Paleoclimatology

Paleoclimatology How do we know anything about the history of Earth’s climate? http://www.ncdc.noaa.gov

What tools do we have? • Rock types (e.g. abundance or lack of carbonate deposits) • Fossil types (animal and plant) • Ice records (chemistry: oxygen, carbon dioxide, etc.) • Speleothems (growth rings, chemistry) • Tree Rings (growth rings, fire history) • Packrat Middens (preserved vegetation) • Lake cores (pollen, organics) • Marine sediment cores (chemistry, remains of small, temperature-sensitive organisms) And on and on….

So what? By understanding what drives the observed changes in these systems, we can interpret Earth’s climatic history. Through these paleoclimate proxies, scientists have been able to gather information from various records around the world, both terrestrial and marine. (Paleoclimate proxies are physical samples that have been preserved and serve as an indirect measurement of something, i.e. rainfall, temperature, etc.) It has been determined that Milankovitch Cycles are responsible for large scale (and cyclic) changes in Earth’s climate…

http://deschutes.gso.uri.edu Milankovitch Cycles change in shape of orbit (more circular or more oval) 100 k.y. change in tilt of 23.5 degrees 41 k.y. Earth spins like a top, and its orientation changes through time 19 and 23 k.y.

A combined look at Milankovitch Cycles http://www.global-greenhouse-warming.com

What else forces climate change? • Distribution of land mass • Change in atmospheric content of greenhouse gases • Solar activity • Volcanic eruptions *these force change on different time scales*

Let’s get back to proxies for paleoclimate reconstruction… specifically: oxygen isotopes! Remember: Isotopes are atoms of the same element with different numbers of neutrons. For our purposes, we are considering oxygen-18 (18O) and oxygen-16 (16O). 16O = 8 protons, 8 neutrons 18O = 8 protons, 10 neutrons Which one has the greater mass? How do you know?

Let’s think about fractionation during precipitation. Certain physical processes cause isotopes in a system to fractionate (separate). Let’s focus on precipitation and evaporation. This happy little raincloud contains H2O… which means that 18O and 16O are present in the water molecules. Do you like to carry heavy things? The raincloud doesn’t, either. Do you think that the water molecules being removed from the cloud through the precipitation tend to have more 16O or 18O?

What about fractionation during evaporation? Dry air moving over water will collect water through evaporation. Do you like to pick up heavy things? Not so much? Well… Which isotope will be preferentially taken up into the atmosphere? The lighter isotope is associated with the more kinetic phase, and the heavier isotope is associated with the less kinetic phase…. SO: Heavy isotopes sit in beanbag chairs and eat Cheetos, lighter isotopes wear sweatbands and run around like crazy.

What does this have to do with paleoclimate proxies? If the Earth is relatively cool and more glaciers form, think about how these processes will affect ocean water, as well as the accumulating snow/ice. Remember, glaciers form from frozen precipitation, not frozen seawater, therefore they have more 16O than the ocean!! Cores of ice and marine sediments provide paleoclimate records. http://commons.wikimedia.org http://www.oceanleadership.org

We measure isotopic ratios and present them in this way: δ= (Rx – Rstd / Rstd) X 1000 where R is the ratio of the heavy to the light isotope in the sample (x) and the standard (std); 18O and 16O lab standards, international standards example: δ18O = -10 ‰SMOW SMOW= “Standard Mean Ocean Water” Enrichment in heavier isotopes leads to a higher (more positive d value --- enrichment in lighter isotopes leads to a lower (more negative) d value. Now you can interpret the data in your lab handout!

Lab Data (GRIP)

Lab Data (SPECMAP)

Lab Data (Vostok)

Paleoclimatology