Constraining Tropical Thermocline Structure using Oxygen Isotope Measurements in Planktonic Foraminiferal Tests from Multiple Species

Foraminiferal 18O is a commonly used proxy that reflects temperature and the isotopic composition of seawater. This proxy has been used extensively with tests of a single species to describe the hydrologic conditions at the calcification depth of that species, for example the 18Oc of G. ruber albus describing sea surface conditions. However, by using multiple species of planktonic foraminifera, this proxy can more holistically describe the upper ocean and the thermocline. This idea is limited by how well the calcification depth of those species is constrained; understanding the calcification habitat of those species leads to a better picture of the past thermocline. In this work, we first establish apparent calcification depth (ACD) estimates for five tropical species of planktonic foraminifera (G. ruber albus, T. sacculifer, G. tumida, N. dutertrei, and P. obliquiloculata) using primarily existing 18O data. Using World Ocean Atlas 2013 (WOA13) temperature and salinity data, we predict foraminiferal 18O using different assumptions such as calcifying at a specific position in the local annual mean thermocline. From this, we find that G. tumida calcifies independently of the thermocline, while N. dutertrei and P. obliquiloculata calcify deeper in locations with deep thermoclines. With these results, we can then start to develop methods for reconstructing the upper ocean density structure. Using an exponential functional form for the shape of the thermocline beneath a surface mixed layer, we can fit modern foraminiferal data and find best fitting parameters in this form. Comparing it to the profile predicted from the WOA13 climatologies gives us insight into how these data can constrain plausible thermocline profiles in the past. We validate this method for the modern tropical Pacific and show that it has the potential to reconstruct the tropical thermocline during the past.