biological remains geochemistry and stable isotopes for the reconstruction of environmental and hydrological changes in the holocene lakes from north sahara abstract a detailed reconstruction of the holocene palaeoenvironments and palaeohydrology is presented for wwamp and lacustrine calcareous sediments which lie in closed depressions along the northern margin of the great western erg algeria it is based on a multidisciplinary approach using numerous techniques the chronological framework is established on the basis of 23 14 c dates on inorganic calcite and mollusc shells ranging from ca 9300 to 3000 yr b p and on an isolated 14 c age of ca 10 900 yr b p the significance of various environmental markers is discussed biological remains are abundant and diversified about 150 taxa including diatoms chrysophytes charophytes ostracods molluscs and foraminifera they provide information on the water chemistry total salinity ionic ratio ph and partial pressure of carbon dioxide and on the habitats water depth development of aquatic plants ephemerality etc emphasis is given to the total salinity individual organisms and palaeobiological assemblages are ordered as a function of their salt requirements and tolerances calibration based on modern diatom samples allows a tentative estimate of the palaeosalinities which range from fresh to marine like and even hypersaline waters attention is drawn to the possible colonization of marine organisms in continental basins without any connection with the sea stable isotope analyses have been carried out on both inorganic and biogenic mollusc and ostracod shells charophyte gyronites carbonates the 18 o contents demonstrate wide changes in the hydrological balance fluctuations in 13 c contents inform on water mixing residence time and on the importance of biological effects on total dissolved inorganic carbon geochemical analyses were performed on inorganic calcite and on ostracod shells high contents in fe and mn indicate reducing environments during stages of high salinity sr ca and mg ca are used as salinity indicators zn mg ratios suggest that the major water supply was the aquifer of the great western erg today down to about 50 m rather than surface waters from the atlas mountains salinities resulted from the balance between inputs from the water table evaporation rate leaching of pre existing salt crusts and seepage through the lake bottom a reconstruction of the evolution of the palaeosystems through time and space has been attempted with special attention to the instability of the water and ionic balances for different time scales short term fluctuations occurring during a single hydrological cycle annual or seasonal are deduced from the apparent discrepancies between the various indicators found in a given sample long term 10 2 10 3 years changes confirm the existence of a climatic phase being wetter than today from at least ca 9300 and may be from ca 10 900 yr b p to 3000 yr b p for the first time the salinity deduced from diatoms and the 18 o content of the water deduced from that of the carbonate at 25 c are used simultaneously to model the hydrological history of the basins and to estimate the relative humidity of the atmosphere during the holocene major environmental trends recorded from ca 9300 to 3000 yr b p do not show a clear synchronims between the different profiles studied therefore they cannot be interpreted in terms of palaeoclimate but instead demonstrate the importance of local hydrology in the evolution of closed basins associated with groundwater a common situation in arid and semi arid zones

holocene climatic and human influences on lakes of the yucatan peninsula mexico an interdisciplinary palaeolimnological approach we used palaeolimnological techniques to examine effects of holocene climate change and human influence on lakes of the yucatan peninsula mexico the three study lakes are located along a west east transect that represents a gradient of increasing modern precipitation and density of former maya settlement at lake coba an 880 cm sediment core yields a complete record of lacustrine sedimentation that began when the lake first filled 8000 bp as groundwater level rose in response to rising sea level and increased precipi tation diatom ostracod and 18o evidence indicate that lake coba was initially shallow and saline coba presently in the region of greatest rainfall showed more episodic water level changes than lake san jose chulchaca or lake sayaucil high lake level and fresh water were evident at 440 cm 2600 bp followed by a decline in water levels and an increase in total ionic salinity to the present time in a 613 cm core from sayaucil in the intermediate precipitation zone total salini

climate change in the lake valencia basin venezuela 12600 yr bp to present we present a palaeoclimate record for northern south america extending from the latest pleistocene 12600 14c yr bp to present climate reconstruction for the valencia basin venezuela was base

holocene vegetation change in the northern peten and its implications for maya prehistory an 8400 cal yr record of vegetation change from the northern peten guatemala provides new insights into the environmental history of the archaeological area known as the mirador basin pollen loss on ignition and magnetic susceptibility analyses indicate warm and humid conditions in the early to mid holocene evidence for a decrease in forest cover around 4600 cal yr b p coincides with the first appearance of zea mays pollen suggesting that human activity was responsible the period between 3450 cal yr b p and 1000 cal yr b p is characterized by a further decline in forest pollen types includes an abrupt increase in weedy taxa and exhibits the highest magnetic susceptibility values since the early holocene all of which suggest further agricultural disturbance in the watershed a brief drop in disturbance indicators around 1800 cal yr b p may represent the preclassic abandonment of the area changing pollen frequencies around 1000 cal yr b p indicate a cessation of human disturbance which represents the late classic collapse of the southern maya lowlands

giant holocene freshwater microbialites laguna bacalar quintana roo mexico with more than 10 km of total length holocene microbialites in laguna bacalar mexico belong to the largest freshwater microbialite occurrences microbialites include domes ledges and oncolites domal forms can grow to diameters and heights of 3 m microbialites are composed of low magnesium calcite which is to a large extent precipitated due to the metabolic activity of the cyanobacteria homeothrix and leptolyngbya and associated diatoms photosynthesis removes carbon dioxide and triggers carbonate precipitation also an elevated carbonate concentration in lagoon waters derived from dissolution of cenozoic limestone in a karst system supports carbonate precipitation trapping and binding of detrital grains is also observed but is not as common as precipitation bacalar microbialites are largely thrombolitic however stromatolitic sections occur as well the bulk of bacalar microbialites probably formed in the late holocene ca 1 kyr bp until present according to 14c dating microbialites accreted 9 to 8 cal kyr bp however these ages may be too old as a result of a strong hard water effect this effect is seen in 14c ages of living bivalve and gastropod mollusc shells from bacalar lagoon which are 8 to 7 cal kyr bp the modern associated fauna of microbialites is characterized by low diversity and high abundance of the bivalve mollusc dreissena sp and the gastropod pomacea sp the abundant grazing gastropods presumably hamper modern microbialite formation a comparison of bacalar microbialites with other modern microbialite occurrences worldwide shows only a few patterns sizes shapes microbial taxa mineralogy type of accretion and settings including water properties of microbialite occurrences exhibit high variability a trend may be seen in the grazing metazoa which are rare to absent in the marine and brackish examples but apparently present in all the freshwater occurrences of microbialites also freshwater examples are usually characterized by elevated concentrations of carbonate and or calcium ions in the surrounding waters

the arid humid transition in the sahara and the sahel during the last deglaciation at the time of the last glacial maximum the sahara and sahel regions of north africa were extremely dry new records of rainfall show that during the subsequent deglaciation the transition from arid to humid conditions in these regions occurred synchronously in two main steps comparison with other records of palaeoclimate in europe and the north atlantic ocean shows that certain common factors controlled changes in ocean and atmosphere dynamics during the deglaciation

a 2000 year reconstruction of forest disturbance from southern pacific costa rica abstract we reconstruct prehistoric and historic human forest disturbance and vegetation change in southern pacific costa rica in the vicinity of the las cruces biological station and the la amistad international park and biosphere reserve pollen and charcoal in sediments from laguna santa elena reveal a nearly continuous record of human alteration of these tropical forests over the past two millennia the basal portion of the core shows nearly intact premontane forests approximately 1800 cal year b p although there is evidence of human presence on the landscape in the form of maize pollen and charcoal clearing for agriculture resulted in the dominance of disturbance taxa in the watershed beginning at least 1400 cal year b p the pollen record reveals a possible brief hiatus in human occupation of the watershed at approximately 540 cal year b p although secondary succession began to occur in the laguna santa elena watershed prior to that time beginning about 700 cal year b p three eruptions of nearby volcan baru which we date to approximately 610 1080 and 1440 cal year b p apparently had little direct effect on the prehistoric populations in the immediate vicinity of the lake historic and modern land clearance has perpetuated a modern vegetation assemblage of disturbance and successional taxa

climate change on the yucatan peninsula during the little ice age we studied a 5 1 m sediment core from aguada x caamal 20 36 6 n 89 42 9 w a small sinkhole lake in northwest yucatan mexico between 1400 and 1500 a d oxygen isotope ratios of ostracod and gastropod carbonate increased by an average of 2 2 and the benthic foraminifer ammonia beccarii parkinsoniana appeared in the sediment profile indicating a hydrologic change that included increased lake water salinity pollen from a core in nearby cenote san jose chulchaca showed a decrease in mesic forest taxa during the same period oxygen isotopes of shell carbonate in sediment cores from lakes chichancanab 19 53 0 n 88 46 0 w and salpeten 16 58 6 n 89 40 5 w to the south also increased in the mid 15th century but less so than in aguada x caamal climate change in the 15th century is also supported by historical accounts of cold and famine described in maya and aztec chronicles we conclude that climate became drier on the yucatan peninsula in the 15th century a d near the onset of the little ice age lia comparison of results from the yucatan peninsula with other circum caribbean paleoclimate records indicates a coherent climate response for this region at the beginning of the lia at that time sea surface temperatures cooled and aridity in the circum caribbean region increased

orbital and internal forcing of climate on the yucatan peninsula for the past ca 36 ka abstract a 19 6 m sedimentary sequence from lake quexil guatemala is presented as a proxy record of climatic change on the low lying yucatan peninsula over the last ca 36 ka long term climatic fluctuations are attributed to orbital forcing of insolation while abrupt changes during the late glacial period are the result of internal forcing mechanisms that remain imperfectly modelled gcms simulating meltwater cooling of the gulf of mexico may predict some but not all climatic responses in the circum caribbean lake quexil did not experience greater aridity than predicted by orbital forcing during late glacial zones q lg 1 and 2 which were drier only in comparison with holocene conditions q lg 1 appears to have been colder than expected based on insolation alone and is contemporary with the younger dryas event between meltwater pulses 1a and 1b thus other feedbacks also may have cooled surface temperatures in the gulf of mexico such as unique late glacial circulation patterns increased albedo for the florida and yucatan peninsulas or surface temperatures on the north american continent additional terrestrial sequences are needed to improve the chronology and confirm the distribution of sub regional climatic variability in the circum caribbean

paleoflood records from sinkholes using an example from the ebro river floodplain northeastern spain 53 pags 10 figs the definitive version is available at https www cambridge org core journals quaternary research

a multiproxy peat record of holocene mangrove palaeoecology from twin cays belize the extent and function of coastal mangrove ecosystems are likely to be influenced by future changes in sea level multiple proxies of past mangrove ecosystems preserved in a 780 cm long peat core tcc2 taken from twin cays belize record palaeoecological changes since 8000 cal yr bp the proxies included pollen and the stable isotope c n and o compositions of mangrove leaf fragments rhizophora mangle red mangrove has been dominant at this site on twin cays for over 8000 years variations in 13 c and 15n suggest past changes in stand structure between dwarf transition and tall r mangle through the holocene marked changes in the 18o up to 4 of mangrove leaf fragments throughout tcc2 most likely record variations in the proportion of seawater versus precipitation taken up by past mangroves reflecting the degree of inundation of the site with seawater resulting from changes in the rate of holocene sea level rise notably a decline in peat accumulation rate at 7200 cal yr bp correlate

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

evolution of deepwater mixing and weathering inputs in the central atlantic ocean over the past 33 myr the isotopic composition of nd in present day deep waters of the central and northeastern atlantic ocean is thought to fingerprint mixing of north atlantic deep water with antarctic bottom water the central atlantic romanche and vema fracture zones are considered the most important pathways for deep water exchange between the western and eastern atlantic basins today we present new nd isotope records of the deepwater evolution in the fracture zones obtained from ferromanganese crusts which are inconsistent with simple water mass mixing alone prior to 3 ma and require additional inputs from other sources the new pb isotope time series from the fracture zones are inexplicable by simple mixing of north atlantic deep water and antarctic bottom water for the entire past 33 myr the distinct and relatively invariable nd and pb isotope records of deep waters in the fracture zones appear instead to have been controlled to a large extent by contributions from saharan dust and the orinoco amazon rivers thus the previously observed similarity of nd and pb isotope time series from the western and eastern north atlantic basins is better explainable by direct supply of labrador seawater to the eastern basin via a northern pathway rather than by advection of north atlantic deep water via the romanche and vema fracture zones

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

changes in the hydrochemistry of a karstic lake in yucatan mexico the yucatan peninsula is a karstic region with groundwater dependent ecosystems and geological features such as karstic lakes the objective of this research was to calculate the volume of water stored in this section of the aquifer and to describe the changes in the hydrochemistry of the lake punta laguna as a representative of karstic lakes in the yucatan peninsula a bathymetry and calculation of the volume of water stored in the lake resulted in 5 198 505 m3 with a maximum depth measured of 26 6 m physicochemical measurements were completed in surface and three vertical profiles to estimate the mass of nutrients and ions and to describe the current hydrochemistry the lake is an oligotrophic one of mixed typed water chloride calcium magnesium saturated or supersaturated with calcite dolomite and aragonite due to limestone dissolution no evidence obtain regarding the saline intrusion the conclusion that we can derive from the data is that punta laguna is a tropical karstic lake in good condition with low anthropogenic influence evolving hydrochemically speaking the changes over a 20 year period suggest that there has been reverse ion exchange evaporation and sulfate reduction

a 2860 year high resolution pollen and charcoal record from the cordillera de talamanca in panama a history of human and volcanic forest disturbance late holocene vegetational disturbance by human and volcanic activity in the lower montane rain forest region of cordillera de talamanca province of chiriqui in panama have been studied on the sedimentary archive from laguna volcan 1500 m elevation the pollen and charcoal record dated by four radiocarbon ams dates provides evidence of amerindian occupation and strong impact on the lower montane rain forest region since the beginning of the record at 2860 14c yr bp there is evidence of at least three eruptions of volcan baru which affected the study region at 1800 1000 and 500 14c yr bp the first two events had apparently relatively little effects while the latest volcanic eruption which deposited a 20 cm thick tephra layer in the studied lake had a catastrophic impact amerindian life and culture of the study area was destroyed after the last eruption the montane rain forest recovered markedly fires were rare and agriculture activity stopped in the surroundings of the lake suggesting that

cultural and climatic history of coba a lowland maya city in quintana roo mexico abstract lake coba within the archaeological site of coba provides evidence bearing on lowland maya development palynological and geochemical data record multidecadal precipitation cycles from a 8 80 m 8370 yr lake sediment sequence terminating on bedrock late classic sedimentation rates are rapid but an anthropogenically derived colluvium layer is lacking initial vegetation was medium semi deciduous and swamp forest forest clearance began 1650 b c early preclassic and maize first occurred at 850 b c middle preclassic lakeside milpas existed until a d 720 late classic and then were moved from the city center as urbanization intensified and lake coba was diked as a reservoir coba was at most briefly vacated during the classic collapse and was abandoned after a d 1240 although some habitation persisted the paleoecological record matches the archaeological history for coba but pervasive disturbance muted the climatic signal as the late classic drought is barely evident the question whether economic trees were maintained within the city is unresolved maize cultivation allowed the maya to develop a complex society and support a large population but dependence on maize was ultimately doomed by variable rainfall precipitation in extreme years was insufficient to support crops while native vegetation was not directly affected by drought that devastated maya agriculture

a 10 300 14c yr record of climate and vegetation change from haiti abstract pleistocene and holocene vegetation dynamics in the american tropics are inferred largely from pollen in continental lake sediments maritime influences may have moderated climate and vegetation changes on caribbean islands stable isotope 18 o study of a 7 6 m core from lake miragoane haiti provided a high resolution record of changing evaporation precipitation e p since 10 300 14 c yr b p the miragoane pollen record documents climate influences and human impacts on vegetation in hispaniola the 18 o and pollen data near the base of the core indicate cool dry conditions before 10 000 14 c yr b p lake miragoane filled with water in the early holocene as e p declined and the freshwater aquifer rose despite increasing early holocene moisture shrubby xeric vegetation persisted forest expanded 7000 14 c yr b p in response to greater effective moisture and warming the middle holocene 7000 3200 14 c yr b p was characterized by high lake levels and greatest relative abundance of pollen from moist forest taxa climatic drying that began 3200 14 c yr b p may have driven some mesophilic animal species to extinction the pollen record of the last millennium reflects pre columbian taino and european deforestation long term holocene vegetation trends in southern haiti are comparable to trends from continental lowland circum caribbean sites suggesting a common response to regional climate change

evolution of atlantic thermohaline circulation early oligocene onset of deep water production in the north atlantic the flow of deep water masses is a key component of heat transport in the modern climate system yet the role of deep ocean heat transport during periods of extreme warmth is poorly understood the present mode of meridional overturning circulation is characterized by deep water formation in both the north atlantic and the southern ocean however a different mode of meridional overturning circulation operated during the extreme greenhouse warmth of the early cenozoic during which time the southern ocean was the dominant region of deep water formation the combination of general global cooling and tectonic evolution of the atlantic basins over the past 55 m y ultimately led to the development of a mode of overturning circulation characterized by both southern ocean and north atlantic deep water sources the change in deep water circulation mode may in turn have affected global climate however unraveling the causes and consequences of this transition requires a better understanding of the timing of the transition new nd isotope data from the southeastern atlantic ocean indicate that the initial transition to a bipolar mode of deep water circulation occurred in the early oligocene ca 33 ma the likely cause of significant deep water production in the north atlantic was tectonic deepening of the sill separating the greenland norwegian sea from the north atlantic

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

reconstructing the internal structure and long term evolution of hazardous sinkholes combining trenching electrical resistivity imaging eri and ground penetrating radar gpr abstract the approaches aimed at characterising specific damaging sinkholes have received limited attention compared with other ground instability phenomena e g landslides moreover the practicality of the trenching technique in combination with numerical dating and retro deformation analysis for sinkhole site investigations has been barely explored this work illustrates the advantages of combining geomorphic mapping electrical resistivity imaging eri ground penetrating radar gpr and trenching for sinkhole characterisation and shows how the trenching technique contributes to fill significant gaps that neither geomorphic nor geophysical methods can address two large sinkholes 200 m long related to the interstratal karstification of evaporites and generated by contrasting subsidence mechanisms sagging collapse were investigated in the fluvia valley ne spain although gpr data may provide high resolution information on subsidence related stratigraphic and structural features at shallow depth the profiles acquired in the investigated sites with 100 mhz shielded and 40 mhz unshielded antennae provided limited insight into the internal geometry of the sinkholes due to reduced signal penetration related to the presence of conductive clayey material the eri sections satisfactorily imaged the general geometry of the sagging and collapse subsidence structures up to depths higher than 100 m and clearly captured the basal contact of the low resistivity sinkhole fill in the sections with adequate layout and resolution the trenches despite their limited depth ca 5 m allowed us to obtain valuable objective information on several key aspects of the subsidence phenomenon 1 mechanisms deformation style and kinematics progressive versus episodic 2 limits of ground deformation 3 temporal evolution expansion versus contraction 4 chronology and timing of most recent deformation phase 5 rates of subsidence and sedimentation and 6 the role played by subsidence in the development of lacustrine environments and the associated sedimentation patterns

a mid holocene paleoprecipitation record from belize abstract understanding past climate may contribute to a better understanding of future climate change allowing for adaptations to changing water resources high latitude paleoclimate reconstructions reveal a warmer northern hemisphere during the mid holocene yet paleoclimate records from tropical central america are lacking especially seasonally resolved reconstructions needed to resolve seasonal shifts here we reconstruct mid holocene precipitation using high resolution sub annual to biannual stable isotope ratios oxygen and carbon extracted from a speleothem recovered from belize to investigate the frequency and magnitude of precipitation variability we found a slight increase in precipitation during the mid holocene in belize with less variability compared to the late holocene this increase in precipitation may be a result of the expansion of the north atlantic subtropical high nash which strengthens the caribbean lower level jet enhancing westward advection of atmospheric moisture to belize the decrease in precipitation variability could be derived from a northward movement of the intertropical convergence zone itcz placing belize within the bounds of the itcz for a longer period each year time series analysis reveals periodicities of 200 250 years which correspond to the suess solar cycle the additional periods of 100 and 50 years also have origins in solar irradiance the north atlantic oscillation nao and the atlantic multidecadal variability amv also known as the atlantic multidecadal oscillation amo long recognized as important drivers of precipitation variability in the region are present but only at the 90 significance level we posit that the reduced influence of the nao and amv could be caused by the northerly migration of the itcz during the mid holocene

neodymium isotopic composition and rare earth element concentrations in the deep and intermediate nordic seas constraints on the iceland scotland overflow water signature neodymium isotopic composition and rare earth element concentrations were measured in seawater samples from eleven stations in the nordic seas these data allow us to study how the iceland scotland overflow water isow acquires its neodymium signature in the modern ocean the waters overflowing the faroe shetland channel are characterized by nd 8 2 0 6 in good agreement with the only other data point published 19 years ago in the greenland and iceland seas the water masses leading to the formation of the isow display lower neodymium isotopic composition with nd around 11 and 9 respectively since no water masses in the nordic seas are characterized by nd 8 the radiogenic signature of the isow likely reflects inputs from the highly radiogenic norwegian basin basaltic margins jan mayen iceland faroe with nd 7 in addition to the neodymium isotopic composition the rare earth element patterns suggest that these inputs occur via the remobilization which includes resuspension and dissolution of sediments deposited on the margins whereas the neodymium isotopic composition behaves conservatively in the oceans in the absence of lithogenic inputs and can be used as a water mass tracer these results emphasize the role of interactions between sediments deposited on margins and seawater in the acquisition of the neodymium isotopic composition of water masses these results should allow a better use of this parameter to trace the present and the past circulation in the north atlantic

subsidence mechanisms and sedimentation in alluvial sinkholes inferred from trenching and ground penetrating radar gpr implications for subsidence and flooding hazard assessment abstract sinkholes function as small sediment traps that may host valuable stratigraphic records for paleoenvironmental reconstructions and hazard assessments e g subsidence floods hurricanes tsunamis tephra fall out the sedimentological features and completeness of such archives are influenced by the sedimentation and subsidence patterns and rates however karst depressions are frequently treated as static basins unaffected by settlement this work illustrates the practicality of integrated studies combining trenching numerical dating and shallow geophysical techniques gpr for characterizing the subsurface subsidence structure associated with sinkholes and reconstructing their deformational and sedimentary history the approach is applied in two collapse sinkholes located in contrasting geomorphic settings relict terrace and floodplain related to deep seated interstratal karstification of evaporites the analysis of the sinkholes particularly the trenching technique provides practical information for assessing the associated subsidence hazard including the presence of larger cavities at depth the kinematic regime episodic versus progressive evidence of catastrophic displacement fluidization structures and the magnitude and timing of collapse events especially the most recent one the sinkhole located in the floodplain offers the opportunity of analyzing the possibilities and limitations of subsidence sinkholes as recorders of past floods in alluvial environments this depression shows a largely incomplete record attributable to the high frequency of flood events compared to that of the collapse events which create the accommodation space for sediment deposition these limitations could be partially overcome by selecting old sinkholes situated in low terraces and or affected by rapid subsidence

lake qinghai china closed basin like levels and the oxygen isotope record for ostracoda since the latest pleistocene abstract in 1985 a sino swiss limnogeological expedition to china s largest inland lake recovered seismic profiles and piston cores to help calibrate palaeoclimatic models of monsoon and insolation fluctuations over the northern tibet qinghai plateau since the last glacial maximum lake qinghai a perennial lake with an area of 4278 km2 1985 at 3194 m above sea level receives drainage from 30 000 km2 of catchment near the northwest corner of the plateau the 26 5 m deep terminal lake 1985 with brackish alkaline magnesium sulphate rich waters 14 g 1 1 tds has not overflowed since the last glacial but its levels have responded to climatic and hydrologic changes seismic reflection profiles for the sub bottom sediment structures revealed a complex topography with channels relict terraces tectonic tilting and deformation in deposits buried under a 4 6 m packet of mainly flat bedded sediments from which core samples were recovered the lowermost cored material 14 500 yr b p comprises mainly water lain loess deposited in a smaller flood plain lake of up a few metres depth under cold arid climatic conditions ams radiocarbon dating indicates average sedimentation rates for latest pleistocene and holocene deposits in central basin regions of 0 3 0 5 mm yr 1 the 18o values of fossil ostracod valves provide a record of changes in the isotopic chemistry of the lake waters that can be related to the precipitation evaporation budgets which have controlled the palaeo levels for lake qinghai the precipitation evaporation budget for the lake qinghai region is largely determined by the asian monsoon system the lake qinghai sediment record shows that there was no large influx of glacoer melt waters into the lake basin during the post glacial the regional climate remained arid until at least 12 500 yr b p when an incipient monsoon was possibly initiated river transported silts first reached the basin centre at 11 800 yr b p when the lake depth may have increased to 5 m possibly a monsoonal weakening for several centuries after 10 800 yr b p europe younger dryas caused a further period of aridity at 10 200 yr b p the basin began filling towards modern lake dimensions in response to a permanent strengthening of the asian monsoon substantial lake level rises probably characterised the periods 10 200 9800 9500 8500 8300 7200 yr b p and through perhaps a century at about 6800 yr b p decreasing levels probably characterised the intervening intervals short term lake level fluctuations through the mid holocene climatic optimum were minimal indicating a very stable climatic hydrologic regime an isotopic steady state for the lake waters since 3000 500 yr b p obscures the climatic hydrologic regime dependent 18o gradient but there is little stratigraphic evidence to suggest that the lake level has been significantly below its present level since that time three negative 18o excursions for the lake waters may correspond to known glacier advances on the plateau at 3000 2000 and 1500 yr b p during china s neoglacial

environmental change in northern belize since the latest pleistocene cores from the new river lagoon in belize have been analysed using a range of palaeolimnological proxies to reconstruct change since the latest pleistocene the combination of diatom and stable isotope analyses supported by mineralogical and major element data indicate that the new river lagoon has been a freshwater system throughout the period of record for most of this time the lagoon has been hydrologically open but there are indications that it may have become closed in the latest pleistocene or very early holocene this may have been associated with a drier climate and lower sea level mid holocene records are consistent with those from elsewhere in the region indicating a stable and moist climate the late holocene was marked by greater variability but there is no clear evidence of any drying in the late classic coincident with the maya collapse ca ad 900 our results indicate that the continuity of occupation at the maya site of lamanai on the shores of the new river lagoon is consistent with continued access to freshwater during periods of regional climatic variability the importance of conditions at individual sites when considering both system response and human vulnerability to climate change is emphasised copyright 2009 john wiley sons ltd

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

paleoenvironmental evidence for first human colonization of the eastern caribbean abstract identifying and dating first human colonization of new places is challenging especially when group sizes were small and material traces of their occupations were ephemeral generating reliable reconstructions of human colonization patterns from intact archaeological sites may be difficult to impossible given post depositional taphonomic processes and in cases of island and coastal locations the inundation of landscapes resulting from post pleistocene sea level rise paleoenvironmental reconstruction is proving to be a more reliable method of identifying small scale human colonization events than archaeological data alone we demonstrate the method through a sediment coring project across the lesser antilles and southern caribbean paleoenvironmental data were collected informing on the timing of multiple island colonization events and land use histories spanning the full range of human occupations in the caribbean from the initial forays into the islands through the arrival and eventual domination of the landscapes and indigenous people by europeans in some areas our data complement archaeological paleoecological and historical findings from the lesser antilles and in others amplify understanding of colonization history here we highlight data relating to the timing and process of initial colonization in the eastern caribbean in particular paleoenvironmental data from trinidad grenada martinique and marie galante guadeloupe provide a basis for revisiting initial colonization models of the caribbean we conclude that archaeological programs addressing human occupations dating to the early to mid holocene especially in dynamic coastal settings should systematically incorporate paleoenvironmental investigations

stratigraphic evidence for anthropogenically induced coastal environmental change from oaxaca mexico abstract previous interdisciplinary paleoenvironmental and archaeological research along the rio verde valley of oaxaca mexico showed that holocene erosion in the highland valleys of the upper drainage basin triggered geomorphic changes in the river s coastal floodplain this article uses stratigraphic data from sediment cores extracted from laguna pastoria an estuary in the lower rio verde valley to examine changes in coastal geomorphology potentially triggered by highland erosion coastal lagoon sediments contain a stratigraphically and chronologically distinct record of major hurricane strikes during late holocene times three distinct storm facies are identified from sediment cores obtained from laguna pastoria which indicate that profound coastal environmental changes occurred within the region and are correlated with increased sediment supplied from highland erosion the chione laevicardium facies was deposited in an open bay while the mytella barnacle facies and sand facies were deposited in an enclosed lagoon following bay barrier formation we argue that highland erosion triggered major geomorphic changes in the lowlands including bay barrier formation by 2500 cal yr b p these environmental changes may have had significant effects on human populations in the region the lagoon stratigraphy further indicates an increase in mid late holocene hurricane activity possibly caused by increased el nino frequencies

geologic and environmental aspects of surface cementation north coast yucatan mexico the northernmost part of the fresh water aquifer of yucatan mexico is confined near the coast by a thin flat nearly impermeable calcareous layer the authors think it probable that this confining layer is now developing by a process of sedimentation and precipitation of pore filling cement on and near the surface of older limestone at the landward margin of the swamp that extends virtually continuously along the north yucatan coast it is their interpretation that this caco sub 3 cemented layer which is 0 5 to 1 4 m thick developed in the zone of discharge of the fresh water aquifer the presence of sup 14 c 6 0 plus minus 0 4 modern carbon in a sample of aquitard supports the hypothesis that cementation is an ongoing process further support comes from the remarkable regularity between modern mean sea level and the elevation of the landward boundary between the confining zone and normal karst along 250 km of coast confinement of the aquifer produces an elevation of the piezometric surface to about 0 5 m above mean sea level and a concurrent depression of the fresh water salt water interface to an estimated depth of about 18 m below mean sea level at the coast breaching of the confining layer implicit inmore some development schemes for the region could dramatically decrease the thickness of the fresh water lens a valuable water resource the mixing zone beneath the confined part of the aquifer is a chemically active volume that may be vigorously pumped by tides as evidenced by increased salinity of ground water near the coast thus making this zone a likely place for rock water interaction including perhaps dolomite formation less