Millennial-scale variability in western tropical Atlantic surface ocean hydrography during the early Pliocene

We use high-resolution oxygen isotope data (δ18O) from planktonic foraminifera in the western tropical Atlantic Ocean (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 4.13 to 4.24 Ma and subsampled it to obtain an average time step of ∼800 yr. We reconstruct changes in upper ocean hydrography using the δ18O values of Globigerinoides sacculifer, a mixed-layer dweller, and Neogloboquadrina dutertrei, a thermocline dweller. Their oxygen isotopic difference (Δδ18O) is taken as a measure of the mixed layer to thermocline thermal gradient. Time series analysis indicates that significant concentration of variance exists in the G. sacculifer and N. dutertrei at sub-Milankovitch periods of between ∼8 and 4 kyr, and in the Δδ18O record between ∼13 and 8 kyr. Wavelet analysis illustrates that the suborbital variance is only present in the record when the amplitude of the precessional signal is large between 4.24 and 4.20 Ma. In this particular portion of the record, however, we observe positive δ18O excursions in the individual δ18O time series. Thus, the suborbital periods evident in the spectra may reflect harmonics associated with asymmetrical time series. Because the excursions only occur when precession forcing is also strong, we suggest that there is a relationship between the proxy records and climate, although, we cannot conclude that it is cyclical in nature. The Δδ18O record on the other hand is characterized by positive as well as negative excursions. We observe significant concentration of variance close to half precession during the portion of the record when precession forcing is also strong, which we believe reflects a close, although nonlinear, response of the western tropical surface ocean to low-latitude insolation forcing.