Seafloor Inventories of Pb-210, Th-234 and Benthic Biomass as Proxies for Deep Carbon Flux: Placing Export Production at Station ALOHA in a General Oceanic Context

Craig Smith¹, Robert Miller¹, Robin Pope² and David DeMaster²

¹School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822

²Department of MEAS, North Carolina State University

Abstract

In the equatorial Pacific, seafloor inventories of excess Pb-210 and Th-234, as well as the standing crop of benthic macro- and megafauna, are tightly correlated with the annual flux of particulate carbon into near-bottom sediment traps; these variables may thus be useful as proxies for export production in overlying waters. Because of the difficulties in collecting long-term sediment-trap records throughout the Pacific, we are attempting to use these proxies to place production processes at Station ALOHA in a general oceanic context. In August 1992, we collected replicate multiple corers and box corers at the 4800 m deep HOT station to measure seafloor inventories of Pb-210 and Th-234, as well as to evaluate macro- and megabenthic standing crops. Mean excess inventories of Pb-210 and Th-234 at Station ALOHA were surprisingly high, roughly matching those at 5 N, 140 W, an abyssal station substantially influenced by high equatorial production. Macrofaunal abundance and biomass at Station ALOHA were also relatively high; macrofaunal abundance was twice that measured in 1971 in the center of the North Pacific gyre (28 N, 155 W), and both abundance and biomass matched those at 9 N, 140 W, i.e., an abyssal station far from the center of the oligotrophic gyre. The abundance of megabenthos at Station ALOHA also was similar to that measured at 9 N, 140 W. These results are subject to at least four explanations: (1) Export production at Station ALOHA exceeds that in the center of the North Pacific gyre (e.g., due to "island mass effects"); (2) Transport of particulate carbon and sediment particles down the Hawaiian slope enhances benthos and radionuclide inventories beneath Station ALOHA; (3) Export production in the central gyre (both at the HOT and Climax stations) is higher now than in previous decades; and/or (4) The relationships between benthos, radionuclides and particulate carbon flux are too noisy (e.g., due to temporal variability) to allow their simple use as proxies for export production. We will discuss the relative merits of these four explanations.