Extinction and environmental change across the Eocene-Oligocene boundary in Tanzania Paul N. Pearson Ian K. McMillan School of Earth, Ocean, and Planetary Sciences, Cardiff University, Park Place, Cardiff CF10 3YE, UK Bridget S. Wade Department of Geological Sciences, Rutgers University, 610 Taylor Road, Piscataway, New Jersey 08854, USA Tom Dunkley Jones Department of Earth Sciences, University College London, Gower Street, London WCE 6BT, UK Helen K. Coxall School of Earth, Ocean, and Planetary Sciences, Cardiff University, Park Place, Cardiff CF10 3YE, UK Paul R. Bown Department of Earth Sciences, University College London, Gower Street, London WCE 6BT, UK Caroline H. Lear School of Earth, Ocean, and Planetary Sciences, Cardiff University, Park Place, Cardiff CF10 3YE, UK ABSTRACT The Eocene-Oligocene transition (between ca. 34 and 33.5 Ma) is the most profound episode of lasting global change to have occurred since the end of the Cretaceous. Diverse geologi- cal evidence from around the world indicates cooling, ice growth, sea-level fall, and acceler- ated extinction at this time. Turnover in the oceanic plankton included the extinction of the foraminifer Family Hantkeninidae, which marks the Eocene-Oligocene boundary in its type section. Another prominent extinction affected larger foraminifera, which resulted in the loss of some of the world’s most abundant and widespread shallow-water carbonate-secreting organisms. However, problems of correlation have made it diffi cult to relate these events to each other and to the global climate transition as widely recorded in oxygen and carbon iso- tope records from deep-sea cores. Here, we report new paleontological and geochemical data from hemipelagic sediment cores on the African margin of the Indian Ocean (Tanzania Drill- ing Project Sites 11, 12 and 17). The Eocene-Oligocene boundary is located between two prin- cipal steps in the stable-isotope records. The extinction of shallow-water carbonate producers coincided with an extended phase of ecological disruption in the plankton and preceded maxi- mum glacial conditions in the early Oligocene by ~200 k.y. Keywords: mass extinction, Eocene, Oligocene, foraminifera, nannofossils. INTRODUCTION such as Sites 744 (Zachos et al., 1994) and 1218 tributing to a sharp decline in shallow-water car- The Eocene-Oligocene boundary is defi ned at (Coxall et al., 2005) do not contain hantkeninids. bonate production (e.g., Kiessling et al., 2003). its Global Stratotype Section and Point (GSSP) These considerations have led to recent calls for However, very few sections are complete across at Massignano in Italy at a level that corre- the formal Eocene-Oligocene boundary to be the boundary, and none has well-defi ned plank- sponds to the extinction of the Hantkeninidae moved to a stratigraphic level or biostratigraphic tonic biostratigraphy, so it is has been unclear (Premoli Silva and Jenkins, 1993). Unfortu- horizon that is more easily correlated to the how rapid the extinction was and how it cor- nately, carbonate preservation is poor in the global climatic changes revealed by the isotope related to the stage boundary and/or global cli- type section, and stable-isotope stratigraphy is data (van Mourik and Brinkhuis, 2005). mate transition. unreliable (Bodiselitsch et al., 2004), so there It has long been known that a variety of long- is uncertainty as to how the formal boundary lived and distinctive larger benthic foraminifera NEW DRILL CORES level relates to global environmental changes. disappeared near the end of the Eocene (e.g., We address these problems of correlation An important drill core through the Eocene- Glaessner, 1945). Following a detailed review by analyzing a newly discovered Eocene- Oligocene boundary at Deep Sea Drilling of shallow-water carbonate sections in the Oligocene boundary section in the Kilwa Group Project (DSDP) Site 522 in the South Atlantic Indian and Pacifi c Oceans, Mediterranean, and of Tan zania, drilled at Tanzania Drilling Project shows that the extinction of the Hantkeninidae Americas, Adams et al. (1986) suggested that (TDP) Sites 11, 12, and 17. The sediments are preceded the most positive oxygen isotopic val- there had been a rapid mass extinction. Major hemipelagic clays with accessory debris fl ows ues, which correspond to the early Oligocene groups to disappear included the Discocyclini- deposited in a bathyal outer-shelf or slope envi- glacial maximum (Liu et al., 2004; Oberhänsli dae, Asterocyclinidae, and some Nummulitidae. ronment in an estimated 300–500 m of water et al., 1984; Zachos et al., 1994), but the micro- Because these groups were so abundant and (Nicholas et al., 2006, 2007). The area has subse- fossils are rare and fragmentary because of dis- widespread, it is plausible that their extinction quently been uplifted, and the sedimentary suc- solution (Poore, 1984). Other important sections had an effect on the global carbon cycle by con- cession is exposed on land. Analysis of organic © 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, February February 2008; 2008 v. 36; no. 2; p. 179–182; doi: 10.1130/G24308A.1; 3 fi gures; Data Repository item 2008042. 179 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/36/2/179/3534940/i0091-7613-36-2-179.pdf by guest on 26 September 2021 biomarkers (van Dongen et al., 2006) shows that datums and geochemical tie-points to generate A B the Kilwa Group clays have never been deeply an age model and correlate the record to deep- buried. They are characterized throughout by sea sites (see the GSA Data Repository1). The excellent preservation of carbonate microfossils age model is on the time scale of Berggren et (foraminifera and nanno fossils) (Fig. 1); hence, al. (1995) with the E/O boundary at 33.7 Ma. the material is ideal for geochemical analysis A minor unconformity occurs in the lower- (Pearson et al., 2001, 2007). The cores also con- most Oligocene at the two more northerly sites tain larger forami nifera that were transported (TDP 17 and 11), resulting in ~12 m and 3 m μ μ 100 m 100 m across the narrow continental shelf to the site of eroded or missing section, respectively. The CDof deposition, both as adult specimens in debris southerly site (TDP Site 12) appears complete fl ows and as juveniles dispersed through the across the Eocene-Oligocene boundary, which background clay sediment (Fig. 1B). occurs in monotonous mudstone facies. Correlation between the sites was achieved using a series of microfossil markers, which BIOTIC TURNOVER allowed us to construct a composite depth scale. Detailed studies of the microfossil biostratig- We used a combination of biostratigraphic raphies will be published elsewhere but can 1 μm1 μm be summarized here. The Shannon diversity Figure 1. Microfossils from Tanzania Drilling Project (TDP) Sites 12 and 17 showing excel- index for planktonic foraminifera (Fig. 2B) lent preservation. A: Planktonic foraminifer Cribrohantkenina infl ata (sample TDP17/39/4, 31–39 cm) showing typical “glassy” transparent test (taken in refl ected light with Leica 1GSA Data Repository item 2008042, age model , DFC 480 camera mounted on a Leica MZ 16 microscope using Earth Basic image software, stable isotope data, and platinum group element exposure 180 ms). B: Scanning electron microscope (SEM) image of juvenile larger ben- data, is available online at www.geosociety.org/ thic foraminifer Discocyclina (sample TDP17/39/4, 31–39 cm). C: SEM of coccosphere of pubs/ft2008.htm, or on request from editing@ Cyclicargolithus fl oridanus (sample TDP 12/47–2). D: SEM of Pontosphaera multipora and geosociety.org or Documents Secretary, GSA, P.O. other coccoliths (sample TDP 12/26/2, 62 cm). Box 9140, Boulder, CO 80301, USA. spp. sp.” sp Plankton Plankton diversity sp. A B Numm. extinctions (Shannon Index) Numm. D Tanzania T. ampliapertura δ18O (‰ VPDB) F Events Age (Ma) 1.2 1.6 2.0 2.4 2.8 -3.5-3.0 -2.5 -2.0 -1.5 Depth tie- points (mcd) Epoch 32.8 ODP Site 1218 Asterocyclinidae Discocyclinidae ‘discoidal’ S. orbitoideus “Biplanispira Heterostegina Operculina ‘lenticular’ Pellatispira C. orbitoideus ODP Site 744 (26.28) 33.0 TDP17 44.16 nannoplankton 33.2 TDP Site 17 TDP12 nannoplankton Early Oligocene 33.4 glacial maximum 96.64 STEP 2 33.6 Hantkeninidae Eocene-Oligocene 102.70 Boundary Turborotalia STEP 1 107.40 spp. Hantkeninidae 33.8 Site 522 Isotope shift P. papillatum Plankton Ecological disturbance extinctions Benthic foram extinction 130.35 34.0 D. saipanensis TDP12 TDP Site 12 planktonic 34.2 foraminifera ODP Site 522 Late EoceneLate Early Oligocene 34.4 34.6 (205.71) C Stratigraphic range of larger ‘reef’ foraminifera 00.5 1 1.5 2 2.5 E Deep-sea δ18O (‰ VPDB) Figure 2. Biotic and geochemical events across Eocene-Oligocene boundary compared to deep-sea records. A: Plankton extinction levels. B: Shannon diversity index for planktonic foraminifera (red diamonds, Tanzania Drilling Project [TDP] Site 12) and calcareous nanno fossils (black circles, TDP Site 12; gray circles, TDP Site 17; lines show three-point moving average). C: Chronostratigraphic ranges of larger benthic foraminifera, compiled from TDP Sites 11, 12, and 17. Legend: thick bars = common, thick dotted bars = frequent, thin dashed lines = rare, Numm = Nummulites, S. orbitoideus = Spiroclypeus orbitoideus , C. orbitoideus = Cycloclypeus sp. D: Planktonic foraminifer oxygen isotope record from Turborotalia ampliapertura, 212–150 μ (light-green diamonds, TDP Site 12; dark-green diamonds, TDP Site 17). VPDB—Vienna Peedee belemnite. E: Deep-sea benthic foraminifer isotope records (purple diamonds, Ocean Drilling Program [ODP] Site 1218 from Coxall et al. [2005]; red diamonds, ODP Site 744 from Zachos et al. [1994]; black diamonds ODP Site 522 from Zachos et al. [1994] with one point removed as suspect [S. Bohaty, 2007, personal commun.]). Hantkenina extinction with sampling bracket is from Poore (1984).