Massive Bloom Fed by Elevated Iron of Possible Hydrothermal Origin in the Pacific Sector of the Southern Ocean

Hydrothermal vent emissions may be an overlooked source of iron fueling primary production in surface waters in the Southern Ocean. Here we report a bloom of unprecedented magnitude and longevity in the otherwise HNLC waters of the ACC in the Pacific sector of the Southern Ocean. The bloom was visible in satellite images for three full months (from December 2013 through February 2014) and covered an area of ~270,000 km². The bloom was marked by depth-integrated Chl a in excess of 300 mg m^-2 with production rates >1000 mg C m^-2 d^-1 resulting in a reduction in pCO2 down to ~200 μatm. The bloom was initially fed by elevated iron that had most likely been entrained in surface waters through deep winter mixing, and the bloom was sustained by a high flux of iron across the ferricline, which balanced the depth-integrated growth rates measured in situ. We propose that the elevated iron (visible in depth profiles down to 2000 m) feeding this bloom is hydrothermal in origin due to the absence of other iron sources in the region as well as the position of the bloom relative to two active hydrothermal vent fields. We posit that deep waters with elevated iron concentrations are brought to the surface through the combined effects of the position of the southern boundary of the ACC (a known upwelling front) relative to the active vent fields, enhanced upwelling due to flow topography interactions as the ACC intersects the Australian Antarctic Ridge, and convective mixing caused by hot emissions from the vent itself. Based on satellite imagery from 1998-2018, we further show that the bloom described here is a recurring feature in the region, visible in both Chl a and NPP climatology images, and we further propose that this is due to the perennial input of iron to surface waters from the active hydrothermal vent fields along the Australian Antarctic Ridge.