Paleoclimate: oxygen isotopes in foraminifera and corals The JOIDES Resolution drillship

Important paleoclimate archives Deep-sea sediments1ky – 1Myforaminifera CaCO 3 oxygen isotopes Ice cores1y – 100ky ice oxygen isotopes, thickness, CO 2 Corals1y – 1kyCaCO3 oxygen isotopes Tree rings1y – 1kythickness, oxygen isotopes? Cave speleothems1y – 100kyCaCO3 oxygen isotopes

Oxygen Isotopes: the rock star of paleoclimate Oxygen has three stable isotopes (same # protons, diff. # neutrons): 16 O 99.76% 17 O 0.04% 18 O 0.2% When calcium carbonate (CaCO 3 ) is formed from seawater (H 2 O), the oxygen isotopes are fractionated  there is a change in the 18 O/ 16 O from liquid to solid This fractionation is temperature- dependent: warmer = lower 18 O/ 16 O cooler = higher 18 O/ 16 O

Using oxygen isotopes (  18 O) in paleoclimate Remember, carbonate δ 18 O values are a dual function of: 1)the δ 18 O of seawater 2)the temperature of the seawater (sensitivity = 0.2 per mil per 1°C) We measure oxygen isotope ratios on a mass spectrometer, and report changes relative to a standard. Changes are small (0.1% or less) so we report them in units of “per mil”. Applications: 1) track changes in seawater δ 18 O on glacial-interglacial timescales (sediments) 2) track changes in seawater temperature on interannual to 1,000 timescales (corals)

Glacial-interglacial changes in whole-ocean  18 O today LGM, 20,000ypb FACT: precipitation is significantly depleted in 18 O so… ice is depleted in 18 O so… building large ice sheets during the Last Glacial period left a lot of 18 O in the ocean and… we can see this whole-ocean δ 18 O change in foraminifera calcite Why do we use benthic (deep) forams for this?

Milankovitch cycles: orbital controls on glacial-interglacial timing FACT: deep-sea δ 18 O records tell us that ice grows and shrinks in cycles of 100ky, 40ky, and 20ky

Corals: The geologic record of El Nino-Southern Oscillation CORALS from the tropical Pacific record ENSO in the δ 18 O of their skeletons This live coral is ~50yrs old This fossil coral grew from ~ A.D. CORALS from 125 thousand years ago contain the signature of ENSO

Why study the El Niño-Southern Oscillation? 1. It is the largest source of year-to-year global climate variability. 2. It carries negative societal consequences, both economic and humanitarian. 3. Improved forecasts minimize negative societal consequences. 4. It’s a fascinating, mysterious, and complex natural phenomenon.

Average tropical Pacific conditions - trade winds blow from East to West - warm water piles up in West Pacific, driving deep atmospheric convection - cold, nutrient-rich water is brought to the surface in the East Pacific (upwelling) strong trade winds cool in East warm in West This system is tightly coupled

Peruvian fisherman (1800’s): named mysterious warm, nutrient-poor waters “El Niño” Sir Gilbert Walker (1924): named East-West seesaw in sea-level pressure the “Southern-Oscillation” Jacob Bjerknes (1969): explains feedbacks that link the “El Niño” and “Southern Oscillation” phenomena Tropical Pacific conditions during El Niño trade winds weaken upwelling slows Eastern Pacific warms The Bjerknes Feedback:

trade winds strengthen upwelling intensifies Eastern Pacific cools …and the reverse for La Niña

El Niño Impacts - these departures from “normal” climate carry serious economic and social costs - improved ENSO forecasts minimize the costs - La Nina’s effects are roughly the opposite of El Nino’s effects - the impacts are not confined to the tropical Pacific El Niño even affects Atlantic hurricanes…. El Nino yearsLa Nina years

ENSO affects temperatures and rainfall in Georgia

and there’s one happening RIGHT NOW!

Reconstructing El Nino-Southern Oscillation events with coral  18 O in the Central Tropical Pacific (CTP) During El Niño events, positive SST and precipitation anomalies both contribute to negative coral  18 O anomalies in the CTP, so we can measure coral  18 O continuously back in time to reconstruct past ENSO activity, compare it to today’s activity SST and rainfall anomalies during the 1982 El Nino

Fossil corals collected during fieldtrips in 1998, 2000, and 2005 Beached fossil corals range from gravel-sized to 2m-diameter (~ yrs). rare longer cores: ENSO & decadal variability common short cores: mean climate

Palmyra 40 cores U/Th dated 28 cores undated Christmas 18 cores U/Th dated 63 cores undated Fanning33 cores undated The Line Islands Fossil Coral Collection

17 th Century Spliced Coral  18 O Records splice - absolute  18 O values agree (give consistent picture of mean climate) - large, well-reproduced El Niño events in mid-17th century

Line Islands coral δ 18 O records to date -large trend towards warmer, wetter conditions post-1970AD -largest El Nino events of millennium in 17 th century (not today!) -cool/dry conditions during Europe’s “Medieval Warm Period”