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The proposal aims to deliver quantitative estimates of temperature, hydrology, and upwelling changes over the last 50+ years by applying multiple geochemical proxies to a suite of coral cores from the central tropical Pacific. The geochemical proxies that have been analyzed on these cores to date include: oxygen isotopes (d18O) as a proxy for temperature and hydrology, Sr/Ca ratios as a proxy of coral temperature, and Cd and Ba/Ca ratios as potential upwelling proxies. Graduate student Intan Suci-Nurhati has performed all the analyses for this project, under PI Cobb's supervision, and in collaboration with Dr. Bill McDonough (U. Maryland), Dr. Chris Charles (SIO-UCSD), and Dr. Rob Dunbar (Stanford). In our first project report, I reviewed our results from last year, summarizing the evidence for a shift in tropical Pacific climate towards an "El Nino-like" background climate state characterized by warmer and wetter conditions in the central tropical Pacific (Nurhati, Cobb et al., 2009). In the last progress report, I summarized the results from a major oceanographic cruise we led to the central tropical Pacific in October 2008, which provided a suite of new coral material that we are currently analyzing. In this narrative report, I present a century-long reconstruction of central tropical Pacific SST (via coral Sr/Ca) and hydrology (via coral-based seawater d18O) from Palmyra Island (Nurhati, Cobb et al., in prep). Our recent results confirm that this long record captures new decadal-scale dynamics in the central tropical Pacific, as well as anthropogenic climate change (Di Lorenzo, Cobb et al., submitted). A PRF-funded effort to document the effects of diagenesis on coral Sr/Ca and d18O in modern and living corals was conducted by Cobb lab undergraduate Hussein Sayani (Sayani et al., submitted). Graduate student Nurhati, together with the PI, have also collaborated with Rutgers PhD student Michele LaVigne and Prof. Rob Sherell, who have been analyzing P/Ca ratios in the Line Island corals to reconstruct upwelling (LaVigne et al., 2010).

--A new 112yr-long record of temperature and hydrology from the central tropical Pacific

Following on the success of our 30yr-long records from northern Line Islands corals (Nurhati et al., 2009), we have extended the same multi-proxy approach to the 112yr-long Palmyra coral. A new coral Sr/Ca record measured at sub-monthly resolution via ICPOES is an excellent proxy for sea-surface temperature at the site (Figure 1). The 112yr-long coral-based SST record faithfully captures variability associated with the El Nino-Southern Oscillation (ENSO), as well as prominent decadal-scale variability with an amplitude comparable to ENSO variability (Figure 2b-d; Nurhati et al., in prep). The strong decadal-scale variability captured in the coral SST record is closely linked to a recently-discovered Pacific basin-scale mode of climate variability called the North Pacific Gyre Oscillation (NPGO). The close correspondence between the coral SST record and the NPGO confirms that the central tropical Pacific is very sensitive to Pacific decadal-scale variability (PDV), and might even be the origin of PDV (Di Lorenzo et al., submitted). The coral SST record suggests that anthropogenic warming has amounted to less than 1oC over the 20th century, in line with the global average warming observed over this period.

We reconstruct central tropical Pacific hydrological trends by removing the Sr/Ca-based SST variations from the coral d18O record to yield a record of d18O of seawater (d18Osw), widely accepted as a proxy for hydrological balance (Figure 2e-f). Even though hydrology and temperature are generally closely-related in the deep tropics (because warm waters drive enhanced vertical convection), the coral d18Osw record bears little resemblance to the coral SST record. The coral d18Osw record is characterized by prominent decadal- scale variability that is strongly correlated to the Pacific Decadal Oscillation (PDO), a close relative of the NPGO but with a distinctly different temporal evolution. So even though central tropical Pacific SST are driven primarily by the NPGO, hydrology seems to respond to the PDO, indicating a decoupling of SST and hydrology on lower- frequencies – a conclusion that has clear implications for the hydrological responses to ongoing anthropogenic warming in the tropical Pacific. Indeed, there is a prominent trend in coral d18Osw towards more precipitation in the late 20th century, indicating a shift in the location and/or strength of large-scale convection over this period. Firm conclusions about this large anthropogenic freshening await the completion of the d18Osw records from nearby Fanning and Christmas records, which are currently underway.

--Effects of diagenesis on coral paleoclimate reconstructions

Undergraduate Hussein Sayani has recently completed a thorough investigation of the morphology and geochemistry of altered phases in modern and fossil corals from the central tropical Pacific, using funding from the ACS-PRF grant to PI Cobb (Sayani et al., submitted). In this work, we find evidence for large offsets in coral d18O and Sr/Ca in both modern and fossil corals, and use Scanning Electron Microscopy and Secondary Ion Mass Spectrometry (in collaboration with Dr. Anne Cohen, WHOI) to constrain the morphology and the micro-chemistry of these subtle changes to the coral skeleton. In general, we find that most alteration is associated with "cool" artifacts in coral Sr/Ca and d18O, except in the rare cases of calcite precipitation, in which case coral Sr/Ca is significantly depleted (indicating artificially "warmer" temperatures). An important implication from this work is that the Palmyra modern coral is not compromised by diagenesis, so the climatic reconstructions discussed above are accurate representations of 20th century climate variability in the central tropical Pacific.

--Conference presentations for Year 3 acknowledging ACS-PRF:

Cobb "Chemical Oceanography: Past and Future",2009

Nurhati World Ocean Conference,2009

Nurhati PAGES 1st Young Scientists Meeting,2009

Sayani Fall AGU Meeting,2009

--Publications acknowledging ACS-PRF: (* indicates student author)

Di Lorenzo, Cobb et al, Central Pacific El Ni–o and decadal climate change in the North Pacific. Nature Geoscience, in prep.

LaVigne*, Cobb et. al, Coral skeleton P/Ca proxy for seawater phosphate: Multi-colony calibration with a contemporaneous seawater phosphate record, Geochimica et Cosmochimica Acta. Volume 74(4): 1282-1293, 2010.

Nurhat, Cobb et al, Low frequency sea-surface temperature and hydrology variability in the central tropical Pacific: anthropogenic trends versus natural variability. Journal of Climate, in prep.