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1 the Solid Phase of Marine Sediments

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1 the Solid Phase of Marine Sediments 1 The Solid Phase of Marine Sediments DIETER K. FÜTTERER 1.1 Introduction Another, more modern approach conceives the ocean sediments as part of a global system in which the sediments themselves represent a The oceans of the world represent a natural de- variable component between original rock source pository for the dissolved and particulate prod- and deposition. In such a rather process-related ucts of continental weathering. After its input, the and globalized concept of the ocean as a system, dissolved material consolidates by means of bio- sediments attain special importance. First, they logical and geochemical processes and is depos- constitute the environment, a solid framework for ited on the ocean floor along with the particulate the geochemical reactions during early diagenesis matter from weathered rock. The ocean floor de- that occur in the pore space between the particles posits therefore embody the history of the conti- in the water-sediment boundary layer. Next to the nents, the oceans and their pertaining water aqueous phase, however, they are simultaneously masses. They therefore provide the key for under- starting material and reaction product, and standing Earth’s history, especially valuable for procure, together with the porous interspaces, a the reconstruction of past environmental condi- more or less passive environment in which tions of continents and oceans. In particular, the reactions take place during sediment formation. qualitative and quantitative composition of the sedimentary components reflect the conditions of their own formation. This situation may be more or 1.2 Sources and Components of less clear depending on preservation of primary Marine Sediments sediment composition, but the processes of early diagenesis do alter the original sediment composi- tion, and hence they alter or even wipe out the Ocean sediments are heterogeneous with regard primary environmental signal. Hence, only an to their composition and also display a consider- entire understanding of nature and sequence of able degree of geographical variation. Due to the processes in the course of sediment formation and origin and formation of the components various its diagenetic alteration will enable us to infer the sediment types can be distinguished: Lithoge- initial environmental signal from the altered nous sediments which are transported and dis- composition of the sediments. persed into the ocean as detrital particles, either Looking at the sea-floor sediments from a as terrigenous particles – which is most fre- geochemical point of view, the function of par- quently the case – or as volcanogenic particles ticles, or rather the sediment body as a whole, i.e. having only local importance; biogenous sedi- the solid phase, can be quite differently ments which are directly produced by organisms conceived and will vary with the perspective of or are formed by accumulation of skeletal frag- the investigator. The “classical” approach – ments; hydrogenous or authigenic sediments simply applying studies conducted on the which precipitate directly out of solution as new continents to the oceans – usually commences formations, or are formed de novo when the par- with a geological-sedimentological investigation, ticles come into contact with the solution; finally, whereafter the mineral composition is recorded in cosmogenic sediments which are only of second- detail. Both methods lead to a more or less overall ary importance and will therefore not be consid- geochemical description of the entire system. ered in the following. 1 1 The Solid Phase of Marine Sediments 1.2.1 Lithogenous Sediments rock and the weathering conditions of the catch- ment area, it will accordingly vary with each river The main sources of lithogenous sediments are ul- system under study. Furthermore, the mineral timately continental rocks which have been bro- composition is strongly determined by the grain- ken up, crushed and dissolved by means of physi- size distribution of the suspension load. This can cal and chemical weathering, exposure to frost and be seen, for example, very clearly in the suspen- heat, the effects of water and ice, and biological sion load transported by the Amazon River (Fig. activity. The nature of the parent rock and the 1.1) which silt fraction (> 4 - 63 µm) predominantly prevalent climatic conditions determine the inten- consists of quartz and feldspars, whereas mica, sity at which weathering takes place. Information kaolinite, and smectite predominate in the clay about these processes can be stored within the fraction. remnant particulate weathered material, the terri- It is not easy to quantify the amount of sus- genous detritus, which is transported by various pension load and traction load annually dis- routes to the oceans, such as rivers, glaciers and charged by rivers into the oceans on a worldwide icebergs, or wind. Volcanic activity also contrib- scale. In a conservative estimative approach utes to lithogenous sediment formation, however, which included 20 of the probably largest rivers, to a lesser extent; volcanism is especially effective Milliman and Meade (1983) extrapolated this on the active boundaries of the lithospheric amount to comprise approximately 13·109 tons. plates, the mid-ocean spreading ridges and the Recent estimations (Milliman and Syvitski 1992) subduction zones. which included smaller rivers flowing directly into The major proportion of weathered material is the ocean hold that an annual discharge of ap- transported from the continents into the oceans proximately 20·109 tons might even exist. by rivers as dissolved or suspension load, i.e. in Under the certainly not very realistic assump- the form of solid particulate material. Depending tion of an even distribution over a surface area of on the intensity of turbulent flow suspension load 362·106 km2 which covers the global ocean floor, generally consists of particles smaller than 30 mi- this amount is equivalent to an accumulation rate crons, finer grained than coarse silt. As the min- of 55.2 tons km-2 y-1, or the deposition of an approxi- eral composition depends on the type of parent mately 35 mm-thick sediment layer every 1000 years. Fig. 1.1 Grain-size distribution of mineral phases transported by the Amazon River (after Gibbs 1977). 2 1.2 Sources and Components of Marine Sediments Fig. 1.2 Magnitude of annual particulate sediment discharge of the world’s major rivers. The huge amount of sediment discharge in southeast Asia and the western Pacific islands is due to high relief, catchment, precipitation and human activity (Hillier 1995). Most of the sediment transported to the coast- rates in the adjoining oceanic region of the Indo- line by the rivers today is deposited on protected Pacific. coastal zones, in large estuaries, and on the Sediment transport by icebergs which calve shelves; only a rather small proportion of the sedi- from glaciers and inland ice into the ocean at polar ment is transported beyond the shelf edge and and subpolar latitudes is an important process for reaches the bottom of the deep sea. The geo- the discharge and dispersal of weathered coarse graphical distribution of the particulate discharge grained terrigenous material over vast distances. varies greatly worldwide, depending on the geo- Due to the prevailing frost weathering in nival graphical distribution of the respective rivers, climate regions, the sedimentary material which is amount and concentration of the suspended mate- entrained by and transported by the ice is hardly rial. According to Milliman and Syvitski (1992), the altered chemically. Owing to the passive transport amount of suspension load is essentially a func- via glaciers the particles are hardly rounded and tion of the surface area and the relief of the hardly sorted in fractions, instead, they comprise catchment region, and only secondarily does it the whole spectrum of possible grain sizes, from depend on the climate and the water mass of the meter thick boulders down to the clay-size rivers. Apart from these influences, others like hu- fraction. man activity, climate, and geological conditions As they drift with the oceanic currents, melting are the essential factors for river systems in icebergs are able to disperse weathered terrig- southeast Asia. enous material over the oceans. In the southern The southeast Asian rivers of China, Bangla hemisphere, icebergs drift from Antarctica north to Desh, India, and Pakistan that drain the high 40°S. In the Arctic, the iceberg-mediated transport mountain region of the Himalayan, and the rivers is limited to the Atlantic Ocean; here, icebergs of the western Pacific islands (Fig. 1.2), transport drift southwards to 45°N, which is about the just about one half of the global suspension load latitude of Newfoundland. Coarse components discharged to the ocean annually. This must natu- released in the process of disintegration and melt- rally also exert an effect on the sedimentation ing leave behind “ice rafted detritus” (IRD), or 3 1 The Solid Phase of Marine Sediments “drop stones”, which represent characteristic sig- the finer grains come to settle much farther away. nals in the sediments and are of extremely high The relevant sources for eolian dust transport importance in paleoclimate reconstructions. are the semi-arid and arid regions, like the Sahel In certain regions, the transport and the distri- zone and the Sahara desert, the Central Asian bution carried out by sea ice are important pro- deserts and the Chinese loess regions (Pye cesses. This is especially true for the Arctic Ocean 1987). According to recent estimations (Prospero where specific processes in the shallow coastal 1996), a total rate of approximately 1-2·109 tons y- areas of the Eurasian shelf induce the ice, in the 1 dust is introduced into the atmosphere, of course of its formation, to incorporate sediment which about 0.91·109 tons y-1 is deposited into material from the ocean floor and the water the oceans. This amount is, relative to the entire column. The Transpolar Drift distributes the sedi- terrigenous amount of weathered material, not ment material across the Arctic Ocean all the way very significant; yet, it contributes considerably to the North Atlantic.
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