Overview
This lecture covers the origin, types, and deposition processes of biogenous sediments in the ocean, with a focus on siliceous (silica-based) microscopic organisms.
Biogenous Sediment Basics
- Biogenous sediments come from the hard parts (shells, bones, teeth) of living organisms.
- Soft parts are usually eaten or decompose quickly, so only hard parts remain.
- Large organisms (e.g. whale bones) rarely contribute, but their remains can still become sediment.
- Most biogenous sediments come from microscopic organisms.
Microscopic Biogenous Sediments
- Microscopic sediment includes tests (shells) or remains of small dead organisms.
- For sediment to be called "biogenous ooze," it must be at least 30% biogenous tests by weight.
- Biogenous oozes mainly originate from algae and protozoans.
Siliceous (Silica-Based) Tests
- Two main groups produce silica tests: diatoms (algae) and radiolarians (protozoans).
- Diatoms are photosynthetic, planktonic, and found in surface waters where there is sunlight.
- Radiolarians are planktonic, heterotrophic (feed on bacteria/plankton), and found in warm ocean waters.
- Siliceous ooze consists of at least 30% hard parts from silica-secreting organisms.
Deposition and Dissolution of Silica
- Ocean water is under-saturated with silica, so silica tests dissolve slowly over time.
- High production rates of tests (due to favorable environmental conditions) allow some to accumulate as sediment.
- Diatom tests are diverse in shape and fit together in a box-and-lid style.
- Diatoms can form dense blooms when nutrients are abundant.
- Some diatoms are benthic (live on the seafloor) but only in shallow, well-lit areas.
- Dead diatom tests sink slowly, but fecal pellets (from being eaten) help them sink faster (10-15 days).
Uses and Importance of Diatoms
- Diatom shells resemble microscopic crushed glass and are used in filters, abrasives, and pesticides (diatomaceous earth).
- Silica from diatoms has been used in manufacturing, including space shuttles.
Sediment Collection and Analysis
- Sediment traps are used to measure productivity and deposition rates in the ocean.
- Traps have rotating bottles for collecting samples over set intervals.
- Sediment cores provide a record of deposition over time.
Key Terms & Definitions
- Biogenous sediment — sediment made from hard parts of living organisms.
- Test — the hard shell of a microscopic organism.
- Biogenous ooze — seafloor sediment with ≥30% biogenous tests.
- Siliceous ooze — biogenous ooze composed mainly of silica-based tests.
- Diatom — photosynthetic, silica-secreting planktonic algae.
- Radiolarian — heterotrophic, silica-secreting planktonic protozoan.
- Sediment trap — device for collecting and measuring falling sediment in the ocean.
Action Items / Next Steps
- Read about calcium carbonate-based tests for the next class.
- Review the process and importance of sediment traps in oceanography.