The role of effective discharge in the ocean delivery of particulate organic carbon by small, mountainous river systems

Abstract

Recent research has shown that small, mountainous river systems (SMRS) account for a significant fraction of the global flux of sediment and particulate organic carbon (POC) to the ocean. The enormous number of SMRS precludes intensive studies of the sort conducted on large systems, necessitating development of a conceptual framework that permits cross-system comparison and scaling up. Herein, we introduce the geomorphic concept of effective discharge to the problem of source-to-sink POC transport. This idea recognizes that transport effectiveness is the product of discharge frequency and magnitude, wherein the latter is quantified as a power-law relationship between discharge and load (the 'rating curve'). An analytical solution for effective discharge (Q e ) identifies two key variables: the standard deviation of the natural logarithm of discharge (s q ), and the rating exponent of constituent i (b i ). Data from selected SMRS are used to show that for a given river Q e -POC , Q esediment, Q e for different POC constituents (e.g., POC fossil vs. POC modern ) differs in predictable ways, and Q e for a particular constituent can vary seasonally. When coupled with the idea that discharge peaks of small rivers may be coincident with specific oceanic conditions (e.g., large waves, wind from a certain direction) that determine dispersal and burial, these findings have potentially important implications for POC fate on continental margins. Future studies of POC transport in SMRS should exploit the conceptual framework provided herein and seek to identify how constituent-specific effective discharges vary between rivers and respond to perturbations.

Runoff-driven export of particulate organic carbon from soil in temperate forested uplands

“…The larger exponent for tPOC (b = 1.33) compared to SS (b = 1.19) means that 328 relatively more POC is exported at higher discharges than SS, in contrast to the 329 relationships seen in the Waipaoa River (New Zealand) and Alsea River (Oregon) 330 (Wheatcroft et al, 2010). The effect is even more pronounced for nfPOC (b = 1.45) 331 than for tPOC.…”

“…Because distinct pools of organic carbon behave differently, shown by the changing 310 composition of POC at different discharges, their long-term export should be 311 considered independently (Wheatcroft et al, 2010). 312…”

“…c Q e is the effective discharge, as defined by Wheatcroft et al (2010). Q/Q mean is the discharge relative to the average discharge over the period 1983-2011 inclusive (38.6 l s -1 ).…”

“…The exponent for fPOC (b = 1.08) is within error of that for SS, 332 reflecting their shared clastic origin. Differences in the rating curve exponents are 333 mirrored by those in effective discharge (Q e ), the discharge that, on average, 334 transports the largest proportion of a given constituent load (Andrews, 1980;Nash, 335 1994;Wheatcroft et al, 2010). Q e is greatest for nfPOC (corresponding to Q/Q mean of 336 13.4), and lowest for fPOC (5.6).…”

Chemical characteristics of particulate organic matter from a small, mountainous river system in the Oregon Coast Range, USA

“…Notably, the mineral material supplied by SMRS may enhance the sequestration of autochthonous POM in depositional centers (deltas/ shelves) by producing rapid burial rates and abundant mineral surfaces, both of which can minimize postdepositional degradation (e.g., Hartnett et al 1998;Mayer 1994). These effects may be most significant in SMRS because they are characterized by high temporal coherence between elevated discharges and energetic ocean conditions that favor rapid deposition and efficient burial (e.g., Wheatcroft et al 2010).…”