A new method for the paleoreconstruction of the North Atlantic subtropical gyre main thermocline

Abstract

The structure and circulation of the main thermocline of the North Atlantic subtropical gyre is influenced by surface processes in the northeastern Atlantic, where new waters enter the thermocline circulation. Because of this connection between the thermocline and the climate and atmosphere, changes in surface processes will force changes in the thermocline, and vice versa. Detailed thermocline reconstructions are therefore important components of efforts to improve our understanding of the ocean- atmosphere-climate system. Physical properties such as density, salinity, and temperature are conserved in the thermocline circulation, which offers a means of locating the point of surface origin ("outcrop'') of water from a given depth in the thermocline. Outcrop locations of the modern thermocline can be directly determined from hydrographic properties. For thermocline paleoreconstructions, however, a proxy indicator of hydrographic properties is needed. The oxygen stable isotope ratios ([]¹⁸O) of the calcite shells of certain species of benthic and planktonic foraminifera are used as proxy indicators of the physical properties of the water in which the animals lived. However, species-specific biological factors may cause the numerical values of the []¹⁸O of one species to differ from those of another, even when both species lived in the same water mass. In this work, empirical constants are derived which represent the average difference between the []¹⁸O of planktonic (representing source wafers) and benthic (representing depth in the thermocline) foraminifera living in water of the same density (i.e. the same water mass) in the North Atlantic subtropical gyre main thermocline. For the two planktonic species studied in this work, these constants are --0.02[]0.24[] (Globorotalia inflata) and -0.38[]0.18[] (G. truncatulinoides). With some additional work to reduce the variance of these results, it should become possible to match fossil planktonic []¹⁸O data to benthic []¹⁸O values from the same water mass with minimal knowledge of hydrography. This would allow the tracing of waders of ancient thermoclines to their source locations at the ocean surface, yielding a three-dimensional picture of the ancient thermocline and a means of linking past thermocline processes and climate.