“EARLY PALEOGENE ASTROCHRONOLOGY FROM THE AGULHAS PLATEAU AND MENTELLE BASIN RECORDS (IODP EXPEDITIONS 392 AND 369): ORBITAL CALIBRATION FROM THE K-Pg – PETM INTERVAL”

Abstract

The present master's dissertation aimed at conducting a detailed cyclostratigraphic study of gamma-ray logging series (total gamma and thorium, potassium, and uranium spectral channels) from two deep-marine Paleocene sequences spanning from the Cretaceous-Paleogene boundary (~66 Ma) to the Paleocene-Eocene Thermal Maximum (~56 Ma), an interval encompassing the post-extinction biotic recovery, the progressive destabilisation of the global carbon cycle, and a series of orbitally paced hyperthermal events that culminated in the largest rapid warming event of the Cenozoic, recovered during Expeditions 392 (Hole U1579D, Agulhas Plateau, southern Indian Ocean) and 369 (Hole U1514C, Mentelle Basin, offshore western Australia). Preliminary cross-correlation tests between the gamma ray component data series indicated a strong zero-lag correlation of the thorium channel with the total gamma-ray series (|r| = 0.784 and 0.917; holes U1579D and U1514C, respectively) and with the potassium series (|r| = 0.737 and 0.893, respectively), alongside phase offsets and reduced coherence consistent with post-depositional diagenetic remobilisation for the uranium channel, supporting the selection of the thorium series as the most suitable proxy for cyclostratigraphic analysis in these pelagic carbonate sequences. Multi-taper method spectral analyses identified significant spectral peaks exceeding the 99% confidence level at both holes. Long eccentricity (E405) peaks were detected at 7.39, 5.46, and 5.07 m at U1579D, and at 7.04, 6.29, 4.45, and 4.29 m at U1514C. Short eccentricity (e131, e125, e99, e95) peaks were identified at 2.80, 2.57, 2.40, and 2.03 m at U1579D, and at 1.34 and 1.28 m at U1514C. These periodicities are consistent with the predicted spectral ratio for ~61 Ma (405:131:125:99:95:39 = 21.6:6.7:5.1:2.1:1.2:1; Laskar et al., 2004). In conjunction with evolutionary harmonic analysis of the series, it was possible to delimit the frequency band associated with the long eccentricity signal, with a central wavelength ratio of ~5 to 7 m (band limits of ~4.55-8.40 m at U1579D; ~4.24-7.19 m at U1514C). COCO, eCOCO, and TimeOpt analyses corroborate sedimentary accumulation rate estimates of ~1.1-2.1 cm/kyr for U1579D (main peak ~1.5-1.9 cm/kyr; TimeOpt ~2.12 cm/kyr) and ~1.0-1.8 cm/kyr for U1514C (TimeOpt ~1.62 cm/kyr). Short eccentricity cycles are identified but exhibit intermittent frequency on evolutionary harmonic analysis. Obliquity (~39 kyr) is preliminarily identified at U1579D; precession is unresolvable at the 15.2 cm sampling resolution. Taken together, these results allow the construction of astronomical time scales anchored at the K-Pg boundary and calibrated by biostratigraphic datums, recording approximately 25.5 and 25 long eccentricity cycles at U1579D and U1514C, respectively, consistent with the astrochronological framework of 25 cycles for the Paleocene and a PETM age of ~ 55.7-55.9 Ma. A prominent positive thorium anomaly at both holes is constrained to ~58.15 Ma, consistent with the unnamed "*" carbon isotope excursion marking the termination of the Paleocene Oxygen Isotope Maximum. This excursion occurs close to an eccentricity minimum and is not considered a classical hyperthermal; its expression as a positive thorium anomaly at both holes argues for a globally coherent sedimentary signal consistent with carbonate dilution or enhanced clay flux during this global carbon-cycle transition. The stronger orbital signal preservation at U1579D relative to U1514C is attributed to the more homogeneous Nannofossil Chalk lithology at the Agulhas Plateau versus the greater lithological heterogeneity of the Mentelle Basin. Integrated Prediction Error Filter Analysis curves document trend reversals consistent with depositional regime changes at the Cretaceous-Paleogene boundary and near the Early Late Paleocene Event /"*" carbon isotope excursion interval, broadly consistent with the Paleocene barystatic sea level, and possibly providing a wireline-based trend indicator of Paleocene depositional dynamics at the Indian Ocean.