History of the Oceans-Lecture Notes

Summary

This week, we will retrace the last 4,000 million years or so and discuss the evolution of the oceans from their formation to the present conditions. As we march through time, we will keep focusing on major changes in the shapes of the basins, chemistry (and physical properties) of the seawater, and the various ocean inhabitants that have come and gone. The first lecture will introduce the important changes that influence the history of the oceans, focusing in details on the causes and effects. The second lecture will scroll through the general story of the oceans.

Important Changes

In order to fully appreciate the history of the oceans, you need to "arm yourself" with the tools necessary for recognizing the changes (listed below). Think about both what would drive the particular change and how that change would alter the various ocean systems. It is also important to recognize that the different changes affect the oceans at different scales and you need to have the proper reference frame.

Sea Level Changes-Sea levels change on a variety of scales and through a variety of reasons. In addition to obvious impacts on ocean (and nearshore) life, the changes in sea level also affect circulation patterns.

Ocean Openings and Collapses-As plates move apart, new oceans are formed, but somewhere else, another ocean is shrinking. In addition to changes in ocean shapes, the plate tectonic movements also alter the distribution and amount of continental shelves.

Atmospheric Changes-The general composition of the atmosphere has not been drastically changed since the introduction of large amounts of oxygen, about 2.3 billion years ago (see below). However, minor changes in the levels of CO2, O2, O3 (ozone), N2, and other compounds can have drastic affects on weather patterns, biological communities, erosion of rocks, etc.

Stagnation of Ocean Water-During times of poor circulation, the oceans can become stagnant and various biological processes can severely decrease the amount of dissolved oxygen in the seawater. This leads to massive die outs of aerobic ocean critters.

Milankovitch Cycles-Periodic and repetitive changes in the climate due to fluctuations in the position of the Earth in space. Specifically, the changes are due to the variation in the tilt of the Earth's rotational axis, the precession (or rotation) of the rotational axis, and the eccentricity of the orbit. These changes occur on scales ranging from 10's to 100's of thousands of years and may explain the timing of glacial intervals.

Meteorite Impacts-From time to time, large extraterrestrial bodies slam into the earth. This happened much more regularly during the early days of the planet and is (thankfully) much more rare these days. The impacts can have catastrophic effects on the organisms living throughout the oceans.

Evolution and Extinction-Speaking of organisms...many different one have come and gone over the ages and influenced the makeup of the ocean communities.

Milestones

Below is a list of some of the more important events of the last 4 billion years, listed in more or less chronological order:

Segregation of the Mantle-As the mantle segregated, the most volatile elements (mostly water vapor) were released to the surface. Once the planet cooled down, the vapor cooled and turned into liquid, thus the oceans. Salts were introduced early on through chemical dissolution of the rocks. The lighter minerals also moved up to form small "platelets" similar to, but distinct from modern continents.

Formation of "True" Continents-The early platelets contained relatively lower amounts of light elements such as aluminum so true continental crust was not around. During the Late Archean time, about 2.5 billion years ago, large volumes of aluminum and other light elements were brought to the surface in the form of granite. This allowed for "true", large continents to form.

Saturation of O2-The early atmosphere contained little free oxygen. As photosynthesizers evolved and became abundant, they polluted the atmosphere with copious amounts of oxygen, thus allowing more complex respirators to evolve. Evidence for this saturation with O2 is seen in the appearance of oxidized rocks at about 2.2-2.3 billion years ago.

Breakup of Rodinia-The various continental pieces were joined in one or maybe two supercontinents in Late Proterozoic time (about 700 million years ago). Around the beginning of the Cambrian Period (about 550 million years ago), Rodinia began to rift apart, and many of the continental plates migrated toward the equator. This affected the global ocean by setting up new current patterns, and most importantly, creating large amounts of continental shelves in the tropical region.

"Sessiletime, and the Livin' is Easy" -Following the breakup of Rodinia, all of the major phyla of animals evolved during the Cambrian Period. For the following 300 million years, the shallow seas were dominated by sessile, benthic filter feeders, such as sponges and corals.

"Sinsk Event" and other Tales of Stinky Water-At several times in the early Paleozoic (about 500-350 million years ago), the oceans became severely depleted in free oxygen. This is recognized, in part, by global deposits of black, stinky, shale. These events may also coincide with major extinction episodes, although the connections are not always so clear-cut.

The Age of Fish-Beginning in the Devonian time (about 400 million years ago), large armored fish and lobe-finned fishes (sarcoptyerigians) evolved and dominated the oceans. Some of these critters were over 30 feet long! Because they were so abundant, this time is called the Age of Fish.

Formation of Pangaea-Like Rodinia before, the various continental pieces began to assemble again during the Late Paleozoic (about 300 million years ago). Once all of the continents were in one spot, there was essentially one large ocean, called "Panthalassa". During the time of Pangaea, there was a severe decrease in the amount of continental shelf space and, because part of the supercontient was over the South Pole, there were many, repetitive glacial episodes.

Critters on the Move-Following the "mother of all mass extinctions" at the end of the Permian Period, sessile benthic communities were replaced by predominantly motile benthic critters, such as molluscs. Think about how many clams and snails you see on the beaches today vs. sponges...

The Reign of the Terror Lizards!-Next time you sit down to a tasty bowl of "DinoCrunch", pick out all of the swimming critters. No dinosaurs swam in the oceans. However, during the time of Dinosaurs (Mesozoic Era, about 225-65 million years ago), very large and bizarre marine lizards evolved and were found throughout the oceans. Some of the more common critters include the pleisiosaurs and ichthyosaurs.

Tethys Ocean and Black Gold-Paleogeographic maps of Pangaea show it to be in the shape of a "C". The ocean that fills the open part of the "C" is called Tethys. During the Mesozoic (about 200 million years ago), Pangaea began to rift apart in the middle, separating Laurasia (North America and Asia and Europe) from Gondwanaland (South America and Africa and Australia). The Tethys began to flow between the two massive continental amalgamations as a major seaway. The phytoplankton floating in the Tethys became the source of the major oil fields of northern Africa, the Middle East, and the Gulf of Mexico.

Evolution of Calcareous Phytoplankton-During the Cretaceous Period, the evolution of calcareous phytoplankton (coccolithophores) led to the formation of massive chalk deposits. Perhaps their evolution signifies a time of supersaturation with respect to calcium carbonate?

Opening of the Drake Passage-When South America separated from Antarctica about 20 million years ago, the latter became fully separated from all other continents. The persistent westerlies could now blow the surface waters completely around the globe, thus setting up the Antarctic Circumpolar Current, creating a permanent refrigerator for the planet.

Cenozoic Ice Ages-During the last 20 million years or so, there have been repetitive ice ages, in fact, we are still reeling from the last one. Most of the scientists agree that Milankovitch Cycles play a major role in driving these changes, although the distribution of the continents has also helped. Deepsea cores record these events by the relative amounts of isotopes of carbon and oxygen preserved in the shells of marine plankton.

Mediterranean Salt Pan-During times of lowered sea level, water would not flow through the Straits of Gibraltar. The Mediterranean would dry up and massive evaporate deposits formed on the sea floor.

Key Points:

1. Sea levels changes occur at different scales and affect marine life and circulation patterns.

2. Plate tectonic movements alter the size and shapes of the ocean basins and distribution and amount of continental shelves.

3. Changes in the levels of CO2, O2, O3 (ozone), N2, and other compounds can have drastic affects on weather patterns, biological communities, and erosion of rocks, etc.

4. Poor circulation can lead to stagnation of the oceans, causing massive die outs.

5. Milankovitch Cycles record periodic variations in the tilt and precession of the Earth's rotational axis, and the eccentricity of the orbit.

6. Meteorite impacts are one of many hypothesized causes for the extinction and evolution of marine organisms.

7. The oceans are a result of segregation of the mantle and subsequent cooling.

8. During the Late Archean time, massive granitization formed "true", large continents.

9. Photosynthesizers polluted the atmosphere with copious amounts of O2 about 2.2-2.3 billion years ago, thus allowing more complex respirators to evolve.

10. At about the beginning of the Cambrian Period (about 550 million years ago), the supercontinent Rodinia rifted apart affecting the global ocean by setting up new current patterns and tropical continental shelves.

11. During the Paleozoic Era, the shallow seas were dominated by sessile, benthic filter feeders.

12. Occasionally, the oceans have become severely depleted in free oxygen, often coinciding with major extinction episodes.

13. The Devonian Period was the time of large armored and lobe-finned fishes (sarcoptyerigians).

14. The major continental pieces began to assemble again during the Late Paleozoic to form the supercontinent Pangaea and leaving one major ocean, Panthalassa.

15. The sessile benthic communities of the Paleozoic were replaced by predominantly motile benthic critters following the Permian Mass Extinction.

16. Large marine lizards were found throughout the oceans during the Mesozoic Era.

17. As Pangaea began to rift apart, the Tethys began to flow between the two massive continental pieces, and the accumulations of phytoplankton became the source of the major oil fields.

18. Calcareous phytoplankton (coccolithophores) are the source of global chalk deposits during the Cretaceous.

19. The opening of the Drake Passage set up the Antarctic Circumpolar Current, leading to the onset of major glacial periods.

20. Deep sea cores record Cenozoic ice ages that were driven, in part, by Milankovitch Cycles.

21. During times of lowered sea level, massive evaporate deposits formed on the floor of the Mediterranean.