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CRP El Niño member - Mr. Robert Dunbar

Mr. Robert Dunbar

Stanford University
Stanford, USA

Scientific Background

Our group has been studying interannual to centennial climate variability using high resolution marine archives for ~20 years. We have focused on reconstructing the history of tropical sea surface temperatures using the skeletons of reef-building corals, and to a lesser extent, on atmospheric and oceanic variability recorded in lacustrine or continental margin sediments.

Dunbar first began analyzing corals in the late 1970's. At this time he conducted the first stable isotopic studies of a branching coral, using specimens collected from the Gulf of Panama in the Pacific, an ENSO sensitive site, as well as fossil specimens from the west coast of Costa Rica. This work led to several new projects focusing on ENSO variability. We switched our archive from branching corals to massive corals, in particular to specimens in the genera Porites and Pavona, in order to take advantage of multi-century growth records. We developed chronologies extending back 300 years at Secas Island, Panama, and nearly 400 years at Urvina Bay, Galapagos Islands. The Secas record includes near-monthly stable isotopic analyses that show pronounced annual and interannual variability associated with the inter-tropical convergence zone (ITCZ). The ITCZ migrates northward and southward in a highly regular fashion, producing Panama's dry and wet seasons. The pattern is altered during strong ENSO warm mode events, when the ITCZ remains locked at a lower latitude than normal, producing drought in part of Panama and Costa Rica. The Secas record revealed significant ENSO variability as well as interdecadal variability, the origin of which remains unknown.

Our work in the Galapagos revealed that 90% of the coral oxygen isotopic signal was generated by sea surface temperature anomalies (SSTa). At this site SSTa are primarily controlled by the state of the ENSO cycle. A 380 year annually-resoled isotopic record shows significant decade-to-decade and century-to-century variability in the character of ENSO. In particular, we observed an increase in the mean return time for El Niño events going back through time, from about 3.5 years during the most recent decades, to about 6 years during the 1600's. The Galapagos record remains one of the longest continuous coral paleoclimate records from the tropical Pacific Ocean.

After developing some long records of Pacific climate variability, we turned our attention to signal-to-noise and reproducibility issues. We conducted detailed in-situ calibration experiment in the Galapagos Islands, using a series of marked corals, in-situ water temperature and salinity recorders, and hand-collected seawater samples. This work serves to demonstrate the sensitivity of coral isotopic composition to actual measured environmental changes. We also conducted a replication study at Clipperton Island in the eastern Pacific. To assess the use of corals in the development of paleoclimatic reconstructions in this region, we developed oxygen isotope time series from multiple specimens of the massive coral Porites lobata. Six near-monthly isotopic records from different sized (age) colonies where produced for the interval 1986-1994. Our results show that the average oxygen isotopic disequilibrium offset (vital effect) from equilibrium seawater composition for individual corals can vary by up to 0.4 ppmil. However our results also suggest that the vital effect offset is constant over time. Similar offsets are observed in the tops of old (age >100 y) and young (age <10 y) colonies, further suggesting that the biologically mediated vital effect offset does not change as a Porites colony ages. All El Niño events of the past 100 years appear to be recorded by coral skeletal isotopic composition at this site.

In the mid-1990's we expanded our region of interest to include the Indian Ocean, in part to examine ENSO teleconnections insofar as they influence the Indian Ocean monsoon systems. In particular, we found that A 194-year annual record of skeletal oxygen isotopic composition from a coral growing at Malindi, Kenya, preserves a history of sea surface temperature (SST) change that is coherent with instrumental and proxy records of tropical Pacific climate variability over interannual to decadal periods. This variability is superimposed on a warming of as much as 1.3 degrees C since the early 1800s. These results suggest that the tropical Pacific imparts substantial decadal climate variability to the western Indian Ocean and, by implication, may force decadal variability in other regions with strong El Niño/Southern Oscillation teleconnections.

We have also begun examining the origin of climate signals in corals at the crystal-genesis spatial scale. To do this, we are employing micro-scale analytical techniques that have only recently become available. For example, in thermodynamic equilibrium with sea water the Sr/Ca ratio of aragonite varies predictably with temperature and the Sr/Ca ratio in coral have thus become a frequently used proxy for past Sea Surface Temperature (SST). However, biological effects can offset the Sr/Ca ratio from its equilibrium value.

We report high spatial resolution ion microprobe analyses of well defined skeletal elements in the reef-building coral Porites lutea that reveal distinct monthly oscillations in the Sr/Ca ratio, with an amplitude in excess of ten percent. The extreme Sr/Ca variations, which we propose result from metabolic changes synchronous with the lunar cycle, introduce variability in Sr/Ca measurements based on conventional sampling techniques well beyond the analytical precision. These variations can limit the accuracy of Sr/Ca paleothermometry by conventional sampling techniques to about 2 degrees C. Our results may help explain the notorious difficulties involved in obtaining an accurate and consistent calibration of the Sr/Ca vs. SST relationship. Our work in this area is continuing as we start to make use of nanometer-scale secondary ion emission mass spectrometers.


  • Grumet, N.S., N.J. Abram, J.W. Beck, R.B. Dunbar, M.K. Gagan, and T.P. Guilderson,W.S. Hantoro, and B. W. Suwargadi, 2004, Coral radiocarbon records of Indian Ocean water mass mixing and wind-induced upwelling along the coast of Sumatra, Indonesia, J. Geophysical Research, 109, C05003, doi:10.1029/2003JC002087.
  • Meibom, A., J.-P. Cuif, F. Hillion, B.R. Constantz, A. Juillet-Leclerc, Y. Dauphin, T. Watanabe, and R.B. Dunbar, 2004, Distribution of magnesium in coral skeleton, Geophysical Research Letters, 31, doi:10.1029/2004GL021313.
  • Roark, E.B., T.P. Guilderson, S. Flood-Page, R.B. Dunbar, B.L. Ingram, S. Fallon, and M. McCulloch, 2005, Radiocarbon-Based Ages and Growth Rates of Bamboo Corals from the Gulf of Alaska, Geophysical Research Letters, 32, doi:10.1029/2004GL021919.
  • Grumet, N.S., P.B. Duffy, M.E. Wickett, K. Caldeira, and R.B. Dunbar, 2005, Intrabasin Comparison of Surface Radiocarbon Levels in the Indian Ocean Between Coral Records and Three-Dimension Global Ocean Models, Global Biogeochemical Cycles, doi:10.1029/2004GB002289.
  • Marion, G.S., R.B. Dunbar, and D.A. Mucciarone, Kremer, J.M., Lansing, S., and A. Arthawiguna, 2005, Coral skeletal δ15N reveals isotopic traces of an agricultural revolution, Marine Pollution Bulletin, 50, 931–944.
  • Meibom, A., Yurimoto, H., Cuif, J.-P., Constantz, B., Dauphin, Y., Domart-Coulon, I., Houlbreque, F., Constanz, B., Tambutte, E., Tambutte, S., Allemand, D., Wooden, J., and R.B. Dunbar, 2006, Vital effects in coral skeletal composition display strict three-dimensional control, Geophysical Research Letters, 33, L11608, doi:10.1029/2006GL025968.
  • Roark, E.B., Guilderson, T.P., Dunbar, R.B., and B.L. Ingram, 2006, Radiocarbon Based Ages and Growth Rates: Hawaiian Deep Sea Corals, Marine Ecology Progress Series, (Feature Article), 327: 1–14.
  • Meibom, A., Mostefaoui, S., Cuif, J.-P., Dauphin, Y., Houlbreque, F., Dunbar, R.B., and B. Constanz, 2007, Biological forcing controls the chemistry of reef-building coral skeleton, Geophysical Research Letters, 34, L02601, doi:10.1029/2006GL028657.
  • Fleitmann, D., Dunbar, R.B., McCulloch, M., Mudelsee, M., Vuille, M., McClanahan, T., Cole, J., Andrews, C., and D.A. Mucciarone, 2007, The history of soil erosion in Kenya recorded in a 300 year-long coral, Geophysical Research Letters, L04401, doi:10.1029/2006GL028525.
  • Druffel, E.R.M., S. Griffin, S.R. Beaupre, and R.B. Dunbar, 2007, Oceanic Climate and Circulation Changes During the Past Four Centuries From Radiocarbon in Corals, Geophysical Research Letters, 34, L09601, doi:10.1029/2006GL028681, 2007.
  • Meibom, A., Cuif, J.-P., Mostefaoui, S., Dauphin, Y., Houlbreque, F., Meibom, K., and R. Dunbar, 2007, Chemical variations at ultra-structure length-scales in coral skeleton: Towards an understanding of ‘vital’ effects, Geochimica et Cosmochimica Acta, in press.
  • Marion, G.S., Dunbar, R.B., Fine, M., Hoegh-Guldberg, O., and D.A. Mucciarone, 2007, Organic Carbon (δ13C) Signals of Endolithic Algal Blooms and Coral Bleaching, Geophysical Research Letters, submitted.
  • Lorrain, A., Y.-M. Paulet, L. Chauvaud, R.B. Dunbar, D.A. Mucciarone, and M. Fontugne, 2004, δ13C variations in scallop shells: Increasing metabolic carbon contribution with body size? Geochimica et Cosmochimica Acta, 68, 3509-3519.
  • Chauvaud, L., R.B. Dunbar, A. Lorrain, Y.-M. Paulet, G. Thouzeau, F. Jean, J.-M. Guarini, and D.A. Mucciarone, 2005, The shell of the Great Scallop Pecten maximus as a high frequency archive of paleoenvironmental change, Geochemistry, Geophysics, Geosystems, Q08001, doi:10.1029/2004GC000890.
  • Thebault, J., Chauvaud. L., Clavier, J., Guarini, J., Dunbar, R.B., Fichez, R., Mucciarone, D.A., and E. Morize, 2007, Reconstruction of seasonal temperature variability in the tropical Pacific Ocean from the shell of the scallop, Comptopallium radula, Geochimica et Cosmochimica Acta, 71, 918–928.
  • Huchette, S., L. Chauvaud, R.W. Day, R. Dunbar, and J. Clavier, 2007, The ormer (Haliotis tuberculata): a new and promising paleoclimatic tool, Geophysical Research Letters, submitted.
  • Rowe, H., and R.B. Dunbar, 2004, Hydrologic-energy balance constraints on the Holocene lake-level history of lake Titicaca, South America, Climate Dynamics, v. 23, doi: 10.1007/s00382-004-0451-8.
  • Nederbragt, A.J., R.B. Dunbar, A.T. Osborn, A. Palmer, J.W. Thurow, and T. Wagner, 2006, Sediment colour analysis from digital images and correlation with sediment composition, In: Rothwell, R.G. (ed.), New techniques in sediment core analysis, Geolog. Soc. London Special Publication, 267, Geolog. Soc. London, UK, 272 pp.
  • Costa, E., R.B. Dunbar, K.A. Kryc, D.A. Mucciarone, S. Brachfeld, E.B. Roark, P.L. Manley, R.W. Murray, and A. Leventer, 2007, Solar forcing and El Nino-Southern Oscillation (ENSO) influences on productivity cycles interpreted from a late- Holocene high-resolution marine sediment record, Adelie Drift, East Antarctic Margin, U.S. Geological Survey and The National Academies; USGS OF-2007-1047, Short Research Paper 036; doi:10.3133/of2007-1047.srp036.
  • Waldmann, N., Ariztegui, D., Anselmetti, F.S., Austin Jr., J.A.,Dunbar, R., M. Moy, C.M., Recasens, C., in press, Lago Fagnano (Tierra del Fuego, Argentina) - A continuous archive of paleoclimatic change and tectonic activity since the Late Glacial, Geologica Acta, 5, No 4, 2007, 1-10.
  • Thiessen, K., Dunbar, R.B., Rowe, H.R., and Mucciarone, D.A., 2007, Multi-decadal to century scale arid episodes on the northern Altiplano during the middle Holocene, Palaeogeography, Palaeoclimatology, Palaeoecology, in press.
  • Moy, C.M., R.B. Dunbar, P.I. Moreno, J.-P. Francois, R. Villa-Martinez, D.M.
  • Mucciarone, T.P. Guilderson, and R. Garreaud, 2007, Isotopic Evidence for Hydrologic Change Related to the Westerlies in SW Patagonia, Chile During the Last Millennium, Quaternary Science Reviews, submitted.