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Physicomechanical Properties of the Water–Sediment Barrier Zone

Журнал ОКЕАНОЛОГИЯ РАН
Физико-механические свойства барьерной зоны вода-осадок
22.06.2012
АТЛАС подводных потенциально опасных объектов акваторий морей Российской Федерации
АТЛАС подводных потенциально опасных объектов акваторий морей Российской Федерации
23.09.2012
Oceanology
2012, Vol. 52, No. 06, pp. 877 - 883

The complex geoecological works carried out in the St. Anna Trough resulted in the detailed investigation of the water–sediment barrier zone. The physicomechanical properties such as the moisture, the moisture at the flowing boundary, the sedimentation rates, and the strength of the sediments measured and calculated for the upper 10cmthick layer of the bottom sediments were used as the main parameters characterizing the barrier zone. These data served as a basis for developing the model of the barrier zone with defining the lithogenesis stages: protosyngenesis—syngenesis—protodiagenegis—early diagenesis. The quantitative estimates of the environmental physicochemical properties presented in this work characterize each of these stages.


G. I. Ivanova, A. A. Svertilovb, and M. A. Kholmyanskiic

DOI: 10.1134/S0001437012060057
 

INTRODUCTION

С onsidering marine geoecology as a scientific branch that studies the technogenic component in the bottom sediments and its influence on the marine ecosystems, we should primarily discriminate it from the natural component and define the sources, the migration paths, the mechanisms of the transformation and accumulation at different levels of the substance orga nization [3], and the scale of the technogenic impacton the biota during the sedimentation [2]. Dissimilar to terrestrial sediments, the technogenic component in marine sediments is mostly confined to the upper most 50cmthick layer, except for the coastal zone and the development areas of mineral resources. This layer represents a system of lithogeochemical barriers confined to the physicochemical hydrosphere–lithosphere interface.

The analysis of the migration paths and substance transformation mechanisms made it possible to recognize and typify the main lithochemical barriers in the surface bottom sediments with the water–sediment boundary being the most important among them. It is shown that the concentration of any chemical elements in the bottom sediments is determined to a large degree by the physicochemical conditions at this barrier and the intensity of the differently oriented migration of the chemical elements from the water column and lithosphere to the surface layer of the bottom sediments [1, 15], where they may accumulate in the barrier zone.

The physical and geochemical properties of the barrier zone in the bottom layer determine to a considerable degree the content of pollutants and the intensity of their accumulation in the bottom sediments, as well as the character of the benthic biocoenoses, which reflect the state of the bottom ecosystem. The discrimination of the technogenic component in the bottom sediments represents an obligatory condition for correct interpretations concerning the scale of the technogenic pollution of the bottom sediments.

The entire barrier zone is usually 0.5–1.5 m thick. The water–sediment contact is its most important barrier. The thickness of this boundary zone usually ranges from 10 to 15 cm for normal depositional environments of finegrained sediments. Precisely this zone represents the main object of marine geoecological investigations.

The lithogenetic processes in this zone are very specific. Two autonomous lithogenesis stages, i.e., protosyngenesis and syngenesis [5], which determine the multilayer structure of the upper sedimentary layer, are recognizable for the clayey varieties

At the protosyngenesis stage, the sedimentary material is concentrated at the bottom surface in the form of particulate matter characterized by the properties of a Newtonian liquid. The existence of such an unstable substance is explained by the development of the diffused viscous sublayer in the water column at the water–bottom interface [6–8, 13, 14, 17]. The thickness of the suspension layer depends on the bottom’s morphology, the velocity of the bottom currents, and the sedimentation rates. It usually varies from 1 to 3 cm except for the natural sedimentary traps in localized bottom depressions. In fact, this seasonal ephemeral zone is not a real geological body: it flows easily in the form of a “heavy liquid” along slopes, is trans ported by bottom currents, and roiled by waves. In special Russian publications, it is frequently callednailok. In the physicochemical aspect, it represents most likely a diffusion zone between the water column and the bottom, not between the bottom sediments and the water column. In this connection, the samples taken for the geoecological investigations are not representative, since they cannot be considered as characterizing the real bottom. At the same time, there is the opinion that precisely the upper 1cmthick layer of particulate matter should be sampled for geoecological investigations.

At the syngenesis stage,the sediment acquires a mechanical structure despite its flowing consistency and possesses the properties of a solid body, which are controlled by rheological laws. The thickness of these sediments is usually less than 10–15 cm. The syngenetic layer becomes thicker on account of the diffusion layer interacting with the latter through the interstitial solutions being fed by them and combined with the immobile diffusion sublayer of the bottom layer of the hydrosphere, and the protosyngenetic sublayer creates environments tolerant toward biogenic sediments.

Consequently, the surface layer of the bottom sediments is the most important and, likely, the most significant system of barrier zones in the World Ocean, although distinct criteria for its defining and determining its upper boundary, i.e., the bottom’s surface, are still missing.

The thickness of the syngenesis zone depends on the sedimentation rates and the hydrophilic properties of the sedimentary material, which determine its dispersion, and the concentrations of organic matter, which accelerate the structurization of the bottom sediments. The high moisture susceptibility suppresses the consolidation (gravitational compacting) processes.

The cause–effect succession of the sedimentation is considered in [16], where it is shown that it is characterized by the following relations: the water’s depth the content of the pelitic and aleuritic fractions in the sediments the moisture susceptibility the initial moisture (porosity and density) the sedimentation coefficient. At the qualitative level, this succession may be added to by the influence of the bottom currents, the slope angles, the bottom surface exposure relative to the current’s vector, the bioproductivity of the water column, and other factors.

The purpose of this work is to investigate the structure of the surface sedimentary layer using the laws of the physics of dispersion systems and the methods of marine sedimentology for defining in these sediments the zones corresponding to the particular lithogenesis stages and substages. The main concept consists in considering the diagenesis exclusively as a process of the sediment’s transformation into sedimentary rock. With such an interpretation, the increase in the strength of the bottom sediments represents an integral indicator of the diagenesis. Therefore, the sediment’s strength was selected as the main parameter to characterize the bottom layer’s structure and its mechanical properties during these investigations. In addition to the strength, the moisture (We) and moisture susceptibility (WL) of the sediments were also determined.

METHODS

The investigations were carried out during the cruise of the R/V Professor Logachev in the St. Anna Trough located in the Kara Sea [14, 16]. The geoecological works, which included hydrophysical, hydrochemical, hydrobiological, geological, and geochemical investigations, were aimed at the study of the physicomechanical properties of the bottom sediments. Particular attention was paid to the investigation of the water–sediment barrier in order to typify the sedimentation conditions and assess the character and intensity of the physicochemical processes [9–11, 13, 15].

The surface sediments of the St. Anna Trough are represented by pelitic and aleuritic muds. The gray green mud and clay contain small caverns and pockets filled with fluidflowing sediments. The beds of fluid flowing sediments are underlain by lithificates, below which the strength and moisture of the sediments remain practically stable. Such compaction patterns of the sediments indicate that the low water permeability at the boundary between the oxidized and reduced sediments prevents the gravitational compaction of the graygreen muds. A remarkable feature of these sediments is the almost complete isotropy of the interstitial space, which is determined from the “shrink age” of cylindrical samples.

Figure 1 shows a schematic map of the sampling sites in the studied area of the St. Anna Trough. The undisturbed sediments used for the investigation of the physicochemical properties were sampled from a sediment monolith obtained by a box corer (1 m2 in size and 2 m thick) and placed into thinwalled plastic cylinders 145 mm in diameter and 150 mm long. The bulk samples of the bottom suspension and the upper 1cm thick mud layers were taken from the surface of the sediment monolith for the investigation of their chemical composition, moisture (We), and moisture susceptibility (WL). The hydrogen index of the sediments (pH) was measured immediately in the box corer.

The sediment’s strength was measured with a step of 0.5 cm in the uppermost 3 cm thick layer and every 1–3 cm to a depth of 10 cm.

The specific resistance to penetration R = P/S, where Р is the load on the conical sonde of the penetrometer and S is its transverse section, was selected to represent the strength index.


Investigation of the mechanical properties in the surface mud layer. The measurement results were used for constructing plots of the variations in the strength and moisture through the section, which were in turn analyzed by the method of the frequency derivative (dR/dH) and (dW/dH). The extremums of the derivatives are positive for the strength and negative for the moisture, while being correlated with the changes in the sediment’s consistency and the significant increments of the strength.

The analysis of the physicomechanical properties in the upper 10cmthick mud layer reveals that the latter is heterogeneous (Fig. 2).

At depths of 1.5–2.0 and 5–7 cm, the strength increases in a spasmodic manner with a practically linear moisture decrease. The spasmodic strength growths and changes in the physical state of the sediments (liquid flowing plastic) correspond to the succession of the lithogenesis stages in Sval’nov’s model [5]: protosyngenesis syngenesiis protodiagenesis. At these stages, the strength increases from 0.0002 to 0.0005 to 0.005 kPa, respectively (table). The spasmodic increase in the strength is determined by the increased abundance and areas of individual coagulation contacts between the structural elements under the reduction of the interstitial space related to the differences in the particles arrangement and by the discrete changes in the number of individual contacts under the increased concentration of clay particles in the dispersion environments [12].

At the protosyngenesis stage, the sediments form a 1 to 2cmthick suspension layer developed in the bottom depressions under calm hydrodynamic conditions and are characterized by a strength of <2 kPa. The suspension is structureless and demonstrates no properties of a solid body.

The sediments of the syngeneic and protodiageneic layers serve as the bottom surface. The difference between the sediments at these stages is determined by the transition of the interrelations between the structural elements from mostly remote to closer coagulation contacts, which is responsible for the spasmodic strength growth. These sediments exhibit thixotropic properties, and their structure and strength become restored during 2–3 days after mechanical disturbances (bioturbation).

Consequently, the strength variations in the surface layer of the bottom sediments are primarily governed by the laws of the mechanics of dispersion systems, while the surface layer itself represents a selforganizing dispersion system.

The transformation of the sediments from the fluidflowing state into the flowing one is accompanied by blocking of the free interstitial space by hydrate envelopes. This process results in the formation of the first diffusion barrier and the disturbance of the free water exchange between the interstitial solutions and the bottom waters. The decomposition of organic matter below this diffusion barrier reduces the oxygen content in the interstitial solutions to drastically change the physicochemical properties of the sediments and form a physicochemical barrier at the boundary between the syngenetic and protodiagenetic layers due to the organic matter transformations. Therefore, this boundary is frequently marked by the transformation of the oxidized sediments into slightly reduced varieties.

It is shown [10, 11] that the sedimentation rates may be estimated at the qualitative level from the ratio between the water susceptibility and the moisture of the sediments: Kсед =2WL/W0, where Kсед is the parameter of the sedimentation conditions, 2WL is the doubled value of the sediment’s moisture at the fluidity boundary, and W0 is the sediment’s moisture. When Kсед is close to 1, the sedimentation rates are normal, Kсед > 1.3 indicates avalanche sedimentation, and Kсед < 0.8 points to bottom erosion.

As a whole, according to the engineering–geological classification of the mud, the surface bottom sediments of the St. Anna Trough may be attributed to extremely hydrated fluidflowing deepwater mud with medium moisture susceptibility. In its southern part, the sediments from the upper layer are gleyey despite the shallower average water depths. These sediments are characterized by higher moisture susceptibility and sedimentation coefficients as compared with their counterparts in the central and northern parts of the studied area, while the lateral variations in their properties and composition are substantially lower. The lower pH values in the southern part of the studied area, the degradation of the oxidized sedimentary layer, and the increase in the moisture susceptibility of the sediments point to the stable and relatively high sedimentation rates in the southern part of the studied area due to the increased share of biogenic compo nents in the sedimentary material.

The analysis of the presented data on the sedimentation patterns and geochemical features of the surface bottom layer provides grounds for the conclusion that the depositional conditions in the northern and southern parts of the studied area are different. The boundary between them is located at 79° N. In the east and west, the areas with uniform sedimentation conditions are bordered by the 100m isobaths of the trough slopes. The surface layer in the largest part of the studied area is dominated by oxidizing conditions, while, in its southern part, oxidizing–gleyey settings are locally developed. In addition, they are in turn subdivided into smaller geochemical landscapes depending on the degree of hydrodynamic activity in the bottom layer. The bottom surface on the northeastern slope of the trough suggests the influence of a strong bottom current; the remaining part of the studied area is characterized by uniform sedimentation rates with some increase in the axial part of the trough, at the base of its western slope, and in the southern part of the studied area.

Against the background of similar regularities in the sedimentation pattern, the sediments from the northern and southern parts of the studied area exhibit differences in both their moisture susceptibility and compaction patterns at the initial diagenesis stage. The changes in these parameters are documented between 79° N and 80° N, while being slightly masked in the axial part of the trough. The thickness of the oxidized layer in the northern and middle parts of the studied area varies from 15 to 50 cm and is inversely proportional to the water’s depth, while, in its southern part, it never exceeds 5 cm regardless of the water’s depth.

The investigations carried out in the St. Anna Trough allow the following conclusions to be reached.

  1. The methodology applied for the study of the physical properties of the sediments in the surface bottom layer demonstrates its high effectiveness for the assessment of the parameters of the physical sediment properties during geoecological investigations and interpretations of the recent depositional environments.
  2. The composition and moisture susceptibility of the sediments from the upper 3cmthick surface layer are mostly controlled by the water’s depth and the bot tom’s angles. The moisture of the recent sediments is largely determined by their moisture susceptibility, which points to normal depositional environments in the trough.
  3. At the initial lithogenesis stages, the sediment transformation is characterized by the spasmodic growth of the strength of the bottom sediments with the lithogeochemical barriers.

REFERENCES

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