Opening the carbon isotope “vital effect” black box 1. Seasonal temperatures in the euphotic zone

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Abstract
Recent laboratory experiments with Orbulina universa show that the carbon and oxygen isotope signals from this symbiont‐bearing planktonic foraminifera record distinct environmental parameters, irradiance and temperature. Since light levels are correlated with depth in the marine environment, the carbon isotopic composition of O. universa can be utilized to differentiate individuals from different depth habitats. By combining δ18O and δ13C data from individuals in an assemblage, estimates of seasonal temperature ranges are possible for the upper and lower euphotic zone of the ocean. Reconstruction of seasonal temperatures using individual Holocene O. universa from western equatorial Pacific Ocean boxcore ERDC‐83 yields ranges of 23.5° to 29.9°C and 20.1° to 24.5°C for the shallow and deep euphotic zone respectively. These values are in excellent agreement with recent temperature measurements from this region. Analyses of individual, late glacial (≈12,000–13,000 years B.P.) O. universa from this site suggest the seasonal upper euphotic zone temperature range was approximately 2.5°C lower than it is today, a function of cooler summers.

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