Millennial-scale Iron Flux and Ocean Circulation Change Affects
Subantarctic Pacific Carbon Cycling During Marine Isotope Stage 3 (57-29
ka)
Abstract
During Marine Isotope Stage 3 (MIS-3; 57–29 ka) Antarctic ice cores
reveal a glacial climate state punctuated by millennial-scale warming
events and pulses of CO2. Changes in iron-fertilised
export production and ocean circulation/upwelling, interpreted from
South Atlantic sediment cores, suggest that the Southern Ocean is a
conduit for the storage and release of CO2 from the deep
ocean. However, it is unclear whether this occurs throughout the
Southern Ocean as these processes have not previously been investigated
in the southwest Pacific . Here we describe localised iron limitation
linked to glaciation changes in New Zealand, which reduced export
production during early MIS-3 (60–48 ka) and caused decreases/increases
in export production during late MIS-3 (48–29 ka) millennial-scale
warming/cooling. Consistent decreases in foraminifera-bound
δ15N during all MIS-3 warming events may reflect
changes in the supply of nitrate to the subantarctic Pacific, possibly
from increased wind-driven upwelling in the Antarctic and northward
eddy-driven transport and/or shifting SO fronts. Concomitant decreases
in bottom water oxygen and increases in the 14C age of
deep waters suggest that old, nutrient-rich waters influenced upper
circumpolar deep water in the southwest Pacific during warming events.
This signature may reflect an expansion of Pacific Deep Water into the
Southern Ocean as Southern Ocean overturning strengthens.
Iron-limitation of export production, the expansion of Pacific Deep
water, and increased wind-driven upwelling would all work to contribute
to increasing atmospheric CO2 through reduced drawdown,
and increased outgassing from the Pacific carbon reservoir during the
millennial-scale warming events of MIS-3.