Late Paleocene CO<sub>2</sub> drawdown, climatic cooling, and terrestrial denudation in the southwest Pacific

Abstract

Abstract. Late Paleocene deposition of an organic-rich sedimentary facies on the continental shelf and slope of New Zealand and eastern Australia has been linked to short-lived climatic cooling and terrestrial denudation following sea-level fall. Recent studies have confirmed that the organic matter in this facies, termed Waipawa organofacies, is primarily of terrestrial origin, with a minor marine component. It is also unusually enriched in δ13C. In this study we aim to determine the cause or causes of this enrichment. For Waipawa organofacies and its bounding facies in the Taylor White section, Hawkes Bay, paired palynofacies and δ13C analysis of density fractions indicate that the heaviest δ13C values are associated with degraded phytoclasts (woody plant matter) and that the 13C enrichment is partly due to lignin degradation. Compound specific δ13C analyses of samples from the Taylor White and mid-Waipara (Canterbury) sections confirms this relationship but also reveal a residual 13C enrichment of ~ 2.5 ‰ in higher plant biomarkers (n-alkanes and n-alkanoic acids) and 3–4 ‰ in the subordinate marine component, which we interpret as indicating a significant drawdown of atmospheric CO2. Refined age control for Waipawa organofacies indicates that deposition occurred between 59.2 and 58.4 Ma, which coincides with a Paleocene oxygen isotope maximum (POIM) and the onset of the Paleocene carbon isotope maximum (PCIM). This timing suggests that this depositional event was related to global cooling and carbon burial. This relationship is further supported by published TEX86-based sea surface temperatures that indicate a pronounced regional cooling during deposition. We suggest that reduced greenhouse gas emissions from volcanism and accelerated carbon burial related to several tectonic factors and positive feedbacks resulted in short-lived global cooling, growth of ephemeral ice sheets, and a global fall in sea level. Accompanying erosion and carbonate dissolution in deep sea sediment archives may have hidden the evidence of this "hypothermal" event until now.

Variation in organic matter across the Cretaceous-Paleogene boundary in New Zealand supports the “Living Ocean” model of biotic recovery

“…We attribute this discrepancy to the complex organic matter source changes across the K-Pg boundary. This could include a transient pulse of reworked 13 C-enriched kerogen, similar to what has complicated PETM CIEs (Inglis et al, 2019) or strongly altered terrestrial organic matter as observed in the same section during the Paleocene Carbon Isotope Maximum (or Waipara Event) as documented by Hollis et al (2022). Although it is unclear what the specific source of 13 C-enriched organic matter would have been, the impact of reworked organic matter on the fidelity of the bulk TOC carbon isotopic record has been documented previously (Carmichael et al, 2019).…”