Chapter 3. Sources and composition of Marine Sediments

1. Chapter 3. Sources and composition of Marine Sediments • The sediment cycle • Sources of sediment • Sediments and sea water composition • Major sediment types Lithogenous sediments Biogenous sediments Non-skeletal carbonates Hydrogenous sediments • Sedimentation rates

2. 3.1 The Sediment Cycle Fig.3.1 Sources, transport, and destination of marine sediments

3. 3.2 Sources of Sediment- 1. River Input • The dissolved and particulate load of rivers • Particulate load; continental shelf & slope deposits • Dissolved load; precipitates (CaCO3 and opal, SiO2.nH2O) • Deep sea sedimentation rate = 1~20mm/kyr • Slope sedimentation rate = up to 100mm/kyr • Taking the value of 100mm/kyr for 10% of the ocean, and a value of 5mm/kyr for the rest; the average is near 15 mm/kyr.

4. 1. River Input • The ocean floor covers more than twice the area of the land. • Hence, the continents, if they are the ultimate source of all sediment, must wear down at a rate of near 30 mm/kyr. • Another rough calculation may be based on an estimated sediment supply from rivers of 12 km3/yr. • If we distribute this amount on the 362 million km2 of sea floor, we get a sedimentation rate of about 30 mm/kyr, and a corresponding erosion rate of 60 mm/kyr for the continents.

5. 1. River Input • Mechanical weathering dominates in high latitudes. • Chemical weathering (leaching) is favored by high rainfall and temperatures and dominates in tropical areas. • We live in a highly unusual period. • High mountain ranges and the powerful abrasive action of the continental ice masses have greatly increased mechanical erosion for the last several million years.

6. 3.2 Sources of Sediment – 2. Glacier Input • In high latitude, the immense masses of outwash material are brought to the shore. • Calving glaciers can transport both fine and very coarse material far out to sea. • Around Antarctica this type of transport reaches about 40°S. • In the North Atlantic the transport boundary roughly follows the present boundary b/w very cold and temperate waters. • At present about 20% of the sea floor receives at least some ice-transported sediments. Fig.3.2 Distribution of ice-rafted materials in the North Atlantic. {resent limits of drift ice (normal and extreme ) run from around Newfoundland toward Greenland and Iceland. During the last ice age, this limit went form New York straight across to Portugal. Triangles Surface samples; dashes dredge samples; circles core samples. [H. R. Kudrass, 1973, Meteor Forschungserg Reihe C 13:1]

7. 3.2 Sources of Sediment – 3. Input from Wind Fig.3.3 Abundance of haze from dust, over the Atlantic Ocean. Numbers are % of observations. [ G. O. S. Arrhenius, 1963, in M. N. Hill, Sea 3: 695] • Wind can only move the fine material. • The rate of dust fall from the air can best be measured in snowfields and ice cores. • Even in the Antarctic and in Greenland, far from desert sources, the rate is quite appreciable; 0.1~1 mm/kyr. • Exactly how much dust is falling is not known. • Some estimate suggest that much or most of the deep sea clay is derived from wind input. • Such clay accumulates at 1 mm/kyr in the North Pacific and at 2.5 mm/kyr in the Atlantic.

8. 3.2 Sources of Sediment – 4. Volcanic Input • A substantial amount of material is delivered by volcanoes, especially those associated with active oceanic margins. • Volcanoes have a short live, and single eruptions are flash-like events. • Therefore, ash layers can be used for regional stratigraphic purposes • Tephrachronology • 백두산 Fig.3.4 Distribution of volcanic ash produced by two large volcanic explosions in the Aegean Sea, Presumably of Santorini. The lower ash layer marks a prehistoric event (> 25000 yrs.). The upper layer is less than 5000 yrs old; the volcanic explosion creating it may be the one which brought catastrophe to the Minoan culture, sine 36000 year ago. [D. Ninkovich, B. C. Heezen, Nature London 213: 1968. Oceans, Prentice-Hall, New Jersey]

9. 3.3 Sediments and Seawater Composition 3.3.1 Acid-Base Titration • Sea water is a solution of sodium chloride (NaCl). • Sodium and chloride make up 86% of the ions present by weight. • Since the major cations form strong bases, but bicarbonate forms a weak acid, the ocean is slightly alkaline, with pH of near 8. • Cl- > Na+ > SO42- > Mg2+ > Ca2+> K+ > HCO3-

10. 3.3 Sediments and Seawater Composition Table 3.1 Comparison between seawater and river water

11. The salty ocean may be understood as the product of emission of acid gases from volcanoes (hydrochloric, sulfuric and carbonic acid) • and the leaching of common silicate rocks whose minerals have the form [MeSiaAlbOc] • where Me stands for the metals, Na, K, Mg, and Ca, and the remainder makes insoluble silica-aluminum oxides, clay minerals.

12. How stable was the composition of seawater through geologic time? • Seawater salt in the ancient salt deposits • Their composition indicates that sea salt did not vary much over the last 600 million years. • Paleontologic evidence certainly agrees with this. • Already in the early Paleozoic there were organisms whose closest modern relatives have rather narrow salt tolerances; radiolarians, corals, brachiopos, cephalopods, echinoderms.

13. On comparing the average composition of river water with that of seawater, one notes drastic difference. • River water is a solution of calcium bicarbonate and silicic acid, with a small admixture of the familiar salts. • The river influx is irrelevant to the make-up of sea salt. • But steady-state condition

14. 3.3.2 Interstitial Water and Diagenesis • Fine-grained sediments; porosities of 70~90%, while sands have around 50%. • Chemical reactions; diagenesis • Redox reactions • These reactions are the more intense the more organic carbon is present. • Stripping oxygen from dissolved nitrate and from solid iron oxides and hydroxides • Sufate reduction • Methanogenesis • There reactions are all mediated by bacteria.

15. In the process, gases are produced (carbon dioxide, ammonia, hydrogen sulfide), as well as iron sulfide (pyrite). • Methane can react with water (low temperature and high pressure) to make clathrates. • The massive escape of gases from organic-rich muds can produce mud volcanoes. Fig.3.5 Mud volcano. Side scan record from the Black Sea bottom. ( Side scan principle see Fig.4.15). [Courtesy Dr. Glunow, Moscow, UNESCO-IMS-Newsletter 61, Paris]

17. 3.3.3 Residence Time • For steady-state conditions, output must equal input. • The seawater has to rid itself of all new salt coming in, in the same proportions as they are added.

18. Sinks • Calcium carbonate; calcareous skeletons built by organisms • Silica; opaline skeletons • Metals leave the ocean as newly formed minerals such as authigenic clay, oxides, and sulfides and as zeolites, also by hydrothermal alteration at the ridge crest • Sulfur; heavy metal sulfides in anaerobic sediments • Salt; pore waters in the sediments

19. Residence time • The average time a seawater component remains in the water, before going out as sediment • T = A/r • Where A is the amount present, r is the input • Salt age for the ocean, under the assumption that the ocean started out fresh and retained all sodium since … near 100 million years!

21. 3.4 Major Sediment Types • Lithogenous, Hydrogenous, Biogenous Fig.3.6 a, b. Familiar examples of two major types of beach sand a medium sand, lithogenous, La Jolla b coarse sand, biogenous, Hawaii. (Photos: W.H.B.)

23. 3.5 Lithogenous Sediments Fig.3.7 Heavy mineral provinces of the Gulf of Mexico, based on typical mineral associations. Ⅰ East Gulf; Ⅱ Mississippi; Ⅲ Central Texas; Ⅳ Rio Grande; ⅤMexico. carbonate particles are dominant off Mexico and Yucatan. The heavy mineral patterns contain clues about the sources and transport paths of terrigenous sediments. [D. K. Davies, W, R. Moore, 1970, J Sediment Petrol 40: 339]

24. 3.6 Biogenous Sediments

25. Fig.3.8 Recent hemipelagic sediment  from Continental Rise off NW Africa (Cape Verde Rise). SEM photos, bar 20μm. Left Fine-silt fraction (2-6μm) predominantly composed of coccoliths (c) and of some detrital mica (m) and quartz (q); Right coarse silt fraction, mainly detrital grains (q quartz) and foraminifera tests and fragments (f). [Photo courtesy D. Futterer]

26. 3.7 Nonskeletal Carbonates Fig.3.9. Calcareous oolites, produced within tidal zone by algal activity. Upper Air photo of oolite sand bars, Bahama Banks. Note tidal channels.[Photo courtesy D. L. Eicher]. Lower SEM photos of ooids, scale bar 5μm; left slightly etched section. Three secondarily filled borings (b) intersect the concentric laminae of primary oolite coating; right close-up of oolite laminae, showing acicular aragonite needles. [Photos courtesy D. Futterer]

27. Fig.3.10 Present-day dolomite formation. Lagoon on the southern coast of the Persian/Arabian Gulf (a), floor covered with calcareous mud. Intertidal zone (b) and  algal mats of uppermost littoral (c)  rim the lagoon. Intermittently flooded evaporite flats (d, sabkhas) follow landward [After L. V, Illing et al 1965, Soc Econ Paleontol Mineral Spec Publ 13:89, Simplified]

28. Hydrogenous Sediment - Marine evaporites • Form on evaporation of seawater • Where? • Coastal lagoon • Salt seas on the shelves • Early rift oceans in the deep sea • How much salt can be produced by evaporating a 1000m-high column of seawater? • Salt constitutes 3.5% of the weight of the column, its density is about 2.5 times that of water. • About 14m of salt

29. Marine Evaporites • The least soluble salts precipitates first: calcium carbonate (aragonite) and calcium sulphate (gypsum) • To precipitate halite, the brine needs to be concentrated about 10 fold • Many evaporites only contain carbonate and gypsum (or anhydrite), others have thick deposits of halite or of the valuable (and very soluble) potassium salts.

30. 3.8 Hydrogenous Sediments Fig.3.11 Possible models for marine evaporite formation. a Serial fractionation in very shallow and extended basins. Saturation of different salts is reached in a series ocean to land. Terrigenous particles may be supplied from land. Recent example: Adshi-darja Lagoon attached to the Caspian Sea by Kara Bogaz Inlet (chemical conditions there are not fully comparable with open sea). b Serial fractionation and differential preservation in deeper basins divided by sills. Saturation of differents salt is reached in a  series shallow to deep water. Derail Only gypsum is precipitated near the sill. Halite saturation is not reached, because brine sinks down to the basin escaping further evaporation. Sill depths can be considerably reduced by carbonate and /or gypsum precipitation. No Recent example known. [ G. Richter-Bernburg, 1955, Dtsch Geol Ges 105 [4]: 59] 

31. Differential preservation and serial fractionation are the key processes in controlling the chemistry of salt deposits. Fig. 3.12 Model of concentric serial fractionation in the uppermost Miocene underneath the Western Mediterranean. G Gibraltar Region, where the connection with the Atlantic was closed for half a million years; M Mallorca; C Corsica; S Sardinia [K. J. Hsu et al., 1973 in Ryan, W. B. F. et al. eds. Initial Repts. DSDP 13, 695, Washington D. C]

32. Phosphorites • Phosphorites deserve special attention for the reason of this tie-in to ocean fertility, but also because of their economic value • Marine phosphorites vary in composition • A general formula is Ca10(PO4,CO3)6F2-3, with increases in carbonates going parallel to increase in fluoride

33. Phosphorites • Modern phosphorites typically occur in areas of high productivity. • Occurs as nodules or crusts up to head size and as irregularly shaped cakes • In the geologic record, they occur as replacement of previously deposited carbonates, or as mineralization of pre-existing organic matter • The common depth of deposition is on the shelf and upper slope.

34. Phosphorites • The association of geologically young phosphorites deposits with present-day regions of upwelling suggests that the source of the phosphorous is organic matter. • Apparently the algae growing in these regions, in the surface waters, extract the phosphorous from the water, and crustacesn and fish concentrate it further in their bodies and excrement. • During decomposition of organic debris, much phosphate is released to the interstitial water (and also to seawater) • Thus, interstitial waters may become saturated with the phophate mineral apatite. • Precipitation of apatite replacement of pre-existing carbonate mineral, and impregnation of sediment can then proceed.

35. Iron Compounds • Abundant in both oceanic margin sediments and in the deep ea since iron is one of the most abundant elements on Earth • On the slopes, the high supply of organic matter commonly leads to oxygen deficiency, and to sulfate reduction by bacteria, in the uppermost sediment layers. • This process results in H2S formation, and in the precipitation of iron sulfide (pyrite). • In the deep sea, oxygen is generally plentiful, and essentially all iron occurs in its oxidized form, as iron-oxide/hydroxide (goethite)

36. Iron Compounds • An iron-mineral which has been much studies is glauconite • It is greenish silicate common in shallow marine areas. • Chemically it is a poorly crystallized mica, rich in potassium (7-8%) and in iron (20-25%) • Apparently the association with decaying organic matter (fecal pellets, interior of shells) is a necessary condition of growth: part of the iron in glauconite is reduced iron. • A high concentration in interstitial waters (at conditions intermediate between reduction of iron oxide and precipitation of sulfide) appears to be favorable for glauconite formation, as is the presence of the right kind of clay to convert into the glauconite mica.

37. 3.9 Sedimentation Rates • High rates at the edges of the continents, especially in estuaries and marginal basins with river influx • The lower rates in abyssal regions • C. slopes = 40-200mm/kyr • Deep sea = 1-20 mm/kyr • Coral reef = 10m/kyr Fig.3.13 Rates of vertical crustal motion, of denudation, and of sedimentation rates. Scale in mm/1000 years. also called "Bubnoffs" (B), of m/million years. Postglacial sea level rise for comparison: 100 m in 5000 yrs. Note interaction with recently deglaciated land (arrow left) and with coral growth rates (arrows right).

38. Fig.3.14 Annual layers (varves) in the sediment of a bay in the Adriatic Sea (Mljet Island). The photo shows light and dark laminae. One light-dark pair corresponds to one year. The lower boundary of light layers are generally sharp; they are due to  precipitation of carbonate by phytoplanktom, which bloom in early summer (record to the right) as temperature rises. In fall and winter, rains bring terrigenous matter which- together with organic detritus- provides for dark colors. the interpretation of the varves is a complicated matter: recent studies use statistical procedures to reconstruct climatic conditions in detail.(photo E. S.)