626 research outputs found
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><sup>13</sup>C) 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><sup>13</sup>C 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>CO<sub>2</sub>) throughout the last 25,000 years. This suggests that increasing <i>p</i>CO<sub>2</sub> levels during post-industrialization may prompt future declines in regional biodiversity (1–10 species per unit CO<sub>2</sub> 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 <sup>14</sup>C 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)
Compound-specific radiocarbon dating of the varved Holocene sedimentary record of Saanich Inlet, Canada
Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 19 (2004): PA2012, doi:10.1029/2003PA000927.The radiocarbon contents of various biomarkers extracted from the varve-counted sediments of Saanich Inlet, Canada, were determined to assess their applicability for dating purposes. Calibrated ages obtained from the marine planktonic archaeal biomarker crenarchaeol compared favorably with varve-count ages. The same conclusion could be drawn for a more general archaeal biomarker (GDGT-0), although this biomarker proved to be less reliable due to its less-specific origin. The results also lend support to earlier indications that marine crenarchaeota use dissolved inorganic carbon (DIC) as their carbon source. The average reservoir age offset ΔR of 430 years, determined using the crenarchaeol radiocarbon ages, varied by ±110 years. This may be caused by natural variations in ocean-atmosphere mixing or upwelling at the NE Pacific coast but variability may also be due to an inconsistency in the marine calibration curve when used at sites with high reservoir ages.This work was supported by the Netherlands Organization
for Scientific Research (NWO) and NSF grants OCE-9907129 and
OCE-0137005 (Eglinton)
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