on the atmospheric response experiment to a blue arctic ocean we demonstrated atmospheric responses to a reduction in arctic sea ice via simulations in which arctic sea ice decreased stepwise from the present day range to an ice free range in all cases the tropospheric response exhibited a negative arctic oscillation ao like pattern an intensification of the climatological planetary scale wave due to the present day sea ice reduction on the atlantic side of the arctic ocean induced stratospheric polar vortex weakening and the subsequent negative ao conversely strong arctic warming due to ice free conditions across the entire arctic ocean induced a weakening of the tropospheric westerlies corresponding to a negative ao without troposphere stratosphere coupling for which the planetary scale wave response to a surface heat source extending to the pacific side of the arctic ocean was responsible because the resultant negative ao like response was accompanied by secondary circulation in the meridional plane atmospheric heat transport into the arctic increased accelerating the arctic amplification

exploring links between arctic amplification and mid latitude weather 1 this study examines observed changes 1979 2011 in atmospheric planetary wave amplitude over northern mid latitudes which have been proposed as a possible mechanism linking arctic amplification and mid latitude weather extremes we use two distinct but equally valid definitions of planetary wave amplitude termed meridional amplitude a measure of north south meandering and zonal amplitude a measure of the intensity of atmospheric ridges and troughs at 45 n statistically significant changes in either metric are limited to few seasons wavelengths and longitudinal sectors however in summer we identify significant increases in meridional amplitude over europe but significant decreases in zonal amplitude hemispherically and also individually over europe and asia therefore we argue that possible connections between arctic amplification and planetary waves and implications of these are sensitive to how waves are conceptualized the contrasting meridional and zonal amplitude trends have different and complex possible implications for midlatitude weather and we encourage further work to better understand these

dynamical link between the barents kara sea ice and the arctic oscillation abstractthe recent accelerated arctic sea ice decline has been proposed as a possible forcing factor for midlatitude circulation changes which can be projected onto the arctic oscillation ao and or north atlantic oscillation nao mode however the timing and physical mechanisms linking ao responses to the arctic sea ice forcing are not entirely understood in this study the authors suggest a connection between november sea ice extent in the barents and kara seas and the following winter s atmospheric circulation in terms of the fast sea ice retreat and the subsequent modification of local air sea heat fluxes in particular the dynamical processes that link november sea ice in the barents and kara seas with the development of ao anomalies in february is explored in response to the lower tropospheric warming associated with the initial thermal effect of the sea ice loss the large scale atmospheric circulation goes through a series of dynamical adjustment processes the decelerated zonal mean zonal

long term acoustic monitoring at north sea well site 22 4b highlights first study using long term passive acoustic monitoring of methane seeps at well blowout site 22 4b seep acoustic temporal variations correlated with ocean tides major acoustic transient event recorded on 8 december 2011 with high temporal resolution abstract marine seeps produce underwater sounds as a result of bubble formation and fragmentation upon emission from the seabed the frequency content and sound levels of these emissions are related to bubble size distribution and emission flux providing important information on methane release from the seafloor long term passive acoustic monitoring was used to continuously record seep sounds over a 7 month period within the blowout crater at the abandoned well site 22 4b in the central north sea also recorded were water column fluid velocities and near seafloor water conductivity temperature and pressure acoustic signatures were primarily from 1 to 10 khz key features were relatively broad spectral peaks at about 1 0 1 5 2 2 3 1 3 6 and 5 1 khz temporal variations in spectral levels were apparently associated with tides the recordings also documented a series of major episodic events including a large and persistent increase 10 db in overall sound levels and spectral broadening on 8 december 2011 the acoustic temporal pattern of this event was consistent with other recorded large transient events in the literature and the major event was correlated with dramatic changes in other measurements including increased water column fluid velocities increased pressure and decreased salinity indicating real changes in emission flux observed seabed morphology changes reported elsewhere in this special issue also likely were related to this event these data demonstrate the dynamic nature of marine seepage systems show the value of monitoring systems and provide direct supporting evidence for a violent formation mechanism of many widespread seep associated seabed features like pockmarks

observations of backscatter from sand and gravel seafloors between 170 and 250 khz interpreting observations of frequency dependence in backscatter from the seafloor offers many challenges either because multiple frequencies are used for different observations that will later be merged or simply because seafloor scattering models are not well understood above 100 khz hindering the understanding of these observations is the paucity of reported calibrated acoustic measurements above 100 khz this manuscript seeks to help elucidate the linkages between seafloor properties and frequency dependent seafloor backscatter by describing observations of backscatter collected from sand gravel and bedrock seafloors at frequencies between 170 and 250 khz and at a grazing angle of 45 overall the frequency dependence appeared weak for all seafloor types with a slight increase in seafloor scattering strength with increasing frequency for an area with unimodal very poorly to moderately well sorted slightly granular to granular medium sand with significant amounts of shell debris and a slight decrease in all other locations

acoustic estimates of methane gas flux from the seabed in a 6000 km2 region in the northern gulf of mexico seeps of free methane gas escaping the seabed can be found throughout the ocean basins to understand the role of methane gas seeps in the global carbon cycle including both gas added to the atmosphere and that which is dissolved and potentially oxidized in the ocean volume it is important to quantify the amount of methane escaping the seabed few large scale mapping projects of natural methane seeps have been undertaken however and even among these quantitative estimates of flux are rare here we use acoustic mapping techniques to survey 357 natural methane seeps in a large region 6000 km2 of the northern gulf of mexico and outline a general approach for methane seep mapping using a combination of multibeam and split beam echo sounders using additional measurements collected with a remotely operated vehicle rov together with the acoustic mapping results we estimate the total gas flux within the 6000 km2 region to be between 0 0013 and 0 16 tg yr or between 0 003 and 0 3 of the current estimates for global seabed methane seepage rates

a link between reduced barents kara sea ice and cold winter extremes over northern continents the recent overall northern hemisphere warming was accompanied by several severe northern continental winters as for example extremely cold winter 2005 2006 in europe and northern asia here we show that anomalous decrease of wintertime sea ice concentration in the barents kara b k seas could bring about extreme cold events like winter 2005 2006 our simulations with the echam5 general circulation model demonstrate that lower troposphere heating over the b k seas in the eastern arctic caused by the sea ice reduction may result in strong anti cyclonic anomaly over the polar ocean and anomalous easterly advection over northern continents this causes a continental scale winter cooling reaching 1 5 c with more than three times increased probability of cold winter extremes over large areas including europe our results imply that several recent severe winters do not conflict the global warming picture but rather supplement it being in qualitative agreement with the simulated large scale atmospheric circulation realignment furthermore our results suggest that high latitude atmospheric circulation response to the b k sea ice decrease is highly nonlinear and characterized by transition from anomalous cyclonic circulation to anticyclonic one and then again back to cyclonic type of circulation as the b k sea ice concentration gradually reduces from 100 to ice free conditions we present a conceptual model which may explain the nonlinear local atmospheric response in the b k seas region by counter play between convection over the surface heat source and baroclinic effect due to modified temperature gradients in the vicinity of the heating area

arctic sea ice loss in different regions leads to contrasting northern hemisphere impacts to explore the mechanisms linking arctic sea ice loss to changes in midlatitude surface temperatures we conduct idealized modeling experiments using an intermediate general circulation model and with sea ice loss confined to the atlantic or pacific sectors of the arctic barents kara or chukchi bering seas extending previous findings there are opposite effects on the winter stratospheric polar vortex for both large magnitude late 21st century and moderate magnitude sea ice loss accordingly there are opposite tropospheric arctic oscillation ao responses for moderate magnitude sea ice loss however there are similar strength negative ao responses for large magnitude sea ice loss suggesting that tropospheric mechanisms become relatively more important than stratospheric mechanisms as the sea ice loss magnitude increases the midlatitude surface temperature response for each loss region and magnitude can be understood as the combination of an indirect part induced by the large scale circulation ao response and a residual direct part that is local to the loss region

early archaean gneisses from the yilgarn block western australia the yilgarn block is an ancient crustal block which extends over 650 000km2 in the south west of western australia and consists of high grade gneiss and granite greenstone terrains1 being one of the largest segments of archaean crust in the world it is important to studies of early crustal evolution we report here a geochronological study of banded gneisses near mount narryer in the north west of the yilgarn block which has given a rb sr whole rock isochron age of 3 348 43 myr with an initial 87sr 86sr ratio of 0 7037 0 0005 the data indicate that the rocks had a prior crustal residence time of 200 myr the validity of this early archaean age is supported by model sm nd ages of 3 510 myr and 3 630 myr for two of the samples taken together these ages represent the oldest evolutionary sequence so far identified in the yilgarn block and are comparable with old ages from other archaean cratons elsewhere in the world

influence of low arctic sea ice minima on anomalously cold eurasian winters 1 influence of low arctic sea ice minima in early autumn on the wintertime climate over eurasia is investigated observational evidence shows that significant cold anomalies over the far east in early winter and zonally elongated cold anomalies from europe to far east in late winter are associated with the decrease of the arctic sea ice cover in the preceding summer to autumn seasons results from numerical experiments using an atmospheric general circulation model support these notions the remote response in early winter is regarded as a stationary rossby wave generated thermally through an anomalous turbulent heat fluxes as a result of anomalous ice cover over the barents kara seas in late autumn which tends to induce an amplification of the siberian high causing colder conditions over the far east the late winter cold anomalies over eurasia are also reproduced in our experiment which is associated with the negative phase of the north atlantic oscillation

does ocean coupling matter for the northern extratropical response to projected arctic sea ice loss the question of whether ocean coupling matters for the extratropical northern hemisphere atmospheric response to projected late 21st century arctic sea ice loss is addressed using a series of experiments with community climate system model version 4 at 1 spatial resolution under different configurations of the ocean model component no interactive ocean thermodynamic slab ocean and full depth dynamic plus thermodynamic ocean ocean atmosphere coupling magnifies the response to arctic sea ice loss but does not change its overall structure however a slab ocean is inadequate for inferring the role of oceanic feedbacks the westerly winds along the poleward flank of the eddy driven jet weaken in response to arctic sea ice loss accompanied by a smaller magnitude strengthening on the equatorward side with largest amplitudes in winter dynamical and thermodynamic oceanic feedbacks amplify this response by approximately 50 air temperature precipitation and sea level pressure responses also show sensitivity to the degree of ocean coupling

contributions of ice thickness to the atmospheric response from projected arctic sea ice loss author s labe z peings y magnusdottir g abstract 2018 american geophysical union all rights reserved a large ensemble of simulations from a high top atmospheric general circulation model are conducted to compare the atmospheric responses from loss of arctic sea ice thickness and sea ice concentration the response to projected sea ice thickness loss indicates substantial surface warming over the arctic ocean and up to 1 c of cooling in eurasia while the dynamic circulation response from sea ice thickness loss is smaller in magnitude it has a similar spatial anomaly pattern as that due to sea ice concentration loss this pattern resembles the negative phase of the northern annular mode the simulations reveal that sea ice thickness loss enhances the thermodynamic and large scale circulation response from sea ice anomalies these results stress the importance of considering a realistic sea ice thickness distribution in future atmospheric general circulation model sea ice perturbation experiments

archaean fluid assisted crustal cannibalism recorded by low δ 18 o and negative e hf t isotopic signatures of west greenland granite zircon the role of fluids during archaean intra crustal magmatism has been investigated via integrated shrimp u pb 18o and la mc icpms 176hf isotopic zircon analysis six rock samples studied are all from the nuuk region southern west greenland including two 3 69 ga granitic and trondhjemitic gneisses a 3 64 ga granitic augen gneiss a 2 82 ga granodioritic ikkattoq gneiss a migmatite with late neoarchaean neosome and a homogeneous granite of the 2 56 ga qorqut granite complex qgc all zircon grains were thoroughly imaged to facilitate analysis of magmatic growth domains within the zircon analysed there is no evidence for metamictization initial ehf zircon values n 63 are largely sub chondritic indicating the granitic host magmas were generated by the remelting of older un radiogenic crustal components zircon from some granite samples displays more than one 207pb 206pb age and correlated with 176hf 177hf compositions can trace multiple phases of remelting or recrystallization during the archaean model ages calculated using lu hf arrays for each sample indicate that the crustal parental rocks to the granites granodiorites and trondhjemites segregated from a chondrite like reservoir at an earlier time during the archaean corresponding to known formation periods of more primitive tonalite trondhjemite granodiorite ttg gneisses zircon from the 3 69 ga granite the migmatite and qgc granite contains eoarchaean cores with chondritic 176hf 177hf and mantle like 18o compositions the age and geochemical signatures from these inherited components are identical to those of surrounding tonalitic gneisses further suggesting genesis of these granites by remelting of broadly tonalitic protoliths zircon oxygen isotopic compositions n 62 over nine age populations six igneous and three inherited have weighted mean or mean 18o values ranging from 5 8 0 6 to 3 7 0 5 the 3 64 ga granitic augen gneiss sample displays the highest 18o with a mildly supra mantle composition of 5 8 0 6 inherited eoarchaean ttg derived zircon shows mantle like values igneous zircon from all other samples spanning more than a billion years of archaean time record low 18o sub mantle compositions these are the first low 18o signatures reported from archaean zircon and represent low 18o magmas formed by the remelting and metamorphism of older crustal rocks following high temperature hydrothermal alteration by meteoric water meteoric fluid ingress coupled with crustal extension associated high heat flow and intra crustal melting are a viable mechanism for the production of the low 18o granites granodiorites and trondhjemites reported here both high and low 18o magmas may have been generated in extensional environments and are distinct in composition from phanerozoic i type granitic plutonic systems which are typified by increasing 18o during intra crustal reworking this suggests that archaean magmatic processes studied here were subtly different from those operating on the modern earth and involved extensional tectonic regimes and the predominance of remelting of hydrothermally altered crystalline basement

wave maintained annular modes of climate variability abstract the leading modes of month to month variability in the northern and southern hemispheres are examined by comparing a 100 yr run of the geophysical fluid dynamics laboratory gcm with the ncep ncar reanalyses of observations the model simulation is a control experiment in which the ssts are fixed to the climatological annual cycle without any interannual variability the leading modes contain a strong zonally symmetric or annular component that describes an expansion and contraction of the polar vortex as the midlatitude jet shifts equatorward and poleward this fluctuation is strongest during the winter months the structure and amplitude of the simulated modes are very similar to those derived from observations indicating that these modes arise from the internal dynamics of the atmosphere dynamical diagnosis of both observations and model simulation indicates that variations in the zonally symmetric flow associated with the annular modes are forced by eddy fluxes in the free troposphere while

guidelines for reporting zircon hf isotopic data by la mc icpms and potential pitfalls in the interpretation of these data invited technical paper high quality analytical data are essential for the development of sound scientific interpretations to ensure the quality of the data published in chemical geology a new type of contribution has been introduced the invited technical paper when a particular technical issue needing discussion or clarification is identified specialists in the technique will be invited to share their expertise as is true for all articles published in the journal these contributions will be subject to peer review before publication the first topic to be treated concerns in situ hf isotopic measurements in zircon by laser ablation inductively coupled plasma mass spectrometry this is an extremely powerful and promising technique that has been adopted by many laboratories throughout the world however if insufficient care is taken during analysis and data processing inaccurate results can be obtained notably because of the existence of very large isobaric interferences on the isotope of interest to address this issue we have asked christopher fisher jeffery vervoort and john hanchar to provide a set of guidelines that can be adopted to assure that reliable hf isotopic data are obtained by this technique abstract over the past decade the hf isotope composition of zircon as determined in situ by laser ablation multicollector inductively coupled plasma mass spectrometry la mc icpms has been applied increasingly to a wide range of geological problems and has proven to be a valuable analytical tool there has been no uniformity however in the reporting of in situ hf isotopic data and quite often reviewers and readers of papers are not provided with sufficient information to assess data quality the goal of this invited contribution is to provide the non specialist with a brief outline and explanation of what is required for proper presentation and documentation of in situ hf isotopic data from zircon including details of how the challenging large isobaric interference corrections have been made in addition we discuss a number of potential pitfalls vis a vis the assignment of the incorrect age to the measured hf isotope composition non specialists should be aware of these important issues when doing their own analyses and evaluating the analyses done by others

hf isotopes in detrital and inherited zircons of the pilbara craton provide no evidence for hadean continents predictions of large volumes of stabilized continental crust by the early archaean stand in stark contrast to the actual amount of pre 3 5 ga rocks presently exposed on earth s surface the pilbara craton of western australia one of the best preserved paleoarchean crustal blocks on earth is believed to have developed on a cryptic possibly 3 8 ga continental basement if substantiated this could support the notion of a widespread and enduring hadean ca 4 5 4 0 ga felsic intermediate crust to test this and to elucidate the earliest evolution of the pilbara craton we report hf isotope data from previously dated detrital zircon grains and inherited zircon crystals hosted by granitic gneisses the crystallization ages 3 80 3 55 ga of which substantially exceed those of the oldest exposed igneous rocks of the craton 3 52 ga the hf isotope compositions of the ancient zircons analyzed in this study are consistent with most of the earliest components of the pilbara craton being extracted from near chondritic mantle between 3 7 and 3 6 ga with little or no input from significantly older crust these new data suggest either that the pilbara craton developed remote from the isotopic influence of the putative eoarchean to hadean continental masses or that the stabilized volumes of the earliest continents have been overestimated the latter scenario would be consistent with the extreme scarcity of 3 9 ga rocks and minerals and the dominantly chondritic hf isotope composition of the oldest continental rocks in earth s most ancient archaean cratons

chondritic lu hf in the early crust mantle system as recorded by zircon populations from the oldest eoarchean rocks of yilgarn craton west australia and enderby land antarctica abstract a persistent question in early earth investigations is the depletion history of the mantle as recorded in the isotopic compositions of the oldest rocks and minerals here we present new in situ sensitive high resolution ion microprobe shrimp u th pb geochronology and laser ablation multi collector inductively coupled plasma mass spectrometry la mc icpms lu hf tracing data for individual zircons extracted from the oldest rocks of the antarctic and australian continents these are 3 88 and 3 85 ga orthogneisses from the gage ridge and mt sones in enderby land and a 3 73 ga meeberrie gneiss from the yilgarn craton the zircons from each sample show complex u th pb systematics due to the mobile behaviour of pb resulting in several age populations with discordant to reversely discordant arrays on concordia plots in contrast measured 176hf 177hf compositions from the same analysis sites show narrowly defined compositional ranges with 3 or fewer groups for each sample reflecting the resistance of 176hf 177hf in zircons to disturbance even under high temperature granulite conditions the initial 176hf 177hf calculated for the most primitive hf population for each sample using the best age estimate yields near chondritic weighted mean compositions of 1 6 0 5 1 6 0 4 and 0 6 0 7 epsilon units deviations in parts per 10 000 from a chondritic uniform reservoir chur reference at 3877 62 3850 50 and 3731 4 ma respectively these results provide no evidence for significant lu hf fractionation on the early earth expected to result from processes including accretion intra mantle differentiation or massive hadean crustal growth

excess hafnium 176 in meteorites and the early earth zircon record 1 the long lived176lu to 176hf decay system is a powerful tool to understand ancient chemical fractionation events associated with planetary differentiation detrital hadean zircons 3 8 gyr from the jack hills metasedimentary belt of western australia record extremely enriched hf isotope signals suggesting early extraction of a continental crust 4 5 gyr but fail to identify a prevalent complementary depleted mantle reservoir suggesting that crust formation processes in the early earth were fundamentally distinct from today however this conclusion assumes that the hf isotope composition of bulk chondrite meteorites can be used to estimate the composition of earth prior to its differentiation into major silicate reservoirs namely the bulk silicate earth bse we report a176lu 176hf internal mineral isochron age of 4869 34 myr for the pristine sah99555 angrite meteorite this age is 300 myr older than the age of the solar system confirming the existence of an energetic process yielding excess 176hf in affected early formed solar system objects through the production of the 176lu isomer t1 2 3 9 hours thus chondrite meteorites contain excess 176hf and their present day composition cannot be used to infer the lu hf parameters of bse using a revised bse estimate based on the sah99555 isochron we show that earth s oldest zircons preserve a record of coexisting enriched and depleted hafnium reservoirs as early as 4 3 gyr in earth s history with little evidence for the existence of continental crust prior to 4 4 gyr this new view suggests continuous juvenile crustal growth and recycling throughout the hadean and archean eras perhaps analogous to modern plate tectonics

lu hf systematics of the ultra high temperature napier metamorphic complex in antarctica evidence for the early archean differentiation of earth s mantle abstract the napier complex of the east antarctic craton comprises some of the oldest rocks on earth 3 8 billion years old overprinted by an ultra high temperature uht metamorphic event near the archean proterozoic boundary garnet orthopyroxene sapphirine osumilite rutile and a whole rock representing a fully equilibrated assemblage from this uht granulite belt have yielded a lu hf isochron age of 2403 43 ma the first ever determined on a uht mineral assemblage preservation of the uht mineral assemblage in the rock analyzed without any significant retrogression suggests rapid cooling with closure likely to have occurred for the lu hf system at post peak uht conditions near a temperature of 800 c this mineral whole rock isochron yields an initial 176 hf 177 hf ratio corresponding to an e hf value of 14 1 acquired during uht metamorphism such a low value demonstrates that overall uht granulites evolved in a low lu hf environment probably formed when the rocks were first extracted from a highly depleted mantle zircon e hf values we have measured see through the uht metamorphism and show that the source materials for the magmas that formed the napier complex were extremely depleted 5 6 e hf at 3 85 ga relative to the chondritic uniform reservoir chur these results also suggest significant depletion of the early archean mantle in agreement with the early differentiation of the earth that the latest core formation models require

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

unsupported radiogenic pb in zircon a cause of anomalously high pb pb u pb and th pb ages ion microprobe u th pb isotopic analyses of zircons from a granodioritic orthogneiss from the napier complex mount sones enderby land antarctica have identified an unambiguous example of unsupported radiogenic pb in a 3 950 ma old crystal at one 40 m spot on the crystal an unusually large heterogeneity in pb content was found the concentration of radiogenic pb ranging from 5 to 50 percent higher than could have been generated in 3 950 ma by radioactive decay of the co existing u and th this relative excess of radiogenic pb is attributed to pb gain rather than to u and th loss because first the pb content varied by more than the u or th contents and secondly changes in the pb u pb th and pb isotopic composition correlated directly with changes in the pb concentration the individual 207pb 206pb apparent ages ranged from 4 000 ma to 4 145 ma all greater than the inferred age of the crystal a correlation between 207pb 206pb and pb u shows that the pb excess has not resulted from recent pb movement the spot apparently gained radiogenic pb about 2 500 ma ago at the same time as the majority of the other zircons in the rock suffered substantial pb loss the pb movement occurred in response to a discrete geologic event reverse discordance is a phenomenon that must be considered when interpreting zircon u pb ages especially 207pb 206pb ages of single crystals or portions of crystals

split beam echo sounder observations of natural methane seep variability in the northern gulf of mexico a method for positioning and characterizing plumes of bubbles from marine gas seeps using an 18 khz scientific split beam echo sounder sbes was developed and applied to acoustic observations of plumes of presumed methane gas bubbles originating at approximately 1400 m depth in the northern gulf of mexico a total of 161 plume observations from 27 repeat surveys were grouped by proximity into 35 clusters of gas vent positions on the seafloor profiles of acoustic target strength per vertical meter of plume height were calculated with compensation for both the sbes beam pattern and the geometry of plume ensonification these profiles were used as indicators of the relative fluxes and fates of gas bubbles acoustically observable at 18 khz and showed significant variability between repeat observations at time intervals of 1 h 7 5 months active gas venting was observed during approximately one third of the survey passes at each cluster while gas flux is not estimated directly in this study owing to lack of bubble size distribution data repeat surveys at active seep sites showed variations in acoustic response that suggest relative changes in gas flux of up to 1 order of magnitude over time scales of hours the minimum depths of acoustic plume observations at 18 khz averaged 875 m and frequently coincided with increased amplitudes of acoustic returns in layers of biological scatterers suggesting acoustic masking of the gas bubble plumes in these layers minimum plume depth estimates were limited by the sbes field of view in only five instances