temperature and salinity staircases in the northwestern weddell sea abstract temperature and salinity data obtained from the northwestern weddell sea during march 1986 reveal numerous thermohaline staircases in the thermocline separating warm deep water from the overlying colder lower salinity winter water staircases in the upper steeper portion of the thermocline were characterized by layers having vertical extents of 1 5 m layer thicknesses in the deeper weaker portion of the thermocline were far greater sometimes exceeding 100 m the former staircases are referred to as type a and the latter as type b vertical gradients in temperature and salinity decreased abruptly across the boundary between type a and type b staircase regions mean density ratios r were 1 52 and 1 36 over the depth intervals containing type a and type b staircases respectively type a staircases were present at all sites sampled whereas type b staircases were present over approximately the central 50 of the area sampled laboratory derived results show that the observed time and vertical

magma reservoir response to transient recharge events the case of santorini volcano greece igneous bodies are built incrementally by episodic magma injection events that vary in rate and duration such recharge events can either grow an upper crustal magma reservoir or trigger an eruption this bifurcation in behavior remains poorly constrained but is essential to volcanic hazard assessment here we use a numerical model that couples the thermal and mechanical processes in a magma reservoir to study the evolution of the santorini magmatic system greece over the past 20 k y our results constrain the recharge rate and duration that are necessary to trigger an eruption for the known long term average inflow rate of 10 3 km3 yr the size of the chamber and its exsolved volatile content are dominant controls on the critical recharge rate and duration that will trigger an eruption our model successfully reproduces the main features of the minoan eruption and nea kameni activity providing volume estimates for the active part of the current subvolcanic reservoir as well as information regarding the presence of exsolved volatiles thermomechanical models offer a new framework to integrate the historic eruption record with geodetic measurements and provide a context to understand the past present and future of active volcanic centers

the santorini volcanic complex and its relation to the stratigraphy and structure of the aegean arc greece abstract the santorini complex is the most active volcano of the southern aegean volcanic arc its relationship to the surrounding area was examined by using a dense set of seismic reflection profiles three seismic units were recognized in the surrounding area unit a consists of generally strong subparallel continuous reflectors into which two subunits a1 and a2 were distinguished that are unconformable in the upper slopes and the shelves and conformable at lower sectors the age of unit a which consists mainly of volcaniclastic sediments is considered quaternary unit b consists of faulted and slightly folded strong continuous reflectors that pinch out at lower basin margin sectors than unit a its age is mainly pliocene its sediments are probably terrigenous and it was deposited in the basins at the early and middle stages of their formation finally unit c represents the acoustic mainly metamorphic basement the dominant tectonic structure in the area is the ne sw trending normal faults superimposed on earlier e w normal faults the santorini complex includes one caldera at santorini and one crater at columbus about 7 km to the northeast the caldera of santorini consists of four deep basins depth from 290 to 390 m the columbus volcano has a well defined crater with a single basin depth 500 m and was formed in 1650 a d by an underwater explosion the start of volcanic activity of santorini which initiated in the early pleistocene and continues up until today may be related to the change in the normal fault orientation from e w to ne sw and to the unconformity between units a and b

a geophysical interpretation of the 1883 krakatau eruption abstract from the present submarine topography in the vicinity of the krakatau islands it is concluded that the focus of the large explosions was situated to the northwest of the present rakata island the channel between krakatau and sebesi islands was completely blocked by banks of volcanic material immediately after the eruptions and it is suggested that this material was mainly lithic fragments the explosion sequences of krakatau are deduced from the records of sea waves and air waves observed at jakarta the large tsunami was caused by the most violent explosion simultaneously with the largest air waves it is inferred that the origin of the tsunami was a sudden upheaval of the seawater due to a violent explosion and that the height of the tsunami near its source was 30 40 m energy of the explosion is estimated from analyses of the air waves as one order of magnitude greater than that of the 1956 bezymianny eruption thus the 1883 krakatau eruption may have caused explosive removal of more than 10 km3 of material the subsurface structure of krakatau islands after the 1883 eruption is deduced from gravity anomalies it is concluded that at the bottom of the caldera there are deposits of low density in the shape of an inverted cone 8 km in diameter and 1 km in depth from the residual gravity anomaly observed over the caldera one can estimate the mass deficiency there this allows estimates to be made as to the amount of ejecta although large uncertainties remain these data indicate that explosive removal of material was the main process responsible for the disappearance of the northern half of the former rakata krakatau island in the 1883 eruption

effective diffusivities within oceanic thermohaline staircases a scale for the thickness of layers in regular diffusive type thermohaline staircases derived from dimensional analysis is found to collapse oceanic data combining this scale with laboratory derived double diffusive flux laws we formulate effective diffusivities for salt heat and density the diffusivities depend on the turner number r but are independent of the brunt vaisala frequency for r near 1 the diffusivities for salt and heat are approximately equal 10 4 m2 s 1 they decrease roughly as r 4 and r 2 respectively over the oceanic range 1 r 10

on the structure and dynamical features of intrusive layering in the eurasian basin in the arctic ocean 1 numerous conductivity temperature depth data obtained in the arctic basins are analyzed to describe structural features of intrusive layering special attention is paid to large intrusions vertical scale of 40 100 m observed in upper intermediate and deep layers depth ranges of 150 200 200 600 and 600 1000 m respectively the analysis of the intrusions is accompanied by descriptions of the frontal zones where the layering was observed based on observations detailed estimates of frontal zone parameters are presented vertical profiles of temperature and salinity are found to have a well defined sawtooth or cog shape displaying a sequence of relatively thick weak gradient layers where temperature and salinity are decreasing with depth interleaved with relatively thin high gradient sublayers where temperature and salinity are increasing with depth some hypotheses about causes responsible for cog structure existence are discussed intrusions with high amplitude anomalies in the vertical profiles of temperature and salinity are shown to be present at baroclinic fronts based on models of interleaving and data analysis the apparent vertical and lateral diffusivity in the frontal zones of the upper and deep ocean layers are estimated and the slope of unstable modes relative to the isopycnals is examined at the baroclinic front in the situation when both temperature and salinity are stably stratified

properties of thermal staircases off the northeast coast of south america spring and fall 1985 in spring and fall 1985 air expendable bathythermograph surveys of a large area off the northeast coast of south america revealed an extensive field of subthermocline thermal staircases which is probably the site of vigorous salt fingering during both seasons the staircases were shallowest in the south 180 360 m and deepest in the northwest 420 650 m occurring on the average between the 8 and 13 c isotherms typically each staircase consisted of 10 well mixed layers with average thickness of 16 m and average interlayer temperature change of 0 52 thicker layers seemed to be associated with larger temperature changes no seasonal variation in these parameters was observed the interfaces separating the layers had a modal thickness of 2 m with interfaces in the spring tending to be thinner than in the fall the region bounded by 10 14 n 52 57 w tended to have staircases with more layers thicker layers and larger interlayer temperature changes the sharp northeastern staircase field boundary occurs at the confluence of the subtropical underwater and the antarctic intermediate water suggesting that the staircases may be formed in this region and then extend south and west to form the rest of the field

reconstructing the tephra dispersal pattern from the bronze age eruption of santorini using an advection diffusion model the advection diffusion model tephra2 has been used in conjunction with the downhill simplex method dsm and one at a time oat inversion methods to reconstruct the eruption conditions and seasonality consistent with the deposit patterns from the bronze age minoan eruption of santorini we investigated three datasets representing different depositional environments proximal terrestrial distal terrestrial and deep sea core in order to determine source conditions such as plume height erupted mass and grain size and recreate the tephra fall deposit from the plinian co ignimbrite and combined eruptive phases the results of the dsm and oat method agreed adequately well with each other for erupted mass plume height and grain size distribution both approaches were able to successfully recreate the plinian deposit but estimating conditions that created the co ignimbrite and deep sea core dataset were less successful the reduced agreement is the result of the low quantity 6 to 28 deposit points and quality inconsistent deposit depths at localities adjacent to each other of the datasets and the different dynamics between co ignimbrite and plinian columns with the former not well represented in the model different sampling methods between archaeological and volcanological disciplines and post depositional processes which have acted on the tephra deposits since the bronze age can explain the discrepancy between these computed and observed deposits the seasonality of the minoan eruption was investigated by using seasonal wind profiles for winter spring summer and autumn we find that the bronze age eruption of santorini is likely to have during the spring and summer months with a main dispersal axis aligned east crete would have received very little ash fall and the eruption would not have caused much disruption to the life of the inhabitants of the island

what determines the thickness of layers in a thermohaline staircase the article of record as published may be found at http dx doi org 10 1017 s0022112004002290

observational and laboratory insights into salt finger convection improving observational capabilities in the ocean are revealing a substantial body of evidence that the double diffusive instability known as salt fingers is widely present and plays a significant role in oceanic mixing this evidence includes unique fine and microstructure features in particular regions and distinct patterns in the temperature and salinity variations over large areas laboratory experiments have also advanced our understanding of salt fingers the insights gleaned from both laboratory and ocean are reviewed and related to large scale ocean structure and climate modeling

mixing 3d mapping and lagrangian evolution of a thermohaline intrusion abstract observations of the three dimensional structure and evolution of a thermohaline intrusion in a wide deep fjord are presented in an intensive two ship study centered on an acoustically tracked neutrally buoyant float a cold fresh low oxygen tongue of water moving southward at about 0 03 m s 1 out of possession sound washington was observed the feature lay across isopycnal surfaces in a 50 80 m depth range the large scale structures of temperature salinity velocity dissolved oxygen and chlorophyll were mapped with a towed depth cycling instrument from one ship while the other ship measured turbulence close to the float with loosely tethered microstructure profilers observations from both ships were expressed in a float relative lagrangian reference frame minimizing advection effects a float deployed at the tongue s leading edge warmed 0 2 c in 24 h which the authors argue resulted from mixing diapycnal heat fluxes inferred from microstructure were 1 2 orders of magnitude too sm

effect of baroclinicity on double diffusive interleaving although ocean fronts are often baroclinic existing models of double diffusive interleaving have ignored such baroclinic effects as velocity shear and horizontal density gradients to determine the importance of these effects the authors have formulated a linear instability analysis applicable to baroclinic fronts two limiting cases are considered one for fronts with strong vertical and or horizontal shear the other for fronts with weak shear in both limits double diffusive interleaving can be enhanced or suppressed by baroclinicity interleaving motion is enhanced if isopycnals rise toward the fresh side of the front conversely interleaving is suppressed if isopycnals slope downward across the front a significant result is that the salinity gradient along isopycnals is not a good indicator of interleaving strength as an example the model is applied to a mediterranean salt lens the effect of baroclinicity is significant the predicted growth rates are increased by 35 90 the large scale velocity and hydrographic fields indicate that meddy sharon lies somewhere between the high and low shear limits nevertheless the model predictions agree reasonably well with the observed interleaving characteristics

historical bathymetric charts and the evolution of santorini submarine volcano greece historical bathymetric charts are a potential resource for better understanding the dynamics of the seafloor and the role of active processes such as submarine volcanism the british admiralty for example have been involved in lead line measurements of seafloor depth since the early 1790s here we report on an analysis of historical charts in the region of santorini volcano greece repeat lead line surveys in 1848 late 1866 and 1925 1928 as well as multibeam swath bathymetry surveys in 2001 and 2006 have been used to document changes in seafloor depth these data reveal that the flanks of the kameni islands a dacitic dome complex in the caldera center have shallowed by up to 175 m and deepened by up to 80 m since 1848 the largest shallowing occurred between the late 1866 and 1925 1928 surveys and the largest deepening occurred during the 1925 1928 and 2001 and 2006 surveys the shallowing is attributed to the emplacement of lavas during effusive eruptions in both 1866 1870 and 1925 1928 at rates of up to 0 18 and 0 05 km3 a 1 respectively the deepening is attributed to a load induced viscoelastic stress relaxation following the 1866 1870 and 1925 1928 lava eruptions the elastic thickness and viscosity that best fits the observed deepening are 1 0 km and 1016 pa s respectively this parameter pair which is consistent with the predictions of a shallow magma chamber thermal model explains both the amplitude and wavelength of the historical bathymetric data and the present day rate of subsidence inferred from insar analysis

revised chronostratigraphy of recurrent ignimbritic eruptions in dominica lesser antilles arc implications on the behavior of the magma plumbing system ignimbritic eruptions represent catastrophic events due to the magma volume involved and the related consequences on earth s environment in relation with the released gases and the tephra dispersal dominica has been recognized as hosting one of the major ignimbritic eruptions of the last 200 ky in the lesser antilles arc called the roseau tuff but more recent works have evidenced several pumiceous events instead of a single large one here we propose a revised chronostratigraphy of the explosive activity that occurred in the last tens of thousands years based on three field trips new 14c ages detailed lithological and geochemical investigations in particular a precise characterization of trace element glass chemistry this eruptive history reconstruction is mainly based on outcrops along the coast and in the valley since the luxury vegetation in the center of the island mostly precludes sections close to the central volcanic centers we thus confirm that the roseau event has been overestimated and that we may recognize five main ignimbritic events grande savane layou 51 ka grand bay roseau 33 ka grand fond 24 ka we discuss the possible volcanic center at their origin in addition to correlations with some plinian events of lower magnitude that were identified in the roseau valley and in the southern part of dominica this study may help to better constrain the eruptive history of the most active volcanic island of the lesser antilles arc which has important implications on hazard mitigation

mechanics of merging events for a series of layers in a stratified turbulent fluid this study attempts to explain the evolutionary pattern of a series of well mixed layers separated by thin high gradient interfaces frequently observed in stratified fluids such layered structures form as a result of the instability of the equilibrium with uniform stratification and their subsequent evolution is characterized by a sequence of merging events which systematically increase the average layer thickness the coarsening of layers can take one of two forms depending on the realized vertical buoyancy flux law layers merge either when the high gradient interfaces drift and collide or when some interfaces gradually erode without moving vertically the selection of a preferred pattern of coarsening is rationalized by the analytical theory the merging theorem which is based on linear stability analysis for a series of identical layers and strongly stratified interfaces the merging theorem suggests that the merger by erosion of weak interfaces occurs when the vertical buoyancy flux decreases with the buoyancy variation across the step if the buoyancy flux increases with step height then coarsening of a staircase may result from drift and collision of the adjacent interfaces our model also quantifies the time scale of merging events and makes it possible to predict whether the layer merging continues indefinitely or whether the coarsening is ultimately arrested the merging theorem is applied to extant one dimensional models of turbulent mixing and successfully tested against the corresponding fully nonlinear numerical simulations it is hypothesized that the upscale cascade of buoyancy variance associated with merging events may be one of the significant sources of the fine scale 10 m variability in the ocean

spatial variability of turbulent mixing in the abyssal ocean ocean microstructure data show that turbulent mixing in the deep brazil basin of the south atlantic ocean is weak at all depths above smooth abyssal plains and the south american continental rise the diapycnal diffusivity there was estimated to be less than or approximately equal to 0 1 x 10 4 meters squared per second in contrast mixing rates are large throughout the water column above the rough mid atlantic ridge and the diffusivity deduced for the bottom most 150 meters exceeds 5 x 10 4 meters squared per second such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry

a review of oceanic salt fingering theory salt fingering is a potentially important mechanism for mixing different water masses in those parts of the ocean where warm salty waters overlie cooler fresher this review surveys advances in salt fingering theory as they pertain to ocean mixing basic equations are presented along with steady maximum buoyancy flux and fastest growing finger solutions attempts to quantify theoretically finger induced fluxes of heat salt and buoyancy and to understand staircase formation and maintenance are described also considered are the confounding influences of internal wave shear and strain and intermittent internal wave shear driven turbulence which are present in the ocean but in few theories finally outstanding theoretical questions about the role of salt fingers in the ocean are raised

double diffusive interleaving part i linear stability analysis abstract quasi horizontal interleaving between water masses is frequently observed in the frontal regions between different water masses where there are significant compensating isopycnal gradients of temperature and salinity it is believed that these intrusions are driven by double diffusive convection which transports salt heat and density across sharp quasi horizontal interfaces i e across regions of large vertical property gradients stern and other investigators have parameterized the vertical flux of salt in these intrusions by an eddy diffusivity multiplied by the vertical gradient of salinity whereas laboratory experiments show that sharp interfaces tend to occur by double diffusive convection and that the vertical fluxes of properties depend on the property contrasts across the interfaces in this paper it is assumed that the vertical fluxes of double diffusive convection are proportional to the salinity contrast across the sharp interfaces that occur between the quasi horizontal intrusio

origin of thermohaline staircases hypotheses concerning the origin of thermohaline staircases in salt fingering regions are reviewed and assessed one such hypothesis that staircases arise from thermohaline intrusions is developed into a quantitative theory it is shown that growing intrusions evolve toward staircases when the background density ratio lies below a threshold value and nonlinear computations confirm that staircases are viable intrusion equilibria staircase properties such as step heights lateral density ratios and layer slopes lie closest to observed values when salt fingers are assumed not to contribute to shear stress and when turbulent mixing rates are smaller than usual thermocline values

chemical diagenesis of a multicomponent carbonate system 2 stable isotopes abstract this study examines the oxygen and carbon isotopic distribution in the brachiopods crinoids rugose corals and matrix cement from the mississippian burlington limestone iowa and missouri and the silurian read bay formation arctic canada the isotopic data can be reconciled with textural and trace element observations only if the 18o content of ancient oceans differed from that of the present oceans this postulated secular variation in the 18o content of seawater requires a separate evaluation of the mississippian and silurian carbonate components examination of the 18o content of the least altered low mg calcite fossil components of the two formations suggests that the 18o of mississippian seawater was on average about 1 5 lighter and that of silurian seawater was about 5 5 lighter than the 18o of present day ocean water the average diagenetic shift in 18o for the read bay formation which stabilized in a partly closed diagenetic phreatic meteoric system is about 1 for the original low mg calcite and about 2 for the original high mg calcite and aragonite components conversely for the burlington limestone which stabilized in an open diagenetic phreatic meteoric system the average diagenetic shift in 18o is about 3 for the original low mg calcite and about 4 f r the original high mg calcite components the 13c values of the originally low mg calcite or high mg calcite with less than 7 mole mgco3 brachiopods and rugose corals the originally high mg calcite crinoids and the originally aragonite lime mud of the burlington limestone and read bay formation are bimodally distributed this bimodal 13c distribution is independent of geologic age inferred original mineralogy of the component degree of diagenetic alteration and the type of diagenetic meteoric system with components from both the read bay formation and burlington limestone contributing to both 13c groups the mode of the light 13c group is 0 25 and for the heavy 13c group it is 3 25 pdb both 13c groups also show a decrease in 13c of about 1 with increasing diagenetic alteration stabilization

gravity fields and the interpretation of volcanic structures geological discrimination and temporal evolution abstract a review of gravity data reflecting shallow concealed structures on volcanoes demonstrates that valuable information may be derived on the geological development of such structures and on the dynamic evolution of active volcanoes to a first approximation all recorded examples of anomalous gravity fields associated with volcanoes are concentric with and have sources that lie within the volcanic edifice positive anomalies with wavelengths less than 20 km and amplitudes up to ca 30 mgal characterise mainly basaltic volcanoes from various tectonic settings and are caused by a relatively dense intrusive complex magma body which contrasts with its surroundings either because the body is more mafic than average or more likely because near surface previously erupted materials are uncompacted negative anomalies with wavelengths up to 100 km and amplitudes up to ca 60 mgal occur over much larger volcanic calderas many of which have erupted highly silicic pyroclastic ash and pumice uncompacted silicic caldera infill with a possible contribution from low density magma bodies is responsible for the observed anomalies there is some evidence for a continuum of gravity anomaly types that corresponds to the geological evolution of volcanic systems generally from primitive rift or subduction related basaltic andesite to mature high level silicic calderas repeated microgravity observations over active volcanoes enable magma movements variations in magma input and changes in magma density to be monitored closely dynamic modelling of volcanic systems is providing new evidence on the behaviour of concealed magma bodies and could have considerable potential for predicting eruptive events

the relationship between double diffusive intrusions and staircases in the arctic ocean abstractthe origin of double diffusive staircases in the arctic ocean is investigated for the particular background setting in which both temperature and salinity increase with depth motivated by observations that show the coexistence of thermohaline intrusions and double diffusive staircases a linear stability analysis is performed on the governing equations to determine the conditions under which staircases form it is shown that a double diffusive staircase can result from interleaving motions if the observed bulk vertical density ratio is below a critical vertical density ratio estimated for particular lateral and vertical background temperature and salinity gradients vertical background temperature and salinity gradients dominate over horizontal gradients in determining whether staircases form with the linear theory indicating that perturbations to stronger vertical temperature gradients are more likely to give rise to a staircase examination of arctic ocean temperature and salinity measurements

vertical heat transfer in the arctic ocean the role of double diffusive mixing 1 microstructure profiles collected in the central amundsen basin are analyzed in order to quantify the role of double diffusive mixing for vertical heat transfer from the atlantic layer in the profiles a persistent but laterally incoherent thermohaline staircase structure is identified in the 200 260 m depth range the staircase contains homogeneous layers with average height of 1 3 m and thin high gradient interfaces with average temperature and salinity jumps of about 0 065 c and 0 015 respectively when inferred from a commonly used diffusive convection parameterization the average vertical heat flux within the staircase is 0 6 w m 2 this is one order of magnitude larger than the molecular diffusion alone and of the same order as the overall heat loss from the atlantic layer in the deep basins of the arctic ocean the parameterization is evaluated using observed turbulent heat fluxes and is found to overestimate diffusive convective fluxes with up to an order of magnitude especially for weak thermal forcing small temperature jumps in staircase staircases coexist with thermohaline intrusions in the vertical temperature and salinity profiles lomonosov ridge is identified as a potential region for formation of intrusions it is found that salt fingering is the dominant process during intrusion growth whereas diffusive convection is the dominant process in maintaining the intrusions at steady state in the deep basins

enhanced diapycnal mixing by salt fingers in the thermocline of the tropical atlantic diapycnal mixing plays a significant role in the ocean s circulation and uptake of heat and carbon dioxide but has not been quantified in salt finger driven thermohaline staircases we recently performed a tracer release experiment in the western tropical atlantic staircase at approximately 400 m depth the observed dispersion implies an effective diapycnal diffusivity for tracer and salt of 0 8 to 0 9 x 10 4 m2 s temperature microstructure data interpreted in terms of a vertical production dissipation balance yields a smaller effective diffusivity for heat of 0 45 0 2 x 10 4 m2 s consistent with salt fingers and well above the mixing ascribable to mechanical turbulence

geophysical prospecting on the santorini islands a program of geophysical research was carried out as a preliminary stage of study of the santorini volcanic group this area is of remarkable geothermal and volcanological interest and the definition of a volcanological structural model is the starting point for an understanding of the local geodynamic processes

the role of saltwater and waves in continental shelf formation with seaward migrating clinoform continental shelves have generally been interpreted as drowned coastal plains associated with the allogenic effect of sea level variation here without disputing this mechanism we describe an alternative autogenic mechanism for subaqueous shelf formation driven by the presence of dissolved salt in seawater and surface waves we use a numerical model describing flow hydrodynamics sediment transport and morphodynamics in order to do this more specifically we focus on two major aspects 1 the role of saltwater in the subaqueous construction of continental shelves and 2 the transformation of these shelves into seaward migrating clinoforms under the condition of repeated pulses of water and sediment input and steady wave effects but no allogenic forcing such as sea level change in the case for which the receiving basin contains fresh water of the same density as the sediment laden river water the hyperpycnal river water plunges to form a turbidity current that can run out to deep water in the case for which the receiving basin contains sea water but the river contains sediment laden fresh water the hypopycnal river water forms a surface plume that deposits sediment proximally this proximate proto shelf can then grow to wave base after which wave supported turbidity currents can extend it seaward the feature we refer to is synonymous with near shore mud belts

stratigraphic relations of santorini s intracaldera fill and implications for the rate of post caldera volcanism we propose a new volcanological interpretation of santorini s intracaldera fill deposits as revealed by seismic reflection profiles this interpretation along with supplementary gravity and geophysical studies reveals three distinct volcaniclastic units 1 3 from top to bottom these units are attributed to 1 modern infilling sediment 2 shallow marine phreatomagmatic volcanism associated with the relatively recent formation of the kameni islands and 3 downfaulted minoan pyroclastic deposits which formed during caldera collapse towards the end of the bronze age eruption estimates of the volumes of the seismic units and kameni islands yield a dense rock equivalent magma volume of 4 85 0 7 km 3 the average rate of volcanism over the past c 3641 years is estimated at 1 3 10 3 km 3 a 1 and is similar to the rate since the ad 1707 eruption of 1 2 10 3 km 3 a 1 based on historical lava volume estimates

tying down eruption risk 200 years after the eruption of mount tambora the eruption volume remains poorly known as is true for other volcanic eruptions over past millennia we need better records of size and occurrence if we are to predict future large eruptions more accurately

carbon and oxygen isotopic composition of holocene reefal carbonates carbon and oxygen isotopic compositions have been determined for holocene reefal carbonate grains and cements in samples from enewetak bikini bermuda and belize to identify components that exhibit systematic isotopic fractionation relationships these findings are compared with fractionation relationships determined from experimentally precipitated low magnesium calcite lmc aragonite and high magnesium calcite hmc to determine if these relationships are observed in naturally precipitated carbonates compositional variation within individual samples is extreme up to 6 in 13 c pdb and 3 in 18 o pdb experimental fractionation relationships fail to predict the range of observed compositions aragonite cements are enriched 1 0 to 1 4 in 13 c and 1 5 to 2 0 in 18 o relative to calcite equilibrium compositions hmc cements are enriched by 0 3 to 0 4 in 13 c and by 1 8 to 1 9 to 18 o for carbon these results are in agreement with the experimental work of rubinson and clayton for oxygen our results suggest that fractionation factors of both aragonite and hmc may have been underestimated skeletal components exhibit inconsistent fractionation relative to inorganically precipitated marine cements suggesting that marine cement compositions are more reliable indicators of the physicochemical conditions of precipitation than co occurring skeletal carbonates

the global magnitude frequency relationship for large explosive volcanic eruptions abstract for volcanoes as for other natural hazards the frequency of large events diminishes with their magnitude as captured by the magnitude frequency relationship assessing this relationship is valuable both for the insights it provides about volcanism and for the practical challenge of risk management we derive a global magnitude frequency relationship for explosive volcanic eruptions of at least 300 mt of erupted mass or m4 5 our approach is essentially empirical based on the eruptions recorded in the lameve database it differs from previous approaches mainly in our conservative treatment of magnitude rounding and under recording our estimate for the return period of super eruptions 1000 gt or m8 is 17 ka 95 ci 5 2 ka 48 ka which is substantially shorter than previous estimates indicating that volcanoes pose a larger risk to human civilisation than previously thought

seismic imaging of santorini subsurface constraints on caldera collapse and present day magma recharge abstract volcanic calderas are surface depressions formed by roof collapse following evacuation of magma from an underlying reservoir the mechanisms of caldera formation are debated and predict differences in the evolution of the caldera floor and distinct styles of magma recharge here we use a dense active source seismic tomography study to reveal the sub surface physical properties of the santorini caldera in order to understand caldera formation we find a 3 km wide cylindrical low velocity anomaly in the upper 3 km beneath the north central portion of the caldera that lies directly above the pressure source of the 2011 2012 inflation we interpret this anomaly as a low density volume caused by excess porosities of between 4 and 28 with pore spaces filled with hot seawater vents that were formed during the first three phases of the 3 6 ka late bronze age lba eruption are located close to the edge of the imaged structure the correlation between older volcanic vents and the low velocity anomaly suggests that this feature may be long lived we infer that collapse of a limited area of the caldera floor resulted in a high porosity low density cylindrical volume which formed by either chaotic collapse along reverse faults wholesale subsidence and infilling with tuffs and ignimbrites phreatomagmatic fracturing or a combination of these processes phase 4 eruptive vents are located along the margins of the topographic caldera and the velocity structure indicates that coherent down drop of the wider topographic caldera followed the more limited collapse in the northern caldera this progressive collapse sequence is consistent with models for multi stage formation of nested calderas along conjugate reverse and normal faults the upper crustal density differences inferred from the seismic velocity model predict differences in subsurface gravitational loading that correlate with the location of 2011 2012 edifice inflation this result supports the hypothesis that sub surface density anomalies may influence present day magma recharge events we postulate that past collapses and the resulting topographical and density variations at santorini influence magma focusing between eruptive cycles a feedback process that may be important in other volcanoes

caldera formation on santorini and the physiography of the islands in the late bronze age the caldera of santorini is a composite structure with a subsidence history extending over 100 ka or more geomorphological mapping shows that the present day caldera wall is a complex assemblage of cliff surfaces of different ages and that collapse at santorini has repeatedly exhumed earlier caldera cliffs and unconformities cliffs bounding the southern southeastern and northwestern rims of the caldera are morphologically fresh and probably formed during or soon after the minoan eruption in the late bronze age the well scalloped shape of these cliffs is attributed to large scale rotational landslip around the margins of the minoan caldera the deposit from one landslip is preserved subaerially minoan landslips in southeast santorini detached along the basement unconformity exposing a cliff of the prevolcanic island the caldera wall in the north northeast and east preserves evidence for three generations of cliff those of minoan age and two earlier generations of caldera wall the two early calderas can be dated relative to a well established statigraphy of lavas and tuffs the presence of in situ minoan tephra plastered onto the present day caldera wall provides evidence that these ancient caldera cliffs had already been exhumed prior to the minoan eruption field relationships permit reconstruction of the physiography of bronze age santorini immediately before the minoan eruption the reconstruction differs from some previously published versions and is believed to be the most accurate to date bronze age sa ntorini had a large flooded caldera formed 21 ka ago this caldera must have acted as an excellent harbour for the bronze age inhabitants of the island the 3 6 ka minoan eruption deepened and widened the extant caldera the volume of minoan collapse 25 km3 is in good agreement with published estimates for the volume of discharged magma if between 5 and 8 km3 of minoan ignimbrite ponded as intracaldera tuff

caldera development during the minoan eruption thira cyclades greece the well known caldera of thira santorini greece was not formed during a single eruption but is composed of two overlapping calderas superimposed upon a complex volcanic field that developed along a ne trending line of vents before the minoan eruption of 1400 b c thira consisted of three lava shields in the northern half of the island and a flooded depression surrounded by tuff deposits in the southern half andesitic lavas formed the overlapping shields of the north and were contemporaneous with and in many places interbedded with the southern tuff deposits although there appears to be little difference between the composition of magmas erupted differences in eruption style indicate that most of the activity in the northern half of the volcanic field was subaerial producing lava flows whereas in the south eruptions within a flooded depression produced a sequence of mostly phreatomagmatic tuffs many of these tuffs are plastered onto the walls of what appears to have been an older caldera most probably associated with an eruption of rhyodiacitic tephra 100 000 years ago the minoan eruption of about 1400 b c had four distinct phases each reflecting a different vent geometry and eruption mechanism the minoan activity was preceded by minormore eruptions of fine ash 1 the eruption began with a plinian phase from subaerial vent s located on the easternmost of the lave shields 2 vent s grew toward the sw into the flooded depression subsequent activity deposited large scale base surge deposits during vent widening by phreatomagnetic activity 3 the third eruptive phase was also phreatomagmatic and produced 60 of the volume of the minoan tuff this activity was nearly continuous and formed a large featureless tuff ring with poorly defined bedding less

evidence from cosmic ray exposure cre dating for the existence of a pre minoan caldera on santorini greece cosmic ray exposure cre dating was performed on the caldera cliffs of santorini with the aim of detecting cliff segments predating the minoan eruption 17th century bce the methodology involved the determination of in situ produced cosmogenic 36cl concentration in basaltic to rhyodacitic whole rocks cropping out in the cliffs after the samples were processed following the chemical protocol of 36cl preparation for silicate rocks 36cl concentrations were measured by accelerator mass spectrometry ams important challenges during the implementation procedure were related to large amounts of radiogenic 36cl complex modeling of inherited 36cl and dominance of the thermal and epithermal low energy neutron capture production pathway nevertheless quantitative assessments on the basis of the contribution of the low energy neutron capture pathway percent to the total production rate validated the calculated cre dates current cre ages demonstrate that an ancient caldera existed on pre minoan santorini occupying at least the northern half of the modern day caldera

numerical experiments on double diffusive intrusions in the ocean and their relation to laboratory experiments abstract numerical experiments on double diffusive intrusions are reviewed briefly though the number of studies is very limited at present they have undoubtedly an advantage that a heat salt system can be studied without undesired heat loss from the boundaries several possibilities for future numerical experiments are summarized

estimating the volume of tephra deposits a new simple strategy volume determination of tephra deposits is necessary for the assessment of the dynamics and hazards of explosive volcanoes several methods have been proposed during the past 40 years that include the analysis of crystal concentration of large pumices integrations of various thinning relationships and the inversion of field observations using analytical and computational models regardless of their strong dependence on tephra deposit exposure and distribution of isomass isopach contours empirical integrations of deposit thinning trends still represent the most widely adopted strategy due to their practical and fast application the most recent methods involve the best fitting of thinning data using various exponential segments or a power law curve on semilog plots of thickness or mass area versus square root of isopach area the exponential method is mainly sensitive to the number and the choice of straight segments whereas the power law method can better reproduce the natural thinning of tephra deposits but is strongly sensitive to the proximal or distal extreme of integration we analyze a large data set of tephra deposits and propose a new empirical method for the determination of tephra deposit volumes that is based on the integration of the weibull function the new method shows a better agreement with observed data reconciling the debate on the use of the exponential versus power law method in fact the weibull best fitting only depends on three free parameters can well reproduce the gradual thinning of tephra deposits and does not depend on the choice of arbitrary segments or of arbitrary extremes of integration

a new model for atmospheric oxygen over phanerozoic time a mathematical model has been constructed that enables calculation of the level of atmospheric o2 over the past 570 my from rates of burial and weathering of organic carbon c and pyrite sulfur s burial rates as a function of time are calculated from an assumed constant worldwide clastic sedimentation rate and the relative abundance and c and s contents of the three rock types marine sandstones and shales coal basin sediments and other non marine clastics red beds arkoses by our model values of o2 versus time using a constant total sedimentation rate agree with those for variable sedimentation derived from present day rock abundances and estimates of erosional losses since deposition this agreement is the result of our reliance on the idea that any increase in total worldwide sediment burial with consequently faster burial of c and s and greater o2 production must be accompanied by a corresponding increase in erosion and increased exposure of c and s on the continents to o2 consumption via weathering it is the redistribution of sediment between the three different rock types and not total sedimentation rate that is important in o2 control to add stability to the system negative feedback against excessive o2 fluctuation was provided in the modeling by the geologically reasonable assignment of higher weathering rates to younger rocks resulting in rapid recycling of c and s we did not use direct o2 negative feedback on either weathering of c and s or burial of c because weathering rates are assumed to be limited by uplift and erosion and the burial rate of c limited by the rate of sediment deposition the latter assumption is the result of modern sediment studies which show that marine organic matter burial occurs mainly in oxygenated shallow water and is limited by the rate of supply of nutrients to the oceans by rivers results of the modeling indicate that atmospheric o2 probably has varied appreciably over phanerozoic time during the late carboniferous and permian periods o2 was higher than previously because of the rise of vascular land plants and the widespread burial of organic matter in vast coal swamps a large decrease in o2 during the late permian was due probably to the drying up of the coal swamps and deposition of a large proportion of total sediment in c and s free continental red beds sensitivity study shows that major parameters affecting results are the mean c concentration in coal basins and the relative sizes of the reservoirs of young rapidly recycled versus old rocks less sensitivity was found for changes over time in total land area undergoing weathering and the use of direct o2 negative feedback on marine carbon burial good agreement for rates of c burial calculated via our model and via independent models which are based on the use of stable carbon isotopes indicates that the dominant factor that has brought about changes in atmospheric o2 level and the isotopic composition of dissolved inorganic carbon in seawater over phanerozoic time is sedimentation and not weathering or higher temperature phenomena such as basalt seawater reaction

precursory eruptive activity and implied cultural responses to the late bronze age lba eruption of thera santorini greece abstract the late bronze age lba eruption ca 1650 bce of thera santorini was one of the largest known in history burying and destroying a thriving theran cycladian culture that occupied the island and surrounding islands the consequent thick tephra deposit provides detailed information on the eruptive sequence and vent mechanics it also provides details on that culture through burial of towns farmsteads and landscapes the most prominent being the town of akrotiri on the south coast of thera here stratigraphic relationships between volcanic and archaeological deposits constructions clearly indicate this eruption was signaled by seismic and minor eruptive events precursory to the main plinian eruption because no casualties have yet been found beneath the tephra inhabitants had advance notice of the impending disaster by a precursory eruption whose deposits are well preserved both within the archaeological site and in geological exposures throughout southern thera island and escaped by boat yet archaeological sites on nearby islands rarely record an influx of new arrivals at this time accordingly it is suggested here that those escaping were incinerated at sea by pyroclastic density currents that swept across the ocean surface during the second phase of the plinian eruption

sources of uncertainty in the mazama isopachs and the implications for interpreting distal tephra deposits from large magnitude eruptions estimating the area of tephra fallout and volume of large magnitude eruptions is fundamental to interpretations of the hazards posed by eruptions of this scale this study uses the tephra from the caldera forming eruption of mount mazama crater lake or usa to demonstrate the challenges faced when working with large prehistoric tephra deposits and outlines the methodologies required to determine eruption volume and magnitude we combine 250 mazama tephra occurrences reported by a range of disciplines including archaeology paleoclimatology and volcanology with new field studies to better understand the extent of the distal tephra we find that the mazama tephra has been remobilised to varying degrees over the past 7000 years so each tephra locality was appraised for the likelihood that it records primary tephra fallout we designated 45 of the distal 100 km from source tephra sites as suitable for use in the production of isopachs using a spline fitting method the new distal isopachs were then integrated with proximal fallout data and estimates of the ignimbrite volume from previous studies to revise the estimated bulk erupted volume from the climactic mazama eruption to 176 km3 61 km3 dense rock equivalent dre this study demonstrates the importance of collating tephra localities from a range of disciplines and that even remobilised deposits provide valuable information about the extent of the deposit interpreting remobilised deposits can provide insight into post eruptive processes that could potentially pose secondary hazards following large magnitude eruptions we also show that in some circumstances remobilised deposits preserve important physical properties such as grain size

on the parameterization of diapycnal fluxes due to double diffusive intrusions abstract an attempt is made to parameterized the large scale average diapycnal cross isopycnal mixing that presumably occurs in the thermohaline fronts that develop when large scale epipycnal along isopycnal gradients of t and s are stirred along isopycnals by mesoscale eddies it is assumed that double diffusive intrusions develop at the fronts and that their thickness is given by the formula of ruddick and turner 1979 this combined with a crude estimate of the frontal width and a very over simplified model of the eddy field leads to a formula for the average diapycnal diffusivity for salt or some neutral tracer and suggests that the mechanism is important in weakly stratified water with a large epipycnal gradient of salinity the diapycnal eddy diffusivities for temperature is negative for a stably stratified temperature field however the opposite signs of the diapycnal diffusivities for salt and heat are unlikely to lead to observable consequences on account of the dominance in duxes acros

total grain size distribution and volume of tephra fall deposits on 17 june 1996 ruapehu volcano new zealand produced a sustained andesitic sub plinian eruption which generated a narrow tephra fall deposit extending more than 200 km from the volcano the extremely detailed data set from this eruption allowed methods for the determination of total grain size distribution and volume of tephra fall deposits to be critically investigated calculated total grain size distributions of tephra fall deposits depend strongly on the method used and on the availability of data across the entire dispersal area the voronoi tessellation method was tested for the ruapehu deposit and gave the best results when applied to a data set extending out to isomass values of 1 g m 2 the total grain size distribution of a deposit is also strongly influenced by the very proximal samples and this can be shown by artificially constructing subsets from the ruapehu database unless the available data set is large all existing techniques for calculations of total grain size distribution give only apparent distributions the tephra fall deposit from ruapehu does not show a simple exponential thinning but can be approximated well by at least three straight line segments or by a power law fit on semi log plots of thickness vs area 1 2 integrations of both fits give similar volumes of about 4 106 m3 integration of at least three exponential segments and of a power law fit with at least ten isopach contours available can be considered as a good estimate of the actual volume of tephra fall integrations of smaller data sets are more problematic

thermohaline staircases in a caribbean eddy and mechanisms for staircase formation the structure of a cyclonic eddy in the eastern caribbean was observed by caribbean vorticity experiment cruise cavortex i in august 2003 prominent thermohaline staircases are evident in profiles of temperature and salinity at locations that appear to correlate with the presence of lateral property gradients and shear within the eddy although such a connection is less than definitive due to the coarseness of horizontal sampling these observations are examined in light of predictions from two theories for staircase formation aspects of the data are found to be consistent with each raising the possibility that both mechanisms are operating

constraining the landscape of late bronze age santorini prior to the minoan eruption insights from volcanological geomorphological and archaeological findings abstract one of the best known places on earth where volcanology meets archaeology and history is the volcanic island of santorini thira greece it is famous for the cataclysmic late bronze age minoan plinian eruption which destroyed the minoan culture that flourished on the island hosting a central flooded caldera bay and within that the active islands of palaea and nea kameni santorini volcano has been the focus of international research efforts for over one and a half centuries in this paper we summarize recent findings and related ideas about the minoan physiography of the island also known as strongyli from a volcanological geomorphological and archaeological point of view as proposed as early as the 1980s a central caldera bay existed prior to the late bronze age probably characterised by a smaller size and located in the northern part of the present day caldera this earlier caldera bay was formed during the previous plinian eruption called cape riva eruption c 22 000 years ago within the caldera bay a central island pre kameni existed named after the present day kameni islands high precision radioisotopic dating revealed that pre kameni started to grow c 20 000 years ago whereas volcanologists have accepted and refined the caldera concept archaeologists have generally favoured the theory of an exploded central cone instead of a pre existing central caldera however analysis of the flotilla fresco one of the wall paintings found in the bronze age settlement of akrotiri reveals the interior of a late bronze age caldera that may be interpreted as a realistic landscape approximately 3600 years ago the island of strongyli was destroyed during the explosive vei 7 minoan eruption pre kameni was lost by this eruption but its scattered fragments together with other parts of strongyli can be recovered as lithic clasts from the minoan tuffs on the basis of photo statistics and granulometry of the lithic clasts contained in the minoan tuffs complemented by volumetric assessment of the erupted tephra and digital elevation model dem analysis of alternative models for the pre eruptive topography the volume of pre kameni can be constrained between 1 6 and 3 0 km3 whereas the volume of the destroyed portion of the ring island of strongyli between 9 1 and 17 1 km3 of these the larger values are considered more realistic and imply that most of the destroyed part of strongyli was incorporated as lithic components in the minoan tuffs whereas up to 3 km3 of strongyli might have been downfaulted and sunken during caldera formation and is not accounted for in the lithics

double diffusive recipes part ii layer merging events this study explores the dynamics of thermohaline staircases well defined stepped structures in temperature andsalinityprofiles commonlyobservedin regionsofactive doublediffusion theevolutionofstaircases in time is frequently characterized by spontaneous layer merging events these phenomena the authors argue are essentialin regulating theequilibriumlayerthicknessin fullydevelopedstaircases thepatternand mechanics of merging events are explained using a combination of analytical considerations direct numerical simulations and data analysis the theoretical merger model is based on the stability analysis for a series of identical steps and pertains to both forms of double diffusion diffusive convection and salt fingering the conceptual significance of the proposed model lies in its ability to describe merging events without assuming fromtheoutsetspecificpowerlawsfortheverticaltransportofheatandsalt theapproachadoptedbyearlier merging models the analysis of direct numerical simulations indicates that merging models based on the four thirds flux laws offer adequate qualitative description of the evolutionary patterns but are less accurate than models that do not rely on such laws specific examples considered in this paper include the evolution of layers in the diffusive staircase in the beaufort gyre of the arctic ocean

the diffusive regime of double diffusive convection the diffusive regime of double diffusive convection is discussed with a particular focus on unresolved issues that are holding up the development of large scale parameterizations some of these issues such as interfacial transports and layer interface interactions may be studied in isolation laboratory work should help with these however we must also face more difficult matters that relate to oceanic phenomena that are not represented easily in the laboratory these lie beneath some fundamental questions about how double diffusive structures are formed in the ocean and how they evolve in the competitive ocean environment

a linear stability theory of double diffusive horizontal intrusions in a temperature salinity front on suppose que les doigts de sel sont responsables des transports verticaux de salinite et de chaleur le front est toujours instable meme en presence de viscosite les intrusions horizontales sont engendrees par l instabilite et l eau douce et froide descend tandis que l eau salee et chaude monte

intrusions in double diffusively stable arctic waters evidence for differential mixing abstract intrusions like those observed in double diffusively stable regions of the arctic ocean can grow from uniform ambient temperature and salinity gradients if diapycnal mixing of these two components differs assuming this to be the driving mechanism the observed 40 60 m intrusion heights constrain the turbulent diffusivity for heat to be less than about 0 01 cm2 s 1 and the salt to heat turbulent diffusivity ratio to be greater than about 0 6 if the diffusivities are constant observations indicate that the intrusions slope across isopycnals in a sense that is consistent with such a scenario although the along intrusion density ratio is greater than that predicted by linear theory for the fastest growing intrusions numerical solutions for growing intrusions resemble observed temperature and salinity profiles

are eruptions from linear fissures and caldera ring dykes more likely to produce pyroclastic flows turbulent volcanic jets are produced by highly energetic explosive eruptions and may form buoyant plumes that rise many tens of kilometres into the atmosphere to form umbrella clouds or collapse to generate ground hugging pyroclastic flows ash injected into the atmosphere can be transported for many hundreds of kilometres with the potential to affect climate disrupt global air travel and cause respiratory health problems pyroclastic flows by contrast are potentially catastrophic to populations and infrastructure close to the volcano key to which of these two behaviours will occur is the extent to which the mechanical entrainment and mixing of ambient air into the jet by large entraining eddies forming the jet edge changes the density of the air ash mixture low entrainment rates lead to pyroclastic flows and high entrainment rates give rise to buoyant plumes recent experiments on particle laden multi phase volcanic jets from flared and straight sided circular openings suggest that the likelihood for buoyant plumes will depend strongly on the shape and internal geometry of the vent region this newly recognised sensitivity of the fate of volcanic jets to the structure of the vent is a consequence of a complex dynamic coupling between the jet and entrained solid particles an effect that has generally been overlooked in previous studies building on this work here we use an extensive series of experiments on multi phase turbulent jets from analogue linear fissures and annular ring fractures to explore whether the restrictive vent geometry during cataclysmic caldera forming ccf eruptions will ultimately lead a relatively greater frequency of pyroclastic flows than eruptions from circular vents on stratovolcanoes our results understood through scaling analyses and a one dimensional theoretical model show that entrainment is enhanced where particle motions contribute angular momentum to entraining eddies however because the size of the entraining eddies scales approximately with vent width the extent of entrainment is reduced as the vent width becomes small in comparison to its length consequently our work shows that for specified mass eruption rates the high length to width ratio vents typical of ccf events are more likely to produce pyroclastic flows we suggest that the enigmatic trend in the geological record for the largest ccf eruptions to produce pyroclastic flows is an expected consequence of their being erupted through continuous or piece wise continuous caldera ring fractures

tephra in caves distal deposits of the minoan santorini eruption and the campanian super eruption abstract tephra deposits in caves are not only significant as stratigraphic markers the comparatively sheltered position of cave environments protected from rainfall may preserve original distal tephra deposition features unlikely to have survived in the open landscape most reported findings of tephra in caves are from the campanian super eruption which originated in the area of naples italy these findings facilitate evaluation in different caves of facies variability and modes of tephra deposition derived from the same eruption the campanian volcanic event about 40 000 years ago was the largest eruption in europe and the mediterranean region during the late pleistocene another major volcanic event during the quaternary was the more recent santorini eruption in the eastern mediterranean aegean sea during the late minoan cultural period approximately 3600 years ago this was the largest holocene eruption in the region but tephra deposits in caves from this event appear to be very rare we present here the first ever finding of a visible tephra layer from the minoan santorini eruption in a cave the pure tephra situated in the pelekita cave in eastern crete near kato zakros has a thickness of up to 9 cm geochemistry analyses of major elements by electron probe micro analysis epma and trace elements by laser ablation inductively coupled plasma mass spectrometry la icp ms prove the tephra to be derived from the minoan santorini eruption radiocarbon dating also fits the time frame of this eruption our findings indicate that the tephra layer in the pelekita cave is an air fall deposit the lower boundary of the tephra layer is sharp and wavy draping over the underlying irregular cave surface particle size distribution of the tephra is bimodal and typical for suspended aeolian transport the consistently smaller size of heavier feldspar particles adjacent to larger but lighter volcanic glass particles in thin sections of undisturbed samples corroborate emplacement of the tephra inside the pelekita cave from high altitude fallout

layering and double diffusion style convection in europa s ocean abstract we assess the effects of fluid composition and depth dependent hydrostatic pressure on dynamics in a europan ocean primarily composed of aqueous na 2 so 4 and mgso 4 for both a saturated and a dilute ocean we observe a salinity and pressure dependent check on buoyancy in putative upwellings which could act as a mechanism for storing heat in the ocean s base uptake of salt to a warmed parcel of water from the seafloor environment causes upwellings to lose buoyancy before reaching the base of the overlying ice implying a tendency toward ocean stratification we make an analogy to double diffusive convecting systems observed in the red sea and elsewhere in earth s waters using currently understood parameterizations for onset and stability we estimate a range of temperatures and salinities for which double diffusive convection may occur in europa s ocean in the red sea boundary layers separating convecting zones have been observed to move upward as the lower layer acquires heat and salt we examine the possibility of stratification and double diffusive convection as mechanisms for heat storage in europa s ocean and possible correlation with recently inferred changes in ice surface alteration style over the last 30 80 myr

vent development during the minoan eruption 1640 bc of santorini greece as suggested by ballistic blocks abstract the minoan eruption on santorini greece was a major plinian caldera forming event that occurred at ca 1640 bc the eruption is generally divided into four distinct phases 1 plinian phase 2 phreatomagmatic base surge 3 ash flow phase and 4 non welded ignimbrite many ballistic blocks mostly consisting of older lava fragments with diameters up to several meters were ejected during phases 2 and 3 as caldera collapse followed the eruption the precise vent location is unknown but its position during phase 1 has been inferred from the pumice isopachs of the plinian deposits the location of the vent during the following phases has been estimated by measuring thickness variations flow directions within the pyroclastic flows and impact directions of ballistic blocks in this study the vent location has been estimated by analysis of the size distribution of the ballistic blocks phase 2 blocks show no clear size dependant distribution pattern blocks ranging between 0 15 and 1 60 m in diameter are found throughout almost the entire deposit but are concentrated in the southeastern parts and within a circle of 14 km diameter thus suggesting a maximum range of at least 6 7 km the center of that circle is regarded as the most probable ejection region it is proposed that the subaerial vent of phase 1 developed into an fracture opening to the sw along the general tectonic trend very large blocks up to 3 m in diameter or possibly more were also ejected during phase 3 with the largest ones being found only in the northern half of the island group near the caldera rim it therefore appears that at least one other vent opened in the northern half of the caldera possibly as a new sw ne trending fracture

an examination of double diffusive processes in a mesoscale eddy in the arctic ocean temperature and salinity measurements of an atlantic water mesoscale eddy in the arctic ocean s canada basin are analyzed to understand the effects of velocity shear on a range of double diffusive processes double diffusive structures in and around the eddy are examined through the transition from low shear outside the eddy and within its solid body core to high geostrophic shear zones at the eddy flanks the geostrophic richardson number takes large values where a double diffusive staircase is observed and lowest values at the eddy flanks where geostrophic velocity is largest and a well formed staircase is not present a thorpe scale analysis is used to estimate turbulent diffusivities in the flank regions double diffusive and turbulent heat salt and buoyancy fluxes from the eddy are computed and used to infer that the eddy decays on time scales of around 4 9 years fluxes highlight that atlantic water heat within the eddy can be fluxed downward into deeper water layers by means of both double diffusive and turbulent mixing estimated lateral variations in vertical fluxes across the eddy allow for speculation that double diffusion speeds up the eddy decay having important implications for the transfer of atlantic water heat in the arctic ocean

shear and convective turbulence in a model of thermohaline intrusions abstract thermohaline interleaving is an important mechanism for laterally fluxing salt heat and nutrients between water masses interleaving is driven by a release of potential energy resulting from the differing diffusivities of heat and salt in seawater the flows are composed of stacked intrusions that flux more and less buoyant water in opposite directions in this paper the role of shear instability caused by this juxtaposed motion is investigated the model described in walsh and ruddick is modified to include both the effects of shear induced turbulence and an improved convective mixing parameterization shear and convective mixing play a similar and significant role in interleaving dynamics in the absence of either instability cross front fluxes are increased by approximately 30 while in situ observations of horizontal diffusivity resulting from interleaving are not yet precise enough to calibrate the parameterizations independently parameter values based on independent laboratory and num

oceanic thermohaline intrusions observations intrusions are commonly observed in the upper deep and coastal oceans and are closely linked to lateral fluxes of heat salt and momentum this is a review of observations of intrusions and the results of comparisons of properties such as scale slopes microstructure activity and fluxes with theoretical models a summary of estimates of lateral heat fluxes indicates a wide range of lateral diffusivities we conclude by noting that our present knowledge is insufficient to predict the structure length scales and lateral fluxes of thermohaline intrusions with confidence and list a number of unresolved questions suggestions are made for compilation of existing data into a database for exploratory analysis and testing of theoretical hypotheses an outline is given of a potential collaborative field experiment using ctd fluorescent dye and microstructure observations

revised estimates for the volume of the late bronze age minoan eruption santorini greece the late bronze age minoan eruption of santorini greece occurred from within an existing caldera low temperature pyroclastic flow emplacement on shallow slopes outside the caldera can only be consistent with the caldera being filled with eruption products that are not preserved field observations and seismic reflection surveys suggest that this missing material has been downfaulted the volume of the caldera infill is estimated as 18 26 km 3 dense rock equivalent dre increasing the total of the minoan eruption deposits to 78 86 km 3 dre and making it the largest known holocene eruption this study highlights a general mechanism whereby caldera eruption volumes could be significantly underestimated

double diffusive recipes part i large scale dynamics of thermohaline staircases abstractthree dimensional dynamics of thermohaline staircases are investigated using a series of basin scale staircase resolving numerical simulations the computational domain and forcing fields are chosen to reflect the size and structure of the north atlantic subtropical thermocline salt finger transport is parameterized using the flux gradient formulation based on a suite of recent direct numerical simulations analysis of the spontaneous generation of thermohaline staircases suggests that thermohaline layering is a product of the gamma instability associated with the variation of the flux ratio with the density ratio after their formation numerical staircases undergo a series of merging events which systematically increase the size of layers ultimately the system evolves into a steady equilibrium state with pronounced layers 20 50 m thick the size of the region occupied by thermohaline staircases is controlled by the competition between turbulent mixing and double diffusion assuming in acc

causes and consequences of outer core stratification abstract the earth s outer core appears to be compositionally layered exotic mechanisms such as an original chemically layered core preserved from the earth s accretionary period or compositionally different core material delivered by a moon creating impactor are conceivable but require a core whose outermost part has been stratified throughout core history relying on unknowable processes to achieve barodiffusion and core mantle reaction lead to layers significantly thinner than observed we show that a balance of mass transferred from the inner core to the top of the outer core is possible and that the stratification could arise as a byproduct of light element accumulation however if a subadiabatic thermal gradient at the top of the outer core exists that quells radial flow it could serve as a witness of light element accumulation by preventing mixing with the convecting part of the core the temperature difference through a subadiabatic layer could be 80 300 k and carry heat fluxes through the core mantle boundary of 0 5 23 tw given uncertainty in core properties

diffusive vertical heat flux in the canada basin of the arctic ocean inferred from moored instruments 1 observational studies have shown that an unprecedented warm anomaly has recently affected the temperature of the atlantic water aw layer lying at intermediate depth in the arctic ocean using observations from four profiling moorings deployed in the interior of the canada basin between 2003 and 2011 the upward diffusive vertical heat flux from this layer is quantified vertical diffusivity is first estimated from a fine scale parameterization method based on ctd and velocity profiles resulting diffusive vertical heat fluxes from the aw are in the range 0 1 0 2 w m 2 on average although large over the period considered the variations of the aw temperature maximum yields small variations for the temperature gradient and thus the vertical diffusive heat flux in most areas variations in upward diffusive vertical heat flux from the aw have only a limited effect on temperature variations of the overlying layer however the presence of eddies might be an effective mechanism to enhance vertical heat transfer although the small number of eddies sampled by the moorings suggest that this mechanism remains limited and intermittent in space and time finally our results suggest that computing diffusive vertical heat flux with a constant vertical diffusivity of 2 10 6 m2 s 1 provides a reasonable estimate of the upward diffusive heat transfer from the aw layer although this approximation breaks down in the presence of eddies

experiments on salt fingers in a hele shaw cell salt fingers are generated when a slowly diffusing solute overlies a more rapidly diffusing solute in a gravitationally stable fluid the phenomenon is important in many areas of science for example it affects the temperature and salinity structure in the upper half of the ocean the production of crystals when liquid alloys solidify and the distribution of properties in magma chambers below the earth s crust the evolution of a salt finger from initial instability to a fully developed feature is difficult to follow in a fluid because of the close packed array of cells that is formed however when the fluid is contained in a hele shaw cell individual fingers occupy the entire gap width thereby enabling the experimenter to monitor and record the evolution much more easily the instability of an array of wide fingers to disturbances that lead to preferred smaller scales is documented in a sequence of photographs and a qualitative physically consistent argument is offered to explain the instability

eastern arctic ocean diapycnal heat fluxes through large double diffusive steps abstractthe diffusive layering dl form of double diffusive convection cools the atlantic water aw as it circulates around the arctic ocean large dl steps with heights of homogeneous layers of

environmental and physiological influences on isotopic and elemental compositions of brachiopod shell calcite implications for the isotopic evolution of paleozoic oceans abstract brachiopods from the demissa bed middle devonian hamilton group biologically regulated the incorporation of mg sr and na into their shell calcite significant taxonomic differences in the elemental contents of three species athyris spiriferoides mediospirifer audacula and mucrospirifer mucronatus may be related to differences in calcification processes in contrast no significant difference or vital effects were observed between the isotopic values of mediospirifer audacula and athyris spiriferoides from erie 18o 0 03 p 0 949 13c 0 76 p 0 083 and genessee counties 18o 0 39 p 0 471 13c 0 06 p 0 854 this suggests that these brachiopods did not exert a biological control over their isotopic compositions and that their shell calcites reflect ambient physicochemical conditions isotopic compositions in unaltered shell calcites of brachiopods from genessee county which was close to the basin depocentre are heavy for carbon 13c x 5 01 pdb and oxygen 18o x 2 85 pdb compared to the species sampled at the basin s edge erie county 13c x 2 79 pdb 18o x 3 83 pdb there is a significant separation in isotopic values between the deeper and shallower water brachiopods of the basin 18o 0 98 p 0 0005 13c 2 22 p 0 0005 the 18o variation suggests a temperature salinity change with water depth whereas the change in 13c composition probably records an enrichment depletion of organic matter with water depth this observation has significant implications for paleozoic ocean isotopic evolution studies because many global changes in marine 18o and 13c are based on isotopic shifts of similar magnitude