Geochemical Journal, Vol. 53, 2019
Emoto Mariko1, Kozo Takahashi2 and Tasuku Akagi1*
1Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
2Hokusei Gakuen University, Oyachi 2-3-1, Atsubetsu-ku, Sapporo 004-8631, Japan
(Received August 28, 2018; Accepted April 26, 2019)
To understand the contribution of carbonate, oxide, organic and siliceous matter to the vertical transportation of elements, sequential chemical leaching was applied to 11 sediment-trap samples collected in and near the Bering Sea. A total of ten samples representing periods of high to low opal flux were analyzed for elements in carbonate, oxide and silicate. Two samples from high and low opal flux periods were analyzed further for those in organic matter. Siliceous matter may be an important carrier for most elements (Li, Mg, Al, Sc, Ti, V, Cr, Fe, Co, Ge, As, Rb, Zr, Nb, Sn, Sb, Cs, Hf, Ta, W and Th); carbonate may be important for Na, K, Ca, Mn, Ni, Sr, Pd, Ag, Cd, Ba, Pb, Bi and U and organic matter for B, P, Cu and Ga. Carbonate, oxide and siliceous fractions almost equally contribute to transportation of Y and lanthanides (REEs). The concentrations of most elements in the siliceous fraction exhibit hyperbolic relationships against opal flux, where the asymptotes are always non-zero. The non-zero asymptotes lead to two possible interpretations. 1) Diatom frustules are pure opal; when diatom production is extremely high, it is accompanied with supply of terrigenous matter whose amount is proportional to diatom production. 2) Diatom frustules are not pure opal (SiO2), but contain significant amounts of other elements; the observed increase of elemental concentrations at smaller productivity results from selective dissolution of opal or terrigenous matter mixing. We argue that the second interpretation is more plausible, and the composition of diatom frustules is estimated as that of the silicate fraction of the high productive group, with an opal flux greater than 200 mg m-2 day-1. This indicates the potential of diatom frustules as effective vertical carriers of multiple elements.
Key words: settling particles; North Pacific Ocean; elemental composition; sequential leaching; carbonate, oxide, organic, silicate matter