Changes in the depth habitat of the Oligocene planktic foraminifera (Dentoglobigerina venezuelana) induced by thermocline deepening in the eastern equatorial Pacific

Abstract

Understanding planktic foraminiferal depth habitat along with consistent taxonomic concepts is key to accurate reconstruction of paleoceanographic records. The Oligocene‐Pliocene long‐ranging and widely distributed species Dentoglobigerina venezuelana lived in the mixed layer (shallower) during the early Oligocene, whereas the same species calcified at thermocline or subthermocline depths (deeper) during the late Oligocene and Miocene. The exact timing of the species' depth habitat change and its possible relationships with Oligocene climate dynamics remain unknown. Here we reveal isotopic records of D. venezuelana along with the Paragloborotalia siakensis group (a mixed‐layer dweller) by using sediments at Integrated Ocean Drilling Program Site U1334 in the eastern equatorial Pacific throughout the Oligocene. A two‐step depth habitat change of D. venezuelana is apparent: (1) from upper to lower mixed layer (~27.4 Ma) and (2) from lower mixed layer to thermocline depth (~26.3 Ma). In addition, the planktic foraminiferal faunal assemblage experienced a marked change from dominantly thermocline (deeper) species to abundant mixed‐layer (shallower) species, suggesting that depth habitat shifts of D. venezuelana were clearly related to thermocline deepening in the eastern equatorial Pacific. Comparison of the first isotopic shift (~27.4 Ma) at multiple sites (U1334, U1333, and 1218) revealed a southward depth habitat change of D. venezuelana within ~200 kyr, implying overall thermocline deepening with reduced steepness in the eastern equatorial Pacific. We consider that global warming conditions during the late Oligocene likely caused thermocline deepening with upwelling decrease in the eastern equatorial Pacific, guiding D. venezuelana to adapt to greater depths in the water column.

The extinction of Chiloguembelina cubensis in the Pacific Ocean: implications for defining the base of the Chattian (upper Oligocene)

“…We therefore speculate that a localised, temporary expansion of the oxygen minimum zone may have led to this peak in abundance in C9n. The enhanced abundance is coincidental with a period of gigantism in Paragloborotalia opima (Wade et al 2016) and the migration to deeper depths observed in Dentoglobigerina venezuelana at Site U1334 (Matsui et al 2016) and Site 1218 ; Supplementary Fig. S1), which Matsui et al (2016) suggested was coincidental with a deepening of the thermocline.…”

“…The enhanced abundance is coincidental with a period of gigantism in Paragloborotalia opima (Wade et al 2016) and the migration to deeper depths observed in Dentoglobigerina venezuelana at Site U1334 (Matsui et al 2016) and Site 1218 ; Supplementary Fig. S1), which Matsui et al (2016) suggested was coincidental with a deepening of the thermocline. The synchronicity of the peak in C. cubensis, gigantism in P. opima and depth migration of D. vene zuelana suggests that these events may have been driven by complex changes in oceano -graphy, at least on a localised scale through Chron C9n at Site U1334.…”

Vertical thermal gradient history in the eastern equatorial Pacific during the early to middle Miocene: Implications for the equatorial thermocline development

“…Although the δ 13 C values of P. siakensis and D. venezuelana were very similar, slightly higher values of the former species than the latter from 15.9 to 14.6 Ma and from 13.4 to 12.4 Ma (Figure ) still support a shallower habitat of P. siakensis . A previous habitat depth assignment has also reported a mixed‐layer habitat for P. siakensis (50–100 m) [ Keller , ; Hodell and Vayavananda , ; Nathan and Leckie , ; Pearson and Wade , ] and calcification at greater depth (thermocline or subthermocline) by D. venezuelana (100–250 m) [e.g., Gasperi and Kennett , ; Hodell and Vayavananda , ; Nathan and Leckie , ; Matsui et al , ] throughout the early to middle Miocene. We therefore estimated the vertical thermal gradient in the EEP by subtracting the δ 18 O values of P. siakensis from those of D. venezuelana (Figure ).…”

“…• Table S1 Correspondence to: H. Matsui Matsui et al, 2016], and measurements at several sites have elucidated zonal (east to west) and meridional (pole to equator) temperature gradients [e.g., Fedorov et al, 2015]. Thermocline history (vertical and zonal gradients) in the equatorial Pacific can thus be reconstructed from analyses of foraminiferal tests.…”

“…The vertical distribution of planktic foraminifera extends throughout the upper few hundred meters of the water column, and their horizontal distribution ranges from tropical to polar regions [ Hemleben et al , ]. Analyses of multiple species at one location have revealed a vertical thermal gradient in the water column [e.g., Gasperi and Kennett , ; Matsui et al , ], and measurements at several sites have elucidated zonal (east to west) and meridional (pole to equator) temperature gradients [e.g., Fedorov et al , ]. Thermocline history (vertical and zonal gradients) in the equatorial Pacific can thus be reconstructed from analyses of foraminiferal tests.…”