Twentieth century seawater delta O-18 dynamics and implications for coral-based climate reconstruction

TitleTwentieth century seawater delta O-18 dynamics and implications for coral-based climate reconstruction
Publication TypeJournal Article
Year of Publication2018
AuthorsStevenson S., Powell B., Cobb K., Nusbaumer J., Merrifield M., Noone D.
Date Published2018/06
Type of ArticleArticle
ISBN Number2572-4517
Accession NumberWOS:000438667300005
Keywordsconvergence zone; earth system model; el-nino; error analysis; Geology; nino-southern; oceanography; oscillation; oxygen isotopic composition; Paleontology; salinity variations; sea-surface temperature; tropical pacific-ocean; warm pool

The oxygen isotopic composition of tropical coral skeletons (delta O-18) is a crucial source of information on past El Nino/Southern Oscillation behavior. Both temperature and the delta O-18 of the surrounding seawater delta O-18(sw) ) affect coral delta O-18; a linear proportionality with delta O-18(sw) is often used to infer past salinity variations, but the degree to which dynamical influences on delta O-18(sw) may affect that relationship is still unclear. Here we use the isotope-enabled Regional Ocean Modeling System to investigate the dynamics of delta O-18(sw) and salinity variations in different twentieth century climate regimes. The dominant modes of delta O-18(sw) variability are the background trend and eastern/central Pacific El Nino, similar to salinity; likewise, budget analysis reveals a strong impact of ocean dynamics (both advection and vertical mixing/diffusion) on delta O-18(sw) variations during El Nino and La Nina events. These dynamics lead to alterations in the delta O-18(sw) :salinity relationship across the Pacific: the linear approximation is most accurate near the eastern edge of the western Pacific warm pool but with nonlinearities due to large delta O-18(sw) excursions during El Nino. At other sites, the delta O-18(sw) :salinity relationship has substantial scatter and explains relatively little delta O-18(sw) variance. Strikingly, interannual variability can lead to site-specific delta O-18(sw) :salinity regression coefficient changes of up to 0.4 parts per thousand/psu on multiyear time scales. This calls the reliability of coral-based estimates of El Nino/Southern Oscillation extremes into question and highlights the need for increased dynamical understanding as well as sustained in situ observations of delta O-18(sw) and salinity variability. Plain Language Summary Tropical corals record past El Nino and La Nina events through changes in the isotopic ratios of oxygen in their skeletons, which is affected by the isotopic composition of seawater local to the reef. This seawater delta O-18 is thought to be closely related to salinity and is used to reconstruct past salinity variations associated with El Nino. However, this paper uses an isotope-enabled ocean model to show that salinity and seawater delta O-18 have a complicated relationship. The processes affecting the two variables can be quite different in places, and their relationship varies significantly with time; therefore, it may be much more difficult to use coral records to reconstruct past salinity changes (and thus, El Nino impacts) than was previously thought.

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