weakening and strengthening of the indian monsoon during heinrich events and dansgaard oeschger oscillations the dansgaard oeschger oscillations and heinrich events described in north atlantic sediments and greenland ice are expressed in the climate of the tropics for example as documented in arabian sea sediments given the strength of this teleconnection we seek to reconstruct its range of environmental impacts we present geochemical and sedimentological data from core so130 289kl from the indus submarine slope spanning the last similar to 80 kyr elemental and grain size analyses consistently indicate that interstadials are characterized by an increased contribution of fluvial suspension from the indus river in contrast stadials are characterized by an increased contribution of aeolian dust from the arabian peninsula decadal scale shifts at climate transitions such as onsets of interstadials were coeval with changes in productivity related proxies heinrich events stand out as especially dry and dusty events indicating a dramatically weakened indian summer monsoon potentially increased winter monsoon circulation and increased aridity on the arabian peninsula this finding is consistent with other paleoclimate evidence for continental aridity in the northern tropics during these events our results strengthen the evidence that circum north atlantic temperature variations translate to hydrological shifts in the tropics with major impacts on regional environmental conditions such as rainfall river discharge aeolian dust transport and ocean margin anoxia key points intensity of indian monsoon is traced with geochemical and grain size analyses of sediments fluvial versus aeolian sediment input to arabian sea mimics do oscillations during heinrich events the indian monsoon weakened distinctly

early holocene climate recorded in geomorphological features in western tibet cosmic ray exposure ages for formation of perched alluvial terraces and for abandonment of an alluvial debrisflow fan on opposite sides of the tangtse valley the outflow at the northwest end of lake panggong which is in the karakorum range of western tibet provide evidence of a humid period at v11 5 to v7 ka this is consistent with other regional records and supports a controversial chronology for the sedimentary record from lake panggong fan abandonment appears to have occurred at v11 5 ka as the climate presumably became more humid in response to initiation of enhanced monsoonal circulation consistent with previously reported onset of humid conditions in a sedimentary record from the easternmost basin of the lake in contrast the terraces did not form until about 7 ka with downcutting of the transverse valley by overflow from lake panggong this lag can be explained in light of the bathymetry of lake panggong the modern lake consists of five basins separated by shallow sills and outflow through the tangtse valley could not occur until the water level was substantially above its present level the easternmost basin receives the inflow of the major rivers feeding the lake making its chemistry highly sensitive to changes in precipitation however sustained wet conditions are required to fill the basins to the west to the sill depth necessary for overflow through the tangtse valley and resultant downcutting and terrace formation 2003 elsevier b v all rights reserved

hydrogen index and carbon isotopes of lacustrine organic matter as lake level indicators abstract optical and geochemical techniques were applied to organic matter of late quaternary sediments from lakes victoria and rukwa east africa variations in total organic carbon toc and hydrogen content expressed as hydrogen index hi can be related to known changes in lake level and used to identify periods of very low water or subaerial exposure in basinal sequences toc and hi both decline as an exposure surface is approached mainly due to the selective removal of unstable components by bacterial respiration and inorganic oxidation inert organic material of very low hi is typically concentrated at the overlying transgression surface the carbon isotopic composition of the particulate organic matter tends to be less negative in sediments associated with exposure surfaces although weathering may be partially responsible isotopic variations are mainly due to changes in the relative importance of material from plants using c4 photosynthesis such as grasses and sedges growing around the shore and on the exposed lake muds and of material from planktonic algae and terrestrial c3 plants

terrestrial climate signal of the 8200 yr b p cold event in the labrador sea region accelerated melting of greenland ice has raised concern about the future impact of enhanced freshwater discharge on regional climate through its effect on ocean circulation an abrupt cooling event ca 8200 cal yr b p has been linked to meltwater from the decaying north american ice sheet oxygen isotopic analyses of cellulose from subfossil sphagnum mosses isolated from a newfoundland peat core reveal a pronounced anomaly ca 8350 yr b p with a duration of 150 years the maximum estimated 18 o precipitation change 4 53 1 05 vienna standard mean ocean water is the largest observed in the circum north atlantic region the magnitude of change exceeds that predicted by recent paleoclimate simulations comparisons with recent records of surface and deep ocean proxies in the labrador sea and the wider north atlantic region suggest synchroneity however an 200 year delay between the responses of the labrador sea region and the greenland ice sheet to the effects of meltwater release remains to be explained

a 25 ka indian ocean monsoon variability record from the andaman sea recent paleoclimatic work on terrestrial and marine deposits from asia and the indian ocean has indicated abrupt changes in the strength of the asian monsoon during the last deglaciation comparison of marine paleoclimate records that track salinity changes from asian rivers can help evaluate the coherence of the indian ocean monsoon iom with the larger asian monsoon here we present paired mg ca and d 18 o data on the planktic foraminifer globigerinoides ruber white from andaman sea core rc12 344 that provide records of sea surface temperature sst and d 18 o of seawater d 18 osw over the past 25 000 years ka before present bp age control is based on nine accelerator mass spectrometry ams dates on mixed planktic foraminifera mg ca sst data indicate that sst was 31c cooler during the last glacial maximum lgm than the late holocene andaman sea d 18 osw exhibited higher than present values during the lateglacial interval ca 19 15 ka bp and briefly during the younger dryas ca 12 ka bp lower than present d 18 osw values during the bolling allerod ca 14 5 12 6 ka bp and during the early holocene ca 10 8 5 5 ka bp are interpreted to indicate lower salinity reflect some combination of decreased evaporation precipitation e p over the andaman sea and increased irrawaddy river outflow our results are consistent with the suggestion that iom intensity was stronger than present during the bolling allerod and early holocene and weaker during the late glaciation younger dryas and the late holocene these findings support the hypothesis that rapid climate change during the last deglaciation and holocene included substantial hydrologic changes in the iom system that were coherent with the larger asian monsoon r 2007 elsevier ltd all rights reserved

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

holocene aridification of india 1 spanning a latitudinal range typical for deserts the indian peninsula is fertile instead and sustains over a billion people through monsoonal rains despite the strong link between climate and society our knowledge of the long term monsoon variability is incomplete over the indian subcontinent here we reconstruct the holocene paleoclimate in the core monsoon zone cmz of the indian peninsula using a sediment core recovered offshore from the mouth of godavari river carbon isotopes of sedimentary leaf waxes provide an integrated and regionally extensive record of the flora in the cmz and document a gradual increase in aridity adapted vegetation from 4 000 until 1 700 years ago followed by the persistence of aridity adapted plants after that the oxygen isotopic composition of planktonic foraminiferglobigerinoides ruberdetects unprecedented high salinity events in the bay of bengal over the last 3 000 years and especially after 1 700 years ago which suggest that the cmz aridification intensified in the late holocene through a series of sub millennial dry episodes cultural changes occurred across the indian subcontinent as the climate became more arid after 4 000 years sedentary agriculture took hold in the drying central and south india while the urban harappan civilization collapsed in the already arid indus basin the establishment of a more variable hydroclimate over the last ca 1 700 years may have led to the rapid proliferation of water conservation technology in south india

late ordovician global warming the boda event there is substantial evidence for mid ashgillian global warming before the latest ordovician hirnantian glaciation as shown by the movement of previously lower latitude benthic faunas such as trilobites and brachiopods to progressively higher latitudes and by an increase in endemic faunas at low latitudes this is shown by faunal dynamics on several paleocontinents and endorsed by published evidence of warmer water sediments from gondwana and elsewhere we term this global warming the boda event

variability of southwest indian summer monsoon precipitation during the bolling allerod we have generated a high resolution 20 yr 2 3 0 th dated stalagmite oxygen isotope record from timta cave in the western himalaya in india that documents southwest indian summer monsoon ism precipitation variations during the bolling allerod interstadial from 15 2 to 11 7 ka compared with the glacial and younger dryas ism precipitation was enhanced during the bolling allerod ism precipitation was apparently coupled to variations in the east asian monsoon and north atlantic climate on millennial and multicentennial time scales during the deglaciation analyses of a high growth rate interval 2 5 yr resolution encompassing the late bolling early alterod suggest that multidecadal monsoon variability was an important aspect of ism behavior at that time the frequency spectrum of ism precipitation during this time interval is similar to that of the 1 4 c record and other ism precipitation records during the latest holocene this raises the hypothesis that multidecadal climate dynamics during the late boiling early allerod may have been similar to those that operated during the last several millennia even though the boundary conditions of these two time intervals were very different

holocene paleoclimates of india we present a comprehensive summary of the available palaeoclimate records from india and compare the results from different proxies the results indicate i fluctuating lake levels during the early holocene the period of relatively higher lake levels from increased precipitation efficiency was reached only 7 2 6 0 cal kyr bp possibly due to increased contribution from winter rainfall ii the onset of aridity in nw india could have begun as early as 5 3 cal kyr bp subsequently there were multiple wet events but of shorter duration and smaller magnitude than during the mid holocene iii there is evidence of several short term climate events in the proxy record however in the absence of a rigorous chronological framework a detailed regional correlation is not possible at this stage finally a comparison between marine and terrestrial records indicates that episodes of strongest and weakest monsoon winds were not always associated with wettest and driest episodes respectively in the nw indian lakes

abrupt weakening of the summer monsoon in northwest india 4100 yr ago climate change has been suggested as a possible cause for the decline of urban centers of the indus civilization 4000 yr ago but extant paleoclimatic evidence has been derived from locations well outside the distribution of indus settlements here we report an oxygen iso tope record of gastropod aragonite 18 o a from holocene sediments of paleolake kotla dahar haryana india which is adjacent to indus settlements and documents indian summer mon soon ism variability for the past 6 5 k y a 4 increase in 18 o a occurred at ca 4 1 ka mark ing a peak in the evaporation precipitation ratio in the lake catchment related to weakening of the ism although dating uncertainty exists in both climate and archaeological records the drought event 4 1 ka on the northwestern indian plains is within the radiocarbon age range for the beginning of indus de urbanization suggesting that climate may have played a role in the indus cultural transformation

teleconnections between the subtropical monsoons and high latitude climates during the last deglaciation the major deglacial intensification of the southwest monsoon occurred at 11 450 150 calendar years before present synchronous with a major climate transition as recorded in greenland ice an earlier event of monsoon intensification at 16 000 150 calendar years before present occurred at the end of heinrich layer 1 in the atlantic and parallels the initial rise in global atmospheric methane concentrations and the first abrupt climate changes in the antarctic thus the evolution of the monsoonal and high latitude climates show teleconnections but hemispheric asymmetries superimposed on abrupt events the monsoonal climate shows high frequency variability of 1785 1450 and 1150 year oscillations and abrupt climate change seems to occur when at least two of these oscillations are in phase

oblique wind waves generated by the instability of wind blowing over water the growth rates of gravity waves are computed from linear inviscid stability theory for wind velocity profiles that are representative of the mean flow in a turbulent boundary layer the energy transfer to the waves is largely concentrated in an angle to the wind interval that broadens with increasing wind speed and narrows with increasing wavelength at sufficiently high wind speeds and sufficiently short wavelengths the waves of maximum growth rate propagate at an oblique angle to the wind the connection with bimodal directional distributions of observed spectra is discussed

sedimentary records and luminescence chronology of late holocene palaeofloods in the luni river thar desert northwest india river luni is the only well integrated river system in the thar desert of india this river catastrophically flooded due to unusually heavy rainfall in the catchment area during july 1979 in order to establish whether floods of this magnitude have occurred in the recent geological past sedimentary records of palaeofloods occurring in the sindari gorge in the lower luni basin were investigated the principal slackwater flood deposits were observed in a back flooded tributary near bhuka analysis of the slackwater flood deposits preserved in the back flooded tributary and their luminescence dating suggests that the luni river has experienced at least 17 extreme floods during the past millennium evidence from the bhuka site also suggests that no floods comparable in magnitude to the july 1979 megaflood have occurred during this period this observation is in conformity with the palaeoflood record of central india comparison of the long term monsoon rainfall series for the luni basin and the indian region reveals a clear link between the two and indicates that the clustering of large floods in the last few decades and during the medieval warming period is a regional phenomenon associated with wetter conditions this correlates with a regionally extended episode of landscape stability denoted by stabilization of dunes in this region

millennial scale holocene climate variability in the nw china drylands and links to the tropical pacific and the north atlantic abstract a loess paleosol sequence and a lake sediment record from the central hexi corridor in the drylands of nw china show patterns of persistent millennial to centennial scale climate variability during the holocene epoch which can be well correlated with abrupt climatic events reported from the north atlantic and the asian monsoon dominated regimes our results show that the drylands in nw china might have been regulated both by the westerlies and by the east asian summer monsoon easm in a roughly synchronous manner it seems that cold dry intervals in the central hexi corridor may generally correspond to cooling phases in high latitudes of the northern hemisphere and that cold warm wet intervals largely correlate with relatively warm phases in northern high latitudes a candidate mechanism involving nao ao and enso dynamics is tentatively proposed to account for this synchroneity climatic patterns may be spatially different in the nw china drylands in response to the changing predominant control of either the westerlies or the easm depending on the contrast of their relative strengths glacier meltwater discharges associated with alpine warming may be partly responsible at least for the two records discussed in this paper for such spatial heterogeneity of climatic changes during the holocene more work is needed to evaluate the individual roles and combined impacts of the easm the westerlies and alpine glacier meltwater regulation on the nw china dryland dynamics

variation of rb sr ratios in the loess paleosol sequences of central china during the last 130 000 years and their implications for monsoon paleoclimatology abstract rb sr and magnetic susceptibility have been measured in the last interglacial glacial loess profiles at luochuan and huanxian central china a high degree of similarity between the parameters in both profiles suggests that variations of rb sr ratios in the sequence can be regarded as an indicator of east asian summer monsoon strength matching the rb sr record with the specmap 18o curve suggests that the rb sr ratio responds sensitively to changes of the east asian monsoon induced by global ice volume variation

correlation between indian ocean summer monsoon and north atlantic climate during the holocene there has been a number of investigations for the correlation between the asia monsoon and the north atlantic climate for the last glacial however little research has been done for the present interglacial the holocene here we present for the first time a high resolution composite proxy record for the indian ocean summer monsoon spanning around 12 000 years based on the 13c time series of both a single plant species carex mulieensis remains cellulose and the total plant assemblage cellulose in the hongyuan peat bog from the tibet plateau the records show that the strength of the indian ocean summer monsoon had abrupt variations during the last 12 000 years the weakest monsoon occurred in the younger dryas period following rapid strengthening from around 11 200 to 10 800 a bp the monsoon kept a generally strong level for around 5300 years from around 5500 a bp onwards the monsoon strength tended to gradual decrease in addition there are a series of abrupt variation events of the monsoon strength on centennial to millennial time scales which superimpose the general tendency of the monsoon variation in every case when the ice rafted debris events in the north atlantic occurred the summer monsoon strength decreased correspondingly these evidences show that teleconnection between the indian ocean summer monsoon and the north atlantic climate is present not only in the last glacial but also in the holocene which may be linked to abrupt reorganizations of the ocean thermohaline circulation leading to redistribution of energy changing temperature and moisture gradient over the southern subtropical indian ocean and eventually controlling the variability of the indian ocean summer monsoon

abrupt post glacial climate events in west asia and north africa monsoon domains regions beyond the present or past penetration of the indian and african monsoons have experienced several large and abrupt climatic fluctuations over the past 13 14c kyr pollen and lake records from west asia western tibet and rajasthan east africa ethiopia and west africa western sahara sahel and subequatorial africa were selected on the basis of chronological control sensitivity of both site and environmental indicators to climate change the continuity of the record and interdisciplinary control of the palaeoclimatic interpretation conditions wetter than those of today prevailed during the early mid holocene period but major dry spells are recorded at all sites during the intervals 11 0 9 5 kyr bp 8 7 kyr bp and 3 4 kyr bp several records also suggest dry events of minor amplitude around 6 kyr bp potential boundary forcings of insolation and sea surface and tropical land surface conditions are discussed the solar radiation accounts for the general envelop of the post glacial monsoon fluctuations but explains neither the timing nor the amplitude of the short term changes in spite of apparent covariation between fluctuations in sea surface conditions in the north atlantic and the monsoon record no direct mechanism could be found relating the intensity of the oceanic thermohaline conveyor belt to the monsoon strength changes in tropical land surface conditions soil moisture negative feedback and changes in ch4 production from wetlands provide a more satisfactory hypothesis for explaining abrupt reversal events

model data comparison for the 8 2 ka bp event confirmation of a forcing mechanism by catastrophic drainage of laurentide lakes abstract to improve our understanding of the mechanism behind the 8 2 ka bp cooling event we compare proxy evidence with climate model simulations in which the thermohaline ocean circulation is perturbed by a freshwater pulse into the labrador sea both the proxy data and model results show a cooling that is mainly concentrated in the north atlantic region ranging from more than 5 c cooling in the nordic seas to about 0 5 1 c over europe and less than 0 5 c over the subtropical north atlantic data and model also indicate a weakening of the summer monsoon and generally a drier circum north atlantic over the south atlantic ocean the model simulates a slight warming mostly less than 0 5 c which falls within the uncertainty of proxy data and thus could not be confirmed to examine in detail the structure of the 8 2 ka bp event we also compare the modeled climatological evolution at two locations with high quality records revealing a generally consistent picture the good model data agreement confirms the hypothesis that the 8 2 ka bp event was forced by a freshwater induced weakening of the thermohaline circulation other forcings are unlikely since they would result in an alternative geographical distribution and expression of the climate response

evolution of ganges brahmaputra western delta plain clues from sedimentology and carbon isotopes abstract sedimentology carbon isotope and sequence stratigraphic analysis of subsurface sediments from western part of ganges brahmaputra gb delta plain shows that a late quaternary marine clay and fluvial channel overbank sediments of mis 5 and 3 highstands are traceable below the holocene strata during the last glacial maximum lgm sea level lowering of 100 m produced a regional unconformity type 1 represented by palaeosols and incised valley c4 vegetation expanded on exposed lowstand surface in an ambient dry glacial climate at 9 ka transgression inundated the lowstand surface pushing the coastline and mangrove front 100 km inland simultaneous intensification of monsoon and very high sediment discharge 4 8 times than modern caused a rapid aggradation of both floodplain and estuarine valley fill deposits between 8 and 7 ka the hoogli river remaining along its present drainage possibly acted as the main conduit for transgression and sediment discharge that was subsequently abandoned c3 vegetation dominated the delta plain during this time from 7 ka onward progradation of delta plain started and continued till recent this period experienced a mixed c3 c4 vegetation with localized mangroves in the mid holocene to dominant return of c4 vegetation in the late holocene period the study indicates that while the initiation of western part of gb delta occurred at least 1 ka earlier than the global mean delta formation age the progradation started at 7 ka at least 2 ka earlier than thought before the terrestrial vegetation change was modulated by changes in depositional environment specific ecological niches and climate rather than pco 2

holocene climatic instability a prominent widespread event 8200 yr ago the most prominent holocene climatic event in greenland ice core proxies with approximately half the amplitude of the younger dryas occurred 8000 to 8400 yr ago this holocene event affected regions well beyond the north atlantic basin as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane widespread proxy records from the tropics to the north polar regions show a short lived cool dry or windy event of similar age the spatial pattern of terrestrial and marine changes is similar to that of the younger dryas event suggesting a role for north atlantic thermohaline circulation possible forcings identified thus far for this holocene event are small consistent with recent model results indicating high sensitivity and strong linkages in the climatic system

luminescence chronology of late holocene extreme hydrological events in the upper penner river basin south india field stratigraphy sedimentology and optically stimulated luminescence osl dating have been used to reconstruct the southwest monsoon variability in the semi arid region of southern india during the late holocene facies architecture and osl dating of the water lain sediment suggest prevalence of a weak hydrological regime around 3 ka following this a progressive strengthening of monsoon occurred till 2 ka after 2 ka and until 1 ka fluvial activity was nearly dormant indicating weakening of the monsoon presence of high magnitude flood deposits overbank sedimentation and pedogenesis during 1 0 6 ka indicate intensification of the southwest monsoon in the basin the onset of aridity was associated with episodic storm surge events that are manifested in the pond sedimentation and localised aeolian accretion this phase is bracketed between 0 5 ka and 0 2 ka a renewed phase of monsoonal activity was observed in the form of floodplain aggradation between 180 and 90 years ago in the past 70 years no significant change in the monsoon performance has been observed copyright 2007 john wiley sons ltd

magnetic properties of sediments in the bay of bengal and the andaman sea impact of rapid north atlantic ocean climatic events on the strength of the indian monsoon abstract the results of a high resolution mineral magnetic study combined with major element geochemistry analysis oxygen isotopes and 14c ams stratigraphy are reported for deep sea gravity cores md77 169 and md77 180 located in the andaman sea and the bay of bengal respectively core md77 169 covers the last 280 kyr and core md77 180 covers the last 160 kyr in both cores rock magnetic parameters indicate that the magnetic assemblage is dominated by pseudo single domain titanomagnetite grains with grain size variations following a strong 23 kyr periodicity smaller magnetic grain sizes are observed during periods characterized by a strong summer monsoon in addition in core md77 180 we observe a correlation between magnetic grain size and a chemical index of alteration this suggests that these magnetic grain size changes are related to chemical weathering driven by summer monsoon rainfall a comparison of the gisp2 ice core isotopic record and the magnetic grain size record of the bay of bengal shows that rapid temperature variations documented in the ice core dansgaard oeschger cycles and heinrich events during the last glacial period are also present in the magnetic grain size record heinrich events and cold stadial events are characterized by relatively large magnetic grain sizes furthermore heinrich events are characterized by lower values of the chemical index of alteration implying a lower degree of chemical weathering related to significantly drier conditions on the continent we suggest that rapid cold events of the north atlantic heinrich events during the last glacial stages are characterized by a weaker summer monsoon rainfall over the himalaya via an atmospheric teleconnection

concentrations and burial fluxes of organic and inorganic carbon on the eastern margins of the arabian sea abstract eleven sediment cores were analysed from the continental margins of the eastern arabian sea 22 n to 10 n for organic carbon c org nitrogen n and caco 3 contents similar variations are observed in both lateral and the vertical org n and caco 3 ranging from 1 6 0 10 0 82 and 10 81 respectively the c n ratios varying from 6 15 averaging 9 3 indicate that c org is mostly of marine origin the burial fluxes bf of c org and caco 3 are in the range of 0 8 21 and 39 194 g m 2 a 1 respectively the upper limits of burial efficiency be for caco 3 compared to its rain rate of 25 g m 2 a 1 in sediment trap at a depth of 2500 m from a nearby region ranges from 39 to 780 be values greater than 100 in the northern 15 n could be most likely due to redeposition of material from shelf and slope regions as well as influx of detrital carbonates from coastal areas the inverse correlation observed in c org and caco 3 distribution is indicative of post depositional effects due to sulphate reduction associated processes this can lead to dissolution of caco 3 and a better preservation of c org due to faster deposition rate and higher detrital clay content the bf and paleoproductivity calculated using the transfer function of sarnthein et al miner soc spec publ 64 1992 411 show increasing trend between 10 and 2 ka with a significant periodicity of 3000 a corresponding to that observed for monsoon induced productivity

the variation of summer monsoon precipitation in central china since the last deglaciation abstract two stalagmites c996 1 and c996 2 collected from the jiuxian cave in the shaanxi province in central china have been studied with u series dating and stable isotope analysis thirty eight 230 th dating results showed that the stalagmite c996 1 was continuously deposited through the last 8 5 ka bp thousand years before present present 1950 ad and c996 2 was deposited through the last 19 ka bp excluding two growth hiatuses between 8 3 and 5 7 ka bp and 15 4 and 11 9 ka bp with a relatively stable boundary condition we interpret the 18 o of speleothem calcite as most indicative of the amount of summer monsoon precipitation although temperature and other factors might have some minor impact the 18 o records show notable changes within the last glacial maximum lgm resembling other east asian monsoon records such as those from the hulu and sanbao caves suggesting that significant monsoonal climate changes occurred in eastern asia as far north as the qinling mountains during the lgm interval a comparison of our records to precisely dated contemporaneous speleothem records from other caves shows that the increasing trend of 18 o during the holocene commenced as early as 7 5 ka bp in the low latitude monsoonal area i e the hoti cave while at higher latitudes this shift occurred later such as 7 0 ka bp in the dongge cave 5 3 ka bp in the heshang cave 4 7 ka bp in the sanbao cave and 4 5 ka bp in the jiuxian cave these results imply an asynchronous change of the summer monsoon precipitation occurred in east asia during the holocene the asynchrony may be related to the responses of a coupled tropical and subtropical monsoon system to changes of the insolation and the differences in thermal forcing which result from the complex geographical configuration the variation of sea surface temperature sst in the western tropical pacific may also have important impacts on the summer monsoon precipitation changes in central and northern china because it affects the northwestern pacific subtropical high a monsoon front regulator more robust tests are needed to confirm this phenomenon and to evaluate the contribution of different factors in detail

early holocene monsoonal fluctuations in the garhwal higher himalaya as inferred from multi proxy data from the malari paleolake abstract a 4 9 m thick lake sequence formed due to the landslide damming of a stream in the semiarid garhwal himalaya was studied to understand past monsoonal variations in the region the optically stimulated luminescence osl chronology indicates that the lake existed between 12 and 7 ka ago chronologically constrained trends of sand percent organic phosphorus op apatite inorganic phosphorus aip and parameters of environmental magnetism were measured in the paleolake profile measured proxies indicate that the indian summer monsoon ameliorated in the early holocene after 12 ka cooling and it appears that all the proxies from the lake have captured this globally recognized early holocene warming four phases of wet conditions intensified monsoon are recognized at 11 5 ka 11 10 5 ka 10 9 ka and 8 7 ka with maximum uncertainties of 1000 years the wet phases are characterized by high magnetic susceptibility increased op and reduced aip in an attempt to understand the primary forcing of the sharp fluctuations in monsoonal activity in the region we show that changes in magnetic susceptibility match variations of residual atmospheric 14c suggesting a role for solar variability as an explanation of climatic variability

the great ordovician biodiversification event gobe the palaeoecological dimension abstract the great ordovician biodiversification event gobe saw a spectacular increase in marine biodiversity at all taxonomic levels largely within the phyla established much earlier during the so called cambrian explosion the diversification was probably the result of a combination of several geological and biological processes and the positive feedbacks resulting from them the present paper reviews the palaeoecological dimension of the gobe it involved major increases in s and biodiversity largely associated with the rise of the paleozoic evolutionary fauna dominated by suspension feeders and involving a greater occupation of ecospace and more complex ecological structures in the ecological evolutionary units p1 and p2 in the benthos these include more complex food webs than those of the cambrian greater tiering especially above the sediment water interface and the development of guild structures indicating increased competition between taxa for particular resources the ordovician is characterized by a profound change in reef composition with a switch from microbial dominated reefs in the early and middle ordovician to metazoan dominated reefs in the late ordovician increases in complexity of deep water trace fossil assemblages began in the early ordovician and mark the increasing exploitation in that environment and the development of hardgrounds permitted bioerosion and encrusting strategies together with the appearance of cryptic communities within the water column the gobe involved a significant increase in the diversity of the phytoplankton and the development of a diverse zooplankton including planktotrophic larvae from a range of invertebrate clades this revolution in the plankton enabled the establishment of a diverse fauna of pelagic vertebrates molluscs and arthropods and promoted the rise of suspension feeders in the benthos an escalation amongst predators and thus community evolution may also have been a major driver of biodiversification

trilobite diversity in avalonia prior to the end ordovician extinction the peak before the trough abstract the trilobite biodiversity curve for avalonia differs markedly from the global and other regional patterns in having the peak of diversity immediately prior to the hirnantian extinction event rather than in the caradoc or even earlier this reflects both the very wide spectrum of biofacies represented in this part of the succession in avalonia and the immigration of taxa resulting from the closure of the iapetus and tornquist oceans detailed analysis of the faunas demonstrates that avalonia exemplifies the many factors involved in provincial breakdown and their influence on biodiversity furthermore it emphasises the need to understand the palaeoenvironmental context within which such changes took place over 90 trilobite genera are known from the mid ashgill cautleyan and rawtheyan stages of avalonia with half of these making their first appearance on this microcontinent in that interval the wide environmental spectrum reflected in the rock record not only includes the return of the deepest water cyclopygid atheloptic biofacies but also marks the first recorded development in avalonia of the pure carbonate illaenid cheirurid association which was first established in low latitude laurentia much earlier in the ordovician its appearance in avalonia may reflect a combination of the drift of the microcontinent towards tropical latitudes and a possible warming event recruitment into this and other environments was overwhelmingly from low latitudes with the largest cohort having its earlier history in laurentia setting aside the new pure carbonate environment the marked rise in diversity and concomitant provincial breakdown resulted from the insinuation of immigrant taxa into existing communities across the shelf and upper slope rather than from any substantial replacement of the established incumbents the peak of diversity in the mid ashgill made the climatically triggered hirnantian extinction all the more dramatic in avalonia

dominant control of the south asian monsoon by orographic insulation versus plateau heating heat emitted from the tibetan plateau as dry heat and water vapour has long been assumed to be the main driver of the south asian summer monsoon but new work suggests that in fact it is the neighbouring mountains that are the major influence william boos and zhiming kuang use an atmospheric model to show that flattening the tibetan plateau has little effect on the monsoon so long as the himalayas and surrounding mountain ranges remain the plateau does boost rainfall locally along its southern edge but it is the build up of hot moist air over india insulated from colder drier air by the himalayas that drives large scale monsoon circulation the elevation of the tibetan plateau is thought to cause its surface to serve as a heat source that drives the south asian summer monsoon potentially coupling uplift of the plateau to climate changes on geologic timescales here however an atmospheric model is used to show that flattening of the tibetan plateau has little effect on the monsoon provided that the narrow orography of the himalayas and adjacent mountain ranges is preserved the tibetan plateau like any landmass emits energy into the atmosphere in the form of dry heat and water vapour but its mean surface elevation is more than 5 km above sea level this elevation is widely held to cause the plateau to serve as a heat source that drives the south asian summer monsoon potentially coupling uplift of the plateau to climate changes on geologic timescales1 2 3 4 5 observations of the present climate however do not clearly establish the tibetan plateau as the dominant thermal forcing in the region peak upper tropospheric temperatures during boreal summer are located over continental india south of the plateau here we show that although tibetan plateau heating locally enhances rainfall along its southern edge in an atmospheric model the large scale south asian summer monsoon circulation is otherwise unaffected by removal of the plateau provided that the narrow orography of the himalayas and adjacent mountain ranges is preserved additional observational and model results suggest that these mountains produce a strong monsoon by insulating warm moist air over continental india from the cold and dry extratropics these results call for both a reinterpretation of how south asian climate may have responded to orographic uplift and a re evaluation of how this climate may respond to modified land surface and radiative forcings in coming decades

a revised 30 000 year paleoclimatic and paleohydrologic history of lake albert east africa abstract new geochemical and palynological data and a sedimentological reinterpretation of two cores from lake albert allow a revision of the paleoclimatic and paleohydrologic history of northwest uganda over the last 30 000 14 c yr bp our findings document one and possibly three periods of significantly reduced inflow to the nile river system from the white nile based on sediment fabric and mineralogy shifts in percentages of total organic carbon toc and total organic nitrogen ton hydrogen index hi and 13 c and 15 n as well as poor preservation of sporopollenin and siliceous microfossils we recognize two mature paleosols that formed between 18 000 and 12 500 14 c yr bp at the surface of exposed lacustrine muds a thin fossiliferous zone between the two soils documents a brief period of flooding lake albert was below the level of the coring site and thus a closed basin for several thousand years during the late pleistocene around 12 500 14 c yr bp the younger vertisol was flooded by a major rise in lake level relatively deep water conditions were established soon after this transgression but an apparent change in sedimentation rate sometime between 8 000 and 3 400 14 c yr bp may reflect another briefer period of low water which resulted in interruption of sedimentation at the coring site palynological data document a rapid and abrupt shift to drier conditions around lake albert from 11 400 to 9 900 14 c yr bp coincident with the cool younger dryas event of the northern hemisphere geochemical evidence suggests that throughout the terminal pleistocene and early holocene and again in the late holocene the water column was relatively well mixed with little tendency to persistent anoxia

preservation of elemental and isotopic source identification of sedimentary organic matter the amount and type of organic matter in the sediments of lakes and oceans contribute to their paleoenvironmental and paleoclimatological records only a small fraction of the initial aquatic organic matter survives destruction and alteration during sinking and sedimentation selective degradation modifies the character of the surviving small fraction of organic matter which becomes incorporated in bottom sediments organic matter alterations can continue to sub bottom depths of hundreds of meters corresponding to millions of years source and paleoenvironmental information nonetheless remains preserved in the molecular elemental and isotopic compositions of organic matter c n and 13c values of total organic matter in particular appear to retain paleoenvironmental information for multi myr time periods

numerical modeling of fetch limited waves in the gulf of tehuantepec abstract during the gulf of tehuantepec experiment gotex conducted in february 2004 surface wave measurements were collected using a scanning lidar airborne topographic mapper atm on the national science foundation nsf ncar c 130 aircraft during fetch limited conditions with winds speeds ranging from 10 to 25 m s 1 the authors present direct comparisons between the observed evolution of the wave field and numerical simulations using a parameterization of the wave energy dissipation for low and intermediate wavenumbers the dissipation corresponds to the saturation based parameterization by alves and banner however at higher wavenumbers their formulation cannot maintain saturation of the spectrum here the authors use a dissipation term that forces the spectrum to match the empirical degree of saturation and explicitly balances the wind input and the nonlinear energy fluxes all model simulations were carried out with exact computations of the nonlinear energy transfer because of four wav

holocene environmental changes in lake bangong basin western tibet part 1 chronology and stable isotopes of carbonates of a holocene lacustrine core abstract a 12 4 m core collected from the eastern part of lake bangong is analyzed for mineralogy radiometric chronology and stable isotope contents of authigenic inorganic carbonate isotopic compositions of the modern lake waters and of its major tributaries are first presented radiocarbon activity of these waters show that equilibrium with the atmospheric co2 is not reached because of admixture of dead carbon in solution in the system a model is thus developed to correct the radiocarbon ages of fossil material of aquatic origin for this ageing the core provides a continuous holocene record changes in mineralogy of bulk sediments and in 13c and 18o contents of authigenic carbonates reveal extremely large changes in environmental conditions induced by climate changes and or local hydrological factors e g closure opening of the lake system controlled by the altitude of the outflow sill the eastern basin of lake bangong was closed before 9 6 and after 1 2 ka b p in response to aridity on the catchment wet warm conditions attributed to the monsoon influence were established suddenly at 9 6 ka b p and maintained until 6 2 ka b p although minor reversal events occurred between 8 6 and 7 7 ka b p the return towards aridity is non linear dry spells at 6 2 3 9 3 2 and 1 3 ka b p are alternated with short term periods with positive precipitation evaporation balance

mid holocene climate and land sea interaction along the southern coast of saurashtra western india abstract the relict mudflat from the southern saurashtra coast of gujarat was investigated using geochemical and palynological analyses supported by radiocarbon dating to understand whether climate fluctuations and sea level operated in tandem during mid holocene the study revealed that the saurashtra coast experienced relatively wet climatic conditions with simultaneous occurrence of marginally high sea level between 4710 and 2825 cal yr bp subsequently a gradual onset of aridity and lowering of the sea level was observed between 2825 and 1835 cal yr bp and further a slight decrease in aridity is observed after 1835 cal yr bp the present day coastal configuration was probably achieved after around 1500 cal yr bp considering the tectonic instability of saurashtra coast land level changes the effective mid holocene sea level was estimated to be 1 m higher than the present the study demonstrates that sea level changes climate variability and land level changes were coupled during the mid holocene

hydrological evolution during the last 15 kyr in the tso kar lake basin ladakh india derived from geomorphological sedimentological and palynological records abstract the tso kar lake basin in ladakh northwestern himalayas 33 18 n 78 e 4527 m a s l is located close to the modern boundary of effective moisture transport from the indian summer monsoon in order to reconstruct the possible monsoonal impact on the hydrology of the lake basin during the last 15 kyr we studied the geomorphology of the basin and combined the results with multi proxy records from tso kar lake sediments major analyses comprise geochemical data high resolution x ray fluorescence xrf mineral composition xrd aquatic pollen and non pollen palynomorphs npp although the chronological framework based on 41 14c ams dates remains speculative for the lateglacial and late holocene periods our results indicate major hydrological phases linked to regional climate variations and tectonic activity the maximum glacier advance falls within the period between the global lgm and ca 15 kyr bp while the basin remained open and may have drained along the paleo tso kar river tectonic pulses may have triggered terrace development and the closure of the lake the estimated mean vertical displacement rate accounts for 7 9 mm yr considerable summer monsoon moisture supply occurred for the periods around 12 5 kyr bp and between ca 11 5 and 8 6 kyr bp indicated by deposition of more profundal lake facies and the development of aquatic fauna and flora the lake level rise about 11 8 kyr bp was due to intensified glacier melt and summer monsoon effective moisture supply the lake reached its maximum level and extent between 8 5 and 7 kyr bp under warm moist climate conditions permafrost was absent from the basin summer monsoon influence weakened after 8 kyr bp with a short reverse trend at ca 5 6 kyr bp the lake shrank gradually after 7 kyr bp towards its lowest stand at about 4 2 kyr bp with return of permafrost activity subsequent terrace formation was probably amplified by tectonic movement the lake remained at a low level with minor fluctuations the results indicate that the orbitally driven regional climate variability is well reflected in the tso kar basin by hydrological variations considerable summer monsoon moisture seems to have influenced the area only during the early holocene the late quaternary evolution of the tso kar was controlled by the interplay of climate forces local morphodynamics and tectonic impact

a synthesis of abrupt changes in the asian summer monsoon since the last deglaciation we have compiled 36 previously published palaeoclimate records to determine the timing and spatial pattern of century scale abrupt changes in asian monsoon precipitation since the last deglaciation we identify abrupt events from 1 the interpretations of the authors of these records and 2 the more objective moving t test calculation our results indicate that abrupt climatic changes occurred at 11 5 cal ka 4 5 5 0 cal ka and ad 1300 at the start of the holocene 11 5 cal ka asian monsoon precipitation increased dramatically this climatic change is synchronous with an abrupt warming in the north atlantic during the middle holocene there was a time of preferred and widespread weakening in monsoon strength 4 5 5 0 cal ka this result contradicts previous notions of either a gradual trend towards drier conditions or a series of abrupt events that occurred in an unorganized fashion across space and time the middle holocene abrupt event could have been synchronous with an abrupt cooling event in the north atlantic as well as a warming and intensie cation of internannual variability in the tropical pacie c in contrast to previous periods precipitation changes at ad 1300 have a heterogeneous spatial pattern we e nd no conclusive evidence for a change in the asian monsoon at 8 2 cal ka as suggested by several previous studies more high resolution data may be needed to observe this short lived event overall our results attest to the potential for rapid and major shifts in asian monsoon precipitation that may be triggered by variations in other components of the climatic system

high resolution stable isotope stratigraphy of upper ordovician sequences constraints on the timing of bioevents and environmental changes associated with mass extinction and glaciation the two phases of the late ordovician mass extinction are approximately coeval with the periods of rapid climate change associated with the onset and demise of the gondwanan glaciation in this paper we argue that the distinctive late ordovician carbon isotope profile provides a chronostratigraphic ruler against which a sequence of environmental and biotic events may be located the ruler also allows regional and global high resolution correlation of successions representing very different environments cores from the upper ordovician succession of estonia and latvia record a large 1 3 c c a r b o n a t e excursion up to 6 with a similar profile shape the consistent relationship between the chemostratigraphy and biostratigraphy in the baltic region suggests that the isotope profile has a regional chronostratigraphic value the presence of similar profiles in nevada united states suggests that the excursion is a global chronostratigraphic signal this interpretation enables a detailed correlation to be made between upper ordovician shallow marine and basinal sequences that have wholly different faunas successions in the baltic area and in canada that do not display the model profile are interpreted as incomplete reinter pretation of these important successions significantly modifies the global database used to assess the pattern of diversity change during the mass extinction key levels of environmental change have been located against the carbon isotope profile new oxygen isotope data from brachiopod and ostracode calcite set tight limits on the start of the glacial events cooling and sea level fall started at the same stratigraphic level as the start of the carbon isotope excursion the later rise in sea level and fall in oxygen isotope values record the end of the glaciation these restrict the duration of the main glaciation to only 1 5 graptolite zones we propose that models of the carbon cycle should be adapted to be consistent with the temporal relationships between carbon cycling sea level fall and temperature change documented here the chronostratigraphic ruler provided by the carbon isotope profiles is used as a scale to determine the sequence of biotic changes and to allow high resolution correlation of biotic events at different locations this approach identifies regional similarities and differences in the patterns of extinction the main phase of graptolite extinction in the monitor range nevada for example is synchronous with the chitinozoan extinction in the baltic region but chitinozoan taxa survive to higher levels in nevada the benthic faunas in the baltic region demonstrate that the main extinction event corresponded with the beginning of the isotope excursion at the start of the hirnantian the level at which the marine environment started to change rapidly but that there were further extinctions of species within the early hirnantian the cold adapted hirnantia fauna did not appear immediately after the extinction in this area the relationship between the second phase of extinction and the carbon isotope excursion is less clear but the available data suggest that the extinction coincides with a time of rapid environmental change but not at the inception of environmental change as happened in the first phase

paleohydraulics of the last outburst flood from glacial lake agassiz and the 8200 bp cold event abstract during the last deglaciation of north america huge proglacial lakes formed along the southern margin of the laurentide ice sheet the largest of these was glacial lake agassiz which formed about 11 7 14 c kyr and drained into hudson bay about 7 7 14 c kyr 8 45 cal kyr overflow from these lakes was variably directed to the mississippi st lawrence and mackenzie drainage systems and it is thought that switches in routing were accompanied by a response in ocean circulation that produced abrupt climate events when the ice dam across hudson bay finally was breached a massive flood drained lake agassiz which was routed through hudson strait to the labrador sea in terms of stored water volume the largest reservoir was associated with the kinojevis level of lake agassiz for this maximum filling the impounded water volume available to produce floods is estimated as 40 000 151 000 km 3 depending on the location of the ice margin and route used the timing of this rapid release of stored freshwater just precedes the early holocene cooling event at 8 2 cal kyr bp we use the spring hutter theory to simulate flood hydrographs for floods that originate in subglacial drainage conduits and find that flood magnitude and duration are 5 sv and 0 5 yr multiple fillings and floods are possible as are single floods having a complex multipulse structure modelling results suggest that the outburst flood from lake agassiz may have terminated before the lake surface elevation dropped to sea level and that the flood ended when a stable drainage channel was established connecting lake agassiz to the tyrrell sea

boundary layer model results for wind sea growth abstract a numerical model of the boundary layer of the atmosphere above a gravity surface wave is reviewed the model results are used to obtain an expression for the wind input the wave growth due to the wind this is done for wave components that propagate at an arbitrary angle to the wind like other purely theoretical expressions for the wind input the wind input from the boundary layer model is much smaller than the wind input inferred from field experiments to study the growth of wind sea the wind input of the third generation wave model wam is replaced by the wind input from the boundary layer model the original wam used a wind input that was inferred from field experiments for the wave wave interactions the discrete interaction approximation is used while the dissipation is tuned to get an appropriate saturation sea state the balance between wind input dissipation and wave wave interactions in the energy containing range of the wave spectrum in this version of the wam is very different

measurements of the evolution of ocean wave spectra due to bottom friction data from a field experiment in which seven wave measurement instruments were placed in a linear transect across the great australian bight are analyzed to determine the spectral decay which can be attributed to bottom friction since other processes such as atmospheric input nonlinear interactions whitecap dissipation refraction and shoaling will also be active the spectral wave model wam is used as an analysis tool explicitly taking these additional processes into account in the analysis the data show marked spectral decay across the shelf analysis reveals that the bottom friction coefficient c is not constant as often assumed in operational wave models the bottom friction coefficient varies approximately inversely with the wave induced bed velocity consistent with the predictions of previous laboratory data and eddy viscosity models it is possible that percolation may also be important in the region but the available data do not allow the relative contributions of bottom friction and percolation to be determined

chronology and climatic implications of late quaternary glaciations in the goriganga valley central himalaya india abstract goriganga valley which is located in the transition zone between the dry steppe of the tibetan plateau in the north and the sub humid himalayan climate in the south has preserved four events of glaciation with decreasing magnitude the oldest stage i glaciation is represented by a 12 5 km long discontinuous diamictite ridge which terminates north of rilkot 3100 m asl the stage ii glaciation is represented by sub rounded and partially eroded lateral moraines and terminates around martoli village 3240 m asl the stage iii and iv glacial moraines are sharp crested unstable and terminate proximal to the present day glacier at 3640 m asl and 3740 m asl respectively the stage ii moraines have been optically dated between 25 2 ka and 22 1 ka implying that glacier expanded during the global last glacial maximum lgm this is contrary to the suggestion that during last glacial maximum lgm glaciation was limited in extent due to weak indian summer monsoon ism in the monsoon dominated regions of the himalaya we ascribe the lgm glaciations to a combination of the lowered temperature and enhanced mid latitude westerlies following the lgm glaciation goriganga valley experienced two major pulses of deglaciation the older event is dated between 16 and 12 ka and is coeval with the initiation of the ism whereas the younger events 10 8 ka represent the early to mid holocene strengthened ism the stage iii and iv glaciations which terminated proximal to the modern glacier are speculated to occur during the mid holocene and little ice age lia respectively

reconstruction of last glacial to early holocene monsoon variability from relict lake sediments of the higher central himalaya uttrakhand india abstract proglacial lake sediments at goting in the higher central himalaya were analyzed to reconstruct the summer monsoon variability during the last glacial to early holocene sedimentary structures high resolution mineral magnetic and geochemical data suggest that the lacustrine environment experienced fluctuating monsoonal conditions optically stimulated luminescence osl dating indicates that the lake sedimentation occurred before 25 ka and continued after 13 ka during this period goting basin witnessed moderate to strengthened monsoon conditions around 25 ka 23 5 ka 22 5 ka 22 ka 18 ka 17 ka 16 5 ka and after14 5 13 ka the last glacial phase ended with the deposition of outwash gravel dated at 11 ka indicating glacial retreat and the onset of holocene condition additionally centennial scale fluctuations between 16 5 ka and 12 7 ka in the magnetic and geochemical data are seen a close correspondence at the millennial scale between our data and that of continental and marine records from the indian sub continent suggests that goting basin responded to periods of strengthened monsoon during the last glacial to early holocene we attribute the millennial scale monsoon variability to climatic instability in higher northern latitudes however centennial scale abrupt changes are attributed to the result of albedo changes on the himalaya and tibetan plateau

a 20 ka climate record from central himalayan loess deposits the southwest monsoon that dominated central himalaya has preserved loessic silt deposits preserved in patches that are proximal to periglacial areas the occurrence of such silts suggests contemporary prevalence of cold and dry northwesterly winds field stratigraphy geochemistry mineral magnetism infrared stimulated luminescence irsl and radiocarbon dating has enabled reconstruction of an event chronology during the past 20 ka three events of loess accretion could be identified the first two events of loess deposition occurred betweem 20 and 9 ka and were separated by a phase of moderate weathering pedogenesis at the end of this event gave rise to a well developed soil that was bracketed around 9 to 4 ka this was followed by the third phase of loess accretion that occurred around 4 to 1 ka episodes of loess deposition and soil formation are interpreted in terms of changes in the strength of the indian southwest monsoon copyright 2005 john wiley sons ltd

abrupt changes in the asian southwest monsoon during the holocene and their links to the north atlantic ocean during the last ice age the indian ocean southwest monsoon exhibited abrupt changes that were closely correlated with millennial scale climate events in the north atlantic region1 2 3 suggesting a mechanistic link in the holocene epoch which had a more stable climate the amplitude of abrupt changes in north atlantic climate was much smaller and it has been unclear whether these changes are related to monsoon variability here we present a continuous record of centennial scale monsoon variability throughout the holocene from rapidly accumulating and minimally bioturbated sediments in the anoxic arabian sea our monsoon proxy record reveals several intervals of weak summer monsoon that coincide with cold periods documented in the north atlantic region4 including the most recent climate changes from the medieval warm period to the little ice age and then to the present we therefore suggest that the link between north atlantic climate and the asian monsoon is a persistent aspect of global climate

brachiopod diversification during the early mid ordovician an example from the dawan formation yichang area central china systematic and statistical study of brachiopods from the dawan formation of the yichang area western hubei province central china indicates that at generic and specific levels the diversity of early to mid ordovician brachiopods in the study area reached the first acme at the beginning of the corymbograptus deflexusbiozone marked by the diverse sinorthis typica community this diversity maximum of brachiopods occurred approximately one biozone later than that of the upper yangtze platform in taxonomic constituents the main contributors to the diversity maximum were orthoids which experienced a major increase in the number of genera in the upper didymograptellus eobifidus biozone and the lower c deflexus biozone in brachiopod communities or associations orthoids and pentameroids were the dominant elements analyses of early mid ordovician brachiopods from the upper yangtze platform indicate that a high level of brachiopod community organization with temporal and spatial stability was achieved by the sinorthis community which persisted from the d eobifidus to the azygograptus suecicus biozones early arenig in the d eobifidus biozone the sinorthis community thrived across a wide ecological range ba3 to upper ba4 in the siliciclastic facies of the central part of the upper yangtze platform and by the latest d eobifidus interval invaded more carbonate rich substrates in more offshore settings in the northeastern part of the platform the present yichang area

three million years of monsoon variability over the northern sahara we present a 3 million year record of aeolian dust supply into the eastern mediterranean sea based on hematite contents derived from magnetic properties of sediments from ocean drilling program site 967 our record has an average temporal resolution of 400 years geochemical data validate this record of hematite content as a proxy for the supply of aeolian dust from the sahara we deduce that the aeolian hematite in eastern mediterranean sediments derives from the eastern algerian libyan and western egyptian lowlands located north of the central saharan watershed 21 n in corroboration of earlier work we relate dust flux minima to penetration of the african summer monsoon front to the north of the central saharan watershed this would have enhanced soil humidity and vegetation cover in the source regions in agreement with results from green sahara climate models our results indicate that this northward monsoon penetration recurred during insolation maxima throughout the last 3 million years as would be expected this orbital precession scale mechanism is modulated on both short 100 kyr and long 400 kyr eccentricity time scales we also observe a strong expression of the 41 kyr obliquity cycle which we discuss in terms of high and low latitude mechanisms that involve southern hemisphere meridional temperature contrasts and shifts in the latitudes of the tropics respectively we also observe a marked increase in sub milankovitch variability around the mid pleistocene transition 0 95 ma which suggests a link between millennial scale climate variability including monsoon dynamics and the size of northern hemisphere ice sheets

climatic fluctuations during the lia and post lia in the kumaun lesser himalaya india evidence from a 400 y old stalagmite record this paper presents the first stable isotope delta o 18 and delta c 13 data of a similar to 400 years 1590 2006 ad long annual to decadal resolution speleothem record collected from the indian lesser himalaya the data show a variation from 2 7 to 5 9 parts per thousand in delta o 18 and 5 3 to 8 8 parts per thousand in delta c 13 the isotopic analyses indicate that the climate during this period can be divided into two stages a wet phase during the little ice age lia 1590 1850 ad and comparatively dry phase during the post lia after 1850 ad however the record also documents the minor dry events during the lia and a wet episode after the lia within the age uncertainty the dry spells during the lia are linked with the historical drought events in the indian subcontinent and similar latitudes the isotopic record is consistent with a number of previous studies in the areas influenced by the westerlies but appears to be conflicting to the regions dominated by the indian summer monsoon ism this may be due to the possible changes in the strength of westerlies in the study area and added by negative anomaly of north atlantic oscillation nao during the lia c 2012 elsevier ltd and inqua all rights reserved

the southwest indian monsoon over the last 18 000 years previously published results suggest that the strength of the sw indian monsoon can vary significantly on century to millenium time scales an observation that has important implications for assessments of future climate and hydrologic change over densely populated portions of asia we present new well dated multi proxy records of past monsoon variation from three separate arabian sea sediment cores that span the last glacial maximum to late holocene to a large extent these records confirm earlier published suggestions that the monsoon strengthened in a series of abrupt events over the last deglaciation however our data provide a somewhat refined picture of when these events took place and suggest the primacy of two abrupt increases in monsoon intensity one between 13 and 12 5 ka and the other between 10 and 9 5 ka this conclusion is supported by the comparisons between our new marine data and published paleoclimatic records throughout the african asian monsoon region the comparison of data sets further supports the assertion that maximum monsoon intensity lagged peak insolation forcing by about 3000 years and extended from about 9 5 to 5 5 ka the episodes of rapid monsoon intensification coincided with major shifts in north atlantic european surface temperatures and ice sheet extent this coincidence coupled with new climate model experiments suggests that the large land sea thermal gradient needed to drive strong monsoons developed only after glacial conditions upstream of and on the tibetan plateau receded cold north atlantic sea surface temperatures european ice sheets and extensive asian snow cover it is likely that abrupt changes in seasonal soil hydrology were as important to past monsoon forcing as were abrupt snow related changes in regional albedo our analysis suggests that the monsoon responded more linearly to insolation forcing after the disappearance of glacial boundary conditions decreasing gradually after about 6 ka our data also support the possibility that significant century scale decreases in monsoon intensity took place during the early to mid holocene period of enhanced monsoon strength further highlighting the need to understand paleomonsoon dynamics before accurate assessments of future monsoon strength can be made

chronology of late quaternary glaciation and landform evolution in the upper dhauliganga valley trans himalaya uttarakhand india abstract detailed field mapping of glacial and paraglacial landforms supported by optical and radiocarbon dating is used to reconstruct the history of late quaternary glaciation and landform evolution in the trans himalayan region of the upper dhauliganga valley the study identifies four events of glaciations with decreasing magnitude which are termed as purvi kamet stage ia pks ia pks ib pks ii pks iii and pks iv respectively the oldest pks ia and ib are assigned the marine isotopic stgae 3 mis 3 the pks ii to the last glacial maximum mis 2 pks iii dated to 7 9 0 7 ka and the pks iv is dated to 3 4 0 3 ka and 1 9 0 2 ka respectively the largest valley glaciations viz the pks ia occurred during the strengthened summer monsoon corresponding to the mis 3 following this the recessional moraines pks ib represent the gradual decline in summer monsoon towards the later part of mis 3 the valley responded to the global last glacial maximum lgm which is represented by the pks ii moraine implying the influence of strengthened mid latitude westerlies during the lgm the post lgm deglaciation was associated with the onset of summer monsoon and is represented by the deposition of four distinct outwash gravel terraces the early holocene pks iii glaciation occurred around 7 9 0 7 ka and broadly coincides with the early holocene cooling event 8 2 ka this was followed by the deposition of stratified scree deposits and the alluvial fan between 5 5 ka and 3 ka during the mid to late holocene aridity this was followed by marginal re advancement of the valley glacier viz pks iv during the late holocene cool and moist climate although glaciers respond to a combination of temperature and precipitation changes however during the holocene it seems that temperature played a major role in driving the glaciation

late glacial and holocene vegetation indian monsoon and westerly circulation in the trans himalaya recorded in the lacustrine pollen sequence from tso kar ladakh nw india abstract palynological investigation of a 410 cm long core section from tso kar 33 10 n 78 00 e 4527 m a s l an alpine lake situated in the arid ladakh area of nw india at the limit of the present day indian summer monsoon was performed in order to reconstruct post glacial regional vegetation and climate dynamics the area was covered with alpine desert vegetation from ca 15 2 to 14 kyr bp 1 kyr 1000 cal years reflecting dry and cold conditions high influx values of long distance transported pinus sylvestris type pollen suggest prevailing air flow from the west and northwest the spread of alpine meadow communities and local aquatic vegetation is a weak sign of climate amelioration after ca 14 kyr bp pollen data e g influx values of pinus roxburghii type and quercus suggest that this was due to a strengthening of the summer monsoon and the reduced activity of westerly winds the further spread of artemisia and species rich meadows occurred in response to improved moisture conditions between ca 12 9 and 12 5 kyr bp the subsequent change towards drier desert steppe vegetation likely indicates more frequent westerly disturbances and associated snowfalls which favoured the persistence of alpine meadows on edaphically moist sites the spread of chenopodiaceae dominated vegetation associated with an extremely weak monsoon occurred at ca 12 2 11 8 kyr bp during the younger dryas interstadial a major increase in humidity is inferred from the development of artemisia dominated steppe and wet alpine meadows with gentianaceae after the late glacial early holocene transition in response to the strengthening of the summer monsoon monsoonal influence reached maximum activity in the tso kar region between ca 10 9 and 9 2 kyr bp the subsequent development of the alpine meadow steppe and desert steppe vegetation points to a moderate reduction in the moisture supply which can be linked to the weaker summer monsoon and the accompanying enhancement of the winter westerly flow from ca 9 2 to 4 8 kyr bp the highest water levels of tso kar around 8 kyr bp probably reflect combined effect of both monsoonal and westerly influence in the region an abrupt shift towards aridity in the tso kar region occurred after ca 4 8 kyr bp as evidenced by an expansion of chenopodiaceae dominated desert steppe low pollen influx values registered ca 2 8 1 3 kyr bp suggest scarce vegetation cover and unfavourable growing conditions likely associated with a further weakening of the indian monsoon

high resolution holocene monsoon record from the eastern arabian sea through stable oxygen and carbon analyses of rapidly accumulating sediment cores from the eastern arabian sea we show that the excess of evaporation over precipitation e p steadily appears to have decreased during the last 10 000 to 2000 years most probably due to an increasing trend in the summer monsoon rainfall contrary to the land based paleoclimatic data from this region which indicate onset of aridity around 4000 years ago our results are consistent with the hypothesis that significant spatial variability in the monsoon rainfall observed today was persistent during most of the holocene alternatively the trend can be seen as an adjustment between two phases one between 10 000 and 6000 years ago of increasing precipitation and another between 3500 and 2000 years ago after the arid episode we also report a significant 700 year periodicity similar to that reported recently from the south china sea indicating that the centennial millennial scale response of the indian and chinese monsoons to high latitude forcing may be alike

fluvial landforms and their implication towards understanding the past climate and seismicity in the northern katrol hill range western india abstract the tectonically active kachchh peninsula in western india lies in the southwest monsoon trajectory and hence provides a rare opportunity to decipher the temporal changes in climate tectonics interaction in the evolution of the fluvial landforms reconstructions based on geomorphology sedimentology and geochemistry supported by optical chronology suggest that the fluvial aggradation in the region was initiated during the onset of the indian summer monsoon ism after the last glacial maximum lgm the sedimentary characteristics and major elemental concentrations suggest that the sediments are dominated by fluvially reworked miliolites with subordinate contribution from the mesozoic sandstones and shales and were deposited with the initiation of the ism after the lgm temporal changes in facies architecture and major element concentrations suggest a progressive strengthening of the monsoon between 17 and 12 ka this was succeeded by an overall strengthened ism phase with fluctuations after 12 ka and based on the morphology of the fluvial landforms two major events of enhanced uplift can be suggested the geomorphic expression of the older uplift event dated to 17 ka is represented by the beveled mesozoic bedrock surfaces which accommodated the post lgm valley fill aggradation the younger event of enhanced uplift which is assigned to

speleothem based paleoclimate record from northern oman u th age dating and stable isotope measurements of speleothems from hoti cave in northern oman yield paleoclimate information from the region extending to 125 ka the results 1 provide further confirmation of an early holocene wet period in southern arabia extending from some time prior to 9 7 ka and ending at 6 2 ka 2 demonstrate a second period of wetness closely coinciding with the last interglacial period marine isotope stage mis 5e 3 indicate that during mis 5e southern arabia was considerably wetter than during the early holocene and 4 demonstrate that periods of increased monsoon wind strength based on data from marine sediments do not always coincide with evidence of greatly increased precipitation even from nearby continental areas

the 8 2 kyr bp event simulated by a global atmosphere sea ice ocean model seven freshwater perturbation experiments were performed with a global atmosphere sea ice ocean model to study the mechanism behind the 8 2 kyr bp holocene cooling event these experiments differed in initial state and duration of the applied freshwater pulse while the amount of freshwater was kept constant 4 67x10 14 m 3 one of the scenarios with freshwater added to the labrador sea at a rate of 0 75 sv during 20 years resulted in weakening of the north atlantic thermohaline circulation during 320 years and surface cooling varying from 1 to 5 degreesc over adjacent continents this result is consistent with proxy data suggesting that a meltwater induced weakening of the thermohaline circulation caused the event moreover our results indicate that the time scale of the meltwater release and the initial state are important as both have a strong effect on the magnitude and duration of the produced model response

climate change at the 4 2 ka bp termination of the indus valley civilization and holocene south asian monsoon variability 1 planktonic oxygen isotope ratios off the indus delta reveal climate changes with a multi centennial pacing during the last 6 ka with the most prominent change recorded at 4 2 ka bp opposing isotopic trends across the northern arabian sea surface at that time indicate a reduction in indus river discharge and suggest that later cycles also reflect variations in total annual rainfall over south asia the 4 2 ka event is coherent with the termination of urban harappan civilization in the indus valley thus drought may have initiated southeastward habitat tracking within the harappan cultural domain the late holocene drought cycles following the 4 2 ka bp event vary between 200 and 800 years and are coherent with the evolution of cosmogenic 14c production rates this suggests that solar variability is one fundamental cause behind holocene rainfall changes over south asia

sharp decrease in summer monsoon strength 4000 3500 cal yr b p in the central higher himalaya of india based on pollen evidence from alpine peat age constrained pollen data and magnetic susceptibility of an alpine peat profile from the garhwal higher himalaya display a continuous record of climate and monsoon trends for the past 7800 yr about 7800 cal yr b p dominance of evergreen oak quercus semecarpifolia alder alnus and grasses in the pollen record reflect a cold wet climate with moderate monsoon precipitation from 7800 to 5000 cal yr b p vegetation was progressively dominated by conifers indicating ameliorated climate with a stronger monsoon a warm humid climate with highest monsoon intensity from 6000 4500 cal yr b p represents the mid holocene climatic optimum between 4000 and 3500 cal yr b p the abundance of conifers sharply decreased with the greatest increase in evergreen oak this trend suggests progressive cooling with a decrease in the monsoon to its minimum about 3500 cal yr b p two relatively minor cold dry events at ca 3000 and 2000 cal yr b p marked step wise strengthening of the monsoon until ca 1000 cal yr b p after a cold dry episode that culminated ca 800 cal yr b p the monsoon again strengthened and continued until today a sharp decrease in temperature and rainfall at 4000 3500 cal yr b p represents the weakest monsoon event of the holocene record this cold dry event correlates with proxy data from other localities of the indian subcontinent arabian sea and western tibet

inverse phase oscillations between the east asian and indian ocean summer monsoons during the last 12 000 years and paleo el nino abstract the inverse phase variations between the east asian and indian ocean summer monsoons on the interannual timescale result from the el nino southern oscillation activity in the tropical pacific which potentially provides a new way for studying paleo enso here we reconstruct a 12 000 yr proxy record for the east asian summer monsoon from 13 c time series of the plant cellulose of the hani peat bog in the northeastern china the comparison of it with the peat proxy record of the indian ocean summer monsoon suggests that the el nino conditions are coincident with strong east asian summer monsoons weak indian summer monsoons and drift ice events in the north atlantic at both orbital and millennial time scales the orbital scale inverse phase relationships between the monsoons indicate the occurrence of a long term enso like pattern confirming the sensitivity of the monsoon systems associated with enso to insolation forcing the nine inverse phase variations on the millennial time scales may suggest the nine el nino like patterns superimposing the long term enso like pattern the inverse phase variations between the monsoons also show close correspondence to the drift ice events at high northern latitudes in every case when the abrupt ice rafted debris events occurred in the north atlantic the inverse phase relationship established and the el nino like pattern occurred in the tropical pacific correspondingly the discussions on the influence of ocean thermohaline circulation on these global linkages have been made

millennial pulsing of environmental change in southern california from the past 24 k y a record of indo pacific enso events century long variations in coastal california vegetation inferred from polliniferous sediments deposited in the santa barbara basin record a succession of brief 14 c yr b p ka these millennial scale pulses many of which correspond with western great basin hydrological fluctuations are also linked to events of southwest monsoon intensification in southern asia we suggest that these correlative high frequency variations in the asian summer and northeast pacific winter climates may reflect submillennial scale el nino southern oscillation enso type forcing involving water vapor transport over the indian and pacific oceans

holocene forcing of the indian monsoon recorded in a stalagmite from southern oman a high resolution oxygen isotope record from a thorium uranium dated stalagmite from southern oman reflects variations in the amount of monsoon precipitation for the periods from 10 3 to 2 7 and 1 4 to 0 4 thousand years before the present ky b p between 10 3 and 8 ky b p decadal to centennial variations in monsoon precipitation are in phase with temperature fluctuations recorded in greenland ice cores indicating that early holocene monsoon intensity is largely controlled by glacial boundary conditions after approximately 8 ky b p monsoon precipitation decreases gradually in response to changing northern hemisphere summer solar insolation with decadal to multidecadal variations in monsoon precipitation being linked to solar activity

asian summer monsoon instability during the past 60 000 years magnetic susceptibility and pedogenic evidence from the western chinese loess plateau abstract the 28 m high resolution shajinping loess section in lanzhou on the western chinese loess plateau records a 60 ka millennial summer monsoon variation the record shows that asian summer monsoons have rapid episodic pulse enhancements spanning only ca 1 2 ka in high frequency domain and having sub milankovitch cycles of progressive weakening in low frequency domain in the last glaciation soil formation seems to occur with a surprisingly fast response to these summer monsoon enhancements resulting in weakly or moderately developed paleosol sequences both the pattern and timing of the summer monsoon enhancements show that they can be correlated to most major warm dansgaard oeschger episodes and long term cooling bond cycles of the north atlantic climatic records indicating a possible teleconnection between tropic oceanic air masses and the north atlantic climatic system but differences exist for the transition of mis 2 3 and the holocene where extraordinarily heavy dust input events and fairly variable climatic fluctuations occur for the former and latter respectively a westerlies swing model is proposed to interpret this link

holocene environmental changes in bangong co basin western tibet part 2 the pollen record abstract a better knowledge of environmental variations in key regions such as the tibetan plateau which plays a fundamental role in the indian monsoon dynamics and thus on the whole northern hemisphere atmospheric circulation is of great importance we present here a detailed paleoenvironmental study based on pollen analyses of a sediment core from lake bangong results on the modern pollen rain are also discussed as they give valuable information for the interpretation of the fossil pollen data the pollen results of the bangong sequence and their paleoclimatic interpretation are integrated into a chronological framework on the basis of palynostratigraphical methods because of dead carbon in the lake system it has been difficult to establish a reliable chronology of the bangong sequence using a geochemical model for correcting measured 14c ages the pollen record from lake bangong compares well with the record from lake sumxi in western tibet lake bangong core provides a continuous holocene pollen record before 9600 yr b p the dominance of desertic elements in the pollen record reflects an arid environment at about 9600 yr b p an abrupt shift to steppe vegetation suggests the onset of moister conditions and from 9500 to 6200 yr b p proxy data indicate conditions warmer and wetter than today proxy data also indicate short term changes during this interval climate instability is suggested by the development of a local palustral flora which peaks at 7700 yr b p and is interpreted as a lake regression in phase with increase in the percentage of littoral and saline water diatom species between 8600 and 7700 yr b p the first humid pulse culminating at 9000 8500 yr b p is thus followed by drier conditions around 8000 7700 yr b p the interval between 7200 and 6500 yr b p regarded as optimal for vegetation growth from the pollen record corresponds to a second humid pulse a trend towards aridity is observed during the following period with dry events centered around 5500 3900 3200 and 700 yr b p

the significance of large sediment supply active tectonism and eustasy on margin sequence development late quaternary stratigraphy and evolution of the ganges brahmaputra delta abstract borehole data from the ganges brahmaputra delta system unveil an intriguing late quaternary history controlled by immense sediment discharge tectonics and eustasy sea level first intersected a major portion of the lowstand surface 10 000 11 000 cal yr bp marking the onset of sediment trapping and delta growth despite rapid sea level rise 1 cm yr sediment load was sufficient to maintain relative shoreline stability during this time by 7000 cal yr bp accommodation was quickly infilled with slowed sea level rise leading to upstream channel migration and widespread dispersal of sands this forced coastal progradation along the western delta front where the ganges was situated and much of the river s fine grained discharge bypassed the subaerial delta and formed a prograding deltaic clinoform on the shelf concurrently brahmaputra sediments were sequestered to an inland tectonic basin thus starving the adjacent shoreline and leading to transgression along the eastern delta by 5000 cal yr bp the two rivers had migrated or changed course to discharge along the eastern delta shoreline which began to prograde into what is now the modern river mouth estuary present delta configuration appears to have developed by 3000 cal yr bp evolution of the tectonically active ganges brahmaputra g b system reveals important similarities and differences with other deltas overall facies succession follows that of basic models progressing from an alluvial valley to coastal marine delta front to a prograding subaerial delta plain however the timing thickness and controls of these deposits differ immense sediment discharge from the ganges and brahmaputra rivers allowed initial delta growth 2000 3000 yr earlier than the global average despite debouching onto a canyon incised high energy margin subsequently a thick 50 m transgressive systems tract was formed during the early holocene phase of delta aggradation contrasting with extensive shoreline retreat documented along most margins highstand progradation of coastal and shelf sequences ensued by the middle holocene but subsiding inland basins also favored accumulation of a thick to 40 m highstand sequence in the lower floodplains unlike many other delta systems subsidence in the subaerial g b delta is not dominated by compaction but rather by plate driven tectonic processes that generate rates up to 4 mm year overall the huge sediment load tectonic subsidence major seismic events and a nearshore canyon system have led to widespread sediment dispersal and sequence formation across the subaerial delta shelf and deep sea bengal fan throughout the late quaternary

late quaternary intensified monsoon phases control landscape evolution in the northwest himalaya the intensity of the asian summer monsoon circulation varies over decadal to millennial time scales and is reflected in changes in surface processes terrestrial environments and marine sedi ment records however the mechanisms of long lived 2 5 k y intensified monsoon phases the related changes in precipitation distribution and their effect on landscape evolution and sedimen tation rates are not yet well understood the arid high elevation sectors of the orogen correspond to a climatically sensitive zone that currently receives rain only during abnormal i e strength ened monsoon seasons analogous to present day rainfall anom alies enhanced precipitation during an intensified monsoon phase is expected to have penetrated far into these geomorphic threshold regions where hillslopes are close to the angle of failure we as sociate landslide triggering during intensified monsoon phases with enhanced precipitation discharge and sediment flux leading to an increase in pore water pressure lateral scouring of rivers and ov ersteepening of hillslopes eventually resulting in failure of slopes and exceptionally large mass movements here we use lacustrine deposits related to spatially and temporally clustered large land slides 0 5 km 3 in the sutlej valley region of the northwest him alaya to calculate sedimentation rates and to infer rainfall patterns during late pleistocene 29 24 ka and holocene 10 4 ka inten sified monsoon phases compared to present day sediment flux measurements a fivefold increase in sediment transport rates re corded by sediments in landslide dammed lakes characterized these episodes of high climatic variability these changes thus em phasize the pronounced imprint of millennial scale climate change on surface processes and landscape evolution

mechanisms forcing abrupt fluctuations of the indian ocean summer monsoon during the last deglaciation abstract a piston core from the somali upwelling area has been studied at high resolution for its dinoflagellate cyst content variations in cyst association are inferred to reflect changes in indian ocean summer monsoon intensity several abrupt fluctuations in monsoon intensity are detected for the interval between 20 and 10 ka bp comparison of these fluctuations with changes in contrast between 30 n and 30 s july insolation and the 18 o grip ice core values suggests that different mechanisms influenced monsoon intensity at different time intervals a general trend in monsoon intensity follows variations in insolation contrast lagging minimum contrast by 7 5 ka and maximum contrast by 4 ka this phase lag difference can be explained by assuming the existence of an ice snow cover over central asia tibet during glacial times between 18 7 and 12 5 ka bp fluctuations in sw monsoon intensity may be forced by variations in the thermohaline ocean circulation a rapid transition towards strong sw monsoons at 12 5 ka bp is possibly the result of variations in atmospheric circulation and melting of the snow ice fields in central asia tibet variations in glacial interglacial boundary conditions related to temperature change at northern latitudes are likely to have influenced sw monsoon intensity between 12 5 and 10 ka bp whereas after 10 ka bp variations in tropical land surface boundary conditions may be the dominant forcing factor 0 1997 elsevier science ltd all rights reserved

recognition of the trans european suture zone tesz by the palaeobiogeographical distribution pattern of early to middle ordovician acritarchs

high resolution millennial and centennial scale holocene monsoon variability in the higher central himalayas abstract relict lake sediments situated within the transition of the lesser and higher central himalayas show a persistent millennial to centennial scale monsoon variability during the holocene based on high resolution geochemical data supported by radiocarbon dating six phases of enhanced indian summer monsoon ism with varying magnitude have been identified these are dated between 10 000 9600 9500 9200 8600 5800 5000 4200 3500 2400 and 1800 1000 cal yr bp the millennial and multi centennial scale phases of enhanced ism are broadly comparable with the existing continental and marine records from the monsoon dominated region of se asia suggesting sensitivity of the regions to short term climatic perturbations further the study observed that the phases of weakened ism largely correlate with the drift ice record of the northern atlantic implying a coupling between short term solar irradiance induced glacial boundary condition in the northern atlantic and the millennial and multi centennial scale monsoon variability in the central himalaya