Paper Number OS 26A-06 At both sites, the majority of the vertical flux attenuation of components such as carbon occurred with the mesopelagic “Twilight Zone.” The sites did differ in the extent of attenuation. ALOHA showed a more rapid attenuation below the euphotic zone and lower deep carbon flux. This is illustrated by comparing flux ratios at the two site. The ratio of the POC flux just below the euphotic zone and primary production (the “e-ratio”), is approximately the same at both sites, at 6-12%. The “transfer efficiency” (the ratio of POC flux at a deep trap such as at 4000m to that below the euphotic zone at 150m) is markedly different at the two sites, with K2 showing a much higher efficiency. This efficiency difference appears to be determined in the mesopelagic as the ratio of flux at 500m to 150m demonstrates. *D. Karl, unpublished data **M. Honda, unpublished data ALOHAK2 1 st Deployment 2 nd Deployment 1 st Deployment 2 nd Deployment Primary Production m POC Flux m POC Flux m POC Flux m POC Flux0.15* 4000m POC Flux0.12* 1.35**0.69** 150m/P.P. (“e-ratio”)10%8%12%6% 500m Flux/150m Flux20%21%46%55% 2800m Flux/150m Flux10%8% 4000m Flux/150m Flux (“transfer efficiency”) 8%6%26%36% 4000m Flux/P.P.1% 3%2% Vertical Flux of Mass, Pigments and Chemical Components in the Mesopelagic "Twilight Zone" of the North Central and Northwest Pacific Ocean during VERTIGO S.J. Manganini 1, J.E. Andrews 1, C. Bertrand 1, R. Bidigare 2, M. Honda 3, D.M. Karl 2, C.H. Lamborg *1, J. Valdes 1, K.O. Buesseler 1 1 Woods Hole Oceanographic Institution, MS #25, Falmouth, MA United States ( * corresponding author) 2 University of Hawaii, School of Ocean and Earth Science and Technology, Honolulu, HI United States 3 Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka, Japan ALOHA (Northcentral Pacific, N. Pac. gyre) – June 2004 The Sediment Traps Two kinds of sediment traps were used: surface tethered, drogued arrays (STDAs or “Clap Traps” – left) and neutrally bouyant sediment traps (NBSTs – right). One each of the Clap Traps were deployed at the 3 target depths, while 2-3 NBSTs were deployed at each depth. Collection sizes and treatments were the same for both systems. See the accompanying posters by Valdes et al. and Andrews et al. regarding issues of trap accuracy and performance. Abstract The mesopelagic “twilight zone” of the ocean ( m) is recognized as a critically important region in determining the export efficiency of the surface biological pump. This region is, however, distinctly understudied. As part of the VERtical Transport in the Global Ocean (VERTIGO) program, we have deployed a number of types of sediment traps in the twilight zones of two contrasting locations in the North Pacific Ocean: at the oligotrophic north central gyre HOTS time series site ALOHA, just north of Hawaii and at the western subarctic gyre site K2, which is part of the HILATS time series program. These two locations were occupied for several weeks during the summers of 2004 (ALOHA) and 2005 (K2), during which two deployments of our trap systems were completed. During each effort, multiple systems were deployed at 150, 300 and 500 m. Here we present some of the results from our on-going analyses (mass, C, N, P, bSi, PIC, 234 Th, pigments and selected trace metals) of the sediment trap material and place them in the context of the time series programs underway at each site. K2 (Northwest Pacific, subarctic gyre) - August 2005 Fe, Al and Mn fluxes at ALOHA showed approximately crustal ratios, and overall fluxes similar to estimates to dust input to the region. Mn may indicate some enrichment at depth. Of the biomineral metals, Ca and Sr showed decreasing flux with depth, with Ba showed constant or increasing flux with depth. Ba also correlated with Al, but at higher than crustal ratios. Zn and Co were close to their respective detection limits, and the numbers are not likely reliable. The fluxes of phytoplankton pigments is shown in the figure below. Pigments of all classes appeared to degrade faster than mass and other components. However, measurable amounts of most of the pigments were found even at 500m. Using the algorithm of Bidigare and Ondrusek, 1996, we estimated that prymesiophytes contributed the majority of Chl a collected in the traps. This, of course, assumes that the relevant pigments are degrading at comparable rates. Vertical fluxes of major components at Station ALOHA showed small variations from one deployment to another. A large fraction of total mass was made up of organic matter, and showed relatively rapid attenuation. The organic matter was somewhat N-poor, and very P- poor. As at ALOHA, Fe and Al showed approximately crustal ratios. Unlike ALOHA, the fluxes increased with depth. Mn and Ba, too, increased in flux with depth, but both were present above their crustal ratios. Radium isotopes (not shown) suggest continental sources for some metals could be important, e.g., Mn. As with mass and other consistuents, Ca and Sr showed little decrease in flux with depth. Zn and Co gave reliable signals, with Zn fluxes much higher than at ALOHA, consistent with large fluxes of Si. Vertical fluxes of major components at Station K2 showed large variations from one deployment to another for many components. The majority of total mass was made up of biogenic silica, and which drove the relatively modest amount of flux attenuation of other components through the mesopelagic. The organic matter was somewhat N-poor, but closer to Redfield in P than at Station ALOHA. For some components, there were significant differences in fluxes between devices, indicating some particle sorting between the presumably more accurate NBSTs and the hydrodynamically biased CLAP traps (CLAPs generally lower fluxes during the higher flux, 1 st deployment. At both sites, POC and PN of trap material showed a remarkable coherence across device type, preservative type, depth and even between sites and deployments. The slopes of lines fit through these data suggest the C:N ratio of degrading material. The intercepts at these very different sites are also similar (and small), suggesting relatively modest impacts of biomineral ballast phases in controlling OM degradation in the mesopelagic zone. * ** by difference