Temporal Variations of Bomb-Produced Radiocarbon Inventory
in the Pacific Ocean
Principal Investigator:
Tsung-Hung Peng
Collaborating scientists:
Robert Key, Princeton University
Objective:
To obtain the temporal variations of bomb-produced radiocarbon inventory
in the Pacific Ocean for the purpose of providing constraints in calibrating
the three-dimensional global ocean carbon cycle models before such models
can be properly used for predicting the oceanic CO2 uptake.
Rationale:
Because of the complexity and dynamic nature of the oceanic carbon
reservoir, realistic models of the ocean carbon cycle on the basis of
three dimensional ocean general circulation models (OGCM) are essential
tools for gaining an understanding of the CO2 system in the ocean and
its interaction with the atmosphere. Before these models can be used to
determine the CO2 holding capacity of the ocean and the net flux of
oceanic uptake of anthropogenic CO2, they have to be calibrated
with the distribution of geochemical tracers, such as radiocarbon, in the
ocean. The global distribution of bomb 14C in the ocean has
been summarized on
the basis of radiocarbon measurements made during GEOSECS, TTO, and SAVE
ocean survey programs. To obtain the temporal variations of the
distribution of bomb-produced 14C, we need to analyze the radiocarbon
measurements made during the recent WOCE program. Both spatial and
temporal distributions of the bomb radiocarbon are considered as the
most powerful constraints in calibrating and adjusting the architecture
of OGCMs.
Method:
The natural and anthropogenic components of the radiocarbon measurements
from seawater samples can be separated by an improved method, which is
based on a very well-defined relationship between natural radiocarbon
and dissolved silica observed mainly during the GEOSECS survey for
waters beneath 1000 m depth. This relationship is further reconfirmed
by the 14C measurements from large volume samples taken in the deep
waters in the Pacific Ocean during the recent WOCE survey program. The
anthropogenic 14C in the water column is then integrated to
obtain the bomb 14C inventory. For this analysis, results
from upper ocean 14C measurements made along 152o
W, and north of 20o N, in the northeastern Pacific Ocean
during the NOAA's CGC91 cruise are used. More analysis are expected when
more WOCE 14C results become available in the future.
Accomplishment:
Results of analyzing CGC91 radiocarbon data indicate that the bomb 14C
inventory in this part of the ocean has increased by 22% since the
GEOSECS measurements made in 1974. This increase is consistent with the
box-model prediction of 25% for the northern hemisphere ocean. Change
of the surface water bomb 14C values during this period is
insignificant. This feature is also consistent with the model
simulation.
Key reference:
Peng, T.-H., R. Key, and H. G. "Ostlund, Temporal
variations of bomb radiocarbon in the Pacific Ocean.
Marine Chemistry. (in press).
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