Export to the Coastal Zone

The conventional wisdom is that the flux of POC and DOC are each about 0.2 PgC y-1, and DIC is 0.4 PgC y-1 (e.g., Schlesinger and Melack 1981; Degens 1982; Meybeck 1982, 1991; Ittekot 1988; Ittekkot and Laane 1991; Ludwig et al. 1996; Ver at al. 1999). That these analyses converge is not terribly surprising. They are all based on much of the same (very sparse) field data and use variations of the same statistically based interpolation schemes. Let us evaluate these numbers.

Because direct measurements are few, POC flux estimations are typically a product of the flux of total suspended sediments (TSS) and the estimated weight-percent organic C (w%C) associated with the sediment (because the bulk of POC is organic C sorbed to mineral grains). The first problem is an adequate resolution of the TSS flux. Data on TSS are frequently poor and of unknown quality. Many reported data are surface samples, and the depth integrations necessary to accurately characterize sediment flux are on the order of two to three times higher. Additionally, much sediment moves during episodic storm events, when measurements are almost never made.

As summarized by Vorosmarty et al. (2003), estimates of TSS transport to the oceans have ranged from 9 Pg y-1 to more than 58 Pg y-1, with more recent studies converging around 15 to 20 Pg y-1. These estimates are generally based on extrapolations of existing data, which are weighted to the large rivers of passive margins and temperate regions. Milliman and Syvitski (1992) called attention to the much higher yield rates from steep mountainous environments (without directly computing a global total). More recently, Milliman et al. (1999) estimated that the total sediment flux from the East Indies alone (the islands of Borneo, Java, New Guinea, Sulawesi, Sumatra, and Timor), representing about 2 percent of the global land mass, is about 4 Pg y-1, or 20— 25 percent of the current global values. This type of environment (steep relief, draining directly to the oceans) is found elsewhere in the world, so the results are not likely to be unique. New data from Taiwan support these high levels, with isotopic analyses of the C showing that a significant part of the flux is human-driven (Kao and Liu 2002).

To obtain POC flux estimates, these values (and their uncertainties) must be multiplied by w%C. Meybeck (1991) divided particulate carbon into inorganic (PIC) and organic (POC) phases and assumed that high-sediment rivers have very low carbon fractions (0.5 w%C), representative of shale; he essentially does not consider the latter to be "atmospherically derived" and hence discounts it from estimates of fluxes to the ocean. More recent values for w%C tend to be in the 1—2 percent range, and higher for organic-rich systems (Richey et al. 1990; Stallard 1998; Gao et al. 2002).

To account for this range, POC flux can be computed as an ensemble based on different combinations of w%C and TSS fluxes, resulting in a range of 0.3 PgC y-1 to 0.8 PgC y-1, with a "more likely" level of about 0.5 PgC y-1 (depending on assumptions used). Therefore, it is possible that the common estimate of 0.2 PgC y-1 is low and that the overall value lies in the range of 0.2—0.5 PgC y-1. The common estimate for PIC of 0.2 PgC y-1 (Meybeck 1991; Ver et al. 1999) may also be underestimated, if sediment fluxes are higher.

The value of 0.2 PgC y-1 for DOC export may also be low. DOC is also subject to sparse (and questionable) measurements, without the availability of a proxy like TSS for POC. Aitkenhead and McDowell (2000) developed a model of riverine DOC flux as a function of soil C:N. Using this model, they computed a global flux of 0.4 PgC y-1, or twice the common estimate. That is, the total organic C output from fluvial systems may well be approximately double the original estimates, in the ~0.8 PgC y-1 range.

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