alkenone molecular stratigraphy in an oceanic environment affected by glacial freshwater events the sedimentary record of a ratio uk37 of long chain c37 unsaturated alkenones is a useful indicator of glacial interglacial climatic change in the late quaternary northern gulf of mexico where a planktonic foraminiferal 18o caco3 record is complicated by meltwater and or fluvial events williams and kohl 1986 application of a laboratory temperature calibration of the uk37 ratio prahl and wakeham 1987 to a pigmy basin hydraulic piston core record deep sea drilling project core 619 suggested that the minimum glacial surface mixed layer sml temperature was 8 1 c colder than the holocene high sml temperature of 25 6 0 5 c this predicted glacial interglacial temperature difference was significantly larger than the differences predicted by either the foraminiferal 18o or foraminiferal assemblage temperature methods 0 8 2 0 c this large difference may be caused by local prymnesiophyte assemblage changes in response to climatically induced hydrographic changes interglacial periods may be dominated by pelagic prymnesiophyte assemblages while glacial periods may be dominated by neritic assemblages a combined mechanism of both climatically varying prymnesiophyte species assemblage and depth of alkenone biosynthesis may account for the large difference between the temperature methods

sedimentological history of bryant canyon area northwest gulf of mexico during the last 135 kyr marine isotope stages 1 6 a proxy record of mississippi river discharge abstract bryant and eastern canyons are located in northwest gulf of mexico and are characterized by a complex sedimentological history related to glacioeustatic cycles river discharges and interactions between depositional and halokinetic processes this study is based on detailed sedimentological analysis from forty eight long cores from these two canyons this paper determines the evolutionary history of the canyons and assesses the response of sedimentary processes to morphological climatic hydrological and sea level changes during the last glaciation the upper and middle continental slope was supplied with sediments by low density turbidity currents derived from the depositional segregation deposition of the coarsest material in the most proximal locations of large turbidity currents initiated on the outer shelf the lower continental slope was supplied with sediment by westward flowing bottom currents originated from the entrainment of the most diluted wash load and tails of turbidity currents from the mississippi fan bryant and eastern canyon systems were active during the penultimate glaciation marine isotope stage mis 6 and were supplied with sediments by an ancestral shelf margin mississippi river delta gravity flows transported enormous amounts of sediment to the continental slope and abyssal plain of the northwest gulf of mexico the sea level rise at mis 5 led to confinement of river sourced sediments to the widespread continental shelf of the northwest gulf of mexico and consequently to the cessation of gravity flows during the first 40 kyr of mis 5 salt diapirs transformed the canyons into a network of intraslope basins the sea level dropped to the mid shelf during mis 3 and 4 but never reached the shelf break and therefore river sourced sediments remained largely confined to the shelf however seaward sediment transportation was achieved occasionally through turbidity currents related to sediment failures storms and high river discharges four high river discharge events have been identified during this period the first three were centred at 37 45 and 53 cal ka bp the last high river discharge occurred at the end of mis 3 29 4 33 2 cal ka bp and resulted in the deposition of closely spaced mud turbidites over the entire continental slope the laurentide ice sheet lis was restricted north of the upper mississippi river valley during 60 to 30 cal ka bp and therefore the high river discharge events are interpreted as melt water events related to brief southward advancements of the lis which resulted in the flooding of mississippi river the extensive lowering of sea level during the last glacial maximum mis 2 resulted in the almost direct discharge of mississippi river sediments to the upper continental slope leading to the development of abundant turbidity currents eleven wet dry cycles during this period are defined they probably originated from episodic subglacial melt water floods released from southern parts of the lis the last deglaciation event is characterized by the development of a major melt water event at 16 5 13 cal ka bp that resulted in the deposition of distinct organic rich sediments at about 13 cal ka bp the melt water discharges of the lis in north america switched from the mississippi river to either the st lawrence or mackenzie river valleys causing the domination of hemipelagic sedimentation on the continental slope of the northwest gulf of mexico isotopic data indicate that melt water discharges returned to the mississippi river valley at 11 4 cal ka bp the absence of any sedimentological indication on the continental slope of the northwest gulf of mexico of the return of the melt water discharges to the mississippi river is attributed to the confinement of river sourced sediments on the continental shelf due to the rise of the sea level

recent evolution of a bahamian ooid shoal effects of hurricane andrew hurricane andrew a category 4 hurricane having wind velocities of 240 km hr passed north of joulters cays bahamas in a westerly direction on august 23 1992 we documented three sedimentary facies in a 2 7 km 2 study area dominated by mobile ooid sands before the hurricane using aerial photographs surface observations and shallow coring the shoal crest at this locality had aggrading and northward prograding parallel to depositional strike washover bars composed of cross bedded well sorted ooid sands burrowed poorly sorted ooid sands were present seaward of the washover bars whereas poorly sorted ooids and mud occupied a stabilized area bankward of the actively migrating shoal and local areas between washover bars on the crest of the shoal the shoal was crosscut by tidal channels and older washover bars were being dissected by tidal currents although hurricane andrew profoundly changed surface features within the study area its effects will probably be only partly preserved the hurricane eroded washover bars and transported sediment seaward leaving a nearly flat shoal crest overlain by a laterally continuous decimeter thick lens of well sorted ooid sand that thins seaward and bankward post hurricane tidal currents deposited a centimeter thick discontinuous layer of carbonate mud over this lens of well sorted ooid sand and transported ooids seaward off the shoal the well sorted ooid sand layer will most likely be reworked when an actively migrating shoal crest is reestablished although some of this storm deposit may be preserved on the shoal crest where the ooid sand layer was deposited in areas of normally less agitated conditions ooids may also be preserved in finer grained sediments seaward of the shoal as suggested by previous studies mud deposits on the shoal crest may be preserved where buried beneath reestablished washover bars although some of this mud will be removed by reworking during diurnal tides

infrared microspectroscopy analysis of water distribution in deformed and metamorphosed rocks infrared microspectroscopy has been applied to thin sections of various deformed and metamorphosed rocks in order to investigate water content of quartz in these rocks the broad ir band absorbance around 3400 cm 1 probably due to fluid inclusion molecular water h2o was used to calculate water contents deformed granitic rocks from the yanazawa kamimura area near the median tectonic line mtl showed an increase of water content in quartz from about 300 ppm to 2500 ppm toward the mtl with increasing degree of deformation metacherts from sambagawa metamorphic rocks asemigawa route showed a systematic decrease of water content in quartz from about 1000 ppm to 200 ppm with increasing metamorphic degree from chlorite garnet albite biotite to oligoclase biotite zones an archaean metachert from the napier complex granulite facies has only 40 ppm water comparing this with an inuyama unmetamorphosed chert sample having water contents of 3500 to 7000 ppm a starting point of metamorphism the systematic decrease of water in quartz with increasing metamorphic grade may extend from unmetamorphosed cherts to the highest grade metacherts water contents in high pressure metamorphic rocks in japan are mostly of the order of 500 ppm 300 700 ppm except for those from kurosegawa with a water content as high as 1700 ppm these results represent an exploratory analysis of intragranular water contents in quartz in various deformed and metamorphosed rocks further micro ft ir studies of the distribution of water in crustal rocks will provide a quantitative basis for examining the geochemical cycle of water in the earth s crust

the rio caliente ignimbrite analysis of a compound intraplinian ignimbrite from a major late quaternary mexican eruption the rio caliente ignimbrite is a multi flow unit orcompound ignimbrite formed during a major late quaternary explosive rhyolitic eruption of la primavera volcano mexico the eruption sequence of the ignimbrite is complex and it occurs between lower and upper plinian air fall deposits it is therefore anintraplinian ignimbrite air fall layers pyroclastic surge mudflow and fluviatile reworked pumice deposits also occur interbedded between ignimbrite flow units a chaotic near vent facies of the ignimbrite includes co ignimbrite lag breccias segregated from proximal pumice flows the facies locates a central vent but one which could not have been associated with a well defined edifice many of the lithics in the exposed lag breccias and near vent facies of the ignimbrite appear to be fragments of welded rio caliente ignimbrite and indicate considerable vent widening or migration during the eruption nearer vent the ignimbrite is thickest and composed of the largest number of flow units here it is welded and is a simple cooling unit evidence suggests that it was only the larger thicker pumice flows that escaped to the outer parts of the sheet detailed analysis of four flow units indicates that the pumice flows were generally poorly expanded less mobile flows which would be produced by collapse of low eruption columns the analogy of a compound ignimbrite with a compound lava flow is therefore good a compound lava flow forms instead of a simple one when the volumetric discharge rate or intensity is low and in explosive eruptions this predicts lower eruption column heights a corollary is that the ignimbrite has a high aspect ratio

water distribution in low grade siliceous metamorphic rocks by micro ftir and its relation to grain size a case from the kanto mountain region japan abstract infrared microspectroscopy reveals that liquid like water is always present in polycrystalline quartz grains from low grade metamorphic cherts and shale in the chichibu group and the mikabu greenrock complex of the kanto mountains area the water distribution in these rocks is heterogeneous and is related to rock textures in the microcrystalline quartz o h species due possibly to si oh are generally present the liquid like water is also present at grain boundaries and or in sub micron fluid inclusions in the microcrystalline parts on the other hand in quartz grains larger than 20 m in diameter within veins only the liquid like water is present possibly as fluid inclusions trapped during quartz vein formation in the microcrystalline cherty matrix of the metamorphic rocks a general decrease in liquid like water content is observed with increasing metamorphic grade associated with an increase in grain size two representative grain shapes a cube and a regular tetradecahedron having 14 planes are used to estimate the surface area between grains per unit volume with the grain size d m 3 d for cubic and 2 37 d for tetradecahedron grain boundary volumes were then calculated assuming grain boundary widths from 0 5 to 20 nm and normalized by unit volume of the rock the measured ir data on low grade metamorphic cherty rocks fall closely on these curves with grain boundary width of around 10 nm for the both models assuming saturation of h2o at grain boundary the observed decrease in water content with increase in grain size can be rigorously explained by the decrease in grain boundary volume per unit volume these results suggest that liquid like water occurs mainly between grain boundaries in the microcrystalline quartz of low grade metamorphic rocks although this grain boundary water model is the first approximation and requires further details of quantitative grain boundary textures the present model can provide a new approach to understand water distribution in polycrystalline rocks

morphology of intergranular pores and wetting angles in pelitic schists studied by transmission electron microscopy in pelitic schists composed mainly of quartz and albite grains the morphology of intergranular pores which were filled with water was studied by transmission electron microscopy tem although some pores are defined by crystallographic planes f face most of their form has an ideal shape determined by interface tensions between grains and fluid high resolution tem observations demonstrate that pore free regions at grain boundaries are tight even at the nanometer scale showing that the wetting angle is larger than 0 in this rock the pore distribution in two grain junctions can be compared to a necklace microstructure developed by instability of a fluid film along the boundary induced by microcracking wetting angles for pores located at grain edges of quartz and albite decrease in the order albite albite quartz quartz and quartz albite the quartz quartz wetting angle in a calcite free sample is smaller than that in a calcite containing sample this angle also changes due to grain misorientation our results confirm that solid solid and solid fluid interfacial energies control the geometry of intergranular fluid in natural rocks

pacific offshore record of plinian arc volcanism in central america 2 tephra volumes and erupted masses 1 sediment gravity cores collected from the pacific seafloor offshore central america contain numerous distal ash layers from plinian type eruptions at the central american volcanic arc dating back to more than 200 ka in part 1 of this contribution we have correlated many of those ash layers between cores and with 26 tephras on land the marine ash layers cover areas of up to 106 km2 in the pacific ocean and represent a major fraction 60 90 of the erupted tephra volumes because the pacific coast lies within a few tens of kilometers downwind from the volcanic arc combining our own mapping efforts on land and published mapping results with our marine data yields erupted volumes of all major tephras along the arc that range from 1 to 420 km3 recalculated to erupted magma mass the widespread tephras account for 65 of the total magma output at the arc complementing our tephra data with published volumes of the arc volcanic edifices and volcano ages we calculate the long term average magma eruption rates for each volcano moreover we use incompatible element variations to calculate the cumulate masses that were fractionated during variable degrees of differentiation this yields a minimum estimate of long term average magma production rate at each volcano because intrusives without surface expression and losses by erosion are not accounted for peak magma production rates increase from costa rica to guatemala but there is considerable scatter within each region and large differences even between neighboring volcanoes

lack of impact of hurricane michelle on tidal flats andros island bahamas integrated remote sensing and field observations abstract major storms commonly are cited as important geomorphic and sedimentologic agents and their significance is perhaps most emphatically noted in interpretations of modern and ancient tidal flats the purpose of this study was to assess the impact of hurricane michelle on the carbonate tidal flats of andros island bahamas by coupling remote sensing data and field observations although regional modis remote sensing data reveal that the passage of hurricane michelle suspended sediments across the shallow bank integrating ultra high resolution remote sensing data with field observations demonstrates that the hurricane had almost no impact on the andros island tidal flats this lack of impact is surprising because the storm still had hurricane force winds as it passed immediately over some of the focal areas with this hurricane storm strength and intra storm interval were probably not limiting factors instead several influences combined to minimize the storm s impact on both sedimentation and erosion including the low level of water on the bank resulting from ebb tides and in the northwestern area from offshore storm winds and the short duration of the storm resulting from its high forward velocity given the apparent prevalence of widespread storm effects on ancient tidal flats but the spatially limited major geomorphic impacts of storms on many modern systems we suggest that the geologic evolution and preservation of ancient tidal flat successions may have been shaped by mega storms that simply cannot form in the present global climate nonetheless understanding tidal flat dynamics and changes on human timescales will require continued monitoring and assessment of low impact storms such as michelle as well the more sensational hurricanes that produce widespread death destruction and deposition

tephrochronology of marine tephra adjacent to central america geochemical analyses performed on volcanic ash layers from the 3 ocean basins surrounding central america revealed 11 distinct tephra horizons during the past 300 000 yr and the distribution of each tephra was delineated the los chocoyos ash 84 000 yr old and worzel layer l ash 230 000 yr old are the 2 most widespread tephra in the region two tephra y 6 75 000 yr old and w 1 136 000 yr old are restricted to the western gulf of mexico the remaining tephra b 36 000 yr old j 1 135 000 yr old g 190 000 yr old i 212 000 yr old k 270 000 yr old i 2 270 000 yr old and i 6 300 000 yr old are primarily restricted to the eastern equatorial pacific ocean immediately adjacent to central america the age of each layer was determined by correlation to oxygen isotope and calcium carbonate stratigraphy in marine cores or by interpolation or extrapolation of ages from that stratigraphy of the marine tephra two were correlated to terrestrial exposures of silicic volcanic rocks the layer d tephra had been correlated to the los chocoyos ash from the lake atitlan caldera in guatemala the layer i 2 tephra is correlated with the t fall ash from lake amatitlan caldera in guatemala the age of the terrestrial volcanic units was determined by identifying the stratigraphic age of the marine tephra the los chocoyos ash and t fall deposit are dated at 84 000 and 270 000 yr b p respectively geochemical correlation of the marine tephra to other terrestrial sources will facilitate dating of explosive volcanic eruptions in southern mexico central america and northern south america

ftir microspectroscopy of ediacaran phosphatized microfossils from the doushantuo formation weng an south china abstract phosphatized microfossils from ca 580 ma from the doushantuo formation in the weng an region of south china were analyzed by fourier transform infrared ftir microspectroscopy for their chemical characterization two morpho types of phosphatized embryo like fossils megasphaera and megaclonophycus were analyzed together with algal fossils transmission ir spectra of the microfossils have absorption bands of around 2960 cm 1 and 2925 cm 1 indicating the presence of aliphatic hydrocarbon anti symmetric aliphatic ch 3 and aliphatic ch 2 and have an additional band of around 1595 cm 1 probably derived from aromatic moieties aromatic c c in addition ir microscopic mapping shows that aliphatic hydrocarbon and aromatics are distributed inside the embryo like fossils the embryo like fossils appear to show three types of ch 3 ch 2 peak height ratios r 3 2 and aromatic c c ch 2 peak height ratios r c c 2 values 1 high r 3 2 low r c c 2 type r 3 2 0 2 1 0 and r c c 2 0 2 2 low r 3 2 medium r c c 2 type r 3 2 0 2 0 6 and r c c 2 1 4 and 3 low r 3 2 high r c c 2 type r 3 2 0 2 0 6 and r c c 2 1 8 all three types are contained in both megasphaera and megaclonophycus raman spectra for the carbonaceous matter within the rock sample show a similar degree of thermal alteration indicating that the organics were embedded in situ prior to thermal maturation the ir spectroscopic differences might reflect differences in original organic compositions of microorganisms and or immediate post mortem alteration this suggests that the origins of phosphatized embryo like fossils are more diverse than was previously recognized on the basis of their morphology a comparison of r 3 2 and r c c 2 values in the embryo like fossils with those of the algal fossils and the extant microorganisms indicates the possible interpretation that some of the embryo like fossils belong to animal embryo others are algae but none of them originate from bacteria

blackriveran carbonates from the subsurface of the lake simcoe area southern ontario stratigraphy and sedimentology of a low energy carbonate ramp the lowest upper ordovician blackriveran strata in the lake simcoe area represent the base of a major transgressive sequence of peritidal carbonates and siliciclastics from detailed petrographic

topographic and sea level controls on oolite microbialite coralgal reef sequences the terminal carbonate complex of southeast spain the terminal carbonate complex of southeast spain is a miocene messinian unit of oolite microbialite and coralgal reefs deposited in association with glacioeustasy and evaporitic drawdown the relationship between paleotopography and sea level history is useful for prediction of microbialite and oolite reservoir facies in the subsurface four sequences record sea level change with minimum amplitudes of 32 77 m 105 253 ft sequences commonly have local basal stromatolites overlain by local thrombolites ooid grainstone volcaniclastic rich planar bedded ooid grainstone and fenestral ooid grainstone at low substrate positions thrombolite boundstones are thicker and laterally more continuous than at higher positions at intermediate substrate positions relative to sea level history sequences have a build and fill architecture characterized by a relief building phase and a relief filling phase with thin sequences draping paleotopography microbialites dominate during the relative sea level rises and build topographic relief oolites dominate during the relative sea level falls and fill topographic relief at higher substrate positions close to highstand sequences thicken and yield stratigraphic character that is inconsistent with a build and fill model apparently the build and fill model requires an intermediate elevation substrate position and nonoptimal carbonate productivity during rapid sea level change sequences progressively show increasing diversity and more normal marine organisms possibly caused by decreasing aridity lithofacies of the la molata area show evidence of more restricted conditions compared to the la rellana ricardillo area lithofacies likely because la molata was in an embayment these results show that distribution of oolite microbialite and reef facies are predictable given known interaction among sea level paleotopography of the depositional surface substrate and coastline configuration

form function and feedbacks in a tidally dominated ooid shoal bahamas although the general aspects of oolitic depositional systems are well documented seascape scale 10 3 10 6 m 2 patterns of oolitic shoals and the details of processes acting on them are not well understood or quantified to begin to fill this basic gap in understanding this paper describes the morphology and hydrodynamics of lily bank a modern tidally dominated bahamian ooid shoal in this study integrating remote sensing imagery with quantitative geo located bathymetrical hydrological and granulometric data in a geographic information system documents geomorphic and sedimentological patterns and facilitates interpreting these patterns in the context of the processes operating in this system the results of these analyses reveal that parabolic bars up to several kilometres in wavelength and several metres in height form a common morphologic motif although there is considerable variation on that general theme the seascape scale configuration of bars and superimposed sedimentary structures is closely linked to spatial patterns of tidal movements and includes the presence of mutually evasive flood and ebb channels sedimentologically bars are neither homogenous nor random bodies instead granulometric parameters such as sorting and percentage mud vary systematically as shaped by hydro geomorphic controls the best sorted coarsest ooids are on bar crests whereas the finest grains are found in the lower energy deeper interior and flanking regions in short results clearly document hydrodynamic bathymetrical influences on these ooid shoals and their granulometry linkages akin to siliciclastic analogues sedimentological hydrodynamic and geomorphic observations are consistent with a conceptual model for the formation of parabolic bars in which initial irregularities in nonparabolic bars are enhanced through their effect of focusing flow constricted flow leads to higher flow velocities tidal flow velocity asymmetries differential net sediment transport and growth of bathymetrical highs this bathymetrical divergence creates separate paths for flood and ebb tides facilitating emergence of better developed parabolic forms the resultant parabolic geometries and component bedforms appear to be either in dynamic equilibrium with both ebb and flood tide flows or evolving toward that state in exploring patterns and processes within carbonate shoals this study illustrates some of the first documented insights on quantitative details of morphology and dynamics and in the links between geomorphic framework and grain size and sorting in an oolitic carbonate system assuming a

hurricane deposits on carbonate platforms a case study of hurricane irma deposits on little ambergris cay turks and caicos islands the study of modern hurricane deposits is useful in both identifying ancient hurricane deposits in the rock record and predicting patterns of deposition and erosion produced by future storm events hurricane deposits on carbonate platforms have been studied less frequently than have been those along continental coasts here we present observations of the characteristics of deposition and scour caused by hurricane irma on little ambergris cay a small uninhabited island located near the southeastern edge of the caicos platform in the turks and caicos islands hurricane irma passed directly over little ambergris cay on 7 september 2017 as a category 5 hurricane we described and sampled multiple types of hurricane deposits and determined that the washover fans were the best sedimentological records for hurricane conditions as they were subject to very little reworking over time we compared different model predictions of storm tide and wave height with eyewitness reports and distributions of scour examining the washover fans allowed for the construction of a conceptual model for hurricane deposits formed in a high energy storm event on a carbonate platform characteristics of the washover fans were their small size the lack of sedimentary structures and very well sorted sediment the size and distribution of carbonate boulders eroded and transported by the storm are consistent with depth averaged flow velocities in the range of 1 5 5 3 m s the strength of the storm and the low lying topography distinct features of a carbonate platform setting contributed to high levels of sediment bypass and high flow velocities resulting in small unstructured deposits

rapid high resolution 14c chronology of ooids abstract ooids are small spherical to ellipsoidal grains composed of concentric layers of caco 3 that could potentially serve as biogeochemical records of the environments in which they grew such records however must be placed in the proper temporal context therefore we developed a novel acidification system and employed an accelerator mass spectrometer ams with a gas accepting ion source to obtain radiocarbon 14 c chronologies extending radially through ooids within one 8 h workday the method was applied to ooids from highborne cay bahamas and shark bay australia yielding reproducible 14 c chronologies as well as constraints on the rates and durations of ooid growth and independent estimates of local 14 c reservoir ages

fourier transform infrared spectroscopy determinations of intragranular water content in quartz bearing rocks implications for hydrolytic weakening in the laboratory and within the earth abstract fourier transform infrared ftir spectroscopy has been used to measure intragranular water contents of quartz and feldspar within fine grained quartzites granite and naturally deformed mylonites calibrations and tests of methods developed for this application using natural and synthetic quartz standards indicate that hydrogen concentrations down to 30 ppm are detectable within apertured regions 100 200 m in diam and concentrations of 400 ppm measurable to an accuracy of 30 quartzite and novaculite starting materials used in previous studies of mechanical properties contain substantial intragranular water contents 2400 3900 ppm much larger than water concentrations commonly found in large clear natural crystals and more comparable to those of wet synthetic crystals however unlike molecular water within rapidly grown synthetic quartz most of this water is freezable resembling fluid inclusion water of natural milky quartz crystals likewise quartz and feldspar grains within granite and mylonites deformed at greenschist to upper amphibolite facies conditions contain large amounts of water from 500 to 8000 ppm as fine often submicron size fluid inclusions and ir spectra are dominated by a freezable o h absorption band comparisons between dry natural and wet synthetic quartz crystals have formed the basis for our understanding of hydrolytic weakening however mechanisms by which water related defects access dislocation cores within quartzites deformed in the laboratory and mylonites deformed within natural shear zones may differ from those within idealized natural and synthetic single crystals and their mechanical properties may not be directly comparable

tephrochronology of the western gulf of mexico for the last 185 000 years abstract tephra in 31 piston cores from the western gulf of mexico and 7 piston cores from the equatorial pacific were analyzed by electron microprobe six ash layers in the western gulf of mexico were easily distinguished by tio2 feo and cao contents and correlated by geochemistry in order to determine the distribution pattern for each ash layer correlation by geochemistry is an easier more accurate method than biostratigraphic correlation some of the tephras were miscorrelated by biostratigraphy the six tephras were dated by geochemical identification in a piston core with oxygen isotope stratigraphy and the ages are y5 30 000 yr b p y6 65 000 yr b p y8 84 000 yr b p x2 110 000 yr b p w1 136 000 yr b p and w2 185 000 yr b p data from this study corroborated correlations of the y8 tephra in the western gulf of mexico with the d layer in the eastern equatorial pacific ocean none of the other five layers in the gulf of mexico however were found in the pacific ocean the limited distribution of the y5 y6 x2 and w2 ash layers close to mexico indicates possible sources in mexico tephra from the late pleistocene la primavera pumice in mexico however does not correlate with the marine tephra

carbonaceous biosignatures of diverse chemotrophic microbial communities from chert nodules of the ediacaran doushantuo formation the ediacaran doushantuo formation dst is renowned for exceptionally preserved precambrian fossils including metazoans some of these fossils particularly microfossils such as multicellular algae and acanthomorphic acritarchs are preserved in dst chert nodules to better understand the geomicrobiological processes that contributed to the authigenic formation of dst chert nodules and facilitated exceptional fossil preservation we analyzed organic matter in these chert nodules and the surrounding matrix calcareous mudstone using multiple in situ techniques confocal laser raman spectroscopy micro fourier transform infrared spectroscopy ftir and secondary ion mass spectroscopy sims we found strong ultrastructural chemical and isotopic heterogeneities in the organic matter as indicated by the raman spectral parameter i 1350 1600 ranging from 0 49 to 0 88 the infrared spectral index r3 2 from 0 12 to 0 90 and an estimated 13corg sims range of 44 v pdb these micron scale heterogeneities imply that the organic matter preserved in the dst chert nodules is derived from different carbonaceous sources in a diverse microbial ecosystem including eukaryotic and or prokaryotic photoautotrophs as well as chemotrophs involved in the fermentation and probably anaerobic oxidation of organic remains thus the microbial ecosystems in ediacaran ocean waters and sediments were more complex than previously thought and these microbial processes controlled dynamic micro environments in dst sediments where chert nodules were formed and fossils were mineralized the results also show that variations in the relative abundances activities and interactions of co existing microorganisms in dst sediments may have modulated 13corg shifts causing local decoupling between 13corg and 13ccarb as measured in bulk samples

the sources and deposition of organic matter in the late quaternary pigmy basin gulf of mexico abstract the concentration and carbon isotopic composition 13c of sedimentary organic carbon corg n c ratios and terrigenous and marine 13corg endmembers form a basis from which to address problems of late quaternary glacial interglacial climatic variability in a 208 7 m hydraulic piston core dsdp 619 from the pigmy basin in the northern gulf of mexico while interpretations of sedimentary 13corg time series records are often not unique paired analyses of 13corg and n c are consistent with the hypothesis that the corg in the pigmy basin is a climatically determined mixture of c3 photosynthetic terrigenous and marine organic matter confirming the earlier 13corg model of sackett 1964 a high resolution 1 4 2 9 ka sample 13corg record shows that sedimentary organic carbon in interglacial oxygen isotope sub stages 1 and 5a b are enriched in 13c average 1 values are 24 2 1 2 o and 22 9 0 7 o relative to pdb respectively while glacial isotope stage values 2 are relatively depleted 25 6 0 5 concentrations of terrigenous and marine sedimentary organic carbon are calculated for the first time using 13corg and corg measurements and empirically determined terrigenous and marine 13corg endmembers the net accumulation rate of terrigenous organic carbon is 4 3 2 6 times higher in isotope stages 2 4 than in sub stages 1 and 5a b recording higher erosion rates of terrigenous organic material in glacial times the concentration and net accumulation rates of marine and terrigenous corg suggest that the nutrient bearing plume of the mississippi river may have advanced and retreated across the pigmy basin as sea level fell and rose in response to glacial interglacial sea level change

miocene to holocene marine tephrostratigraphy offshore northern central america and southern mexico pulsed activity of known volcanic complexes we studied the tephra inventory of fourteen deep sea drill sites of three dsdp and odp legs drilled offshore guatemala and el salvador legs 67 84 138 and one leg offshore mexico leg 66 marine tephra layers reach back from the miocene to the holocene we identified 223 primary ash beds and correlated these between the drill sites with regions along the volcanic arcs and to specific eruptions known from land in total 24 correlations were established between marine tephra layers and to well known quaternary eruptions from el salvador and guatemala additional 25 tephra layers were correlated between marine sites another 108 single ash layers have been assigned to source areas on land resulting in a total of 157 single eruptive events tephra layer correlations to independently dated terrestrial deposits provide new time markers and help to improve or confirm age models of the respective drill sites applying the respective sedimentation rates derived from the age models we calculated ages for all marine ash beds hence we also obtained new age estimates for eight known but so far undated large terrestrial eruptions furthermore this enables us to study the temporal evolution of explosive eruptions along the arc and we discovered five pulses of increased activity 1 a pulse during the quaternary 2 a pliocene pulse between 6 and 3 ma 3 a late miocene pulse between 10 and 7 ma 4 a middle miocene pulse between 17 11 ma and 5 an early miocene pulse 21 ma

evolution in h 2 o contents during deformation of polycrystalline quartz an experimental study shear experiments were performed in a griggs type apparatus at 800 c and 1 5 gpa at a strain rate of 2 1 10 5s 1 using different starting materials i powder grain size 6 10 m of dry brazil quartz with 0 15 wt added h2o ii dry brazil quartz porphyroclasts grain size 100 200 m devoid of fluid inclusions embedded in the same fine grained powder and iii wet porphyroclasts grain size 100 200 m containing initially a high density of m scale fluid inclusions embedded in the same powder after hot pressing samples were deformed to large shear strains 3 to 4 5 in order for the microstructures and h2o distribution to approach some state of equilibrium the h2o content and speciation in quartz were analyzed by fourier transform infra red ftir spectroscopy before and after the experiments mechanical peak strength is generally lower in experiments with 100 hydrated matrix intermediate in experiments incorporating wet porphyroclasts with a proportion of 30 or 70 and highest in those with dry porphyroclasts all experiments with porphyroclasts show pronounced strain weakening and the strengths of most samples converge to similar values at large strain wet porphyroclasts are pervasively recrystallized during deformation while dry porphyroclasts recrystallize only at their rims and remain weakly deformed recrystallization of the initially fluid inclusion rich porphyroclasts results in a decrease in inclusion abundance and total h2o content while h2o content of initially dry clasts increases during deformation h2o contents of all high strain samples converge to similar values for matrix and recrystallized grains in samples with wet porphyroclasts shear bands with high porosity and fluid contents develop and they host the precipitation of euhedral quartz crystals surrounded by a free fluid phase these high porosity sites are sinks for collecting h2o in excess of the storage capacity of the grain boundary network of the recrystallized aggregate the h2o storage capacity of the grain boundary network is determined as a h2o boundary film of 0 7 nm thickness

towards a high resolution global deep sea chronology for the last 750 000 years abstract variations in the ratio of 18 o 16 o as measured in shells of marine calcareous microfossils are primarily dominated by changes in global ice volume hence these variations provide a set of global time lines in deep sea sediments it is likely that the timing of major changes in oxygen isotope values is strongly influenced if not controlled by variations in the geometry of the earth s orbit since the variation of orbital parameters can be accurately calculated the opportunity exists for transforming this orbital chronology into a geological chronology through careful correlation of oxygen isotope records in a set of deep sea cores from the sub antarctic south atlantic and equatorial pacific we have assembled a composite isotopic section spanning the last 750 000 years with an average sedimentation rate of 2 3 cm 1000 years a new chronology for this time period was developed by adjusting the ages of the oxygen isotope stage boundaries in this composite section so as to extend the consistent phase relationships that exist between variations in oxygen isotope ratios and changes in obliquity and precession during the last 300 000 years to the entire 750 000 year record previously identified difficulties in phase locking precession with the filtered isotopic signal between 365 000 and 465 000 years b p have been resolved with the recognition that precessional variations have an average period of 19 000 years and not 23 000 years during this interval since this new age model yields the best match between variations in obliquity and precession and their corresponding frequency components in the oxygen isotope record we believe that it presents the most accurate chronology yet developed for deep sea sediments with this new age model providing the time control power spectral analyses of south atlantic and sub antarctic chemical and biotic indices show that there is a strong tendency for variance to be concentrated at frequencies corresponding to periods of 100 000 41 000 and 23 000 years

a 400 ka tephrochronological framework for central america from lake peten itza guatemala sediments highlights dating 400 ka paleoclimate record of neotropics revision and new eruptive volumes for large central american eruptions age models for peten itza sediments linking lacustrine ash inventory to eruptions from central america and mexico abstract lake peten itza northern guatemala lies within a hydrologically closed basin in the south central area of the yucatan peninsula and was drilled under the auspices of the international continental scientific drilling program icdp in 2006 at 16 55 n latitude the lake is ideally located for study of past climate and environmental conditions in the neotropical lowlands because of its great depth 160 m lake peten itza has a record of continuous sediment accumulation that extends well into the late pleistocene a key obstacle to obtaining long climate records from the region is the difficulty of establishing a robust chronology beyond 40 ka the limit of 14c dating tephra layers within the lake peten itza sediments however enable development of age depth relations beyond 40 ka ash beds from large magnitude pleistocene to holocene silicic eruptions of caldera volcanoes along the central american volcanic arc cava were found throughout drill cores collected from lake peten itza these ash beds were used to establish a robust chronology extending back 400 ka we used major and trace element glass composition to establish 12 well constrained correlations between the lacustrine tephra layers in lake peten itza sediments and dated deposits at the cava source volcanoes and with their marine equivalents in eastern pacific ocean sediments the data also enabled revision of eight previous determinations of erupted volumes and masses and initial estimates for another four eruptions as well as the designation of source areas for 14 previously unknown eruptions the new and revised sedimentation rates for the older sediment successions identify the interglacial of mis5a between 84 and 72 ka followed by a stadial between 72 and 59 ka that corresponds to mis4 we modified the age models for the lake peten itza sediment sequences extended the paleoclimate and paleoecological record for this neotropical region to 400 ka and determined the magnitude and timing of cava eruptions

delineating and quantifying depositional facies patterns in carbonate reservoirs insight from modern analogs modern carbonate settings have long been used as analogs for interpreting various environments of deposition in subsurface data sets based on well log and core data and have provided examples of how these environments can be interrelated within a larger context this study focuses on carbonate sands in the exuma islands part of the great bahama bank to show how modern settings can be better used for reservoir analogs by emphasizing visualization and quantification techniques that conceptually delineate aspects of potential grainstone reservoir distribution and heterogeneity along the windward margin of the exumas the association of tidal deltas channels and islands forms an exploration scale linear belt of mostly ooid sand approximately 5 10 km 3 1 6 mi wide and 170 km 106 mi long paralleling the platform margin but set back from the platform edge the distribution of carbonate sands in the exumas was assessed with a digital elevation model and satellite imagery by selecting different water depth intervals parts of the sand belt were highlighted for visual analysis and extracted for measurements of size and shape for example 37 of the sand belt comprises shallowest sands in which the largest areas greater than 100 000 m2 1 07 million ft2 are highly sinuous and maintain connectivity the sand belt was also subdivided into various depositional settings flood tidal delta lobes comprise 85 of the sand belt tidal channels and islands add further heterogeneity to the sand belt channels average about 3 km 1 8 mi in length and have regular but locally variable spacing a representative island shows how the syndepositional topography of holocene ridges develops complexly around antecedent pleistocene topography

the arce tephra two subsequent paroxysmal plinian eruptions from coatepeque caldera el salvador abstract the coatepeque volcanic complex in el salvador produced at least four plinian eruptions within the last 80 kyr the eruption of the 72 ka old arce tephra formed the coatepeque caldera and was one of the most powerful explosive eruptions in el salvador hitherto it was thought that the arce tephra had been emplaced only by one mostly plinian eruptive event that ended with the deposition of a thick ignimbrite however our stratigraphic geochemical and zircon data reveal a temporally closely spaced double eruption separated by a gap of only a couple of hundred years and we therefore distinguish lower and upper arce tephras both eruptions produced in the beginning a series of fallout units generated from fluctuating eruption columns and turning wind directions the final phase of the upper arce eruption produced surge deposits by several eruption column collapses before the terminal phase of catastrophic ignimbrite eruption and caldera collapse mapping of the individual tephra units including the occurrences of distal marine and lacustrine ash layers in the pacific ocean the guatemalan lowlands and the caribbean sea result in 25 6 km3 tephra volume areal distribution of 4 105 km2 and eruption column heights between 20 33 km for the lower arce eruption and 40 5 km3 tephra volume including 10 km3 for the ignimbrite distributed across 6 105 km2 and eruption column heights of 23 28 km for the upper arce eruption these values and the detailed eruptive sequence emphasize the great hazard potential of possible future highly explosive eruptions at coatepeque caldera especially for this kind of double eruption

the cultural nature of tephra problematic ecofacts and artifacts and the baru volcano panama abstract this paper presents data from archaeological stratigraphy and lacustrine core samples containing tephra from the baru volcano in western panama the discussion seeks to refine the understanding of medial tephra deposits near baru in relation to the eruption history indicated by geological palaeoecological and archaeological data a primary goal of this fieldwork is the identification and correlation of tephra from major baru eruptions encountered in archaeological contexts by their lithology stratigraphic position and mineralogy without resorting to geochemical fingerprinting the intention is to develop a cost efficient and reproducible means of utilizing tephra as a chronostratigraphic marker in archaeological contexts and examine the intersection of human life with volcanic events and dynamic environments tephra in archaeological contexts can have ambiguous placement between ecofact and artifact as is the case with a small number of crude tephra sculptures in my excavated material and that of a seminal prior study while the research project utilizes visible tephra layers not cryptotephra by all means the tephra data prove to be cryptic engagement with the imperfection entailed in meshing multidisciplinary data sets and different scientific communities with interests in tephra particularly in lesser studied regions is posited as a total view of tephra

marine isotope stage 6 canyon and spillover deposits of the bryant and eastern canyon systems northwest gulf of mexico importance of fine grained turbidites on a delta fed prograding slope abstract a large set of long sediment cores and high resolution seismic reflection data were used to study the sedimentology of the delta fed prograding slope of the northwest gulf of mexico during marine isotope stage mis 6 125 180 ka bryant and eastern canyon systems were the main pathways during this period through which mississippi river sediment was delivered down the continental slope two mis 6 sedimentary environments have been defined on the slope of the northwest gulf of mexico intracanyon and overbank environments the erosional action of turbidity currents in bryant and eastern canyons was focused mainly on the thalweg whereas on the rest of the canyon floors the flows acquired a mainly depositional character resulting in the formation of inner levees and terraces mis 6 deposits in overbank environments consist entirely of thick 50 m widespread 15 20 km and continuous successions of mud turbidites which indicate that spillover processes were dominant along bryant and eastern canyons the total absence of bioturbation structures and hemipelagic sediments in mis 6 overbank deposits indicate high sedimentation rates exceeding 200 600 cm ky the majority of the overbank deposits are normally graded indicating that most of the turbidity currents during mis 6 resulted from sediment failures on the outer shelf and upper continental slope sedimentological observations indicate that a complete mud turbidite sequence t0 8 sequence in the nomenclature of stow and shanmugam 1980 results from a waning low density turbidity current consisting of four successive flow regime stages 1 an initial erosional stage 30 cm s 2 deposition of the coarsest noncohesive fraction silt of the suspended sediment in the flows 30 100 cm s 3 cyclic deposition of cohesive mud and noncohesive silt laminae due to fluctuations in the shear stress in the boundary layer of the flow 12 30 cm s and 4 deposition from a silt depleted and slowly moving 15 cm s sediment cloud

dislocation creep of dry quartz small scale shear zones within the permian truzzo meta granite developed during the alpine orogeny at amphibolite facies conditions in these shear zones magmatic quartz deformed by dislocation creep and recrystallized dynamically by grain boundary migration with minor subgrain rotation recrystallization to a grain size of around 250 750 m consistent with flow at low differential stresses fourier transform infrared ftir spectroscopy reveals very low water contents in the interior of recrystallized grains in the form of discrete oh peaks 20 h 10 6 si and very little broad band absorption 100 h 10 6 si the spectral characteristics are comparable to those of dry brazil quartz in ftir spectra magmatic quartz grains show a broad absorption band related with high water concentrations only in those areas where fluid inclusions are present while other areas are dry drainage of fluid inclusions and synkinematic growth of hydrous minerals indicates that a hydrous fluid has been available during deformation loss of intragranular water during grain boundary migration recrystallization did not result in a microstructure indicative of hardening these ftir measurements provide the first evidence that quartz with extremely low intragranular water contents can deform in nature by dislocation creep at low differential stresses low intragranular water contents in naturally deformed quartz may not be necessarily indicative of a high strength and the results are contrary to implications taken from deformation experiments where very high water contents are required to allow dislocation creep in quartz it is suggested that dislocation creep of quartz in the truzzo meta granite is possible to occur at low differential stresses because sufficient amounts of intergranular water ensure a high recovery rate by grain boundary migration while the absence of significant amounts of intragranular water is not crucial at natural conditions

milankovitch climatic signals in lower triassic olenekian peritidal carbonate successions nanpanjiang basin south china abstract meter scale peritidal carbonate strata from an isolated platform in the nanpanjiang basin of south china were tested for the presence of periodic or quasi periodic climatic signals these signals provide information on factors controlling cyclic sedimentation in the equatorial eastern tethys during the early triassic greenhouse period parasequences are composed of shallowing upward successions of subtidal facies including skeletal packstone and grainstone calcimicrobial boundstone and oolitic packstone and grainstone intertidal laminated lutite and ribbon rock and rare supratidal microbial laminite parasequence stacking patterns display three orders of cyclicity suggesting hierarchical stratigraphic relationships gamma analysis of kominz and bond earth planet sci lett 48 1990 233 244 was used to estimate facies dependent thickness time conversion factors i e s and to construct tuned and untuned time series for two stratigraphic sections spectra of these time series indicate the presence of quasi periodic signals with prominent short eccentricity 94 9 131 2 kyr short obliquity 35 8 kyr and long precessional index 21 2 kyr peaks and minor long eccentricity 412 9 kyr long obliquity 45 3 kyr short precessional index 17 7 kyr and constructional tone 9 7 kyr peaks when calibrated on the tuned spectra thus milankovitch climatic forcing probably greatly influenced sedimentation the calibration indicates that the subtidal facies has a sedimentation rate of 24 6 30 7 cm kyr and the intertidal supratidal facies has a rate of 2 7 6 0 cm kyr the estimated duration of deposition of the two sections is 1139 1423 kyr and corresponds to a stratigraphic completeness of 32 92 the completeness is much greater than that of icehouse stratigraphic records we speculate that variations in carbonate productivity and environmental conditions driven by milankovitch climatic forcing combined with low amplitude sea level fluctuations were likely major controls on cyclic sedimentation furthermore evolutional spectra of the two sections show that dominant milankovitch climatic forcings varied from short eccentricity obliquity to long precessional index during the course of sedimentation some spectral differences between the two sections suggest variations in depositional recording mechanisms of milankovitch climatic signals between platform interior and windward platform margin

distribution of volcanic ash reflectors in the eastern equatorial pacific ocean abstract volcanic ash erupted from vents in central america southern mexico and northern south america has accumulated as discrete layers in sediment of the surrounding ocean basins many of these ash layers form acoustic reflectors which are observed on 3 5 khz seismic records from the eastern equatorial pacific twelve discrete reflectors within 40 m of the seafloor may be traced in the region shallow 20 m can be correlated with ash layers in two dsdp cores and used to indirectly establish that all seismic reflectors in this study are ash layers based on areal distribution patterns most seismic reflectors indicate the direction of possible volcanic sources moreover using an assumed constant sedimentation rate of 3 cm 10 3 yr 1 the average estimated age of each reflector can be calculated all ash reflectors are quaternary in age reflectors 1 to 3 are correlated to marine ash layers d i and l and the assigned ages correlate well with the previously established ages of 83 000 133 000 and 230 000 yrs b p respectively for these layers

investigation of mass wasting on the continental slope and rise abstract a methodology is presented for the investigation of submarine slope instability this approach integrates the results of a variety of tools and methods including multibeam side scan and subbottom acoustic data geotechnical sampling and testing and advanced dating techniques for slides slumps that have the potential for causing tsunamis the use of long large diameter piston corers is a cost effective way of obtaining good quality sediment samples using a multi sensor core logger and geotechnical analyses a baseline of sediment properties is developed from nearby stable unfailed regions which is then compared to more active areas where faulting and instability is suspected clear evidence of past and recent slope failures can often be obtained from the stress history of the sediment derived from consolidation test results and from careful identification of naturally deposited sediments overlying relic failures this methodology was applied for the investigation of slope failures along a study corridor in deepwater gulf of mexico large failures were identified from multibeam data and coring locations were chosen based on subbottom and side scan data obtained with a deep tow system buried failures were identified from detailed bulk density measurements geotechnical test results and paleomagnetic and 14 c dating techniques dating of these failures can provide information regarding the triggering mechanisms of these events which can be used to assess the current risk of new instabilities