INVESTIGATING THERMOCLINE CONTROL ON ENSO IN THE HOLOCENE USING FORAMINIFERA TRACE ELEMENT GEOCHEMISTRY

The El Niño Southern Oscillation (ENSO) is an important climate phenomenon strongly expressed in the Eastern Equatorial Pacific (EEP). ENSO events alter temperature, precipitation, and weather patterns across the globe. However, the mechanisms that control tropical variability and by extension ENSO are not well understood, especially on centennial to millennial timescales. Here we present data from surface and subsurface thermal reconstructions of the EEP over the course of the climatically relevant Holocene, to investigate how tropical variability relates to changes in the region’s thermocline. We analyzed trace elemental ratios in the subsurface foraminifera species Neogloboquadrina dutertrei, which lives within the thermocline at the deep chlorophyll maximum, to reconstruct subsurface temperature. In the subsurface we see an average temperature of 17.3 °C, with multiple fluctuations between 19 °C and 15 °C occurring during the mid-Holocene from 5 to 5.9 kya. Coupling this data with foraminiferal temperature records of the upper water column, we can assess changes in the water column through the last ~10,000 years. Differences are recognized in the EEP surface-subsurface temperature gradient, with a minimum occurring in the mid-Holocene at 5.9 kya and a maximum at 8.3 kya. We investigate the implications of changes in subsurface data and the surface-subsurface gradient during this period. However, apparent interval to interval variability suggests possible non-thermal influence on N. dutertrei. To clarify this, we compare data from N. dutertrei with trace elements from another subsurface foraminifera species Globorotalia tumida. We place our results in the context of existing records of ENSO variability to determine how subsurface changes align with changes in ENSO and investigate possible mechanisms of thermocline control.