in order to determine the paleointensity of the geomagnetic field when they erupted on to the sea floor previously reported fission track ages are 2 000 to 6 000 years for the youngest rocks mainly olivine basalts and 10 000 to 100 000 years for the others mainly plagioclase basalts and pyroxene basalts all but three pillow basalts meet the conditions commonly considered as indicative of quite reliable paleointensity estimates stability of the direction of nrm during its thermal demagnetization constant ratio of nrm trm natural remanent magnetization to thermoremanent magnetization over 50 or more of the original nrm intensity 80 to 94 for 11 specimens and reproducibility of low temperature partial trm ptrm however strong field thermomagnetic measurements indicate that 11 of these 16 samples display a significant increase in curie temperature 15 to 80oc during the paleointensity experiments below 250oc notwithstanding the linearity of the nrm trm plot in this temperature interval this alteration probably due to low temperature oxidation of the specimens seems typical of young pillow basalts and may result in paleointensity estimates which are too high this result shows that excellence of remanence tests nrmtrm linearity and ptrm stability does not ensure the absence of chemical changes during the thellier experiments and therefore the validity of the paleointensity obtained the assumption that reliable paleointensities are obtained when the curie point increase is 10oc or less led to the selection of five of our specimens all from the youngest group their mean paleointensity is 64 2 20 5 xt standard deviation and the corresponding virtual axial dipole moment vadm is 11 5 3 7 x 1022 a m 2 given the variations of the vadm of the earth s magnetic field over the last 6 000 years as established from archeomagnetic studies our paleointensity results suggest that the latest eruptions on the inner floor of the rift valley at 36 8on occurred 1 500 1 000 years ago

magnetic hysteresis in young mid ocean ridge basalts dominant cubic anisotropy magnetic hysteresis data from young mid ocean ridge basalts include samples with saturation remanence to saturation magnetization mrs ms ratios greater than 0 5 the theoretical limit for an assemblage of single domain grains with uniaxial anisotropy under the usual assumption of dominant uniaxial anisotropy the narrow single domain grain size distribution implied by these high mrs ms values is difficult to reconcile with petrographic and remanence data that suggest the presence of larger multidomain grains dominant cubic anisotropy provides a plausible explanation for the high mrs ms ratios and if generally valid requires reinterpretation of granulometric and domain state inferences made from hysteresis data

sorption capacity on lead copper and zinc by clay soils from south wales united kingdom the sorption capacity of five clay soils from south wales in the united kingdom was investigated using two main tests physico chemical and batch equilibrium bet the physico chemical property results show that soil weathered mudrocks mr1 glacial till gt1 and estuarine alluviums nea4 pea3 and cea3 are capable of being used as landfill liners and are chosen for further sorption tests the batch tests i e sorption tests on soil suspension produce adsorption ph curves showing increasing adsorption at higher ph levels the sorption data are also presented using adsorption isotherm curves and are best fitted using the langmuir adsorption equation that yields the maximum adsorption capacity of the soils the sorption capacity of soils are ranked as estuarine alluvium soils glacial till weathered mudrocks estuarine alluvium soils show a good buffering capacity and high sorption capability compared to glacial till and weathered mudrocks the study also discovers that the maximum sorption capacities are highly influenced by the chemical properties of the soils

petrology and rock magnetism of the gabbro of troodos ophiolite abstract in order to determine the nature of remanence carriers of the layered gabbro of the troodos ophiolite cyprus we report optical and electron microscopic observations together with rock magnetic and paleomagnetic experiments above all the study aims to understand and clarify the time of magnetic acquisition relative to the brittle deformation of the oceanic crust manifested by the ridge transform intersection solea graben and the arakapas transform petrographic examination of pyroxene grains revealed isolated magnetite inclusions ranging in size from single domain sd to multi domain md and in addition md pyrrhotite inclusions residing in veins and cracks thermal demagnetization and thermomagnetic procedures indicate two components low and high temperature we argue that the low temperature component the sd magnetite inclusions exsolved in pyroxene are the dominant carriers of magnetic remanence in troodos gabbro the initial formation of these inclusions occurred via exsolution reaction at temperatures between 520 and 850 c above the curie temperature of pure magnetite during the solidification of the magma therefore acquisition of remanent magnetization of the troodos gabbro took place during the earliest stages of crustal accretion before any brittle deformation associated with the spreading ridge and the transform fault occurred

pedosediments of karstic sinkholes in the eolianites of ne yucatan a record of late quaternary soil development geomorphic processes and landscape stability abstract the pedogenetic and geomorphic processes controlling soil development in tropical karstic landscapes are still poorly understood we have discovered and studied thick pedosediments in karst pockets developed in late pleistocene lithified calcareous dunes eolianites in the ne coast of the yucatan peninsula the morphological geochemical and mineralogical characteristics of the pocket fills were interpreted as a record of pedogenesis and geomorphic dynamics that shaped the soil mantle during the holocene sand fraction mineralogy and the zr ti ratio indicated a mixed origin of the soil parent material volcanic and granitic metamorphic components redeposited in the lithoral and coastal environments were identified within the soil matrix primary minerals were transformed by weathering and caused desilicification accumulation of clay hydroxy interlayered vermiculite and kaolinite and dithionite citrate bicarbonate extractable iron oxides fed reworked pedofeatures indicative of clay illuviation and redoximorphic processes were observed this set of processes points to a continuous phase of humid pedogenesis several thousand years long in the early middle holocene however the development of thick recalcified pedosediments in the pockets contrasting with the thin background rendzinas requires active soil redeposition during a late phase of geomorphic activity the available accelerator mass spectrometry ams radiocarbon date indicates that the switch of the erosion sedimentation processes could be related to ancient maya land use impact

comparison of the coe thellier thellier and microwave palaeointensity techniques using high titanium titanomagnetites results from a tertiary basaltic intrusion from the sydney basin new south wales abstract conventional thellier and microwave mw palaeointensity experiments accompanied by rock magnetic tests were conducted on samples from an early tertiary ca 49 ma basaltic intrusion the peats ridge pr basalt from the sydney basin thermomagnetic and low temperature susceptibility measurements suggest that the dominant magnetic mineral is ti enriched titanomagnetite tm65 tm50 and hysteresis results indicate that the majority of samples exhibit psd like behaviour some samples show evidence of alteration during laboratory heating but only at temperatures in excess of 400 c no alteration is observed in the dominant temperature range 70 300 c used for calculation of palaeointensities stepwise thermal demagnetisation studies reveal single component after removal of a small viscous reverse polarity magnetisations in all samples with a mean characteristic remanence chrm direction of d i 197 60 95 5 k 62 coe thellier thellier ctt experiments yielded a high success rate with 68 of samples conforming to standard acceptance criteria microwave mw palaeointensity experiments predominantly using the perpendicular applied field technique produced a greater success rate of 82 mean palaeointensities of 27 1 4 4 t n 21 and 27 4 4 0 t n 28 were obtained from the ctt and mw techniques respectively these means could not be distinguished statistically and the respective quality indicators were similar these results suggest that the new microwave technique for calculating palaeointensities is in this case equivalent to the well established ctt technique this is an intuitive outcome given that rock magnetic analyses suggest that alteration does not occur below the curie temperature of the samples our results therefore provide a positive test for the equivalence of the mw and ctt techniques the virtual dipole moment vdm value for the intrusion is 4 7 0 7 10 22 am 2 suggesting that the dipole field was approximately 59 of the present day value at ca 49 ma this provides a valuable data point from the southern hemisphere for a sparsely covered time period in the global palaeointensity database

geoarchaeology and tropical forest soil catenas of northwestern belize new research on soil catenas under old growth tropical forest in nw belize aims to understand soil formation long term human impacts and slope stability over time we studied areas with intensive ancient maya impacts from 3000 to 1000 years ago and those with little ancient impact to compare how ancient land use influenced slopes after about 1000 years of stable tropical forest cover we characterized the physical chemical and chronological characteristics of soils at crest shoulders backslopes footslopes and depression locations analyzing typical soil parameters and carbon isotopes 13c in dated and undated sequences several of the footslope and depressions sites preserved evidence of buried paleosols with increased 13c dating by four new ams dates and three others we reported earlier from the maya classic period with high amounts of soil organic matter from c4 species in ecosystems where only a few c4 species occur today most crest shoulders and backslopes had no evidence for increased 13c though some crests and mid slope sinks preserved evidence of ancient maya impacts the observation that steep slopes preserved no evidence of c4 species inputs may mean that ancient maya land use maintained forest reserves here alternatively ancient maya land uses caused slope erosion with the 13c signatures detected today resulting from more recent soil development under forest and soil re formation over the last millennium additional evidence that these soils are recent in age includes elevated caco3 sr and low soil magnetic susceptibility showing less leaching compared with older soils in our study other human impacts on soils include increased concentrations of fe and mn terracing and further evidence of substantial topsoil erosion in antiquity

the ancient agricultural landscape of the satellite settlement of ramonal near tikal guatemala abstract soil science methodologies often enrich archaeological reconstructions in the maya lowlands of mesoamerica stable carbon c isotope ratio 13c analysis of soil organic matter som from profiles near ancient archaeological sites has been used to interpret the vegetative histories related to ancient maize agriculture due to distinct photosynthetic processes the decomposed organic matter from the cultivation of maize can be distinguished from som derived from native forest detritus the recalcitrant nature of humic substances allows for this distinction to be preserved through time this study evaluates som 13c from 98 soil profiles near tikal guatemala to identify areas of ancient maya agriculture and the staple crops used ancillary physical and chemical properties of the soil profiles were examined to facilitate and to supplement the interpretation of the isotope data most of the soils analyzed in this study are shallow haprendolls with limited c isotopic evidence of prehistoric vegetation changes the deeper well developed argiudolls and hapludolls contained strong evidence of vegetation changes associated with ancient maize agriculture areas with strong 13c signatures of ancient c4 vegetation corresponded with foot and toeslope locations high clay content and low phosphorus p levels the shallow backslope soils lacked significant evidence of ancient agriculture the connections between profile location settlement isotope data and p levels enable the identification of both agriculture production zones and enhanced understanding of stable c isotope dynamics in soils within the unique physiographic and archaeological surroundings of the maya lowlands

importance of titanohematite in detrital remanent magnetizations of strata spanning the cretaceous paleogene boundary hell creek region montana intermediate composition titanohematite fe2 ytiyo3 with 0 5 y 0 7 is seldom the focus of paleomagnetic study and is commonly believed to be rare in nature while largely overlooked in magnetostratigraphic studies intermediate titanohematite has been identified as the dominant ferrimagnetic mineral in an array of late mesozoic and early cenozoic laramide clastic deposits throughout the central united states intermediate titanohematite is ferrimagnetic and has similar magnetic properties to titanomagnetite except its unique self reversing property due to these similarities and with detrital remanent magnetizations masking its self reversing nature intermediate titanohematite is often misidentified in sedimentary deposits past studies relied upon nonmagnetic techniques including x ray diffraction and electron microprobe analysis while these techniques can identify the presence of intermediate titanohematite they fail to test whether the mineral is the primary recorder to facilitate the identification of intermediate titanohematite in sedimentary deposits we characterize this mineral using low temperature magnetometry and high temperature susceptibility experiments and present a new identification technique based on titanohematite s self reversing property for sediments that span the cretaceous paleogene boundary hell creek region montana results from the self reversal test indicate that the majority of remanence is held by minerals that become magnetized parallel to an applied field but that intermediate self reversing titanohematite y 0 53 0 63 is an important ancillary carrier of remanence while earlier literature suggests that intermediate titanohematite is rare in nature reanalysis using specialized rock magnetic techniques may reveal that it is more abundant in the rock record particularly within depositional basins adjacent to calc alkaline volcanics than previously thought

a review of human and natural changes in maya lowland wetlands over the holocene abstract in the maya lowlands of mexico belize and guatemala two main types of wetlands have played important roles in human history bajos or intermittently wet environments of the upland interior yucatan and perennial wetlands of the coastal plains many of the most important maya sites encircle the bajos though our growing evidence for human wetland interactions is still sparse the deposits of these wetlands record two main eras of slope instability and wetland aggradation the pleistocene holocene transition as rainfall increased and forests eclipsed savannas and the maya preclassic to classic as deforestation land use intensity and drying increased the ancient maya adapted with terraces around these bajo margins but retracted in the late preclassic in some areas the perennial wetlands of the coastal plains have different histories and the first conceptual model of human wetland interaction described intensive wetland agriculture in the preclassic through classic based on raised fields and canals but a second model arose that interpreted the wetland stratigraphy and canals as more indicative of natural aggradation by accelerated erosion and gypsum precipitation that buried archaic and preclassic fields and there was little classic era use we present new data on a third and fourth model in this study the third is a hybrid of the models one and two including the archaic to preclassic aggradation of the second model and the first model s classic period fields and canals as piecemeal attempts by the maya to adapt to these and other environmental changes the fourth conceptual model describes a very late terminal classic preplanned project on a floodplain these wetland fields were short lived aggraded rapidly but with some reoccupation in the postclassic all of these new models display the burgeoning evidence for intricate maya interactions with wetlands and the diversity of evidence from the relatively few studies underscores the infancy of our understanding of maya interaction with tropical wetlands

steady and non steady state magma chambers below the east pacific rise volcanic rocks collected on flowline traverses on the flanks of the east pacific rise epr document changes in axial magma chemistry with time providing a record of the thermal history of axial magma chambers amc we present data for a sample set of closely spaced 1 2 km samples along epr flowlines at three localities out to 800 ka showing both steady constant average temperature and non steady state behavior of magma chambers on time scales of 200 500 ka though based on only three symmetrical traverses so far it appears that magmatically robust ridge locations 11 20 n and 9 30 n have steady state chambers whereas a magmatically starved axis 10 30 n shows large temperature changes with time these observations provide a new petrologic perspective to the ongoing debate regarding the significance and causes of morphologic variations along the axis of the east pacific rise

intrusion of lamprophyre dyke and related deformation effects in the host rock salt a case study from the loule diapir portugal abstract a rock salt lamprophyre dyke contact zone sub vertical ne sw strike was investigated for its petrographic mechanic and physical properties by means of anisotropy of magnetic susceptibility ams and rock magnetic properties coupled with quantitative microstructural analysis and thermal mathematical modelling the quantitative microstructural analysis of halite texture and solid inclusions revealed good spatial correlation with ams and halite fabrics the fabrics of both lamprophyre and rock salt record the magmatic intrusion plastic flow and regional deformation characterized by a nw se trending steep foliation ams and microstructural analysis revealed two deformation fabrics in the rock salt 1 the deformation fabrics in rock salt on the nw side of the dyke are associated with high temperature and high fluid activity attributed to the dyke emplacement 2 on the opposite side of the dyke the emplacement related fabric is reworked by localized tectonic deformation the paleomagnetic results suggest significant rotation of the whole dyke probably during the diapir ascent and or the regional tertiary to quaternary deformation

correcting distorted paleosecular variation in late glacial lacustrine clay abstract an undisturbed horizontal chronostratigraphic marker horizon of laminated red glacio lacustrine clay crops out over 25 000 km 2 in northern ontario canada the primary clastic hematite laminae possess two stable vector components of magnetization we sampled a 1 6 m vertical section at overlapping 2 cm intervals in cubic specimens 8 cm 3 n 106 precisely oriented in geographic coordinates which permitted measurement of inclination and declination alternating field demagnetization 12 17 steps per specimen isolated a characteristic chrm primary component coercivity 40 mt approximately 30 shallower than the mean geomagnetic field inclination at this latitude with distorted paleosecular secular variation psv a lower coercivity overprint 20 40 mt is 6 shallower than the present geomagnetic field and similarly inclined to the depositional remanence deflection when the clay was re sedimented in the laboratory we believe this angle to be representative of the inclination of the deflected primary remanence inclination during deposition caused by the magnetic anisotropy of the clay using this as a proxy for the initial inclination the chrm hard component inclinations were restored to their original values assuming vertical compaction which averages to 51 the restored inclinations are compatible with site latitude and the restored secular variation loops centre reasonably on the geographic north pole the structural correction based on homogeneous vertical shortening over simplifies the reality of heterogeneous particulate flow in which grain rotations depend on shape size and packing and effectively combines influences of compaction and depositional settlement nevertheless this correction makes the psv data more useful and interpretable in terms of paleopole migration the more logical correction technique using anisotropy of anhysteretic remanence aarm jackson m j banerjee s k marvin j a lu r gruber w 1991 detrital remanence inclination errors and anhysteretic remanence anisotropy quantitative model and experimental results geophys j int 104 95 103 is foiled here due to the high coercivity of the remanence bearing grains and their unknown orientation distribution and shape distribution

magnetization during low temperature oxidation of seafloor basalts no large scale chemical remagnetization difficulty in removing the effects of grain size dependence have hampered interpretations of magnetic property changes during low temperature oxidation of titanomaghemite for the present study samples were collected perpendicular to cracks in seafloor basalts to produce subsamples of essentially constant grain size but varying degree of low temperature oxidation within a few centimeters coarse grained samples and those showing evidence of high temperature oxidation were excluded increasing curie temperature was used as the most reliable measure of increasing maghemitization intensity of natural remanent magnetization jnrm consistently decreased with increasing low temperature oxidation indicating that any increase in jnrm with age of oceanic crust must be due to other factors coercive force hc increased until curie temperatures reached 250 c and then in samples exhibiting single domain behavior decreased drastically because coercive force depended on degree of oxidation in this complex manner chemical remanent magnetization crm in even slightly oxidized grains should have a different coercivity than the original thermal remanent magnetization trm most deep sea drilling project samples in this study have experienced tectonic rotations or field reversals within 1 0 m y of extrusion thus field controlled crm if present should have different coercivities and a discrete direction and be discernible during alternating field af demagnetization alternating field demagnetizations showed no evidence for significant acquisition of field controlled crm viscous remanent magnetization vrm was the dominant secondary magnetization in samples with several components of remanence in very fine grained basalts vrm was difficult to remove by af demagnetization and may be the source of increased dispersion in highly oxidized oceanic basalts mechanisms which may in part be responsible for coercivity and vrm changes are 1 an increase in the critical size of domain transitions during low temperature oxidation and 2 incipient phase separation into fe rich and ti rich areas the fe rich areas being superparamagnetic the latter mechanism explains the increase in saturation magnetization js and concomitant decrease in jnrm when curie temperatures are above 360 c

low temperature oxidation in morb of titanomagnetite to titanomaghemite a gradual process with implications for marine magnetic anomaly amplitudes mid ocean ridge basalt morb samples varying in age from recent to jurassic were selected for electron microscopic and rock magnetic studies our observations indicate that the degree of oxidation of titanomagnetite in morb increases only gradually with sample age the titanomagnetite in recent morb 100 ka both electron microscopic observations and rock magnetic data support the notion that low temperature oxidation of titanomagnetite to titanomaghemite in morb is a gradual process moreover the rate of maghemitization is controlled by many factors on both macroscopic and microscopic scales including regional oceanic crustal structures lithological features grain size and surrounding matrices pillow lavas and the tops of massive flows tend to have higher degrees of oxidation than the interiors of massive flows owing to higher porosity and permeability of the former in contrast interstitial glass can protect fine titanomagnetite grains from alteration the natural remanent magnetization nrm intensity of morb varies as a function of age magnetic granulometry concentration of magnetic materials and the degree of alteration fine grained morb samples typically have higher nrm intensity the nrm intensity appears to decrease substantially with increasing degree of maghemitization and hence with age the envelope of the smoothed magnetic anomaly amplitudes resembles the change in nrm intensity for morb samples of the last 30 million years but the underlying assumption that the highest degree of maghemitization observed in the samples of a given age is entirely responsible for these intensity changes is not supported maghemitization of the micrometer sized and larger grains is probably only partly responsible whereas a significant contribution to nrm intensities is inferred from those submicrometer sized titanomagnetite grains that remained protected from oxidation by the surrounding matrix of interstitial glass

magnetization of axial lavas from the southern east pacific rise 14 23 s geochemical controls on magnetic properties although the spatial association of iron rich lavas and high amplitude magnetic anomalies is well documented a causal link between enhanced iron content and high remanent magnetization has been difficult to establish here we report magnetic data from approximately 250 samples with 8 16 feo total iron as feo from the southern east pacific rise epr that provide strong support for the presumed geochemical dependence of remanent intensity the limited age range 0 6 ka of axial lavas from this ultrafast spreading ridge 150 mm yr full rate effectively minimizes variations resulting from time dependent changes in geomagnetic intensity or low temperature alteration systematic sampling relative to the chilled margin illustrates that substantial grain size related variations in magnetic properties occur on a centimeter scale both microprobe data and curie temperatures suggest that the average groundmass titanomagnetite composition in the southern epr samples is approximately constant modal modified ulv6spinel content 0 67 over a wide range of lava compositions saturation magnetization and saturation remanence are highly correlated with feo r 0 73 and 0 83 respectively indicating that more iron rich lavas have higher abundances of otherwise similar titanomagnetite we show that there is a good correlation between natural remanent magnetization nrm and feo provided that sufficient specimens are used to determine the average nrm of a sample r 0 63 because the range of iron contents in mid ocean ridge basalts is limited the best fit slope 4 44 a m per feo in an ambient field of 0 030 mt should provide reasonable bounds on the equatorial magnetization of submarine lavas 10 a m at 8 5 feo and 50 a m at 16 feo finally we demonstrate that along axis variations in nrm closely parallel geochemical changes along the southern epr where magnetization values deviate significantly from those predicted from the range of measured feo contents these discrepancies may reflect additional unrecognized geochemical variability

low temperature magnetic behavior of titanomagnetites titanium substitution has a significant effect on the temperature dependence 5 300 k of low temperature saturation remanence and of initial susceptibility in synthetic and natural titanomagnetite fe3 xtixo4 0 x 0 4 and electronic and lattice relaxation phenomena over the entire titanomagnetite compositional range these features produce diagnostic magnetic behavior useful for titanomagnetite identification in natural samples moreover the apparent similarities between the thermal decay of low temperature remanence of titanomagnetite with pyrrhotite pure magnetite and superparamagnetic phases can lead to complications in interpreting low temperature remanence data when titanomagnetite is present

magnetic state of 10 40 ma old ocean basalts and its implications for natural remanent magnetization abstract the natural remanent magnetization nrm of ocean basalts giving rise to the pattern of marine magnetic anomalies is known to be of comparatively low intensity for about 20 ma old oceanic crust the aim of this study is to detect possible peculiarities in the rock magnetic properties of ocean basalts of this age and to establish a link between magnetomineralogy rock magnetic parameters and the low nrm intensity ocean basalts covering ages from 0 7 to 135 ma were selected for rock magnetic experiments and their room temperature hysteresis parameters curie temperature and temperature dependence of saturation magnetization ms t was determined and complemented by reflected light microscopy the majority of samples is magnetically dominated by titanomagnetite and titanomaghemite with increasing oxidation state with age for these a strong dependence of hysteresis parameters on the age of the samples is found the samples have a minimum in saturation magnetization and a maximum in magnetic stability in the age interval ranging from approximately 10 to 40 ma coinciding with the age interval of low nrm intensity the observed change in saturation magnetization is in the same order as that for the nrm intensity a further peculiarity of the titanomaghemites from this age interval is the shape of their ms t curves which display a maximum above room temperature neel p type and sometimes a self reversal of magnetization below room temperature neel n type these special rock magnetic properties can be explained by titanomagnetite low temperature oxidation and highly oxidized titanomaghemites in the age interval 10 40 ma a corresponding measurement of the nrm at elevated temperature allows to identify a maximum in nrm intensity above room temperature for the samples in that age interval this provides evidence that the nrm is equally carried by titanomaghemites and that the low nrm intensities for about 20 ma old ocean basalts are caused consequently by the low saturation magnetization of these titanomaghemites

impact of forest clearance on the soils of tropical cone karst the precise nature and effects of forest clearance followed by a two to three year cycle of shifting cultivation and subsequent secondary re growth was observed in an area of lowland evergreen forest over cone karst topography in central belize the surface soil properties varied in their response to disturbance but a number of significant changes were measured by comparing the pre clearance and post cultivation soil analyses four groups of soil properties were identified 1 organic properties which suffered a marked decrease over the summit and upper slope areas and which showed a tendency to increase towards the foot 2 alkaline status which demonstrated an increase on the upper slopes 3 nutrient cations whose levels from more limited evidence were generally lower after clearance particularly in the summit and upper slope areas and 4 soil physical properties where both the fine fraction and soil depth were drastically affected by clearance the proportion of clay diminished on the upper slopes and increased at the footslope zone whilst soil depth thinned at the summit and increased markedly over the foreland the scale of disturbance is so considerable that it has led to recommendations to limit cultivation and clearance pressure in these vulnerable environments

determination of lattice parameter oxidation state and composition of individual titanomagnetite titanomaghemite grains by transmission electron microscopy a transmission electron microscopy tem method higher order laue zone holz diffraction has been applied to measurement of accurate lattice parameters of individual titanomagnetite and titanomaghemite grains lattice parameters and cation composition data are obtained with tem analytical electron microscopy aem for individual grains smaller than 0 1 m in diameter a suite of ocean drilling program deep sea drilling project ocean floor basalts containing titanomagnetite titanomaghemite with a wide range of oxidation degree z were employed to establish the relationship between lattice parameter a z and ti content x for naturally occurring samples standardization was accomplished with measurements on grains sufficiently large to allow the determination of absolute oxygen contents by electron microprobe analyses which directly yielded z values the probe data confirm that the maghemitization process in ocean floor basalts is one involving loss of iron and indicate a nearly linear relationship between lattice parameters and oxidation degrees for a given x value except for the smallest and largest values of z the standardized relation permits determination of z for individual submicrometer sized grains using only measured a values and cation compositions as determined by holz diffraction and aem

variable ti content and grain size of titanomagnetite as a function of cooling rate in very young morb transmission electron microscopy observations and rock magnetic measurements on a pillow from the new flow extruded in 1993 at the juan de fuca ridge demonstrate that ti content in large up to 40 m titanomagnetite grains varies as a function of the cooling rate large grains in the interior of the pillow have a narrow composition range of approximately x 0 6 where x represents the ulvospinel content in the solid solution series with magnetite in contrast the titanomagnetite grains near the pillow rim are progressively smaller and have a broad composition range average x 0 45 within 0 5 cm of the rim a significant portion of the grains is single domain sd to superparamagnetic sp and appears to have little or no ti this lower ti content provides a ready explanation for the higher curie temperature near the glassy margin of the pillow moreover determination of the oxidation state of the titanomagnetite does not show low temperature oxidation anywhere within the pillow indicating no rapid alteration to titanomaghemite as previously suggested for this very young mid ocean ridge basalt morb in order to explain the curie temperatures submicrometer titanomagnetite in interstitial glass in the interior of the pillow also shows variable x values these grains make contributions to susceptibility that show up as high up to 580 c curie temperatures in thermomagnetic analyses and they appear to be sd to sp the variation of x values for the titanomagnetite in the morb is consistent with thermodynamic and kinetic considerations because crystallization of titanomagnetite in the morb is controlled by processes depending on cooling rate and crystal melt fractionation

low geomagnetic field intensity during the kiaman superchron thellier and microwave results from the great whin sill intrusive complex northern united kingdom 1 coe modified thellier cmt and microwave mw paleointensity experiments were conducted on samples from the multi phase late carboniferous ca 301 ma to early permian ca 270 ma great whin sill complex statistically comparable results were obtained with both techniques from high quality data forty three samples 25 with cmt and 18 with mw yielded mean paleointensity values of 12 0 5 8 t and 16 5 8 2 t for the early and late intrusive phases respectively corresponding to vdms of 1 6 0 8 1022 and 2 1 1 1 1022 am2 when combined with previous data the results suggest a generally low dipole moment during the krs reducing from 75 of the present value at 301 ma to 20 27 of the present value later in the superchron these data imply either a decoupling between processes controlling reversal rate and field strength or a more complex intriguing time lag relationship more absolute palaeointensity results are needed to further evaluate these alternatives

recovery of the forest ecosystem in the tropical lowlands of northern guatemala after disintegration of classic maya polities we employed paleolimnological methods to investigate tropical forest recovery and soil stabilization that followed abandonment of agricultural systems associated with disintegration of classic maya polities ca a d 800 1000 we used lithological geochemical magnetic and palynological data from sediment cores of lake peten itza in the maya lowlands of northern guatemala sediment core chronology was developed using radiocarbon dates on terrestrial wood and charcoal fragments our results indicate that in the absence of large human populations and extensive farming activities peten forests recovered under humid climate conditions within a span of 80 260 yr soil stabilization postdates pollen evidence of forest regrowth stratigraphically and required between 120 and 280 yr we conclude that the tropical forest ecosystem in the watershed of lake peten itza had been reestablished by the early postclassic period a d 1000 1200

variations in magnetization intensity and low temperature titanomagnetite oxidation of ocean floor basalts the remanent magnetization of the oceanic crust exhibits a systematic long term variation which correlates with the amplitudes of marine magnetic anomalies after a sharp initial decrease of natural remanent magnetization intensity a minimum is reached at 20myr followed by a gradual increase up to ages of 120 myr the progressive sea floor alteration of the magnetic minerals carrying the crustal magnetism is proposed as a cause for this behaviour

separating geometry from stress induced remanent magnetization in magnetite with ilmenite lamellae from the stardalur basalts iceland

deciphering the inorganic chemical record of ancient human activity using icp ms a reconnaissance study of late classic soil floors at cancuen guatemala abstract inorganic chemical analysis of soil floors using inductively coupled plasma mass spectrometry icp ms was undertaken to provide information on the nature and location of past human activity in the ancient maya city of cancuen guatemala the use of icp ms to detect trace and ultra trace elemental enrichment of two excavated soil floors from the late classic period is the first study of its kind in mesoamerican archaeology geochemical background of the site was established by the analysis of palaeosols and nearby undisturbed off site soil profiles robust statistical methods used in the study clearly distinguished the level of anthropogenic enrichment across the former floors many elements measured showed only minor departures 10 20 from the site s background soil chemistry the greatest levels of elemental enrichment were detected in the rare earth elements mercury and gold the latter is of particular interest considering the consensus that gold was absent from the world of the classic period maya comparisons of the spatial pattern of mercury enrichment with lithic and archaeological data show strong linkages to past industrial and ritual activities elevated rare earth element concentrations were recorded broadly across both soil floors and are considered to be related to concentrated human occupation in antiquity

geochemical modeling and low frequency geoelectrical methods to evaluate the impact of an open dump in arid and deltaic environments in mexico open dumps that are maintained by the municipality but provide no covering of waste are not uncommon further disposal at these sites is often performed by burning the aim of this study was to determine the leachate plume from an open dump located in a depositional deltaic environment with arid climate low rainfall and where the water table is about 2 m below the surface the methodology comprised analysis of groundwater monitoring wells and geophysical and geochemical prospecting techniques the 3d geoelectric interpretation shows a typical area of these depositional environments with low resistive values 10 20 m associated with the presence of sands and clays interbedded however there is a very low resistivity zone associated with the dump s impact which reaches values below 5 m and it is located where the disposal and burning of wastes occurred another zone with values above 16 m appears as a limit for the advance of the saline this is interpreted as a sandy lenses area these sandy lenses with higher porosity transport aquifer s water thus the dump is in direct contact by this conduct with clean groundwater piper diagrams constructed with the chemical data analysis reveal that the groundwater in the area corresponds to the chlorinated and or sulfated sodium type showing the impact caused by the dump the monitoring well np8 on site with the highest dissolved solids content behaves anomalously and belongs to the more conductive zone according to the geoelectric profiles the subsoil geochemical behavior is influenced by the seasonal water table variations provoking the dissolution of burned and unburned wastes but the effects of slow flows in the direction of the regional flow are not discarded although the most impacted area within the dump is characterized to a depth of 10 m there is a slow flow in the direction of the regional flow that has been moving pollutants out of the dumpsite during its almost 20 years of operation the results of this study provide valuable information for the authorities to carry out an appropriate restoration project

impacts of the ancient maya on soils and soil erosion in the central maya lowlands abstract many studies across the central and southern maya lowlands of belize guatemala honduras and mexico have produced records of land degradation mostly sedimentation and soil erosion during the ancient maya period from before 1000 bc to the maya collapse of c ad 900 this paper provides new data from two sites blue creek and cancuen synthesizes more than a decade of the authors research in guatemala belize and mexico and synthesizes other findings from this region these research projects analyzed more than 100 excavations in upland and depression sites cored lakes and wetland sediments and studied sediments in the field and laboratory using radiocarbon dating a battery of soil chemistry tests stratigraphic analysis magnetic susceptibility elemental analyses and artifact identification our objective was to date when sedimentation and soil erosion occurred identify stable surfaces and correlate them with the state of knowledge about past land use these findings indicate three general epochs of accelerated soil erosion and identified two major paleosols the three waves of soil erosion occurred in the preclassic period c 1000 bc to ad 250 the late classic ad 550 to 900 and in the last several decades the major paleosol eklu um in these sites is a well developed mollisol or vertisol that started forming in the early holocene and was buried in either the preclassic or classic periods ad 250 to 900 at some sites the eklu um paleosol lies beneath sediments with a fainter paleosol which in turn lies buried below classic period and later sediments this picture shows higher than expected soil erosion linked to the region s first pioneer farmers in the preclassic and less than expected soil erosion in the late classic when population peaked and land use was the most intensive in other regions like cancuen guatemala however most soil erosion occurred during the maya late classic ad 550 830 erosion here was intense but short lived depressions record 1 3 m of aggradation in two centuries a third epoch of accelerated soil loss and aggradation arose with the rapid land use changes brought by new pioneers during the last several decades

crossover plots a useful method for plotting sirm data in paleomagnetism the effective grain size of magnetic minerals and therefore domain type controls the saturation isothermal remanent magnetization acquisition and subsequent demagnetization response and can be used to create crossover plots for paleomagnetic specimens where sirm is plotted as a function of the field applied using a logarithmic scale this paper presents templates of such plots for magnetite pyrrhotite hematite and goethite onto which sample data can be plotted for conventional paleomagnetic specimens to characterize their magnetic mineralogy and effective grain size

ancient maya impacts on the earth s surface an early anthropocene analog abstract the measure of the mayacene a microcosm of the early anthropocene that occurred from c 3000 to 1000 bp comes from multiple late quaternary paleoenvironmental records we synthesized the evidence for maya impacts on climate vegetation hydrology and the lithosphere from studies of soils lakes floodplains wetlands and other ecosystems maya civilization had likely altered local to regional ecosystems and hydrology by the preclassic period 3000 1700 bp but these impacts waned by 1000 bp they altered ecosystems with vast urban and rural infrastructure that included thousands of reservoirs wetland fields and canals terraces field ridges and temples although there is abundant evidence that indicates the maya altered their forests even at the large urban complex of tikal as much as 40 of the forest remained intact through the classic period existing forests are still influenced by ancient maya forest gardening particularly by the large expanses of ancient stone structures terraces and wetland fields that form their substrates a few studies suggest deforestation and other land uses probably also warmed and dried regional climate by the classic period 1700 1100 bp a much larger body of research documents the maya impacts on hydrology in the form of dams reservoirs canals eroded soils and urban design for runoff another metric of the mayacene are paleosols which contain chemical evidence for human occupation revealed by high phosphorus concentrations and carbon isotope ratios of c 4 species like maize in the c 3 dominated tropical forest ecosystem paleosol sequences exhibit maya clays a facies that reflects a glut of rapidly eroded sediments that overlie pre maya paleosols this stratigraphy is conspicuous in many dated soil profiles and marks the large scale maya transformation of the landscape in the preclassic and classic periods some of these also have increased phosphorous and carbon isotope evidence of c 4 species we synthesize and provide new evidence of maya period soil strata that show elevated carbon isotope ratios 13 c indicating the presence of c 4 species in typical agricultural sites this is often the case in ancient maya wetland systems which also have abundant evidence for the presence of several other economic plant species the mayacene of c 3000 to 1000 bp was thus a patchwork of cities villages roads urban heat islands intensive and extensive farmsteads forests and orchards today forests and wetlands cover much of the maya area but like so many places these are now under the onslaught of the deforestation draining and plowing of the present anthropocene

oxide mineral formation during the serpentinization of a cr spinel seam insights from rock magnetic experiments 1 serpentinization of ultramafic rocks results in the growth of serpentine and magnetite following hydration of magmatic olivine cr spinel seams in ultramafic intrusions that also contain olivine might produce magnetite during the serpentinization process however hysteresis and temperature dependence of susceptibility experiments on serpentinized chromitite 80 cr spinel from the dawros peridotite western ireland reveal the presence of small quantities of a fe ti oxide phase likely titanomaghemite reflected light petrography reveals that the oxides occur as 0 1 m to 20 m grains that are typically intergrown with serpentine it is suggested that the formation of the minute quantities of low susceptibility high curie point titanomaghemite occurred by low temperature alteration of the magnetite produced during serpentinization the presence of such low quantities of the oxide phase may be either a function of the very high mg content of the magmatic olivine or the decoupling of oxide growth from the process of serpentinization where magnetite formed later than the reaction of olivine with the infiltrating fluids our data highlight the usefulness of rock magnetic techniques to the investigation of oxide mineral phases in serpentinized ultramafic rocks

geochemical processes controlling the groundwater transport of contaminants released by a dump in an arid region of mexico the vado carranza dump located in the mexicali valley baja california northwest mexico was operated for more than 15 years receiving about 30 tons day of solid wastes the disposed wastes were periodically burned in open air the presence of a shallow aquifer in the area makes the groundwater vulnerable to contamination processes the purpose of this study was the evaluation of heavy metals content cu cd ni pb and zn in soil and groundwater in the vicinity of this dump the results indicate high content of metals in soil mainly at a superficial level with the highest concentrations in the areas where burning of wastes occurred elevated concentrations of cadmium and copper were detected in groundwater with the highest concentrations occurring in monitoring wells located in the north side of the dump downward of groundwater flow although the high content of metals in soil can be attributed to the burning of waste other sources of pollution could be the agricultural irrigation in the vicinity of the dump the program phreeqc was used to model the geochemical evolution of groundwater results suggest that evaporation of the contaminated waters circulating below the landfill is one of the key processes that explain the increased concentration of contaminants in groundwater and its seasonal variations as groundwater flows away from the dump evaporation can concentrate the chemicals making the water more toxic these results are important because they illustrate processes that are likely to occur in landfills located in other desert areas of the world

preliminary geochemical assessment of limestone resources and stone use at maya sites in the three rivers region belize the carbonate bedrock of northwestern belize is poorly understood from the standpoint of both geochemistry and the use of stone in prehispanic maya sites for buildings and monuments the friable nature of the rock in this topographically rugged area makes it especially difficult to distinguish monuments from bedrock spall as little carving if any survives and identification rests on location and positioning the research presented here analyzed 63 limestone samples collected from two sites in the three rivers region of belize icp ms and icp aes were used to characterize the major minor and trace element chemistry of the limestone bedrock of the region and determine the amount of geochemical variability another important objective was attempting to trace the movement of monument stone and determining whether it was imported from outside of the sites bedrock quarries and possible monuments were all sampled for these purposes bedrock proved to be similar across wide areas however at chawak but o ob along the flank of the rio bravo changes downslope in mg concentration suggest a leaching of the bedrock by meteoric waters based on differences in porosity at maax na a hilltop site in contrast such leaching is not as apparent many monuments at both sites were found to be composed of stone similar in chemistry to the local bedrock including several of the identifiable stelae however our analyses also revealed that a few monuments at maax na were made of material with a different chemical composition apparently from stone imported to the site these results suggest that the maya deliberately selected certain types of limestone for certain purposes and may even have traded in non local rock overall the methods used in this pilot study indicate there is real potential in more intensive regional assessments of the materials used at archaeological sites even in areas where the local stone does not have a distinctive geochemical signature

on the use of magnetic transient hysteresis in paleomagnetism for granulometry 1 parameters derived from magnetic hysteresis experiments have been used as granulometric indicators in paleomagnetism in practice discriminating the superparamagnetic sp fraction from the coarse grain magnetites in parameter ratio plots is often inconclusive to overcome this ambiguity transient hysteresis th has been recently proposed we have carried out micromagnetic simulations to provide a fundamental rationale for the use of th for granulometry we found that magnetic th results from the difference of magnetization configuration between ascending and descending hysteresis loops as a result of self demagnetization the descending branch has a tendency to keep a more uniform e g flower like configuration while the ascending branch prefers a less uniform e g vortex like structure according to our simulations th increases as the grain size increases and as the aspect ratio decreases we also carried out th measurements for well defined synthetic and natural samples it is notable that th from the simulation for samples with aspect ratio q 1 5 agrees well with the experimental observations for annealed magnetites of smaller sizes in general small th is a clear indication for the absence of complex magnetized structures adding th analysis to the hysteresis loop measurements requires a minor effort yet provides strong constraint on grain size

modification of the magnetic mineralogy in basalts due to fluid rock interactions in a high temperature geothermal system krafla iceland summary active high temperature 150 c geothermal areas like the krafla caldera ne iceland often show distinct magnetic lows in aeromagnetic anomaly maps suggesting a destruction of magnetic minerals by hydrothermal activity the main alteration processes in such an environment are low temperature oxidation 350 c maghemitization and fluid rock interactions we investigated the rock magnetic properties natural remanent magnetization nrm magnetic susceptibility and their temperature and field variation and the mineralogy using x ray diffraction microscopic methods and electron microprobe analyses of two drill cores kh1 and kh3 from the rim of the krafla caldera the drill cores have distinctly lower nrm values average 3 a m 1 compared to younger surface basalts average 20 a m 1 along with a large variation in magnetic susceptibility 1 3 10 7 4 9 10 5 m3 kg 1 the secondary mineral assemblage sulphides sphene rutile and chlorite indicates an alteration within the chlorite smectite zone for both cores without depth zoning optical miscroscopy in combination with the bitter technique and backscatter electron microscopy along with the thermomagnetic analyses allow distinguishing two different magnetomineralogical groups of titanomaghemite 1 titanomaghemite with intermediate titanium concentration and probably high vacancy concentration and 2 titanomaghemite with low titanium concentration and low vacancy concentration the mineral assemblages textures and magnetic properties deduced from the mentioned magnetic measurements indicate two stage transformation mechanism 1 dissolution of titanium at low ph under oxidizing conditions the ulvospinel component of titanomagnetite and ilmenite forms rutile or sphene and fe2 migrates out of the spinel lattice forming titanomaghemite 2 formation of pyrite and dissolution of remaining titanomaghemite under reducing and acidic conditions the latter mechanism produces ghost textures all titanomaghemite is transformed and only their former grain shapes are preserved with only paramagnetic minerals left and ferrimagnetic minerals nearly dissolved this mechanism could explain the significant magnetization loss which is seen in many local magnetic anomaly lows within the oceanic crust and volcanic islands like iceland or hawaii the production of nanoporous textures in titanomaghemites is suggested as a mechanism for the enhancement of the magnetic susceptibility values related to the hydrothermal alteration of krafla

the duration of a yellowstone super eruption cycle and implications for the age of the olduvai subchron abstract small volume rhyolitic eruptions preceding and following a caldera forming eruption can provide insights into the tempo of eruption cycles and timing of magmatic recharge in this contribution high precision 40 ar 39 ar eruption ages were obtained on the three effusive eruptions bracketing the huckleberry ridge tuff which comprise yellowstone s first volcanic cycle these dates are supplemented with detailed paleomagnetic and rock magnetic analyses to resolve discrepancies with previous reported stratigraphy the huckleberry ridge tuff 2 08 ma was preceded by an eruption at 2 14 ma and followed by eruptions at 1 98 and 1 95 ma all of which occurred during four distinct periods of geomagnetic instability within the matuyama chron the first volcanic cycle of yellowstone has now been constrained to within a 200 kyr timespan or half of the previously proposed duration and similar to the duration of volcanic activity for caldera forming systems in the jemez volcanic field the maximum duration for magmatic recharge for the first yellowstone volcanic cycle is no greater than 100 kyr and likely closer to 40 kyr furthermore the combined 40 ar 39 ar eruption ages and paleomagnetic results provide polarity anchors for the pre olduvai excursion and olduvai subchron which are often used as tie points in studies of early pleistocene hominin evolution

nrm intensity of altered oceanic basalts across the mar 21 n 0 1 5 ma a record of geomagnetic palaeointensity variations summary during cruise tammar may 1996 of r v nadir and submersible nautile 66 basaltic rock samples were collected along two long cross sections encompassing the oceanic crust between 0 and 1 5 myr at the centre of the mid atlantic ridge segment located at 21 n this dense sampling provides a means to investigate natural remanent magnetization nrm variability with age using rock magnetic studies high field and k t experiments palaeofield intensity determinations by the original thellier method together with the good geophysical and geological knowledge of this peculiar segment nrm intensities range from 1 3 to 25 4 a m 1 but do not display any rapid exponential decay with age as expected despite the scatter they seem to present short wavelength variations consistent on both flanks of the two lines because of the relative uniformity in grain size i e single domain sd ulvospinel content i e x 0 6 and amount of magnetic minerals in tammar samples the observed across axis nrm intensity variations may be due either to oxidation degree variations or to geomagnetic field intensity changes curie temperatures display no increase towards the flanks nor a clear relationship with nrm intensities suggesting that oxidation degree is not the major process controlling the nrm variations half of the collected samples either fresh or highly altered provide good quality palaeointensity determinations depending on the alteration degree of samples two major types of nrm trm diagrams are observed for relatively fresh materials only one line segment and the major part of nrm is used for field intensity calculations for moderately or highly altered specimens two line segments are observed with the first most probably corresponding to thermal demagnetization of the original remanence and the second to the product resulting from chemical modification i e inversion despite the difference in the amount of nrm that can be thermally demagnetized samples of similar age but of different non stoichiometric degree give coincident palaeofield strengths palaeofield values from 15 to 62 t i e vadm from 3 4 to 13 6 1022 a m2 are in good agreement with the global palaeointensity database and display coherent short wavelength undulations with age consistent assuming a 1 km wide neo volcanic zone and steady symmetrical spreading with the relative palaeointensity record deduced from sediment core analysis guyodo valet 1999 comparison between nrm and field intensity variations with age reveals a remarkable coincidence between both signals suggesting that the nrm intensities are more sensitive to the field intensity than to the alteration degree these observations suggest that magnetic particles are titanomaghemites generated during the initial magma cooling and storing a durable record of the geomagnetic field intensity the type of remanence is either 1 a chemical remanent magnetization that closely mimics the properties of the initial nrm or 2 a thermoremanent magnetization the maghemitization precedes the trm acquisition or 3 a mixture of both

the hysteresis properties of multidomain magnetite and titanomagnetite titanomaghemite in mid ocean ridge basalts abstract hysteresis measurements have been carried out on a suite of ocean floor basalts with ages ranging from quaternary to cretaceous approximately linear yet separate relationships between coercivity bc and the ratio of saturation remanence saturation magnetization mrs ms are observed for massive doleritic basalts with low ti magnetite and for pillow basalts with multi domain titanomagnetites with x 0 6 even when the morb has undergone low temperature oxidation resulting in titanomaghemite the parameters are still distinguishable although offset from the trend for unoxidized multidomain titanomagnetite the parameters for these iron oxides with different titanium content reveal contrasting trends that can be explained by the different saturation magnetizations of the mineral types this plot provides a previously underutilized and non destructive method to detect the presence of low titanium magnetite in igneous rocks notably morb

physical interpretation of hysteresis loops micromagnetic modeling of fine particle magnetite 1 hysteresis measurements have become an important part of characterizing magnetic behavior of rocks in paleomagnetic studies theoretical interpretation is often difficult owing to the complexity of mineral magnetism and published data sets demonstrate remanence and coercivity behavior that is currently unexplained in the last decade numerical micromagnetic modeling has been used to simulate magnetic particles such simulations reveal the existence of nonuniform remanent states between single and multidomain known as the flower and vortex configurations these suggest plausible explanations for many hysteresis measurements yet fall short of explaining high saturation remanence high coercivity data such as those commonly observed in fine grained submarine basalts in this paper we review the theoretical and experimental progress to date in understanding hysteresis of geological materials we extend numerical simulations to a greater variety of shapes and sizes including random assemblages of particles and shapes more complex than simple rods and cubes our simulations provide plausible explanations for a wide range of hysteresis behavior

human and natural impacts on fluvial and karst depressions of the maya lowlands abstract this paper begins to differentiate the major drivers and chronology of erosion and aggradation in the fluvial and fluviokarst landscapes of the southern and central maya lowlands we synthesize past research on erosion and aggradation and add new data from water soils radiocarbon dating and archaeology to study the quantity timing and causes of aggradation in regional landscape depressions geomorphic findings come from many excavations across a landscape gradient from upland valleys karst sinks and fans into the coastal plain floodplains and depressions findings from water chemistry show that sources in the uplands have low quantities of dissolved ions but water in the coastal plains has high amounts of dissolved ions often nearly saturated in calcium and sulfate we found significant geomorphic complexity in the general trends in upland karst sinks in a few instances sediments preserve late pleistocene paleosols buried 2 3 m though many more have distinct middle to late holocene paleosols buried 1 2 m after c 2300 bp maya early to late preclassic from 2300 1100 bp late preclassic to classic periods the landscape aggraded from five main mechanisms river flooding climatic instability accelerated erosion ancient maya landscape manipulation and gypsum precipitation from a rise in a water table nearly saturated in calcium and sulfate ions evidence exists for two or three high magnitude floods possibly driven by hurricanes moreover lake core and geophysical studies from the peten lakes region have shown high rates of deposition of silicate clays maya clays starting and peaking during the maya preclassic and continuing to be high through the late classic the main driver on upland karst depressions the peten lakes upland valleys and fans was accelerated soil erosion but water table rise probably driven by sea level rise was the main driver on the wetlands of the coastal plain because the aggraded sediments here are dominantly composed of gypsum precipitated from the groundwater this latter mechanism represents a little recognized mechanism of aggradation over a large region these large scale environmental changes occurred during periods of intensive ancient maya land use and climatic instability both of which may have contributed to erosion by increasing runoff despite these geomorphic changes ancient maya farmers adapted in several key cases