Survey Measurement of Total Organic Carbon
in the Equatorial Pacific Ocean
Introduction
Measurements of total organic carbon (TOC) were made from 12°N to
12°S along 140°W aboard the R/V Thomas Thompson during
the EqPac Spring Survey cruise (Feb-Mar) and the Fall Survey cruise (Aug-Sep)
of 1992. In addition, samples for shore-based analysis were collected from
10°N to 10°S along 140°W in April of 1992 aboard the
NOAAS Malcolm Baldrige and from 10°N to 10°S along
140°W, 125°W, 110°W and 95°W (14°S to 3°N)
during September through December of 1992 aboard the NOAAS
Discoverer. All samples were analyzed by direct injection into a
custom built high temperature combustion TOC analyzer.
Analytical
The measurement of organic carbon is a problem that has long challenged
marine organic geochemists. Shipboard measurements have proven especially
difficult owing to the motion sensitivity of the CO2 detector. Recent
advances in both the analyzer design and detector stability now make the
at-sea determination of DOC possible. The Li-Cor solid-state infra-red CO2
detector and a computer based data collection and peak integration program
allowed for instrument operation in varying sea-states without deterioration
of signal nor loss of analytical precision. A description of this
instrument, noting improvements over the Suzuki design, can be found in
Peltzer and Brewer (1993) and
Peltzer, et al. (1996).
This new hardware solved only some of the problems. Operating, quality
control and quality assurance protocols were also developed during the
US-JGOFS Equatorial Pacific Ocean cruises in 1992 to measure and correct for
the instrument blank; to determine instrument response factors on a regular
basis; and to eliminate routine inadvertent contamination. As a result of
these protocols, typical relative standard deviation of replicate analyses
was ± 2%. The instrument response factor was measured twice daily using
three reference compounds in seawater: glucose, potassium hydrogen phthalate,
and glucosamine. Typically, the difference between these response factors
varied by < ± 5% from the mean of all calibrations. The instrument
blank was measured frequently using carbon-free distilled water, every 6-10
samples. Initially, the blank was large (> 25 µMC), but decreased
rapidly. After several days of use, the instrument blank "stabilized"
between 12-15 µMC. All TOC concentrations reported are corrected for
this instrument blank. Due to the higher precision and stability of this
instrument, seasonal and spatial variations in mixed-layer TOC concentrations
can now be observed.
Because none of these samples were filtered and in order to be consistent
with the older literature, it is more proper to call them total organic
carbon as opposed to dissolved organic carbon (DOC). However, given the very
low surface particulate organic carbon (POC) concentrations, it is apparent
that > 90-95% of the TOC is DOC at the surface. In the deep water, the
fraction of the TOC that is POC is even smaller.
Results
The DOC concentrations observed were similar to those reported by earlier
investigators using the traditional wet chemical techniques of persulfate
oxidation or UV photolysis. The recently reported higher DOC concentrations
were not observed. However, due to the higher precision and accuracy of the
current method, statistically significant changes in DOC concentration in
response to physical forcing, or as a result of seasonal variations in
productivity were observed. Mixed layer TOC varied with latitude from about
80 µMC (observed at both 12°N and 12°S) to a minimum of ~60-65
µMC at the equator. Deep water (1000m) concentrations showed little
variation across the transect. A mean concentration of 38 ± 2 µMC
was observed for all the samples obtained from this depth. Moreover,
repeated deep-water measurements several months apart at the same location
are within 1-2 µMC demonstrating the long-term stability of the
instrument and the consistency of the calibration protocols.
Seasonal variations in mixed-layer DOC concentration are clearly visible
in the four transects along 140°W (see the figures below). Variations
in both the TOC concentration in the upwelling zone and the north-south
location of the upwelling zone were observed. Part of these changes were a
reflection of changes in physical forcing due to the presence of an El
Niño - Southern Oscillation (ENSO) event early in 1992 and a return to
more climatologically normal conditions later in the year. Also visible in these
sections, and especially in the east-west equatorial section, is the shoaling
of the upwelling zone as one proceeds from 140°W to 95°W.
Estimates of the rate of production of TOC in the upper ocean along
140°W have been calculated from simple box models (Peltzer and Hayward,
in press). At the equator, TOC export fluxes averaged 8 ± 4 mmol C /
m² · day and also varied seasonally. Off the axis of the equatorial
upwelling zone considerable variation was observed in the TOC production
flux. North of the equator (between 5°N and 9°N), the TOC export
flux was small, varing between -1 to 5 mmol C / m² · day. South of
the equator (between 5°S and 9°S), the TOC export flux was nearly
as large as in the central axis of the upwelling zone, averaging 6 ±
3 mmol C / m² · day. These export fluxes account for 50-75% of the
total carbon deficit and are consistent with other estimates and model predictions.
Figures
Vertical sections of TOC contours for each of the transects are available.
Select the section you wish to preview from the table below. These sections
have been converted to "gif" format [~20KB] to reduce the size of the files
and to decrease access times. If you desire a full resolution post-script
file [~2MB] of the transect, send your request via email to:
etp3@mbari.org.
On-line Data Access
The TOC data for each of the transects is available on-line:
Acknowledgements
This research was supported by National Science Foundation grants OCE 91-15201
and OCE 92-03953 with funds provided by the Office of Global Programs,
National Oceanic and Atmospheric Administration. We thank program managers
Neil Anderson and Jim Todd for their guidance and encouragement. We gratefully
acknowledge the captains, crew and technicians of the R/V Thomas G. Thompson,
NOAAS Malcolm Baldrige and NOAAS Discoverer for their help
and support. We also thank chief scientists Jim Murray, Dick Barber and Dick
Feely for cruise planning and leadership. Finally, we would like to thank
the younger members of the scientific party and recently experienced
trans-hemispherical travelers for their highly imaginative and entertaining
speculation regarding the origin of the deepwater TOC anomalies.
References
- Peltzer, E. T. and P. G. Brewer (1993). Some practical aspects of
measuring DOC: sampling artifacts and analytical problems with marine
samples. Marine Chemistry 41: 243-252. (Abstract)
- Peltzer, E. T., B. Fry, P. H. Doering, J. H. McKenna, B. Norrman and U.
L. Zweifel (1996). A comparison of methods for the measurement of
dissolved organic carbon in natural waters. Marine
Chemistry 54: 85-96.
(Abstract)
- Peltzer, E. T. and N. A. Hayward (1996). Spatial and temporal
variability of total organic carbon along 140°W in the equatorial Pacific
ocean in 1992. Deep-Sea Research II 43: 1155-1180.
(Abstract with
figures)
This page was last updated on 21 April 1999 by Edward Peltzer, MBARI.
