The Scotia Sea and the Drake Passage as an orographic barrier for the Antarctic Circumpolar Current

It is shown on the basis of the data of the Russian Academy of Sciences expeditions in 2003–2010, the historical CTD database, the WOCE climatology, and the satellite altimetry that the area of the Scotia Sea and the Drake Passage is even a greater significant orographic barrier for the eastward Antarctic Circumpolar Current (ACC) than was previously thought. It is the current concept that this barrier is the most important for the ACC; it consists of three obstacles: the Hero Ridge with the Phoenix Rift, the Shackleton Ridge, and the North Scotia Ridge with the relatively shallow eastern part of the Scotia Sea. Despite the fact that all three obstacles are permeable for the layer of the Circumpolar Bottom Water (CBW; 28.16 < γ ⁿ < 28.26) being considered the lower part of the circumpolar water, the circulation in this layer throughout the Scotia Sea and the Drake Passage quite substantially differs from the transfer by the surface-intensified ACC jets. Herewith, the upper CBW boundary is the lower limit of the circumpolar coverage of the ACC jets. This result is confirmed by the near zero estimate of the total CBW transport according to the three series of the LADCP measurements on the sections across the Drake Passage. It is shown that the transformation (cooling and freshening) of the CBW layer, which occurs owing to the flow of the ACC over the Shackleton Ridge, is associated with the shape and location of the ridge in the Drake Passage. The high southern part of this ridge is a partially permeable screen for the eastward CBW transport behind which the colder and fresher waters of the Weddell Sea and the Bransfield Strait of the same density range as the CBW penetrate into the ACC zone. The partial permeability of the Shackleton Ridge for the CBW layer leads to the salinization of this layer on the eastern side of the ridge and to the CBW’s freshening on the western side of this ridge, which is observed across the entire Drake Passage.     

Geotechnical characterization of slump scars,eroded slopes,and correlated deposits on continental margins: Gulf of Lions,Western Mediterranean sea

Abstract

Vertical variations of geotechnical properties in the uppermost sediment layers characterize the main sedimentary processes acting on the construction and destruction of progressive‐type continental slopes. In the Gulf of Lions, the original thicknesses and distribution of the uppermost sedimentary layers of the continental slope and rise, which consist of Holocene muds overlying Pleistocene muds, have been greatly modified by erosion and several kinds of slope failure processes. Each process is typified through sets of geotechnical properties measured in the eroded or slumped sections and in the associated sediment accumulations.

In slump scars, the water‐rich Holocene muds lie on fine, overconsolidated, Pleistocene muds with high plasticity and low shear strength. In bottom current‐eroded slopes, where modern sedimentation is extremely reduced, the Pleistocene muds frequently outcrop and may sometimes be overlain by a very thin layer of Holocene muds. The Pleistocene muds of eroded slopes are overconsolidated and more silty and less plastic than the Pleistocene muds from slopes affected by slope failure, their shear strength being 10 times greater.

Deposits at the toe of slumps are very often formed by several superposed three‐layer units (triplets of interstratified Holocene, transitional, and Pleistocene layers) issued from retrogressive slumping occurring in the slump scars above their head area. The main body of each layer is then relatively undisturbed, showing the usual burial geotechnical gradients due to overburden pressure (i.e., decrease of water content and increase of unit weight and shear strength). At the toe of bottom current‐eroded slopes, a thick and homogeneous layer of Holocene muds overlies the Pleistocene muds; this Holocene layer has unappreciable burial depth gradient of its geotechnical parameters because of a high rate of modem and continuous deposition.     

Water, Salt, Phosphorus and Nitrogen Budgets of the Japan Sea

Water, salt, phosphorus and nitrogen budgets of the Japan Sea have been calculated by box model analysis using historical data. Average residence time of the Tsushima Warm Current Water in the upper 200 m is 2.1 years and that of the Japan Sea Proper Water is 90 years. The salt flux from the Tsushima Strait balances those through the Tsugaru and Soya Straits. Average residence times of phosphorus and nitrogen from the Tsushima Strait are 2.2 years and 1.6 years, respectively, in the upper 200 m of the Japan Sea. Total nitrogen/total phosphorus ratios of riverine load, the Tsushima Warm Current water and the water in the Japan Sea are 16.4, 16.6 and 11.3, respectively. This suggests that denitrification is dominant in the Japan Sea. This revised version was published online in July 2006 with corrections to the Cover Date.     

Trench-fill submarine-fan facies associations of the Upper Cretaceous Chugach terrane,southern Alaska

Turbidites of the Upper Cretaceous Chugach terrane of southern Alaska were deposited in a trench during northward-directed subduction. The fault-bounded outcrop belt of the Chugach terrane is about 2000-km long and 100-km wide and was accreted to Alaska during the Cenozoic. Turbidites are at least 5000 m thick, are extensively deformed, have been regionally metamorphosed, and have been intruded by anatectic granites. Facies associations indicate an east-to-west progression from inner-fan to middle-fan, outer-fan, fan-fringe, and basin-plain deposits. To the north is a marginal trench-slope facies association and a basin.     

Trench-fill submarine-fan facies associations of the Upper Cretaceous Chugach terrane, southern Alaska

Turbidites of the Upper Cretaceous Chugach terrane of southern Alaska were deposited in a trench during northward-directed subduction. The fault-bounded outcrop belt of the Chugach terrane is about 2000-km long and 100-km wide and was accreted to Alaska during the Cenozoic. Turbidites are at least 5000 m thick, are extensively deformed, have been regionally metamorphosed, and have been intruded by anatectic granites. Facies associations indicate an east-to-west progression from inner-fan to middle-fan, outer-fan, fan-fringe, and basin-plain deposits. To the north is a marginal trench-slope facies association and a basin. Margin setting represents fan and/or source area     

A large encrusting clionaid sponge in the Eastern Mediterranean Sea

An excavating sponge, which covers extensive areas of limestone rock, has been found at several locations of the Eastern Mediterranean Sea. This zooxanthellate clionaid, brown with yellow oscula, may have an extension of several square meters under the β-form, similar to what has been described in coral reef areas. It has been observed at 3–30 m depth, generally in clear water in the Ionian Sea, Crete, Cyprus and Lebanon. The gross morphology and the spicule characters are described from all the collected specimens. The spicule complement, with variable tylostyles and rare, sometimes absent, thin spirasters, is compared with that of Cliona parenzani Corriero & Scalera-Liaci 1997 from the Apulian coast (Ionian Sea, Mediterranean Sea), and of several Caribbean and Indo-Pacific clionaid species, which display the same morphology and rather similar spicule characters. The sponge is tentatively identified as C. parenzani , although there are some differences with the type specimens in the highly variable spiculation, suggesting that it may represent a species complex similar to what is known for other species in the Caribbean and Indo-Pacific areas. Although the sponge appears to be closely related to large brown spreading clionaids from tropical areas and was previously practically overlooked in the Eastern Mediterranean, it does not appear to be a recent introduction. It is suggested that it could be a survivor in the warmer area of the Mediterranean of an ancient thermophilous fauna, which did not survive in the colder Western Basin during glacial episodes of the Pleistocene.     

东营市东辛2-4井磁性地层划分对比研究

本文通过东辛2—4井岩心样品的古地磁学研究,整段岩心呈现以正极性为主的混合极性特征。自上而下划分的布容、松山、高斯和吉尔伯特极性时和布容极性时内清晰显示出的哥德堡和布莱克亚时的界限分别位于井深28.0m、161.0m、274.5m和452.0m。据黄河口南北5个钻孔磁性地层的划分对比,长江口、苏北盆地和东营地区,吉尔伯特、高斯和布容极性时的沉积速度较快,松山极性时较慢或近于缺失;黄河口以北至河北地区,高斯极性时地层不发育,吉尔伯特、松山和布容极性时则较厚,表明它们所处的构造单元、地质基础和沉积环境有较大的不同。     

Dietary Shifts in the Queen Triggerfish, Balistes vetula, in the Absence of its Primary Food Item, Diadema antillarum

Abstract. The long-spined sea urchin Diadema antillarum has been reported to be the major food item of the queen triggerfish, Balistes vetula in the Caribbean. This sea urchin has undergone a massive mortality on reefs throughout the Caribbean during 1983. The dietary habits of twenty-three queen triggerfish from patch reef habitats in Belize were examined. It was found that crabs and chitons now form the major dietary items when D. antillarum is not available. It is suggested that this predator, which was previously specialized on a single food item, is able to shift resource utilization in the absence of its primary food source, and that the queen triggerfish can capture diverse prey from a wide variety of habitats.     

福州市马尾地区第四纪地层划分及海相层分析

通过对该区钻孔剖面贡料,结合~(14)C、古地磁、孢粉及微体古生物等资料综合分析研究,对该区第四纪地层提出划分意见和自第四纪晚期以来,地层中存在着典型两期海侵所形成的海相层。     

Cryptic diversity and phylogeography of the island‐associated barnacle Chthamalus moro in Asia

Pleistocene glaciations were among the important historic events that shaped the population structures of marine organisms. Genetic studies of different marine fauna and flora have demonstrated the effect of Pleistocene glaciations on taxa that reside in marginal seas. However, how marine island species responded to Pleistocene glaciations remains relatively unstudied, especially in Asia. Genetic analyses based on the island‐associated barnacle Chthamalus moro collected from 14 sites in Asia reveal that C. moro comprises three distinct lineages, with COI divergence ranging from 3.9 to 8.3%. Population genetic analyses on respective lineages reveal signs of demographic expansion within the Pleistocene epoch at different times. The Ogasawara lineage, which has a more oceanic distribution, expanded the earliest, followed by the population expansion of the Ryukyu and Southern lineages that inhabit islands closer to the continent. The data suggest that the inhabitants of outer islands may have been less affected by Pleistocene glaciations than those that reside closer to the continent, as the former were able to maintain a large, stable, effective population size throughout the late Pleistocene.     

浙江宁波地区第四纪自然环境变迁

第四纪自然环境的变迁,作为地球环境演変过程的最新一幕,对人类的生存和发展有着重要的意义。近年来,我们对中国东部沿海地区第四纪地质和海岸演化进行了研究,通过沉积物中孢粉和微体古生物等资料,对古气候及植被变化、海陆变迁等自然环境演变问题作了初步分析。宁波地区的研究是其中的一部分。 宁波滨海平原,地处东海之滨、杭州湾南岸,三面环山,一面临海,面积仅850平方公里。本区的第四纪地质调查主要是由浙江省地质局等有关单位进行的。我们利用浙江省地质局第六地质大队提供的钻探岩芯祥品,作了孢粉、微体古生物、软体动物化石分析,并委托中国科学院地球化学研究所C14实验室,作了部分样品的测年。本文总结已获得的资料,提出宁波地区第四纪自然环境演变的初步认识。     

Caribbean current variability and the influence of the Amazon and Orinoco freshwater plumes

The variability of the Caribbean Current is studied in terms of the influence on its dynamics of the freshwater inflow from the Orinoco and Amazon rivers. Sea-surface salinity maps of the eastern Caribbean and SeaWiFS color images show that a freshwater plume from the Orinoco and Amazon Rivers extends seasonally northwestward across the Caribbean basin, from August to November, 3–4 months after the peak of the seasonal rains in northeastern South America. The plume is sustained by two main inflows from the North Brazil Current and its current rings. The southern inflow enters the Caribbean Sea south of Grenada Island and becomes the main branch of the Caribbean Current in the southern Caribbean. The northern inflow (14°N) passes northward around the Grenadine Islands and St. Vincent. As North Brazil Current rings stall and decay east of the Lesser Antilles, between 14°N and 18°N, they release freshwater into the northern part of the eastern Caribbean Sea merging with inflow from the North Equatorial Current. Velocity vectors derived from surface drifters in the eastern Caribbean indicate three westward flowing jets: (1) the southern and fastest at 11°N; (2) the center and second fastest at 14°N; (3) the northern and slowest at 17°N. The center jet (14°N) flows faster between the months of August and December and is located near the southern part of the freshwater plume. Using the MICOM North Atlantic simulation, it is shown that the Caribbean Current is seasonally intensified near 14°N, partly by the inflow of river plumes. Three to four times more anticyclonic eddies are formed during August–December, which agrees with a pronounced rise in the number of anticyclonic looper days in the drifter data then. A climatology-forced regional simulation embedding only the northern (14°N) Caribbean Current (without the influence of the vorticity of the NBC rings), using the ROMS model, shows that the low salinity plume coincides with a negative potential vorticity anomaly that intensifies the center jet located at the salinity front. The jet forms cyclones south of the plume, which are moved northwestward as the anticyclonic circulation intensifies in the eastern Caribbean Sea, north of 14°N. Friction on the shelves of the Greater Antilles also generates cyclones, which propagate westward and eastward from 67°W.     

Paleomagnetic results from deep-sea sediment of the Korea Deep Ocean Study (KODOS) area (northern equatorial Pacific) and their paleodepositional implications

Paleomagnetic properties of sediment cores were examined to reconstruct paleodepositional conditions in the Korea Deep Ocean Study (KODOS) area, located in the northeastern equatorial Pacific. The studied KODOS sediments have a stable remanent magnetization with both normal and reversed polarities, which are well correlated with the geomagnetic polarity timescale for the late Pliocene and Pleistocene. Average sedimentation rates are 1.56 and 0.88 mm/kiloyear for the Pleistocene and late Pliocene, respectively. Clay mineralogy and scanning electron microscope analyses of the sediments indicate that terrestrial material was transported to the deep-sea floor during these times. The variations of sedimentation rates with age may be explained by the onset of the northern hemisphere glaciation and subsequent climatic deterioration during the Pliocene and Pleistocene. For the Pleistocene, an increasing sedimentation rate implies that input of terrestrial materials was high, and also a high input of biogenic materials was detected as a result of increased primary production in the surface water. The down-core variations in paleomagnetic and rock-magnetic properties of the KODOS sediments were affected by dissolution processes in an oxic depositional regime. As shown by magnetic intensity and hysteresis parameters, the high natural remanent magnetization (NRM) stability in the upper, yellowish brown layers indicates that the magnetic carrier was in pseudo-single domain states. In the lower, dark brown sediments, only coarse magnetic grains survived dissolution and the NRM was carried by more abundant, multi-domain grains of low magnetic stability. The down-core variation of magnetic properties suggests that the KODOS sediments were subjected to dissolution processes resulting in a loss of the more stable components of the magnetic fraction with increasing core depth.     

Controls on the development of valleys,canyons, and unconfined channel–levee complexes on the Pleistocene Slope of East Kalimantan,Indonesia

Shallow 3D seismic data show contrasting depositional patterns in Pleistocene deepwater slopes of offshore East Kalimantan, Indonesia. The northern East Kalimantan slope is dominated by valleys and canyons, while the central slope is dominated by unconfined channel–levee complexes. The Mahakam delta is immediately landward of the central slope and provided large amounts of sediments to the central slope during Pleistocene lowstands of sea level. In the central area, the upper slope contains relatively straight and deep channels. Sinuous channel–levee complexes occur on the middle and lower slope, where channels migrated laterally, then aggraded and avulsed. Younger channel–levee complexes avoided bathymetric highs created by previous channel–levee complexes. Levees decrease in thickness down slope. Relief between channels and levees also decreases down slope.North of the Mahakam delta, siliciclastic sediment supply was limited during the Pleistocene, and the slope is dominated by valleys and canyons. Late Pleistocene rivers and deltas were generally not present on the northern outer shelf. Only one lowstand delta was present on the northern shelf margin during the upper Pleistocene, and sediments from that lowstand delta filled a pre-existing slope valley complex and formed a basin-floor fan. Except for that basin-floor fan, the northern basin floor shows no evidence of sand-rich channels or fans, but contains broad areas with chaotic reflectors interpreted as mass transport complexes. This suggests that slope valleys and canyons formed by slope failures, not by erosion associated with turbidite sands from rivers or deltas. In summary, amount of sediment coming onto the slope determines slope morphology. Large, relatively steady input of sediment from the Pleistocene paleo-Mahakam delta apparently prevented large valleys and canyons from developing on the central slope. In contrast, deep valleys and canyons developed on the northern slope that was relatively “starved” for siliciclastic sediment.     

Current titles in marine geology

The asymmetrical Astoria Fan (110 × 180 km) developed off the Columbia River and Astoria submarine canyon during the Pleistocene. Morphology, stratigraphy, and lithology have been outlined for a Pleistocene turbidite, and a Holocene hemipelagic sedimentary regime to generate geologically significant criteria for comparison with ancient equivalent deposits. Both gray silty clay of the Late Pleistocene and olive-gray clay of the Early Holocene are interrupted by turbidites. The few deeply incised fan valleys of the more steeply sloping upper fan contain thick, muddy and very poorly sorted sand and gravel beds that usually have poorly developed internal sedimentary structures. The numerous shallower fan valleys and distributaries of the flatter middle and lower fan contain thick, clean, and moderately sorted medium to fine sands that are vertically graded in texture, composition and well-developed internal sedimentary structures. Tuffaceous turbidites (containing Mazama ash, 6600 B.P.) can be traced as thick deposits (ca. 30–40 cm) throughout the Astoria Channel system and as thin correlative interbeds (ca. 1–2 cm) in interchannel areas. Similarly, sand/shale ratios are high throughout the fan valleys and the middle and lower fan areas of distributaries, but are low in the upper-fan interchannel areas.These depositional trends indicate that high-density turbidity currents carry coarse traction loads that remain confined in upper but not lower fan valleys. Fine debris selectively sorts out from channelized flows into overbank suspension flows that spread over the fan and deposit clayey silt. A high content of mica, plant fragments, and glass shards (if present) characterizes deposits of the overbank flows, a major process in the building of upper fan levees and interchannel areas.In the Late Pleistocene, turbidity currents funneled most coarse-grained debris through upper channels to depositional sites in middle and lower fan distributaries that periodically shifted, anastomosed and braided to spread sand layers throughout the area. At this time, depositional rates were many times greater (>50 cm/1000 years) than in the Holocene (8 cm/1000 years).During the Holocene rise of sea level, the shoreline shifted, the Columbia River sediment was trapped, and turbidity-current activity slackened from one major event per 6 years in the Late Pleistocene, to one per 1000 years in the Early Holocene, to none since the Mt. Mazama eruption (ca. 6600 B.P.). Turbidites became muddier and deposited as thick beds within main channels, in part explaining Holocene deposition rates three times greater there (25 cm/1000 years) than in interchannel regions. Turbid-layer debris, funneled through channel systems and trapped from flows off the continental terrace, also contributed to rapid sedimentation in valleys; however, less than 2% of the suspended sediment load of the Columbia River has been trapped in fan valleys during the Holocene.By the Late Holocene, continuous particle-by-particle deposition of hemipelagic clay with a biogenous coarse fraction was the predominant process on the fan. These hemipelagites contain progressively more clay size and less terrigenous debris offshore, and are finer grained, richer in planktonic tests and dominated by radiolarians compared to the foraminiferal-rich Pleistocene clays. The hemipelagic sedimentation of interglacial times, however, is insignificant compared to turbidite deposition of glacial times.     

元谋盆地的第四纪红土

本文阐述了元谋红土的分布规律，物理化学性质，红土化作用时期以及红土对环境的解释。按成因观点，元谋红土可分风化壳型红土，地下水型红土，坡积红土三种。中更新世中期，晚更新世早期，全新世最佳期，是元谋红土的主要形成特期。作为岩石圈，大气圈，生物圈相互作用的产物，红土的形成发育，与盆地的第四纪发育历史密切相关。同时，红土也记录了盆地过去的环境演化，因而有可能提取红土储藏的信息重塑第四纪环境。     

Tectonics, basin subsidence mechanisms, and paleogeography of the Caribbean-South American plate boundary zone

Using a mega-regional dataset that includes over 20,000 km of on- and offshore 2D seismic lines and 12 wells, we illustrate three different stages of fault formation and basin evolution in the Caribbean arc-South American continent collisional zone. Transpressional deformation associated with oblique collision of the Caribbean arc migrates diachronously over a distance of ∼1500 km from western Venezuela in Paleogene time (∼57 Ma) to a zone of active deformation in the eastern offshore Trinidad area. Each diachronous stage of pre-, syn-, and post-collisional basin formation is accompanied by distinct patterns of fault families. We use subsidence histories from wells to link patterns of long-term basinal subsidence to periods of activity of the fault families.

Stage one of arc-continent collision

Initial collision is characterized by overthrusting of the south- and southeastward-facing Caribbean arc and forearc terranes onto the northward-subducting Mesozoic passive margin of northern South America. Northward flexure of the South American craton produces a foreland basin between the thrust front and the downward-flexed continental crust that is initially filled by clastic sediments shed both from the colliding arc and cratonic areas to the south. As the collision extends eastward towards Trinidad, this same process continues with progressively younger foreland basins formed to the east. On the overthrusting Caribbean arc and forearc terranes, north-south rifting adjacent to the collision zone initiates and is controlled by forward momentum of southward-thrusting arc terranes combined with slab pull of the underlying and subducting, north-dipping South American slab. Uplift of fold-thrust belts arc-continent suture induces rerouting of large continental drainages parallel to the collisional zone and to the axis of the foreland basins.

Stage two

This late stage of arc-continent collision is characterized by termination of deformation in one segment of the fold-thrust belt as convergent deformation shifts eastward. Rebound of the collisional belt is produced as the north-dipping subducted oceanic crust breaks off from the passive margin, inducing inversion of preexisting normal faults as arc-continent convergence reaches a maximum. Strain partitioning also begins to play an important role as oblique convergence continues, accommodating deformation by the formation of parallel, strike-slip fault zones and backthrusting (southward subduction of the Caribbean plate beneath the South Caribbean deformed belt). As subsidence slows in the foreland basins, sedimentation transitions from a marine underfilled basin to an overfilled continental basin. Offshore, sedimentation is mostly marine, sourced by the collided Caribbean terranes, localized islands and carbonate deposition.

Stage three

This final stage of arc-continent collision is characterized by: 1) complete slab breakoff of the northward-dipping South American slab; 2) east-west extension of the Caribbean arc as it elongates parallel to its strike forming oblique normal faults that produce deep rift and half-grabens; 3) continued strain partitioning (strike-slip faulting and folding). The subsidence pattern in the Caribbean basins is more complex than interpreted before, showing a succession of extensional and inversion events. The three tectonic stages closely control the structural styles and traps, source rock distribution, and stratigraphic traps for the abundant hydrocarbon resources of the on- and offshore areas of Venezuela and Trinidad.     

加勒比海三重双向嵌套三维环流模型的应用

对三重双向嵌套海洋环流三维数值模型系统(CANDIE),用尤卡坦海峡实测横断面流速和海温等值线、1990年代根据卫星跟踪漂移轨迹推求得到的15m水深处的时间平均流场、实测平均海温过程和西加勒比海体积输送流函数等资料进行了验证,结果表明所建三重双向嵌套模型系统结构与参数合理,模型能较好地模拟西加勒比海的环流特征如加勒比流、巴拿马-哥伦比亚漩涡和该地区温盐的季节性变化。模型被用来计算小模型计算域(伯利兹大陆架)珊瑚礁地区不同水深处悬浮颗粒的运动轨迹线、颗粒的保留与耗散。近表层耗散在Lighthouse Reef Atoll和Glovers Reef Atolls以东相对较强,在内大陆架特别在Inner Channel内和South Water Cay附近相对较低。为了测试伯里兹大陆架海表面珊瑚礁之间的水动力连接特性,计算了位于Turneffe Islands Atoll和GloversReef Atolls珊瑚礁的上游颗粒来源区(源)和下游颗粒耗散区(汇),在30天内到达这两处环形珊瑚礁的近表面颗粒的可能来源区包括该两者周围的浅水区域、两者之间的深水区域和洪都拉斯Bay Islands的沿岸水域,Turneffe和Glovers Reef Atolls 30天下游耗散区分别覆盖了伯里兹大陆架的中部和南部水域。     

Sedimentation processes on the continental rise of northeastern South America

A section of the continental rise of northeastern South America northeast of the Orinoco delta contains physiographic features built by the interaction of southward-flowing North Atlantic Deep Water and turbidity currents generated in the Orinoco region during the last Pleistocene glacials. A sedimentary outer ridge of low relief (Demerara Outer Ridge) trends northeast along the rise and a field of westward-migrating sediment waves trending north-northwest is superimposed on the outer ridge. The sediment waves have a maximum amplitude and wavelength of 20 m and 4 km, respectively. Seismic profiler records indicate that the outer ridge was probably built during the Pleistocene. A major turbidity-current pathway adjacent to the outer ridge on the north supplied sediment to the southward-flowing North Atlantic Deep Water which then deposited this sediment down-stream on the outer ridge and formed the sediment waves. Piston cores from the outer ridge contain numerous silt—sand beds and appear to be contourites. The cores consist primarily of gray hemipelagic clay of a Late Wisconsin age and have high ($\text{>}\text{10}\text{cm}\text{1000}\text{yrs}$) sedimentation rates. In contrast, cores from the continental rise north of the turbidite channel are brown clays with relatively low sedimentation rates ($\text{3.0}\text{cm}\text{1000}\text{yrs}$) and do not contain silt—sand contourites.     

The geochemistry of lead in rivers,estuaries and the continental shelf of the Southeastern United States

Transport by southeastern rivers has insignificant influence on the lead concentration of southeastern shelf waters. If the rate of mobilization and transport of lead by these rivers is representative of uncontaminated fluvial transport during the Pleistocene, only about 5% of the prehistoric output of lead to the North Atlantic can be accounted for by river input.Lead concentrations in southeastern shelf waters are similar to those of North Atlantic Water from the upper 1000 m, which is probably similar to the intrusion source water for the shelf. Atmospheric inputs to the shelf of the same magnitude as observed for the Western North Atlantic are difficult to reconcile given the residence time of shelf waters and their lead concentration unless the rate of loss of lead to shelf sediments is about the same as the atmospheric flux.