monsoonal variability in the northeastern arabian sea during the past 5000years geochemical evidence from laminated sediments abstract a high resolution laminated sediment record from the oxygen minimum zone in the northeastern arabian sea off pakistan was studied to reconstruct climatic variations to infer the monsoon driven moisture history in the northeastern arabian sea during the past 5000 years we have used three types of independant proxies 1 varve thickness 2 stable isotope ratios of planktic foraminifera and 3 inorganic geochemical composition on the basis of these proxies we have subdivided the sedimentary record into five intervals the first time interval from 5000 to 3900 yr bp period i is dominated by summer monsoon activity deduced from ti al zr al k al and sr ca ratios and oxygen isotope data of planktonic foraminifera slightly enhanced winter monsoon conditions can be inferred for the period from 3900 to about 3000 yr bp period ii maximum precipitation during enhanced winter monsoon activity at about 3100 yr bp was followed by an onset of aridification at 3000 yr bp which continued until about 2000 2200 yr bp period iii the textural mineralogical and geochemical data suggest that this aridification was a gradual century scale process rather than an abrupt event comparison to late holocene records in nw and sw india the near east and tropical africa supports our interpretation that period iii from 3000 to 2000 yr bp is characterized by a gradual change in the atmospherical moisture budget over much of this area reduced monsoonal rainfall may be linked to variations of eurasian snow cover as deduced from varve thickness and element to al ratios period iv from 2000 to about 1500 yr bp is characterized by a summer monsoon dominated sequence with higher precipitation than during period iii the 18o record shows an abrupt shift at 2000 yr bp coinciding with an increase of varve thickness which suggests a cooling of surface waters in the northern arabian sea present day conditions in the makran area were established at 1500 yr bp period v based on inorganic geochemical data and meteorological records at several stations in the makran we conclude that period v is dominated by precipitation during the winter season here declined rainfall associated with the summer monsoon may be related to a less northward position of the intertropical convergence zone itcz during summer and a stronger influence of winter monsoon linked phenomenona

is there a persistent control of monsoon winds by precipitation during the late holocene past variations in the south asian summer monsoon southwest monsoon swm have been inferred earlier mainly using wind proxies in marine sediment cores from the upwelling dominated western arabian sea here we report precipitation proxies i e high resolution stable oxygen isotope variations of two different species of planktonic foraminifera namely gs sacculifer and gs ruber in an ams 14c dated sediment core from the monsoon runoff dominated eastern arabian sea the above data sets reveal that during the past 1800 years and perhaps up to 2800 years resolution of the wind record is too low for comparison reductions in monsoon wind strength in the western arabian sea appear to be persistently accompanied by aridity over india it clarifies that past fluctuations in swm precipitation over the indian subcontinent followed the wind intensity records from the western arabian sea on centennial time scales we further provide the first paleoclimatic evidence for the control exerted by swm precipitation over the swm wind strength via latent heat release

high resolution holocene environmental changes in the thar desert northwestern india sediments from lunkaransar dry lake in northwestern india reveal regional water table and lake level fluctuations over decades to centuries during the holocene that are attributed to changes in the southwestern indian monsoon rains the lake levels were very shallow and fluctuated often in the early holocene and then rose abruptly around 6300 carbon 14 years before the present 14c yr b p the lake completely desiccated around 4800 14c yr b p the end of this 1500 year wet period coincided with a period of intense dune destabilization the major harrapan indus civilization began and flourished in this region 1000 years after desiccation of the lake during arid climate and was not synchronous with the lacustral phase

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

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

late holocene climate in the lower narmada valley gujarat western india inferred using sedimentary carbon and oxygen isotope ratios we report the late holocene climate and vegetation of the lower narmada valley gujarat western india as inferred from the stable carbon and oxygen isotopic composition 13c and 18o of sediment

granulometric characterization and evaluation of annually banded mid holocene estuarine silts welsh severn estuary uk coastal change sea level and climate abstract holocene silts salt marshes and highest intertidal supratidal peats are superbly exposed on a 15 km coastal transect which reveals two laterally extensive units of annually banded silts beds 3 7 associated with three transgressive regressive silt peat cycles early sixth early fourth millennium bc bed 3 in places is concordantly and gradationally related to peats above and below but in others transgresses older strata bed 7 also grades up into peat but everywhere overlies a discordance the banding in bed 3 at three main and two minor sites was resolved and characterized texturally at high resolution 2 5 5 mm contiguous slices using laser granulometry ls230 with pids and a comprehensive scheme of data assessment most of bed 3 formed very rapidly at peak values of several tens of millimetres annually in accordance with modelled effects of sea level fluctuations on mature marshes bed concordant and gradational and on marshes growing up after coastal erosion and retreat bed with discordant base using data from the modern severn estuary the textural contrast within bands and its variation between bands points to a variable but overall milder mid holocene climate than today the inter annual variability affected marsh dynamics as shown by the behaviour of the finely divided plant tissues present given local calibration the methodology is applicable to other tidal systems with banded silts in britain and mainland northwest europe

identification of multiple loess units within modern soils of clay county nebraska abstract modem soils in south central nebraska may have formed in parent material made up of more than one loess unit the aerial extent of these soils could be as much as 1 6 million ha the presence of lithologic discontinuities within these soils complicates the explanation of their genesis the objectives of this study were to determine if multiple loess units exist in the modem soils and to identify properties that could be used to differentiate between the loess units four pedons representing an extensive soil and landscape map unit were sampled and particle size distribution cation exchange capacity extractable bases organic carbon content and mineralogy were determined results indicated that at least two loess deposits associated with the bignell loess of holocene age and the peoria loess of late wisconsinan age were present within the pedons studied these loess units can be differentiated on the basis of the following properties particle size distribution chemistry volcanic ash distribution and clay mineralogy differences in these properties within the pedons can be attributed to changes in sediments carried by the platte river the assumed source of the loess

a mid to late holocene record of palaeoclimatic changes from nal sarovar a palaeodesert margin lake in western india with a view to obtain palaeoclimatic data from a climatically sensitive region we have studied core samples from nal sarovar a large shallow lake lying within the palaeodesert margin of thar in western india a combination of c n ratios and 13c on a radiocarbon dated core section have been used as climatic proxies a high resolution record extending back to ca 6 6 ka bp has been reconstructed the data indicate that in the past climate has varied from drier to wetter than present on time scales of few hundreds to few thousands of years there are however significant differences on the timing of these wet and dry periods when compared with the available data from lakes farther north in rajasthan further it looks unlikely that during the 6 6 ka the catchment areas of nal sarovar experienced such a significant increase in rainfall as has been suggested for the rajasthan lakes interestingly drier periods in nal sarovar data appear to correlate well with periods of glacier expansion in eurasia indicating that the palaeoclimatic variations recorded in nal sarovar are a regional feature 1997 john wiley sons ltd

evidence of enhanced winter precipitation and the prevalence of a cool and dry climate during the mid to late holocene in mainland gujarat india the kothiyakhad sedimentary sequence of mahi estuary in mainland gujarat india contains valuable information on late to mid holocene climatic conditions as inferred by phytolith palynofacies magnetic susceptibility and clay mineralogical studies three distinct climatic regimes ie phase i ii and iii were established phase i 3660 3400 yr bp shows a gradual weakening of sw monsoonal activity though overlapped by enhanced western disturbances which led to the development of cool climatic conditions the coupled effect of sw monsoon and enhanced winter precipitation produced improved hydrological conditions which supported the agrarian society of the indus valley civilization until the beginning of phase ii 3400 3000 yr bp during phase ii the sw monsoon was in a state of severe recession leading to severe drought like conditions other than for a brief but intensely warm and humid pulse recorded at 3320 yr bp associated with sw monsoonal activity in phase iii 3000 2850 yr bp sw pre

mangrove pollen of indonesia and its suitability as a sea level indicator we investigated the mangroves of southeast sulawesi indonesia to assess their potential as proxies for reconstructing sea level during the holocene initial investigations confirmed that the mangrove species demonstrate zonations parallel to the shoreline and are dominated by the family rhizophoraceae with avicennia heritiera and sonneratia also important constituents of the mangroves we investigated the vertical distributions of pollen assemblages at three sites partial cca analysis demonstrated that at all three sites elevation was a significant control on the distribution of pollen assemblages in surface samples we combined the three contemporary transects to develop a regional transfer function to explain the relationship between the surface assemblages and elevations using the maximum likelihood ml method the developed transfer function indicated mangrove pollen can be used as a precise indicator of past sea levels with an error of 0 22 m the transfer function was applied to two fossil cores from the wakatobi marine national park and evaluated using the modern analogue technique both cores showed similar patterns in changes of palaeo mangrove elevation and had modern analogues in the contemporary training set we conclude that mangrove pollen is a suitable proxy for reconstructing sea level in tropical environments

pollen analysis of mangrove sediments as a sea level indicator assessment from tongatapu tonga increasing necessity for precision in holocene sea level records calls for definitive sea level indicators to be assessed these are reviewed by use of examples from the s w pacific and the technique of pollen analysis of mangrove sediments evaluated as a high resolution sea level indicator example is given how a fully calibrated sea level reconstruction can be gained from holocene mangrove peat units from tongatapu tonga by application of features established from a modern analogue of surface samples to interpret fossil pollen patterns

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

sedimentary record of late holocene relative sea level change and tectonic deformation from the guerrero seismic gap mexican pacific coast studies of the coastal sedimentary record have allowed both the reconstruction of relative sea level changes and the determination of local rates and magnitudes of tectonic deformation particularly in tectonically active areas despite their successful use elsewhere studies of this type are much less common for the mexican pacific coast which parallels the cocos north america subduction plate boundary stratigraphic geochemical and microfossil data from sediments in laguna mitla the pacific coast of guerrero mexico document late holocene sea level changes induced by tectonic activity in the mexican subduction zone three major events are identified first the formation of the lagoon by c 4630 yr bp as indicated by a freshwater to brackish peat second a relative sea level rise or land subsidence as indicated by a shift from a freshwater marginal lagoon environment to a marine setting preceded by a marine inundation represented by a sand unit possibly a tsunami deposit by c 3400 yr bp and finally a return to lagoonal conditions indicating a drop in relative sea level or coastal uplift by c 2300 yr bp the laguna mitla stratigraphy indicates general coastal subsidence or relative sea level rise of c 1 mm yr we argue that these relative sea level land level changes have been induced by tectonic activity associated with the mexican megathrust a plausible explanation for the 3400 yr bp marine inundation is probably a tsunami produced by a large seismic event accompanied by coastal subsidence discrete fining upward fine to coarse sand units with an erosional basal contact medium to poor sorting and clay mud rip up clasts an increase in na and sr elemental concentrations indicative of a marine origin and the landward extent of the sands support a tsunamigenic source for these deposits however these apparent tsunami deposits require further study to determine their lateral extent and to assess whether they can be correlated from one site to another this study demonstrates the applicability of a multiproxy sedimentary approach in interpreting relative sea level land level changes and to derive data on related earthquake and tsunami events in tropical coastal lagoons

mid late holocene monsoonal variations from mainland gujarat india a multi proxy study for evaluating climate culture relationship a multi proxy study involving palynology phytoliths sedimentology clay mineralogy carbon isotopes and magnetic mineralogy was carried out on wadhwana lake sediments from sub humid zone of mainland gujarat to determine the mid holocene climatic fluctuations and its possible impact on the harappan culture the 1 03 m sediment profile of wadhwana lake shows five paleoclimatic phases the study reveals high lake stand during phase i 7500 5560 cal yr bp a considerable cool and moist climatic condition during phase i is inferred due to the presence of pollen belonging to wet evergreen taxa and high phytolith climatic index ic later part of phase i show gradual replacement of evergreen to deciduous pollen taxa decrease in ic value and dominance of smectite over kaolinite indicating a reduction in wet climatic condition due to decline in the precipitation and prevalence of seasonally dry climate however a large variety of madhuca meliaceae asteraceae pollen taxa abundant fresh water algae chrozophora and ceralia pollen in this period provides evidence of intensified arboriculture and agricultural activity low lake level and dry climate have been documented during phase ii 5560 to 4255 cal yr bp and are synchronous with the lake records of western indian region high values of phytolith aridity index iph high primary minerals increase in the i 13c values provide evidence for excessive dry climatic conditions at 4255 cal yr bp phase iii shows a gradual strengthening of sw monsoon after 3500 cal yr bp phase iv shows a short pulse of dry climatic conditions 3238 to 2709 cal yr bp followed by somewhat similar to present day climate for phase v the study concludes that onset of dry climate after 5500 cal yr bp is a regionally spread synchronous event that has been documented in several lake records of western india it is surmised that the urbanization in harappan civilization in north west india was coincident with the initial phase of declining rainfall of mid holocene the emergence of cultural complexity of harappan civilization should be seen as an initial adaptation to earliest phase of environmental deterioration and its subsequent decline is probably linked with the changing seasonality pattern and excessive dry climate of later phase 4200 4255 cal yr bp of mid holocene

major and trace element geochemistry of bay of bengal sediments implications to provenances and their controlling factors major and trace elements of sediments from a 13 m long piston core sk187 pc33 from the western bay of bengal have been investigated in this study to infer about the changes in provenances and related controlling factors during the last glacial interglacial period factor analysis of these geochemical dataset ascertains dominant role of riverine supply of sediments in regulating the geochemistry of sk187 pc33 sediments the al normalized major k and ti and trace elemental cu and cr ratios of these marine sediments fall within the ranges observed for their major provenances viz ganga brahmaputra and the godavari krishna gk rivers and depth profiles of these ratios showed significant variations with synchronous excursion at around the last glacial maxima lgm implying a change in their provenances with relatively reduced himalayan contribution during this climatic event relatively higher cia and ti ca ratios with lower k al ratios of these sediments during the holocene hint intense chemical weathering in the river basins in response to strengthened monsoon during this period inverse model calculation of al normalized elemental ratios of the sediments estimated an average sediment contribution of 66 13 and 34 13 from the himalayan and the peninsular indian rivers to the core site respectively consistent with the depth profiles of elemental ratios the estimated sediment contributions from the himalayan rivers are observed to decrease by 20 from the himalayan rivers particularly that of the ganga to the studied location during the lgm lowering of sediment supply from the himalaya ganga during the lgm is due to weakening of south west monsoon and reduction in available exposure area for weathering due to extent of glacier cover outcomes of this study underscore the strong linkage between erosion and climate

late quaternary sediments minerals and inferred geochemical history of didwana lake thar desert india abstract variations in clastic sediment texture mineralogy of both evaporites formed at the surface and precipitates formed below the lake floor and the relative chemical activities of the major dissolved components of the chemical precipitates have allowed reconstruction of the history of salinity and water level changes in didwana lake thar desert india hypersaline conditions prevailed at about the last glacial maximum with little evidence of clastic sediments entering the lake between ca 13 000 and 6000 b p the lake level fluctuated widely the lake alternately hypersaline and fresh and clastic sediments were delivered to the lake at a low rate deep water conditions occurred ca 6000 b p and clastic influx increased abruptly the water level dropped towards 4000 b p when the lake dried briefly since 4000 b p the lake has been ephemeral with a lowered rate of sedimentation and mildly saline conditions rather like those of today this sequence of changes documented in the lake parallels changes in vegetation recorded in published pollen diagrams from both the thar and the arabian sea correlation of the various lines of evidence suggests that the climate of the last glacial maximum at didwana was dry and windy with a weak monsson circulation the monsson was re established between ca 13 000 and a little before 6000 b p and when winter rainfall increased ca 6000 b p the lake filled to its maximum depth

influence of the intertropical convergence zone on the east asian monsoon a palaeoclimate record spanning the past 16 000 years with nearly annual time resolution has been obtained from lake huguang maar in china the magnetic properties and titanium content of the lake sediments are thought to reflect changes in the east asian winter monsoon strength thus providing a complement to palaeoclimate archives that record the strength of the rain bearing summer monsoon the records point to an anti correlation between winter and summer monsoon strength best explained by migration of the intertropical convergence zone itcz a belt of low pressure air at the equator interestingly the decline of china s tang dynasty and the mayan civilization broadly coincide with drought periods recorded in lake huguang maar and cariaco basin sediments respectively did major shifts in the position of the itcz catalyse simultaneous events in civilizations on opposite sides of the pacific ocean the asian australian monsoon is an important component of the earth s climate system that influences the societal and economic activity of roughly half the world s population the past strength of the rain bearing east asian summer monsoon can be reconstructed with archives such as cave deposits1 2 3 but the winter monsoon has no such signature in the hydrological cycle and has thus proved difficult to reconstruct here we present high resolution records of the magnetic properties and the titanium content of the sediments of lake huguang maar in coastal southeast china over the past 16 000 years which we use as proxies for the strength of the winter monsoon winds we find evidence for stronger winter monsoon winds before the bolling allerod warming during the younger dryas episode and during the middle and late holocene when cave stalagmites suggest weaker summer monsoons1 2 3 we conclude that this anticorrelation is best explained by migrations in the intertropical convergence zone similar migrations of the intertropical convergence zone have been observed in central america for the period ad 700 to 900 refs 4 6 suggesting global climatic changes at that time from the coincidence in timing we suggest that these migrations in the tropical rain belt could have contributed to the declines of both the tang dynasty in china and the classic maya in central america

stratigraphical and palynological appraisal of the late quaternary mangrove deposits of the west coast of india the organic deposits derived from the mangrove swamps form reliable stratigraphic markers within the late quaternary sequence of kerala konkan basin three generations of such deposits have been identified the older one is dated to around 43 000 40 000 14c yr b p with a few dates beyond the range of radiocarbon the younger ones date from the middle holocene to latest pleistocene 10 760 4540 14c yr b p and the late holocene 4000 14c yr b p pollen analyses confirm that the deposits are mostly derived from the mangrove vegetation peat accumulation during the period 40 000 28 000 14c yr b p can be correlated with the excess rainfall 40 100 greater than modern values of the asian summer monsoon the low occurrence of mangrove between 22 000 and 18 000 14c yr b p can be attributed to the prevailing aridity and or reduced precipitation associated worldwide with last glacial maximum because exposure surfaces and ferruginous layers are commonly found in intervals representing this period the high rainfall of 11 000 4000 14c yr b p is found to be the most significant as the mangrove reached an optimum growth around 11 000 14c yr b p but with periods of punctuated weaker monsoons from the present and previous studies it has been observed that after about 5000 or 4000 14c yr b p the monsoons became gradually reduced leading to drying up of many of the marginal marine mangrove ecosystems a case study of hadi profile provided an insight to the relevance of magnetic susceptibility to record the ecological shift in late holocene

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

mid holocene sedimentation and landscape evolution in the western great rann of kachchh india abstract this study is an attempt to reconstruct the history of sedimentation and landform evolution in the western great rann of kachchh field stratigraphy sedimentology geochemistry optical and radiocarbon dating suggest dominance of tidal flat sedimentation with subordinate contribution coming from the southern draining streams during 5 5 to 2 ka variability in the geochemical proxies are interpreted as a surrogate for tidal energy viz zr and cr which along with the major elemental ratios tio 2 al 2 o 3 and k 2 o al 2 o 3 indicate enhanced tidal energy condition during 5 5 to 5 ka and around 3 ka a close correspondence of the major and trace elements with those of the indus river sediment and the dominance of illite suggest the indus river was a major contributor for western rann sedimentation during the mid holocene these sediments were routed through the kori creek during marginally high sea level until around 2 2 ka and the present landscape was largely sculptured after the 1819 allah bund earthquake

holocene sea levels and palaeoenvironments malay thai peninsula southeast asia sedimentological and palynological investigations of great songkhla lakes east coast of the malay thai peninsula southeast asia reveal sedimentary sequences rich in palynomorph assemblages dominated by pollen of mangroves and freshwater swamps compared with other regions in southeast asia the assemblages are of relatively low diversity geochronological data indicate that the great songkhla lakes record one of the earliest mangrove environments in southeast asia 8420 8190 cal yr bp which are subsequently replaced by a freshwater swamp at 7880 7680 cal yr bp owing to the decline of marine influence sea level observations from great songkhla lakes and other areas of the malay thai peninsula reveal an upward trend of holocene relative sea level from a minimum of 22 m at 9700 9250 cal yr bp to a mid holocene high stand of 4850 4450 cal yr bp which equates to a rise of c 5 5 mm yr the sea level fall from the high stand is steady at c 1 1 mm yr geophysical modelling shows that hydroisostasy

evidence for solar forcing on the indian monsoon during the last millennium abstract solar influence on the intensity of the indian monsoon is demonstrated using a sediment core from the eastern arabian sea dating back to 1200 yr through pattern matching as well as spectral analysis of proxy records of monsoon and solar activity the intensity of the indian monsoon is found to have decreased during periods of solar minima during the last millennium periodicities of 200 20 105 15 and 60 10 yr are observed in the proxy records coinciding with those known for solar cycles the 60 yr periodicity observed in the instrumental rainfall data appears to be of solar origin and supports the hypothesis of solar control on the indian monsoon on a multi decadal time scale evidence for the presence of a coupled atmospheric forcing for the indian and east african monsoons on a centennial time scale is also seen

1300 km long late pleistocene holocene shelf edge barrier reef system along the western continental shelf of india occurrence and significance abstract a detailed analysis of echosounding and side scan sonar data collected from the western continental margin of india has revealed the presence of prominent shelf edge reefs concentrated mostly on the central and the southern parts their depth of occurrence varies between 85 and 136 m the reefs are 1 12 m high and 0 1 2 6 km wide av 700 m morphologically they may be classified into 1 simple and 2 complex types the former are single and broad or narrow av width 350 m while the latter are generally massive av width 950 m with several superimposed peaks sub bottom profiles indicate the presence of paleolagoons this reef system more than 1000 km long trends nnw sse i e parallel to subparallel to the present day shoreline it is surmised that coral algal reef growth commenced with the advent of the holocene transgression and favorable antecedent topography and continued until early holocene subsequently rapid sea level rise drowned the reefs the shelf edge reefs therefore are part of relict submerged barrier reef system and reflect late pleistocene early holocene shoreline

holocene submarine terraces on the western continental shelf of india implications for sea level changes abstract detailed echosounding side scan sonar and high resolution seismic reflection profiling on the western continental shelf of india have revealed a series of submarine terraces at different depths between the outer fringe of the inner shelf and the shelf break being more common between 11 and 20 n the terraces are prominent between water depths of 50 and 115 m and occur at six distinct levels 1 55 60 m 2 65 70 m 3 75 80 m 4 85 90 m 5 95 100 m and 6 110 115 m they are categorized into 1 wave cut terraces 2 coral algal reef induced terraces and 3 palaeo beach barrier terraces the reef induced terraces occur predominantly in the carbonate rich province 20 13 n while both the wave cut and palaeo beach barrier terraces are discernible in the terrigenous regime which is located north and south of these latitudes despite their genetic differences their evolution is ascribed to reef growth progradation and wave activity during the lowstands of sea level and reflect different phases of stillstands of the late pleistocene to mid holocene age

buried salt marsh edges and tide level cycles in the mid holocene of the caldicot level gwent south wales uk combining stratigraphical sedimentological and biofacial foraminifera approaches a high resolution time constrained study has been made of a mid holocene c 5800 4900 14c yrs bp estuarine su

reconstruction of the variability of the southwest monsoon during the past 3 ka from the continental margin of the southeastern arabian sea from temporal variation in 18o in globigerinoidesruber and g sacculifer and geochemical indices of weathering erosion chemical index of alteration al and ti we infer rapid southwest monsoon swm deterioration with dwindling fluvial and detrital fluxes at ca 450 650 1000 and 1800 2200 cal a bp during the late holocene we have evaluated the role of solar influx reconstructed and high latitude climate variability archived in grip and gisp 2 cores on swm precipitation broadly our 18o climate reconstruction is concordant with grip and gisp 2 and supports a teleconnection through atmospheric connection between the swm and the north atlantic climate albeit temporal extents of the little ice age and medieval warm period from high latitude are not entirely coeval moreover there is a humid climate and enhanced precipitation during the terminal stages of the little ice age the medieval warming ca ad 800 1300 is not synchronous either and is punctuated by an arid event centred at 1000 a bp although the delineation of the specific influence of solar influx on swm precipitation is elusive we surmise that swm precipitation is a complex phenomenon and local orography along southwestern india may have a role on the entrapment of moisture from the southwest trade winds when these hit land copyright 2009 john wiley sons ltd

a mid late holocene flood record from the alluvial reach of the mahi river western india abstract flooding of rivers in india is linked with the peak monsoons investigating the linkage between monsoonal patterns and flood history of various rivers is therefore of fundamental importance in the indian context in the present study the slackwater deposits in the alluvial reaches of the mahi river basin western india have been documented these occur in the ravines incised during the early holocene on an alluvial surface comprising sediments of late pleistocene age the slackwater deposits occur at elevations up to 20 m from the present river level and extend to about 500 m inland the carbonate rich sediments forming the ravine cliffs have provided bank stability and the dissections in the ravines have helped in the accumulation of slackwater deposits due to backflooding of the floodwater from the main channel recent gullies have incised the sediments and exposed deposits related to major flood events the best exposures of slackwater deposits have been observed at dodka the sediment succession of the slackwater deposits is dominated by bedsets and laminasets of silt and sand separated by colluvial sediments four events of flood deposition occurred during the mid to late holocene two units of slackwater deposits swd 2 and 4 have been dated by irsl at 4 6 1 ka and 1 7 0 5 ka the stratigraphy of these deposits indicates that the first two slackwater units swd1 and swd2 have resulted due to flooding in a regime of intense monsoon the other two units however represent extreme high magnitude floods in a period of low average precipitation

geochemical signatures of late holocene paleo hydrological changes from phulera and pokharan saline playas near the eastern and western margins of the thar desert india abstract stratigraphical mineralogical geochemical and optical dating methods were used to reconstruct paleo hydrological changes in two playas phulera 500 mm a and pokharan 200 mm a in near extremum climatic regions of the thar desert sediment successions in shallow profiles from phulera and pokharan contain three and four stratigraphic units respectively each with characteristic geochemical properties these units reflect changes in chemical weathering detrital input salinity and provide a measure of the changes in precipitation i e monsoon through time sediments from pokharan suggest short rainfall events during ca 6 6 4 ka relatively stable fresh water higher and persistent rainfall regime during 4 2 3 ka and a hyper saline low rainfall condition during 2 3 1 1 ka sediments at phulera record hyper saline low rainfall lacustrine conditions during 1 4 ka higher abundance of gypsum in pokharan 2 3 1 1 ka and proto dolomite in phulera 2 3 1 4 ka are nearly synchronous and reflect enhancement of salinity selenite crystals in pokharan and large desiccation cracks in buried horizons at phulera reflect desiccation of playas at ca 2 ka both playas progressively became less saline after 1 4 ka given the regional nature of this record these changes are attributed to fluctuation of the monsoon over the indian sub continent

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

planforms of self consistently generated plates in 3d spherical geometry 1 in the past decade several studies have documented the effectiveness of plastic yielding in causing a basic approximation of plate tectonic behavior in mantle convection models with strongly temperature dependent viscosity strong enough to form a rigid lid in the absence of yielding the vast majority of such research to date has been in either two dimensional or three dimensional cartesian geometry in the present study mantle convection calculations are performed to investigate the planform of self consistent tectonic plates in three dimensional spherical geometry the results are compared to those of similar calculations where a three dimensional cartesian geometry is used we found that when yield stress of the lithosphere is low 20 mpa a great circle subduction zone forms at low intermediate yield stresses 100 mpa plates spreading centers and subduction zones formed and were destroyed over time at high intermediate yield stresses 200 mpa two plates form separated by a great circle boundary that is a spreading centre on one side and a subduction zone on the other side at high yield stresses 400 mpa a rigid lid was observed the great circle subduction zone and the rigid lid are stable over time while at intermediate yield stresses some episodic behavior is observed the spherical cases showed a higher more earth like toroidal poloidal ratio of the surface velocity field than the cartesian cases

mid holocene mangrove succession and its response to sea level change in the upper mekong river delta cambodia abstract middle holocene vegetation and mangrove successions are clearly evident in the palynological records of two cores from the upper mekong river delta in cambodia spanning from 9 4 to 6 3 cal ka bp the cores mainly record a transgressive sequence from floodplain freshwater marsh to tidal flat which was overlain by mangrove corresponding to the decelerated sea level rise at 8 3 cal ka bp pioneer mangrove species sonneratia alba and sonneratia caseolaris appeared in the sediments and then was replaced by regressive mangrove succession containing upward increasing abundances of rhizophora apiculata and bruguiera spp high salinity and flooding tolerant community s alba was developed at the western core site psg at 8 2 cal ka bp and the eastern core site pk at 7 5 cal ka bp the time difference of s alba appearance between the two sites might be resulted from the complexity of sedimentary environment where a higher sediment supply was provided to the western floodplain than to the eastern floodplain after 7 5 cal ka bp aggradational stacking of intertidal sediments of which the thickness is larger than the present maximum tidal range may have resulted from continuous sea level rise during 7 5 7 0 cal ka bp