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46207-AC2
Coral records of late 20th century tropical Pacific upwelling: investigating a potential climate-carbon feedback

Kim Cobb, Georgia Institute of Technology

Year 2 Progress Report:  "Coral records of late 20th century tropical Pacific upwelling:  investigating a potential climate-carbon feedback"

PI:  Kim Cobb

Date:  4/26/09

            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).

Figure 1. Map of cruise track for Southern Line Islands Expedition in October, 2008.

The Georgia Tech Southern Line Islands coral cruise, October 2008

The primary objectives of the cruise (partially funded by PRF funding) was to survey corals in the southern Line Islands group (Caroline, Malden and Flint Islands; see Figure 1) and retrieve living coral cores to reconstruct 20th century tropical Pacific climate. Samples collected from the cruise complement our ongoing PRF-funded work from the northern Line Islands group (Palmyra, Fanning and Christmas Islands) in investigating the role of the tropical Pacific climate in the current greenhouse forcing (Nurhati et al., submitted). Along the cruise track (Figure 1), we conducted seawater sampling every 1û latitude and continuous underway sea-surface temperature logging.

Long temperature and hydrology records from northern Line Island corals:  Sr/Ca and d18Osw

            Our previous work (Nurhati et al., submitted) demonstrates the power of combining the two most robust coral climate proxies, namely Sr/Ca ratios and oxygen isotopic values (d18O), to reconstruct the temperature and hydrological history of the central tropical Pacific over the last thirty years. Our approach emphasizes regional reproducibility (by constructing coral geochemical records from three neighboring islands) and site reproducibility (by constructing multiple coral geochemical records from the same site). Our results indicate that the tropical Pacific has undergone a shift towards "El Nino-like" conditions over the last decades – a trend that we believe is related to anthropogenic greenhouse forcing and thus will continue in coming decades.

            Ongoing work with corals from the same northern Line Islands sites that were used in Nurhti et al. (submitted) focus on extending the coral climate proxy records to the early 20th century. This research will allow us to better interpret the climatic signals uncovered in the short, 30yr records already completed – allowing for the clear identification of any anthropogenic-related signals (which presumably could be detected by comparing the early and late 20th century portions of the coral geochemical timeseries). Preliminary data from Palmyra Island (6N, 162W) is shown in Figure 2. By comparing the coral d18O and Sr/Ca records over the 20th century, we can gain insights into the history of warming (Sr/Ca) versus freshening (d18O-Sr/Ca) over this time period. The Sr/Ca record indicates that late 20th century warming is not unprecedented in the Palmyra coral record (a result that will be tested with additional coral Sr/Ca records from southern neighbors Fanning and Christmas Islands) (Figure 2). However, the d18O record indicates that late 20th century freshening is unprecedented, and likely reflects a shift in the Inter-tropical Convergence Zone that has occurred since ~1970AD.

Figure 2. Long records of coral d18O (temperature and precipitation) and Sr/Ca (temperaure only) from Palmyra Island (6N, 162W). The Palmyra record will extend to 1886AD, while other Northern Line Island coral cores from Fanning (4N, 160W) and Christmas (2N, 157W) extend to 1923AD and 1938AD, respectively. We will complete these additional long d18O and Sr/Ca records in the next year.

In summary, our coral geochemical work has yielded new insights into the recent history of tropical Pacific climate, far beyond the reach of sparse instrumental records in this remote region. Our tasks for the next year (a 1 year no-cost extension has been requested) include completing the long coral timeseries from the northern Line Islands, and investigating the new coral climate records from the southern Line Islands.

References:

Nurhati, I.S.*, Cobb, K.M., Charles, C.D., Dunbar, R.B. Late 20th century trend warming and freshening in the central tropical Pacific, Submitted to Geophysical Research Letters.

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