Lecture 24 - Causes of Long-Term Climate Change (p.4)

Q: What are the two major reasons for long-term sea level changes throughout Earth’s history?

(1) Changes in the amount of water in the oceans due to climate change (ice sheet growth/melt, thermal expansion). (2) Changes in the volume of ocean basins and their capacity to hold water, driven by sea floor spreading rates.

Q: How much would sea level change if all Antarctic ice melted? What about Greenland ice?

Antarctic ice holds approximately 66 m of sea level equivalent; Greenland ice holds approximately 6 m.

Q: Why do ocean ridges subside with time?

Heating of rocks at mid-ocean ridges causes thermal expansion; as the crust moves away from the ridge and cools, it contracts and subsides. All ocean ridges show a similar age-vs-depth profile.

Q: How do sea floor spreading rates affect sea level?

Fast spreading produces more young, thermally expanded ocean floor (“fat” ridge profile), reducing basin volume and raising sea level. Slow spreading produces a “thin” profile, increasing basin volume and lowering sea level.

Q: By how much can sea floor spreading rates change sea level?

Approximately 200 to 300 m.

Q: What happens to shelf sediments at low vs. high sea levels?

At low sea levels, shelf sediments are eroded. At high sea levels, shelf sediments are deposited worldwide and often preserved in the geological record.

Q: Why is the existence of shelf sediments in the geological record a useful proxy?

Their presence indicates periods of high sea levels, which in turn indicate high sea floor spreading rates.

Q: How does the collision of continents affect sea level?

It increases the area and volume of ocean basins (by removing a continent from the basin), resulting in a drop in sea level. For example, the collision of India and Asia caused an approximately 40 m sea level drop.

Q: What are first-order sea level changes thought to be a function of?

The expansion and contraction of ocean basins resulting from heat flux from the mantle.

Q: When were first-order sea level minima, and when was the major maximum?

Minima occurred at the Precambrian–Cambrian boundary, the Permo-Triassic boundary, and today. The major maximum was the mid-Cretaceous, when sea level was 200–300 m higher than today.

Q: Why is there more disagreement about sea level and spreading rate reconstructions further back in time?

The record becomes sparser, uncertainty increases, and there is a lack of direct measurements, leading to diverging interpretations.

Q: How far back can sea floor spreading rates be reconstructed with relatively high certainty?

Only the last 200 million years, because older ocean crust has been subducted and destroyed.

Q: How do records of ocean crust production relate to granitic pluton emplacement in North America?

They agree well — periods of high sea floor spreading correspond to abundant granitic intrusions, while low spreading periods show few granitic plutons.

Q: What are the two main sources that make up ocean water composition?

River water (contributing Ca²⁺, HCO₃⁻, and SO₄²⁻) and mid-oceanic ridge brines from hydrothermal vents (black smokers).

Q: How does mid-ocean ridge activity affect Ca²⁺ in ocean water?

Ca²⁺ is released at mid-ocean ridges, so high spreading rates produce relatively high Ca concentrations and low spreading rates produce relatively low Ca concentrations.

Q: How does mid-ocean ridge activity affect Mg²⁺ and SO₄²⁻ in ocean water?

Mg²⁺ and SO₄²⁻ are removed at mid-ocean ridges, so high spreading rates result in comparatively low Mg and SO₄ in ocean water, while low spreading rates result in comparatively high Mg and SO₄.

Q: What are evaporites, and what is a common example?

Evaporites are minerals formed from evaporated sea water. A common example is halite (NaCl).

Q: What are oolites?

Small, round particles in which CaCO₃ is deposited concentrically around a nucleus.

Q: When do MgSO₄-rich evaporites occur, and what do they indicate?

They occur only during the late Precambrian, Carboniferous–Permian, and Neogene–Quaternary — all periods of low sea floor spreading rates. Most other evaporites are MgSO₄-poor and KCl-rich.

Q: What are “aragonite seas” and “calcite seas”?

They describe periods when non-skeletal marine limestones were predominantly aragonitic or calcitic in composition, reflecting changes in ocean chemistry through the Phanerozoic.

Q: When did aragonite seas occur?

Precambrian–Cambrian, Carboniferous–Permian, Triassic–Jurassic, and Paleogene/Neogene–Quaternary.

Q: When did calcite seas occur?

All periods not listed as aragonite seas (broadly the Ordovician–Devonian and much of the Cretaceous).

Q: What three parameters strongly influence whether calcite or aragonite precipitates from sea water?

The Mg/Ca ratio of the solution, the ionic strength of the solution (relatively constant at 0.7 for ocean water), and temperature (which varied by no more than ~10 °C through the Phanerozoic).

Q: How does the Mg/Ca ratio of ocean water determine carbonate mineralogy?

Low Mg/Ca ratios favor calcite precipitation; moderate to high Mg/Ca ratios favor aragonite precipitation.

Q: What is the connection between MgSO₄-rich evaporites, aragonite seas, and sea floor spreading rates?

All three indicators align — periods of low spreading rates have high Mg and SO₄ in ocean water (producing MgSO₄-rich evaporites) and high Mg/Ca ratios (favoring aragonite precipitation), providing independent evidence for low MOR activity.