lateral injection of oxygen with the bosporus plume fingers of oxidizing potential in the black sea saline and warm mediterranean water flowing through the bosporus strait maintains a permanent pycnocline with vertical separation of oxic o 2 suboxic absence of o2 and h2s and anoxic h2s zones in the black sea the stable suboxic zone implies restricted vertical mixing of the upper oxic and lower anoxic layers and limited vertical flux of oxygen that cannot balance the upward flux of sulfide we report data that directly confirm massive lateral injections 200 km from the bosporus of oxygen enriched waters of the bosporus plume created by the mixing of shallow cold intermediate layer black sea water with mediterranean water these plume waters are laterally injected into the oxic layer and more importantly into the suboxic and anoxic layers over several small vertical scales fingers of 5 m at water densities st from 15 0 to 16 4 o2 injection oxidizes mn ii to mn iii iv which then oxidizes h2s the onset of h2s detection occurs in deeper waters in the southwest 170 m st 16 4 relative to the west central black sea 110 m st 16 2 and coincides with increased mno2 and s8 formation in the southwest

grain size variations in pelagic carbonate oozes from the walvis ridge se atlantic ocean odp site 1265 a low resolution miocene record of carbonate sedimentation and preservation abstract a record of variations in winnowing and dissolution was established for miocene 19 7 ma deep sea sediments at site 1265 from the walvis ridge south atlantic in this study we apply a new preservation index based on the abundance of coarse calcareous silt fraction 10 63 m the miocene record of this silt preservation index corresponds well with the conventional foraminiferal preservation index and variations in sand content preservation ranging from good to moderate during the miocene indicates that site 1265 is located above and close to the foraminiferal lysocline grain size mass accumulation rates of carbonate and organic carbon records indicate that changes in productivity and carbonate production play a minor role in variations of calcium carbonate concentration the miocene variation in carbonate content is caused mostly by a combination of winnowing and dissolution significant decreases of preservation at 17 3 16 3 13 8 13 0 11 6 and 10 4 ma coincide with miocene glacial events mi events suggesting that dissolution was caused by an increase of southern component water scw influence which occurred as a response to the intensification of antarctic ice sheet development the scw acts as the major deep water source during the global miocene cooling at 10 4 ma a change to overall better preservation points to a weakening of scw that occurred as a response to the initiation or strengthening of north atlantic deep water nadw

seismic stratigraphy and depositional signatures of the maldive carbonate system indian ocean abstract as documented by industrial seismic reflection profiles and well data the cenozoic stratigraphic evolution of the maldive carbonate system shows three distinct intervals characterized by their own specific depositional signatures late palaeocene volcanics generated by the reunion hotspot filled deeper pull apart structures and created a flat topography that promoted the onset of the maldive carbonate system in the early eocene some of the initial structural depressions remained as deep internal seaways surrounded by reefal margins until the late oligocene the eocene early oligocene stratigraphic signature is essentially aggradational and corresponds to a transgressive cycle associated with a second order sea level rise in the mid oligocene a major sea level fall exposed the neritic carbonate system and restricted sedimentation to the former deep internal basins during the next transgression a neritic carbonate system re established itself on the newly flooded platform and was finally drowned near the oligocene miocene boundary the main effect of this second order transgressive cycle was the transformation of the central troughs into one single deep basin in the neogene the palaeo maldive inner sea when a reef framework was established in the early miocene near the present location of the platform edge systematic bank margin progradation toward the central inner sea basin became enhanced this neogene progradational phase is explained as the response to a long term sea level fall associated with a second order regressive cycle finally the late pliocene pleistocene depositional signature is mainly aggradational and corresponds to the regional modification from flat topped carbonate banks to atoll physiographies the antecedent karst theory suggests that increased karstification was triggered by high frequency eustatic and climatic changes in the last 3 0 ma

glacial versus interglacial sedimentation rates and turbidite frequency in the bahamas the southern tongue of the ocean is a 1300 m deep flat floored basin in the bahamas that receives large amounts of sediment from the carbonate platforms surrounding it on three sides we have examined five 8 13 m long piston cores and determined bulk sedimentation rates turbidite frequency and turbidite accumulation rates for the past two glacial and interglacial periods the mean of bulk sedimentation rates is four to six times higher in interglacial periods average accumulation rates of recognizable turbidites are higher by a factor of 21 to 45 and interglacial turbidite frequency is higher by a factor of 6 to 14 sediment composition indicates that increased interglacial rates are due to higher accumulation of platform derived material additional data from other bahamian basins as well as published material from the caribbean strongly suggest that highstand shedding is a general trend in pure carbonate depositional systems carbonate platforms without a siliciclastic component export more material during highstands of sea level when the platform tops are flooded and produce sediment the response of carbonate platforms to quaternary sea level cycles is opposed to that of siliciclastic ocean margins where sediment is stored on the inner shelf during highstands and passed on to continental rises and abyssal plains during lowstands of sea level

monsoon induced partial carbonate platform drowning maldives indian ocean multibeam maps and high resolution seismic images from the maldives reveal that a late miocene to early pliocene partial drowning of the platform was linked to strong sea bottom currents in the upper miocene to holocene currents shaped the drowned banks the current moats along the bank edges and the submarine dune fields bottom currents in the maldives are driven by the monsoon it is proposed that the onset and the intensification of the monsoon during the neogene provoked platform drowning through injection of nutrients into surface waters since the late miocene topographically triggered nutrient upwelling and vigorous currents switched the maldives atolls into an aggradational to backstepping mode which is a growth pattern usually attributed to episodes of rising sea level

the middle pleistocene transition characteristics mechanisms and implications for long term changes in atmospheric pco2 abstract the emergence of low frequency high amplitude quasi periodic 100 kyr glacial variability during the middle pleistocene in the absence of any significant change in orbital forcing indicates a fundamental change internal to the climate system this middle pleistocene transition mpt began 1250 ka and was complete by 700 ka its onset was accompanied by decreases in sea surface temperatures ssts in the north atlantic and tropical ocean upwelling regions and by an increase in african and asian aridity and monsoonal intensity during the mpt long term average ice volume gradually increased by 50 m sea level equivalent whereas low frequency ice volume variability experienced a 100 kyr lull centered on 1000 ka followed by its reappearance 900 ka although as a broad band of power rather than a narrow persistent 100 kyr cycle additional changes at 900 ka indicate this to be an important time during the mpt beginning with an 80 kyr event of extreme sst cooling followed by the partial recovery and subsequent stabilization of long term north atlantic and tropical ocean ssts increasing southern ocean sst variability primarily associated with warmer interglacials the loss of permanent subpolar sea ice cover and the emergence of low frequency variability in pacific ssts and global deep ocean circulation since 900 ka ice sheets have been the only component of the climate system to exhibit consistent low frequency variability with the exception of a near universal organization of low frequency power associated with marine isotope stages 11 and 12 all other components show an inconsistent distribution of power in frequency time space suggesting a highly nonlinear system response to orbital and ice sheet forcing most hypotheses for the origin of the mpt invoke a response to a long term cooling possibly induced by decreasing atmospheric pco2 none of these hypotheses however accounts for the geological constraint that the earliest northern hemisphere ice sheets covered a similar or larger area than those that followed the mpt given that the mpt was associated with an increase in ice volume this constraint requires that post mpt ice sheets were substantially thicker than pre mpt ice sheets indicating a change in subglacial conditions that influence ice dynamics we review evidence in support of the hypothesis that such an increase in ice thickness occurred as crystalline precambrian shield bedrock became exposed by glacial erosion of a thick mantle of regolith this exposure of a high friction substrate caused thicker ice sheets with an attendant change in their response to the orbital forcing marine carbon isotope data indicate a rapid transfer of organic carbon to inorganic carbon in the ocean system during the mpt if this carbon came from terrigenous sources an increase in atmospheric pco2 would be likely which is inconsistent with evidence for widespread cooling apparently rapid carbon transfer from terrestrial sources is difficult to reconcile with gradual erosion of regolith a more likely source of organic carbon and nutrients which would mitigate pco2 rise is from shelf and upper slope marine sediments which were fully exposed for the first time in millions of years in response to thickening ice sheets and falling sealevels during the mpt modeling indicates that regolith erosion and resulting exposure of crystalline bedrock would cause an increase in long term silicate weathering rates in good agreement with marine sr and os isotopic records we use a carbon cycle model to show that a post mpt increase in silicate weathering rates would lower atmospheric pco2 by 7 12 ppm suggesting that the attendant cooling may have been an important feedback in causing the mpt

quaternary slope development of the western leeward margin of the great bahama bank abstract to date work on the great bahama bank s western leeward margin has centred chiefly on seismic scale expressions of carbonate sequences and systems tracts however periplatform slope sediments also exhibit very well developed cyclicity on scales of decimetres to several metres it is these small scale high frequency cycles within the larger scale facies successions of the quaternary which form the main topic of this paper previous studies have shown that the small scale cycles correlate to the orbitally forced high frequency sea level changes therefore these cycles should indicate how sea level has affected the slope development and thus platform margin evolution during this period through detailed high resolution sequence stratigraphy of the great bahama bank s leeward margin obtained via 18o isotope and mineralogical xrd analyses confined by u th dating and nannofossil bioevents a greater understanding of the bedding geometries within the pleistocene holocene seismic sequences and clues as to the nature of the slope development has been achieved the high resolution seismic profiles indicate that since the plio pleistocene change in geometry in which the great bahama bank developed into a rimmed platform continued steepening and subsequent progradation of the leeward margin has typified slope development during the quaternary which is described as an accretionary slope however on the basis of our observations we conclude that only the early to lower middle pleistocene section isotope stages 45 20 and the holocene isotope stage 1 of the leeward margin is accretionary this indicates that a degree of erosion and or by passing has occurred on the leeward margin since the lower middle pleistocene isotope stage 19 during the first part of this period isotope stages 19 12 erosion and or by passing occurred in the middle to lower slope regions and toe of slope by the end of the upper middle to late pleistocene phase isotope stages 11 2 erosion also occurred on the upper slope this erosion by currents at the toe of slope and oversteepening of the upper and middle slopes have led to back cutting upslope and resulted in the progressive retreat of the toe of slope towards the platform to the east however the rise in sea level since the last glacial maximum to its present day level has allowed high productivity on the platform top during the holocene and the deposition of a thick sediment wedge on the slope and sedimentation across the entire leeward flanks this has led to the redevelopment of an accretionary slope and continued westward progradation of the great bahama bank s western leeward margin

wind variability over the northern indian ocean during the past 4 million years insights from coarse aeolian dust iodp exp 359 site u1467 maldives abstract the lithogenic fraction of carbonate drift sediments from iodp exp 359 site u1467 maldives provides a unique record of atmospheric dust transport over the northern indian ocean during the past 4 myr grain size data provide proxies for dust flux controlled by source area aridity as well as wind transport capacity wind speed entrainment and long range transport of dust in the medium to coarse silt size range is linked to the strength of the arabian shamal winds and the occurrence of convective storms which prolong dust transport dust flux and the size of dust particles increased between 4 0 and 3 3 ma corresponding to the closure of the indonesian seaway and the intensification of the south asian monsoon there is no clear trend in dust flux between 3 3 and 1 6 ma whereas wind transport capacity decreased between 1 6 ma and the recent dust flux increased and shows higher variability especially during the last 500 kyr transport capacity increased between 1 2 and 0 5 ma and slightly decreased since then frequency analysis shows that dust transport varies on orbital timescales with eccentricity control being the most prominent 400 kyr throughout the record 100 kyr between 2 0 and 1 3 ma and since 1 0 ma higher frequency cycles obliquity and precession are more pronounced in wind transport capacity than in the amount of dust this indicates that the amount of coarse dust in sediments from the maldives as a far field site is more prone to changes in transport mechanisms than to changes in dust source area aridity

controls on grain size patterns in periplatform carbonates marginal setting versus glacio eustacy the importance of grain size as a fundamental property controlling other physically derived properties is well known recent grain size studies on periplatform sediments of the caribbean and western north atlantic have shown that grain size variability is coupled with changes in carbonate platform productivity and export in response to pleistocene holocene glacio eustacy these findings showed that periplatform sediments are dominated by fine grained material during sea level highstands and coarser material during sea level lowstands this paper addresses the question of grain size patterns in periplatform settings and their relationship to glacio eustacy furthermore it considers whether these patterns are applicable to different types of margin settings accretionary erosional here existing data pleistocene holocene from the accretionary western leeward margin of great bahama bank odp leg 166 and new data from the erosional eastern windward margin odp leg 101 are used the results show that the grain size patterns of both margins are controlled by late neogene sea level fluctuations the accretionary leeward margin sediments indicate the same grain size pattern as previously described in the literature however the erosional windward margin shows a reverse pattern with coarser sediments in interglacials and finer deposits in glacials this proposes that the leeward margin grain size pattern is controlled by the productivity export mode of the platform this is indicated by the platform top derived fine muds transported off bank to form a thick sediment wedge on the low angled slope during interglacials in contrast on the windward margin the sediment flux pattern is controlled by the impact of mass transport processes this is indicated by the increased occurrence of turbidites during interglacials that might be more frequent at this time because the steep angled erosional slope is less apt for the deposition of excess fine material this material is eroded and or bypasses the slopes and is redeposited in the periplatform basin setting

giant pockmarks in a carbonate platform maldives indian ocean abstract circular structures and depressions in carbonate platforms are known to represent karst chimneys or sinkholes which form as a response to rock solution this formation mechanism is plausible for shallow water carbonates which lie in the reach of meteoric diagenesis or fresh water lenses circular structures which occur in deeper waters however need an alternative interpretation such an example of sea floor depressions in more than 300 m deep waters occurs in the inner sea of the maldives carbonate platform in the indian ocean the structures were mapped with multibeam and parasound multi channel seismics were used to link the depressions with structures at depth the circular depressions have diameters of up to 3000 m and depths of up to 180 m the craters are interpreted as pockmarks formed through the venting of gas and fluids gas and fluid lenses below the pockmarks are reflected by bright spots in the seismic sections as well as a reduction of the instantaneous frequency these areas at depth are linked to chimneys connected to faults and drowned oligocene carbonate banks a model is presented that relates the different forms and sizes of the structures to distinct development stages of sea floor deformation to one process early stages of gas and fluid migration into the shallow part of the sedimentary succession induce formation of dome shaped bodies initial gas and fluid escape to the sea floor is reflected by the formation of sand volcanoes and aligned small pockmarks active pockmarks are the deepest and have the shape of truncated cones in cross section mature pockmarks are characterized by erosion of the flanks of the structure by bottom currents late stage pockmarks are bowl shaped in cross section and are to different degrees filled by drift sediments packages of strata revealing high reflection amplitudes and high interval velocities interpreted as microbially mediated carbonate precipitates underlie some of the pockmarks the pockmarks in the maldives show that circular structures other than solution related features can be abundant in carbonate platform deposits and that such structures may be more abundant in the geological record of carbonate platforms as previously thought pockmarks in the maldives indicate that the archipelago is an example of a hydrocarbon system which consists of an isolated oceanic carbonate platform overlying a volcanic basement and lacustrine source rocks

relationship between late pleistocene sea level variations carbonate platform morphology and aragonite production maldives indian ocean a piston core from the maldives carbonate platform was investigated for carbonate mineralogy grain size distributions calcium carbonate content and organic carbon the sedimentary record was linked to late pleistocene sea level variations using an age model based on oxygen isotopes obtained from planktonic foramanifera nannofossil biostratigraphy and 14c age determinations the correlation between the sedimentary record and late pleistocene sea level showed that variations in aragonite and mud during the past 150 000 years were clearly related to flooding and sea floor exposure of the main lagoons of the atolls of the maldives carbonate platform platform flooding events were characterized by strongly increased deposition of aragonite and mud within the inner sea of the maldives exposure events in contrast can be recognized by rapid decreases in the values of both proxy records the results show that sediments on the maldives carbonate platform contain a continuous record of pleistocene sea level variations these sediments may therefore contribute to a better understanding of regional and even global sea level changes and yield new insights into the interplay between ocean currents and carbonate platform morphology

reworked paleogene to early neogene planktic foraminifera implications of an intriguing distribution at a late neogene prograding margin bahamas abstract a rich uniquely distributed assemblage 80 species of reworked paleogene to early neogene 66 15 ma planktic foraminifera is documented within 470 m of late neogene 5 7 1 6 ma borehole limestone from the great bahama bank west margin tests vary in size from 0 13 to 1 0 mm and are unusually well preserved where obtainable from the rock the relict species constitute an estimated 20 to 50 of all taxa paleocene faunas occur in upper miocene host rocks eocene species occur in upper miocene and basal upper pliocene rocks and mixed oligocene and early miocene taxa occur throughout the section including the uppermost pliocene i e the oldest groups disappear successively uphole each host rock interval is separated by major disconformities this record provides a substantial proxy for 1 a probable source feature an epipelagic marine tectonic wall escarpment or outcrop composed of pelagic and hemipelagic sediments 2 a probable tectonically active source area northern cuba or southern bahamas and 3 a variety of events that span 60 myr and have wide ranging implications from local progressive isolation of the oldest faunas to regional changes in water depth indicated by the faunas to global hiatuses that correlate with one or more major sequence boundaries in the atlantic bahamas and gulf of mexico as well as proposed eustatic lowstands seven unidentified foraminiferal biozones correspond to a proposed early eocene composite condensed section on the eustatic curve and may have regional implications the relict faunas also have significant implications for the timing and types of paleogene and early neogene sedimentation or nondeposition sea level fluctuations slumping or changing depth of effective marine erosion and regional late neogene events that occurred over 4 myr age of host rocks the direction of the source area from the secondary emplacement site and the estimated percentages of relict faunas indicate that not all the microfossils in the late neogene borehole sediments and by extrapolation therefore along the entire west margin of the platform were locally derived the amount of a possible attendant sedimentary component is not known

global dynamics of a temperate valley glacier mer de glace and past velocities deduced from forbes bands transverse profiles and velocities which have been measured on the ablation zone of mer de glace more or less continuously since 1891 contradict the weertman nye theory of glacier kinematic waves faint broad waves which undoubtedly result from fluctuations in the balance travel down the glacier faster than this theory predicts this theory having first been completed by taking changes in width with time and with distance down the glacier into account on the other hand velocity fluctuations are synchronous and more or less the same over the entire length studied 6 km these discrepancies result from bottom friction being of the solid type i e independent of sliding velocity friction should also be almost insensitive to discharge in subglacial waterways since in the steady state energy for keeping them open not entirely flooded and at atmospheric pressure is superabundant the sliding velocities at all cross profiles are thus controlled by some areas where the body of the glacier suffers strong deformation because the valley shape is far from cylindrical one such controlling zone exists on mer de glace owing to the existence of a subglacial transverse shoulder a new perturbation equation and a new rough expression for wave velocity are given intervals between forbes bands were plotted on seven aerial surveys between 1939 and 1979 progressive tilting of the slices of blue dusty ice from the position from which these dark bands proceed and progressive lowering of their exposed edge must be taken into account this analysis confirms the validity of our simple model for velocity fluctuations and allows us to estimate the entire series since the year 1888

controls and development of late quaternary periplatform carbonate stratigraphy in walton basin northeastern nicaragua rise caribbean sea the late quaternary sedimentary record in periplatform oozes and muds deposited on the northern nicaragua rise results from the interplay of four main controls 1 input of pelagic carbonates and bank derived fine aragonite and magnesian calcite 2 input of siliciclastic sediments 3 dispersal and removal of sediments by the caribbean current and 4 partial seafloor dissolution of metastable carbonates high accumulation rates of the calcite coarse sediment fraction throughout the study area demonstrate that planktonic foraminiferal productivity peaked during interglacial stages neritic carbonate productivity also peaked during interglacial stages when bank and shelf tops along the nicaragua rise were submerged within the photic zone because the bank and shelf tops remained mostly exposed during glacial stages the surface area available for the neritic carbonate productivity was drastically reduced to a narrow band along the margins of pedro bank and the southern shelf of jamaica a large volume of siliciclastic sediments transported from the coastal area of south america and the eastern and western regions of jamaica were deposited during glacial stages within the eastern and western deep extensions of walton basin this finding contrasts with the input pattern of siliciclastic sediments in walton basin itself where the largest input of siliciclastic sediments occurred at each of the interglacial climatic optima corresponding to intervals of maximum sea level transgression finally cores in water depths exceeding 1100 m display during interglacial stages aragonite accumulation rates systematically lower than rates in cores from water depths shallower than 1100 m this indicates that some bank derived aragonite and magnesian calcite has been partially removed by seafloor dissolution in areas from water depths exceeding 1100 m results from this study on the northeastern nicaragua rise indicate that not only do offbank transport and water column saturation state influence the late quaternary record but also that the variable strength of the caribbean current and the proximity of sources for siliciclastic sediments have played a major role in the development of the late quaternary periplatform high resolution stratigraphy

new arabian sea records help decipher orbital timing of indo asian monsoon abstract a recent study suggested that indian monsoonal proxies commonly used in the arabian sea in general productivity proxies could be impacted by changes in the atlantic overturning rate amoc throughout a control on the nutrient delivery into the euphotic zone this oceanic mechanism could lead to a misunderstanding between the indian summer monsoon sm and orbital forcing and could confuse a direct comparison with other archives derived from other monsoonal sub systems such as east asian or african records here we analyze three independent proxies bromine foraminifera assemblages and grain size extracted from a marine sediment core md04 2861 covering the last 310 ka and retrieved in the northern arabian sea near the makran margin an area influenced by summer and winter indian monsoon the grain size proxy deals with the regional continental climate through fluvial and eolian processes it cannot be linked to changes in nutrient content of amoc and present the same phase relationship timing than the other sm proxies this demonstrates that the productivity signals bromine in the northern arabian sea are mainly controlled by sm dynamics and not amoc modulated nutrients at orbital scale changes we thus build a multi proxy record of sm variability i e sm stack using statistical tools principal component analysis further compiled on an age model constructed independently from orbital tuning we find that strong sm lag by 9 1 ka the nh summer insolation maximum minimum of precession june 21 perihelion and obliquity maximum in the precession band and by 6 1 3 ka in the obliquity band these results are consistent with previous studies based on marine and terrestrial records in both indian and asian regions except asian speleothems our study supports the hypothesis that internal climate forcing decreased ice volume together with the increase of latent heat export from the southern indian ocean set the timing of strong indo asian summer monsoons within both the precession and obliquity cycle the external forcing direct sensible heating initiate monsoonal circulation strong indian winter monsoon wm occurs between ice maxima and northern hemisphere sensible heat minima indicating that both act to strengthen wm circulation the summer and winter monsoons are in antiphase in the precession band suggesting that the two systems are dynamically linked

glacial interglacial indian summer monsoon dynamics the modern indian summer monsoon ism is characterized by exceptionally strong interhemispheric transport indicating the importance of both northern and southern hemisphere processes driving monsoon variability here we present a high resolution continental record from southwestern china that demonstrates the importance of interhemispheric forcing in driving ism variability at the glacial interglacial time scale as well interglacial ism maxima are dominated by an enhanced indian low associated with global ice volume minima in contrast the glacial ism reaches a minimum and actually begins to increase before global ice volume reaches a maximum we attribute this early strengthening to an increased cross equatorial pressure gradient derived from southern hemisphere high latitude cooling this mechanism explains much of the nonorbital scale variance in the pleistocene ism record

quaternary aragonite cycles and oxygen isotope record in bahamian carbonate ooze carbonate ooze in bahamian troughs displays cyclic variation in the content of aragonite a 10 m core devoid of turbidites shows 3 5 cycles each defined by a sharp increase followed by a gradual decrease of aragonite the aragonite curve matches closely the oxygen isotope curve of planktic foraminifera from the same core aragonite highs correspond to light oxygen isotope values and thus to interglacial stages content of terrigenous material is low during interglacials and higher during glacial periods intermittent flooding and exposure of the bahama banks is most probably not the cause of the cycles because the latest increase in aragonite precedes bank flooding by 8 000 yr and because the cycles are distinctly asymmetric with rapid increase and gradual decrease of aragonitic content carbonate dissolution cycles tied to the glacial rhythm of earth9s climate are a more likely explanation of the variations in aragonite content

the maldives a giant isolated carbonate platform dominated by bottom currents abstract the maldives a 900 km north south trending paired chain of atolls that surround a ca 100 km wide central basin the inner sea represent a giant isolated carbonate platform in the indian ocean new reflection seismic multibeam and acoustic doppler current profiler data show that its development exhibits a distinct change from sea level to mainly bottom current controlled sedimentation at the end of the middle miocene the post middle miocene intra basinal fill consists of 9 mega units that have been formed and shaped by strong bottom currents entering the inner sea between the inter atoll passages at the end of the middle miocene the maldives represent a rimmed platform with the inner sea forming an empty bucket a connection to the indian ocean existed only in the northeastern part of the platform namely the kardiva channel probably concomitant with the onset of the indian monsoon the current pattern around the maldives changed significantly resulting in the formation of channels in the rimmed platform margin the latter process was encouraged by local drowning of large platform parts and additionally by bottom current forced submarine erosion that widened the channels to passages disintegrating the platform into separated atolls of different sizes the reworked material was dumped into the inner sea where it formed large contourite fans along slope bottom currents entering the inner sea from the north further redistributed the material leading to a southward shift of these sedimentary bodies in this way the inner sea was filled from west to east mega units 1 5 starting with mega unit 6 the opening of a southern gateway introduced a prominent northward flow of bottom waters in the inner sea it leads to the deposition of giant elongated drifts at the eastern flank of the basin filling it from east to west because the current swept away most of the material around the atolls the system was not able to prograde and the steady subsidence was compensated by aggradation accordingly a fundamental characteristic of a current controlled carbonate system is that aggradation occurs irrespective of platform sediment export or differently worded the rate of change in accommodation is always positive and greater than the rate of sediment supply adcp measurements in the winter monsoon season show two water masses in the inner sea one at the surface 0 150 m and a second below it that reaches its sea floor by the use of the grain size distribution of odp leg 716 located in the inner sea as proxy for the flow intensity of the bottom water we recognized an inverse proportional relationship to the monsoon strength from late upper miocene to present some of the current controlled deposits form sigmoidal prograding clinoforms preferably at the edges of the atolls adjacent to the passages where the velocity of the current entering the inner sea is high the anatomy of these prograding wedges is similar to sea level controlled clinoforms described for large isolated carbonate platforms in the sequence stratigraphic concept however our results document that they are not influenced by sea level fluctuations because of the high current velocities involved in their deposition they predominately consist of coarse sand sized carbonate fragments exhibiting large pore space and high permeability therefore these drift deposits comprise a possible exploration target

nonstationary phase of the plio pleistocene asian monsoon paleoclimate records indicate that the strength of the asian summer monsoon is sensitive to orbital forcing at the obliquity and precession periods 41 000 and 23 000 years respectively and the extent of northern hemisphere glaciation over the past 2 6 million years the timing phase of strong monsoons has changed by 83 degrees in the precession and 124 degrees in the obliquity bands relative to the phase of maximum global ice volume inferred from the marine oxygen isotope record these results suggest that one or both of these systems is nonstationary relative to orbital forcing

seismic expression of quarternary climatic cycles in the peri platform carbonate ooze of the northern bahamas stratigraphic physical and acoustic properties of peri platform carbonate sediments in an 18 m long piston core core 2218 655 m from northwest providence channel bahamas have been investigated the sediments range in age from recent to about 1 m y old there is a close correlation between changes of compressional wave velocity and quaternary climate as indicated by 18 o velocity correlates negatively to aragonite content water content and porosity whereas it correlates positively to grain size saturated bulk density and observed degree of cementation changes in the degree of cementation exert a particularly strong influence on velocity variations all parameters correlate to quaternary climate sea level fluctuations during glacial low sea level stages the aragonite content water content water content and porosity decrease but grain size saturated bulk density and the degree of cementation increase all of these increase velocity in general water content porosity and the aragonite content decrease slightly with core depth but grain size saturated bulk density degree of cementation and velocity increase slightly two processes may contribute to these trends 1 shallow subsurface diagenesis upon burial and 2 variations of both the interglacial production of bank top sediments and the intensity of glacial sea floor diagenesis major changes in velocity are associated with transitions from glacial to interglacial stages acoustic reflectors correlate one to one with the velocity changes occurring at isotopic stage boundaries 7 8 9 10 and 11 12 at the site of core 2218 deeper reflectors have an interference origin because velocity changes are more closely spaced the reflectors that correlate with isotopic stage boundaries can be used to study the detailed history of quaternary peri platform carbonate deposition and place it into the context of global climate sea level cycles

physical chemical and microbial regimes in an anoxic fjord nitinat lake new observations of physical biological and chemical parameters were obtained between july 2003 and february 2005 in nitinat lake british columbia a permanently anoxic seawater fjord these observations include continuous profiles of nitrate and bisulfide using a new instrume nt for in situ ultraviolet spectrophotometry anoxic seawaters in this 200 m deep basin can be found as shallow as 7 m however nitinat lake is surprisingly dynamic with a deep water renewal time of only about 5 yr and significant spatial and temporal variability exchange with the ocean is limited by the size of the linking channel and during low tides the outflow is subject to an hydraulic control this nonlinearity causes a relatively large fortnightly cycle in water level and explains flooding events associated with heavy rainstorms in addition the restricted flow makes deep water renewal gradual so that anoxic conditions are preserved intermediate waters 5 40 m in depth are renewed alternately by oxygenation from surface mixing in winter and by upwelling of anoxic water and horizontal inflow of subducted ocean water in summer the horizontal inflow results in a subsurface region of suboxic water in mid fjord during summer no suboxic regime is evident at either the river end of nitinat lake where oxygen and sulfide rich regions adjoin in a nitrate depleted water column or at the ocean end of nitinat lake where oxygen and nitrate rich regions sometimes overlap with sulfide rich regions maximum phytoplankton levels near the surface are high in both winter and summer a second peak in biological activity appears near the anoxic boundary possibly as a result of sulfur oxidizing bacteria sulfate reducing bacteria are also present in anoxic water in both seasons and just above the oxic anoxic interface in winter and just above the suboxic anoxic interface in summer

high production and highstand shedding from deeply submerged carbonate banks northern nicaragua rise abstract because the shallow isolated carbonate banks on the northern nicaragua rise the nicaragua honduras and southern jamaica carbonate shelves and many other modern carbonate banks worldwide are covered by an average of 20 to 30 m of water and by a thin blanket of coarse carbonate sediments other carbonate sedimentologists have considered these banks good examples of incipiently drowned or even drowned carbonate banks however based on recent research on the northern nicaragua rise we can demonstrate that these banks currently are healthy producers of large volumes of periplatform sediments fine aragonite and magnesian calcite which are exported almost entirely to the deep surrounding slopes these sediments which were deposited during the past 9000 10 000 years form peripl tform wedges on the middle and upper slopes which are seen clearly on 3 5 khz seismic profiles radiocarbon ages of the wedge surface sediment range between 230 and 610 ybp and provide clear evidence for the contemporaneous production of sediments on the shallow bank and shelf and their instantaneous export to the upper slopes for the last 5000 years sedimentation rates ranged from 2000 mm ky off pedro bank to 1300 mm ky off jamaica these rates are somewhat lower than but of the same order of magnitude as sedimentation rates on the western leeward slopes of great bahama bank deposition of the metastable carbonate sediments discussed in this paper occurred as soon as sea level rose to flood the bank and shelf which flank walton basin following the last glacial lowstand they com rise a wedge shaped package of transgressive and highstand sediments on the upper and middle slopes of pedro bank and the southern jamaica shelf

the deepwater exchange cycle in indian arm british columbia abstract a silled fjord on the british columbia coast indian arm was used to study the exchange processes controlling deepwater exchange in fjords bottom water renewal took place during the winter of 1984 85 hydraulic control of the exchange is exerted over the long sill which restricts access to the fjord the maximum density at the sill occurs during neap tides no distinct peaks in the velocity at the sill are observed during periods of inflow reduced vertical mixing over the long sill during periods of neap tides sets the timing of the bottom water exchange during each inflow over the 1984 85 winter about 20 of the water in the fjord was replaced these exchanges occurred over periods of 5 10 days between the exchange years diffusion of the water in the basin is important in conditioning the water for the next exchange event the vertical diffusion coefficient k t was determined from an analysis of the salinity data and was found to be related to the buoyancy frequency n by the relationship k v n 1 6

chemoautotrophy in the redox transition zone of the cariaco basin a significant midwater source of organic carbon production during the cariaco time series program microbial standing stocks bacterial production and acetate turnover were consistently elevated in the redox transition zone rtz of the cariaco basin the depth interval 240 450 m of steepest gradient in oxidation reduction potential anomalously high fluxes of particulate carbon were captured in sediment traps below this zone 455 m in 16 of 71 observations here we present new evidence that bacterial chemoautotrophy fueled by reduced sulfur species supports an active secondary microbial food web in the rtz and is potentially a large midwater source of labile chemically unique sedimenting biogenic debris to the basin s interior dissolved inorganic carbon assimilation 27 159 mmol c m 22 d 21 in this zone was equivalent to 10 333 of contemporaneous primary production depending on the season however vertical diffusion rates to the rtz of electron donors and electron acceptors were inadequate to support this production therefore significant lateral intrusions of oxic waters mixing processes or intensive cycling of c s n mn and fe across the rtz are necessary to balance electron equivalents chemoautotrophic production appears to be decoupled temporally from short term surface processes such as seasonal upwelling and blooms and potentially is more responsive to longterm changes in surface productivity and deep water ventilation on interannual to decadal timescales findings suggest that midwater production of organic carbon may contribute a unique signature to the basin s sediment record thereby altering its paleoclimatological interpretation

orbital scale variation of primary productivity in the central equatorial indian ocean maldives during the early pliocene abstract we investigate the response of the central equatorial indian ocean odp site 716 maldives upwelling system to orbital forcing between 3 64 and 4 19 ma this time interval in the early pliocene was characterized by global average temperatures considerably warmer than present and reduced zonal and meridional sea surface temperature gradients across the major oceans chlorin a degradation product of chlorophyll and a potential indicator of primary productivity was previously shown to record orbital scale variations at this site to confirm that chlorin records primary productivity and to understand the mechanism controlling the orbital variations we compare changes in the planktic foraminiferal assemblages with the chlorin content of the sediment cross wavelet analysis demonstrates the in phase relationship between the abundance of thermocline species and chlorin content indicating that upwelling and a shallower thermocline drive the productivity changes the aragonite content in periplatform carbonates is known to reflect variations in sea level and or climate with high values generally indicating sea level highstands the aragonite content in the studied interval shows precession and eccentricity astronomical cycles whereas the primary productivity proxies show only precessional cycles the negative correlation between aragonite content and the relative abundance of thermocline species on the precessional band implies that the primary productivity increased during sea level lowstands this phase coupling is consistent with quaternary productivity records from the central indian ocean that were proposed to be controlled by orbital induced variations of the el nino southern oscillation enso we therefore propose that a similar mechanism was active in the early pliocene despite major changes in the global boundary conditions

intermediate depth circulation of the indian and south pacific oceans measured by autonomous floats as part of the world ocean circulation experiment 306 autonomous floats were deployed in the tropical and south pacific ocean and 228 were deployed in the indian ocean to observe the basinwide circulation near 900 m depth mean velocities seasonal variability and lateral eddy diffusivity from the resultant 2583 float years of data are presented area averages local function fits and a novel application of objective mapping are used to estimate the mean circulation patterns of mean circulation resemble those at the surface in both basins well developed subtropical gyres twice as strong in the indian ocean as in the pacific feed western boundary currents tropical gyres are separated by eastward flow along the equator in both hemispheres of both basins although the indian subcontinent splits the north indian tropical gyre the antarctic circumpolar current acc and west wind drifts are prominent in both basins generally tending slightly southward but deviating to the north behind the del cano kerguelen and campbell plateaus and of course south america remarkably the eastern boundaries of the southern subtropical gyres in all three basins apparently occur in the ocean interior away from land the indian ocean s subtropical gyre and perhaps part of the south atlantic s reaches east to a retroflection just upstream of the campbell plateau south of new zealand seasonal variability at 900 m is focused around the equator with weaker variability found near certain bathymetric features there is a remarkable agreement between the observed seasonable variability and that predicted by the jet propulsion laboratory jpl estimating the circulation and climate of the ocean ecco data assimilating numerical model aside from seasonal effects eddy variability is greatest along the equator in tropical and subtropical western basins and along the acc integrals of velocity across regional passages tasman sea mozambique channel provide useful reference for hydrographic analyses of transport across whole ocean basins however the uncertainty associated with the appropriate continuity relation for horizontal flow e g geostrophy vs nondivergence is comparable to the mean flow

sources of methane to an arctic lake in alaska an isotopic investigation sources of dissolved methane ch4 at toolik lake alaska include both diffusion from lake sediments and groundwater entering the lake from its perimeter here we use hydrogen and oxygen isotopes in water h2o carbon and hydrogen isotopes in ch4 and carbon isotopes in dissolved inorganic carbon dic to calculate the relative importance of lake sediment and groundwater discharge as sources of dissolved ch4 to toolik lake we also resolve the relative importance of the source contribution spatially within the lake and determine the processes controlling ch4 concentrations in groundwater surrounding the lake our findings from a mixing model based on isotopes in ch4 suggest that groundwater is a more important source of ch4 at the perimeter of the lake where the water to air flux is high additionally we find on the local scale that high groundwater methane concentrations may be better linked to areas around the lake where rain is the dominant source of water to the active layer indicating that changes in precipitation and active layer thaw depth will impact methane concentrations in the active layer and ultimately the groundwater associated flux to toolik lake

current and sea level signals in periplatform ooze neogene maldives indian ocean abstract periplatform ooze is an admixture of pelagic carbonate and sediment derived from neritic carbonate platforms compositional variations of periplatform ooze allow the reconstruction of past sea level changes periplatform ooze formed during sea level highstands is finer grained and richer in aragonite through the elevated input of material from the flooded platform compared to periplatform ooze formed during the episodes of lowered sea level in many cases however the sea floor around carbonate platforms is subjected to bottom currents which are expected to affect sediment composition i e through winnowing of the fine fraction the interaction of sea level driven highstand shedding and current impact on the formation of periplatform ooze has hitherto not been analyzed to test if a sea level driven input signal in periplatform ooze is influenced or even distorted by changing current activity an integrated study using seismic hydroacoustic and sedimentological data has been performed on periplatform ooze deposited in the inner sea of the maldives the miocene to pleistocene succession of drift deposits is subdivided into nine units limits of seismostratigraphic units correspond to changes or turnarounds in grain size trends in cores recovered at odp site 716 and neoma site 1143 for the pleistocene it can be shown how changes in grain size occur in concert with sea level changes and changes of the monsoonal system which is thought to be a major driver of bottom currents in the maldives a clear highstand shedding pattern only appears in the data at a time of relaxation of monsoonal strength during the last 315 ky results imply 1 that drift sediments provide a potential target for analyzing past changes in oceanic currents and 2 that the ooze composition bears a mixed signal of input and physical winnowing at the sea floor

orbital and suborbital variability in the equatorial indian ocean as recorded in sediments of the maldives ridge odp hole 716a during the past 444 ka this study is aimed at understanding past 444 ka record of climate variability in the equatorial indian ocean using high resolution records of planktic and benthic foraminifera and pteropods from ocean drilling program hole 716a maldives ridge southeastern arabian sea in total 892 samples of 10 cm 3 volume from 444 ka old sequence were analysed at 1 cm intervals to generate census data of the foraminiferal fauna and pteropods the percent and detrended time series of mixed layer species globigerinoides ruber and globigerinoides sacculifer and thermocline species neogloboquadrina dutertrei benthic foraminifera cymbaloporetta squammosa sphaeroidina bulloides and uvigerina proboscidea and pteropods from odp hole 716a reveal significant changes in wind intensity during the past 444 ka an abrupt decrease in the cymbaloporetta squammosa population at c 300 ka across mis 8 9 suggests a weakening of equatorial wind intensity which could be linked to indian monsoon and may have driven pronounced changes in the oxygen minimum zone in the maldivian region these changes were contemporaneous with the mid brunhes climatic event the beginning of aridity in the indonesian australian region and the onset of a humid phase in equatorial east africa as observed in several oceanic and continental records this strengthens a connection between equatorial indian ocean wind intensity the indian monsoon and indonesian australian african climates