A review of modern coral δ18O and Δ14C proxy records

This paper is a review of published modern coral δ18O and Δ14C isotopic records that are at least 30 and 20 years long, respectively. The data are presented to show basin-scale trends in both of these proxy records on decadal-to-centennial timescales. The goal was to qualitatively integrate the general inter-annual-to-centennial timescale variability revealed in these data, as well as the statistical and modeling output results that have been produced using these coral records. While many review papers typically include a representative subset of the data available, this review aims to include as much of the available data as possible. In general, coral δ18O records show a long-term warming and/or freshening throughout the tropical oceans, and agree with the NOAA Extended Reconstruction Sea Surface Temperature 2 (ERSST) on decadal timescales. In the western Pacific, it is most likely a freshening of the seawater δ18O that dominates the signal. El Niño Southern Oscillation (ENSO) variability dominates most δ18O records either by varying local seawater temperature or salinity, depending on the regional oceanography/climatology. Outside of the Pacific, ENSO affects seawater temperature and salinity via atmospheric or oceanic teleconnections. Post-bomb coral Δ14C records collectively show that the uptake of 14C has been greatest in gyre-water fed sites, followed in descending order by western boundary current areas, equatorial upwelling regions, and eastern tropical Pacific upwelling sites. These surface water Δ14C values indicate the proportion of surface water and/or the residence time of water at the surface at a given location, and can be used to model water mass mixing rates. Such models have only begun to be run and show that the amount of eastern Pacific water entering the central South Pacific increases during El Niños and that the Indonesian throughflow is supplied year-round by the North Pacific. Comparing ocean circulation models with coral Δ14C-modelled circulation enables researchers to explore the mechanisms that drive seawater Δ14C variability and fine-tune their models. In addition, our comparison between the rate of coral Δ14C increase between 1960 and 1970 and total anthropogenic CO2 uptake rates show general agreement, demonstrating the value of coral records in understanding past carbon fluxes. Overall, coral δ18O and Δ14C proxy records represent natural archives of seawater conditions and are critical for studying the natural variability in local and regional patterns within, and teleconnection patterns between, the tropics, extra-tropics, temperate, and Polar Regions on intra-annual-to-centennial timescales.