late quaternary palaeoenvironment of northern guatemala evidence from deep drill cores and seismic stratigraphy of lake peten itza long sediment cores were collected in spring 2006 from lake peten itza northern guatemala in water depths ranging from 30 to 150 m as part of an international continental scientific drilling program project the sediment records from deep water consist mainly of alternating clay gypsum and carbonate units and in at least two drill sites extend back 200 kyr most of the lithostratigraphic units are traceable throughout the basin along seismic reflections that serve as seismic stratigraphic boundaries and suggest that the lithostratigraphy can be used to infer regional palaeoenvironmental changes a revised seismic stratigraphy was established on the basis of integrated lithological and seismic reflection data from the basin from ca 200 to ca 85 ka sediments are dominated by carbonate clay silt often interbedded with sandy turbidites indicating a sediment regime dominated by detrital sedimentation in a relatively humid climate at ca 85 ka an exposure horizon consisting of gravels coarse sand and terrestrial gastropods marks a lake lowstand or partial basin desiccation indicating dry climate conditions from ca 85 to ca 48 ka transgressive carbonate clay sediments overlain by deep water clays suggest a lake level rise and subsequent stabilization at high stage from ca 48 ka to present the lithology is characterized by alternating clay and gypsum units gypsum deposition correlates with heinrich events i e dry climate whereas clay units coincide with more humid interstadials

climate forcings on vegetation of the southeastern yucatan peninsula mexico during the middle to late holocene abstract climate and vegetation history from the yucatan peninsula southeastern mexico are inferred from a mangrove sediment core deposited between the middle and late holocene 5600 1700 cal yr b p in the rio hondo delta fossil pollen and concentrations of ca and fe and ca fe ratio in sediments are used to record changes in vegetation and climate palaeoecological and palaeoclimatic interpretations obtained from pollen abundances and associations and fe ca ratio coincide with dynamics of major global forcings of climate change like itcz enso and global cooling mesic conditions enabled tropical forest expansion during the middle holocene 5600 3650 cal yr b p although there were periodic dry episodes at 5200 cal yr b p and at 4300 cal yr b p that caused disturbance and enabled herbaceous vegetation to expand changes in sedimentation and a gradual change from semi evergreen to dry tropical forest occurred at 3650 cal yr b p with increasing enso activity and southward migration of the itcz during transition of the middle to late holocene the driest period and lowest forest cover occurred between 2600 and 2000 cal yr b p data show that over the last two millennia influence of the enso on southeastern mexico is stronger compared to other proxy records of climate variability from the caribbean region

the soil leakage ratio in the mudu watershed china soil leakage in karst areas is an important geomorphic agent of environmental interest there are several viewpoints concerning soil leakage ratios one viewpoint states that the soil leakage ratio is high in karst regions whereas others consider that it is very low this paper s objective is to describe the soil leakage phenomena qualitatively and quantitatively in the chongqing karst china by conventional methods such as monitoring runoff plots and paint traces drawn on piles and walls found within runoff plots and by measuring the activity of the isotope 137cs at small medium and large spatial scales this paper s results show that soil filled karst cracks occurred at a small spatial scale with soil leakage ratios ranging from about 3 to about 34 at two abandoned quarries and six soil profiles with rock karst crack development the soil leakage ratio was almost zero at the medium spatial scale the soil leakage ratio was 4 50 and the surface soil erosion ratio was 95 50 of the total soil erosion at the large spatial scale it is concluded that soil leakage ratios in the study area were low at a large spatial scale and that cracks in karst were filled with soil at small spatial scales this paper s results indicate that soil leakage phenomena vary depending on the processes acting at the small medium and large spatial scales

great blue hole lighthouse reef belize a continuous annually resolved record of common era sea surface temperature atlantic multidecadal oscillation and cyclone controlled run off abstract the holocene 1900 ce is more likely the result of the suess effect anthropogenic impact of the industrial revolution on carbon isotopes composition than another reflection of a tc peak activity interval

soil geomorphology and wet cycles in the holocene record of north central mexico abstract the distinction between the impact of climatic periodicities or land use practices on soil erosion is an important issue for pre hispanic and colonial mexico that question can best be addressed by first documenting the dynamics of changing wet cycles during the holocene in the central mexican region between the northern limits of pre hispanic agriculture and its southern margins in northwestern chihuahua consequently the laguna project targeted a 125 000 km 2 sector of north central mexico 250 km from north to south and 500 km from east to west from saltillo to durango some 40 sedimentary profiles with multiple cumulic soils were studied in the field and laboratory supported by 163 conventional 14 c and ams dates on charcoal and soil humates we distinguish 1 wet floodplains with humic paleosols redox phenomena reflecting high water tables channel ponding sequences and interbedded tufas that imply aquifer recharge sustained base flow and mainly low energy conditions and 2 high energy pulses of discharge that mobilized cobble gravels or forced channel entrenchment gullying and were tied to episodic excessive rains that promoted valley and slope instability in between such wet cycles and recurrent disequilibrium events climate was similar to today probably less humid with limited geomorphologic change or slow soil formation wet cycles were rare at the end of the pleistocene but prominent during the holocene disequilibrium proxies became common and dramatic after 2500 bp the drainages from the eastern and western sierra madres responded in phase but varied in detail around ad 1050 1200 natural erosion led to loss of soil organic carbon as alternating severe droughts and heavy rains destroyed the ground cover and led to ecological aridification well before arrival of spanish miners and settlers the evidence that human activity triggered pre hispanic or colonial erosion in central mexico should therefore be re evaluated global comparisons and interpretations are discussed but with caution since no single theory can explain the whole of the record the soil geomorphology geoarchive of north central mexico primarily is an environmental history of alternating wet cycles rather than of sustained wet or dry climates the critical differences between soaking and excessive rains with their respective impacts may be due to switching between winter and summer storm categories

a 3400 year paleolimnological record of prehispanic human environment interactions in the holmul region of the southern maya lowlands abstract the timing magnitude and drivers of late holocene environmental change in the holmul region of the southern maya lowlands are examined by combining paleoenvironmental and archeological data environmental proxy analyses on a 3350 cal yr lacustrine sediment record include pollen charcoal loss on ignition magnetic susceptibility and elemental geochemistry archeological evidence is derived from extensive settlement surveys conducted near the study site results indicate nearby settlement and agricultural activity taking place in an environment characterized by open forest from around 3350 to 950 cal yr bp the fire history shows a dramatic increase in burning during the classic period possibly reflecting changing agricultural strategies a distinct band of carbonate deposited from 1270 to 1040 cal yr bp suggests decreased hydrologic input associated with drier conditions abrupt changes in proxy data around 940 cal yr bp indicate a cessation of human disturbance and local abandonment of the area

a 4000 year lacustrine record of environmental change in the southern maya lowlands peten guatemala a 4000 yr sediment core record from lake salpeten guatemala provides evidence for maya induced forest clearance and consequent soil erosion between 1700 cal yr b c and 850 cal yr a d radiocarbon ages of wood seeds and charcoal support an age depth model with average errors of 110 cal yr relatively low carbonate 18o values between 1300 and 400 cal yr b c coincide with pollen evidence for forest loss consistent with increased surface and groundwater flow to the lake minimum 18o values between 400 cal yr b c and 150 cal yr a d suggest a high lake level as do 14c dated aquatic gastropods as much as 7 5 m above the present lake stage high lake levels resulted from reduced evaporation to precipitation ratios increased hydrologic input caused by anthropogenic deforestation or both the preclassic abandonment 150 a d and early classic late classic boundary 550 a d are marked by relatively high 18o values indicating reduced lake levels oxygen isotope composition increased further coincident with the terminal classic maya demographic decline between 800 and 900 a d this period of high 18o may have been caused by the greater aridity that has been documented in northern yucatan lakes or by decreased hydrologic input to the lake as a consequence of forest recovery reduced soil erosion after 850 cal yr a d coincided with the terminal classic maya demographic decline and permitted forest recovery and resumption of organic sedimentation

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

evidence for volcanic ash fall in the maya lowlands from a reservoir at tikal guatemala powder x ray diffraction and petrographic analyses of reservoir sediments from tikal guatemala have identified significant quantities of decomposed volcanic ash in the form of smectite and euhedral bipyramidal quartz crystals x ray fluorescence trace element content analysis was used to eliminate distant sahara sahel and antilles sources the zr y and ni cr ratios of reservoir sediment from tikal are consistent with a source from central american volcanism e g guatemalan and salvadoran ams radiocarbon dating of the smectite and crystalline quartz rich reservoir sediments show that volcanic ash fell during the preclassic classic and postclassic maya cultural periods it may now be possible to develop an effective chronology of ash fall at tikal and the greater peten published by elsevier ltd

quantification of soil erosion rates related to ancient maya deforestation we used seismic and sediment core data to quantify soil erosion rates for the past 6000 yr in the closed catchment of lake salpeten in the tropical lowlands of northern guatemala the region was affected by ancient maya land use from before ca 1000 b c to a d 900 this period of human impact coincided with deposition in the lake of a detrital unit maya clay as much as 7 m thick that contrasts sharply with the relatively organic rich gyttja deposited both before and after maya occupation of the watershed the greatest soil loss with mean sustained values of 1000 t km2yr 1 occurred in the middle and late preclassic periods 700 b c to a d 250 associated with relatively low maya population densities soil erosion slowed during the period of maximum population density in the late classic period a d 550 830 indicating a decoupling between human population density and soil erosion rate the most rapid soil loss occurred early during initial land clearance suggesting that even low numbers of people can have profound impacts on lowland tropical karst landscapes

climate drying and associated forest decline in the lowlands of northern guatemala during the late holocene abstract palynological studies document forest disappearance during the late holocene in the tropical maya lowlands of northern guatemala the question remains as to whether this vegetation change was driven exclusively by anthropogenic deforestation as previously suggested or whether it was partly attributable to climate changes we report multiple palaeoclimate and palaeoenvironment proxies pollen geochemical sedimentological from sediment cores collected in lake peten itza northern guatemala our data indicate that the earliest phase of late holocene tropical forest reduction in this area started at 4500 cal yr bp simultaneous with the onset of a circum caribbean drying trend that lasted for 1500 yr this forest decline preceded the appearance of anthropogenically associated zea mays pollen we conclude that vegetation changes in peten during the period from 4500 to 3000 cal yr bp were largely a consequence of dry climate conditions furthermore palaeoclimate data from low latitudes in north africa point to teleconnective linkages of this drying trend on both sides of the atlantic ocean

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

erosion creep collapse mechanism of underground soil loss for the karst rocky desertification in chenqi village puding county guizhou china carbonate rocks distribute widely in china the total area of the carbonate rocks is about 3 430 000 km 2 and the exposed area of the carbonate is approximately 13 of china s territory in 2003 soil loss in yunnan guizhou and guangxi provinces reached 179 600 km 2 which is almost 40 1 of the total area causing rocky desertification in this study the erosion creep collapse mechanism of underground soil loss for the karst rocky desertification in chenqi village puding county guizhou province is proposed the mechanism occurs under the following geological environment slope surface undulation underlying bedrock surface fluctuation and thin and inhomogeneous soil overlying overlying soil generation by bedrock weathering underground karst development and large groundwater depth and lying water table under the bottom of soils the erosion creep collapse mechanism of underground soil loss in the karst slopes is explained as follows power loss due to human cultivation activities that destroy the soil structure hydraulic force formed by rainfall infiltration wet dry cycle generated by rainfall erosion effect caused by rainfall penetration creeping and flowing of plastic stream soil and collapse the erosion creep collapse mechanism of underground soil loss has seven steps disturbance of soils filled in underground karst cave by human activities internal soil erosion and partial collapse caused by hydraulic power internal free surface formation within the soil in the filled karst cave internal soil creeping soil pipe formation soil pipe collapse and ground surface collapse and filling soil loss develops slowly and sudden transportation occurs by collapse soil loss can be explained by the proposed mechanism and soil loss can be prevented by controlling soil collapse

man and climate in the maya lowlands abstract a 15 m sedimentary core from lake salpeten provides the first complete holocene sequence for the lowlying peten district guatemala today lake salpeten is a brackish calcium sulfate lake near saturation surrounded by tropical semievergreen forest the basal pollen record depicts sparse juniper scrub surrounding a lake basin that held ephermal pools and halophytic marshes the lake rapidly deepened to 27 m in the early holocene and may have been meromictic because nearly 2 m of gypsum mush was deposited mesic forests were quickly established and persisted until the maya entered the district 3000 yr ago and caused extensive deforestation any climatic information contained in the pollen record of the maya period is thus masked but a regional pollen sequence linked to the archaeological record is substantiated because environmental disturbance was pervasive local intensification of occupation and population growth are seen as an increased deposition of pollen of agricultural weeds and colluviation into the lake while the classic maya collapse is marked by a temporary decline in compositae pollen effects of perturbations induced by the maya persist in the pollen and limnetic record 400 yr after the spanish conquest

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

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

terminal classic drought in the northern maya lowlands inferred from multiple sediment cores in lake chichancanab mexico we present new density records from sediment cores taken along a depth transect in lake chichancanab mexico the data reveal in great detail the climatic events that comprised the terminal classic drought tcd and coincided with the demise of classic maya civilization between ca 750 and 1050 ad in shallow water cores the tcd is marked by a condensed gypsum horizon in deeper water sections it is expanded and represented by numerous interbedded gypsum and organic rich strata the tcd was not a single two century long megadrought but rather consisted of a series of dry events separated by intervening periods of relatively moister conditions we estimate the tcd occurred between ca 770 and 1100 ad and included an early phase ca 770 870 ad and late phase ca 920 1100 ad the intervening 50 year period ca 870 920 ad was relatively moister each dry phase is represented by multiple gypsum density bands interbedded with organic rich sediment that indicate alternating dry and wet conditions spectral analysis revealed significant periods around 213 50 and 27 years despite uncertainty regarding the ages of these events their pattern is robust and generally consistent with other proxy records under the same climate regime such as the marine cariaco basin off northern venezuela science 299 2003 1731

icaz 2006 zooarchaeology of the late pleistocene early holocene in the americas and zooarchaeological evidence of the ancient maya and their environment

impacts of pre vs postcolonial land use on floodplain sedimentation in temperate north america abstract this paper evaluates the relative importance of anthropogeomorphic sedimentation on floodplains in the mid latitudes of north america before and after the arrival of euroamericans geohistorical and geoarchaeological theories have emerged that have conflicts concerning the relative effectiveness of pre and postcolonial anthropogeomorphic change in north america many geoarchaeologists paleoecologists historians and cultural geographers have concluded that intensive precolonial land use resulted in substantially humanized landscapes prior to the arrival of colonists this presents a geomorphic paradox to fluvial geomorphologists and river restorationists who interpret buried floodplain soils as evidence of landscape stability evidence for these theories is reviewed from a geomorphic perspective while making distinctions between ecological and geomorphic change and recognizing spatial and temporal variability in some locations mid millennial land use generated erosion and floodplain sedimentation subsequent indigenous population declines and reductions in land use intensity may have been followed by precolonial geomorphic stability and pedogenesis an abandoned landscape hypothesis that floodplain soils developed on precolonial anthropogenic alluvium and were subsequently buried should be considered where indigenous populations and land use activities were high variability is also noted in postcolonial floodplain sedimentation many stratigraphic studies show that rates of sedimentation increased greatly after colonization in response to land clearance for agriculture timbering and mining an inventory of studies that document postcolonial floodplain sediment shows strong spatial patterns most studies of postcolonial sedimentation cluster in the eastern usa from the mid atlantic southward and in the mississippi valley from wisconsin to mississippi but many occurrences of mining sediment are scattered throughout the west few of the studies reviewed in the west document anthropogenic floodplain sediment from agriculture or logging although more thorough regional inventories could change that vast areas of temperate north america do not have historical deposits that are well represented in the scientific literature so assumptions that legacy sediment on floodplains is ubiquitous are ill founded spatial and temporal variations in pre and postcolonial floodplain sedimentation are complex and more research is needed to fill in the knowledge gaps

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

conditions for hydrofracture and the fluid permeability of accretionary wedges abstract the dewatering of fine grained sediment piles in accretionary wedges is controlled by zones of active deformation episodic hydrofracture in these zones enhances permeability enabling fluid flow to be faster than the speed of underthrusting here i investigate the mechanics of simultaneous shear failure and hydrofracture in muddy rocks as a function of the faulting mode the physical properties of model rock types are chosen to represent uncemented mud or cherty marly mudstones that have undergone significant diagenesis during thrust faulting hydrofracture always requires pore fluid pressures in excess of the lithostatic load whereas shallow level wrenching or normal faulting permits hydrofracture at hydrostatic pressures significantly lower than lithostatic load all three faulting modes are expected to occur in an accretionary wedge attempting to dynamically maintain its critical taper the model computations indicate that in the upper 80 120 m of a section of soft mudstone hydrofracture inevitably accompanies wrench or normal faulting hard mudstones inevitably hydrofracture to depths of up to 1400 m during normal faulting these differences in mechanical behaviour have profound consequences for fluid venting capabilities of accretionary wedges they influence hydrocarbon trapping and play a role in the localisation of mud diapirs and mud volcanism

erosion in mediterranean mountain landscapes during the last millennium a quantitative approach based on lake sediment sequences iberian range spain abstract land degradation and soil erosion are key environmental problems in mediterranean mountains characterized by both a long history of human occupation and strong seasonality of hydrological regimes experimental monitored catchments have identified main controlling factors on erosion dynamics but because of the short time span of the instrumental measurements they do not integrate the complete temporal and spatial variability of these environments to obtain some long term series of mediterranean landscape evolution we propose a novel strategy based on multi proxy analyses of lake sediments aimed to quantify sediment delivery and erosion dynamics karstic lakes in the iberian range central spain provide an opportunity to reconstruct the depositional evolution of the watersheds and to evaluate the response to both the variable anthropogenic impacts and climate forcings during the last 1000 years the most significant periods of increased sediment delivery 26 483 t km yr 1 in the iberian range sites took place during the 15th 16th and 18th 19th centuries as a result of large land use changes and land clearing positive synergies between increasing human population and more humid periods led to peaks in sediment deliver during some phases of the little ice age recent changes associated with the rural exodus and land abandonment during mid 20th century are more clearly shown in larger watersheds by decreased sediment delivery to the lakes due to the re vegetation of the landscape 6 200 t km yr 1 this new integrated approach based on long term lake sequences provides a longer perspective of erosion processes in mediterranean watersheds and validates the main factors identified in monitoring surveys land cover and surface formations erodibility in both settings lakes and experimental catchments most sediment delivery occurs during flooding events measured and reconstructed values show comparable large ranges underlying the punctuated nature of sediment dynamics in mediterranean landscapes at decadal and centennial scales

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

hydrogeochemistry of the northern barbados accretionary complex transect ocean drilling project leg 110 detailed studies of the major element geochemistry oxygen and hydrogen isotope ratios of pore fluids and the 87sr 86sr ratio of dissolved strontium have made it possible to unravel physical and chemical processes that affect the pore fluid chemistry in a transect of drill holes across the northern barbados accretionary complex these processes include 1 alteration of volcanic ash buried in the pleistocene pliocene sediment column 2 alteration of underlying basalts of layer 2 of the oceanic crust 3 movement of fluids from deep in the accretionary complex along fault zones particularly the decollement and minor permeable layers these fluids from deeper in the complex are characterized by low chloride concentrations and increased 18o h2o values presumably as a result of dehydration of smectite interlayers and 4 mixing processes involving the migrating fluids cause incongruities in the geochemical anomalies of these fluids

a late holocene paleoenvironmental reconstruction from agua caliente southern belize linked to regional climate variability and cultural change at the maya polity of uxbenka abstract we report high resolution macroscopic charcoal pollen and sedimentological data for agua caliente a freshwater lagoon located in southern belize and infer a late holocene record of human land use climate interactions for the nearby prehistoric maya center of uxbenka land use activities spanning the initial clearance of forests for agriculture through the drought linked maya collapse and continuing into the historic recolonization of the region are all reflected in the record human land alteration in association with swidden agriculture is evident early in the record during the middle preclassic starting ca 2600 cal yr bp fire slowly tapered off during the late and terminal classic consistent with the gradual political demise and depopulation of the uxbenka polity sometime between ca 1150 and 950 cal yr bp during a period of multiple droughts evident in a nearby speleothem record fire activity was at its lowest during the maya postclassic ca 950 430 cal yr bp but rose consistent with increasing recolonization of the region between ca 430 cal yr bp and present these data suggest that this environmental record provides both a proxy for 2800 years of cultural change including colonization growth decline and reorganization of regional populations and an independent confirmation of recent paleoclimate reconstructions from the same region

butzer down under debates on anthropogenic erosion in early colonial australia abstract the arrival of europeans in the new world and the rapid deforestation for agriculture and settlement that shortly followed resulted in large and rapid increases in soil erosion and sediment aggradation in australia as in parts of north america and other new world sites this event dates to the last several centuries of the common era a period for which we have surprisingly little historical or scientific data capable of informing debates on the impact of european colonisation on soils and erosion these research themes and debates were central to the work of karl butzer who undertook field and archival research in australia to test existing ideas of the timing and magnitude of landscape response to colonisation in this paper the work and impact of butzer in australia is reviewed as part of ongoing scholarship on the impact of colonisation on landscapes to which butzer contributed butzer and helgren 2005 geomorphic records of soil erosion and historical accounts of land use and impact from eastern australia show that european settlement land clearing and agriculture resulted in rapid and often severe increases in erosion in the late eighteenth and early nineteenth centuries mirroring the experience of anthropogenic soil loss experienced in other parts of the world it is postulated that the geomorphic response to european settlement in australia is more complex than researchers have previously noted that in some contexts continues to influence catchment processes decades and centuries after colonisation

relation between permeability and effective stress along a plate boundary fault barbados accretionary complex in situ bulk permeability was measured in a borehole that intersected the decollement zone a low angle detachment fault between the north american and caribbean plates permeability measurements were made at a variety of fluid pressure conditions defining a quantitative relation between bulk permeability and effective stress for this plate boundary fault zone the bulk permeability in this zone changed by several orders of magnitude as a consistent function of fluid pressure this relation may help to explain the dynamics of fluid fault interactions and the transient nature of hydrologic processes during deformation at convergent margins

climate variability on the yucatan peninsula mexico during the past 3500 years and implications for maya cultural evolution climate variability on the yucatan peninsula during the past 3500 yrs is reconstructed from the measurement of 18o in monospecific ostracods and gastropods in a 6 3 m sediment core from lake punta laguna mexico this late holocene record is divided into three periods based on changes in mean 18o values from 3310 to 178514c yr b p period i low mean 18o values indicate relatively wet conditions i e low evaporation to precipitation ratio e p mean oxygen isotopic values increased 178514c yr b p and the interval between 1785 and 93014c yr b p period ii was distinctly drier than the periods before or after the climate during the latter part of period ii was persistently dry with exceptionally arid events centered at 1171 1019 and 94314c yr b p equivalent to 862 986 and 1051 a d this interval of frequent drought was recorded at several other localities in mexico and central america and coincided with the collapse of classic maya civilization following the last arid event 18o values decreased abruptly at 93014c yr b p beginning of period iii signaling a return to wetter conditions that have generally prevailed to the present with the exception of a dry episode centered at 55914c yr b p 1391 a d the paleoclimatic record from punta laguna provides evidence that multi decadal and millennial scale changes ine poccurred on the yucatan peninsula during the late holocene these wet dry episodes may have influenced cultural evolution in mesoamerica

the floating gardens of chan cahal soils water and human interactions abstract ancient maya wetland agriculture has been studied since the 1960s but there are still too few multiproxy excavations to adequately portray this phenomenon this paper builds on multidisciplinary and multiproxy research near the ancient maya site of chan cahal near blue creek belize with two new excavations four new cores 22 new ams dates and a new review of the site s water chemistry and archeology a long term study of the site s water chemistry has shown its pervasive impacts on wetland formation and limits to agricultural uses we argue that a reservoir in the midst of the wetlands became inundated above a paleosol during the earliest maya occupation in the middle preclassic was dredged out during a time of population expansion sometime in the late preclassic to early classic and was filled again with maya clay during and after the classic period vibracores through two wetland fields and one canal corroborated our earlier model of preclassic through classic inundation of a paleosol by peat formation flood sands clay deposition and gypsum precipitation these cores produced new evidence for field and canal construction starting in the late preclassic a later addition was a platform added at the very end or after the main occupation ended in the terminal classic a new excavation at sayap ha on the coastal plain wetlands 7 m lower than chan cahal experienced a similar history of inundation and aggradation here maya farmers reclaimed these wetlands with canals and raised fields late in the late or terminal classic and abandoned them soon thereafter the sayap hah fields thus parallel the bop fields 9 km south since both were built and abandoned at a late stage as a comparison with other wetland field studies the stable c isotopes of soil humin through soil profiles showed a strong classic period to present influence of c 4 species at sayap hah and an increase in c 4 inputs in the upper 25 cm at both sayap hah and the chan cahal platform perhaps due to tropical pasture grasses prevalent here for the last 50 years

an extended and higher resolution record of climate and land use from stalagmite mc01 from macal chasm belize revealing connections between major dry events overall climate variability and maya sociopolitical changes abstract the stalagmite mc01 was recovered from macal chasm cave on the vaca plateau of belize in 1995 and an initial paleoclimate interpretation was published in 2007 additional uranium thorium ages have extended the paleoenvironmental record back from 3250 to 5250 cal yr bp and the stable isotope 18o and 13c record is dramatically improved by 660 new values a series of major dry events mdes evident in stable isotopes ultraviolet stimulated luminescence and petrography began 3100 cal yr bp and the initiation of these events coincides with an increase in el nino dominance and southern shift in the intertropical convergence zone three mdes centered at 1750 cal yr bp 200 ce 1100 cal yr bp 850 ce and 850 cal yr bp 1100 ce and found in other regional climate records coincide with maya sociopolitical changes residuals from regression of 13c versus 18o are interpreted as a proxy for maize cultivation and land clearing with residual values gradually increasing at the start of preclassic period settlement 3950 cal yr bp 2000 bce peaking after 2250 cal yr bp 300 bce during major maya development in the late preclassic and classic periods and dropping to pre preclassic values after regional land abandonment 850 cal yr bp 1100 ce regional maya population growth and cultural expansion may have been aided by abnormally low precipitation variability as stable isotope variability suggests the late preclassic through the late classic was the most stable precipitation regime of the past 4000 years this additional research on mc01 complements other regional paleoenvironmental records that suggest that mdes coincided with disruptions in maya society from the preclassic through the postclassic periods although it is clear that not all significant sociopolitical changes can be attributed to the mdes these events likely played an antagonistic role in social stability

margin for error anthropogenic geomorphology of bajo edges in the maya lowlands abstract many early maya cities developed along the edges of large structural or karst depressions bajos this topographic position aided growing populations to more effectively capture and store rainwater a necessity for year round occupation of interior portions of the maya lowlands of mexico and central america ancient maya forest clearance on sloping terrain led to accelerated soil loss and the aggradation of the bajo margins these newly created margins of colluvial lands became a focus of subsequent intensive agriculture and helped underwrite further urban expansion we document this long term landscape transformation with data derived principally from field investigations at tikal guatemala and yaxnohcah mexico but with reference to other maya centers in the elevated interior region eir data are derived from field investigations interpretation of lidar imagery and laboratory analyses we present a model of three variants of bajo margin landscape change with differences attributable to topography lithology hydrology and cultural processes we present preliminary data on crops that were cultivated on bajo margin soils we further describe how agriculture was adapted to evolving bajo margins as evidenced by systems of field walls terraces and ditches

barbados ridge hydrogeologic tests implications for fluid migration along an active decollement hydrogeologic tests were conducted at a sealed borehole penetrating the decollement of the barbados ridge accretionary complex at low excess pore pressures p p p h p l p h 0 0 to 0 36 where p p pore pressure p h hydrostatic pressure and p l lithostatic pressure estimated permeabilities were comparable to those of similar unfractured sediment these tests complement shipboard packer tests completed at higher fluid pressures 0 5 to 1 0 during ocean drilling program odp leg 156 together the test results suggest a 4 to 5 order of magnitude permeability increase as fluid pressure varied from hydrostatic 0 to lithostatic 1 however unlike the results of the shipboard packer tests the test results presented here exhibit no evidence of a relationship between permeability and pore pressure the combined findings from the two sets of hydrogeologic tests indicate that significant permeability increases can occur within the decollement at pore pressures below lithostatic pressure

climate change in lowland central america during the late deglacial and early holocene the transition from arid glacial to moist early holocene conditions represented a pro found change in northern lowland neotropical climate here we report a detailed record of changes in moisture availability during the latter part of this transition 11 250 to 7500 cal yr bp inferred from sediment cores retrieved in lake peten itza northern guatemala pollen assemblages demon strate that a mesic forest had been largely established by 11 250 cal yr bp but sediment properties indicate that lake level was more than 35 m below modern stage from 11 250 to 10 350 cal yr bp during the preboreal period lithologic changes in sediments from deep water cores 50 m below modern water level indicate several wet dry cycles that suggest distinct changes in effective moisture four dry events designated pbe1 4 occurred centred at 11 200 10 900 10 700 and 10 400 cal yr bp and correlate with similar variability observed in the cariaco basin titanium record and glacial meltwater pulses into the gulf of mexico after 10 350 cal yr bp multiple sediment proxies suggest a shift to a more persistently moist early holocene climate comparison of results from lake peten itzawith other records from the circum caribbean demonstrates a coherent climate response during the entire span of our record furthermore lowland neotropical climate during the late deglacial and early holocene period appears to be tightly linked to climate change in the high latitude north atlantic we speculate that the observed changes in lowland neotropical precipitation were related to the intensity of the annual cycle and associated displacements in the mean latitudinal position of the intertropical convergence zone and azores bermuda high pressure system this mechanism operated on millennial to submillennial timescales and may have responded to changes in solar radiation glacial meltwater north atlantic sea ice and the atlantic meridional overturning circulation moc copyright 2005 john wiley sons ltd

reprint of impacts of mayan land use on laguna tuspan watershed peten guatemala as seen through clay and ostracode analysis abstract most of the cities built by the mayas in the peten area in the central yucatan peninsula were abandoned 1200 to 1000 years ago the phenomenon is sometimes un appropriately called the collapse of the maya civilization its main causes are still debated ranging from climatic according to the occurrence of severe or modest droughts to societal in the form of environmental mismanagement of the environment in both processes it is inferred that stress triggered the formation in many peten lake sediments of erosional clay deposits known as maya clays this work presents a high resolution multi proxy study of maya clays in lacustrine sediments from laguna tuspan near the archaeological site of la joyanca micropaleontological ostracodes mineralogical clay minerals and geochemical bulk elemental composition and stable isotopes in organic carbon records reveal three different phases of soil erosion throughout the last 5300 years the oldest phase from 5281 to 2998 cal yr bp i e 3331 1048 bc is characterized by successive natural and moderate soil erosion deposits which follow climatic variations recorded in the american tropical belt the time interval between 2998 and 1281 cal yr bp i e 1048 bc and ad 661 contains four distinct erosional layers which according to clay mineralogy are indicative of both increased erosion of the regolith and strong soil loss the most recent also the most massive deposit of maya clay ends around 1281 cal yr bp ad 661 that is some 200 years before the so called maya collapse in the peten area recent archeological fieldwork studies indicate that a population mobility took place into the city of la joyanca from its hinterland by the early late classic period ca ad 600 that is at the end or just after this erosion episode and well before the occurrence of the terminal classic postclassic ad 800 1250 drastic climatic changes shifts in environmental management by the local society and timing of urbanization may explain environmental changes better than droughts per se

terrestrial sediment yield and the lifetimes of reservoirs lakes and larger basins water reservoirs lakes and larger basins including their drainage areas represent sedimentologically closed to semi closed denudation accumulation systems the mean rates of mechanical denudation drme and clastic sedimentation srme are related by the ratio of the drainage lake area ad al if the latter is known drme or the specific sediment yield sy in t per km2 a can be calculated from srme or vice versa the best data for modern sy mainly come from the sediment fills of artificial reservoirs small drainage areas of mountainous regions show sy values up to two orders of magnitude higher than lowlands and approximately one order higher than larger regions of mixed relief this is also true of arid to semi arid zones which often provide approximately as much sediment sy as humid temperate and even tropical zones of comparable relief lithology and climate river runoff also may play some role for sy from catchments of limited size the importance of these factors is exemplified by perialpine lakes and two east african lakes sediment yields gained from some large reservoirs compare well with long term denudation rates derived from geological studies e g the tarbela dam reservoir along the indus river in many other cases human activities have raised sy by factors of 2 10 locally up to 100 artificial reservoirs in mountainous regions with sy in the range of 300 2000 t per km2 a tend to become filled within several tens to hundreds of years some have even shorter lifetimes perialpine lakes of the alps and british columbia are strongly affected by delta prograding and have lifetimes mostly between 15 and 40 ka closed lake systems in deep morphological depressions lake bonneville aral sea northern caspian sea have a high potential for sediment storage up to the level of spillover and therefore can persist over long time periods basins with markedly subsiding basin floors lakes of the east african rift zone the southern caspian sea and the black sea both on oceanic crust can survive for many ma in the future despite relatively high terrigenous input

roman driven cultural eutrophication of lake murten switzerland abstract land cover transformations have accompanied the rise and fall of civilizations for thousands of years exerting strong influence on the surrounding environment soil erosion and the associated outwash of nutrients are a main cause of eutrophication of aquatic ecosystems despite the great challenges of water protection in the face of climate change large uncertainties remain concerning the timescales for recovery of aquatic ecosystems impacted by hypoxia this study seeks to address this issue by investigating the sedimentary record of lake murten switzerland which witnessed several phases of intensive human land use over the past 2000 years application of geophysical and geochemical methods to a 10 m long sediment core revealed that soil erosion increased drastically with the rise of the roman city of aventicum 30 ce during this period the radiocarbon age of the bulk sedimentary organic carbon oc increasingly deviated from the modeled deposition age indicating rapid flushing of old soil oc from the surrounding catchment driven by intensive land use enhanced nutrient delivery resulted in an episode of cultural eutrophication as shown by the deposition of varved sediments human activity drastically decreased towards the end of the roman period 3rd century ce resulting in land abandonment and renaturation recovery of the lake ecosystem from bottom water hypoxia after the peak in human activity took around 50 years while approximately 300 years passed until sediment accumulation reached steady state conditions on the surrounding landscape these findings suggest that the legacy of anthropogenic perturbation to watersheds may persist for centuries

abundance of smectite and the location of a plate boundary fault barbados accretionary prism isolating the respective roles of factors responsible for the initiation and localization of fault zones remains one of the more important goals of research in neotectonics the frontal decollement zone of the northern barbados accretionary prism provides several important clues as to how the interwoven variables of clay mineralogy fluid flow chemical interactions and sediment physical properties affect strain localization this plate boundary fault is centered at a lithologic contact between miocene claystone and fine grained oligocene turbidites the fault zone is nearly 40 m thick at ocean drilling program site 948 and its upper part passes through smectite rich deposits a sharp minimum in percent smectite and a maximum in percent illite mark the base of the decollement there is a consistent increase in percent smectite with distance above the base of the decollement but the top of the decollement is poorly defined by clay mineralogy the intrinsic mechanical weakness of strata with abundant smectite group clays probably influences where the fault tip propagates into the undeformed stratigraphy of the atlantic abyssal plain a second inherited parameter is the local abundance of radiolarians which contribute to higher than normal porosities sediment shear strength also decreases because pore pressure within the fault zone is significantly greater than hydrostatic the principal cause of excess pore pressure seems to be updip fluid advection in theory however decreases in pore fluid salinity and porosity collapse should increase the amount of physicochemical stress generated by expandable clay minerals the imported fluid is unusually low in salinity because it has migrated from zones of deeper seated dehydration reactions if fresher pore water migrates to the propagating tip of the decollement its arrival should increase smectite swelling and reduce the shear strength of the mudstone even more the location and evolution of the decollement therefore are controlled by a complicated interplay of static factors inherited from the abyssal atlantic stratigraphy and dynamic factors associated with episodic fluid flow and changing fluid chemistry

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

heterogeneous hydrofracture development and accretionary fault dynamics ocean drilling program observations at many accretionary wedges indicate that fluid flow in low permeability sediments is focused along faults rich in clay minerals recent ring shear experiments show that shear failure generally reduces permeability in muddy faults to values well below the surrounding sediments and that very little increase occurs in the fault permeability even when continued failure proceeds under steadily reduced effective loads it is thus unlikely that observed focused flow occurs through the matrix of muddy faults and we suggest that focused flow can only be explained by the development of open fracture systems the lithostatic fluid pressures necessary for hydrofracturing in weak sediments cannot however exist throughout the imbricate thrust system and basal decollement zone because a zero basal shear stress condition would result in regional instability and accretionary wedge collapse we propose that the regional hydrofracture networks must propagate heterogeneously leaving relatively strong asperity regions at sublithostatic fluid pressures to balance forces in the wedge in three dimensions

friction experiments on saturated sediments and their implications for the stress state of the nankai and barbados subduction thrusts the nankai and barbados forearcs have low stress subduction thrusts the sediments entering the subduction zone and namely the material in the decollement zones have been well characterized by numerous deep sea drilling legs and studies of the recovered cores nankai has high heat flow and significant amounts of illite while barbados is a smectite dominated system based on results from ring shear 2 mpa normal stress and direct shear 30 mpa tests on marine sediments and mineral standards this translates into a residual frictional resistance of r 0 25 and r 0 11 in clay horizons respectively such values agree with theoretical estimates from critical wedge theory nankai b 0 16 0 26 and barbados b 0 06 0 09 and fault spacing geometries from seismic profiles nankai b 0 12 0 23 and barbados b 0 11 0 19 maximum pore pressure ratios of 0 85 and 0 73 for nankai and barbados respectively allow us to estimate effective shear stresses as a function of friction coefficient and density of the sediment gouge to reach only 10 mpa or less in the frontal 50 km of the decollement zone respectively our data support the contention that fluid pressure transients and sediment composition contribute equally to the weakness along plate boundary faults down to the seismogenic zone with the first probably dominating the shallow decollement shear velocity stepping tests show that the clay dominated gouges strengthen velocity irrespective of the clay mineralogy and hence suggest that clay transformation does not affect the onset of seismogenesis

on conducting the modified slug test in tight formations the method introduced by bredehoeft and papadopulos 1980 for conducting a modified slug test in tight formations does not assure the condition of approximate equilibrium necessary at the start of the test in addition compressibility in the shut in well can be significantly larger than the compressibility of water which bredehoeft and papadopulos indicate should be used in the calculations errors that may result from using the original test procedure are discussed and a modified procedure and testing arrangement are presented

borehole observations of fluid flow from south chamorro seamount an active serpentinite mud volcano in the mariana forearc abstract a sealed borehole observatory cork was deployed on south chamorro seamount an active serpentinite mud volcano in the mariana forearc to explore subduction related processes on a non accretionary convergent plate margin formation fluid was overpressured relative to ambient hydrostatic conditions fluid flowed from the borehole at 0 2 l s when the observatory was opened to recover instruments 2 yr after it was installed the chemical composition of the formation fluid is similar to that extrapolated from trends in pore water data collected during ocean drilling program leg 195 when the observatory was established reduced sulfur is present in this highly alkaline ph 12 4 formation fluid indicative of microbial activity at or below the depth of the screened casing 149 202 m below the seafloor discharge from the open borehole continued for 37 days until the observatory was resealed this discharge requires significant permeability at depth 6 10 14 m2 zones of high permeability may be associated with the formation of headwall scarps consistent with numerous slumps on the southeastern flank of the seamount and likely shape a geochemical environment suitable for an active microbial community

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

the tectonic fabric of the barbados ridge accretionary complex abstract a map showing near surface structure and other surficial geological features on the barbados ridge accretionary complex has been constructed from gloria long range side scan sonographs detailed bathymetric surveys using seabeam and single and multichannel seismic reflection sections the map shows major variations in the size orientation and style of structures across the accretionary complex that are principally related to the increasing thickness of the complex and the sediment drape upon it and variations along the complex that derive from the northward thinning of sediment accreted from the ocean floor and the effects of ridges in the oceanic basement the development of crossfaulting is mainly associated with basement ridges mud diapirism is abundant in the southern part of the complex and is characteristic of that part of the complex into which turbidites from the orinoco submarine fan are accreted slumping on a variety of scales is another feature of the complex the map is presented at a scale of 1 1063415 4 inches per degree of longitude and also shows bathymetry at 200 m contour intervals

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

wildfire effects on vadose zone hydrology in forested boreal peatland microforms summary peatland vulnerability to wildfire disturbance has been shown to vary as a function of hummock and hollow microforms and vadose zone hydrology with low lying hollow microforms most susceptible to deep combustion of peat to better understand how this microform induced pattern of burning alters vadose water storage pore water pressure and water table relationships we examined a paired burned and unburned peatland in the boreal plain region of north central alberta water table response to rain events increased significantly after wildfire resulting in a more variable unsaturated zone thickness that was more responsive to smaller rain events water storage losses in the vadose zone occurred primarily at depths greater than 15 cm large peat surface water loss occurred in hummock microforms in the early spring due to the presence of unsaturated frozen peat at depth likely a result of a vapour gradient from the unfrozen peat into the frozen peat underneath during this period the loss of water storage in the vadose zone satisfied up to 25 of daily evaporative demand compared to only 3 5 during ice free periods a similar but less severe drying was observed late in summer with burned hummocks the most vulnerable with high pore water pressures the enhanced surface drying observed is a precursor to high pore water pressure conditions that inhibit sphagnum regeneration our observations point to a paradox where the hummocks being most resistant to combustion are themselves most prone to high pore water pressures following wildfire the harsher hummock environment may contribute to the observed delay in post fire sphagnum regeneration in hummocks compared to hollows

seismic stratigraphy of the blue hole lighthouse reef belize a late holocene climate and storm archive abstract five seismic units may be identified in the 8 m thick holocene sediment package at the bottom of the blue hole a 120 m deep sinkhole located in the atoll lagoon of lighthouse reef belize these units may be correlated with the succession of an existing 5 85 m long sediment core that reaches back to 1 385 kyrs bp the identification of seismic units is based on the fact that uniform fine grained background sediments show weak reflections while alternating background and coarser grained event storm beds exhibit strong reflections in the seismic profiles the main source of sediments is the marginal atoll reef and adjacent lagoon area to the east and north northeasterly winds and storms transport sediment into the blue hole as seen in the eastward increase in sediment thickness i e the eastward shallowing of the blue hole previous assumptions of much thicker holocene sediment packages in the blue hole could not be confirmed so far close to 6 m long cores were retrieved from the blue hole but the base of the sedimentary succession remains to be recovered the nature of the basal sediments is unknown but mid holocene and possibly older pleistocene sinkhole deposits can be expected the number of event beds identified in the blue hole n 37 during a 1 385 kyr long period and the number of cyclones listed in historical databases suggest that only strong hurricanes categories 4 and 5 left event beds in the blue hole sedimentary succession storm beds are numerous during 1 3 0 9 kyrs bp and 0 8 0 5 kyrs bp

hydrogeologic properties of a thrust fault within the oregon accretionary prism two sets of hydrogeologic tests conducted at ocean drilling program odp hole 892 on the oregon accretionary prism provided the opportunity to determine hydrogeologic properties of an active accretionary prism fault zone the first set of tests consisted of shipboard packer tests conducted during odp leg 146 fall 1992 while the second set of tests were constant drawdown and constant discharge tests conducted in fall 1993 using the submersible alvin pressure response during the first set of tests suggests that fractures remained open until excess fluid pressure relative to hydrostatic dropped below 0 315 to 0 325 mpa 0 53 to 0 54 where pore pressure hydrostatic lithostatic hydrostatic analysis of the packer test data suggested an apparent background pressure of 0 25 mpa 0 42 to 0 50 because the borehole had been open for 12 hours prior to the packer tests formation pore pressures may have exceeded this value prior to drilling of the borehole these overpressures dissipated by the time the second set of tests were conducted one possible explanation for this decay is that the borehole may provide a vertical conduit between the overpressured zone and overlying or underlying sediments that had previously been hydraulically separated from the overpressured zone the second set of tests were conducted at pressures 0 019 mpa or 0 03 below that estimated to maintain open fractures and yielded transmissivities 1 to 2 orders of magnitude less than estimated for the packer tests when fractures were open constraints on fluid flow rate along the fault are provided by observed displacement in a bottom simulating reflector bsr at its intersection with the fault zone the closed fracture transmissivities are insufficient to produce flow rates capable of displacing the bsr therefore open fracture transmissivities under conditions of elevated pore pressure are inferred to be necessary for the observed bsr displacement in addition calculated rates of specific discharge through the fault zone are 2 to 3 orders of magnitude lower than discharge measured at an associated seafloor vent site fluid flow must become spatially or temporally focused as it moves up the fault zone toward the seafloor

the ability of rock physics models to infer marine in situ pore pressure pore fluid pressure is an important parameter defining the mechanical strength of marine sediments obtaining high spatial resolution in situ pore pressure measurements in marine sediments however is a challenge and as a result only a handful of in situ pore pressure measurements exist at scientific drill sites integrating rock physics models with standard iodp odp measurements provides a potentially widely applicable approach for calculating in situ pore pressure here we use a rock physics approach to estimate in situ pore pressure at two scientific ocean drill sites where in situ pressure is well constrained odp site 1173 used as reference for normal hydrostatic fluid pressures and odp site 948 where previous studies infer high fluid pressures 0 45 0 95 where the pore pressure ratio is defined as the pore pressure above hydrostatic divided by the difference between the largest principal stress and hydrostatic stress our analysis indicates that the rock physics method provides an accurate low precision method for estimating in situ pore pressure at these drill sites and sensitivity analysis indicates this method can detect modestly high 0 6 pore pressure at the 95 confidence level this approach has broad applicability because it provides an inexpensive high resolution meter scale method for retrospectively detecting and quantifying high pore pressure at any drill site where quality wireline logs and ocean drilling data exist

progressive and regressive soil evolution phases in the anthropocene soils have a substantial role in the environment because they provide several ecosystem services such as food supply or carbon storage agricultural practices can modify soil properties and soil evolution processes hence threatening these services these modifications are poorly studied and the resilience adaptation times of soils to disruptions are unknown here we study the evolution of pedogenetic processes and soil evolution phases progressive or regressive in response to human induced erosion from a 4000 year lake sediment sequence lake la thuile french alps erosion in this small lake catchment in the montane area is quantified from the terrigenous sediments that were trapped in the lake and compared to the soil formation rate to access this quantification soil processes evolution are deciphered from soil and sediment geochemistry comparison over the last 4000 years first impacts on soils are recorded at approximately 1600 yr cal bp with the erosion of surface horizons exceeding 10 t km 2 yr 1 increasingly deep horizons were eroded with erosion accentuation during the higher middle ages 1400 850 yr cal bp reaching 1000 t km 2 yr 1 and leading to the remobilization of carbonated and poorly weathered material hence rejuvenating soil development erosion exceeded the soil formation rate and constituted a regression in the development of soils the tolerable erosion limit is thus defined for erosion from 25 to 30 t km 2 yr 1 beyond this limit the sustainability of the agroecosystem is limited and ecosystem services decrease afterwards pedogenesis evolved again from progressive 700 300 yr cal bp to regressive 300 yr cal bp today phases erosion was less important during the last 700 years than during the middle ages but with the same weathering stages indicating that soils were deeply affected during the middle age and have yet not recovered our results highlight the importance of the human factor in the pedogenesis over last millennia and suggest that the studied agro ecosystem entered the anthropocene 1400 years ago

transient fluid flow through the toe of the barbados accretionary complex constraints from ocean drilling program leg 110 heat row studies and simple models thirty four sediment and mud line temperatures were collected from six drill holes on ocean drilling program odp leg 110 near the toe of the barbados accretionary complex when combined with thermal conductivity measurements from sediment cores and results from earlier surveys these data delineate the complicated thermal structure on the edge of this convergent margin heat flow values at the seafloor of 92 192 mw m sup 2 are 80 300 higher than those predicted by standard heat flow versus age models for oceanic crust but are compatible with earlier seafloor measurements made in this area at the same latitude heat flow tends to decrease downhole at four sites suggesting the presence of heat sources within the sediments these results are consistent with the flow of warm fluids through the complex along high permeability conduits including thrust faults the major decollement zone and sandy intervals simple calculations suggest that this fluid flow is transient occurring on time scales of tens of thousands of years fluid flow velocities along the decollement zone estimated with a simple thermal model are about 10 sup minus 7 m s 100 times faster than predicted by numerical steady state analyses the estimated maximum sediment permeability within the decollement zone which was basedmore on the above fluid flow velocity is about 10 sup minus 12 m sup 2 also 100 times higher that that calculated numerically high heat flow in the vicinity of 15 degree 30 prime n and not elsewhere along the deformation front suggests that the leg 110 drill sites may be situated over a prism water discharge zone with dewatering more active here than elsewhere along the accretionary complex less

fluid regime in faulting deformation of the waratah fault zone australia as inferred from major and minor element analyses and stable isotopic signatures abstract geochemical studies have been conducted on the waratah fault zone a steep ne sw trending brittle fault zone in the southeastern lachlan fold belt australia strike slip movement along the fault juxtaposed an early devonian turbidite sequence liptrap formation against heavily veined early devonian limestones the fault zone consists of the faulted liptrap formation and faulted limestones and a fault core composed of a fault melange zone and strongly brecciated limestones the fault zone records extensive mineralogical and stable isotope alteration within the deformed turbidites of the liptrap formation and within the limestone sequence for the liptrap formation a volume loss of 28 was calculated for the transition from unaltered sediments to the fault melange zone the volume loss in the limestone gouge is much higher 60 the calculated fluid rock ratios in the limestone sequence are at least an order of magnitude lower than in the fault melange zone stable isotopic and rare earth elemental data document the infiltration of near surface meteoric water during brittle deformation and alteration of the fault zone the interaction of fluids with contrasting lithologies led to different alteration processes affecting fault weakening and or strengthening the cyclic change between large scale fluid infiltration due to brittle failure open system and fault rock cementation closed system may represent a combined conduit barrier system in the limestone sequence within the liptrap formation no cementation took place as the conditions for quartz precipitation were unsuitable hence the fault system remains open in respect to fluids conduit system only additionally the permanent fluid infiltration dissolved silica and altered detrital feldspar resulting in subsequent porosity increase within the fault melange zone our results are consistent with investigations in modern accretionary complexes where major decollements and subsidiary faults have been shown to be important fluid conduits

deepwater fold and thrust belt classification tectonics structure and hydrocarbon prospectivity a review abstract deepwater fold and thrust belts dwftbs are classified into near field stress driven type 1 systems confined to the sedimentary section and type 2 systems deformed by either far field stresses alone or mixed near and far field stresses dwftbs can occur at all stages of the wilson cycle up to early stage continent continent collision type 1 systems have either weak shale or salt detachments they occur predominantly on passive margins but can also be found in convergent related areas such as the mediterranean and n borneo examples include the niger and nile deltas the west coast of africa and the gulf of mexico type 2 systems are subdivided on a tectonic setting basis into continent convergence zones and active margin dwftbs continent convergence zones cover dwftbs developed during continent arc or continent continent collision and those in a deepwater intracontinental setting e g w sulawesi makassar straits active margins include accretionary prisms and transform margins the greatest variability in dwftb structural style occurs between salt and shale detachments and not between tectonic settings changes in fold amplitude and wavelength appear to be more related to thickness of the sedimentary section than to dwftb type in comparison with shale salt detachment dwftbs display a lower critical wedge taper more detachment folds long and episodic duration of deformation and more variation in vergence structures unique to salt include canopies and nappes accretionary prisms also standout from other dwftbs due to their relatively long continuous duration rapid offshore propagation of the thrust front and large amount of shortening in terms of petroleum systems many similar issues affect all dwftbs these include the oceanward decrease in heat flow offshore increase in age of mature source rock and causes of trap failure e g leaky oblique and frontal thrust faults breach of top seal by fluid pipes one major difference between type 1 and type 2 systems is reservoir rock high quality continent derived quartz rich sandstones are generally prevalent in type 1 systems more diagenetically reactive minerals derived from igneous and ophiolitic sources are commonly present in type 2 systems or many are simply poor in well developed turbidite sandstone units however some type 2 systems particularly those adjacent to active orogenic belts are partially sourced by high quality continent derived sandstones e g nw borneo s caspian sea columbus basin in some cases very high rates of deposition in accretionary prisms adjacent to orogenic belts coupled with uplift due to collision results in accretionary prism related fold belts that pass laterally from sub aerial to deepwater conditions e g s caspian sea indo burma ranges the six major hydrocarbon producing regions of dwftbs worldwide gulf of mexico niger delta nw borneo brazil west africa s caspian sea stand out as differing from most other dwftbs in certain fundamental ways particularly the very large volume of sediment deposited in the basins and or the great thickness and extent of salt or overpressured shale sdetachments

lake level changes in central patagonia argentina crossing environmental thresholds for lateglacial and holocene human occupation the role and extent of climate as a cause of the expansion and decline of human cultures is still debatable it is clear however that human environment interactions are enhanced and interplay more closely in climatically sensitive areas such as around hydrologically closed basins lago cardiel is located at 498 s in the very arid rain shadow east of the andes providing an exceptionally receptive system to changes in hydrological balance results of a geophysical survey combined with sedimentological and geochemical studies provide a continuous lateglacial holo cene record of substantial water level changes these variations combined with archaeological results from the catchment area offer a unique possibility to explore the pattern of peopling within this remote area of the globe and its possible relation to climate change human occupation in patagonia is well documented towards the andes throughout the entire holocene archaeological data from the lago cardiel basin however show an apparent lack of human activity during the first part of this period which coincides with well constrained high lake levels our results show an intriguing coincidence between low lake level and increasing human occupation suggesting that the lago cardiel basin has focused human use during intervals with relatively lower effective moisture such as during the late pleistocene but its evidence may have been submerged this interpretation is confirmed by archae ological remains from lago strobel another perennial lake with a comparable catchment located in the same climatic region and thus sharing the same climatic history as lago cardiel copyright 2009 john wiley sons ltd

permeability of a decollement zone results from a two well experiment in the barbados accretionary complex fault zones are hypothesized to play a major role in focusing fluid expulsion from accretionary complexes a small number of previous investigations have examined the hydrogeologic properties of these fault zones using borehole packer or submersible based slug and constant rate flow tests while these single well experiments yielded the first data on the permeability of active fault zones they could only investigate a small area around the borehole and could not ensure reliable determination of formation storage properties recent data from an unplanned experiment in the decollement zone of the barbados accretionary prism demonstrate the potential for multiple well hydrogeologic tests pressure data from a sealed borehole show a clear signal of drilling activity 45 m away analysis of the pressure response suggests that decollement zone permeability lies between 1 0 and 1 2 x 10 14 m 2 this permeability is 2 4 orders of magnitude greater than overlying and underlying sediment and thus appears sufficient to focus fluid flow along the decollement zone this inadvertent test evaluated fault zone hydrogeologic parameters over a larger radius and longer timescale and at pressures closer to in situ than previous single well tests

a 400 ka tephrochronological framework for central america from lake peten itza guatemala sediments highlights dating 400 ka paleoclimate record of neotropics revision and new eruptive volumes for large central american eruptions age models for peten itza sediments linking lacustrine ash inventory to eruptions from central america and mexico abstract lake peten itza northern guatemala lies within a hydrologically closed basin in the south central area of the yucatan peninsula and was drilled under the auspices of the international continental scientific drilling program icdp in 2006 at 16 55 n latitude the lake is ideally located for study of past climate and environmental conditions in the neotropical lowlands because of its great depth 160 m lake peten itza has a record of continuous sediment accumulation that extends well into the late pleistocene a key obstacle to obtaining long climate records from the region is the difficulty of establishing a robust chronology beyond 40 ka the limit of 14c dating tephra layers within the lake peten itza sediments however enable development of age depth relations beyond 40 ka ash beds from large magnitude pleistocene to holocene silicic eruptions of caldera volcanoes along the central american volcanic arc cava were found throughout drill cores collected from lake peten itza these ash beds were used to establish a robust chronology extending back 400 ka we used major and trace element glass composition to establish 12 well constrained correlations between the lacustrine tephra layers in lake peten itza sediments and dated deposits at the cava source volcanoes and with their marine equivalents in eastern pacific ocean sediments the data also enabled revision of eight previous determinations of erupted volumes and masses and initial estimates for another four eruptions as well as the designation of source areas for 14 previously unknown eruptions the new and revised sedimentation rates for the older sediment successions identify the interglacial of mis5a between 84 and 72 ka followed by a stadial between 72 and 59 ka that corresponds to mis4 we modified the age models for the lake peten itza sediment sequences extended the paleoclimate and paleoecological record for this neotropical region to 400 ka and determined the magnitude and timing of cava eruptions

wetland geomorphology and paleoecology near akab muclil rio bravo floodplain of the belize coastal plain abstract to understand wetland geomorphology and paleoecology we collected a 2 6 m sediment core from a flooded swamp adjacent to the maya archaeological site of akab muclil in the maya lowlands of northwestern belize the site of akab muclil has a known occupation that persisted from early maya classic 1700 1350 bp through the terminal maya classic 1180 1050 bp and into the postclassic 1050 450 bp and lies near a vast network of ancient maya canal and field systems we analyzed this core using a combination of paleoecological and geochemical techniques to determine the history of land use and natural change over time within this wetland accelerator mass spectrometry dating pollen charcoal analysis micropaleontology geochemical analysis and magnetic susceptibility provide a suite of methods from which we interpret the geomorphic and ecological history of this wetland system four ams dates from the length of the core provide us with an age model that runs from 1675 cal bp through the maya classic onward to the present at the base of this system soil composition and chemistry provide evidence that the system changed from a seasonally wet terrestrial soil to a perennially wet swamp as the basal mollisol soil lies buried by peats and calcareous sediments this shift to a perennial wetland could be related to ancient maya water management or a natural geomorphic change though we suspect the former because of nearby ancient maya large scale geomorphic and hydrological manipulation in the form of intensive canalization and agriculture evidence of ancient maya uses and impacts including sedimentation zea mays pollen and high charcoal counts occur from the lowest levels of the sequence through the classic and into the postclassic period above this level the strata change to stable peats laminated deposits of light gray dark gray gypsum authigenic carbonate and layers of fibrist peat with little evidence of human impact until recent increases in charcoal and phosphorous this study compared with other regional studies indicates a later transition from terrestrial to wetland later human impacts in the postclassic and a geomorphic impact record closely tied to the history of the adjacent site rather than broader land use trends