Sourcing the iron in the naturally fertilised bloom around the Kerguelen Plateau: particulate trace metal dynamics

[摘要] The KEOPS2 project aims to elucidate the role of natural Fe fertilisation onbiogeochemical cycles and ecosystem functioning, including quantifying thesources and processes by which iron is delivered in the vicinity of theKerguelen Archipelago, Southern Ocean. The KEOPS2 process study used anupstream high-nutrient, low-chlorophyll (HNLC), deep water (2500 m), reference station to compare with ashallow (500 m), strongly fertilised plateau station and continued theobservations to a downstream, bathymetrically trapped recirculation of thePolar Front where eddies commonly form and persist for hundreds ofkilometres into the Southern Ocean. Over the Kerguelen Plateau, meanparticulate (1–53 μm) Fe and Al concentrations (pFe = 13.4 nM, pAl = 25.2 nM)were more than 20-fold higher than at an offshore(lower-productivity) reference station (pFe = 0.53 nM, pAl = 0.83 nM).In comparison, over the plateau dissolved Fe levels were only elevated by afactor of ~ 2. Over the Kerguelen Plateau, ratios of pMn / pAland pFe / pAl resemble basalt, likely originating from glacial/fluvial inputsinto shallow coastal waters. In downstream, offshore deep-waters, higherpFe / pAl, and pMn / pAl ratios were observed, suggesting loss of lithogenicmaterial accompanied by retention of pFe and pMn. Biological uptake ofdissolved Fe and Mn and conversion into the biogenic particulate fraction oraggregation of particulate metals onto bioaggregates also increased theseratios further in surface waters as the bloom developed within therecirculation structure. While resuspension of shelf sediments is likely tobe one of the important mechanisms of Fe fertilisation over the plateau,fluvial and glacial sources appear to be important to areas downstream ofthe island. Vertical profiles within an offshore recirculation featureassociated with the Polar Front show pFe and pMn levels that were 6-fold and3.5-fold lower, respectively, than over the plateau in surface waters, thoughstill 3.6-fold and 1.7-fold higher respectively than the reference station.Within the recirculation feature, strong depletions of pFe and pMn wereobserved in the remnant winter water (temperature-minimum) layer near 175 m,with higher values above and below this depth. The correspondence betweenthe pFe minima and the winter water temperature minima implies a seasonalcycle is involved in the supply of pFe into the fertilised region. Thisobserved association is indicative of reduced supply in winter, which iscounterintuitive if sediment resuspension and entrainment within the mixedlayer is the primary fertilising mechanism to the downstream recirculationstructure. Therefore, we hypothesise that lateral transport of pFe fromshallow coastal waters is strong in spring, associated with snow melt andincreased runoff due to rainfall, drawdown through summer and reduced supplyin winter when snowfall and freezing conditions predominate in the Kerguelenregion.

[学科分类] 地球化学与岩石