Distribution of Carbonate Crusts, Slabs and Edifices 



The two ROV surveys clearly showed, for the first time, that 

 authigenic carbonates occur in a great variety of configurations 

 over an extensive area of the upper slope and outer continental 

 shelf off northern Oregon. Previous dredge and dart core samples 

 from this region of the Oregon margin occasionally recovered 

 calcareous mudstones, but no carbonate slabs (Kulm and Fowler 

 1974; Kulm et al. 1984). Direct observation with the ROV and 

 submersible allows mapping of the distribution pattern of the 

 carbonates to locate the most likely areas of active fluid 

 venting. 



Composition of Carbonates 



The carbonate mineral composition of the irregular edifice 

 is quite different from that of the three other chimneys 

 recovered from the same area on the continental shelf. The 

 former structure consists largely of aragonite and Mg-calcite 

 while the latter structures consist of dolomite. This wide range 

 in carbonate composition was also reported from the several 

 venting areas on the continental slope off Oregon ( Ritger et al. 

 1987). 



The carbonate edifice recovered in the Aloha trawl is highly 

 depleted in carbon-13 (average -51.29) compared to the conical 

 and cylindrical carbonate chimneys (average -19.77) recovered by 

 the commercial trawling vessel Kodiak from the same area of the 

 outer continental shelf. Both the oxygen and carbon isotopes 

 suggest that the sources of fluids for the two types of chimney 

 structures are different. There appears to be a rather broad 

 range in carbonate mineralogy and stable isotopic composition in 

 both the shelf chimneys and the slope chimneys. In the next 

 field program in 1988, we shall determine under what geochemical 

 and geological conditions each of the different chimney 

 compositions occurs in the active venting areas on both the shelf 

 and slope. We shall attempt to sample the fluids being expelled 

 at each vent site as described below. 



Fluid Flow Measurements 



Preparations were made prior to the cruise to sample the 

 fluids flowing from any of the venting structures (e.g., 

 carbonate chimneys, slabs, edifices) that might be found on the 

 shelf. A new automated rosette water sampler enclosed in a 

 benthic chamber was constructed by the investigators for sampling 

 these fluids. Unfortunately, only one submersible dive was 

 completed in the northern shelf and this was before the chimney 

 and slab structures were discovered by the ROV. While this 

 water sampling device was not used on this cruise, it was 

 deployed successfully two weeks later at a vent site on the lower 



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