ALBERT

Description: Instrumental climate observations provide robust records of global land and ocean temperatures during the twentieth century. Unlike for temperature, continuous salinity observations in the surface ocean are scarce prior to 1970, and the magnitude of salinity changes during the twentieth century is largely unknown. Surface ocean salinity is a major component in climate dynamics, as it influences ocean circulation and water mass formation. Here we present an annually resolved reconstruction of salinity variations in the surface waters of the western subtropical North Pacific Ocean since 1873, based on bimonthly records of d18O, Sr/Ca, and U/Ca in a coral from the Ogasawara Islands. The reconstruction indicates that an abrupt regime shift toward fresher surface ocean conditions occurred between 1905 and 1910. Observational atmospheric data suggest that the abrupt freshening was associated with a weakening of the winds that drive the Kuroshio Current system and the associated subtropical gyre circulation. We note that the abrupt early-twentieth-century freshening in the western subtropical North Pacific precedes abrupt climate change in the northern North Atlantic by a few years. The potential for abrupt regime shifts in surface ocean salinity should be considered in climate predictions for the coming decades.

Description: Paleotemperature estimates based on coral Sr/Ca have not been widely accepted because the reconstructed glacial-Holocene shift in tropical sea-surface temperature (~4-6°C) is larger than that indicated by foraminiferal Mg/Ca (~2-4°C). We show that corals over-estimate changes in sea-surface temperature (SST) because their records are attenuated during skeletogenesis within the living tissue layer. To quantify this process, we microprofiled skeletal mass accumulation within the tissue layer of Porites from Australasian coral reefs and laboratory culturing experiments. The results show that the sensitivity of the Sr/Ca and d18O thermometers in Porites will be suppressed, variable, and dependent on the relationship between skeletal growth rate and mass accumulation within the tissue layer. Our findings help explain why d18O-SST sensitivities for Porites range from -0.08 per mil/°C to -0.22 per mil/°C and are always less than the value of -0.23 per mil/°C established for biogenic aragonite. Based on this observation, we recalibrated the coral Sr/Ca thermometer to determine a revised sensitivity of -0.084 mmol/mol/°C. After rescaling, most of the published Sr/Ca-SST estimates for the Indo-Pacific region for the last ~14,000 years (-7°C to +2°C relative to modern) fall within the 95% confidence envelope of the foraminiferal Mg/Ca-SST records. We conclude that two types of calibration scales are required for coral paleothermometry; an attenuated Porites-specific thermometer sensitivity for studies of seasonal to interannual change in SST and, importantly, the rescaled -0.084 mmol/mol/°C Sr/Ca sensitivity for studies of 20th-century trends and millennial-scale changes in mean SST. The calibration-scaling concept will apply to the development of transfer functions for all geochemical tracers in corals.

Description: The Pacific Decadal Oscillation (PDO), the leading mode of sea surface temperature (SST) anomalies in the extratropical North Pacific Ocean, has widespread impacts on precipitation in the Americas and marine fisheries in the North Pacific. However, marine proxy records with a temporal resolution that resolves interannual to interdecadal SST variability in the extratropical North Pacific are extremely rare. Here we demonstrate that the winter Sr/Ca and U/Ca records of an annually-banded reef coral from the Ogasawara Islands in the western subtropical North Pacific are significantly correlated with the instrumental winter PDO index over the last century. The reconstruction of the PDO is further improved by combining the coral data with an existing eastern mid-latitude North Pacific growth ring record of geoduck clams. The spatial correlations of this combined index with global climate fields suggest that SST proxy records from these locations provide potential for PDO reconstructions further back in time.

Description: [1] Previous studies have demonstrated the potential for the Li content of coral aragonite to record information about environmental conditions, but no detailed study of tropical corals exists. Here we present the Li and Mg to Ca ratios at a bimonthly to monthly resolution over 25 years in two modern Porites corals, the genus most often used for paleoclimate reconstructions in the tropical Indo-Pacific. A strong relationship exists between coral Li/Ca and locally measured SST, indicating that coral Li/Ca can be used to reconstruct tropical SST variations. However, Li/Ca ratios of the skeleton deposited during 1979–1980 do not track local SST well and are anomalously high in places. The Mg/Ca ratios of this interval are also anomalously high, and we suggest Li/Ca can be used to reconstruct tropical SST only when Mg/Ca data are used to carefully screen for relatively rare biological effects. Mg/Li or Li/Mg ratios provide little advantage over Li/Ca ratios, except that the slope of the Li/Mg temperature relationship is more similar between the two corals. The Mg/Li temperature relationship for the coral that experienced a large temperature range is similar to that found for cold water corals and aragonitic benthic foraminifera in previous studies. The comparison with data from other biogenic aragonites suggests the relationship between Li/Mg and water temperature can be described by a single exponential relationship. Despite this hint at an overarching control, it is clear that biological processes strongly influence coral Li/Ca, and more calibration work is required before widely applying the proxy.

Description: We reconstructed a 237-year record of SST and salinity using a coral core collected at Bicol, southern Luzon, Philippines, which is at the northern edge of the western Pacific warm pool. A 2.5-m-long core (SWGM03–01) was collected from a Porites sp. colony at 6 m below mean sea level on 16 March 2002. Temporal resolution of the Sr/Ca analyses for the most recent 46 years of the record was approximately monthly, whereas for the rest of about 190 years, the resolution was mostly bimonthly. The temporal resolution of the d18O analyses was bimonthly throughout most of the core.

Description: A 2.5-m-long core (SWGM03–01) was collected from a Porites sp. colony at 6 m below mean sea level on 16 March 2002. The core was drilled vertically from the top of the colony and then cut into 7-mm-thick slabs in the laboratory. The δ18Ocoral values of samples covering the period from 1890 to 2002 was analyzed using an online system comprising an IsoPrime Isotope-ratio mass spectrometer (GV Instruments Ltd.) coupled to a Multicarb automatic sample treatment system at the Geological Survey of Japan (GSJ), whereas the δ18Ocoral values of samples predating 1890 were determined by the Center for Advanced Marine Core Research at Kochi University (KCC) using a same system used at GSJ. All δ18Ocoral data were normalized to Vienna Pee Dee Belemnite (V-PDB) using the NBS-19 standard (δ18O = +2.2‰) from the U.S. National Institute of Standards and Technology. The International Atomic Energy Agency standard IAEA-603 (δ18O = –2.37 ± 0.04‰) was also used for some measurements conducted at KCC. The standard deviations of replicate δ18O measurements of the standards during the mass spectrometer runs were 0.04‰ (1s) and 〈 0.10‰ (1s) at GSJ and KCC, respectively. Concentrations of Ca and Sr were measured by inductively coupled plasma optical emission spectrometry (ICP-OES) using an IRIS Advantage system (Thermo Electron Co., Ltd) at GSJ and an Agilent 720 system (Agilent Technologies) at Okayama University. Each sample was first weighed (100 ± 10 µg) and then dissolved in 2% HNO3. Sr/Ca analyses of samples covering the period after 1883 were conducted at GSJ, and samples predating 1890 were analyzed at Okayama University, respectively, with a 6-year overlap to confirm measurement consistency. The analytical precision (RSD) of the Sr/Ca measurements was 〈 0.37% (1s) made at both facilities. Measurements of Sr/Ca were mostly performed on microsamples, every 0.8 mm and 1.6 mm for the first 46 years and the remainder of the record, respectively. Given that mean growth rate is approximately 8.0 mm/yr, temporal resolution of the Sr/Ca analyses for the most recent 46 years of the record was approximately monthly, whereas for the rest of about 190 years, the resolution was mostly bimonthly. The temporal resolution of the d18O analyses was bimonthly throughout most of the core. An age model for the period before 1982 was constructed using the Sr/Ca ratios with Sr/Ca maxima (indicating relatively cool SSTs) corresponding to growth mainly in winter (January or February). Other Sr/Ca ratios in this time interval were then converted to the time domain by simple linear interpolation. For the period 1982–2002 in which Sr/Ca analyses were conducted with higher time resolution, the precise timing of the coolest SSTs was determined using IGOSS weekly SSTs then converted to the time domain. This age model for the entire core was confirmed and fine-tuned by counting annual density bands on the X-radiographs of the coral core and applied to d18O record as well.

Description: We reconstructed a 237-year record of SST and salinity using a coral core collected at Bicol, southern Luzon, Philippines, which is at the northern edge of the western Pacific warm pool. A 2.5-m-long core (SWGM03–01) was collected from a Porites sp. colony at 6 m below mean sea level on 16 March 2002. Temporal resolution of the Sr/Ca analyses for the most recent 46 years of the record was approximately monthly, whereas for the rest of about 190 years, the resolution was mostly bimonthly. The temporal resolution of the d18O analyses was bimonthly throughout most of the core.