• Recall from Chapter 8 that the 41,000 year cycle of change in Earth's tilt influences higher latitudes the most. At low and mid-latitudes, the 23,000 year cycle of change due to precession is dominant.
• Because monsoons occur at low and mid-latitudes, the 23,000 year precession cycle should control monsoon strength.
• Fig. 9-5 show the 23,000 year cycle using lake levels across north Africa. Notice the time period between the highest peak and lowest peak. It is approximately 100,000 years! (Does that number ring a bell?)
• See Fig. 9-6A. Today, the typical Mediterranean circulation features cold, salty surface water (chilled by cold air in winter) that sinks to deeper layers. This surface water is rich in oxygen, so when it sinks, it carries this oxygen to the deeper waters below. Today, the sea floor sediment shows a well-oxygenated ocean basin because the sediment is composed of silty mud that contains shells of plankton and bottom-dwelling sea life.
• Fig. 9-6B shows the result of fresh water intrusion by runoff from the Nile River due to strong monsoons. The sea-floor was covered with sediments rich in iron sulfides. This sediment is referred to as "stinky mud" and forms in an oxygen-deprived environment. Seen in Fig. 9-7.
• Fig. 9-8 shows how a strong monsoon could increase Nile River runoff and it also shows that the Nile River covered a larger area in the past.
• Fig. 9-9 shows evidence of fresh water diatoms in the sediment buried in the Atlantic Ocean off the coast of western Africa. These diatoms could not have formed in the ocean so they must have come from fresh water lakes in northern Africa. These diatoms appear in the ocean sediment at 23,000 year cycles.
• Fig. 9-10 shows a 5000 - 6000 year time lag between the lake level peak and the diatom peak. When monsoons are strongest, lake levels are highest. As the monsoon weakens over time, the lake slowly dries out. Once the lake is dried out (about 5000 years later) winds can blow the lake sediment out to the Atlantic Ocean.
• During a weak monsoon period such as today, the SE Trade Winds are very strong which drives water away from the African coast. This causes nutrient-rich colder water below to upwell to the surface.
• During a strong monsoon period, the SE Trade Winds are much weaker which results in warmer water near the African coast. See Fig. 9-11.
• There are "cold-water" plankton and "warm-water" plankton. Fig. 9-13 shows how these two types of plankton have varied on a 23,000 year cycle.
• Fig. 9-15 shows that the Newark Basin was located in the tropics about 200 My ago. Fig. 9-16 shows that lakes on Pangaea occasionally dried out. This figure is from a basin in Connecticut that existed at the same time as the Newark Basin.
• When lakes were deep (100 m or more), sediment tended to be gray or black muds containing large amounts of organic carbon. When lakes were shallow or dried out sediment tended to be red or purple because of oxidation due to contact with air and they typically contain footprints of dinosaurs.
• Fig. 9-17 shows lake depth with time using sediment cores from the Newark Basin. Three time scales are observed:
  1. 20,000 years every 4-5 m of core depth. (Orbital precession)
  2. 100,000 years every 20-25 m of depth. (Eccentricity changes)
  3. 400,000 years every 90-100 m of depth. (Eccentricity changes)

Helpful Links:

The Monsoon Climate
Orbital Monsoon Hypothesis - .PDF file (869 Kb)
Geology of the Newark Rift Basin