Sea Level Change and Climate
Sea Level Changes:
Short time scale and oscillatory: tide, seasonal, El Nino (cms to meters)
EUSTATIC Sea Level Change
Glacial Ice Volume (100s of meters)
Change in size of ocean basins (100s of meters) due to sea floor spreading changes-faster
spreading means higher Mid-ocean ridge = higher sea level (long time scale change)
Glacial Record on Continents is incomplete (4 cycles!)
Sedimentary record in the oceans much more complete
OXYGEN ISOTOPES
Atoms have ATOMIC # = # protons in nucleus. All atoms of a given element have the same
number of protons. ISOTOPES of elements have a different # of neutrons, and therefore a
different Mass Number.
Main Oxygen isotopes
18
O (0.24%) has 8 protons and 8 neutrons, 16O (99.76%) has 8 protons and 10 neutrons
Subtle differences in how these atoms behave in the world. 16O is lighter and therefore
more easily evaporated.
δ18O is a measure of the relative abundance of these two isotopes.
Positive values have more 18O and negative values have less 18O than normal seawater
When glaciers form, more 16O is stored in glaciers, and more 18O is stored in the ocean,
so when we measure the shells of organisms that make their shells from seawater, +
values indicate that it is colder and there are more glaciers, and – values represent warm
times with fewer glaciers.
When we look at the sediment record in the ocean, we see a series of Ice ages (around 10
in the past 1,000,000 years)
Fluctuations over the past 1 million years agree with Milutin Milankovitch’s hypothesis about
solar radiation, based on three changes in the orbit of the earth around the sun:
Eccentricity: How elliptical the orbit of the earth is around the sun (100,000 year cycle)
Tilt: How much the earth’s axis is tilted (varies from 21.5 to 24.5 degrees every 40,000
yrs.)
Wobble: The earth’s rotation axis changes its orientation, so the seasons “switch” on a
23,000 year cycle
What is the past and current volume of ice?
Antarctic: 14 million km2 to 12.5 million
Arctic: 13 million km2 to 147,000 km2 (99% drop)
44 million km2 to 15 million km2 today (most in Antarctica)
We’ll talk about Isostatic Rebound More on Monday
Isostatic Rebound of the land from the removal of significant volume (and wt.) of ice.
Up to 1000 meters of rebound (0.9 g/cm3 vs. 2.7 g/cm3) for crust
Slow rate of uplift.
Also a small subsidence of ocean floor due to increased wt. of water
100 meters over the past 18,000 years
Scandanavia: rebound is about 9 mm/year-net uplift of about 30-35 meters, more in other areas
like Hudson Bay.
Amount of rebound decreases southward due to later, thinner ice sheets
GREAT LAKES Example: tilt N to S in 3000 years they may drain to the MISS. R. instead of
North.
Rapid SL rise in the first few thousand years post ice age: 10mm/yr. Sand bars and barriers
DROWNED-didn’t have time to migrate landward