More Like this

1. Coral Oxygen Isotopic Records Capture the 2015/2016 El Niño Event in the Central Equatorial Pacific

   https://doi.org/10.1029/2021GL094036  

   O’Connor, Gemma K. ; Cobb, Kim M. ; Sayani, Hussein R. ; Atwood, Alyssa R. ; Grothe, Pamela R. ; Stevenson, Samantha ; Baum, Julia K. ; Chen, Tianran ; Claar, Danielle C. ; Hitt, Nicholas T. ; et al ( December 2021 , Geophysical Research Letters)

   Coral oxygen isotopes (δ¹⁸O) from the central equatorial Pacific provide monthly resolved records of El Niño‐Southern Oscillation activity over past centuries to millennia. However, calibration studies usingin situdata to assess the relative contributions of warming and freshening to coral δ¹⁸O records are exceedingly rare. Furthermore, the fidelity of coral δ¹⁸O records under the most severe thermal stress events is difficult to assess. Here, we present six coral δ¹⁸O records andin situtemperature, salinity, and seawater δ¹⁸O data from Kiritimati Island (2°N, 157°W) spanning the very strong 2015/16 El Niño event. Local sea surface temperature (SST) anomalies of +2.4 ± 0.4°C and seawater δ¹⁸O anomalies of −0.19 ± 0.02‰ contribute to the observed coral δ¹⁸O anomalies of −0.58 ± 0.05‰, consistent with a ∼70% contribution from SST and ∼30% from seawater δ¹⁸O. Our results demonstrate that Kiritimati coral δ¹⁸O records can provide reliable reconstructions even during the largest class of El Niño events.

    

   more » « less
2. Reproducibility of Coral Mn/Ca‐Based Wind Reconstructions at Kiritimati Island and Butaritari Atoll

   https://doi.org/10.1029/2020GC009398  

   Sayani, Hussein R. ; Thompson, Diane M. ; Carilli, Jessica E. ; Marchitto, Thomas M. ; Chapman, Alice U. ; Cobb, Kim M. ( March 2021 , Geochemistry, Geophysics, Geosystems)

   Global surface temperatures during the twentieth century are characterized by multidecadal periods of accelerated or reduced warming, which are thought to be driven by Pacific decadal variability, specifically changes in trade‐wind strength. However, the relationship between trade‐wind strength and global surface warming remains poorly constrained due to the scarcity of instrumental wind observations. Previous work has shown that corals growing at Tarawa Atoll (1.3°N, 173°E) incorporate dissolved Mn flushed from lagoon sediments by El Niño‐related westerly wind events (WWEs), providing records of both westerly wind variability and trade‐wind strength (on decadal time scales). Here, we explore the utility of this novel coral Mn/Ca‐wind proxy at two nearby islands that also feature west‐facing lagoons. Short coral Mn/Ca records from Butaritari (3°N, 173°E) and Kiritimati (2°N, 157.5°W) track WWEs, albeit with some intercolony variability in the magnitude and timing of the signal. Variability in coral Mn/Ca signal intensity among records from Butaritari suggests that wind‐driven mixing of the sediment Mn reservoir may be finite and/or localized. At Kiritimati, a coral growing outside the lagoon shows higher Mn/Ca concentrations during the 1997/1998 El Niño event, suggesting that nearshore sediments may be an overlooked dissolved Mn reservoir. Taken together, these results highlight a need for additional studies of Mn reservoir variability within and across atolls that hold promise for recording WWEs. These results also suggest that Mn/Ca records from multiple coral colonies and sites are needed to generate robust coral‐based wind reconstructions, particularly from sites with unknown or complex Mn transport pathways.

    

   more » « less
3. Accuracy and Reproducibility of Coral Sr/Ca SIMS Timeseries in Modern and Fossil Corals

   https://doi.org/10.1029/2021GC010068  

   Sayani, Hussein R. ; Cobb, Kim M. ; Monteleone, Brian ; Bridges, Heather ( August 2022 , Geochemistry, Geophysics, Geosystems)

   Coral strontium‐to‐calcium ratios (Sr/Ca) provide quantitative estimates of past sea surface temperatures (SST) that allow for the reconstruction of changes in the mean state and climate variations, such as the El Nino‐Southern Oscillation, through time. However, coral Sr/Ca ratios are highly susceptible to diagenesis, which can impart artifacts of 1–2°C that are typically on par with the tropical climate signals of interest. Microscale sampling via Secondary Ion Mass Spectrometry (SIMS) for the sampling of primary skeletal material in altered fossil corals, providing much‐needed checks on fossil coral Sr/Ca‐based paleotemperature estimates. In this study, we employ a set modern and fossil corals from Palmyra Atoll, in the central tropical Pacific, to quantify the accuracy and reproducibility of SIMS Sr/Ca analyses relative to bulk Sr/Ca analyses. In three overlapping modern coral samples, we reproduce bulk Sr/Ca estimates within ±0.3% (1σ). We demonstrate high fidelity between 3‐month smoothed SIMS coral Sr/Ca timeseries and SST (R = −0.5 to −0.8;p < 0.5). For lightly‐altered sections of a young fossil coral from the early‐20th century, SIMS Sr/Ca timeseries reproduce bulk Sr/Ca timeseries, in line with our results from modern corals. Across a moderately‐altered section of the same fossil coral, where diagenesis yields bulk Sr/Ca estimates that are 0.6 mmol too high (roughly equivalent to −6°C artifacts in SST), SIMS Sr/Ca timeseries track instrumental SST timeseries. We conclude that 3–4 SIMS analyses per month of coral growth can provide a much‐needed quantitative check on the accuracy of fossil coral Sr/Ca‐derived estimates of paleotemperature, even in moderately altered samples.

    

   more » « less
4. BOMB-PRODUCED RADIOCARBON ACROSS THE SOUTH PACIFIC GYRE—A NEW RECORD FROM AMERICAN SAMOA WITH UTILITY FOR FISHERIES SCIENCE

   https://doi.org/10.1017/RDC.2021.51  

   Andrews, Allen H ; Prouty, Nancy G ; Cheriton, Olivia M ( December 2021 , Radiocarbon)

   ABSTRACT Coral skeletal structures can provide a robust record of nuclear bomb produced 14 C with valuable insight into air-sea exchange processes and water movement with applications to fisheries science. To expand these records in the South Pacific, a coral core from Tutuila Island, American Samoa was dated with density band counting covering a 59-yr period (1953–2012). Seasonal signals in elemental ratios (Sr/Ca and Ba/Ca) and stable carbon (δ 13 C) values across the coral core corroborated the well-defined annual band structure and highlighted an ocean climate shift from the 1997–1998 El Niño. The American Samoa coral 14 C measurements were consistent with other regional records but included some notable differences across the South Pacific Gyre (SPG) at Fiji, Rarotonga, and Easter Island that can be attributed to decadal ocean climate cycles, surface residence times and proximity to the South Equatorial Current. An analysis of the post-peak 14 C decline associated with each coral record indicated 14 C levels are beginning to merge for the SPG. This observation, coupled with otolith measurements from American Samoa, reinforces the perspective that bomb 14 C dating can be performed on fishes and other marine organisms of the region using the post-peak 14 C decline to properly inform fisheries management in the South Pacific. 

   more » « less
5. Developing a Coral Proxy System Model to Compare Coral and Climate Model Estimates of Changes in Paleo‐ENSO Variability

   https://doi.org/10.1029/2019PA003836  

   Lawman, A. E. ; Partin, J. W. ; Dee, S. G. ; Casadio, C. A. ; Di Nezio, P. ; Quinn, T. M. ( July 2020 , Paleoceanography and Paleoclimatology)

   Coral records of surface‐ocean conditions extend our knowledge of interannual El Niño–Southern Oscillation (ENSO) variability into the preinstrumental period. That said, the wide range of natural variability within the climate system as well as multiple sources of uncertainties inherent to the coral archive produce challenges for the paleoclimate community to detect forced changes in ENSO using coral geochemical records. We present a new coral proxy system model (PSM) of intermediate complexity, geared toward the evaluation of changes in interannual variance. Our coral PSM adds additional layers of complexity to previously published transfer functions of sensor models that describe how the archive responds to sea surface temperature (SST) and salinity. We use SST and salinity output from the Community Earth System Model Last Millennium Ensemble 850 control to model coral oxygen isotopic ratios and SST derived from Sr/Ca. We present a detailed analysis of our PSM using climate model output for sites in the central and southwest Pacific before extending the analyses to span the broader tropical Pacific. We demonstrate how variable growth rates, analytical and calibration errors, and age model assumptions systematically impact estimates of interannual variance and show that the relative magnitude of the change in interannual variance is location dependent. Importantly, however, we find that even with the added uncertainties in our PSM, corals from many circum‐Pacific locations are broadly able to capture decadal and longer (decadal+) changes in ENSO variability. Our code is publicly available on GitHub to facilitate future comparisons between model output and coral proxy data.

    

   more » « less