Keywords:Living benthic foraminifera, Cold seeps, Hidaka Trough, Isotope geochemistry
This study present ecological and isotopic data on living benthic foraminifera sampled from Calyptogena, mud volcano and aragonite sites in cold seep area of Hidaka Trough, northwestern Pacific. Sediment samples for this research were collected with three different methods including grab sampler, pilot corer and ROV push corer during Daiichi Kaiyomaru expedition in July 2017. Living benthic foraminifera standing stocks range between 2 per gram at mud volcano site (pilot 4) and 138 per gram at Calyptogena site (ROV push core 3B). Shannon Index (H') is minimal (1.20) at Calyptogena site (Pilot 1) and maximal (3.77) at aragonite site (ROV push core 6), while Evenness indices only show minor variability between three sites. Perforate calcareous species dominate benthic foraminifera faunas at all sites, accounting for >93% of the living community. Their contribution is highest at Calyptogena site, where they account for 99% of benthic population. Agglutinated foraminifera represent between 0.3% and 6% of foraminifera assemblages. Of the perforate species, Stainforthia fusiformis, Nonionella stella, Nonionella globosa, Nonionella labradorica, Uvigerina akitaensis, Elphidium batialis, Globobulimina auriculata, Chilostomellina fimbriata, and Brizalina pacifica are dominant at all sites. The isotope composition of the dissolved inorganic carbon (DIC) at three sites in Hidaka area indicates anaerobic oxidation of methane by bacteria with δ13CDIC values as low as -49‰ in the upper part of the sediments. The isotope data for living and dead benthic foraminifera in each site showed that each species has its own range of isotope signatures. In addition, differences in δ13C values for living benthic foraminifera of a given species were observed within a same location or between different locations. For instance, live specimens of G. auriculata showed δ13C values ranging from -0.95‰ at mud volcano site (Grab 1707) to -2.56‰ at Calyptogena site (Grab 1705B). These differences in δ13C values also observed in other species such as U. akitaensis, B. spissa, C. fimbriata, E. batialis, and N. labradorica. In spite of extremely low pore water δ13CDIC values (-4 to -49‰) in the study area, the δ13C values of living benthic foraminifera are not significantly lighter than those reported previously in non-seep sediments, and are within the range expected from local organic matter decomposition (0 to -3‰). However, carbon isotope value of fossil foraminifera found deeper in the sediments showed more negative than living δ13C values. For example, fossil specimens of E. batialis at Calyptogena site (ROV push core 3A) showed δ13C value that were 5.95‰ lighter than its living δ13C value. In addition, the δ13C signatures of dead or recently dead foraminifera species (U. akitaensis, C. fimbriata, N. labradorica, E. batialis) in pilot 1 at Calyptogena site were extremely depleted as low as -33.95‰. The apparent δ13C disequilibrium between living benthic foraminifera calcite and pore water DIC might be suggests that in seep area of Hidaka Trough, living benthic foraminifera mostly calcify during periods of low methane discharge or during intermittent of seawater flow into the sediments and overprinting of the original isotopic composition of foraminifera by overgrowth or recrystallization at or below the sediment surface might cause extreme 13C depletion observed in fossil foraminifera. Heterogeneity of isotopic values may result either from genetic or biological differences between the populations or from differences in environmental isotopic influences such as pore water differences. Understanding the processes involved in the disequilibrium between benthic foraminifera isotopic composition and the pore water DIC where they were found will provide reliable proxies for paleoceanographic reconstructions.