birth growth and morphologic evolution of the laghetto cinder cone during the 2001 etna eruption we have undertaken detailed observations of the formation of the laghetto cinder cone a new cone that formed during a 2 week period of intense activity in piano del lago on the upper slopes of mount etna in summer 2001 we describe the events leading to the formation of a small graben the formation of pit craters on the base of the graben the onset of phreatomagmatic activity a transition to intense strombolian activity and a return to phreatomagmatic activity as the eruption came to an end we discuss the reasons for these transitions and describe the morphological development of the cone during these events arcuate cracks on the southern part of the cone were related to withdrawal of magma at the end of the eruption other slope instabilities that developed during the eruption include the formation of small radial grain flows on the outer flanks of the cone and the collapse into the crater of part of the crater rim some of the failure planes we observed were first identified using a flir tm 695 thermal infrared camera this is the first time that infrared thermography has been used to detect instability of volcanic structures results obtained during this test case demonstrate that thermal cameras are a very useful tool for studies of volcanic instability

a comparison of shear and buoyancy driven planetary boundary layer flows abstract planetary boundary layer pbl flows are known to exhibit fundamental differences depending on the relative combination of wind shear and buoyancy forces these differences are not unexpected in that shear instabilities occur locally while buoyancy force sets up vigorous thermals which result in nonlocal transport of heat and momentum at the same time these two forces can act together to modify the flow field in this study four large eddy simulations less spanning the shear and buoyancy flow regimes were generated two correspond to the extreme cases of shear and buoyancy driven pbls while the other two represent intermediate pbls where both forces are important the extreme cases are used to highlight and quantify the basic differences between shear and convective pbls in 1 flow structures 2 overall statistics and 3 turbulent kinetic energy tke budget distributions results from the two intermediate les cases are used to develop and verify a velocity scaling and a tke budget mode

formation of venusian canali considerations of lava types and their thermal behaviors because liquid water is unstable at present venusian surface conditions the discovery of channels termed canali on venus thousands of kilometers long was not predicted low viscosity lavas that remain fluid for several thousand kilometers are considered to be the canali forming agents possible compositions of venusian canali forming lavas include komatiite and high fe ti lunar type basalts results of analytical and numerical models of these lavas reveal that total cooling is more efficient on venus than on earth suggesting that venusian lavas rapidly form insulating crusts and thus that the canali lavas were essentially tube fed the models also reveal that thermal erosion should be less efficient on venus than on earth suggesting that venusian channels are either the product of mechanical rather than thermal erosion or constructional processes

thermal erosion by laminar lava flows thermal erosion by hot lavas during their laminar flow over cold ground is investigated both theoretically and experimentally an analysis of the steady forced convective heat transfer by a laminar channel flow at large peclet numbers is presented which is used to determine the final steady state erosion velocity as well as the thicknesses and timescales of the associated thermal boundary layers in both the lava and ground the initial transient period involving the growth and remelting of a basal chill layer is also quantified laboratory experiments are described in which hot molten wax flowed over and thermally eroded an underlying bed of solidified wax the experimental observations of chill formation and thermal erosion are found to be in excellent agreement with the theoretical predictions the theory is applied to predict rates of thermal erosion consistent with those observed in recent flows of basaltic lava in hawaii and of carbonatite lava in tanzania

germanium isotopic variations in igneous rocks and marine sediments a new technique for the precise and accurate determination of ge stable isotope compositions has been developed and applied to silicate rocks and biogenic opal the analyses were performed using a continuous flow hydride generation system coupled to a mc icpms samples have been purified through anion and cation exchange resins to separate ge from matrix elements and eliminate potential isobaric interferences variations of 74ge 70ge ratios are expressed as 74ge values relative to our internal standard and the long term external reproducibility of the data is better than 0 2 for sample size as low as 15 ng of ge data are presented for igneous and sedimentary rocks and the overall variation is 2 4 in 74ge representing 12 times the uncertainty of the measurements and demonstrating that the terrestrial isotopic composition of ge is not unique co variations of 74ge 70ge 73ge 70ge and 72ge 70ge ratios follow a mass dependent behaviour and imply natural isotopic fractionation of ge by physicochemical processes the range of 74ge in igneous rocks is only 0 25 without systematic differences among continental crust oceanic crust or mantle material on this basis a bulk silicate earth reservoir with a 74ge of 1 3 0 2 can be defined in contrast modern biogenic opal such as marine sponges and authigenic glauconite displayed higher 74ge values between 2 0 and 3 0 this suggests that biogenic opal may be significantly enriched in light isotopes with respect to seawater and places a lower bound on the 74ge of the seawater to 3 0 this suggests that seawater is isotopically heavy relative to bulk silicate earth and that biogenic opal may be significantly fractionated with respect to seawater deep sea sediments are within the range of the bulk silicate earth while mesozoic deep sea cherts opal and quartz have 74ge values ranging from 0 7 to 2 0 the variable values of the cherts cannot be explained by binary mixing between a biogenic component and a detrital component and are suggestive of enrichment in the light isotope of diagenetic quartz further work is now required to determine ge isotope fractionation by siliceous organisms and to investigate the effect of diagenetic processes during chert lithification

factors controlling the silicon isotope distribution in waters and surface sediments of the peruvian coastal upwelling we present the first systematic study of the silicon isotope composition in the water column 30sisi oh 4 and in diatoms 30sidiatom from the underlying surface sediments in a coastal upwelling region the surface waters upwelling on the shelf off peru are mainly fed by southward flowing subsurface waters along the coast which show a mean 30sisi oh 4 of 1 5 the concentration of dissolved silicic acid si oh 4 increases towards the south in these waters and with increasing water depth suggesting lateral mixing with water masses from the south and intense remineralisation of particulate biogenic silica bsio2 in the water column and in the surface sediments surface waters in the realm of the most intense upwelling between 5 s and 15 s have only marginally elevated 30sisi oh 4 values 30sisi oh 4 1 7 with respect to the source si isotope composition whereas further north and south where upwelling is less pronounced surface waters are more strongly fractionated 30sisi oh 4 up to 2 8 due to the stronger utilisation of the smaller amounts of available si oh 4 the degree of si oh 4 utilisation in the surface waters along the shelf estimated from the si oh 4 concentration data ranges from 51 to 93 the 30sidiatom values of hand picked diatoms in the underlying surface sediments vary from 0 6 to 2 0 which is within the range of the expected fractionation between surface waters and diatoms the fractionation signal in the surface waters produced during formation of the diatoms is reflected by the 30sidiatom values in the underlying sediments with the lowest 30sidiatom values in the main upwelling region the silicon isotope compositions of bsio2 30sibsio2 from the same surface sediment samples are generally much lower than the 30sidiatom signatures indicating a significant contamination of the bsio2 with biogenic siliceous material other than diatoms such as sponge spicules this shift towards lighter 30sibsio2 values by up to 1 3 compared to 30sidiatom signatures for the same surface sediment samples potentially biases the interpretation of 30si paleorecords from sediments with low bsio2 concentrations and thus the reconstruction of past si oh 4 utilisation in surface waters

opening the gateways for diatoms primes earth for antarctic glaciation the abrupt onset of antarctic glaciation during the eocene oligocene transition 33 7 ma oi1 is linked to declining atmospheric pco2 levels yet the mechanisms that forced pco2 decline remain elusive biogenic silicon cycling is inextricably linked to both long and short term carbon cycling through the diatoms siliceous walled autotrophs which today account for up to 40 of primary production it is hypothesised that during the late eocene a sharp rise in diatom abundance could have contributed to pco2 drawdown and global cooling by increasing the proportion of organic carbon buried in marine sediment diatom and sponge silicon isotope ratios 30si are here combined for the first time to reconstruct the late eocene early oligocene ocean silicon cycle and provide new insight into the role of diatom productivity in antarctic glaciation at odp site 1090 in the southern ocean a 0 6 rise in diatom 30si through the late eocene documents increasing diatom silicic acid utilisation with high near modern values attained by the earliest oligocene a concomitant 1 5 decline in sponge 30si at odp site 689 on the maud rise tracks an approximate doubling of intermediate depth silicic acid concentration in the high southern latitudes intermediate depth silicic acid concentration peaked at 31 5 ma coincident with the final establishment of a deepwater pathway through the tasman gateway and drake passage these results suggest that upwelling intensification related to the spin up of a circum antarctic current may have driven late eocene diatom proliferation organic carbon burial associated with higher diatom abundance and export provides a mechanism that can account for pco2 drawdown not only at but also prior to antarctic glaciation as required by a pco2 threshold mechanism for ice sheet growth

micrometer scale chemical and isotopic criteria o and si on the origin and history of precambrian cherts implications for paleo temperature reconstructions abstract oxygen and silicon isotopes in cherts have been extensively used for the reconstruction of seawater temperature during the precambrian these reconstructions have been challenged because cherts can have various origins hydrothermal sedimentary volcanic silicification and their isotopic compositions might have been reset by metamorphic fluid circulation existing criteria used to assess the pristine sedimentary origin of a chert are based on petrography criterion 1 chert is composed mostly of microquartz on the bulk oxygen isotopic composition criterion 2 bulk 18 o has to be close enough to the maximum 18 o value previously measured in other cherts of the same age and on the presence of a large 18 o range at the micrometer scale criterion 3 18 o range of 10 at 2 m however these criteria remain incomplete in determining precisely the origin and degree of preservation of ancient cherts we report in situ si and o isotope compositions and trace element concentrations in seven chert samples ranging from 1 88 to 3 5 ga in age correlations between 30 si and al 2 o 3 and k 2 o tio 2 reveal that microquartz is of three different origins i e diagenetic hydrothermal or silicification moreover chert samples composed mostly of diagenetic microquartz show a large range of 30 si at the micrometer scale 1 7 4 5 consistent with the large range of 18 o previously found in the gunflint diagenetic cherts we propose two further quantitative criteria to assess the origin state of preservation and diagenetic history of cherts criterion 4 uses trace element concentrations coupled with 30 si to ascribe the origin of cherts among three possible end members diagenetic hydrothermal and silicified criterion 5 is the presence of a large range of 30 si in pure diagenetic microquartz in the seven samples analyzed in this study only one from the gunflint iron formation at 1 88 ga passes all the criteria assessed here and can be used for paleo temperature reconstructions the criteria outlined here will help guide to the identification of pristine diagenetic cherts and or to better constrain seawater paleo temperature reconstructions by taking into account the effect of diagenesis

large eddy simulation of flow and turbulence at the steep topography of luzon strait steep generation sites for topographic internal gravity waves can also be sites of turbulence the present work uses turbulence resolving large eddy simulation les for tidal flow over multiscale steep bathymetry patterned after a portion of the west ridge of luzon strait at 20 6 n oceanic values of the slope criticality froude number excursion number and aspect ratio are matched the amplitude and phasing of velocity overturns and dissipation are similar to observations made at a deep mooring however a tidal phase is noted in which there is a discrepancy that could arise from the resonant interaction with waves generated at the east ridge that is not included in the model turbulent dissipation originates from several mechanisms including breaking lee waves during flow reversal downslope jets critical slope boundary layer high mode internal wave beams off slope lee wave breaking and wave breaking in a valley between subridges the model baroclinic energy budget is closed with negligible residual and the local turbulence loss of 23 5 is large

middle eocene to early oligocene paleoceanography of the antarctic ocean maud rise odp leg 113 site 689 change from a low to a high productivity ocean the sediment record from the middle eocene about 45 ma to the early oligocene about 33 ma on maud rise site 689 weddell sea paleodepth 1400 1650 m has been studied by means of a coarse fraction analysis in order to reconstruct variations in surface water productivity and bottom water masses productivity has been estimated using carbonate dissolution and accumulation rates of radiolaria diatoms benthonic and planktonic foraminifers sponge debris ostracods and echinoids accumulation rates of bolboforma spp vary parallel to these proxies productivity increased with time three episodes were found in which productivity increased sharply and productivity varied cyclically with a period of 430 ky the middle eocene ca 45 44 37 26 ma was a period of generally low productivity no opal was deposited and carbonate preservation was excellent during periods of a slight increase in productivity accumulation rates of benthonic and planktonic foraminifers ostracods and echinoids increased in spite of an increase in carbonate dissolution in the period ca 41 62 43 07 ma productivity rose in three distinct pulses clinoptilolite formed from biogenous opal is a common mineral in these sediments these three productivity maxima coincide with a mica rich unit mica probably originated from erosion on antarctica and was transported by an intermediate water depth current in the late eocene ca 36 9 34 95 ma productivity increased considerably and opal skeletons were deposited carbonate dissolution increased but planktonic foraminifer accumulation rates were not markedly decreased in periods of higher productivity because productivity increase exceeded or equalled rises in dissolution rates increasing opal accumulation rates coincided with increases in the rates for benthonic foraminifers sponge debris ostracods and echinoids in the late eocene early oligocene period 34 95 33 05 ma a further increase in productivity led to a strong dissolution of planktonic foraminifers in high productivity periods the productivity increase is probably due to the increasing pole equator temperature gradient which led to stronger atmospheric and oceanic turnover and mixing in the oceans in the uppermost late eocene two unusual layers suggest changes in bottom water masses affecting the seafloor at site 689 a sediment interval with especially well preserved carbonate microfossils underlies an interval that has poor carbonate preservation despite evidence of low productivity these intervals suggest a change in bottom water conditions from warm saline deep water wsdw which favors carbonate preservation to antarctic bottom water aabw which promotes carbonate dissolution cooling of surface waters over a period of 300 ky led to a thickening of the deep aabw layer and an upward shift of the overlying wsdw until the aabw finally reached the top of the maud rise in the area of site 689 this period of aabw influence on top of the maud rise lasted 200 ky

mixing by shear instability at high reynolds number 1 shear instability is the dominant mechanism for converting fluid motion to mixing in the stratified ocean and atmosphere the transition to turbulence has been well characterized in laboratory settings and numerical simulations at moderate reynolds number it involves rolling up i e overturning of the density structure within the cores of the instabilities in contrast measurements in an energetic estuarine shear zone reveal that the mixing induced by shear instability at high reynolds number does not primarily occur by overturning in the cores rather it results from secondary shear instabilities within the zones of intensified shear separating the cores this regime is not likely to be observed in the relatively low reynolds number flows of the laboratory or in direct numerical simulations but it is likely a common occurrence in the ocean and atmosphere

a first look at the distribution of the stable isotopes of silicon in natural waters the first data on the distribution of the stable isotopes of silicon in marine and freshwater systems are reported both marine and riverine 30si are more positive than 30si of igneous rocks suggesting isotopic fractionation during weathering and clay formation and or biomineralization the 30si value for dissolved silicic acid from several ocean basins is 1 1 0 3 n 69 more positive values are found in surface waters due to discrimination against 30si during biogenic opal formation by diatoms the deep pacific ocean is depleted in 30si relative to the deep atlantic by roughly 0 4 30si of riverine dissolved silicon averages 0 8 0 3 n 8 the biologic fractionation of silicon isotopes coupled with effects associated with thermohaline circulation appears to play a significant role in controlling the distribution of silicon isotopes in the sea the average 30si value of inputs of silicon to the ocean most likely falls between 0 3 and 0 9 and that of the outputs falls between 0 9 to 1 9 probably in the neighborhood of 1 0

erosion by flowing lava field evidence erosion of substrate materials by melting or mechanical means has been suggested in active lava flows on earth and other planets although there are many references to lava erosion on earth unambiguous evidence is rare geological relationships commonly cited as evidence of downcutting by lava can be explained without recourse to erosion in order to assess possible erosion by flowing lava we carried out field studies of tube fed basalt flows sheet flows of the columbia river basalt province crb and precambrian komatiites unequivocal evidence for thermal erosion melted dacite substrate was found at the cave basalt lava tube mount st helens for which fluid dynamic analysis indicates laminar flow although erosion was enhanced in areas of locally steep slopes possibly as a result of localized turbulence other lava tubes in our study display strong but inconclusive evidence for erosion komatiite flows display good evidence for erosion of their substrate possibly in a turbulent regime but assessment of the extent of erosion is hampered by limited and disrupted exposures no evidence for thermal erosion was found in the crb our findings suggest that an erosional origin for planetary sinuous rilles and canali would be favored by high reynolds number flows high mass flux low viscosity lava steep slopes and substrates having a lower melting temperature than the lava or low mechanical strength e g regolith

estimation of volcanic eruption conditions for a large flank event on elysium mons mars mars orbiter camera image 5 23 m pixel and topographic data from the mars orbiter laser altimeter data are used to analyze a channel on the upper northern flank of the elysium mons volcano mars we show that this channel is best explained as the product of the thermal erosion of the preexisting surface by a high speed turbulent lava flow fed from a fissure eruption we deduce the volume eruption rate 3400 13 000 m3 s dense rock equivalent the total volume of erupted lava 29 39 km dense rock equivalent and the duration of the activity 40 130 days perhaps significantly the erupted lava volume is essentially identical given the uncertainties in the analysis to the volume of the observed caldera 31 km3 which has implications for volume limited eruptions on mars and potentially for the unusual steep upper slopes of the volcano the eruption that produced the channel appears to have been typical for elysium mons and implies that the other lava channels on the upper flank of the volcano may also have taken 1 4 months to have formed

high temperature silicon isotope geochemistry abstract silicon si is the defining element of silicate reservoirs yet despite its dominance in major earth processes there is still no clear understanding of how much is hosted in earth s core how the enriched continental crust forms or even if the crust is isotopically different from the mantle because of a long history of weathering erosion and subduction with the advent of multiple collector inductively coupled plasma mass spectrometry it has become relatively straightforward to explore small 100 ppm level mass dependent variations in si isotopic composition resulting from high temperature fractionation and to develop new isotopic fingerprints of magmatic processes and source regions this paper reviews the technique developments the new data and the veracity of current interpretations only a small si isotopic effect is associated with basalt formation via mantle melting however there now is compelling evidence based on a considerable number of samples 100 that the silicate earth is isotopically fractionated by 100 200 ppm per amu to a heavy composition relative to that of chondrites and also all differentiated stony meteorites this could reflect variability in the circumstellar disc but this is not well supported by data for enstatite chondrites which are isotopically light the most plausible current explanation is that si is a light element in earth s core and that differences in the bond stiffness between silicate and metal hosted si resulted in substantial fractionation interestingly the moon has the same si isotope composition as earth s mantle which is hard to explain unless the moon s atoms were mainly derived from earth differentiated magmatic sequences such as those of hekla iceland display a systematic relationship between isotopic composition and si content more complex magmatic suites such as i and s type granites reveal a range of isotopic compositions not well correlated with chemical composition similar effects are found in lower crustal granulite facies xenoliths nonetheless the overall composition of the continental crust is only slightly heavy relative to the mantle in fact the effect is barely resolvable therefore the amounts of surface dissolved heavy si removed to the mantle via weathering over time have been small or these losses have been balanced by subduction of isotopically light clay

sims analyses of silicon and oxygen isotope ratios for quartz from archean and paleoproterozoic banded iron formations banded iron formations bifs are chemical marine sediments dominantly composed of alternating iron rich oxide carbonate sulfide and silicon rich chert jasper layers isotope ratios of iron carbon and sulfur in bif iron bearing minerals are biosignatures that reflect microbial cycling for these elements in bifs while much attention has focused on iron banded iron formations are equally banded silica formations thus silicon isotope ratios for quartz can provide insight on the sources and cycling of silicon in bifs bifs are banded by definition and microlaminae or sub mm banding are characteristic of many bifs in situ microanalysis including secondary ion mass spectrometry is well suited for analyzing such small features in this study we used a cameca ims 1280 ion microprobe to obtain highly accurate 0 3 and spatially resolved 10 lm spot size analyses of silicon and oxygen isotope ratios for quartz from several well known bifs isua southwest greenland 3 8 ga hamersley group western australia 2 5 ga transvaal group south africa 2 5 ga and biwabik iron formation minnesota usa 1 9 ga values of d 18 o range from 7 9 to 27 5 and include the highest reported d 18 o values for bif quartz values of d 30 si have a range of 5 from 3 7 to 1 2 and extend to the lowest d 30 si values for precambrian cherts isua bif samples are homogeneous in d 18 o to 0 3 at mm to cm scale but are heterogeneous in d 30 si up to 3 similar to the range in d 30 si found in bifs that have not experienced high temperature metamorphism up to 300 c values of d 30 si for quartz are homogeneous to 0 3 in individual sub mm laminae but vary by up to 3 between multiple laminae over mm to cm of vertical banding the scale of exchange for si in quartz in bifs is thus limited to the size of microlaminae or less than 1 mm we interpret differences in d 30 si between microlaminae as preserved from primary deposition silicon in bif quartz is mostly of marine hydrothermal origin d 30 si 0 5 but silicon from continental weathering d 30 si 1 was an important source as early as 3 8 ga

the riverine silicon isotope composition of the amazon basin we present here the first large scale study of riverine silicon isotope signatures in the amazon basin the amazon and five of its main tributaries were studied at different seasons of the annual hydrological cycle the 30si signature of the dissolved silicon dsi exported to the estuary weighted for dsi flux for the period considered is estimated at 0 92 a river cross section shows the homogeneity of the amazon river regarding dsi concentration and isotope ratio the biogenic silica bsi concentration measured in surface water from all rivers is generally small compared to the dsi reservoir but large variations exist between rivers very low isotope signatures were measured in the upper rio negro 30si 0 05 0 06 which we explain both by an equilibrium between clay formation and dissolution and by gibbsite formation the si isotope fractionation in the andean tributaries and the amazon main stem can be explained by clay formation and follow either a rayleigh or a batch equilibrium fractionation model our results also suggest that the formation of 2 1 clays induces a fractionation factor similar to that of kaolinite formation

stable silicon isotopes of groundwater feldspars and clay coatings in the navajo sandstone aquifer black mesa arizona usa abstract we present some of the first analyses of the stable isotopic composition of dissolved silicon si in groundwater the groundwater samples were from the navajo sandstone aquifer at black mesa arizona usa and the si isotope composition of detrital feldspars and secondary clay coatings in the aquifer were also analyzed silicon isotope compositions were measured using high resolution multi collector inductively coupled mass spectrometry hr mc icp ms nu1700 nuplasma hr the quartz dominated bulk rock and feldspar separates have similar 30 si of 0 09 0 04 and 0 15 0 04 95 sem respectively and clay separates are isotopically lighter by up to 0 4 compared to the feldspars from isotopic mass balance considerations co existing aqueous fluids should have 30 si values heavier than the primary silicates positive 30 si values were only found in the shallow aquifer where si isotopes are most likely fractionated during the dissolution of feldspars and subsequent formation of clay minerals however 30 si decreases along the flow path from 0 56 to 1 42 representing the most negative dissolved si isotope composition so far found for natural waters we speculate that the enrichment in 28 si is due to dissolution of partly secondary clay minerals and low temperature silcretes in the navajo sandstone the discovery of the large range and systematic shifts of 30 si values along a groundwater flow path illustrates the potential utility of stable si isotopes for deciphering the si cycling in sedimentary basins tracing fluid flow and evaluating global si cycle

silicon isotopic fractionation in marine sponges a new model for understanding silicon isotopic variations in sponges abstract the silicon si isotope 30 si composition of deep sea sponges from near antarctica subantarctic waters tasmania seamounts and subtropical waters north of new zealand vary widely between 0 87 and 3 40 vs nbs28 depth profiles show that sponge 30 si compositions trend to lower values with increasing depth this is exemplified by sponges from the tasmania seamounts where 30 si varies from 0 87 to 3 13 over a depth range from 100 m to 1200 m these changes in 30 si of sponges are inconsistent with a rayleigh type isotope fractionation model requiring constant 30 si fractionation between sponge and seawater we conclude that overall si isotope fractionation 30 si 30 si sponge 30 si seawater is influenced by seawater si concentration with more fractionated lower isotope values being associated with sponges collected from waters high in si we invoke and fit a model whereby the 30 si fractionation varies as a function si influx and efflux using this model it appears that 30 si fractionation during transport into the sponge is constant at 1 34 the model also shows asymptotic behaviour with 30 si trending towards a maximum of 6 02 at very high si concentrations these results suggest that the 30 si composition of fossil spicules may be useful for reconstruction paleo si concentrations during the past

factors influencing lava substrate heat transfer and implications for thermomechanical erosion we develop numerical simulations of basaltic lava flowing laminarly over a basalt substrate in order to examine the details of the lava dynamics and thermal boundary layers and to understand the implications for substrate heating as the initial stage of a larger study of thermomechanical erosion in different planetary environments we aim to understand why erosion occurs on earth why erosion features are not ubiquitous given the high temperatures involved and whether it is a plausible mechanism for the formation of planetary channels such as lunar sinuous rilles and venusian canali here we confine our attention to terrestrial lavas with well known properties and eruption parameters with relatively simple computational fluid dynamic simulations most closely representing tube fed hawaiian basalts for which erosion has been documented we demonstrate the importance of incorporating several key factors in models of lava flow substrate heat transfer which have commonly been neglected in previous treatments by addressing the interaction of the flow dynamics and heat transfer in the lava our work suggests that the development of a temperature gradient in the base of the lava even for undeveloped flow has a significant influence on substrate temperature the sensitivity of the lava substrate interface temperature to the thermophysical properties of the lava and substrate suggests that a delicate balance is required for partial melting to occur thus it might take weeks of continuous flow to initiate partial melting of the substrate at distances of several kilometers from the vent these durations exceed the periods of stability typical of lava flowing in tubes pauses blockages surges and breakouts frequently disrupt the flow however natural irregularities in the flow dynamics or substrate topography might help to initiate and maintain substrate melting on shorter timescales by disturbing the intimately coupled dynamic and thermal boundary layers although a purely thermal mechanism cannot be ruled out our findings support the premise that mechanical erosion may play a key role in reports of erosion based on field evidence

biogenic silica a neglected component of the coupled global continental biogeochemical cycles of carbon and silicon on geological time scales 106 years the global geochemical cycles of carbon and silicon are coupled by the drawdown of atmospheric co2 through chemical weathering of ca and mg silicate minerals in continental rocks rivers transport the soluble products of weathering cations alkalinity and silicic acid to the oceans where they are utilized by marine ecosystems on decadal to glacial interglacial time scales however large biotic fluxes and storages of si within terrestrial and freshwater ecosystems need to be taken into account recent studies have emphasized the importance of si accumulating plants which deposit significant amounts of amorphous hydrated silica in their tissues as opal phytoliths these include grasses sedges palms some temperate deciduous trees and conifers and many tropical hardwoods landscapes dominated by accumulator plants such as tropical rainforests grasslands herbaceous wetlands and bamboo forests act as silica factories important silica bioengineers in freshwater ecosystems comprise diatoms sponges and chrysophytes this paper reviews the biological role of si in higher plants the impact of vegetation on rates of chemical weathering the fluxes of si through catchment ecosystems lakes and rivers and the potential contribution of new geochemical and isotopic tracers to si biogeochemistry multiproxy investigations of lake sediments will provide novel insights into past variations in si biocycling from terrestrial to aquatic realms on 10 106 year time scales copyright 2008 john wiley sons ltd

the silicon isotope composition of ethmodiscus rex laminated diatom mats from the tropical west pacific implications for silicate cycling during the last glacial maximum the cause of massive blooms of ethmodiscus rex laminated diatom mats ldms in the eastern philippine sea eps during the last glacial maximum lgm remains uncertain in order to better understand the mechanism of formation of e rex ldms from the perspective of dissolved silicon dsi utilization we determined the silicon isotopic composition of single e rex diatom frustules 30sie rex from two sediment cores in the parece vela basin of the eps in the study cores 30sie rex varies from 1 23 to 0 83 average 1 04 a range that is atypical of marine diatom 30si and that corresponds to the lower limit of reported diatom 30si values of any age a binary mixing model upwelled silicon versus eolian silicon accounting for silicon isotopic fractionation during dsi uptake by diatoms was constructed the binary mixing model demonstrates that e rex dominantly utilized dsi from eolian sources i e asian dust with only minor contributions from upwelled seawater sources i e advected from subantarctic mode water antarctic intermediate water or north pacific intermediate water e rex utilized only 24 of available dsi indicating that surface waters of the eps were eutrophic with respect to silicon during the lgm our results suggest that giant diatoms did not always use a buoyancy strategy to obtain nutrients from the deep nutrient pool thus revising previously proposed models for the formation of e rex ldms

siliceous deep sea sponge monorhaphis chuni a potential paleoclimate archive in ancient animals abstract the deep sea sponge monorhaphis chuni forms giant basal spicules which can reach lengths of 3 m they represent the largest biogenic silica structures on earth that is formed from an individual metazoan the spicules offer a unique opportunity to record environmental change of past oceanic and climatic conditions a giant spicule collected in the east china sea in a depth of 1110 m was investigated the oxygen isotopic composition and mg ca ratios determined along center to surface segments are used as geochemical proxies for the assessment of seawater paleotemperatures calculations are based on the assumption that the calculated temperature near the surface of the spicule is identical with the average ambient temperature of 4 c a seawater temperature of 1 9 c is inferred for the beginning of the lifespan of the monorhaphis specimen the temperature increases smoothly to 2 3 c to be followed by sharply increased and variable temperatures up to 6 10 c in the outer part of the spicule the inferred seawater temperature is about 4 c the lifespan of the spicule can be estimated to 11 000 3000 years using the long term trend of the inferred temperatures fitted to the seawater temperature age relationships since the last glacial maximum specimens of monorhaphis therefore represents one the oldest living animals on earth the remarkable temperature spikes of the ambient seawater occurring 9500 3100 years b p are explained by discharges of hydrothermal fluids in the neighborhood of the spicule the irregular lamellar organization of the spicule and the elevated mn concentrations during the high temperature growth are consistent with a hydrothermal fluid input

ageostrophic secondary circulation at a submesoscale front and the formation of gravity currents abstractlarge eddy simulations are performed to investigate the development of the ageostrophic secondary circulation asc and associated transport in a submesoscale front based on the observatio

lava channel formation via the viscoplastic indentation of hot substrates 1 lava channels that appear to have been incised into the underlying substrate are commonly ascribed to thermal or thermomechanical erosion here we show that where the associated lava flow moves on a hot and soft substrate the channel may also be ascribed to viscoplastic indentation of the substrate a simple model of viscoplastic indentation leads to distinct predictions that can be readily tested in the field for example two broad wedge shaped levee like features should rise from the substrate during indentation such features appear to have risen alongside of a channel that was incised into a hot and soft recently emplaced basaltic substrate during the 2001 eruption on mount etna

the isotope composition of inorganic germanium in seawater and deep sea sponges abstract although dissolved concentrations of germanium ge and silicon si in modern seawater are tightly correlated uncertainties still exist in the modern marine ge cycle germanium stable isotope systematics in marine systems should provide additional constraints on marine ge sources and sinks however the low concentration of ge in seawater presents an analytical challenge for isotopic measurement here we present a new method of pre concentration of inorganic ge from seawater which was applied to measure three ge isotope profiles in the southern ocean and deep seawater from the atlantic and pacific oceans germanium isotopic measurements were performed on ge amounts as low as 2 6 ng using a double spike approach and a hydride generation system coupled to a mc icp ms germanium was co precipitated with iron hydroxide and then purified through anion exchange chromatography results for the deep i e 1000 m depth pacific ocean off hawaii nearby loihi seamount and the deep atlantic off bermuda bats station showed nearly identical 74 70ge values at 3 19 0 31 2sd n 9 and 2 93 0 10 2sd n 2 respectively vertical distributions of ge concentration and isotope composition in the deep southern ocean for water depth 1300 m yielded an average 74 70ge 3 13 0 25 2sd n 14 and ge si 0 80 0 09 mol mol 2sd n 12 significant variations in 74 70ge from 2 62 to 3 71 were measured in the first 1000 m in one station of the southern ocean near sars seamount in the drake passage with the heaviest values measured in surface waters isotope fractionation by diatoms during opal biomineralization may explain the enrichment in heavy isotopes for both ge and si in surface seawater however examination of both oceanographic parameters and 74 70ge values suggest also that water mass mixing and potential contribution of shelf derived ge also could contribute to the variations combining these results with new ge isotope data for deep sea sponges sampled nearby allowed us to determine a ge isotope fractionation factor of 0 87 0 37 2sd n 12 during ge uptake by sponges although ge has long been considered as a geochemical twin of si this work underpins fundamental differences in their isotopic behaviors both during biomineralization processes and in their oceanic distributions this suggests that combined with si isotopes ge isotopes hold significant promise as a complementary proxy for delineating biological versus source effects in the evolution of the marine silicon cycle through time

biogeochemical cycling in the bering sea over the onset of major northern hemisphere glaciation the bering sea is one of the most biologically productive regions in the marine system and plays a key role in regulating the flow of waters to the arctic ocean and into the subarctic north pacific ocean cores from iodp expedition 323 to the bering sea provide the first opportunity to obtain reconstructions from the region that extend back to the pliocene previous research at bowers ridge south bering sea has revealed stable levels of siliceous productivity over the onset of major northern hemisphere glaciation nhg c 2 85 2 73 ma however diatom silica isotope records of oxygen 18odiatom and silicon 30sidiatom presented here demonstrate that this interval was associated with a progressive increase in the supply of silicic acid to the region superimposed on shift to a more dynamic environment characterized by colder temperatures and increased sea ice this concluded at 2 58 ma with a sharp increase in diatom productivity further increases in photic zone nutrient availability and a permanent shift to colder sea surface conditions these transitions are suggested to reflect a gradually more intense nutrient leakage from the subarctic northwest pacific ocean with increases in productivity further aided by increased sea ice and wind driven mixing in the bering sea in suggesting a linkage in biogeochemical cycling between the south bering sea and subarctic northwest pacific ocean mainly via the kamchatka strait this work highlights the need to consider the inter connectivity of these two systems when future reconstructions are carried out in the region

silicon isotope fractionation by marine sponges and the reconstruction of the silicon isotope composition of ancient deep water the silicon isotope composition d30si of biogenic opal provides a view of the silica cycle at times in the past reconstructions require the knowledge of silicon isotope fractionation during opal biomineralization the d30si of specimens of hexactinellid sponges and demosponges growing in the modern ocean ranged from 1 2 to 3 7 n 6 corresponding to the production of opal that has a d30si value 3 8 0 8 more negative than seawater silicic acid and a fractionation factor a of 0 9964 this is three times the fractionation observed during opal formation by marine diatoms and terrestrial plants and is the largest fractionation of silicon isotopes observed for any natural process on earth the d30si values of sponge spicules across the eocene oligocene boundary at ocean drilling program site 689 on maud rise range from 1 1 to 3 0 overlapping the range observed for sponges growing in modern seawater

paragenesis of silicified mid paleozoic and mid cenozoic corals based on petrography and silicon isotopic analyses abstract petrographic analyses of silicified coral fossils from the upper ordovician cutter formation and silurian fusselman formation of new mexico and the paleogene lower neogene tampa member and suwannee limestone of florida show that septal voids of these coral fossils are filled with megaquartz and septal walls are replaced by chalcedonic microspherulites and microquartz the boundaries of the microquartz and spherulites of the septal walls conform to accommodation space left by the megaquartz that filled the septal voids these textures suggest that megaquartz filled the septal voids prior to silicification of the septal walls the measured 30si compositions of silica defining the septal walls and silica that fills the septal voids range from 5 23 to 1 03 the void filling silica has isotopically light si and the septal wall replacement si is isotopically heavier the silicon isotopic data are consistent with distillation of a finite silica reservoir which favors incorporation of lighter silicon isotopes in earlier formed silica by rayleigh fractionation distillation of light isotopes during progressive silica precipitation is also documented in vug filling silica thus silica replacement textures and silica isotopic compositions of samples from different geologic ages and locations regardless of crystal sizes or degree of cementation of the primary porosity in the corals support early void filling silica precipitation followed by septal wall silica precipitation in essentially closed system environments