dynamical mechanisms for african monsoon changes during the mid holocene 1 during the mid holocene about 6 ka bp the african monsoon was stronger and extended farther north than in present day climate associated with differences in orbital parameters affecting insolation earlier research suggests that land surface vegetation interaction and atmosphere ocean coupling may be important to the northward extension of the holocene monsoon rainfall but different climate models produce different results in terms of rainfall distribution this study examines the dynamical mechanisms that modulate the orbital forcing effect on the african monsoon convection a series of simulations with 6 ka bp orbital parameters modified land surface albedo that mimics grassland extending over north africa and alterations of the model dynamics are used it is found that the ventilation mechanism inflow of low moist static energy air from oceanic or poleward regions is a key process in setting the northern rainfall boundary for the african monsoon in particular advection and diffusion of less moist air oppose the increase of moisture that would be required to meet the increased convective instability criteria set by the warmer tropospheric air above the continent while changes in heat fluxes into the atmospheric column favor convection in principle the ventilation mechanism limits the poleward extent of the region that can actually convect

a multi model analysis of the role of the ocean on the african and indian monsoon during the mid holocene we investigate the role of the ocean feedback on the climate in response to insolation forcing during the mid holocene 6 000 year bp using results from seven coupled ocean atmosphere general circulation models we examine how the dipole in late summer sea surface temperature sst anomalies in the tropical atlantic increases the length of the african monsoon how this dipole structure is created and maintained and how the late summer sst warming in the northwest indian ocean affects the monsoon retreat in this sector similar mechanisms are found in all of the models including a strong wind evaporation feedback and changes in the mixed layer depth that enhance the insolation forcing as well as increased ekman transport in the atlantic that sharpens the atlantic dipole pattern we also consider changes in interannual variability over west africa and the indian ocean the teleconnection between variations in sst and sahelian precipitation favor a larger impact of the atlantic dipole mode in this region in the indian ocean the strengthening of the indian dipole structure in autumn has a damping effect on the indian dipole mode at the interannual time scale

new estimates of carbon transfer to terrestrial ecosystems between the last glacial maximum and the holocene ice core records of atmospheric co2 show an 80 ppm rise between the last glacial maximum lgm and the mid holocene during a corresponding world wide expansion of the terrestrial biomass and changes in ocean chemistry therefore the absolute amount of carbon transferred to the atmosphere probably from the oceans remains uncertain to address this issue i evaluated changes in terrestrial ecosystem carbon storage and isotopic fractionations between the lgm and the mid holocene using a process based terrestrial carbon cycle model forced with two general circulation model gcm simulations of each interval the results indicate that global carbon storage in terrestrial ecosystems vegetation and soils increased by 668 gt c during the last glacial interglacial transition a value within the range obtained from a revised global carbon isotope mass balance analysis 550 680 gt c and consistent with independent estimates from the marine isotopic record

the role of oceanic forcing in mid holocene northern hemisphere climatic change previous model investigations suggested that changes in orbital forcing and feedbacks associated with northward expansion of the boreal forest were both required to explain the full magnitude of enhanced high latitude northern hemisphere summer warming at 6 ka thus implying that biospheric feedbacks decreased planetary albedo associated with forest expansion may be large in the future foley et al 1994 tempo testing earth system models with paleo observations 1996 before the magnitude of past biospheric and other feedbacks can be estimated with confidence however the role of realistic high latitude oceanic forcing i e sea surface temperature sst increases and sea ice reductions should also be considered here we review existing paleoceanographic observations that suggest portions of the north atlantic were up to 4 c warmer than today at 6 ka we then combine our estimates of mid holocene north atlantic sst and sea ice conditions with new climate model simulations to suggest that a significant portion of high latitude summer warming at 6 ka in the northern hemisphere can be accounted for by altered orbital forcing ssts and sea ice relative to today our results underscore the importance of incorporating realistic boundary conditions when using paleoenvironmental data to evaluate the climate system s sensitivity to altered forcing and suggest that high latitude oceanic feedbacks were major contributors to enhanced high latitude summer warming in the northern hemisphere at 6 ka

reconstructing biomes from palaeoecological data a general method and its application to european pollen data at 0 and 6 ka biome models allow the results of experiments with atmospheric general circulation models to be translated into global maps of potential natural vegetation the use of biome models as a diagnostic tool for palaeoclimate simulations can yield maps that are directly comparable with palaeoecological pollen and plant macrofossil records provided these records are biomized i e assigned to biomes in a consistent way this article describes a method for the objective biomization of pollen samples based on fuzzy logic pollen types taxa are assigned to one or more plant functional types pfts then affinity scores are calculated for each biome in turn based on its list of characteristic pfts the pollen sample is assigned to the biome to which it has the highest affinity subject to a tie breaking rule modern pollen data from surface samples reflecting present vegetation across europe are used to validate the method pollen data from dated sediment cores are then used to reconstruct european vegetation patterns for 6 ka the reconstruction shows systematic differences from present that are consistent with previous interpretations the method has proved robust with respect to human impacts on vegetation and provides a rational way to interpret combinations of pollen types that do not have present day analogs the method demands minimal prior information and is therefore equally suitable for use in other regions with richer floras and or lower densities of available modern and fossil pollen samples than europe

general circulation model study on the green sahara during the mid holocene an impact of convection originating above boundary layer 1 the mechanism maintaining widespread vegetation over the sahara 6000 years before present 6kbp is investigated with an atmospheric general circulation model gcm focusing on an impact of convection originating above the planetary boundary layer pbl the multiple cloud base arakawa schubert scheme mcb is implemented in the gcm and it is shown that mcb significantly strengthens the impact of vegetation over the sahara on precipitation in this region if it is assumed that the whole sahara was covered by vegetation at 6kbp the total precipitation is sufficient to maintain vegetation over the south and northwest sahara high valued moist static energy air ranges from surface to the middle troposphere over the northern half of intertropical convergence zone itcz in summer because of strong northward moisture transport in the lower troposphere convection originating above the pbl prevails in this region especially when the pbl top is lowered between night and early morning and strengthens the intrusion of the itcz into the sahara then moisture is further transported northward by the african wave disturbances enhancing precipitation over the northwestern sahara

arid and humid phases in southern spain during the last 4000 years the zonar lake record cordoba a multiproxy study of sediment cores from zonar lake 37 29 00 n 4 41 22 w 300 m a s l supported by 11 14c ams dates provides the first high resolution centennial scale reconstruction of past h

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

land surface feedbacks and palaeomonsoons in northern africa we ran a sequence of climate model experiments for 6000 years ago with land surface conditions based on a realistic map of palaeovegetation lakes and wetlands to quantify the effects of land surface feedbacks in the saharan region vegetation induced albedo and moisture flux changes produced year round warming forced the monsoon to 17 25 n two months earlier and shifted the precipitation belt 300 km northwards compared to the effects of orbital forcing alone the addition of lakes and wetlands produced localised changes in evaporation and precipitation but caused no further extension of the monsoon belt diagnostic analyses with biome and continental hydrology models showed that the combined land surface feedbacks although substantial could neither maintain grassland as far north as observed 26 n nor maintain lake megachad 330 000 km

simulations of northern hemisphere ice sheet retreat sensitivity to physical mechanisms involved during the last deglaciation a 3 dimensional thermomechanical ice sheet model is used to simulate the evolution of the geometry of northern hemisphere ice sheets through the last deglaciation the ice sheet model is forced by a time evolving climatology provided by the linear interpolation through time of climate snapshots simulated by the lmd5 3 atmospheric general circulation model agcm at different periods of the last deglaciation 21 15 9 6 and 0 kyr bp the agcm is driven by insolation atmospheric co2 content ice sheet configuration and sea surface temperatures although our approach is able to produce the complete continental ice retreat our simulated deglaciation presents a phase lag with reconstructions based on observational evidences this suggests that physical mechanisms related to climate forcing and or ice sheet internal dynamics are not properly represented the influence of millennial scale forcing feedback mechanisms between ice sheet elevation and surface mass balance and parameterization of the ice flow is also tested through a set of sensitivity experiments the rapid variability has a strong impact on the evolution of the ice volume because of nonlinear effects in temperature mass balance relationships fennoscandia appears to be strongly sensitive to the small scale ice sheet instability both ice sheets are to some extent sensitive to an increased basal sliding

palaeovegetation in china during the late quaternary biome reconstructions based on a global scheme of plant functional types two previous reconstructions of palaeovegetation across the whole of china were performed using a simple classification of plant functional types pfts now a more explicit global pft classification scheme has been developed and a substantial number of additional pollen records have become available here we apply the global scheme of pfts to a comprehensive set of pollen records available from china to test the applicability of the global scheme of pfts in china and to obtain a well founded reconstruction of changing palaeovegetation patterns a total of 806 pollen surface samples 188 mid holocene mh 6000 14c yr bp and 50 last glacial maximum lgm 18 000 14c yr bp pollen records were used to reconstruct vegetation patterns in china based on a new global classification system of pfts and a standard numerical technique for biome assignment biomization the biome reconstruction based on pollen surface samples showed convincing agreement with present potential natural vegetation coherent patterns of change in biome distribution between mh lgm and present are observed in the mh cold and cool temperate evergreen needleleaf forests and mixed forests temperate deciduous broadleaf forest and warm temperate evergreen broadleaf and mixed forest in eastern china were shifted northward by 200 500 km cold deciduous forest in northeastern china was replaced by cold evergreen needleleaf forest while in central northern china cold deciduous forest was present at some sites now occupied by temperate grassland and desert the forest grassland boundary was 200 300 km west of its present position temperate xerophytic shrubland temperate grassland and desert covered a large area on the tibetan plateau but the area of tundra was reduced treeline was 300 500 m higher than present in tibet these changes imply generally warmer winters longer growing seasons and more precipitation during the mh westward shifts of the forest shrubland grassland and grassland desert boundaries imply greater moisture availability in the mh consistent with a stronger summer monsoon during the lgm in contrast cold deciduous forest cool temperate evergreen needleleaf forest cool mixed forests warm temperate evergreen broadleaf and mixed forest in eastern china were displaced to the south by 300 1000 km while temperate deciduous broadleaf forest pure warm temperate evergreen forest tropical semi evergreen and evergreen broadleaf forests were restricted or absent from the mainland of southern china implying colder winters than present strong shifts of temperate xerophytic shrubland temperate grassland and desert to the south and east in northern and western china and on the tibetan plateau imply drier conditions than present

aliphatic hydrocarbons and triterpenes of the congo deep sea fan abstract hydrocarbons were analyzed in sediments from the congo river deep sea fan from the congo river and in sinking particles collected by sediment traps 40 m above the sediment studied sites encompassed three lobes of decreasing age of formation along the canyon sites a f and c and a another lobe system disconnected from the active channel since 4 ka site e terrestrial long chain odd n alkanes were dominant in all sediments of the lobe system unsaturated terpenoids sourced by higher plants such as gammacerene lupene ursene and oleanene were also detected at site c characterized by high accumulation rates 10 20 cm yr 1 the organic matter spends less time in the oxic layer than at other sites and high phytadiene concentrations 10 17 g g oc 1 evidenced recent terrestrial and phytoplanktonic remains reworked in anaerobic conditions in these sediments organic carbon normalized concentrations of terrestrial alkanes and terpenoids were several fold higher than in the lobe sediments with lower accumulation rates sites a and f arguing for a more rapid degradation of terrestrial hydrocarbons than bulk organic carbon in the first steps of pre diagenesis ample variations in the contributions of biomarkers from higher plants ferns bacteria and angiosperms indicate an heterogeneous contribution of the soil and vegetation detritus delivered to the congo lobe sediments lower concentrations in terrestrial hydrocarbons at site e 45 km away from the active canyon indicated that river particles are still admixed to the dominant marine organic matter diploptene and hop 7 21 ene have a dual origin from terrestrial and marine microorganisms scatter in their relationship to gammacerene argues for a contribution of marine microorganisms in addition to soils sourced microorganisms the close distribution patterns of diploptene hop 21 ene hop 7 21 ene and neohop 13 18 ene is in line with the hypothesis of sequential clay catalyzed isomerisation of bacterial hopenes terrestrial biomarker accumulation fluxes at site c are one order of magnitude higher than vertical pelagic flux demonstrating the magnitude of the inputs delivered through turbiditic transport in the submarine canyon crude oil contamination was evidenced at the disconnected site e ucm c 21 to c 26 tricyclic diterpanes cpi and in smaller amounts in some sediments from sites a and c it may be related to marine crude oil extraction and transport a short chain mode of alkanes with an even predominance is evidenced in sediments of the lobe complex and likely sources crude oil microorganisms and ferns are discussed

quantitative reconstructions of annual rainfall in africa 6000 years ago model data comparison precipitation are based on 85 pollen sites 14 c dated at 6000 500 years b p and distributed over the whole of africa to improve the reliability of the pollen based climate reconstruction two methods are used the modern analogues technique mat and the plant functional types pft methods we then conduct a model data comparison for five distinct regions allowing an evaluation of model outputs the sahara sahel the eastern sahara western equatorial africa east africa and madagascar the pollen inferred reconstructions are compared with 21 mid holocene simulations yielded by atmospheric general circulation models agcms and coupled ocean atmosphere vegetation models oavgcms the large scale feature of the hydrological changes is shown to be well captured by most of the models data show that during the mid holocene the sahara was considerably wetter than today 200 to 700 mm yr the results reinforce the conclusion that the agcms significantly underestimate this precipitation increase in the sahara whereas the oavgcm simulations are in accordance with the data our results show that vegetation and ocean feedbacks do not have a strong impact in the intertropical zone and that models fail to properly reproduce the climatic conditions in east africa and madagascar the model data comparison also suggests that the lengthening of the dry season during boreal winter in the west equatorial region is a robust feature although the pollen based reconstruction shows no change or only slight drying there

mid holocene east asian summer climate as simulated by the pmip2 models abstract in the present study datasets derived from twelve coupled ocean atmosphere general circulation models oagcms in the paleoclimate modelling intercomparison project phase two were used to analyze the east asian summer climate during the mid holocene about 6000 calendar years ago on the whole the oagcms reproduced warmer and wetter summer climate conditions in east asia during the mid holocene the multi model ensemble showed that in east asia the regionally averaged summer surface air temperature sat increased by 0 89 c summer precipitation was 5 8 higher and an obviously strengthened southerly wind corresponded to a strong summer monsoon in the mid holocene when compared to preindustrial levels the data model comparison in china reveals a good agreement between the oagcms results and the reconstructed changes in the summer sat in east china during the mid holocene in north china the simulated sat anomalies are 0 5 c lower overall than reconstruction in contrast the oagcms fail to capture the strongest warming in the southern qinghai tibetan plateau on the other hand the simulated precipitation agrees well with proxy data except for in the central parts of china where the simulated summer precipitation disagrees in sign with reconstruction in addition there is a large spread among the simulations particularly over and around the qinghai tibetan plateau and inter model discrepancies are larger for precipitation than for sat as a whole

qualitative assessment of pmip3 rainfall simulations across the eastern african monsoon domains during the mid holocene and the last glacial maximum abstract in this paper we compare a compilation of multiproxy records spanning the eastern african margin with general circulation model simulations of seasonal precipitation fields for the mid holocene and the last glacial maximum lgm carried out as part of the third phase of the paleoclimate modelling intercomparison project pmip3 results show good agreement during the mid holocene the 6k experiment with palaeodata and model outputs correlating well and indicating that changes in insolation drove a stronger northern african monsoon north of 0 5 s during the terminal african humid period and a weaker southeast african monsoon for the lgm the 21k experiment however significant discrepancies exist both between model simulations and between existing palaeodata and simulated conditions both in terms of direction and amplitude of change none of the pmip3 simulations reflect the pattern inferred from the palaeodata two major discrepancies have been identified to explain this 1 the limited sensitivity of the southern monsoon domain to the colder temperatures of the indian ocean 2 c and 2 the absence of changes in the dynamic of the indian ocean walker circulation over the entire basin despite the exposure of the sahul and sunda shelves that weakened convection over the indo pacific warm pool during the lgm these results indicate that some major features of the atmospheric and oceanic teleconnections between the different monsoon regions require further consideration as models evolve

origin and distribution of terrestrial organic matter in the nw mediterranean gulf of lions exploring the newly developed bit index the branched and isoprenoid tetraether bit index is based on the relative abundance of nonisoprenoidal glycerol dialkyl glycerol tetraethers gdgts derived from organisms living in terrestrial environments versus a structurally related isoprenoid gdgt crenarchaeol produced by marine crenarchaeota the bit index varies between 0 and 1 representing marine and terrestrial organic matter om end members respectively hopmans et al earth planet sci lett 224 107 116 2004 in this study the applicability of the bit index to trace terrestrial om is tested in combination with other organic parameters toc c n ratio 13corg total lipid and n alkane in the gulf of lions a river dominated continental margin of the western mediterranean we analyzed a variety of soils and riverbed sediments from the continent as well as surface sediments from the shelf and canyons the bit index in soils and riverbed sediments shows high values 0 9 while it varies between 0 02 and 0 83 in marine sediments decreasing seaward from the inner shelf to the slope for marine surface sediments high bit values are associated with lower 13corg values as well as higher toc contents and higher n alkane concentrations our results confirm that the bit index can be applied in coastal marine environments in order to characterize terrestrial om as proposed by hopmans et al 2004 therefore the bit index is a useful addition to the proxies presently available for studying the origin and distribution of om in continental margins and especially valuable in multiproxy studies

ice age paleotopography a gravitationally self consistent theory of postglacial relative sea level change is used to infer the variation of surface ice and water cover since the last glacial maximum lgm the results show that lgm ice volume was approximately 35 percent lower than suggested by the climap reconstruction and the maximum heights of the main laurentian and fennoscandian ice complexes are inferred to have been commensurately lower with respect to sea level use of these ice age boundary conditions in atmospheric general circulation models will yield climates that differ significantly from those previously inferred on the basis of the climap data set

a novel proxy for terrestrial organic matter in sediments based on branched and isoprenoid tetraether lipids we propose a novel tracer for terrestrial organic carbon in sediments based on the analysis of tetraether lipids using high performance liquid chromatography mass spectrometry hplc ms analysis of terrestrial soil and peats shows that branched tetraether lipids are predominant in terrestrial environments in contrast to crenarchaeol the characteristic membrane lipid of non thermophilic crenarchaeota which is especially abundant in the marine and lacustrine environment based on these findings an index was developed the so called branched and isoprenoid tetraether bit index based on the relative abundance of terrestrially derived tetraether lipids versus crenarchaeol this bit index was applied to surface sediments from the angola basin where it was shown to trace the outflow of the congo river furthermore analyses of particulate organic matter from the north sea showed relatively higher bit indices in water column particulate organic matter near large river inputs a survey of globally distributed marine and lacustrine surface sediments shows that the bit index in these environments correlates with the relative fluvial input of terrestrial organic material making this index generally applicable the new proxy allows the rapid assessment of the fluvial input of terrestrial organic material in immature sediments up to 100 ma old

progress in paleoclimate modeling this paper briefly surveys areas of paleoclimate modeling notable for recent progress new ideas including hypotheses giving a pivotal role to sea ice have revitalized the low order models used to simulate the time evolution of glacial cycles through the pleistocene a prohibitive length of time for comprehensive general circulation models gcms in a recent breakthrough however gcms have succeeded in simulating the onset of glaciations this occurs at times most recently 115 kyr b p when high northern latitudes are cold enough to maintain a snow cover and tropical latitudes are warm enhancing the moisture source more generally the improvement in models has allowed simulations of key periods such as the last glacial maximum and the mid holocene that compare more favorably and in more detail with paleoproxy data these models now simulate enso cycles and some of them have been shown to reproduce the reduction of enso activity observed in the early to middle holocene modeling studies have demonstrated that the reduction is a response to the altered orbital configuration at that time an urgent challenge for paleoclimate modeling is to explain and to simulate the abrupt changes observed during glacial epochs i e dansgaard oescher cycles heinrich events and the younger dryas efforts have begun to simulate the last millennium over this time the forcing due to orbital variations is less important than the radiance changes due to volcanic eruptions and variations in solar output simulations of these natural variations test the models relied on for future climate change projections they provide better estimates of the internal and naturally forced variability at centennial time scales elucidating how unusual the recent global temperature trends are

the last forests on antarctica reconstructing flora and temperature from the neogene sirius group transantarctic mountains abstract fossil bearing deposits in the transantarctic mountains antarctica indicate that despite the cold nature of the continent s climate a tundra ecosystem grew during periods of ice sheet retreat in the mid to late neogene 17 2 5 ma 480 km from the south pole to date palaeotemperature reconstruction has been based only on biological ranges thereby calling for a geochemical approach to understanding continental climate and environment there is contradictory evidence in the fossil record as to whether this flora was mixed angiosperm conifer vegetation or whether by this point conifers had disappeared from the continent in order to address these questions we have analysed for the first time in sediments of this age plant and bacterial biomarkers in terrestrial sediments from the transantarctic mountains to reconstruct past temperature and vegetation during a period of east antarctic ice sheet retreat from tetraether lipids mbt cbt palaeothermometer we conclude that the mean continental summer temperature was ca 5 c in agreement with previous reconstructions this was warm enough to have allowed woody vegetation to survive and reproduce even during the austral winter biomarkers from vascular plants indicate a low diversity and spatially variable flora consisting of higher plants moss and algal mats growing in microenvironments in a glacial outwash system abietane type compounds were abundant in some samples indicating that conifers most likely podocarpaceae grew on the antarctic continent well into the neogene this is supported by the palynological record but not the macrofossil record for the continent and has implications for the evolution of vegetation on antarctica

role of the biosphere in the mid holocene climate of west africa received 26 june 2001 revised 2 december 2001 accepted 5 march 2002 published 16 january 2003 1 in previous studies a zonally symmetric synchronously coupled biosphereatmosphere model zonalbam which includes explicit representation of ecosystem dynamics has been developed and validated based on current conditions over the region of west africa here we use zonalbam to study the response of the coupled biosphereatmosphere system to changes in the earth s orbital forcing during the middle holocene 6k yrs bp and the relative contribution of vegetation feedbacks simulations in which vegetation conditions were fixed to the current distribution show that an orbitally induced increased seasonality in insolation for the middle holocene by itself results in a 1 1 northward shift in the location of the southern margin of the sahara as compared to current solar forcings when vegetation is allowed to be dynamic a 2 4 northward shift is simulated however when dynamic vegetation is initialized to palaeovegetation a 5 1 northward shift is simulated bringing results more consistent with palaeoevidence based on previous studies on the role of the gradient of moist static energy on the dynamics of large scale tropical circulations a mechanism for the enhancement of the summer monsoon circulation has been developed our results suggest that multiple equilibria could have coexisted over the region of west africa during the middle holocene furthermore based on previous studies on the current climate over the region we hypothesize that transitions between the different equilibria could have taken place during the middle holocene causing the southern desert margin to migrate between 18 1n and 21 4n and shaping climate variability index terms 0315 atmospheric composition and structure biosphere atmosphere interactions 3322 meteorology and atmospheric dynamics land atmosphere interactions 3344 meteorology and atmospheric dynamics paleoclimatology 9305 information related to geographic region africa keywords west africa holocene vegetation dynamics monsoon climate change insolation

what have we learnt from palaeoclimate simulations there has been a gradual evolution in the way that palaeoclimate modelling and palaeoenvironmental data are used together to understand how the earth system works from an initial and largely descriptive phase through explicit hypothesis testing to diagnosis of underlying mechanisms analyses of past climate states are now regarded as integral to the evaluation of climate models and have become part of the toolkit used to assess the likely realism of future projections palaeoclimate assessment has demonstrated that changes in large scale features of climate that are governed by the energy and water balance show consistent responses to changes in forcing in different climate states and these consistent responses are reproduced by climate models however state of the art models are still largely unable to reproduce observed changes in climate at a regional scale reliably while palaeoclimate analyses of state of the art climate models suggest an urgent need for model improvement much work is also needed on extending and improving palaeoclimate reconstructions and quantifying and reducing both numerical and interpretative uncertainties

sources and distributions of tetraether lipids in surface sediments across a large river dominated continental margin glycerol dialkyl glycerol tetraether gdgt based proxies are increasingly used in modern carbon cycling and palaeoenvironmental investigations it is therefore crucial to examine the robustness sources transport and degradation of all gdgt based proxies in continental margins where sedimentation rates and extent of carbon cycling are high we have analyzed the distributions of gdgts in surface sediments from the lower yangtze river and east china sea ecs shelf the results revealed multiple sources and complex shelf processes that govern the distributions the isoprenoid gdgt inferred sea surface temperatures ssts are robust and reflect the satellite derived annual mean ssts on the shallow ecs shelf confirming an origin from surface water column dwelling crenarchaeota the input from methanogen sourced isoprenoid gdgts is significant in the river surface sediments but they are almost absent from the ecs shelf branched gdgts are also abundant in the river sediments but ca 95 are degraded in the yangtze estuary a much greater extent than observed for other terrigenous organic matter om proxies there is also evidence for production of branched gdgts in the oxic ecs shelf water column and the anoxic sediments waters of the lower yangtze river as a result branched gdgt based proxies in the lower river and ecs surface sediments do not reflect the catchment environmental conditions the effective degradation in the estuary and widespread aquatic contributions of branched gdgts improves our understanding of how to use branched gdgt based proxies in marginal seas

an evaluation of the simulated water balance of eurasia and northern africa at 6000 y bp using lake status data changes in the water balance of eurasia and northern africa in response to insolation forcing at 6000 y bp simulated by five atmospheric general circulation models have been compared with observations of changes in lake status all of the simulations show enhancement of the asian summer monsoon and of the high pressure cells over the pacific and central asia and the middle east causing wetter conditions in northern india and southern china and drier conditions along the chinese coast and west of the monsoon core all of the models show enhancement of the african monsoon causing wetter conditions in the zone between ca 10 20 n four of the models show conditions wetter than present in southern europe and drier than present in northern europe three of the models show conditions similar to present in the mid latitude continental interior while the remaining models show conditions somewhat drier than present the extent and location of each of the simulated changes varies between the models as does the mechanism producing these changes the lake data confirm some features of the simulations but indicate discrepancies between observed and simulated climates for example the data show 1 conditions wetter than present in central asia from india to northern china and mongolia indicating that the simulated asian monsoon expansion is too small 2 conditions wetter than present between ca 10 30 n in africa indicating that the simulated african monsoon expansion is too small 3 that northern europe was drier but the area of significantly drier conditions was more localized around the baltic than shown in the simulations 4 that southern europe was wetter than present apparently consistent with the simulations but pollen data suggest that this reflects an increase in summer rainfall whereas the models show winter precipitation and 5 that the mid latitude continental interior was generally wetter than present

mapping eastern north american vegetation change of the past 18 ka no analogs and the future the method of modern analogs and an extensive data base of modern and fossil pollen data were used to generate a new series of paleovegetation maps for eastern north america spanning the past 18 ka the maps illustrate the continuous nature of climate induced vegetation change and the development after about 10 ka of modern regional vegetation patterns before the holocene vegetation biomes without modern analogs were widespread in response to climate conditions without modern analogs and to a lesser extent to the rapidity of climate change over the last glacial interglacial transition this geological perspective suggests that possible future climate changes could force similarly complex changes in natural vegetation including the development of biomes without modern analogs

constraints on the application of the mbt cbt palaeothermometer at high latitude environments svalbard norway abstract branched glycerol dialkyl glycerol tetraethers gdgts are membrane lipids of unknown bacteria that are ubiquitous in soil and peat two indices based on the distribution of these lipids in soils the cyclization of branched tetraethers cbt and the methylation of branched tetraethers mbt indices have been shown to correlate with soil ph and mean annual air temperature mat and soil ph respectively and can be used to reconstruct mat in palaeoenvironments to verify the extent to which branched gdgts in marine sediments reflect the distribution pattern on land and whether these proxies are applicable for palaeoclimate reconstruction in high latitude environments with a mat of

viscoelastic deformation of lunar impact basins implications for heterogeneity in the deep crustal paleo thermal state and radioactive element concentration 1 diverse geological characteristics found for the three major lunar provinces i e the feldspathic highlands terrane fht the south pole aitken terrane spat and the procerallum kreep terrane pkt strongly suggest their distinctly different thermal histories quantitative differences among these provinces in their early thermal histories and crustal radioactive element concentrations however are highly unknown one of the few observables that retain a record of the ancient lunar thermal structure is the viscoelastic state of impact basins this study investigates the long term evolution of basin structures using global lunar gravity field data obtained by kaguya tracking and derives constraints for 1 the paleo thermal state of impact basins and for 2 crustal column averaged radioactive element concentrations for each province our calculation results indicate that impact basins in the central anorthositic region of the fht i e the fht an require a very cold interior dt dr 20 k km 1 on the surface this result strongly suggests that the deep portion of the thick farside highlands crust is highly depleted in radioactive elements th 0 5 ppm indicating that the th rich spa basin floor crust is clearly different from the lower crust underneath the fht an and cannot be accounted for by simple exposure of the lower crust our analysis also indicates that the observed basin structure allows as high as 6 ppm of column averaged th concentration in the crust inside the pkt these results indicate that radioactive element concentrations deep in the crust probably vary greatly region by region similarly to those observed on the surface

soil feedback drives the mid holocene north african monsoon northward in fully coupled ccsm2 simulations with a dynamic vegetation model we explore climate vegetation interactions in mid holocene north africa with a suite of community climate system model ccsm2 simulations the ccsm includes synchronously coupled atmosphere ocean sea ice land and vegetation models the ccsm s present day precipitation for north africa compares well with simulations of other models and observations mid holocene data reveal a wetter and greener sahara compared to the present the ccsm exhibits a greater closer to the expected precipitation increase than other models and in response grasses advance from 18 75 to 22 5 n in much of north africa precipitation is enhanced locally by the northward advance of grasses but suppressed regionally mainly due to an insufficient albedo decrease with the expansion of vegetation prior studies have always lowered the surface albedo with the expansion of vegetation in north africa in the ccsm s mid holocene simulations the albedo decreases more because wetter soils are simulated darker than drier soils than due to expanding vegetation these results isolate albedo as the key ingredient in obtaining a positive precipitation vegetation feedback in north africa two additional simulations support this conclusion in the first simulation the desert s sandy soil textures are changed to loam to represent increased organic matter soil water retention and grass cover increase albedo decreases somewhat precipitation responds with a small yet widespread increase in the second simulation a darker soil color is prescribed for this region now the monsoon advances north about 4 these results illustrate a north african monsoon highly sensitive to changes in surface albedo and less sensitive to changes in evapotranspiration

thermoluminescence and new 14c age estimates for late quaternary loesses in southwestern nebraska abstract loess of late quaternary age mantles most of nebraska south of the platte river valley at least five late quaternary loesses are recognized from oldest to youngest one or more undifferentiated pre illinoian loesses the loveland loess the gilman canyon loess which exhibits a well developed soil and rests unconformably on the sangamon soil the peoria loess capped by the brady soil and the bignell loess which is distributed discontinuously previous research shows that the loveland loess is illinoian the gilman canyon loess and peoria loess are wisconsin and the bignell loess is holocene we present here the first thermoluminescence tl age estimates and new 14 c ages for these late quaternary loesses at two key sections in southwestern nebraska the eustis ash pit and the bignell hill road cut tl age estimates from all samples collected from eustis ash pit and bignell hill were internally consistent tl and 14 c age estimates from these two sections generally agree and support previous age determinations the tl age estimate on loveland loess indicates deposition at 163 ka tl and radiocarbon age estimates indicate that gilman canyon loess believed to be deposited during the farmdale interstade first began to accumulate at about 40 ka the lower part of the gilman canyon loess is 36 ka at eustis and the middle of the unit is 30 ka at bignell hill the lower and upper parts of the peoria loess give age estimates of 24 ka and 17 ka respectively tl age estimates for deposition of the bignell loess are 9 ka near the base in agreement with radiocarbon age estimates and 6 ka immediately below the modern soil substantiating its holocene age comparisons of tl age estimates with 18 o and insolation curves which show loess deposition during interglacial and interstadial as well as glacial periods indicate that loess deposition on the great plains can occur under a variety of climatic conditions

tracking sedimentation from the historic a d 2011 mississippi river flood in the deltaic wetlands of louisiana usa management and restoration of the mississippi river deltaic plain southern united states and associated wetlands require a quantitative understanding of sediment delivery during large flood events past and present here we investigate the sedimentary fingerprint of the 2011 mississippi river flood across the louisiana coast atchafalaya delta terrebonne barataria and mississippi river delta basins to assess spatial patterns of sedimentation and to identify key indicators of sediment provenance the sediment deposited in wetlands during the 2011 flood was distinguished from earlier deposits based on biological characteristics primarily absence of plant roots and increased presence of centric planktonic diatoms indicative of riverine origin by comparison the lithological bulk density organic matter content and grain size and chemical stable carbon isotopes of bulk organic matter properties of flood sediments were nearly identical to the underlying deposit flood sediment deposition was greatest in wetlands near the atchafalaya and mississippi rivers and accounted for a substantial portion 37 to 85 of the annual accretion measured at nearby monitoring stations the amount of sediment delivered to those basins 1 1 1 6 g cm 2 was comparable to that reported previously for hurricane sedimentation along the louisiana coast 0 8 2 1 g cm 2 our findings not only provide insight into how large scale river floods influence wetland sedimentation they lay the groundwork for identifying previous flood events in the stratigraphic record

branched and isoprenoid tetraether bit index traces water content along two marsh soil transects surrounding lake qinghai implications for paleo humidity variation abstract the bit index represents the relative abundances of branched glycerol dialkyl glycerol tetraethers bgdgts and the isoprenoid gdgt crenarchaeol while bgdgts are produced mainly by soil anaerobic bacteria crenarchaeol is known to be a biomarker for aerobic ammonia oxidation by chemolithoautotrophic thaumarchaeota particularly in the open ocean or lakes thus the index in marine and lacustrine settings has been widely used as a proxy for soil input here we have investigated gdgt distribution along two transects extending from the lake shore marsh to upland soils on the ne qinghai tibetan plateau the results show that soil water content swc correlates positively with concentration of bgdgts and negatively with concentration of crenarchaeol consequently there is a significant positive correlation between swc and the bit index our study highlights a new potential application of bit as a humidity proxy in loess soil and peat deposits

mid holocene climates of the americas a dynamical response to changed seasonality simulations of the climatic response to mid holocene 6 ka bp orbital forcing with two coupled ocean atmosphere models foam and csm show enhancement of monsoonal precipitation in parts of the american southwest central america and northernmost south america during northern hemisphere summer the enhanced onshore flow that brings precipitation into central america is caused by a northward displacement of the inter tropical convergence zone driven by cooling of the equatorial and warming of the northern subtropical and mid latitude ocean ocean feedbacks also enhance precipitation over the american southwest although the increase in monsoon precipitation there is largely driven by increases in land surface temperature the northward shift in the equatorial precipitation band that causes enhanced precipitation in central america and the american southwest has a negative feedback effect on monsoonal precipitation in northern south america the simulations demonstrate that mid holocene aridity in the mid continent of north america is dynamically linked to the orbitally induced enhancement of the summer monsoon in the american southwest with a spatial structure wet in the southwest and dry in the mid continent similar to that found in strong monsoon years today changes in winter precipitation along the west coast of north america in central america and along the gulf coast caused by southward displacement of the westerly storm tracks indicate that changes in the northern hemisphere winter monsoon also play a role in regional climate changes during the mid holocene although the simulations with foam and csm differ in detail the general mechanisms and patterns are common to both the model results thus provide a coherent dynamical explanation for regional patterns of increased or decreased aridity shown by vegetation lake status and aeolian data from the americas

regional energy budget control of the intertropical convergence zone and application to mid holocene rainfall shifts in the latitude of the intertropical convergence zone a region of intense tropical rainfall have often been explained through changes in the atmospheric energy budget specifically through theories that tie rainfall to meridional energy fluxes these quantitative theories can explain shifts in the zonal mean but often have limited relevance for regional climate shifts such as a period of enhanced precipitation over saharan africa during the mid holocene here we present a theory for regional tropical rainfall shifts that utilizes both zonal and meridional energy fluxes we first identify a qualitative link between zonal and meridional energy fluxes and rainfall variations associated with the seasonal cycle and the el nino southern oscillation we then develop a quantitative theory based on these fluxes that relates atmospheric energy transport to tropical rainfall when applied to the orbital configuration of the mid holocene our theory predicts continental rainfall shifts over africa and southeast asia that are consistent with complex model simulations however the predicted rainfall over the sahara is not sufficient to sustain vegetation at a level seen in the palaeo record which instead requires an additional large energy source such as that due to reductions in saharan surface albedo we thus conclude that additional feedbacks such as those involving changes in vegetation or soil type are required to explain changes in rainfall over africa during the mid holocene shifts in the latitude of the tropical rainfall band are constrained by meridional energy fluxes calculations show that combining zonal and meridional energy fluxes can explain past regional rainfall variations like the african humid period

hydrologic response of the crow wing watershed minnesota to mid holocene climate change in this study we have integrated a suite of holocene paleoclimatic proxies with mathematical modeling in an attempt to obtain a comprehensive picture of how watersheds respond to past climate change a three dimensional surface water groundwater model was developed to assess the effects of mid holocene climate change on water resources within the crow wing watershed upper mississippi basin in north central minnesota the model was first calibrated to a 50 yr historical record of average annual surface water discharge monthly ground water levels and lake level fluctuations the model was able to reproduce reasonably well long term historical records 1949 1999 of water table and lake level fluctuations across the watershed as well as stream discharge near the watershed outlet the calibrated model was then used to reproduce paleo groundwater and lake levels using climate reconstructions based on pollen transfer functions from williams lake just outside the watershed computed declines in mid holocene lake levels for two lakes at opposite ends of the watershed were between 6 and 18 m simulated streamflow near the outlet of the watershed decreased to 70 of modern average annual discharge after 200 yr the area covered by wetlands for the entire watershed was reduced by 16 the mid holocene hydrologic changes indicated by these model results and corroborated by several lake core records across the crow wing watershed may serve as a useful proxy of the hydrologic response to future warm dry climatic forecasts ca 2050 made by some atmospheric general circulation models for the glaciated midwestern united states

comparison of north american pollen based temperature and global lake status with cccma agcm2 output at 6 ka abstract we present new paleoclimate reconstructions for the time period of 6000 yr bp 6 ka that permit a quantitative comparison to corresponding mapped patterns of the newest version of the canadian cccma agcm2 climate model july temperature reconstructions were estimated using the modern analog technique employing fossil pollen records from the north american pollen database we present a new method of optimizing critical choices within the application of the mat through modern calibration the pattern of 6 ka temperature anomalies generated by the cccma agcm2 generally agree with those of the pollen based reconstruction particularly in the presence of a band of positive temperature anomalies across the continent from the yukon through the great lakes region and along the st lawrence this agreement suggests that both the model and data are reconstructing a contraction in the polar vortex over north america at 6 ka major areas of disagreement between the pollen based reconstruction and model simulation are also areas where the data are sparse and edge effects produce less reliable reconstructions the cccma agcm2 simulates drier conditions in a west east band stretching from alaska through the canadian prairies and midwest united states in agreement with low lake level observations at 6 ka simulated and reconstructed precipitation evaporation are also in agreement over the southwest united states with the exception of north central africa the cccma agcm2 fails to capture the northward and eastern extension of the afro asian monsoon belt at 6 ka general agreement between cccma agcm2 p e anomalies and lake status change is found throughout europe at 6 ka

mid holocene hydrologic model of the shingobee watershed minnesota abstract a hydrologic model of the shingobee watershed in north central minnesota was developed to reconstruct mid holocene paleo lake levels for williams lake a surface water body located in the southern portion of the watershed hydrologic parameters for the model were first estimated in a calibration exercise using a 9 yr historical record 1990 1998 of climatic and hydrologic stresses the model reproduced observed temporal and spatial trends in surface groundwater levels across the watershed mid holocene aquifer and lake levels were then reconstructed using two paleoclimatic data sets ccm1 atmospheric general circulation model output and pollen transfer functions using sediment core data from williams lake calculated paleo lake levels based on pollen derived paleoclimatic reconstructions indicated a 3 5 m drop in simulated lake levels and were in good agreement with the position of mid holocene beach sands observed in a williams lake sediment core transect however calculated paleolake levels based on ccm1 climate forcing produced only a 0 05 m drop in lake levels we found that decreases in winter precipitation rather than temperature increases had the largest effect on simulated mid holocene lake levels the study illustrates how watershed models can be used to critically evaluate paleoclimatic reconstructions by integrating geologic climatic limnologic and hydrogeologic data sets

dust sources and deposition during the last glacial maximum and current climate a comparison of model results with paleodata from ice cores and marine sediments mineral dust aerosols in the atmosphere have the potential to affect the global climate by influencing the radiative balance of the atmosphere and the supply of micronutrients to the ocean ice and marine sediment cores indicate that dust deposition from the atmosphere was at some locations 2 20 times greater during glacial periods raising the possibility that mineral aerosols might have contributed to climate change on glacial interglacial time scales to address this question we have used linked terrestrial biosphere dust source and atmospheric transport models to simulate the dust cycle in the atmosphere for current and last glacial maximum lgm climates we obtain a 2 5 fold higher dust loading in the entire atmosphere and a twenty fold higher loading in high latitudes in lgm relative to present comparisons to a compilation of atmospheric dust deposition flux estimates for lgm and present in marine sediment and ice cores show that the simulated flux ratios are broadly in agreement with observations differences suggest where further improvements in the simple dust model could be made the simulated increase in high latitude dustiness depends on the expansion of unvegetated areas especially in the high latitudes and in central asia caused by a combination of increased aridity and low atmospheric co2 the existence of these dust source areas at the lgm is supported by pollen data and loess distribution in the northern continents these results point to a role for vegetation feedbacks including climate effects and physiological effects of low co2 in modulating the atmospheric distribution of dust

possible role of atmosphere biosphere interactions in triggering the last glaciation we coupled a global biome model iteratively with an atmospheric general circulation model to study the possible role of vegetation in the climate system at the time of glacial inception 115 000 years ago orbital forcing alone was not sufficient to initiate glaciation when other components of the climate system were kept as present atmospheric composition oceans biosphere and cryosphere summers were however cold enough to induce major vegetation shifts in high northern latitudes southward migration of the boreal forest tundra limit helped to create favourable conditions for continental ice sheet growth with increasing snow depth and duration in labrador arctic canada and northern western fennoscandia these results support a role for biogeophysical feedback in initiating glaciations

a high resolution record of holocene rainfall variations from the western chinese loess plateau antiphase behaviour of the african indian and east asian summer monsoons tropical monsoons are key mechanisms for transfer of heat and moisture to higher latitudes here we present a high resolution terrestrial proxy summer monsoon record for the southeast asian monsoon from a rapidly accumulating holocene loess soil sequence in the western chinese loess plateau we use magnetic and clastic grain size proxies to make quantitative estimates of holocene rainfall and identify variations in winter monsoon intensity our record reveals cyclical millennial and multimillennial rainfall changes as with the northwest african southwest asian monsoon records a short arid interval at 12 5 to 11 5 ka bp the younger dryas and subsequent summer monsoon intensification are recorded however at 6 ka bp the southeast asian summer monsoon weakened when the northwest african southwest asian monsoons strengthened and then from 5 ka bp intensified when northwest africa southwest asia became dry these antiphase monsoonal relationships may reflect competition between sea surface temperature changes and solar forcing the intensity of the southeast asian winter monsoon intensified from 9 ka bp onwards varying in its phase relationships with summer monsoon intensity after 2 25 ka bp extreme climatic instability is indicated by both climate proxies

simulated changes in extreme temperature and precipitation events at 6 ka abstract we have employed a high resolution regional climate model rcm to test the sensitivity of extreme climate events to 6 ka orbital forcing using western north america as a case study calculated differences 6 ka control in annual extreme precipitation event frequency were mostly positive throughout the rcm domain as were differences in extreme event contribution to total annual precipitation these annual mean differences were driven by changes in the seasonal distribution of extreme precipitation events at 6 ka which was in turn linked to changes in the daily variability of upper level zonal wind speeds over the northeast pacific positive differences in the frequency of extreme maximum daily temperature values occurred inland in the rcm domain with peak differences of 28 days year over the great basin likewise days in which the maximum daily temperature exceeded 32 c were 24 more frequent in the 6 ka integration than in the control integration and heat waves lasted up to 12 days longer mean first and last freeze dates were generally earlier in the 6 ka integration along the pacific coast and over the southern great basin and later over the northern great basin and interior of the pacific northwest we propose that by exerting controls on paleoclimate archives unique from changes in the mean climate state such changes in the frequency and duration of extreme climate events could have played a substantial role in shaping the mid holocene paleoclimate record of the region and that extreme climate events may have been similarly important in other regions and during other time periods

stable carbon isotopes as potential sea level indicators in salt marshes north carolina usa we compared the use of 13c values and c n ratios from salt marsh sediments to reconstruct relative sea level rsl with an established approach using foraminifera analysis of bulk organic sediment and plant samples collected along transects at two sites in north carolina usa demonstrates that sediment 13c values can be used to distinguish between spartina alterniflora dominated low marsh c photosynthetic pathway 13c values from 17 6 to 16 1 and juncus roemerianus dominated high marsh c 3 photosynthetic pathway 13c values from 28 2 to 21 8 environments juncus roemerianus plants undergo little decompositional change in 13c average 0 8 although there is a clear difference between spartina alterniflora tissue and bulk organic sediments approximately 4 c n ratios on bulk organic sediment from freshwater upland and salt marsh environments converge during early diagenesis rendering them of little use in reconstructing rsl the utility of 13c values as a sea level indicator is limited by the elevational range of c4 plants making it difficult to recognize salt marsh subenvironments and improve the precision of rsl reconstructions furthermore juncus roemerianus dominated high marsh and freshwater upland sediments cannot be adequately distinguished with 13c values

late quaternary environmental change of yellow river basin an organic geochemical record in bohai sea north china abstract bulk geochemical characterization total organic carbon grain size distribution carbon isotope composition and molecular biomarkers lignin phenols straight chain aliphatic hydrocarbons glycerol dialkyl glycerol tetraethers were analyzed for a 21 m core from the bohai sea north china spanning ca 21 ka bp these paleo proxies presented remarkable differences between the late glacial period and the holocene reflecting continental and coastal environments respectively two peat layers were deposited during the period of ca 9000 8460 yr bp thereafter the core site has been consistently covered by seawater until recent reclamation of land from the sea the occurrence of a total organic carbon maximum from ca 6000 3800 yr bp was attributed to delivery of organic carbon enriched sediments via the yellow river consistent with increased vegetation density and higher development of soil under warm and humid mid holocene climate conditions the distributions of lignin phenol compositions and c 31 c 29 n alkane ratio suggested the largest expansion of woody plants between ca 5300 and 4000 yr bp corresponding to the extremely favorable climatic conditions since ca 3800 yr bp an abrupt increase in the c 31 c 29 n alkane ratio suggested higher abundance of grasses consistent with a drying climate trend after the mid holocene since our coastal sediments close to the yellow river outflow contain catchment integrated environmental signals of the river basin molecular proxies demonstrate that the variability of vegetation distributions in the holocene is a widespread phenomenon in those areas adjacent to yellow river basin

distributions of branched gdgts in a tropical lake system implications for lacustrine application of the mbt cbt paleoproxy branched glycerol dialkyl glycerol tetraether gdgt lipids are abundant and ubiquitous in lake sediments potentially allowing for a paleolimnological application of the so called mbt cbt proxy methylation index of branched tetraethers cyclization ratio of branched tetraethers to investigate the origin and characteristics of these compounds in lacustrine environments we examined the distributions of gdgts in soils river sediments and lake sediments from lake towuti on the island of sulawesi indonesia we found significant differences in the degree of methylation and cyclization expressed by way of the mbt and cbt indices between the soil samples and the aquatic samples suggesting that there may be in situ production of gdgts in the aquatic environment based on these findings we urge caution in the application of the mbt cbt paleoproxy to lake sediments and encourage more rigorous study of these compounds in freshwater environments

the application of δ13c toc and c n geochemistry to reconstruct holocene relative sea levels and paleoenvironments in the thames estuary uk we examined the use of d 13 c toc and c n geochemistry of sedimentary organic matter to reconstruct former sea levels and paleoenvironments in the absence of suitable microfossil data the modern distribution of d 13 c toc and c n of 33 vegetation and 74 surface sediment samples collected from four coastal wetlands in the thames estuary and norfolk uk are described the d 13 c toc and c n geochemistry of sediments varied in relation to the input of in situ vascular vegetation versus allochthonous particulate organic matter and algae which was controlled primarily by tidal inundation we reviewed published and unpublished studies to produce an english database of vegetation n 257 and sediment n 132 d 13 c toc and c n geochemistry four elevation dependent environments in the database had statistically distinct d 13 c toc and c n values 1 tidal flat low marsh d 13 c 24 9 1 2 toc 3 6 1 7 c n 9 9 0 8 2 middle marsh high d 13 c 26 2 1 0 toc 9 8 6 7 c n 12 1 1 8 3 reed swamp d 13 c 27 9 0 7 toc 36 5 11 5 c n 13 9 1 2 and 4 fen carr d 13 c 29 0 0 6 toc 41 6 5 7 c n 17 4 3 1 the d 13 c toc and c n geochemistry database was applied to a holocene sediment core collected from the thames estuary to produce three new sea level index points and one limiting date illustrating the utility of d 13 c toc and c n values to reconstruct holocene relative sea levels copyright 2015 john wiley sons ltd

simulated climate and biomes of africa during the late quaternary comparison with pollen and lake status data new compilations of african pollen and lake data are compared with climate ccm1 ncar boulder and vegetation biome 1 2 gsg lund simulations for the last glacial maximum lgm and early to mid holocene emh the simulated lgm climate was ca 4 c colder and drier than present with maximum reduction in precipitation in semi arid regions biome simulations show lowering of montane vegetation belts and expansion of southern xerophytic associations but no change in the distribution of deserts and tropical rain forests the lakes show lgm conditions similar or drier than present throughout northern and tropical africa pollen data indicate lowering of montane vegetation belts the stability of the sahara and a reduction of rain forest the paleoenvironmental data are consistent with the simulated changes in temperature and moisture budgets although they suggest the climate model underestimates equatorial aridity emh simulations show temperatures slightly less than present and increased monsoonal precipitation in the eastern sahara and east africa biome simulations show an upward shift of montane vegetation belts fragmentation of xerophytic vegetation in southern africa and a major northward shift of the southern margin of the eastern sahara the lakes indicate conditions wetter than present across northern africa pollen data show an upward shift of the montane forests the northward shift of the southern margin of the sahara and a major extension of tropical rain forest the lake and pollen data confirm monsoon expansion in eastern africa but the climate model fails to simulate the wet conditions in western africa

the response of northern hemisphere extratropical climate and vegetation to orbitally induced changes in insolation during the last interglaciation abstract the last interglaciation substage 5e provides an opportunity to examine the effects of extreme orbital changes on regional climates we have made two atmospheric general circulation model experiments p t approximated the northern hemisphere seasonality maximum near the beginning of 5e p t approximated the minimum near the end of 5e simulated regional climate changes have been translated into biome changes using a physiologically based model of global vegetation types major climatic and vegetational changes were simulated for the northern hemisphere extratropics due to radiational effects that were both amplified and modified by atmospheric circulation changes and sea ice feedback p t showed mid continental summers up to 8 c warmer than present mid latitude winters were 2 4 c cooler than present but in the arctic summer warmth reduced sea ice extent and thickness producing winters 2 8 c warmer than present the tundra and taiga biomes were displaced poleward while warm summer steppes expanded in the mid latitudes due to drought p t showed summers up to 5 c cooler than present especially in mid latitudes sea ice and snowpack were thicker and lasted longer polar desert tundra and taiga biomes were displaced equatorward while cool summer steppes and semideserts expanded due to the cooling a slight winter warming in mid latitudes however caused warm temperate evergreen forests and scrub to expand poleward such qualitative contrasts in the direction of climate and vegetation change during 5e should be identifiable in the paleorecord

how well can we simulate past climates evaluating the models using global palaeoenvironmental datasets abstract global syntheses of palaeoenvironmental data are required to test climate models under conditions different from the present data sets for this purpose contain data from spatially extensive networks of sites the data are either directly comparable to model output or readily interpretable in terms of modelled climate variables data sets must contain sufficient documentation to distinguish between raw primary and interpreted secondary tertiary data to evaluate the assumptions involved in interpretation of the data to exercise quality control and to select data appropriate for specific goals four data bases for the late quaternary documenting changes in lake levels since 30 14 c kyr bp the global lake status data base vegetation distribution at 18 14 c kyr and 6 14 c kyr bp biome 6000 aeolian accumulation rates during the last glacial interglacial cycle dirtmap and tropical terrestrial climates at the last glacial maximum the lgm tropical terrestrial data synthesis are summarised each has been used to evaluate simulations of last glacial maximum lgm 21 calendar kyr bp and or mid holocene 6 cal kyr bp environments comparisons have demonstrated that changes in radiative forcing and orography due to orbital and ice sheet variations explain the first order broad scale in space and time features of global climate change since the lgm however atmospheric models forced by 6 cal kyr bp orbital changes with unchanged surface conditions fail to capture quantitative aspects of the observed climate including the greatly increased magnitude and northward shift of the african monsoon during the early to mid holocene similarly comparisons with palaeoenvironmental datasets show that atmospheric models have underestimated the magnitude of cooling and drying of much of the land surface at the lgm the inclusion of feedbacks due to changes in ocean and land surface conditions at both times and atmospheric dust loading at the lgm appears to be required in order to produce a better simulation of these past climates the development of earth system models incorporating the dynamic interactions among ocean atmosphere and vegetation is therefore mandated by quaternary science results as well as climatological principles for greatest scientific benefit this development must be paralleled by continued advances in palaeodata analysis and synthesis which in turn will help to define questions that call for new focused data collection efforts

orbital suppression of wind driven upwelling in the california current at 6 ka 1 marine records clearly show activity in the california current cc system to have been altered during the mid holocene however the specific nature of cc activity during this period remains ambiguous as proxies that suggest cooler than present sea surface temperatures co occur in mid holocene marine sediments with multiple proxies that suggest decreased coastal upwelling to test the sensitivity of wind driven upwelling in the cc to mid holocene orbital forcing we have employed a high resolution regional climate model rcm the seasonality of modern cc wind driven coastal upwelling simulated by the rcm is in strong agreement with observational data further we show that changes in the seasonality of solar insolation induced by mid holocene milankovitch forcing decrease early and peak season coastal upwelling in the cc along with increasing late season coastal upwelling these results are in agreement with several marine and terrestrial proxies suggesting that relative to present cc activity during the mid holocene was characterized by a longer and less vigorous upwelling season with decreased seasonal contrast

reconciling divergent trends and millennial variations in holocene temperatures cooling during most of the past two millennia has been widely recognized and has been inferred to be the dominant global temperature trend of the past 11 700 years the holocene epoch however long term cooling has been difficult to reconcile with global forcing and climate models consistently simulate long term warming the divergence between simulations and reconstructions emerges primarily for northern mid latitudes for which pronounced cooling has been inferred from marine and coastal records using multiple approaches here we show that temperatures reconstructed from sub fossil pollen from 642 sites across north america and europe closely match simulations and that long term warming not cooling defined the holocene until around 2 000 years ago the reconstructions indicate that evidence of long term cooling was limited to north atlantic records early holocene temperatures on the continents were more than two degrees celsius below those of the past two millennia consistent with the simulated effects of remnant ice sheets in the climate model community climate system model 3 ccsm3 ccsm3 simulates increases in growing degree days a measure of the accumulated warmth above five degrees celsius per year of more than 300 kelvin days over the holocene consistent with inferences from the pollen data it also simulates a decrease in mean summer temperatures of more than two degrees celsius which correlates with reconstructed marine trends and highlights the potential importance of the different subseasonal sensitivities of the records despite the differing trends pollen and marine based reconstructions are correlated at millennial to centennial scales probably in response to ice sheet and meltwater dynamics and to stochastic dynamics similar to the temperature variations produced by ccsm3 although our results depend on a single source of palaeoclimatic data pollen and a single climate model simulation they reinforce the notion that climate models can adequately simulate climates for periods other than the present day they also demonstrate that amplified warming in recent decades increased temperatures above the mean of any century during the past 11 000 years

sources and distributions of branched and isoprenoid tetraether lipids on the amazon shelf and fan implications for the use of gdgt based proxies in marine sediments branched glycerol dialkyl glycerol tetraethers brgdgts in river fan sediments have been used successfully to reconstruct mean annual air temperature maat and soil ph of the congo river drainage basin however in a previous study of amazon deep sea fan sediments the reconstructed maats were ca 10 c colder than the actual maat of the amazon basin in this study we investigated this apparent offset by comparing the concentrations and distributions of brgdgts in amazon river suspended particulate matter spm and sediments to those in marine spm and surface sediments the riverine brgdgt input was evident from the elevated brgdgt concentrations in marine spm and surface sediments close to the river mouth the distributions of brgdgts in marine spm and sediments varied widely but generally showed a higher relative abundance of methylated and cyclic brgdgts than those in the river since this difference in brgdgt distribution was also found in intact polar lipid ipl derived brgdgts which were more recently produced the change in the marine brgdgt distribution was most likely due to marine in situ production consequently the maats calculated based on the methylation of branched tetraethers mbt and the cyclisation of branched tetraethers cbt were lower and the cbt derived ph values were higher than those of the amazon basin however spm and sediments from stations close to the river mouth still showed mbt cbt values that were similar to those of the river therefore we recommend caution when applying the mbt cbt proxy it should only be used in sediment cores that were under high river influence the influence of riverine derived isoprenoid gdgt isogdgt on the isogdgt based tex86 temperature proxy was also examined in marine spm and sediments an input of riverine isogdgts from the amazon river was apparent but its influence on the marine tex86 was minor since the tex86 of spm in the amazon river was similar to that in the marine spm and sediments

impact of river channel shifts on tetraether lipids in the rhone prodelta nw mediterranean implication for the bit index as an indicator of palaeoflood events we tested the applicability of the bit branched and isoprenoid tetraether index as a proxy for palaeoflood events in the river dominated continental margin of the gulf of lions nw mediterranean we compared the concentrations of branched glycerol dialkyl glycerol tetraethers br gdgts and crenarchaeol in suspended particulate matter spm collected downstream in the rhone river as well as in surface sediments and a ca 8m piston core from the rhone prodelta the core covered the last 400 yr with four distinct intervals recording the river influence under natural and man induced shifts in four main channels of the river mouth bras de fer grand rhone pegoulier and roustan the results indicate that there are mixed sources of br gdgts and crenarchaeol in the prodelta complicating applicationof the bit index as an indicator of continental organic carbon input and thus as a palaeoflood proxy however the sedimentary bit record for the period when continental material was delivered by the river more directly to the core site roustan phase 1892 to present mimics the historical palaeoflood record this shows the potential of the bit index as a palaeoflood proxy provided that the delivery route of the continental material by rivers to the core sites remains constant over time the study also highlights the idea that shifts in river channels should be taken into account for the use of the bit index as a palaeoflood proxy

the past is a guide to the future comparing middle pliocene vegetation with predicted biome distributions for the twenty first century during the middle pliocene the earth experienced greater global warmth compared with today coupled with higher atmospheric co 2 concentrations to determine the extent to which the middle pliocene can be used as a test bed for future warming we compare data and model based middle pliocene vegetation with simulated global biome distributions for the mid and late twenty first century the best agreement is found when a middle pliocene biome reconstruction is compared with a future scenario using 560 ppmv atmospheric co 2 in accordance with palaeobotanical data all model simulations indicate a generally warmer and wetter climate resulting in a northward shift of the taiga tundra boundary and a spread of tropical savannahs and woodland in africa and australia at the expense of deserts our data model comparison reveals differences in the distribution of polar vegetation which indicate that the high latitudes during the middle pliocene were still warmer than its predicted modern analogue by several degrees however our future scenarios do not consider multipliers associated with long term climate sensitivity changes in global temperature and thus biome distributions at higher atmospheric co 2 levels will not have reached an equilibrium state as is the case for the middle pliocene by the end of this century

organic matter distribution in the modern sediments of the pearl river estuary we determined biomarker concentrations and distributions for surface sediments from 54 sites in the pearl river estuary china we focus on a suite of four biomarker based indicators for relative terrestrial to marine organic matter om source the branched isoprenoid tetraether bit index the ratio of high low molecular weight n alcohols c26 34 c16 18 c26 34 an analogous ratio for n fatty acids and the c29 steroids c29 steroids brassicasterol ratio all four exhibit the same terrestrial to marine transition seen in previous bulk 13c studies but with an abrupt decrease in the relative terrestrial contribution across the delta front to pro delta transition concentrations of terrestrially derived biomarkers show no systematic decrease across the transition instead the decrease in the proportion of terrestrial om is due to a decrease in the sedimentation rate and associated terrestrial om burial across the delta toe this suggests that diagenetic controls on the fate of terrestrial om such as increased biodegradation where sedimentation rate is low are subordinate to sedimentological processes biomarker derived temperature values are cooler than expected for the lower pearl river catchment suggesting that the dominant component of the terrestrial om is derived from the cooler upland regions of the catchment the dominance of input from more distal terrain with greater topographic relief is evidence for the importance of geomorphological control on terrigenous om transport collectively the results demonstrate the importance of sedimentological processes in the supply deposition and transport of terrestrial om

benthic ostracode δ13c as sensor for early holocene establishment of modern circulation patterns in central europe abstract shells from adult specimen of the benthic ostracodes limnocytherina sanctipatricii and leucocythere mirabilis selected from a 8 7 m long piston core provide continuous stable oxygen and carbon records for the past approximately 16 ka oxygen isotopes from both species show identical values and track the general north atlantic and european temperature history since deglaciation in great detail values of ostracode 18o values suggest that about 16 cal ka the average annual air temperatures were about 11 c colder than today carbon isotopic values from both species of ostracodes are similar during the lateglacial and early holocene and show an overall decrease from 4 to 7 that is probably related to an increase in photosynthetic productivity in the water column as suggested by an increase in organic carbon delivering 13c depleted organic matter to the bottom waters carbon pump about 9 cal ka only l mirabilis 13c values decreased about 2 5 within 300 years higher 13c variability and ecological evidence suggests that l mirabilis represents a summer signal whereas l sanctipatricii displays a more subdued annual average after about 7 cal ka another 1 5 decrease for both species accompanied by an increase in magnetic susceptibility a decrease in carbonate content and more positive bulk carbonate isotope values followed suggesting higher detrital clastic input into the lake in order to provide a possible mechanism explaining the negative l mirabilis 13c values sediment pore water profiles of o2 and ch4 in short cores collected from sites distal to proximal to the alpine rhine river delta were inspected sediments in cores from more proximal sites to the rhine delta become anoxic at shallower sediment depth due to the decay of high allochthonous organic carbon input to the sediment which greatly increases concentrations of methane in pore waters closer to the rhine inflow when methane is oxidized close to the sediment water interface 13c depleted carbon is added to pore water dic that is then available for incorporation into ostracode shells this mechanism suggests that about 9 cal ka the oxygen supply to the bottom waters especially in summer decreased this stimulated methanogenesis close to the sediment water interface and provided 13c depleted carbon to benthic dwellers independent evidence for methanogenesis is provided by the increase in concentration of tetrahymanol after about 9 cal ka coincident with the decrease in 13c of l mirabilis we suggest that about 9 cal ka the northward retreat of the northern hemisphere ice sheets and consequently the polar front left the alpine region affected by a more oceanic climate characterized by warmer winters as they occur today especially during the positive north atlantic oscillation index phase more frequently incomplete mixing of the water column may have shifted the decay of organic matter faster to anaerobic conditions in surficial sediments especially during summer by about 7 cal ka the north atlantic region had probably warmed sufficiently to increase precipitation in central europe and consequently detrital clastic runoff to lake constance

drastic changes in the distribution of branched tetraether lipids in suspended matter and sediments from the yenisei river and kara sea siberia implications for the use of brgdgt based proxies in coastal marine sediments abstract the distribution of branched glycerol dialkyl glycerol tetraethers brgdgts in soils has been shown to correlate with ph and mean annual air temperature because of this dependence brgdgts have found an application as palaeoclimate proxies in coastal marine sediments based on the assumption that their distribution is not altered during the transport from soils to marine systems by rivers to study the processes acting on the brgdgt distributions we analysed the full suite of brgdgts including the recently described 6 me brgdgts in both the suspended particulate matter spm of the siberian yenisei river and the spm and sediments of its outflow in the kara sea the brgdgt distribution in the spm of the yenisei river was fairly constant and characterized by high abundances of the 6 me brgdgts reflecting their production at the neutral ph of the river water however the brgdgt distribution showed marked shifts in the marine system firstly in the yenisei river mouth the fractional abundance of the 6 me brgdgts decreases sharply the brgdgt signature in the yenisei river mouth possibly reflects brgdgts delivered during the spring floods that may carry a different distribution also coastal cliffs were shown to contain brgdgts and to influence especially those sites without major river inputs e g khalmyer bay further removed from the river mouth in situ production of brgdgts in the marine system influences the distribution however also the fractional abundance of the tetramethylated brgdgt ia increases resulting in a distribution that is distinct from in situ produced signals at similar latitudes svalbard we suggest that this shift may be caused by preferential degradation of labile riverine in situ produced brgdgts and the subsequent enrichment in less labile soil material the offshore distribution indeed agrees with the brgdgt distribution encountered in a lowland peat this implies that the offshore kara sea sediments possibly carry a soil dominated signal indicating potential for palaeoclimate reconstructions at this site both in the river system and coastal cliffs brgdgts were much more abundant than crenarchaeol an archaeal isoprenoid gdgt resulting in high 0 93 branched and isoprenoid tetraether bit index values moving downstream in the marine sediments a decrease in brgdgt concentrations coeval with an increase in crenarchaeol resulted in decreasing bit index values this decrease correlates with changes in bulk proxies for terrigenous input 13corg c n confirming the use of the bit index to trace the delivery of river transported and coastal cliff derived terrigenous organic matter

tracing soil organic carbon in the lower amazon river and its tributaries using gdgt distributions and bulk organic matter properties abstract in order to trace the transport of soil organic carbon oc in the lower amazon basin we investigated the distributions of crenarchaeol and branched glycerol dialkyl glycerol tetraethers gdgts by analyzing riverbed sediments and river suspended particulate matter spm collected in the solimoes amazon river mainstem and its tributaries the branched and isoprenoid tetraether bit index a proxy for river transported soil oc into the ocean was determined from the distributions of these gdgts the gdgt derived parameters were compared with other bulk geochemical data i e c n ratio and stable carbon isotopic composition the gdgt derived and bulk geochemical data indicate that riverine spm and riverbed sediments in the lower amazon river and its tributaries are a mixture of c3 plant derived soil oc and aquatic derived oc the branched gdgts in the spm and riverbed sediments did not predominantly originate from the high andes soils 2500 m in altitude as was suggested previously however further constraint on the soil source area of branched gdgts was hampered due to the deficiency of soil data from the lower montane forest areas in the andes our study also revealed seasonal and interannual variation in gdgt composition as well as soil oc discharge which was closely related to the hydrological cycle by way of a simple binary mixing model using the flux weighted bit values at obidos the last gauging station in the amazon river we estimated that 70 80 of the poc pool in the river was derived of soil oc however care should be taken to use the bit index since it showed a non conservative behaviour along the river continuum due to the aquatic production of crenarchaeol further investigation using a continuous sampling strategy following the full hydrological cycle is required to fully understand how soil derived gdgt signals are transformed in large tropical river systems through their transport pathway to the ocean

quantifying the role of biosphere atmosphere feedbacks in climate change coupled model simulations for 6000 years bp and comparison with palaeodata for northern eurasia and northern africa the lmd agcm was iteratively coupled to the global biome1 model in order to explore the role of vegetation climate interactions in response to mid holocene 6000 y bp orbital forcing the sea surface temperature and sea ice distribution used were present day and co2 concentration was pre industrial the land surface was initially prescribed with present day vegetation initial climate anomalies differences between agcm results for 6000 y bp and control were used to drive biome1 the simulated vegetation was provided to a further agcm run and so on results after five iterations were compared to the initial results in order to identify vegetation feedbacks these were centred on regions showing strong initial responses the orbitally induced high latitude summer warming and the intensification and extension of northern hemisphere tropical monsoons were both amplified by vegetation feedbacks vegetation feedbacks were smaller than the initial orbital effects for most regions and seasons but in west africa the summer precipitation increase more than doubled in response to changes in vegetation in the last iteration global tundra area was reduced by 25 and the southern limit of the sahara desert was shifted 2 5 n north to 18 n relative to today these results were compared with 6000 y bp observational data recording forest tundra boundary changes in northern eurasia and savana desert boundary changes in northern africa although the inclusion of vegetation feedbacks improved the qualitative agreement between the model results and the data the simulated changes were still insufficient perhaps due to the lack of ocean surface feedbacks

holocene temperature trends in the extratropical northern hemisphere based on inter model comparisons large uncertainties exist in holocene climate estimates especially for the early holocene when large scale reorganization occurred in the climate system to improve our understanding of these uncertainties we compare four holocene simulations performed with the loveclim ccsm3 hadcm3 and famous climate models the simulations are generally consistent for the large scale northern hemisphere extratropics while the multi simulation consistencies are heterogeneous on the sub continental scale consistently simulated temperature trends are found in greenland northern canada north eastern and north western europe and central west siberia these holocene temperatures show a pattern of an early holocene warming mid holocene warmth and gradual decrease towards the pre industrial in winter and the extent of early holocene warming varies spatially with 9 c warming in northern canada compared with 3 c warming in central west siberia in contrast mismatched temperatures are detected in alaska the warm early holocene winter in loveclim primarily results from strongly enhanced southerly winds induced by the ice sheets in eastern siberia the intense early holocene summer warmth anomaly in ccsm3 is caused by large negative albedo anomalies due to overestimated snow cover at 0 ka in the arctic cool winter conditons in famous can be attributed to extensive sea ice coverage probably due to simplified sea ice representations thus the holocene temperature trends in these regions remain inconclusive

mechanistic controls on diverse fates of terrestrial organic components in the east china sea terrestrial carbon transferred from the land to sea is a critical component of the global carbon cycle a range of geochemical proxies has been developed to fingerprint the fate of terrestrial organic matter tom in marine sediments however discrepancies among different proxies limit our ability to quantify and interpret the terrestrial signals in marine sediments with consequences for the investigation of both the modern carbon cycle and past environmental change to mechanistically understand these discrepancies we examined the distributions of a range of terrestrial proxies and their aquatic counterparts i e marine proxies in the yangtze river east china sea yr ecs shelf system where tom experiences extensive modification during transport and burial tom proxies in the yr ecs system collectively fit a power law model but with distinct attenuation rates the a values for individual molecular proxy groups among a range of tom proxies the modeled a values decrease in the order soil marker bhps triterpenols lignin hmw n alkanols branched gdgts hmw n alkanes for biomarkers and rsoil bit tomiso for proxies tracing tom rapid loss of tom components through dissociation in the narrow estuary followed by oxidation over the wide open shelf are best described by power curves inherent chemical reactivity i e the number of functional groups responses to hydraulic sorting and in situ production regulate the individual attenuation rates of them chemical reactivity plays the most important role on proxy behavior supported by a strong correlation between a values and standard molal gibbs energies both physical protection and chemical reactivity fundamentally control the overall behavior of tom components with the relative importance being setting dependant the former is relatively important in the estuary whereas the later is the primary control over the open shelf moreover regional variation of different marine counterparts is also significant over the river ecs shelf system seemingly regulated by regional nutrient distributions therefore for tom estimates using molecular ratio approaches the specific behavior of individual terrestrial components and marine counterparts and the physical biological and chemical characteristics of depositional settings all need to be considered

high resolution holocene record in the southeastern bay of biscay global versus regional climate signals abstract core ks10b north east atlantic 43 50 n 2 03 w water depth 550 m was recovered at a location of high sediment deposition and hence includes a high resolution palaeoclimatic record for the southeastern bay of biscay over the last 9000 cal yr bp the effect of global and regional climatic forcing factors e g orbital forcing north atlantic oscillation nao and their subsequent control on water temperature sea level hydrology and continental influence were investigated by studying planktic and benthic foraminiferal faunas 150 m sedimentology and stable isotopes results indicate probable episodic long term incursions of the warm and salty iberian poleward current ipc into the bay of biscay during the holocene by the intermittent presence of subtropical globigerinoides ruber in the otherwise temperate fauna these incursions seem to be triggered by negative nao like conditions our data show five main climatic periods which are in general agreement with the literature data on the climatic variability in the north atlantic and the iberian peninsula the early holocene 9000 7400 cal yr bp is characterized by low sea level significant downslope transport eutrophic benthic settings and high surface water productivity these environmental characteristics are probably due to the general wet and warm climate under a prevailing negative nao increasing precipitation and river runoff and favouring the incursion of the ipc into the bay of biscay the climatic optimum appears between 7400 and 6000 cal yr bp under a generally positive nao index in the following 6000 3500 cal yr bp surface water cooling is matching a steep decline of the northern hemisphere summer insolation meanwhile benthic settings are changing progressively towards present day trophic conditions the time interval from 3500 to 1800 cal yr bp encompasses the iberian roman humid period characterized by warm and humid conditions and increasing primary production under a prevailing negative nao index the last 1800 years were characterized by stable cool conditions reflecting a change towards a positive nao state

climate of east africa 6000 14c yr b p as inferred from pollen data this paper presents a spatial reconstruction of climate in east africa at 6000 14c yr b p two different approaches using pollen data have been used the standard best modern analogues method and the new plant functional type method based on groups of pollen taxa both methods have been applied to 32 fossil pollen spectra dated at 6000 14c yr b p for each site we have estimated two climatic parameters annual precipitation and mean annual temperature three bioclimatic parameters ratio of actual to potential evapotranspiration and the mean temperature of the coldest and warmest months results show that the temperature lapse rate was less steep at 6000 14c yr b p than it is today the climate was wetter north of 3 s precipitation anomalies 50 mm yr and slightly drier farther south precipitation anomalies 50 mm yr the ratio of actual to potential evapotranspiration also indicates conditions similar to or wetter than today north of 3 s anomalies 10 the climate was warmer than today by 2 c north of the equator and was cooler southward 1 c although both methods indicate similar climatic patterns the plant functional type method provides more consistent results

mid to late holocene climate change an overview the last 6000 years are of particular interest to the understanding of the earth system because the boundary conditions of the climate system did not change dramatically in comparison to larger glacial interglacial changes and because abundant detailed regional palaeoclimatic proxy records cover this period we use selected proxy based reconstructions of different climate variables together with state of the art time series of natural forcings orbital variations solar activity variations large tropical volcanic eruptions land cover and greenhouse gases underpinned by results from general circulation models gcms and earth system models of intermediate complexity emics to establish a comprehensive explanatory framework for climate changes from the mid holocene mh to pre industrial time the redistribution of solar energy due to orbital forcing on a millennial timescale was the cause of a progressive southward shift of the northern hemisphere nh summer position of the intertropical convergence zone itcz this was accompanied by a pronounced weakening of the monsoon systems in africa and asia and increasing dryness and desertification on both continents the associated summertime cooling of the nh combined with changing temperature gradients in the world oceans likely led to an increasing amplitude of the el nin o southern oscillation enso and possibly increasingly negative north atlantic oscillation nao indices up to the beginning of the last millennium on decadal to multi century timescales a worldwide coincidence between solar irradiance minima tropical volcanic eruptions and decadal to multi century scale cooling events was not found however reconstructions show that widespread decadal to multi century scale cooling events accompanied by advances of mountain glaciers occurred in the nh e g in scandinavia and the european alps this occurred namely during the little ice age lia between ad w1350 and 1850 when the lower summer insolation in the nh due to orbital forcing coincided with solar activity minima and several strong tropical volcanic eruptions the role of orbital forcing in the nh cooling the southward itcz shift and the desertifi cation of the sahara are supported by numerous model simulations other simulations have suggested that the fingerprint of solar activity variations should be strongest in the tropics but there is also evidence that changes in the ocean heat transport took place during the lia at high northern latitudes with possible additional implications for climates of the southern hemisphere sh

intercomparison of simulated global vegetation distributions in response to 6 kyr bp orbital forcing abstract the response of ten atmospheric general circulation models to orbital forcing at 6 kyr bp has been investigated using the biome model which predicts equilibrium vegetation distribution as a diagnostic several common features emerge a reduced tropical rain forest as a consequence of increased aridity in the equatorial zone b expansion of moisture demanding vegetation in the old world subtropics as a consequence of the expansion of the afro asian monsoon c an increase in warm grass shrub in the northern hemisphere continental interiors in response to warming and enhanced aridity and d a northward shift in the tundra forest boundary in response to a warmer growing season at high northern latitudes these broadscale features are consistent from model to model but there are differences in their expression at a regional scale vegetation changes associated with monsoon enhancement and high latitude summer warming are consistent with palaeoenvironmental observations but the simulated sh

the influence of continental ice sheets on the climate of an ice age the climate influence of the land ice that existed 18 000 years before present 18k b p is investigated by use of a general circulation model of the atmosphere coupled with a static mixed layer ocean simulated climates are obtained from two versions of the model one with the land ice distribution of the present and the other with that of 18k b p in the northern hemisphere the tropospheric flow field is strongly influenced by the laurentide ice sheet and features a split flow straddling the ice sheet with a strong jet stream forming the southern branch the northern branch of the flow brings very cold air over the north atlantic ocean where thick sea ice is maintained the distribution of sea surface temperature sst difference between the two experiments in the northern hemisphere resembles the difference between the sst at 18k b p and at present as estimated by the climap project 1981 the 18k b p ice sheets have very little influence upon atmospheric temperature and sst in the southern hemisphere this is because the interhemispheric heat transport hardly changes as the loss of heat energy due to the reflection of solar radiation by continental ice sheets in the northern hemisphere is almost completely counterbalanced by the in situ reduction of upward terrestrial radiation hydrologic changes in the model climate are also found with statistically significant decreases in soil moisture occurring in a zone located to the south of the ice sheets in north america and eurasia these findings are consistent with some geological evidence of regionally drier climates from the last glacial maximum

climate in northern eurasia 6000 years ago reconstructed from pollen data using a climatic calibration based on the scores of the plant functional types pfts calculated for 1245 surface pollen spectra the climate at 6 ka bp has been reconstructed for a set of 116 pollen spectra from the former soviet union and mongolia the results are presented as maps of climatic anomalies and maps of probability classes showing the significance of these differences from the modern climate the reconstructed patterns are spatially coherent but have confidence levels that vary from region to region due to the often large error ranges at 6 ka the winters were more than 2oc warmer than today north of 50on with a high significance east of the urals summers were also more than 2oc warmer than today with a high level of confidence north of the polar circle and in central mongolia in the mid latitudes of siberia in northern kazakhstan and around the black and the caspian seas 6 ka summers were significantly cooler than today the reconstructed moisture availability ratio of actual to equilibrium evapotranspiration was more than 10 higher than today in the ukraine southern russia and northern mongolia and more than 10 lower than today in central mongolia this pattern corresponds partly with that of the water budget annual precipitation minus evaporation reconstructed from lake level records 1999 elsevier science b v all rights reserved

evaluation of coupled ocean atmosphere simulations of the mid holocene using palaeovegetation data from the northern hemisphere extratropics we have used the biome4 biogeography biochemistry model and comparison with palaeovegetation data to evaluate the response of six ocean atmosphere general circulation models to mid holocene changes in orbital forcing in the mid to high latitudes of the northern hemisphere all the models produce a a northward shift of the northern limit of boreal forest in response to simulated summer warming in high latitudes the northward shift is markedly asymmetric with larger shifts in eurasia than in north america b an expansion of xerophytic vegetation in mid continental north america and eurasia in response to increased temperatures during the growing season c a northward expansion of temperate forests in eastern north america in response to simulated winter warming the northward shift of the northern limit of boreal forest and the northward expansion of temperate forests in north america are supported by palaeovegetation data the expansion of xerophytic vegetation in mid continental north america is consistent with palaeodata although the extent may be over estimated the simulated expansion of xerophytic vegetation in eurasia is not supported by the data analysis of an asynchronous coupling of one model to an equilibrium vegetation model suggests vegetation feedback exacerbates this mid continental drying and produces conditions more unlike the observations not all features of the simulations are robust some models produce winter warming over europe while others produce winter cooling as a result some models show a northward shift of temperate forests consistent with though less marked than the expansion shown by data and others produce a reduction in temperate forests elucidation of the cause of such differences is a focus of the current phase of the palaeoclimate modelling intercomparison project

comparison of modeled and reconstructed changes in forest cover through the past 8000 years eurasian perspective reproducing the tree cover changes throughout the holocene is a challenge for land surface atmosphere models here results of a transient holocene simulation of the coupled climate carbon cycle model climber2 lpj driven by changes in orbital forcing are compared with pollen data and pollen based reconstructions for several regions of eurasia in terms of changes in tree fraction the decline in tr ee fraction in the high latitudes suggested by data and model simulations is driven by a decrease in summer temperature over the holocene the cooler and drier tr end at the eastern side of the eurasian continent in mongolia and china also led to a decrease in tree cover in both model and data in contrast the holocene trend towards a cooler climate in the continental interior kazakhstan is accompanied by an increase in woody cover ther e a relatively small reduction in precipitation was likely compensated by lower evapotranspiration in comparison to the monsoon affected regions in general the model data comparison demonstrates that climate driven changes during the holocene result in a non homogeneous pattern of tree cover change across the eurasian continent for the eifel region in germany the model suggests a relatively moist and cool climate and dense tree cover the holzmaar pollen r ecord agrees with the model for the intervals 8 3 ka and 1 7 1 3 ka bp but suggests great reduction of the tree cover 3 2 ka and after 1 3 ka bp when highly developed settlements and agriculture spread in the region

signatures of tetraether lipids reveal anthropogenic overprinting of natural organic matter in sediments of the thames estuary uk intertidal foreshore sediments from a 110 km stretch of the thames estuary were analysed for glycerol dialkyl glycerol tetraethers gdgts to track soil organic matter om input and evaluate the impact of coastal urbanisation on their distribution concentration of branched br gdgts ranged from 1 to 15 g g organic carbon oc and crenarchaeol ranged from 0 6 to 19 g g oc an overall decrease in brgdgts was observed from the inner thames brentford to the outer thames isle of grain suggesting a drop in soil om input towards the sea in contrast crenarchaeol concentration was highest around east london rather than towards the open sea such elevated crenarchaeol concentration occurred in the section of the river most influenced by anthropogenic pollution such as discharge points for london s major sewage treatment plants docks and power plants the non systematic spatial distribution of crenarchaeol was also reflected in the branched isoprenoid tetraether bit index the highest bit values occurred upstream and in close proximity to salt marshes 0 8 1 whereas the lowest values 0 3 0 5 were towards the sea however unusually low values 0 4 were observed in the river section that had high crenarchaeol concentration in contrast bulk 13c values were insensitive to london s anthropogenic influence this suggests that the natural systematic decrease in bit index in the estuary is overprinted by london s anthropogenic activity between deptford creek and tilbury we therefore advise caution when interpreting the bit index for sediments in close proximity to megacities discharging industrial and municipal waste that can become incorporated into the near surface sedimentary record

transient simulations of holocene atmospheric carbon dioxide and terrestrial carbon since the last glacial maximum 1 conflicting hypotheses are investigated for the observed atmospheric co2 increase of 20 ppm between 8 ka bp and pre industrial time the carbon component of the bern carbon cycle climate bern cc model which couples the lund potsdam jena dynamic global vegetation model to an atmosphere ocean sediment component is driven by climate fields from time slice simulations of the past 21 ka with the hadley centre unified model or the ncar climate system model the entire holocene ice core record of co2 is matched within a few ppm for the standard model setup and results are broadly consistent with proxy data of atmospheric 13 co2 mean ocean d 13 c and pollen data within their uncertainties our analysis suggests that a range of mechanisms including calcite compensation in response to earlier terrestrial uptake terrestrial carbon uptake and release sst changes and coral reef buildup contributed to the 20 ppm rise the deep sea d 13 c record constrains the contribution of the calcite compensation mechanism to 4 10 ppm terrestrial carbon inventory changes related to climate and co2 forcing the greening of the sahara peat buildup and land use have probably influenced atmospheric co2 by a few ppm only the early holocene co2 decrease is quantitatively explained by terrestrial uptake and calcite compensation in response to terrestrial uptake during the glacial interglacial transition the recent hypothesis by ruddiman 2003 that anthropogenic land use caused a 40 ppm co2 anomaly over the past 8 ka preventing the climate system from entering a new glacial would imply an anthropogenic emission of 700 gtc and a decrease in atmospheric d 13 co f 0 6 permil this is not compatible with the ice core d 13 c record and would require an upward revision of land use emission estimates by a factor of 3 to 4 index terms 0322 atmospheric composition and structure constituent sources and sinks 0315 atmospheric composition and structure biosphere atmosphere interactions 1615 global change biogeochemical processes 4805 1610 global change atmosphere 0315 0325 keywords carbon cycle modeling carbon dioxide holocene

dirtmap the geological record of dust atmospheric dust is an important feedback in the climate system potentially affecting the radiative balance and chemical composition of the atmosphere and providing nutrients to terrestrial and marine ecosystems yet the potential impact of dust on the climate system both in the anthropogenically disturbed future and the naturally varying past remains to be quantified the geologic record of dust provides the opportunity to test earth system models designed to simulate dust records of dust can be obtained from ice cores marine sediments and terrestrial loess deposits although rarely unequivocal these records document a variety of processes source transport and deposition in the dust cycle stored in each archive as changes in clay mineralogy isotopes grain size and concentration of terrigenous materials although the extraction of information from each type of archive is slightly different the basic controls on these dust indicators are the same changes in the dust flux and particle size might be controlled by a combination of a source area extent b dust emission efficiency wind speed and atmospheric transport c atmospheric residence time of dust and or d relative contributions of dry settling and rainout of dust similarly changes in mineralogy reflect a source area mineralogy and weathering and b shifts in atmospheric transport the combination of these geological data with process based forward modelling schemes in global earth system models provides an excellent means of achieving a comprehensive picture of the global pattern of dust accumulation rates their controlling mechanisms and how those mechanisms may vary regionally the dust indicators and records of terrestrial and marine palaeoenvironments dirtmap data base has been established to provide a global palaeoenvironmental data set that can be used to validate earth system model simulations of the dust cycle over the past 150 000 years

evolution and variability of the asian monsoon system state of the art and outstanding issues the asian monsoon is comprised of the indian and east asian subsystems these two components are linked to one another in varying degrees by regions of strong sensible heating indo asian landmass and strong latent heat export the western pacific warm pool and the southern subtropical indian ocean variability within the indian and east asian subsystems interactions among them and the extent to which they interact with other climate phenomena e g enso are current topics of modern and paleoclimate research this work provides an overview of past and current paleomonsoon research on tectonic to interannual time scales with a primary focus on marine sediment records while past and current work has contributed greatly to our understanding of paleomonsoon variability at all time scales additional efforts are required to make further progress in two critical areas 1 additional efforts are needed in terms of proxy development and evaluation requiring concerted efforts at long term sediment trap deployments in key monsoon influenced regions as well as development of adequate and widely available core top databases these are necessary to assess the impact of modern oceanographic and seafloor processes on potential monsoon proxies 2 additional effort is also needed in acquiring a sufficient geographic distribution of downcore records to assess linkages among the subsystems and their role in the context of extratropical climate change these records should have high sedimentation rates including varved sections this requires substantial survey support to identify the most appropriate coring and drilling targets c 2004 elsevier ltd all rights reserved