Late Quaternary polycystine radiolarian datum events in the Sea of Okhotsk

Five radiolarian datum events have been recorded in the Late Quaternary sediment section from the Sea of Okhotsk, using age assignments based on oxygen-isotope stratigraphy: the last occurrence datum (LOD) of Stylacontarium acquilonium (at about 329 ka), the LOD of Spongodiscus sp. (at about 287 ka), the LOD of Amphimelissa setosa (at about 72 ka), the LOD of Lychnocanoma nipponica sakaii (at about 28 ka), and the L. nipponica sakaii acme event (at about 72 ka). The LODs of S. acquilonium, Spongodiscus sp. and A. setosa appear to be synchronous in comparison with those in the Subarctic North Pacific, but the LOD and the acme event of L. nipponica sakaii show an apparent diachroneity with the corresponding North Pacific events.     

晚第四纪南海北部陆坡沉积物常量元素比值对气候变冷事件的指示意义

南海北部陆坡柱状沉积物的常量元素统计分析表明, 沉积物中的Ti、Al、Fe、K、Mn、Mg代表了陆源元素组合;而消除粒度效应的陆源元素比值Al/Ti、Fe/Ti和K/Ti等的变化曲线与浮游有孔虫氧同位素曲线对比表明, 在气候变冷事件(Heinrich事件、Younger Dryas事件、PME斜室普林虫低值等事件)期间, 陆源常量元素比值明显降低。这一对应特征, 一方面体现了气候变冷事件的发生对陆源区化学风化强度的制约, 另一方面也说明与指示长时间尺度气候相似, 陆源元素比值对短时间尺度的气候变化也具有明显的指示作用。     

Late Quaternary changes in depositional processes along the western margin of the Indus Fan

Detailed sedimentological, geochemical and isotopic analyses were carried out on sediment samples from ODP Site 720A on the Indus Fan, Arabian Sea. High values of calcium carbonate associated with low values of Al and Ti from 0 to 375 ka, and low values of calcium carbonate along with high values of Al and Ti from 375 to 525 ka represent two distinct sedimentary sequences. The sediments deposited from 525 to 375 ka correspond to a turbidite sequence, characterized by a high terrigenous input of coarse-grained sediments composed mostly of sand and silt. The sediments deposited from 375 ka to the present day comprise a pelagic sequence, consisting of pelagic material and clay. The major turbidity flow between 375 and 525 ka resulted in the greatest development of the Indus Fan during the late Quaternary. Most of the active channels were buried by 375 ka, followed by deposition of mainly pelagic sequences since then. Enrichment of an Indus-derived Himalayan clay mineral assemblage (illite and chlorite) in both the turbiditic and pelagic sequences reveals that the source and supply of clay minerals to the Indus Fan were the same during pre- and post-turbidite deposition. At ODP Site 720A, Al, Ti and terrigenous material do not show any systematic changes with respect to glacial and interglacial periods, suggesting that sea-level changes are not directly responsible for the terrigenous material supply to this site. Rather, a major switch in distributary channels away from the western margin of the Indus Fan is suggested.     

Late Quaternary evolution of coral reefs on a cool-water carbonate margin: the Abrolhos Carbonate Platforms, southwest Australia

Late Quaternary coral reefs have developed on the southwestern Australian margin, which has otherwise been characterised by cool-water carbonates since the Eocene. The Houtman Abrolhos coral reefs are at the limits of existence, extending, with the assistance of the Leeuwin Current, a poleward-flowing, warm water stream, into a region dominated by more temperate communities. Coring in the Easter Group reefs, supported by high precision dating, by both U/Th TIMS and ¹⁴C methods, has shown vigorous coral growth, with reefs over 26 m thick in the Holocene and over 15 m thick in the Last Interglacial. Each of the three Abrolhos platforms consists of a central platform composed of Last Interglacial reefs, about which windward and leeward Holocene reefs developed asymmetrically. Reef, peritidal and eolian facies comprise the emergent Last Interglacial limestones which are extensively calcretized, with reef facies up to 5 m above MSL. The Last Interglacial high stand lasted for at least 10 ka from 130 to 120 ka, and possibly 15 ka, from 132 to 117 ka. Holocene reef facies are also emergent by 0.5 m, and are overlain by peritidal and storm ridge facies in an upward-shallowing sequence. Windward (10 m thick) and leeward (26 m thick) Holocene reefs in the Easter Group show contrasting lithofacies. The wave-exposed windward reefs consist of a slow-growing association of coralline algal bindstones and coral framestones, whereas fast-growing coral framestones dominate the more protected leeward reefs. The leeward reefs commenced growth 10,000 years ago and grew to the present sea level by 6500 years ago, generating Holocene constructional topography consisting of ‘blue-hole’ terrain in the leeward parts of the platforms.     

Pockmarks and seafloor instability in the Olbia continental slope (northeastern Sardinian margin, Tyrrhenian Sea)

The seafloor morphology and the subsurface of the continental slope of the Olbia intraslope basin located along the eastern, passive Sardinian margin (Tyrrhenian Sea) has been mapped through the interpretation of high-resolution multibeam bathymetric data, coupled with air-gun and sparker seismic profiles. Two areas, corresponding to different physiographic domains, have been recognized along the Olbia continental slope. The upper slope domain, extending from 500 to 850 m water depth, exhibits a series of conical depressions, interpreted as pockmarks that are particularly frequent in seafloor sectors coincident with buried slope channels. In one case, they are aligned along a linear gully most likely reflecting the course of one of the abandoned channels. The location of the pockmarks thus highlights the importance of the distribution of lithologies within different sedimentary bodies in the subsurface in controlling fluid migration plumbing systems. A linear train of pockmarks is, however, present also away from the buried channels being related to a basement step, linked to a blind fault. Two bathymetric highs, interpreted as possible carbonate mounds, are found in connection with some of the pockmark fields. Although the genetic linkage of the carbonate mounds with seafloor fluid venting cannot be definitively substantiated by the lack of in situ measurements, the possibility of a close relationship is here proposed. The lower slope domain, from 850 m down to the base of the slope at 1,200 m water depth is characterized by a sudden gradient increase (from 2° to 6°) that is driven by the presence of the basin master fault that separates the continental slope from the basin plain. Here, a series of km-wide headwall scars due to mass wasting processes are evident. The landslides are characterized by rotated, relatively undeformed seismic strata, which sometimes evolve upslope into shallow-seated (less than 10 m), smaller scale failures and into headless chutes. Slope gradient may act as a major controlling factor on the seafloor instability along the Olbia continental slope; however, the association of landslides with pockmarks has been recognized in several continental slopes worldwide, thus the role of over-pressured fluids in triggering sediment failure in the Olbia slope can not be discarded. In the absence of direct ground truthing, the geological processes linked to subsurface structures and their seafloor expressions have been inferred through the comparison with similar settings where the interpretation of seafloor features from multibeam data has been substantiated with seafloor sampling and geochemical data.     

Quaternary contourite drifts of the Western Spitsbergen margin

The study of contourite drifts is an increasingly used tool for understanding the climate history of the oceans. In this paper we analyse two contourite drifts along the continental margin west of Spitsbergen, just south of the Fram Strait where significant water mass exchanges impact the Arctic climate. We detail the internal geometry and the morphologic characteristics of the two drifts on the base of multichannel seismic reflection data, sub-bottom profiles and bathymetry. These mounded features, that we propose to name Isfjorden and Bellsund drifts, are located on the continental slope between 1200 and 1800 m depth, whereas the upper slope is characterized by reduced- or non-deposition. The more distinct Isfjorden Drift is about 25 km wide and 45 km long, and over 200 ms TWT thick. We revise the 13 years-long time series of velocity, temperature, and salinity obtained from a mooring array across the Fram Strait. Two distinct current cores are visible in the long-term average. The shallower current core has an average northward velocity of about 20 cm/s, while the deeper bottom current core at about 1450 m depth has an average northward velocity of about 9 cm/s. We consider Norwegian Sea Deep Water episodically ventilated by relatively dense and turbid shelf water from the Barents Sea responsible for the accumulation of the contourites. The onset of the drift growth west of Spitsbergen is inferred to be about 1.3 Ma and related to the Early Pleistocene glacial expansion recorded in the area. The lack of mounded contouritic deposits on the continental slope of the Storfjorden is related to consecutive erosion by glacigenic debris flows. The Isfjorden and Bellsund drifts are inferred to contain the record of the regional palaeoceanography and glacial history and may constitute an excellent target of future scientific drilling.     

Evidence for multiple Quaternary ice advances and fan development from the Amundsen Gulf cross-shelf trough and slope,Canadian Beaufort Sea margin

The seismic stratigraphy and sedimentary architecture of the Amundsen Gulf Trough and adjacent slope, Canadian Beaufort Sea margin, are investigated using a grid of 2-D seismic reflection data. The inner-shelf of the Amundsen Gulf Trough is interpreted to be composed predominantly of exposed or near-surface bedrock, overlain by a spatially-discontinuous veneer of glacimarine to open-marine sediment. There is a seaward transition from exposed bedrock on the inner-shelf to a thick (up to 500 m) outer-shelf prograding wedge of acoustically semi-transparent sediment. Eight seismic sequences, divided into four megasequences, are described from the outer-shelf stratigraphy. Eight till sheets are identified from Megasequences A to C, providing evidence for at least eight Quaternary ice-stream advances through the Amundsen Gulf Trough to the shelf break. A trough-mouth fan with a minimum volume of about 10,000 km³ is present on the adjacent slope. The Amundsen Gulf ice stream probably represented the most northwesterly marine-terminating ice stream of the Laurentide Ice Sheet through much of the Quaternary, providing a major route for ice and sediment transfer to the Arctic Ocean. The youngest till sheet within the Amundsen Gulf Trough, Megasequence D, was probably deposited by a subsidiary ice stream, the Anderson ice stream, subsequent to retreat of the last, Late Wisconsinan Amundsen Gulf ice stream. This provides evidence of dynamic ice-sheet behaviour and the reorganisation of the northwest Laurentide Ice Sheet margin during the last deglaciation. A number of buried glacigenic landforms, including palaeo-shelf break gullies and a grounding-zone wedge with a volume of 90 km³, are described from the Amundsen Gulf Trough stratigraphy. Lateral grounding-zone wedges are identified at the northern and southern lateral margins of the Amundsen Gulf and M'Clure Strait troughs, respectively, and are interpreted to have been formed roughly contemporaneously by ice streams in Amundsen Gulf and M'Clure Strait.     

Late Quaternary stable isotope record and meltwater discharge anomaly events to the south of the Antarctic Polar Front,Drake Passage

Marine isotope stages (MISs) 1 to 5 were identified in the planktonic ¹⁸O record in sediment core DP00-02 just south of the Antarctic Polar Front in the Drake Passage, Antarctica. The oxygen isotope record, based on Neogloboquadrina pachyderma sinistral, is correlated with the contemporaneous global ¹⁸O stratigraphy. Marked deviations from the global climate curve suggest a local/regional overprint, particularly during MIS 3 which is considered a colder time period in the ocean record than MIS 1 and MIS 5 during the last interglacial. The comparison shows that negative ¹⁸O shifts in core DP00-02 during MIS 3 are larger than mean global changes which show a shift equal to or smaller than 0.5. The isotope shift, exceeding the glacial-interglacial ice volume effect, probably resulted from changes in the isotope composition of seawater, which is linearly related to decreases in salinity rather than to increases in sea-surface temperature. Increased ice-rafted debris (IRD) content during this interval indicates a strong influx of IRD from melting ice shelves and icebergs, which may be related to upwelling of warmer circumpolar deep water.     

渤海地区晚第四纪环境演化与第四纪滨海相地下卤水的形成

由于气候变化和新构造运动,渤海曾发生多次海陆演化,从而形成了第四纪地下卤水。间冰期的海侵提供了形成地下卤水的水源,气候波动产生了海水浓缩的条件,而冰期海退过程中,陆源物质覆盖于海相沉积上,保存了地下卤水。对环境演化在“潮滩生卤”和“冰冻生卤”过程中重要作用的分析结果表明,在第四纪环境演变的过程中,两种生卤作用在地质时期卤水矿床形成中均具有重要意义,并且只要存在海水物源,就有可能形成地下卤水。     

Erosion and slope instability on Horizon Guyot,Mid-Pacific Mountains

Seismic-reflection profiles, sediment cores, and current velocities were assessed to study the impact of erosion and sediment redistribution on the pelagic sediment cap of Horizon Guyot, a flat-topped submarine volcanic ridge in the Mid-Pacific Mountains. These processes seem to concentrate their effect around the rim of the sediment cap. Sediment slumping occurs on the northwest perimeter of the guyot's sediment cap. Slope stability analysis suggests that if overconsolidation on Horizon Guyot is the result of current reworking or if local undercutting by bottom currents steepens the sea floor declivity, the sediment cap may be unstable during infrequent earthquake loading, transporting sediment from the guyot summit to the abyssal sea floor.     

辽东湾晚第四纪层序地层

通过对辽东湾高分辨率浅地层剖面声学地层与典型钻孔沉积地层的对比分析,揭示了研究区晚第四纪MIS5以来的地层层序。辽东湾高分辨率浅地层剖面自下而上划定的6个声学地层单元(SU₅、SU₄、SU₃、SU₂、SU₁²、SU₁¹)与钻孔岩芯划分的6个沉积地层单元(DU₅、DU₄、DU₃、DU₂、DU₁²、DU₁¹)具有良好的对应关系。分别与MIS4期、MIS2期低海面时期的沉积间断密切相关的两个层序界面R5、R3,将辽东湾识别出的地层单元自下而上划分为3个层序(SQ3、SQ2、SQ1)。其中SQ3仅识别出上部的海侵体系域与高水位体系域,对应MIS5期海平面相对较高时期的滨浅海相沉积(DU₅);SQ2自下而上由低水位体系域(MIS...     

Late Quaternary chronology of the southern Black Sea basin

During New Euxinian time when sea level dropped below the sill connecting the Black and Marmara seas, the Black Sea became isolated and freshwater sediments were deposited. Now it is a semieuxinic basin with the oxic/anoxic boundary at 100–150 m. The seasonal changes in sedimentation are preserved in the form of laminated sequences. The counting of varves in southeastern Black Sea cores show the chronology of the O₂/H₂S interface. The age of the Holocene sapropel along the eastern margin ranges from 4000 to 1000 yr BP in deep water and 2500—1000 yr BP in shallower water. Sapropel formation started at 3650 yr BP at a water depth of 2200 m.     

Late Quaternary Seismic Stratigraphy of the Eastern Bengal Shelf

An ultra-high-resolution seismic study of the eastern Bengal Shelf with the parametric narrow-beam echosounder Parasound allows the interpretation of late Quaternary depositional patterns in terms of seismic stratigraphy. Accommodation space was still present on the outer shelf during the last lowstand, where a prograding delta developed in the western survey area. Oolitic beach ridges were later formed on top of this lowstand delta. Farther east, large parts of the shelf were exposed to subaerial erosion and a river system extended seaward across the area. A subaqueous highstand delta prograded southwards following the maximum transgression about 7,000 years ago. Its foreset beds exhibit acoustic voids very likely generated by sediment liquefaction, possibly caused by episodic energetic events such as major cyclones and/or earthquakes. Bottomset sediments extend seaward close to the shelf break in the west, whereas no Holocene sediments cover the outer shelf in the east.     

Fluid flow related features as an indicator of potential gas hydrate zone: western continental margin of India

Multichannel seismic reflection data from the continental margin of western India suggest the potential presence of fluid expulsion features, which may or may not be associated with gas hydrates. No typical bottom simulating reflector was observed on the reflection seismic section. As a result we look for other evidence in seismic sections in a small corridor of the western continental margin of India in order to establish the presence of gas hydrates. We study features including venting through the seafloor, pockmarks, sea floor collapse, faults acting as migration paths for fluid flow, transparent gas-charged sediment, reduction in amplitude strength, diapirism and mud-volcano. Presence of all these gas-escape features on a seismic section implies the probable presence of methane within the zone of hydrate stability field.     

Paleoenvironmental changes of the Yellow Sea during the Late Quaternary

Based on stable isotope data and carbon and sulfur elemental analyses, the sedimentary environment of the Yellow Sea was significantly influenced by the sea level changes during the Late Quaternary. At the low sea level stand when the sea level was lower by 56?m, the salinity of seawater was reduced to about 7.6‰, and the sedimentation rate in the central part of the Yellow Sea was three times higher than the present rate. The high C/S ratio during the low sea level stand is strong evidence that sedimentation took place in a lower salinity environment than exists at present.     

Paleoenvironmental changes of the Yellow Sea during the Late Quaternary

 Based on stable isotope data and carbon and sulfur elemental analyses, the sedimentary environment of the Yellow Sea was significantly influenced by the sea level changes during the Late Quaternary. At the low sea level stand when the sea level was lower by 56 m, the salinity of seawater was reduced to about 7.6‰, and the sedimentation rate in the central part of the Yellow Sea was three times higher than the present rate. The high C/S ratio during the low sea level stand is strong evidence that sedimentation took place in a lower salinity environment than exists at present. Received: 25 September 1997 / Revision reveived: 15 June 1998     

Morphology and Late Quaternary sedimentation in the Gulf of Oman Basin

The morphology of the Gulf of Oman Basin, a 3,400 m deep oceanic basin between Oman and southern Pakistan and southern Iran, ranges from a convergent margin (Makran margin) along the north side, a passive type (Oman margin) along the south side, translation types along the basin's west (Zendan Fault-Oman Line) and east (Murray Ridge) sides and a narrow continental rise and a wide abyssal plain in the centre of the basin. Sediment input into the basin during the Late Quaternary has been mainly from the north as a result of the uplift of the Coast Makran Mountains in the Late Miocene-Pliocene. Today most of this detritrus is deposited on the shelf and upper continental slope and perched basins behind the fold/fault ridges on the lower slope. The presence of fans and channels on the continental rise on the north side of the basin indicate, however, that continental derived debris was, and possibly is, being transported to the deep-sea by turbidity currents via gaps in the ridges on the lower slope. In addition to land derived terrigenous sediments, the basin deposits also contain biogenic (organic matter and calcium carbonate), eolian detritus and hydrates and authigenic carbonates from the tectonic dewatering of the Makran accretionary wedge. The eolian sediment is carried into the Gulf of Oman Basin from Arabia and the Mesopotamia Valley by the northwesterly Shamal winds. This type of detritus was particularly abundant during the glacial arid periods 21,000–20,000 and 11,000 (Younger Dryas) years ago when exposure of the Persian (Arabian) Gulf increased the area of dust entrainment and shifted the position of the source of the eolian sediments closer to the basin.     

Physiographic features on the northern Gulf of Mexico continental slope

The continental slope of the northern Gulf of Mexico is diapirically controlled and is comprised of coalescing salt sheets, salt withdrawal basins, salt ridges, salt tongues and sills, and submarine canyons. Bathymetric information from single-beam data has resulted in several published maps. Many of the map areas have been remapped, using multibeam surveys, by the US National Ocean Service, and names have been given to the major physiographic features. The multibeam program was discontinued before complete coverage of the slope was accomplished. We provide charts of the remaining areas with names of features that have been accepted by the US Board of Geographic Names.