Direct measurement of riverine particulate organic carbon age structure

Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 39 (2012): L19703, doi:10.1029/2012GL052883.Carbon cycling studies focusing on transport and transformation of terrigenous carbon sources toward marine sedimentary sinks necessitate separation of particulate organic carbon (OC) derived from many different sources and integrated by river systems. Much progress has been made on isolating and characterizing young biologically-formed OC that is still chemically intact, however quantification and characterization of old, refractory rock-bound OC has remained troublesome. Quantification of both endmembers of riverine OC is important to constrain exchanges linking biologic and geologic carbon cycles and regulating atmospheric CO2 and O2. Here, we constrain petrogenic OC proportions in suspended sediment from the headwaters of the Ganges River in Nepal through direct measurement using ramped pyrolysis radiocarbon analysis. The unique results apportion the biospheric and petrogenic fractions of bulk particulate OC and characterize biospheric OC residence time. Compared to the same treatment of POC from the lower Mississippi-Atchafalaya River system, contrast in age spectra of the Ganges tributary samples illustrates the difference between small mountainous river systems and large integrative ones in terms of the global carbon cycle.This work was partially supported by U.S. National Science Foundation (NSF) Cooperative Agreement OCE-228996 to NOSAMS and NSF grants OCE-0851015 & OCE-0928582 to VG.2013-04-0

Short communication: Massive erosion in monsoonal central India linked to late Holocene land cover degradation

Soil erosion plays a crucial role in transferring sediment and carbon from land to sea, yet little is known about the rhythm and rates of soil erosion prior to the most recent few centuries. Here we reconstruct a Holocene erosional history from central India, as integrated by the Godavari River in a sediment core from the Bay of Bengal. We quantify terrigenous fluxes, fingerprint sources for the lithogenic fraction and assess the age of the exported terrigenous carbon. Taken together, our data show that the monsoon decline in the late Holocene significantly increased soil erosion and the age of exported organic carbon. This acceleration of natural erosion was later exacerbated by the Neolithic adoption and Iron Age extensification of agriculture on the Deccan Plateau. Despite a constantly elevated sea level since the middle Holocene, this erosion acceleration led to a rapid growth of the continental margin. We conclude that in monsoon conditions aridity boosts rather than suppresses sediment and carbon export, acting as a monsoon erosional pump modulated by land cover conditions

Chemical weathering outputs from the flood plain of the Ganga

Transport of sediment across riverine flood plains contributes a significant but poorly constrained fraction of the total chemical weathering fluxes from rapidly eroding mountain belts which has impor- tant implications for chemical fluxes to the oceans and the impact of orogens on long term climate. We report water and bedload chemical analyses from the Ganges flood-plain, a major transit reservoir of sediment from the Himalayan orogen. Our data comprise six major southern tributaries to the Ganga, 31 additional analyses of major rivers from the Himalayan front in Nepal, 79 samples of the Ganga collected close to the mouth below the Farakka barrage every two weeks over three years and 67 water and 8 bedload samples from tributaries confined to the Ganga flood plain,. The flood plain tributaries are characterised by a shallow d 18 O - dD array, compared to the meteoric water line, with a low dD excess from evaporative loss from the flood plain which is mirrored in the higher dD excess of the mountain rivers in Nepal. The stable-isotope data confirms that the waters in the flood plain tributaries are domi- nantly derived from flood plain rainfall and not by redistribution of waters from the mountains. The flood plain tributaries are chemically distinct from the major Himalayan rivers. They can be divided into two groups. Tributaries from a small area around the Kosi river have 87 Sr/ 86 Sr ratios > 0.75 and molar Na/Ca ratios as high as 6. Tributaries from the rest of the flood plain have 87 Sr/ 86 Sr ratios <0.74 and most have Na/Ca ratios <1. One sample of the Gomti river and seven small adjacent tributaries have elevated Na concentrations likely caused by dissolution of Na carbonate salts. The compositions of the carbonate and silicate components of the sediments were determined from sequential leaches of floodplain bedloads and these were used to partition the dissolved cation load between silicate and car- bonate sources. The 87 Sr/ 86 Sr and Sr/Ca ratios of the carbonate inputs were derived from the ace- tic-acid leach compositions and silicate Na/Ca and 87 Sr/ 86 Sr ratios derived from silicate residues from leaching. Modelling based on the 87 Sr/ 86 Sr and Sr/Ca ratios of the carbonate inputs and 87 Sr/ 86 Sr ratios of the silicates indicates that the flood plain waters have lost up to 70% of their Ca (average ~ 50%) to precipitation of secondary calcite which is abundant as a diagenetic cement in the flood plain sedi- ments. 31% of the Sr, 8% of the Ca and 45% of the Mg are calculated to be derived from silicate miner- als. Because of significant evaporative loss of water across the flood plain, and in the absence of hy- drological data for flood plain tributaries, chemical weathering fluxes from the flood plain are best calculated by mass balance of the Na, K, Ca, Mg, Sr, SO 4 and 87 Sr/ 86 Sr compositions of the inputs, comprising the flood plain tributaries, Himalayan rivers and southern rivers, with the chemical dis- charge in the Ganga at Farakka. The calculated fluxes from the flood plain for Na, K, Ca and Mg are within error of those estimated from changes in sediment chemistry across the flood plain (Lupker et al., 2012, Geochemica Cosmochimica Acta). Flood plain weathering supplies between 33 and 48% of the major cation and Sr fluxes and 58% of the alkalinity flux carried by the Ganga at Farakka which compares with 24% supplied by Himalayan rivers and 18% by the southern tributaries

NaV_2O_5 as a quarter-filled ladder compound

A new X-ray diffraction study of the one-dimensional spin-Peierls compound \alpha-NaV_2O_5 reveals a centrosymmetric (Pmmn) crystal structure with one type of V site, contrary to the previously postulated non-centrosymmetric P2_1mn structure with two types of V sites (V^{+4} and V^{+5}). Density functional calculations indicate that NaV_2O_5 is a quarter-filled ladder compound with the spins carried by V-O-V molecular orbitals on the rungs of the ladder. Estimates of the charge-transfer gap and the exchange coupling agree well with experiment and explain the insulating behavior of NaV_2O_5 and its magnetic properties.Comment: Final version for PRL, value of U correcte

The influence of biogenic emissions from Africa on tropical tropospheric ozone during 2006: a global modeling study

We have performed simulations using a 3-D global chemistry-transport model to investigate the influence that biogenic emissions from the African continent exert on the composition of the troposphere in the tropical region. For this purpose we have applied two recently developed biogenic emission inventories provided for use in large-scale global models (Granier et al., 2005; LathiSre et al., 2006) whose seasonality and temporal distribution for biogenic emissions of isoprene, other volatile organic compounds and NO is markedly different. The use of the 12 year average values for biogenic emissions provided by LathiSre et al. (2006) results in an increase in the amount of nitrogen sequestrated into longer lived reservoir compounds which contributes to the reduction in the tropospheric ozone burden in the tropics. The associated re-partitioning of nitrogen between PAN, HNO3 and organic nitrates also results in a similar to 5% increase in the loss of nitrogen by wet deposition. At a global scale there is a reduction in the oxidizing capacity of the model atmosphere which increases the atmospheric lifetimes of CH4 and CO by similar to 1.5% and similar to 4%, respectively. Comparisons against a range of different measurements indicate that applying the 12 year average of LathiSre et al. (2006) improves the performance of TM4_AMMA for 2006 in the tropics. By the use of sensitivity studies we show that the release of NO from soils in Africa accounts for between similar to 2-45% of tropospheric ozone in the African troposphere, similar to 10% in the upper troposphere and between similar to 5-20% of the tropical tropospheric ozone column over the tropical Atlantic Ocean. The subsequent reduction in OH over the source regions allows enhanced transport of CO out of the region. For biogenic volatile organic C1 to C3 species released from Africa, the effects on tropical tropospheric ozone are rather limited, although this source contributes to the global burden of VOC by between similar to 2-4% and has a large influence on the organic composition of the troposphere over the tropical Atlantic Ocean

Weak ferromagnetism and field-induced spin reorientation in K2V3O8

Magnetization and neutron diffraction measurements indicate long-range antiferromagnetic ordering below TN=4 K in the 2D, S=1/2 Heisenberg antiferromagnet K2V3O8. The ordered state exhibits ``weak ferromagnetism'' and novel, field-induced spin reorientations. These experimental observations are well described by a classical, two-spin Heisenberg model incorporating Dzyaloshinskii-Moriya interactions and an additional c-axis anisotropy. This additional anisotropy can be accounted for by inclusion of the symmetric anisotropy term recently described by Kaplan, Shekhtman, Entin-Wohlman, and Aharony. This suggests that K2V3O8 may be a very unique system where the qualitative behavior relies on the presence of this symmetric anisotropy.Comment: 5 pages, 4 ps figures, REVTEX, submitted to PR

Assessing the blank carbon contribution, isotope mass balance, and kinetic isotope fractionation of the Ramped Pyrolysis/Oxidation instrument at NOSAMS

Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Radiocarbon 59 (2017): 179-193, doi:10.1017/RDC.2017.3.We estimate the blank carbon mass over the course of a typical Ramped PyrOx (RPO) analysis (150 to 1000 °C; 5 °C×min-1) to be (3.7 ± 0.6) μg C with an Fm value of 0.555 ± 0.042 and a δ13C value of (-29.0 ± 0.1) ‰ VPDB. Additionally, we provide equations for RPO Fm and δ13C blank corrections, including associated error propagation. By comparing RPO mass-weighted mean and independently measured bulk δ13C values for a compilation of environmental samples and standard reference materials (SRMs), we observe a small yet consistent 13C depletion within the RPO instrument (mean – bulk: μ = -0.8 ‰; ±1σ = 0.9 ‰; n = 66). In contrast, because they are fractionation-corrected by definition, mass-weighted mean Fm values accurately match bulk measurements (mean – bulk: μ = 0.005; ±1σ = 0.014; n = 36). Lastly, we show there exists no significant intra-sample δ13C variability across carbonate SRM peaks, indicating minimal mass-dependent kinetic isotope fractionation during RPO analysis. These data are best explained by a difference in activation energy between 13C- and 12C-containing compounds (13–12ΔE) of 0.3 to 1.8 J×mol-1, indicating that blank and mass-balance corrected RPO δ13C values accurately retain carbon source isotope signals to within 1 to 2‰.J.D.H. was partly supported by the NSF Graduate Research Fellowship Program under grant number 2012126152; V.V.G. was partly supported by the US National Science Foundation (grants OCE- 0851015 and OCE-0928582), the WHOI Coastal Ocean Institute (grant 27040213) and an Independent Study Award (grant 27005306) from WHOI; G.S. and P.K.Z. were supported by the WHOI Postdoctoral Scholar Program with funding provided by NOSAMS (OCE-1239667)

Diverse soil carbon dynamics expressed at the molecular level

The stability and potential vulnerability of soil organic matter (SOM) to global change remains incompletely understood due to the complex processes involved in its formation and turnover. Here we combine compound-specific radiocarbon analysis with fraction-specific and bulk-level radiocarbon measurements in order to further elucidate controls on SOM dynamics in a temperate and sub-alpine forested ecosystem. Radiocarbon contents of individual organic compounds isolated from the same soil interval generally exhibit greater variation than those among corresponding operationally-defined fractions. Notably, markedly older ages of long-chain plant leaf wax lipids (n-alkanoic acids) imply that they reflect a highly stable carbon pool. Furthermore, marked 14C variations among shorter- and longer-chain n-alkanoic acid homologues suggest that they track different SOM pools. Extremes in SOM dynamics thus manifest themselves within a single compound class. This exploratory study highlights the potential of compound-specific radiocarbon analysis for understanding SOM dynamics in ecosystems potentially vulnerable to global change