Geo-Marine Letters, Vol. 5:51-53(1985) GEOq ARINE Letters

© 1985 Springer-Verlag New York lnc,

What Does Percent Organic Carbon in Sediments Measure?

Larry j. Doyle and Robert M. Garrels

bepartment of Marine Science, University of South Florida, 140 Seventh Avenue South, St. Petersburg, FL 33701

Abstract

Weight percent total organic carbon (TOC) is one of the most commonly tl~ed descriptors for both Recent and ancient sediments. It is commonly °nSidered to be approximately proportional to organic carbon accumula- n rates and, therefore, a gauge of those rates. Weight percent TOC is

tit~ Used to judge hydrocarbon source rock potential and primary produc- ~%' of the overlying water column. On the basis of data from both the . eeent and the entire Phanerozoic, we conclude that weight percent TOC 18 an Unreliable measure of the rate of deposition of organic carbon and that arguments and conclusions based on that relationship are suspect.

Introduction

The purpose of this short note is to question the value of the ~ rnmon usage of the measurement of total organic carbon 0C). TOC is most often expressed as a percentage of the

.dry Weight of the sediment. It is commonly assumed that an lrlCrease in the rate of organic carbon deposition is reflected by an increase in percent TOC [1-4]. Axiomatically, through this relationship, percent TOC has been used as a measure of hydrocarbon source rock potential for various deposits [4] arid as a measure of the primary productivity of the overlying ~Vater column [5].

We postulate that percent TOC is an unreliable measure 0f the rate of deposition of organic carbon and that argu- raents and conclusions based on that supposed relationship are SUspect. In the following section, we will provide three ~ Xamples to illustrate our point; one from the Recent, one torn the Tertiary, and one which covers the whole of the

Phanerozoic.

EXamples

Sedimentation rates of organic carbon can be calculated from Weight percent TOC multiplied by total sedimentation rate

at known densities. This step has the advantage of directly determining the parameter used in interpretation and of elim- inating the "masking effect" which has been used to excuse perturbations from the expected positive correlation between percent TOC and its sedimentation rate [4,6]. In Figure 1, the log percent TOC is plotted against the log of the TOC sedimentation rate in cm/1000 years for Recent sediments from different environments. Strictly speaking, the compar- ison should involve a density factor. However, our assump- tion of a constant density should not materialy affect the gen- eral relationships. Several groupings are apparent, as well as obvious contradictions to the concept of positive correlation of percent TOC with TOC accumulation rate. Grouped with low TOC and low TOC accumulation rate are erosional shelves and deep-sea sediments. A second grouping of modern de- positional shelves, slopes, estuaries, and the Black Sea shows no obvious trends between percent TOC and TOC accumu- lation rate. A third group includes unusual sedimentary sys- tems, depositional areas such as offshore Peru, some North Sea fjords, Malaysian Peat deposits, and the Cape Lookout Bight. No obvious correlation is apparent in this group either. A fourth group is made up of the deltas of major rivers. These have the highest accumulation rates of TOC, yet their per- cent TOC is modest in comparison with that in depositional shelves, slopes, and estuarine sediments.

We turn now to an example from the ancient; the Tertiary Monterey Formation of California. Figure 2 is constructed from data provided by Carolyn lssacs of the U.S.G.S., Menlo Park, Califomia. It shows weight percent TOC plotted against accumulation rate TOC in mg/cm2/yr . No positive corre- lation is obvious. In fact if anything, an inverse relationship is the case.

Finally, turning to the picture of the entire Phanerozoic time, we offer an independent method of measuring sedi-

52

ne

0 g /

• PeAr

c~P[

n u ~ c o ~ s r t l l ~ u ~ m e z

-~ f o 1 z LOG TOC SED RATE IN c m / l o 0 0 YRS

Figure 1. tog weight percent TOC plotted against TOC sedimentation rate in cm/lO00 years from many sources [1,9-17], and Harry Roberts, per- sonal communication. Sediment density is assumed to be constant.

1.2

1.0 o 0 I - .8

t -

c9 o -- I ,4

0 . L ~ _L 8 -~7 - - - t5 - f 3 - 12 - r l - 1 , , . - ' .6 - . - 1 4 , . .

LOG TOC ACCUMULATION RATE mg/cm2/y r

Figure 2. Log weight percent TOC plotted against log TOC accumulation rate in mg/cm~/yr for the California Monterey Foundation. (Data provided by Carolyn Issacs, U.S.G.S., Menlo Park, California.)

0 0 I -

t -

1 .2

1 . 0 -

.8

.6

.4

.2--

2

0 % 0

#

• •

• l I I ..... I I

3 4 5 6

ORGANIC C BURIAL RATE

(UNITS OF 101eMOLSIIO6yRS)

Figure 3. Percent TOC plotted against organic carbon burial rate (units of 10 ~" re• l / t0" yr) for the whole of Phanerozoic time. (Data for the ordinate are from Ronov [8]. Data for the abscissa are from Veizer [7].)

GEO-MARINE LETTERS

mentation rate of organic carbon to support our argument- The ~I~C of marine carbonates averaged on a global scale for long time periods can be used to estimate the variation in the total flux of organic carbon to the sediments. As the depOS- ited ratio of is•topically light organic carbon to inorganic carbon increases, isotope mass balance requires that ~~C of inorganic carbon increase. Figure 3, based on data from Veizer [7], shows this relationship for the Phanerozoic plotted against the percent TOC as determined by Ronov [8] for the san-ae time periods. Although there is considerable uncertainty in the actual numbers produced by various methods of model" ing the carbonate-organic carbon system, a pattern of posi- tive correlations" is not obvious.

Discussion and Conclusions

Let us reflect for a moment on the nature of the available data. Numbers of TOC measurements and amounts of ac- companying sedimentary information are disparate among the depositional environments of the deep sea and the continental margins. TOC numbers for red clay and the median deep sea in Figure 1 are based on thousands of DSDP measurements with concomitant well-defined sedimentation rates. Data fror~ continental margins are much more haphazard. Most studies do not include both TOC weight percent measurements arid sedimentation rates. Measurements are often not broadly rep" resentative, but are from specific environments and study areas of limited scope.

Sedimentation rates must also be viewed in the context of location. Rates for the deep sea are probably consistent for long time periods, on the order of 10 3 to 10 6 years. Rates o~ margins are more variable. Accumulation rates for deltas and other areas of rapid sediment influx vary with the period of measurement because of change in input and the effects of rapid sediment build-up on the bottom configuration. There" fore, sedimentation rates calculated over periods of dayS, months, years, and centuries may all be quite different, and numbers can not be extrapolated to longer time periods with confidence. Diagenesis affects TOC measurements dramati" cally with depth and at the surface. Measurements taken at the surface are quite different from the final organic content of a sedimentary deposit.

Consideration of all these factors demonstrates that reli" ance on percent TOC numbers alone is often misleading and that rates of total organic carbon accumulation should be cal- culated. These factors also dictate that sedimentation rate de- terminations must become a routine part of continental mar" gin sediment•logic studies.

References

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VOL. 5, NO. 1, 1985 53

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Manuscript received 18 September 1984; revision received 26 December 1984.