7.3. High-Nutrient, Low-Chlorophyll (HNLC) Regions
127
A fraction of the redu ced carbo n is removed from communication with
the atmo sphere when it sinks out of surface waters under the influence of
gravity, and is remin eralized in the deep oce an . The stoichiometry of the
reaction represents, on ave rage , the compos ition of the sinking biogenic
material. Ratios of the major ino rganic nutrients in the ocea n interior are
set by the net result of the redox reaction of carbo n fixation and remineralization.
The con stant stoichiometry of marine export production , often called the
Redfield ratio , is used to estimate carbo n fluxes from changes in surfacewater nutrient concentrations (Redfield et al. 1963). In the modem ocean,
the yield of this generalized reaction is thought to be limited by the supply
of N (in the form of N0 3
- ) to the euphotic zone because upwelled N-tonutrient ratios are on average below the Redfield values (Anderson &
Sarmiento 1994; Falkowski 1997). Although the specific causes for variations in atmospheric CO 2 on glacial-interglacial time scales remain unknown, one way to explain the glacial increase of the oce anic carbon inventory and resulting reducti on of atmospheric CO 2 levels is through
increased biological pumping of carbo n (increased exp ort production) to
the deep oce an (Martin 1990).
7.3. High-Nutrient, Low-Chlorophyll (HN LC) Regions
In certain areas of today's ocea n, such as the Southe rn Ocean and the
Equatorial Pacific, the presence of excess surface nutrients and the virtual
abse nce of phytoplankton biomass imply that the biological pump is not
ope rating to its full potential (Martin 1990). These areas are referred to as
high nutrient-low chloro phyll (HNLC) regions. A portion of the decrease in
glacial atmospheric CO 2 could be accounted for if the biological pump
we re able to draw down these excess nutrients and associa ted inorganic
carbon from the HNLC. In fact, organic carbon preserved in the sediments
indicates that export production from HNLC surface waters has covaried
with atmospheric CO 2 levels during the Quatern ary (Berger et al. 1989).
One way to increase the efficiency of the biological pump (expo rt production) would be to enhance local levels of primary produ ction .
The iron hypothesis was advanced in order to address why and how
productivity in HNLC region s co uld have increa sed so remarkably during
the last glaciation. Martin (990) proposed that the redu ction of atmospheric CO 2 during the past glaciation event was caused by increased biological produ ction du e to grea ter aeolian supply of Fe to ocean surface
water . During the last glaciation, Fe fluxes to the ocea n surface were approximately 50 times higher than present-day fluxes, and there is good
correspo ndence between the supply of Fe and atmos phe ric CO 2 inferred
from ice-core records (Petit et al. 1999). The hypothesis assumes that in
certain HNLC areas of the present-day ocean, ph otosynthetic fixation of in-
127
A fraction of the redu ced carbo n is removed from communication with
the atmo sphere when it sinks out of surface waters under the influence of
gravity, and is remin eralized in the deep oce an . The stoichiometry of the
reaction represents, on ave rage , the compos ition of the sinking biogenic
material. Ratios of the major ino rganic nutrients in the ocea n interior are
set by the net result of the redox reaction of carbo n fixation and remineralization.
The con stant stoichiometry of marine export production , often called the
Redfield ratio , is used to estimate carbo n fluxes from changes in surfacewater nutrient concentrations (Redfield et al. 1963). In the modem ocean,
the yield of this generalized reaction is thought to be limited by the supply
of N (in the form of N0 3
- ) to the euphotic zone because upwelled N-tonutrient ratios are on average below the Redfield values (Anderson &
Sarmiento 1994; Falkowski 1997). Although the specific causes for variations in atmospheric CO 2 on glacial-interglacial time scales remain unknown, one way to explain the glacial increase of the oce anic carbon inventory and resulting reducti on of atmospheric CO 2 levels is through
increased biological pumping of carbo n (increased exp ort production) to
the deep oce an (Martin 1990).
7.3. High-Nutrient, Low-Chlorophyll (HN LC) Regions
In certain areas of today's ocea n, such as the Southe rn Ocean and the
Equatorial Pacific, the presence of excess surface nutrients and the virtual
abse nce of phytoplankton biomass imply that the biological pump is not
ope rating to its full potential (Martin 1990). These areas are referred to as
high nutrient-low chloro phyll (HNLC) regions. A portion of the decrease in
glacial atmospheric CO 2 could be accounted for if the biological pump
we re able to draw down these excess nutrients and associa ted inorganic
carbon from the HNLC. In fact, organic carbon preserved in the sediments
indicates that export production from HNLC surface waters has covaried
with atmospheric CO 2 levels during the Quatern ary (Berger et al. 1989).
One way to increase the efficiency of the biological pump (expo rt production) would be to enhance local levels of primary produ ction .
The iron hypothesis was advanced in order to address why and how
productivity in HNLC region s co uld have increa sed so remarkably during
the last glaciation. Martin (990) proposed that the redu ction of atmospheric CO 2 during the past glaciation event was caused by increased biological produ ction du e to grea ter aeolian supply of Fe to ocean surface
water . During the last glaciation, Fe fluxes to the ocea n surface were approximately 50 times higher than present-day fluxes, and there is good
correspo ndence between the supply of Fe and atmos phe ric CO 2 inferred
from ice-core records (Petit et al. 1999). The hypothesis assumes that in
certain HNLC areas of the present-day ocean, ph otosynthetic fixation of in-