Invited ReviewCenters of organic carbon burial and oxidation at the land-ocean interface
Introduction
Mitigating and adapting to global environmental change associated with land-use and climate are among society’s grand challenges for the future. A corresponding challenge in carbon (C)-cycle research is the determination of global inventories and fluxes in a heterogeneous, dynamic world. Rates and inventories are uneven spatially and temporally. Scientists often gravitate to regions where inventories, rates and/or fluxes are the highest because they provide a greater than average accounting per unit area relative to sampling effort. Here, we adopt the convention of McClain et al. (2003), who defined biogeochemical hot spots as “patches that show disproportionately high reaction rates relative to the surrounding matrix” and hot moments as “short periods of time that exhibit disproportionately high reaction rates relative to longer intervening time periods.” The hot spots and/or moments provide useful insights in to processes, all the more necessary if we are to understand how the Earth System will respond to future anthropogenic perturbations. In this review, hot spots and moments within the continental margin, a zone that has been characterized itself as a global hot spot in terms of C-sequestration over geological timescales, will be discussed from the perspective of the controls on C-burial and/or oxidation, as well as the character of the organic C reaching sedimentary systems.
Section snippets
Background: a primer on OC burial in the coastal ocean margin
The major hot spot for the burial of much of the organic carbon (OC) in the global ocean is the coastal margin. In particular, river delta and non-deltaic shelf regions bury an estimated 114 Tg C/year and 70 Tg C/year, respectively, with only ca. 6 Tg C/year buried in the open ocean (Burdige, 2005). While there has been longstanding general agreement that continental shelves represent the largest sink of both terrestrial (OCterr) and marine (OCmar) OC in the global ocean (Berner, 1982, Hedges,
Our approach
The behavior of OC in terms of whether it is oxidized or ultimately preserved via burial in the seabed has often been described to be a result of the “labile” or “refractory” nature of the OC (Bianchi, 2011, Schmidt et al., 2011). These terms, which are commonly used to describe differences in the quality of OC as a food resource for heterotrophs, may only be valid in the context of the ambient environment and over a particular timescale of observations (Burdige, 2006). In reality, the
Linkages between depositional environments, OC and mineral-aggregate associations and hydrodynamic sorting
The OC entering coastal hotspots is typically characterized as a variable mixture of vascular plant, algal debris, diagenetically-altered material (e.g. aged soil C and resuspended sedimentary OC), and petrogenic OC (primarily on active margins) derived from the weathering of sedimentary rocks (Blair and Aller, 2012). These components reside across a range of particle density and size and thereby are susceptible to partial separation via hydrodynamic sorting prior to final burial (Wakeham et
Linkages between residence time and bulk OC age, abundance and source
The average composition of the OC that is transported to the coastal ocean strongly reflects properties of the watershed and the residence time of OC and sediments therein (Blair and Aller, 2012). In general, residence times in small, mountainous drainage basins characteristic of active margins and rapidly eroding source regions are less than the residence times in low-gradient river systems with extensive lowlands, the latter often characteristic of passive tectonic margins. This results in
Lignin biomarkers as tools for assessing the fate of terrestrial OC
Lignin may be the most extensively studied terrestrial biomarker because of its abundance. It is the second most abundant biopolymer on Earth, constituting ca. 20–30% of vascular plant tissue (Kirk and Farrell, 1987), and mechanisms of its breakdown and preservation are thus an important aspect of Earth’s carbon cycle. In fact, the high rate of carbon burial in coastal systems has been in large part attributed to delivery of terrestrial plant-and soil-sourced OC that has been stabilized by its
Future questions and directions
How do biomarkers and isotopes reflect differences in the spatio-temporal evolution of organic matter signals – from pre-aging, deposition, redistribution and burial in “hotspot” environments and during “hot moment” events? What additional information can be gained from biomarkers, which in many cases only represent a relatively small fraction of the TOC, beyond that obtained from bulk measurements? How can serial oxidation methods (e.g. ramped PyrOx) be used to bridge the gap between bulk and
Overview
- 1.
Coastal systems represent potential hot spots and hot moments of carbon burial and oxidation at the land-ocean interface. To better understand how the Anthropocene is altering the mineralization and burial of OC in different zones of the coastal ocean, more emphasis needs to be made on the spatial heterogeneity, as demonstrated in Critical Zone Observatory (CZO) work (largely in terrestrial systems).
- 2.
The relationship between %OC, average grain size and surface area in surface sediments continues
Acknowledgements
We would to thank the reviewers for their helpful comments, which greatly improved this manuscript. T.S.B. would also like to thank the Jon and Beverly Thompson Chair for providing funds that were in part, used to support work on this manuscript. Finally, we would like to thank Dr. Rui Bao for his help with modifications to Fig. 4.
References (255)
- et al.
The abundance and significance of a class of large, transparent organic particles in the ocean
Deep Sea Research Part I: Oceanographic Research Papers
(1993) Mobile deltaic and continental shelf muds as suboxic, fluidized bed reactors
Marine Chemistry
(1998)- et al.
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
(2004) - et al.
Carbon remineralization in the Amazon-Guianas tropical mobile mudbelt: a sedimentary incinerator
Continental Shelf Research
(2006) - et al.
Changes in organic matter–mineral interactions for marine sediments with varying oxygen exposure times
Geochimica et Cosmochimica Acta
(2007) - et al.
Quantifying the degradation of organic matter in marine sediments: a review and synthesis
Earth-Science Reviews
(2013) - et al.
Molecular indicators of the sources and transformations of dissolved organic matter in the Mississippi River plume
Organic Geochemistry
(2001) - et al.
The effect of grain size and surface area on organic matter, lignin and carbohydrate concentration, and molecular compositions in Peru Margin sediments
Geochimica et Cosmochimica Acta
(1997) - et al.
The science of hypoxia in the Northern Gulf of Mexico: a review
Science of the Total Environment
(2010) - et al.
Sources of terrestrially-derived organic carbon in lower Mississippi River and Louisiana shelf sediments: implications for differential sedimentation and transport at the coastal margin
Marine Chemistry
(2002)