1. River-dominated margins and the carbon cycle

2. Outline I.Introduction to the carbon cycle– short-term and long-term II.The role of continental margins in the long- term carbon cycle III.Variability of OC sources to rivers IV.Sediment dispersal processes and the fate of riverine carbon on the margins

3. The Carbon Cycle– short term and long term When we discuss the carbon cycle, we are referring to the transfer of carbon between the atmosphere, hydrosphere (e.g., the oceans), lithosphere (the solid earth), and biosphere (living things) These transfers occur on a wide range of time scales– it is useful to separate the short-term carbon cycle from the long-term cycle http://nas-sites.org/americasclimatechoices/more-resources-on- climate-change/climate-change-lines-of-evidence-booklet/evidence- impacts-and-choices-figure-gallery/figure4/

4. The short-term carbon cycle The short-term carbon cycle refers to transfers of C that occur over time- scales of days to thousands of years It involves the up-take of CO 2 by plants on land and phytoplankton in the oceans, the transfer of C to soils by dropping of leaves, root growth, respiration, etc., the consumption of plants by herbivores, and of herbivores by carnivores It also involves the release of CO 2 due to the decomposition of plants and animals by microorganisms, and the exchange of CO 2 between the oceans and atmosphere http://scied.ucar.edu/carbon-cycle

5. The long-term carbon cycle The long-term carbon cycle involves transfer of C over time-scales of millions of years, to and from the rocks that comprise the solid Earth These transfers include the burial of organic carbon and carbonate (inorganic carbon) in marine sediments and eventually into the rock record Figure used with permission from Steven Petsch, Umass Amherst

6. The long-term carbon cycle The long term carbon cycle also includes the weathering of silicate minerals near the Earth’s surface, which converts CO 2 to dissolved HCO 3 - and ultimately leads to precipitation of carbonate minerals The subduction of sediments and rocks containing carbonate and organic carbon and the release of CO 2 from subduction and metamorphism are additional components of the long-term carbon cycle Figure used with permission from Steven Petsch, Umass Amherst

7. Carbon burial on the continental margins The continental margins play a particularly key role in particulate organic carbon (POC) burial– it is estimated that about 90% of all POC burial on Earth today takes place on the margins, including in deltas, non-deltaic areas of continental shelves, and on the continental slopes. Average annual particulate organic carbon flux [gC m −2 y −1 ] deposited on the ocean bottom between 1998 and 2001 Geophysical Research Letters Volume 32, Issue 1, L01602, 6 JAN 2005 DOI: 10.1029/2004GL021346 http://onlinelibrary.wiley.co m/doi/10.1029/2004GL021 346/full#grl19030-fig-0001 Volume 32, Issue 1, http://onlinelibrary.wiley.co m/doi/10.1029/2004GL021 346/full#grl19030-fig-0001

8. Carbon burial on the continental margins It is important to realize that a very small proportion of the POC that is delivered to the seabed survives to be buried (<1%) This small proportion, which we can consider as a “slow leak” from the short-term to the long-term carbon cycle, however, has played a key role in regulating atmospheric composition and climate over geologic history, and is the raw ingredient for fossil fuels (oil, gas), on which our society depends Raymond, 2005 Redrawn from Ruddiman, 2001

9. River-dominated margins River-dominated continental margins are especially important sites of particulate organic carbon burial Factors that promote POC burial in these settings include:  Large fluxes of organic matter from terrestrial environments  Large supplies of nutrients, leading to productive water columns  Recalcitrant nature of some of the organic matter  Rapid burial in sediments, minimizing exposure to oxidants  Occlusion by/sorption to mineral particles, inhibiting degradation

10. River-dominated margins are not all alike! Recent research has revealed that the amount, composition, and age of particulate organic carbon on river-dominated margins depends on the characteristics of both the river catchments and the adjacent continental shelves

11. Sources of particulate organic carbon to rivers The particulate organic carbon carried by rivers to the oceans is a mixture of material from a number of sources It ranges from young and easily degraded material (e.g., fresh water algae) to old and very resistant to breakdown (e.g. kerogen or rock carbon)

12. Riverine POC = A complex mixture Riverine primary productivity Riverine primary productivity Plant debris Plant debris Soil carbon Soil carbon Rock carbon Rock carbon (kerogen) (kerogen) Less reactive Chichagof Island, NOAA

13. Less reactive The average radiocarbon ( 14 C) content of river particulate organic carbon (POC) reflects the “recipe,” i.e. the proportions of young (labile) and old (refractory) The mixture also differs within each river, especially with different flows Sediment yield from Milliman and Farnsworth, 2011; F m data compiled from numerous sources by L. Leithold (see Mini Lesson list of references)

14. Passive vs. active margin rivers In general, the mixtures of POC discharged by large passive margin rivers differ from those discharged by small, active margin rivers Large passive margin rivers typically have large flood plains where sediments and associated carbon may reside for 1000’s of years Particulate organic carbon discharged from such rivers is typically derived from soils with a range of ages Amazon Flood Plain

15. Passive vs active margin rivers Active margin rivers typically have only small flood plains Material is commonly introduced to the channels during storm events, when mass wasting (e.g. landslides) may mobilize sedimentary bedrock, thin and relatively young soils, and biomass (e.g., trees) Eel River, California during flood (photo courtesy of Jeff Borgeld)

16. Blair and Aller 2012 Large, low relief catchments, thick soils Large, low relief catchments, thick soils Large flood plains, with extensive storage Large flood plains, with extensive storage Small catchments with steep slopes, frequent mass wasting, thin soils Small catchments with steep slopes, frequent mass wasting, thin soils Minimal flood plain storage Minimal flood plain storage modern terrestrial C kerogen C modern marine C modern + aged terrestrial C Modified from Blair and Aller, 2012 Courtesy of Lonnie Leithold

17. The fate of riverine POC on the margins The fate of riverine POC on the margins depends in part on sediment dispersal processes Rapid burial in sediments decreases the exposure time of POC to oxygen in the water column and uppermost seabed, and favors high “burial efficiencies” In contrast, extensive reworking of sediments and POC by currents and waves results in low burial efficiencies High burial efficiencies offshore from small mountainous rivers (SMRs) are likely related to rapid, episodic deposition, as well as the recalcitrance of the OC (e.g., large components of rock carbon)

18. Two contrasting systems POC burial has been studied extensively on two very different margins as part of the NSF Margins S2S program Two Margins S2S focus sites: Fly River/Gulf of Papua (GoP), Papua New Guinea Waipaoa Sedimentary System, North Island, New Zealand

19. COMPARISON OF DISPERSAL SYSTEMS Fly Waipaoa Climatic settingwet tropicaltemperate Tectonic settingforeland basinforearc basin Basin size75,000 km 2 2,200 km 2 Sediment Dischargelarge (10 8 t/y)moderate (10 7 t/y) Lowland floodplainlong tidal reachtrivial tidal reach River mouthdelta, mangrovesstrand plain http://malumnalu.blogspot.com/2013/01/a- magical-journey-up-fly-river.html Courtesy of Lonnie Leithold

20. COMPARISON OF DISPERSAL SYSTEMS Fly Waipaoa Discharge eventsunrelated to oceancorrelated to ocean Gravity flowssaltwateralso freshwater (fluid mud)(hyperpycnal flows) Shelf sedimentationprograding basin infill clinoforms http://web.vims.edu/margins/

21. The Fly sedimentary system The Fly River, Papua New Guinea, is a large, tropical, passive margin system that begins in the Finisterre mountains, and flows across a broad floodplain built on a subsiding foreland basin The Fly River discharges to the Gulf of Papua, where strong tidal currents disperse sediment across a prograding, subaqueous delta http://malumnalu.blogspot.com/2013/01/a- magical-journey-up-fly-river.html

22. The Fly System POC discharged from the Fly River is derived mostly from soils POC burial efficiencies are low on the topset regions of the subaqueous delta, where extensive reworking and bypass of sediments takes place In contrast, terrestrial POC is being efficiently buried on actively prograding portions of the subaqueous delta (see, e.g., Goni et al., 2006, 2008) Modified from Aller and Blair, 2004

23. The Waipaoa System The Waipaoa River is a small river located on the east coast of the North Island of New Zealand It sits within the forearc area of the Hikurangi subduction zone The Waipaoa is one of the muddiest rivers in the world– with a sediment yield of 6800 tons km -2 y -1 The large sediment yield is related to steep slopes, erodible, tectonically- crushed lithologies, episodically intense rainfall, and anthropogenic deforestation The Waipaoa has only a small flood plain

24. POC in the Waipaoa system The POC discharged by the Waipaoa River is a mixture of young plant debris, soil organic matter, and rock carbon (kerogen) Both terrestrial POC and marine OC are efficiently buried on the adjacent margin The high burial efficiency appears to be related to high sedimentation rates in subsiding offshore depocenters as well as to the recalcitrant nature of some of the OC discharged (e.g., kerogen) Redrawn from Brackley et al., 2010 Apportionment of POC buried in the Waipaoa system to various sources

25. Summary Continental margins are important sites of organic carbon burial, serving as a link between short-term and long-term components of the carbon cycle The sources, age, and reactivity of the POC buried on margins is variable and depends in part on tectonic setting

26. Sources Aller, Robert C., and Neal E. Blair. "Early diagenetic remineralization of sedimentary organic C in the Gulf of Papua deltaic complex (Papua New Guinea): Net loss of terrestrial C and diagenetic fractionation of C isotopes." Geochimica et Cosmochimica Acta 68, no. 8 (2004): 1815-1825. Berner, R.A., 2004, The Phanerozoic carbon cycle: Oxford University Press, New York, 150 pp. Blair, N.E., and Aller, R.C., 2012, The fate of terrestrial organic carbon in the marine environment: Annual Review of Marine Science 4: 401-423. Brackley et al., 2010, Dispersal and transformation of organic carbon across an episodic, high sediment discharge continental margin, Waipaoa Sedimentary System, New Zealand: Marine Geology 270, 202-212. Goni, M.A., et al., 2006, Distribution and sources of particulate organic matter in the water column and sediments of the Fly River Delta, Gulf of Papua (Papua New Guinea): Estuarine, Coastal, and Shelf Science 65, 225-245. Goni, M.A., et al., 2008, Terrigenous organic matter in sediments from the Fly River delta- clinoform system (Papua New Guinea): Journal of Geophysical Res., Earth Surface 113, 1-27. Milliman, J.D., and Farnsworth, K.L., 2011, River Discharge to the Coastal Ocean: Cambridge University Press, 384 pp. Raymond, P.A., 2005, Carbon cycle: The age of the Amazon's breath Nature 436, 469-470 Nature 436, 469-470 Ruddiman, W.F., 2010, Earth’s Climate Past and Future, WH Freeman Pub.