REACTIVE TRACE-METALS IN THE STRATIFIED CENTRAL NORTH PACIFIC

Vertical concentration profiles of the dissolved and suspended particu late phases were determined for a suite of reactive trace metals, Al, Fe, Mn, Zn, and Cd, during summertime at a station in the center of th e North Pacific gyre. During summer the euphotic zone becomes stratifi ed, forming a shallow (0-25 m), oligotrophic, mixed layer overlying a subsurface (25-140 m), strongly-stratified region. The physical, biolo gical, and chemical structure within the euphotic zone during this per iod enhanced the effect of atmospheric inputs of Al, Fe, and mn on mix ed layer concentrations. For example, the concentration of dissolved F e in the surface mixed layer was eighteen times that observed at a dep th of 100 m. The observed aeolian signature of these metals matched th at predicted from estimates of atmospheric input during the period bet ween the onset of stratification and sampling. The distributions of su spended particulate Al, Fe, and Mn all exhibited minima in the euphoti c zone and increased with depth into the main thermocline. Particulate Al and Fe were then uniform with depth below 1000 m before increasing in the near bottom nepheloid layer. Average particulate phase concent rations in intermediate and deep waters of the central North Pacific w ere 1.0, 3.1, and 0.055 nmol.kg(-1) for Al, Fe, and Mn, respectively. The distribution of particulate Cd exhibited a maximum within the subs urface euphotic zone. Particulate zinc also exhibited a surface maximu m, albeit a smaller one. Concentrations of particulate Zn and Cd in in termediate and deep waters were 17 and 0.2 pmol.kg(-1). Substantial in terbasin differences in particulate trace metals occur. Concentrations of suspended particulate Al, Fe, and Mn were three to four times lowe r in the central North Pacific than recently reported for the central North Atlantic gyre, consistent with differences in atmospheric input to these two regions. Concentrations of suspended particulate Cd and Z n were enriched in the North Pacific relative to the North Atlantic, a n observation consistent with their assimilation by plankton. Reactive trace metals exhibit a range of biogeochemical behaviors that can be characterized by two endmembers, nutrient-type and scavenged-type. Nut rient-type metals, best exemplified by Zn and Cd, are primarily remove d from surface waters by biogenic particles and then remineralized at depth. Internal biogeochemical cycles together with physical mixing an d circulation patterns control the distributions of nutrient-type meta ls. Scavenged-type metals, best exemplified by Al, continue to be remo ved onto particles in intermediate and deep waters as well as at the s urface. External inputs, such as the deposition of aeolian dust, contr ol the concentrations and distributions of scavenged-type metals. Othe r metals, such as Fe, exhibit a mixture of the characteristic behavior s of these two endmembers.