Reconstructing the Variability of the El Niño-Southern Oscillation During Marine Isotope Stage 5 at ODP Site 1240 Using Individual Foraminifera Analyses of Globigerinoides ruber (white)

Abstract:

Today, the El Niño-Southern Oscillation (ENSO) determines the extent of the eastern tropical Pacific cold tongue on a sub-decadal timescale and influences sea-surface temperature (SST) and thermocline depth in the Eastern Equatorial Pacific (EEP). Thermocline depth in the EEP also regulates the strength of upwelling and influences the amount of CO₂ exchange between the low-latitude ocean and atmosphere. Efforts to reconstruct the predominant ENSO state (El Niño or La Niña) and the variability of ENSO events over the late Pleistocene using marine archives are limited by the lack of proxies on glacial-interglacial timescales. A promising approach applied in prior studies in the EEP measured the variance in δ¹⁸O of Globigerinoides ruber (white) individuals at the Holocene and at the Last Glacial Maximum.¹ The δ¹⁸O of this near-surface dweller reflects the temperature in its environment over its weeks long lifespan. The δ¹⁸O variance within a discrete period is thought to represent the variability in SST, which indicates the frequency and strength of ENSO events. This study will explore orbital influences on ENSO, using individual foraminifera analyses on the δ¹⁸O of G. ruber (w) from Ocean Drilling Program (ODP) Site 1240 (2921 m; 0˚1.311’N, 86˚27.758’W) during Marine Isotope Stage (MIS) 5, a time of strong precessional influence. ODP 1240 is located in the modern zone of equatorial upwelling in the EEP, where SST varies as a result of ENSO events. This proxy will be employed through MIS 5 to investigate if and how variations in Earth's orbital parameters, notably precession, influence ENSO variability in the EEP. In addition, measurements of the bulk δ¹⁸O of seven species of planktonic foraminifera, representing different depth habitats at this site, will be used to characterize the vertical δ¹⁸O_carbonate gradient over MIS 5. This δ¹⁸O_carbonate gradient, predominantly determined by seawater temperature today, will be used to reconstruct the position of the local thermocline. Determining whether or not ENSO variability and the structure of the thermocline changed in connection with orbital variations during the sub-stages of MIS 5 will further our understanding of how low-latitude climate and ocean processes influence global climate.

¹Koutavas and Joanides (2012), Paleoceanography, 27(4).