Utilising Deep-Sea Coral Geochemistry and Climate Modelling to Reconstruct Aotearoa New Zealand’s Baseline Ocean Dynamics

The oceans play a major role in modulating Earth’s climate and supporting healthy ecosystems. Globally, the marine environment is experiencing unprecedented changes at an ever-evolving pace. As an island nation with one of the largest Exclusive Economic Zones (EEZ) in the world, Aotearoa New Zealand is heavily reliant on its oceans for the climate and ecosystem services provided. Modern instrumental and satellite data have revealed changes in both physical and biogeochemical processes in the ocean in the form of a ‘spin-up’ of the South Pacific subtropical gyre and declining rates of primary production. But to forecast how these processes might continue to change in the future, records are needed prior to the instrumental and satellite era to understand how they have naturally varied in the past. This thesis examined the natural variability inherent in the water circulation and nutrient dynamics of the ocean around New Zealand over the last millennium through 6 chapters. This was accomplished through high resolution paleoreconstructions from deep-sea black corals and climate modelling. The first chapter covers background information on New Zealand climatology and oceanography with discussions on two modes of natural climate variability, the Southern Annular Mode (SAM) and El Niño Southern Oscillation (ENSO). Deep-sea black corals are presented as paleoarchives of choice for this study due to their global distribution, long lifepans, and ability to be radiometrically dated. The following two black coral proxies are also discussed: radiocarbon (14C) for to reconstruct water circulation and stable nitrogen isotopes (δ15N) for nutrient dynamics. In combination with the paleoreconstructions, two climate models were used and are introduced in this chapter: the New Zealand Earth System Model (NZESM) and University of Victoria Earth System Climate Model (UVic).

The modelling work is presented in Chapter 2 and provides a look at the first attempt to add nitrogen isotope modelling capabilities to the NZESM. The NZESM is used in conjunction with the UVic model to interrogate the impact on ENSO on surface ocean δ15N spatial patterns. This research builds upon work from a previous study which indicated there was a connection between δ15N values and ENSO. The climate model results revealed high variability in δ15N spatial patterns in response to ENSO forcing.

Chapter 3 details radiometrically dating ten new black coral samples from two genera (Leiopathes and Antipathella) using radiocarbon and uranium-thorium disequilibrium methods. Measured coral ages ranged from 71 years (Antipathella fiordensis) up to 2221 years (Order Antipatharia). Radial growth rates were also calculated for the base of the skeletons with ranges from 2.6 (Leiopathes sp.) – 126.7 µm/yr (A. fiordensis). Comparison between growth rate patterns over the corals’ lives also revealed an early growth rate pattern. Five of the ten corals were selected for paleoreconstructions and these had age-depth models constructed for them.

The fourth chapter presents water circulation reconstructions using paired measurements of radiocarbon and uranium-thorium to generate radiocarbon reservoir ages. Reconstructions revealed little variability, and therefore suggest no major changes in current strength or South Pacific subtropical gyre size around New Zealand between 916 – 1859 A.D. However, anomalously young radiocarbon measurements at the outer edge of some of the coral skeletons indicates that post-1950 bomb radiocarbon may have penetrated into the skeleton.

Nitrogen isotopes are revisited in Chapter 5 using bulk and compound specific δ15N of amino acids (CSIA-N-AA) to assess natural variability in nutrient dynamics. Bulk records range from 7.29 to 13.58 covering the period between 411 – 2009 A.D. with some corals showing prominent multi-decadal variability. CSIA-N-AA results reveal high levels of microbial reworking of particulate organic matter the corals ingested and calculated trophic positions of coral skeletons range from 1.41 – 2.83. Two Leiopathes spp. corals from the Bay of Plenty region were collected from the same seamount (~ 0.28 km apart) with lives that overlapped between 1657 to 1775 A.D. allowing for a δ15N replication study. The corals showed different magnitudes and trends in δ15N during their period of overlap, and it is hypothesized this is due to differences in microbial reworking of material ingested by the corals. This indicates that more reproducibility studies are needed to understand how reliable δ15N records from black corals are. Four out of the six coral paleorecords showed some correlation (or anti-correlation) with the ENSO index. These results combined with the modelling work from Chapter 2 give credibility to the idea that ENSO has some effect on the nitrogen isotopes in the southwest Pacific, but that it is not a primary driver as the response is variable by region. The last chapter concludes the thesis with a reflection on the major results and limitations of different portions of the work, while offering suggestions for next steps and future research directions. Finally, collaboration with tangata whenua (people of the land; Māori) partners is presented with details of kanohi ki te kanohi (face-to-face) meetings and informational flyers shared on the project mahi (work). Ngā kohuki (takeaways), created by the wider project team, are presented to assist future research with navigating relationships with tangata whenua.