Net, actual, and potential sediment-water interface NH4 fluxes in the northern Gulf of Mexico (NGOMEX): Evidence for NH4 limitation of microbial dynamics

A new approach, combining ¹⁵NH₄⁺ isotope dilution and continuous-flow techniques, provided estimates of “actual” and “net” NH₄⁺ flux and sediment NH₄⁺ demand (SAD) at the sediment-water interface (SWI) of sites in the northern Gulf of Mexico (NGOMEX). The sites included a hypoxic site (C6), two sites with intermediate oxygen levels (B7 and F5), and a normoxic site (CT). Control cores without isotope addition and other cores from the same site treated with ¹⁵NH₄⁺ labeled overflowing water differentiated between net and actual regeneration flux and actual vs. potential uptake flux of NH₄⁺. Experiments were conducted in 2008 before (July) and after (September) two successive hurricanes (Gustav and Ike) and in January and August, 2009. Actual regeneration was significantly higher than net flux at most sites. Net flux did not differ significantly in most sites/dates, but the actual regeneration, and the actual and potential uptake, showed temporal and spatial variation; the flux at the hypoxic site was more active than non-hypoxic sites. SAD, the difference between potential and actual NH₄⁺ uptake flux, was higher at the hypoxic site than at non-hypoxic sites before and after the hurricanes in 2008 and during the hypoxia season in 2009. SAD related negatively to bottom water DO values. Conclusions: (1) net flux often underestimated actual regeneration, (2) hurricane activity decreased N dynamics, and (3) microbial N limitation status at the hypoxic site related to NH₄⁺ removal processes that were independent of oxygen (e.g., anaerobic heterotrophic uptake or anammox). These results indicate a rather consistent NH₄⁺ demand at the SWI during the hypoxic season and suggest that reduced nitrogen may limit microbial dynamics in the region.     

Recent sediment dynamics in hadal trenches: Evidence for the influence of higher-frequency (tidal,near-inertial) fluid dynamics

In addition to high hydrostatic pressure, scarcity of food is viewed as a factor that limits the abundance and activity of heterotrophic organisms at great ocean depths, including hadal trenches. Supply of nutritious food largely relies on the flux of organic-rich particulate matter from the surface ocean. It has been speculated that the shape of hadal trenches helps to ‘funnel’ particulate matter into the deeper parts of the trench, leading to sediment ‘focussing’ and improved benthic food supply. Here we investigate for five Northwest Pacific trenches the efficiency of sediment focussing by evaluating ratios of measured (sediment-derived) and expected (water-column-derived) sedimentary inventories of the naturally occurring and radioactive particulate-matter tracer ²¹⁰Pb_xs. The sites comprise a broad range of surface-ocean productivity and physical-oceanographic regimes. Across the five trench-axis settings the inventory ratio varies between 0.5 and 4.1, with four trench-axis settings having ratios>1 (sediment focussing) and one trench-axis setting a ratio<1 (sediment winnowing). Although the fluid- and sediment-dynamical forcing behind sediment focussing remains unclear, this study finds evidence for another mechanism that is superimposed on, and counteracts, the focussing mechanism. This superimposed mechanism is related to higher-frequency (tidal, near-inertial) fluid dynamics. In particular, there is evidence for a strong and negative relation between the intensity of propagating internal tides and the extent of sediment focussing in the trench-axis. The relation can be approximated by a power function and the most intense drop in sediment focussing already occurs at moderate internal-tide intensities. This suggests that propagating internal tides may have a subtle but significant influence on particulate-matter dynamics and food supply in hadal trenches in particular, but possibly also in the deep seas in general. A mechanism for the influence of internal tides on sediment dynamics is proposed.     

Biodiversity and community composition of sediment macrofauna associated with deep-sea Lophelia pertusa habitats in the Gulf of Mexico

Scleractinian corals create three-dimensional reefs that provide sheltered refuges, facilitate sediment accumulation, and enhance colonization of encrusting fauna. While heterogeneous coral habitats can harbor high levels of biodiversity, their effect on the community composition within nearby sediments remains unclear, particularly in the deep sea. Sediment macrofauna from deep-sea coral habitats (Lophelia pertusa) and non-coral, background sediments were examined at three sites in the northern Gulf of Mexico (VK826, VK906, MC751, 350–500 m depth) to determine whether macrofaunal abundance, diversity, and community composition near corals differed from background soft-sediments. Macrofaunal densities ranged from 26 to 125 individuals 32 cm⁻² and were significantly greater near coral versus background sediments only at VK826. Of the 86 benthic invertebrate taxa identified, 16 were exclusive to near-coral habitats, while 14 were found only in background sediments. Diversity (Fisher’s α) and evenness were significantly higher within near-coral sediments only at MC751 while taxon richness was similar among all habitats. Community composition was significantly different both between near-coral and background sediments and among the three primary sites. Polychaetes numerically dominated all samples, accounting for up to 70% of the total individuals near coral, whereas peracarid crustaceans were proportionally more abundant in background sediments (18%) than in those near coral (10%). The reef effect differed among sites, with community patterns potentially influenced by the size of reef habitat. Taxon turnover occurred with distance from the reef, suggesting that reef extent may represent an important factor in structuring sediment communities near L. pertusa. Polychaete communities in both habitats differed from other Gulf of Mexico (GOM) soft sediments based on data from previous studies, and we hypothesize that local environmental conditions found near L. pertusa may influence the macrofaunal community structure beyond the edges of the reef. This study represents the first assessment of L. pertusa-associated sediment communities in the GOM and provides baseline data that can help define the role of transition zones, from deep reefs to soft sediments, in shaping macrofaunal community structure and maintaining biodiversity; this information can help guide future conservation and management activities.     

基于多尺度资料的体系域划分及其意义——以渤海海域沙垒田凸起东南部东二段为例

不同体系域砂体分布和成藏特征存在明显的差别,正确划分体系域对于油气勘探,特别是隐蔽油气藏勘探具有重要意义。相对于海相地层,由于陆相层序本身的复杂性,并不发育明显的坡折带,在进行体系域划分时,不同学者有不同的划分方案,无论在井上还是地震上很难把握各体系域界面的位置。针对这个问题,结合渤海海域沙垒田凸起东南部东二段体系域划分,提出利用多种不同尺度资料层层控制来划分低位域、湖泛域和高位域的思路方法。这种从尺度较大资料到较小尺度资料层层控制来划分体系域的思路方法,可以避免直接从井上来进行划分导致的错误结论,同时相对湖平面变化曲线法的引用,向建立渤海东营组层序划分的标准迈向了坚实一步。     

Temporal Change of Runoff and Sediment Load and their Differential Response to Human Activities：A Case Study for a Well-vegetated Mountain Watershed of Southern China

This study examined the temporal trends of runoff and sediment load and their differential response to human activities in the Lishui river,a tributary of the Yangtze river in southern China.The long-term observation data at four gauging stations,generally involving two periods from 1954 to 1985 and from 2007 to 2011,were used.We detected no significant temporal trend for both the annual runoff volume（Q） and the annual suspended Sediment Load（SL） over more than 30 years before 1985.The flow duration curves and the Suspended Sediment Concentration（SSC） also hold constant before 1985.Compared with the period before 1985,SL has decreased by about 80% though Q remains unchanged for the period after 2007.Detailed examination shows that the flow duration curves after 2007 have changed with a significant decrease in the high-flow component,which acts as a major cause for the decreasing SL.In addition,SSC has decreased by several times,which also contributes to the decrease in SL after 2007.Both decreases in high-flow discharges and in SSC can be linked with recent human activities,mainly including vegetation establishment and dam constructions.The constant Q and the decreasing SL are also reported for the main stream of the Yangtze River and other major rivers in southern China,although they are orders of magnitude larger than our study area in drainage area size.The present study highlights the importance of high-flow discharges on SL and suggests that the use of SL is more appropriate to reflect environmental change than Q.     

General Table of Contents(Vol.11,2014)

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山东省近岸海域沉积物类型及变化趋势

文章依据山东省近岸海域表层沉积物调查资料,分析了沉积物粒度的时空变化特征及其影响因素。结果显示:2020年,山东省近岸海域表层沉积物粒径总体呈由近岸向远海逐渐变小的趋势,其中,粉砂含量最高,平均66.45%;砂次之,平均26.78%;黏土含量较低,平均6.77%。沉积物不同粒级组分具有明显的空间分布差异性,类型包含粉砂、砂质粉砂、粉砂质砂和砂,以粉砂为主,占调查站位的61.9%。2016—2020年,山东省近岸海域表层沉积物整体呈现粗化的趋势,其中,日照、青岛、潍坊和滨州近岸海域粗化趋势较为明显,东营次之,烟台和威海粗化程度最低。沉积物粒度时空特征的主要影响因素包含物源、区域水动力条件(如,渤海和黄海海流、山东省沿岸流等)及沿海海岸工程建设情况,具体的影响机制仍需进一步调查研究。本研究成果可以为山东省近岸海域底质类型变化分析和海岸带保护与利用提供参考。     

Observations of sea ice influence on the littoral sediment exchange,North Zealand,Denmark

Nielsen, Niels: Observations of sea ice influence on the littoral sediment exchange, North Zealand, Denmark. Geografisk Tidsskrift 88:61–67. Copenhagen. 1988.

In 1985 and 1986 monthly surveys of sediment volumes were carried out on a sandy beach and its nearshore platform at the north coast of Zealand. Both winters in the research period had temperatures below average, and consequently abundant sea ice formations. ‘Ice winters’ in open Danish waters occur statistically about 1/3 of the winters. The appearance of sea ice resulted in marked loss of sediment from the whole littoral zone, but by far the largest net erosion could be detected on the nearshore plane. An important factor is assumed to be the interaction of waves with the icefoot and ice pile-ups. For the beach itself, ice-glazing of the beach surface and interstitial frost in the foreshore sediments caused wave erosion within and just off the swash zone due to variations of the percolation parameter. This dynamic was observed during both the freeze-up and thaw-up periods. On the backshore, wind-derived forms characterized the winter beach, but here the net sediment budget was less affected.     

Parameterized expressions for an improved Rouse equation

The equilibrium profile of suspended sediment concentration （SSC） in steady, uniform turbulent flow has been extensively studied since the proposition of Rouse equation （Rouse, 1937）. Many researchers keep studying and attempting to improve the Rouse equation by modifying the mixing length concept. Bose and Dey （2009） improved Rouse equation based on the modified sediment diffusivity, however, their proposed sediment diffusivity has two parameters, the depth modification factor α and the reciprocal of the sediment Schmidt number β = 1/S, and the corresponding expressions were not given properly in their paper. This paper aims to parameterize the relevant coefficients, the results shows： （1） α is parameterized from a reasonable approximation by the analysis of surface boundary condition of SSC; （2） considering some effect factors on the settling velocity, β can be expressed by an extended and modified Montes-Ippen formula, and its related parameters are calibrated by substantial data about the relationship between the primary Rouse parameter Z. = ω/ku and its counterpart Z＇ = ω/βku.. Through verification with some experimental and field datasets, and comparison with related formula, the results show that the sediment diffusivity and SSC profiles have a good agreement with measured data used the present parameterized expressions of a and,8, so the present improved Rouse equation is reasonable, and can be applied in practical applications. Finally, an approximation and simplified expression of the improved Rouse equation is deduced by using the approach of perturbation, which can be applied easily for the estimation of suspended sediment transport rate.     

Fully coupled methods for multiphase morphodynamics

We present numerical methods for a system of equations consisting of the two dimensional Saint–Venant shallow water equations (SWEs) fully coupled to a completely generalized Exner formulation of hydrodynamically driven sediment discharge. This formulation is implemented by way of a discontinuous Galerkin (DG) finite element method, using a Roe Flux for the advective components and the unified form for the dissipative components. We implement a number of Runge–Kutta time integrators, including a family of strong stability preserving (SSP) schemes, and Runge–Kutta Chebyshev (RKC) methods. A brief discussion is provided regarding implementational details for generalizable computer algebra tokenization using arbitrary algebraic fluxes. We then run numerical experiments to show standard convergence rates, and discuss important mathematical and numerical nuances that arise due to prominent features in the coupled system, such as the emergence of nondifferentiable and sharp zero crossing functions, radii of convergence in manufactured solutions, and nonconservative product (NCP) formalisms. Finally we present a challenging application model concerning hydrothermal venting across metalliferous muds in the presence of chemical reactions occurring in low pH environments.