Extinction and Environmental Change Across the Eocene-Oligocene Boundary in Tanzania

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 geological 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 isotope 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 maximum 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 nannofossils) (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 foraminifera 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). F: Stratigraphical summary of events (mcd—meters composite depth).

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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 shows moderately diverse assemblages up to Plankton ca. 34 Ma, after which diversity declined toward extinctions δ13C (‰VPDB) Age (Ma) the Eocene-Oligocene boundary at 33.7 Ma. Epoch Calcareous nannofossils Planktonic foraminifera 0 0.5 1 1.5 Depth tie- points (mcd) We recognize the boundary by the coordinated 32.8 extinction of ﬁ ve planktonic foraminifer species in the Family Hantkeninidae (Hantkenina 33.0 44.16 and Cribrohantkenina; see Coxall and Pear- son, 2006). Apart from a single specimen from 33.2

a higher level that may have been reworked, O1 all ﬁ ve species disappear within a 25 cm sampling gap, equivalent to a sudden extinction 33.4 in less than 5000 yr. This extinction is more 96.64 sharply deﬁ ned than at the GSSP (Coccioni, NP21 1988), possibly because poor preservation and 33.6 Hantkenina spp. low abundances obscure the pattern there. The 102.70 Turborotalia 107.40 Eocene-Oligocene boundary event is preceded 33.8 spp.

by the extinction or dramatic diversity reduc- Pemma papillatum tion in the Turborotalia cerroazulensis group, 34.0 130.35 which occurs 5.2 m below. In the age model Discoaster saipanensis for the Massignano stratotype, these extinction events are separated by ~65 k.y. (Berggren and 34.2

Pearson, 2005). Other planktonic foraminifer EoceneLate Early Oligocene events recorded in Tanzania that have been found E15/E16 34.4 elsewhere (Nocchi et al., 1986; Boersma and TDP Site 12 TDP Site 17 Premoli Silva, 1986; Molina et al., 2006) are the ODP Site 1218 NP19/NP20 sudden dwarfi ng of Pseudohastigerina micra, 34.6 ODP Site 522 which occurs precisely at the Eocene-Oligocene ODP Site 744 boundary, and a delayed diversification of Dentoglobigerina spp. in the lower Oligocene. 0 0.5 1 1.5 2 Calcareous nannoplankton show clear evi- Cibs. spp. δ13C (‰VPDB) dence of ecological disruption through the Figure 3. Carbon isotope stratigraphy across Eocene-Oligocene boundary in Tanzania com- Eocene-Oligocene boundary interval. The extinc- pared to deep-sea records. VPDB—Vienna Peedee belemnite; ODP—Ocean Drilling Program; tions of Discoaster saipanensis and Pemma TDP—Tanzania Drilling Project; mcd—meters composite depth. papillatum are followed by a distinct shift toward less diverse assemblages across the boundary (Fig. 2B). These changes are mainly the result of stepwise abundance declines (<200 k.y.) but not instantaneous. Neverthe- the most complete deep-sea benthic foramini- of warm-water taxa, some of which fi nally less, it constitutes a major crisis in car bonate fer isotope records (Oberhänsli et al., 1994; became extinct later in the Oligocene. The platform ecology on a greater scale than had Zachos et al., 1996; Coxall et al., 2005). As in nannofossil diversity decline lags that seen in happened for tens of millions of years. the deep-sea sites, the oxygen isotope record planktonic foraminifera. The extinction of such widespread and shows two main steps that lead in to the most The cores also record a series of extinctions numerous shallow-water carbonate pro ducers positive values of the early Oligocene glacial and originations among the larger forami- may itself have had a signifi cant impact on maximum. The overall magnitude of the oxy- nifera (Fig. 2C). Specimens of Discocyclina, Earth systems. If the extinctions were linked gen isotopic changes is less than shown by Asterocyclina, and a discoidal morphology of to a reduction in shelf-carbonate production the benthic records, especially at the second Nummulites are ubiquitous in the Eocene of rates, possible effects might have included a main step, suggesting that high-latitude cool-

Tanzania, up to and including the lower part transient reduction in atmospheric CO2, global ing as well as ice growth was responsible for of the cores described here, but they disappear cooling, increased ocean alkalinity, and a deep- generating the benthic records. This possibility close to the boundary level. The extinction of ening of the oceanic carbonate compensation is further examined using trace-element geo- the Discocyclinidae seems to slightly precede depth (CCD). All these are characteristic of chemistry by Lear et al. (2008). the Eocene-Oligocene boundary. It may itself the Eocene-Oligocene transition (Zachos et al., No microtektites or other evidences for extra- be preceded by the extinction of the Astero- 2001; Coxall et al., 2005). terrestrial impact were observed in any of our cyclinidae, although that group is generally samples. To more fully investigate the possibil- rare in the cores, and so sampling effects limit GEOCHEMISTRY AND GLOBAL ity of impact, we analyzed platinum group ele- our ability to precisely defi ne the sequence of CORRELATION ment concentrations from 14 samples from TDP events. A discoidal morphotype of Nummulites We constructed a stable-isotope stratigraphy Site 12, but concentrations were close to aver- is replaced by a new lenticular form very close from overlapping intervals of TDP Sites 17 and age crust values throughout the succession (see to the boundary, and rare specimens of Cyclo- 12 using shells of the mixed-layer planktonic online GSA Data Repository). clypeus and (very briefl y) Pellatispira also foraminifer Turborotalia ampliapertura (data The fi rst main step in the oxygen isotope appear at this level. The record suggests that given in the online GSA Data Repository, see record corresponds quite closely to the extinc- the turnover of larger foraminifera postulated footnote 1). The oxygen (Fig. 2D) and carbon tion level of the Discocyclinidae (benthic by Adams et al. (1986) was relatively rapid (Fig. 3) isotope records are similar in form to foram i nif era) and Turborotalia cerroazulensis

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