Marine Micropaleontology 78 (2011) 50–64

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Marine Micropaleontology

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Impact of the Middle Miocene climate transition on elongate, cylindrical foraminiferain the subtropical Pacific

Katherine Johnson a,⁎, Bruce W. Hayward a, Ann Holbourn b

a Geomarine Research, 49 Swainston Road, Auckland, New Zealandb Institute of Geosciences, Christian Albrechts University, D-24118 Kiel, Germany

⁎ Corresponding author. Current address: DepartmEastern Illinois University, 600 Lincoln Ave., Charleston581 7270.

E-mail address: kjohnson4@eiu.edu (K. Johnson).

0377-8398/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.marmicro.2010.09.008

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 March 2010Received in revised form 27 September 2010Accepted 27 September 2010

Keywords:Deep-sea benthic foraminiferaMiddle Miocene coolingExtinctionSubtropical Pacific

Fifty-eight species of elongate, cylindrical benthic foraminifera (here referred to as the Extinction Group)belonging to genera that became extinct during the mid-Pleistocene Climate Transition (MPT), weredocumented (~50 kyr resolution) through the early middle Miocene (15–13 Ma) in two sites on oppositesides of the subtropical Pacific Ocean (ODP Sites 1146, South China Sea; ODP Site 1237, southeast Pacific). Thestudy was undertaken to investigate the response of the Extinction Group (Ext. Gp) to the major coolingduring the middle Miocene Climate Transition (MCT) to look for clues that might explain the causes of theextinction during the glacials of the mid-Pleistocene Climate Transition. Ext. Gp faunal differences betweenthe two sites (attributed to regional and bathymetric differences in food supply to the seafloor) are greaterthan those that occurred through the 2 myr time span at either site. The middle Miocene Climate Transitionwas not an interval of enhanced species turnover or a decline in Ext. Gp abundance, in contrast to the majorextinctions that occurred during the mid-Pleistocene Climate Transition. Distinct changes in the compositionof the Ext. Gp faunas did occur through this time (more pronounced in Site 1237). At both sites the pre-middleMiocene Climate Transition faunas were transformed into their post-middle Miocene Climate Transitioncomposition during the period of major cooling (14.0–13.7 Ma). During this transition interval the faunalcomposition swung back and forth between the two end member faunas. These faunal changes are attributedto changes in productivity (decrease in South China Sea, increase in southeast Pacific), brought about bymajorchanges in global climate and continental aridity.

ent of Geology–Geography,, IL 61920 USA. Tel.: +1 217

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

1.1. Rationale for study

Studies in recent decades have shown that a group of elongate,cylindrical benthic foraminifera became extinct during the cycliccoolings of the mid-Pleistocene Climate Transition (e.g., Schönfeld,1996; Hayward, 2001, 2002; Hayward et al., 2007; Kawagata et al.,2007). The precise causes of this “Last Global Extinction” in the deepsea are unknown but appear to be related to the periods ofincreasingly severe glacial coolings. To further investigate the possiblecauses of this extinction and its apparent correlation with increasedglobal cooling we have formulated this study to investigate the impactof another known period of major global cooling on this ExtinctionGroup (Ext. Gp.) of benthic foraminifera. The chosen interval is thelargest cooling event in the Neogene: the middle Miocene ClimateTransition (MCT).

Our research addresses the following questions:

1. Was there increased faunal turnover (especially extinctions)during the rapid cooling of the MCT, similar to that during theMPT glacial periods? If so, how significant was it?

2. Were the Ext. Gp faunas similar on either side of the subtropicalPacific during the middle Miocene? If not, what were the likelyreasons for the differences?

3. Were the faunal composition and/or population structure of theExt. Gp. impacted by the MCT? If so, what were the changes, andwere there differences in the responses between the sites?

4. Does the extensive environmental proxy data from the two studysites provide further insights into the causes of the documentedchanges in the Ext. Gp through this MCT interval?

1.2. The last global extinction fauna

During the late Pliocene-middle Pleistocene (2.6–0.55 Ma), ~20%(19 genera, 95 species) of cosmopolitan, deep-sea (500–4000 m),benthic foraminiferal species (excluding unilocular taxa), becameextinct (e.g., Lutze, 1979; Weinholz and Lutze, 1989; Schönfeld, 1996;Hayward, 2001, 2002; Kawagata et al., 2005, 2006; Hayward et al.,

Table 1Extinction Group genera listed by family.

Nodosariidae Pleurostomellidae Stilostomellidae

Cribronodosaria Amplectoductina SiphonodosariaGlandulonodosaria Ellipsoidella StilostomellaMucronina Ellipsoidina StrictocostellaOrthomorphina Ellipsoglandulina

EllipsopolymorphinaNodosarellaPleurostomella

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2006, 2009; O'Neill et al., 2007). During this Last Global ExtinctionEvent (LGE), species from three families were impacted (Table 1),with one family, the Stilostomellidae (30 species) disappearingcompletely, and all but one species in the Pleurostomellidae becomingextinct (Hayward et al., 2007). Specific foraminiferal morphologies(elongate, cylindrical, often uniserial tests) and apertural types (e.g.,small rounded, dentate, cribrate, or lunate) were preferentially

Fig. 1. Locations of ODP S

affected by the LGE. Peak extinctions of this group of foraminifera(76 species) occurred during the mid-Pleistocene Climate Transition(MPT, 1.2–0.55 Ma). Factors that have been inferred to have beenresponsible for the decline of the Ext. Gp. include increasedventilation of deep and intermediate water and/or decline in specificfood sources during glacial coolings (Thomas, 1985, 1986, 1992, 2007;Thomas and Vincent, 1987, 1988; Kawagata et al., 2005, 2006, 2007;Hayward et al., 2006, 2008, 2009; O'Neill et al., 2007).

Species of the Ext. Gp genera and families were a more dominantcomponent of the deep-sea benthic foraminifera in the Paleogene,comprising up to 50%–70% of the fauna in the middle-late Eocene(Thomas, 2007; Hayward et al., 2010b). One hypothesis has been thatthe LGE was the final event in the decline and loss of the mainconstituents of the Greenhouse World deep-sea benthic foraminiferalfauna (Thomas, 2007; Hayward et al., 2007, 2010b). As the Earthcooled and the modern ocean circulation pattern evolved since thelate Eocene, the quantity of dead phytoplankton food reaching the seafloor has increased, its composition has changed (increased diatom

ites 1146 and 1237.

Fig. 2. Age model, stable oxygen and carbon isotope data of benthic foraminifera, reflectance (L*), magnetic susceptibility, carbonate weight %, absolute abundance of total benthicand Extinction Group foraminifera, specimens g−1 sed (BF g−1 sed, Ext.G g−1 sed), Ext. Gp foraminifera as a percentage of the total benthic foraminifera (Ext.Gp%), Ext. Gp speciesrichness (Ext. Gp S), linear sedimentation rates, cm−2 kyr−1 (LSR), and accumulation rate of total benthic and Ext. Gp foraminifera, specimens cm−2 kyr−1 (BFAR, Ext. Gp AR) forODP Site 1237 (Holbourn et al., 2005, 2007). Reflectance, magnetic susceptibility, and carbonate weight % from the ODP Janus database. The gray band indicates MCT (~14.2-13.8 Ma)A. ODP Site 1146. B. ODP Site 1237.

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component), and its supply has become more seasonally pulsed(Thomas, 2007); one or more of these factors drove the decline of theExt. Gp fauna.

1.3. Middle Miocene climate

Climate changed dramatically during the course of the Miocene.Temperatures warmed and ice volume was reduced during themiddle Miocene climatic optimum (17–14.7 Ma). This event wasfollowed by a pulsed cooling phase, during which the Antarctic IceSheet expanded, the oceans cooled and climate markedly altered(Woodruff and Savin, 1989, 1991; Holbourn et al., 2005, 2007). Theexpansion of the East Antarctic Ice Sheet in the middle Miocenerepresents a vital step in the development of the Neogene Icehouse(Miller et al., 1991; Flower and Kennett, 1993, 1994; Zachos et al.,2001).

Proxy evidence has been employed to tease apart details of themiddle Miocene climate system and shows multiple drivers andevents to be crucial to understanding the cooling from ~15–13 Ma.Studies using benthic foraminiferal δ18O and Mg/Ca ratios have beenutilized to investigate the global cooling in the middle Miocene (Savinet al., 1985; Woodruff and Savin, 1985; Billups and Schrag, 2002;Shevenell and Kennett, 2004; Holbourn et al., 2005, 2007; Smart et al.,2007). Deep-sea δ18O values exhibited a ~1‰ increase across themiddle Miocene Climate Transition (MCT) at 14.2–13.8 Ma (e.g.,Shackleton andKennett, 1975; Zachos et al., 2001; Flower andKennett,1995; Shevenell et al., 2004; Holbourn et al., 2005; Smart et al.,2007). It has been estimated that ice volume accounts for ~70% ofthe 1‰ δ18O change (Miller et al., 1991; Shevenell et al., 2004). Globaldeep-water temperatures are believed to have decreased by ~1.5–

2.5 °C across the middle MCT excursion (Miller et al., 1991; Wrightet al., 1992; Flower and Kennett, 1995). The terrestrial realm was alsoaffected by the cooling and witnessed increased continental aridityand expansion of C4 grasslands (Retallack, 2001; Jia et al., 2003).

Increased middle Miocene primary productivity is supported bydeposition of organic-rich sediment around the circum-Pacific, as wellas foraminiferal stable carbon isotopes. The “Monterey Hypothesis”was proposed by Vincent and Berger (1985) to explain the positivecarbon-isotope excursion from 16.9 to 13.5 Ma. Vincent and Berger(1985) suggested that the “Monterey” excursion was a result ofincreased carbon burial in organic-rich sediments around the Pacific.Carbon burial in sedimentary reservoirs along regions with coastalupwelling would then lead to drawdown of atmospheric carbon, thuscausing the global cooling. Relatively high foraminiferal δ13C valuesfrom the middle Miocene are also inferred to be associated withhigher levels of open ocean primary productivity (Woodruff andSavin, 1985). The long-lasting “Monterey” excursion cannot beexplained by marine carbon reservoirs alone, a large portion may bedue to terrestrial organic carbon burial in the form of lignite depositsworldwide at this time (Diester-Haass et al., 2009).

A transition in the pacing of δ18O fluctuations occurred at 13.9–13.8 Ma (Holbourn et al., 2005, 2007). Variations in δ18Omainly followobliquity between 14.7 and 13.9 Ma (Holbourn et al., 2005), whereasδ18O exhibits a prominent 100-kyr variability after 13.8 Ma. Recordsfrom the southwest Pacific Ocean support the expansion of the EAIStowards the end of the climatic optimum (~15 Ma) (Shevenell et al.,2008). Atmospheric heat and moisture supply to the Antarctic appearto have played a major role in the middle Miocene EAIS expansionduring an interval in which the Southern Ocean was believed to berelatively warm (Holbourn et al., 2005; Shevenell et al., 2008).Following the switch to eccentricity-pacing after 13.8 Ma, improveddeep-water ventilation and enhanced production of southern-sourcedeep and intermediate waters occurred (Holbourn et al., 2007).

While the middle Miocene and middle Pleistocene are bothcharacterized by changes in orbital pacing from 41-kyr to 100-kyrcycles, the details are quite different. As compared to the mid-Pleistocene orbital pacing change, the middle Miocene 100-kyr cyclesdo not exhibit higher amplitude variations from that of the 41-kyrcycles (Holbourn et al., 2005). Also, in the middle Miocene, thetransition from 41-kyr to 100-kyr periodicity occurred during orbefore ice-sheet expansion without a substantial time lag (Holbournet al., 2005). In addition, middle Miocene 100-kyr cycles show a clearphase relationship and similar amplitude modulation to eccentricity,as well as 125-kyr and 95-kyr components, unlike the middlePleistocene 100-kyr cycles (Holbourn et al., 2005).

2. Study sites and their regional settings

The present study was undertaken to investigate the response ofthe Ext. Gp. to the dynamic paleoceanographic and climatic changesthat occurred during the middle Miocene. ODP Sites 1237 and 1146were chosen due to their well-constrained chronostratigraphyspanning the middle Miocene Climate Transition (Holbourn et al.,2005). These sites, located in the South China Sea (1146) and in thesoutheastern Pacific Ocean atop Nazca Ridge off Peru (1237)(Fig. 1),allow for correlation of paleoceanographic events across the subtrop-ical Pacific (Holbourn et al., 2005).

2.1. ODP Site 1146, South China Sea

ODP Site 1146 (19°27.40′N, 116°16.37′E; water depth: 2092 m) islocated within the northern South China Sea (SCS; Fig. 1). The SCS isone of the world's largest marginal seas in the tropics, located at thenorthwesternmargin of the tropical Pacific Ocean. The easternmarginof the SCS is characterized by broad continental shelves that extendmore than 100 km south off the coast of China (18–23°N). Along the

Fig. 4. Stratigraphic records of the absolute abundance of the common Ext. Gp species (specimens/10-g sediment) and the benthic stable oxygen isotope record from Site 1146(Holbourn et al., 2005, 2007). Gray band indicates MCT (~14.2–13.8 Ma). The asterisk (*) indicates sample in excess of 80 specimens/10-g sediment.

54 K. Johnson et al. / Marine Micropaleontology 78 (2011) 50–64

continental shelf, the Red and Pearl Rivers deliver terrestrially derivedsediment to the northern SCS. The SCS is heavily influenced bymonsoons (both winter and summer) and local upwelling.

Today, South China Sea deep-water characteristics are controlled bythe inflowof Pacificwater through the Bashi Strait, between Taiwan andLuzon Island. Site 1146 is located above the current sill depth at BashiStrait (2600 m) (Shipboard Scientific Party, 2000). Transport throughthe Bashi Strait is complicated,withwater exiting the South China Sea atintermediate water depths and entering from the Pacific at the surfaceand in deeper layers (Chao et al., 1996; Chen andHuang, 1996; Chen andWang, 1998; Qu, 2002; Yuan, 2002). Deep-water entering from thePacific sinks as it crosses the strait, due to its cooler, less salineproperties(Wyrtki, 1961), forcing upwelling in other regions (notably, SW ofTaiwan and just off of Vietnam) of the South China Sea (Nitani, 1972;Broecker et al., 1986; Chao et al., 1996). The deep upwellingmeans thatthe residence time of deep waters within the South China Sea isrelatively short (~50–150 years) (Chao et al., 1996). Site 1146 lies abovethe modern lysocline (~3000 m) (Wang and Lin, 2004).

Middle Miocene sediment from ODP Site 1146 is clay, nannofossilclay, and clayey nannofossil ooze (Shipboard Scientific Party, 2000).Sedimentation rates averaged 2.96-cm/kyr during 15–13 Ma (Figs. 2and 3).We used the astronomically tuned agemodel based on benthicforaminiferal δ18O of Holbourn et al. (2005, 2007).

2.2. ODP Site 1237, Southeast Pacific Ocean

ODP Site 1237 (16°0.421′S, 76°22.685′W; water depth: 3212 m) islocated 140 km off the coast of Peru in the southeast Pacific Ocean

(Fig. 1). The site sits on a flat bench on the eastern side of the aseismicNazca Ridge (Shipboard Scientific Party, 2003) and is located todaybeneath the Peru Current, which is a broad, slow current with anaverage transport of 15–20 Sv (1 Sv=106 m3 s−1) (Wooster andReid, 1963). The coastal region is known for upwelling, but it isconfined to within 50–60 km of the shoreline (Strub et al., 1998).During the middle Miocene, the site was situated further to the westand remained in an oligotrophic setting. The Peru–Chile trench is theprincipal conduit for bottom waters entering the Peru Basin from thesouth (Mantyla, 1975; Laird, 1971; Lonsdale, 1976).

Site 1237 today is a transition zone between oxygen-rich, nutrientdepleted Circumpolar Deep Water that enters the Peru Basin throughthe Peru–Chile Trench and nutrient-rich Pacific Central Water(Shipboard Scientific Party, 2003). Circumpolar Deep Water in theeastern Peru Basin today is more saline and more oxygenatedthan that in the western Pacific, due to mixing with North AtlanticDeep Water (NADW) leaking through Drake Passage in the Pacific(Lonsdale, 1976). At Site 1237, primary productivity has increasedinto the present; it is estimated that primary productivity 30 millionsyear ago at the location was less than one-third today's values(Shipboard Scientific Party, 2003). Site 1237's paleodepth was locatedclose to the lysocline (Holbourn et al., 2007).

Middle Miocene sediment in Site 1237 is nannofossil ooze(Shipboard Scientific Party, 2003). Sedimentation rates were low, asexpected in a pelagic setting, with an average of 1.20 cm/kyr during15–13 Ma (Figs. 2 and 3). There were two short periods of increasedsedimentation: 13.91–13.80 Ma (av. rate of 1.87 cm/kyr) and 13.33–13.25 Ma (av. rate of 1.74 cm/kyr). We used the astronomically tuned

Fig. 5. Stratigraphic records of the absolute abundance of the common Ext. Gp species (specimens/10-g sediment) and the benthic stable oxygen isotope record from Site 1237(Holbourn et al., 2005, 2007). The gray band indicates MCT (~14.2–13.8 Ma).

55K. Johnson et al. / Marine Micropaleontology 78 (2011) 50–64

age model based on benthic foraminiferal δ18O from the compositemiddle Miocene section from Holes 1237B and C of Holbourn et al.(2005, 2007).

Table 2Environmental and other proxies used in our analyses.

Age Age in Millions of Years

BFAR Benthic foraminiferal accumulation rate=number of tests persq cm per kyr (using LSR); proxy for increased food supplyand phytoplankton productivity

δC13 Carbon isotopes ratios of benthic foraminiferaδO18 Oxygen isotope ratios of benthic foraminiferaExt. Gp AR Extinction Group accumulation rate=number of tests per sq

cm per kyr (using LSR)Ext. Gp% Extinction Group tests as a percentage of total benthic

foraminiferal testsIBFAR Benthic foraminiferal accumulation rate using nonlinear

interpolated sedimentation rates from Holbourn unpublisheddata.

IExt. Gp AR Extinction Group accumulation rate using nonlinearinterpolated sedimentation rates from Holbourn unpublisheddata.

ISR Nonlinear interpolated sedimentation rates in cm per kyrfrom Holbourn unpublished data.

L* Reflectivity; proxy for carbonate percentage and bioclasticsediment

LSR Linear sedimentation rate in cm per kyr from Holbourn et al.,2007.

Magnetic Suscept. Magnetic Susceptibility; proxy for terrigenous sedimentCibs* Cibicides tests per gram of sediment N250 μm (unpublished

data from Holbourn)Uvig* Uvigerina tests per gram sediment N250 μm (unpublished

data from Holbourn)

3. Materials and methods

3.1. Foraminiferal samples and processing

Sixty-seven samples (36 from Site 1146, 31 from Site 1237) of thosestudied by Holbourn et al. (2005) were selected to document the historyof the Ext. Gp. benthic foraminifera (Figs. 4, 5) through the interval 15–13Ma (sample resolution ~50 kyr; see online Appendix B). Sampleprocessing methods are described in Holbourn et al. (2005).

Table 3Summary of benthic foraminiferal statistics at each study site.

Property Site 1146 Site 1237

Number of samples studied 31 36Number of Ext. Gp tests counted 8463 2900Number of Ext. Gp species 45 45Number of nodosariid Ext. Gp species 13 9Number of pleurostomellid species 18 22Number of stilostomellid species 14 14Number of Ext. Gp species/sample 7-20, 13 (av.) 7-19, 12 (av.)Dominant Ext. Gp species Siphonodosaria

lepidulaSiphonodosarialepidula

Strictocostellaspinata

Siphonodosariasubspinosa

Strictocostellamatanzana

Ext. Gp% of total benthic forams 8%–27%, 18% (av.) 4%–19%., 11% (av.)Benthic foram specimens/g sediment 15–139, 43 (av.) 18–91, 38 (av.)Benthic foram specimens/cm2/kyr 123 (av.) 42 (av.)Ext. Gp specimens/g sediment 3–41, 7 (av.) 1–6, 4 (av.)Ext. Gp specimens/cm2/kyr 21 (av.) 4 (av.)

Fig. 6. Number of highest occurrence records of Ext. Gp species per 100 kyr through themiddle Miocene interval of 15–13 Ma in Sites 1146 and 1237.

Fig. 7. Unconstrained cluster analysis (chord dissimilarity coefficient) dendrogram ofMiddle Miocene samples in Sites 1146 and 1237 based on the relative abundances ofspecies within the Ext. Gp of benthic foraminifera. Samples are labeled by age inmillions of years and Site (P=1237, C=1146).

56 K. Johnson et al. / Marine Micropaleontology 78 (2011) 50–64

All Ext. Gp (Table 1) specimens were picked from three sizefractions (N250 μm, 150–250 μm, and 63–150 μm) of thewashed sandresidues. In every sample, 100% of the N250 μm and 150–250 μmfractions was investigated, except for one sample (Site 1146,13.97 Ma), where only 50% was searched. Within the smallest sizefraction (63-150 μm), between 6.25% and 100% was searched,depending on the quantity of sediment and richness of specimens.Species richness in this study refers to the number of Ext. Gp species ina sample. Ext. Gp census counts (see online Appendix B) weresubsequently multiplied up to the number of specimens in the totalsample and standardized as specimens per g of bulk sediment(absolute abundance). For cluster and correspondence analyses thecensus count data were standardized as relative abundances of theExt. Gp fauna in each sample. The accumulation rate (specimens percm2 per kyr) was calculated for the Ext. Gp (Ext.GpAR) and totalbenthic foraminifera (BFAR) using linear sedimentation rates (Fig. 2).Total benthic foraminiferal data was from the earlier study ofHolbourn et al. (2005). For linear correlation, accumulation rates(IExt.GpAR, IBFAR) were also calculated according to the interpolatedsedimentation rates shown in Fig. 3. The plots of these are so similar tothose of the Ext.GpAR and BFAR of Fig. 2, they are not shown.

All Ext. Gp specimens were mounted on faunal slides andidentified to species level where possible. Faunal slides are housedin the collections of the Institute of Geological and Nuclear Sciences,Lower Hutt, New Zealand.

3.2. Correlation coefficients

Correlation coefficients between the relative abundances of Ext.Gp taxa and environmental proxies (Table 2) were calculated usingthe software package PAST (Hammer et al., 2001). Pearson's r-valuesrange from −1 to 1; a value of 0 indicates no correlation. P values,probability that variables are uncorrelated, were used to assess thesignificance of the correlation. Only correlation coefficients with Pvalues of less than 0.05, or having more than 95%confidence ofstatistical correlation have been used (see online Appendix C).

3.3. Cluster analyses

Hierarchical cluster analysis was performed on relative abundancedata using MVSP v. 3.13 (Kovach Computing Services, 2008). Thealgorithm used was the Unweighted Pair–Group Mean Average(UPGMA). Cluster analysis was run on both unconstrained andstratigraphically constrained (within each site) data sets, using thechord dissimilarity index.

3.4. Detrended canonical correspondence analyses

Detrended Canonical Correspondence Analyses (DCCA) wereundertaken separately for each site to investigate the relationshipbetween the faunal composition (relative abundance) of the Ext. Gp

and environmental drivers using MVSP software package (KovachComputing Services, 2008). The environmental proxy data included inthe Canonical Correspondence analyses is listed in Table 2.

Fig. 8. Mean relative abundance and standard deviation of the common Ext. Gp species (N2% at either site) and their occurence (% of samples in which they occur) in the middleMiocene of Sites 1146 and 1237.

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4. Results

4.1. Middle Miocene Extinction Group diversity and abundance

A total of 58 Ext. Gp species in 14 genera are recorded from themiddle Miocene of the two study sites (Table 1). The number of Ext.Gp species in each sample fluctuated between 7 and 20 (Table 3). Thefluctuations in number of species were more marked in Site 1146, butthere was no temporal trend of increasing or decreasing speciesrichness (Fig. 2). In Site 1237 however there was a slight decrease inspecies richness from a mean of ~14 species prior to the MCT to amean of ~10 species post 13.2 Ma. This decline in species richnessoccurred in two steps at ~14.0 and ~13.3 Ma (Figs. 2 and 6).

When the highest occurrence of each Ext. Gp species through thestudied time interval in each site is examined (Fig. 6), it is clear thatthere were few local withdrawals of species from either site prior tothe MCT (N14.2 Ma). During the MCT (14.2–13.8 Ma), and especiallythe latter half there were elevated numbers of local disappearances ofExt Gp species from both sites and a second peak of these at Site 1237at 13.5–13.3 Ma. Highest occurrence records after 13.3 Ma reflectproximity to the top of the studied interval and not reality.

At Site 1146, the relative abundance of the Ext Gp (as a percentageof total benthic foraminifera) fluctuated between 10 and 30% (Fig. 2),but there was no significant decline or increase through the studiedmiddle Miocene interval. At Site 1237, the Ext. Gp relative abundancefluctuated between 5% and 18% of benthic foraminifera with nosignificant change across the MCT. There was a perceptible declinehowever, from 18% to 4–8% during the latter part of the studiedinterval (13.6–13.0 Ma) at a timewhen the accumulation rates of boththe Ext. Gp and total benthic foraminifera also increased substantially(Fig. 2).

4.2. Extinction Group differences between sites

The unconstrained cluster analysis on the full Ext. Gp relativeabundance data from both sites together distinguishes two high levelclusters—one comprising all samples from Site 1146 and the othermost samples from Site 1237 (Fig. 7). The reasons for this dichotomybecome clear when the mean relative abundances of individualspecies at each site are compared (Fig. 8). Three of the five mostdominant species (N8% mean relative abundance of Ext. Gp) in Site1146 (Strictocostella spinata, S. scharbergana, Pleurostomella brevis)were significantly more abundant and more frequently present(occurrence values N86%) than in Site 1237, and three of the fivemost dominant species (N8% mean rel. abund.) in Site 1237(Siphonodosaria subspinosa, Pleurostomella subnodosa, Strictocostella

matanzana) were significantly more abundant and more frequentlypresent than in Site 1146 (Fig. 8). The other two dominant species(Siphonodosaria lepidula, Pleurostomella tenuis) were common to bothsites at similar relative abundance levels. Both sites had similarspecies richness at the site (45 spp.) and sample (12–13 spp.) levels(Table 3), with 31 species in common and the remaining 27, mostlyrare, species occurring in only one of the sites. Stilostomellidspecimens were more abundant in Site 1146 (63% mean relativeabundance v. 53% in Site 1237), whereas pleurostomellids andnodosariids were more abundant in Site 1237 (Fig. 9). In commonwith other study sites, the shallower (1146) site had more nodosariidspecies and the deeper site (1237) had more pleurostomellid species(Hayward et al., 2010a,b).

In terms of abundance, the Ext. Gp generally comprised a greaterproportion of the benthic foraminiferal fauna in Site 1146 (mean 18%)than in Site 1237 (mean 11%) (Fig. 9). Site 1146 also had higherlevels of Ext. Gp absolute abundance and accumulation rate (7specimens g−1 sed; 21-cm−2 kyr−1) than Site 1237 (4 specimens g−1

sed; 4-cm−2 kyr−1), as did the total benthic fauna (Table 3).

4.3. Extinction Group changes in Site 1146

On the constrained cluster analysis dendrogram for Site 1146(Fig. 10), we recognize three clusters (C1–C3). C1 (15.0–14.46 Ma)and C2 (14.36–14.02 Ma) are separated by unclustered sample(14.42 Ma), and C2 and C3 (13.64–12.96 Ma) are separated by aninterval of poorly clustered or unclustered samples (13.97–13.69 Ma).On the DCCA ordination (Fig. 11), C1 and C3 plot quite separately withC2 in between. There was a clear shift in faunal composition with agefrom right to left on the ordination with the largest shifts occurringbetween the clusters at ~14.4, 14.0 and 13.7 Ma (Fig. 10).

The Ext. Gp faunal shifts from C1 through C2 to C3 were largelyprogressive changes in the relative abundance of some of the commonspecies, rather than wholesale changes in species composition(Fig. 12). The single dominant species in C1, S. spinata (mean rel. ab.44%) declined to co-dominance with S. lepidula (each 20-26%) in C3(Figs. 4 and 12). Also declining in relative abundance through timewere Orthomorphina perversa, Pleurostomella acuta, Strictocostellamodesta and S. pseudoscripta. Increasing in relative abundance fromC1 to C3 were S. lepidula, and many species of Pleurostomella(alazanensis, bolivinoides, brevis, and subnodosa)(Figs. 4 and 12). Thefauna of C2 was generally halfway between C1 and C3 in composition(Fig. 12), except that it contained peak relative abundances of P. tenuisand P. elliptica.

Accompanying the faunal changes through the middle Miocene inthis site were overall increases in oxygen isotope values, magnetic

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enth

icfo

ram

inif

era

Stilostomellidae

Site 1237

Site 1146

Site 575

Site 574

Site 573

0

10

20

30

40

12.5 13 13.5 14 14.5 15 15.5

Millions of Years Ago

Extinction Group

Fig. 9. Stratigraphic record of the relative abundance of the families Stilosotmellidae,Pleurostomellidae, uniserial Nodosariidae (includes extinct and extant species) and thetotal Extinction Gropu at subtropical Pacific Ocean sites 1146, 1237 [this study, as wellas equatorial Pacific sites 575, 574, 573 from (Thomas, 1985)].

Fig. 10. Constrained cluster analysis dendrogram generated for Site 1146 using chorddissimilarity coefficient and the data set comprising relative abundances of specieswithin the Ext. Gp of benthic foraminifera. Clusters (C1–C3) have been selected by uswith guidance from the DCCA ordination (Fig. 11). Arrows indicate significant changesin fauna with the ages next to them in millions of years. Samples are labeled by age inmillions of years and Site (C=1146).

58 K. Johnson et al. / Marine Micropaleontology 78 (2011) 50–64

susceptibility and carbonate percentage, and decreases in carbonisotopes and sedimentation rates (Figs. 2 and 3). Hardly exhibitingany significant change through time were reflectivity, and thefluctuating accumulation rates (specimens cm−2 kyr−1) of the Ext.Gp and total benthic foraminiferal tests, and the Ext. Gp% of the totalbenthic foraminifera (Figs. 2 and 10).

4.4. Extinction Group changes in Site 1237

On the constrained cluster analysis dendrogram for Site 1237(Fig. 13), we recognize three clusters (P1–P3). Clusters P1 (14.98–14.33 Ma) and P2 (14.20–14.07 Ma) are separated by one unclusteredsample (14.26 Ma). P2 and P3 (13.76–12.99 Ma) are separated by aninterval of poorly clustered samples (14.01–13.82 Ma). As in Site1146, clusters 1 and 3 plot quite separately on the DCCA ordination(Fig. 14), with cluster 2 lying between. Therewas a clear shift in faunalcomposition with age from right to left on the ordination with thelargest shifts occurring between the clusters at ~14.3, 14.0 and13.8 Ma (Fig. 13).

The changes through time in the Ext. Gp fauna at this site weremore substantial than in Site 1146. From P1 to P3, two of the co-dominant species (Strictocostella matanzana and S. spinata) disap-peared and there were declines in the relative abundance of P. acutaand P. bolivinoides (Figs. 5 and 12). Replacing these species wasincreased relative abundance of S. lepidula, which became the singledominant in C3 and increases in P. elliptica and P. subnodosa. The

transition cluster, P2, while being partly intermediate in compositionbetween P1 and P3, also had some distinctive features such as thehighest relative abundance of Cribronodosaria, Orthomorphina and S.subspinosa (Fig. 12).

Accompanying the faunal changes through the middle Miocene inthis site were overall increases in oxygen isotope values, sedimenta-tion rate, reflectivity and the accumulation rate of Ext. Gp and totalbenthic foraminiferal tests, along with decreases in carbon isotopesand the Ext Gp.% of the total benthic foraminiferal fauna (Figs. 2 and14). Magnetic susceptibility and carbonate % exhibited no significanttrends through time.

Fig. 11. Two-dimensional DCCA ordination of samples (given as age in Myrs) and common Ext. Gp species (abbreviations in Fig. 8 from the middle Miocene of Site 1146 based on therelative abundance of species within the Ext. Gp. Sample clusters from the constrained cluster analysis dendrogram (Fig. 10) are circled. Abbreviations for environmental proxy labelson vector arrows are given in Table 2. Lower spidergram tracks faunal changes through time on the ordination.

59K. Johnson et al. / Marine Micropaleontology 78 (2011) 50–64

5. Interpretation

5.1. Impact of the MCT on the biodiversity and relative abundance of theExtinction Group

All Ext. Gp species recorded in this study have global firstoccurrence records prior to the middle Miocene (Hayward et al.,2010a,b). Thus there are no evolutionary speciation events in thestudied middle Miocene interval at these two sites. The majority ofExt. Gp species became extinct during the early and middlePleistocene, but a few appear to have disappeared earlier (Haywardet al., 2007, 2010a,b). Four of the Ext. Gp species in this study(Cribronodosaria laeve, Nodosarella cf. balearica, Pleurostomella win-tereri, Strictocostella pseudoscripta) currently have known highestglobal occurrence levels in the middle Miocene. Three of these havetime ranges extending right through our studied interval in one orboth of our two sites (see online Appendix B), with only the firsthaving its highest occurrence in this study and globally during theMCT (at 13.97 Ma in Site 1146). Therefore there is evidence of perhapsonly one species becoming extinct during this middleMiocene coolingevent, and it was not an interval of elevated species turnover incontrast to the 76 species that disappeared during the MPT glacialcoolings (Hayward et al., 2007).

The global relative abundance of the Ext. Gp through the Cenozoic,as revealed in nine deep-sea sites (Thomas, 2007; Hayward et al.,2010b), increased through the Paleocene and early Eocene Green-houseWorld to a peak of up to 60%–70% during the warmmiddle-lateEocene. This was followed by a gradual decline in Ext. Gp relative

abundance that paralleled the Earth's cooling and culminated in theextinction of this group of benthic foraminifera during the coldestglacials of the MPT (Hayward et al., 2007; Kawagata et al., 2007). Inour two study sites there was no significant decline in the Ext. Gp'srelative abundance during the major cooling of the MCT (14.2–13.8 Ma). There was, however, a significant decline at Site 1237between 13.6 and 13.0 Ma, soon after the MCT. This decline coincidedwith a significant increase in the accumulation rate of both the Ext. Gpand total benthic foraminifera at this site, although the latter group'sincrease was greater (Fig. 2). Studies of total benthic foraminiferalfaunal changes through the middle Miocene have been interpreted asindicating increased seasonality of food supply to the deep-sea floor(e.g. Thomas, 2007). Therefore, this change may indicae that Ext. Gp.species were out-competed by opportunistic foraminifera duringtimes of increased, possibly seasonally episodic food supply.

5.2. Causes of Extinction Group differences between sites

A number of previous studies in the late Pliocene-early Pleistocene(e.g., Kawagata et al., 2005, 2006; Hayward et al., 2006; O'Neill et al.,2007) have shown a strong positive correlation between absoluteabundance and accumulation rate of the Ext. Gp with a group of otherelongate benthic foraminifera (e.g. Bolivina, Bulimina, Globobulimina,Trifarina, Uvigerina) that are considered to be a good proxy for highorganic carbon food supply with low oxygen bottom conditions. If thisrelationship is applied to the middle Miocene in the SubtropicalPacific we could infer that the higher abundances of the Ext. Gp in Site1146 were due to greater food supply and less bottom oxygen than at

Fig. 12. Mean relative abundance and standard deviation of the common Ext. Gp species (N2% in any cluster) in each of the three clusters recognized on the constrained clusteranalysis dendrograms for Sites 1146 and 1237 (Figs. 10 and 13).

60 K. Johnson et al. / Marine Micropaleontology 78 (2011) 50–64

Site 1237, especially after 14.7 Ma when deep-water ventilationincreased at Site 1237 (Holbourn et al., 2007). This explanation iscompatible with the greater proximity of the South China Sea Site1146 to land and its shallower water depth (2100 m) than SoutheastPacific Site 1237 (3200 m). The rain of organic food flux to the seafloorgenerally decreases with increasing depth (Berger and Wefer, 1990).The δ13C gradient between the two sites intensified between 14.7-13.9 Ma, due to regional productivity and ventilation differencesbetween the sites. After 13.9 Ma, the δ13C was consistently offset by~0.5 ‰ between the two sites with Site 1146 having lower values,reflecting the higher regional paleoproductivity (Holbourn et al.,2007). Deviations to lower δ13C values occurred at Site 1146 from14.3–14.1 Ma (Holbourn et al., 2007). Holbourn et al. (2007) hassuggested the deviation reflects the presence of a northern sourceintermediate-water at Site 1146 during this time. The carbon isotopesignal from Site 1237 is probably a better reflection of global carbonreservoir changes than that from Site 1146, because of its pelagicsetting and greater proximity to the source of deep-water formationin the Southern Ocean (Holbourn et al., 2007). Reduced carbonatedissolution occurs at Site 1237 during middle Miocene cool phases,which may suggest enhanced production of well-ventilated deepwater and the decreased influence of corrosive deep water from anorthern source (Holbourn et al., 2007). Carbonate dissolution wasinvestigated in detail by Holbourn et al. (2005, 2007) performed X-rayfluorescence measurements on the archive halves of core from Site1237; Fe maxima were interpreted as intervals of increased carbonatedissolution caused by poor deep water ventilation. They describedeccentricity-paced carbonate dissolution cycles during warmer inter-vals, as revealed by δ18O, Fe, and coarse fraction data (Holbourn et al.,2005, 2007). The Miocene dissolution cycles during warm intervalsare likely due to reduced production of Southern Ocean deep waterand increased corrosive deep-water from the north (Holbourn et al.,2007).

The reasons for the inter-site differences in the faunal compo-sition of the Ext. Gp are not so well documented, but may also beexplained by lower food flux and greater ventilation at the deeper,southeast Pacific site (1237). Global studies on the geographic andbathymetric distribution of the Ext. Gp in the Late Pliocene-MiddlePleistocene (e.g. Hayward et al., 2007) have shown that nodosariidmembers (esp. Mucronina) were more species-rich at shallowerbathyal depths than abyssal and that the reverse was true for the

pleurostomellids. This pattern was also true in the Subtropical Pacificin the middle Miocene (Table 3). At the species level in the latePliocene-middle Pleistocene, the common species P. tenuis and S.lepidula had extremely broad bathymetric tolerances which are alsoevident in this middle Miocene study. Two dominant species (P.subnodosa, S. matanzana) that are far more abundant in our deepermiddle Miocene site (Site 1237) and one of the dominant species (S.scharbergana) that is far more abundant in our shallower middleMiocene site (Site 1146) had similar depth preferences in later times(e.g. Hayward et al., 2007). Two of the most commonmiddle Miocenespecies however, S. spinata (shallower site) and S. subspinosa (deepersite) declined dramatically in abundance after the Miocene and wererare to absent globally by the late Pliocene and Pleistocene (Haywardet al., 2007), but were presumably also strongly influenced by depth-related differences in food supply.

5.3. Causes of Extinction Group faunal changes in the Middle Miocene

At both sites, the strongest vectors (including age) on the DCCAordinations (Figs. 11 and 14) are parallel or subparallel to axis 1 withthe oldest samples consistently plotted on the right and the youngestconsistently plotting on the left. Thus the major faunal changes in thestudied interval (15–13 Ma) in the middle Miocene occur throughtime and are unrelated to any shorter term, maybe climate-related,cycles. In both sites the other consistently strong vector, in addition toage, that parallels axis 1 is the overall increase in δ18O values throughtime, reflecting the major oxygen isotope shift, Mi-3 (Miller et al.,1991), during the middle Miocene climate transition (MCT) at 14.2–13.8 Ma.

Both sites show a similar pattern on the ordination spidergramsthat track the Ext. Gp faunal changes through time (Figs. 11 and 14).Each site has two pre-MCT faunal clusters (C1–C2, P1–P2, on theright) having distinctly different compositions from the one post-MCTcluster (C3, P3, on the left). The pre- and post-MCT faunas wereseparated by an interval of unclustered or weakly clustered samplesthat coincided with the younger part of theMCT (14–13.7 Ma), duringwhich the faunal composition swings back and forth between the twocompositions on the DCCA ordination spidergrams (Figs. 11 and 14).The subdivision of the pre-MCT faunas at both sites into two clusterscould be interpreted as being produced by the start of the faunalchanges that occurred across theMCT at 14.4 Ma in Site 1146 (onset of

Fig. 13. Constrained cluster analysis dendrogram generated for Site 1237 using chorddissimilarity coefficient and the data set comprising relative abundances of specieswithin the Ext. Gp of benthic foraminifera. Clusters (P1–P3) have been selected by uswith guidance from the DCCA ordination (Fig. 14). Samples are labeled by age inmillions of years and Site (P=1237).

61K. Johnson et al. / Marine Micropaleontology 78 (2011) 50–64

C2) and at 14.3 Ma in Site 1237 (onset of P2). A more stable post-MCTfaunal composition (C3, P3)was established at both sites by ~13.7 Ma.

The changes in faunal composition of the Ext. Gp that occurredacross the MCT were more marked in the middle abyssal, southeastPacific Site 1237 than in the upper abyssal, South China Sea Site 1146.Despite the considerable differences in their faunas, two of thedominant species at each site show parallel changes in relativeabundances across the MCT—S. lepidula increased and S. spinata

decreased. As all species within the Ext. Gp are no longer living, wehave no modern data that might suggest what changes in theirseafloor environments on either side of the Subtropical Pacificproduced the observed changes in dominance and composition atthis time. To find clues that might help explain the faunal changes weexamine our proxy environmental data (Fig. 2).

In the Site 1146 DCCA (Fig. 11), the length and direction of theproxy vectors suggest that the terrigeneous component (magneticsusceptibility) of the sediment increased across the MCT (r=-0.65,p=1.95×10−5 with age, Appendix C) while linear sedimentationrate (r=0.4, p=.009), probably the biocarbonate component (BFAR,reflectivity) declined. Increased terrigenous input to the South ChinaSea during this period was probably due to higher sediment inputfrom rivers, as well as dust from central Asia. Evidence indicates thatdeserts within central Asia were large enough by the Miocene(~22 Ma) to produce loess sequences (Guo et al., 2002). Guo et al.(2002) also asserted that there were periods of high dust accumu-lation from 15–13 Ma, which may indicate temporary instabilities ofclimate or intensification of aridification in the dust source regions ofcentral Asia. Other studies support the enhancement of aridity andEast Asian monsoon activity during the period 15–13 Ma (Ramsteinet al., 1997; Clift et al., 2002; Guo et al., 2002; Jia et al., 2003; Spiceret al., 2003; Sun and Wang, 2005; Harris, 2006). In this case, thegreater inputs of terrigenous sediment may decrease the photic zoneand thus phytoplankton productivity. This could have resulted in adecrease or change in character of the food supply to the seafloor,which could have caused the subtle Ext. Gp changes.

In the Site 1237 DCCA (Fig. 14) the proxy vectors indicatesignificant increases across the MCT in sedimentation rate, reflectivityand accumulation rates of the Ext. Gp and total benthic foraminiferaltests (rN0.6, pb .0003with age, Appendix C) withminimal decrease inthe terrigeneous component (magnetic susceptibility). These changesclearly suggest increased benthic foraminiferal and phytoplanktonproductivity through this part of the middle Miocene—the opposite ofthe conclusion for the South China Sea. The increased sedimentationrates (mainly bioclastic) were most likely due to strengthening oftrade winds, cooling of waters, and increased nutrient upwellingthrough this interval. However, a decrease in dissolution due toimproved bottom water ventilation (CCD drop) may play a role in theincreased sedimentation rates, as well ( Holbourn et al., 2005, 2007).

Examination of the correlation coefficients (Table 3) for the fourmost common species with significant changes in relative abundanceacross the MCT in Site 1237 show that all (P. subnodosa, S. lepidula, S.matanzana, S. spinata) are significantly correlated with the oxygenisotopes and accumulation rate of the Ext. Gp. The disappearance of S.spinata through the MCT is significantly correlated (r=−0.6,p=0.0006) with an increase in the absolute abundance of tests ofthe benthic foraminiferal genus Cibicides. Increasing absolute abun-dance of Cibicides may indicate seafloor conditions with highervelocity bottom currents; species in this genus are epifaunal andgenerally prefer conditions with some current action that winnowsaway mud (Mackensen et al., 1985; Corliss, 1985). Increased bottomcurrent strength would result in increased ventilation, whereasincreased productivity usually causes decreased ventilation. En-hanced production of well-ventilated deep-water, cooling associatedwith the MCT, and decreased amounts of corrosive northern com-ponent bottomwater at this location, as supported by proxy evidence,may have played a role in the changing Ext. Gp population structure(Holbourn et al., 2005, 2007). Maybe it was changes in the characterand amount of food supply together with ventilation changes thatcaused the significant Ext. Gp faunal changes across the MCT.

Our rationale for choosing to study the interval from 15–13 Mawas to investigate what changes might have occurred within the Ext.Gp population structure, especially across the large oxygen isotopeshift from 13.91–13.84 Ma (Mi-3 of Miller et al., 1991). At both sites,we found significant shifts in population structure at 14.4–14.3 Ma,

Fig. 14. Two-dimensional DCCA ordination of samples (given as age in Ma) and common Ext. Gp species (abbreviations in Fig. 8) from the Middle Miocene of Site 1237 based on therelative abundance of species within the Ext. Gp. Sample clusters from the constrained cluster analysis dendrogram (Fig. 13) are circled. Abbreviations for environmental proxy labelson vector arrows are given in Table 2. Lower spidergram tracks faunal changes through time on the ordination.

62 K. Johnson et al. / Marine Micropaleontology 78 (2011) 50–64

14.0 Ma, and 13.7 Ma. Environmental change most likely explains thereorganization of the seafloor communities at these specific times.

At both sites, the carbon isotope signal exhibits a cyclic sawtoothpattern prior to the rapid and large positive shift in both carbon andoxygen isotope values at 13.91–13.84 Ma (Holbourn et al., 2007). Thecyclicity of these major trends in the carbon isotope record was400 kyr (Holbourn et al., 2007). Changes in carbon cycling are clearlyimportant to understanding climate evolution during the Mioceneand its impact on the deep-sea biota.

During the period 14.4–14.3 Ma, Site 1146 witnessed negativedeviations in the δ13C record, as well as increased reflectance values,carbonate weight percentages and lower terrigenous input (Fig. 2).We interpret this result as a time of increased paleoproductivity atthis site, which could have forced a change in population structure.This is supported by increased values of BFAR and Ext.GpAR (Fig. 2).However, it has also been postulated that the negative deviations inthe δ13C record could be due to incursions of northern sourcewater, aswell (Holbourn et al., 2007).

At Site 1237, there was an increase in carbon-isotope values,reflectance and carbonate-weight percentages between 14.4–14.3 Ma(Fig. 2). There is also a peak in themagnetic susceptibility at 14.36 Ma,signalling eolian hematite flux to the site. Site 1237 saw a slightincrease in BFARs and Ext.GpARs during the period 14.4–14.3 Ma, alsoreflecting a time of increased paleoproductivity and food supply in thesoutheast Pacific that could have impacted the community structureof the Ext. Gp (Fig. 2).

Population structure changes at 14.0 Ma and 13.7 Ma at both sitesare perhaps easier to understand, because they are on either side ofthe large positive oxygen and carbon excursions at 13.91–13.84 Ma.

Some expansion of the East Antarctic Ice Sheet (EAIS) is believed tohave occurred before the large oxygen isotope shift (13.91-13.84 Ma)(Holbourn et al., 2007; Shevenell et al., 2008). However, there was arapid increase in ice volume between 13.87–13.84 Ma due to inferredlow summer insolation over Antarctic and declining carbon dioxidelevels (Holbourn et al., 2005). The most striking feature of the middleMiocene δ13C curve is the positive excursion beginning at 13.92 Ma.The peak maximum was centered at ~13.7 Ma, which coincided withour Ext. Gp faunal changes within the subtropical Pacific. This peakrepresents the final and most pronounced δ13C increase and pCO2

decrease within the “Monterey Excursion” (Holbourn et al., 2005).This peak was coincident with inferred maximum ice volume andslight warming in surfacewaters at high southern latitudes (Shevenellet al., 2004).

A confluence of climatic forcing factors appears to have lead to achange in boundary conditions, which in turn caused the middleMiocene expansion of the EAIS and global cooling (Holbourn et al.,2005). Some of the factors contributing to this change in boundaryconditions included: increased moisture and poleward heat supply,changing orbital pacing, and fluctuations in atmospheric CO2 (Holbournet al., 2005; Shevenell et al., 2008). These resulting climatic changes, inturn, affected the paleoenvironment of the Ext. Gp communities.

6. Conclusion

1. Was there increased faunal turnover (especially extinctions)during the rapid cooling of the MCT, similar to that during theMPT glacial periods? If so, how significant was it?

63K. Johnson et al. / Marine Micropaleontology 78 (2011) 50–64

• No new Ext. Gp species appeared and one (Cribronodosaria laeve)appears to have become extinct during the MCT, in contrast tothe global extinction of 76 Ext. Gp species during the MPT.

2. Were the Ext. Gp faunas similar on either side of the subtropicalPacific during the middle Miocene? If not, what were the likelyreasons for the differences?• The middle Miocene Ext. Gp faunas in Sites 1146 and 1237 showa greater difference in composition between the two sites oneither side of the Subtropical Pacific, than they do through timeat either site. The inter-site faunal differences were probably dueto a lower food supply in the southeast Pacific Site 1237, whichmay have been partly due to lower nutrient supply to the surfacewaters and also to the greater depth of the site.

• We infer that MCT changes in the Ext. Gp faunas were a result ofdecreased productivity and food supply to the seafloor in SouthChina Sea Site 1146 and to increased productivity and foodsupply in the southeast Pacific Site 1237.

3. Were the faunal composition and/or population structure of theExt. Gp impacted by the MCT? If so, what were the changes, andwere there differences in the responses between the sites?• There was no significant change in the relative abundance of theExt. Gp across theMCT at either site, in contrast to its progressivedecline from peak occurrences in the warm Eocene (up to 60%–70% of benthic foraminifera) to its disappearance during theglacials of the MPT.

• At both sites the composition of the Ext. Gp faunas changedacross the MCT, with far more dramatic changes in the relativeabundances of the dominant species in the southeast Pacific thanSouth China Sea.

• At both sites the pre-MCT faunas were transformed into theirpost-MCT composition during the period 14.0–13.7 Ma. Duringthis transition interval the faunal composition fluctuated backand forth between the two end member faunas.

4. Does the extensive environmental proxy data from the two studysites provide further insights into the causes of the documentedchanges in the Ext. Gp through this MCT interval?

• Overall, environmental changes associated with the major globalcooling during the MCT did not impact the Ext. Gp to anywherenear the extent that would have been predicted based on thegroup's record in theMPT. This implies that whatever the changeswere that caused the Last Global extinction in the deep sea, theydid not occur orwere ineffective 13 myrs earlier (during theMCT).

Supplementarymaterials related to this article can be found onlineat doi:10.1016/j.marmicro.2010.09.008.

Acknowledgements

We thank the Ocean Drilling Program (ODP) for providing samplesfor this study. ODP is sponsored by the US National ScienceFoundation and participating countries under the management ofthe Joint Oceanographic Institutions. We are grateful to Ashwaq Sabaaand Hugh Grenfell for help and advice during the course of this studyand for suggesting improvements to the manuscript. We thankDrs. Ellen Thomas, Christopher Smart, and Akira Tsujimoto for theirthorough reviews. This study was funded by the Marsden Fund of theRoyal Society of New Zealand.

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