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We are examining the middepth silica and excess helium-3 maxima in the northeast Pacific on neutral surfaces in order to deduce circulation of and communication between the subtropical and subpolar gyres in the northeast Pacific. The middepth silica maximum at about 2000 - 2500 m has values between 170 to 180 µmol/kg thoughout the subpolar and northern subtropical gyres in contrast to the excess helium-3 maximum at approximately the same depth which has high values in the subtropical gyre (20-25%) but almost background levels in the subpolar gyre (<20%). Analysis on neutral surfaces shows that the middepth silica maximum actually occurs on different neutral surfaces in different geographical areas. The excess helium-3 maximum occurs on the same neutral surface that coincides with the silica maximum only where the excess helium-3 values are large. The sources for the silica maximum appear to be a hydrothermal source associated with the high excess helium-3, remineralization in the water column and from the sediments.
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