habitat characterization of deep water coral reefs in la gaviera canyon aviles canyon system cantabrian sea abstract surveys conducted at the complex aviles canyon system southern bay of biscay in order to identify vulnerable habitats and biological communities revealed the presence of noteworthy deep water coral reefs in one of the tributaries of the system la gaviera canyon the aim of the present study is to determine why this deep sea canyon provides suitable environmental conditions for corals to grow this hanging canyon is characterized by an irregular u shaped floor with two narrow differentiated flanks sand ripples and rocky outcrops structured in diverse w e directed steps are observed on the canyon floor suggesting intense hydrodynamic activity accordingly high frequency near bottom current and thermal structure profiles showed that there occur strong shifts in currents hydrography behaving as front like features at each tidal cycle these involve the sudden increase of along axis velocities to over 50 cm s and vertical velocities of over 5 cm s in each tidal cycle associated with the passage of sharp thermal fronts and thermal inversions suggesting overturning a year long near bottom current record showed events with near bottom velocities well over 1 m s lasting for several days three cold water coral settings were distinguished a dense coral reef located on stepped rocky bottoms of the eastern and western flanks carbonate mounds 20 30 m high located on the canyon floor and a cluster of shallower water dead coral framework at the head sector of the canyon video and still images from a towed sled and rov verified the presence of dropstones and rippled sand sheets surrounding the mounds and revealed changes in the coral population alive or dead total or patchy coverage in coral reef and carbonate mound areas the dominant species of the reef are lophelia pertusa and madrepora oculata which considerably increase the habitat s complexity and biodiversity in relation to other facies described in the canyon the presence of living cold water reefs is directly related to a high energy environment at depths between 700 and 1200 m in the levels between the lower bound of eastern north atlantic central water enacw and the core of mediterranean water mw such level matches the water density range 27 35 27 65 kg m 3 which has been identified as limits for cold water coral distribution in the north atlantic

precise determination of the open ocean 234u 238u composition uranium has a long residence time in the open oceans and therefore its salinity normalized u concentration and 234u 238u activity ratio expressed herein as 234u the deviation from secular equilibrium are assumed to be uniform the marine 234u 238u activity ratio is currently in radioactive disequilibrium and shows a 15 excess of 234u with respect to the secular equilibrium value due to continuous input from riverine sources knowledge of the marine 234u and how it has evolved through the quaternary is important for validating age accuracy in the u series dating of marine carbonates which is increasingly relied upon for providing a chronological basis in paleoclimate research however accurate and precise measurements of 234u are technically difficult thus existing compilations of the open ocean 234u value vary by up to 10 and the assumed uniformity in the oceanic 234u remains to be confirmed using mc icpms techniques and a suite of multiple faraday cups instead of the typical configurations based on a combined faraday cup multiplier array a long term reproducibility of better than 0 3 2 is achieved for 234u measurements applying these very high precision techniques to open ocean seawater samples an average 234u of 146 8 0 1 2 m n 19 is obtained these high precision seawater measurements yield an external reproducibility of better than 0 4 2 and show that the open oceans have a uniform 234u on the sub level these new data constrain the vertical mixing time of the open oceans to less than 1000 years

optical dating of aeolian dynamism on the west african sahelian margin abstract the sahelian margin of west africa is widely recognised as an area of recent environmental catastrophe and human suffering arising from food shortage and land degradation associated with prolonged drought the propensity of this region to suffer drought has been related using environmental data collected during the period of instrumental records to a combination of low mean annual rainfall levels and a high degree of rainfall variability which relates to sea surface temperature anomalies in the adjacent tropical atlantic ocean despite the significant environmental and human consequences of such droughts there is a paucity of long term environmental data for the west african sahel aeolian dune reactivations in this area are a potentially highly useful environmental archive of past periods of extended drought conditions which may have resulted in localised or widespread dune reactivation here we describe the initial results from an ongoing programme of research which seeks to develop a detailed record of past dune reactivations in mali we find evidence for repeated holocene dune reactivation events and a significant number of reactivations which commenced at the time of onset of the last major drought cycle in the early 1970s we obtain ages as young as 20 30 years for some significant dune units thickness up to 1 m and describe the results of experiments which test the performance of our dating exercise we specifically test for the significance of preheat temperature on single aliquot regeneration sar equivalent dose determinations and recycling ratios neither are found vary significantly as a function of preheating optical dating of sand sized quartz could provide a useful tool for palaeogeographical mapping of ancient and historical dune reactivations in this region and elsewhere

palaeomagnetic evidence for unification of the north and west australian cratons by ca 1 7 ga new results from the kimberley basin of northwestern australia summary a palaeomagnetic study of the elgee formation red siltstones and shales in the palaeoproterozoic kimberley basin of northwestern australia has been carried out all seven sampling sites revealed an extremely stable magnetic remanence carried by haematite the age of the formation is confined by precise shrimp u pb ages of early diagenetic xenotime from rocks both above and below it to be 1704 7 14 ma but this may represent a minimum age the youngest detrital zircon grains in the underlying formation provide a maximum age of 1786 14 ma for the formation the extreme stability of the remanence the dissimilarity of the remanent direction from expected younger palaeomagnetic directions and the lack of regional overprint in the 1790 4 ma hart dolerite just north of the study region support a primary origin for the remanence a marginally positive fold test also supports a primary origin the mean direction of d 92 2 i 14 9 a 95 6 4 gives a palaeopole at 4 4 s 210 0 e with dp 3 3 dm 6 5 this pole a previously reported palaeopole from the hart dolerite and ca 1700 ma overprint poles from the pilbara craton all agree with palaeopoles of similar ages from the mcarthur basin of northern australia palaeomagnetic results thus suggest that the north and west australian cratons were possibly joined together by approximately 1 7 ga

labrador sea water in the eastern north atlantic part i a synoptic circulation inferred from a minimum in potential vorticity abstract a synoptic distribution of labrador sea water lsw in the eastern north atlantic is determined from a regularly sampled but sparse 3 resolution survey covering 39 to 54 n 11 to 34 w during spring 1991 the core of lsw can be defined by a minimum in potential vorticity pv using property values at this minimum the authors infer the circulation of lsw in addition using a known source function for salinities at the core of lsw estimates are able to be made of lsw vintages the authors then compared the synoptic circulation to historical data youngest 1986 vintage lsw crosses the mid atlantic ridge to the eastern basin between 48 and 51 n at 34 w this water then flows north to the iceland basin and eastward to the rockall trough where it was found to be of 1978 vintage tongues of low salinity low temperature and high oxygen extend southward on the eastern side of the mid atlantic ridge indicating that lsw also flows southward in the eastern basin at the southern edge of the s

gas hydrate crystals may help build reefs during a recent cruise in the porcupine basin off southwest ireland we discovered two extensive and hitherto largely unsuspected deep water reef provinces including a giant cluster of hundreds of buried mounds the ring shapes of many reefs suggest that they are caused by an axial fluid expulsion at the sea bed a transient flow well confined in space and time we are exploring various hypotheses but a stimulating avenue for research is opened by a glacially controlled growth pulse and subsequent decay of a shallow layer of gas hydrates as a methane buffer and probably indirectly as a ground for overlying biological communities

sedimentation patterns on a cold water coral mound off mauritania an unconformity bound glacial sequence 135 cm thick of a coral bearing sediment core collected from the flank of a cold water coral mound in the banda mound province off mauritania was analysed in order to study the relation between coral framework growth and its filling by hemipelagic sediments u series dates obtained from the cold water coral species lophelia pertusa were compared to c 14 dates of planktonic foraminifera of the surrounding matrix sediments the coral ages ranging from 45 1 to 32 3 ka bp exhibit no clear depositional trend while on the other hand the c 14 dates of the matrix sediment provide ages within a much narrower time window of 3000 yrs 34 6 31 8 cal ka bp corresponding to the latest phase of the coral growth period in addition high resolution computer tomography data revealed a subdivision of the investigated sediment package into three distinct parts defined by the portion and fragmentation of corals and associated macrofauna as well as in the density of the matrix sediments grain size spectra obtained on the matrix sediments show a homogeneous pattern throughout the core sediment package with minor variations these features are interpreted as indicators of redeposition based on the observed structures and the dating results the sediments were interpreted as deposits of a mass wasting event namely a debris flow during this event the sediment unit must have been entirely mixed resulting in averaging of the foraminifera ages from the whole unit and giving randomly distributed coral ages in this context for the first time mass wasting is proposed to be a substantial process of mound progradation by exporting material from the mound top to the flanks hence it may not only be an erosional feature but also widening the base of the mound thus allowing further vertical mound growth

oceanic density fronts steering bottom current induced sedimentation deduced from a 50 ka contourite drift record and numerical modeling off nw spain abstract how the various bottom near hydrographic and sedimentary processes control the formation of contourite drifts i e of bottom current related confined deep sea depocenters usually remains widely speculative this study uses a transect of six sediment cores and a sediment echosounder profile across a whole contourite system off nw spain to address the sediment dynamics responsible for the depositional pattern this mounded patch type contourite drift 18 km long 20 km wide with a 150 m deep channel moat has formed around an 800 m high structural obstacle deposition on the contourite drift in the past was characterized by alternating calm and high energy bottom flow conditions calm conditions last glacial period 27 17 cal ka bp late holocene times process based numerical modeling demonstrates that pulse like oceanic density fronts traveling within the transition zone of two water masses labrador sea water mediterranean outflow water provide a powerful mechanism for contouritic deposition rather than the core of a water mass itself these gravity driven density fronts lead to local re suspension of sands stored inside the drift s moat and to subsequent upward transport towards the drift s crest here the oceanic density fronts produce additional km scale eddies these migrating eddies provide an efficient mechanism for further widespread sediment re distribution in comparison with paleoceanographic reconstructions a downward migration or expansion of the mediterranean outflow water by about 300 m led most probably to such temporary contouritic sand deposition we finally propose a conceptual model to explain how seafloor obstacles redirect and perturbate bottom currents this model proposes not only a sharp contact between two water masses but also the transition zone between those as an important high energy regime offering oceanic density fronts a travel medium these fronts are strong enough to distribute fine sands across highly pronounced seabed topography on the respective time scale the moat itself seems to act as the main source for those sands making a remote source and a long distance sediment transport unnecessary

thermoluminescence dating of loess horizons in wiesbaden graselberg and wallertheim by the quartz inclusion method abstract two examples of thermoluminescence dating of loess sections in loess regions of west germany are described the samples are both from the upper wurmian loess with marker horizons of volcanic tephra the total bleach regeneration method and the additive dose method were used for determination of the paleodose ed the annual dose was calculated using data from measurements with neutron activation analyses atomic emission spectroscopy and alpha counting

signatures of the mediterranean outflow from a north atlantic climatology 1 salinity and density fields using historical data from the national oceanic data center the climatology of the eastern north atlantic basin has been investigated for the purpose of detailing the mediterranean outflow water in terms of its salinity density and flow patterns part 1 of this work is a descriptive analysis of the fate of the mediterranean water once it flows out of the strait of gibraltar tracing the salinity and density signatures high resolution maps of the climatological outflow are presented with an emphasis on the continuity of the water from its source from the climatological fields a continuous signal of mediterranean water is tracked northward to 50 20 n yet its westward advection is limited to the tagus basin recirculations of mediterranean water in the gulf of cadiz and in the bay of biscay deduced from property signals are also detailed iorga and lozier this issue present absolute velocity fields from a diagnostic model constrained by geostrophic dynamics conservation of mass and no flux boundary conditions

cold water corals are they hydrocarbon seep related numerous cold water ahermatypic coral reefs so called lophelia reefs up to 30m high 150m wide and 400m long occur at waterdepths from 100 to 350 m off mid norway after the discovery of relatively high interstitial methane concentrations 204 ppb in sediments collected at the base of one of these reef structures a causative link between the presence of the reefs and local micro seepage through seabed of light hydrocarbons is here suggested shallow seismic records gathered across this and other reef colonies off mid norway also indicate the existence of gas charged sediments below the reefs

vivaldi 1991 a study of the formation circulation and ventilation of eastern north atlantic central water abstract a synoptic hydrographic data set comprising 32 full depth ctd casts and 2500 ctd seasoar profiles to 500 m is used to describe the s properties and circulation of central water east of the mid atlantic ridge and between 39 n and 54 n eastward transport of 20 10 6 m 3 s 1 in the north atlantic current turns entirely northwards to the west of 54 n 20 w this transport consists in the upper layers of western north atlantic water freshened at temperatures below 10 c by mixing with subarctic intermediate water northern and southern branches of the north atlantic current are well defined and both turn northwards west of 20 w a further 10 10 6 m 3 s 1 of eastern north atlantic water forms and recirculates anticyclonically to the west of spain south of the north atlantic current and north of 40 n eastern north atlantic water is most weakly stratified east of 20 w and there is clear correlation between weakly stratified pycnostads and positive salinity anomalies relative to western north atlantic water thus eastern north atlantic water is a winter mode water in which strong winter cooling has increased the density and hence also the salinity anomaly at a given temperature near the southern entrance to the rockall trough there is evidence that salinities are also increased by mediterranean water influence circulation south of the north atlantic current is complex there is no evidence for direct ventilation southwards across 40 n where water properties s potential vorticity and cfc 113 and historical data all indicate westward ventilation east of 24 w with weak southward ventilation occurring further west in the vicinity of the azores the circulation pattern suggested is remarkably similar to that proposed by helland hansen and nansen in 1926 the eastern north atlantic geophysiske publicajoner 4 1 76 with anticyclonic circulation of colder eastern north atlantic water north of 40 n meeting warmer water from south of 40 n circulating cyclonically north of the azores current the distribution of pycnostads and s properties between 20 w and 35 w north of the azores indicates alternate bands of western and eastern north atlantic water moving eastward and westward respectively including evidence for westward motion immediately south of the southern branch of the north atlantic current possibly by westward propagation of anticyclonic eddies containing deep pycnostads

changes in fossil assemblage in sediment cores from mingulay reef complex ne atlantic implications for coral reef build up ct scanning and descriptions of sedimentary sequences have been conducted to describe the internal structure and define the evolutionary sequence of events leading to the build up of cold water coral reef constructed by the azooxanthellate scleractinian lophelia pertusa two vibrocores containing coral clasts and associated fauna embedded in a matrix of marine sediment were taken at the mingulay reef complex an inshore 120 190 m water depth l pertusa reef off western scotland the cores were scanned using computed x ray tomography ct scan with the aim of 1 reconstructing and quantifying the relative abundance and preservation of coral fragments in the cores 2 identifying distinct reef facies and hence 3 providing data that could lead to a conceptual model of reef growth u series dates were obtained on l pertusa clasts from the two sediment cores to provide a temporal framework of the reef build up after exploring a range of methods for processing ct scans three simple parameters were used to classify different strata within the cores the overall density of the cores the number of fossil clasts and the sum of the area of clasts detected by the ct scan these parameters were validated by comparing with density measurements from a multi sensor core logger mscl and physical dissection and analysis of fossils in the cores from the ct scan analyses three distinct reef facies were evident coral rich sediment rich and shelly coral hash layers the succession and rapid cycle of reef framework forming l pertusa development disturbance and recovery allow us to provide a model of cold water coral reef build up including shifts in biodiversity physical chemical biological processes and the sedimentary regime

geomorphology of the iberian continental margin abstract the submarine features and processes around the iberian peninsula are the result of a complex and diverse geological and oceanographical setting this paper presents an overview of the seafloor geomorphology of the iberian continental margin and the adjacent abyssal plains the study covers an area of approximately 2 3 million km 2 including a 50 to 400 km wide band adjacent to the coastline the main morphological characteristics of the seafloor features on the iberian continental shelf continental slope continental rise and the surrounding abyssal plains are described individual seafloor features existing on the iberian margin have been classified into three main groups according to their origin tectonic and or volcanic depositional and erosional major depositional and erosional features around the iberian margin developed in late pleistocene holocene times and have been controlled by tectonic movements and eustatic fluctuations the distribution of the geomorphological features is discussed in relation to their genetic processes and the evolution of the margin the prevalence of one or several specific processes in certain areas reflects the dominant morphotectonic and oceanographic controlling factors sedimentary processes and the resulting depositional products are dominant on the valencia catalan margin and in the northern part of the balearic promontory strong tectonic control is observed in the geomorphology of the betic and the gulf of cadiz margins the role of bottom currents is especially evident throughout the iberian margin the galicia portuguese and cantabrian margins show a predominance of erosional features and tectonically controlled linear features related to faults

ice volume and sea level during the last interglacial during the last interglacial period 125 000 years ago sea level was at least several meters higher than at present with substantial variability observed for peak sea level at geographically diverse sites speculation that the west antarctic ice sheet collapsed during the last interglacial period has drawn particular interest to understanding climate and ice sheet dynamics during this time interval we provide an internally consistent database of coral u th ages to assess last interglacial sea level observations in the context of isostatic modeling and stratigraphic evidence these data indicate that global eustatic sea level peaked 5 5 to 9 meters above present sea level requiring smaller ice sheets in both greenland and antarctica relative to today and indicating strong sea level sensitivity to small changes in radiative forcing

recent sediment transport and deposition in the nazare canyon portuguese continental margin processes pathways and fluxes of sediment transport and deposition in the nazare submarine canyon portuguese continental margin were investigated by water column profiling of suspended particulate matter recording of near bottom currents and suspended particulate matter fluxes with benthic landers and by analysis of surface sediment cores the results show that fine grained suspended sediment from the adjacent shelf is captured in the upper canyon and actively transported through the canyon to the abyssal plain under influence of tidal currents and intermittent sediment gravity flows tidal currents are actively resuspending and transporting sediment in the upper to middle canyon as reflected by high concentration of suspended particles in bottom waters high horizontal and vertical sediment fluxes in the bottom water layer and high sediment accumulation rate on the seabed diminishing strength of tidal currents leads to focussed sediment deposition in the middle canyon further transport of sediments toward the lower canyon takes place predominantly by sediment gravity flows occurring on yearly or longer timescale turbidity currents strong enough to transport sand sized material to the lower canyon occur only on centennial or longer timescales

sedimentary processes and cold water coral mini mounds at the ferrol canyon head nw iberian margin abstract the western iberian margin has a complex morphology controlled by both geological and oceanographic processes compared to the submarine canyons in the southern part of this margin e g nazare and setubal the canyons in the northern part e g ferrol and a coruna have received little attention this study maps the geomorphological features around the ferrol canyon head and combines them with oceanographic observations to infer the sedimentary and oceanographic processes active in this area furthermore the occurrence of a cold water coral cwc mini mound province near the canyon head and the ortegal spur pockmark field is investigated with regard to seepage processes oceanographic conditions and anthropogenic impact the ferrol canyon head and outer ortegal spur are characterized by erosional erosional and abraded surfaces contourite channels and furrows depositional contourite drifts and sediment waves and mixed contourite terrace features indicating a dominant control of bottom currents on the sedimentary processes bottom currents are related to the interaction of the canyon head topography with both the mediterranean outflow water mow slope current and with m2 internal tides associated to the interface pycnocline between eastern north atlantic central water enacw and mow due to this interaction the ferrol canyon head is subject to active sediment resuspension and forms a potential source area for the intermediate nepheloid layers along the nw iberian upper slope the cwc mini mounds 1 9 2 7 m high and 75 141 m in diameter occur between 400 and 560 m depth seismic facies signatures indicative of fluid flow and morphometric analysis implies that larger cwc mini mounds adjacent to the pockmark field initiated by colonization of pre existing pockmarks while smaller more clustered mounds developed independently the observed mini mounds are relict features likely related to their position outside of the contemporary enacw mow interface which favours cwc growth through 1 increased bottom currents enhancing food supply and 2 the presence of a potential density envelope 27 35 27 65 kg m3 proposed to control coral larvae transport along the ne atlantic margin alternatively the lack of contemporary coral growth might be related to habitat destruction by bottom trawling however a preliminary age constraint reveals cwc growth occurred during the early holocene coeval to relict cwc mini mounds near canyon heads on the armorican margin their existence suggests a regional shift in the ne atlantic density profile where the enacw mow interface occurred up to 200 m above its contemporary position

thermoluminescence dating of palaeodunes in northeast nigeria abstract to obtain a broad view of the ages of dunes in borno state nigeria 23 samples were dated using quartz grains results from seven selected samples are discussed in this paper for all these samples problems arose in the evaluation of the annual doses as the contents of u th and k are low in total three independent methods for the determination of these radioactive isotopes were tested the results sometimes differ by more than a factor of two this leads to the general conclusion that the thermoluminescence ages of samples with small u th and k contents are not very reliable the data from alpha counting are preferable for the evaluation of ages

bedform velocity matrix the estimation of bottom current velocity from bedform observations a wide variety of bedforms both depositional and erosional in origin has been recognized on the deep seafloor and attributed to the influence of bottom currents these range in scale from those visible in bottom photographs centimeter to decimeter to those recorded with seafloor bathymetric imaging meter to kilometer in many cases it has been possible to provide some quantification of substrate grain size and flow velocity responsible for each bedform type we have synthesized both our own and published data in order to present a bedform velocity matrix which facilitates the estimation of bottom current velocity based on bedform type despite imperfections we believe this to be a valuable model for assessing strength and variability of bottom currents that can have a significant influence on the siting of submarine cables pipelines and other seafloor installations

dune ages in the sand deserts of the southern sahara and sahel in this paper we aim to document the history of aeolian processes within the southern sahara as part of the inqua dune atlas we review available luminescence ages for sand dunes across the southern sahara to develop an improved understanding of the dune chronology on a regional basis and attempt to correlate periods of sand accumulation this was achieved by analysing dune age by country as well as by latitude and longitude the results show a very patchy spatial distribution of dune ages with large gaps that encompass some of the largest sand seas despite these gaps some related patterns in dune morphology and stratigraphy appear to be consistent between northern nigeria and southern mali where older linear dunes are distinct from younger late holocene transverse and barchanoid dunes elsewhere in mauretania linear dunes with different orientations appear to have accumulated at different times most likely in response to changes in atmospheric circulation regional climatic changes are identified where dunes are transgressed by lake deposits within endorehic basins we identify four locations where dune accumulation is terminated by lacustrine transgressions two of which in lake chad and the bodele depression occur shortly after the last glacial maximum lgm the third example at gobiero in niger occurred later in the early holocene around 8 4 ka and a fourth marks a later transgression of palaeolake megachad after 4 7 ka larger scale latitudinal and longitudinal distributions in dune ages across the southern sahara do not show any consistent patterns though this may be due to the small sample size relative to the study area in addition local variations in external controls such as wind regime rainfall vegetation and sand supply need to be considered sometimes on a site by site basis limiting the analysis to dune ages determined using the single aliquot regenerative dose sar protocol indicates a lack of dune preservation during the lgm and the younger dryas times associated with increased dust input to the oceans which is assumed to indicate increased aeolian activity the sar dune dates suggest that preservation of dunes at the onset of succeeding humid intervals is an important component of the dune record the most striking examples of this phenomenon occur where dunes are preserved within endorehic basins by lacustrine transgressions

do norwegian deep water coral reefs rely on seeping fluids hundreds of large coral reefs up to 45 m high exist a on the continental shelf and b on morainic threshold ridges in fjords of northern mid and southwest norway they occur in water depths between 400 and 40 m and contain a large variety of megafauna the most common frame building coral is lophelia pertusa l the oldest reef found to date is 8600 calendar years old determined by radiocarbon dating of buried lophelia skeleton even though many of these reefs have been known to science for over 200 years there is as yet no viable and unifying hypothesis to explain their existence in deep cool waters other than perhaps the hydraulic theory presented by the author in 1990 it states that primary producers mainly bacteria are locally formed and concentrated at reef locations due to seepage of light hydrocarbons mainly methane on the continental shelf and nutrient rich groundwater in the fjords the hydraulic theory is supported by the following topographic reflection seismic and geochemical indicators for the continental shelf reefs a seaward dipping sedimentary permeable strata enhanced acoustic seismic reflectors adjacent pockmark craters locally elevated light hydrocarbon sediment content locally elevated seawater methane content for the fjord reefs b threshold morainic substratum rather than adjacent hardrock substratum sub surface dipping sedimentary layers h2s smell of near surface sediment samples these indicators or reef associated observations have been documented by german british and norwegian researchers over the last 10 15 years however all the listed indicators do not necessarily occur at each of the respective shelf and fjord reef locations simultaneously

the imprint of methane seepage on the geochemical record and early diagenetic processes in cold water coral mounds on pen duick escarpment gulf of cadiz abstract the diagenetic history and biogeochemical processes in three cold water coral mounds located in close proximity to each other on pen duick escarpment in the gulf of cadiz were examined the influence of ascending methane rich fluids from underlying sediment strata delineated two mound groups alpha and beta mound showed evidence for the presence of a sulfate methane transition zone smtz at shallow depth whereas gamma mound appeared to lack a shallow smtz in the methane influenced alpha and beta mound upward diffusion of hydrogen sulfide from the shallow smtz caused extensive pyritization of reactive iron phases as indicated by values for the degree of pyritization 0 7 this secondary pyritization overprinted the sulfur isotope composition of sulfides formed during organoclastic sulfate reduction the almost complete consumption of reactive iron phases by upward diffusing sulfide limited dissimilatory iron reduction to the top layer in these mounds while organic matter in the pyritized zones below was primarily degraded by organoclastic sulfate reduction hydrogen sulfide produced during sulfate reduction coupled to the anaerobic oxidation of methane aom diffused upward and induced aragonite dissolution as evidenced in strongly corroded corals in alpha mound this mound has been affected by strong fluctuations in the depth of the smtz as observed by distinct layers with abundant diagenetic high mg calcite with a 13 c depleted carbon isotope composition in the non methane influenced gamma mound low sulfate reduction rates elevated concentrations of dissolved iron and solid phase iron speciation indicated that organic matter mineralization was driven by dissimilatory iron reduction and organoclastic sulfate reduction coupled to oxidative sulfur cycling the latter process led to 34 s depletion in pyrite of up to 70 relative to pore water sulfate