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Coupling of marine and continental oxygen isotope records during the Eocene-Oligocene transition Coupling of marine and continental oxygen isotope records during the Eocene-Oligocene transition

Nathan D. Sheldon1,†, Stephen T. Grimes2, Jerry J. Hooker3, Margaret E. Collinson4, Melanie J. Bugler2, Michael T. Hren5, Gregory D. Price2, and Paul A. Sutton2 1Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA 2School of Geography, Earth & Environmental Sciences, Plymouth University, Drake Circus, Plymouth, Devon, PL4 8AA, UK 3Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, UK 4Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK 5Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269, USA

ABSTRACT INTRODUCTION can be difficult to link marine and terrestrial records of the Eocene-Oligocene transition for While marine records of the Eocene-Oligo­ The Eocene-Oligocene transition, onset at two reasons: (1) Continental successions are cene transition indicate a generally coherent ca. 34 Ma, represents a climatic regime change most often preserved in endorheic basins, far response to global cooling and the growth from “greenhouse” conditions to “icehouse” from marine incursions that would make direct of continental ice on Antarctica, continental conditions. Physical evidence (various; e.g., age comparison possible, and (2) oxygen iso- records­ indicate substantial spatial variabil- Davies et al., 2012) indicates the initiation tope records from continental interiors can be ity. Marine Eocene-Oligocene transition rec­ of large-scale Antarctic glaciation during the complicated by a variety of non-temperature- ords are marked by an ~+1.1‰ foraminiferal Eocene-Oligocene transition, with permanent related factors (e.g., changing circulation δ18O shift, but continental records­ rarely re- ice sheets formed on Antarctica for the first patterns, orographic effects) or may respond cord the same geochemical signature, mak- time between 34.0 and 33.65 Ma, peaking at the to local, rather than global hydrologic cycle ing both correlation and linking of causal onset of an oxygen isotopic event known as Oi-1 drivers (e.g., Sheldon et al., 2012). Thus, sec- mechanisms between marine­ and conti- (Miller et al., 1991; Zachos et al., 1996, 2001; tions in continental strata that span the Eocene- nental records challenging. Here, a new Coxall et al., 2005; Pälike et al., 2006, supple- Oligocene­ transition and that are well cali- high-resolution continental δ18O record, ment), which is an ~+1.1‰ shift that reflects brated to the geomagnetic polarity time scale derived from the freshwater gill-breathing a combination of temperature and salinity/ and to marine geochronology are rare. gastropod ­Viviparus lentus, is presented ice-volume change (Zachos et al., 2008). Both The English Hampshire Basin Solent Group from the Hampshire Basin, UK. The Solent benthic (Coxall et al., 2005; Coxall and Wil- (Fig. 1) was deposited in a coastal floodplain Group records marine incursions and has son, 2011) and planktonic (Pearson et al., 2008) environment. It has documented magneto- an established magnetostratigraphy, mak- foraminifera­ record a coherent spatial and tem- stratigraphy, sequence stratigraphy, mamma- ing it possible to correlate the succession poral isotopic response, which implies global- lian and charophyte biostratigraphy, and brief directly with marine records. The V. lentus scale changes to Earth’s oceans and global-scale calcareous nannoplankton events (Hooker, δ18O record indicates a penecontemporane- climatic cooling. 1987, 2010; Sille et al., 2004; Gale et al., 2006; ous, higher-magnitude shift (>+1.4‰) than In contrast, continental records are spatially Hooker et al., 2009) that allow good calibra- marine records, which reflects both cool- variable (Sheldon, 2009; Zanazzi et al., 2015), tion to marine records through the entire late ing and a source moisture compositional with regional differences in the magnitude Eocene (Priabonian) and early Oligocene shift consistent with the growth of Antarc- of temperature change (e.g., Zanazzi et al., (Rupelian). The depositional rate of the Solent tic ice. When combined with “clumped” 2007 vs. Retallack, 2007), the magnitude of Group strata is high, ranging from ~3 to 10 cm isotope measurements from the same suc- precipitation change (Sheldon and Retallack, k.y.–1 (Hooker et al., 2009), which facilitates cession, about half of the isotopic shift can 2004; Abels et al., 2011), and even whether high-resolution sampling. The only significant be attributed­ to cooling and about half to precipitation increased or decreased (Sheldon hiatus was during the glacial maximum follow- source moisture change, proportions similar et al., 2009). The widely recognized positive ing the Oi-1 event, caused by major sea-level to ­marine forami­niferal records.­ Thus, the isotope excursion seen in marine foraminiferal fall. Hren et al. (2013) recently published a new record indicates strong hydro­logical d18O records of the Eocene-Oligocene transi- “clumped isotope” paleotemperature recon- cycle connections between marine and mar- tion glaciation is rarely found in terrestrial struction of the Eocene-Oligocene transition ginal continental environments during the records (cf. Zanazzi et al., 2007; Zanazzi and from the Solent Group based on the proso- Eocene-Oligocene transition not observed Kohn, 2008), which instead often show no branch gastropod Viviparus lentus (Solander), in continental interior records. discernible shift in d18O (e.g., Sheldon et al., where they found: (1) the magnitude of mean 2012) or a delayed shift relative to marine annual air temperature change (~4–6 °C) †[email protected] records (Zanazzi et al., 2007). In addition, it was comparable to North Atlantic sea-surface

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GeologicalFor Society permission of to America copy, contact Bulletin [email protected], v. 1XX, no. XX/XX 1 © 2015 Geological Society of America Geological Society of America Bulletin, published online on 14 September 2015 as doi:10.1130/B31315.1

Sheldon et al. 53N 50N Bay London 0W C. Whitecli ff Isle of Wight 5W 10 km Newport N D. Hamstead Ledge End Paleogene Cretaceous Bouldnor foreshore and cliff al. (2009). Map is modified from Daley (1999). The time scale used here and in Figures 3–4 is that of and in Figures The time scale used here Daley (1999). al. (2009). Map is modified from B. Sconce

E. and F. A. Colwell Bay / Cli ff A E C B F D al. (2006, supplement). Stratigraphic abbreviations: BAR—Bartonian; Becton S.—Becton Sand Formation; Bemb. Lst—Bem - al. (2006, supplement). Stratigraphic abbreviations:

S.B

Lin. Mbr Upper Colwell Bay Member Pälike et Figure 1. Sample locations and their stratigraphic context. The extent of sections selected for sampling at sites A–F is shown against the against is shown A–F sites sampling at sections selected for extent of The context. stratigraphic their locations and Sample 1. Figure et stratigraphic column of Hooker bridge Limestone Formation; Hamst.—Hamstead; Hath.—Hatherwood Limestone; L.—lower; Lin.—Linstone Chine; Mbr—Member; Bay Member. S.B.—Seagrove Marls Cli ff End Member Hamst. Member Lacey ’s Farm Totland Totland Bay Member Member Hampshire Basin Osborne Becton S. Cranmore Member Member L. Hamst. Bemb. Lst Hath. Mbr Fishbourne Member Bembridge 1n 1n 2n 2r 1n 15r 13r 16n1r 15n 13n 16n 12r 16n 16n 17n

Mag- chron C NAINOBAIRP RUPELIAN

BAR Stage Ma 32 34 36 37 33 35

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Coupling of marine and continental oxygen isotope records during the Eocene-Oligocene transition temperature changes, and (2) growing-season temperatures for V. lentus were essentially 60N Isle of constant prior to the Eocene-Oligocene tran- Montana Wight sition but dropped by nearly 10 °C following Oregon Ebro Basin Nebraska Spain Oi-1. An earlier climate study of the Solent 30N Group from the combined proxies of mammal tooth enamel, pulmonate gastropod (Lymnaea longiscata Brongniart) shells, fish otoliths, IODP 1218 and charophyte gyrogonites at six horizons (Grimes et al., 2005) also found little evidence TDP 12/17 for variability in summer temperatures prior to the Eocene-Oligocene transition. However, 30S both of those previous studies had limited data for the early Oligocene. 60S To build upon these low-resolution isotope- derived climate studies and to provide greater data coverage for the early Oligocene, we ana- Figure 2. Locations of marine and terrestrial Eocene-Oligocene transition sites. Marine lyzed the d18O composition of shell carbon- comparative sites include Integrated Ocean Drilling Project Site 1218 (IODP; e.g., Coxall­ ate of V. lentus at numerous horizons through et al., 2005) and Tanzanian Drilling Project Sites 12 and 17 (TDP; e.g., Pearson et al., 3 m.y. (35.4–32.4 Ma) of the Solent Group 2008). North America terrestrial sites were reviewed in Sheldon (2009), and the Ebro Basin (Bugler, 2011). We then compared this new, (Spain) site was described in Sheldon et al. (2012). Late Eocene paleogeographic base map denser paleoclimatic record (with ~750 k.y. is used with permission from Ron Blakey, Colorado Plateau Geosystems (original reference: leading up to Oi-1 having a frequency inter- Blakey, 2008). val of ~35 k.y.) with marine oxygen isotope records and with paleoclimatic records from other continental Eocene-Oligocene transition sites (Fig. 2).

using mesh sizes of 2 mm, 1 mm, and 250 µm. 90 °C for ~1 h. The CO2 produced was analyzed SAMPLE LOCATIONS AND Each size fraction was oven dried at 25 °C. on an Isoprime mass spectrometer with a Gilson METHODOLOGY Between 7 and 15 randomly selected V. lentus multiflow carbonate auto-sampler at Plymouth fragments were picked from the >2 mm size University. The results were calibrated against Six localities representing different parts of fraction and cleaned in an ultrasonic bath. Each Vienna Peedee belemnite (VPDB) using the the late Eocene (Priabonian) to earliest Oligo­ individual fragment (n = 483) was crushed using international standards NBS-19, IAEA-CO-8, cene (Rupelian) Solent Group stratigraphy of an agate mortar and pestle. Between 0.30 mg and IAEA-CO-9. Five NBS-19 standards were the Hampshire Basin (Fig. 1; supplemental and 0.50 mg aliquots of powder were placed also evenly distributed throughout the individ- data and Table S11) were examined. In total, into vials for isotope analysis. The remainder ual isotope runs to correct for daily drift. The 40 stratigraphic horizons containing abundant was retained for repeat analyses and X-ray dif- mean standard deviation on replicate analy- shells of the freshwater prosobranch (gill- fraction (XRD) in order to check for any altera- ses of individual samples was on the order of breathing) gastropod V. lentus were sampled, tion of the original aragonite mineralogy. 0.2‰ for d13C and 0.2‰ for d18O. Analytical including both complete and fragmentary, but To determine the preservation state of shells results are given in supplemental Table S1 (see identifiable, shells. Modern Viviparus gastro- from individual horizons, powder from 3 to 5 footnote 1). pods inhabit only clean, slowly moving or stable V. lentus fragments from each horizon was ana- The lithostratigraphy used here (Figs. 1, 3, water bodies, and their habitat tracks the avail- lyzed either at Plymouth University or at Royal and 4) follows Hooker et al. (2009). Calibration ability of aquatic environments (Boss, 1978). Holloway, University of London (RHUL). At of the Solent Group to the geomagnetic polarity Fossil Viviparus gastropods have been used Plymouth University, the samples were analyzed scale (Vandenberghe et al., 2012) also follows previously in studies of both warming (Schmitz by Bugler, using a Phillips PW1792 X-ray diffrac- Hooker et al. (2009), but the time scale used is and Andreasson, 2001) and cooling (Hren tometer (XRD) with High-Score Plus identifica- that of Pälike et al. (2006, supplement). Cali- et al., 2013) Paleogene paleoclimate events, tion software. The V. lentus powder was aligned bration is based on combined biostratigraphy, and modern Viviparus gastropod species reli- along a metal plate, which was then placed into magnetostratigraphy, and sequence stratigraphy, ably record changing environmental conditions a sealed chamber within the XRD. A Cu anode allowing for correlation between both terrestrial (Bugler, 2011). source was used with generator settings of 30 kV and marine records. The highest resolution was Bulk sediment samples were air dried, dis- and 40 nA. Expert High Score was used to ana- obtained where biostratigraphic events coin- aggregated in warm water, and then wet sieved lyze the X-ray chromatography produced. At cided with either sequence boundaries or mag- RHUL, samples were analyzed courtesy of Dr. netic polarity shifts, or both. Thus, in terms of 1GSA Data Repository item 2015314, supple- Dave Alderton using a Philips Analytical XRD samples used in this study, the most accurately mental information consists of: 1) GPS locations for PW3710 machine with PC-APD diffraction soft- positioned points are at the boundaries of nor- sampling sites, 2) a supplemental table of isotopic ware. Samples were scanned between 20° to 50° mal and reversed polarity zones (where these are data and 3) a supplemental table that summarizes paleoclimatic results from other continental sites, is (2q) using a copper tube anode. recorded), the base of the Lacey’s Farm Mem- available at http://​www​.geosociety​.org​/pubs​/ft2015​ For isotope analysis, the carbonate powders ber, the base of the Bembridge Limestone For- .htm or by request to editing@​geosociety​.org. were reacted with 100% phosphoric acid at mation, and top of the lower Hamstead Member.

Geological Society of America Bulletin, v. 1XX, no. XX/XX 3 Geological Society of America Bulletin, published online on 14 September 2015 as doi:10.1130/B31315.1

Sheldon et al. 0 –4.0 3.00 ~10° South –2.0 0– Sites 12 and 17 O (‰ VPDB) lentus shell fragments plot - 18 δ Tanzanian Drilling Project Tanzanian 1.00 al., 2005; see also Coxall and Wilson, Wilson, al., 2005; see also Coxall and

O values represents ±1 σ (standard devia - represents values O 18 δ O record from V . from O record 18 2008). Abbreviations: mfs—maximum flooding mfs—maximum Abbreviations: 2008). ~8° North al.,

ODP site 1218 ODP B 2.00 2008; Lear et Lear 2008; al.,

2009). Shaded area around the mean the around area Shaded 2009). al.,

Viviparus Viviparus with Oi-1 glacial Hiatus associated maximum sea level fall (‰ VPDB) –1.50 –2.50 –3.50 carb. O 18 δ Solent Group, UK: Shift 0.50 –0.50 sb3 sb:tlb sb4 mfs sb5/6/7 mfs mfs mfs Oi-1 EOT Onset A S.B Lin. Mbr Upper Colwell Bay Member Marls Cliff Cliff End Member Hamst. Member Lacey’s Lacey’s Farm Member Hampshire Basin Osborne Cranmore Member Member L. Hamst. Bemb. Lst Hath. Mbr Fishbourne Member 2011) and from Tanzanian Drilling Project Sites 12 and 17 (Pearson et (Pearson 17 and 12 Sites Drilling Project Tanzanian from and 2011) tion) from the mean. (B) Isotope results from the Ocean Drilling Program (ODP) Site 1218 (Coxall et the Ocean Drilling Program from the mean. (B) Isotope results tion) from Figure 3. Comparative climate data for the Solent Group and marine records. (A) The mean δ (A) and marine records. the Solent Group 3. Comparative climate data for Figure ted against the Solent Group stratigraphy (Hooker et (Hooker stratigraphy Group Solent the against ted surface (marked by maximum salinity fauna within sequence); sb—sequence boundary; tlb—top log bed; EOT—Eocene-Oligocene transi - 1. Peedee belemnite; others as in Figure VPDB—Vienna tion; Bembridge 1n 2n 1n 15r 13r 16n1r 15n 13n 16n 12r 16n

Mag- chron C N RUPELIAN PRIABONIA Stage Ma 32 34 36 33 35

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Coupling of marine and continental oxygen isotope records during the Eocene-Oligocene transition 6 21 mperature (°C) Maians, Spain 81 Te O 18 6 21 mperature (°C) Oregon, USA 81 Te DE MAP (mm/yr) Isle of Wight, UK 400 600 800 1000 12001400 C al. (2005) and mean annual air temperatures calculated temperatures al. (2005) and mean annual air

AT O record as in Figure 3. Shaded area around the mean δ around 3. Shaded area as in Figure O record mp. 18 °C) are given for the oldest paleosol and are the same for all the same for the oldest paleosol and are given for °C) are MA

Te δ al. (2009). (D) Paleosol-derived mean annual air temperature temperature al. (2009). (D) Paleosol-derived mean annual air Summer

} 47 Δ °C; E, ±2.5

Temperature (°C) Isle of Wight, UK lentus Lymnaea V. + + Charophyte + Otolith Mammal teeth B 10 20 30 40 Hiatus associated with Oi-1 glacial maximum sea level fall (‰ VPDB) carb. O 18 Isle of Wight, UK δ al. (2012). Representative error bars (D, ±4 error al. (2012). Representative

Shift al. (2013) using the April to October water-air temperature transfer function of Hren and Sheldon (2012). (C) Paleosol-derived function of Hren transfer temperature water-air April to October al. (2013) using the

0.50 –0.50 –1.50 –2.50 –3.50 A

S.B

Lin. Mbr Figure 4. Comparative climate data for the Solent Group and other terrestrial sites from the Solent Group (A–C) and the John Day Basin the Solent Group sites from terrestrial and other the Solent Group 4. Comparative climate data for Figure lentus (A) Viviparus Basin (Spain, E), respectively. D) and the Ebro (Oregon, by Hren et by Hren mean annual precipitations calculated for the Solent Group by Sheldon et the Solent Group calculated for mean annual precipitations . (B) Summer season temperatures calculated by Grimes et season temperatures ±1 σ . (B) Summer values represents (Solent Group) near-coastal from While the records sake. clarity’s for data points, but they have not been plotted throughout of the other - settings show cooling associated with the Eocene-Oligocene transition and a general temporal correspon and continental margin (Oregon) little change during the Eocene-Oligocene sites (e.g., Spain) show relatively intermontane continental interior dence with the marine realm, temperature; annual air MAAT—mean Peedee belemnite; MAP—mean annual precipitation; VPDB—Vienna Abbreviations: transition. 1. others as in Figure calculated for the John Day Basin by Sheldon (2009). (E) Paleosol-derived mean annual air temperature calculated for Maians, Ebro Basin, Maians, Ebro calculated for temperature the John Day Basin by Sheldon (2009). (E) Paleosol-derived mean annual air calculated for Spain, by Sheldon et Upper Colwell Bay Member Marls End Member Cli ff Hamst. Member Lacey ’s Farm Member Hampshire Basin Osborne Cranmore Member Member L. Hamst. Bemb. Lst Hath. Mbr Fishbourne Member Bembridge 1n 2n 1n 15r 13r 16n1r 15n 13n 16n 12r 16n

Mag- chron C RUPELIAN PRIABONIAN Stage Ma 32 34 36 33 35

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Sheldon et al.

The hiatus during the Oi-1 glacial maximum is then back to –1.7‰ at 33.7 Ma. At Oi-1, d18O tal isotope record, the mechanisms that control estimated as being the interval (Fig. 3) between values shift to +0.1‰ (basal upper Hamstead the isotopic composition of the water source the top log bed (tlb) sequence boundary and the Member) before a gradual recovery toward from which V. lentus precipitates are more com- subsequent maximum flooding surface of the more depleted values. The timings of these plicated. These factors include (1) evaporation, Nematura bed (i.e., basal meter of the upper shifts are broadly similar to marine isotope (2) changes in the isotopic ratio of precipitation, Hamstead Member; Hooker et al., 2009), iden- records, with the Solent Group excursions and and (3) change in sources of precipitation. These tified as belonging to sequence TA4.4 of Haq recoveries each roughly 0.5‰ larger than the additional factors, the last two of which may be et al. (1987). Between these points, resolution is marine isotope records. The pattern of a minor linked to changes in global ice volume, proba- less precise and assumes constant sedimentation followed by a major positive shift resembles bly explain the greater magnitude of the isotope rate. Sampling from the base of the Seagrove the succession Eocene-Oligocene transition 1 shifts seen in the continental realm. Bay Member to the top of the lower Hamstead (precursor event) and Oi-1 in marine sites (e.g., In the previous section, an ecological argu- Member, representing ~750 k.y. (Eocene-Oligo­ Coxall et al., 2005; Coxall and Wilson, 2011; ment against evaporation playing a dominant cene transition) leading up to Oi-1, averages herein, Fig. 3B). role in controlling the V. lentus d18O record was a frequency interval of ~35 k.y. The sampling presented. In other continental Eocene-Oligo­ frequency in the last ~250 k.y. subset aver- DISCUSSION cene transition successions (Fig. 2), locally ages ~18 k.y. evaporative conditions are indicated by aridi- Suitability of Using V. lentus as a fication (Sheldon and Retallack, 2004; Abels RESULTS Continental Oxygen Isotope Proxy et al., 2011; Passchier et al., 2013; Boardman and Secord, 2013), by shifts in the assemblage XRD analyses of 173 V. lentus fossil frag- Viviparus lentus, which has been used suc- of paleosol types (Terry, 2001), and by shifts in ments from 40 horizons (22 spanning the cessfully before in paleoclimate research the trace fossil assemblage (Sheldon and Hamer, Eocene-Oligocene transition and Oi-1) indicate (Schmitz and Andreasson, 2001; Hren et al., 2010). None of these features is observed in the preservation of aragonite consistent with that 2013), is an ideal freshwater paleoclimate Hampshire Basin, and rather than aridification of another freshwater gastropod, L. longiscata, proxy because modern Viviparus species col­ through the Eocene-Oligocene transition, wet- from the same clay-dominated lacustrine strata onize a wide range of habitats, including rivers, ter conditions have been reconstructed (Fig. 4C; (Grimes et al., 2005). This, in addition to the streams, ponds, and lakes (Boss, 1978). They Sheldon et al., 2009) using the same reconstruc- preservation of three discrete carbonate layers, are also gill breathers (prosobranchs) and, there- tion technique that indicated aridification in as viewed in numerous cross sections through fore, are found in permanent water bodies up to other regions (supplemental Table S2 [see foot- representative shells (Bugler, 2011; Hren et al., 20 m deep (Boss, 1978). Their dependence on note 1]). Furthermore, the most pronounced evi- 2013), strongly suggests that the V. lentus fossil dissolved oxygen for respiration makes them dence for evaporative enrichment in the Hamp- 18 fragments are not diagenetically recrystallized intolerant of polluted water (Strayer, 1990), so shire Basin is at ca. 34.4 Ma, based on d Owater and therefore should preserve their original iso- they are rarely found in stagnant water bodies values calculated using clumped isotope data topic signal. These results are also supported by (e.g., shallow ponds; Jokinen, 1983), where (Hren et al., 2013). In addition, some of the previous high-resolution single-shell microsam- significant evaporation can occur. These eco- paleosols in the Hampshire Basin stratigraphic pling that indicated that the V. lentus shells of logical parameters, in addition to evidence for sequence also include various redoximorphic the Solent Group preserve subannual seasonal the maintenance of primary aragonite and features such as drab-haloed root traces and rare variability in d18O (Hren et al., 2013). a three-layered shell structure, all suggest that Fe-Mn nodules that are consistent with episodic The d18O record from V. lentus is shown in the isotope record that has been generated from or even continuous inundation (Sheldon et al., Figures 3 and 4. At each of the 40 stratigraphic V. lentus is unaffected by diagenesis or evapora- 2009). Thus, in contrast to some other conti- levels, the d18O data points are a mean from the tive enrichment. nental records that are potentially compromised analyses of at least 7 V. lentus shell fragments, by local evaporative conditions (e.g., Nebraska; and the associated errors (shaded region) are the Factors Controlling Shifts in the Sheldon, 2009; Boardman and Secord, 2013; standard deviation (±1s) from the mean. Bugler V. lentus δ18O Record cf. Zanazzi et al., 2015), the Hampshire Basin et al. (2009), based upon micromilled isotope record appears likely to be a high-fidelity profiles of a number of modern­ Viviparus con- The magnitude of the d18O shifts differs sig- recorder of environmental conditions. tectus (Millet) shells, demonstrated that d18O nificantly between the continental V. lentus and At the same time, if the d18O shifts (Figs. 3A values could vary cyclically by up to 1.9‰, the marine foraminiferal records. The marine and 4A) were due to temperature change alone, primarily as a response to changes in seasonal d18O shifts are typically of the order 0.4‰– then they are above, or at the very upper limit of, temperature. Similar-magnitude d18O cycles 1.0‰, while those from the Hampshire Basin those previously recorded. For example, using were also shown by Hren et al. (2013) in a are >1.4‰ (Fig. 3). Oxygen isotope values the Viviparus genus-specific equation (Bugler smaller number of whole V. lentus shells from observed in the marine realm represent a com- et al., 2009), and assuming no change in the the Solent Group, which suggests that the varia­ bination of temperature and the oxygen isotopic d18O of freshwater, temperature shifts across the bility reported here from the analysis of multi­ composition of seawater, which is controlled Eocene-Oligocene transition would be between ple shell fragments is likely to be related to sea- mainly by changes in the global ice volume. 10 °C and 13 °C. These reduce to 6–8 °C using sonal changes. In the marine realm, a well-mixed ocean is a general inorganic aragonite equation (Gross- Early in the Eocene-Oligocene transition, assumed (various; e.g., Zachos et al., 2001), man and Ku, 1986) or if the likely season of d18O values from V. lentus shells are variable but and, consequently, shifts in the oxygen isotopic the majority of carbonate formation is used generally cluster around –1.5‰ (Figs. 3A and composition of seawater are related largely or to adjust the temperature (Hren and Sheldon, 4A). Just prior to Oi-1 (upper half of the lower entirely to ice volume and temperature changes. 2012). In contrast, a sea surface temperature Hamstead Member), they shift to –0.6‰ and While temperature plays a role in the continen- (SST) change of ~5 °C across the Eocene-

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Coupling of marine and continental oxygen isotope records during the Eocene-Oligocene transition

Oligocene transition has been reconstructed not temperature, were responsible for the d18O between the type of proxy employed (i.e., paleo- recently based on high-latitude marine records peaks and troughs during the ~1 m.y. preceding sol records can indicate either cooling or stabil- of TEX86 (tetraether index of tetraethers consist- the Eocene-Oligocene transition. ity, depending on the locality), which suggests K′ ing of 86 carbon atoms) and U 37 (unsaturation Observations of modern V. contectus in that this environmental heterogeneity is real.

index of C37 alkenones; Liu et al., 2009). Simi- ­England indicate that shell growth is dominated Summarizing from various studies, two general larly, Hren et al. (2013) documented a 4–6 °C by summer growth (Bugler et al., 2009). Ecol- trends have emerged: (1) Proximity to a marine drop in mean annual air temperature across the ogy of V. lentus preserved in the Hampshire moisture source matters, and (2) the magni- Eocene-Oligocene transition (Fig. 4B) using Basin was likely similar, even if the overall tude of climatic response is larger at higher V. lentus as in this study, and “clumped isotope” climate state was warmer during the Eocene- paleolatitudes. In comparing a number of paleo- analyses, which are insensitive to evaporation Oligocene transition. Lake water temperatures precipitation reconstructions, Sheldon et al. or to changes in source-water d18O. Thus, the are very highly correlated with air temperatures, (2012) recognized that records from endorheic discrepancy between the apparent magnitude of but the precise relationship varies quantitatively basins indicated little response, whereas basins temperature change indicated by the new d18O according to latitude and also to the length of that were exorheic and located close to their record if due to temperature alone (6–13 °C, season that is considered (Hren and Sheldon, marine moisture source (e.g., Hampshire Basin) depending on equation) and the “clumped iso- 2012). Thus, while it is difficult to ascertain recorded a strong response. This is similar to tope” record (4–6 °C) likely indicates a shift in exactly what part of the year is represented by the observation that modern coastal precipita- source-water d18O toward relatively enriched the Hampshire Basin record, it is likely most tion stations record d18O of precipitation that values, which accompanied the drop in tem- reflective of summer growing-season lake con- more closely mirrors changes in surface ocean perature. Because source-water evaporative ditions. Serial isotope sampling of V. ­lentus water d18O than inland sites, due to continen- enrichment was unlikely to have been signifi- from the Hampshire Basin indicates similar tality effects (Dansgaard, 1964). Similarly, cant (see earlier herein), the shift in source- season ranges before, during, and after the terrestrial temperature response to the Eocene- water d18O toward enriched values instead likely Eocene-Oligocene transition, but with different Oligocene transition (Fig. 4) appears to reflect records enrichment in meteoric water due to absolute values during the Eocene-Oligocene paleolatitude, where higher-paleolatitude sites ice-sheet growth on Antarctica. Assuming that transition, suggesting substantial reorganiza- (Fig. 2; e.g., Hampshire Basin; Blakey, 2008) the “clumped isotope” record (Hren et al., 2013) tion of the hydrologic cycle (Bugler, 2011; Hren indicate higher-magnitude cooling as compared and high-latitude marine SST record (Liu et al., et al., 2013). While the large sustained d18O to relatively lower-paleolatitude sites (e.g., 2009) reflect the “true” temperature change, shift recorded in the Hampshire Basin is not Oregon), a pattern which mimics marine SST then roughly half of the d18O shift recorded in recorded in European endorheic basin records records (Liu et al., 2009). Thus, when combin- the new Hampshire Basin V. lentus record is (Maians, Spain; Fig. 4D), similar reorganization ing both patterns, a midlatitude endorheic basin due to temperature change, and half is due to of the hydrologic cycle was previously identi- site such as the Ebro Basin (Fig. 4E; Sheldon ice-volume change. These relative proportions fied from high-magnitude variability in paleosol et al., 2012) shows little or no climatic response are similar to what has been inferred for marine carbonate d18O values (Sheldon et al., 2012). to the Eocene-Oligocene transition, whereas a foraminiferal d18O records (Zachos et al., 2001; Because the poles cooled more rapidly during higher-latitude coastal plain site such as the Isle Coxall et al., 2005; Gallagher and Sheldon, the Eocene-Oligocene transition than lower-lati­ of Wight (Figs. 4A and 4B; this study) records 2013). In addition, the marine SST (Liu et al., tude sites (e.g., Liu et al., 2009), the changing the highest-magnitude changes, larger even than 2009) and “clumped isotope” records (Fig. 4B) ocean and air temperature gradients may have other sites located relatively close to the ocean both indicate a post–Eocene-Oligocene transi- changed the moisture source amounts more at (e.g., Oregon). In Oregon, the Cascades uplift tion rebound to slightly warmer conditions that higher latitudes as well. Although records from postdated the Eocene-Oligocene transition and is mirrored by marine d18O records and by the other sites are needed to confirm this pattern, causes a rain shadow at present that was not a Hampshire Basin V. lentus d18O record (Figs. it suggests that ice-sheet growth on Antarctica factor in the Paleogene. Furthermore, accre- 3A and 4A), and by reconstructed atmospheric was connected to Northern Hemisphere climate tion of the Coast Range generally postdates the 11 pCO2 based on the d B composition of plank- through cooling of North Atlantic Ocean waters Eocene, so central Oregon was much closer to tonic foraminifera (Pearson et al., 2009). and alteration of regional hydrology. the coastline at the time of deposition than it is Additional support for interpretation of the at present (Fig. 2). enriched d18O values during the Eocene-Oligo- Comparison with Other Terrestrial cene transition and Oi-1 as representing both ice Paleoclimate Records CONCLUSIONS volume and temperature is provided by sequence stratigraphy. The highest d18O values and lowest As discussed in the introduction, terrestrial XRD data, shell structural preservation, and clumped isotope temperatures postdate the top records of the Eocene-Oligocene transition ecology of nearest living relatives suggest that log bed (tlb) sequence boundary (Fig. 3), where indicate varied responses and varied magni- V. lentus from the UK Solent Group yields a an unconformity indicates major sea-level fall. tudes of response (supplemental Table S2 [see d18O record that has not been affected by dia- In contrast, lower in the succession, pre–Eocene- footnote 1]; Fig. 4). For example, records from genesis. Based on paleosol proxy evidence, Oligocene transition (Fig. 3A), high d18O values the Hampshire Basin (this study; Hren et al., precipitation increases up sequence and does coincide with sequence boundaries (Lacey’s 2013), Nebraska (Zanazzi et al., 2007; but see not shift in amount in parallel with the V. lentus Farm and Seagrove Bay Members), while shifts also Boardman and Secord, 2013), and Oregon d18O record, further indicating that evaporation toward lower values coincide with maximum (Sheldon and Retallack, 2004; Gallagher and is unlikely to be a major factor affecting these flooding surfaces (Fishbourne and Bembridge Sheldon, 2013) indicate significant cooling, continental oxygen isotope values in the Solent Marls Members), where temperatures recorded while records from Montana (Retallack, 2007) Group during the Eocene-Oligocene transition. by clumped isotopes are relatively stable (Fig. and Spain (Sheldon et al., 2012) indicate little or The oxygen isotope record derived from 4B). This suggests that changes in ice volume, no cooling. There are no systematic differences V. lentus records a positive d18O shift across

Geological Society of America Bulletin, v. 1XX, no. XX/XX 7 Geological Society of America Bulletin, published online on 14 September 2015 as doi:10.1130/B31315.1

Sheldon et al.

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Geological Society of America Bulletin

Coupling of marine and continental oxygen isotope records during the Eocene-Oligocene transition

Nathan D. Sheldon, Stephen T. Grimes, Jerry J. Hooker, Margaret E. Collinson, Melanie J. Bugler, Michael T. Hren, Gregory D. Price and Paul A. Sutton

Geological Society of America Bulletin published online 14 September 2015; doi: 10.1130/B31315.1

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