hummocky cross stratification and megaripples a geological double standard abstract hummocky cross stratification in ancient sedimentary deposits is considered to be diagnostic of inner shelf storm deposits the geometry of the cross strata sets implies a low relief oval bedform of a few meters spacing orientation is poorly defined or random hummocky cross strata sets have been attributed to the action of strong storm wave surges megaripples of 2 5 m spacing are typical responses of the inner atlantic shelf of north america to storm flows side scan sonar records have shown that megaripples on the inner shelf are commonly hummocky oval to circular in plan view and lack asymmetry box cores and vibracores reveal gently inclined to horizontal lamination as the characteristic primary structure the megaripples am produced by combined flow storm currents these are alongcoast geostrophic flows with a mean flow component of 20 60 cm sec 1 a high frequency wave orbital current of similar or weaker intensity is present but its role in megaripple formation is subordinate to that of the mean flow the high velocity coastal margin of the flow is often jetlike and downwelling in nature and is characterize by a down current decrease in sand flux and rapid deposition of sand the bottom boundary layer of such flows is characterized by dense sediment suspensions 40 60 mgl 1 and is highly turbulent due to the interaction of the mean flow and wave orbital current components eddy diffusivity up to 200 cm2 sect at 40 cm above the bed sand transport in this coastal boundary zone is obliquely offshore but neither the density of the sediment suspension nor storm surge ebb appear to be important driving forces it is difficult to account for either modern hummocky megaripples or ancient hummocky cross stratification as responses to purely oscillatory flows combined flow currents are the typical response of mostmodern shelves and epicontinental seas to storms and presumably were so in the past hummocky cross stratified beds are generally too thick 20 80 cm to be accounted for by in situ resuspension they are more likely to have been despoited by combined flow currents in areas where these currents were experiencing a downstream velocity decrease and sediment deposition throughout much of the storm s duration evidence for strong scour in swales between hummocks and the smoothly rounded surface of the hummocks suggests a transitionlike bed regime measurements on the atlantic shelf likewi e suggest a transitionlike regime whose boundary layer is characterized by lower velocity than the transitional regime of unidirectional flow in flumes but more intense turbulence and a higher suspended sand concentration the lack of normal cross bedding in bothmodern and ancient hummocky bedforms indicates that these features did not migrate this characteristic may be a consequence of the complex trajectories followed by near bottom water particles in response to combined flow currents

shelf construction in a foreland basin storm beds shelf sandbodies and shelf slope depositional sequences in the upper cretaceous mesaverde group book cliffs utah the upper cretaceous campanian kenilworth member of the blackhawk formation mesaverde group is part of a series of strand plain sandstones that intertongue with and overstep the shelfal shales of the western interior basin of north america analysis of this section at a combination of small sedimentological and large stratigraphical scales reveals the dynamics of progradation of a shelf slope sequence into a subsiding foreland basin four major lithofacies are present in the upper mancos and kenilworth beds of the book cliffs a lag sandstone and channel fill shale lithofacies constitutes the thin basal transgressive sequence which rests on a marine erosion surface it was deposited in an outer shelf environment shale interbedded sandstone and shale and amalgamated sandstone lithofacies were deposited over the transgressive lag sandstone lithofacies as a wave dominated delta and its flanking strand plains prograded seaward analysis of grain size and primary structures in kenilworth beds indicates that there are four basic strata types which combine to build the observed lithofacies the fine to very fine grained graded strata of the interbedded facies are tempestites deposited out of suspension by alongshelf storm flows geostrophic flows there is no need to call on cross shelf turbidity currents density underflows to explain their presence very fine to fine grained hummocky strata are likewise suspension deposits created by waning storm flows but were deposited under conditions of more intense wave agitation on the middle shoreface cross strata sets in this region are bed load deposits that accumulated on the upper shore face in the surf zone lag strata are multi event bed load deposits that are the product of prolonged storm winnowing they occur on transgressive surfaces while the graded beds are tempestites in the strict sense all four classes of strata are storm deposits the distribution of strata types and their palaeocurrent orientations suggests a model of the kenilworth transport system driven by downwelling coastal storm flows and probably by a northeasterly alongshore pressure gradient the stratification patterns shift systematically from upper shoreface to lower shoreface and inner shelf lithofacies partly because of a reduction in fluid power expenditure with increasing water depth but also because of progressive sorting which resulted in a decrease in grain size in the sediment load delivered to successive downstream environments the kenilworth member and an isolated outlier the hatch mesa lentil constitute a delta prodelta shelf depositional system their rhythmically bedded lenticular sandstone and shale successions are a prodelta shelf facies and may be prodelta plume deposits major upper cretaceous sandstone tongues in the book cliffs are underlain by erosional surfaces like that beneath the blackhawk formation which extend for many tens of kilometres into the mancos shale these surfaces are the boundaries of upper cretaceous depositional sequences the sequences are large scale genetic stratigraphic units they result from the arranging of facies into depositional systems the depositional systems are in turn stacked in repeating arrays which constitute the depositional sequences the anatomy of these foreland basin sequences differs

geostrophic versus friction dominated storm flow palaeocurrent evidence from the late permian brotherton formation england the late permian brotherton formation of northeastern england is a 0 75 m thick carbonate sequence which exhibits abundant erosively amalgamated hummocky cross stratification gutter casts and shallow scour fills suggesting deposition in a storm dominated platformal shelf setting the outcrop ca 15 m thick is divisible into three facies associations a basal transgressive interval 4 5 m thick facies association a which passes upwards into a thinly bedded argillaceous condensed section 1 m thick facies association b which in turn rapidly grades upwards into a 8 10 m thick regressive highstand succession of hummocky cross stratified amalgamated scour fills facies association c the dominance of hummocky cross stratification in all facies associations would suggest that post storm deposition and fairweather reworking was largely under oscillatory currents however the erosional palaeocurrent data indicate that facies associations a and b were deposited following combined oscillatory and geostrophically balanced shore parallel flows in contrast facies association c was deposited by infilling the scours of friction dominated offshore directed and downwelling jet like flows combined with oscillatory wave currents the dominance of geostrophic or friction dominated flows in the study area is interpreted to be controlled by relative sea level changes during transgressive conditions increasing accommodation permitted rotation of the downwelling pressure driven currents in the regressive phase particularly in the accommodation limited late highstand bottom friction was important in inhibiting flow rotation it is suggested that the paucity of evidence for geostrophic flows in the ancient record may in part be attributed to the palaeocurrent indicators in sand grade substrates being restricted to thin transgressive intervals or surfaces whilst in the volumetrically more important regressive deposits palaeocurrent data are largely contained within facies from the friction dominated zone

the last glaciation in central magellan strait southernmost chile abstract glacial landforms and drift stratigraphy in central magellan strait southernmost chile document repeated fluctuations during the last glacial cycle of outlet lobes from an ice cap centered over the southern andes the lobes developed comparatively low gradient profiles because of low basal shear stresses over soft deformable beds and this made them sensitive to even small scale changes in the mass balance such low profiles and rapid calving in deep proglacial lakes during deglaciation may have made the magellan ice lobe particularly responsive to climatic fluctuations during the last glacial cycle and to advance and retreat over considerable distances study of the glacial landforms and drift stratigraphy has led to the identification of at least five glacier advances to limits at and south of the segunda angostura fragments of mollusc shells contained in basal till indicate marine incursions between some advances thus documenting extensive deglaciation a partial chronology based on amino acid studies and radiocarbon dating suggests that five of these advances occurred during the last glacial cycle the most extensive advances may have culminated during substages of marine isotope stage 5 substage 5b or 5d and or during stage 4 slightly less extensive advances occurred between ca 28 000 and 14 000 yr b p

the influence of tributary flow density differences on the hydrodynamic behavior of a confluent meander bend and implications for flow mixing abstract the goal of this study is to evaluate the influence of tributary flow density differences on hydrodynamics and mixing at a confluent meander bend a detailed field characterization is performed using an acoustic doppler current profiler adcp for quantification of the 3d flow field flow discharge and bathymetry as well as ctd measurements conductivity temperature depth to characterize the patterns of mixing satellite images of the confluence taken at complementary times to the field surveys were analyzed to evaluate the confluence hydrodynamics at different flow conditions the results illustrate the differences in hydrodynamics and mixing length in relation to confluences with equal density tributaries at low density differences and higher discharge ratio qr between the two rivers the flow is similar to equi density confluent meander bends in contrast at high density differences low qr the tributary flow is confined to near the confluence but the density difference causes the flow to move across channel in this case the density difference causes the lateral spread of the tributary flow to be greater than at a greater qr when the density difference is less these results illustrate the potential importance of density differences between tributaries in determining the rate and spatial extent of mixing and sediment dispersal at confluent meander bends

late quaternary evolution of a subantarctic paleofjord tierra del fuego lago roca lapataia valley 54 50 s 68 34 w is a paleofjord that was occupied by a valley glacier system during the glacial maximum of the late pleistocene estimated ca 18 20 ka bp deglaciation began before 10 080 270 bp the marine fauna in several marine terraces found in the area shows that early middle holocene climatic conditions were basically the same as at present species found are characteristic of cold and shallow waters although minor temperature fluctuations cannot be ruled out for this period a recent radiocarbon date of 7518 58 bp on chlamys patagonica nz 7730 confirms that lago roca was transformed into a fjord ca 7500 8000 bp the sea reached its maximum level of 8 10 m a s l around 6000 bp and at 4000 4500 bp was at least above 6 1 m a s l later when sea level fell lago roca was occupied by fresh water and was no longer tidal the relative land sea positions during this period are a consequence of combined eustatic and neotectonic processes

late quaternary vegetation and climate of southern tierra del fuego vegetation and climate over approximately the past 13 000 yr are reconstructed from fossil pollen in a 9 4 m mire section at caleta robalo on beagle channel isla navarino 54 56 s 67 38 w southern tierra del fuego fifty surface samples reflecting modern pollen dispersal serve to interpret the record chronologically controlled by nine radiocarbon dates fossil pollen assemblages are empetrum gramineae gunnera tubuliflorae zone 3b 13 000 11 850 yr b p gramineae empetrum assorted minor taxa zone 3a 11 850 10 000 yr b p nothofagus gramineae tubuliflorae polypodiaceae zone 2 10 000 5000 yr b p nothofagus empetrum zone 1b 5000 3000 yr b p and empetrum nothofagus zone 1a 3000 0 yr b p assemblages show tundra under a cold dry climate zone 3 followed by open woodland zone 2 as conditions became warmer and less dry and later with greater humidity and lower temperatures by closed forest and the spread of mires zone 1 comparisons drawn with records from antarctica new zealand tasmania and the subantarctic islands demonstrate broadly uniform conditions in the circumpolar southern hemisphere the influences of continental and maritime antarctic air masses were apparently considerable in tierra del fuego during cold late glacial time whereas holocene climate was largely regulated by interplay between maritime polar and maritime tropical air

progradation geometries of carbonate platforms examples from the triassic of the dolomites northern italy various types of progradation of triassic carbonate platforms are described from the dolomites of the southern alps the internal and external geometric relationships are exposed in spectacular natural sections and moreover their scale 500 1000 m of thickness is such that they can be compared with features found in seismic profiles the different types of progradation are controlled by a number of factors which normally interact with each other these factors include rate of basinal sedimentation rate of subsidence width of the platform depth of the surrounding basin and eustatic variations of sea level progradation is not a continuous process but episodic moments of massive debris input during which the platform advances alternate with long periods of negligible progradation during which basinal sediments accrete and onlap the toe of slope upper boundary relationships of the prograding platforms include offlap toplap and erosional truncation lower boundary relationships are horizontal climbing and descending progradations a variety of phenomena and circumstances have caused the cessation of progradation of the triassic platforms they include volcanism collapse of margins drowning rapid relative rise of sea level subaerial exposure relative fall of sea level and probably a natural decay of the system in the triassic of the dolomites two main progradation models can be put forward in the ladinian model progradation took place simultaneously with aggradation relative rise of sea level whereas the characteristic feature of the carnian model is toplap relative stillstand of sea level

sea level pressure variability around antarctica since a d 1750 inferred from subantarctic tree ring records a tree ring chronology network recently developed from the subantarctic forests provides an opportunity to study long term climatic variability at higher latitudes in the southern hemisphere fifty long 1911 1985 homogeneous records of monthly mean sea level pressure mslp from the southern latitudes 15 65 s were intercorrelated on a seasonal basis to establish the most consistent long term trans polar teleconnections during this century variations in summer mslp between the south america antarctic peninsula and the new zealand sectors of the southern ocean are significantly correlated in a negative sense r 0 53 p 0 001 climatically sensitive chronologies from tierra del fuego 54 55 and new zealand 39 47 were used to develop verifiable reconstructions of summer november to february mslp for both sectors of the southern ocean these reconstructions which explain between 37 and 43 of the instrumentally recorded pressure variance indicate that inverse trends in mslp from diametrically opposite sides of antarctica have prevailed during the past two centuries however the strength of this relationship varies over time differences in normalized mslp between the new zealand and the south america antarctic peninsula sectors were used to develop a summer trans polar index stpi which represents an index of sea level pressure wavenumber one in the southern hemisphere higher latitudes tree ring based reconstructions of stpi show significant differences in large scale atmospheric circulation between the nineteenth and the twentieth centuries predominantly negative stpi values during the nineteenth century are consistent with more cyclonic activity and lower summer temperatures in the new zealand sector during the 1800s in contrast cyclonic activity appears to have been stronger in the mid twentieth than previously for the south american sector of the southern ocean recent variations in mslp in both regions are seen as part of the long term dynamics of the atmosphere connecting opposite sides of antarctica a detailed analysis of the mslp and stpi reconstructions in the time and frequency domains indicates that much of the interannual variability is principally confined to frequency bands with a period around 3 3 3 6 y cross spectral analysis between the stpi reconstruction and the southern oscillation index suggests that teleconnections between the tropical ocean and extra tropical mslp variations may be influencing climate fluctuations at southern latitudes

on the mixing of rivers with a difference in density the case of the negro solimoes confluence brazil abstract confluences are common components of all riverine systems and characterized by converging streamlines and potential mixing of separate flows the fluid dynamics of confluences possess a highly complex structure with several common types of flow features observed an investigation was carried out in both low and relatively high flow conditions at the confluence of the negro and solimoes rivers brazil which ranks among the largest river junctions on earth during this field research acoustic doppler velocity profiling adcp and water quality sampling were applied to investigate hydrodynamics sediment transport and mixing characteristics at this confluence it was found that the location and the width of the mixing interface were closely related to changes of the discharge ratio between the tributaries due to both seasonal and annual variations second a timescales analysis was applied to comparatively identify the contribution to mixing at the negro solimoes confluence of four processes difference in 1 velocity and 2 density between the rivers 3 bed friction including form drag and 4 change in channel width the analysis demonstrated that adjustments of confluence hydrodynamics and morphodynamics can modify the relative importance of each contribution immediately downstream of the junction the effects of differences in velocity and density were comparable while farther downstream the latter was predominant and the role of bed friction was significant at the end this study suggests that mixing at the negro solimoes confluence can be explained as a combination of the four hydrologic and morphologic processes

hummocky cross stratification tropical hurricanes and intense winter storms most previous workers have inferred a storm origin for hummocky cross stratification which typically occurs in shallow marine deposits on the modern earth the only storms capable of profoundly affecting shallow marine depositional environments are severe tropical cyclones hurricanes and mid latitude winter wave cyclones intense winter storms this paper examines the palaeogeographic distribution including palaeolatitude and palaeogeographic setting of 107 occurrences of hummocky cross stratification ranging in age from the proterozoic to recent in each of these stratigraphic units both palaeolatitude and palaeogeography are consistent with a direct storm influence associated with the passage of hurricanes or winter storms directly over the site of deposition this palaeogeographic evidence lends support to the inferred storm origin for hummocky cross stratification further the distribution of the structure suggests that most occurrences 73 were generated by tropical hurricanes the remaining 27 being generated by intense mid latitude winter storms the preferential generation of hummocky cross stratification by hurricanes is consistent with 1 the known differences in the nature of the bottom flows generated by the two major storm types and 2 the inferred nature of the flows which form hummocky cross stratification hurricanes couple less effectively with the water column than do intense winter storms due to this ineffective coupling hurricane generated bottom flows tend to be oscillatory or multidirectional dominant with only minor unidirectional components of motion in contrast intense winter storms generally do couple effectively with the water column generating bottom flows which possess a dominant or significant unidirectional component most previous workers have suggested that hummocky cross stratification forms under oscillatory or multidirectional dominant flow thus it is conceptually reasonable that the vast majority of ancient occurrences of hummocky cross stratification were probably hurricane generated as suggested by the aforementioned palaeogeographic distribution the proterozoic palaeozoic neogene and quaternary were times when global climate was similar to that of today the distribution of hummocky cross stratification deposited during these times suggests that both hurricanes and intense winter storms occupied latitudinal belts during these times which were essentially identical to those occupied by their modern counterparts the mesozoic and palaeogene were non glacial times when global climate was much warmer than that of today the distribution of hummocky cross stratification deposited during this interval suggests that hurricanes occurred more frequently at higher latitudes during non glacial times than they do at present the possibility of a broadened hurricane belt during the mesozoic and palaeogene is consistent with climatic considerations a limited number of mesozoic and palaeogene rock units containing hummocky cross stratification were deposited in palaeogeographic settings that preclude a direct hurricane influence these examples were deposited in the middle latitudes suggesting that intense winter storms continued to form hummocky cross stratification in the middle latitudes during these much warmer times some previous workers have suggested that tsunamis may be capable of generating hummocky cross stratification the palaeogeographic distribution of the structure does not support an origin due to tsunamis lacustrine examples of hummocky cross stratification reported herein are the first known non marine occurrences they suggest that storm effects strongly influence the sedimentary record of some lakes

fuegian floating tree ring chronology from subfossil nothofagus wood the discovery of subfossil nothofagus wood buried in several peat bogs of argentine tierra del fuego is reported a provisional fragmented floating tree ring width chronology covering the last 1400 years has been obtained statistical and spectral analysis revealed no significant differences between the present and floating tierra del fuego tree ring chronologies some cyclical growth peculiarities found in both modern and floating chronologies are discussed

human dispersal and climatic conditions during late pleistocene times in fuego patagonia abstract a brief review of the evidences for a late glacial cooling episode in fuego patagonia and of the arguments used to suggest or reject the existence of a younger dryas chronozone is presented from this review it becomes clear that harsher climatic conditions were characteristic of at least several centuries during late glacial times however the timing and magnitude of that episode is difficult to establish due to scale and sampling problems resulting from the use of data obtained by different disciplines this review is followed by a discussion of the implications of this deterioration for human occupation which was in the process of initial installation in selected regions of fuego patagonia

the geochemical signature of suspended sediments in the parana river basin implications for provenance weathering and sedimentary recycling abstract the parana river basin is one of the largest hydrological systems in south america the present study focuses on the mineralogical and chemical composition of the suspended sediments exported by the parana river to the atlantic ocean with the aim of analyzing their provenance the chemical weathering signature and the likelihood of sedimentary recycling the particulate matter of the middle parana river and its main tributaries i e the paraguay and the upper parana rivers is mostly composed of quartz plagioclase k feldspar and clays such as illite and kaolinite different geochemical approaches indicate that the suspended sediments transported by the parana river preserve the chemical signature of its sources and its composition is not significantly modified during transport these sediments are mainly supplied by acidic arc sources located in the andean headwaters of the bermejo and pilcomayo rivers and transported by the paraguay river and by tholeiitic basalts outcropping in the headwaters of the upper parana river the incomplete mixing of both main tributaries produces a transverse geochemical asymmetry in the particulate material of the middle parana river which was detected 32 km downflow the confluence the suspended load transported by the paraguay river which includes the contributions from the bermejo and pilcomayo rivers indicates incipiently to moderately weathering whereas the particulate matter exported by the upper parana river reveals a higher degree of chemical alteration this is the result of the different lithology and climatic regimes that prevail in the headwaters of both tributaries the weathering signature of the middle parana river s suspended load resembles that of the paraguay river which in turns supplies most of the particulate matter through the bermejo river this work also shows that the suspended load exported by the parana river basin has a mixed origin where the chemical signatures from young materials derived from undifferentiated volcanic rocks and from recycled materials affected by intracrustal differentiation can be distinguished

correlation and dating of quaternary littoral zones along the patagonian coast argentina six areas of the patagonian coast argentina were investigated in order to determine the number characteristics geomorphology and development of quaternary littoral zones it became apparent that utilizing dl ratios of aspartic acid and leucine of various molluscan species would be the most useful in correlation relative age dating and estimating the ages of these zones the oldest littoral zone is found at elevations between 24 and 41 m above mean sea level and is judged to be older than oxygen isotope substage 5e in age based on relatively high amino acid ratios and extrapolated from non linear kinetic models an intermediate aged littoral zone may be present at some locations based upon beach ridges or platforms varying in elevation between 16 and 28 m above mean sea level dl ratios are generally lower than those for the oldest zone but show a greater variation this zone may represent the substage 5e sea level stand well defined young beach ridges 8 12 m above mean sea level are found in most locations and have been 14c dated and verified by amino acid ratios as being holocene the presence of quaternary aged emerged littoral zones at roughly the same elevation suggest that the glacio eustatic contribution is the primary cause of the high sea level stands whereas secondary variations are attributed to other factors

pleistocene glaciation in the southern lake district of chile relative age criteria permit deposits of successive andean glacier advances in the southern lake district of chile to be divided into four mappable drift sheets the oldest two of which overlie tertiary bedrock along the eastern flank of the cordillera de la costa only the youngest drift llanquihue is datable by radiocarbon during the most extensive ice advance of the last glaciation the lago llanquihue glacier was about 95 km long and reached an estimated maximum thickness of between 1000 and 1300 m glacier equilibrium lines at that time lay about 1000 m below their present level and rose eastward with a gradient of about 5 m km successive ice advances in the lago llanquihue basin which resulted in construction of end moraines and associated outwash plains beyond the lake margin culminated sometime before about 20 000 yr ago and between 20 000 and 19 000 yr ago a later readvance inferred from the sedimentary record of lake level fluctuations in the basin had begun by about 15 000 yr ago and culminated shortly after 13 000 yr ago a comparable but less closely dated record of ice advances is found northwest of seno reloncavi and on isla chiloe deglaciation following the latest advance is likely to have been rapid for the major glacier lobes fronted on deep water bodies that would have promoted extensive calving

chronology of a probable neotectonic pleistocene rock avalanche cordon del plata central andes mendoza argentina abstract placetas amarillas 1 rock avalanche located at 32 43 s 69 25 w was geomorphologically stratigraphically and chronologically studied this extraordinary event dammed a secondary gully causing the formation of a barrier lake indicated by relict lacustrine and diatomite deposits stratigraphically its deposit is overlain by alluvial fans where three tephra layers are intercalated dating using ar ar method of the middle ash level at 350 80 ka determined a middle pleistocene or older age for this event furthermore the younger age of placetas amarillas 2 is established by relative dating techniques such as soil development and rock varnish development the ages established in this research indicate that placetas amarillas 1 may be temporally correlated with the tigre dormido rock avalanche suggesting that the occurrence of these rock avalanches may be related to regional neotectonic activity nevertheless climatic conditions are not underestimated as the tigre dormido event occurred before formation of an outwash terrace related to a glaciation of early middle pleistocene age

quaternary glaciations of the southern andes abstract the southern andes comprise the southernmost portion of the andean cordillera beginning at the edge of the puna altiplano lat 27 s and ending at isla de los estados lat 55 s the late cainozoic glacial record of these mountains spans the interval from the late miocene to the present and is one of the most complete to be found anywhere in the world this has arisen for several reasons i the conterminous mountain ice cap extended to the piedmont zone on both flanks of the range where the sedimentary and morphological record has been well preserved ii periodic volcanism mainly from monogenetic fissure eruptions of basalt east of the range and from central tephra producing cones along the mountain crest has provided opportunity for the preservation and radiometric dating of interbedded glacial deposits iii a tectonically induced interval of stream incision in the mid pleistocene and simultaneous uplift has preserved glacial sediments on interfluves iv in the chilean lakes region west of the mountains late quaternary glaciers terminated in a well vegetated landscape thus creating scope for radiocarbon dating of interbedded and incorporated organic materials consequently the last glaciation in the llanquihue area of chile is one of the best dated sequences in south america thus the llanquihue glaciation is proposed as the south american equivalent of the wisconsin and weichsel glaciations of north america and north west europe respectively

palaeoenvironmental conditions during the early peopling of southernmost south america late glacial early holocene 14 8 ka b p abstract environmental conditions in southernmost south america the patagonian andes and the extra andean plains during the early peopling of the region in late glacial early holocene times are herein described different glacial advances or stabilization phases in valley glaciers occurred several millennia after the last glacial maximum lgm as a consequence of regional cold climatic episodes according to clapperton 1993 two advances during the late glacial took place ca 15 14 14 c ka and 12 10 14 c ka b p a cold event during this last period probably correlative with the younger dryas episode in the northern hemisphere has been observed in the palynological record a tundra environment dominated until ca 13 10 14 c ka b p afterwards a gramineous steppe with forest refuges developed the transition steppe forest lasted to the middle holocene when the nothofagus forest took over the arrival of man in southernmost south america ca 13 14 c ka b p or even before probably took place while glaciers were still present in most andean valleys sea level was much lower than today and tundra environments dominated the extra andean plains moreover peopling in the isla grande de tierra del fuego would have occurred not later than 11 8 14 c ka b p when the present magellan straits valley was still occupied by a meltwater discharge braided stream fed by the receding magellan glacier

distribution of dinoflagellate cysts and other aquatic palynomorphs in surface sediments from the beagle channel southern argentina article i nfo palynological analyses of 22 surface sediment samples from the beagle channel tierra del fuego argentina were performed in order to investigate the distribution of dinocyst assemblages and other aquatic pal ynomorphs and to explore their relationships with sea surface conditions which are regionally characterized by cold and low salinity conditions 4 9 c and 27 33 5 respectively results show relatively low dinocyst concentrations 253 to 5568 cysts g and species diversity 19 taxa identified the assemblages are mostly composed of protoperidiniaceae and appear typical of a marginal marine environment with surface waters characterized by low to moderate salinity and high nutrient content due to river inputs the assemblages are thus compatible with the occurrence of freshwater to brackish water taxa botryococcus braunii botryococcus sp polyasterias sp halodinium sp and radiosperma corbiferum the dinocyst assemblages are dominated by brigantedinium spp echinidinium spp and selenopemphix quanta accompanied by cysts of pentapharsodinium dalei islandinium minutum votadinium spinosum polykrikos kofoidii and polykrikos schwartzii the assemblages from the beagle channel show similarities with those of high latitude regions of the northern hemisphere however some taxa which were exclusively reported from relatively warm environments in the northern hemisphere occur in relatively high percentages in the beagle channel e g echinidinium delicatum and votadinium spinosum the absence of selenopemphix antarctica in the bea gle channel is consistent with its known ecological affinities since it characterizes environments marked by seasonal sea ice cover

late quaternary environmental changes in southernmost south america reflected in marine calcareous macro and microfossils abstract late quaternary environmental changes especially sea level fluctuations and glacial advances and retreats affected the distribution of the benthic marine fauna which inhabited the beagle channel in southernmost south america to evaluate these changes calcareous macro and microfossils obtained from holocene marine sediments along a sector of the western side of the beagle channel have been used for a multi proxy approach in total 119 species were recovered 65 mollusks 24 bivalves 35 gastropods and 6 chitons 33 foraminifera and 21 ostracods other preserved calcareous groups which appeared in minor proportions were brachiopods and cirripeds the calcareous fossil assemblages from the early holocene represent an ecosystem transition which started with vacant niches first occupied by opportunistic species and or eurytopic taxa these were then replaced by more diverse fauna assemblages associated with the proliferation of habitats which occurred under truly marine conditions during the mid holocene when holocene calcareous fauna is compared with its pleistocene counterparts from the same region some differences in the proportion of taxonomic groups and taxa composition are noted these partly follow the heterogeneity of habitats in the beagle channel and water masses of different origin which have either greater or less influence throughout the late quaternary but they can also be interpreted in association with environmental changes after the last glacial maximum in conclusion the data provide insights into both spatial and temporal changes and show evidence of a gradual transformation from a brackish to a marine environment caused by the holocene sea transgression it also documents late quaternary faunistic changes associated with topographic and oceanographic local conditions in this microbasin

cold climatic episode of younger dryas age in tierra del fuego pronounced climatic change during the last glacial maximum is recognized as global in extent and has been attributed to orbital forcing1 not readily explained in a global context because of the equivocal nature of the evidence are the late glacial climatic events of short 100 1 000 yr duration the cold climate of the younger dryas chron 11 000 10 000 yr bp for example is known to have been particularly pronounced over the north atlantic and parts of europe and north america2 3 whereas in the southern hemisphere cold conditions for that time are not well established here we present evidence for late glacial climatic cooling in southernmost south america from studies of radiocarbon dated glacial and pollen stratigraphy at sites along the beagle channel in tierra del fuego where a forest tundra tension zone in the region is especially sensitive to shifting climate we conclude that the cold episode corresponds with the time of the younger dryas in europe when a drop in temperature ascribed most recently to colder sea surface temperatures and production rate of deep water in the north atlantic4 5 apparently occurred worldwide

climate and chronology of antarctica and adjacent south america over the past 30 000 yr abstract well dated records of paleoclimate essential for developing general circulation models of the atmosphere during the quaternary are limited in the subpolar southern hemisphere for antarctica where dating is particularly restricted records from distal south america provide a measure to assess antarctic ice sheet chronology and to establish the timing of past climatic events of the continental and insular land masses in the middle latitudes south america lies closest to antarctica and most reflects conditions in the polar region close interval dating of a pollen record at harberton 54 52 s 67 53 w in southern tierra del fuego strengthens implications drawn from paleoecological data and from fluctuations of glaciers to show a climatic parallel with isotope temperature climate data developed from antarctic ice cores over the time span of approximately 30 000 yr since before the full glacial coldest conditions both in antarctica and south america appear centered at 20 000 yr b p warming of climate occurred in the late glacial after 15 000 yr b p interrupted by a cold episode between about 13 000 and 10 500 yr b p with maximum warmth at 9000 yr b p in the early holocene after which temperatures were lower varying only moderately until the present this deglacial pattern although inconsistent in other southern hemisphere records is reproducible on a global scale

mixing and circulation at the confluence of two rivers entering a meandering reservoir 1 a field data set collected under different conditions is analyzed to characterize the spatial arrangement of two large inflows ebro and segre with distinct physical chemical characteristics as they join at the upstream end of ribarroja reservoir in northern spain given the short average residence time of water in the reservoir the spatial arrangement of the rivers at their confluence and their mixing rates are likely the drivers of the stratification patterns observed near the dam in winter inflows have similar densities 0 o 10 5 and their spatial distribution is largely determined by inertial forces and in particular by the discharge ratio downstream of the confluence both rivers flow side by side and largely unmixed over long distances in summer with 0 of o 10 3 the flow fields at the confluence are largely controlled by buoyancy forces atmospheric forcing during strong wind events and centrifugal forces caused by the meandering shape of the reservoir induce significant tilting of the isotherms leading to localized high mixing rates mixing in general though is weak at this time of the year in fall and early winter density differences are largely controlled by conductivity differences between the incoming flows the warmer ebro water with larger thermal inertia flows beneath the colder segre water the spatial arrangement of the inflows is largely controlled by the discharge ratio and mixing between sources is strong likely as a result of mixed water being denser than either of the incoming flows

classification of sedimentary furrows and a model for furrow initiation and evolution sedimentary furrows are longitudinal bed forms which form in fine grained cohesive sediments of the deep sea floor as well as in similar sediments in estuaries and in large lakes deep sea furrows have been observed where bottom currents often flow in one direction at 5 to 20 cm sec currents are much stronger greater than 50 cm sec in furrowed estuarine environments but sediment accumulation rates are higher a review of furrows studied in a variety of locations and environments suggests that these bed forms fall into two broad morphological classes those developed in muddy cohesive sediments tend to have steep walls and flat floors while those developed in carbonate rich cohesive sediments are broad and have gently sloping walls at abyssal depths furrows can often be detected by their distinctive echo patterns on 3 5 khz echo sounders a series of similar evenly spaced hyperbolic echoes which often have their apices tangent to the sea floor however different furrow cross sections may generate distinctive hyperbolic echo patterns a model for furrow initiation and development in muddy cohesive sediments suggests that furrows develop in depositional environments swept by recurring directionally stable and episodically strong currents furrows are initiated when secondary circulations in the boundary layer align coarse generally light debris in sand ribbon like bed forms as the coarse light sediments move along the sea floor they abrade small longitudinal troughs into the muds the grooves may fill in during intervals between strong current events if sedimentation rates are too high once a furrow is established on the sea floor further development may be controlled by the balance between sediment deposition and erosion where deposition occurs continuously and erosion occurs only episodically during brief periods of increased current strength narrow furrows width 1 5 to 1 15 of spacing appear to develop where deposition exceeds erosion whereas wide furrows width 1 2 of spacing develop where erosion exceeds or equals deposition dissolution may play an important role in the formation and evolution of furrows in carbonate rich cohesive sediments secondary circulation patterns over established furrows may be strongly controlled by bed form topography

mixing processes in the amazon river at the confluences of the negro and solimoes rivers encontro das aguas manaus brazil the confluence of the rivers negro and solimoes gives birth to the amazon river near manaus brazil at their confluence these two rivers with their very different physical and geochemical characteristics provide an interesting example of the mixing of waters along a reach of approximately one hundred kilometres the purpose of this article is to describe and explain the spatial configuration of the solimoes negro rivers mixing zone based on a systematic sampling survey carried out in september 1997 using a special sampler and an acoustic doppler current profiler acoustic doppler current profiler the waters of the negro river differ from those of the solimoes river in their lower velocity 0 3 vs 1 m s 1 conductivity 8 vs 80 s cm 1 at 25 c turbidity 5 vs 80 ntu ph 5 5 vs 7 0 and higher temperatures by 1 c due to their higher density the waters of the solimoes river slide under those of the negro river and consequently negro river waters are located at the surface close to the left bank and solimoes river waters are located at the bottom close to the right bank the contact between the waters of the two rivers changes from a clearly defined vertical boundary to a diffuse horizontal boundary as they move downstream the complete mixing process takes more than 30 h and 100 km copyright 2009 john wiley sons ltd

the geochemical filter of large river confluences abstract this study reports the behaviour and fate of major and trace elements in solid and solution in the encontro das aguas mixing zone in the amazonian basin this area results from the confluence of acidic organic rich rio negro and andean sediment rich rio solimoes the differences existing between these two water masses are likely to modify the partitioning of elements between solid and solution or to induce losses by physical or chemical removal to account for lateral and vertical heterogeneities in the river sections the fluxes of elements in dissolved and solid loads were estimated as accurately as possible using an interpolation technique a general loss of suspended particulate matter spm of 23 is measured 80 km after the confluence but the various elements and their associated hosted minerals are not affected in a similar way sispm caspm naspm or srspm mostly associated with coarse phases quartz and ca na feldspars are preferentially lost refractory elements cospm crspm tispm alspm nispm other alkali a and alkali earth ae elements kspm mgspm baspm fespm or organic carbon oc are lost to a lesser extent due to their occurrence in finer fractions such as clays smectite illite or kaolinite oxides and particulate organic matter pom in solution a ae and cudiss mostly originating from the rio solimoes behave conservatively by contrast refractory elements coming mostly from the rio negro exhibit important losses from 10 for doc to 60 for codiss changes in ph and ionic strength during the mixing may induce a sorption of these elements to particles supplied by the rio solimoes a clustering and or a settling of nanokaolinites phases or a sorption of aldiss fediss codiss mndiss or zndiss by dissolved organic matter dom the differential elemental settling at a river confluence could thus induce a phase shift between element supply and export out of a watershed the physical and chemical processes reported here for a river confluence could be applied in other mixing zones such as estuaries or mining exhausts

holocene sea level changes along the strait of magellan and beagle channel southernmost south america abstract radiocarbon dated marine sediments from five coastal sites along the strait of magellan and beagle channel in southernmost chile permit construction of a curve of relative sea level fluctuations during the holocene morphologic and stratigraphic data point to coastal submergence during the early holocene as the sea rose to a maximum level at least 3 5 m higher than present about 5000 yr ago progressive emergence then followed during the late holocene data from widely separated localities define a smooth curve the form of which is explainable in terms of isostatic and hydroisostatic deformation of the crust resulting from changing ice and water loads apparently anomalous data from one site located more than 100 km behind the outer limit of the last glaciation may reflect isostatic response to deglaciation the sea level curve resembles one derived by clark and bloom 1979 in proceedings of the 1978 international symposium on coastal evolution in the quaternary sao paulo brasil pp 41 60 sao paulo using a spherical earth model both in amplitude and in the timing of the maximum submergence

extending the antarctic meteorological record using ice sheet temperature profiles abstract two vertical ice temperature profiles from locations in the antarctic peninsula unaffected by meltwater are presented a simple time dependent heat diffusion advection model is used to infer broad surface temperature variations in the antarctic peninsula over the century prior to the start of local meteorological records air temperature records from two representative meteorological stations faraday and halley are used to determine the ice surface boundary conditions in the later stages of the model runs earlier temperature histories are then devised to provide best fits between the modeled and observed ice temperature profiles results of the modeling suggest that the surface temperature in the antarctic peninsula dropped by 1 6 0 2 c between the beginning of the nineteenth century and the start of the faraday record in 1944 on time scales of several decades the faraday air temperature record began during a period colder than the average and temperatures in the early 1800s were probably

geomorphological evidence of different glacial stages in the martial cirque fuegian andes southernmost south america abstract the geomorphological features of a cirque partially occupied by receding ice bodies are presented as evidences of different glacial stages in the fuegian andes summits since the last glaciation ios 2 a glacierization scheme involving i regional glacier accumulation basin ii glacier individualization from a main regional ii a stillstand during the general retreat and iii two advancing stages are suggested geomorphic and sedimentary units named martial i ii iii and iv have been distinguished as a consequence of the glacial scheme unfortunately the lack of datable materials has impeded establishment of an absolute chronology recognized glacier advances in the region are discussed the palaeoclimatic framework for the surroundings based on previous studies is presented

carbonate platform facies models various types of carbonate platforms are characterized by distinctive profiles facies and evolutionary sequences ramps may be homoclinal or distally steepened and may have fringing or barrier shoal water complexes of ooid pellet sands or skeletal banks homoclinal ramps pass seaward into deeper water without major break in slope and lack deep water breccias distally steepened ramps may be low energy and characterized by widespread shallow subwave base mud blankets or high energy with coastal beach dune complexes and widespread skeletal sand blankets slope facies may contain abundant breccias of slope derived clasts rimmed shelves have relatively flat tops and marked break in slopes at the high energy shallow shelf edge where they pass into deep water such shelves may be aggraded with peritidal facies extending over much of the shelf or incipiently drowned depending on magnitude of sea level fluctuations they may be accretionary or bypass types that include gullied slope escarpment and high relief erosional forms intrashelf basins occur on some shelves controlling distribution of reservoir and source beds isolated platforms are surrounded by deeper water and may be located on rifted continental margins or on submarine volcanoes most have high relief rimmed margins platforms that have been subjected to rapid sea level rise may be incipiently drowned and characterized by raised rims elevated patch or pinnacle reefs and widespread subwave base carbonate or fine clastic blankets completely drowned shelves develop where the shelf is submerged to subphotic depths terminating shallow water deposition and commonly resulting in blanketing of the shelf by deeper water facies some margins show extensive down to basin faulting that is contemporaneous with carbonate deposition or associated with thick prograding clastic sequences the various types of platforms change in response to variations in sedimentation subsidence or sea level rise and may form distinctive evolutionary sequences the relatively few models presented appear to accommodate most geological examples some of which contain major reservoir facies

depositional mechanisms of storm sedimentation in the triassic durrenstein formation dolomites italy abstract the durrenstein formation triassic italy represents an overall shallowing upward sequence formed by vertical accretion in response to a long term sea level stillstand the investigated upper part of the durrenstein formation represents an inner and an outer shelf the lithologic composition and sedimentary structures of the inner shelf poorly sorted onco intrasparites greenish pelites and dismicrites reflect a deposition under a low sedimentation rate above fair weather base interrupted by significant sediment reworking during storms the outer shelf consists of poorly sorted bioclastic calcarenites hummocky cross bedding and channels typically occur in this facies which formed below fair weather base sedimentary structures indicate a deposition by high energy storms responsible for an offshore transport of sediment the directions of storm currents inferred from paleocurrent measurements nw se ne sw interpreted respectively as an alongshore and an offshore directed storm current fit the basic storm pattern described by swift et al 1983 the offshore directed secondary flow in the bottom layer of storm current formed storm cut gutters probably as a result of a helical flow pattern a fining upward and a coarsening upward sequence have been recognized the fining upward sequence which occurs in the lower portions of the measured stratigraphic sections formed in the outer shelf in response to offshore storm currents the coarsening upward sequence occurs in the upper parts of the measured sections and often directly overlies the fining upward sequence it represents a deposition under fair weather conditions in the inner shelf vertical relationships between sequences and lithologies are in accordance with a gradual infilling of the basin by vertical aggradation

a 400 years isotope record of the antarctic peninsula climate a 400 year deuterium record has been obtained from a 154 3 m ice core drilled on dalinger dome james ross island antarctic peninsula based on a comparison between the isotope profile and the temperature data over the recent period aristarain et al 1986 an interpretation is proposed in terms of temperature changes the warmest part of this proxy record occurs around 1850 with as a salient feature a temperature decline of 2 c from that time up until present conditions this feature at odds with the long timescale warming trend recorded for both hemispheres over the same period likely reflects a regional characteristic related to the lack of a high latitude low latitude link in southern hemisphere circulation patterns

post last glacial maximum evolution of a fjord type lake based on high resolution seismic data the lago roca acigami southern tierra del fuego argentina chile

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

mixing dynamics at the confluence of two large rivers undergoing weak density variations simulations of tracer experiments conducted with a three dimensional primitive equation hydrodynamic and transport model are used to understand the processes controlling the rate of mixing between two rivers ebro and segre with distinct physical and chemical properties at their confluence upstream of a meandering reservoir ribarroja reservoir mixing rates downstream of the confluence are subject to hourly scale oscillations driven partly by changes in inflow densities and also as a result of turbulent eddies that develop within the shear layer between the confluent rivers and near a dead zone located downstream of the confluence even though density contrasts are low at most o 10 1 kg m 3 difference among sources and almost negligible from a dynamic point of view compared with inertial forces they are important for mixing mixing rates between the confluent streams under weakly buoyant conditions can be of up to 40 larger than those occurring under neutrally buoyant conditions the buoyancy effects on mixing rates are interpreted as the result of changes in the contact area available for mixing distortion of the mixing layer for strong density contrasts though when the contact area between the streams becomes nearly horizontal larger density differences between streams will lead to weaker mixing rates as a result of the stabilizing effect of vertical density gradients

reconstruction of the late holocene paleoenvironment of the western beagle channel argentina based on a palynological analysis abstract palynological analysis was performed on sediment samples from arroyo baliza site 54 51 s 68 33 w western sector of the beagle channel tierra del fuego argentina in order to reconstruct the palaeoenvironmental history during the holocene marine transgressive regressive cycle in the beagle channel the palynological analysis reveals the presence of a closed canopy nothofagus forest development in the coastal areas under a cold wet and variable climate conditions during the late holocene the marine assemblage composition shows similarities to those modern assemblages recorded in the surface samples from the beagle channel characterised by the dominance of peridiniales taxa suggesting environmental conditions during the late holocene comparable to today the increasing salinity during middle holocene was accompanied by an increase in number and diversity of the microplanktonic assemblages suggesting the gradual installation of a marine environment as it exists today in the beagle channel

shallowing upward cycles in pelagic troughs upper jurassic subbetic southern spain abstract abundant calcareous tempestites with chert and hummocky cross stratification hcs occur in upper jurassic successions milanos formation of the subbetic betic cordillera southern spain these calcareous tempestite beds are calcisiltites and very fine calcarenites with peloids and bioclasts and have thicknesses between 25 and 75 cm beds show symmetrical wave ripple lamination near the top and are usually capped with wave ripples according to palaeogeographic reconstructions these strata were deposited in a subsiding trough in the pelagic domain of the southern iberian continental margin which was the northwestern most border of the tethys calcisiltite and calcarenite beds deposited in this pelagic trough form small scale to large scale shallowing upward cycles shallowing upward cycles with calcareous tempestites in the lower part and oolitic limestone beds in the upper part have also been recognized in some sectors of the subbetic pelagic basin the large scale cycle occurs in the upper part of the jarropa radiolarite formation which directly underlies upper oxfordian kimmeridgian and tithonian strata of the milanos formation two types of large scale shallowing upward cycles have been recognized according to the presence or absence of thick units of oolitic limestone with large scale cross stratification the water depth during deposition ranged from below but near storm wave base at the base of the large scale cycles to very shallow carbonate platform at the top

deglacial paleoclimate of the american sector of the southern ocean late glacial holocene records from the latitude of canal beagle 55 s argentine tierra del fuego abstract paleoclimate at the time of deglaciation 14 000 10 000 14 c yr b p in different sectors of the southern ocean has followed a continuous warming trend but also warming broken by a transitory episode the younger dryas stadial event during which on the order of a millennium colder conditions prevailed evidence is from antarctic ice cores deep sea cores glacier fluctuations and pollen and beetle records reasons for the apparent nonuniformity include differential sensitivity of each sector to climatic change uneven dating control and the variable forcing response nature of biota used as climatic indicators establishing the late glacial paleoclimate of the southern ocean is of significance as it bears on comprehending the character of deglaciation in a global context with the object of investigating further the sequence of deglacial climatic events pollen stratigraphic studies at canal beagle at the southern extremity of south america were extended westward to the vicinity of ushuaia and lapataia supplementing work done previously at caleta robalo puerto harberton and bahia moat at ushuaia pollen frequency and influx records of the late glacial as well as the holocene chronologically controlled by 25 14 c age determinations are from four sphagnum empetrum mires results obtained at sites dating from 12 500 to 12 000 yr b p show late glacial climatic variability most indicative among tundra communities of grass and dwarf shrub heath is a drop both in frequency and influx of the singular arboreal component nothofagus at the time of the younger dryas when estimated summer january temperature was 3 c lower than at present at ushuaia in general terms the late glacial pattern is not unlike the northwestern european sequence sensu walker 1995 which exhibits a late glacial interstadial 13 000 11 000 yr b p followed by a younger dryas stadial 11 000 10 000 yr b p within limits of the chronology in the american sector there is no indication as shown by marine cores from the southern ocean and by antarctic ice cores that deglacial climatic events at higher latitudes of the southern hemisphere exemplified by the younger dryas lead similar events in the northern hemisphere uninterrupted warming evident in the marine and ice cores occurred after 11 600 11 500 yr b p at canal beagle in contrast the trend toward steadily milder climate indicated by continuous expansion of nothofagus frequency followed in the early holocene after 10 000 yr b p early holocene nothofagus woodland containing grass and composites replaced tundra under a warmer and drier set of conditions coupled with burning estimated summer temperature averaged 1 c higher than today and annual precipitation 100 mm less pollen influx remained generally low until about 5000 yr b p when the rate of nothofagus greatly increased together with increase in the rate of empetrum a characteristic mire component the implication is of late holocene spread of both closed forest and mire the result of greater precipitation and lower temperatures while burning abated summer temperatures evidently varied within about 0 5 c and precipitation by 100 mm compared with the present renewed albeit fluctuating glacial activity is recorded after 5000 yr b p under a variable cooler and more humid late holocene climatic regime

rates and patterns of thermal mixing at a small stream confluence under variable incoming flow conditions confluences are important locations for river mixing within drainage networks yet few studies have examined in detail the dynamics of mixing within confluences this study examines the influence of momentum flux ratio the scale of the flow cross sectional area and the density differences between incoming flows on thermal mixing at a small stream confluence results reveal that rates and patterns of thermal mixing depend on event specific combinations of the three factors the mixing interface at this confluence is generally distorted towards the mouth of the lateral tributary by strong helical motion associated with curvature of flow from the lateral tributary as it aligns with the downstream channel as the momentum flux from the lateral tributary increases mixing is enhanced because helical motion from the curving tributary flow expands over the width of the downstream channel the cross sectional area of the flow is negatively correlated with mixing rates suggesting that the amount of mixing over a fixed distance downstream of the confluence is inversely related to the scale of the flow density differences are not strongly related to rates of mixing results confirm that mixing rates within the region of confluent flow interaction can be highly variable among flow events with different incoming flow conditions but that in general length scales of mixing are short and rates of mixing are high at this small confluence compared with those typically documented at large river confluences copyright 2015 john wiley sons ltd

gravel drift and wind effects on the macrotidal san sebastian bay tierra del fuego argentina abstract in northeastern tierra del fuego the holocene mean sea level msl fluctuation reworked much pre wisconsin glacial drift into sand gravel beaches and spits and sand mud tidal flats for the last 7000 years sea level has been falling a southwards wave induced longshore transport of sand and gravel has constructed the 17 km long el paramo spit which partly encloses the wide san sebastian bay semidiurnal tides with a range of up to 10 m enter the bay from the south causing a dominance of flood currents and wave action in the southern part of the bay ebb currents predominate in the north this segregation of flows conditioned sedimentation and is thus responsible for the south north transition of welded gravel ridges sand flats mixed flats and mud flats on the southern coast of the bay wave action constructed discrete sets of gravel ridges which diverted the san mar in river inlet northwards over the tidal flats north of this inlet where gravels are not yet available storms produce chenier systems discrete deposits of shells and sands the lengths of which progressively shortened as the inlet of the bay became narrower and the bay infilled because storms affect both the bay and the atlantic beach washover fans cut these cheniers and the gravel ridges on the narrower part of the spit the fossil gravel ridges in the northern and southern parts of san sebastian bay are not the only supratidal environments fossil and active marshes evolve conditioned by the strong and persistent wind action seasonal ponds are produced by intense deflation salicornia marsh occurs as circular structures 2 3 m diameter clustered over the supratidal flats on the intertidal mud flats wind causes deflation during neap tides and silting during spring tides north of the bay the eastward migration of straight tidal channels or rills is conditioned by winds the effects of strong and persistent winds from the pacific influence not only the tidal flats but also the gravel spit if ocean waves induce a southward drift of gravels within the bay the wind induced gravel drift is to the north during winter there are no aeolian effects over the iced tidal flats

holocene raised beaches along the northern coast of the beagle channel tierra del fuego argentina abstract raised beaches developed during holocene times along the northern beagle channel coast these deposits contain a rich marine fauna especially shelled organisms such as mollusks four holocene marine sites along the northern coast of the beagle channel have been analysed from west to east they are bahia ensenada playa larga bahia brown and cutalataca airfield they were studied in relation to relative altitude sediment type faunal composition and radiocarbon dates the area is characterized by several discontinuous terraces with elevations varying from about 1 5 to 10 m above sea level their possible origin 1 due to tectonic uplifting in comparison with raised beaches of the northern part of tierra del fuego and 2 resulting from glacio isostatic recovery taking into account the ice retreat is discussed seventeen species of mollusks eight bivalves nine gastropods were collected from the study sites all of them are living species from the beagle channel and have been interpreted as representative of holocene paleocommunities in this channel these species show no evidence of substantial climatic changes during the holocene although minor temperature fluctuations cannot be ruled out yet

pleistocene glaciolacustrine sedimentation at lago fagnano andes of tierra del fuego southernmost south america a pleistocene glaciolacustrine sequence is exposed at the southeastern end of lago fagnano fuegian andes argentina these sediments are interpreted as proglacial delta facies which rest upon a till the proglacial delta consists of i bottomset beds composed mainly of greyish blue silts with dropstones ii foreset beds mostly cross stratified gravel and coarse sands and iii uppermost fine grained lacustrine deposits that include diatomites and peat layers overlying horizontally stratified gravel beds may be interpreted as glaciofluvial plain deposits pollen and spore analysis of fossil peat beds indicates regional steppe conditions immediately after the deposition of the foreset beds represented by predominant gramineae and empetrum a very low content of tree pollen grains nothofagus spp compared to modem surface samples supports the interpretation of steppe tundra environments the palaeoenvironmental setting at the time when the delta was built was much colder and drier than today at lago fagnano this is confirmed by the diatom analysis which suggests a fresh water cool temperate low energy environment the palaeoenvironmental analysis confirms the sedimentological interpretation of the studied units as formed in an ice contact proglacial lake during stadial termination or interstadial conditions 4c dating of the peaty lacustrine deposits interbedded between the upper gravels and the foreset beds suggests an early wisconsinan age oxygen isotope stage 4 for this sequence although a pre wisconsinan age stage 6 cannot be ruled out 1997 elsevier science ltd

middle late holocene palynology and marine mollusks from archipielago cormoranes area beagle channel southern tierra del fuego argentina abstract the palynology and marine mollusks of a marine sequence from rio ovando 54 51 s 68 35 w archipielago cormoranes beagle channel has been studied in order to reconstruct paleoenvironmental conditions during the middle late holocene the dinoflagellate cyst assemblages from rio ovando sequence reflect fjord estuarine environments close to terrestrial ice field affected by glacier meltwater discharge and characterized by short term oscillations of sea surface water parameters the base of the section which is dated at about ca 4160 14 c yr b p 4736 cal yr b p palynological subzone ro 2c is characterized by a high species diversity of dinocyst and mollusks and it is immediately followed by an interval palynological subzone ro 2b characterized by the presence of the echinidinium islandinium complex and the monospecific mytilus mollusk assemblage this subzone registers an inverse correlation between nothofagus dombeyi type and echinidinium islandinium complex concentration values during ca 4160 14 c yr b p 4736 cal yr b p 4064 14 c yr b p 4540 cal yr b p suggesting a variable climatic condition probably related to neoglacial episodes occurred in the southern patagonia andes during this interval the pollen assemblages permit direct correlations with the onshore palynostratigraphy from southern of tierra del fuego the high percentages of nothofagus dombeyi type recorded throughout most of the profile strongly suggest the presence of a closed forest confirming the existence of a variable cool and wet climate for the archipielago cormoranes area during the middle late holocene

storm surge ebb origin of hummocky cross stratified units of the andrews mountain member campito formation lower cambrian white inyo mountains eastern california the andrews mountain member of the campito formation consists of complexly interbedded and regionally metamorphosed feldspathic quartzites siltstones and mudstones of latest precambrian and early cambrian age extensive outcrops of the member occur in the white inyo mountains of eastern california the andrews mountain member accumulated in a shallow water offshore depositional system within the broadly subsiding cordilleran miogeocline the sedimentology of the member suggests that both storm and fairweather processes were involved in its formation the following idealized sequence of sedimentary structures is recognized within individual beds 1 an erosional base with localized flute and load casts 2 fine grained sands containing parallel and low angle cross lamination hummocky cross stratification rare penecontemporaneous deformation and internal channels and discontinuities 3 these free sands grade into silts and very fine sands containing climbing ripples wave current ripples flaser bedding and locally abundant bioturbation and 4 the uppermost portions of the bed grade into bioturbated muds with cruziana ichnofossil assemblages and local occurrences of linguoid symmetrical and interference ripples the sedimentary structures in these sequences reflect deposition by high energy storm events the flows generated by these events are interpreted to be the product of the ebb of large storm surges an inferred scenario of storm deposition can be reconstructed for the andrews mountain member shelf 1 initial erosion and suspension of fairweather sediments in response to long period storm waves 2 as a low pressure system moves onshore and dissipates a sediment charged return bottom flow is generated that flows across the shelf and further erodes and entrains material 3 fine sands are deposited as parallel laminated and hummocky cross stratified units as the competence and capacity of the flow decreases 4 attenuation of the surge leads to low flow regime deposition of climbing ripples and where deposition is within storm wave base wave current ripples and flaser bedding 5 with the resumption of fairweather conditions softbodied benthic communities recolonize muddy substrates which are locally reworked by tidal and oceanic currents rather than depositing sand sheets over extensive areas multiple storm surges are inferred to have moved sand incrementally across the andrews mountain member shelf thinning fining upward and thickening coarsening upward sequences may reflect changes in both the intensity of the flows and the proximity of a local sand source

past human populations and landscapes in the fuegian archipelago southernmost south america this contribution discusses possible relationships between human populations and holocene environmental deterioration phenomena cold arid pulses and volcanic eruptions in the fuegian archipelago south america based on summed probability distributions of archaeological dates paleoenvironmental information geospatial data and archaeological evidence during the first millennia after peopling only the hudson ca 7700 cal yr bp and the first monte burney ca 8600 cal yr bp eruptions might have played a role in human dispersion particularly a more intense human occupation around the beagle channel and long distance interactions are proposed as risk buffer strategies related to the hudson eruption a cooling phase and a demographic growth at ca 5500 cal yr bp might have favored more dispersed spatial occupations and a subsistence diversification in the beagle channel in the northern steppes the second monte burney eruption ca 4300 cal yr bp and an arid episode ca 2600 cal yr bp are proposed as the main triggers for changes in land use patterns long network interactions and subsistence strategies even though occupation changes in the fuegian archipelago coexist with environmental deterioration episodes after 1500 cal yr bp demographic processes and the european colonization most likely explain this trend similarities between the steppe ecotone and forest occupation curves suggest common behavioral patterns across the holocene

late holocene climatic changes in tierra del fuego based on multiproxy analyses of peat deposits a ca 1400 yr record from a raised bog in isla grande tierra del fuego argentina registers climate fluctuations including a medieval warm period although evidence for the little ice age is less clear changes in temperature and or precipitation were inferred from plant macrofossils pollen fungal spores testate amebae and peat humification the chronology was established using a 14c wiggle matching technique that provides improved age control for at least part of the record compared to other sites these new data are presented and compared with other lines of evidence from the southern and northern hemispheres a period of low local water tables occurred in the bog between a d 960 1020 which may correspond to the medieval warm period date range of a d 950 1045 generated from northern hemisphere tree ring data a period of cooler and or wetter conditions was detected between ca a d 1030 and 1100 and a later period of cooler wetter conditions estimated at ca cal a d 1800 1930 which may correspond to a cooling episode inferred from law dome antarctica

storm influenced inner shelf sand lobes in the caradoc ordovician of shropshire england abstract a sequence through the middle caradoc ordovician of shropshire shows the horderley sandstone with thick beds of hummocky cross stratified sandstone succeeded by the cheney longville flags consisting of interbedded mudstones siltstones and sandstones some of which also exhibit hummocky bedding in the upper formation the mudstones are commonly bioturbated but also have many wave rippled silty horizons the interbedded thick siltstones and sandstones all lack current ripples and appear to have been rapidly deposited as a result of storm episodes the siltstone and sandstone beds can be separated into two types type 1 beds are massive or faintly planar laminated and have good lateral continuity type 2 beds are lenticular coarser well laminated and have hummocky bedding the f rmer are believed to have been deposited from suspension and the latter from powerful bottom traction currents flowing offshore during storm surges two varieties of hummocky bedding have been recognized within the sequence the more usual type has laminae mantling a hummocky erosional topography a second variety confined to the cheney longville flags has laminae that accreted upwards from a planar base to produce undulating bedforms wavelength up to 3 m wave height up to 15 cm that have symmetrical internal laminae that are form concordant the bedforms resemble wave ripples in cross section but are about an order of magnitude larger than usual wave ripples in the cheney longville flags the argillaceous sediments and their fauna show virtually no change through 80 m of the succession the type 1 beds however increase in proportion upwards whereas the type 2 beds show two strong maxima because the overall environment apparently remained nearly constant and there is no evidence for progradation we interpret the two developments of type 2 beds as reflecting two localized lobes of sand carried offshore into an inner shelf environment by strong laterally restricted offshore currents analogous to but on a larger scale and more powerful than rip currents the independent upward increase in type 1 beds might reflect regional changes in paleogeography which caused a progressive exposure of the shoreface to an oceanic wave climate

carbonates and relative changes in sea level abstract in the geologic record some of the most accurate gauges of changes in sea level are the sediment type geometry and diagenesis of carbonate shelves and platforms this is because carbonates frequently occur at or very near sea level and are usually less compacted than siliciclastics world wide changes in relative sea level the sum of eustatic sea level changes sedimentation and crustal movements have occurred repeatedly and cyclicly through geologic time producing characteristic responses in carbonates 1 1 relative rises in sea level usually caused by the cumulative effect of tectonic subsidence and eustatic rise may result in the following 1 1 a drowned carbonate reefs or platforms here carbonate growth potential is exceeded by relative sea level rise and is characterized by shallow water sediments overlain by hardgrounds and or deep water sediments some of which may be condensed sequences 1 2 b platforms where only the fast growing rim and patches of the interior are able to match sea level rise while the remainder of the platform is drowned temporarily 1 3 c platforms which keep up and maintain a flat top at sea level and contain shallow water sediments whose thickness at the least matches the height of the sea level rise if terrigenous supply is limited prograding sheets of shelf carbonate occur frequently capped by supratidal evaporites with prograding shelf margin carbonate clinoforms and turbidites if terrigenous supply is high the shelf carbonates encroach on deltaics 2 2 relative drops in sea level caused by crustal uplift or by subsidence being outpaced by a eustatic drop in sea level cause karst and soil development over shelves and platforms deposition of deep water evaporites in adjacent semi enclosed basins and in open marine basins the deposition of deltaic and aeolian clastics that bypassed the shelf falls are accompanied by platform wide fresh water diagenesis during relative sea level rises marine diagenesis is common in the subtidal portions of the shoaling upward carbonates and fresh water diagenesis and dolomitization and sulphate deposition is common in their intertidal and supratidal portions the stratigraphic significance of these reponses to relative sea level change is that many are tied to eustatic events and so are predictable within a basin of deposition

late and postglacial vegetation and fire history in southern patagonia and tierra del fuego abstract two 17 000 yr old peat bog records from low elevation sites in tierra del fuego harberton 54 54 s and southern patagonia rio rubens 52 4 48 s and one c 14 000 cal yr bp record from the upper treeline in tierra del fuego paso garibaldi 54 43 s were analyzed for pollen charcoal and plant macrofossils to reconstruct changes in regional and local vegetation fire frequency and bog hydrology respectively past environmental changes in both lowland records and in the upper treeline record are interpreted in terms of variations in effective moisture during the late glacial period effective moisture changes at both low and high elevation sites were interpreted from comparable shifts between mesic herb rich grasslands and arid empetrum heath or shrub grassland with abundant disturbance indicators the late glacial effective moisture changes were primarily driven by temperature changes during the early holocene expansion of open nothofagus woodlands in the lowlands in present day areas of dense forest was related to a marked precipitation increase however precipitation must have remained highly variable with century long periods of increased summer drought as evidenced by repeated intervals when bogs dried out and fire frequency was high up slope shifts of the nothofagus forest andean tundra ecotone at 11 000 and 9000 cal yr bp also appear to reflect precipitation increases precipitation variability however must not have affected the upper treeline environments as no fires were recorded and the present day nothofagus forest had become established after 9000 cal yr bp upper treeline apparently was below present from 8000 to 7000 and 2500 to c 400 cal yr bp during those times low elevation environments did not register change which suggests that the upper treeline may have been affected by lower temperatures after c 5000 cal yr bp fires became rare in the lowlands suggesting a shift to an equable precipitation regime with only minor intervals of summer drought no simultaneous change was recorded at the upper treeline thus for the late glacial and early holocene the upper and lower treeline environments apparently responded similarly to primarily moisture changes only during the mid and late holocene environmental changes at high and low elevations differed suggesting responses to different climate signals precipitation in the lowlands and temperature at high elevation

quaternary of tierra del fuego southernmost south america an updated review abstract the quaternary of tierra del fuego is represented by glacial glaciofluvial glaciolacustrine marine and aeolian deposits six drift units have been described the oldest dating from the late pliocene these are starting from the oldest the rio grande sierra de los frailes cabo virgenes punta delgada primera angostura and segunda angostura drifts neoglacial and little ice age events are represented in cirques and higher mountain valleys marine deposits and raised beaches were formed during middle pleistocene late pleistocene and holocene interglacial stages after the definitive ice retreat 10 ka ago vegetation changed from tundra and cold steppe to subantarctic forest environments marine deposits reflect also colder conditions than the present ones aeolian processes prevailed in northern tierra del fuego where semiarid conditions and the frequent westerlies favoured the retransportation of finer materials coming from pre existing deposits before the opening of the magellan straits earliest human colonization occurred in northern tierra del fuego ca 11 ka bp in tundra like environmental conditions pedestrian hunters of camelids and foxes co existed with pleistocene fauna that became extinct during late glacial earliest holocene times the steppe area inland and atlantic coast was successively occupied since then until recent times on the other hand the beagle channel coasts were occupied since 6 ka bp by hunter gatherer groups adapted to maritime littoral conditions finally the easternmost area of the island was inhabited at least since 1 5 ka bp the human settlements in these latter areas occurred under environmental conditions similar to the present ones when the fuegian forest was definitively established

fate of particulate copper and zinc isotopes at the solimoes negro river confluence amazon basin brazil abstract the behaviour and fate of copper cu and zinc zn at river confluences is poorly understood although chemical and physical processes during mixing of compositionally different tributaries might condition metal availability and fluxes to the ocean to identify and quantify the effect of such processes in river mixing zones particulate cu and zn isotope signatures 65cuspm and 66znspm were measured along cross sections and vertical profiles at the largest river confluence in the world the encontro das aguas mixing zone of the amazon river where the organic rich negro river meets the sediment rich solimoes river the negro river suspended sediments with highly variable cu and zn concentrations as well as 65cuspm and 66znspm are mostly influenced by organic matter and by the lateritic cover of the watershed the solimoes river suspended sediments more homogeneous in cu and zn concentration and isotope composition across the river section reflect the signature of weathered silicate rich sediments derived from the andes the solimoes river supplies the majority of the suspended cu and zn to the amazon river and despite important flux losses across the confluence 35 for cu and 27 for zn 65cuspm and 66znspm show a conservative behaviour during the mixing in the dissolved load cu concentrations and 65cudiss mostly supplied by the solimoes river behave conservatively whereas zn derived mainly from the negro river suffers an important loss 58 that can be attributed to zn adsorption onto the suspended sediments from the solimoes river this transfer does not induce a significant 66znspm shift in the amazon river suspended sediments therefore cu and zn isotope ratios in the suspended sediments behave conservatively through this confluence which mixes two very chemically contrasted rivers our findings thus suggest that the riverine isotopic information on the sources of particulate cu and zn is preserved during tributary mixing

repeated subaerial exposure of subtidal carbonate platforms triassic northern italy evidence for high frequency sea level oscillations on a 104 year scale triassic carbonate platforms in northern italy ladinian latemar carnian durrenstein norian dolomia principale consist of hundreds of stacked meter scale cycles each made of a subtidal unit with a cap rock that carries vadose cements pisoids and tepee structures these diagenetic caps are interpreted to be due to subaerial exposure of the subtidal sediments the absence of peritidal deposits rules out progradation as the cause of the exposure i e these are not ginsburg autocycles thus the repeated exposure must be the result of relative sea level oscillations which based on the number of cycles and time intervals involved occurred with an average frequency in the 104 year range we postulate a combination of small scale high frequency eustasy and local tectonic movements e g differential subsidence in the norian and infrequent pulses of uplift in the ladinian to explain the cyclicity and its internal variations

glacial geomorphology of the pleistocene lake fagnano ice lobe tierra del fuego southern south america abstract a regional geomorphological study is presented of the southern and eastern coast of lake fagnano one of the most extensive glacial areas of tierra del fuego island at the southernmost tip of south america a palaeoglacial reconstruction is made based on the location of erosional and depositional glacial landforms the outlet glacier flowing eastwards from the darwin cordillera fuegian andes chile had more than 50 tributary glaciers an alpine type landscape including aretes cirques truncated spurs and hanging valleys developed in the western region of the present lake whereas a piedmont type landscape including lateral moraines glaciofluvial and glaciolacustrine terraces and an ice disintegration landscape developed in the eastern region the glacier spread over the low ranges and lowlands through three different lobes and was drained by four main outwash basins directly into the atlantic ocean the ice covered area is estimated at 4000 km 2 the maximum length of the main lobe at 132 km and the general slope at 8 four terminal positions of the glacier were recognized and related to the inutil bay and beagle channel glacial areas located to the north and south respectively 14 c dates from basal peats show that most of the area especially the easternmost part and the southern coast were free of ice by 12 300 years b p fossil peat contained in the lower basal till deposits yield 14 c dates of 31 000 48 200 years b p indicating that a glacial advance occurred in the area prior to the last glacial maximum ca 25 000 23 000 cal years b p