Carbon burial on continental margins—Controls from Source to Sink

Introduction

The continental margins are major sites of carbon burial and thus play a critical role in the long term carbon cycle, the Earth’s atmospheric chemistry, climate and the formation of fossil fuels. In fact, it has been estimated that approximately 90% of carbon burial in the oceans takes place in continental margin settings. Although a small portion (<1%) of the organic carbon deposited on the seabed is preserved, margins tend to have greater burial efficiencies than other parts of the ocean. This means that a great-than-average proportion of the organic carbon that is delivered to the seabed on the margins survives to be buried and preserved over the long term. The comparatively greater burial efficiencies on margins are thought to be related to the composition of the organic matter deposited in these regions (relatively resistant biomolecules), rapid burial in sediments (short exposure times to oxygen and other oxidants), and association with mineral particles (protection against degradation by physical occlusion or sorption).

Particulate organic carbon (POC) in continental margin sediments is derived from both from terrestrial and marine sources and comes in many different “flavors.” The POC discharged by rivers to the oceans, for example, is derived from fresh water algae, terrestrial plants (e.g., leaves, needles, wood fragments), soil, and sedimentary rocks. Marine POC is derived primarily from phytoplankton (especially algae) in the photic zone (lighted surface waters of the ocean). Organic carbon deposited on the seabed may range in age from less than one year (e.g., algae), to thousands of years (e.g. soil carbon), to many millions of years (rock carbon). The younger material typically includes many compounds that are relatively easily broken down (are more “labile” or “reactive”), whereas older material typically is dominated by more resistant compounds (are more “recalcitrant” or “less reactive”). Burial of these different forms of POC has implications for the carbon cycle in general, including the formation of fossil fuels. The burial of young POC derived from algae and plants, for example, removes organic carbon from the short-term carbon cycle, thus impacting CO2 levels in the atmosphere and ocean. The re-burial of rock carbon that is millions of years old, in contrast, has little impact on the short-term OC cycle. Over the longer term, marine algae are the most common precursors to petroleum, whereas terrestrial plant debris is more typically a precursor to natural gas.

Understanding the variability of the types of organic carbon buried on different margins is clearly important for furthering our understanding of the carbon cycle and its response to anthropogenic and natural environmental changes. Recent research has revealed that the composition of POC buried on various continental margins depends on characteristics of both the terrestrial and marine segments of sedimentary systems. This exercise is intended to introduce you to both parts—the rivers that are sources of POC to the margins, and the shelf sedimentary deposits that are sinks for that terrestrial material as well as marine carbon formed in the water column.

Part 1: Controls on the particulate carbon discharged by rivers

In the introductory lesson on rivers and their discharge of sediment to the oceans, you explored some small and large rivers, their tectonic settings, and the size of their floodplains. In this exercise you will examine some data on the age of particulate carbon discharged from these rivers, as measured using 14C (also known as radiocarbon). It is important to realize that the 14C-age of riverine POC reflects a mean age—that is, a mixture of material of a range of ages.

The plot below is based on some of the data provided in the accompanying Excel file titled “River OC data.” Note that the data provided have been compiled from many research papers and include %POC, the weight percent particulate carbon in suspended sediments recovered at various times from each of the rivers, and Fmod, the fraction modern carbon in the sample, a measure of its 14C content relative to a 1950 standard. An Fmod value of 0 signifies that the sample contains carbon with a mean age greater than about 50,000 years and that it therefore contains negligible 14C (the half-life of 14C is 5730 years). Note that some samples have Fmod values greater than 1.0, reflecting the incorporation of 14C released to the atmosphere by above-ground nuclear weapons testing in the 1950’s and 60’s and peaking in 1963. The plot below depicts one interesting way to look at these data. Here, sediment yield for each river (see summary table in accompanying exercise) is plotted against Fmod. Because the sediment yield ranges over a wide range of values, it is convenient to show it on a log scale.

Figure 2: Relationship between the 14C-age of riverine particulate organic carbon (expressed as Fmod) and the sediment yield in a range of drainage basins (see references in accompanying spreadsheet).

Questions:

1) Why do you think river carbon tends to have a lower Fmod (i.e., is older) in rivers draining areas with higher sediment yields? How does this trend relate to the likely mixtures of OC from different sources discussed in the introduction?

2) What do you think accounts for the variability in ages of Fmod sampled in individual rivers? Are there some conditions under which a river might tend to be carrying somewhat older or younger particulate organic carbon?

3) Based on these data, and your grouping of the rivers in the introductory exercise, what can you conclude about the relationship between the age of river POC and tectonic setting?

4) One of the most important features of rivers that govern the time it takes sedimentary particles (including particulate organic carbon) to reach the oceans is the extent of their flood plains. Flood plains may store materials derived from headwater regions for thousands of years before they are re-eroded and sent down river channels toward the ocean. During this lowland storage, soils develop on top of floodplain sediments and organic carbon derived from rocks and soils in steep, upland areas may degraded and replaced by more recent, plant-derived OC. In the introductory exercise you examined the size of floodplains in some different river systems. Does the extent of floodplains explain some of the differences we see in the age of POC discharged from different rivers? Choose two rivers that transport young POC and two that discharge relatively old POC—how would you characterize their floodplains?

Part 2: Marine sediment dispersal and carbon burial

Now that we have looked at the variability of the age of POC discharged from different rivers, we will examine how the transport (dispersal) of river sediment away from river mouths affects the fate of that material in the oceans. Walsh and Nittrouer (2009) summarized 5 different types of sedimentary systems (Figure 2) based on patterns of offshore sediment accumulation, and showed that these patterns are related to river sediment supply, wave and tidal energy, and shelf width. Estuarine Accumulation Dominated (EAD) systems are those where rivers with low sediment loads drain into large, unfilled estuaries that capture the majority of the sediment load. Proximal Accumulation Dominated (PAD) systems are deltaic systems with rapid rates of sediment accumulation close to river mouths, whereas in systems with Subaqeous Deltaic Clinoforms (SDC), sediment typically bypasses the proximal area due to intense tidal reworking and is transported seaward in dense suspensions (fluid muds). As a result, it accumulates tens to hundreds of kilometers from the river mouth. In MDD (Marine Sediment Dispersal) systems, large waves and/or strong tidal currents efficiently disperse sediment away from river mouths to areas of rapid sediment accumulation on the shelf, slope, or in submarine canyons. Finally, Canyon Capture (CC) systems are those where the majority of sediment discharged by rivers funneled directly to the deep sea via submarine canyons, even during the present sea-level highstand. The classification of sedimentary systems we studied in the introductory exercise is shown in Table 1.

Figure 2: Types of river sediment dispersal systems on continental margins (from Walsh and Nittrouer, 2009)

Table 1: Classification of sediment dispersal systems examined in this exercise

River Dispersal System TypeAmazon SDCGanges/Brahmaputra SDCCongo CCFly/Strickland SDCHudson EADLanyang MDDLi-Wu MDDMississippi PADPearl EADPotomac EADRhone PADSanta Clara MDDWaipaoa MDDWaiapu MDD

How does the classification of sediment dispersal systems relate to carbon burial on the margins? We would expect that the fate of river carbon on the margins would be related to its initial composition (including its age and hence “reactivity”). Burial of both terrestrial (river) and marine carbon on the margins is also related to its exposure time to oxygen (and other oxidants) in the surface mixed layer in the seabed, which is in turn inversely related to sediment accumulation rate (see Figure below from Hartnett et al. (1998)).

Figure 3. Relationship between the length of time that sedimentary organic carbon is exposed to oxygen in the uppermost seabed and organic carbon burial efficiency, based on results from sediments on the continental margin offshore from northwestern Mexico and Washington State (from Hartnett et al., 1998)

The figure below, adapted from a recent paper by Blair and Aller (2012) summarize data on the burial efficiency particulate organic carbon on continental margins, including on the continental shelves offshore from many of the rivers that we have examined in this exercise.

A few things to notice:

1) The plot shows a field of data from “typical” continental shelves where the water column is well oxygenated and where sediment accumulation rates vary over several orders of magnitude.

2) The plot shows three fields of data from areas where burial efficiencies differ from typical conditions. These shelf areas include:

a. Margins where the water column is relatively poor (depleted) in dissolved oxygen. These areas are typically near zones of upwelling, where high concentrations of nutrients drive high rates of primary productivity. The decay of organic matter in the water column consumes oxygen and renders these areas of low O2 concentration. Some examples of such margins include those seaward of Peru, Chile, and Pakistan.

b. The topset areas of some deltas, where sediment is extensively reworked (deposited and resuspended multiple times) before it is bypassed to deeper water.

c. The depocenters offshore from small mountainous rivers (SMRs) such as the Waipaoa, Waiapu, and Taiwanese Rivers

Figure 4: Organic carbon burial efficiency in continental margin environments (Aller and Blair (2012) and references cited therein).

For you to do:

1) Add the following data to Figure 4:

Margin Range of sediment accumulation rates

(g cm-2 yr-1)

Burial efficiency of terrestrial carbon

(%)Amazon (topsets) 1 to 8 28Congo (slope adjacent to canyon) 2 48Congo (slope adjacent to canyon) 3 50Fly (topsets) 0.9 to 2 26Ganghes-Brahmaputra 1 to 15 50 to 90Rhone 0.07 19Rhone 0.07 25Rhone 7 80Waipaoa 0.2 to 0.8 55Waiapu 0.8 to 2 90

2) Answer the following questions:A. How would you explain the relationship between organic carbon burial efficiency (% C

preserved) and sediment accumulation rates on continental shelves where the water column is well oxygenated?

B. How would you explain the difference between organic carbon burial efficiency in oxygenated and oxygen-depleted settings on the continental margins?

C. In which types of dispersal systems are the highest carbon burial efficiencies observed? What is the most likely explanation for high carbon burial efficiencies in those systems?

D. What are possible explanations for the high burial efficiency of organic carbon seaward of small mountainous rivers? What factors in both the drainage area (source) and on the continental margins (sink) might contribute to this high efficiency?

E. What is the probable explanation for relatively low burial efficiency in some deltaic environments?

F. The data for the Amazon and Fly systems in table 2 come from the topset areas of the subaqueous deltas. Are there parts of those deltas where you might expect to find higher burial efficiencies? Where and why?

G. How might human activities affect the burial efficiency of carbon on the margins, and what impacts might that have on global climate?

H. Which types of margins might be most conducive to petroleum formation? What is your reasoning?

I.

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