Airborne measurements of dimethylsulfide,sulfur dioxide,and aerosol ions over the Southern Ocean South of Australia

Vertical distributions of dimethylsulfide (DMS), sulfur dioxide (SO₂), aerosol methane-sulfonate (MSA), non-sea-salt sulfate (nss-SO₄ ^2-), and other aerosol ions were measured in maritime air west of Tasmania (Australia) during December 1986. A few cloudwater and rainwater samples were also collected and analyzed for major anions and cations. DMS concentrations in the mixed layer (ML) were typically between 15–60 ppt (parts per trillion, 10^–12; 24 ppt=1 nmol m^–3 (20°C, 1013 hPa)) and decreased in the free troposphere (FT) to about <1–2.4 ppt at 3 km. One profile study showed elevated DMS concentrations at cloud level consistent with turbulent transport (cloud pumping) of air below convective cloud cells. In another case, a diel variation of DMS was observed in the ML. Our data suggest that meteorological rather than photochemical processes were responsible for this behavior. Based on model calculations we estimate a DMS lifetime in the ML of 0.9 days and a DMS sea-to-air flux of 2–3 mol m^–2 d^–1. These estimates pertain to early austral summer conditions and southern mid-ocean latitudes. Typical MSA concentrations were 11 ppt in the ML and 4.7–6.8 ppt in the FT. Sulfur-dioxide values were almost constant in the ML and the lower FT within a range of 4–22 ppt between individual flight days. A strong increase of the SO₂ concentration in the middle FT (5.3 km) was observed. We estimate the residence time of SO₂ in the ML to be about 1 day. Aqueous-phase oxidation in clouds is probably the major removal process for SO₂. The corresponding removal rate is estimated to be a factor of 3 larger than the rate of homogeneous oxidation of SO₂ by OH. Model calculations suggest that roughly two-thirds of DMS in the ML are converted to SO₂ and one-third to MSA. On the other hand, MSA/nss-SO₄ ^2- mole ratios were significantly higher compared to values previously reported for other ocean areas suggesting a relatively higher production of MSA from DMS oxidation over the Southern Ocean. Nss-SO₄ ^2- profiles were mostly parallel to those of MSA, except when air was advected partially from continental areas (Africa, Australia). In contrast to SO₂, nss-SO₄ ^2- values decreased significantly in the middle FT. NH₄ ⁺/nss-SO₄ ^2- mole ratios indicate that most non-sea-salt sulfate particles in the ML were neutralized by ammonium.     

The use of magnetic field excursions in stratigraphy

The use of magnetic field excursions in stratigraphy is difficult primarily because the excursion field is complex and not dominantly dipolar. In contrast with a reversal, which is a global event, an excursion can be evidenced at one location but not another. Although this does not by itself rule out the use of excursions in stratigraphy, it does limit the geographic area over which they may be correlated. We recommend, somewhat conservatively, that excursions can be used to correlate between sedimentary cores separated by angular distances of less than 30° on Earth's surface. Correlation between cores separated by more than 45° should not be attempted.     

The ODP color digital imaging system: Color logs of quaternary sediments from the Santa Barbara basin,site 893

Digital color logs of cores from leg 146, holes 893A and 893B, have been taken from images captured during January 1993, within days after the cores were split and described. The images were captured and color analyses performed on the Ocean Drilling Program (ODP) color digital imaging system, which was assembled from relatively inexpensive, off‐the‐shelf components. The images were used to calculate sedimentation rates by fitting chronological data from hole 893A to void‐corrected depths determined by eliminating all voids mapped from the images as >1 cm in length measured downcore. Color measurements were made at intervals between 0.22 and 1.0 mm in length, and then Commission Internationale de l'Eclairage (CIE) 1931 chromaticity values were computed. Results plotted within CIE chro‐maticity space lie in close proximity to the Munsell 5Y hue plane, confirming that the instrumental color analysis technique has produced results consistent with those of the human observers who described colors visually. Significant periodicities occurring at 12, 17, 31, and 90 years correlate with sunspot activity cycles, suggesting that color variations may reflect global climatic forcing functions. Linear correlation between color data sets from holes 893A and 893B suggests that as much as 1.2 m of material present at or near the top of hole 893B may not have been recovered from hole 893A, and that there is a 70‐cm depth discrepancy in the opposite direction at 51.5 m below seafloor (mbsf). The article recommends that the 1931 CIE chromaticity system be used routinely for describing colors of geological materials because it readily accommodates the mathematical manipulations required for statistical and time‐series analyses and avoids the subjectivity and other weaknesses inherent in the Munsell Color System.