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

George, Simon C.1, Ausín, Blanca2, Childress, Laurel B.3, Röhl, Ursula4, Thomas, Deborah J.5, Hollis, Christopher J.6, and the IODP Expedition 378 Science Party

1Department of Earth and Environmental Sciences, Macquarie University, Sydney, Australia 2Department of Geology, University of Salamanca, Spain 3International Ocean Discovery Program, Texas A&M University, USA 4MARUM, University of Bremen, Germany 5College of Geosciences, Texas A&M University, USA 6GNS Science, New Zealand

The Cenozoic era in the South Pacific is poorly known through rather sparse drilling. 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. This high-southern latitude site builds on the legacy of Deep Sea Drilling Project (DSDP) Site 277, a single, partially spot cored hole, providing a unique opportunity to refine and augment existing reconstructions of the past ~66 My of climate history. Multiple cored intervals were likely recovered from the Eocene–Oligocene transition (EOT), the Middle Eocene Climatic Optimum (MECO) and other Eocene Thermal Maximum events, and the Paleocene–Eocene Thermal Maximum (PETM). Expedition 378 also discovered a new expanded Paleocene siliciclastic formation (Unit V) that had never been drilled before, with an unknown basal age.

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 from the Campbell Plateau. The recovered sections comprise five lithostratigraphic units. About 4 m of Pleistocene foraminifer-rich nannofossil ooze (Unit I) overlies an expanded sequence (~200 m thick) of late Oligocene–early Oligocene nannofossil ooze with foraminifers (Unit II). The nannofossil ooze in Unit II gradually transitions into nannofossil chalk in Unit III over 50 m from ~175 to 225 mbsf. Lithification of carbonates continues downcore and results in limestone, categorized as Unit IV. Finally, 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.

Headspace gas analyses for the uppermost 480 m of Site U1553 indicated very low 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.


Biography

Professor Simon George is an organic geochemist and marine geoscientist. He works especially on research areas to do with the geochemical record of the early evolution of life, petroleum geochemistry, marine geoscience, and bioremediation in cold climates.

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