Millennial-Scale Variability of Western Equatorial Atlantic Ocean Thermocline Depth: Evidence From the Early Pliocene

We use high-resolution oxygen isotope data from planktonic foraminifera in the Western Equatorial Atlantic (Ocean Drilling Program Leg 154, Site 925) to investigate millennial-scale climate variability during an interval of relative climate warmth, the early Pliocene. For this purpose, we have chosen a 100 kyr long time interval from 4.1 Ma to 4.2 Ma and sub-sampled it to obtain an average time step of 700 yr. We reconstruct changes in mixed layer depth using the difference in oxygen isotope values (Δ δ ¹⁸O) between Globigerinoides sacculifer, a mixed-layer dweller, and Neogloboquadrina dutertrei, a thermocline dweller. Time series analysis of the Δ δ ¹⁸O records indicates that temporal changes in mixed layer depth occur at sub-Milankovitch periods of between 10-5 kyr. Similar variability during this interval of time has been observed in lacustrine sediments from the Mediterranean region (Steenbrink et al., 2003). Our results suggest that millennial-scale variability may be a pervasive climate signal during this interval of relative global warmth. As mixed layer depth in the western tropical Atlantic is a function of trade wind strength, we believe that millennial-scale variability in mixed layer depth during the early Pliocene is also tied to changes in wind patterns. We speculate that the millennial-scale changes in mixed layer depth at site 925 reflect millennial-scale variability in trade wind strength and direction perhaps linked to the interaction between low latitude insolation and the North African monsoon.