Antarctic glaciation recorded in Early Miocene New Zealand foraminifera

Marine Micropaleontology 92–93 (2012) 52–60

Contents lists available at SciVerse ScienceDirect

Marine Micropaleontology journal homepage: www.elsevier.com/locate/marmicro

Antarctic glaciation recorded in Early Miocene New Zealand foraminifera Kenichi Fukuda a, 1, Daniel B. Thomas b, 2, Russell D. Frew a, R. Ewan Fordyce b,⁎ a b

Department of Chemistry, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand Department of Geology, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand

a r t i c l e

i n f o

Article history: Received 22 December 2010 Received in revised form 9 May 2012 Accepted 12 May 2012 Available online 29 May 2012 Keywords: foraminifera geochemistry Mg/Ca thermometer palaeoclimate palaeotemperature

a b s t r a c t New Zealand sedimentary sequences are important repositories of southern temperate palaeoenvironmental data, as may be interpreted from biogenic chemical signals preserved in marine microfossils. Calibration curves for Mg/Ca ratios versus water temperatures were established using modern benthic foraminifera, Notorotalia and Cibicides. Notorotalia is a long-ranged endemic benthic genus with a good record in shelf sediments, while Cibicides allows comparisons with similar studies elsewhere. The resulting correlations were T (°C) = ln(Mg/Ca [mmol/mol] / 1.64) × 10.89 for Cibicides spp., and T (°C) = ln(Mg/Ca [mmol/mol] / 0.44) × 5.71 for Notorotalia spp. Well-preserved Early Miocene Notorotalia and Cibicides were collected for paired Mg/Ca and δ 18O analysis from a 3.6 m section of the Mount Harris Formation spanning an estimated 60 ka and dating from about 17.7 Ma (Globoconella zealandica zone, roughly middle Burdigalian, Early Miocene) within the local Altonian Stage (15.9–18.7 Ma). Mg/Ca bottom-water palaeotemperature estimates from Cibicides and Notorotalia gave concordant results: 13.3 ± 1.0 °C for Notorotalia spinosa, 15.5 ± 3.0 °C for Cibicides spp. Estimates of oxygen isotopic composition for Altonian sea water (δ18Opalaeo-sw) were − 0.4 ± 0.4‰, suggesting the presence of small ice sheets on Antarctica. The method used to generate such results has far reaching implications for reconstructing δ 18Opalaeo-sw, and should allow Antarctic ice volume history to be finely resolved from New Zealand sequences. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Oxygen isotope measurements from calcareous marine invertebrates (δ18Ocarbonate, hereafter δ 18Oc) have been widely employed as a proxy for palaeotemperature (e.g. Miller et al., 1987; Billups and Schrag, 2002; Hollis et al., 2009). The δ 18Oc depends on the water temperature during carbonate formation and also on the δ 18O of the water in which the carbonate was formed. The δ 18Oc composition of marine invertebrates is dependent on salinity, and hence global ice volume, and currently binds reliable palaeotemperature estimates to independent estimates of polar ice volume (e.g. Shackleton, 1967). The dual signals of salinity and ambient temperature contained as δ18Oc within biogenic carbonates have historically proven inseparable without independent proxies (e.g. alkenone saturation, faunal abundance; Elderfield and Ganssen, 2000). One recently-favoured approach for establishing the ambient temperature of growth in foraminifera is the assessment

⁎ Corresponding author. Tel.: + 64 3 479 7510; fax: +64 3 479 7527. E-mail addresses: [email protected] (K. Fukuda), [email protected] (D.B. Thomas), [email protected] (R.D. Frew), [email protected] (R.E. Fordyce). 1 Present address: Nakashima Propeller Co. Ltd., Development Group, R&D Division, Okayama, Japan. 2 Present address: Department of Vertebrate Zoology, MRC-116, National Museum of Natural History, Smithsonian Institution, P.O. Box 37012, Washington, D.C. 20013, USA. 0377-8398/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.marmicro.2012.05.002

of the Mg/Ca ratio within test carbonate: incorporation of Mg into the biogenic calcite lattice is influenced by ambient temperature during calcification (Nürnberg et al., 1996; Rosenthal et al., 2000). The relationship between Mg concentration of a foraminiferal test and temperature is not universal across all species (Toyofuku et al., 2000; Dissard et al., 2010), thus requiring interpretations of fossil data to be grounded by studies of modern representatives of particular fossil lineages. Many of the extant foraminifera from modern-day New Zealand have a significant fossil record (e.g. Hornibrook et al., 1989). The Cenozoic biostratigraphy of New Zealand is richly resolved (Hornibrook et al., 1989; Morgans et al., 1999; Graham et al., 2000; Cooper, 2004), with excellent documentation of foraminiferal localities. The New Zealand Cenozoic marine record, which accumulated in mid-temperate southern latitudes, is readily accessible in onland outcrops of thin Palaeogene strata that accumulated in a passive margin setting, and much thicker Neogene strata that reflect development of an active plate boundary. Due to such attributes, New Zealand amongst southern landmasses is a Rosetta stone for understanding Southern Hemisphere and Antarctic-related climate signals (Zachos et al., 2008; Field et al., 2009) resulting from the breakup of Gondwana. As elsewhere, interpretation of the New Zealand palaeoclimate record would be greatly assisted by the ability to tease temperature from salinity signals stored in calcareous microfossils. Here we evaluate the efficacy of recovering temperature and salinity signals from modern and related fossil foraminifera using a paired Mg/Ca and δ 18Oc approach.

K. Fukuda et al. / Marine Micropaleontology 92–93 (2012) 52–60

2. Materials and methods 2.1. Geologic setting Modern foraminifera were collected from South Pacific and Southern Ocean sediments (localities detailed in Section 3.1; Fig. 1). Fossil foraminifera were extracted from an outcrop of the Mount Harris Formation at Pukeuri, North Otago, New Zealand (44°46′S, 171°01′E, or NZMS 260 reference J41/544733; Fig. 1). The Mount Harris Formation is a soft, massive, light yellow-brown calcareous sandy siltstone to silty sandstone that accumulated in a quiet setting below the storm wave base, probably on the outer shelf (discussed below). All samples collected for the present study included the planktonic foraminifera Globoconella zealandica, a zonal species with a first appearance datum in Chron C5En (18.06–18.52 Ma; Morgans et al., 2002; Gradstein et al., 2004) that marks the middle of the New Zealand Altonian Stage (Early Miocene, 15.9–18.7 Ma: Hollis et al., 2010). The Altonian Stage spans the later Burdigalian and earlier Langhian Stages (Cooper, 2004). Further, two Lentipecten sp. (scallop — Pectinidae) shell samples within the Pukeuri sequence had 87Sr/86Sr compositions of 0.708625±0.000011 (shell 10 cm immediately below foraminiferal sample 4) and 0.708633±0.000010 (shell level with foraminiferal sample 7, about 50 cm along bedding plane) (D.A. Teagle and M. Cooper, pers. comm.), corresponding to 17.74 Ma and 17.65 Ma, respectively, and each rounding to 17.7 Ma (McArthur et al., 2001). 2.2. Samples Modern benthic foraminifera specimens representing tropical to subpolar latitudes were obtained from sediments within Fijian and New Zealand waters (Fig. 1). Specimens of Cibicides were collected from Kadavu Passage (18°11′S, 178°25′E, 135 m below sea level, BSL), Three Kings Plateau (34°20′S, 172°30′E, 100 m BSL), Challenger Plateau (42°14′S, 169°30′E, 1125 m BSL), Taieri Bight, Otago (46°10′S, 170°23′E, 66 m BSL), Enderby Island (50°50′S, 166°30′E, 115 m BSL) and Campbell Island (52°32′S, 169°10′E, 15 m BSL). Samples of Notorotalia were collected from Three Kings Plateau (34°20′S, 172°30′E, 100 m BSL), two sites within Blueskin Bay, Otago (45°44′S, 170°41′E, 20 m BSL and 45°46′S, 171°05′E, 207 m BSL), Enderby Island (50°30′S, 166°19′E, 2–3 m BSL) and Auckland Island (50°32′S, 166°12′E, 2–3 m BSL). Kadavu Passage samples were collected during the 2nd Joint National Fisheries University–University of the South Pacific Fisheries and Oceanography Research

53

Cruise (3–7 December 1997) by the Koyo-Maru (commercial vessel) using a Van Veen grab sampler. Otago Shelf samples were collected as grab samples by the R/V Munida (19 September 2001) (University of Otago research vessel) and a 6 m alloy runabout (05 November 2003). Samples from Three Kings and Challenger Plateaus were collected by the R/V Tangaroa (20 June 1978) (NIWA National Institute of Water and Atmospheric Research Ltd. research vessel) and Westbay (6–10 November 1995) (commercial deep-sea trawler) using an anchor dredge. Shallow water samples from Auckland, Campbell and Enderby Islands were provided by J. Guise (University of Canterbury), and were supplemented by samples collected in the early 1900s and housed at Canterbury Museum, New Zealand (Chapman, 1909). Sea surface temperatures (SST) or bottom water temperatures (BWT) for the Kadavu Passage, Challenger Plateau, Otago Shelf and Auckland Island sites were collated from earlier studies (Heath, 1984; Pickering and Suda, 2003; Van Hale, 2003). Temperature for the Three Kings Plateau sample site was estimated from oxygen isotopes derived from carbonate (unpublished thesis: Fukuda, 2002), with data for the Campbell and Enderby Island sites taken from the National Institute of Water and Atmosphere (NIWA) archive. Fossil foraminifera were collected from a 3.6 m thick sequence of the Mount Harris Formation, with 2 cm slices taken at 33 cm intervals (12 foraminiferal sample horizons and 1 Sr sample horizon, numbered as J40/f 278 to J40/f 290 in the Geological Society of New Zealand Fossil Record File). The Mount Harris Formation at the Pukeuri locality yields wellpreserved foraminifera, commonly with translucent (glassy) tests that retain micron-level detail under scanning electron microscopy, with promise of limited diagenetic alteration. Based on sedimentation rates discussed for the Mount Harris Formation (62.5 m Ma− 1; e.g. Morgans et al., 1999, using the name Bluecliffs Silt), the sampled interval at Pukeuri is expected to span approximately 60 ka. The present study provides a detailed description of a thin sequence spanning a 60 ka interval about 17.7 Ma ago. 2.3. Modern sample extraction Decanted oceanic sediments containing modern foraminifera were saturated with Rose Bengal stain (BDH Chemicals) solution (2 g L− 1 ethanol) for 12 h to distinguish between recent living and relict foraminifera (Walton, 1952). Samples were thoroughly dried at 40 °C, allowing pink (protoplasm-stained) specimens of Cibicides corticatus, Cibicides dispars,

Fig. 1. Locality map for specimens collected and analysed in this study. A) Modern samples were collected from Fiji (Fi), Three Kings Plateau (Th), Challenger Plateau (Ch), Otago Shelf (Ot) and Auckland (Au), Enderby (En) and Campbell (Ca) Islands. Fossil samples were collected from Pukeuri (Pu), North Otago, New Zealand. B) Pukeuri is currently located north of Oamaru at 45.0°S, and was located at 43.3°S during the middle of the local Altonian Stage (15.9–18.7 Ma) (King, 2000).

54

K. Fukuda et al. / Marine Micropaleontology 92–93 (2012) 52–60

Cibicides marlboroughensis, Notorotalia depressa, Notorotalia finlayi, Notorotalia hornibrooki, Notorotalia inornata, Notorotalia olsoni and Notorotalia zealandica to be collected and stored for later analysis.

generally sufficed for analysis, but 2 specimens of Cibicides spp. were required to quantify Mg. Mg/Ca ratios were calculated using only data with a precision (relative standard deviation) better than 5%.

2.4. Fossil extraction The Mount Harris Formation sediment samples (~100–300 g) were dried at 60 °C overnight, weighed, and soaked in a dispersion solution containing ~10 g analytical grade sodium hexametaphosphate (NaPO3)6 (BDH Chemicals) dissolved in ~500 ml distilled water. Sediment and dispersion solution were transferred to a stainless steel pot and diluted with excess distilled water before being boiled for 30 min. Disaggregated samples were then washed (distilled water) through a 63 μm stainless steel or brass sieve. Collected sediment (>63 μm fraction) was then dried at 60 °C overnight and subsequently weighed, allowing the mass of the eluted mud fraction to be determined. Well preserved (not-infilled, minimal surface alteration) Notorotalia spinosa and Cibicides spp. were picked from the >63 μm fraction and stored for later analysis. 2.5. Siliceous sand and carbonate weight percent Subsamples of the Mount Harris Formation sediment (~100–300 g) were dried, weighed and boiled prior to sieving to remove the mud fraction (following the fossil extraction method above). Samples were then oven dried (60 °C) for approximately 12 h before again being weighed, and were then soaked in excess 0.075 M HNO3 to dissolve carbonate. The residue was filtered, washed, dried and reweighed. Percentages of both siliceous sand and calcareous material were calculated relative to the initial dry weight of each subsample. 2.6. Planktonic fraction Between 50 and 100 g of the prepared >63 μm fraction of each Mount Harris Formation sediment sample was resieved using a 150 μm mesh (e.g. Morgans et al., 2002). The >150 μm fraction was then split repeatedly until a few hundred (desirably >100 and b300) complete foraminifera remained. All foraminifera were then picked from each sample using a Zeiss DR or SV11 binocular microscope, and were transferred to slides for counting and identification. 2.7. Mg/Ca measurement Prior to analysis, modern and fossil specimens were cleaned as follows. Individual specimens were crushed between cleaned glass plates, with ~30 μg of crushed sample transferred into a 0.5 ml plastic vial with doubly distilled water (Milli-Q®). Each sample was sonicated within the plastic vial for 1 min; supernatant water was then removed by vacuum pipette and replaced with Milli-Q®, before a second minute of sonication. Samples were rinsed with Milli-Q® and sonicated three times, followed by two sequences of rinse and sonication with methanol, and a final rinse and sonication with Milli-Q®. Approximately 2 ml alkaline peroxide solution (0.1% H2O2 in 0.1 M NaOH) were added to each sample, which were then boiled for 10 min, and sonicated for 30 s. Alkaline peroxide solution was replaced with Milli-Q® for a set of four rinses. Each sample was then subjected to a dilute acid leach (0.25 ml 0.001 M HNO3; 30 s with sonication) before two final rinses with Milli-Q®. The level of contamination and cleaning efficacy of each step was evaluated through concurrent analyses of a marble laboratory standard. To track the fidelity of foraminiferal carbonate, representative N. spinosa specimens were imaged before and after the acid cleaned glass plate step using a Cambridge 360 scanning electron microscope (SEM) (Department of Anatomy, University of Otago). Cleaned samples were dissolved in 0.4 ml 0.075 M HNO3. Mg/Ca ratios were determined using inductively coupled plasma optical emission spectrometry (ICP-OES, Thermo Jarrell Ash) (Department of Chemistry, University of Otago). Single specimens of Notorotalia

2.8. Stable isotope measurements Oxygen isotopic composition of test carbonate (δ18Oc) was established for modern Cibicides sp. and Notorotalia spp., as well as N. spinosa from the Mount Harris Formation. Samples were cleaned prior to analysis: specimens were transferred into plastic vials with Milli-Q® and sonicated for 1 min, followed by sonication in methanol. Samples were further sonicated in 12.5% NaClO to remove organic material and rinsed by being sonicated in Milli-Q®. Samples were then oven-dried (40 °C) before being crushed. Between 700 μg and 1 mg of crushed foraminiferal powder was reacted with 104% H3PO4 at 25 °C, with the evolved CO2 gas cryogenically-purified and analysed using a Europa Scientific 20–20 isotope ratio mass spectrometer in dual-inlet mode. Oxygen and carbon isotope values were reported relative to Vienna Peedee Belemnite (VPDB) standard via National Bureau of Standards (NBS) 19 and a laboratory standard (δ18O=−2.45‰). A correction to the oxygen isotopic composition of seawater (δ18Osw; −0.27‰) was used for converting Vienna Standard Mean Ocean Water (VSMOW) to VPDB (Hut, 1987). Palaeoseawater isotopic compositions (δ18Opalaeo-sw) are here reported relative to VSMOW. 3. Results 3.1. Mg/Ca measurements Mg/Ca ratios were measured in modern Notorotalia spp. from four sample localities (Table 1): four specimens from Enderby Island (2.1 ± 0.4 mmol/mol), five from Auckland Island (2.3 ± 0.6 mmol/mol), 16 from Otago Shelf (2.7 ± 0.3 mmol/mol) and 10 from Three Kings Plateau (6.7 ± 0.9 mmol/mol) (mean ± 95% confidence interval; online Supplementary material). Mg/Ca ratios were also measured in modern Cibicides spp. from six localities (Table 1): five from Challenger Plateau (2.1± 0.2 mmol/mol), five from Enderby Island (2.6± 0.28 mmol/ mol), four from Campbell Island (3.9± 1.3 mmol/mol), six from Otago Shelf (5.3± 0.4 mmol/mol), six from Three Kings Plateau (6.3 ± 1.3 mmol/mol) and five from Kadavu Passage (12.9 ± 1.3 mmol/mol) (mean± 95% confidence interval; online Supplementary material). The Mg/Ca ratios of 53 N. spinosa and 23 Cibicides sp. specimens from the Mount Harris Formation at Pukeuri, North Otago were also measured (Fig. 2; online Supplementary material). N. spinosa gave relatively consistent Mg/Ca ratios across all 12 sampling horizons, ranging between 2.5±0.2 mmol/mol (n=4, horizon 10) and 3.8± 0.2 mmol/mol (n =5, horizon 8), with an average for all sample horizons of 3.3±0.2 mmol/mol. Mg/Ca ratios from Cibicides spp. demonstrated considerable variation both within and between sample horizons, with an average of 5.0±1.1 mmol/mol from a range of 3.3±1.0 mmol/mol (n=2, horizon 2) to 8.4 (n=1, horizon 1) mmol/mol. Samples were cleaned prior to Mg/Ca analysis, and cleaning efficacy was checked using scanning electron microscope (SEM) images collected from fossil Notorotalia spp. before and after the weak acid leach. Images showed that most secondary carbonates were removed. The Mg/Ca ratio of a marble laboratory standard did not change with subsequent cleaning steps. Further, Rose Bengal stain did not alter the Mg/Ca ratio of foraminifera (unstained Notorotalia sp.=2.7 mmol/mol σ=0.3, n=4; stained Notorotalia sp.=2.63 mmol/mol, n=1). Elderfield et al. (2006) also noted that Rose Bengal stain does not affect the Mg/Ca ratio. 3.2. Stable isotope measurements Oxygen isotopic ratios (δ 18Oc) for modern Cibicides sp. and Notorotalia spp., as well as fossil N. spinosa, were determined in

K. Fukuda et al. / Marine Micropaleontology 92–93 (2012) 52–60

55

Table 1 Sample and locality data for modern Cibicides and Notorotalia. Locality

Latitude (°S)

Longitude (°E)

Species

Cibicides Challenger Plateau Challenger Plateau Challenger Plateau Challenger Plateau 16 km north of Enderby Is. Campbell Island Taieri Bight, Otago Three Kings Plateau Kadavu Passage, Fiji

42°14′ 42°14′ 42°14′ 42°14′ 50°50′ 52°32′ 46°10′ 34°20′ 18°11′

169°30′ 169°30′ 169°30′ 169°30′ 166°30′ 169°10′ 170°23′ 172°30′ 178°25′

Cibicides sp. C. corticans C. dispars C. marlboroughensis C. dispars C. dispars C. corticans C. marlboroughensis C. dispars

Notorotalia Enderby Island Auckland Island Otago Shelf Otago Shelf Three Kings Plateau Three Kings Plateau Three Kings Plateau

50°30′ 50°32′ 45°44′ 45°46′ 34°20′ 34°20′ 34°20′

166°19′ 166°12′ 170°41′ 171°05′ 172°30′ 172°30′ 172°30′

Notorotalia sp. Notorotalia sp. Notorotalia sp. N. zealandica N. hornibrooki N. inornata N. olsoni

combination with Mg/Ca analyses. Four Cibicides specimens from the Three Kings Plateau sampling site exhibited a mean δ 18Oc value of 0.6 ± 0.3‰ (mean ± 95% confidence interval; ‰VSMOW). Four specimens of Notorotalia sp. from the Blueskin Bay locality along the Otago Shelf had δ 18Oc values of 1.5 ± 0.2‰, and two Notorotalia sp. specimens from Auckland Island exhibited values of 1.38 ± 0.03‰. Early-stage diagenesis was assessed by calculating the oxygen isotopic composition of sea water (δ 18Osw) from ambient growth temperature and oxygen isotopic composition of test carbonate (δ 18Oc), and comparing the calculated and expected VSMOW values (e.g. Horibe and Oba, 1972). All δ 18Osw values were within the expected range for VSMOW. The oxygen isotopic compositions of N. spinosa from Mt. Harris Formation ranged from 0.2 ± 0.1‰ (n = 2, horizon 4) to 0.8‰ (n = 1, horizon 12), and averaged 0.5 ± 0.2‰ (n = 23, mean ± 95%

Fig. 2. Mg/Ca ratios for Cibicides and Notorotalia at 12 sample horizons from Pukeuri, North Otago, New Zealand (60 ka span around 17.7 Ma). Lentipecten sp. (scallop — Pectinidae) shell was used to establish the two absolute dates (Sr isotope measurements).

N

Depth (m)

Annual mean water temperature (°C)

1 1 2 1 4 4 7 6 5

1125 1125 1125 1125 115 15 66 100 135

4.7 4.7 4.7 4.7 5.5 7.5 10.5 15.0 23.0

4 5 10 6 2 1 7

2–3 2–3 20 207 100 100 100

8.5 8.5 11.6 10.1 15.0 15.0 15.0

confidence interval; ‰VSMOW) (Fig. 3; online Supplementary material). Analytical precisions (1 sd) determined by replicate analyses of a marble laboratory standard were ± 0.04‰. 3.3. Environmental indices Weight of sand and carbonate fractions within the Mount Harris Formation were established as percentages for the 12 sample horizons at Pukeuri, North Otago (Fig. 4). Sand weight percent (wt.%) ranged from 30 to 57 wt.% and averaged 40 ± 8 wt.% (Fig. 4). The amount of carbonate within the sediment was relatively constant, averaging 1.6 ± 0.9%. Sand fraction was predominantly angular quartz (very fine sand) with less abundant microfossils (foraminifera). The percent of planktonic foraminifera in the >150 μm fraction of each sample averaged about 30% and ranged between 25.7% and 36.1% (Fig. 4). G. zealandica, which is diagnostic for the middle Altonian stage (Morgans et al., 2002), occurred as large but rare specimens in all 12

Fig. 3. Oxygen isotopic composition of Notorotalia spinosa collected from 12 sample horizons at Pukeuri, North Otago, New Zealand (60 ka span around 17.7 Ma).

56

K. Fukuda et al. / Marine Micropaleontology 92–93 (2012) 52–60

Fig. 4. Summary of palaeoenvironmental proxies collected from 12 sample horizons at Pukeuri, North Otago, New Zealand (60 ka span around 17.7 Ma).

sediment samples (1–10% by number of the planktonics, or ~0.5–2.5% of total foraminifera). The persistence of a globorotaliid planktonic species suggests ongoing minor influx of deeper and/or more-oceanic waters. 3.4. Ocean temperatures Correlations between the Mg/Ca ratio and water temperature were established for modern species of Notorotalia and Cibicides (Fig. 5). Average annual temperatures for each collection locality were treated as ambient growth temperatures for foraminifera. Kadavu Passage bottom water temperature (BWT) had a seasonal average value of 23.0 °C (Pickering and Suda, 2003). Challenger Plateau BWT measured from sediment pore-water was 4.7 °C and is considered seasonally stable (Heath, 1984). Otago Shelf BWT ranged from 9.2 to 12.1 °C, with the Blueskin Bay locality averaging 10.1 °C and the Taieri Bight locality averaging 10.5 °C (unpublished thesis: Van Hale, 2003). Three Kings Plateau BWT was estimated to be 15.0 ± 0.9 °C from earlier oxygen isotope analyses of modern Cibicides sp. and Quinqueloculina sp. (unpublished thesis; Fukuda, 2002). Seasonally averaged BWT values for Auckland, Campbell and Enderby Islands were 8.5, 7.5 and 5.5 °C respectively (data from the National Institute of Water and Atmospheric Research, New Zealand (NIWA), Sea Surface Temperatures Archive).

details of both adjustments are provided in the online Supplementary material. Adjusting the calibration curves for an estimated seawater Mg/Ca composition at 17.7 Ma gives T (°C) = ln(Mg/Ca/ 1.614)× 10.89 for Cibicides spp. and T (°C)= ln(Mg/Ca / 0.313) × 5.71 for Notorotalia spp. When applied to foraminifera from the Mount Harris Formation, average bottom palaeotemperatures were calculated to be 13.3 ± 1.0 °C for N. spinosa and 15.5 ± 3.0 °C for Cibicides spp. (Fig. 6). 3.6. Ancient seawater composition The oxygen isotopic composition for palaeoseawater (δ18Opalaeo-sw) was estimated for each of the 12 Mount Harris Formation sample horizons from Mg/Ca temperature values and oxygen isotopic compositions of Notorotalia sp. tests (δ18Oc) (Fig. 7). The calculation followed the equation of Horibe and Oba (1972), rearranged to solve for seawater composition: δ18Opalaeo-sw =−((4.34+(−4.342 −(4×0.16×(17.0− T[°C]))0.5 /(2×0.16))+δ18Oc. Estimated oxygen isotopic compositions averaged − 0.4 ± 0.4‰ (VSMOW) within a range of − 0.6 ± 0.1‰ (VSMOW) (horizon 10) to 0.1 ± 0.1‰ (horizon 8; online Supplementary material).

3.5. Mg/Ca temperature calibration

4. Discussion

Calibration curves for predicting temperature from the Mg/Ca ratio were fitted as exponential functions (e.g. Rosenthal et al., 1997): T (°C)= ln(Mg/Ca [mmol/mol] / 1.64) × 10.89 for Cibicides spp., and T (°C)= ln(Mg/Ca [mmol/mol] / 0.44) × 5.71 for Notorotalia spp. (Fig. 5). The uptake of Mg into foraminiferal calcite is influenced by the Mg/Ca ratio of seawater, and calibration curves developed for modern foraminifera require two adjustments before use with fossils (MedinaElizalde et al., 2008). Both adjustments affect the pre-exponential exponent, a constant that describes Mg uptake into calcite (Medina-Elizalde et al., 2008). For the calibration curves given above, the preexponential exponent is 1.64 for Cibicides spp. and 0.44 for Notorotalia spp. The first adjustment accounts for the percentage difference between the modern seawater Mg/Ca ratio (4.96 mol/mol; Fantle and DePaolo, 2006) and an estimated composition for ocean water 17.7 Ma ago (3.52 mol/mol; after Fantle and DePaolo, 2006). In essence, less Mg is incorporated into foraminiferal calcite because there is less Mg in the ocean. The second adjustment concerns the rate of Mg uptake into calcite and uses a relationship derived from experimental data with inorganic calcium carbonate (Mucci and Morse, 1983). Essentially, the amount of Mg incorporated into foraminiferal calcite increases at an increasing rate when exposed to higher concentrations of Mg in seawater (Ries, 2006). The second adjustment is minor compared to the first;

4.1. Mg/Ca temperature calibration Intergeneric variations in Mg levels within foraminifera test carbonate may reflect, for example, vital effects (Boyle, 1995); such variations reduce the confidence of palaeotemperature estimates from a calibration curve not tailored to specific taxa. To test intergeneric variation between living taxa we analysed Mg/Ca ratios in Cibicides sp. and Notorotalia sp. from six localities, with the expectation that uptake of Mg in both genera would be influenced by the same environmental conditions. Mechanisms for calcification of foraminifera are poorly understood and may vary between taxa (ter Kuile, 1991; Dissard et al., 2010). A Mg/Ca ratio of 3 mmol/mol was incorporated at 11.0 °C by Notorotalia spp. and 6.6 °C by Cibicides spp. in our study. In contrast, the same Mg/Ca ratio measured from Hoeglundina elegans, Oridorsalis umbonatus, and Neogloboquadrina crassaformis would indicate growth temperatures of 7.5 °C (Rosenthal et al., 2006), 9.6 °C (Lear et al., 2002) and 23.5 °C (Regenberg et al., 2009) respectively. Calcification is fast, suggesting that foraminifera with different life spans acquire different Mg/Ca values (Lear et al., 2000; Dissard et al., 2010). Differences in the Mg/Ca values recorded from modern Cibicides spp. and Notorotalia spp. in this study were significant, supporting the need for genusspecific calibration curves. Notorotalia can be a common genus in New

K. Fukuda et al. / Marine Micropaleontology 92–93 (2012) 52–60

57

Fig. 5. Mg/Ca temperature curves calibration from modern A) Cibicides spp. and B) Notorotalia spp. T (°C) = ambient growth temperature (°C), Mg/Ca = measured Mg/Ca ratio (mmol/mol).

Zealand Cenozoic marine continental shelf strata, and the temperature calibration we present may increase the accessibility of Notorotalia to future Mg/Ca studies. Our curves developed from modern taxa enabled us to confidently determine the palaeotemperature of ambient growth in fossil Notorotalia and Cibicides. The Notorotalia calibration curve was the better resolved of the two, demonstrating greater sensitivity and fewer uncertainties than for the Cibicides curve, particularly between 10 and 15 °C.

36% of each sample, suggesting an outer shelf to upper slope depositional environment (Hayward, 1986). Furthermore, the presence of large G. zealandica suggests a small but persistent input of oceanic water into an otherwise shelf-based setting. Overall, the environmental proxies indicate an outer shelf depositional environment of 100–200 m water depth. The microfaunal and sedimentary characteristics are consistent with the lithology of the sediment, which is a massive calcareous sandy siltstone, and suggests that the Mount Harris Formation accumulated in a quiet setting with no significant traction currents.

4.2. Depositional environment 4.3. Mg/Ca palaeotemperature estimates Microfaunal and sedimentary characteristics from Pukeuri were collated to determine local depositional environment. The weight percent (wt.%) of sand was measured, and mean grain sizes noted visually with a binocular microscope, as possible indicators of change in water depth through the 12 sampled horizons. The persistent presence of very fine sand indicated a continuous terrigenous clastic source; massive bedding indicated deposition below the storm wave base, and there were no changes to suggest deepening beyond the shelf, or shallowing towards mid to inner shelf settings. Weight percent of the carbonate fraction within the sediment samples was treated as an index of oceanicity (wt.% CaCO3 = 1.6 ± 0.9%), and further indicated an environment with rapid clastic deposition. The percentage of planktonic foraminifera in each sample provided another index of oceanicity (e.g. Hayward et al., 1999; Morgans et al., 2002). Planktonics comprised between 25 and

Fig. 6. Bottom water temperatures calculated from the Mg/Ca ratios of Cibicides spp. and Notorotalia spinosa collected from 12 sample horizons at Pukeuri, North Otago, New Zealand (60 ka span around 17.7 Ma). Temperature calibration equations were adjusted for an estimated Mg/Ca ratio of seawater at 17.7 Ma (3.52 mol/mol; Fantle and DePaolo, 2006).

Early Miocene foraminifera from Pukeuri were collected and analysed. Mg/Ca ratios from N. spinosa and Cibicides spp. were interpreted using correlation curves derived from modern descendants (and adjusted for ancient seawater Mg/Ca composition), to produce high-resolution palaeotemperature values for an estimated 60 ka span around 17.7 Ma (G. zealandica range; Morgans et al., 2002). Average palaeotemperature estimates from the two genera were concordant, although Cibicides sp. demonstrated greater variation than Notorotalia sp. (Fig. 6). Such variation may reflect the temperature capture-period of the different genera, with data from Notorotalia representing a timeaveraged annual water temperature while Cibicides may only reflect a short-term water temperature (seasonal temperature). Measurements

Fig. 7. Oxygen isotopic composition (δ18O VSMOW) of outer shelf bottom water calculated using paired Mg/Ca temperature and δ18Oc values. Data were from Notorotalia spinosa collected from 12 sample horizons at Pukeuri, North Otago, New Zealand (60 ka span around 17.7 Ma).

58

K. Fukuda et al. / Marine Micropaleontology 92–93 (2012) 52–60

from single chambers of individual specimens of modern Notorotalia spp. showed steady changes in Mg/Ca value, indicating calcite growth during changing temperatures, and hence growth during multiple seasons (data not shown). In contrast, multiple Mg/Ca measurements from individual, modern Cibicides spp. showed little variation (data not shown). The intergeneric differences observed here reinforce the need for genus-specific Mg/Ca thermometric curves calibrated from modern descendants. Bottom water palaeotemperature estimates for Pukeuri were approximately 14 °C at palaeolatitude 43.3°S (equivalent to modern day Christchurch, New Zealand, based on palaeogeographic reconstructions by King, 2000). Our estimates are slightly lower than those previously reported for northern New Zealand during the Early Miocene: 17–22 °C (23.8 to 15.9 Ma, marine shelf; Devereux, 1967); 18–27 °C (~20 to ~17 Ma, coral reef; Hayward, 1977); 15–18 °C (27.7 to 19.0 Ma, marine shelf; Nelson and Burns, 1982). Indeed, our Early Miocene temperature is slightly warmer than a previous estimate for southern New Zealand (i.e. 10.4±0.3 °C, 21.7 to 19.0 Ma, marine shelf; Buening et al., 1998). Other New Zealand studies have generally identified the Altonian as a time of temperate to warm climate. Beu and Maxwell (1990) noted that Altonian molluscs from the Mount Harris Formation resembled a warm-temperate fauna, and Hornibrook (1992) used foraminifera assemblages to suggest that Altonian oceans around New Zealand were warm to subtropical. Our results suggest that Early Miocene New Zealand outer shelf bottom temperatures were slightly warmer than at present. Further, our bottom water estimates for the southern Zealandia Shelf are within the range of contemporaneous sea surface temperatures estimated from Ocean Drilling Program (ODP) site 1171C (48°30′S, 149°07′ E; Shevenell et al., 2004); these temperatures are substantially warmer than contemporaneous pelagic bottom water temperatures estimated from ODP sites 747 (55°S, 77°E; Billups and Schrag, 2003) and 1171C (~3 to 7 °C; Billups and Schrag, 2002; Shevenell et al., 2004). Our findings suggest that the modern temperature gradient observed between the southern New Zealand Shelf (10 °C; this study) and Antarctic bottom water (10 °C; Elderfield et al., 2010) was present during the Altonian, despite absolute temperatures being 3–4 °C cooler at present. Note that our calculated temperatures are corrected for the Early Miocene Mg/Ca seawater values summarised by Fantle and DePaolo (2006); different Mg/Ca values for ancient seawater could have a minor effect on temperature values (e.g. up to 3 °C warmer; Fantle and DePaolo, 2006; Cramer et al., 2011). The seawater temperatures calculated by Billups and Schrag (2003) and Shevenell et al. (2004) were not corrected for an Early Miocene Mg/Ca seawater value. The effect of diagenetic alteration is of utmost concern for interpretation of signals from geochemical proxies. The growth of secondary carbonate and dissolution of foraminiferal calcite may alter original Mg/Ca ratios (Lorens et al., 1977; Rosenthal and Boyle, 1993; Brown and Elderfield, 1996; Erez, 2003). SEM images of surface and internal details confirmed that the sampled fossils were clean and intact after the cleaning procedures (Fig. 8). Agreement between palaeotemperature estimates from the 12 sample horizons gives confidence that the geochemical signals from the Mount Harris Formation foraminifera are primary and not a result of alteration. 4.4. Ancient sea water Mg/Ca temperature estimates and oxygen isotopic compositions of N. spinosa test carbonate (δ18Oc) were used to calculate the oxygen isotopic composition of seawater (δ18Opalaeo-sw) during the Altonian (e.g. Horibe and Oba, 1972). δ18Opalaeo-sw is an important parameter that has provided insight into the fluctuation of continental ice volume, and the salinity and sea level of ancient oceans (e.g. Miller et al., 1987; Hollis et al., 2009). Sedimentological and micropalaeontological evidence suggest that the Mount Harris Formation at Pukeuri accumulated in an open marine shelf setting; there is no independent evidence of significant fresh water input from the gradually emerging but generally

low-relief Zealandia. Because major ocean currents comparable to those of modern New Zealand were established before the Altonian (Nelson and Cooke, 2001), the water at Pukeuri was likely sourced from the Southern Ocean, with a global oxygen isotopic signal littleinfluenced by major freshwater inputs within the 60 ka window reported here. The palaeo-seawater isotopic compositions averaged −0.4 ± 0.4‰ (VSMOW) throughout the studied sequence. Our results are consistent with δ18Opalaeo-sw values calculated from Altonian aged foraminifera at ODP sites 747 (Billups and Schrag, 2003) and 1171C (Shevenell et al., 2008). If the values here are considered to represent the global average for δ18Opalaeo-sw during the Early Miocene then the signal implies reasonably ice-free conditions or the presence of only small ice sheets (e.g. Kennett, 1986; Zachos et al., 2001, 2008). Palaeoseawater values may represent a more local feature (e.g. localised water mass or current), but no other finely-sampled δ18Opalaeo-sw Miocene values for New Zealand are available for comparison. Note that our palaeo-seawater isotopic compositions are influenced by an estimate of Early Miocene seawater Mg/Ca (i.e. Fantle and DePaolo, 2006), and different estimates may give slightly different δ18Opalaeo-sw values. Sedimentary and microfaunal proxies indicated that the palaeoenvironmental setting was stable during relatively ice-free conditions, although small fluctuations between sample horizons 5 and 8 (Anomaly 1) and between sample horizons 10 and 12 (Anomaly 2) may reflect increases in ice volume (Fig. 7). The increase in calculated δ18Opalaeo-sw value (~0.7‰) of Anomaly 1 suggests a relative fall in sea level and is supported by an increased sand fraction, reduction of the amount of carbonate and fewer planktonic foraminifera (Fig. 4). The brevity of the δ18Opalaeo-sw departure suggests that Anomaly 1 represents a short period of rapid glacio-eustatic sea level change. In contrast, the percentage of planktonic foraminifera increased across Anomaly 2, indicating a decoupling or lag between δ 18Opalaeo-sw and the relative abundance of planktonic foraminifera. Irrespective, the similarity in δ18Opalaeo-sw excursions between the two anomalies suggests that Anomaly 2 is also a response to a small glaciation event within the Early Miocene. 5. Conclusions Calibration curves were produced for benthic foraminifera by correlating Mg/Ca with known water temperatures. Intergeneric variations were found between calibration curves developed for Cibicides spp. and Notorotalia spp., with the Notorotalia calibration curve more sensitive than the Cibicides curve between 10 and 15 °C. The modern calibration curves were adjusted for ancient Mg/Ca seawater values and applied to fossil species of each genus from a 60 ka span about 17.7 Ma at Pukeuri, New Zealand. Palaeotemperature estimates from the two different curves were concordant, although estimates from N. spinosa (13.3 ± 1.0 °C) were relatively constant while estimates from Cibicides (15.5 ± 3.0 °C) were more varied. Both palaeotemperature estimates are consistent with warm-temperate oceans, indicating that Altonian bottom waters at the Pukeuri locality were slightly warmer than the present day Otago Shelf. Oxygen isotopic composition was estimated for Altonian sea water (δ 18Opalaeo-sw) using a paired Mg/Ca ratio and oxygen isotopic approach. δ18Opalaeo-sw featured two small fluctuations that were hypothesised to represent minor glaciations during otherwise relatively ice-free conditions. Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.marmicro.2012.05.002. Acknowledgements We thank K. Swanson and J. Guise (University of Canterbury), N. Hiller (Canterbury Museum), B.A. Marshall (Museum of New Zealand), and O. Gussman (University of Otago) for providing modern materials. M. Uddstorm (NIWA) and B. Dickson (Portobello laboratory, University

K. Fukuda et al. / Marine Micropaleontology 92–93 (2012) 52–60

59

Fig. 8. Scanning electron micrographs of fossil Notorotalia from horizon 2 of the Pukeuri study section. Images show foraminifera surfaces A) without cleaning, B) after sonication and rinsing, and C) after all cleaning steps including acid leaching. The full suite of cleaning procedures was effective at removing secondary minerals.

of Otago) provided us with seasonal water temperature archive records. We thank R. Dewdney (Department of Geology, University of Otago) for helping to prepare and pick samples. We thank B.W. Hayward, an anonymous reviewer, and the Editor for constructive comments. Support from Department of Chemistry, Geology and Marine Science are gratefully acknowledged. Financial support for this study was provided by a University of Otago Postgraduate Scholarship. References Beu, A.G., Maxwell, P.A., 1990. Cenozoic Mollusca of New Zealand. New Zealand Geological Survey Paleontological Bulletin 58 (518 pp.). Billups, K., Schrag, D.P., 2002. Paleotemperatures and ice volume of the past 27 Myr revisited with paired Mg/Ca and 18O/16O measurements on benthic foraminifera. Paleoceanography 17, 3.1–3.11. Billups, K., Schrag, D.P., 2003. Application of benthic foraminiferal Mg/Ca ratios to questions of Cenozoic climate change. Earth and Planetary Science Letters 209, 181–195. Boyle, E.A., 1995. Limits on benthic foraminiferal chemical analyses as precise measures of environmental properties. Journal of Foraminiferal Research 25, 4–13. Brown, S.J., Elderfield, H., 1996. Variations in Mg/Ca and Sr/Ca ratios of planktonic foraminifera caused by postdepositional dissolution: evidence of shallow Mg dependent dissolution. Paleoceanography 11, 543–551. Buening, N., Carlson, S.J., Spero, H.J., Lee, D.E., 1998. Evidence for the Early Oligocene formation of a proto-subtropical convergence from oxygen isotope records of New Zealand Paleogene brachiopods. Palaeogeography, Palaeoclimatology, Palaeoecology 138, 43–68. Chapman, F., 1909. Report on the foraminifera from the subantarctic islands of New Zealand. In: Chilton, C. (Ed.), The subantarctic islands of New Zealand. Reports on the geophysics, geology, zoology and botany of the islands lying to the south of New Zealand, based mainly on observations and collections made during an expedition in the Government Steamer “Hinemoa” (Captain J. Bollons) in November, 1907. Philosophical Institute of Canterbury, Christchurch, pp. 312–371. The New Zealand geological timescale. In: Cooper, R.A. (Ed.), Institute of Geological and Nuclear Science Monograph 22 (284 pp.). Cramer, B.S., Miller, K.G., Barrett, P.J., Wright, J.D., 2011. Late Cretaceous–Neogene trends in deep ocean temperature and continental ice volume: reconciling records of benthic foraminiferal geochemistry (δ18O and Mg/Ca) with sea level history. Journal of Geophysical Research 116, C12023, http://dx.doi.org/10.1029/2011JC007255.

Devereux, I., 1967. Oxygen isotope paleotemperature measurements on New Zealand Tertiary fossils. New Zealand Journal of Science 10, 988–1011. Dissard, D., Nehrke, G., Reichart, G.J., Bijma, J., 2010. The impact of salinity on the Mg/Ca and Sr/Ca ratio in the benthic foraminifera Ammonia tepida: results from culture experiments. Biogeosciences 7, 81–93. Elderfield, H., Ganssen, G., 2000. Past temperature and δ18O of surface ocean waters inferred from foraminiferal Mg/Ca ratios. Nature 405, 442–445. Elderfield, H., Yu, J., Anand, P., Kiefer, T., Nyland, B., 2006. Calibrations for benthic foraminiferal Mg/Ca paleothermometry and the carbonate ion hypothesis. Earth and Planetary Science Letters 250, 633–649. Elderfield, H., Greaves, M., Barker, S., Hall, I.R., Tripati, A., Ferretti, P., Crowhurst, S., Booth, L., Daunt, C., 2010. A record of bottom water temperature and seawater δ18O for the Southern Ocean over the past 440 kyr based on Mg/Ca of benthic foraminiferal Uvigerina spp. Quaternary Science Reviews 29, 160–169. Erez, J., 2003. The source of ions for biomineralization in foraminifera and their implications for paleoceanographic proxies. In: Dove, P.M., De Yoreo, J.J., Weiner, S. (Eds.), Biomineralization. : Reviews in Mineralogy and Geochemistry, 54. Mineralogical Society of America, Washington, DC, pp. 115–149. Fantle, M.S., DePaolo, D.J., 2006. Sr isotopes and pore fluid chemistry in carbonate sediment of the Ontong Java Plateau: calcite recrystallization rates and evidence for a rapid rise in seawater Mg over the last 10 million years. Geochimica et Cosmochimica Acta 70, 3883–3904. Field, B.D., Crundwell, M.P., Lyon, G.L., Mildenhall, D.C., Morgans, H.E.G., Ohneiser, C., Wilson, G.S., Kennett, J.P., Chanier, F., 2009. Middle Miocene paleoclimate change at Bryce Burn, southern New Zealand. New Zealand Journal of Geology and Geophysics 52, 321–333. Fukuda, K., 2002. Deducing the Cenozoic paleotemperature by using Mg/Ca ratios in benthic foraminifera. MSc thesis, University of Otago, Dunedin. Gradstein, F.M., Ogg, J.G., Smith, A.G., 2004. A Geologic Time Scale 2004. Cambridge University Press, Cambridge. 610 pp. Graham, I.J., Morgans, H.E.G., Waghorn, D.B., Trotter, J.A., Whitford, D.J., 2000. Strontium isotope stratigraphy of the Oligocene–Miocene Otekaike Limestone (Trig Z section) in southern New Zealand: age of the Duntroonian/Waitakian stage boundary. New Zealand Journal of Geology and Geophysics 43, 335–347. Hayward, B.W., 1977. Lower Miocene corals from the Waitakere Ranges, North Auckland, New Zealand. Journal of the Royal Society of New Zealand 7, 99–111. Hayward, B.W., 1986. A guide to paleoenvironmental assessment using New Zealand Cenozoic foraminiferal faunas. New Zealand Geological Survey Report 109 (73 pp.). Hayward, B.W., Grenfell, H.R., Reid, C.M., Hayward, K.A., 1999. Recent New Zealand shallow water benthic foraminifera: taxonomy, ecologic distribution, biogeography and use in paleoenvironmental assessment. Institute of Geological and Nuclear Science Monograph 21 (258 pp.).

60

K. Fukuda et al. / Marine Micropaleontology 92–93 (2012) 52–60

Heath, R.A., 1984. The depth of the mixed layer as an indicator of oceanic circulation around New Zealand. New Zealand Journal of Marine and Freshwater Research 18, 83–92. Hollis, C., Handley, L., Crouch, E.M., Morgans, H.E.G., Baker, J.A., Creech, J., Collins, K.S., Gibbs, S.J., Huber, M., Schouten, S., Zachos, J.C., Pancost, R.D., 2009. Tropical sea temperatures in the high-latitude South Pacific during the Eocene. Geology 37, 99–102. Hollis, C.J., Beu, A.G., Crampton, J.S., Crundwell, M.P., Morgans, H.E.G., Raine, J.I., Jones, C.M., Boyes, A.F., 2010. Calibration of the New Zealand Cretaceous–Cenozoic timescale to GTS2004. GNS Science report 2010/43. 20 pp. Horibe, Y., Oba, T., 1972. Temperature scales of aragonite–water and calcite–water systems. Fossils 23/24, 69–74. Hornibrook, N. de B., 1992. New Zealand Cenozoic marine paleoclimates: a review based on the distribution of some shallow water and terrestrial biota. In: Tsuchi, R., Ingles, J.C. (Eds.), Pacific Neogene Environment, Evolution and Events. University of Tokyo Press, Tokyo, pp. 83–106. Hornibrook, N. de B., Brazier, R.C., Strong, C.P., 1989. Manual of New Zealand Permian to Pleistocene foraminiferal biostratigraphy. New Zealand Geological Survey Paleontological Bulletin 69, 45p. Hut, G., 1987. Consultant's Group Meeting on Stable Isotope Reference Samples for Geochemical and Hydrological Investigations, September 16–18 1985, Vienna. International Atomic Energy Agency, Vienna. 43 pp. Kennett, J.P., 1986. Miocene to early Pliocene oxygen and carbon isotope stratigraphy in the southwest Pacific, Deep Sea Drilling Project Leg 90. In: Kennett, J.P., von der Borch, C.C. (Eds.), Initial Report of the Deep Sea Drilling Project Leg, 90. United States Government Printing Office, Washington, DC, pp. 1383–1411. King, P.R., 2000. Tectonic reconstructions of New Zealand: 40 Ma to the Present. New Zealand Journal of Geology and Geophysics 43, 611–638. Lear, C.H., Elderfield, H., Wilson, P.A., 2000. Cenozoic deep-sea temperatures and global ice volumes from Mg/Ca in benthic foraminiferal calcite. Science 287, 269–272. Lear, C.H., Rosenthal, Y., Slowey, N., 2002. Benthic foraminiferal Mg/Ca-paleothermometry: a revised core-top calibration. Geochimica et Cosmochimica Acta 66, 3375–3387. Lorens, R.B., Williams, D.F., Bender, M.L., 1977. The early nonstructural chemical diagenesis of foraminiferal calcite. Journal of Sedimentary Petrology 47, 1602–1609. McArthur, J.M., Howarth, R.J., Bailey, T.R., 2001. Strontium isotope stratigraphy: Lowess version 3: best fit to the marine Sr-isotope curve for 0–509 Ma and accompanying look‐up table for deriving numerical age. Journal of Geology 109, 155–170. Medina-Elizalde, M., Lea, D.W., Fantle, M.S., 2008. Implications of seawater Mg/Ca variability for Plio-Pleistocene tropical climate reconstruction. Earth and Planetary Science Letters 269, 584–594. Miller, K.G., Fairbanks, R.G., Mountain, G.S., 1987. Tertiary oxygen isotope synthesis, sea level history, and continental margin erosion. Paleoceanography 2, 1–19. Morgans, H.E.G., Edwards, A.R., Scott, G.H., Graham, I.J., Kamp, P.J.J., Mumme, T.C., Wilson, G.J., Wilson, G.S., 1999. Integrated stratigraphy of the Waitakian–Otaian stage boundary stratotype, Early Miocene, New Zealand. New Zealand Journal of Geology and Geophysics 42, 581–614. Morgans, H.E.G., Scott, G.H., Edwards, A.R., Graham, I.J., Mumme, T.C., Waghorn, D.B., Wilson, G.S., 2002. Integrated stratigraphy of the lower Altonian (Early Miocene) sequence at Tangakaka Stream, East Cape, New Zealand. New Zealand Journal of Geology and Geophysics 45, 145–173. Mucci, A., Morse, J.W., 1983. The incorporation of Mg2+ and Sr2+ into calcite overgrowths: influences of growth rate and solution composition. Geochimica et Cosmochimica Acta 47, 217–233. Nelson, C.S., Burns, D.A., 1982. Effect of sampling interval on the resolution of oxygen isotopic paleotemperature trends — an example from the New Zealand early Miocene. New Zealand Journal of Geology and Geophysics 25, 77–81.

Nelson, C.S., Cooke, P.J., 2001. History of oceanic front development in the New Zealand sector of the Southern Ocean during the Cenozoic — a synthesis. New Zealand Journal of Geology and Geophysics 44, 535–553. Nürnberg, D., Bijma, J., Hemleben, C., 1996. Assessing the reliability of magnesium in foraminiferal calcite as a proxy for water mass temperatures. Geochimica et Cosmochimica Acta 60, 803–814. Pickering, T.D., Suda, Y. (Eds.), 2003. Report of the 2nd Joint National Fisheries University–University of the South Pacific Fisheries and Oceanography Research Cruise on board Koyo-Maru, Kadavu Passage, December 1997. : Marine Studies technical report. University of the South Pacific. 69 pp. Regenberg, M., Steph, S., Nürnberg, D., Tiedemann, R., Garbe-Schönberg, D., 2009. Calibrating Mg/Ca ratios of multiple planktonic foraminiferal species with δ18O-calcification temperatures: paleothermometry for the upper water column. Earth and Planetary Science Letters 278, 324–336. Ries, J.B., 2006. Mg fractionation in crustose coralline algae: geochemical, biological, and sedimentological implications of secular variation in the Mg/Ca ratio of seawater. Geochimica et Cosmochimica Acta 70, 891–900. Rosenthal, Y., Boyle, E.A., 1993. Factors controlling the fluoride content of planktonic foraminifera: an evaluation of its paleoceanographic applicability. Geochimica et Cosmochimica Acta 57, 335–346. Rosenthal, Y., Boyle, E.A., Slowey, N., 1997. Temperature control on the incorporation of magnesium, strontium, fluorine, and cadmium into benthic foraminiferal shells from Little Bahama Bank: prospects for thermocline paleoceanography. Geochimica et Cosmochimica Acta 61, 3633–3643. Rosenthal, Y., Lohmann, G.P., Sherrell, R.M., 2000. Incorporation and preservation of Mg in Globigerinoides sacculifer: implications for reconstructing the temperature and 18 O/16O of seawater. Paleoceanography 15, 135–145. Rosenthal, Y., Lear, C.H., Oppo, D.W., Linsley, B.K., 2006. Temperature and carbonate ion effects on Mg/Ca and Sr/Ca ratios in benthic foraminifera: the aragonitic species Hoeglundina elegans. Paleoceanography 21, PA1007. Shackleton, N., 1967. Oxygen isotope analyses and Pleistocene temperatures reassessed. Nature 215, 15–17. Shevenell, A.E., Kennett, J.P., Lea, D.W., 2004. Middle Miocene Southern Ocean cooling and Antarctic cryosphere expansion. Science 305, 1766–1769. Shevenell, A.E., Kennett, J.P., Lea, D.W., 2008. Middle Miocene ice sheet dynamics, deep-sea temperatures, and carbon cycling: a Southern Ocean perspective. Geochemistry, Geophysics, Geosystems 9, Q02006. ter Kuile, B., 1991. Mechanisms for calcification and carbon cycling in algal symbiontbearing foraminifera. In: Lee, J.J., Anderson, R. (Eds.), Biology of Foraminifera. Academic Press, London, pp. 255–284. Toyofuku, T., Kitazato, H., Kawahata, H., Tsuchiya, M., Nohara, M., 2000. Evaluation of Mg/Ca thermometry in foraminifera: comparison of experimental results and measurements in nature. Paleoceanography 15, 456–464. Van Hale, R., 2003. The Stable Isotope Oceanography of the Otago Shelf. PhD thesis, University of Otago, Dunedin. Walton, W.R., 1952. Techniques for recognition of living foraminifera. Contributions from Cushman Foundation for Foraminiferal Research 3, 56–60. Zachos, J.C., Pagani, M., Sloan, L., Thomas, E., Billups, K., 2001. Trends, rhythms, and aberrations in global climate 65 Ma to Present. Science 292, 686–693. Zachos, J.C., Dickens, G.R., Zeebe, R.E., 2008. An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics. Nature 451, 279–283.