hurricanes and climate in the caribbean during the past 3700 years bp a multiproxy analysis of lacustrine sediments cored in grand case pond at saint martin north of the lesser antilles archipelago reveals three distinct climatic periods for the last 3700 years from 3700 to 2500 yr cal bp and from 1150 yr cal bp to the present carbonate mud deposition occurred in connection with pond lowstands these periods were also punctuated by severe drought events marked by gypsum laminae and hurricane landfalls leading to marine sand inputs into the pond the intermediate time interval from 2500 to 1150 yr cal bp is typified by black organic mud deposition suggesting that hypoxic to anoxic conditions prevailed at the pond bottom these were probably linked with a perennial pond highstand and reflect more uniform and wetter climatic conditions than today the carbon isotopic composition of the ostracod perissocytheridea bisulcata shows that the lowest 13c values are recorded during the hypoxic periods as a consequence of bacterial recycling of isotopically depleted org

the belize margin revisited 1 holocene marine facies belize is best known for its 260 km long barrier reef and associated lagoon reef shoals but also exhibits a complete transition from near shore siliciclastics to pure carbonate deposits across a narrow shelf lagoon and in addition has three of the very few occurrences of caribbean atolls published holocene facies relationships on the belize shelf have been semi quantitative in that they are based on quantitative point count thin section data in the northern shelf lagoon and on qualitative estimates of thin section composition in the southern shelf lagoon this contrast has been rectified by point counting southern shelf lagoon thin sections and incorporating the result with published point count data from the northern shelf supplemented by additional information to produce a modified factor analysis facies distribution for the entire shelf the resulting nine facies are extended seaward to include previously published analysis of sediment on the three belize off shelf atolls resulting in a total of 11 facies in addition the carbonate mud distribution has been mapped over both the shelf and atolls with the not unexpected result that in places the distribution pattern clearly indicates a barrier platform source of lagoon deposited carbonate mud dunham s limestone classification terminology has been used as a basis for description to make these relationships more useful to those reconstructing ancient limestone depositional environments

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

tropical sea surface temperatures for the past four centuries reconstructed from coral archives most annually resolved climate reconstructions of the common era are based on terrestrial data making it a challenge to independently assess how recent climate changes have affected the oceans here as part of the past global changes ocean2k project we present four regionally calibrated and validated reconstructions of sea surface temperatures in the tropics based on 57 published and publicly archived marine paleoclimate data sets derived exclusively from tropical coral archives validation exercises suggest that our reconstructions are interpretable for much of the past 400 years depending on the availability of paleoclimate data within and the reconstruction validation statistics for each target region analysis of the trends in the data suggests that the indian western pacific and western atlantic ocean regions were cooling until modern warming began around the 1830s the early 1800s were an exceptionally cool period in the indo pacific region likely due to multiple large tropical volcanic eruptions occurring in the early nineteenth century decadal scale variability is a quasi persistent feature of all basins twentieth century warming associated with greenhouse gas emissions is apparent in the indian west pacific and western atlantic oceans but we find no evidence that either natural or anthropogenic forcings have altered el nino southern oscillation related variance in tropical sea surface temperatures our marine based regional paleoclimate reconstructions serve as benchmarks against which terrestrial reconstructions as well as climate model simulations can be compared and as a basis for studying the processes by which the tropical oceans mediate climate variability and change

observations of a squall line and its near environment using high frequency rawinsonde launches during vortex2 abstract rawinsonde data were collected before and during passage of a squall line in oklahoma on 15 may 2009 during the second verification of the origins of rotation in tornadoes experiment vortex2 nine soundings were released within 3 h allowing for unprecedented analysis of the squall line s internal structure and nearby environment four soundings were released in the prestorm environment and they document the following features low level cooling associated with the reduction of solar isolation by a cirrus anvil abrupt warming 1 5 k in 30 min above the boundary layer which is probably attributable to a gravity wave increases in both low level and deep layer vertical wind shear within 100 km of the squall line and evidence of ascent extending at least 75 km ahead of the squall line the next sounding was released 5 km ahead of the squall line s gust front it documented a moist absolutely unstable layer within a 2 km deep layer of ascent with vertical air velocity of approximately 6 m s 1

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

reconstructing relative flooding intensities responsible for hurricane induced deposits from laguna playa grande vieques puerto rico extreme coastal flooding primarily during hurricane strikes has deposited sand rich layers in laguna playa grande a back barrier lagoon located on the island of vieques puerto rico silici clastic grain size distributions within these overwash deposits fine landward away from the barrier and toward the mainland a simple advective settling model can explain this pattern of lateral sorting and is used to constrain the relative magnitude of past flooding events a deposit associated with the a d 1928 san felipe hurricane is used as a modern analogue to test the technique which produces reasonable estimates for wave heights that exceed the barrier during the event a 5000 yr reconstruction of local flooding intensity is developed that provides a measure of the competence for each overwash event to transport coarser grained sediment a fixed distance into the lagoon this reconstruction indicates that although the laguna playa grande record exhibits large scale changes in hurricane frequency on centennial to millennial time scales the magnitude of these events has stayed relatively constant over the last 5000 yr no evidence exists for an anomalously large hurricane or tsunami event with a competence for sediment transport greater than historical hurricane events

timing and climate forcing of volcanic eruptions for the past 2 500 years volcanic eruptions contribute to climate variability but quantifying these contributions has been limited by inconsistencies in the timing of atmospheric volcanic aerosol loading determined from ice cores and subsequent cooling from climate proxies such as tree rings here we resolve these inconsistencies and show that large eruptions in the tropics and high latitudes were primary drivers of interannual to decadal temperature variability in the northern hemisphere during the past 2 500 years our results are based on new records of atmospheric aerosol loading developed from high resolution multi parameter measurements from an array of greenland and antarctic ice cores as well as distinctive age markers to constrain chronologies overall cooling was proportional to the magnitude of volcanic forcing and persisted for up to ten years after some of the largest eruptive episodes our revised timescale more firmly implicates volcanic eruptions as catalysts in the major sixth century pandemics famines and socioeconomic disruptions in eurasia and mesoamerica while allowing multi millennium quantification of climate response to volcanic forcing

holocene lagoonal development in the isolated carbonate platforms off belize abstract thirty one vibracores were taken in interior lagoons of glovers reef lighthouse reef and turneffe islands three isolated carbonate platforms offshore belize central america holocene facies successions overlying the pleistocene limestone bedrock begin with soils followed by mangrove peats and marine carbonate sediments of lagoonal origin the soils formed on top of subaerially exposed pleistocene limestone before the holocene transgression mangrove peats developed during initial flooding of the platforms glovers ca 8 5 ky lighthouse ca 7 ky turneffe ca 6 ky bp as water depths increased reefs colonized platform margins lagoonal circulation improved thereby promoting carbonate production the basal lagoonal carbonate sediments are characterized by shell beds and or halimeda packstones grainstones mollusk dominated wackestones and packstones follow upsection in glovers and lighthouse reefs at present open circulation prevails in glovers and lighthouse lagoons in contrast organic rich halimeda wackestones and packstones dominate the turneffe islands holocene succession the main lagoon area of turneffe is enclosed by mangroves and restricted circulation prevails factors that explain the differences in geomorphology circulation and facies are variations in depth of antecedent topography and degree of exposure to waves and currents the thickness of holocene lagoon sediments may exceed the maximum core length of 6 m in all atolls holocene sedimentation rates average 0 6 m ky with highest rates in turneffe 0 82 m ky followed by lighthouse 0 53 m ky and glovers 0 46 m ky like in many other isolated carbonate platforms and atolls lagoon floor sedimentation did not keep pace with rising sea level leading to unfilled accommodation space at present glovers has an 18 m deep lagoon while lighthouse and the main turneffe lagoon are 8 m deep it is unlikely that the lagoons will be completely filled during the holocene sea level highstand cycle this observation should be kept in mind when using cycle thickness as a proxy for eustatic sea level change in fossil carbonate platforms

atlantic seasonal hurricane frequency part i el nino and 30 mb quasi biennial oscillation influences abstract this is the first of two papers on atlantic seasonal hurricane frequency in this paper seasonal hurricane frequency as related to e1 nino events during 1900 82 and to the equatorial quasi biennial oscillation qbo of stratospheric zonal wind from 1950 to 1982 is discussed it is shown that a substantial negative correlation is typically present between the seasonal number of hurricanes hurricane days and tropical storms and moderate or strong 15 cases el nin off the south american west coast a similar negative anomaly in hurricane activity occurs when equatorial winds at 30 mb are from an easterly direction and or are becoming more easterly with time during the hurricane season this association of atlantic hurricane activity with el nino can also be made with the southern oscillation index by contrast seasonal hurricane frequency is slightly above normal in non el nino years and substantially above normal when equatorial stratospheric winds blow from a westerly direction and or are bec

siliciclastic foundations of quaternary reefs in the southernmost belize lagoon british honduras analyses of a closely spaced lattice of seismic profiles from the southernmost belize lagoon combined with logs of nearby drill holes reveal that the quaternary late pleistocene plus holocene reefs and carbonate sediments are growing on siliciclastic sediments which have an alluvial and delta like morphology the residual relief of these coastal plain deposits localized initial coral growth or an accumulation of calcareous skeletal material favored sites for these were elevations such as levees and bars of remnant channels and deltaic lobes

persistent positive north atlantic oscillation mode dominated the medieval climate anomaly the medieval climate anomaly mca was the most recent pre industrial era warm interval of european climate yet its driving mechanisms remain uncertain we present here a 947 year long multidecadal north atlantic oscillation nao reconstruction and find a persistent positive nao during the mca supplementary reconstructions based on climate model results and proxy data indicate a clear shift to weaker nao conditions into the little ice age lia globally distributed proxy data suggest that this nao shift is one aspect of a global mca lia climate transition that probably was coupled to prevailing la nina like conditions amplified by an intensified atlantic meridional overturning circulation during the mca

the belize margin revisited 2 origin of holocene antecedent topography the importance of antecedent topography in dictating holocene facies patterns has been generally recognized there is however disagreement as to origin or lithology of the antecedent topography particularly with respect to the siliciclastic or carbonate nature of the underlying topography and structural patterns to help resolve these problems published and unpublished information have been compiled to produce a structural fabric map of onshore and offshore belize that includes a new geologic map of the country the map along with illustrated seismic lines demonstrates the occurrence of a number of nne trending transpressional faults in which landward directed thrusting is consistently displayed along with tectonic inversion offshore wells in conjunction with the seismic lines document the inversion as post eocene suggesting a similar age for transpressional fault movement presumably the landward directed thrusting reflects the opposing force of eastward directed subduction along the western margin of central america relative to the westward seafloor spreading of the caribbean cayman ridge the belize faults show little current seismicity but nevertheless the resulting structures have affected quaternary carbonate deposition as evidenced on an illustrated seismic line by both seaward and landward bi directional progradation of the reef margin from an underlying structural high the structural influence on the development of holocene antecedent topography is further suggested by the occurrence of a bouguer gravity plateau in the same shelf area that marks the occurrence of the belize lagoon rhomboid shelf atolls the youngest documented lithology of 12 illustrated offshore exploration wells is miocene carbonate in some wells the carbonate is interpreted as extending into the pliocene and holocene although no age diagnostic criteria are in evidence in other wells siliciclastics of unknown age and thickness are identified as overlying neogene carbonates the regional distribution and age of onshore limestones suggests that unroofing of the cretaceous carbonate cap of the maya mountains siliciclastic sediment source did not occur until late in neogene time perhaps no earlier than late pliocene consequently the maya mountains could not have been a major offshore source of siliciclastics until the quaternary information on the lithology of the immediately underlying pre holocene is provided by limited penetration core data and shallow resolution seismic lines these show that antecedent pleistocene limestones beneath the holocene reefs were deposited around 130 000 b p isotope stage 5e the thickness of the overlying holocene shelf margin reef capped carbonates increases along depositional strike from a few meters in the north to more than 25 m in the south in contrast piston cores and seismic data from the southern shelf lagoon collectively document the occurrence of antecedent siliciclastic topography the southward dip of both carbonate and siliciclastic antecedent surfaces is presumably a reflection of increasing southerly subsidence increasing antecedent erosional relief in the same direction reflects the pronounced southerly increase in paleo rainfall that presumably paralleled that of present day rainfall

paleoclimate proxy perspective on caribbean climate since the year 1751 evidence of cooler temperatures and multidecadal variability 1 annually resolved coral 18o and sr ca records from southwestern puerto rico are used to investigate caribbean climate variability between 1751 and 2004 c e mean surface ocean temperatures in this region have increased steadily by about 2 c since the year 1751 with sr ca data indicating 2 1 0 8 c and 18o data indicating 2 7 0 5 c coral geochemical records from across the tropics demonstrate that regional variability is important for understanding climate variations at centennial time scales a strong multidecadal salinity signal in the oxygen isotope data correlates with observed multidecadal temperature variations in the northern hemisphere instrumental wind and precipitation data indicate that the most recent coral isotopic variations are caused by expansion and contraction of the steep regional salinity gradient forced by trade wind anomalies through meridional ekman transport the timing of the fluctuations suggests that the multidecadal scale wind and surface circulation anomalies might play a role in atlantic temperature variability and meridional overturning circulation but further work is needed to confirm this suggestion

1400 yr multiproxy record of climate variability from the northern gulf of mexico a continuous decadal scale resolution record of climate variability over the past 1400 yr in the northern gulf of mexico was constructed from a box core recovered in the pigmy basin northern gulf of mexico proxies include paired analyses of mg ca and 18 o in the white variety of the planktic foraminifer globigerinoides ruber and relative abundance variations of g sacculifer in the foraminifer assemblages two multi decadal intervals of sustained high mg ca indicate that gulf of mexico sea surface temperatures ssts were as warm or warmer than near modern conditions between 1000 and 1400 yr b p foraminiferal mg ca during the coolest interval of the little ice age ca 250 yr b p indicate that sst was 2 2 5 c below modern sst four minima in the mg ca record between 900 and 250 yr b p correspond with the maunder sporer wolf and oort sunspot minima suggesting a link between changes in solar insolation and sst variability in the gulf of mexico an abrupt shift recorded in both 18 o calcite and relative abundance of g sacculifer occurred ca 600 yr b p the shift in the pigmy basin record corresponds with a shift in the sea salt sodium ssna record from the greenland ice sheet project 2 ice core linking changes in high latitude atmospheric circulation with the subtropical atlantic ocean

a tree ring based reconstruction of the atlantic multidecadal oscillation since 1567 a d 1 we present a tree ring based reconstruction of the atlantic multidecadal oscillation amo which demonstrates that strong low frequency 60 100 yr variability in basin wide 0 70 n sea surface temperatures ssts has been a consistent feature of north atlantic climate for the past five centuries intervention analysis of reconstructed amo indicates that 20th century modes were similar to those in the preceding 350 yr and wavelet spectra show robust multidecadal oscillations throughout the reconstruction though the exact relationships between low frequency sst modes higher frequency 7 25 yr atmospheric modes e g north atlantic oscillation arctic oscillation and terrestrial climates must still be resolved our results confirm that the amo should be considered in assessments of past and future northern hemisphere climates

isotope paleontology growth and composition of extant calcareous species abstract isotope paleontology uses the isotopic composition of fossil remains of organisms to make inferences about the physical surroundings of growth of the organisms especially temperature and to obtain clues about life history and modes of growth in calcareous fossils oxygen isotopes are mainly used in the former and carbon isotopes in the latter however since physical surroundings and organism response are intimately associated both types of information are contained in each of the isotopic signals to explore the potential of isotope paleontology and to provide a basis for reconstruction a broad range of extant organisms has been studied taking the pioneering work of epstein and associates as a starting point results are summarized for a representative sampling of these studies with emphasis on work at the laboratories of the authors from the mid seventies to the present the organisms considered are nannoplankton benthic algae planktonic and benthic deep sea foraminifera larger foraminifera sponges corals bryozoans polychaetes arthropods bivalves gastropods cephalopods and vertebrates fish otoliths the survey broadly suggests that regarding oxygen isotopes materials tend to be precipitated close to equilibrium with the surrounding seawater as postulated by urey and that for carbon isotopes disequilibrium is the rule life spans growth rates differential seasonal growth and age of reproductive activity can be extracted under favorable circumstances from individual shells and skeletal parts in detail the interpretation of isotope records of individual shells is quite complicated and simple models will fail to give satisfactory results in many or most cases

the structural and sedimentological controls on the reoccupation of quaternary incised valleys belize southern lagoon a grid of high resolution single channel seismic data from the belize southern lagoon has documented the development of several generations of stacked late pleistocene 400 ka holocene incised valley fills a feedback mechanism between tectonics reefal buildups and incised valley location is apparent in the data incised valleys are recognized as medium to high amplitude reflections that truncate reflections produced by older strata on the seismic data the holocene valley fills in the belize southern lagoon contain three seismic facies units from bottom to top 1 a basal unit characterized by moderately high amplitude chaotic reflections or progradational patterns that downlap onto the valley floor 2 a middle unit characterized by continuous horizontal near transparent reflections that drape the basal unit and onlap the valley walls and 3 an upper unit that consists predominantly of transparent to low amplitude low angle downlapping reflections the middle and upper units are separated by an erosional surface based on core evidence and seismic facies analysis the basal middle and upper seismic and sedimentary facies units are interpreted as fluvial and carbonate sands estuarine muds and marine sands and muds respectively the erosional surface separating the upper and middle units is interpreted to be a ravinement surface that marks the transition from brackish estuarine conditions to open marine conditions faulting decreases and reefal buildups increase upsection the oldest valleys mimic structural trends and flow through areas currently occupied by extensive carbonate reefs rapid high amplitude late pleistocene sea level rises may have led to wides pread reef development creating a template that subsequent incisions reoccupation valleys followed regardless of structural trends the intimate relationship among tectonics sea level change carbonate reef buildups and incised valley reoccupation serves as a model for understanding ancient tropical mixed carbonate siliciclastic systems accelerator mass spectrometer 14c dates of sediment core samples marine gastropods marine mollusks barnacles worm tubes peat and oysters indicate all the sediment cored 1 4 m in this study is holocene in age between 4740 60 and 11 230 90 14c age this radiocarbon dating consistently links the erosional surface between the upper and middle units with the intersection of the regional sea level curve nonmarine marine boundary

underestimation of the authigenic fraction of cu and ni in organic rich sediments abstract there are two possible techniques to estimate the degree of authigenic or non lithogenic metal enrichment in sediments first the authigenic metal fraction can be calculated according to m auth m total m al background al total while m is the metal of interest and al is aluminum second a metal enrichment factor ef is calculated according to ef m al total m al background both techniques rely on the appropriate use of the m al ratio of the lithogenic background for the latter the m al ratio of average shale or crust is widely applied as lithogenic background in marine geochemistry however as we show here the lithogenic background based on average shale or crust is too high for cu and ni as a consequence the authigenic fraction of cu and ni in sediments and suspended particulate matter spm is significantly underestimated as shown for examples from various environments upwelling areas of peru chile and the gulf of california the norwegian kyllaren fjord and the german wadden sea by correlating cu al and ni al with toc in the samples the apparent m al ratio of the background can be estimated a prerequisite for this technique is high linearity and high quality of correlation r 2 0 8 otherwise the lithogenic background cannot be exactly determined the resulting higher authigenic level of cu and ni may promote their use as productivity indicators while the status of cu and ni may shift from moderately to highly enriched when the enrichment is assessed via efs

a centennial scale variability of tropical north atlantic surface hydrography during the late holocene abstract sea surface temperature sst and sea surface salinity sss fluctuations in the northeastern caribbean have been reconstructed through the last 2000 yr using an artificial neural network and 18 o analyses of planktonic foraminifera a warmer period prevailed in the ne caribbean from ad 700 950 which may reflect the occurrence of stronger and or more frequent el nino events a 2 c cooling of winter ssts from ad 1400 to 1550 coincides with the occurrence of reduced solar output the sporer event episodes of lower ssss with marked minima at the onsets of the dark ages in europe ad 500 600 and little ice age ad 1400 are cyclically recurrent at intervals of 200 400 yr and coincide with drier periods in mexico this may indicate that the tropical atlantic evaporation precipitation budget and ssss are affected by a centennial scale modulation involving the freshwater export import from into the atlantic ocean coeval changes recorded in the deep north atlantic circulation indicate that low latitude sss anomalies may be advected polewards by the north atlantic current system thus affecting deep ocean convection and strength of the thermohaline circulation

an 8 century tropical atlantic sst record from the cariaco basin baseline variability twentieth century warming and atlantic hurricane frequency 1 we present the first direct comparison and calibration of a downcore foraminiferal mg ca record to historical instrumental sea surface temperature sst mg ca measured on the planktic foraminifer globigerina bulloides from a cariaco basin sediment core strongly correlate with spring march may instrumental ssts between a d 1870 and 1990 a mg ca sst equation is derived and a paleo sst record is presented spanning the last 8 centuries an interval that includes the end of the medieval warm period and the little ice age the long term record displays a surprising amount of variability the temperature swings are not necessarily related to local upwelling variability but instead represent wider conditions in the caribbean and western tropical atlantic the mg ca sst record also captures the decadal and multidecadal variability observed in records of global land and sea surface temperature anomalies and atlantic tropical storm and hurricane frequency over the late nineteenth and twentieth centuries a divergence between the sst proxy record and atlantic storm frequency around 1970 appears to reflect a fundamental change in atlantic hurricane behavior noted in historical data on average twentieth century temperatures are not the warmest in the entire record but they do show the largest increase in magnitude and fastest rate of sst change over the last 800 a

florida current surface temperature and salinity variability during the last millennium surface water d 18 ow near dry tortugas increased 0 4 during the course of the little ice age lia 1200 1850 a d equivalent to a salinity increase of 0 8 1 5 on the great bahama bank where surface waters are influenced by the north atlantic subtropical gyre d 18 ow increased by 0 3 during the last 200 years although a portion 0 1 of this shift may be an artifact of anthropogenically driven changes in surface water sco2 the remaining d 18 ow signal implies a 0 4 1 increase in salinity after 200 years b p the simplest explanation of the d 18 ow data is southward migration of the atlantic hadley circulation during the lia scaling of the d 18 ow records to salinity using the modern low latitude d 18 ow s slope produces an unrealistic reversal in the salinity gradient between the two sites only if d 18 ow is scaled to salinity using a high latitude d 18 ow s slope can the records be reconciled variable atmospheric 14 c paralleled dry tortugas d 18 ow suggesting that solar irradiance paced centennial scale migration of the inter tropical convergence zone and changes in florida current salinity during the last millennium citation lund d c and w curry 2006 florida current surface temperature and salinity variability during the last millennium paleoceanography 21 pa2009 doi 10 1029 2005pa001218

geochemical and geophysical implications of the radiocarbon calibration abstract a precise and accurate chronological framework is crucial to study the dynamics of a variety of phenomena which occurred during the last 45 000 years although the 14 c dating method has been widely applied since the 1950s it is recognized that the atmospheric 14 c 12 c ratio has not been stable during the past in order to calculate accurate ages these fluctuations have to be corrected by means of a calibration curve obtained by comparing raw 14 c measurements with true calendar ages provided by independent dating methods the calibration curve obtained so far is characterized by a long term trend with raw 14 c ages being significantly younger than calendar ages during most of the last 45 000 years abrupt 14 c shifts which occurred over centuries to millennia are superimposed on this long term trend of decreasing atmospheric 14 c 12 c ratio to a certain extent it is possible to outline the different causes of atmospheric 14 c variations by considering complementary information obtained from other cosmogenic nuclides studied at different latitudes 1 most high frequency changes in the atmospheric 14 c 12 c ratio are linked to magnetic fluctuations of solar origin as revealed by studying the last three centuries for which direct observations of the sun are available a similar conclusion is derived by comparing 14 c 12 c events with 10 be and 36 cl concentration maxima in polar ice cores 2 the long term trend shift of 14 c ages is due to a long period of decreased shielding effect of the geomagnetic dipole field which occurred over the interval between 10 000 and 40 000 years bp this interpretation is supported by paleomagnetic measurements performed on volcanic and sedimentary rocks and by 10 be and 36 cl analysed in low and high latitude records 3 a prominent and rapid atmospheric 14 c 12 c excursion occurred between 13 000 and 11 500 cal yr bp thus corresponding to the younger dryas cold period by contrast with the 14 c variability mentioned above which is linked to 14 c production changes this so called 14 c age plateau is probably due to an abrupt variation in the rates of exchange within the global carbon cycle this interpretation is supported by independent geochemical proxies and by numerical modelling of the carbon cycle

solar modulation of cosmogenic nuclide production over the last millennium comparison between 14c and 10be records for about the last 30 years it has been recognized that the high frequency component of the tree rings 14c 12c record is dominated by the modulation of the cosmic ray flux by the solar wind in particular it has been demonstrated that the three most recent periods of low sunspot occurrence were characterized by high values of atmospheric 14c 12c during the last millennium other periods of high 14c 12c values were observed but their solar origin is still debatable in the present work we compare these fluctuations with an independent record of cosmogenic 10be measured in ice from the south pole to check the solar origin of the observed 14c 12c variations in order to compare quantitatively the results obtained on 10be and 14c it is necessary to take into account the different behaviour of these two cosmogenic isotopes and especially the damping effect of the carbon cycle in the case of 14c as an input to a 12 box numerical model we used the relative fluctuations of the 10be concentrations record measured in south pole ice and converted it into a synthetic 14c record we took into account the fact that 10be modulation is enhanced in polar regions due to the orientation of the geomagnetic field as expected the fluctuations of the modelled 14c record are much smaller a factor of 20 than those observed for the raw 10be record in addition the variations are smoother and shifted in time by a few decades the 10be based 14c variations closely resemble the 14c measurements obtained on tree rings r 0 81 in particular it is easy to identify periods of maximal 14c 12c which correspond to solar activity minima centred at about 1060 1320 wolf 1500 sporer 1690 maunder and 1820 dalton yr a d cross correlation calculations suggest that there is no significant lag between the 10be based 14c and the tree ring 14c records our study strongly suggests the dominance of the solar modulation on the cosmonuclide production variations during the last millennium

late holocene high resolution palaeoclimatic reconstruction inferred from sebkha mhabeul southeast tunisia relations between climate change and landscape evolution during the last two millennia in southeastern coastal tunisia have been documented using high resolution reconstruction of flood history and fire activity in the sebkha mhabeul core the age model based on tephrochronology indicates that the core extends from roman to modern times and encompasses the well defined climatic periods of the last two millennia this record provides a first palaeoecological palaeoclimatic high resolution reconstruction in north africa using a cross disciplinary approach with both physical grey scale intensity quartz particles and biological charcoal and pollen indicators the flood history shows four wet dry cycles ca ad 550 950 950 1300 1300 1570 and 1570 1870 of different duration major hydrological instabilities are concentrated during the medieval climate anomalies and the early little ice age between ad 1000 and 1550 direct correlation between climate and fire cannot be established suggesting that the fire history of the sebkha environment is mainly influenced by human activity this study demonstrates the great value of sebkhas as palaeoenvironmental archives

temperature dependent variation in the distribution of tetraether membrane lipids of marine crenarchaeota implications for tex86 paleothermometry 1 recently a new geochemical temperature proxy the tex86 was introduced this proxy is based on the number of cyclopentane moieties in the glycerol dialkyl glycerol tetraethers gdgts of the membrane lipids of marine crenarchaeota which changes as a response to temperature however until now only sediment data have been used to establish this proxy and experimental work is missing we performed mesocosm studies with marine crenarchaeota incubated at temperatures ranging from 5 to 35 c and salinities of 27 and 35 to test the validity of the tex86 proxy growth of marine crenarchaeota in these mesocosms was evident from the substantial increase in the concentration of marine crenarchaeotal membrane lipids with amounts up to 3400 ng l with increasing temperature an increase in the number of cyclopentane moieties in the crenarchaeotal membrane lipids was observed different salinities did not show any effect on the gdgt distribution the tex86 showed a significant linear correlation to incubation temperature tex86 0 015 t 0 10 r2 0 79 this equation has a similar slope to the correlation obtained from core tops but differs in the intersection tex86 0 015 t 0 28 r2 0 92 this difference is mainly determined by the smaller amount of the regioisomer of crenarchaeol in the incubation series compared to core top samples these incubation experiments indicates that water temperature is indeed the major controlling factor for the membrane distribution of marine crenarchaeota and confirms that the tex86 proxy depends on a physiological response to regulate membrane fluidity

holocene development of three isolated carbonate platforms belize central america abstract locally operating factors such as topography of the reef basement and exposure to waves and currents rather than regionally effective factors such as the post glacial sea level rise in the western atlantic explain the different holocene developments of the three isolated carbonate platforms glovers reef lighthouse reef and turneffe islands offshore belize a series of nne striking tilted fault blocks at the passive continental margin forms the deep basement of the belize reefs glovers and lighthouse reefs are located on the same fault block while turneffe islands is situated west of lighthouse reef on an adjacent fault block the three platforms are surrounded by deep water and have surfacebreaking reef rims significant differences exist between platform interiors glovers reef has only 0 2 of land and an 18 m deep well circulated lagoon with over 800 patch reefs lighthouse reef has 3 of land and a well circulated lagoon area patch reefs are aligned along a nne striking trend that separates a shallow western 3 m and a deeper eastern 8 m lagoon turneffe islands has 22 of land that is mainly red mangrove interior lagoons are up to 8 m deep and most have restricted circulation and no patch reefs surface sediments are rich in organic matter in contrast the northernmost part of turneffe islands has no extensive mangrove development and the well circulated lagoon area has abundant patch reefs holocene reef development was investigated by means of 9 rotary core holes that all reached pleistocene reef limestones and by radiometric dating of corals maximal holocene reef thickness reaches 11 7 m on glovers reef 7 9 m on lighthouse reef and 3 8 m on turneffe islands factors that controlled holocene reef development include the following 1 holocene sea level the margin of glovers reef was flooded by the rising holocene sea ca 7500 ybp that of lighthouse reef ca 6500 ybp and that of turneffe islands between 5400 and 4750 ybp all investigated holocene reefs belong to the keep up type even though the three platforms were flooded successively and hence the reefs had to keep pace with different rates of sea level rise 2 pre holocene topography pleistocene elevation and relief are different on the three platforms this is the consequence of both tectonics and karst different elevations caused successive reef initiation and they also resulted in differences in lagoon depths variations in pleistocene topography also explain the different facies distribution patterns on the windward platforms that are located on the same fault block on lighthouse reef tectonic structures are clearly visible such as the linear patch reef trend that is aligned along a pleistocene fault on glovers reef only short linear trends of patch reefs can be detected because the pleistocene tectonic structures are presumably masked by the higher holocene thickness the lower pleistocene elevation on glovers reef is probably a consequence of both a southward tectonic tilt and stronger karstification towards the south related to higher rainfall 3 exposure to waves and currents glovers reef lighthouse reef and the northernmost part of turneffe islands receive the maximum wave force as they are open to the caribbean sea adjacent lagoons are well circulated and have luxuriant patch reef growth and no extensive mangrove development by contrast most of turneffe islands is protected from the open caribbean sea by lighthouse reef to the east and is only exposed to reduced wave forces allowing extensive mangrove growth in these protected areas