内蒙古苏尼特左旗晚泥盆世弧背前陆盆地的发现及构造意义

兴蒙造山带位于中亚造山带东段,是研究地壳生长与古亚洲洋演化的热点区域。内蒙古中部苏尼特左旗地区位于兴蒙造山带中段,区内分布的上泥盆统色日巴彦敖包组磨拉石建造是古亚洲洋闭合的重要证据之一,前人对其岩石组合、地层层序与沉积环境进行了报道,并指出其属于晚泥盆世的前陆盆地,但未就前陆盆地的属性、分类及其与造山带的空间关系展开详细研究。因此,本研究选择该区东北部的昌特敖包剖面,利用野外实测、锆石U-Pb同位素定年、沉积相分析等方法查明该碎屑沉积的地层层序、形成时代与物质来源,在此基础上补充完善了兴蒙造山带北造山带二级构造单元的划分方案。研究结果表明,昌特敖包剖面碎屑沉积属上泥盆统色日巴彦敖包组,而非前人认为的二叠纪大石寨组地层,其碎屑物质主要来自南部的宝力道岛弧与北部兴安-爱力格庙地块。沉积环境分析表明昌特敖包剖面色日巴彦敖包组主要为冲积扇相沉积,上部过渡到干旱气候火山活动背景下的滨浅海相沉积。综合昌特敖包剖面的沉积、年代、物源及大地构造位置特征,可以推断其代表晚泥盆世造山带的弧背前陆盆地。由此,苏尼特左旗地区北造山带的构造单元在前人划定的前陆变形带、混杂带、周缘前陆盆地、岛弧岩浆岩、同碰撞花岗岩以北,还存在弧背前陆盆地。本研究结果为恢复中亚造山带的构造演化过程提供了关键沉积学证据。     

Spatial characterization of water quality in Florida Bay and Whitewater Bay by multivariate analyses: Zones of similar influence

We apply an objective statistical analysis to a 6-yr, multiparameter dataset in an effort to describe the spatial dependence and inherent variation of water quality patterns in the Florida Bay-Whitewater Bay area. Principal component analysis of 16 water quality parameters collected monthly over a 6-yr period resulted in live principal components (PC) that explained 71.8% of the variance of the original variables. The “organic” component (PC₁) was composed of TN, TON, APA, and TOC; the “inorganic N” component (PCII) contained NO₂, NO₃, and NH₄ ⁺, the “phytoplankton” component (PC_III) was made up of turbidity, TP, and Chl a; DO and temperature were inversely related (PC_IV); and salinity was the only parameter included in PC_V. A cluster analysis of mean and SD of PG scores resulted in the spatial aggregation of 50 fixed monitoring stations in Florida Bay and Whitewater Bay into six zones of similar influence (ZSI) defined as Eastern Florida Bay. Core Florida Bay, Western Florida Bay, Coot Bay, the Inner Mangrove Fringe, and the Outer Mangrove Fringe. Marked differences in physical, chemical, and biological characteristics among ZSI were illustrated by this technique. Comparison of medians and variability of parameter values among ZSI allowed large-scale generalizations as to underlying differences in water quality in these regions. For example. Fastern Florida Bay had lower salinity, TON, TOC, TP, and Chl a than the Core Bay as a function of differences in freshwater inputs and water residence time. Comparison of medians and variability within ZSI resulted in new hypotheses as to the processes generating these internal patterns. For example, the Core Bay had very high TON, TOC, and NH₄ ⁺ concentrations but very low NO₃ ^?, leading us to postulate the inhibition of nitrification via CO production by TOC photolysis. We believe that this simple, objective approach to spatial analysis of fixed-station monitoring datasets will aid scientists and managers in the interpretation of factors underlying the observed parameter distribution patterns. We also expect that this approach will be useful in focussing attention on specific spatial areas of concern and in generating new ideas for hypothesis testing.     

Determination of diffusion coefficients associated with the transport of210Pb radionuclides in lake bed sediments

An iterative least-squares optimization technique is utilized in conjunction with a one-dimensional representation of the mass transport equation to generate theoretical²¹⁰Pb concentration/depth profiles beneath the water-sediment interface that are best-fit approximations to directly measured²¹⁰Pb concentration/depth profiles at various locations within the Great Lakes system. The outputs of such an optimization analysis are the diffusion coefficientsD _M (molecular) andD _B (bioturbation) associated with the transport of²¹⁰Pb radionuclides in lake bed sediments. For all stations studied, the estimated values ofD _B are consistently larger than the estimated values ofD _M , emphasizing the importance of accounting for the effects of bioturbation in the modelling of contaminant transport through lake bed sediments.     

Geometry and dynamics of braided channels and bars under experimental density currents

Submarine channels convey turbidity currents, the primary means for distributing sand and coarser sediments to the deep ocean. In some cases, submarine channels have been shown to braid, in a similar way to rivers. Yet the strength of the analogy between the subaerial and submarine braided channels is incompletely understood. Six experiments with subaqueous density currents and two experiments with subaerial rivers were conducted to quantify: (i) submarine channel kinematics; and (ii) the responses of channel and bar geometry to subaerial versus submarine basin conditions, inlet conditions and the ratio of ‘flow to sediment’ discharge (Q _w/Q _s). For a range of Q _w/Q _s values spanning a factor of 2·7, subaqueous braided channels consistently developed, were deeper upstream compared to downstream, and alternated with zones of sheet flow downstream. Topographic analyses included spatial statistics and mapping bars and channels using a reduced‐complexity flow model. The ratio of the estimated depth‐slope product for the submarine channels versus the subaerial channels was greater than unity, consistent with theoretical predictions, but with downstream variations ranging over a factor of 10. For the same inlet geometry and Q _w/Q _s, a subaqueous experiment produced deeper, steeper channels with fewer channel threads than its subaerial counterpart. For the subaqueous cases, neither slope, nor braiding index, nor bar aspect ratio varied consistently with Q _w/Q _s. For the subaqueous channels, the timescale for avulsion was double the time to migrate one channel width, and one‐third the time to aggrade one channel depth. The experiments inform a new stratigraphic model for submarine braided channels, wherein sand bodies are more laterally connected and less vertically persistent than those formed by submarine meandering channels.     

A simple empirical optical model for simulating light attenuation variability in a partially mixed estuary

Representation of the subsurface light field is a crucial component of pelagic ecosystem and water quality models. Modeling the light field in estuaries is a particularly complicated problem due to the significant influence of high concentrations of dissolved and particulate matter that are derived from both terrestrial and estuarine sources. The goal of this study was to develop a relatively simple but effective way to model light attenuation variability in a turbuid estuary (Chesapeake Bay, United States) in a coupled physical-biological model. We adopted a simple, nonspectral empirical approach. Surface water quality data (salinity was used as a proxy of chromophoric dissolved organic matter [CDOM]) and light measurements from the Chesapeake Bay Program were used to determine the absorption coefficients in a linear attenuation model using regression methods. This model predicts K_c (specific attenuation due to phytoplankton/chlorophylla [chla]), K_t (specific attenuation due to total suspended solids), and K_s (a function of specific attenuation coefficients of CDOM in relation to salinity). The Bay-wide fitted relation between the light attenuation coefficient and water quality concentrations gives generally good estimates of total light attenuation, K_d. The direct inclusion of salinity in the relationship has one disadvantage: it can yield negative values for K_d at high salinities. We developed two separate models for two different salinity regimes. This approach, in addition to solving the negative K_d problem, also accounts for some changes in specific light absorption by chla, seston (nonphytoplankton particulate matter), and CDOM that apparently occur in different salinity regimes in Chesapeake Bay. The resulting model predicts the statistical characteristics (i.e., the mean and variance) of K_d quite accurately in most regions of Chesapeake Bay. We also discuss in this paper the feasibility and caveats of using K_d converted from Secchi depth in the empirical method.     

Fractal analysis of the fracture strength of lava dome material based on the grain size distribution of block-and-ash flow deposits at Unzen Volcano, Japan

A fractal theory of rock fragmentation is applied to block-and-ash flow deposits from the Fugendake dome, Unzen Volcano, Kyushu, Japan, in order to analyze the material strength and the energy required for size reduction of the source lava dome. Two fractal dimensions h and D_s, which are mutually interchangeable, represent the relative strength and energy for particles reduced to a given size. They can be theoretically estimated from the power relations of a reference grain size to the cumulative mass and number of fragments smaller than that size. The Unzen–Fugendake block-and-ash flow deposits have been further modified by size sorting and secondary fragmentation that occurred during flowage, so that the h value decreases (or D_s value increases) with increasing distance from the source. Coarse, reversely graded deposits are, however, found to retain the original size population relatively well. The D_s values estimated from deposits of this type are compatible with those previously reported from decompression–fragmentation experiments conducted on the same dome material. The employed fractal approach could thus give insights into the potential mode of dome collapse that generates block-and-ash flows.     

Measurement of seasonal variability of hydro-environmental characteristics of Kuwait Bay

To fill in the large existing data gap, this study presents results of a comprehensive data set from Kuwait Bay (KB), showing the horizontal and vertical distribution of its prominent hydrodynamic variables (i.e., water temperature, seawater salinity, seawater density) and water quality variables (i.e., chlorophyll-a concentration, turbidity, dissolved oxygen (DO) concentration, DO saturation, apparent oxygen utilization, photosynthetic active radiation). Field measurements were carried out between 11 September 2014 and 17 August 2015 covering number of monitoring stations in the entire bay. The results revealed significant seasonal variations and apparent three-dimensional features of the measured variables exemplifying the necessity of not considering the bay as a well-mixed water body in the future oceanographic studies anytime of the year. However, well-mixed conditions (with the Brunt-Vaisala frequency of about 0.0002 s^?1) were existent only in the winter. Bay’s water column began stratifying in late spring, and these were significantly intensified (with the Brunt-Vaisala frequency of about 0.0008 s^?1) during the summer and early autumn. The stratification, together with the increase oxygen demand for decomposition processes and decrease in DO solubility in summer, led to the formation of a lower DO water mass (daytime DO concentration <4.2 mg l^?1) at the lower layers. Results also suggested that the water of the KB is more transparent (indicated by lower turbidity) compared to the adjacent sea water. Measurement of light intensity along the water column indicated that the light extincts rapidly (i.e., the light reduced to approximately 10 % of the surface values at water depth of 2–6 m) in KB and many times absent at the water near the seabed. This study also suggested that the KB can be classified at a year-round negative, hypersaline, inverse, and hyperpycnal estuary.     

Analysis of Critical Excavation Depth for a Jointed Rock Slope Using a Face-to-Face Discrete Element Method

Summary The critical excavation depth of a jointed rock slope is an important problem in rock engineering. This paper studies the critical excavation depth for two idealized jointed rock slopes by employing a face-to-face discrete element method (DEM). The DEM is based on the discontinuity analysis which can consider anisotropic and discontinuous deformations due to joints and their orientations. It uses four lump-points at each surface of rock blocks to describe their interactions. The relationship between the critical excavation depth D _s and the natural slope angle α, the joint inclination angle θ as well as the strength parameters of the joints c _r ,φ _r is analyzed, and the critical excavation depth obtained with this DEM and the limit equilibrium method (LEM) is compared. Furthermore, effects of joints on the failure modes are compared between DEM simulations and experimental observations. It is found that the DEM predicts a lower critical excavation depth than the LEM if the joint structures in the rock mass are not ignored.     

Relations between load and settlement of circular foundations on or in a dense sand expressed by a function of diameter and depth

When load acts on a circular foundation on or in a dense sand, average contact pressure on the lower surface of the foundation is q and settlement of the foundation is s. Diameter and depth of the foundation are B and D_f. When the sand, B and D_f are given, we can know the relation between q and s/B by, e.g. a loading test, i.e. the relation is determined by B and D_f for the sand. Using the results of numerical analyses, we express a relation between q and s/B up to s=0.1B by functions of a single variable which is a linear combination of B and D_f. Consequently when two foundations have different B's and different D_f's but have the same value of the variable, the relations are the same. Then we examine whether the functions can express the results of eleven tests of model foundations of wide range of B and/or D_f. In all the tests, the relations are expressed with sufficient accuracy. Copyright © 2005 John Wiley & Sons, Ltd.     

The stability of sublittoral, fine-grained sediments in a subarctic estuary

The erodibility of natural estuarine sediments was measured in sit along a longitudinal transect of Manitounuk Sound, Hudson Bay, using the benthic flume Sea Carousel. Sedimentation processes along the transect varied from continuous, rapid, post-glacial sedimentation in the inner Sound, to glacial outcrops and seabed reworking of the outer Sound. The grain size and physical bulk properties reflect changes in depositional environment and correlate with sediment erosion threshold stress (τ_c), erosion rate (E), erosion type and still-water mass settling rate. There was a steady increase in τ_c (0·8–2·0 Pa) with distance down the Sound in parallel with the decreasing sedimentation rate (0·003–0·001 m yr^?1) and increasing sediment bulk density (1650–2010 kg m^?3). The near-surface friction coefficient varied up to 68° in proportion to the clay content of post-glacial material. Glacial sediments were characterized by variable results and generally higher friction coefficients. Seabed erosion in Sea Carousel began with surface creep of loose aggregates, pellets and organic debris. This was followed by Type I bed erosion at rates that varied between 0·0002 and 0·0032 kg m^?2 s^?1 (mean 0·0015). Type I peak erosion rate was inversely related to applied bed shear stress (τ_o). Type II erosion succeeded Type I, often after a broad transitional period. Simulations of suspended sediment concentration in Sea Carousel were made using four commonly used erosion (E) algorithms. The best results were obtained using Krone's dimensionless ratio relationship: E=M(τ_o/τ_c-1). Simulations were highly sensitive to the definition of erosion threshold with sediment depth [τ_c(z)]. Small errors in definition of τ_c(z) caused large errors in the prediction of suspended sediment concentration which far exceeded differences between the methods tested.     

Effect of local soil conditions on dynamic ground response in the southern coast of Izmir Bay,Turkey

The city of Izmir, located at the western end of Turkey, has experienced many strong earthquakes throughout its history. The southern coast of Izmir Bay, one of the most densely populated areas of Izmir, is located on deep alluvial sediments. It is important to determine the effect of local soil conditions on dynamic ground response in the study area, where thick loose water-saturated alluvial sediments exist. A database including geotechnical and geological information on the study area is constructed. Majority of the site is classified as D and E according to NEHRP provisions. Dynamic site response analyses are performed with EERA by utilizing the field and laboratory test results and earthquake time histories of moderate-scale earthquakes such as 1977 Izmir (M_L = 5.3), 2003 Urla (M_d = 5.6), and 2005 Uzunkuyu-Urla (M_L = 5.9), which occurred in and nearby Izmir. In addition, a scenario ground motion generated by the Izmir Fault with a magnitude of 6.5, having an average distance of 10 km to the study area, is also considered. The output data obtained from the dynamic site response analyses are evaluated, and maps displaying variation in dynamic parameters on ground surface are prepared for the southern coast of Izmir Bay, Turkey. Consequently, the dynamic analyses performed with the soil models constituted for the study area verified the damage occurred in a close distance event of 1977 Izmir earthquake. The scenario earthquake resulted in peak ground accelerations more than 0.6 g at the eastern and western ends of the study area. However, long distance events resulted in spectral amplifications by up to 5 times. With this study, it is emphasized that local soil conditions should be evaluated individually in the area of interest. Generation of a site-specific design spectrum is recommended for the areas located on deep alluvial sediments.     

Trace metal and radionuclide pollution in marine sediments of the Aegean Sea (Izmir Bay and Didim)

To determine radioactivity and trace metal levels, surface sediments were collected from two important areas (?zmir Bay and Didim) in the Aegean Sea region of Turkey, and were analyzed for concentrations of ²¹⁰Po, ²¹⁰Pb and trace metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn). The average ²¹⁰Po and ²¹⁰Pb massic activities in sediments varied in the range of 24 ± 5 to 126 ± 6 Bq kg^?1 dry wt. and 18 ± 3 to 59 ± 4 Bq kg^?1 dry wt., respectively. Izmir Bay exhibited the highest polonium activities in sediments, likely due to specific sedimentation processes and other sediment characteristics. The trace metal results showed that the Izmir Bay is facing trace metal pollution. The metal concentrations in sediment samples are low compared to those from the other neighboring marine environments.     

Spatial and temporal variation of suspended sediment concentration versus turbidity in the stream Harşit Watershed,NE Turkey

Suspended sediment related studies based on direct measurements for Turkish rivers and streams are very few for various reasons such as cost, time, or fewer operational sediment gauging stations, and thus the researchers tend to look for indirect methods. Turkey is a developing country and in need of many investments, which resulted in engineering and management modifications in its river and stream systems. This paper investigates the spatial and temporal variations of suspended sediment concentration (SSC, mg/L) versus turbidity (NTU) in the stream Har?it watershed having a length of the main branch at 143 km and catchment area of 3,280 km², Eastern Black Sea Basin of Turkey. Subsequently, the effect of the all kinds of anthropogenic activities was evaluated, namely, dam operation, levee construction, municipal wastewater discharge, sand–gravel mining on SSC, and turbidity in the watershed. In situ turbidity monitoring and water sampling studies were semimonthly conducted at ten stations from March 2009 to February 2010. On a semimonthly basis, it was revealed that SSC and turbidity values having significant correlation varied spatially and temporarily. The municipal wastewater discharge from the city of Gümü?hane together with the decreasing flow rate revealed itself with a significant increase in the turbidity and SSC, especially in the summer months. Torul Dam having a reservoir volume of 168 hm³ could trap 78.6 % of the SSC and could remove 66.6 % of the turbidity, thanks to its long hydraulic residence time; however, Kürtün Dam released the suspended sediment as a result of its sluiceway tunnel operation. Sand–gravel mining activities between the last two stations severely affected the water quality by increasing the SSC and turbidity.     

Vertical Profiles of Major Organic Geochemical Constituents and Extracellular Enzymatic Activities in Sandy Sediments of Aransas and Copano Bays, TX

Vertical distribution patterns of organic geochemical constituents and the enzymes aminopeptidase and β-glucosidase provide insights about the nature and reactivity of sediment organic matter in the sandy sediments of two shallow “South Texas” estuaries. Sediment total organic carbon (TOC) δ¹³C values indicated that the organic matter (OM) was derived more from a mixture of seagrass and phytoplankton than from terrigenous OM. Down-core amounts of TOC and total nitrogen (TN) were <0.2% of dry weight, respectively. Enzyme activities were highest near surface and ranged from 25 to 1 μM/h for aminopeptidase as compared to 5 to 0.2 μM/h for glucosidase. In Aransas Bay, aminopeptidase activity correlated with sediment TN content (r _s = 0.30) and β-glucosidase with TOC content (r _s = 0.27). In Copano Bay, aminopeptidase correlated with TOC, TN, and carbohydrate content (r _s = 0.89, 0.90, and 0.83, respectively). Variations of glucosidase activity also related positively to TOC, TN, and total carbohydrate content (r _s = 0.68, 0.77, and 0.48, respectively). Overall, enzyme activities in these low OM, sandy sediments resembled those for other benthic marine environments.     

Imaging of Vp, Vs, and Poisson’s ratio anomalies beneath Kyushu, southwest Japan: Implications for volcanism and forearc mantle wedge serpentinization

We determine detailed 3-D V_p and V_s structures of the crust and uppermost mantle beneath the Kyushu Island, southwest Japan, using a large number of arrival times from local earthquakes. From the obtained V_p and V_s models, we further calculate Poisson’s ratio images beneath the study area. By using this large data set, we successfully image the 3-D seismic velocity and Poisson’s ratio structures beneath Kyushu down to a depth of 150 km with a more reliable spatial resolution than previous studies. Our results show very clear low V_p and low V_s anomalies in the crust and uppermost mantle beneath the northern volcanoes, such as Abu, Kujyu and Unzen. Low-velocity anomalies are seen in the mantle beneath most other volcanoes. In contrast, there are no significant low-velocity anomalies in the crust or in the upper mantle between Aso and Kirishima. The subducting Philippine Sea slab is imaged generally as a high-velocity anomaly down to a depth of 150 km with some patches of normal to low seismic wave velocities. The Poisson’s ratio is almost normal beneath most volcanoes. The crustal seismicity is distributed in both the high- and low-velocity zones, but most distinctly in the low Poisson’s ratio zone. A high Poisson’s ratio region is found in the forearc crustal wedge above the slab in the junction area with Shikoku and Honshu; this high Poisson’s ratio could be caused by fluid-filled cracks induced by dehydration from the Philippine Sea slab. The Poisson’s ratio is normal to low in the forearc mantle in middle-south Kyushu. This is consistent with the absence of low-frequency tremors, and may indicate that dehydration from the subducting crust is not vigorous in this region.     

A numerical simulation of residual circulation in Tampa Bay. Part I: Low-frequency temporal variations

The residual (time-average) salinity and circulation in a numerical ocean model of the Tampa Bay estuary are shown to experience significant temporal variation under realistic forcing conditions. A version of the Estuarine Coastal Ocean Model developed for Tampa Bay with 70 by 100 horizontal grid points and 11 sigma levels is examined for the years 2001–2003. Model output variables are averaged over the entire time of the simulation to generate long-term residual fields. The residual axial current is found to be dominated by the buoyancy-driven baroclinic circulation with an outflow (southwestward) at the surface and to the sides of the shipping channel, and an inflow (northeastward) usually occurring subsurface within or above the shipping channel. Averages over 30 d are used to examine variations in the residual fields. During the simulation the average surface salinity near the head of Tampa Bay varies with the freshwater inflow, from 12‰ to 33%. At the bay mouth salinity varies from 30%. to 36%.. A localized measure of the baroclinic circulation in the shipping channel indicates the residual circulation can vary strongly, attaining a magnitude triple the long-term mean value. The baroclinic circulation can be disrupted, going to near zero or even reversing, when the buoyancy-driven flow is weak and the surface winds are to the northeast. Three time periods, representing different environmental conditions, are chosen to examine these results in detail. A scaling argument indicates the relative strength of buoyancy versus wind as ΔρgH²(LC _Dω²)⁻¹, where δρ is head-to-mouth density difference across the bay,g is gravitational acceleration,H is depth,L is bay length,C _D is the surface wind drag coefficient, andw is wind speed. Tampa Bay is usually in the buoyancy dominated regime. The importance of winds in the weak-buoyancy case is demonstrated in an additional simulation without wind stress.     

An experimental investigation of the partitioning of REE between garnet and liquid with reference to the role of defect equilibria

The partitioning of samarium and thulium between garnets and melts in the systems Mg₃Al₂-Si₃O₁₂-H₂O and Ca₃Al₂Si₃O₁₂-H₂O has been studied as a function of REE concentration in the garnets at 30 kbar pressure. Synthesis experiments of variable time under constant P, T conditions indicate that garnet initially crystallizes rapidly to produce apparent values of D _Sm (D _Sm=concentration of Sm in garnet/concentration of Sm in liquid) which are too large in the case of pyrope and too small in the case of grossular. As the experiment proceeds, Sm diffuses out of or into the garnet and the equilibrium value of D _Sm is approached. Approximate values of diffusion coefficients for Sm in pyrope garnet obtained by this method are 6 × 10^–13 cm² s^–1 at 1,300 ° C and 2 × 10^–12 cm² s^–1 at 1,500 ° C, and for grossular, 8.3 × 10^–12 cm² s^–1 at 1,200 ° C and 4.6 × 10^–11 cm² s^–1 at 1,300 ° C. The equilibrium values of D _Sm have been reversed by experiments with Sm-free pyrope and Sm-bearing glass, and with Sm-bearing grossular and Sm-free glass.Between 12 ppm and 1,000 ppm Sm in pyrope at 1,300 ° C and between 80 ppm and >2 wt.% Tm in pyrope at 1,500 ° C, partition coefficients are constant and independent of REE concentration. Above 100 ppm of Sm in garnet at 1,500 ° C, partition coefficients are independent of Sm concentration. At lower concentrations, however, D _Sm is dependent upon the Sm content of the garnet. The two regions may be interpreted in terms of charge-balanced substitution of Sm₃Al₅O₁₂ in the garnet at high Sm concentrations and defect equilibria involving cation vacancies at low concentrations. At very low REE concentrations (< 1 ppm Tm in grossular at 1,300 ° C) D_REE garnet/liquid again becomes constant with an apparent Henry's Law value greater than that at high concentrations. This may be interpreted in terms of a large abundance of cation vacancies relative to the number of REE ions.The importance of defects in the low concentration region has been confirmed by adding other REE (at 80 ppm level) to the system Mg₃Al₂Si₃O₁₂-H₂O at low Sm concentrations. These change D _Sm in the defect region, demonstrating their role in the production of vacancies.Experiments on a natural pyropic garnet indicate that defect equilibria are of importance to REE partitioning within the concentration ranges found in nature.     

Some new aspects of two-dimensional turbidity currents*

A theoretical consideration of two dimensional underflows and surge-type turbidity currents results in a general momentum equation. A number of formulae in current use are special cases of this equation, among which are the modified Chézy equation and Bagnold's criterion for autosuspension. Five dimensionless parameters are included: the Richardson number Ri (defined as the inverse square of the Froude number), the friction coefficient c_f, the slope β, the dimensionless settling velocity of the sediment V_s/u and the changes in flow height with distance dD/dx. The latter is mainly a measure of the dilution by entrainment of ambient water. For chalk powder experiments on surge type turbidity currents and on the initial front of continuous underflows the momentum equation is shown to be correct. Values for Ri range from about 1.5 at 0° slope to about 0.75 at 5° and are slightly to substantially lower than values from earlier authors. The two types of turbidity currents investigated show close similarity. A surprising attribute is their strong dilution even at very low-angle slopes. Pelitic sedimentation is possible from the upper, dilute part of the currents, graded intervals found at the base of turbidites can be explained as bedload deposits from the lowermost, concentrated layer of the current; hydraulic jumps are expected to be rare in surge-type turbidity currents and fronts of incipient underflows.     

Hydrogen isotopes in individual alkenones from the Chesapeake Bay estuary

Hydrogen isotope ratios of individual alkenones from haptophyte algae were measured in suspended particles and surface sediment from the Chesapeake Bay (CB) estuary, eastern USA, in order to determine their relationship to water δD values and salinity. δD values of four alkenones (MeC_37:2, MeC_37:3, EtC_38:2, EtC_38:3) from particles and sediments were between −165‰ and −221‰ and increased linearly (R² = 0.7-0.9) with water δD values from the head to the mouth of the Bay. Individual alkenones were depleted in deuterium by 156-188‰ relative to water. The MeC₃₇ alkenones were consistently enriched by ∼12‰ relative to the EtC₃₈ alkenones, and the di-unsaturated alkenones of both varieties were consistently enriched by ∼20‰ relative to the tri-unsaturated alkenones. All of the increase in alkenone δD values could be accounted for by the water δD increase. Consequently, no net change in alkenone-water D/H fractionation occurred as a result of the salinity increase from 10 to 29. This observation is at odds with results from culture studies with alkenone-producing marine coccolithophorids, and from two field studies, one with a dinoflagellate sterol in the CB, and one with a wide variety of lipids in saline ponds on Christmas Island, that indicate a decline in D/H fractionation with increasing salinity. Why D/H fractionation in alkenones in the CB showed no dependence on salinity, while D/H fractionation in CB dinsoterol decreased by 1‰ per unit increase in salinity remains to be determined. Two hypotheses we consider to be valid are that (i) the assemblage of alkenone-producing haptophytes changes along the Bay and each species has a different sensitivity to salinity, such that no apparent trend in α_{alkenone-water} occurs along the salinity gradient, and (ii) greater osmoregulation capacity in coastal haptophytes may result in a diminished sensitivity of alkenone-water D/H fractionation to salinity changes.     

Seasonal and long-term trends in the water quality of Florida Bay (1989–1997)

Analysis of 6 yr of monthly water quality data was performed on three distinct zones of Florida Bay: the eastern bay, central bay, and western bay. Each zone was analyzed for trends at intra-annual (seasonal), interannual (oscillation), and long-term (monotonic) scales. the variables TON, TOC, temperature, and TN∶TP ratio had seasonal maxima in the summer rainy season; APA and Chla, indicators of the size and activity of the microplankton tended to have maxima in the fall. In contrast, NO₃ ⁻, NO₂ ⁻, NH₄ ⁺, turbidity, and DO_sat, were highest in the winter dry season. There were large changes in some of the water quality variables of Florida Bay over the study period. Salinity and TP concentrations declined baywide while turbidity increased dramatically. Salinity declined in the eastern, central, and western Florida Bay by 13.6‰, 11.6‰, and 5.6‰, respectively. Some of the decrease in the eastern bay could be accounted for by increased freshwater flows from the Everglades. In contrast to most other estuarine systems, increased runoff may have been partially responsible for the decrease in TP concentrations as input concentrations were 0.3–0.5 μM. Turbidity in the eastern bay increased twofold from 1991 to 1996, while in the central and western bays it increased by factors of 20 and 4, respectively. Chla concentrations were particularly dynamic and spatially heterogeneous. In the eastern bay, which makes up roughly half of the surface area of Florida Bay, Chla declined by 0.9 μg l⁻¹ (63%). The hydrographically isolated central bay zone underwent a fivefold increase in phytoplankton biomass from 1989 to 1994, then rapidly declined to previous levels by 1996. In western Florida Bay there was a significant increase in Chla, yet median concentrations of Chla in the water column remained modest (∼2 μg l⁻¹) by most estuarine standards. Only in the central bay did the DIN pool increase substantially (threefold to sixfold). Notably, these changes in turbidity and phytoplankton biomass occurred after the poorly-understood seagrass die-off in 1987. It is likely the death and decomposition of large amounts of seagrass biomass can at least partially explain some of the changes in water quality of Florida Bay, but the connections are temporally disjoint and the process indirect and not well understood.