Overview of Marine Sedimentation

• chemistry and mineralogy (reflecting origin of particles)
• grain size(s) (reflecting energetics of transport, deposition)
• degree of compaction and/or cementation (resulting from burial (physical) and diagenesis (chemical)).

The material deposited in marine sediments has its origin on the continents:

• rivers
  • suspended load: 180 x 10 g/y
  • dissolved load: 40 x 10 g/y (corrected for cyclic salt component)
• wind
  • atmospheric dust: 5 x 10 g/y

Eq 13-2:

These include quartz, feldspar, and clay minerals such as kaolinite and montmorillonite. These clay minerals are products of chemical weathering of primary igneous materials and typically very fine-grained .

Transport of Sediments

Physical processes dominate at ocean margins, where they transfer particles eroded from the land to the sea floor.

Active sedimentation processes (where the sediment modifies the properties and behavior of the suspension) include mass wasting and density currents. Such deposition tends to mask other sedimentary processes. The giant landslides surrounding the Hawaiian islands and the abyssal plains of the Atlantic are striking deep-sea examples of active sedimentation. Because active sedimentation depends on gravitational energy, it does not extend seaward of the trenches along convergent plate boundaries (but can affect the entire ocean basin off passive margins).

Passive sedimentationprocesses are those in which the sediment is carried by but does not modify the normal thermohaline circulation. Examples:

• Hemipelagic sedimentation, where it appears that fine sediment moves along isopycnal surfaces high in the water column. This process is poorly understood (for example, the role of internal waves in resuspending sediment, the role of squirts and jets in transporting sediment offshore and the role of biota in sediment deposition have yet to be quantified). Hemipelagic sedimentation produces a fringe of terrigenous (land-derived) deposits up to a few hundred kilometers wide around land masses. These deposits are draped uniformly over the sea floor topography
  
  
• "Drift" deposition along the path of bottom currents. Prominent in the high latitude North Atlantic along the flanks or ridges. Sediment in transit to drift deposits creates a near-bottom "nepheloid layer." Paleoceanography, 9 (6), (December 1994) has a dozen papers on drift deposits.
  
• Eolian (wind transported) sedimentation. Prominent where major wind systems cross semi-arid source areas (ephemeral lakes) or active ash-generating volcanoes. Prominent in the North Pacific (Figure MS-1), North Atlantic and Arabian Sea
  
  
  
  Figure MS-1. Quartz concentration in North Pacific sediments (carbonate-free basis) showing effects of eolian transport in westerlies and northeast trades.
  
  
• Ice rafting. Restricted to high latitudes, but can carry coarse particles into the subarctic gyres, far from land.

Sediments formed by physical processes have distinctive acoustic signatures of military interest. Hence they have been much studied during the past 50 years.

"Passive" sediments record the history of deep currents, volcanism, aridity, wind trajectories, and iceberg abundances and trajectories. Drift deposits, which can accumulate at hundreds of meters per million years, yield some of the highest resolution paleoceanographic records.

Biological processes dominate sediment formation in areas of high productivity that receive little terrigenous material. The equatorial Pacific and Southern Ocean are examples.

CaCO

• foraminiferal ooze
  • organism: forams, a protozoan
    most common source is genus Globigerina

  • mineral form: calcite
• coccolith ooze
  • organism: algae coccolithophoridae
  • mineral form: calcite
• pteropod ooze
  • organism: planktonic molluscs
  • mineral form: aragonite

• diatom ooze
  • organism: algal diatoms
  • mineral form: opal
• radiolarian ooze
  • organism: radiolaria (protozoans)
  • mineral form: opal

Benthic organisms also modify the historical record by actively mixing ("bioturbating") the most recently deposited sediments. Bioturbation, which is addressed later in the course, is effectively a low-pass filter that supresses or eliminates records of events that create layers of sediment thinner than the depth of mixing. Rapidly deposited or anoxic sediments provide the only deep-sea records capable of resolving events shorter than about a millennium.

Chemical processes dominate sedimentation only in deep, low productivity areas shielded from terrigenous material. Precipitates from hydrothermal solutions emanating from mid-ocean ridges are prominent along the flanks of the East Pacific Rise in the South Pacific. Authigenic deposits (formed by the very slow precipitation of oxyhydroxides and silicates? from normal seawater) form distinctive sediments in the central South Pacific as well as ferromanganese nodules (Figure MS-2) and crusts on any surfaces where other sediments are absent or accumulating extremely slowly.

Figure MS-2. Deep sea ferromanganese nodules on the floor of the South Pacific Ocean (individual nodules are 5-10 cm diameter).

Distribution of Sediments

Figure 13-3, from NOAA/NGDC, additional information is available concerning the underlying database Distribution of Recent (Surface) Sediments

The global distribution of surface sediment types is shown here:

Figure 13-4, from (2)

Except for areas near to the continents, which are dominated by hemipelagic contiental margin sediments and muds (and at high latitudes, glacial marine sediments), the predominant sediments in the deep ocean are biogenic oozes and/or fine-grained clays.

The distribution of biogenic sediments responds to oceanic conditions in three ways:

controls on primary productivity, especially those related to the intensity of upwelling

The particle compositions exhibits increasing silica/carbonate ratio as upwelling rate increases; there are also regional effects (temperature, light) on species distributions

corrosiveness of deep ocean waters to (settling particles and) surficial sediments

Waters at all depths are undersaturated with respect to solid amorphous silica; the driving force for dissolution decreases as deep water ages and develops higher dissolved silica concentration. (Because waters at all depths are undersaturated, organisms must spend energy to precipitate amorphous silica.). The siutation is more involved for calcium carbonate, with surface waters supersaturated and deep waters undersaturated.

topography

Most of the dissolution of biogenic debris occurs at seafloor. Thus the topography influences the temperature and pressure conditions under which dissolution occurs.

Figure 13-5, from (20)

Near to the ridge at which the basement rock is created, metalliferous sediment (of hydrothermal origin) dominates the supply of material. Once away from this source of sediment, carbonate material raining from above accumulates. As the plate subsides, the dissolution of carbonate increases such that this material is no longer preserved and only residual clay material accumulates (at a much lower sedimentation rate). As the site passes beneath the equator, the rate of supply of biogenic silica exceeds the rate of dissolution and it accumulates. Further north beyond the equatorial band of productivity, red clay dominates again.

Stratigraphy

Sediments place events in time. Their study from this perspective is termed stratigraphy: age relations of rock strata (layers). The objectives of stratigraphic studies are to

• establish markers of a sequence of events and thus relative ages of events,
• correlate these markers of events either regionally or globally, and,
• assign absolute ages (the basis of these age assignments is radiometric dating of materials) or a chronology to these events.

Earth's history is divided into ages:

Figures 13-6ab

In ocean sediments, the Cenozoic and Mesozoic ages are represented. These ages can be further divided into periods and epochs and stages:

Figure 13-7

Figure 13-8

(Figures 13-6, 7 and 8 courtesy of the University of California at Berkeley Museum of Paleontology, their copyright notice. Their web server offers an extensive discussion of geological time in relation to evolution of the species.)

In establishing time scales for ocean sediments, stratigraphic correlations can be made in a number of ways:

lithostratigraphy

boundaries between rocks of different type or character (variant: seismic stratigraphy recognizing these boundaries from seismic reflection profiles)

biostratigraphy

based on fossil content (presence, absence of particular indicator species)

chronostratigraphy

based on ages of materials as established by radiometric methods

magnetostratigraphy

based on magnetic reversals recorded in sediments

stable isotope stratigraphy

based on oxygen isotopic composition of carbonate material, a signal dominated by global state of ice storage on the continents during glacial times

Indicators of Changes in Ocean Conditions

Our interest in Pleistocene sediments is derived from the record of climate change they contain. There are two important classes of signals:

Deposition Rates

Signals indicating the extent to which material is deposited and preserved. These include absolute rates of net deposition and indices of changing proportions of materials (e.g., Si vs. C, specific species)

Chemical Signatures

Signals involving properties of sedimentary materials which reflect conditions of original deposition, either directly or as a proxy for some other property. Some examples are oxygen isotopic composition reflecting ice storage on the continents (and to a lesser extent temperature of deposition); Sr/Ca reflecting the temperature of deposition (and to a lesser extent upwelling), and Cd/Ca reflecting upwelling.

Background Information on Sediment Classification

Figure 13-1

Figure 13-2, from the Geology 202 Pages at University of British Columbia

Marine sediments are further classified:

• median grain size <5µm: pelagic deposit (clays and oozes)
  • biogenic component <30%: calcareous or siliceous clay
  • biogenic component >30%
    • calcareous component dominant
      • carbonate component <67%: marl ooze
      • otherwise: chalk ooze
    • siliceous component dominant: diatom or radiolarian ooze
  • median grain size >5µm: hemipelagic deposit (muds)