Terri M. Rust

Controls on the carbon isotopic composition of southern ocean phytoplankton

Carbon isotopic compositions of suspended organic matter and biomarker compounds were determined for 59 samples filtered from Southern Ocean surface waters in January 1994 along two north-south transects (WOCE SR3 from Tasmania to Antarctica, and across the Princess Elizabeth Trough (PET) east of Prydz Bay, Antarctica). Along the SR3 line, bulk organic matter show generally decreasing 13C contents southward, which are well correlated with increasing dissolved molecular carbon dioxide concentrations, CO2(aq). This relationship does not hold along the PET transect. Using concentrations and isotopic compositions of molecular compounds, we evaluate the relative roles of several factors affecting the δ13C of Southern Ocean suspended particulate organic matter. Along the WOCE SR3 transect, the concentration of CO2(aq) plays an important role. It is well described by a supply versus demand model for the extent of cellular CO2 utilization and its associated linear dependence of isotopic fractionation (EP) on the reciprocal of CO2(aq). An equally important factor appears to be changes in algal assemblages along the SR3 transect, with their contribution to isotopic fractionation also well described by the supply and demand model, when formulated to include the cell surface/volume control of supply. Changes in microalgal growth rates appear to have a minor effect on EP. Along the PET transect, algal assemblage changes and possibly changes in microalgal growth rates appear to strongly affect the carbon isotopic variations of suspended organic matter. These results can be used to improve the formulation of modern carbon cycle models that include phytoplankton carbon isotopic fractionation.

Mechanisms of nitrous oxide production in the subtropical North Pacific based on determinations of the isotopic abundances of nitrous oxide and di-oxygen

Abstract In this study, we compare stable isotopic compositions of di-oxygen (O 2 ) and nitrous oxide (N 2 O) in two depth profiles at the well-characterized deep water station ALOHA (A Long-term Oligotrophic Habitat Assessment) in the subtropical North Pacific gyre to attain an understanding of the mechanisms of N 2 O production. The δ 18 O of O 2 varied from values indicative of an atmospheric origin near the surface ( 24.7‰ ), to minimum values reflective of a predominance of photosynthesis over respiration between the surface and 200 m (as low as 22.2 ‰ ), to maximum values as high as 36.6‰ in association with the O 2 minimum near 800 m. A similar pattern of isotopic variation was evident in the δ 18 O of N 2 O, however, values were enriched by approximately 20‰ . The similar pattern of variation in δ 18 O with depth is consistent with an origin of O in N 2 O from dissolved O 2 via the nitrification of intermediate compounds NH 2 OH or NO. Between the depths of 350 and 500 m, however, the distinction in the isotopic composition of N 2 O and O 2 was reduced to as little as 12‰ . This decrease in the difference between the δ 18 O of N 2 O and that of O 2 with depth indicates either a reduction in the magnitude of isotopic discrimination during nitrification or a contribution of O in N 2 O from water via the reduction of NO 2 − . Two mechanisms of N 2 O production via nitrification may, therefore, occur in the subtropical Pacific; release from the nitrification of NH 2 OH or NO at most depths and reduction of NO 2 − between 350 and 500 m. In that, the carbon flux decreases markedly over a similar depth interval at this locale (Karl, D.M., Knauer, G.A., Martin, J.H., 1988. Downward flux of particulate organic matter in the ocean: A particle decomposition paradox. Nature 332, 438–441), this distinct mechanism of N 2 O production between 350 and 500 m may be associated with the mineralization of organic matter from sinking particles. Low O 2 or anoxic micro-environments within particles within this depth interval may be maintained by lower ambient O 2 than at the surface and high rates of microbial activity supported by the mineralization of organic matter. Such conditions may facilitate an environment conducive to N 2 O production via NO 2 − reduction.

Carbon kinetic isotope effect accompanying microbial oxidation of methane in boreal forest soils

Atmospheric methane (CH4) oxidation occurs in soils at sites in the Bonanza Creek L.T.E.R. near Fairbanks, Alaska, USA, at rates ⩽2 mg CH4 m−2 d−1; the maximum CH4 oxidizing activity is located in loess at a depth of ∼15 cm. Methane, carbon dioxide, and stable isotope (δ 13C-CH4, δ 13C-CO2) depth distributions were measured at two sites: South facing Aspen (AS2) and North facing Black Spruce (BS2). The combined effects of diffusion and oxidation are similar at both sites and result in a CH4 concentration decrease (1.8–0.1 ppm) and a δ 13C-CH4 increase (−48% to −43%) from the soil surface to 60–80 cm depth. Isotope flux ratio and diffusion-consumption models were used to estimate the kinetic isotope effect (KIE); these results agree with the observed top-to-bottom difference in δ 13C-CH4, which is the integrated result of isotope fractionation due to diffusion and oxidation. The KIE for CH4 oxidation determined from these measurements is 1.022–1.025, which agrees with previous KIE determinations based on changes in headspace CH4 concentration and δ 13C-CH4 over time. A much lower soil respiration rate in the North facing Black Spruce soils is indicated by fivefold lower Soil CO2 concentrations. The similarity in CH4 oxidation at the two sites and the differences in inferred soil respiration at the two sites suggest that soil CH4 oxidation and soil respiration are independent processes. The soil organic matter responsible for the CO2 flux has a δ 13C estimated to be −27 to −28%.

Highly elevated methane in the eastern tropical North Pacific and associated isotopically enriched fluxes to the atmosphere

During the May - June, 2000 Eastern Pacific Redox Experiment (EPREX) we examined the dynamics of methane (CH4) in the eastern tropical North Pacific (ETNP), a large region of high surface-ocean productivity fueled by coastal upwelling. We discovered that (I) the ETNPcontains by far the largest pool of CH4 yet discovered in the open ocean; (2) CH4 production in the upper half of this subsurface pool is associated with the decomposition of locally produced sinking particulate matter; (3) the deeper half of this pool is from a coastal source; (4) advection and oxidation of the upper pool leads to the heavy CH4 isotopic values seen at midwater in the North Pacific subtropical gyre; and (5) the ETNP is a source of isotopically enriched CH4 to the atmosphere. Our results suggest that other oceanic areas of upwelling-induced anoxia may be sites of significant atmospheric input of isotopically heavy CH 4 •

Methane distribution and cycling in Tomales Bay, California

Cycling of methane (CH4) in Tomales Bay, a 28-km2 temperature estuary in northern California with relatively low inputs of organic carbon, was studied over a 1-yr period. Water column CH4 concentrations showed spatial and temporal variability (range=8–100 nM), and were supersaturated with respect to the atmosphere by a factor of 2–37. Rates of net water column CH4 production-oxidation were determined by in situ experiments, and were not found to be significantly different from zero. Fluxes across the sediment-water interface, determined by direct measurement using benthic chambers, varied from −0.1 μmol m−2 d−1 to +16 μmol m−2 d−1 (positive fluxes into water). Methane concentrations in the two perennial creeks feeding the bay varied annually (140–950 nM); these creeks were a significant CH4 source to the bay during winter. In addition, mass-balance calculations indicate a significant additional inter CH4 source, which is hypothesized to result from storm-related runoff from dairy farms adjacent to the bay. Systemwide CH4 budgets of the 16-km2 inner bay indicate benthic production (110 mol d−1) and atmospheric evasion (110 mol d−1) dominated during summer, while atmospheric evasion (160 mol d−1) and runoff from dairy farms (90 mol d−1) dominated during winter.

Impact of land runoff on water quality in an Hawaiian estuary

Abstract Water quality studies were carried out for six-week periods during the summers of 1991 and 1992 in the Ala Wai canal, an artificial estuary heavily impacted by land runoff on the island of Oahu in the Hawaiian Islands. Circulation in the upper reaches of the estuary is greatly restricted by a sill of accumulated sediment which has washed in from the surrounding watershed. Both the biomass of phytoplankton and photosynthetic rates increased from the mouth to the head of the estuary. Gross photosynthetic rates near the head of the estuary were comparable to net production rates reported from intensive algal cultures. Pigment analyses indicated that the phytoplankton community was dominated by diatoms and dinoflagellates, with dinoflagellates most abundant at the head and diatoms at the mouth of the estuary. Productivity indices declined from 15 to 4 g C g −1 chl a h −1 between the mouth and the head of the estuary, presumably due to light limitation effects. Respiration rates were relatively uniform throughout the estuary. Although physical processes significantly reduced the magnitude of diel O 2 changes which would otherwise have resulted from photosynthesis and respiration, O 2 concentrations at the head of the estuary nevertheless fell as low as 3 mg liter −1 near the bottom during the day. A complete carbon budget for the system indicates that the estuary is heterotrophic, with a photosynthesis/respiration ratio of about 0·6. Of the organic inputs to the system, 70% are respired, 25% are buried in the sediments, and about 5% wash out of the estuary. In terms of allochthonous nitrogen loading, the Ala Wai canal appears to be one of the most heavily fertilized estuaries in the world.

The origin of organic matter in the Martian meteorite ALH84001

Stable carbon isotope measurements of the organic matter associated with the carbonate globules and the bulk matrix material in the ALH84001 Martian meteorite indicate that two distinct sources are present in the sample. The delta 13C values for the organic matter associated with the carbonate globules averaged -26% and is attributed to terrestrial contamination. In contrast, the delta 13C values for the organic matter associated with the bulk matrix material yielded a value of -15%. The only common sources of carbon on the Earth that yield similar delta 13C values, other then some diagenetically altered marine carbonates, are C4 plants. A delta 13C value of -15%, on the other hand, is consistent with a kerogen-like component, the most ubiquitous form of organic matter found in carbonaceous chondrites such as the Murchison meteorite. Examination of the carbonate globules and bulk matrix material using laser desorption mass spectrometry (LDMS) indicates the presence of a high molecular weight organic component which appears to be extraterrestrial in origin, possibly derived from the exogenous delivery, of meteoritic or cometary debris to the surface of Mars.