P/B RATIOS IN THE DEEP SEA SEDIMENTS OF THE OCEAN—A PROPOSAL FOR COUNTING ERRORS OF THE DEEP SEA BENTHIC FORAMINIFERA

The ratios of the planktonic foraminifera to the benthic foraminifera (P/B ratios) in the recent sediments were shown by Bandy (1953) and were used recently to estimate the water depth of the sediments (Mego Vela et al., 2014). In this study, we investigate the P/B ratios in the fractions larger than 63µm of the deep sea sediments, in the Neogene sequences of DSDP Site 590 (water depth 1299m), Site 591 (2131m), and Site 206 (3196m) in the Tasman Sea, southwest Pacific. The P/B ratios in the early Miocene and early middle Miocene are higher than other ratios at Site 591, and are approximately 5000 and 6000 respectively. The ratios decreased, with average P/B ratios ranging approximately from 400 to 1000. Similar trends are recognized at Sites 590 and 206. The conspicuous decrease of the P/B ratios was contemporaneous with the well-known early Middle Miocene oxygen isotope shift (Kurihara and Kennett, 1986). The deep sea benthic foraminifera are very scarce in the deep sea sediments.

Little has been discussed about the counting error of the deep sea benthic foraminifera since the beginning of the Deep Sea Drilling Project in 1968. There is the difficulty to count the deep sea benthic foraminifera correctly, in addition to the high P/B ratios. Kurihara and Yabumoto (2015) explained that the deep sea benthic foraminiferal assemblages larger than 63µm are suitable to recognize the oceanic events or changes of the ocean, but the deep sea benthic foraminiferal assemblages larger than 125µm and 149µm are not suitable for that purpose. Approximately 73% of the specimens in the deep sea benthic foraminiferal assemblage are included in the fractions between 63µm and 125µm.The planktonic foraminifera contain large-sized specimens. These indicate the difficulty to count all deep sea benthic foraminifera. We propose that “blind samples” are necessary to be distributed to the scientists engaged in the deep sea benthic foraminifera, as was conducted in the study of the fission track dating, and then to compare the results of the analysis of the samples in order to decrease the counting error of the deep sea benthic foraminifera by developing their own method of study. We consider that these procedures will surely become necessary and contribute to the development of the study of paleoceanography in the 21^st century.