Hydrocarbons in a new early Paleocene sedimentary section recovered from the Campbell Plateau, south of New Zealand, by IODP Expedition 378

International Ocean Discovery Program (IODP) expedition 378 “South Pacific Paleogene Climate” took place offshore New Zealand from January to February 2020. Expedition 378 recovered the first continuously cored, multiple-hole Paleogene sedimentary section from the southern Campbell Plateau at Site U1553. Coring at Site U1553 reached a maximum depth of 584.3 mbsf and recovered a 581.16 m long sedimentary succession of deep-sea pelagic sediment of Pleistocene and Oligocene to early Paleocene age. The bottom ~100 m of the sediment column contains siliciclastic Unit V, characterized by alternating mudstone, sandy mudstone, and very fine to medium-grained sandstone. The base of the unit was not drilled. Headspace gas analyses for the uppermost 480 m of Site U1553 indicated negligible hydrocarbon concentrations, suggesting the lack of biogenic and/or thermogenic gas production or their upward migration. A sudden increase in methane concentration occurred at the transition from Unit IV to Unit V. The methane increase was accompanied by the detection of thermogenic hydrocarbons (C₂, C₃, and C₄), suggesting in situ methane production, possibly by microbial activity, and upward migration of thermogenic gas. Additionally, the deeper Unit V cores had a strong hydrocarbon odour on the catwalk and after core splitting, and fluoresced under UV light. A sample of the deepest core in Hole D was placed in a glass vial immediately after on-board core splitting and covered with acetone, so as to obtain a signature of these hydrocarbons. This solvent mixture was analysed by gas chromatography-mass spectrometry, which has revealed the presence of a bimodal distribution of n-alkanes (maxima at n-C₁₄ and n-C₂₀). The sample also contains methylalkanes, isoprenoids, alkylcyclohexanes, alkylbenzenes, alkylnaphthalenes, alkylphenanthrenes, other polycyclic aromatic hydrocarbons, and high molecular weight biomarkers including hopanes and steranes. The distribution of these compounds is consistent with a mixed signature, from firstly a mature migrated hydrocarbon phase, and secondly from indigenous immature hydrocarbons from the rock matrix.