The radiocarbon age of organic carbon in marine surface sediments

Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 74 (2010): 6788-6800, doi:10.1016/j.gca.2010.09.001.Long-term carbon cycling and climate change are strongly dependent on organic carbon (OC) burial in marine sediments. Radiocarbon (14C) has been widely used to constrain the sources, sinks, and processing of sedimentary OC. To elucidate the dominant controls on the radiocarbon content of total organic carbon (14CTOC) accumulating in surface sediments we construct a box model that predicts 14CTOC in the sediment mixed layer (measured as fraction modern, Fm). Our model defines three distinct OC pools (“degradable,” “semi-labile,” and “refractory”) and assumes that 14CTOC flux to sediments is exclusively derived from surface ocean primary productivity, and hence follows a “generic” surface ocean dissolved inorganic carbon (DIC) bomb curve. Model predictions are compared to a set of 75 surface sediment samples, which span a wide geographic range and reflect diverse water column and depositional conditions, and for which sedimentation rate and mixed layer depth are well characterized. Our model overestimates the Fm value for a majority (65%) of these sites, especially at shallow water depths and for sites characterized by depleted δ13CTOC values. The model is most sensitive to sedimentation rate and mixed-layer depth. Therefore, slight changes to these parameters can lead to a match between modeled and measured Fm values at many sites. Because of model sensitivity, slight changes in sedimentation rate and mixed layer depth can allow predictions to match measured Fm at many sites. Yet, in some cases, we find that measured Fm values cannot be simulated without large and unrealistic changes to sedimentation rate and mixed layer depth. These results point to sources of pre-aged OC to surface sediments and implicate soil-derived terrestrial OC, reworked marine OC, and/or anthropogenic carbon as important components of the organic matter present in surface sediments. This approach provides a valuable framework within which to explore controls on sedimentary organic matter composition and carbon burial over a range of spatial and temporal scales.This work was supported by NSF grants OCE-0526389 (W. Martin), OCE-0851350 and OCE-0402533 (T. Eglinton), as well as WHOI Senior Scientist Chair and Independent Study Award funds (T. Eglinton)

The exposure history of the Apollo 16 site: An assessment based on methane and hydrolysable carbon

Nineteen soils from eight stations at the Apollo 16 landing site have been analyzed for methane and hydrolysable carbon. These results, in conjunction with published data from photogeology, bulk chemistry, rare gases, primordial and cosmogenic radionuclides, and agglutinate abundances have been interpreted in terms of differing contributions from three components-North and South Ray Crater ejecta and Cayley Plains material

Isotopic variance among plant lipid homologues correlates with biodiversity patterns of their source communities

<div><p>Plant diversity is important to human welfare worldwide, and this importance is exemplified in subtropical and tropical [(sub)tropical] African savannahs where regional biodiversity enhances the sustaining provision of basic ecosystem services available to millions of residents. Yet, there is a critical lack of knowledge about how savannahs respond to climate change. Here, we report the relationships between savannah vegetation structure, species richness, and bioclimatic variables as recorded by plant biochemical fossils, called biomarkers. Our analyses reveal that the stable carbon isotope composition (<i>δ</i>¹³C) of discrete sedimentary plant biomarkers reflects vegetation structure, but the isotopic range among plant biomarkers–which we call LEaf Wax Isotopic Spread (LEWIS)–reflects species richness. Analyses of individual biomarker <i>δ</i>¹³C values and LEWIS for downcore sediments recovered from southeast Africa reveal that the region’s species richness mirrored trends in atmospheric carbon dioxide concentration (<i>p</i>CO₂) throughout the last 25,000 years. This suggests that increasing <i>p</i>CO₂ levels during post-industrialization may prompt future declines in regional biodiversity (1–10 species per unit CO₂ p.p.m.v.) through imminent habitat loss or extinction.</p></div

On the stratigraphic integrity of leaf-wax biomarkers in loess paleosols

Paleoenvironmental and paleoclimate reconstructions based on molecular proxies, such as those derived from leaf-wax biomarkers, in loess-paleosol sequences represent a promising line of investigation in Quaternary research. The main premise of such reconstructions is the synsedimentary deposition of biomarkers and dust, which has become a debated subject in recent years. This study uses two independent approaches to test the stratigraphic integrity of leaf-wax biomarkers: (i) long-chain n-alkanes and fatty acids are quantified in two sediment-depth profiles in glacial till on the Swiss Plateau, consisting of a Holocene topsoil and the underlying B and C horizons. Since glacial sediments are initially very poor in organic matter, significant amounts of leaf-wax biomarkers in the B and C horizons of those profiles would reflect postsedimentary root-derived or microbial contributions. (ii) Compound-specific radiocarbon measurements are conducted on n-alkanes and n-alkanoic (fatty) acids from several depth intervals in the loess section "Crvenka", Serbia, and the results are compared to independent estimates of sediment age. <br><br> We find extremely low concentrations of plant-wax n-alkanes and fatty acids in the B and C horizons below the topsoils in the sediment profiles. Moreover, compound-specific radiocarbon analysis yields plant-wax ¹⁴C ages that agree well with published luminescence ages and stratigraphy of the Serbian loess deposit. Both approaches confirm that postsedimentary, root-derived or microbial contributions are negligible in the two investigated systems. The good agreement between the ages of odd and even homologues also indicates that reworking and incorporation of fossil leaf waxes is not particularly relevant either

Fermentable sugar profile as an alternative to Apparent Attenuation Limit for selection in Barley Breeding

R.L. Fox, S.J. Logue and J.K. Eglinto

An evaluation of 14C age relationships between co-occurring foraminifera, alkenones, and total organic carbon in continental margin sediments

Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 20 (2005): PA1016, doi:10.1029/2004PA001103.Radiocarbon age relationships between co-occurring planktic foraminifera, alkenones and total organic carbon in sediments from the continental margins of Southern Chile, Northwest Africa and the South China Sea were compared with published results from the Namibian margin. Age relationships between the sediment components are site-specific and relatively constant over time. Similar to the Namibian slope, where alkenones have been reported to be 1000 to 4500 years older than co-occurring foraminifera, alkenones were significantly (~1000 yrs) older than co-occurring foraminifera in the Chilean margin sediments. In contrast, alkenones and foraminifera were of similar age (within 2σ error or better) in the NW African and South China Sea sediments. Total-organic-matter and alkenone ages were similar off Namibia (age difference TOC-alkenones: 200-700 years), Chile (100-450 years), and NW Africa (360-770 years), suggesting minor contributions of pre-aged terrigenous material. In the South China Sea total organic carbon is significantly (2000-3000 yrs) older due to greater inputs of pre-aged terrigenous material. Age offsets between alkenones and planktic foraminifera are attributed to lateral advection of organic matter. Physical characteristics of the depositional setting, such as sea-floor morphology, shelf width, and sediment composition, may control the age of co-occurring 2 sediment components. In particular, offsets between alkenones and foraminifera appear to be greatest in deposition centers in morphologic depressions. Aging of organic matter is promoted by transport. Age offsets are correlated with organic richness, suggesting that formation of organic aggregate is a key process.GM and MK acknowledge financial support from the WHOI postdoctoral scholarship program. This work was funded by NSF grant OCE-0327405

Collective behavior of composite active particles

We describe simulations of active Brownian particles carried out to explore how dynamics and clustering are influenced by particle shape. Our particles are composed of four disks, held together by springs, whose relative size can be varied. These composite objects can be tuned smoothly from having a predominantly concave to a convex surface. We show that even two of these composite particles can exhibit collective motion which modifies the effective Peclet number. We then investigate how particle geometry can be used to explain the phase behavior of many such particles

Collective behavior of composite active particles

We describe simulations of active Brownian particles carried out to explore how dynamics and clustering are influenced by particle shape. Our particles are composed of four disks, held together by springs, whose relative size can be varied. These composite objects can be tuned smoothly from having a predominantly concave to a convex surface. We show that even two of these composite particles can exhibit collective motion which modifies the effective Peclet number. We then investigate how particle geometry can be used to explain the phase behavior of many such particles

Carbon dynamics in the western Arctic Ocean : insights from full-depth carbon isotope profiles of DIC, DOC, and POC

© The Author(s), 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Biogeosciences 9 (2012): 1217-1224, doi:10.5194/bg-9-1217-2012.Arctic warming is projected to continue throughout the coming century. Yet, our currently limited understanding of the Arctic Ocean carbon cycle hinders our ability to predict how changing conditions will affect local Arctic ecosystems, regional carbon budgets, and global climate. We present here the first set of concurrent, full-depth, dual-isotope profiles for dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and suspended particulate organic carbon (POCsusp) at two sites in the Canada Basin of the Arctic Ocean. The carbon isotope composition of sinking and suspended POC in the Arctic contrasts strongly with open ocean Atlantic and Pacific sites, pointing to a combination of inputs to Arctic POCsusp at depth, including surface-derived organic carbon (OC), sorbed/advected OC, and OC derived from in situ DIC fixation. The latter process appears to be particularly important at intermediate depths, where mass balance calculations suggest that OC derived from in situ DIC fixation contributes up to 22% of POCsusp. As in other oceans, surface-derived OC is still a dominant source to Arctic POCsusp. Yet, we suggest that significantly smaller vertical POC fluxes in the Canada Basin make it possible to see evidence of DIC fixation in the POCsusp pool even at the bulk isotope level.The 2008 JOIS hydrographic program was supported by Fisheries and Oceans Canada, the Canadian International Polar Year Office, and the US National Science Foundation (OPP-0424864; lead-PI Andrey Proshutinsky)

Fluvial organic carbon cycling regulated by sediment transit time and mineral protection

Rivers transfer terrestrial organic carbon (OC) from mountains to ocean basins, playing a key role in the global carbon cycle. During fluvial transit, OC may be oxidized and emitted to the atmosphere as CO2 or preserved and transported to downstream depositional sinks. The balance between oxidation and preservation determines the amount of particulate OC (POC) that can be buried long term, but the factors regulating this balance are poorly constrained. Here, we quantify the effects of fluvial transit on POC fluxes along an ~1,300 km lowland channel with no tributaries. We show that sediment transit time and mineral protection regulate the magnitude and rate of POC oxidation, respectively. Using a simple turnover model, we estimate that annual POC oxidation is a small percentage of the POC delivered to the river. Modelling shows that lateral erosion into POC-rich floodplains can increase POC fluxes to downstream basins, thereby offsetting POC oxidation. Consequently, rivers with high channel mobility can enhance CO2 drawdown while management practices that stabilize river channels may reduce the potential for CO2 drawdown