Southwest Pacific Paleoceanography During the Late Holocene: 3000 years of ocean circulation and marine biogeochemistry reconstructed from New Zealand deep-sea black corals
Authors:   , Hitt Nicholas 
Published: March 2021

<p>The global climate results from interactions between the ocean and atmosphere. Ocean gyres are perhaps one of the most significant interactions; they regulate temperature, salinity and nutrient flow across the ocean basins. Gyres transport warm, tropical waters to higher latitudes and cold waters to lower latitudes and act as the dominant heat-transport mechanism in the Earth’s climate system. They also influence spatial patterns in marine primary production by distributing nutrients between the equator and poles. However, gyre circulation in the subtropics has been strengthening, leading to marine heat waves, changing biogeochemistry and reducing primary production since the early 1900s. These changes are often interpreted as a consequence of anthropogenic climate change. However, ocean circulation and primary production can exhibit natural variations on a variety of timescales. Could these recent changes be a part of a long-term natural cycle or a product of anthropogenic change?
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<b>This research aims to reconstruct South Pacific Gyre (SPG) circulation and biogeochemistry using a suite of New Zealand black corals. The primary research goal is determining if there is a precedent for the ocean changes observed over the instrumental period. Black corals are an ideal paleoceanographic archive for this work; they provide high-resolution, multi-millennial records of biogeochemistry and ocean circulation within their skeletons, derived using radiocarbon (14C) and stable isotopes (d13C and d15N). In this thesis, I show that late Holocene SPG strength has been highly variable and the relationship between circulation and biogeochemistry is timescale dependent.
</b><p>The black coral radiocarbon records suggest late Holocene SPG circulation has been controlled by westerly wind strength. Our records show the SPG exhibits natural variability on multi-centennial and millennial timescales that corresponds to the variability within the Southern Annular Mode (SAM) and the El Niño-Southern Oscillation (ENSO). The black coral circulation record shows that the modern gyre circulation is not without precedent over the last 3000 years.
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<p>The black coral d13C and d15N records show significant variability on multi-decadal to multi-centennial timescales. Multi-centennial variability in black coral d13C and d15N appears to be driven by sea surface temperature (SST), nitrogen fixation rates and wind-driven upwelling and is possibly forced by the mean state of the Southern Oscillation Index and ocean circulation strength. A trend in black coral d13C over the last 1500 years also suggests a shift in phytoplankton community structure towards larger and faster growing phytoplankton. These records also reveal a shift in mean coral d13C and d15N between the 0-2000BP and 2000-3000BP period, the latter corresponding to a period of stronger gyre circulation inferred from the radiocarbon records.
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<p>This work shows that: 1) New Zealand’s black corals are a promising archive for studying paleoceanography; they can extend instrumental ocean records and fill the gap between traditional southwest Pacific paleoceanographic proxy records (tropical corals, sediment cores); 2) SPG circulation has been highly variable over the last 3000 years; circulation is controlled by atmospheric patterns (e.g. SAM) on multi-centennial to millennial timescales; 3) Gyre circulation is only one of many forcing factors on southwest Pacific primary production and marine biogeochemistry; comparisons between the ∆R, d13C and d15N proxies show that variations in SPG biogeochemical patterns and productivity are likely driven by local dynamics such as phytoplankton community structure, SST, upwelling and gyre circulation. Finally, this research demonstrates the key role that a distributed set of deep-sea coral paleoceanographic reconstructions could play in characterizing the dynamical variability in southwest Pacific Ocean circulation, biogeochemistry and primary production. This information is critical for detecting and attributing past and future anthropogenic impacts on the southwest Pacific Ocean.
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