Late Paleocene CO₂ drawdown, climatic cooling and terrestrial denudation in the southwest Pacific

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 confirm 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 13C. In this study we address the cause of this enrichment. For Waipawa organofacies and its bounding facies in the Taylor White section, Hawke's Bay, paired palynofacies and carbon isotope analysis of heavy liquid-separated density fractions indicate that the heaviest δ13C values are associated with degraded phytoclasts (woody plant matter) and that the 13C enrichment may be partly due to lignin degradation. Compound-specific stable carbon isotope analyses of samples from the Taylor White and mid-Waipara (Canterbury) sections display similar trends and further reveal a residual 13C enrichment of ∼ 2.5 ‰ in higher plant biomarkers (long chain n-alkanes and fatty acids) and a ∼ 2 ‰–5 ‰ change in subordinate marine biomarkers. Using the relationship between atmospheric CO2 and C3 plant tissue δ13C values, we determine that the 3 ‰ increase in terrestrial δ13C may represent a ∼ 35 % decrease in atmospheric CO2. Refined age control for Waipawa organofacies indicates that deposition occurred between 59.2 and 58.5 Ma, which coincides with an interval of carbonate dissolution in the deep sea that is associated with a Paleocene oxygen isotope maximum (POIM, 59.7–58.1 Ma) and the onset of the Paleocene carbon isotope maximum (PCIM, 59.3–57.4 Ma). This association suggests that Waipawa deposition occurred during a time of cool climatic conditions and increased 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, due to tectonic factors, 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.