Abstract
Geological paleoclimate reconstructions can be used to reduce the uncertainty of the Equilibrium Climate Sensitivity (ECS) which describes the amount of warming Earth will experience in response to a doubling of PCO2. Current IPCC assessments indicate the ECS is “likely to lie in the range of 2 °C to 4.5 °C, with a most likely value of about 3 °C”. Paleoclimate reconstructions of the Eocene Climatic Optimum may allow us to calibrate the ECS for very high atmospheric CO2 conditions (around 600 - 1000 ppm) and mean annual surface air temperature of 6-8 °C warmer than today. The Orepuki-1 (ORE-1) sedimentary drill core, recovered from Orepuki, Southland, provides an opportunity for an Eocene paleoclimate reconstruction, as it comprises the basal Eocene-age fluvio-deltaic Beaumont Coal Formation and overlying lacustrine Orauea Mudstone, potentially providing a window into the E-O transition and Southland, New Zealand’s climate state. This study aimed to generate the first biostratigraphically constrained magnetic polarity stratigraphy for Eocene-aged South Island sediments, and to reconstruct surface temperatures using organic geochemistry.
Here we present results of a paleomagnetic study which aimed to provide a chronostratigraphic framework for a paleoclimate reconstruction of Southland, New Zealand during the Eocene, using the 200 m long ORE-1 sedimentary drill core. Sedimentary logging identified the Beaumont Coal Formation as extending from 147.0 m to 198.50 m, and the Orauea Mudstone from 97.28 m to 147.0 m, with a conformable contact. An unconformity at 97.28 m overlying sequence was inferred to be the Pliocene-aged fluvial deposits of the Orepuki Formation, which extended from 97.28 m to 40.40 m, overlain by Quaternary age fluvial and beach terrace deposits from 40.40 m to the top of the core at 30.00 m, however there were no biostratigraphic indicators to confirm this. A total of 336 samples of the ORE-1 core were demagnetised thermally to generate a magnetic polarity stratigraphy for the ORE-1 core, which was dominated by normal polarity with very few reversed polarity intervals. The late Eocene to early Oligocene portion of the magnetic polarity timescale is dominated by reversed polarity chronozones. Hysteresis loops and IRM curves were generated for 110 samples, and thermomagnetic analyses were undertaken for nine samples throughout the ORE-1 core and magnetite was found to be the main remanence carrier. Above 140 m the ORE-1 sediments appear to have a stable magnetisation which is carried by detrital magnetite, however, after 140 m the magnetisation is unstable in places, likely due to diagenetic alteration of magnetite to greigite, so correlation is tenuous.
Sporomorphs examined from the basal Beaumont formation place these sediments in the Nothofagidites matauraensis Assemblage Zone, suggesting a latest Eocene (Runangan) age, however the
most feasible age model correlation places the lower 97.28 m - 199 m of the ORE-1 core within the Kaiatan Stage of the middle Eocene because of the thick normal polarity intervals and high expected sedimentation rate. This suggests that either (i) these sediments have been remagnetised post-deposition, (ii) the sediments were deposited rapidly during a normal polarity interval and the thin reversed polarity intervals are previously unrecognised short polarity changes, known as cryptochrons, or (iii) the sediments were deposited during the Kaiatan and there is a need for the calibration of Eocene palynological zones to be reassessed for the South Island of New Zealand. The Pliocene-aged sediments in the upper section of the ORE-1 core produced a reliable magnetic polarity stratigraphy but could not be correlated with the GPTS due to a lack of biostratigraphic indicators.
This study demonstrates that the construction of a magnetostratigraphy for Paleogene terrestrial sediments using a minerals industry drill core is not straightforward due to a combination of factors, but it is possible. I suggest that a locally calibrated biostratigraphy for the South Island is crucial for calibration with an Eocene/Oligocene magnetostratigraphy and there is a need to find new age diagnostic species with shorter age range than what is currently available. This study was partially interrupted by the Covid-19 lockdown in 2020 and 2021, which restricted access to laboratories, and delayed the establishment of the organic geochemical facility at the University of Otago Department of Geology. Ten ORE-1 samples were processed for biomarker geochemical analysis, which will be undertaken in November 2021 by Dr. Catherine Beltran.