9/11/25 Marine Science Chapter 4 Pt. 2 Sediment Environments and Biogenous/Calcareous Siliceous Oozes

Introduction to Marine Sediments

  • Sediments are distributed in standard environments and described in two broad categories: neuritic (nearshore, lithogenous) and pelagic (deep-water, more cosmopolitan).
  • Transport processes can be non-selective (e.g., landslides or turbidity currents) and move a mix of particle sizes to the same location.
  • Goal: describe sediment distributions in a standard way to understand environments and their processes.

Neuritic vs Pelagic Environments

  • Neuritic sediments = shallow-water deposits close to land; dominantly lithogenous (derived from land).
    • Composed of sands, clays, silts from weathered terrestrial rocks.
    • Deposited relatively quickly with high accumulation rates due to large land sediment sources and rapid river input.
  • Pelagic sediments = deeper-water deposits farther from land; typically on the continental shelf/slope and abyssal plain.
    • Deposited slowly and are very fine-grained.
    • Most sediments from land do not reach pelagic zones; only wind-blown dust or the finest clays reach there.
    • Pelagic sediments include a large biogenous component.

Nearshore Neuritic Lithogenous Sediments (Examples)

  • Beach deposits: quartz-rich sands, medium texture, well sorted, mainly wave-deposited.
  • Continental shelf deposits: sediments that extend farther from shore; form layered sediment cores used to reconstruct climate history via relict sediments and cores recovered by coring ships.
  • Continental slope and rise deposits: graded bedding from turbidity currents (turbidites).
  • Glacial deposits: ice rafting; icebergs carry rocks, sand, silt and drop them as they melt, creating larger rocks on the shelf and sometimes the abyssal plain.

Pelagic Sediments: Lithogenous and Biogenous Components

  • Pelagic lithogenous sources include wind-blown dust, clay transported by deep-ocean currents, and sporadic volcanic ash events (large eruptions).
  • Pelagic sediments are generally very fine-grained and accumulate slowly due to low surface productivity and weak sediment supply.
  • Biogenous sediments become a major component in pelagic environments.
  • The life cycle of organisms in surface oceans is slow in some areas, leading to very slow sediment deposition on the seafloor (e.g., a few centimeters per thousand years in some pelagic regions).
  • Biogenous deposition is governed by productivity, dissolution, and dilution (see below).

Abyssal Clay vs Oozes

  • Abyssal clay definition: sediments with at least 70% clay-sized lithogenous particles from continental sources.
  • Oozes (biogenous sediments) are distinct: must contain at least 30% hard remains of biogenic test material (siliceous or calcareous).
  • In pelagic regions with low productivity and strong lithogenous input, abyssal clays dominate; in high-productivity areas, biogenous oozes can dominate.

Biogenous Sediments: Macroscopic vs Microscopic

  • Macroscopic biogenous sediments: visible to the naked eye (e.g., beaches in Florida/The Caribbean dominated by shells).
  • Microscopic biogenous sediments (deep ocean): tiny shells or tests from microscopic organisms; form biogenous oozes.
  • Biogenous ooze name derives from origin: ooze-like consistency due to very fine particles.
  • Major composition types: calcareous oozes (calcium carbonate) and siliceous oozes (silica).
  • Two main chemical compositions:
    • Calcium carbonate (calcareous) oozes
    • Silica (siliceous) oozes

How Biogenous Sediments Sink to the Seafloor

  • Tests (skeletons) sink after the organisms die, but sinking speeds depend on size and density.
  • Very fine tests (10–50 microns) sink slowly; horizontal currents can move them before burial.
  • Predation and fecal pellet formation accelerate sinking:
    • If a heterotrophic zooplankton (e.g., copepods, krill) eats tests and excretes fecal pellets, sinking times reduce dramatically (≈10–15 days to reach the seafloor).
    • Pellets are larger and denser, speeding sedimentation.

Siliceous Biogenous Sediments (Siliceous Ooze)

  • Composed of silicate tests from two silica-secreting plankton groups:
    • Diatoms: photosynthetic algae with silica shells; form ornate glass-like tests.
    • Radiolarians: protozoans with silica tests; typically larger than diatoms.
  • Siliceous ooze requires at least 30% hard remains of siliceous organisms.
  • High productivity regions near continental margins foster diatom-rich ooze due to nutrient upwelling.
  • Silica dissolves in seawater over long times; burial beneath subsequent sediment layers preserves siliceous ooze for millions of years.
  • Diatomaceous earth: lithified silica ooze from diatoms; used in pesticides, water filtration, absorbents, and other industrial applications due to sharp silica morphology.

Calcareous Biogenous Sediments (Calcareous Ooze)

  • Composed primarily of calcium carbonate tests from calcite-secreting organisms:
    • Coccolithophores: tiny, photosynthetic algae that build calcium carbonate coccolith plates; form calcareous ooze when abundant.
    • Foraminifera: larger heterotrophic protozoans with calcium carbonate tests; also contribute to calcareous ooze.
  • Calcareous ooze definition: at least 30% hard remains of calcareous secreting organisms.
  • Key difference from siliceous ooze: depth-related dissolution drives distribution via the Calcite Compensation Depth (CCD).

Calcite Compensation Depth (CCD)

  • CCD is the depth in the ocean where calcium carbonate readily dissolves; below this depth, dissolution dominates over supply.
  • Depth at which dissolution rate equals the supply rate of calcium carbonate:
    • At depths shallower than the CCD: carbonate can accumulate as calcareous oozes.
    • At depths deeper than the CCD: carbonate dissolves faster than it can accumulate, so calcareous ooze cannot form.
  • Typical CCD depth is about
    • DCCD ≈ 4.5 km.D_{CCD} \,\approx \,4.5 \,\text{km}.
  • Implications:
    • Warm, shallow, well-supplied oceans favor calcareous ooze accumulation near the surface and along mid-ocean ridges that rise above the CCD.
    • Deep, cold regions (abyssal plains) are undersaturated in CaCO3 and do not accumulate calcareous ooze there.
    • Relict calcareous ooze can be found in deep areas if it was deposited when those regions were shallower or covered by other sediments later.

Distribution Related to Mid-Ocean Ridges

  • Mid-ocean ridges are relatively shallow (above the CCD) and often enriched in calcareous organisms.
  • Calcareous ooze tends to surround mid-ocean ridges due to favorable depth and productive environments.
  • As seafloor spreads away from ridges and depths exceed the CCD, calcareous ooze deposition declines and is overlain by abyssal clays or siliceous oozes elsewhere.

Chalk and Neuritic Carbonate Deposits

  • When calcareous ooze lithifies, it forms carbonate rocks, including chalk.
  • Chalk examples: White Cliffs of Dover, UK; formed largely from coccolith-rich ooze.
  • In neuritic regions, biogenic sediment accumulation is generally limited by lithogenous input, but there are carbonate-rich exceptions:
    • Caribbean/Florida shell beaches with abundant carbonate sands.
    • Stromatolites in nearshore carbonate settings, produced by cyanobacteria that lay down carbonate layers; fossil stromatolites show ancient life and modern analogs exist (e.g., Shark Bay stromatolites).
    • Stromatolites: fine carbonate layers built by cyanobacteria; ancient organisms still forming today.

Summary: Environmental Controls on Ooze Formation

  • Siliceous oozes form in cool, highly productive waters with abundant siliceous organisms; accumulate near margins where upwelling nutrients are high.
  • Calcareous oozes form in shallow warm waters where CaCO3 is stable and the CCD is not reached; they tend to accumulate around mid-ocean ridges where water is shallow enough for CaCO3 to persist.
  • CCD depth governs where calcareous ooze can accumulate; deeper regions rely on non-calcareous sediments or relict calcareous layers.
  • Productivity, dissolution, and dilution govern biogenous sediment distributions:
    • Productivity: higher surface production yields more biogenous material sinking to the seafloor.
    • Dissolution: deeper waters dissolve CaCO3; siliceous materials are more resistant but also subject to dissolution over very long timescales.
    • Dilution: high lithogenous input reduces the relative abundance of biogenous material.

Practical Implications and Real-World Connections

  • Diatomaceous earth is mined for various industrial uses due to its microstructure and properties.
  • Chalk formations are linked to coccolithophore-rich ooze and can form significant sedimentary rocks under suitable tectonic and sedimentation conditions.
  • The Iron Age of marine sediments includes evidence from neritic stromatolites and other carbonate structures that inform the history of life and ocean chemistry.
  • Understanding ooze distributions helps interpret past ocean productivity, climate changes, and plate tectonics (e.g., relict calcareous ooze indicates former shallower depths).

Research and Practical Example Problems

  • Clicker concept check: How are ooze sediments different from abyssal clays?
    • Answer: Oozes are at least 30% biogenous test material; abyssal clays are at least 70% fine clay-sized lithogenous particles from continents.
  • CCD definition check:
    • The calcite compensation depth is the depth at which the rate of calcium carbonate dissolution equals the rate of calcium carbonate supply. At depths greater than the CCD, carbonate will dissolve faster than it is supplied, so calcareous ooze cannot accumulate.
  • Distribution reasoning problem (example from the lecture): You are on a research cruise heading east from Japan. At ~1,500 km east of Japan you encounter the Shatsky Rise, a tall volcanic plateau, with sediments and a depth reading of ~2250 m2250 \,\text{m}. What kind of sediment would you expect?
    • Answer: Carbonate ooze. Rationale: Far from lithogenous clay sources (limited lithogenous input), the plateau rises above the CCD, enabling CaCO3 to accumulate; warm tropical to temperate waters around the plateau favor calcareous organisms; thus carbonate ooze dominates over abyssal clay or siliceous ooze in this setting.

Connections to Later Topics

  • Hydrogeneous and cosmogenous marine sediments (to be covered later) and their role in ocean chemistry and resource extraction.
  • Resources extracted from seafloor sediments (e.g., diatomaceous earth, calcareous rocks).

Key Definitions and Formulas

  • Abyssal clay: sediment with at least 70% clay-sized lithogenous particles from continental sources.
  • Oozes (biogenous sediments): sediment with at least 30% hard remains of biogenic tests (siliceous or calcareous).
  • Calcite Compensation Depth (CCD): depth at which the dissolution rate of calcium carbonate equals the supply rate; below this depth, CaCO3 dissolves faster than it accumulates.
    • D<em>CCD:depth where R</em>diss=Rsup.D<em>{CCD}: \text{depth where } R</em>{diss} = R_{sup}.
  • Sinking times (typical)::
    • Micro tests (10–50 µm): tens of years to sink from surface to seafloor unless aided by aggregation.
    • Fecal pellets (larger, denser): ~10–15 days to reach seafloor.

Final Note

  • The distribution of pelagic sediments is a balance of productivity, dissolution, and dilution by lithogenous inputs, with distinct regional patterns shaped by ocean circulation, nutrient upwelling, and plate tectonics. The roles of siliceous vs calcareous organisms and their environmental preferences drive where siliceous and calcareous oozes accumulate, while the CCD imposes a major depth-dependent constraint on calcareous ooze formation. In neritic zones, lithogenous input dominates, though carbonate beaches and stromatolites show notable biogenic carbonate production locally. This integrated view helps interpret ocean history, resource potential, and global sedimentary processes.