Progress in Deep-Sea Sedimentology

C H A P T E R

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Progress in Deep-Sea Sedimentology ¨neke,† and A.J. Van Loon‡ Thierry Mulder,* Heiko Hu Contents 1 2 3 5 5 11 12 14 14 14 16 22

1. Introduction 1.1. Scope of the book 2. What are Deep-Sea Sediments? 3. Tools Used for Deep-Sea Sediment Investigations 3.1. Geophysics 3.2. Geotechnic tools 3.3. Sediment sampling 3.4. Submersible systems 3.5. Current meters and particle traps 3.6. Laboratory analyses 4. Structure of the Book References

1. Introduction In this book, all marine domains extending seaward of the shelf break are considered as deep-sea. This domain represents 63.6 % of the Earth’s surface (the ocean in its entirety covers 361
106 km2 or 70.8% of the Earth’s surface, including continental shelves). From a stricter geological point of view, the oceanic domain would begin at the boundary between the high-density (3.25 on average), usually thin (5 km in average) oceanic crust and the thick (30 km on average) low-density (2.7 in average) continental crust. A transitional crust may exist in between. The study of deep-sea sediments benefited greatly from recent improvements in technologies. These improvements have been driven by academic needs (most of the sea floor remains unexplored in detail and most of the topography of abyssal plains has not been mapped with accurate tools) and * Universite´ de Bordeaux, UMR CNRS 5805 EPOC, Avenue des Faculte´s, 33185 Talence Cedex, France { Institut fu¨r Geographie und Geologie, Universita¨t Greifswald, Jahn-Strasse 17a, D–17487 Greifswald { Geological Institute, Adam Mickiewicz University, Mako´w Polnych 16, 61–606 Poznan, Poland Developments in Sedimentology, Volume 63 ISSN 0070-4571, DOI: 10.1016/S0070-4571(11)63001-X

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2011 Elsevier B.V. All rights reserved.

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by economic needs, such as the demand for mineral deposits (metal-bearing nodules, exploration of ultra-deep offshore oil). These newly-developed technologies benefited from both in situ data collection and data interpretation in laboratory. In terms of data collection, this includes: – – – – –

Sea-floor morphology (multibeam bathymetry), subsurface investigation (seismic tools), high-resolution echosounders, 3-D tools, sampling gear (interface corer). In terms of data interpretation in the laboratory, this includes:

– core scanners for measurement of geotechnical and physical properties, X-ray, geochemistry, – the development of biological tracers and biomarkers for palaeoenvironmental reconstruction, – the improvement and development of stratigraphic tools and dating methods based on radiogenic and non-radiogenic elements (especially for the Quaternary), the development of micro-lithostratigraphy (IRD, tephra recognition) and magnetostratigraphy.

1.1. Scope of the book The chapters of this book have the following objectives: – to explain the formation and supply of sedimentary particles by continental erosion (river load, ice or wind transport), coastal erosion, currentinduced winnowing, through volcanic and authigenic processes, and by means of biogenic productivity; – to describe the way the sediments are transported from the source area (continental edge, slope, surface water) to the accumulation zone in the deep-sea; – to present the early geochemical transformations affecting the particles in the water column or the sediments as soon as they are produced and accumulate on the sea floor; – to show how sediments are preserved on the sea floor despite erosion and dissolution; – to present the characteristic features and main changes in worldwide ocean sedimentation with focus on “modern” oceans that have been formed since the disintegration of Pangaea (Mesozoic-Cenozoic); – to discuss major changes in biogenic productivity, sea-water chemistry, and external controls of deep-sea sedimentary processes, depending on long-term trends in ocean history;

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– to present the academic (e.g., palaeoclimatic studies), societal (e.g., natural hazards) and industrial interests (e.g., the exploration for mineral resources) in the study of deep-sea sediments.

2. What are Deep-Sea Sediments? The sea-water environment can be subdivided into shallow (epicontinental) seas and deep seas. The morphology of modern oceans and marginal seas is based on the water depth and on changes in the slope gradient (Fig. 1.1). Using a classical cross-section through a passive continental margin, the shallowest environment is the continental shelf (or platform), which extends in the continental domain from the shoreline to the shelf break. It represents 26
106 km2 (7.2% of the marine area). In this area, the sea-floor gradient is < 0.5 . In offshore direction, the water depth extends down to 100–110 m such as on the north-western African margin (Seibold and Hinz, 1974) or 200 m on most of the continental margins, including the northern European and the North-American Atlantic margins. Its extent can be from several hundreds of kilometres (1500 km for the Siberian shelf, > 600 km for the southern Argentina–Patagonian Shelf) to a few kilometres (off Nice in the Mediterranean). Active continental margins, such as the South-American Pacific margin, are usually only a few kilometres wide. The continental shelf is exposed to numerous oceanographic processes that are absent in deep seas. Most of them are related to atmospheric processes. They include swell and storm waves that generate oscillatory motions in the water column (producing specific sedimentary structures such as hummocky cross-stratifications), tides, shallow contour currents, as well as shelf and coastal currents, including littoral drift. The continental shelf is separated from the continental slope by the shelf break, which is defined by a change in the slope gradient. The slope steepens from a gradient < 1 on the shelf to 3–5 in average along the slope, to sometimes more than 20 in areas where canyons are incised the slope and the shelf. Further downslope, it passes into the continental rise at a water depth of about 2500 m. The continental slope corresponds approximately to the bathyal zone (200–3000 m). On the rise, the slope gradient decreases to 1–2 and the relief becomes smoother. Because of this change, the continental rise is the preferential area for final deposition of terrigenous sediment that bypassed the shelf and slope area. Together, the continental shelf, slope and rise form the continental margin. The margin can be passive and tectonically quiescent (North Atlantic margin) or active and tectonically dynamic (circum-Pacific margins). At about 5000 m water depth, the rise passes into the abyssal plain. Abyssal plains represent the largest oceanic domains with a mean water

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deep sea littoral zone

bathyal zone

abyssal zone

hadal zone

coastline shelf

0m slope 2000 m average ocean depth ridge

rise

abyssal plain

4000 m 6000 m 8000 m 10,000 m

trench active margin

passive margin ocean continent

continent LITHOSPHERE

oceanic crust

continental crust

mantle

Figure 1.1 Cross-section through an ocean, showing the various deep-sea environments and domains. Lithosphere includes upper part of upper mantle plus oceanic or continental crust. (A multi-colour version of this figure is on the included CD-ROM.)

depth of  3800 m. Abyssal plains are “flat” at a large scale. A closer look reveals, however, that their “flatness” is disrupted by tectonic and volcanic features: transform faults at different scales and strike-like faults with hanging walls of several hundreds of metres or even several kilometres in height and related local sedimentary basins. There are, in addition, hot-spot-related volcanic mounds and islands, volcano alignments forming the oceanic ridges, channels and thick and extensive accumulations of sediments forming drifts, and levees, gypsum diapirs; there are also dissolution structures. The continental rise and abyssal plains constitute the abyssal domain (3000– 6000 m). Only 2% of the total ocean surface is deeper than 6000 m (hadal domain). In subduction areas, the presence of a subduction trench generates the deepest oceanic environments, down to 11,020 m (Mariana Trench). There, the presence of an accretionary prism can generate important reliefforming processes, such as mud diapirs and volcanoes (which may be related to the upward motion of deep fluids) and pockmarks, which are due to liquefaction related to fluid escape. The sediments in the deep-sea consist of (1) clastic particles derived from eroded rocks and sediments outcropping either on the emerged continents or previously deposited in a marine environment, (2) particles formed by

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volcanic eruptions, (3) particles formed by living organisms, including organic matter, skeletal hard parts of calcareous, opaline or phosphatic composition, and faecal particles, and (4) particles formed by chemical precipitation of the elements contained in the salty sea water (average concentration of dissolved salts in sea water is 35.5 g/l). Most of chemical processes include microbiotic reactions and are thus grouped under the term “biochemical processes”. The term “sedimentation” describes the process of accumulating sediments in the form of layers or beds and includes all events that take place during particle formation (by weathering, erosion or biogenic production), through transport to final deposition of the sedimentary particles. It also includes all the consolidation processes (such as dewatering) occurring either during the deposition or shortly after, as well as the associated biochemical and chemical changes occurring in the sediment just after deposition and favouring particle bonding (cementation) through a variety of processes summarized under the term “diagenesis” that finally transforms the (soft) sediment into an (indurated) rock. Sedimentation also includes biological processes that rework sediments early after deposition (bioturbation) and that favour early diagenesis through improvement of fluid circulation. Despite wind and atmospheric transport, which are responsible for a small part of oceanic sediment-particle transport (wind-driven dust, volcanic ashes), water should be considered as the main agent of particle transport to and within the deep-sea.

3. Tools Used for Deep-Sea Sediment Investigations 3.1. Geophysics The deep-sea can be investigated by both indirect and direct measurements from a boat or a vessel. During indirect measurements, a signal (usually acoustic) is emitted towards the sea-floor. It can be reflected at the seawater/sea-floor interface or it penetrates into the sediment before it is reflected at a bedding plane or any other disconformity. Whatever the path is, parts of the signal come back to the boat and are recorded to be subsequently processed and studied (Fig. 1.2). During direct measurements, a submersible or an ROV (Remotely Operated Vehicle) is sent along the sea floor, and a sampling device or any probe penetrates into the sea floor. In all cases, the quality and reproducibility of measurements along the sea floor have been drastically improved during the last decade because of the enhanced positioning with the development of the GPS (Global Positioning System).

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A

B

Figure 1.2 Geophysical research of the deep-sea floor. (A) Principle of multibeam bathymetry survey (modified from Ifremer’s internet website) (http://www.ifremer.fr/ anglais/). (B) The French side-scan sonar SAR (Syste`me Acoustic Remorque´ ¼ Acoustic Towed System) operated by Genavir. Picture by T. Mulder. (A multi-colour version of this figure is on the included CD-ROM.)

3.1.1. Tools measuring bathymetry Multibeam echosounders allow measuring the bathymetry (direct distance between the acoustic source and the sea floor) on a strip parallel to the boat track with a width of typically 120 –150 , in order to provide high-precision (0.5 m resolution) bathymetric maps (Figs. 1.2A and 1.3). Because of the high density of data collected within a survey, this tool is well-suited to provide 3-D views of the sea-floor topography. The insonified stripe has a width that corresponds to approx. 5–7 times the water depth. Most of the multibeam bathymetry gears are permanently embedded on the boat hull and a few are trailed behind the boat. Most of them can be operated at high speed (10 knots  18.5 km per hour). At the same time, the sounder provides a backscatter of the sea floor that can be related to its sedimentary characteristics (e.g., grain size, porosity, water content) (Table 1.1). The systems operate at frequencies varying from 12 to 500 kHz.

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A

2D bathymetry

SAR Huelva Channel

C

AD

IZ

R

ID

G

E

36 ⬚ 20 ′N

36 ⬚ 15 ′N 2 km

7 ⬚02 ′W

B 36 ⬚ 08 ′N

2 km

6 ⬚57 ′W

Slope gradient map

SAR

Failure

36 ⬚ 06 ′N 7 ⬚09 ′W

7 ⬚05 ′W

Figure 1.3 Example of Simrad EM 300 bathymetry and corresponding SAR image in the Gulf of Cadiz. (A) Cadiz Channel. (B) Slump along the giant contouritic levee. (A multi-colour version of this figure is on the included CD-ROM.)

3.1.2. Side-scan sonar A side-scan sonar (Figs. 1.2B and 1.3) is a deep-towed acoustic system that are used mainly to map the morphology and composition of the sea floor. This equipment is essential to identify small (metre-range) sedimentary features. They either record the returned signal from an acoustic beam transmitted by the tool, or the backscatter from the sea floor. The backscatter signal is a function of the topography and particularly of the sea-floor slope, which influences the angle of incidence and the nature of the sea floor. The main types of a side-scan sonar devices used for sedimentological investigation operate at frequencies from 65 to 500 kHz and are listed in Table 1.2. 3.1.3. Seismic tools Artificial seismics are based on the measurement of the travel time of acoustic waves generated by a non-natural source. We will restrict ourselves here to seismic reflection, which is the method most used in sedimentary

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Table 1.1 Main features of multibeam echosounders (from Masson, 2003).

Echosounder

Frequency (Hz)

Maximum swath width (km) Resolution (m)

Low frequency

12–24

20

Middle frequency 300 High frequency 100–1000

4–5 1

Water depth (m)

7 (cross-track), 60– > 2500 200 (along track) 1000–2500 0.2–0.4% of water 5–800 depth

Table 1.2 Main features of a side-scan sonar (from Masson, 2003).

Side-scan sonar

Frequency Swath (Hz) (km)

Low frequency

6–12

Middle frequency 30 High frequency

10–500

Resolution (m)

Towing speed (knots)

10 up to 45 few 10’s (cross-track), 10’s–100’s (along track) 2–6 1–2 (cross track),