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.

EqPac 1992 Figures

MonthShipDepth RangeLatitudeLongitudeStationsTOC
FebruaryThompson0 - 600m12°N - 12°S140°WTransect-1Section-1
AprilBaldrige0 - 600m10°S - 10°N140°WTransect-2Section-2
AugustThompson0 - 600m12°N - 12°S140°WTransect-3Section-3
SeptemberDiscoverer0 - 600m10°N - 10°S140°WTransect-4Section-4
SeptemberDiscoverer0 - 600m10°S - 10°N125°WTransect-5Section-5
OctoberDiscoverer0 - 600m10°N - 10°S110°WTransect-6Section-6
NovemberDiscoverer0 - 600m14°S - 3°N95°WTransect-7Section-7
NovemberDiscoverer0 - 600m5°S & 13°SCoastalCoastalNA
Aug - NovBoth0 - 600mEquator140°W - 95°WNAEq-Section


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


This page was last updated on 21 April 1999 by Edward Peltzer, MBARI.