Professor Mandia's Extra Help

CHAPTER 7

Global Climate Change since 55 Myr Ago

See Fig. 1. Earth has dramatically cooled over the last 55 Myr.
The deep ocean is a climate archive that has endured for this entire 55 Myr period so it contains evidence of the climate change.
Foraminifera (single-cell sea creatures ranging in size between 100 micrometers and 20 cm in length) form shells made of Calcite, or CaCO₃. The oxygen in these shells is made up of two different isotopes (¹⁸O and ¹⁶O) which is taken directly from the ocean water.
When these shells fall to the sea floor they become a permanent record of the ratio of ¹⁶O vs. ¹⁸O at the time they existed.
¹⁶O makes up 99.8% of all the Earth's oxygen while ¹⁸O takes up only .2% of the available oxygen. ¹⁶O is a lighter form of oxygen than ¹⁸O.
Concentrations of these two isotopes are directly related to: 1) the total amount of ice existing on Earth's surface and 2) the temperature of the ocean in which the shells formed.
δ¹⁸O values indicate the relative abundance of ¹⁸O vs. ¹⁶O. Positive δ¹⁸O values indicate more ¹⁸O than normal while negative δ¹⁸O values indicate less ¹⁸O than normal. (THIS IS VERY IMPORTANT TO UNDERSTAND!)
As water temperature increases, the δ¹⁸O value in shells decreases. For example, in warmer tropical water today, the δ¹⁸O values are about -1% while in the colder deep waters below, the value is about +3.5%.
Lighter ¹⁶O evaporates from tropical water more easily than ¹⁸O, so evaporation leaves the tropical oceans with more ¹⁸O. Additionally, more ¹⁸O is removed during condensation and precipitation leaving the water vapor in the atmosphere richer in ¹⁶O and the tropical oceans even more enriched with ¹⁸O. The end result is that ¹⁶O moves poleward more easily than ¹⁸O. During icehouse eras, this ¹⁶O can end up in polar ice sheets which "permanently" removes it from the oceans. Therefore, when there are ice sheets present on Earth, the δ¹⁸O values of the oceans should be higher.
Fig. 7-7 shows the long-term trend of δ¹⁸O values in deep water which shows a global cooling over the past 55 Myr (δ¹⁸O values increasing to present day.)

Global Climate Cooling: Was it BLAG?

If BLAG explains the cooling, there should be evidence of slower-spreading rates from 55 Myr ago until present day. Recall that slower BLAG rates means less CO₂ is added to the atmosphere which would cause a cooling trend.
Fig. 7-9 shows that the spreading rate was slower until about 15 Myr ago. In the last 15 Myr, the rate has increased. However, ice in the northern hemisphere first appeared in the last 15 Myr!
In summary, the BLAG hypothesis may have caused the global cooling before 15 Myr ago, but it cannot explain the cooling since then.

Global Climate Cooling: Was it Uplift Weathering?

Weathering removes CO₂ which means it is a cooling factor. If Uplift Weathering is to explain the global cooling over the past 55 Myr, it must show a trend of MORE weathering since 55 Myr ago. Therefore, it must show:
1. #2 must be causing unusually large rates of weathering today.
Fig. 7-11 shows the Tibetan Plateau which supports #1. The collision occurred about 55 Myr ago and continues uplifting today.
Fig. 7-12, 7-13 show sediment flow into the Indian Ocean. The increase in sediment shows rapid erosion (which exposes fresh material) along the sides of the Himalayas. This tends to support conclusion #2.
Fig. 7-14 shows the amount of sediments in rivers which is related to the rates of chemical weathering. The figure suggests that conclusion #3 might be accurate because chemical weathering rates are highest near the Himalayas and Andes Mts.

Global Climate Cooling: Was it Ocean Heat Transport?

This hypothesis suggests that oceanic gateways might have changed the amount and type of ocean water moving between large ocean basins. These gateways could have altered the heat and salt carried by ocean currents.
Fig. 7-8 (B) shows that 65 Myr ago, Australia and Antarctica were attached which allowed warmer ocean currents to move toward Antarctica, thus keeping it warm. When these two land masses separated, it allowed ocean water to just circle around Antarctica which caused a cooling trend and a subsequent glaciation.
Fig. 7-15 shows Drake's Passage which formed around 20-25 Myr ago which formed when South American and Antarctica separated. However, climate models have shown that the climate of Antarctica would not have changed with either an "open" or "closed" passage.
See Fig. 7-16. An alternative hypothesis is that uplift in Central America caused the closing of the Central America Seaway. Warm, salty water would instead be redirected toward the poles which would decrease the amount of sea ice. (Saltier water is harder to freeze.) Less sea ice meant that there would be more liquid ocean water exposed to the air which would result in more land-based ice sheets when this increased moisture precipitated out.
However, GCMs have shown that this warmer water would have inhibited ice sheets so the hypothesis is NOT supported by climate modeling.

Causes of Brief Tectonic-Scale Climate Change

Volcanoes release sulfur dioxide SO₂ which combines with water and sulfuric acid to form sulfate aerosols. Sulfate aerosols block incoming sunlight. Fig. 7-18 shows how eruptions can cause a short-term cooling of climate.
Volcanoes that erupt near the equator cause a greater cooling effect than those at higher latitudes because low-latitude volcanoes can emit sulfates into both hemispheres.
Burial of organic carbon is another source of short-term climate change. More buried carbon means less CO₂ in the air which means a cooler climate. Burial of carbon can be increased by:
1. Changes in wind direction along a coast that cause upwelling and carbon production. This carbon is then buried in sediment when the plankton die off.
2. An increase in organic carbon and nutrients delivered to the oceans by run-off where it eventually becomes buried in sediment.
3. A change to a wetter climate on flatter continental margins which would cause more swampy conditions. Swamps cause organic matter to deposit out into sediment.
The jury is still out on this one!

Uplift Hypothesis Summary: Positive vs. Negative Feedback

Fig. 7-19 shows a negative feedback of enhanced uplift.
Increased uplift causes greater weathering in those uplifted regions. That causes a lower global CO₂ level. That reduces temperature, precipitation, and vegetation. That causes a decrease in chemical weathering. That causes a reduction in CO₂ removal which reduces some of the initial cooling due to uplift.
Fig. 7-20 shows a positive feedback of enhanced uplift.
Uplift causes greater weathering which causes global cooling. More ice exists in a cooler climate so there is increased glacier ice on Earth. More glacier ice means there will be more rock grinding which exposes fresh material. More fresh material means a greater weathering rate which causes even more CO₂ to be removed from the air. Less CO₂ increases the global cooling.

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