Vertical distributions of sulfur species simulated by large scale atmospheric models in COSAM: Comparison with observations

A comparison of large‐scale models simulating atmospheric sulfate aerosols (COSAM) was conducted to increase our understanding of global distributions of sulfate aerosols and precursors. Earlier model comparisons focused on wet deposition measurements and sulfate aerosol concentrations in source regions at the surface. They found that different models simulated the observed sulfate surface concentrations mostly within a factor of two, but that the simulated column burdens and vertical profiles were very different amongst different models. In the COSAM exercise, one aspect is the comparison of sulfate aerosol and precursor gases above the surface. Vertical profiles of SO₂, SO²⁻₄, oxidants and cloud properties were measured by aircraft during the North Atlantic Regional Experiment (NARE) experiment in August/September 1993 off the coast of Nova Scotia and during the Second Eulerian Model Evaluation Field Study (EMEFSII), in central Ontario in March/April 1990. While no single model stands out as being best or worst, the general tendency is that those models simulating the full oxidant chemistry tend to agree best with observations although differences in transport and treatment of clouds are important as well.     

Closed-system marine burial diagenesis: isotopic data from the Austin Chalk and its components

ABSTRACT
The carbon and oxygen isotopic composition of the Austin Chalk was examined in cores representing a range of depths from surface to 3000 m in order to document the effects of burial diagenesis on carbon and oxygen isotopic composition. Low magnesium calcite oysters were separated (from 500 um wide areas) and analysed to estimate the starting composition of Cretaceous marine sediment. These gave an average value of -2·5%δ¹⁸O; + 2·0%δ¹³C (PDB). The compositions of micrite, intergranular cement, and fracture cement were analysed, and their deviation from this original marine composition was evaluated to document the progression of chalk diagenesis. Interestingly, micrite exhibits only minor variation in composition from marine values despite present burial depth ranges in excess of 3000 m. The average deviation from δ18O marine is less than 1·5. Furthermore, intergranular cement and particularly fracture cements, which occur only in the deepest cores and which clearly post-date micrite lithification, are generally indistinguishable from micrite in composition. Isotopic compositions exhibit no correlation with depth of burial despite abundant petrographic evidence of deep burial diagenesis. This uniformity in composition is interpreted as reflecting a closed, rock-dominated diagenetic system in which the compositions of precipitated carbonate cements were controlled by the composition of dissolving carbonates during lithification. As such, the composition of burial cement is not representative of the rock-water temperatures during precipitation.
Thus, in the context of isotopic analyses from other carbonate systems, unless the degree of openness of the diagenetic system is known, oxygen isotopic signatures of cements cannot directly be converted to the rock-water temperatures at which they were precipitated unless the composition of the ambient porefluid is also known.