Changes of Oligocene planktic foraminiferal depth habitat in the eastern equatorial Pacific (IODP Site U1334 and U1333)

Abstract:

Planktic foraminifera thrive worldwide and calcify from the mixed layer (surface) through the thermocline (intermediate) to the sub-thermocline (deep). The accuracy of paleoenvironmental reconstruction using planktic foraminifera largely depends on the knowledge of their depth habitat. While fossile planktic foraminiferal depth habitat can be estimated from oxygen-carbon isotope ratio of calcite, little is known about Oligocene planktic foraminifera. Among them planktic foraminifera Dentoglobigerina venezuelana calcifies within the mixed layer during the early Oligocene but inhabits at thermocline or below from the late Oligocene to the early Pliocene. However, no continuous isotopic record of D. venezuelana through the Oligocene have been achieved owing to poor carbonate recovery in order to confirm the depth habitat shift of D. venezuelana from the mixed layer to the thermocline. Here we reveal D. venezuelana depth habitat change throughout Oligocene using deep sea sediment deposited in the eastern equatorial Pacific, IODP U1333 and U1334. We identified planktic foraminiferal assemblage and measured oxygen-carbon isotope of D. venezuelana along with Paragloborotalia siakensis group (mixed layer species). By comparison of D. venezuelana isotopes with those of Pg. siakensis group, studied interval is divided into following 3 phases. From the early to middle Oligocene (Phase I: ~28.7 to 27.4 Ma), D. venezuelana probably inhabited at upper mixed layer while Pg. siakensis group calcified within lower mixed layer. During middle to late Oligocene (Phase II: ~27.4 to 26.3 Ma), the depth habitat of D. venezuelana and Pg. siakensis group were indistinguishable with each other at around lower mixed layer. From late Oligocene to earliest Miocene (Phase III: ~26.3 to 23.0 Ma), D. venezuelana again deepened the depth habitat to thermocline depth while Pg. siakensis group continued to inhabit within lower mixed layer. The two-step depth habitat change accompanied planktic foraminiferal assemblage shift from thermocline species dominance to mixed-layer species prevalence, which suggested depth habitat change was related to thermocline deepening in the eastern equatorial Pacific.