Coral records of natural and anthropogenic climate change

Fossil coral drilling on Christmas Island (2N, 157W). We use a hydraulic drilling rig to recover 3” diameter cores. These corals turned out to be 3-6 thousand years old.
Kim and tech extraordinaire Jordan Watson drilling a Porites coral core on Fanning Island (4N, 160W). They recovered ~60 yrs of growth in a 3-ft-long core. Credit: Zafer Kizilkaya.
Fossil coral heads on Palmyra Island (6N, 162W). We core these heads and back in the lab, we date them with Uranium-Thorium dating and analyze their oxygen isotopic composition for climate reconstruction. Credit: Jordan Watson.
An aerial view of Fanning Island, (4N, 160W). The Gilbertese name is Tabueran, or “sacred footprint”. The entire island is only 3-4km wide.
Jessica Carilli and Jordan Watson drilling fossil corals with eager assistants on Fanning Island (Tabueran).
Kim and Kristy Dahl trying to finish the last of the day’s fossil coral drilling on Palmyra Island.
Jess Conroy and Hussein Sayani comb a fossil coral rubble field on Christmas Island for Porites cobbles that we can use for a new reconstruction of tropical Pacific climate over the last several millennia.
Tropical Pacific climate change represents one of the largest uncertainties in future climate projections, in part because instrumental records of climate from this region rarely extend beyond the 1950’s. As tropical Pacific temperature variations have ripple effects all over the globe, as witnessed during El Nino and La Nina events, better constraints on tropical Pacific climate projections would increase the accuracy of regional climate projections in many areas of the world.

In my lab, we generate records of past tropical Pacific climate from corals, working across a broad-range of timescales that are relevant to anthropogenic climate change. By measuring the oxygen isotopic (δ18O) content of the coral skeletal aragonite every 1mm down-core, we obtain timeseries that reflect changes in past ocean temperature and/or changes in the oxygen isotopic content of the seawater (the latter linearly related to salinity, and thereby used as a proxy for hydrological balance). Our long-term sites in the central equatorial Pacific are extremely sensitive to temperature and rainfall changes associated with the El Nino-Southern Oscillation (ENSO), so corals from these sites provide exceptional archives of past ENSO activity.

We use so-called “modern” corals, drilled from large living colonies underwater, to calibrate our coral oxygen isotope proxy against instrumental climate data, and to reconstruct the evolution of 20th century climate. We use so-called “fossil corals”, drilled or collected from long-dead colonies scattered along beaches at our sites, to reconstruct climate of the more distant past (back 7,000 yrs so far). With monthly-resolution and a large menu of environmentally-sensitive geochemistries to choose from, corals hold immense potential to significantly reduce the uncertainties associated with future tropical climate and environmental change.

Some highlights from our coral publications:

  • characterization of a particularly pronounced decadal-scale climate signal in the Palmyra coral δ18O record (Cobb et al., 2001; now recognized as the “North Pacific Gyre Oscillation” (Di Lorenzo, Schneider, Cobb et al., 2008)).
  • presentation of coral δ18O records spanning large segments of the last millennium, greatly expanding our understanding of tropical Pacific climate change during this time (Cobb et al., 2003)
  • reconstruction of monthly-resolved surface water radiocarbon in the central tropical Pacific over the last millennium, showing evidence for significant upwelling intensification roughly 700-800yrs ago, a potentially globally significant change whose causes are unknown (Zaunbrecher et al., 2010)
  • uncovered a large negative trend in seawater δ18O (implying more precipitation) likely associated with anthropogenic climate change, consistent with climate model projections that the “wet get wetter” under increased atmospheric CO2 (Nurhati et al., 2011)
  • greatly expanded the range of paleo-ENSO variability over the last 7,000yrs, in the process discovered that 20th century ENSO strength is 20% higher than the natural variability baseline (95% significant), suggesting that present-day ENSO may be intensifying under global warming (Cobb et al., 2013)

Publications

Cobb, K.M., N. Westphal, H. Sayani*, E. Di Lorenzo, H. Cheng, R.L. Edwards, C.D. Charles, Highly variable El Nino-Southern Oscillation throughout the Holocene, Science DOI: 10.1126/science.1228246, 2013.

Nurhati, I.*, K.M. Cobb, E. Di Lorenzo, Decadal-scale SST and salinity variations in the central tropical Pacific: signatures of natural and anthropogenic climate change. Journal of Climate doi: 10.1175/2011JCLI3852.1, 2011.

Sayani, H.*, K.M. Cobb, A. Cohen, E. Crawford, L.K. Zaunbrecher, I.S. Nurhati, Diagenetic effects on the morphology and geochemistry of modern and young fossil coral skeletons, Geochimica Cosmochimica Acta 75: 6361-6373, 2011.

Zaunbrecher, L.K.*, K.M. Cobb, J.W. Beck, C.D. Charles, E.R.M. Druffel, R.G. Fairbanks, S. Griffin, H. Sayani, Coral records of central tropical Pacific radiocarbon variability during the last millennium , Paleoceanography 25, PA4212, doi:10.1029/2009PA001788, 2010.

Nurhati, I.S.*, Cobb, K.M., Charles, C.D., Dunbar, R.B. Late 20th century warming and freshening trend in the central tropical Pacific, Geophysical Research Letters doi:10.1029/2009GL040270, 2009.

Cobb, K.M., C.D. Charles, R.L. Edwards, H. Cheng, M. Kastner. El Nino-Southern Oscillation and tropical Pacific climate during the last millennium. Nature 424: 271-276, 2003.

Cobb, K.M., C.D. Charles, R.L. Edwards, H. Cheng, M. Kastner. U/Th-dating living and young fossil corals from the central tropical Pacific. Earth and Planetary Science Letters 210(1-2): 91-103, 2003.

Cobb, K.M., D.E. Hunter, C.D. Charles. A central tropical Pacific coral demonstrates Pacific, Indian, and Atlantic decadal climate connections. Geophys. Res. Lett. 28(11): 2209-2212, 2001.

Presentations

Testing late 20th century ENSO variability against new coral records of natural variability (15min) AGU Fall Meeting, 2012.

People

Collaborators

Funding Sources

Field support