Inferring source regions and
transport pathways of iron in the
Southern Ocean from Satellite
chlorophyll data
IAPSO Symposium, June 2015, Prague
R.M. Graham, A.M. de Boer,
E. van Sebille, K. E. Kohfeld
C. Schlosser
Introduction
• Iron is a limiting
nutrient over large
areas of the Southern
Ocean.
• Changes in the supply
of iron to the Southern
Ocean could have
major impacts on
primary production
and atmospheric
carbon dioxide
concentrations.
Part 1: Where are modern day shelf
sediment iron sources located?
Part 2: Is upwelling or horizontal
advection the more important supply
mechanism of iron at ocean fronts?
Part 1: Where are modern day shelf
sediment iron sources located?
Part 2: Is upwelling or horizontal
advection the more important supply
mechanism of iron at ocean fronts?
Results presented are from:
Graham, de Boer, E. van Sebille, Kohfeld, Schlosser,
Inferring source regions and transport pathways of iron in the Southern
Ocean from Satellite chlorophyll data. Deep Sea Research-I. (in press)
Modern day iron Sources
in the Southern ocean
Iceberg iron source
Dust iron source
Sediment iron source
[Wadley et al., 2014]
Shelf sediment iron source is assumed to
be inversely proportional to ocean depth /
large over all shallow bathymetry.
Iceberg iron source
Dust iron source
Sediment iron source
[Wadley et al., 2014]
Methods
• We infer the location of major iron sources by
identifying where chlorophyll concentrations
are enhanced, making the assumption surface
waters are iron limited.
• We challenge the theory that there is a large
iron flux over all areas of shallow bathymetry.
• Highest
chlorophyll
concentrations
upon continental
shelf.
 Shelf sediment
iron supply?
• Chlorophyll
concentrations peak
along coastal
margins. Not uniform
over the entire
continental shelf.
 Shelf sediment iron
source is located along
coastlines, rather than
spread over entire
shelf?
• Chlorophyll
concentrations
enhanced around
islands, but not
seamounts.
 Shelf sediment
iron source is only
active near coastal
margins, not all
shallow bathymetry?
COASTAL
Chlorophyll concentration
VS distance from coastline
40S – 60S
< 1000 m water depth
< 500 km to a coastline
Red = > 5 grid points
• All mean annual chlorophyll concentrations >
2mg/m3 occur within 50 km of a coastline.
OPEN OCEAN
Chlorophyll concentration
VS ocean depth
40S – 60S
< 2000 m water depth
> 500 km to a coastline
Red = > 5 grid points
• Chlorophyll concentrations not visibly enhanced
over shallow bathymetry located more than 500
km from a coastline.
Part 1: Summary
• Little evidence from satellite chlorophyll data
of a large shelf sediment iron flux from all
areas of shallow bathymetry.
• Instead, shelf sediment iron flux is located
around coastal margins.
• Parameterisation of shelf sediment iron flux in
biogeochemical models should be revised.
Part 2:
Horizontal advection
VS upwelling
• Upwelling at ocean fronts is thought to deliver
iron to the ocean surface, leading to enhanced
productivity.
• Upwelling where fronts pass over bathymetric
features is thought to be particularly
important.
• Mean annual chlorophyll concentrations are low
in regions where model upwelling velocities are
large, such as Drake Passage.
• Crozet Islands
• South Georgia
• Chlorophyll concentrations are enhanced at
ocean fronts downstream of islands.
• Chlorophyll concentrations
enhanced along contours of
sea surface height
extending off the
continental shelf.
• Iron from coastal regions is
entrained into boundary
currents and advected into
the Sub-Antarctic Zone
along the Subtropical front.
Summary
• Major shelf sediment iron source located along
coastal margins… NOT all shallow bathymetry.
• Western boundary currents and ocean fronts
advect iron long distances into the open ocean
from source regions along continental and
island coastlines.
Contact Rob: [email protected]
Graham, de Boer, E. van Sebille, Kohfeld, Schlosser,
Inferring source regions and transport pathways of iron in the Southern
Ocean from Satellite chlorophyll data. Deep Sea Research-I. (in press)