Ann P. McNichol

Origins of lipid biomarkers in Santa Monica Basin surface sediment: a case study using compound-specific Δ14C analysis

Abstract Compound-specific Δ 14 C values are reported for 31 different lipid biomarker molecules obtained from Santa Monica Basin and Santa Barbara Basin surface sediments. These organic compounds represent phytoplanktonic, zooplanktonic, bacterial, archaeal, terrestrial higher plant, and fossil carbon sources. The lipid classes include the following: long-chain n -alkanes, fatty acids (as methyl esters; FAMEs), n -alcohols, C 30 midchain ketols and diols, sterols, hopanols, and C 40 isoprenoid side chains from the ether-linked glycerols of Archaea . The data show that the carbon source for the majority of the biomarkers is marine euphotic zone primary production or subsequent heterotrophic consumption of this biomass. A small amount of benthic incorporation of 14 C-depleted dissolved inorganic carbon was identified for the bacterial hopanols and C 15 linear and branched-chain fatty acids. However, there is no apparent uptake of 14 C-depleted dissolved inorganic carbon in Santa Monica Basin by the bacteria, including filamentous Beggiatoa spp., that produce C 18:1ω7 fatty acid. Two of the lipid classes did not reflect carbon originally fixed by marine photoautotrophs. These were the n -alkanes, for which the Δ 14 C data are consistent with mixed fossil carbon and contemporary terrestrial higher plant sources, and the archaeal isoprenoids, for which the Δ 14 C data are consistent with chemoautotrophic growth below the euphotic zone.

Microscale AMS 14C measurement at NOSAMS

Techniques for making precise and accurate radiocarbon accelerator mass spectrometry (AMS) measurements on samples containing less than a few hundred micrograms of carbon are being developed at the NOSAMS facility. A detailed examination of all aspects of the sample preparation and data analysis process shows encouraging results. Small quantities of CO (sub 2) are reduced to graphite over cobalt catalyst at an optimal temperature of 605 degrees C. Measured (super 14) C/ (super 12) C ratios of the resulting targets are affected by machine-induced isotopic fractionation, which appears directly related to the decrease in ion current generated by the smaller sample sizes. It is possible to compensate effectively for this fractionation by measuring samples relative to small standards of identical size. Examination of the various potential sources of background (super 14) C contamination indicates that the sample combustion process is the largest contributor, adding ca. 1 mu g of carbon with a less-than-modern (super 14) C concentration.

TIC, TOC, DIC, DOC, PIC, POC — unique aspects in the preparation of oceanographic samples for 14C-AMS

Abstract The radiocarbon content of discrete carbon pools (total (T), dissolved (D), and particulate (P) inorganic (I) and organic (O) carbon (C)) is a useful tracer of carbon cycling within the modern and past ocean. The isolation of different carbon pools in the ocean environment and conversion to CO 2 presents unique analytical problems for the radiocarbon chemist. In general, isolation and preparation of inorganic carbon presents few problems; dissolved carbon is easily extracted by acidifying the sample and stripping with an inert gas. Carbon is also readily isolated from particulate carbonate samples; in this case, CO 2 is prepared by hydrolysis of the substrate with phosphoric acid. The isolation and preparation of organic carbon presents a much greater problem. Dissolved organic carbon (DOC) must first be isolated from DIC and then oxidized in the presence of very high salt concentrations. We present results from a closed-tube combustion method in which the DIC-free seawater is evaporated to dryness, transferred to a clean combustion tube, and oxidized overnight at 550°C. Combustion of total organic carbon (TOC) in sediments with a high inorganic carbon content is also difficult. Removal of CaCO 3 with acid leaves severely deliquescent salts which, if not thoroughly dried, cause combustion tubes to explode. Removal of the salts by rinsing can also remove significant amounts of organic matter. Finally, we present results from a local coastal region.

Reconstructing the oceanic 13C Suess Effect

The anthropogenic δ13C change for the time period 1968 to 1991 was determined based on calculations of the preformed 13C/12C of dissolved inorganic carbon (DIC) distributions on isopycnal surfaces in the main thermocline of the Pacific, North Atlantic and South Indian Oceans. The time rate of change of preformed δ13C (the 13C Suess effect) along isopycnals was calculated using CFC-derived water ages and yields a time history of the surface water δ13C change at the isopycnal outcrop location. The surface ocean Suess effect recorded on isopycnals decreased with increasing outcrop latitude from approximately −0.2‰ decade−1 within the subtropics to around −0.1‰ decade−1 in the subpolar oceans. In the Pacific Ocean these surface δ13C change rate reconstructions agree, both in magnitude and meridional trend, with direct observations of surface ocean δ13C changes reported from time series measurements and from comparisons of surface water δ13C of DIC measurements in 1970 and 1993. A global ocean average surface δ13C rate of change of −0.15 ± 0.04 ‰ decade−1 is determined, which is slightly smaller than a previous time series data and model-based estimate (−0.171‰ decade−1 , [Bacastow etal., 1996]). Depth integrations of the 13C reconstructions in the Pacific, Indian, and Atlantic Oceans, when combined with these previous individual depth profile comparisons and Geochemical Ocean Sections Study bomb 14C inventories [Quay et al, 1992], imply a global depth-integrated δ13C change rate of −9.7 ± 2.4‰ m yr−1 over the time period 1970–1990. These results imply a net oceanic CO2 uptake rate of 1.9 ± 0.9 Gt C yr−1 over the time period 1970–1990 when applied to an atmospheric 13CO2 and 12CO2 budget.

Woce AMS radiocarbon. I: Pacific Ocean results (p6, p16 and p17) : 14C cycling and the oceans

AMS radiocarbon results from the World Ocean Circulation Experiment in the Pacific Ocean show dramatic changes in the inventory and distribution of bomb-produced 14 C since the time of the GEOSECS survey (8/73-6/74). Near-surface Δ 14 C values for the eastern portion of both the northern and southern subtropical gyres decreased by 25-50‰, with the change being greater in the north. Equatorial near-surface values have increased by ca. 25‰. Changes in the 250-750-m depth range are dramatically different between the northern and southern basins. The intermediate and mode waters of the southern basin have increased by as much as 75‰ since GEOSECS. Waters of similar density in the northern hemisphere are not exposed to the Southern Ocean circulation regime and are significantly less ventilated, showing maximum changes of ca. 50‰.

An organic tracer for surface ocean radiocarbon

The Δ14C of surface water dissolved inorganic carbon (DIC) in the Southern California Bight was compared to Δ14C as recorded by the sterols in Santa Monica and Santa Barbara Basin sediments. All of the C26, C27, C28, and C29 sterols as well as dinosterol had 14C concentrations equal to surface water DIC, indicating that all of the major sterols were derived from phytoplanktonic production. There is no detectable terrestrial component. Their tracer capability was confirmed by comparing the “bomb 14C”-derived change in surface water Δ14CDIC with the change in Δ14Csterol. The “prebomb” Δ14CDIC was −82‰, and prebomb sterols averaged −75±19‰. The Δ14C value in 1996 was +71‰. Eighteen measurements representing eight different sterols from the sediment-water interface of both Santa Monica and Santa Barbara Basins averaged +62±23‰. When three of these values were eliminated because of suspected contamination, the remaining data averaged +71 ±12‰. The entire compound class could serve as an excellent proxy for the 14C concentration of ocean surface waters.

THE FIRST TRANS-ARCTIC 14C SECTION: COMPARISON OF THE MEAN AGES OF THE DEEP WATERS IN THE EURASIAN AND CANADIAN BASINS OF THE ARCTIC OCEAN

We present Δ14C data collected during three cruises to the Arctic Ocean that took place in the summers of 1987 (POLARSTERN cruise ARK IV/3), 1991 (ARCTIC 91 Expedition), and 1994 (Arctic Ocean Section 94). The cruise tracks of these three expeditions cover all major basins of the Arctic Ocean (Nansen, Amundsen, Makarov and Canada basins), and can be combined to a trans-Arctic section reaching from the Barents Sea slope to the southern Canada Basin just north of Bering Strait. The section is based on 17 stations covering the entire water column (about 250 data points). The combined Δ14C data set was produced from a mixture of large volume samples measured by low-level counting and small volume samples measured by Accelerator Mass Spectrometry (AMS). We use the Δ14C section, together with previously published Δ14C data from single stations located in several basins of the Arctic Ocean, to derive mean “ages” (isolation times) of the deep waters in the Arctic Ocean. We estimate these mean “ages” to be ≈ 250 years in the bottom waters of the Eurasian Basin and ≈ 450 years in the Canadian Basin Deep Water. A remarkable feature of the Δ14C section is the homogeneity in the 14C distribution observed in the deep Canadian Basin. Within the measurement precision of about ±2‰ (LV) to about ±5‰ (AMS), we cannot detect significant horizontal or vertical Δ14C gradients below 2000 m depth between the northern boundary of the Makarov Basin and the southern margin of the Canada Basin. There is no statistically significant difference between samples measured by AMS and by low-level counting.

Carbon cycling in coastal sediments: 1. A quantitative estimate of the remineralization of organic carbon in the sediments of Buzzards Bay, MA

Seasonal remineralization rates of organic carbon are calculated in the top 20–30 cm of biologically irrigated, organic-rich sediments of Buzzards Bay, MA. Six cores were collected over a period of two years, and the pore water concentrations of the following species were measured: dissolved inorganic carbon (ΣCO2), PO3−4, ΣH2S, Alk, and Ca2+. Overall, these constituents showed large gradients with depth, which are larger in summer than in winter. Remineralization rates in the sediments were estimated by applying a non-local exchange, vertical molecular diffusion, reaction model to the ΣCO2 depth profiles. The major processes affecting the pore water concentration of ΣCO2 described in the model are diffusion, irrigation, and the oxidation of organic carbon. The calculated remineralization rates varied seasonally with the high of 7.5 × 10−9 mol/L-sec observed in August 84 and the low (0.6 × 10−9) in December 1983. The remineralization rates were dependent on the amount of irrigation in the sediments. It was possible to calculate remineralization rates between 0 and 20 cm because the amount of irrigation was well-characterized at this site. We calculated that 69 gC/m2 are oxidized annually and 5–33 gC/m2-yr are buried. It appears that there is a highly reactive portion of organic matter which is oxidized at the sediment water interface. Examination of the Alk and dissolved Ca2+ profiles indicates that there was significant production of acid which dissolved CaCO3 in the spring and early summer.

Fossil Sources of Ambient Aerosol Carbon Based on 14C Measurements Special Issue of Aerosol Science and Technology on Findings from the Fine Particulate Matter Supersites Program

Organic and elemental carbonaceous material constitute significant portions of PM10 and PM2.5 mass on a global basis and are typically 35% and 10%, respectively, in urban areas of the southeastern United States. The organic portion of this is a complex mixture of primary compounds and secondary materials formed in situ from primary emissions. These materials derive from a variety of sources that are dynamically changing in time and space, making difficult the quantitative characterization of their sources. Results of measurements of the 14C content of organic aerosol particles (PM10 and TSP) sampled at Look Rock, TN are reported in this work with the aim of better estimating bounds to fossil and biogenic sources that contribute to PM mass concentrations. The fraction of fossil-derived carbon in these particles has been estimated as 1 minus the fraction of “modern” carbon, the latter by definition biologically synthesized in equilibrium with atmospheric, cosmic ray-derived 14CO2. Fossil carbon fractions vary in a wide range from less than 10 to about 60% during spring, summer, and fall seasons at this ridge-top site near the Great Smoky Mountains National Park. The implications of these findings on control strategies for the organic carbon fraction of PM2.5 mass are discussed.

WOCE Radiocarbon IV: Pacific Ocean Results; P10, P13N, P14C, P18, P19 & S4P

The World Ocean Circulation Experiment, carried out between 1990 and 1997, provided the most comprehensive oceanic survey of radiocarbon to date. Approximately 10,000 samples were collected in the Pacific Ocean by U.S. investigators for both conventional large volume p counting and small volume accelerator mass spectrometry analysis techniques. Results from six cruises are presented. The data quality is as good or better than previous large-scale surveys. The 14 C distribution for the entire WOCE Pacific data set is graphically described using mean vertical profiles and sections, and property-property plots.