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Progress on this project for the period ending August 31, 2010 has been excellent. Our work in the central Equatorial Pacific has been augmented by preliminary analyses of an independent record from another coral colony located at Christmas Island, but separated from our first coral by many kilometers.  These new analyses confirm the decrease of 50-60% in coral P/Ca over the course of the 1980's.  The magnitude of these decadal drop exceeds those associated with the 1982-83 and 1987 El Nino periods.  We propose that the shift is consistent with a connection, though with a 6-year lag, to the 1976-77 Pacific climate regime shift.  In addition, in the past year, we have extended our studies to another study region of interest, the Arabian Sea upwelling region, and have initiated new measurements of nitrogen isotope ratios in corals to complement the P/Ca work. The project has supported portions of the research of three female graduate students, one female undergraduate intern, and has fully supported one postdoc.  The following is a summary of our findings to date.

1. Our work on the P/Ca proxy calibration, using samples of Pavona gigantea and Porites lobata grown in the Gulf of Panama, is now published in Geochimica et Cosmochimica Acta.  This work demonstrates that P/Ca reflects ocean phosphate concentrations over short periods when compared to contemporaneously collected seawater samples, and over longer mean periods in corals sampled from a range of ocean nutrient conditions.  This result has allowed us to pursue down-core work with more confidence, although we feel that the quantitative details of the calibration may be colony-specific to a degree as well as species-specific.  Further opportunities for calibration verification will be pursued in the coming year, in parallel with our focus on generating seawater histories by sampling many successive years through a core, typically at ~monthly intervals.

2. We have acquired a second Christmas Island coral core (Evans core) that allows us to confirm with an independent record the findings from the first core (Cobb core).  As a recap, the Cobb core is a Porites core from Christmas Island (2°N, 157°W) analyzed for P/Ca over a period covering 1978-1995.  It reveals an apparent mode shift in central Equatorial surface nutrient concentrations, occurring during the 1980's.   This is a large effect, with seawater phosphate decreasing an estimated 60% from the late 1970's to much lower and more stable values in the early 1990's, whereas the P/Ca decreases during interannual El Nino events are only 30-40%.  Because this is an absolutely novel finding (monitoring of nutrients in this region did not begin until the late 1990's), with large implications for decadal scale variations in the circulation of the Pacific, we decided to try to confirm it using the Evans core.  The preliminary analyses to date show a slightly smaller response for the 1980's (~50% decrease), but largely confirm the findings from the Cobb coral.  We now feel we are sufficiently confident of this record that we are near completion of a new manuscript for submission to Nature.  We feel our cautious approach has been rewarded.  Now we are working to complete the records in the Evans core, not only to fill in all samples since the 1970's but to extend the overall record back to ~1940.  Just recently, former grad student and continuing collaborator Michele LaVigne (now a postdoc at UC Davis) has located samples of micro-atoll corals from Christmas Island, and we will attempt to general a parallel record from these corals.  This will not only provide multiple replication of our record, it will test the utility of micro-atolls as paleo-archives in the future, and bring the P/Ca record up to 2007, so we can compare it to seawater nutrient measurements in this region.  Michele presented the updated Christmas Island work at the International Conference on Paleoceanography (Aug. 2010, La Jolla).

3. Through the work of MS student Livia Montone, and in collaboration with Rick Mortlock and Rick Fairbanks of Rutgers Earth and Planetary Sciences Department, we have generated an entirely new P/Ca (and Sr, Mg, U, B, Ba and oxygen isotope) record from a coral drilled in 1996 from off the coast of Oman, in the Arabian Sea upwelling region.  We reasoned that this Porites record would provide another test of the P/Ca proxy, since the strong seasonal upwelling varies the nutrient concentrations in surface waters over quite a large range.  The record currently extends from 1980 to 1996 (~20 samples per year) and shows beautiful Sr/Ca variations that follow extremely well the sea surface temperature (SST) variations as recorded by satellite AVHRR and by ship observations, and therefore hold promise for reconstructing SST, and therefore upwelling intensity, back to before the remote sensing era (the core extends to ~1960, and other nearby cores are available.  Somewhat to our surprise, the P/Ca variations do not seem to following the upwelling history closely, but seem at this location to be related to lateral mixing processes with near-shore coastal water.  We found, however, that Ba/Ca variations seem to reach maxima twice per year, in a pattern that suggests these signals are tracking biological productivity.  This work formed the MS degree of Livia Montone (defended July 2010), and is being continued by postdoc Dan Sinclair.  Dan presented the results to date at the International Conference on Paleoceanography (Aug. 2010) and we expect to submit two manuscripts on the Oman coral work before the end of 2010.

4. We have initiated a new component to our research plan that will complement our work on P/Ca proxy development and application.  With Daniel Sigman at Princeton, and his graduate student, we are starting to pursue the goal of determining Nitrogen isotope ratios on the small fraction of organic matter bound within the matrix of coral skeleton.  We hypothesize that these data will reflect the relative utilization of upwelled nitrate and the degree of N contribution from N fixation. Preliminary results look promising, and we will be pursuing this approach in the coming year.