Southeastern Atlantic Deep-Water Evolution During the Late-Middle Eocene to Earliest Oligocene (Ocean Drilling Program Site 1263 and GEOSPHERE; V

Research Paper

GEOSPHERE Southeastern Atlantic deep-water evolution during the late-middle Eocene to earliest Oligocene (Ocean Drilling Program Site 1263 and GEOSPHERE; v. 12, no. 3 Deep Sea Drilling Project Site 366) doi:10.1130/GES01268.1 Samantha J. Langton1,*, Nathan M. Rabideaux1,†, Chiara Borrelli2, and Miriam E. Katz1 6 figures; 3 tables; 2 supplemental files 1Department of Earth & Environmental Sciences, Rensselaer Polytechnic Institute, Jonsson-Rowland Science Center, 1W19 110 8th Street, Troy, New York 12180, USA 2Department of Earth & Environmental Sciences, University of Rochester, 227 Hutchison Hall, P.O. Box 270221, Rochester, New York 14627, USA

CORRESPONDENCE: katzm@rpi.edu

CITATION: Langton, S.J., Rabideaux, N.M., Borrelli, ABSTRACT Miller et al., 1992, 2005, 2008a, 2008b, 2009; Diester-Haass and Zahn, 1996; C., and Katz, M.E., 2016, Southeastern Atlantic deep-water evolution during the late-middle Eo- Zachos et al., 1996; Lear et al., 2004, 2008; Coxall et al., 2005; Katz et al., 2008; cene to earliest Oligocene (Ocean Drilling Program Comparison of new benthic foraminiferal δ18O and δ13C records from Ocean Wade and Pearson, 2008; Pusz et al., 2011; Wade et al., 2012; Bijl et al., 2013). The Site 1263 and Deep Sea Drilling Project Site 366): Drilling Program (ODP) Site 1263 (Walvis Ridge, southeast Atlantic, 2100 m transition from warmer to colder high-latitude ocean temperatures from the Geosphere, v. 12, no. 3, p. 1032–1047, doi:10.1130 /GES01268.1. paleodepth) and Deep Sea Drilling Project (DSDP) Site 366 (Sierra Leone Rise, early Eocene to the Oligocene (ca. 50–33 Ma) is evident in many paleoceano eastern equatorial Atlantic, 2200–2800 m paleodepth) with published data graphic records (e.g., Browning et al., 1996; Diester-Haass et al., 1996; see Received 18 September 2015 from Atlantic and Southern Ocean sites provides the means to reconstruct the compila tions of Cramer et al., 2009, 2011). Southern Ocean deep-water tem- Revision received 2 February 2016 development of deep-water circulation in the southeastern Atlantic from the peratures peaked during the early Eocene and subsequently cooled during the Accepted 7 April 2016 late-middle Eocene to the earliest Oligocene. Our comparison shows that in middle Eocene to the early Oligocene by as much as 5–10 °C (e.g., Mackensen Published online 5 May 2016 the late-middle Eocene (ca. 40 Ma), the South Atlantic was characterized by a and Ehrmann, 1992; Zachos et al., 2001; Diekmann et al., 2004; Cramer et al., homogeneous thermal structure. Thermal differentiation began ca. 38 Ma. By 2011; Bohaty et al., 2012). The earliest documented ephemeral Antarctic glacia- 37.6 Ma, Site 1263 was dominated by Southern Component Water; at the same tion occurred in the late-middle Eocene (ca. 37.3 Ma; Scher et al., 2014), followed time, warm saline deep water filled the deeper South Atlantic (recorded at by additional ephemeral glaciations and warming and cooling cycles leading southwest Atlantic ODP Site 699, paleodepth 3400 m, and southeast Atlantic into continental-scale Antarctic glaciation in the early Oligocene (e.g., Miller ODP Site 1090, paleodepth 3200 m). The deep-water source to eastern equa- et al., 1991; Zachos et al., 1992; Thomas, 1992; Cramer et al., 2011). torial Site 366 transitioned to Northern Component Water ca. 35.6–35 Ma. Pro- Deep-water circulation patterns during this transition changed as key tec- gressive cooling at Site 1263 during the middle to late Eocene and deep-water tonic gateway configurations changed (Exon et al., 2004; Stickley et al., 2004; thermal stratification in the South Atlantic may be attributed at least in part to Scher and Martin, 2006; Livermore et al., 2007; Cramer et al., 2009; Katz et al., the gradual deepening and strengthening of the proto–Antarctic Circumpolar 2011; Borrelli et al., 2014; Borrelli and Katz, 2015), including the progressive Current from the late-middle Eocene to the earliest Oligocene, as the Drake openings of the Drake (Cox, 1989; England, 1992; Toggweiler and Bjornsson, and Tasman gateways opened. Our isotopic comparisons across depth and 2000; Livermore et al., 2004, 2007; Sijp and England, 2004; Eagles et al., 2006) latitude provide evidence of the development of deep-water circulation simi- and Tasman (Exon et al., 2004; Stickley et al., 2004; Sijp et al., 2011, Bijl et al., lar to modern-day Atlantic Meridional Overturning Circulation. 2013) passages that ultimately resulted in the development of the Antarctic Circumpolar Current (ACC). Initial opening of the Drake Passage began in the middle Eocene (ca. 50 Ma; Livermore et al., 2007), with circulation as deep as INTRODUCTION 1000 m by ca. 41 Ma (Scher and Martin, 2006; Livermore et al., 2007). Onset of spreading at the West Scotia Ridge in the latest Eocene–early Oligocene al- The late-middle Eocene to earliest Oligocene (ca. 38–33 Ma) was a period of lowed the development of eastward-flowing deep currents through the Drake transition to large-scale glaciation on Antarctica and cooling of the oceans (e.g., Passage (Livermore et al., 2007). Shallow westward flow through the Tasman Miller and Fairbanks, 1985; Barron et al., 1991; Ehrmann and Mackensen, 1992; gateway began in the middle Eocene (Bijl et al., 2013), progressing to deep flow by the latest Eocene–earliest Oligocene (Scher et al., 2015); deep eastward *Now at Environmental Products and Services of Vermont, 210 Wembly Road, New Windsor, flow through the Tasman gateway began ca. 30 Ma (Scher et al., 2015). New York 12553, USA For permission to copy, contact Copyright †Now at Department of Geosciences, Georgia State University, 24 Peachtree Center Ave., NE, The cooling of Southern Component Water (SCW; deep-water mass origi Permissions, GSA, or [email protected]. Atlanta, Georgia 30302, USA nating in the Southern Ocean) coincided with the progressively developing

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ACC in response to the gradual opening of the Drake and Tasman Passages Drilling Project (DSDP) Site 366 (eastern equatorial Atlantic, 2853 m present (e.g., Livermore et al., 2007; Cramer et al., 2009; Sijp et al., 2011; Bijl et al., depth, 2200–2800 m paleodepth) to published data sets from the Atlantic and 2013); this is consistent with models that show the formation of cold SCW Southern Oceans (Table 1; Figs. 1 and 2). We compare the circulation changes with the progressive deepening of the Drake and Tasman Passages (e.g., to gateway changes in order to examine how the development of the proto- Toggweiler and Samuels, 1995; Toggweiler and Bjornsson, 2000; Nong et al., ACC affected deep-water circulation in the southeastern Atlantic, as has been 2000; Sijp and England, 2004, 2005). The opening of the Drake Passage and suggested for other regions including the Southern Ocean, northern Atlantic, developing proto-ACC (defined as the precursor to the modern ACC; the and northwestern Pacific (e.g., Diester-Haass and Zahn, 1996; Diekmann et al., proto-ACC was shallower than the modern current because of the more con- 2004; Scher and Martin, 2006; Via and Thomas, 2006; Cramer et al., 2009; Katz stricted Drake and Tasman gateways; e.g., Borrelli et al., 2014) also has been et al., 2011; Borrelli et al., 2014; Borrelli and Katz, 2015). linked to the production of Northern Component Water (NCW; deep-water mass originating in the northern North Atlantic; e.g., Scher and Martin, 2008; Katz et al., 2011; Borrelli et al., 2014) that originated in the late-middle Eocene MODERN SOUTH ATLANTIC OCEAN CIRCULATION from the Labrador Sea (Borrelli et al., 2014). In addition, the Tethys closed to the North Atlantic during the middle Eocene, which prohibited export of The modern ocean is driven by thermohaline circulation (e.g., Gordon, low-latitude warm saline deep water to the Atlantic Ocean from the western 1986; Broecker, 1991; Jayne and Marotzke, 2001), wind-driven surface mix- Tethys (Oberhänsli, 1992). ing (Wyrtki, 1961; Schmitz, 1995; Rahmstorf, 2003), and eddy diffusivity (e.g., In the marine record, d18O values of foraminifera are used to interpret paleo Holzer and Primeau, 2006). The Meridional Overturning Circulation (MOC) in temperature and global ice volume (e.g., Miller and Fairbanks, 1987; Cramer the present-day ocean is driven primarily by wind-induced upwelling that is et al., 2009), while d13C values are indicative of paleoproductivity, water-mass most intense along the ACC and causes the displacement of deep water to aging, and the oceanic carbon cycle (e.g., Kroopnick, 1985; Lynch-Stieglitz the surface, allowing formation of new deep water through downwelling in et al., 1995). The analysis of benthic foraminifera that record isotopic changes the North Atlantic and Southern Ocean (e.g., Toggweiler and Samuels, 1998). provides a snapshot of global climate and deep-water circulation from the time The South Atlantic plays an important role in the global balance of the modern they formed their tests. Intersite comparisons between benthic foraminiferal Atlantic Meridional Overturning Circulation (AMOC) through mixing, advec- d18O and d13C records are used to interpret the source region of deep-water tion, and subduction of water masses that enter this basin from all depths and production and the flow path of deep-water masses (e.g., Curry and Lohmann, oceans (Garzoli and Matano, 2011). The AMOC is potentially a strong influence 1982; Oppo and Fairbanks, 1987; Miller, 1992; Wright et al., 1992; Zachos et al., on today’s global climate because of its large interbasin transfer of heat (Gar- 2001; Cramer et al., 2009). zoli and Matano, 2011). The global changes in deep-water circulation at million-year time scales Today, Site 1263 (Walvis Ridge) is influenced by a combination of Circum- associated with the shift from virtually ice-free conditions during the early- polar Deep Water (CDW) mixed with North Atlantic Deep Water from the north middle Eocene to the development of continental-scale Antarctic glaciation in and west, and CDW that travels north directly from the ACC (Fig. 1; Friedrichs the early Oligocene has not been documented fully in the South Atlantic. Here et al., 1994; Speer et al., 1995; Stramma and England, 1999). The mid-Atlantic, we investigate changes in deep-water circulation in the southeastern Atlantic Walvis, and Agulhas Ridges have constrained deep-water circulation in the during this transition by comparing new benthic foraminiferal stable oxygen southeastern Atlantic since ca. 130 Ma (O’Connor and Duncan, 1990; Müller and carbon isotopes (d18O and d13C) from Ocean Drilling Program (ODP) Site et al., 1993; Stramma and England, 1999; Fig. 1). From north to south, these 1263 (Walvis Ridge, 2700 m present depth, 2100 m paleodepth) and Deep Sea ridges mark the boundaries for the Angola, Cape, and Agulhas Basins.

TABLE 1. PALEODEPTHS OF DEEP SEA DRILLING PROJECT AND OCEAN DRILLING PROGRAM SITES DISCUSSED IN THE TEXT Middle to late Eocene paleodepth Location (m) Citation ODP Site 1263 Walvis Ridge~2100 Zachos et al. (2004) DSDP Site 366 Sierra Leone Rise ~2200–2800 Shipboard Scientiﬁc Party (1978) ODP Site 689 Maud Rise ~1400 Kennett and Stott (1990) ODP Site 699 East Georgia Basin ~3400 Mead et al. (1993) ODP Site 1090 Agulhas Ridge~3200 Pusz et al. (2009, 2011) ODP Site 1053 Blake Nose ~1500 Katz et al. (2011); Borrelli et al. (2014) Note: ODP—Ocean Drilling Program; DSDP—Deep Sea Drilling Project.

60°W 30° 0° 30°E 60° 60°

1053 30° 30° 30°N 30°N 1053 366 366 0°0 0° NADW 0 0° 1263 RFZ 30° 30° Angola 699 1090 689 Brazil 1263 60° 60° 30° 30° Rio Grande Ridge Agulhas 35 Ma reconstruction alvis Rise W Cape Latitude Argentine Figure 2. Ocean Drilling Program sites used in this study (Site 366—Deep Sea Drilling Proj- 1090 ect) are shown on an Eocene paleogeographic map (35 Ma; http://www .odsn.de/cgi-bin/make _map.pl). Sites are color coded to match data in Figures 5 and 6. 699 60°S CDW 60°S earlier than previously reported (Exon et al., 2004; Stickley et al., 2004) for the ACC late Eocene (Bijl et al., 2013). Plate tectonic models indicate that the Drake Pas- 689 sage began to open ca. 50 Ma and may have contributed to cooling and growth of ice sheets in Antarctica during the middle Eocene (Livermore et al., 2007; Cramer et al., 2011). Before a deep and continuous ACC was established, flow of the shallower proto-ACC was likely controlled by tectonic changes that either 60°W 30° 0° 30°E constricted or enabled Drake Passage throughflow (Livermore et al., 2004, 2007). Longitude In addition, surface water originating from the Pacific and Tethys may have downwelled and traveled as deeper currents into the North and South Atlantic Figure 1. Modern deep-water circulation in the South Atlantic Basin, including North Atlan- Basins (Kennett and Stott, 1990; Barrera and Huber, 1993; Mead et al., 1993; tic Deep Water (NADW, green), the Agulhas Current (red), and Circumpolar Deep Water (CDW, green) paths in the South Atlantic (details in text). Current flow patterns are adapted from Fried Thomas, 2005; Scher and Martin, 2006; Via and Thomas, 2006). The closing of richs et al. (1994), Schmitz (1995), Speer et al. (1995), Stramma and England (1999), and Garzoli the Tethys may have occurred as early as 35 Ma (Allen and Armstrong, 2008), and Matano (2011). ACC—Antarctic Circumpolar Current; RFZ—Romanche Fracture Zone. Sites and would have restricted flow from the Tethys into the North Atlantic and used in this study are shown (Site 366 is Deep Sea Drilling Project; others are Ocean Drilling Program). Site color corresponds to color of data and trend lines in Figures 5 and 6. Map is changed the water-mass characteristics in the southeastern Atlantic (Allen adapted from Google, Map Data, MapLink (www.maplink.com/), Tele Atlas (2013; ‘s-Hertogen- and Armstrong, 2008). This restriction of deep-water flow from the Tethys to bosch, Netherlands, www.tomtom.com). the North Atlantic may have amplified Tethyan flow eastward into the Indian Ocean, much like the modern-day Red Sea Intermediate Water into the Agulhas Current (e.g., Roman and Lutjeharms, 2009). The closing of the Tethys to the DEEP-WATER CIRCULATION FROM THE MIDDLE EOCENE North Atlantic, combined with the development of the proto-ACC, established TO LATE OLIGOCENE a tectonic configuration similar to today. The ACC is an important component of modern-day AMOC by controlling wind-driven mixing, a key component of Tectonic changes have affected oceanographic circulation at >105 yr time meridional ocean circulation (Kuhlbrodt et al., 2007; Garzoli and Matano, 2011); scales. The separation of Antarctica from South America and Australia led to the however, the role of the ACC in setting the strength of the northern cell of formation of the ACC, which may have caused thermal isolation of the Southern the modern MOC is still a matter of debate (Kuhlbrodt et al., 2007). In addition, Ocean as early as the latest Eocene by preventing the flow of warm surface the Agulhas leakage, which consists of rings broken off from the Agulhas Cur- currents from the subtropical gyres to Antarctica (Toggweiler and Bjornsson, rent at the Agulhas retroflection off the southern tip of the African peninsula, 2000; Barker, 2001; Exon et al., 2004; Sijp and England, 2004, 2005). Paleoceano can affect AMOC by altering the flow of warm, salty Indian Ocean water into graphic evidence indicates that surface flow through the Tasman Passage was in the South Atlantic (van Leeuwen et al., 2000; Speich et al., 2007; Biastoch et al., place during the middle Eocene and resulted in cooling of the Southern Ocean 2009; Garzoli and Matano, 2011).

In this paper we explore the evolution of deep-water flow and early AMOC Earth and Planetary Sciences at Rutgers University (New Jersey) (Supplemen- at the Walvis Ridge (eastern South Atlantic) in the late-middle Eocene to the ear- tary Tables 1 and 21; Fig. 3). Foraminifera were reacted in phosphoric acid at liest Oligocene (40–33 Ma), during the early stages of the opening of the Drake 90 °C for 15 min in an automated peripheral attached to a Micromass Optima and Tasman Passages and development of the ACC. We monitor changes in mass spectrometer. water masses at the Walvis Ridge by comparing benthic foraminiferal d18O and The equations listed here were used for the interspecies calibrations for d13C from Sites 1263 and 366 to published records from other regions within Cibicidoides spp. with O. umbonatus and N. truempyi for analyses from Site the Atlantic Ocean (Table 1; Figs. 1 and 2). Together, these new and published 1263. Equations to calibrate N. truempyi (Sites 1263 and 689) d18O and d13C, records provide the means to reconstruct deep-water circulation in the Atlantic and O. umbonatus d18O (Site 1263), are from Katz et al. (2003). The equation to Ocean and provide evidence of meridional overturning circulation in the South calibrate O. umbonatus d13C (Site 1263) is from Shackleton et al. (1984) because Atlantic as it responded to the development of the proto-ACC during the period it provides the best fit to theCibicidoides spp. d13C data as compared to the leading up to the large-scale glaciation of Antarctica. calibration in Katz et al. (2003). These calibrated values are within the range of the Cibicidoides values (Fig. 3). Other published data used in this study utilized Cibicidoides, and needed no calibration. METHODS (1) O. umbonatus: 18O: y = x + 0.28 ODP Site 1263, Hole B (Site 1263; 28°30′S, 2°45′E) was drilled at 2717 m d 18 18 water depth on the Walvis Ridge (Figs. 1 and 2). The paleodepth for this location such that Cibicidoides spp. d O = (O. umbonatus d O) – 0.28 13 from the middle Eocene to early Oligocene was ~2100 m, with general lithology d C: y = x – 1.00 13 13 consisting of nannofossil ooze, chalky nannofossil ooze, foraminifera-bearing such that Cibicidoides spp. d C = (O. umbonatus d C) + 1.00 nannofossil ooze, and clay-bearing nannofossil ooze (Zachos et al., 2004). DSDP (2) N. truempyi: Site 366 (Figs. 1 and 2) is located in the eastern equatorial Atlantic on the Sierra d18O: y = 0.89x – 0.10 Leone Rise, 4°40.70 N, 19°51.10 W, ~800 km west of Sierra Leone with a present ′ ′ such that Cibicidoides spp. d18O = [(N. truempyi d18O) + 0.10]/0.89 water depth of 2853 m (Shipboard Scientific Party, 1978). The Site 366 paleolati d13C: y = x – 0.34 tude during the Eocene was ~2°N, with a paleodepth of 2200–2800 m during the such that Cibicidoides spp. d13C = (N. truempyi d13C) + 0.34. middle Eocene to early Oligocene (Miller et al., 1989). The general lithology of this section at Site 366 consists of nannofossil ooze and/or chalk, pelagic clay, At Site 366, we primarily used C. eocaenus because of its high abun- and siliceous limestone (Shipboard Scientific Party, 1978). dance and excellent preservation. Well-preserved C. praemundulus, C. bradyi, C. grimsdalei, and C. mexicanus were substituted in some samples with limited specimens of C. eocaenus. Burial depth of the section studied at Site 366 DepthAge, Ma Age, Ma O18 PDBC13 PDBtaxon O18 PDBC13 PDB (mcd) (2004 GTS) (2012 GTS) Oridorsalis & Nuallides not adjusted to Sample Preparation (~395–475 m below seafloor, mbsf) is shallower than the ~500–550 mbsf at adjusted to Cibicidoides Cibicidoides 18 86.64 33.01 32.90 1.99 1.32 Cibicidoides which burial diagenesis may begin to affect d O values, with the potential to 86.79 33.03 32.92 2.12 1.58 Cibicidoides 86.94 33.05 32.94 2.20 1.23 Cibicidoides 87.09 33.07 32.96 2.00 1.29 Cibicidoides A total of 336 samples from ODP Site 1263B were obtained at 15 cm inter- overprint the primary signal with anomalously low values (Miller and Fair- 87.24 33.10 32.99 1.99 1.17 Cibicidoides 87.39 33.12 33.01 2.14 1.47 Cibicidoides 18 87.54 33.14 33.03 1.88 1.40 Cibicidoides vals and 157 samples were obtained from DSDP Site 366. Samples from Site banks, 1987). The absence of anomalous d O values in the lowest part of our 87.69 33.16 33.05 2.24 1.51 Cibicidoides 18 87.84 33.18 33.07 1.92 1.44 Cibicidoides 366 were soaked in a sodium metaphosphate solution for no more than 24 h to record is consistent with preservation of the primary d O signal (Fig. 3B). 87.99 33.20 33.09 2.15 1.50 Cibicidoides 88.14 33.22 33.11 2.32 1.55 Cibicidoides 88.29 33.25 33.14 2.34 1.76 Cibicidoides disaggregate the sediment. Samples from Site 1263 were soaked overnight in Stable isotope analyses for Site 366 were conducted using a GV Instru-

Adjusted Adjusted deionized water. All samples were rinsed through a 63 µm stainless steel sieve ments IsoPrime dual inlet mass spectrometer at the University of South Caro Age, Ma Age, Ma Adjusted Age, Ma Age, Ma (2004 (2012 Depth (2004 (2012 Depth (mbsf) GTS) GTS) O18 PDBC13 PDB (mbsf) GTS) GTS) with tapwater and then rinsed with deionized water. The >63 µm fraction was lina. Stable isotope values are reported versus Vienna Peedee belemnite by 395.89 32.93 32.83 1.46 1.15 400.54 33.0632.95 398.53 33.11 33.00 1.37 1.03 403.18 33.2333.12 dried in an oven (~50 °C) overnight. Each sample was dry sieved and benthic analyzing NBS-19 and an internal lab standard during each automated run. 399.93 33.21 33.10 1.51 1.08 403.93 33.2733.17 404.35 33.51 33.44 1.34 1.77 407.933.53 33.46 18 13 404.60 33.53 33.46 1.23 1.74 408.15 33.5333.46 foraminifera were picked from the >150 µm fraction. The selected foraminifera We use the published values for NBS-19 of –2.20 and 1.95‰ for d O and d C, 404.85 33.55 33.49 1.49 1.78 408.433.54 33.48 404.85 33.55 33.49 1.54 1.20 408.433.56 33.50 were sonified in deionized water for 1–2 s to remove residual clays. respectively (Coplen, 1995). The 1s precision of the standards analyzed for Site 405.10 33.57 33.51 1.38 1.49 408.65 33.5633.50 405.32 33.58 33.52 1.42 1.57 408.87 33.5733.51 18 13 405.57 33.60 33.55 1.37 1.90 409.12 33.5933.53 At Site 1263, we used the most abundant of the Cibicidoides species, Cibici 1263 are 0.08‰ and 0.05‰ for d O and d C, respectively (Coplen, 1995). The 405.83 33.62 33.57 1.69 1.68 409.38 33.6033.55 406.07 33.63 33.58 1.54 1.68 409.62 33.6233.57 406.33 33.65 33.60 1.57 1.99 409.88 33.6333.58 doides praemundulus, for isotopic analyses; C. havanensis, C. bradyi, C. grims 1s precisions of the standards analyzed for Site 366 are 0.07‰ and 0.04‰ for dalei, C. mexicanus, or C. robertsonianus were substituted as needed. Nuttall d18O and d13C, respectively (Supplementary Table 2). 1Supplemental Tables 1 and 2. Isotope specimen ides truempyi and Oridorsalis umbonatus were analyzed from samples that Where they overlap, we compared our data from Site 366 to those of Miller sample data from Sites 1263 and 366. Please visit were devoid of Cibicidoides. Two to five well preserved, ultrasonically cleaned et al. (1989) from the same site, and duplicated their results for a subset of http://dx .doi.org/10 .1130/GES01268.S1 or the full- text article on www.gsapubs.org to view the Supple- Cibicidoides, N. truempyi, or O. umbonatus specimens from each sample from samples ranging from the latest Eocene to earliest Oligocene. The duplicate mental File. Site 1263 were analyzed in the stable isotope laboratory in the Department of analyses yielded similar isotopic values, with a mean deviation between Miller

18 δ O ‰ δ13C ‰ 18 13

A δ O ‰ δ C ‰ B Core Recovery 2 1.5 1 0.5 0 –0.5 00.5 11.5 22.5 3 2.5 2 1.5 1 0.5 0 –0.5 00.5 11.5 22.5 80 80 Cibicidoides spp. Cibicidoides spp. 400 400 90 O. umbonatus 90 N. truempyi

100 100 420 420 )

110 110

120 120 440 440 Depth (mcd ) Depth (mbsf 130 130

140 140 460 460

150 150 3 2.5 2 1.5 1 0.5 0 –0.5 00.5 11.5 22.5 480 480 2.5 2 1.5 1 0.5 0 –0.5 00.5 11.5 22.5

Figure 3. (A) Ocean Drilling Program Site 1263B Cibicidoides spp. δ18O and δ13C data, with Nuttallides truempyi and Oridorsalis umbonatus δ18O and δ13C data adjusted to Cibicidoides spp., plotted versus depth in core (mcd—meters composite depth). Adjusted δ18O and δ13C values are within reasonable agreement with the Cibicidoides data. (B) Deep Sea Drilling Project Site 366 Cibicidoides spp. δ18O and δ13C data plotted versus depth in core (mbsf—meters below seafloor). Core recovery is shown to highlight the reason for data gaps.

et al. (1989) data and our new analysis of 0.174‰ d18O and 0.138‰ d13C. Our base of magnetochron C13n was identified in Hole 1263B (Bowles, 2006) and higher resolution data set adds detailed structure to the lower resolution Miller is used in our age model, supported by the oxygen isotope correlation with et al. (1989) record, and shows that it was subject to signal aliasing as a result Site 689. The highest occurrence (HO) of the nannoplankton Ericsonia formosa of low resolution. was used as the uppermost datum in the core (Fig. 4A) because it is well constrained (Blaj, 2009). Age Models We identified the last occurrence of Hantkenina spp. in our samples from Site 1263 at 98.89 m composite depth (mcd), updating the depth of 104.05 mcd The ages for datums in all age models were updated to the 2004 geological provided by the ODP Site 1263 initial report, which was based on lower reso- time scale (2004 GTS; Gradstein et al., 2005; Tables 2A–2F). We use this time lution samples (Zachos et al., 2004). The age of this datum was updated from scale to allow comparisons to published data from the compilation of Cramer 33.7 to 33.9 Ma from the 1995 geologic time scale (Cande and Kent, 1995) to et al. (2009). Ages for the planktonic foraminiferal datums are from Wade et al. the 2004 GTS (Gradstein et al., 2005; Wade et al., 2011); the use of this datum in (2011) and the ages of late Eocene to early Oligocene calcareous nannoplank- the age model is supported by the d18O correlation between Sites 1263 and 689 ton are from Blaj (2009). (Fig. 5). The planktonic foraminifer Turborotalia cerroazulensis was not used in our age model because specimens were rare, with poorly defined first and Site 1263 last occurrences at Site 1263 (Zachos et al., 2004); in addition, its range was sporadic and not ubiquitous among various global locations (Berggren and Most of the sediment in Hole 1263B from the middle Eocene to lower Pearson, 2005). The unreliability of T. cerroazulensis at Site 1263 is supported Oligocene is dominated by nannofossil ooze, which did not preserve a strong by the fact that if it is included in the age model, it results in an unreasonably magnetic signal in much of this section (Zachos et al., 2004). Nonetheless, the old age for Oligocene oxygen isotope event 1 (Oi-1; e.g., Miller et al., 1991,

TABLE 2A. DATUMS USED TO DEVELOP THE AGE MODELS FOR OCEAN DRILLING PROGRAM SITE 1263 Depth Age (mcd) (Ma) Datum Used in age model Hole used for datum Reference 83.62 33.266 T C13n 1263A Bowles (2006) 86.00 32.919 HO Ericsonia formosa x 1263B Blaj (2009) 91.74 33.738 B C13n x 1263B Bowles (2006) 98.05 34.000 HO Turborotalia cerroazulensis 1263B Zachos et al. (2004) 98.89 33.900 HO Hantkenina spp. x 1263B This study 104.30 34.435 HO Discoaster saipanensis 1263B Zachos et al. (2004) 104.05 34.500 HO Globigerinatheka index 1263A Zachos et al. (2004) 113.80 35.800 HO Globigerinatheka semiinvoluta 1263A Zachos et al. (2004) 124.20 39.000 HO Acarinina primitiva 1263B Zachos et al. (2004) 124.23 36.930 HO Chiasmolithus grandis 1263B Zachos et al. (2004) 138.47 39.370 MECO x 1263B This study 148.01 40.440 T C19n x 1263B Zachos et al. (2004) Note: Core depths are in meters composite depth (mcd). HO—highest occurrence; LO—lowest occurrence; T—top of magnetochron; B—base. MECO—Middle Eocene Climatic Optimum.

TABLE 2B. DATUMS USED TO DEVELOP THE AGE MODELS TABLE 2C. DATUMS USED TO DEVELOP THE AGE MODELS TABLE 2D. DATUMS USED TO DEVELOP THE AGE MODELS FOR DEEP SEA DRILLING PROJECT SITE 366 FOR OCEAN DRILLING PROGRAM SITE 689 FOR OCEAN DRILLING PROGRAM SITE 699 Depth Age Depth Age Datum Depth Age (mbsf) (Ma) Datum Reference (mbsf) (Ma) (magnetochron) (mbsf) (Ma)Datum 382.34 32.00 HO Pseudohastigerina spp. Miller et al. (1989) 90.20 28.186 T C10n.1n 253.64 31.26 B hiatus 406.33 33.65 δ13C maximum This study 92.10 28.715 B C10n.2n 284.55 33.738 B C13n 462.87 38.00 E14-E15 boundary Krasheninnikov and 99.80 29.451 T C11n.1n 329.20 36.000 LO Isthmolithus recurvus Pflaumann (1978) 104.50 30.627 T C12n 355.20 39.400 MECO 116.70 33.266 T C13n Note: mbsf—meters below seafloor. HO—highest occurrence. E is planktonic Note: mbsf—meters below seafloor. MECO—Middle Eocene 120.20 33.738 B C13n foraminiferal biozone. Climatic Optimum. LO—lowest occurrence; C—magnetochron; 124.10 34.782 T C15n B—base. Data from Mead et al. (1993). Ages updated to the 2004 134.20 36.276 B C16n.2n geological time scale (Gradstein et al., 2005). 144.40 37.235 B C17n.1n 197.50 49.500 hiatus Note: mbsf—meters below seafloor. Data from Bohaty and Zachos (2003); Diester-Haass and Zahn (1996). Ages updated to the 2004 geological time scale (Gradstein et al., 2005). T—top; B—base.

TABLE 2E. DATUMS USED TO DEVELOP THE AGE MODELS TABLE 2F. DATUMS USED TO DEVELOP THE AGE MODELS FOR OCEAN DRILLING PROGRAM SITE 1053 FOR OCEAN DRILLING PROGRAM SITE 1090 Depth Age Depth Age Datum (mbsf) (Ma) Datum (mbsf) (Ma) (magnetochron) 0.00 33.900 HO Hantkenina 118.37 29.740 B C11n 10.78 34.780 T C15n 119.36 30.627 T C12n 19.52 35.040 B C15n 124.89 31.116B C12n 80.73 36.000 LO Isthmolithus recurvus 139.25 33.266 T C13n.1n 144.83 37.240 B C17n.1n 146.64 33.738 B C13n.2n 160.41 37.550 B C17n.2n 155.70 34.782 T C15n.1n 172.57 37.700 HO Morozovella spinulosa 158.10 35.043 B C15n.2n 182.16 38.000 LO Globigerinatheka semiinvoluta 161.60 35.404 T C16n.1n 162.90 35.567 B C16n.1n Note: mbsf—meters below seaﬂoor. HO—highest occurrence; LO—lowest 164.60 35.707 T C16n.2n occurrence. C—magnetochron; T—top; B—base. Data from Borrelli et al. (2014). Ages updated to the 2004 geological time scale (Gradstein et al., 2005). 167.79 36.276 B C16n.2n 172.85 36.600 B C16n.3n Note: Ages updated to the 2004 geological time scale (Gradstein et al., 2005); mbsf—meters below seaﬂoor. T—top; B—base. Data from Pusz et al. (2009, 2011).

Age (Ma) A (Zachos et al., 2004; Bowles, 2006) and its agreement with the biostratigraphy 31 32 33 34 35 36 37 38 39 40 41 42 80 of the core (Tables 2A–2F; Fig. 4A) (Zachos et al., 2004). Our age model is sup- HO E. formosa C13n (T) ported by the correlation of the d18O increase at Sites 1263 and 689 ca. 37.3 Ma, 90 6.85 C13n (B) which is consistent with the Priabonian d18O event (Scher et al., 2014) and 44.14 HO T. cerroazulensis 100 HO Hantkenina spp. changes in radiolarian assemblages at Southern Ocean sites (Pascher et al., HO D. saipanensis HO G. index 2015). These correlations support the use of fairly widespread datums at Site 110 1263, indicating a fairly constant sedimentation rate for ~6 m.y. HO G. semiinvoluta 7.24 120 LO I. recurvus The ages of several datums, including Chiasmolithus grandis, Globigeri HO C. grandis HO A. primitiva natheka semiinvoluta, and Nannotrina spp., were converted to the 2004 GTS Depth (mcd ) 130 and support the datums chosen for the Site 1263 age model; however, they MECO 140 were not used as tie points in the age model because they are not well con- 8.92 strained at Site 1263 due to the wide sampling interval and possible reworking C19n (T) 150 (Zachos et al., 2004).

B Age (Ma) Site 366

Core Recovery 32 33 34 35 36 37 38 39 380 HO Pseudohastigerina spp. The age model for DSDP Site 366 (Fig. 4B; Table 1) is based on the highest occurrence of Pseudohastigerina spp. (Krasheninnikov and Pflaumann, 1978; 390 14.54 Miller et al., 1989), adjusted to the 2004 GTS, a tie point with the average age 400 of the peaks in d13C records from Sites 1263 and 689 at the Eocene-Oligocene peak carbon isotope value transition, and the base of the E15 planktonic foraminiferal biozone (Wade 410 et al., 2011). Sparse age control datums may result in uncertainty, especially in 420 the 420–450 m below seafloor (mbsf) interval. Coring gaps may add additional uncertainty: protocol is to move all recovered sediment to the top of the drilled 430 13.00 interval, even though it is possible that the recovered sediment should have Depth (mbsf ) been placed lower in the drilled interval. This is an issue especially for cores 440 11 (423.0–432.5 mbsf, recovery 1.65 m), 12 (432.5–442.0 mbsf, recovery 5.8 m), 450 and 13 (442.0–452.5 mbsf, recovery 3.4 m; Fig. 4B). To explore the impact of following the convention of moving all recovered sediment to the top of each 460 E15(B) drilled interval, we moved each sediment core to the bottom of each drilled in- 470 terval and ran an alternate age model. This also resulted in moving the datums used in the age model, and we emphasize that because each core was moved Figure 4. (A) Ocean Drilling Program Site 1263 age-depth plot (mcd—meters composite depth) (see Table 2A). Datums used in age model are shown in red and are connected. Unused datums a different amount (because each core gap is different), the resulting changes are in black. HO—highest occurrence, LO—lowest occurrence, T—top, B—base. Sedimentation are nonlinear, with some data from some cores becoming older than the origi rates (m/m.y.) were calculated by linear interpolation between datums, and are shown with nal, while others become younger. A comparison of both versions of the Site associated line segment. C—magnetochron; MECO—middle Eocene climatic optimum; T. —Tu r borotalia; G.—Globigerinatheka; E.—Ericsonia; D.—Discoaster; A.—Acarinina; C.—Chiasmo 366 isotope record with the other isotope records used in this study shows lithus; I—Isthmolithus. (B) Deep Sea Drilling Project Site 366 age-depth plot (mbsf—meters that the timing of some of the Site 366 isotope changes shift, but the overall below seafloor) (see Table 2B). Core recovery is shown to highlight the reason for data gaps. interpretations of the data as discussed in the following remain unchanged Sedimentation rates (m/m.y.) were calculated by linear interpolation between datums, and are 2 shown with associated line segment. E15(B)—base of planktonic foraminiferal biozone. (Supplementary Fig. 1 ).

Published Isotopic Records 2008b; Zachos et al., 1996; Katz et al., 2008). The d18O excursion associated with the middle Eocene climatic optimum (MECO) was also used as a datum in The age models from Sites 1090 (Pusz et al., 2009, 2011), 1053 (Borrelli et al., the age model because it is a well-constrained global event, identified in Hole 2014), and 699 (Mead et al., 1993) were updated to the 2004 GTS from the 2 Supplemental Figures 1 and 2. Comparison of Site 1263B by Bohaty et al. (2009) and updated to GTS 2004 (Gradstein et al., 2005) middle Eocene to the early Oligocene in order to compare results to Sites 1263 366 d18O and d13C records. Please visit http://dx .doi .org/10 .1130/GES01268 .S2 or the full-text article on in this study. The top of magnetochron C19n was used for the lowest tie point (this study) and 689 (Diester-Haass and Zahn, 1996; Bohaty et al., 2012; the age www.gsapubs.org to view the Supplemental File. for the Site 1263 age model because of its clear signal within the sediments model was updated to the 2004 GTS used by Cramer et al., 2009).

δ 18 O ‰ δ 13 C ‰ Tectonic events 3 2.5 2 1.5 1 0.5 0 -0.5 0 0.51 1.5 2 2.5 s e 33

Oligocen Oi-1 34 34.2 Ma Intensified deepening and bottom current West Scotia Ridge Initial spreading of the

Figure 5. δ18O and δ13C records from Ocean Drilling Program (ODP) Walvis Ridge Site 35 1263 (this study; Nuttallides truempyi and Oridorsalis umbonatus values calibrated to Cibicidoides spp. samples; see late

35.8 Ma Increased deepening and onset of bottom current s text and Fig. 2), Sierra Leone Rise Deep

36 e Sea Drilling Project Site 366 (this study), Southern Ocean ODP Site 689 (Kennett and Stott, 1990; Diester-Haass and Zahn, 36.6 Ma 1996), South Atlantic ODP Sites 699 (Mead

Age (Ma ) et al., 1993) and 1090 (Pusz et al., 2009,

37 e 2011), and western North Atlantic Site 1053 (Katz et al., 2011; Borrelli et al., 2014). The Gradstein et al. (2005) geologic time scale 37.6 Ma Eocen is used for the age model. Dashed lines Initial minor deepening indicate key time periods (see Results and 38 Discussion in text). Stable isotope data 38.2 Ma are compared to tectonic gateway event

e reconstructions (Livermore et al., 2007; crustal stretching, subsidenc Stickley et al., 2004). MECO—Middle Eo-

middl cene Climatic Optimum; Oi-1—Oligocene spreading 39 Dove Basin oxygen isotope event 1.

MECO

40 3 2.5 2 1.5 1 0.5 0 -0.5 00.5 11.5 22.5 DrakeTasman

1263 Cibicidoides spp. 689 1263 O. umbonatus 699 1263 N. truempyi 1053 1090 366

The d18O minima from Sites 689 and 1263 record the MECO (Figs. 5 RESULTS and 6). Recalibration of the Site 689 age model from the 1995 GTS to the 2004 GTS shows that MECO occurred at 39.4 Ma, which is supported by Site 1263 Benthic Foraminiferal δ18O well-constrained biostratigraphic datums (Cande and Kent, 1995; Gradstein et al., 2005; Cramer et al., 2009). This age for the MECO was used as a tie Site 1263 d18O showed variability similar to that of Site 689 (Diester-Haass point with the d18O minima from Site 1263. Constant sedimentation rates and Zahn, 1996; Bohaty et al., 2012) from the middle Eocene to the early Oligo were assumed between datums, and linear interpolation was used to deter- cene, with small offsets at times (Fig. 5). A >~1.0‰ increase in d18O over ~400 k.y. mine sample ages (Fig. 4). followed the MECO event at 39.4 Ma at Site 1263. The d18O values at Sites 1053

Latitude Latitude Latitude 60°S 30° 0° 30°N 60°S 30° 0° 30°N 60°S 30° 0° 30°N

1000 39 Ma Mixed Ocean 38 Ma 37 Ma A B δ13C ~ 1.5 C SCW warm NCW cooling SCW δ18O ~ 1.2 1500 18 18 warm δ13C ~ 1.1 cold 13 δ O ~ 0.5 δ13O ~ 1 18 δ C ~ 1.5 δ C ~ 0.7 18 δ O ~ 0.5 18 13 2000 δ O ~ 0.7 δ O ~ 1.5 δ C ~ 1.3 δ13C ~ 1.1 ?mix δ18O ~ 0.9 13 coldest 2500 δ C ~ 1.1 13 δ13C ~ 0.5 δ C ~ 0.7 aleodepth (m) Figure 6. Deep-water evolution based on P 3000 benthic foraminiferal stable isotope data. WSDW The δ18O and δ13C (‰) changes at Ocean 3500 δ18 O ~ 0.7 Drilling Program Sites 1263, 689, 1053, 699, δ13 C ~ 1.3 and 1090 and Deep Sea Drilling Project Site 366 from 39 Ma to 34 Ma in paleodepth 1000 36 Ma warm NCW 35 Ma warm NCW 34 Ma warm NCW versus paleolatitude at 1 m.y. intervals D 18 E SCW 18 F 18 18 SCW 18 δ O ~ 0.6 are shown. Solid lines designate different δ O ~ 1 δ O ~ 0.7 δ O ~ 1.3 δ O ~ 0.5 SCW 13 13 water masses; dashed lines designate pos- 1500 δ C ~1.2 δ C ~ 1 18 coldest 13 cold 13 δ O ~ 1.7 13 sible mixing of water masses. See Figure 4 cold δ C ~ 1.6 δ C ~ 1 δ13C ~ 0.7 δ C ~ 0.7 18 caption for references. SCW—Southern 2000 13 18 δ O ~ 1.6 18 δ C ~1.3 13 mix δ O ~ 1.4 Component Water, NCW—Northern Com- δ O ~ 1.3 δ C ~ 1.4 18 13 mix 18 13 δ O ~ 1 δ C ~ 0.7 δ O ~ 1 ponent Water, WSDW—warm saline deep δ C ~ 0.4 13 cold 13 2500 coldest coldest δ C ~ 1 δ C ~ 0.7 water. 18 18 18 leodepth (m) δ O ~ 0.7 δ O ~ 1 δ O ~ 0.8 13 13 13 Pa 3000 δ C ~ 0.7 δ C ~ 0.1 δ C ~ 0.5 WSDW WSDW WSDW 3500 δ18O ~ 0.5 δ18O ~ 0.8 δ13C ~ 1.3 δ13C ~ 0.8

Site 1263 Walvis Ridge Site 1090 Agulhas Ridge Site 689 Maud Rise Site 1053 Blake Nose Site 699 East Georgia Basin Site 366 Sierra Leone Rise

and 699 diverged from Site 1263 beginning ca. 38.2 Ma. The d18O values at Sites lower than Site 1263. The d18O values at Sites 1263, 366, 689, and 1090 peaked 1263 and 366 from 38 to 37.6 Ma were between the higher d18O values at Site ca. 33.7 Ma. Site 699 d18O remained ~0.5‰ lower and Site 366 d18O remained 689 and the lower d18O values at Sites 1053 and 699. The d18O records from Sites ~1‰ lower than at Site 1263 in the earliest Oligocene. 689 and 1263 from ca. 37.6–35.8 Ma were within 0.2‰ of each other (Fig. 5). From ca. 35.8 to 34.2 Ma, the d18O at Site 1263 was 0.1‰–0.3‰ higher than at Site 689. The d18O values for this period of time were determined directly from Site 1263 Benthic Foraminiferal δ13C Cibicidoides spp. and from d18O adjusted from O. umbonatus specimens. The d18O values at Site 366 were high and similar to Site 1263 from 36.7 The record of benthic foraminiferal d13C at Site 1263 was most similar to to 35.8 Ma. Site 366 values decreased to a range of ~1‰–0.5‰ from ca. 35 to the records from Sites 1053 and 699, while the d13C records from Sites 689, 34.3 Ma, similar to the d18O records at Sites 1053, 699, and 1090. These records 1090, and 366 were less similar to Site 1263 during the time span covered were 0.3‰–0.8‰ lower than Site 1263 until ca. 34 Ma. Just before Oi-1, all sites here (Fig. 5). The Site 1263 and Site 699 d13C values were generally 0.5‰ to displayed a rapid and brief decrease in d18O values by 0.5‰–1.5‰ ca. 33.9– >1‰ higher than at Site 689 from 39.6 to 36.2 Ma (Fig. 5); ca. 36.2 Ma, Site 689 33.8 Ma. All of the d18O records discussed here increased beginning ca. 34 Ma, values became similar to those of Sites 1263 and 1053. Sites 1053 and 1263 had which was the start of Oi-1, but Sites 366, 1090, and 699 were ~0.2‰–0.5‰ similar d13C records from ca. 37.5 to ca. 35.6 Ma, but with greater short-term

variability at ODP Site 1053. The d13C at Site 689 increased by >0.5‰ from 36.8 et al., 2005; tropical western Atlantic ODP Sites 1258 and 1260, Sexton et al., to 36.2 Ma and was similar to Site 1263 during this time and until ca. 35.2 Ma. 2006; northwestern Atlantic Site 1053, Katz et al., 2011; Borrelli et al., 2014; The d13C at Site 1090 was significantly lower than the other sites from ca. 36.2 Atlantic and Indian sector of the Southern Ocean, ODP Sites 689, 738, and 748, to 34 Ma. Site 1263 d13C values became ~0.2‰–0.5‰ higher than those of Sites Bohaty and Zachos, 2003). 1053 and 689 from ca. 35 to 34.2 Ma, with convergence of these records ca. 34.2 Ma. The variability at Sites 689 and 1263 was similar from 34 to 33 Ma, although Site 1263 d13C was >0.5‰ higher than at Site 689. The d13C at Sites 689 Site 366 Benthic Foraminiferal δ13C and 1263 increased and peaked between 33.7 and 33.6 Ma. The peak values at Sites 1263 and 689 were ~2.3‰ and ~1.6‰, respectively. The earliest part of the Site 366 d13C record ca. 39–38.8 Ma had values that were ~0.5‰–1‰ lower than at Site 689 and ~1‰–1.5‰ lower than at Site 1263 (Fig. 5). Site 366 d13C values are the lowest of any of the sites from 36.2 to Site 366 Benthic Foraminiferal δ18O 35.7 Ma. The d13C remained low throughout the majority of this record (0.5‰– 1.0‰), and the trend did not resemble other sites until the latest Eocene to The benthic foraminiferal d18O at Site 366 in the oldest part of this record earliest Oligocene, when the d13C values increased at all sites shown here (ca. 39–38.8 Ma) was similar to Sites 1263 and 689, ~1‰ (Fig. 5). Values ranged by ~1.5‰. from ~0.5‰ to 1‰ from ca. 38.1 to 37.2 Ma. These values were ~0.2‰–0.5‰ lower than at Sites 689 and 1263 and were similar to d18O at Sites 1053 and 699. The d18O at Site 366 was ~1.3‰–1.5‰ from ca. 36.8 to 35.6 Ma and was DISCUSSION similar to values at Site 1263. The d18O at both of these sites was ~0.1‰–0.2‰ higher than values at Site 689 for much of this time. Due to coring gaps and We compare the Walvis Ridge Site 1263 and Sierra Leone Rise Site 366 ben- the uncertainty of the Site 366 age model (see preceding), we are unable to thic foraminiferal d18O and d13C (this study) to coeval published records from precisely locate the timing of the d18O increase at Site 366 for this million-year the Atlantic and Southern Oceans (Fig. 5). In the following sections, we explore period. The d18O values at Site 366 were low and more similar to those of Sites the evolution of South Atlantic deep-water circulation during the late-middle 1053, 1090, and 699 during the latter part of the record, from ca. 35 to 34 Ma. Eocene to the earliest Oligocene within the context of these isotopic relation- The high variability in d18O and d13C values in portions of the records shown ships. For purposes of discussion, we separate time intervals into segments in this study (both new and published data) has been documented at middle that reflect major variations in the deep-water characteristics at Site 1263: and late Eocene sites in different basins (e.g., northern Pacific ODP Site 884, ca. 40–38.2 Ma, ca. 38.2–37.6 Ma, ca. 37.6–36.6 Ma, and ca. 36.6 to the earliest Pak and Miller, 1995; Borrelli and Katz, 2015; subtropical Pacific ODP Site 1209, Oligocene (Figs. 5 and 6). A summary of the interpretations from our results is Dawber and Tripati, 2011; equatorial Pacific ODP Sites 1218 and 1219, Tripati provided in Table 3.

TABLE 3. SUMMARY OF THE RESULTS AND INTERPRETATIONS OF THIS STUDY Age interval (Ma) Results Interpretations 36.60–33.00 Highest δ18O and δ 13C values at ODP Sites 1263, 689, and DSDP Site 366. SCW at Sites 1263 and 366 at 36.6 Ma, replaced by NCW at Site 366 by 35 Ma, Lowest δ 18O values at Sites 1053, 699, and 1090. Site 366 δ 18O values indicating greater NCW production and/or a decrease in SCW production around decreased, becoming similar to Site 1053 by 35.6–35 Ma. this time.

37.60–36.60 Highest δ 18O values at Site 1263, relatively high values at Site 689, and Greater SCW production with stratiﬁcation of colder water at greater depths and in relatively low values at Sites 366, 1053, and 699. δ18O values at Site the equatorial Atlantic. WSDW still present at greater depths in the South Atlantic 366 increased to values similar to Site 1263 at the end of this interval. Sites 699 and 1090 (mixed with PW at Site 1090). Warm NCW at Site 1053. Higher Relatively low δ13C values at Sites 689 and 366. productivity in the ﬂow paths to Sites 689 and 366 than at other sites listed.

38.20–37.60 δ18O values at Sites 699, 366, and 1053 were lower than at Site 689. δ18O Possible mixing of water masses in the southeastern Atlantic. Warm water masses and δ13C values at Site 1263 were intermediate between these sites. present at Sites 1053 (NCW), 699 (WSDW), and 366 (NCW or WSDW).

40.00–38.20 δ18O values at Sites 1263, 689, and 699 were similar; δ13C values were Well-mixed ocean with higher local productivity affecting Site 689 δ13C. lower at Site 689 than at other sites. Note: DSDP—Deep Sea Drilling Project; all other sites are Ocean Drilling Program (ODP); SCW—Southern Component Water; NCW—Northern Component Water; WSDW— warm saline deep water; PW—Paciﬁc water.

MECO to 38.2 Ma: Onset of Deep-Water Differentiation 2006) (Supplementary Fig. 2). In this scenario, considering that the Site 1263 paleodepth was deeper than Site 689 and shallower than Site 1090, the water From the beginning of MECO ca. 39.6 Ma until ca. 38.2 Ma, the Walvis Ridge mass at Site 1263 might represent the upper branch of a deep-water mass Site 1263 and Sierra Leone Rise Site 366 d18O records were very similar to composed of SCW and Pacific seawater. In contrast, Via and Thomas (2006) the Southern Ocean Site 689 and East Georgia Basin Site 699, including the suggested that the Walvis Ridge was bathed by a single Southern Ocean in- MECO d18O minimum of ~0.1‰ ca. 39.4 Ma (Figs. 5 and 6). This indicates a termediate and/or deep-water mass until the early Oligocene; however, the

uniform deep-water mass bathing several regions of the Atlantic at several low-resolution eNd records from Sites 1262 and 1263 for this time (Supplemen-

paleodepths, as also shown by neodymium isotope data (eNd) from the Walvis tary Fig. 2) do not allow us to draw a firm conclusion about the possibility of Ridge (Sites 1262 and 1263; Via and Thomas, 2006) (Supplementary Fig. 2). In Pacific influence at Site 1263.

contrast, eNd data from the Agulhas Ridge (Site 1090; Scher and Martin, 2006) NCW began to form in the Labrador Sea during the late-middle Eocene (Supplementary Fig. 2) show the influence of Pacific seawater at this location (Borrelli et al., 2014). This deep-water mass is characterized at Site 1053 by starting ca. 41.3 Ma (data not included in Supplementary Fig. 2). Without ben- lower d18O and higher d13C than the SCW at Site 689 (Borrelli et al., 2014). The thic foraminiferal stable isotope data from Site 1090, we can only note that intermediate d18O and d13C values at Site 1263 during that time may represent the Atlantic-Pacificd 18O offset was minimal at this time, consistent with fairly mixing of NCW and SCW. Mixing of these water masses at Site 1263 is con- uniform deep-water temperatures (Cramer et al., 2009). sistent with a meridional circulation component and wind-driven advection in The d18O at Sites 1263, 689, and 699 increased after MECO, indicating uni- the southeastern Atlantic. The restriction of the Tethys seaway to the Atlantic form deep-water cooling; d13C at Site 1263 was ~0.5‰–1.0‰ higher than at Site may have driven meridional transport from the western North Atlantic to the 689 at that time (Figs. 5 and 6A), consistent with paleoproductivity proxies that eastern South Atlantic, and wind-driven advection may have been possible indicate high productivity at Site 689 (Diester-Haass and Zahn, 1996) and with with the initiation of the proto-ACC as the Drake Passage opened (Allen and the location of Site 1263 beneath the South Atlantic gyre at that time. Armstrong, 2008). Alternatively, the warmer deep water at Site 1263 may have resulted from 38.2–37.6 Ma: Deep-Water Mixing in the Southeastern Atlantic SCW mixing with warm saline deep water (WSDW) that originated from the high-salinity warm waters of the Tethys seaway (Kennett and Stott, 1990; Mead The highest d18O was recorded at Maud Rise Site 689 (Kennett and Stott, et al., 1993; Wright and Miller, 1993; Scher and Martin, 2004). Tethyan WSDW 1990) and likely resulted from Southern Ocean cooling in response to the had low d18O and was probably warmer, saltier, and denser than the overlying 18 onset of thermal isolation with the development of the proto-ACC, which ac- high d O SCW (Kennett and Stott, 1990; Mead et al., 1993); eNd values at Site

companied gateway openings (e.g., Barker, 2001; Exon et al., 2004; Scher and 689 are consistent with Tethyan eNd values at that time (Scher and Martin, Martin, 2006; Cramer et al., 2009; Katz et al., 2011; Sijp et al., 2011; Borrelli et al., 2004). Temperature and salinity calculations based on d18O records establish 2014; see Introduction herein for details). Site 689 also had lower d13C, which the density structure that supports the presence of this warmer, more saline is consistent with (1) paleoproductivity proxies that indicate high productivity deep water underlying colder, fresher deep water in this region in the Eocene; at Site 689 (Diester-Haass and Zahn, 1996), and (2) the location of Site 1263 this warm saline deep water may have formed through mixing of cold high- beneath the South Atlantic gyre at that time. From 38.2 to 37.6 Ma, the d18O and latitude sourced deep water with WSDW sourced from an evaporative semi d13C values at Walvis Ridge Site 1263 were slightly lower than at Site 689, and enclosed basin (Mead et al., 1993). higher than at Blake Nose Site 1053 and East Georgia Basin Site 699 (Figs. 5 The concomitant formation of relatively warm NCW and cooling of SCW be- and 6B). This may indicate that Site 1263 was bathed with (1) SCW that was ginning ca. 38 Ma is a response to the progressive development of the (proto-) slightly warmer than recorded at Site 689, or (2) SCW mixed with a warmer ACC with the opening of the Drake and Tasman Passages; this isolated surface deep-water mass, possibly from the Pacific (Scher and Martin, 2006), North waters to the south of the ACC from the warmer subtropical gyre, cooled the Atlantic, or Tethys Sea (Figs. 5 and 6B). source region of SCW, and resulted in cooling of SCW (e.g., Toggweiler and Site 1263 could have been bathed by SCW mixed with Pacific seawater. Bjornsson, 2000; Sijp and England, 2004, 2005; Livermore et al., 2007; Cramer Site 1263 d18O values from 38.2 to 37.6 Ma are similar to Site 1218 d18O values et al., 2009; Katz et al., 2011; Bijl et al., 2013; Borrelli et al., 2014). In addition,

(Lear et al., 2004) and eNd data are similar to those from Site 1090 (Scher and small-scale Antarctic glaciation (Browning et al., 1996; Scher et al., 2014) and Martin, 2006), consistent with an influx of radiogenic waters from the Pacific the development of the proto-ACC (Cramer et al., 2011) may have contributed into deep-water formation regions of the Southern Ocean as a consequence to increasing d18O at Sites 689 and 1263. The isotopic evidence presented here of the opening of the Drake Passage (Scher and Martin, 2006) (Supplemen- shows that deep water began to cool only at these sites during that time, which tary Fig. 2). Site 689 recorded a much weaker response to the influx of Pacific indicates that cold deep water was not being produced in large enough quanti- water into the Southern Ocean, possibly indicating the isolation of Pacific-influ- ties to displace WSDW at all depths and locations, while WSDW was still being enced deep water to below the depth of Maud Rise (Site 689; Scher and Martin, supplied to the equatorial Atlantic and South Atlantic (Fig. 6B).

Ca. 37.6–36.6 Ma: Initial Cooling of Southeastern Atlantic Deep Water analogous to the modern AMOC driven by the ACC in the South Atlantic Ocean (Garzoli and Matano, 2011). It is also consistent with modeling studies that find Site 1263 is characterized by a slightly higher d18O compared to all other that the Drake Passage opening initiates ACC flow and AMOC-like circulation sites from ca. 37.6 to 36.6 Ma (Figs. 5 and 6C), indicating cold SCW at Site (Toggweiler and Bjornsson, 2000; Fyke et al., 2015). 1263, whereas the deep-water mass in the northwestern Atlantic (Site 1053) likely originated in the Labrador Sea (Borrelli et al., 2014). The warm deep- water mass at Site 699 may have been derived from WSDW (Mead et al., 1993), 36.6–33 Ma: SCW Deep-Water Stratification possibly originating in the Tethys and entering the South Atlantic via the Indian Ocean, similar to the warm, salty Indian Ocean water that enters the South From ca. 36.6 to 35.8 Ma, the southeastern Atlantic deep-water mass at Atlantic today (van Leeuwen et al., 2000; Speich et al., 2007; Biastoch et al., Sites 1263 and 366 recorded d18O that ranged from ~0.0‰ to 0.2‰ higher than 2009; Garzoli and Matano, 2011). The d18O values at the Sierra Leone Rise Site at Maud Rise Site 689, consistent with SCW flowing through the southeast- 366 from ca. 37.6 to 37.2 Ma are similar to those at Sites 1053 and 699, and then ern Atlantic at least as far north as the equatorial region (Figs. 5 and 6D). The increase by ca. 36.8 Ma, becoming similar to d18O values at Sites 689 and 1263, 0.2‰ higher d18O values at Sites 1263 and 366 likely reflect the slightly colder, indicating that SCW dominated the equatorial Atlantic by that time. denser waters in the deeper portion of SCW. The d18O at Site 1263 continued to The reduced d13C gradient among all sites at 37.6–37 Ma (Fig. 5) indicates be slightly higher than any other published record until ca. 34.2 Ma (Figs. 6E, that there was a possible overall increase in productivity within source waters, 6F). The d18O values at Site 366 decreased to a range of ~1.2‰–0.5‰ between in the overlying water column, or along the flow paths for Sites 1263, 699, ca. 35.6 and 35 Ma (across a coring gap), which may reflect mixing with the and 1053 at that time. The similar high d13C values at Site 1263, East Georgia warmer NCW-sourced water recorded at Site 1053. These circulation changes Basin Site 699, and Site 1053 from ca. 37 to 36 Ma, coupled with higher d18O are consistent with the progressive strengthening of the proto-ACC (e.g., Exon (~1.0‰–1.7‰) at Site 1263, suggests that the deep-water masses at these sites et al., 2004; Cramer et al., 2009; Katz et al., 2011). Similar deep-water stratifica- derived from different surface waters that were exposed to uniform changes in tion is present in the modern Atlantic Ocean (Talley et al., 2011). carbon cycling (e.g., Billups et al., 2002). The d18O values at the deeper south Atlantic Sites 1090 and 699 were within The isotopic records presented here reflect the development of at least 4 a range of ~0.4‰–0.7‰ from ca. 36 to 35.4 Ma (Figs. 5 and 6). These low values deep-water masses by ca. 37.6 Ma. This is consistent with hypotheses that the indicate that there was either a warm deep-water mass restricted to greater proto-ACC contributed to cold deep-water formation and deep-water circula- depths than at Sites 689 and 1263, or the cold South Atlantic water was con- tion changes as early as ca. 38 Ma (Cramer et al., 2009; Borrelli et al., 2014). fined to west of Site 699 and a warm deep-water mass flowed into the South 18 These deep-water masses include (1) warm NCW at Site 1053 with low d O Atlantic from the Indian Ocean (Figs. 6D, 6E). However, we note that eNd values and high d13C (Borrelli et al., 2014); (2) cold SCW at Site 689 with high d18O and suggest that Pacific seawater was present at Site 1090 (Scher and Martin, 2006) low d13C (Diester-Haass and Zahn, 1996; Bohaty and Zachos, 2003); (3) WSDW (Supplementary Fig. 2); taken together with the d18O record (Pusz et al., 2009, at Site 699 with low d18O and high d13C (Mead et al., 1993); and (4) cold SCW at 2011), this may indicate mixing of WSDW and Pacific seawater at that time. Site 1263 with high d18O and high d13C (this study), possibly as a consequence The d13C at Site 1263 is higher than at Site 689 (Fig. 5) during most of the of mixing between SCW and NCW, WSDW, or Pacific seawater, as suggested period investigated, indicating that productivity was lower in the Argentine 13 by Site 1090 eNd values (Scher and Martin, 2006) (Supplementary Fig. 2). and Brazil Basins in the South Atlantic. The increase in d C at Site 689 ca. The cold deep water at Site 1263 may have shared a SCW source with Site 36 Ma may indicate a reduction of productivity in this area. This may reflect 689; however, high productivity at Site 689 may have lowered the d13C values. the intensification of flow through the Drake Passage, enhanced upwelling, By ca. 37.6 Ma, the deep-water mass at the Walvis Ridge (Site 1263) changed and the concentration of nutrients along the northern edge of the proto- from a mixture of SCW with NCW, WSDW, or Pacific seawater to being primar- ACC, which may have affected productivity at Agulhas Ridge Site 1090, as ily SCW sourced (Fig. 5). In the eastern equatorial Atlantic (Site 366), mixing evidenced by low d13C values, without reaching East Georgia Basin Site 699 of NCW and/or WSDW mixing with SCW eventually shifted to SCW dominance (Fig. 6). High primary production is also supported at Site 1090 by barite, car- by ca. 37–36.8 Ma (a coring gap prevents precise placement of this transition). bonate, and phosphorous accumulation (Anderson and Delaney, 2005) and This progressive change may have resulted from greater cold water produc- opal accumulation (Diekmann et al., 2004; Anderson and Delaney, 2005). In tion in the Southern Ocean, consistent with previously published models addition, low d13C at Site 366 from ca. 36.2–35.8 Ma likely resulted from oxida- showing progressively developing proto-ACC in response to gradual opening tion of terrestrial-sourced 12C-enriched organic carbon (Wagner, 2000). Based of the Drake and Tasman Passages, deeper and stronger flow of the proto-ACC, on our isotopic comparisons, 3 distinct water masses existed in a stratified and thermal isolation of the Southern Ocean (e.g., Cramer et al., 2009; Katz ocean by 35 Ma (Fig. 6E): (1) cold SCW at Sites 689 and 1263; (2) warm NCW at et al., 2011; Bijl et al., 2013). These progressive circulation changes are consis- Sites 1053 and 366; and (3) WSDW at Sites 699 and 1090, with Pacific seawater tent with the development of a weak early AMOC (Figs. 5, 6B, and 6C). This is influence at Site 1090.

The d18O and d13C data from Walvis Ridge Site 1263 converge with records Gateway openings and closings in the Eocene–early Oligocene affected from Site 689 in the latest Eocene, ca. 34.2 Ma (Fig. 6F). Limited data show a SCW production and ocean circulation in the South Atlantic Ocean. With a brief d18O decrease of ~0.5‰–1‰ ca. 34 Ma at Sites 1263, 366, 689, 699, and closed Drake Passage, the eastern South Atlantic received all of its deep water 1053; this may indicate that cold deep-water production was interrupted for from the Southern Ocean with little vertical stratification, as observed in our

a very short period, revealing an interruption or slowing of the developing stable isotope comparisons and eNd isotope records (Thomas, 2005; Via and meridional overturning circulation, and that deep-water production and circu- Thomas, 2006). Our stable isotope comparisons show that there was restruc- lation was unstable. However, additional high-resolution data are needed to turing of Atlantic deep-water circulation as gateways opened and the proto- better constrain the d18O decrease and to investigate the possible instability in ACC deepened in the late-middle to late Eocene, with development of warm meridional overturning circulation at this time. NCW and cold SCW, WSDW that may have entered the South Atlantic from the An ~1.5‰ increase in d18O at all sites occurred from the late Eocene to early Indian Ocean, and progressive shifts in deep-water mixing. The d13C records Oligocene, including the Eocene-Oligocene transitions 1 and 2 and the Oi-1 show further complexity that indicates modification of deep-water masses by isotope event, as a consequence of ~2–4 °C cooling (e.g., Katz et al., 2008; Lear productivity. et al., 2008; Pusz et al., 2011) and associated large-scale Antarctic glaciation Our isotope comparisons indicate the development of deep-water circula- that caused a 55–70 m eustatic fall (e.g., Kennett and Stott, 1990; Diester-Haass tion patterns similar to modern AMOC, supporting models that show how early and Zahn, 1996; Zachos et al., 2001; Katz et al., 2008; Miller et al., 2009; Cramer AMOC could have been driven by proto-ACC flow (Toggweiler and Samuels, et al., 2011). At Oi-1, Site 1090 d18O and d13C values shifted toward Site 689 and 1995). The combined openings of the Drake and Tasman passages may have 1263 values, consistent with an intensification of the proto-ACC or a reduction enhanced wind-driven overturning circulation, which may have been an early in the Tethys deep-water source. analog of modern AMOC (Toggweiler and Bjornsson, 2000; Sijp et al., 2011; Fyke et al., 2015). This is analogous to the modern AMOC driven by the ACC in the South Atlantic Ocean (Garzoli and Matano, 2011). SUMMARY AND CONCLUSIONS

Beginning ca. 38.2 Ma, d18O at Maud Rise Site 689 diverged from East ACKNOWLEDGMENTS Georgia Basin Site 699, Sierra Leone Rise 366, and North Atlantic Site 1053; We thank Richard Mortlock and James Wright (Rutgers University, New Jersey) and Eric Tappa Walvis Ridge Site 1263 d18O values were intermediate between these values, (University of South Carolina) for their help with stable isotope analyses. This research was supported by National Science Foundation (NSF) grant OCE 09-28607/09-27663 (B.S. Cramer, M.E. indicating possible mixing of water masses. This divergence was likely in re- Katz). We are grateful to Ben Cramer, Robbie Toggweiler, Ken Miller, and Yair Rosenthal for useful sponse to increased flow through the Drake and Tasman Passages, which initi- discussions and to Brianna King and Kyle Monahan for sample processing. This research used ated thermal isolation of surface waters around Antarctica, the source area of samples provided by the International Ocean Discovery Program (IODP). This paper benefitted from the thoughtful comments and suggestions of two anonymous reviewers. SCW. There was a simultaneous warming in the North Atlantic (Borrelli et al., 2014) and production of WSDW at low latitudes (Kennett and Stott, 1990; Mead et al., 1993; Wright and Miller, 1993; Scher and Martin, 2004). REFERENCES CITED Our comparisons indicate that by ca. 37.6 Ma there were at least 4 deep- Allen, M.B., and Armstrong, H.A., 2008, Arabia-Eurasia collision and the forcing of mid-Cenozoic water masses in the Atlantic and Southern Oceans: (1) warm NCW with low global cooling: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 265, p. 52–58, doi:10 d18O and high d13C (Site 1053, Borrelli et al., 2014); (2) cold SCW with high d18O .1016/j.palaeo.2008.04.021. and low d13C (Site 689, Diester-Haass and Zahn, 1996; Bohaty and Zachos, Anderson, L.D., and Delaney, M.L., 2005, Middle Eocene to early Oligocene paleoceanog 18 13 raphy from Agulhas Ridge, Southern Ocean (Ocean Drilling Program Leg 177, Site 1090): 2003); (3) WSDW with low d O (Site 366 low d C, this study; Site 699 with high Paleoceanography, v. 20, PA1013, doi:10.1029/2004PA001043. d13C, Mead et al., 1993); and (4) cold SCW with high d18O and high d13C (Site Barker, P.F., 2001, Scotia Sea tectonic evolution: Implications for mantle flow and palaeocircula- 1263, this study). tion: Earth-Science Reviews, v. 55, p. 1–39, doi:10.1016/S0012-8252(01)00055-1. Barrera, E., and Huber, B.T., 1993, Eocene to Oligocene oceanography and temperatures in the At 36.6 Ma, the deep-water masses at Sites 1263 and 366 were colder than Antarctic Indian Ocean, in Kennett, J.P., and Warnke, D.A., eds., The Antarctic paleoenvi- the water mass at Site 689, likely reflecting the slightly colder, denser lower ronment: A perspective on global change: Part 2: American Geophysical Union Antarctic portion of SCW; this indicates strong SCW production that dominated the Research Series Volume 60, p. 49–65, doi:10 .1029 /AR060p0049 . southeast and equatorial Atlantic by that time. The strengthening of the proto- Barron, J.A., Larsen, B., and Baldauf, J.G., 1991, Evidence for late Eocene to early Oligocene Antarctic glaciation and observations of late Neogene glacial history of Antarctica: Results ACC is the only known mechanism that may have led to colder water produc- from Leg 119, in Barron, J., et al., Proceedings of the Ocean Drilling Program, Scientific tion from the Southern Ocean reaching the depths of Sites 1263 and 366. By results, Volume 119: College Station, Texas, Ocean Drilling Program, p. 869–891, doi:10 .2973 ca. 35 Ma, the d18O at Site 366 was substantially lower than at Sites 1263 and /odp.proc.sr.119.194.1991. Berggren, W.A., and Pearson, P.N., 2005, A revised tropical to subtropical Paleogene planktonic 689; this may have resulted from increased production of NCW and/or change foraminiferal zonation: Journal of Foraminiferal Research, v. 35, p. 279–298, doi:10 .2113 /35 of deep-water flow patterns into the equatorial Atlantic. .4.279.

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