latest jurassic early cretaceous events and the late cimmerian unconformity in north sea area each of several regional or local unconformities may have been identified locally as the late cimmerian unconformity a supposed major break at the base of the valhall formation or roedby formation over most of the north sea the base of the valhall formation is isochronous and conformable with underlying sediments it is detected on seismic reflection profiles because it represents a widespread facies change marking the late ryazanian transgression most of the unconformities are of eustatic origin and the effects of eustatic sea level changes were never completely masked by local tectonics thus in the modeling of individual oil fields the possibility of sedimentary breaks occurring can be predicted in part by reference to regional or eustatic events modified journal abstract

segmentation and coalescence of cenozoic carbonate platforms northwestern great bahama bank seismic profiles over the northwestern great bahama bank reveal that it is formed by the coalescence of three smaller platforms and significant lateral progradation this conclusion is contrary to previous models which assumed that the bank is the result of continuous accumulation on one huge buildup in late cretaceous to early tertiary time a north south trending depression the straits of andros separated an eastern platform andros bank from a western platform bimini bank initially the straits of andros had dimensions similar to the modem tongue of the ocean but was progressively filled from east to west in early to middle tertiary time a second depression the bimini embayment formed within bimini bank by folding it had a maximum depth of about 470 m this depression was also filled from east to west in addition prograding systems built the western margin of bimini bank more than 25 km westward into the straits of florida

seismic reflection evidence for a dangerous grounds miniplate no extrusion origin for the south china sea the collision of india and asia has caused large strike slip faults to form in east asia resulting in the extrusion of crustal blocks toward the southeast since the eocene as a result of the indentation of rigid india into asia it has been suggested that the south china sea opened as a result of relative motion between a rigid indochina sundaland block and china alternative models propose that rifting and seafloor spreading were driven by trench forces to the south we test these competing models by analysis of seismic reflection profiles across the boundary between sundaland and the southern rifted margin known as the dangerous grounds we show that the southern boundary of the dangerous grounds is a subduction zone that jammed in the middle miocene to the west the dangerous grounds are bounded by a strike slip zone also active until similar to 16 ma that becomes diffuse south of the now inactive south china sea seafloor spreading center we place the western edge of the dangerous grounds just to the east of the natuna arch lupar line the west baram line is confirmed as originating as a major strike slip fault within the dangerous grounds and is continuous with the red river fault zone because the dangerous grounds were independent of sundaland until similar to 6 ma its motion cannot have been constrained by motion of this block making extrusion impossible as a mechanism to rift the south china sea se motion by both the dangerous grounds and sundaland suggests subduction forces were the primary trigger for plate motions our reconstruction places a similar to 280 km upper limit on the motion on the red river fault and a similar to 1400 km width to the paleo south china sea

mapping bathymetry and depositional facies on great bahama bank satellite imagery and an extensive set of water depth measurements have been used to map and critically evaluate the magnitude and patterns of bathymetry across great bahama bank descriptions of previously collected sediment samples were combined with satellite imagery to map and refine the interpreted distribution of surficial carbonate sediments depositional facies data reveal that 60 of great bahama bank lies in 5 m or less of water the deep portion occurs mainly in a generally east west trending area in the southern portion of the platform the re evaluation of the facies reveals that great bahama bank is essentially a very grainy platform with muddier accumulations primarily in the lee of andros island this area of great bahama bank also experiences currents related to an excursion of the florida current onto the platform top possibly enhancing sediment mud production through the generation of whitings sediment equivalents to mudstones wackestones and mud rich packstones cover 8 5 and 14 respectively of the platform top whereas sediment equivalents to mud poor packstones grainstones and rudstones account for 20 45 and 3 of the surface area boundstones reefs were not specifically mapped in this study due to the resolution of the mapping there is a poor relationship between the occurrence of the depositional texture and water depth in that the grainier sediment types are abundant across the full range of water depths the most abrupt lateral facies changes portrayed on the facies maps are observed leeward of islands areas which also hold the highest diversity in facies type the majority of the islands on the platform align with the north west south east strike of the platform margin and these islands in turn exert control on the shape and orientation of facies belts that develop in proximity to them for this reason regions of the platform that contain principal islands host facies belts that align with the principal axis of the platform whereas for regions lacking islands the facies belts adopt an east west trend consistent with prevailing winds and currents there is a clear trend that the wide southern portion of the platform hosts the most continuous expanses of grain rich sediments

aeolian particles in marine cores as a tool for quantitative high resolution reconstruction of upwelling favorable winds along coastal atacama desert northern chile upwelling areas play a major role in ocean biogeochemical cycles and ultimately in global climate especially in higly productive regions as the south eastern pacific this work is based on the analysis of the aeolian lithic particles accumulated in laminated sediments off mejillones 23 s in the eastern boundary humboldt current system it proposes a high resolution quantitative reconstruction of the upwelling favorable southerly wind strength in the past 250 years comparing its variability with changes in organic carbon export preserved changes to the sea bottom the increase of the intensity and variability in fluxes of particles larger than 35 m and 100 m since the second half of the 19th century and during the 20th century confirms a general strengthening of southerly winds in the region spectral analysis on the complete time series of yearly depositional fluxes indicates that sedimentary variability can be explained by a combination of interannual enso to decadal pdo oscillations similar to the ones yielded by the analysis of the interdecadal pacific oscillation index however when applied separately to the lithic fluxes of the first and last centuries of the time series the method shows that relative to the one of the interannual mode of variability the influence of the decadal mode has increased in the recent period based on the presence absence of particles with sizes larger than 35 100 m each year of the time series is classified as a low wind 12 m s year from the ad 1754 1820 period to the ad 1878 1998 one the proportion of low and intermediate wind years decreased from 12 and 74 to 3 and 68 respectively whereas the proportion of strong wind years increased from 14 to 29 for these periods the mean organic carbon also increased 22 stating the strong relation between export preservation productivity rate and southerly wind intensity in the recent period from ad 1950 on for which the oceanic nino index is available the strong wind years ad 1982 1983 1994 and 1997 correspond to large values of this index suggesting that constructive interferences that result from the interplay between interannual and decadal oscillations modes might explain in part the reinforcement of the winds along the north chilean coast

holocene lagoonal development in the isolated carbonate platforms off belize abstract thirty one vibracores were taken in interior lagoons of glovers reef lighthouse reef and turneffe islands three isolated carbonate platforms offshore belize central america holocene facies successions overlying the pleistocene limestone bedrock begin with soils followed by mangrove peats and marine carbonate sediments of lagoonal origin the soils formed on top of subaerially exposed pleistocene limestone before the holocene transgression mangrove peats developed during initial flooding of the platforms glovers ca 8 5 ky lighthouse ca 7 ky turneffe ca 6 ky bp as water depths increased reefs colonized platform margins lagoonal circulation improved thereby promoting carbonate production the basal lagoonal carbonate sediments are characterized by shell beds and or halimeda packstones grainstones mollusk dominated wackestones and packstones follow upsection in glovers and lighthouse reefs at present open circulation prevails in glovers and lighthouse lagoons in contrast organic rich halimeda wackestones and packstones dominate the turneffe islands holocene succession the main lagoon area of turneffe is enclosed by mangroves and restricted circulation prevails factors that explain the differences in geomorphology circulation and facies are variations in depth of antecedent topography and degree of exposure to waves and currents the thickness of holocene lagoon sediments may exceed the maximum core length of 6 m in all atolls holocene sedimentation rates average 0 6 m ky with highest rates in turneffe 0 82 m ky followed by lighthouse 0 53 m ky and glovers 0 46 m ky like in many other isolated carbonate platforms and atolls lagoon floor sedimentation did not keep pace with rising sea level leading to unfilled accommodation space at present glovers has an 18 m deep lagoon while lighthouse and the main turneffe lagoon are 8 m deep it is unlikely that the lagoons will be completely filled during the holocene sea level highstand cycle this observation should be kept in mind when using cycle thickness as a proxy for eustatic sea level change in fossil carbonate platforms

carbonate systems along nutrient and temperature gradients some sedimentological and geochemical constraints research over the past several decades has clearly demonstrated that changes in the ocean environment have had major impacts on carbonate systems changes in climate ocean circulation and seafloor spreading rates have influenced temperature and seawater chemistry including carbonate saturation state and nutrient availability and thereby have determined boundary conditions for the biota that form carbonate platforms in turn the biota determine accumulation rates and facies zonations thus controlling platform geometry and facies dynamics in the first section of this paper we examine how nutrient availability influences carbonate facies associations we first discuss the role of temperature and nutrient gradients in the modern ocean and their influence on biotic associations then we discuss how carbonate sedimentation can be characterized along nutrient gradients in the second section we review proxies currently used to reconstruct paleoproductivity in open ocean environments and discuss their applicability to neritic carbonate systems we highlight the variety of existing proxies and their limitations and suggest that multiple lines of evidence are needed for valid interpretations our short review discusses sedimentological biogenic and geochemical proxies that can be used to reconstruct past nutrient fluxes and to constrain paleoceanographic controls over the distribution of carbonate associations however it also reveals that more data and case studies are needed that integrate shallow and deep water carbonate sequences and elucidate the links between temperature vs nutrient supplies changes and facies in ancient carbonate sequences

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 paradox of drowned reefs and carbonate platforms shallow water carbonate platforms and reefs are drowned when tectonic subsidence or rising sea level outpaces carbonate accumulation and benthonic carbonate production ceases drowned platforms are common in the geologic record but they present a paradox if one considers rates of processes involved during the early holocene when sea level rose at rates of 6 000 to 10 000 m yr mm 1 000 yr most reefs and platforms were outpaced by the rising sea during the late holocene with sea level rising 500 to 3 000 m yr in the atlantic caribbean area reefs and platforms started to recover built to sea level and prograded seaward 1 000 m yr is thus a conservative estimate of the average growth potential of modern reefs and platforms independently accumulation rates of prograding platforms in the geologic record suggest growth potential in excess of several hundred microns per year the growth potential of 1 000 m yr exceeds any relative rise of sea level caused by long term processes in the geologic record newly formed ocean crust subsides at a maximum of 250 m yr basin subsidence averages 10 to 100 m yr and sea level rises due to increased sea floor spreading amount to less than 10 m yr rapid pulses of relative rise of sea level or reduction of benthic growth by deterioration of the environment remain the only plausible explanations of drowning the geologic record shows examples of both of these processes global mass extinctions of reefs and platforms occurred in the middle cretaceous eustatic rise due to submarine volcanism or desiccation of a small ocean basin and the late devonian global crisis of ocean environment extraterrestrial cause drowning controlled by regional tectonics prevailed in the jurassic and early cretaceous of the tethyan realm and the drowning of mesozoic platforms in the western north atlantic seems to have been dictated by plate tectonic drift to higher latitudes

analyzing spatial patterns in modern carbonate sand bodies from great bahama bank abstract three carbonate sand bodies on great bahama bank which show a range of depositional facies patterns typifying modern deposits as well as their ancient counterparts are quantitatively interrogated to broaden our perspective of the types of information that can be derived from studies of modern environments rimming the southern end of tongue of the ocean toto is the broadest expanse of high energy sands found in the bahamas characterized by narrow sandbars separated by wide deep channels and a lack of islands a variation of the tidal bar motif with broader and more irregular sandbars relatively narrow channels and few small islands occurs at the northern end of exuma sound schooners sands associated with tidal channels and the numerous islands of the exumas chain along the western edge of exuma sound occur primarily as flood tidal deltas the geometry of sandbars that inhabit the three sand bodies is quantified using landsat remote sensing and assembled along with optically derived bathymetric surfaces into a gis this database is quantitatively examined using a suite of morphometric tools to compare and contrast geometric character of sandbars within and between the three sand bodies considered are properties such as the size shape complexity distribution orientation and topography of the individual sandbars profiles and spatial analysis tools enable sandbar and channel spacing position relative to the platform margin connectedness separation distances and density to be characterized as has been previously reported for reef dominated environments certain aspects of the geometry of the three systems are found to behave in a systematic and hence predictable manner though important mathematical differences are revealed between the scaling of reefal landscapes and the grainy geobodies considered by this study because the three sand bodies are disparate in their overall extent and depositional settings e g orientation prevalence of islands this predictable behavior has the potential to impart considerable insight to the characterization of grainstone systems

the uncertainties involved in extracting amplitude and frequency of orbitally driven sea level fluctuations from shallow water carbonate cycles in outcrops shallow water carbonates often form thick vertically stacked metre scale exposure bounded depositional packages here called carbonate cycles because the facies within carbonate cycles can be tied precisely to water depths they are considered ideal for reconstructing past sea level changes there is however increasing evidence that these depositional cycles are unreliable recorders of both the frequency and the amplitude of orbitally driven sea level fluctuations carbonate cycles record only a fraction of the amplitude of a sea level cycle a portion of the rise and nothing of the fall cores through the pleistocene cycles on great bahama bank illustrate this shortcoming although the amplitudes of the last nine sea level changes are each a hundred metres or more the thickness of the cycles varies from a few metres to ca 15 m the lack of correlation between sea level amplitude and cycle thickness is not eliminated for cycles deposited during times of lower sea level amplitudes for example the cretaceous upper cretaceous cycles on the maiella platform margin document the irregularly filled accommodation space and the resultant variability in cycle thickness and frequency uncertainties in assessing the frequencies of sea level changes from shallow water carbonate cycles are caused by missed beats and metre scale oscillations of sea level within highstands that potentially produce cycles of very short duration the random amplitude variability during the last 57 glacio eustatic sea level changes illustrates the difficulty of assessing missed beats where a sea level fluctuation is not recorded because the sea level rise does not reach the platform top missed beats are also produced by the depositional topography that is created by irregularly filled accommodation space as a result variable numbers of cycles are deposited across the platform further complicating orbital frequency analyses are decametre scale oscillations of the sea level during highstands the amplitude of these sub orbital sea level oscillations up to 17 m within the last interglacial marine isotope stage 5e are sufficient to expose shallow platforms like great bahama bank and subsequently produce an additional depositional cycle with similar facies successions the combined effects of missed beats and oscillations within highstands are likely to produce cycles and hiatuses of variable duration that are difficult to extract from the rock record consequently estimates of the orbital forcing mechanisms and frequencies from ancient shallow water carbonate cycles carry large uncertainties

meridional overturning circulation in the south china sea envisioned from the high resolution global reanalysis data glba0 08 the pattern of meridional overturning circulation moc in the south china sea scs is studied using a numerical lagrangian tracing method with the hycom ncoda global 1 12 degrees analysis glba0 08 data the scs moc has a sandwich structure which consists of a layer of stronger clockwise circulation above 500 m depth a counterclockwise layer in the mid layer between 500 and 1000 m depth and a weaker clockwise layer below 1000 m the deep below 1000 m depth clockwise layer is divided into three cells namely the deep southern moc cell dsmoc the deep middle moc cell dmmoc and the unclosed deep northern moc cell dnmoc the inflow through the luzon strait is the main source for the scs mocs the upper layer luzon strait inflow dominates the upper scs moc structure but has relatively less contribution to the dnmoc whereas the deep layer luzon strait inflow mainly influences the dnmoc and it mostly rises near 18 degrees n the inflow through the taiwan strait mainly contributes to the upper layer moc moreover inflows from the mindoro and karimata straits contribute negatively to the upper moc but play a significant role on the dsmoc the backward integration of lagrangian trajectories further validates that the scs deep water comes not only from the deep inflow but also from the entrainment of the middle and upper layer inflow through the luzon strait in the scs basin there are three northwest southeast tilted zones where tracers upwell which correspond to the three deep moc cells one possible mechanism for these upwelling zones is the interaction between the continental slope trapped waves and the westward planetary rossby waves

deep south china sea circulation 1 the analysis of an updated monthly climatology of observed temperature and salinity from the u s navy generalized digital environment model reveals a basin scale cyclonic circulation over the deep south china sea scs the cyclonic circulation lies from about 2400 m to the bottom the boundary current transport of the cyclonic circulation is around 3 0 sv our results suggest that the cyclonic circulation is mainly forced by the luzon overflow with bottom topography playing an important role the structures of potential temperature salinity and potential density in the deep scs are consistent with the existence of the cyclonic circulation specifically low salinity water is found in the interior region west of luzon island and surrounded by saline pacific water in boundary current regions to the north west and southwest our results show the potential density distribution and the corresponding cyclonic circulation in deep scs are primarily controlled by salinity variations in the deep basin

disobedient sediments can feedback on their transportation deposition and geomorphology abstract most sediments are obedient to the winds waves and currents which direct their transportation and deposition it has long been recognized however that the grain size and or grain kind of sediments can feedback on the processes of their own transportation deposition and geomorphology as well as that of succeeding deposits this note is to review three examples of marine sediments in which a single grain size or grain kind produces multiple feedbacks tidal bars of holocene ooid sands on great bahama bank are an example of multiple feedbacks of one grain kind on tidal currents wave action and accumulations these feedbacks are responsible for the distinctive pattern of elongated bars and channels which in turn amplify tidal currents the near constant movement of grains on the shallow bars and in the channels is where pellet nuclei are coated to form ooids thus the development and growth of bars feeds back to produce more ooids regional encrinites which consist predominantly of the disarticulated highly porous skeletons of crinoids are common from ordovician to jurassic during blooms of these filter feeders the resulting grains which are equivalent to quartz grains a tenth of their size can be entrained by currents as low as a knot ca 0 5 m s the resulting mobile substrate deters other invertebrates taphonomic feedback and results in the prevalence of layering produced by traction transport of low velocity the belt of mud extending for some 1600 km between the amazon and orinocco rivers is a special example of the feedback of mud on depositional processes sedimentary structures and geomorphology of the accumulations the clay rich mud from the amazon produces fluid mud which dampens and transforms wave action from the open sea to promote its own accumulation in giant bars in the inner shelf and shoreline with a variety of familiar laminations the result is a wedge of mud rich deposits some 24 m thick and 30 km wide capped with cheniers of sand which has prograded seaward in some 6000 years

coral reef carbonate record of the pliocene pleistocene climate transition from an atoll in the south china sea abstract as a natural laboratory for testing various paleoclimatological and paleoceanographic hypotheses the pliocene pleistocene stratigraphic sequence from a full coring major scientific drilling well ck2 with total length 928 75 m in yongle atoll of xisha islands northern south china sea was studied the petrography of studied section of the core was dominated by reef carbonate limestones which contain abundant shallow marine organisms e g coralline algae corals and foraminifera which almost consisted of pure carbonates and preserved the primary geochemical message since pliocene according to the isolation of well ck2 and the provenance of elements the systematic changes and abrupt points of isotope and element geochemical compositions rare earth element parameters sio2 content al ti ratio etc from coral reef carbonate record presented the elevated terrigenous influences related to the east asian monsoon after 2 6 ma bp as part of the pliocene pleistocene climate transition our results demonstrated that the continental weathering controlled by the east asian summer monsoon dominated the surface seawater compositions of study area before 2 6 ma bp when the winter monsoon was not strong enough to exert significant influences then the effects of eolian dust associated with the intensity of east asian winter monsoon the precipitation driven by summer monsoon and the global temperature which played different roles at different periods after 2 6 ma bp should be considered our results confirmed that the potentials and significances of shallow water carbonate deposition from coral reefs and provided a unique perspective to shed light on the carbonate record of climate changes

neogene carbonate platform development in the nw south china sea litho bio and chemo stratigraphic evidence abstract the variations of lithostratigraphy foraminifera geochemical elements and organic biomarkers in well xk1 from the xisha paracel islands reveal the history of carbonate platform development since the early miocene in the south china sea our results indicate that reef growth was more active in the middle miocene than in any earlier periods decreased in the late miocene revitalized in the pliocene and active again in the pleistocene interglacial periods unlike those limestone rich intervals the late miocene succession in well xk1 is characterized by dolomitized limestones with a low deposition rate 30 60 m ma post compaction these late miocene rocks also contain relative high mgo 20 more ree abundant algae and some small sized foraminifera but rare larger benthic foraminifera very low land sourced brgdgts 0 0 5 ng 20 g low bit index 0 1 0 5 and low tex86 derived sea temperature 20 25 c indicating limited active reef growth in relative cooler mesophotic environments at 30 m water depths similar dolomitized limestones of late miocene age also occur in other carbonate platform areas including the nansha spratly islands in the southern south china sea together these south china sea records provide regional evidence of a recession in global carbonate platform development starting from the late miocene while carbonate platform reduction in other regions may have been triggered by global cooling and changing oceanic chemistry accelerated basin subsidence coupled with monsoon cooling are considered as the main factors causing the reef decline in the late miocene south china sea

seismic characteristics of a reef carbonate reservoir and implications for hydrocarbon exploration in deepwater of the qiongdongnan basin northern south china sea our analysis of approximately 40 000 km of multichannel 2 d seismic data reef oil field seismic data and data from several boreholes led to the identification of two areas of reef carbonate reservoirs in deepwater areas water depth 500 in of the qiongdongnan basin qdnb northern south china sea high resolution sequence stratigraphic analysis revealed that the transgressive and highstand system tracts of the mid miocene meishan formation in the beijiao and ledong lingshui depressions developed reef carbonates the seismic features of the reef carbonates in these two areas include chaotic bedding intermittent internal reflections chaotic or blank reflections mounded reflections and apparent amplitude anomalies similar to the seismic characteristics of the lh11 1 reef reservoir in the dongsha uplift and island reef of the salawati basin indonesia which house large oil fields the impedance values of reefs in the beijiao and ledong lingshui depressions are 8000 9000 g cc x m s impedance sections reveal that the impedance of the lh11 1 reef reservoir in the northern south china sea is 800010000 g cc x m s whereas that of pure limestone in bd23 1 1 is 10000 g cc x m s the mid miocene paleogeography of the beijiao depression was dominated by offshore and neritic environments with only part of the southern beijiao uplift emergent at that time the input of terrigenous sediments was relatively minor in this area meaning that terrigenous source areas were insignificant in terms of the beijiao depression reef carbonates were probably widely distributed throughout the depression as with the ledong lingshui depression the combined geological and geophysical data indicate that shelf margin atolls were well developed in the beijiao depression as in the ledong lingshui depression where small scale patch or pinnacle reefs developed these reef carbonates are promising reservoirs representing important targets for deepwater hydrocarbon exploration c 2008 elsevier ltd all rights reserved

growth and demise of cenozoic isolated carbonate platforms new insights from the mozambique channel seamounts sw indian ocean although long term evolutions of isolated shallow water carbonate platforms and demise episodes leading to guyot formation have been the subject of numerous studies during the last decades their driving processes are still the subject of active debates the mozambique channel sw indian ocean is characterized by several flat topped seamounts ranging from 11 s to 21 s in latitudes based on a comprehensive geomorphologic study and on dredged samples analysis we show that these features correspond to tropical isolated shallow water carbonate platforms coupling strontium isotopy and foraminifera biostratigraphy well constrained chronostratigraphy results indicate that shallow water carbonate production started in the mozambique channel during distinct cenozoic periods ranging from paleocene to early miocene our data also demonstrate that these carbonate platforms were subsequently characterized by different evolutions locally marked by tectonic and rejuvenated volcanism while some of them kept developed until present days forming modern carbonate systems some others were drowned during late neogene and subsided to form guyots although different factors can be discussed tectonic and volcanism appear as good potential triggers for demise episodes during late miocene early pliocene times chronology and location of this geodynamical activity tend to emphasize influence of east african rift system until southern mozambique channel

drowning history of a miocene carbonate platform zhujiang formation south china sea abstract the identification and interpretation of drowning events in the geologic record can aid significantly to the reconstruction of the depositional tectonic and eustatic history of a study area and often improve reservoir and seal prediction in carbonate rocks the differentiation between drowned platforms showing a record of continuous deepening and those with a record of exposure followed by rapid deepening remains however problematic the zhujiang carbonate platform liuhua 11 1 field south china sea study shown here provides an example of an integrated approach combining high resolution geochemistry microfacies analyses and foraminiferal biostratigraphy in order to improve the reconstruction of environmental conditions prior during and after platform demise and drowning the zhujiang carbonate platform displays the following vertical succession of four facies types i skeletal grain facies with a miogypsinid lepidocyclinid dominated fauna deposited in a moderately deep heterostegina sp and spiroclypeids possibly capped by a subaerial exposure surface well rounded rhodoids representing a mesotrophic lagoon dominate the upper portions of the rhodoid facies iv pelagic marine shales of the hanjiang formation burying the carbonate platform after drowning this facies succession in combination with geochemical evidence suggests a deepening upward trend this trend might have been interrupted by transient subaerial exposure but no evidence for meteoric diagenesis was found at the drowning unconformity topping the carbonate platform instead microfacies analyses suggest that platform demise may be related to progressive changes in environmental conditions including increasing nutrient levels and or decreasing temperature up core towards the drowning unconformity these findings are of significance for those concerned with miocene carbonate factories and more specifically the demise of carbonate platforms in general

bahamian carbonate platform development in response to sea level changes and the closure of the isthmus of panama in this paper we show that the development of the sediment architecture at the leeward toe of slope of great bahama bank ocean drilling project leg 166 bahama transect during the last 6 ma is not only a response to sea level fluctuations but also to major paleo oceanographic and climatic changes a major sequence boundary close to the miocene pliocene boundary dated at 5 6 5 4 ma is interpreted to reflect a major sea level drop that was followed by a sea level rise which led to the re flooding of the mediterranean sea at the end of the messinian and increasing sea surface temperatures at great bahama bank distinct erosional horizons occurred during the pliocene dated at 4 6 and 3 3 3 6 ma related to sea level change and the intensification of the gulf stream when the emergence of the isthmus of panama reached a critical threshold the gulf stream brings warm saline and nutrient poor waters to the bahamas starting at the early late pliocene boundary at 3 6 ma this paleo oceanographic reorganization in combination with enhanced sea level fluctuations associated with the late pliocene main intensification in northern hemisphere glaciation since 3 2 ma led to 1 a gradual change from a ramp type to a flat topped type morphology and 2 a change from a skeletal to a non skeletal dominated sedimentary system mainly peloidal increased sea level fluctuations during the second half of the pleistocene led to an intensified high stand shedding depositional pattern within the surrounding basins

early history of the atlantic ocean and gas hydrates on the blake outer ridge results of the deep sea drilling project leg 76 leg 76 of the deep sea drilling project achieved two major scientific objectives the first objective was met at site 533 where on the blake outer ridge gas hydrates were identified by geophysical geochemical and geological studies gas hydrate decomposition produced a volumetric expansion of 20 1 of gas volume to pore fluid volume this expansion exceeded by about a factor of four the volume of gas that could be released from solution in pore water under similar conditions the gas hydrate includes methane ethane propane and isobutane but apparently excluded normal butane and higher molecular weight hydrocarbons as predicted from gas hydrate crystallography for the first time marine gas hydrates were tested with a pressure core barrel the second objective was achieved when coring at site 534 in the blake bahama basin sampled the oldest oceanic sediments yet recovered the sequence of oceanic basement and overlying sediments documents the geologic history of the early stages of the opening of the north atlantic ocean in detail the oldest oceanic sediments are red claystones and laminated green and brown claystones of middle callovian age this finding supports the interpretation that the beginning of the modern north atlantic occurred in the early callovian 155 m y b p as much as 20 m y later in time than often previously thought

palynofacies investigation of callovian middle jurassic sediments from dsdp site 534 blake bahama basin western central atlantic abstract during the extension of deep sea drilling project dsdp leg 76 a new and previously unpenetrated lithological unit composed mainly of claystones was cored above basalt basement at site 534 in the blake bahama basin the callovian part of the new unit contains interbedded black shales which were hitherto unexpected in this part of the section this paper presents a brief palynological examination of lithofacies kerogen relationships in these sediments and shows that their organic content is almost entirely a function of the re deposition of terrestial and marine organic matter versus the ambient redox conditions of the depositional environment allochthonous organic matter inputs are highest in the interbedded turbidites and decline progressively toward the pelagic black shales in which marine organic matter is comparatively well preserved the significance of various kerogen and palynomorph indices are discussed the study emphasizes the absolute necessity for sedimentologically aware sampling in all palynological and geochemical work on lithologically heterogeneous sequences

facies model on the modern isolated carbonate platform in the xisha archipelago south china sea abstract the xuande atoll with a discontinuous reef rim caused by platform drowning is a typical ellipse isolated platform on the northern slope of the south china sea based on the data of drilling holes the analysis of surface sediments and high resolution acoustic data six sedimentary facies are identified in the xuande atoll including reef outer slope reef front reef flat aeolianite sand island back reef slope and lagoon facies map shows that the elevated reef rimmed margins with islands are more noticeable on the windward of the platform with an elevated and discontinuous reef rimmed margin as well as coarse grained sediment in the interior platform xuande can serve as a case of a leaky bucket atoll which is characterized by initially drowning scenarios the facies architecture of the xuande atoll is evidently influenced by complex and interacting parameters including antecedent topography sea level wind driven currents water depth and tectonic subsidence different controlling factors exist in the drowning of the windward and leeward sides of the platform margin the study of modern depositional settings in the xuande atoll will provide insight on predictions for ancient carbonate provinces

recent sedimentary facies of isolated carbonate platforms belize yucatan system central america abstract distribution of recent sedimentary facies is significantly different in four closely spaced isolated carbonate platforms off the coasts of belize and yucatan se mexico these differences are largely controlled by variations in submarine topography which are likely the result of differential subsidence and karstification from platform to platform a variety of depositional styles and patterns is found in platform interiors that developed during the same holocene sea level rise composition and texture of surface sediment samples were used to define four major sediment types and to distinguish nine depositional environments within the platforms facies maps are based on analysis of 454 sediment samples and high resolution satellite imagery glovers reef is characterized by a circular facies distribution the nearly continuous reef margin is composed of a coral red coralline algae halimeda grainstone mixed nonskeletal peloidal skeletal wackestones and packstones are present in shallow lagoon parts 5 m water depth behind the margin the deep interior from 5 to 18 m water depth is characterized by mollusk foram halimeda wackestones with over 860 patch reefs of coral red algae halimeda mollusk packstones lighthouse reef also has a coral grainstone and coral red algae halimeda grainstone rim but the facies distribution in the interior is strongly asymmetrical the 8 m deep eastern lagoon is floored by mollusk foram halimeda wackestones and packstones the 3 m deep western lagoon has the mixed peloidal skeletal wackestone to grainstone facies a linear trend of coalescing patch reefs and skeletal grainstone facies separates the two lagoons banco chinchorro is similar to lighthouse reef inasmuch as facies distribution patterns of the interior are strongly asymmetrical mollusk foram halimeda wackestones grainstones are present in eastern lagoon areas whereas shallow 5 m western parts of the lagoon are composed of mixed peloidal skeletal grainstones packstones and wackestones most of turneffe islands is protected on the east by lighthouse reef accordingly turneffe has a narrower reef and skeletal grainstone rim than the other two platforms restricted interior lagoons are up to 8 m deep and are enclosed by wide mangrove rims and surface sediments dominated by halimeda rich wackestone rich in organic matter coral patch reefs are absent in contrast the unprotected northernmost part of turneffe has a wide reef and grainstone rim and the open lagoon area consists of mollusk foram halimeda wackestones packstones with abundant patch reefs

the impact of eustatic sea level fluctuations temperature variations and nutrient level changes since the pliocene on tropical carbonate platform xisha islands south china sea abstract sedimentological paleontological and geochemical data from core xk 1 in the xisha islands south china sea allow a detailed documentation of the evolution of the reef bank system from the early pliocene to the late pleistocene and of the aeolianite deposits from the late pleistocene to the holocene a bank characterized by heterozoan carbonates occurred until 1 7 ma between 1 7 and 0 2 ma the system turned into a reef dominated by photozoan carbonates after 0 2 ma the aeolianite deposits composed of heterozoan skeletal grains occupied this area a pliocene to pleistocene decrease in the abundance of planktonic foraminifera and some benthic foraminifer species generally associated with deep water settings indicates a long term shallowing upwards trend resulting from the large scale eustatic sea level falls seven exposure horizons are recorded in core xk 1 due to high frequency sea level changes the thickness of the reef body in the individual cycles increased after the middle pleistocene transition this is interpreted as a consequence of the increase in duration of the short term sea level highstands the formation of heterozoan aeolianite deposits after 0 2 ma is probably caused by climate cooling related with the intensification of east asian winter monsoon the rapid increase in coral abundance at 1 7 ma correlates well with a sharp decline in nutrient level recorded by decreases in geochemical nutrient indexes values declines in bryozoan and red algae rhodolith abundance and decreases in the abundance of foraminifer species indicative of elevated nutrient level this correlation indicates that the carbonate factory turnovers were also affected by nutrient level fluctuations which could be linked to a paleoceanographic reorganization these results demonstrate that together with sea level fluctuations also paleoceanographic changes had a significant effect on the pliocene holocene tropical carbonate system

architecture development and geological control of the xisha carbonate platforms northwestern south china sea newly acquired seismic data allow improved understanding of the architecture and evolution of isolated carbonate platforms on the continental slope of the northern south china sea the xisha carbonate platforms were initiated on a basement high the xisha uplift in the early miocene and have remained active up to the present their distribution is limited to pre existing localized fault bounded blocks within the xisha uplift so individual platforms were small in size at the beginning of the miocene however during the middle miocene the platform carbonate factories flourished across an extensive area with 55 900 km 2 the platforms began to backstep in response to a relative sea level rise in the late miocene platform edge reefs patch reefs pinnacle reefs atoll reefs and horseshoe reefs all developed on various platforms the distribution of platform carbonates shrank significantly during pliocene quaternary time to isolated carbonate platforms represented today by xuande atoll and yongle atoll tectonics and eustasy were the two main controls on platform development tectonics controlled both the initial topography for reef growth and the distribution of platforms including those that survived the drowning event associated with the late miocene rapid relative sea level rise eustasy controlled high frequency carbonate sequence development crown copyright c 2014 published by elsevier b v all rights reserved

carbonate platform facies models various types of carbonate platforms are characterized by distinctive profiles facies and evolutionary sequences ramps may be homoclinal or distally steepened and may have fringing or barrier shoal water complexes of ooid pellet sands or skeletal banks homoclinal ramps pass seaward into deeper water without major break in slope and lack deep water breccias distally steepened ramps may be low energy and characterized by widespread shallow subwave base mud blankets or high energy with coastal beach dune complexes and widespread skeletal sand blankets slope facies may contain abundant breccias of slope derived clasts rimmed shelves have relatively flat tops and marked break in slopes at the high energy shallow shelf edge where they pass into deep water such shelves may be aggraded with peritidal facies extending over much of the shelf or incipiently drowned depending on magnitude of sea level fluctuations they may be accretionary or bypass types that include gullied slope escarpment and high relief erosional forms intrashelf basins occur on some shelves controlling distribution of reservoir and source beds isolated platforms are surrounded by deeper water and may be located on rifted continental margins or on submarine volcanoes most have high relief rimmed margins platforms that have been subjected to rapid sea level rise may be incipiently drowned and characterized by raised rims elevated patch or pinnacle reefs and widespread subwave base carbonate or fine clastic blankets completely drowned shelves develop where the shelf is submerged to subphotic depths terminating shallow water deposition and commonly resulting in blanketing of the shelf by deeper water facies some margins show extensive down to basin faulting that is contemporaneous with carbonate deposition or associated with thick prograding clastic sequences the various types of platforms change in response to variations in sedimentation subsidence or sea level rise and may form distinctive evolutionary sequences the relatively few models presented appear to accommodate most geological examples some of which contain major reservoir facies

paleotemperature of the last interglacial period based on δ18o of strombus bubonius from the western mediterranean sea abstract the objective of the present study is to quantify sea surface temperature sst by measuring 18o in shells of strombus bubonius first we present data obtained from modern shells collected at different locations in the gulf of guinea where strombus bubonius is now geographically restricted islands of corisso bioko and pagalu next we give the results of our investigations of fossils found in two classical mediterranean sites monastir in tunisia and palma nova a pleistocene deposit on the island of majorca both sites have been assigned an age corresponding to the last interglacial period lip by stratigraphic correlation with terraces dated by the u th method the sampling scheme followed precisely the growth spiral of the mollusk shell in order to obtain time series of 3 4 years depending on the size of the specimen as expected the 18o records generally exhibit a cyclic pattern which can attributed of the seasonality of sst 18o sst s reconstructed for the modern shells are in rough agreement with sst values obtained from climatological maps which suggests that strombus shells may be useful in paleotemperature reconstructions the difference between 18o maxima and minima isotopic seasonality is generally in aggreement with the modern sst seasonality range inferred from climatological data this 18o sst seasonal temperature difference ranges from 2 c to 6 c for the modern shells from the gulf of guinea and from 7 c to 9 c for the mediterranean fossils which is not significantly different from modern seasonality at these locations the 18o records obtained from the fossils suggest that the sst s were higher by several c during the lip however the magnitude of this temperature difference is difficult to quantify precisely due to our lack of knowledge of the sea surface salinity sss distribution during the lip we chose two different working hypotheses 1 that the mediterranean sss s were the same as today and 2 that the sss s were typical for periods of sapropel deposition mean temperature differences of about 7 c and 3 c can be deduced under the first and the second working hypotheses respectively the second value is in agreement with other temperature proxies from marine and terrestrial environments which lends support to the second hypothesis

seismic characteristics and development of the xisha carbonate platforms northern margin of the south china sea the xisha carbonate platforms which include the modern day xisha atoll occur upon the northern continental margin of the south china sea scs in this study we identify the seismic characteristics about various sedimentary facies of the carbonate platform and different types of reef in this area based on the 2 d seismic data reefs show high amplitue moundy continuous reflection at the top and weak chaotic reflections inside carbonate platforms show high amplitude continuous reflection at the top and high reflection alternative with weak reflection the above analysis provided information about the influence of fluctuations in relative sea level tectonic movements paleotopography and the development mechanism of the carbonate platform based on the seismic data and data from drillholes xichen 1 xiyong 1 we propose a schematic sedimentary model of the xisha carbonate platforms at the northern margin of the scs and outline five stages of development for the carbonate platform its sedimentary evolution consists of initial establishment stage development stage exposure stage drowning stage and large atoll reefs development stage we also propose that several phases tectonisms supply proper environment and structural position for carbonate platform development they can also destroyed the exist platforms besides eustasy change was also the main influential factor on the development of the platforms c 2010 elsevier ltd all rights reserved

the transient nature of the empty bucket model of reef sedimentation abstract the geometry of modern reefs has been likened to an empty bucket with the narrow reef rim comprising the container pail that encloses the water filled bucket while the analogy is valid the assumption that this constitutes a final geometry is not examples from the indian pacific and atlantic oceans individually and collectively demonstrate current filling of the lagoons by reef perimeter sand aprons since sea level only approximated its present position 3 to 6 ky ago it follows that the observed extent of modern highstand lagoon filling is minimal if the filling process was to continue unabated the result would be a flat topped bank but during the pleistocene low stands of sea level aborted the filling process extrapolating these relationships to the geologic record suggests that the empty bucket model of unfilled reef lagoon space is greatest during icehouse high frequency glacial fluctuations of sea level alternatively filled buckets in which cycle thickness approximates accommodation space are probably largely restricted to greenhouse time intervals the co occurrence of quaternary high amplitude frequency sea level changes with rapidly growing scleractinian corals probably enhanced the occurrence of empty bucket morphologies

organic facies of middle cretaceous black shales in deep north atlantic the middle cretaceous black shales of the deep north atlantic consist of alternating layers of sediment rich in total organic carbon toc that contains abundant amorphous types of organic matter and sediment poor in toc that contains mostly woody and coaly types of organic matter the amorphous materials are derived mostly from marine organisms in the eastern north atlantic off africa in the caribbean and on the demerara rise off surinam and from higher land plants in the western north atlantic and off europe marine amorphous material is important off africa because this was an area of upwelling and highly productive surface water amorphous marine organic matter was deposited across the entire north atlantic basin in the cenomanian in response to an ocean wide upwell ng event connected with the opening of the equatorial atlantic the difference between the toc rich and background toc layers reflects deposition of the former under reducing conditions which allowed the preservation of labile amorphous materials of marine and terrestrial orgin and of the latter under oxidizing conditions when nearly all that was preserved was refractory organic material

the leaking bucket of a maldives atoll implications for the understanding of carbonate platform drowning abstract seismic and multibeam data as well as sediment samples were acquired in the south male atoll in the maldives archipelago in 2011 to unravel the stratigraphy and facies of the lagoonal deposits multichannel seismic lines show that the sedimentary succession locally reaches a maximum thickness of 15 20 m above an unconformity interpreted as the emersion surface which developed during the last glacial sea level lowstand such depocenters are located in current protected areas flanking the reef rim of the atoll or in infillings of karst dolinas much of the 50 m deep sea floor in the lagoon interior is current swept and has no or very minor sediment cover erosive current moats line drowned patch reefs whereas other areas are characterized by nondeposition karst sink holes blue holes and karst valleys occur throughout the lagoon from its rim to its center lagoonal sediments are mostly carbonate rubble and coarse grained carbonate sands with frequent large benthic foraminifers halimeda flakes red algal nodules mollusks bioclasts and intraclasts some of them glauconitic as well as very minor ooids finer grained deposits locally are deposited in current protected areas behind elongated faros i e small atolls which are part of the rim of south male atoll the south male atoll is a current flushed atoll where water and sediment export with the open sea is facilitated by the multiple passes dissecting the atoll rim with an elevated reef rim and tower like reefs in the atoll interior it is an example of a leaky bucket atoll which shares characteristics of incipiently drowned carbonate banks or drowning sequences as known from the geological record

development of the east asian monsoon mineralogical and sedimentologic records in the northern south china sea since 20 ma we here reconstruct the past change of the east asian monsoon since 20 ma using samples from ocean drilling program odp site 1146 in the northern south china sea based on a multi proxy approach including a monomineralic quartz isolation procedure identification of clay minerals by x ray diffraction xrd and grain size analysis of isolated terrigenous materials terrigenous supply to odp site 1146 was dominated by changes in the strength of multiple sources and transport processes grain size data modeled by an end member modeling algorithm indicate that eolian dust from the and asian inland and fluvial input have contributed on average 20 and 80 of total terrigenous material to odp site 1146 respectively specifically about 40 53 of the total quartz feldspar and only 6 11 of the total clay is related to eolian supply at the study site detailed analysis of the sedimentary environment and clay minerals combined with previous studies shows that smectite originates mainly from luzon kaolinite from the pearl river and illite and chlorite from the pearl river taiwan and or the yangtze river the proportion and mass accumulation rate mar of the coarsest end member em1 interpreted as eolian dust ratios of illite chlorite smectite quartz feldspar and mean grain size of terrigenous materials at odp site 1146 were adopted as proxies for east asian monsoon evolution the consistent variation of these independent proxies since 20 ma shows three profound shifts in the intensity of east asian winter monsoon relative to summer monsoon as well as aridity of the asian continent occurred at similar to 15 ma similar to 8 ma and the youngest at about 3 ma in comparison the summer monsoon intensified contemporaneously with the winter monsoon at 3 ma the phased uplift of the himalaya tibetan plateau may have played a significant role in strengthening the asian monsoon at similar to 15 ma 8 ma and 3 ma c 2007 elsevier b v all rights reserved

on long term net flow over great bahama bank abstract a 398 day time series of middepth current measurements is combined with available wind and bottom pressure measurements and historical salinity data to characterize long term net flow patterns over great bahama bank between the tongue of the ocean and exuma sound the progressive vector diagram constructed from current measurements indicates a resultant flow of just over 2 cm s 1 toward 002 semidiurnal tides dominate the instantaneous current a one dimensional numerical model is used to investigate the local response to regional forcing results suggest that the long term net flow is a response to the combined effect of wind driven and density driven flow toward the northwest and a tide induced residual current toward the east southeast aided by a quasi steady barotropic pressure gradient toward the north northeast the slope of the sea surface is specified according to its ability to reproduce measurements under steady state conditions the model reproduces the middepth flow and indicates

seismic geomorphology and evolution of early mid miocene isolated carbonate build ups in the timor sea north west shelf of australia abstract analysis of extensive 2d and 3d seismic data reveals the presence of about sixty isolated carbonate build ups icbs of early mid miocene age spreading over a wide area of the northwestern bonaparte basin individual build ups are 100 m thick with an average diameter of 3 km integration of full volume 3d seismic attribute analysis with extensive biostratigraphic and lithological data from exploration wells allows examining the evolution of icbs and their associated morphologies at high resolution the development of icbs took place above a seismically flat surface the stratigraphic architecture of the build ups typically consists of 1 mid burdigalian initiation tf1 cn2 2 lateral expansion during the late burdigalian cn3 and 3 backstepping and drowning during the langhian tf2 cn4 this is followed by sub aerial exposure that probably corresponds to a major sea level fall during the serravallian only small patch reefs pinnacles developed afterwards during the late miocene the observed growth phases of the icbs correlate with global sea level fluctuations and major changes in global climate and oceanography the role of local tectonics in the initiation and demise of these build ups appears as minimal growth of the northwestern bonaparte basin icbs is synchronous with the establishment of tropical carbonate factories elsewhere along the australia s north west shelf that suggest an acme of reef development during the early to middle miocene

cenozoic carbonates in southeast asia implications for equatorial carbonate development cenozoic equatorial carbonate production was at its most extensive and diverse in the seas of southeast asia yet the carbonates of this region remain poorly documented this paper presents the first published review of carbonate deposition during the cenozoic collating sedimentological data for carbonate successions throughout the region current models of warm water carbonate platform evolution sedimentation and facies distribution inspired by studies of the classic sub tropical areas provide inadequate analogues for the evaluation of modern or cenozoic equatorial carbonates in southeast asia the equatorial carbonates differ in being dominated by bioclasts coated grains and associations with evaporites or dolomites are rare the carbonate systems occur in a range of depositional settings which were often affected by coeval tectonism siliciclastic input or volcanism an understanding of the carbonate depositional environments spatial facies distributions and controls on deposition and diagenesis is essential in order to characterize equatorial carbonate development and to evaluate their considerable economic potential

foraminiferal isotopic evidence for monsoonal activity in the south china sea a present lgm comparison abstract the relationship between planktonic and benthic foraminiferal stable isotope values and oceanographic conditions and factors controlling isotopic variations are discussed on the basis of oxygen and carbon isotopic analyses of 192 modern surface and last glacial maximum lgm samples from the south china sea scs the harmonic variation of benthic 18o in surface sediments with water depth and temperature implies that the temperature is the main factor influencing benthic 18o variations planktonic 18o fluctuates with sea surface temperature sst and salinity sss the n s temperature gradient results in planktonic 18o decreasing from the northeast to the south cool saline waters driven by the winter monsoon are interpreted to have been responsible for the high 18o values in the northeast scs the east asian monsoons not only bring nutrients into the south china sea and maintain high nutrient concentration levels at the southwestern and northeastern ends which cause depleted 13c both in planktonic surface and benthic bottom samples but also reduce planktonic benthic 18o differences the distribution of 18o and 13c in the surface and lgm samples are strikingly similar indicating that the impact of sst and sss has been maintained and nutrient inputs mainly from the northeastern and southwestern ends have been controlled by monsoons since the lgm comparisons of the modern and lgm 18o indicate a difference of about 3 6 c in bottom water temperature and a large surface to bottom temperature gradient during the lgm as compared to today

sea level changes and carbonate platform evolution of the xisha islands south china sea since the early miocene abstract analyses of the biogeochemical branched and isoprenoid tetraether bit index and elemental geochemistry from well xk1 provide insight into the development of the xisha islands carbonate platform in the south china sea scs since the early miocene bit is the ratio of branched glycerol dialkyl glycerol tetraethers bgdgts to isoprenoid glycerol dialkyl glycerol tetraethers igdgts which are derived from meteoric and marine environments respectively bit serves as a novel proxy for tracking sea level changes the bit curve of well xk1 is characterized by low high low high alternating stages indicating the superposition of sea level changes on carbonate platform evolution in the scs following the rapid expansion of the scs in the early stages of the early miocene biogenic reefs began to form and bit oscillated intensely the scs started to regress after its initial expansion during the middle miocene at which time global sea level was also falling significantly the combination of both tectonic activity and eustatic variation has resulted in the deposition of strata with reef beach facies which are characterized by high bit values relative sea level reached its lowest point in the late stages of the middle miocene 11 6 ma subsequent sea level rise between the late miocene and pliocene induced a negative shift in bit and lagoonal facies formed under optimal warm marine conditions periodic exposure caused the reefs of the xisha islands to suffer from erosion during the pleistocene which is consistent with a second elevation in the bit index however modern trends show increasing regional water depths again major and trace element curves na si p b ga mo zn as well as the associated ratios na2o k2o na2o sio2 b ga ti sr zr sr al sr th u accurately record sea level variations several significant discontinuities correlate with sea level shifts and provide important details about the geomorphology sedimentation patterns and diagenesis types on the scs carbonate platform the agreement between organic and inorganic geochemical data demonstrates that our reconstructions of scs sea level changes and modes of carbonate platform development are robust our findings also demonstrate that carbonate evolution in the xisha islands was shaped by relative sea level changes implying that sea level fluctuations in the scs were controlled by global eustatic factors in addition to regional tectonic subsidence

onset of asian desertification by 22 myr ago inferred from loess deposits in china the initial desertification in the asian interior is thought to be one of the most prominent climate changes in the northern hemisphere during the cenozoic era1 2 3 4 but the dating of this transition is uncertain partly because desert sediments are usually scattered discontinuous and difficult to date here we report nearly continuous aeolian deposits covering the interval from 22 to 6 2 million years ago on the basis of palaeomagnetic measurements and fossil evidence a total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils this new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of asia by the early miocene epoch at least 14 million years earlier than previously thought3 5 regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in asia

strontium isotope stratigraphy and paleomagnetic age constraints on the evolution history of coral reef islands northern south china sea understanding the history of the response of coral reefs to past climate changes can provide valuable information for predicting the future response of modern reefs however dating such ancient biotic carbonate is still challenging because of its sensitivity to diagenetic alteration processes scarcity of well preserved fossils and low magnetic mineral content there have been a long debates about the origin and evolutionary history of coral reefs in the northern south china sea mainly due to the lack of direct and reliable age constraints this provides us with a good opportunity to verify the practicability of different dating approaches especially the strontium sr isotope analysis of bulk carbonate here we retrieved a 972 55 m long core from the xisha islands to provide a credible chronologic constraint on the carbonate platform evolution the lithostratigraphy strontium isotope stratigraphy and magnetostratigraphy were analyzed throughout the whole reef sequence the lithostratigraphic results show that the 873 55 m reef sequence developed on an ancient volcaniclastic basement and experienced multiple evolutionary phases the sr 87 sr 86 results of all 100 bulk carbonate samples vary from 0 708506 to 0 709168 and show a monotonic increase with decreasing depth except for a few outliers trace element criteria and stable isotope delta o 18 and delta c 13 methods were applied to these bulk carbonate samples and results imply that the primary or near primary seawater sr 87 sr 86 values were likely preserved although different degrees of diagenetic alteration occurred in addition the paleomagnetic results indicate 10 normal polarity and eight reversed polarity magnetozones based on the sr 87 sr 86 ratios of the selected 58 samples and paleomagnetic reconstruction of polarity reversals the bottom of the reef sequence is dated to 19 6 ma and the observed polarity chronozones extend from chron c6 19 722 18 748 ma at 866 60 m to present at the top based on the new data we propose a new chronologic framework for the evolutionary history of the reef islands where 1 the reefs initiated in the early miocene 19 6 ma and were drowned until 16 26 ma 2 during 16 26 10 66 ma lagoon to lagoon slope environments prevailed 3 the lagoon environment progressively transformed into a reef crest environment from 10 66 to 4 36 ma and 4 36 to 1 59 ma and 4 the reef started to be drowned again during 1 59 0 ma our study provides a new and reliable chronologic constraint on the general evolutionary history of the reef islands in the northern south china sea furthermore the sr 87 sr 86 results from bulk carbonate indicate that strontium isotope stratigraphy is a powerful dating tool only when rigid sample selection sequential leaching procedures and strict trace element and isotopic criteria are applied

comparison of milankovitch periods between continental loess and deep sea records over the last 2 5 ma abstract a high resolution east asian winter monsoon proxy record reconstructed from the baoji loess section in china shows two major shifts in climate modes over the past 2 5 ma one occurring at about 1 7 1 6 ma bp and the other at about 0 8 0 5 ma bp the 1 7 1 6 ma shift is characterized by a rather abrupt transition of winter monsoon variability from various periodicities to dominant 41 ka cycles and accompanied by a substantial increase in intensity of winter monsoon winds as manifested by an increase in average loess grain size the 0 8 0 5 ma event shows a relatively gradual transition from constant 41 ka cycles to predominant 100 ka climatic oscillations with a significant increase in amplitude the 0 8 0 5 ma shift matches that registered in deep sea 18 o records whereas the 1 7 1 6 ma shift is absent in global ice volume changes this comparison suggests that at about 1 6 ma bp the ice sheets in the northern hemisphere may have reached a critical size sufficient to modulate changes in the global climate system the discrepancy of climate cyclicity between loess and deep sea records over the 2 5 1 6 ma interval suggests that the older matuyama climate evolution cannot be understood simply by a regular 41 ka cycle model on a global scale more long proxy records derived from continental deposits are needed

seismic stratigraphy of the central south china sea basin and implications for neotectonics coring logging data and physical property measurements from international ocean discovery program expedition 349 are integrated with and correlated to reflection seismic data to map seismic sequence boundaries and facies of the central basin and neighboring regions of the south china sea first order sequence boundaries are interpreted which are oligocene miocene middle miocene late miocene miocene pliocene and pliocene pleistocene boundaries a characteristic early pleistocene strong reflector is also identified which marks the top of extensive carbonate rich deposition in the southern east and southwest subbasins the fossil spreading ridge and the boundary between the east and southwest subbasins acted as major sedimentary barriers across which seismic facies changes sharply and cannot be easily correlated the sharp seismic facies change along the miocene pliocene boundary indicates that a dramatic regional tectonostratigraphic event occurred at about 5ma coeval with the onsets of uplift of taiwan and accelerated subsidence and transgression in the northern margin the depocenter or the area of the highest sedimentation rate switched from the northern east subbasin during the miocene to the southwest subbasin and the area close to the fossil ridge in the southern east subbasin in the pleistocene the most active faulting and vertical uplifting now occur in the southern east subbasin caused most likely by the active and fastest subduction obduction in the southern segment of the manila trench and the collision between the northeast palawan and the luzon arc timing of magmatic intrusions and seamounts constrained by seismic stratigraphy in the central basin varies and does not show temporal pulsing in their activities

composition and diagenesis of pleistocene aeolianites at shidao xisha islands implications for palaeoceanography and palaeoclimate during the last glacial period abstract pleistocene aeolianites on shidao xisha islands are one of the rare aeolianite successions found in asia there the aeolianites which unconformably overlie pleistocene coral reefs marine isotope stage 5e are 21 8 m thick and formed largely of grains derived from heterozoan organisms including foraminifera predominantly amphistegina geniculate red algae echinoderms and molluscs the aeolian sands were probably derived from a subtropical shallow subtidal environment the variation in the distribution of metastable aragonite and high mg calcite hmc divides the aeolianite succession in core xk 1 into three intervals which are unit 1 11 5 21 8 m unit 2 3 11 5 m and unit 3 upper 3 m of core the entire aeolianite succession underwent diagenetic alterations in relatively closed diagenetic environments with low water rock ratios compared to the aeolianites in units 1 and 3 however the aeolianites in unit 2 display lower amount of hmc lower sr and mg concentrations and more negative 18o values of bulk samples this evidence demonstrates that unit 2 underwent more extensive diagenetic alteration probably due to the invasion of larger volumes of meteoric water correlation of the aeolianites in xk 1 to the aeolianites in surface exposures and subsurface core indicates that most shidao aeolianites were deposited in mis 3 the dry cold climate combined with strong onshore winds blew the exposed marine sediments onshore where they formed the aeolian dunes the palaeosols present in the shidao aeolianite succession indicate multiple episodes of subaerial exposure with relatively more humid climate collectively the results imply that aeolianites in ancient rock record can be used to examine the variations in palaeoceanography and the palaeoclimate