Holocene tropical Pacific dynamics revealed by trace elemental variations in mixed-layer foraminifera: signs of changing eastern equatorial Pacific seasonality or upper water column structure?

The surface and subsurface temperatures of the eastern equatorial Pacific (EEP) cold tongue region are closely tied to tropical Pacific dynamics and to the El Niño Southern Oscillation (ENSO). Uncovering the paleoclimate history of the EEP is key to understanding Holocene ENSO change, but reconstructions of EEP variability are often complex and integrate both seasonal and interannual variability. While sub-annually resolved corals may be able to differentiate these components, such records from the EEP are sparse, short, and discontinuous. Ocean sediment records can provide continuous records, but have been limited in their ability to differentiate seasonal and inter-annual variability changes. Here we examine mixed-layer foraminifera to explore whether variations in the trace elemental ratios of closely-related species/morphotypes can address the question of seasonality change and enhance our understanding of the evolution of Holocene ENSO. We analyzed Mg/Ca ratios in EEP foraminifera including the mixed-layer dwelling Globigerinoides ruber (sensu stricto) and the closely related form G. elongatus, which has historically been classified as a morphotype of G. ruber (sensu latu). Evidence suggests G. ruber ss calcification temperatures in the EEP are slightly warmer, commonly interpreted as a shallower calcification environment. However, the geochemical evidence is also consistent with a warm-season bias, which, combined with a potential cold-season preference for G. elongatus / G. ruber sl could provide evidence to reconstruct EEP seasonal change. We find that the Mg/Ca difference between these forms varies through the Holocene, with the smallest difference at mid-Holocene (from 3-6 ka) when reduced tropical Pacific variability is recorded by multiple proxies and simulated in models. We explore whether our data show reduced seasonality or alterations to the upper-water column thermal structure. However, as EEP seasonality and upper-water column structure are both related to tropical Pacific dynamics, the variations observed in our record point toward mid-Holocene alterations to tropical dynamics. These results are put in the context of our current understanding of EEP variability and ENSO changes, and we examine how these insights can enhance our insights into ENSO evolution through the Holocene.