J Orchardo

Organic carbon oxidation and benthic nitrogen and silica dynamics in San Clemente Basin, a continental borderland site

Organic carbon oxidation rates in San Clemente Basin were determined by benthic chamber experiments using the Bottom Lander, along with studies of pore water chemistry. Non-steady-state diagenetic models are developed for interpreting concentration-time data from the benthic chamber experiments. O2, NO3−, and SO42− are all important oxidants for organic carbon at our study site. Regenerated fixed nitrogen was consumed by NO3− reduction. There is a flux of NO3− into the sediments, and the benthic flux of NH4+ is undetectable. The total rate at which fixed nitrogen is removed from the oceans at this site is about twice the flux of PON to the sea floor. SiO2 fluxes calculated from interfacial pore water gradients are in satisfactory agreement with those determined using the Lander. Most silica dissolution must therefore occur within the sediments, although interstitial profiles show that little dissolution occurs below 1 cm depth.

A high precision isotope ratio mass spectrometry method for measuring the O2N2 ratio of air

Studies of the distribution of O2 in air will inform us about critical problems in the global carbon cycle which are not readily accessed by other measurements, including the rate of seasonal net production in the oceans on a hemispheric scale, the rate at which the oceans are taking up anthropogenic CO2 and the net rate of change of the continental biomass. In this paper, we outline a method for measuring the O2N2 ratio of air to a standard error of ± 6 per meg (± 0.006%.) for a sample analyzed in quadruplicate, corresponding to ± 1.2 ppm V O2 in air out of 210,000. The method involves measuring the ratio of 16O2 to 15N14N by isotope ratio mass spectrometry. Potential and actual problems with this method include fractionation as sample and reference gases are introduced to the mass spectrometer, mass spectrometric nonlinearity, effects of imbalance of sample and reference ion currents on the measured isotopic ratio, isobaric interferences at masses 28 and 29 due to the formation of CO+ from CO2 in the source and zero enrichments. We discuss the magnitude of errors introduced by these factors and outline the relevant corrections. The ultimate mass spectrometric uncertainty is about ±2 per meg (±0.4 ppmV) for a 1 h instrumental analysis. Overall precision is currently limited by fractionation as sample and reference gases are introduced into the mass spectrometer. A considerable improvement in precision may be possible.

Benthic organic carbon degradation and biogenic silica dissolution in the central equatorial Pacific

Abstract Shipboard whole-core squeezing was used to measure pore water concentration vs depth profiles of NO3−, O2 and SiO2 at 12 stations in the equatorial Pacific along a transect from 15°S to 11°N at 135°W. The NO3− and SiO2 profiles were combined with fine-scale resistivity and porosity measurements to calculate benthic fluxes. After using O2 profiles, coupled with the NO3− profiles, to constrain the C:N of the degrading organic matter, the NO3− fluxes were converted to benthic organic carbon degradation rates. The range in benthic organic carbon degradation rates is 7–30 μmol cm−2 y−1, with maximum values at the equator and minimum values at the southern end of the transect. The zonal trend of benthic degradation rates, with its equatorial maximum and with elevated values skewed to the north of the equator, is similar to the pattern of primary production observed in the region. Benthic organic carbon degradation is 1–2% of primary production. The range of benthic biogenic silica dissolution rates is 6.9–20 μmol cm−2 y−1, representing 2.5–5% of silicon fixation in the surface ocean of the region. Its zonal pattern is distinctly different from that of organic carbon degradation: the range in the ratio of silica dissolution to carbon degradation along the transect is 0.44–1.7 mol Si mol C−1, with maximum values occurring between 12°S and 2°S, and with fairly constant values of 0.5–0.7 north of the equator. A box model calculation of the average lifetime of the organic carbon in the upper 1 cm of the sediments, where 80 ± 11% of benthic organic carbon degradation occurs, indicates that it is short: from 3.1 years at high flux stations to 11 years at low flux stations. The reactive component of the organic matter must have a shorter lifetime than this average value. In contrast, the average lifetime of biogenic silica in the upper centimeter of these sediments is 55 ± 28 years, and shows no systematic variations with benthic flux.

Production and respiration rates in the Arabian Sea during the 1995 Northeast and Southwest Monsoons

Abstract In this paper we examine the relationships among oxygen, carbon and nitrogen production and respiration rate measurements made in the Arabian Sea during the 1995 Northeast (NEM) and Southwest (SWM) Monsoons. Increased biological production characterized the SWM, with rates 12–53% higher than the NEM. In most cases, we found remarkable similarity in production rates during the two monsoons and an absence of strong spatial gradients in production between nearshore and offshore waters, especially during the SWM. Daily 14C and total 15N production underestimated gross C production, and at the majority of stations 14C and total 15N production were either the same as net C production or between gross and net C production. Moreover, new production (15NO3), scaled to carbon, was substantially less than net C production. Approximately 50% of the PO14C was metabolized during the photoperiod, with smaller losses (7–11%) overnight. The simplest explanation for the discrepancy between gross and total 15N production and between net C and new production was the loss of 15N-labeled particulate matter as dissolved organic matter. Partitioning of metabolized gross C production into respiratory and dissolved pools showed distinct onshore–offshore distributions that appeared to be related to the composition of the phytoplankton assemblage and probably reflected the trophodynamics of the ecosystem. The percentage of gross C production released as dissolved organic carbon (DOC) was highest in the nearshore waters where diatoms dominated the phytoplankton assemblage, while community respiration was a more important fate for production further offshore where picoplankton prevailed. In general, stations that retained more gross C production as net production (i.e., high net C/gross C ratios) had higher rates of DOC production relative to community respiration. Locations where community respiration exceeded DOC production were characterized by low rates of net C production and had low net C/gross C ratios. In those ecosystems, less net C production was retained because higher metabolic losses reduced gross C production to a greater extent than at the more productive sites.