5.3 Marine Sediments

Marine sediments blanket most of the ocean floor and serve as the geological archive of Earth's climate, ocean chemistry, and biological history. They are classified by origin: terrigenous, biogenic, hydrogenous, and cosmogenous. Their distribution is controlled by proximity to land, biological productivity, water depth, and ocean chemistry.

Sediment Classification

Terrigenous (Lithogenous) Sediments

Derived from weathering and erosion of continental rocks. Includes clay (<4 μm), silt (4–63 μm), and sand (63–2000 μm). Transported by rivers, wind (aeolian dust, e.g., Saharan dust across the Atlantic), glaciers (ice-rafted debris), and turbidity currents. Dominates near continental margins and in high-latitude regions. Sedimentation rates: 1–100 cm/kyr on shelves, decreasing offshore.

Biogenic (Biogenous) Sediments

Composed of skeletal remains of marine organisms. Calcareous ooze(>30% CaCO₃): foraminifera and coccolithophore tests; covers ~48% of the deep seafloor. Siliceous ooze (>30% SiO₂): diatom frustules (high latitudes) and radiolarian tests (equatorial). Biogenic sediments dominate the open ocean where terrigenous input is low.

Hydrogenous (Authigenic) Sediments

Precipitate directly from seawater or pore water. Manganese nodules(polymetallic nodules) grow at ~1–10 mm/Myr on the abyssal floor, rich in Mn, Fe, Ni, Cu, Co. Phosphorites form on continental shelves in upwelling regions. Metal-rich sediments precipitate near hydrothermal vents.

Cosmogenous Sediments

Extraterrestrial material: micrometeorites (cosmic spherules), tektites (impact ejecta). Volumetrically insignificant (<1%) but scientifically important as time markers (e.g., iridium anomaly at the K–Pg boundary).

Carbonate Compensation Depth (CCD)

Calcium carbonate dissolves more readily under high pressure and low temperature. The lysocline is the depth where dissolution becomes significant (~3500–4000 m), while the CCD is where the rate of dissolution equals the rate of supply (~4500 m in the Pacific, ~5000 m in the Atlantic). Below the CCD, the seafloor is free of calcareous sediment.

Carbonate Saturation State

$$\Omega = \frac{[\text{Ca}^{2+}][\text{CO}_3^{2-}]}{K_{sp}}$$

When $\Omega < 1$, seawater is undersaturated and CaCO₃ dissolves. $K_{sp}$ increases with pressure, so deep water is more corrosive. The CCD is deeper in the Atlantic because North Atlantic Deep Water is younger and less CO₂-rich than Pacific deep water.

Calcite vs Aragonite: Aragonite (denser CaCO₃ polymorph) dissolves at shallower depths (~2000 m) than calcite (~4500 m). Pteropod ooze (aragonite shells) is thus restricted to shallow depths. The CCD has fluctuated through geological time.

Stokes Settling Velocity & Sedimentation Rates

The terminal settling velocity of a spherical particle in a viscous fluid (low Reynolds number) is given by Stokes' law:

$$w_s = \frac{2}{9}\frac{(\rho_p - \rho_f)\,g\,r^2}{\mu}$$

where $\rho_p$ is particle density, $\rho_f$ is fluid density, $r$ is particle radius, and $\mu$ is dynamic viscosity. Fine clay (1 μm) takes ~50 years to settle through 4 km of ocean, while sand (100 μm) settles in hours.

Pelagic

0.1–5 mm/kyr

Deep ocean, far from land

Hemipelagic

1–20 cm/kyr

Continental margins

Coastal/Deltaic

0.1–100 cm/yr

Estuaries, river deltas

Paleoceanographic Proxies

Sediment cores are time capsules of ocean history. Chemical and isotopic analyses of microfossil shells (foraminifera, diatoms) provide proxies for past ocean conditions:

$\delta^{18}\text{O}$ — Temperature and Ice Volume

Oxygen isotope ratio in foraminiferal calcite reflects both water temperature and global ice volume:

$$T(°\text{C}) = 16.9 - 4.38\,(\delta^{18}\text{O}_c - \delta^{18}\text{O}_w) + 0.10\,(\delta^{18}\text{O}_c - \delta^{18}\text{O}_w)^2$$

(Epstein et al., 1953; modified by Shackleton)

Mg/Ca Ratio — Temperature

Mg incorporation into calcite is temperature-dependent: $\text{Mg/Ca} = B\,\exp(A \cdot T)$ where$A \approx 0.09$ and $B$ is species-specific. Combined with $\delta^{18}\text{O}$, this separates temperature from ice-volume effects.

Alkenone Unsaturation Index (U'₃₇)

Organic geochemical proxy from coccolithophore lipids, linearly correlated with SST:$T = (U^{K'}_{37} - 0.044) / 0.033$. Robust across 1–28 °C. Preserved in sediments for millions of years.

Sediment Cores and Stratigraphy

Deep-sea sediment cores (obtained by piston coring or ocean drilling) provide continuous records spanning millions of years. Stratigraphic correlation uses biostratigraphy (first/last appearance of microfossil species), magnetostratigraphy (magnetic reversals), and chemostratigraphy ($\delta^{18}\text{O}$ stages). The marine isotope stages (MIS) system numbers glacial and interglacial periods back to MIS 104 (~2.6 Ma).

Piston Core

Free-fall corer, typically 10–30 m penetration. Rapid recovery with minimal disturbance of the sediment column.

Ocean Drilling (IODP)

Rotary drilling from a research vessel. Can penetrate hundreds of metres into sediment and basement rock for deep time records.

Python: Stokes Settling, CCD Plot & Proxy Calibration

Python

!/usr/bin/env python3

script.py66 lines

#!/usr/bin/env python3
"""marine_sediments.py - Stokes settling, CCD model, d18O calibration."""
import numpy as np
import matplotlib.pyplot as plt

# ── 1. Stokes Settling Velocity ──
def stokes_velocity(d_um, rho_p=2650, rho_f=1025, mu=1.08e-3):
    """
    Stokes settling velocity (m/s).
    d_um : particle diameter in micrometres
    rho_p : particle density (kg/m^3), default quartz
    rho_f : fluid density (kg/m^3)
    mu    : dynamic viscosity (Pa s)
    """
    r = d_um * 1e-6 / 2.0
    g = 9.81
    return (2.0 / 9.0) * (rho_p - rho_f) * g * r**2 / mu

diameters = np.logspace(-1, 3, 200)    # 0.1 to 1000 um
ws = stokes_velocity(diameters)
settling_time_days = 4000.0 / (ws * 86400)

fig, axes = plt.subplots(1, 3, figsize=(18, 5))

ax = axes[0]
ax.loglog(diameters, ws * 86400, 'b-', lw=2)
ax.set_xlabel('Particle Diameter (um)')
ax.set_ylabel('Settling Velocity (m/day)')
ax.set_title('Stokes Settling Velocity')
ax.axvline(4, color='brown', ls='--', label='Clay/Silt boundary')
ax.axvline(63, color='orange', ls='--', label='Silt/Sand boundary')
ax.legend(fontsize=8); ax.grid(True, alpha=0.3, which='both')

# ── 2. CCD vs Latitude (schematic) ──
lat = np.linspace(-80, 80, 200)
ccd_pacific = 4500 - 500 * np.exp(-lat**2 / 400)
ccd_atlantic = 5000 - 300 * np.exp(-lat**2 / 600)

ax = axes[1]
ax.plot(lat, ccd_pacific, 'r-', lw=2, label='Pacific CCD')
ax.plot(lat, ccd_atlantic, 'b-', lw=2, label='Atlantic CCD')
ax.fill_between(lat, 6000, ccd_pacific, color='sienna', alpha=0.2, label='No CaCO3')
ax.invert_yaxis()
ax.set_xlabel('Latitude')
ax.set_ylabel('Depth (m)')
ax.set_title('CCD vs Latitude')
ax.legend(fontsize=8); ax.grid(True, alpha=0.3)

# ── 3. Synthetic d18O Record (Milankovitch cycles) ──
age_kyr = np.linspace(0, 800, 1000)
d18O_record = (3.5
    + 1.0 * np.sin(2 * np.pi * age_kyr / 100)
    + 0.3 * np.sin(2 * np.pi * age_kyr / 41)
    + 0.15 * np.sin(2 * np.pi * age_kyr / 23))

ax = axes[2]
ax.plot(age_kyr, d18O_record, 'purple', lw=1)
ax.invert_yaxis()
ax.set_xlabel('Age (kyr BP)')
ax.set_ylabel('d18O (permil)')
ax.set_title('Synthetic Benthic d18O Record')
ax.grid(True, alpha=0.3)

plt.tight_layout()
plt.savefig('marine_sediments.png', dpi=150)
plt.show()

Click Run to execute the Python code

Code will be executed with Python 3 on the server

Fortran: 1D Sedimentation Model with Dissolution & Bioturbation

This program models carbonate accumulation on the seafloor. The governing equation is:

$$\frac{\partial C}{\partial t} = D_b \frac{\partial^2 C}{\partial z^2} - w \frac{\partial C}{\partial z} + S - \lambda C$$

where $C$ is CaCO₃ concentration, $D_b$ is bioturbation diffusivity,$w$ is burial velocity, $S$ is the carbonate rain rate, and $\lambda$ is the dissolution rate constant.

Fortran: 1D Sedimentation Model with Dissolution & Bioturbation

Fortran

============================================================

program.f9067 lines

program sedimentation_1d
  ! ============================================================
  ! 1D sedimentation model with dissolution + bioturbation
  ! ============================================================
  implicit none
  integer, parameter :: dp = selected_real_kind(15, 307)
  integer, parameter :: nz = 200, nt = 50000
  real(dp) :: C(0:nz), C_new(0:nz)
  real(dp) :: Db, w, S_rain, lambda_diss
  real(dp) :: dz, dt, zmax, z
  real(dp) :: depth_water, CCD, lysocline
  integer  :: i, n

! Parameters
  zmax        = 0.5_dp               ! 50 cm sediment column (m)
  dz          = zmax / real(nz, dp)
  dt          = 1.0_dp               ! 1 year time step
  Db          = 1.0d-8               ! bioturbation diffusivity (m^2/yr)
  w           = 2.0d-5               ! sedimentation rate 2 cm/kyr (m/yr)
  S_rain      = 0.03_dp              ! CaCO3 rain rate (frac/yr at surface)
  depth_water = 4200.0_dp            ! water depth (m)
  CCD         = 4500.0_dp
  lysocline   = 3800.0_dp

! Dissolution rate depends on water depth
  if (depth_water > CCD) then
    lambda_diss = 0.05_dp
  else if (depth_water > lysocline) then
    lambda_diss = 0.01_dp * (depth_water - lysocline) / (CCD - lysocline)
  else
    lambda_diss = 0.0_dp
  end if

! Initial condition: uniform CaCO3 = 0.5
  C = 0.5_dp
  C(0) = min(1.0_dp, 0.5_dp + S_rain)

! Time integration (explicit finite difference)
  do n = 1, nt
    do i = 1, nz - 1
      C_new(i) = C(i) + dt * ( &
        Db * (C(i+1) - 2.0_dp*C(i) + C(i-1)) / (dz*dz) &
        - w * (C(i) - C(i-1)) / dz &
        - lambda_diss * C(i) )
      C_new(i) = max(0.0_dp, min(1.0_dp, C_new(i)))
    end do
    C_new(0)  = min(1.0_dp, 0.5_dp + S_rain)
    C_new(nz) = C(nz)
    C = C_new
  end do

! Output final profile
  open(unit=10, file='sediment_profile.dat', status='replace')
  write(10,'(A)') '# Depth_cm  CaCO3_fraction'
  do i = 0, nz
    z = real(i, dp) * dz * 100.0_dp
    write(10,'(F8.2, 1X, F8.5)') z, C(i)
  end do
  close(10)

write(*,'(A,F6.1,A)') 'Water depth: ', depth_water, ' m'
  write(*,'(A,ES10.3)') 'Dissolution rate: ', lambda_diss
  write(*,'(A,F6.4)') 'Surface CaCO3: ', C(0)
  write(*,'(A,F6.4)') 'Bottom CaCO3:  ', C(nz)
  write(*,'(A)') 'Profile written to sediment_profile.dat'

end program sedimentation_1d

Click Run to execute the Fortran code

Code will be compiled with gfortran and executed on the server

Global Sediment Distribution Patterns

The distribution of marine sediments on the ocean floor reflects the interplay of biological productivity, water depth (relative to the CCD), proximity to continental sources, and ocean circulation patterns. Understanding these patterns is essential for interpreting the geological record and for assessing marine mineral resources.

Pacific Ocean

Dominated by siliceous ooze (diatom ooze in the north, radiolarian ooze at the equator) and pelagic red clay at abyssal depths below the CCD (~4200–4500 m). Calcareous ooze occurs on the flanks of the East Pacific Rise and on shallower seamounts. Terrigenous sediment is confined to narrow continental margins due to the ring of subduction trenches that trap continental runoff.

Atlantic Ocean

Calcareous ooze dominates because the Mid-Atlantic Ridge is relatively shallow and the CCD is deeper (~5000 m) due to younger, less CO₂-enriched deep water. Terrigenous sediment extends far from the passive margins, forming extensive abyssal plains (Sohm, Hatteras). Turbidite deposits are common near submarine canyon mouths. The Amazon Fan is one of the largest deep-sea fans on Earth.

Indian Ocean

Mixed sediment types. The Bengal Fan (from the Ganges–Brahmaputra) is the world's largest submarine fan, extending 3000 km. Calcareous ooze predominates on ridges and plateaus. Siliceous ooze is found south of the Polar Front near Antarctica.

Southern Ocean

Diatom ooze forms a wide belt around Antarctica (the "diatom ooze belt"), reflecting the extremely high siliceous productivity driven by upwelling along the Antarctic Divergence. Ice-rafted debris (IRD) from icebergs is common and provides a record of ice sheet dynamics through geological time.

Key Equations Summary

Stokes Settling (Reynolds Number Check)

$$Re = \frac{\rho_f\,w_s\,d}{\mu}$$

Stokes' law is valid when $Re < 0.5$. For larger particles, drag corrections (e.g., Oseen) are needed.

Bioturbation Diffusion

$$D_b = \frac{1}{2}\frac{(\Delta x)^2}{\Delta t}$$

Typical values: $D_b \sim 10^{-10}$ to $10^{-8}\;\text{m}^2/\text{s}$, mixing the upper ~10 cm of sediment over centuries.

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