[M-IS06] Evolution and variability of the Tropical Monsoon and Indo-Pacific climate during the Cenozoic Era

[MIS06-03] Orbital-Scale Controls on Eastern Equatorial Pacific Oxygen Minimum Zone Variability During the Early Pleistocene

*Shraddha Tukaram Band¹, Yu-Hsin Huang¹, Arpita Biswas¹, Kuan-yu Jow¹, Haojia Abby Ren³, Chuan-Chou Shen², Jeroen Groeneveld¹ (1.Institute of Oceanography, National Taiwan University, Taipei City, Taiwan ROC, 2.High-precision Mass Spectrometry and Environment Change Laboratory (HISPEC), Department of Geosciences, National Taiwan University, Taipei, Taiwan (ROC), 3.Department of Geosciences, National Taiwan University, Taipei City, Taiwan (R.O.C.))

Keywords:Eastern Equatorial Pacific, Oxygen Minimum Zone, Foraminifera-bound d15N , Obliquity and Precession, early Pleistocene , G. Hexagonus

Early Pleistocene tropical Pacific climate variability has primarily been inferred from sea surface temperature (SST) reconstructions. During glacial-interglacial cycles of the early Pleistocene, SST variability in the tropical Pacific was predominantly influenced by obliquity periodicity (~41 kyr). The strong dependence on high latitude climate variability through ocean-atmosphere tunneling undermines the role of local insolation in driving the biogeochemical changes of the tropical Pacific driven by upwelling.
On geological timescales, variations in the oxygen minimum zone (OMZ) are linked to easterly wind intensity, which is reflected in the expansion and contraction of the cold tongue, as well as to denitrification. Both easterly wind intensity and denitrification exhibit strong relationships with obliquity cycles, aligning with glacial-interglacial variations. In contrast, equatorial upwelling is primarily governed by precession-driven changes in local insolation. Modern observations indicate that the strength of equatorial upwelling can be inferred from the degree of nutrient utilization at the equator. To understand the contribution of these processes in modulating the OMZ in the EEP, we employed a multiproxy approach at IODP Site U1338, spanning the early Pleistocene.
To reconstruct the degree of equatorial upwelling based on nutrient utilization, we developed a foraminifera-bound d15N (FB-d15N) record using the planktonic species Trilobatus sacculifer. Additionally, we used the percentage abundance of the oxygen-sensitive foraminifer species Globorotaloides hexagonus as a proxy for OMZ strength, while the surface-subsurface water column temperature gradient reconstruction served as a proxy for upwelling intensity from the same site. Our record encompasses the first three major glacial-interglacial cycles (Marine Isotope Stages (MIS)96-100) following the onset of Northern Hemisphere glaciation at 2.75 Ma. Our multiproxy analysis reveals that during glacial periods characterized by strong obliquity control, the OMZ intensifies. Similarly, stronger subsurface deoxygenation is observed during low local insolation or precession maxima periods in the Northern Hemisphere. We conclude that subsurface ocean deoxygenation is most pronounced during glacial periods and intervals of low local insolation.