Source-to-sink sedimentary systems and global carbon burial: A river runs through it

    Source to sink sedimentary systems and the global C-cycle: A river runs through it Elana L. Leithold, Neal E. Blair, Karl W. Wegmann PII: DOI: Reference:

S0012-8252(15)30057-X doi: 10.1016/j.earscirev.2015.10.011 EARTH 2183

To appear in:

Earth Science Reviews

Received date: Revised date: Accepted date:

8 October 2014 22 September 2015 27 October 2015

Please cite this article as: Leithold, Elana L., Blair, Neal E., Wegmann, Karl W., Source to sink sedimentary systems and the global C-cycle: A river runs through it, Earth Science Reviews (2015), doi: 10.1016/j.earscirev.2015.10.011

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Source to Sink Sedimentary Systems and the Global C-Cycle: A River Runs Through It

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Elana L. Leithold, Neal E. Blair, and Karl W. Wegmann Abstract

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Source to sink sedimentary systems serve a key function in the global carbon cycle and are the primary

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locations for organic carbon burial. The age and character of the carbon that is buried at the terminal

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ends of these systems reflects the sources and transformations of the organic carbon (OC) throughout

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their linked terrestrial and marine segments. Profound differences are observed between large passive

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and small active margin systems. Large passive margin systems are characterized by large floodplains

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and relatively broad shelves where OC has protracted exposure to oxidants. Rapid burial in prograding,

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subaqeous deltaic clinoforms or bypass to submarine fans, however, leads to high burial efficiency of

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terrestrial biospheric OC in some passive margin settings. The OC in small active margin systems, in

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contrast, follows relatively short pathways from headwaters to seabed. This rapid transit, facilitated by

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the important role of storm-driven transport in such settings, leads to high OC burial efficiencies. The

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study of OC sources and transformations in contemporaneous source to sink sedimentary systems

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informs interpretations about the systems in which OC was buried in the geologic past, their

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stratigraphic records of environmental change, and their potential to produce petroleum resources.

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Introduction

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The sediment routing or “source to sink” systems that extend from mountainous environments, across

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the continental margins, and to the deep sea are the primary locations for the erosional and

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depositional processes that constitute Earth’s sedimentary cycle. They are also the nexus for a major

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portion of the Earth’s long-term carbon cycle (Figure 1). Source to sink sedimentary systems are the

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principle sites of the oxidation of organic carbon contained in uplifted and weathered sedimentary rocks

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and of the burial of organic carbon in marine sediments, two processes that over geologic time scales of

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ACCEPTED MANUSCRIPT tens of thousands to millions of years are key mechanisms in the transference of carbon dioxide from

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the solid Earth (lithosphere) to the atmosphere and back again. As such, these systems serve as

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important controls on long-term trends in atmospheric carbon dioxide and oxygen levels and climate

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(Berner, 1982, 2004; Galy et al., 2011).

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Most geoscientists are familiar with the influence of plate tectonic setting on sedimentary cycling, and

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the attribution of sediment properties to past tectonic activity. As an example, well-rounded and sorted

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pure quartz sands (e.g., quartz arenites) typically reflect prolonged histories of weathering and transport

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in low gradient, stable continental and marginal marine settings (e.g., Dickinson and Suczek, 1979). In

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contrast, poorly sorted, angular sands composed of diverse minerals and rock fragments (e.g. lithic

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arenites and wackes) are common to active margins where steep, rapidly eroding catchments are linked

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to offshore environments of rapid sediment accumulation such as submarine fans. Perhaps less familiar

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is the concept that the character of the organic carbon buried in sediments and sedimentary rocks also

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reflects its origin and history in the various segments of source-to-sink systems, as strongly mediated by

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tectonic setting. The character of organic carbon, for example whether or not it is relatively young

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material derived from extant ecosystems or ancient, highly altered material released by comminution of

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sedimentary rocks, has fundamental implications for the carbon cycle as well as for the generation of

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fossil fuels. In this review we follow the source and fate of particulate organic carbon through the

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terrestrial and marine components of sedimentary systems, drawing on an expanding body of research

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in both large, continental scale systems that discharge to passive margins, as well as small, mountainous

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systems located along active margins (Table 1).

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Sources of OC to S2S systems

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The organic carbon transported through sedimentary systems includes both dissolved and particulate

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materials, operationally defined by filtration with a usual cutoff at 0.45 microns and typically assumed to

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ACCEPTED MANUSCRIPT be present in roughly equal amounts in river systems. This assumption may be biased by the focus of

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dissolved organic carbon studies towards passive margin rivers that characteristically have lower

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particulate loads. Here our focus is on the particulate fraction (POC) because it is the material that is

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ultimately buried with the potential to enter the long-term carbon cycle. Particulate organic carbon in

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sedimentary systems is composed of material from different sources, having a range of ages and

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reactivities (Figure 2). It includes fractions relatively recently fixed via photosynthesis, sometimes

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referred to as biospheric carbon. This material includes fragments of terrestrial vascular plants (e.g.

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leaves, wood, pollen), the remains of freshwater algae and heterotrophically recycled material.

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Biospheric carbon also includes components of the soil carbon pool that were derived from vegetative

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sources and biogeochemically altered during storage on land for up to thousands of years before

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erosion releases it to sedimentary systems. Marine primary production contributes biospheric carbon to

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sediments in the ocean.

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Organic carbon in S2S systems also includes much older, more recalcitrant material. Organic carbon in

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sedimentary and meta-sedimentary rocks, sometimes referred to as fossil, rock, or petrogenic carbon is

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material fixed by photosynthetic organisms in the geologic past (typically millions of years ago), buried

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in sediments on land or in the ocean, and lithified along with mineral grains. In sedimentary rocks, most

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petrogenic carbon is in the form of kerogen, defined as that part of the sedimentary organic carbon that

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is insoluble in common aqueous alkaline and organic solvents (Tissot and Welte, 1984). If the

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sedimentary rocks have been metamorphosed, petrogenic carbon may be in the form of graphite. As

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discussed further below, petrogenic carbon forms a major portion of the organic carbon (OC)

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transported through some S2S systems and in some cases a major portion of the OC that is buried in

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their down-system segments.

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ACCEPTED MANUSCRIPT The burial of OC of variable age and reactivity has implications for the global C cycles. The burial of

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biospheric OC represents a long-term sink for CO2, whereas the reburial of petrogenic OC prolongs its

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residence in the lithosphere and attenuates the potential variability of atmospheric chemistry (Hedges,

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1992).

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The fate of OC in S2S systems

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Because of rapid cycles of production and oxidation, only a small portion of the OC that enters

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sedimentary systems survives to be buried in marine sediments. The S2S system can be viewed as a

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network of biogeochemical reactors that facilitate the cycling of OC and are interconnected by sediment

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generation, transport and deposition processes (Blair et al., 2004). Soils and surface sediments are the

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principal reactors in which local primary production and upslope sources of OC are mixed, degraded

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and/or aged (Fig. 3). The extent to which the inputs are modified from their original compositions and

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aged is dependent on their residence times within the biogeochemical reactor and their reactivity.

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Reactor characteristics or ‘design’ also influence the nature of the OC exported downstream (Aller,

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2004). The factors that influence the evolution of OC as it travels from source to sink are discussed in

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greater detail below.

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Soils as an example reactor and the conditional reactivity of OC

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The subaerial weathering of the regolith to produce soils is accompanied by a complex suite of C-cycling

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reactions that are not well understood in detail (Schmidt et al., 2011). Soil OC is derived primarily from

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the colonizing ecosystem and the soil parent material. The parent material OC may be virtually non-

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existent as is the case for soils developing on tephras, or a potentially (and often unrecognized)

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important component in soils weathered from sedimentary rocks, alluvium and colluvium (Blair et al.,

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2010; Blair et al., 2003). The vast majority of soil OC studies have focused on the fate of the plant-

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derived material because without question this is the source of the largest flux of organic matter into

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ACCEPTED MANUSCRIPT the system (Schmidt et al., 2011). However, the plant-derived material by virtue of its freshness is

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expected to cycle more rapidly than the older parent material, thus the parent material OC could

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represent a significant portion of the C pool below the litter layer and O-horizon in some situations. This

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becomes important when one considers deeply excavating mass wasting processes, such as landsliding,

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gully formation and bank failure, as sources of sediment and OC to downstream locations. OC delivered

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to a floodplain, for example, will bear witness to the upstream history of the sediment. The soil that

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develops on the floodplain adds to the recorded history and may not completely replace it (Blair et al.,

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2010).

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Determining how the various OC pools are integrated within a soil or parcel of sediment is a daunting

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task that has been accomplished with relatively low resolution because of the complexity of the

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mixtures. Discerning the relative contributions of different OC sources requires an understanding of

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their reactivities. The oxidation of OC is conventionally described by pseudo-first order models, such as

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(1)

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The rate coefficient, k, is in part dependent on the intrinsic characteristics of the compound or material.

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Heteroatom (O, N, P) -rich, hydrolyzable biochemicals (proteins, polysaccharides) are intrinsically more

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unstable than large, polyaromatic structures such as those found in kerogen, the principal form of

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sedimentary rock OC. The relative activation energies for bond cleavage are a factor in this regard. There

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is also a conditional element to k. Kerogen can persist for billions of years when in the subsurface yet be

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oxidized in thousands of years or less when exposed in outcrops (Petsch et al., 2000). Environmental

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conditions, such as oxidant and water availability, and temperature will influence k either directly, as in

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the well-established temperature dependence of k, or indirectly via controlling the biotic and

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geochemical portions of the biogeochemical reactor.

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ACCEPTED MANUSCRIPT The association with high surface area minerals (clays, iron hydroxides) and their aggregates has

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emerged as a potential conditional control of k in soils wherein OC degradation appears to be retarded

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(Hedges and Oades, 1997; Schmidt et al., 2011). The mechanism is uncertain but protection of the OC

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from enzymatic attack by absorption on mineral surfaces and/or encapsulation within an OC-mineral

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aggregate has been hypothesized (Hedges and Oades, 1997; Schmidt et al., 2011). This cannot be the

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sole mechanism for long-termed OC sequestration because of the ephemeral nature of many mineral

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aggregates. The seemingly more recalcitrant phases of the soil OC are microbial in certain

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characteristics, such as C/N ratio, thus secondary heterotrophic processes may also play a role (Hedges

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and Oades, 1997). These in concert with purely chemical side reactions may produce materials that are

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intrinsically unreactive in the subsoil environment. Exhumation and transport to the surface can

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reactivate previously unreactive OC thus posing a major complexity in deconvoluting prior OC

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signatures. Removal of OC from a stabilizing environment can partially erase the organic geochemical

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record of the particles.

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Classic organic geochemical signatures of terrestrial plants are transformed by the soil reactor. The

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environment. Subaerial C3 plant δ13C values, ~-24 to -35‰, and those of C4 plants, -9 to -13‰, bracket

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marine phytoplankton values, -18 to -23‰ in most temperate to tropical climates (Collister et al., 1994;

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Farquhar et al., 1989; O'Leary, 1981). Subsurface soil δ13C values are frequently more positive (13C-

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enriched) by several per mil relative to the surface (Ehleringer et al., 2000). Microbial cycling of OC with

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a release of 13C-depleted respired CO2 is a likely source of the 13C-enrichment (Blair et al., 1985;

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Ehleringer et al., 2000). Another commonly employed terrestrial OC indicator, the elemental C/N (or

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N/C) ratio, can also be erased via soil diagenesis. Vascular plant organic matter, rich in cellulose and

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lignin, has C/N ratios that are typically 20-500 and is easily recognized relative to marine phytoplankton

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(~5-8)(Hedges et al., 1997). Microbial processing of the organic matter, whether in soils or sediments,

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C/12C ratio (reported as δ13C) of terrestrial plants may be the most recognizable in the marine

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ACCEPTED MANUSCRIPT causes C/N ratios to converge to values of ~8-12, which may reflect the elemental composition of low-

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reactivity microbial macromolecules, such as N-rich cell-wall components (Schreiner et al., 2014).

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OC in segments of S2S systems

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Mountains and uplands—Mountainous and upland regions are the source of a large proportion of both

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the sediment and organic carbon transported through S2S systems. Globally, steep, wet mountains are

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estimated to contribute about 62% of the sediment delivered to the oceans, although they cover about

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14% of the ocean-draining land surface (Milliman and Farnsworth, 2011; Larsen et al., 2014). On a single

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catchment basis, the importance of mountainous headwaters to sediment production is also apparent.

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About 80-90% of the sediment flux through the Amazon River, for example, is thought to derive from

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the Andes (Gibbs, 1967; Meade et al., 1985; Richey et al., 1986; Wittmann et al., 2011), which constitute

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slightly less than 8% of the total basin area.

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Sediment produced in mountainous areas by physical and chemical weathering is delivered from

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hillslopes to channel networks by processes including soil creep, sheet wash, gullying, debris flows, and

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landslides. The same processes also mobilize particulate organic carbon (POC) from above- and below-

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ground biomass, soils, and sedimentary rocks. In general, POC in near-surface sources such as biomass

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and litter tends to be more concentrated, younger, and more reactive (labile) than that contained in

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deeper soil horizons and sedimentary rocks (Figure 2). As a result, the relative importance of these

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various erosional processes over time and space is a fundamental control on the amount and character

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of the POC transported in and through headwater streams (e.g., Leithold et al., 2006). Data from a wide

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range of rivers show that the fraction of POC in riverine total suspended solids (TSS) decreases with

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increasing TSS concentrations (e.g. Ludwig et al., 1996; Figure 4), reflecting the fact that high turbidity is

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accommodated by erosion into deeper, OC-poor soil horizons and bedrock. The mobilization of these

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ACCEPTED MANUSCRIPT deeper, older biospheric and petrogenic sources of OC in areas of more rapid erosion is also reflected by

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the observed relationship of POC radiocarbon age to basin-averaged sediment yield (Figure 5).

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Mountainous regions are commonly characterized by high rates of precipitation and highly variable

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hydrographs. Low frequency, high magnitude events that activate erosion and transport processes in

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steep mountain catchments are typically responsible for a disproportionately large portion of sediment

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and POC fluxes in these areas. In the Liwu catchment in Taiwan, for example, Hilton et al. (2008)

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estimated that on decadal time scales 77-92% of biospheric POC transport occurs during cyclone-

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triggered floods that take place about three times annually. During a two-year study of the Eel River in

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northern California, Goñi et al. (2013) found that about 40% of water transport, 88% of the total

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suspended sediment, and 80-85% of the POC flux occurred during 29 days of high river discharge

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associated with winter storms.

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In some mountainous/upland settings where rocks are competent, slopes are moderate, and vegetation

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cover is thick, surficial erosion may dominate even when precipitation is heavy and river discharge is

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high, so that recent biomass is the primary source of POC mobilized during storms (e.g., Hattin et al.,

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2012; Smith et al., 2014). In many steep catchments, however, debris flows and landslides are triggered

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during such events, delivering OC from deeper horizons. In the Southern Alps of New Zealand, Hilton et

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al. (2011) used landslide inventories for 13 river catchments to estimate the mobilization of 7.6 ± 2.9

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tons km-2yr-1 of biospheric OC by landsliding over a 40 year period. About 30% of this material was

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delivered to river channels and presumably exported from the catchments, while the remainder was

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retained on hillslopes. In a suite of the 11 mountainous catchments in Taiwan, Hilton et al. (2012)

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documented a substantially higher average annual rate of landslide-driven transfer of 21 ± 10 tons km-2

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of biospheric POC from hillslopes to streams, with the catchment areas having slopes above the typical

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ACCEPTED MANUSCRIPT thresholds for landsliding (>35o) making the largest contributions. In their two-year study, a strong

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positive correlation of biospheric POC concentrations and river discharge was documented.

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During extreme events and where bedrock is either friable or pervasively fractured, the triggering of

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bedrock-involved landslides transfers petrogenic OC to the sedimentary system as well. In the 11

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mountainous catchments of Taiwan mentioned above, biospheric OC consitituted only 30% of the POC

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in transport, with the remaining 70% composed of petrogenic OC (Hilton et al., 2010, 2011 a, 2012).

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Similarly, in the Andean headwaters of the Amazon River in Peru, Clark et al. (2013) found that up to

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80% of the total POC transported was petrogenic.

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A fraction of the petrogenic carbon contained in the uplifted sedimentary rocks of some mountainous

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catchments is oxidized in situ, while the remainder is transferred to lower reaches of S2S systems.

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Petrogenic OC oxidation is hypothesized to occur along with the near-surface oxidative weathering of

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soils (Petsch et al., 2000; Bolton et al., 2006). In the headwaters of mountainous rivers in Taiwan, Hilton

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et al. (2014) showed that petrogenic OC oxidation rates are positively related to decadal suspended

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sediment yield, demonstrating the strong control of physical erosion rates on the fate of petrogenic OC

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oxidation and CO2 release during weathering. In these mountainous catchments, however, short

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residence times in soils means that >80% of the export of petrogenic OC released by denudation

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(weathering plus erosion) occurs in solid particulates rather than as CO2.

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Lowlands

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Sediment and POC leaving the mountainous headwaters of S2S systems enters relatively low gradient

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river reaches of variable extent. One of the most important differences between small active margin

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large passive margin rivers and is the size of their floodplains (Figures 6, 7; Table 1). Small active margin

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rivers have steep gradients and small flood plains that provide minimal storage for river sediments. The

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typical transit time for riverine particulate matter from mountainous headwaters to the ocean is

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ACCEPTED MANUSCRIPT therefore short and there is little opportunity for OC fractions transported from the mountains to be

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degraded, or for new fractions to be added. As a result, particulate OC discharged to the oceans from

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small mountainous rivers bears the characteristics of its headwaters (Leithold et al., 2006).

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Although large rivers such as the Amazon, Ganges-Brahmaputra, and MacKenzie, as well as the

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moderately-sized Fly originate in high relief mountain belts, they flow for long distances across foreland

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basins that accommodate thick accumulations of sediments and underlie vast, relatively flat floodplains

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(Figure 7, Table 1). Sediments and particulate organic carbon in floodplains undergo multiple cycles of

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deposition and erosion (Meade et al., 1984; Allison et al., 1998; Dunne et al., 1998) and may be stored

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for thousands of years before discharge to the ocean (Alin et al., 2008; Galy and Eglinton, 2011; Bouchez

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et al., 2012; Goñi et al., 2014). The floodplains of the Amazon, Ganges-Brahmaputra, and Fly Rivers

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serve as conveyor-belt style “reactors” where OC is deposited during high flow and subsequently

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decomposed and/or aged. New OC is added from lowland vegetation and phytoplankton in floodplain

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lakes (Moreira-Turq et al., 2012). Older, more refractory POC is preferentially preserved as it moves

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through the floodplain, so that the material discharged to the ocean is significantly aged. Biospheric

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POC in the lower Amazon at Obidos (750 km upstream from the Atlantic Ocean) has a mean 14C-age of

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~1400 years old (Bouchez et al., 2014), whereas that in the lower Ganges-Brahmaputra is about 3000

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years old (Galy and Eglinton, 2011). Low density, vascular plant debris discharged to the Fly delta has a

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mean 14C age of ~2800 ybp (Goni et al., 2008). The age of POC discharged from Arctic Rivers reflects

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even longer storage times. Feng et al. (2013), for example, estimated that 47-77% of the biospheric

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carbon discharged by four rivers in the Russian Arctic (the Yenisey, Lena, Indigirka, and Kolyma)

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originates in deep permafrost horizons, contributing to bulk 14C ages between ~1500 and 7500 ybp. The

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proportion of the ancient biospheric OC in the sediments discharged by these rivers has increased by 3-

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6% between 1985 and 2004, reflecting climate change-induced thawing and mobilization of old

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permafrost in the drainage basins.

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ACCEPTED MANUSCRIPT Continental Shelf

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Rivers deliver roughly 200 Tg of particulate OC to the oceans annually (Ludwig et al., 1996; Galy et al.,

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2007; Schlunz and Schneider, 2000), where 55-80% is "remineralized" along the continental margins by

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bacterial respiration, converting it back to inorganic carbon and producing dissolved nutrients (Burdige

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2005). The remainder, along with POC originating from marine production, is buried in marine

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sediments that are partitioned in approximately equal portions between deltaic depocenters (~40-50%)

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and non-deltaic continental margins settings (Berner, 1982; Hedges and Keil, 1995; Burdige, 2005, Blair

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and Aller, 2012).

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As in the upland and lowland segments of S2S systems, the fate of OC on the continental shelf depends

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on how readily it is degraded (related to its composition and mineral associations) and to its exposure to

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oxidants. These dependencies are well illustrated by a plot of OC burial efficiency (percentage of organic

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carbon preserved) in a range of S2S systems (Figure 8). Shelves with low water-column O2 levels show

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high burial efficiencies, reflecting the reduced OC degradation rates under low O2 conditions. For shelves

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with normal water-column O2 levels, there is a strong relationship of burial efficiency to sediment

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accumulation rates. With progressively higher rates of sediment accumulation, the exposure time to

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oxygen and other oxidants in the upper mm’s to cm’s of the seabed is progressively reduced.

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Sediment accumulation rates in the shelf segments of S2S systems depend on sediment supply, shelf

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accommodation, and processes of sediment dispersal (Walsh and Nittrouer, 2007). Many small

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mountainous rivers on active margins, for example, discharge moderate amounts of sediment to narrow

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shelves (Somme et al., 2009, Figure 9). In these environments, large waves and/or high tidal ranges

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typically promote efficient dispersal of sediment away from river mouths (Walsh and Nittrouer, 2007).

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Because very high suspended sediment concentrations in these settings may be achieved at the river

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mouths and on the inner shelf during episodic floor/storm events, hyperpycnal river flows and wave-

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ACCEPTED MANUSCRIPT supported sediment gravity flows may occur in these settings (e.g., Milliman and Kao, 2005; Traykovski

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et al., 2000, 2007). These processes provide a means of rapid transfer of fluvial sediment and POC to

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relatively deep water, where it commonly accumulates in tectonically-influenced depocenters on the

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middle and outer shelf and beyond (Leithold and Hope, 1999; Blair et al., 2003; Brackley et al., 2010). All

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of these processes conspire to rapidly bury organic carbon on active margin shelves. The resulting low

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oxygen exposure times as well as the intrinsic recalcitrance of the petrogenic OC that may comprise a

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substantial portion of the POC discharged from mountainous rivers results in some of the highest burial

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efficiencies yet measured (Fig. 8, Blair and Aller, 2012). On the Waiapu shelf off the North Island of New

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Zealand, as an extreme example, the burial efficiency of riverine C within the mid-shelf depocenter is

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approximately 90% (Blair and Aller, 2012). The biospheric terrestrial carbon appears marginally reactive

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in shallow, sandy sediments but is otherwise preserved in muddy facies (Blair et al 2013, Thompson,

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2009). The petrogenic fraction, which represents ~80% of the riverine POC, shows no sign of oxidative

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loss.

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Along many passive margins, in contrast, where shelves are wide (Figure 9, Table 1) and large rivers

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discharge abundant sediment to the ocean, depositional topography is built by the accumulation of

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sediment close to the river mouth to form a shoreline bulge (e.g., Mississippi, Rhone, and MacKenzie

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River Deltas). Alternatively, especially where tidal energy is high, depocenters are displaced farther

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seaward to form what are termed subaqueous deltaic clinoforms (e.g., Amazon, Fly, Ganges-

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Brahmaputra (Bengal) deltas).

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OC burial efficiencies are variable in different parts of these systems. In shallow “topset” environments,

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repeated suspension of sediment by tides and waves may lead to the formation of dense suspensions

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known as fluid muds (e.g., Trowbridge and Kineke, 1994; Sheremet et al., 2011) that behave analogously

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to fluidized bed reactors (Aller, 1998; Blair and Aller, 2012), so that 70-80% of the terrestrial OC and

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ACCEPTED MANUSCRIPT about 90% of the locally produced marine OC is remineralized. On the other hand, areas of rapid

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deposition on these deltas may be sites of high terrestrial OC burial efficiency (Galy et al., 2007, Goñi et

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al., 2008, 2014). Offshore from the Fly River, for example, where a 100-km wide clinoform extends

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seaward across the northern Gulf of Papua, most of the terrestrial OC appears to be deposited on the

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seabed without significant degradative losses (Goñi et al., 2014).

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Deeper sea

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Sediment and organic carbon is dispersed from the shelves to the deeper ocean by diffusive (e.g.

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nepheloid layers) and advective processes (dense water cascades, turbidity currents) (Nittrouer and

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Wright, 1994). Where sedimentation rates are slow, and oxygen exposure times are long, OC burial

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efficiencies may be low. Distal turbidites of the Madeira Abyssal Plain, at the base of the Canary Islands

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shelf at 5400 m water depth, for example, have been the subject of a number of studies of organic

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matter degradation due to long-term oxygen exposure (e.g., Prahl et al., 1997; Huguet et al., 2008).

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Where sedimentation rates are high, as in more proximal settings where turbidity currents are frequent

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and submarine fans are formed, OC burial should be more efficient.

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Submarine canyons serve to funnel sediment from shelves and slopes to the basin floor, where

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submarine fans that scale to the size of their linked terrestrial drainage basins are built (Somme et al.,

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2009). Submarine canyon heads on active margins incise into narrow shelves and therefore tend to be

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located closer to river mouths than those on passive margins; this results in sediment and carbon bypass

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of the continental shelf and more rapid transport to the deep sea. The head of Eel Canyon offshore

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from northern California, for example, lies within 12 km from the mouth of the Eel River and is a zone of

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rapid sediment accumulation (Mullenbach et al., 2000, 2004; Puig et al., 2004). Recent observations

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from the Eel Fan, about 20 km farther seaward, indicate that turbidites, rich in wood debris, have been

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deposited at recurrence intervals of 36 years in the late Holocene, and with greater frequency during

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ACCEPTED MANUSCRIPT the Pleistocene deglaciation and early Holocene (Paull et al., 2014). Similarly, the Sepik River in

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northern Papua New Guinea discharges to the Bismarck Sea, where a steep and narrow continental shelf

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is 10 km at its widest point. The head of the Sepik Canyon is located within the river mouth itself, and

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the 1000 m depth contour lies within 16 km of the coast (Kineke et al., 2000; Burns et al., 2008). Burns

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et al. (2008) estimate that over half of the POC discharged from the river is deposited on the slope or

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directly delivered to the deep sea via the canyon.

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Although the bulk of sediment discharged from rivers on passive margins is stored on continental

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shelves during the present-day highstand of sea level, on some passive margins, including offshore from

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the Mississippi and Ganges-Brahmaputra Rivers, submarine canyons are incised into sediments

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deposited during previous cycles of shoreface-shelf progradation that now allow bypass of fluvial

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sediment to basin floor fans. The subaqueous delta of the Ganges-Brahmaputra River, for example, is

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incised by the Swatch of No Ground, a 160-km-long submarine canyon whose head lies at 20 m water

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depth within 30 km of the present coastline (Kuehl et al., 2005; Galy et al., 2013). Prograding foresets of

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the subaqueous delta directly feed into the canyon head, where extremely high rates of sediment

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accumulation (>15-100 cm y-1) have been measured (Kuehl et al., 1997; Kudrass et al., 1998; Michels et

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al., 2003). Over longer time scales (102-103 years), mass wasting and turbidity currents in the Swatch of

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No Ground have funneled sediment to the Bengal Fan, the largest submarine fan in the world.

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Biomarker and isotopic studies demonstrate that OC in Bengal Fan sediments is almost identical to that

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in the lower reaches of the fluvial system, indicating that selective degradation of terrestrial OC or

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addition of marine OC is negligible during transfer to the fan (Galy et al., 2007; 2013). The

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extraordinarily high burial efficiency of terrestrial POC on the Bengal Fan (approaching 100%) is

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attributed to very high rates of sediment accumulation on the shelf, as well as to low O2 concentrations

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in the Bay of Bengal (Galy et al., 2013).

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ACCEPTED MANUSCRIPT OC in ancient S2S systems—stratigraphic trends and hydrocarbon potential

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An understanding of the potential sources and transformations of OC throughout the linked terrestrial

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and marine components of S2S systems provides a rich foundation for interpreting stratigraphic records

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of past environments and environmental change. Keil et al. (1997), for example, examined organic

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carbon buried in Pleistocene-Holocene sediments of the Amazon Fan and provided evidence for the

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effects of different sediment and carbon transport pathways during different stands of sea-level. Low

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organic carbon to mineral surface area ratios (OC/SA) characterize sediments deposited on the fan

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during interglacial sea-level highstands, suggesting conditions similar to those at present in the Amazon

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system, where low burial efficiencies of terrestrial organic matter are related to continual resuspension

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of fluid muds on the broad Amazon shelf. In contrast, higher OC/SA ratios were interpreted to reflect

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greater burial efficiency of terrestrial OC during times of glaciation and sea-level lowstand when there

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was a more direct path from the Amazon River to the fan.

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Stratigraphic changes in the age and/or composition of OC buried in marine sediments have been shown

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to record changes in OC sources and processing in the linked terrestrial segments of S2S systems. Kao et

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al. (2008), for example, showed that the supply of petrogenic OC to the Okinawa Trough offshore from

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Taiwan has been higher during the Holocene Epoch than it was during late Pleistocene glaciation. Kao et

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al. (2008) interpreted this trend to reflect enhanced physical weathering in the small mountainous

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catchments of Taiwan under wetter Holocene climate conditions. On a slightly shorter time scale,

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Leithold et al. (2013) interpreted Holocene changes in the radiocarbon age of various terrestrial OC

340

fractions buried on the Waipaoa shelf offshore from New Zealand to indicate temporal changes in

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erosional processes in the Waipaoa river catchment. A mid-Holocene episode during which the age of

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terrestrial carbon buried on the shelf increased by ~ 1000 years relative to that buried earlier was

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interpreted to reflect a stormy period when mass wasting in the mountainous catchment occurred at an

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ACCEPTED MANUSCRIPT elevated rate, mobilizing larger proportions of aged biospheric (soil) and petrogenic carbon relative to

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younger, surficial biospheric OC (Fig. 10).

346

Hydrocarbon production in S2S systems

347

The river-ocean margin is a particularly attractive target for conventional hydrocarbon exploration

348

because of the common stratigraphic association of organic C-bearing mudstones that can serve as

349

source rocks and permeable sandstones that act as reservoirs. Not surprisingly, many of the world’s

350

major hydrocarbon finds are associated with large rivers with notable examples being the Niger River -

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Gulf of Guinea, the Mackenzie River - Beaufort Sea, and the Mississippi River - Gulf of Mexico paleo-S2S

352

systems (Konyukhov, 2012; Yunhua, 2012).

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The conceptual model for the evolution of POC from source to sink has specific relevance to petroleum

354

and gas exploration insofar as the identification of potential source rocks requires an understanding of

355

the composition of the POC that has been buried in marine depocenters. Source rocks containing

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organic matter derived from terrigenous vascular plant matter are gas-prone because the buried OC has

357

a relatively low aliphatic (long-chained hydrocarbon) character (Baudin et al., 2010). These types of

358

sediments will be found preferentially in nearshore environments. The replacement of terrigenous OC

359

with that derived from marine productivity as particles transit offshore from high turbidity zones and

360

into deeper more quiescent regions of the seabed leads to oil-prone source rocks. Nutrients delivered by

361

the river can fuel the marine production (Yunhua, 2012). Environments that offer low C-burial

362

efficiencies, such as the C-incinerating mobile mud belts of the Amazon shelf, would be poor choices for

363

source rocks. The foreset beds of the same deltas, on the other hand, would be logical targets because

364

of the rapid burial of local marine production. Historically, exploration has focused on shallow

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environments because of the belief that OC burial fluxes in deep water were inadequate to form quality

366

source rocks. This problem is circumvented by turbidity currents in river-submarine canyon networks

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ACCEPTED MANUSCRIPT that provide an effective mode of transport of fine-grained sediments and OC to deepwater

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environments, especially during lower stands of sea level. Deep sea fans have drawn interest as

369

potential hydrocarbon sources as exploration has focused further offshore (Weimer and Link, 1991;

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Baudin et al 2010; Petter et al., 2012).

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The presence of recycled (or reworked) kerogen in a source rock is an issue that requires attention

372

during exploration. Recycled kerogen has a lower hydrocarbon generation potential because of its past

373

burial and catagenetic history. If the presence of recycled OC is not considered in petroleum generation

374

kinetic models, the potential of the source rock may be overestimated. The vitrinite reflectance method,

375

which assays the optical reflectance of individual microfossils (macerals) of plant debris, is often used to

376

detect recycled material (Senftle et al., 1993). The S2S OC model specifically identifies the necessary

377

ingredients for consistently recycling kerogen – a watershed with an underlying lithology composed of

378

mudstones, and steep overall altitudinal gradients that limit lowland processing of sedimentary OC.

379

These conditions are more easily met by small river systems on active margins but may not be limited to

380

those.

381

Summary and Conclusions

382

Source to sink sedimentary systems serve a key function in the global carbon cycle and are the primary

383

location of long-term organic carbon burial. The age and character of the OC that is buried at the

384

terminal ends of these systems reflects the sources and transformations of OC throughout the linked

385

terrestrial and marine segments. Profound differences are observed between large passive margin and

386

small active margin systems.

387

Sediment and OC transported through passive margin S2S systems such as the Amazon, Ganges-

388

Brahmaputra, and Fly have prolonged residence in terrestrial lowlands and the bulk biospheric OC

389

discharged tends to be significantly aged (thousands of years old) and refractory. This material may be

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ACCEPTED MANUSCRIPT further subject to extensive exposure to oxidants on broad continental shelves, resulting in areas of low

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OC burial efficiency seaward of passive margin rivers. Rapid burial in prograding, subaqeous deltaic

392

clinoforms or bypass to submarine fans, however, leads to high burial efficiency of terrestrial biospheric

393

OC in some passive margin settings.

394

Small active margin systems such as the Eel and Santa Clara in the U.S., Waipaoa and Waiapu in New

395

Zealand, and Liwu in Taiwan, are collectively important sources of sediment and OC to the oceans. Mass

396

wasting on the steep slopes of the river catchments results in high rates of erosion, and the extensive

397

mobilization of petrogenic carbon as well as biospheric carbon. These materials follow relative short

398

pathways from headwaters to the seabed, with minimal storage in lowland environments. Once the

399

sediment and OC is discharged at the river mouths, rapid burial in tectonically controlled depocenters

400

on narrow, steep shelves or direct bypass to the deeper sea via submarine canyons is common. This

401

rapid transit, facilitated by the important role of storm-drive sediment transport in these settings, leads

402

to very high OC burial in general, and to the burial of relatively young biospheric carbon specifically.

403

The study of OC sources and transformations in contemporaneous S2S systems potentially informs

404

interpretations about the systems in which OC was buried in the geologic past. Sea-level variations, for

405

example, will impact the pathways that sediments and OC follow through S2S systems and the extent to

406

which transformations occur in lowland terrestrial and shelf environments. Similarly, tectonic uplift and

407

climate changes may impact both the relative importance of biospheric and petrogenic OC sources and

408

their transit times to offshore sites of burial. All of these long-term variations in S2S OC cycling will

409

impact the petroleum potential of offshore deposits as well as the organic geochemical signals of

410

environmental change that they contain.

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Table 1. Characteristics of some S2S systems on active and passive margins* Latitude of river mouth

Longitude of river mouth

Tectonic Setting

Climate zone

Lanyang

Taiwan

24.709508°

121.834480°

Active

Subtropical Subtropical

Li-Wu Eel Santa Clara

24.135370° 40.638564 34.230031°

121.664497° -124.315979 -119.261505°

Active Active Active

Pacific

-38.711486°

177.940325°

Active

Haast Amazon

Taiwan US US New Zealand New Zealand New Zealand Brazil

Pacific Phillipine Sea / Pacific Pacific Pacific

Brahmaputra

Bangladesh Bangladesh Papua New Guinea US

Mississippi

US Dem. Republic of Congo/Rep. of Congo

Congo

178.488757°

Active

-44.185800o 0.623405°

168.192244o -49.860234°

Active Passive

23.891927°

89.723627°

Maximum elevation (m)

Discharge (km3/y)

Sediment yield (tons km-2 y-1)**

Approx. Shelf Width (km)

2.8

Suspended sediment discharge (106 tons/y)** 6.5

73

3500

7222

4.5

0.6 9.5 4.4

55 320 130

3700 2300 2000

0.99 7.7 0.12

6.8 19 3

11333 2000 682

6 18 15

2.6

90

1200

1.8

15

5769

30

2.2

90

2000

2.8

35

15909

25

0.93 6300

100 6400

3000 5500

6 6300

5.9 1200

6344 190

12 300

670

2600

5500

630

540

806

165

980

2200

7000

490

520

165

76 36

620 490

3600 1600

180 15

110 (80) 0.2

531 1447 (1053) 6

2975

5900

4348

490

210 (400)

71(134)

150

4416

4700

4115

1300

43

10

80

1805

1738

3435

310

100

55

140

644

2129

2368

120

10

16

700

2580

5539

3351

620

4.1(13)

2(5)

450

0.9

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-37.773065°

Temperate Tropical Subtropical

PT ED

Ganges Fly/ Strickland Hudson

Pacific Tasman Sea Atlantic Bay of Bengal Bay of Bengal Gulf of Papua Atlantic

Passive

Subtropical

23.830407°

89.540420°

Passive

Tropical

-8.595155° 40.718140°

143.614055° -74.030875°

Passive Passive

Gulf of Mexico

28.992543°

-89.143362°

Passive

Atlantic

-6.078666°

12.454001°

Passive

CE

Waiapu

Length (km)

Temperate

AC

Waipaoa

Temperate Subtropical Temperate

Drainage area (103km2)

PT

Ocean

RI

Country

SC

Basin

MacKenzie

Canada

Arctic

69.063467°

-136.114435°

Passive

Kolyma

Russia

Arctic

69.596965°

161.649353°

Passive

Yenisei

Russia

Arctic

70.986064°

83.308328°

Passive

Temperate Temperate to subtropical Tropical

Cold (Arctic) Cold (Arctic) Cold (Arctic)

*Data on rivers are from Milliman and Farnsworth (2011). Shelf width was measured using GeoMapApp (http://www.geomapapp.org, Ryan et al., 2009). **Values in parentheses are before significant anthropogenic modification of the catchment

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Figure 1. The global carbon (C) cycle (after Berner, 1994, 2004). The global C cycle comprises a series of nested cycles that operate on timescales of moments (biochemical) to billions of years (subduction and seafloor spreading). The cycling of carbon in Source-to-Sink (S2S) systems (outlined with dashed line) occupies a unique niche because it crosscuts timescales by incorporating contemporary primary production on land and at sea (not shown), the aging of OC on land in soils over millennia, the exhumation and incomplete weathering of petrogenic C (kerogen) that is millions of years old, followed by the delivery, dispersal and ultimate burial of the various C-cycle products in the ocean. The S2S system is the primary mechanism that links the nested C-cycles. Figure 2. The spectrum of age and reactivity of organic carbon in source-to-sink sedimentary systems

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Figure 3. The network of biogeochemical reactors that facilitate the cycling of OC in source-to-sink sedimentary systems (after Blair et al., 2004). Organic carbon in each reservoir may be input from upstream reservoirs via erosion and transport, or produced in situ. Longer residence times in reservoirs prolong exposure to oxidants and tend to favor OC degradation and turnover. Bypassing of reservoirs avoids change. Source to sink systems display a continuum of behaviors, with passive margin systems typically characterized by extensive OC residence and turnover in floodplains and shelf environments and small mountainous systems commonly characterized by minimal residence of OC in upland, lowland, and shelf reservoirs.

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Figure 4. Suspended sediment concentrations vs. weight percent particulate carbon (POC) in the Ishikari, Ganga, Brahmaputra, Zengjiang, Congo, Umpqua, and Eel rivers. Decreasing POC with increasing suspended sediment loads reflects the dilution of organic matter concentrated in more surficial horizons with mineral matter eroded from deeper soil horizons and bedrock. Data are from Alam et al. (2007), Coynel (2005), Galy et al. (2007), Gao et al. (2007), and Goñi et al. (2013). Figure 5. The radiocarbon age of suspended POC, expressed as Fraction Modern (Fmod), vs. basinaveraged sediment yield for a range of rivers in tropical and temperate environments. Fmod is the deviation of the 14C/12C ratio of a sample from “modern,” defined as 95% of the radiocarbon concentration in AD 1950. The trend indicates that with progressively greater rates of erosion, OC is mobilized in greater proportions from deeper, older soils horizons and bedrock. Data are from Alin et al. (2008), Bouchez et al. (2010), Cathalot et al. (2013), Galy et al., (2008), Galy and Eglinton (2011), Hedges et al.(1986), Hilton et al. (2008, 2010), Hossler and Bauer (2012), Kao and Liu (1996), Komada et al.(2004), Leithold et al. (2006), Mayorga et al. (2005) Raymond et al. (2004), Rosenheim et al. (2013), Spencer et al. (2012), and Wei et al.(2010). Figure 6. Hypsometric curves for large passive and small active margin rivers that record the distribution of ground surface area within a catchment with respect to elevation above sea level (Strahler, 1952). The hypsometric curves display cumulative-frequency profiles representing the statistical distribution of the area (a) within a catchment relative to the total catchment area (A) that lies within 100 m intervals of elevation (h) relative to the maximum catchment elevation (H). The distribution of elevations as a function catchment area were obtained from 90-m Shuttle Radar Topography Mission (SRTM) data (Jarvis et al., 2008) Figure 7. Comparison of flood plain area for A) the Amazon basin, a large passive margin river system, to (B) the Waipaoa (North Island - New Zealand), (C) Santa Clara (California – USA), and (D) Li Wu (Taiwan) all of which are relatively small, active margin rivers (see Table 1). Floodplains are identified as contiguous valley-bottoms zones with gradients of < 0.01° (1%) as determined from the 90 m SRTM 29

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Figure 8. The percentage of organic carbon deposited on continental shelves that is buried as a function of sediment accumulation rate. “SMRs” refers to shelves offshore from small mountainous rivers. Data are from Aller (1998), Aller et al. (2008), Blair et al. (2003, 2010), Canfield (1994), Coynel (2005), Galy et al.(2007), Goñi et al. (2005), Henrichs and Reeburgh (1987), Huh et al. (2009), Kao et al. (2006), Spencer et al. (2014), and Thompson et al. (2009).

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Figure 9: Topographic profiles of the continental shelves and upper slopes offshore of large passive and small active-margin rivers. The bathymetric profiles were constructed from the General Bathymetric Chart of the Oceans (GEBCO) global 30-arc second digital elevation model dataset (The GEBCO_08 Grid, version 20100927, http://www.gebco.net).

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Figure 10: Stratigraphic trends in the 14C-age of clay-associated OC and discrete plant-debris buried on the middle continental shelf offshore from the Waipaoa River, North Island, New Zealand compared to the depositional age of sediments as determined by tephrochronology. Leithold et al. (2013) interpreted the burial of relatively older OC fractions on the shelf between about 5000 and 3000 years ago to reflect a stormy period, during which mass wasting mobilized a larger proportion of older biospheric and petrogenic carbon in the river catchment relative to younger, more surficial sources of biospheric OC.

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ACCEPTED MANUSCRIPT Acknowledgements

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Numerous scientific colleagues, including students and support personnel, assisted over the years in the river, delta, and margins research that helped provide the perspectives for this review, including the AmasSeds, Ocean Margins, STRATAFORM, TROPICS, and MARGINS S2S programs. Particular thanks to C. Nittrouer, who headed several of these efforts. This research is partially supported by the NSF S2S project OCE-0646159. Additional support was provided by the NSF GeoPrisms program (award #1144483) and the Critical Zone Observatory for Intensively Managed Landscapes (IML-CZO; EAR1331906).

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