Enhanced ventilation of the North Atlantic subtropical gyre thermocline during the last glaciation

Abstract

THE ocean's intermediate^1–6 and deep^7,8 circulation are both known to have differed during the last glaciation from those of today, but little is known about the history of the subtropical gyres. Variations in gyre processes should be closely linked to variations in global climate, as gyre circulation is driven by air–sea interactions and reflects the climate at the ocean surface. The gyres are also a significant reservoir of carbon and nutrients, so that gyre processes affect the distribution of carbon and nutrients in the oceans and thus atmospheric CO₂. Here we use measurements of δ¹³C and δ¹⁸O in foraminifera from the Bahamas to produce a detailed reconstruction of nutrient and temperature profiles in the thermocline during the last glaciation. The thermocline of the glacial North Atlantic subtropical gyre lacked the oxygen minimum that is characteristic of the modern ocean, and was depleted in nutrients, indicating greater, more uniform thermocline ventilation at that time. Thus not only intermediate waters^1–5 but the entire upper water column within the glacial North Atlantic was depleted of nutrients. Moreover, glacial thermocline waters were cooler, had a steeper temperature gradient and a shallower base. Both the ventilation and the thermal structure of the glacial thermocline are consistent with what is known of the glacial climate^9–14.