Negar Haghipour

Online 13C and 14C gas measurements by EA-IRMS–AMS at ETH Zürich

Studies using carbon isotopes to understand the global carbon cycle are critical to identify and quantify sources, sinks, and processes and how humans may impact them. 13C and 14C are routinely measured individually; however, there is a need to develop instrumentation that can perform concurrent online analyses that can generate rich data sets conveniently and efficiently. To satisfy these requirements, we coupled an elemental analyzer to a stable isotope mass spectrometer and an accelerator mass spectrometer system fitted with a gas ion source. We first tested the system with standard materials and then reanalyzed a sediment core from the Bay of Bengal that had been analyzed for 14C by conventional methods. The system was able to produce %C, 13C, and 14C data that were accurate and precise, and suitable for the purposes of our biogeochemistry group. The system was compact and convenient and is appropriate for use in a range of fields of research.

Temporal and spatial variability of particle transport in the deep Arctic Canada Basin

To better understand the current carbon cycle and potentially detect its change in the rapidly changing Arctic Ocean, we examined sinking particles collected quasi-continuously over a period of 7 years (2004–2011) by bottom-tethered sediment trap moorings in the central Canada Basin. Total mass flux was very low (<100 mg m−2 d−1) at all sites and was temporally decoupled from the cycle of primary production in surface waters. Extremely low radiocarbon contents of particulate organic carbon and high aluminum contents in sinking particles reveal high contributions of resuspended sediment to total sinking particle flux in the deep Canada Basin. Station A (75°N, 150°W) in the southwest quadrant of the Canada Basin is most strongly influenced while Station C (77°N, 140°W) in the northeast quadrant is least influenced by lateral particle supply based on radiocarbon content and Al concentration. The results at Station A, where three sediment traps were deployed at different depths, imply that the most likely mode of lateral particle transport was as thick clouds of enhanced particle concentration extending well above the seafloor. At present, only 1%–2% of the low levels of new production in Canada Basin surface waters reaches the interior basin. Lateral POC supply therefore appears to be the major source of organic matter to the interior basin. However, ongoing changes to surface ocean boundary conditions may influence both lateral and vertical supply of particulate material to the deep Canada Basin.

Comprehensive radiocarbon analysis of benzene polycarboxylic acids (BPCAs) derived from pyrogenic carbon in environmental samples

Compound-specific radiocarbon analysis (CSRA) of benzene polycarboxylic acids (BPCAs) yields molecular-level, source-specific information necessary to constrain isotopic signatures of pyrogenic carbon. However, the purification of individual BPCAs requires a multistep procedure that typically results in only microgram quantities of the target analyte(s). Such small samples are highly susceptible to contamination by extraneous carbon, which needs to be minimized and carefully accounted for in order to yield accurate results. Here, we undertook comprehensive characterization and quantification of contamination associated with molecular radiocarbon ( 14 C) BPCA analyses through systematic processing of multiple authentic standards with both fossil and modern 14 C signatures at various concentrations. Using this approach, we precisely apportion the contribution of extraneous carbon with respect to the four implemented subprocedures. Assuming a constant source and quantity of extraneous carbon we correct and statistically evaluate uncertainties in resulting 14 C data. Subsequently, we examine the results of triplicate analyses of reference materials representing four different environmental matrices (sediment, soil, aerosol, riverine natural organic matter) and apportion their BPCA sources in terms of carbon residues derived from biomass or fossil fuel combustion. This comprehensive approach to CSRA facilitates retrieval of robust 14 C data, with application in environmental studies of the continuum of pyrogenic carbon.

Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils.

Microbes eat rocks and leave carbon dioxide The reaction of atmospheric carbon dioxide (CO2) with silicate rocks provides a carbon sink that helps counterbalance the release of CO2 by volcanic degassing. However, some types of rocks contain petrogenic organic carbon, the oxidation of which adds CO2 to the atmosphere, counteracting the drawdown by silicates. Hemingway et al. present evidence from the rapidly eroding Central Range of Taiwan showing that microbes oxidize roughly two-thirds of the petrogenic organic carbon there and that the rate of oxidation increases with the rate of erosion. Science, this issue p. 209 The oxidation of organic carbon in rapidly eroding mountain soils is microbially mediated. Lithospheric organic carbon (“petrogenic”; OCpetro) is oxidized during exhumation and subsequent erosion of mountain ranges. This process is a considerable source of carbon dioxide (CO2) to the atmosphere over geologic time scales, but the mechanisms that govern oxidation rates in mountain landscapes are poorly constrained. We demonstrate that, on average, 67 ± 11% of the OCpetro initially present in bedrock exhumed from the tropical, rapidly eroding Central Range of Taiwan is oxidized in soils, leading to CO2 emissions of 6.1 to 18.6 metric tons of carbon per square kilometer per year. The molecular and isotopic evolution of bulk OC and lipid biomarkers during soil formation reveals that OCpetro remineralization is microbially mediated. Rapid oxidation in mountain soils drives CO2 emission fluxes that increase with erosion rate, thereby counteracting CO2 drawdown by silicate weathering and biospheric OC burial.

Global-scale evidence for the refractory nature of riverine black carbon

Wildfires and incomplete combustion of fossil fuel produce large amounts of black carbon. Black carbon production and transport are essential components of the carbon cycle. Constraining estimates of black carbon exported from land to ocean is critical, given ongoing changes in land use and climate, which affect fire occurrence and black carbon dynamics. Here, we present an inventory of the concentration and radiocarbon content (∆14C) of particulate black carbon for 18 rivers around the globe. We find that particulate black carbon accounts for about 15.8 ± 0.9% of river particulate organic carbon, and that fluxes of particulate black carbon co-vary with river-suspended sediment, indicating that particulate black carbon export is primarily controlled by erosion. River particulate black carbon is not exclusively from modern sources but is also aged in intermediate terrestrial carbon pools in several high-latitude rivers, with ages of up to 17,000 14C years. The flux-weighted 14C average age of particulate black carbon exported to oceans is 3,700 ± 400 14C years. We estimate that the annual global flux of particulate black carbon to the ocean is 0.017 to 0.037 Pg, accounting for 4 to 32% of the annually produced black carbon. When buried in marine sediments, particulate black carbon is sequestered to form a long-term sink for CO2.Particulate black carbon in rivers can have ages of up to 17,000 14C years before it is sequestered in the oceans, according to an inventory of particulate black carbon in 18 rivers across the globe.

Correlation of fluvial terraces and temporal steady-state incision on the onshore Makran accretionary wedge in southeastern Iran: Insight from channel profiles and 10Be exposure dating of strath terraces

We describe and summarize the geomorphology of fluvial terraces along the four major rivers draining the central, onshore Makran accretionary wedge in Iran, and describe uplifted marine terraces on the coast of this region. Thirty-five strath terraces at different sites were dated using in situ–produced cosmogenic 10 Be concentrations from surfaces and depth profiles. These new measurements reveal abandonment ages between ca. 15 and 380 ka. The age distribution allows determining the chronology of terrace levels and establishing regional correlations between two major regional levels. The geographically widespread correspondence suggests that these two levels result from a regional, climatically driven force. Systematic dating also provides evidence for time and spatial variations in incision rate, which enables distinguishing between a regionally uniform incision rate of 0.3–0.4 mm/a and higher local incision and/or uplift (~0.8 mm/a) rates. These spatial changes are consistent with where localized tectonic activity is recognized in the field. We also dated, using 14 C in shells, four uplifted marine terraces on the coast of the study area. Comparing incision rates derived from strath terraces (0.3 mm/a) with published uplift estimates from marine terraces (0.2 mm/a) reveals that fluvial rivers responded to a regional, long-term incision and surface uplift pattern. This rate reflects the tectonically steady state of the wedge on a regional scale. Locally high incision rates delineate active faults and folds, indicating that perfect steady state is unlikely on short length scales.

Publisher Correction: Global-scale evidence for the refractory nature of riverine black carbon

In the version of this Article originally published, the units of the x and y axes in Fig. 3a were incorrectly given as ‘mg km–2 yr–1’; the correct units are ‘Mg km–2 yr–1’. These errors have now been corrected in the online versions.

Tectonically-triggered sediment and carbon export to the Hadal zone

Sediments in deep ocean trenches may contain crucial information on past earthquake history and constitute important sites of carbon burial. Here we present 14C data on bulk organic carbon (OC) and its thermal decomposition fractions produced by ramped pyrolysis/oxidation for a core retrieved from the >7.5 km-deep Japan Trench. High-resolution 14C measurements, coupled with distinctive thermogram characteristics of OC, reveal hemipelagic sedimentation interrupted by episodic deposition of pre-aged OC in the trench. Low δ13C values and diverse 14C ages of thermal fractions imply that the latter material originates from the adjacent margin, and the co-occurrence of pre-aged OC with intervals corresponding to known earthquake events implies tectonically triggered, gravity-flow-driven supply. We show that 14C ages of thermal fractions can yield valuable chronological constraints on sedimentary sequences. Our findings shed new light on links between tectonically driven sedimentological processes and marine carbon cycling, with implications for carbon dynamics in hadal environments.Within sediments in deep ocean trenches an earthquake record may be observed. Here, the authors present 14C data on bulk organic carbon (OC) and thermal decomposition from a sediment core in the Japan Trench and match OC values with known earthquake events.

Organic Carbon Aging During Across‐Shelf Transport

Compound-specific radiocarbon analysis was performed on different grain-size fractions of surficial sediments to examine and compare lateral transport times (LTTs) of organic carbon. C aging of long-chain leaf wax fatty acids along two dispersal pathways of fluvially derived material on adjacent continental margins implies LTTs over distances of ~30 to 500 km that range from hundreds to thousands of years. The magnitude of aging differs among grain size fractions. Our finding suggests that LTTs vary both temporally and spatially as a function of the specific properties of different continental shelf settings. Observations suggest that C aging is widespread during lateral transport over continental shelves, with hydrodynamic particle sorting inducing age variations among organic components residing in different grain sizes. Consideration of these phenomena is of importance for understanding carbon cycle processes and interpretation on sedimentary records on continental margins. Plain Language Summary The radiocarbon age of organic matter accumulating in sediments on continental margins can shed light on processes that are involved in carbon cycling in these dynamic marine environments. However, it has proven challenging to constrain the role of hydrodynamic processes on observed radiocarbon ages of sedimentary organic matter. In this study, we examine the radiocarbon ages of source-specific biomarker compounds in different grain-size fractions of surface sediments from two shelf systems in the North Pacific Ocean. We find different patterns of aging among grain size fractions that are attributed to the contrasting hydrodynamic characteristics of the continental shelves. The aging of organic matter during lateral transport across continental shelves emerges as a widespread phenomenon, comprising an important facet of the marine carbon cycle.

Improved Method for Isolation and Purification of Underivatized Amino Acids for Radiocarbon Analysis

We have improved a method for isolation and purification of individual amino acids for compound-specific radiocarbon analysis (CSRA). To remove high-performance liquid chromatography (HPLC) eluent blanks from isolated amino acid fractions prior to the radiocarbon (Δ14C) measurement, each fraction was filtered through a membrane filter and then washed with diethyl ether twice. Radiocarbon measurements on standard amino acids processed and purified with the above method using elemental analyzer-accelerator mass spectrometry resulted in Δ14C values that were in strong agreement ( R2 = 0.998) with the original Δ14C value of each amino acid standard. From these measurements, we calculate dead and modern carbon contamination contributions as 1.2 ± 0.2 and 0.3 ± 0.1 μgC, respectively, which are consistent with direct assessments of HPLC procedural blanks of 1.0 ± 0.8 μgC per sample. These contamination constraints allow correction of measured Δ14C values for accurate and precise CSRA and are widely applicable to future archeological and biogeochemical studies.