Eighth Edition
by Harold L. Levin

Chapter 6 - page 7

Life on Earth: What do Fossils Reveal?

Fossils Indicate Past Environments

Fundamental Concepts of Ecology and Paleoecology

Paleoecologists are particularly interested in the marine (ocean) ecosystem because marine life dominates the fossil record.

1. Ecology = The interrelationship between organisms and their environment.
2. Paleoecology = Ancient ecology; the study of the interaction of ancient organisms with their environment.
   In large part, paleoecology depends on comparisons of ancient organisms with living organisms.
   We use modern analogs to help us interpret something about the way in which the fossil animals lived and related to their environment.
3. Ecosystem = The organisms and their environment - the entire system of physical, chemical, and biological factors influencing organisms.
4. Habitat = The environment in which the organism lives.
5. Niche = The way in which the organism lives - its role or lifestyle.
6. Community = the association of several species of organisms in a particular habitat (the living part of the ecosystem).
7. Paleocommunity = An ancient community.

Trophic level

The organisms living in the ocean ecosystem are commonly grouped according to their method of obtaining nutrients, or their trophic level.

1. Primary Producers or autotrophs - produce their own food through photosynthesis, and supply food and energy for other organisms. In the marine ecosystem, many of the producers are plankton, including tiny diatoms and other photosynthetic organisms.
2. Consumers or heterotrophs - cannot produce their own food and must eat.
1. Herbivores - heterotrophs that eat plants
2. Carnivores - heterotrophs that eat herbivores and other carnivores
3. Other feeding modes:
1. Decomposers and Transformers - bacteria and fungi which break down organic matter converting it into a form which can be utilized by other organisms (nutrients)
2. Parasites - derive nutrition from other organisms without killing them
3. Scavengers - derive nutrition from dead organisms

The Marine Ecosystem

The ocean may be divided into two realms:

1. Pelagic realm = The water mass lying above the ocean floor. It can be subdivided into:
1. Neritic zone = The water overlying the continental shelves.
2. Oceanic zone = The water seaward of the continental shelves.

2. Benthic realm = The bottom of the sea, which includes:
1. Supratidal zone = Above the high tide line
2. Littoral zone (or intertidal zone) = Between the high and low tide lines
3. Sublittoral zone (or subtidal zone) = Continuously submerged zone, from low tide line to the edge of the continental shelf (about 200 m deep)
4. Bathyal zone = (200 - 4000 m deep)
5. Abyssal zone = (4000 - 6000 m deep)
6. Hadal zone = (more than 6000 m deep) - The extreme depths found in the deep sea trenches. Note that the deepest point in the oceans is in the Marianas Trench, 11,033 m deep.

Classification of marine environments.

Within the pelagic realm, animals may have varying modes of life:

1. Plankton - Small plants and animals that float, drift, or swim weakly
1. Phytoplankton - Plants and plant-like plankton, such as diatoms and coccolithophores
2. Zooplankton - Animals and animal-like plankton, such as foraminifera and radiolaria

2. Nekton - Swimming animals that live within the water column

3. Benthic organisms or benthos - Bottom dwellers, which may be either:
1. Infaunal - Living beneath the sediment surface; they burrow and churn and mix the sediment, a process called bioturbation
2. Epifaunal - Living on top of the sediment surface

Types of Seafloor Sediments in the Marine Environment

1. Terrigenous sediment
   Mineral grains from weathered continental rocks
   Fine-grained sediment (clay, mud)
   Accumulates slowly (5,000 to 50,000 years to deposit 1 cm)
   Color may be black, red or brown
2. Biogenous sediment
   Biological origin - primarily shells and skeletons of microscopic plankton
   1. Calcareous oozes
      Remains of foraminifera, coccolithophores, and pteropods (planktonic gastropods or snails)
      May form chalk in waters less than about 4,000-5,000 m.
   2. Siliceous oozes
      Remains of radiolarians and diatoms
      Typically accumulate in colder, deeper ocean waters where other sediments are lacking, or where high nutrient levels lead to abundances of siliceous organisms.
      May form diatomite or chert
   3. Phosphatic material
      From bones, teeth and scales of fish.

3. Hydrogenous sediment
   Minerals that precipitate from sea water by chemical reactions. (Called authigenic or diagenetic minerals.)
   Example: Manganese nodules

Carbonate Compensation Depth (CCD)

The carbonate compensation depth (or CCD) is a particular depth in the oceans (ranging from about 4000-5000 m), which affects where calcareous oozes may or may not accumulate.

1. Above the CCD (in water shallower than 4,000-5,000 m), the water is warmer, and precipitation of CaCO₃ occurs.
   Calcarous plankton can be found in the water column, and on the seafloor.
   Bottom sediments can consist of calcareous sediments forming chalk or limestone.
2. Below the CCD (below about 4,000-5,000 m), water is colder, and CaCO3 tends to dissolve . Dissolution of calcium carbonate occurs because the cold waters hold more carbon dioxide in solution, which increases their acidity.
   Tiny shells of CaCO₃ dissolve, and do not accumulate on the bottom if water is deeper than the CCD.
   In water depths below the CCD, the bottom sediments consist of:
   1. Clay
   2. Silica shells of plankton (diatoms, radiolarians)
3. At the CCD, the rate of precipitation of CaCO₃ = the rate of dissolution of CaCO₃

Carbonate compensation depth (or CCD). Calcareous ooze is deposited above the CCD. Siliceous ooze and clay are deposited below the CCD.