Three Centuries of Southwest Pacific Gyre Biogeochemistry Reconstructed from a New Zealand Black Coral

Abstract:

Oceanic gyre circulation is the dominant heat and nutrient transport mechanism in the global oceans. Gyre circulation has been intensifying globally over the 20^th century, particularly along western boundary currents due to increased westerly wind stress. In the South Pacific Gyre (SPG), this has manifested as a strengthening of the southwest subtropical gyre currents. The strengthening has transported warm, macronutrient-poor subtropical waters further south, which has led to marine heat waves, a change in nutrient distributions and net productivity, and has been shown in some regions to impact marine biodiversity. However, instrumental records of SPG circulation and biogeochemistry have only existed since the early 1900s and natural ocean changes can last centuries. Assessing the impact of 20^th century circulation changes on marine biogeochemistry therefore requires longer records.

Deep-sea black corals faithfully record surface biogeochemistry by incorporating the δ¹³C and δ¹⁵N isotopic signatures of surface exported particulate organic matter. They can live for millennia and can be dated to a decadal resolution using U-Th dating techniques. Here we present a Suess-corrected δ¹³C and δ¹⁵N reconstruction spanning 1684 – 2009 CE derived from an eastern New Zealand Antipathes sp. coral. We show coupled multidecadal δ¹³C and δ¹⁵N variability (R = 0.71; p < 0.01) and a significant negative trend in δ¹³C and δ¹⁵N from 1900 CE to present. This trend corresponds with gyre intensification evident in 20^th century paleo and instrumental data records.

The isotope records suggest an increase in 20^th century bioavailable nitrogen and net primary production. Primary production along eastern New Zealand is supported by upwelled nitrate; thus, the negative isotope trends imply increased upwelling along eastern New Zealand that is concurrent with the 20^th century gyre intensification. SPG intensification would cause warmer waters to penetrate further south around New Zealand. This would inhibit upwelling along eastern New Zealand due to increased vertical stratification which contradicts the trends in our isotope records. Therefore, while instrumental studies suggest the gyre has pushed more warm waters around southern New Zealand, our data suggests it may not have been penetrating as far as southeast New Zealand.