10:45 AM - 11:00 AM
[BPT07-07] Ontogenetic stable isotope records of Eocene planktic foraminifers: Comparison to modern symbiotic species observation
Keywords:Eocene, Planktic foraminifer, Carbonate stable isotopes, Photosymbiosis
Three Eocene species recovered by IODP Exp. 342 (U1407) were analyzed to examine ontogenetic isotopic profiles; Morozovella, Acarinina (symbiotic), and Subbotina (asymbiotic). Each individual test was dissected into chambers by using a micro-blade, then analyzed by the customized continuous-flow mass spectrometry system that can measure micro-volume carbonate as small as a single chamber.
The δ13C values of Morozovella and Acarinina tended to show the ontogenetic positive shifts until the penultimate chamber. Then the final chamber showed more 13C-depleted δ13C value drastically by 0.5–1.0 ‰. In contrast to these species, Subbotina showed comparatively 13C-depleted δ13C values whole through its ontogeny. We can say that the progressive 13C-enrichment of the chambers δ13C with growth, excluding the last chamber, seen in Morozovella and Acarinina probably reflected the increase of the effect of symbiont photosynthesis. The contrasting relatively constant δ13C values through ontogeny seen in Subbotina is in good agreement with the known asymbiotic nature of this species. The notable thing is that the last chamber of the two symbiotic species showed comparable δ13C value to that of Subbotina. It indicates that the two symbiotic species had already lost their symbionts, or their photosynthesis was not active at the time of the last chamber calcification.
Our recent culture experiments of modern species (Globigerinoides sacculifer and Globigerinella siphonifera) to investigate symbiont photosynthesis through ontogeny showed that the chlorophyll content of the foraminifers, thus the symbiont content, drastically decreased to almost zero at the time or just before the gametogenesis. It sometimes occurred during the last chamber calcification. These modern observations indicate that the more 13C-depleted δ13C value in the final chamber in Eocene symbiotic species could reflect the digestion or lysis of symbionts. If so, it can be said that each individual had to acquire the symbionts from the environment at some time during the ontogeny as the modern obligate symbiotic species do.