Chapter 8. Deep-Sea Sediments – Patterns, Processes, and Stratigraphic Methods

1. Chapter 8.Deep-Sea Sediments – Patterns, Processes, and Stratigraphic Methods • Inventory and Overview • Red Clay and Clay Minerals • Calcareous ooze • Siliceous ooze • Turbidites

2. 8.1 Background Fig.8.1 Recovery of deep-sea sediment by box corer. The coring device is a steel box with sharp edges which is pressed into the sea floor by heavy weights on top. The box is closed by a shovel which rotates around two bolts just above the box. The shovel is pushed downward and sideward when the box is pilled out, by pulling on the arm opposite the shovel. The frame with the three legs steadies the corer before it penetrates the sediment. The many lines visible in the photo are used to prevent the heavy device from swinging on deck. This type of box corer was first used by H. E. Reineck. [photo T. Walsh. S. I. O.]

3. 8.2 Inventory and Overview 8.2.1 Sediment types and patterns Table.8.1 Classification of deep sea sediments. [W. H. Berger 1974 in C. A. Burk and C. L. Drake The geology of continental margins. Springer Heidelberg Berlin New York]

4. Sediment types • Pelagic clays are extremely fine-grained • lithogenous and volcanogenic deposits • Oozes consist of biogenous minterals: shells of planktonic foraminufera, radiolarians, coccolithophores, and diatoms. • Hemipelagic deposits are the same as clays and oozes except with large ad mixtures of shelf-derived sediment and of continental material.

5. Fig.8.2 Sediment cover of the deep-sea floor. The chief sediment types or facies are pelagic clay and calcareous ooze. [W. H. Berger,1974, in C. A. Burk, C. L. Drake (eds) The geology of continental margins. Springer, Heidelberg]

6. The major facies boundary in deep sea deposits is the calcite compensation depth, that is, the boundary between calcareous and noncalcareous sediments. • Essentially, the calcareous facies characterizes the oceanic rises and elevated platforms, while the Red Clay facies is typical for the deep basins. • Thus overall pattern is depth-controlled. • Superimposed on this pattern are the siliceous deposits, which accumulate below areas of high fertility; that is, the oceanic margins, the equatorial belt and the polar front regions.

7. 8.2.2 Biogenous Sediments Dominate Fig.8.3 Shell-bearing planktonic organisms. Clockwise from upper left Siliceous diatom (× 600), centric warm-water form; siliceous radiolarian (× 180); calcareous warm-water foraminiferGlobigerinoidessacculifer (× 55); tropical subsurface foraminifer Globorotalia menardii (× 28); organic-walled tintinnid (× 480); calcareous coccolithophore (× 2100) with interlocking platelets ("coccoliths") [Diatom microphoto by H.-J. Schrader; all others; SEM photo by C. Samtleben and U.Pfaumann]

8. About one half of the deep sea floor is covered by oozes. • The organisms producing the shells drift passively with ocean currents. • Except for some of the radiolarians, virtually all plankton organisms making sediment live in surface waters. • Coccolithophores and diatoms need light for photosynthesis. • This is also true for many planktonic foraminifera, because of symbiotic algae.

9. The shells and skeletons of foraminifera and radiolarians probably have several functions, including protection and trapping food. • Shells are much heavier than water, and tend to pull the organism down, away from sunlight and food. • To counter this tendency, some skeletons are highly perforate, or delicate. • Lipids and gas inclusions may increase buoyancy. • Sinking tendency also is decreased by small size, and by the growing of spines and other proturberances.

10. Controlling Factors • In summary, the most important factors controlling the composition of biogenous deep sea sediments are fertility and depth. • Fertility controls the supply of plankton remains, while depth controls the dissolution of carbonate (through pressure and water mass chemistry).

11. Controlling Factors Fig.8.4 Distribution of major facies in a depth-fertility frame, based on sediment patterns in the eastern central Pacific. Numbers are typical sedimentation rates in mm/1000 yr(which is the same as m/million yr). [Source as for Fig.8.2]

12. 8.2.3 Sedimentation rates • Maximum values for terrigenous muds off river mouths (several meters/thousand years), • intermediate ones for calcareous ooze (several centimeters / thousand years), • very low ones for Red Clay (a few millimeters / thousand years)

13. 8.2.4 Thickness of Deep Sea Sediments • The sedimentary columns for typical basins in the Atlantic are only about 500 m thick, and in the Pacific, a mere 300m.

14. 8.3 Pelagic Rain 8.3.1 Importance of Fecal Transport • Sediment trap • Much of the pelagic rain consists of fecal pellets. The pellets derive from copepods, salps, krill (in the Antarctic), and other grazing organisms. • The accelerated sinking allows even the smallest particles to arrive at the seafloor within a week or two. • If left to settle on their own in case of fine particles, they would not arrive at all.

15. 8.3.2 Flux of Organic Matter • The proportion of the primary production that leaves the photic zone experiences substantial losses during settling. • Due to break-up of pellets and aggregates from scavenging and decay. • The amount of organic material in traps decreases by the same factor as the depth increases. Fig.8.5 Fluxes of particulate organic carbon found in sediment traps. as fraction of the productivity of overlying waters.

16. 8.3.3 Seasonality of Flux Fig.8.6 a-c. Seasonal and interannual variations in particle flux as observed in traps. a Sargasso Sea (W. G. Deuser); b Gulf pf Alaska (S. Honjo); c Bransfield Strait, Antarctica (G. Wefer et al.). Photo shows krill fecal string and close-up of particle (coccosphere) on surface of string. Photo courtesy G. Wefer

17. 8.4 Red Clay and Clay Minerals 8.4.1 Origin of Red Clays: the Questions • Red Clay is uniquely restricted to the deep sea environment. • The bulk of the components are extremely fine-grained, and the coarse silt and sand fractions consist of particles originating in the ocean: hydrogenous minerals, volcanogenic debris, ferromanganese concretions, and traces of biogenous particles such as fish teeth, arenaceous forams, and in some cases, spicules and radiolarians.

18. What is the source of Red Clay? (three questions in one) • the ultimate source: to what extent is the clayey fraction is pelagic clays derived from in situ decomposition of volcanic material, and what is the contribution from continents and other sources? • transportation: what is the relative importance of transport by wind versus transport by rivers and ocean currents in bringing continent-derived clay particles to their site of deposition? • chemical reactions between clay and seawater: the what extent are degraded clays from the continents "upgraded" by reactions with seawater, and what proportion of deep sea clays may be considered a "precipitate" from seawater?  This third problem includes the concept of reverse weathering, which describes a process leading to the uptake of cations (Na+, K+) by clays?

19. 8.4.2 Composition of Red Clay • Clay minerals: montmorillonite, illite, chlorite, kaolinite, and mixed-layer derivatives • Lithogenous minerals: feldspar, pyroxene, quartz • Hydrogenous (or authigenic) minerals: zeolite and ferromanganese oxides and hydroxides

20. Concerning the lithogenous and hydrogenous minerals; their parent rocks (e.g., basic or acidic volcanics, terrigenous rocks), • The distribution of quartz in the North pacific suggests eolian transport from desert belts. • The clay minerals warrant our special attention, since they make up the bulk of the finest size class (≈2/3 of the clay size fraction). • the clay size fraction in turn provides some 90% of "pure" Red Clay

21. Fig.8.7 Structure of clay minerals. The thickness of a complete layer is measured in Angstrom units (Å = 10-8 cm), using X-ray diffraction techniques. Note the basic similarity of the clay minerals.

22. Montmorillonite (or smectite) layers • An aluminous octahedral layer is sandwiched between tetrahedral layers. • Abundances of Mg2+ (also Fe2+) and Al3+ in the octahedral layer, and of Al3+ and Si4+ in the tetrahedral layer are such tat there is a small net negative charge. • This charge is balanced by exchangeable cations between the "sandwiches". • The cations are hydrated, thus introducing variable amounts of water into the interlayer positions. • Montmorillonite is a weathering product of volcanic rock.

23. Illite • Belonging to the mica group • A fine-grained degraded muscovite • An octahedral layer sandwiched between tetrahedral layers • There is only Al3+ in the octahedral layer, and the ratio of Si to Al in the tetrahearal layer is exactly 3 to 1. • The net negative charge of the "sandwich"is balanced by nonhydrated, firmly bound K+ ions fitting in the holes left by the hexagonal arrangement of the corners of the silica tetrahedron.

24. Chlorite • sandwiches, but now bound by an additional octahedral layer, the so-called brucite layer • a common constituent of low-grade metamorphic rocks which are widely exposed on glacially eroded shield areas and provide much of the material for glacial deposits.

25. Kaolinite • Alternating tetrahedral and octahedral layers • A product of intense chemical weathering, and represents an insoluble aluminosilicate residue remaining after cations are stripped from feldspars and other minerals by extensive leaching.

26. 8.4.3 Distribution of Clay Minerals The clay mineral which are most abundant in deep sea clay are montmorillonite and illite Fig.8.8 Clay mineral distribution on the ocean floor. The map shows the dominant mineral in the fraction less than 2㎛. Mixture indicates that no one clay mineral exceeds 50% of the total.

27. Distribution of Clays • Their distributions suggest that montmorillonite has important sources in oceanic volcanism, at least in the Pacific, while illite is largely derived from the continents. • The remaining two important clay minerals, kaolinite and chlorite, also are land-derived, kaolinite from chemical weatheringin the tropics and chlorite from physical weathering in high latitudes.

28. It is surprising that the clay mineral provinces are so clearly defined. • Clay settles exceedingly slowly. • Bramlette (1961) suggested that the removal of fine particles by filter-feeding planktonic organisms, and subsequent settling in fecal pellets. • both biogenous matter and inorganic clay particles are quickly filtered out from surface waters by planktonic organisms, and incorporated into fecal pellets. • In addition, near the ocean margins, bottom-near redistribution of clay minerals within hemipelagic sediment may play an important role in spreading the various clay types while retaining coherent patterns of distribution.

29. 8.5 Calcareous Ooze 8.5.1 Depth Distribution • The rivers which feed the oceans are essentially dilute solutions of calcium bicarbonate and silica. • To balance this input, the ocean precipitates calcium carbonate.

30. Shell-building organisms find their way to the sea floor, where they are preserved on the more elevated parts and dissolved on the deeper ones, because the undersaturation of seawater increases with pressure and decreasing temperature Fig.8.9 a, b Depth distribution of calcareous deep sea sediments. a Idealized bathymetric zonation of deep-sea deposits, produced by increasing dissolution of carbonate with depth. [According to J. Murray, J. Hjort, 1912. The depths of the ocean. Macmillan, New York] Pteropods are pelagic snails with aragonitic shells. b Generalized depth profiles for carbonate content in deep-sea sediments. [R. R. Revelle, 1944, Carnegie Inst Wash Publ 556]

31. Atlantic vs. Pacific • The Atlantic has higher carbonate percentages at all depths. • Ultimately this difference is due to the effects of deep ocean circulation, which fills the deep Atlantic with calcite-saturated waters (NADW), and leaves much of the Pacific undersaturated. • The greatest depth is in the North Atlantic (>5.5 km), where the deep water is young and supersaturated with calcite down to about 4.5 km. • The shallowest CCD levels are in the northern North Pacific, where deep waters are old and rich in excess CO2. • Here waters are undersaturated for much of the water column below 1 km depth.

32. 8.5.2 Dissolution patterns in the deep sea • CCD (Carbonate Compensation Depth) • The CCD is the particular depth level at any one place in the ocean where the rate of supply of calcium carbonate to the sea floor is balanced by the rate of dissolution, so that there is no net accumulation of carbonate. • ACD (Aragonite Compensation Depth) • CCD (Calcite Compensation Depth)

33. 8.5.2 Dissolution patterns in the deep sea • Global dissolution patterns on the sea floor is represented by mapping the calcite compensation depth (CCD) Fig.8.10 Topography of the CCD surface, that is, the depth in kilometers below which little or no carbonate accumulates.

34. 8.5.3 Patterson's Level and the Lysocline Fig.8.11 a, b. Dissolution of carbonate as a function of depth. a Peterson's experiment. Polished calcite spheres were exposed in a line kept taut by a large buoy  submerged below the surface. The diagram shows the weight loss.[M.N.A.Peterson. 166, Science, 154:1542]. b Differential dissolution and the lysocline. At the shallower depths on the sea floor, usually above 3000m of so, pelagic foraminifers are well preserved. Below a critical level, preservation rapidly deteriorates with depth. The boundary between well-preserced and poorly preserved foraminifers on the sea floor is the lysocline. It is closely associated with the critical level of Peterson, and also with the saturation level. in areas of low productivity.[W. H. Berger, 1985, Episodes 8:163] • Patterson (1542) showed a drastic increase of dissolution rates below 3500 m in the central Pacific.

35. Lysocline • Another CCD-like level which can be mapped to describe dissolution patterns is the lysocline. • The concept of the lysocline was introduced to denote a contour-following boundary zone between well-preserved and poorly-preserved foraminiferal assemblages on the floor of the central Atlantic Ocean and on that of the South Pacific. • The lysocline marks the top of the Antarctic Bottom Water.

36. Conceptual Model Fig.8.12 Conceptual model for the origin of the CCD and its relationship to the lysocline. Increased carbonate supply at the equator depresses the CCD, as seen in Fig.8.10. [Berger et al., J Geophys Res 81:2617]

37. Figure 1. Generalized diagrams illustrating the relative position of calcite and aragonite solubility profiles in the modern tropical ocean and the variation in temperature with depth. The major zones of digenesis are plotted to the right.

38. Atlantic vs. Pacific

39. 8.5.4 Dissolution Patterns near continents • The carbonate line does not simply follow depth contours. • For example, high fertility along the Pacific equator leads to a depression of the CCD, by some 500m. • Paradoxically, high fertility raises the CCD in the margin areas around continents. • In the fertile areas of the ocean margins, the high supply of organic matter leads to highly increased benthic activity as well as to the development of much CO2 in interstitial waters, producing carbonic acid. • Thus, calcite shells are attacked even at depths of a few hundred meters on continental slopes. • In the equatorial areas of the central Pacific, on the other hand, increased fertility leads to an increased supply of calcareous shells which goes well beyond the increased supply of organic matter in this region.

40. Dissolution during settling • Fecal pellet transport is important for coccoliths. • For foraminifera, the evidence indicated that all but the small ones reach the sea floor, once they start to fall. • Examination of surfacial sediments showed the presence of many delicate foraminifera, mixed with heavily corroded resistant ones and their fragments. • Net tows at water depths below the level corresponding to the regional CCD level contained delicate forms, as well as the aragonite shells of pteropods, which were not found in the sediment.

41. 8.5.5 Why is there a CCD? • The ultimate reason why abyssal waters dissolve calcite is that organisms supply calcium carbonate to the sea floor in excess of the amount that can be sedimented over the long run. • A dynamic steady state is maintained. • Through geologic time, an overall increase in productivity leads to an overall increase in dissolution, and vice versa.

42. 8.5.6 A Global Experiment • An amount equivalent to 20% of the CO2 already present in the atmosphere has been added within this century. • Over the next few centuries, ten or twenty times of the CO2 in the atmosphere could be added to the air-ocean system. • In the long run, the ocean floor will neutralize most of the industrial CO2 through the dissolution of carbonate: • CO2 +H2O +CaCO3 → Ca2+ + 2HCO3- • The industrial CO2-pulse will produce a hiatus on the sea floor. • A thickness of about 1 m of carbonate sediment will have to be dissolved from the carbonate-bearing sea floor.

43. Future projection Fig.8.13 a, b. CO2 increase in the atmosphere and projected carbonate dissolution on the sea floor. a Atmospheric CO2increase as seen in ice cores from Siple Station, Antarctica, and in the data of N. D. Keeling. from Nauna Loa in Hawaii.[U. Siegenthaler, H. Oeschger, 1987, Tellus 39: 140.] b Calculated factor by which future atmospheric CO2 will exceed preindustrial CO2 if present trends of CO2 input continue till oil and coal are used. [Note that carbonate dissolution on the sea floor does not prevent a strong initial rise of CO2. [R. B. Bacastow, C. D. Keeling, 1979, U S Dept Energy Conf 770385:72]

44. A Prediction • A doubling of the CO2 content would increase the global average temperature by about 2°C. • Such a doubling might come within the next 50 years or so. • What might be the effect of such a temperature increase? • Nobody knows the answer.

45. 8.6 Siliceous Ooze 8.6.1 Composition and distribution Fig.8.14 Assemblage of modern radiolarians from sediments recovered in the equatorial Pacific.[Microphoto W. H. B.]

46. Siliceous Ooze • Distribution, production, and dissolution patterns of the siliceous deposits • Remains of diatoms, silicoflagellates and radiolarians, and sponge spicules, all of which are made of opal, a hydrated form of amorphous silicon oxide. • Diatom oozes are typical for high latitudes, diatom muds for pericontinental regions, and radiolarian oozes for equatorial areas.

47. The siliceous deposits typically occur in areas of high fertility; that is, in regions of surface water with relatively high phosphate values. Fig.8.15 Flux of siliceous fossil to the sea floor.[W. H. Berger, J. C. Herguera, in P. G. Falkowski. A. D. Woodhead. eds, 1992, Primary productivity and biogeochemical cycles in the sea. Plenum Press, New York]

48. Re-deposition Process • The silica frustules are light and easily transported, and the activity of benthic animals which tends to resuspend fine sediment is especially pronounced in fertile areas. • Thus, aided by bottom currents and gravity, siliceous frustules tend to accumulate in local and regional depressions.

49. 8.6.2 Controlling factors • the rate of production of siliceous organisms in the overlying waters • the degree of dilution by terrigenous, volcanic, and calcareous particles • the extent of dissolution of the siliceous skeletons

50. The rate of production of siliceous organisms in the overlying waters - maximum in coastal regions • This leads to the formation of a silica ring around each ocean basin. • Silica belts are provided by the latitudinally arranged oceanic divergences which are a result of atmospheric circulation. • The regions of divergence have nutrient-rich surface waters. • Such areas are rich in grazing zooplankton, which pack the siliceous frustules into fecal pellets, thus accelerating delivery to the sea floor.