Isotopic composition of a large photosymbiotic foraminifer: Evidence for hypersaline environments across the Great Australian Bight during the late Pleistocene

Sedimentary Geology 213 (2009) 113–120

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Sedimentary Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s e d g e o

Isotopic composition of a large photosymbiotic foraminifer: Evidence for hypersaline environments across the Great Australian Bight during the late Pleistocene John M. Rivers a,⁎, T. Kurt Kyser b, Noel P. James b a b

ExxonMobil, CORP-WGR-764, 396 West Greens Road, Houston, TX 77210, United States Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario, Canada, K7L 3N6

a r t i c l e

i n f o

Article history: Received 24 July 2008 Received in revised form 16 November 2008 Accepted 28 November 2008 Keywords: Marginopora Stable Isotope Australia Southern Ocean Salinity

a b s t r a c t Analysis of carbon and oxygen isotopic compositions of large benthic foraminifera tests (Marginopora vertebralis) that lived in the Great Australian Bight during the late Pleistocene, reveal that the tests are enriched by 1 to 3‰ in both 18O and 13C relative to modern specimens from the same region. The intolerance of M. vertebralis for cool waters negates lower ocean water temperature as an explanation for such high δ18O values. The oxygen isotopic compositions are thus interpreted to reflect tests secreted in hypersaline waters of up to 56 ppt salinity, concentrated from seawater by evaporation. M. vertebralis thrives today in waters of similar salinity at Shark Bay, Western Australia. The Pleistocene sedimentary assemblage supports an interpretation that environments broadly similar to those in outer modern-day Shark Bay were wide spread across the Great Australian Bight during portions of marine isotope stages 2, 3 and 4. The high δ13C values of the Pleistocene M. vertebralis are interpreted to reflect enhanced photosynthetic activity that depletes dissolved carbonate in 12C in such shallow, saline settings. These hypersaline environments formed during periods of lower sea-level when shallow-waters (b 20 m depth) extended from the shoreline over ~ 100 km across what is currently a relatively deep shelf. This study indicates that shelf bathymetry was a critical determinant of past environments of deposition across the Great Australian Bight. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The southern continental margin of Australia is a vast cool–water carbonate depositional realm. Carbonate sediments blanketing the margin are palimpsest, a mixture of grains deposited during marine isotope stage (MIS) 1 (the Holocene epoch) with Pleistocene sediments deposited during MIS 2, 3 and 4 (James et al., 1997, 2001; Rivers et al., 2007). Rivers et al. (2007) detailed the composition and disposition of the late Pleistocene grains, and showed that during MIS 2, 3 and 4, much of the expansive Great Australian Bight (GAB) (Fig. 1) was an area of shallow, warm, well-lit, marine-grass-bank environments similar to those in local inboard embayments today. One conspicuous constituent of the Pleistocene sea-grass-bank assemblage was the large photosymbiotic foraminifer Marginopora vertebralis. This study documents the carbon and oxygen isotopic composition of M. vertebralis tests collected from Pleistocene sediments across the GAB, as well as M. vertebralis tests recovered from the westernmost portion of the study area where they live today. The chemical make up of Pleistocene sediments has the potential to elucidate oceanographic conditions in the GAB during previous sea-level stands. These older grains, however, have been diagenetically altered on the seafloor by infilling of skeletal pores with Mg-calcite micritic cements. To evaluate ⁎ Corresponding author. E-mail address: [email protected] (J.M. Rivers). 0037-0738/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2008.11.008

the effect of such diagenesis, the δ18O and δ13C values of seafloor cement from southern Australian sediment have also been measured. The purpose of this study is to determine whether the isotopic composition of the older M. vertebralis tests reflect original oceanography, and if so to elucidate the late Pleistocene oceanographic conditions of GAB. 2. Background 2.1. The Great Australian Bight The GAB is part of the passive margin of southern Australia and extends from Cape Pasley, Western Australia, eastward to the southern tip of the Eyre Peninsula (Fig. 1). James et al. (2001) detailed the sedimentological and oceanographic attributes of the region. The shelf surface is generally deep (N50 m), open, unrimmed, and predominantly mantled by heterozoan carbonate sands. The GAB is a highenergy, storm-dominated realm with N2.5 m-high swells emanating from the southwest (Davies, 1970; Wright et al., 1982). Winter westerlies (May–September) drive the warm-water Leeuwin current and the warm, saline Southern Australian Current across the region. Such flows weaken during summer months as winds blow northwestward, allowing Sub-Antarctic Water to locally upwell onto the shelf (e.g. Griffin et al., 1997). The GAB is bathed in ocean water with surface temperatures ranging from 15 °C in inboard regions during the

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J.M. Rivers et al. / Sedimentary Geology 213 (2009) 113–120

Fig. 1. Map showing the Great Australian Bight with bathymetry contours and sample sites. Living populations of Marginopora vertebralis dwell at Esperance, WA, in water depths of less than 20 m. Water depth contours are in meters.

winter to highs of 22 °C in the central GAB during the summer (James et al., 2001). Because southern Australia has a mostly arid to semiarid climate (Hesse et al., 2004), fluvial input is limited, and some waters in shallow portions of the margin are characterized by seasonally high salinity. 2.2. Marginopora vertebralis: description and distribution M. vertebralis, a large (up to 2 cm in diameter or more) photosymbiotic foraminifera belonging to the Soritidae family (Loeblich and Tappan, 1964), is known to inhabit tropical to subtropical (N17 °C) waters of normal to hypersaline salinities (Murray, 1973). The discoidal test of this protist is composed of numerous chambers surrounding a proloculus, with marginal chambers having irregularly arranged apertural pores (Barker, 1960; Loeblich and Tappan, 1964; Murray, 1973; Saraswati, 2004). The test is covered with a thin, transparent layer of carbonate that encloses the protoplasm, which hosts zooxanthellate symbionts. In shallow waters (~ 10 m) where maximum concentration of symbionts occurs, reproductive maturity is realized in 2 years, at which point the test reaches its maximum size (Ross, 1972, 1974). The distribution of this foraminifer is wide with its presence reported in equatorial regions of the Pacific and Indian oceans (e.g. Barker, 1960; Murray, 1973; Coulbourn and Resig, 1975; Hallock, 1984; Hohenegger, 1994). It has been recognized living on the western margin of Australia, the eastern margin of Australia (probably confined to the Queensland coast), as well as at Esperance on the southern Australian margin (Fig. 1) (Cann and Clarke, 1993). On the southern margin, M. vertebralis tests have also been reported eastward of Esperance (Cann and Clarke, 1993) in sediments on the Great Australian Bight (GAB) as well as Spencer Gulf and the Lincoln shelf (Fig. 1), but there is some uncertainty as to whether these occurrences reflect living populations. M. vertebralis are reported to have been deposited in the saline shallow embayment of Spencer Gulf during the last interglacial (MIS 5e) (Cann et al., 2000). Whereas most occurrences of M. vertebralis on the deeper open GAB shelf have been reported as stained lithoclasts (termed relict grains by James et al. (1997) and Rivers et al. (2007)), “fresh” tests have been identified by Li et al. (1996), who interpreted them as Holocene and reflecting the current oceanographic regime. At Esperance, Western Australia, the site closest to the GAB where M. vertebralis are confirmed to be living (Cann and Clarke, 1993), annual ocean water temperatures vary between 16 and 20 °C and salinities are

normal (~36 ppt). In tropical latitudes, M. vertebralis are known to live to depths of 50 m (Ross, 1972), but at Esperance they are observed living at depths as shallow as 2 m and no deeper than 20 m (Cann and Clarke, 1993). In this area they are found most commonly attached to algal turf and seagrass banks on protected sides of islands. At Shark Bay, on the western Australian margin, Davies (1970) reported their occurrence amongst Posidonia seagrass at depths b20 m. The waters they inhabit vary in temperature from 17–25 °C and are generally hypersaline (up to 56 ppt salinity). 2.3. Previous isotopic studies of Marginopora vertebralis The isotopic composition of tropical M. vertebralis have been previously reported by Wefer and Berger (1980), Wefer et al. (1981) and Langer (1995). Wefer and Berger (1980) and Wefer et al. (1981) showed that the δ18O values of this foraminifera vary as expected with isotopic composition of sea water and water temperature, but that once Mg content was taken into account, their shells are depleted in 18O with respect to equilibrium by ~1.5‰. Their δ13C values were likewise found to be lower than the expected equilibrium values, by as much as ~2.5‰, and this was attributed to the incorporation of metabolic CO2 in shell construction. 3. Methods Foraminifera were picked from seafloor sediment samples obtained during cruises of the CSIRO R.V. Franklin in 1995 and 1998 (James et al., 2001) (Table 1). Whole tests ranging in size from 5 to 10 mm in diameter were chosen for isotopic analysis. Specimens included those from living populations at Esperance, Western Australia, which displayed no dissolution features (Fig. 2A) as well as samples of the most pristine white M. vertebralis tests from the GAB having empty chambers but commonly with some dissolution features (Fig. 2B), and relict M. vertebralis tests from the GAB that are highly abraded, cement-filled and brown-stained specimens (Fig. 2C). The GAB M. vertebralis were collected from depths between 50 m and 100 m. All samples were treated for 24 h in bleach to remove organic material, washed three times in deionized water, dried, pulverized, and analyzed at the Queen's Facility for Isotope Research (QFIR). GAB seafloor cements sampled from infilled chambers of gastropods and isolated from cemented grain aggregates from similar depths were also analyzed (Table 2). Separating cement from relict foraminfera for analysis was not possible because the individual

J.M. Rivers et al. / Sedimentary Geology 213 (2009) 113–120 Table 1 Locations, depths, and isotopic compositions of Marginopora vertebralis from the Great Australian Bight Sample number

Latitude

Longitude

Depth (m)

δ18O

δ13C

Interpreted age (approximate)

esp 2/5 esp 2/5 esp 2/8 esp 2/8 esp 4/12 esp 4/12 esp 4/16 esp 4/16 esp 4/17 ACM_072 GAB 032 GAB 093 ACM_093 GAB 055 ACM_069 ACM_080 GAB 059 ACM_059 GAB_053 ACM_078 GAB_058 ACM_094 ACM_089 GAB_052 ACM_050 ACM_058 GAB_070 GAB_037 GAB_085 ACM_088 ACM_082 GAB_086 ACM_095 ACM_81 ACM_048 GAB_065R ACM_072R GAB_068R ACM_080R ACM_059R GAB_053R ACM_078R ACM_052R GAB_086R ACM_095R GAB_036R

33° 55.00′ S 33° 55.00′ S 33° 55.00′ S 33° 55.00′ S 33° 55.00′ S 33° 55.00′ S 33° 55.00′ S 33° 55.00′ S 33° 55.00′ S 32° 18.03′ S 32° 40.05′ S 34° 31.59′ S 31° 57.89′ S 33° 16.79′ S 32° 41.02′ S 32° 03.91′ S 32° 50.08′ S 32° 53.39′ S 33° 26.30′ S 32° 12.24′ S 33° 08.22′ S 32° 30.03′ S 32° 16.01′ S 33° 36.00′ S 33° 38.17′ S 33° 18.02′ S 33° 54.87′ S 32° 59.03′ S 34° 16.38′ S 32° 49.02′ S 32° 52.09′ S 34° 26.19′ S 32° 56.14′ S 32° 22.05′ S 34° 26.03′ S 33° 03.50′ S 32° 18.03′ S 33° 37.90′ S 32° 03.91′ S 32° 53.39′ S 33° 26.30′ S 32° 12.24′ S 33° 12.88′ S 34° 26.19′ S 32° 56.14′ S 33° 10.16′ S

121° 53.00′ E 121° 53.00′ E 121° 53.00′ E 121° 53.00′ E 121° 53.00′ E 121° 53.00′ E 121° 53.00′ E 121° 53.00′ E 121° 53.00′ E 133° 06.99′ E 128° 28.90′ E 122° 58.42′ E 130° 27.98′ E 125° 18.16′ E 133° 24.93′ E 131° 43.07′ E 125° 58.31′ E 133° 48.25′ E 125° 04.98′ E 132° 27.70′ E 125° 57.73′ E 130° 28.24′ E 130° 51.92′ E 125° 11.00′ E 134° 10.49′ E 133° 32.05′ E 124° 23.07′ E 127° 11.19′ E 124° 00.12′ E 128° 52.00′ E 131° 15.68′ E 123° 48.54′ E 130° 28.23′ E 131° 31.00 E 134° 00.16′ E 124° 22.92′ E 133° 06.99′ E 124° 22.99′ E 131° 43.07′ E 133° 48.25′ E 125° 04.98′ E 132° 27.70′ E 134° 16.46′ E 123° 48.54′ E 130° 28.23′ E 127° 39.69′ E

10 10 10 10 10 10 10 10 10 50 55 55 56 56 57 60 62 62 62 63 65 66 68 71 71 72 73 76 80 87 89 90 94 95 96 43 50 59 60 62 62 63 65 90 94 100

0.6 0.7 0.4 0.5 0.5 0.8 0.3 0.9 0.9 1.7 2.4 1.9 2.0 2.0 1.6 2.1 2.2 2.7 2.1 3.1 1.3 2.2 2.7 1.7 2.7 2.0 1.8 1.9 2.2 1.6 2.0 2.5 1.6 2.3 1.8 1.8 1.9 1.6 2.7 1.5 1.0 1.5 2.1 2.1 2.4 1.6

2.5 2.7 3.2 3.1 2.4 2.6 2.2 2.4 1.9 3.2 3.8 3.7 4.5 4.1 3.9 3.7 4.0 4.6 3.7 4.6 4.2 4.2 4.6 3.7 4.4 3.9 3.8 4.3 5.0 3.8 3.7 4.5 3.4 4.4 3.3 3.1 3.8 2.7 3.9 3.6 2.7 3.8 4.0 3.4 4.0 3.7

Holocene Holocene Holocene Holocene Holocene Holocene Holocene Holocene Holocene MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 2 MIS 3 or 4 MIS 3 or 4 MIS 3 or 4 MIS 3 or 4 MIS 3 or 4 MIS 3 or 4 MIS 3 or 4 MIS 3 or 4 MIS 3 or 4 MIS 3 or 4 MIS 3 or 4

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foraminifera and cement (both ~ 12 mol% by XRD) by increasing δ18Oeq by 0.7‰ (Tarutani et al., 1969). The calculated δ18Oeq values range from −0.1 to +0.7‰ VPDB across the Esperance temperature range. Calcite–HCO−3 fractionation of 13C has no significant temperature dependency across a seawater temperature range of 10–40 °C (Romanek et al., 1992). The δ13C equilibrium values for calcite were calculated using the equation from Jimenez-Lopez et al., (2006): δ13 Ceq = 0:82 + δ13 CHCO3 + 0:024x per molek MgCO3 :

ð2Þ

δ13C values for DIC of southern Australia shelf waters range between +0.2 and +0.7‰ (Rahimpour-Bonab et al., 1997), which would give a range for calcite with 12 mol% MgCO3 of +1.3 and +1.8‰. Five samples of white M. vertebralis (~10 tests per sample) from the GAB were radiocarbon dated, and their dates reported as years before present (yr BP) (Table 3). Multiple tests were used in each sample in order to acquire a suitable amount of gas for dating. Using the radiocarbon calibration program of Stuiver and Reimer (1993) and the calibration data of Hughen et al. (2004), these dates were corrected for the southern Australia surface-water marine reservoir effect. A Δr value of 137 ± 86 was used in the calibration program, as was calculated by Cann et al. (2006) for southern Australian ocean surface waters. The Δr is equivalent to that determined by Gillespie and Polach (1979). Both conventional and calibrated, marine-reservoir corrected dates are reported with a 1σ error. 4. Results

chambers are minute. δ13C and δ18O values were measured on CO2 released from dissolution of 0.5 mg of sample in 100% H3PO4 after 4 h of reaction at 72 °C. Replicate analyses using these procedures and interlaboratory comparisons are within 0.1‰ for both δ13C and δ18O. Carbon and oxygen isotopic compositions are reported in standard δ notation in units of per mil relative to Vienna Peedee Belemnite (VPDB) standard (Tables 1 and 2) whereas all waters are relative to Vienna Standard Mean Ocean Water (VSMOW). δ18O equilibrium values for calcite from marine waters were calculated using Kim and O'Neil (1997) as arranged by Peeters et al. (2002):

Of the five groups of samples of unstained GAB M. vertebralis that were carbon-dated, three have corrected ages of 7630 ± 140, 9960 ± 280 and 12,090 ± 200, indicating their age of deposition to be near the end of MIS 2 or the very early Holocene. Two samples have corrected dates of 17,280 ± 160 and 21,420 ± 350, implying deposition during the last glacial maximum (Table 3). The stable isotopic composition of Holocene M. vertebralis from Esperance, late Pleistocene M. vertebralis from the GAB, as well as GAB intergranular and intragranular cements are shown in Tables 1 and 2 and on Fig. 3 along with the calculated range of δ18O and δ13C values for Mg-calcite precipitated from southern Australian waters at Esperance temperatures. δ18O values for Holocene foraminifera range between 0‰ and +1‰, most within the expected equilibrium field for local waters. The δ18O values for Pleistocene GAB foraminifera and GAB Mg-calcite cements range mostly between 1‰ and 3‰, significantly higher than Holocene foraminifera. δ13C values for all M. vertebralis are higher than the calculated equilibrium values. Holocene foraminifera from Esperance range between +1.9‰ and +3.2‰, on average ~1‰ higher than equilibrium values for these waters (Fig. 3). The δ13C values of cements from the GAB range from 1.6‰ to 3.1‰, similar to Holocene foraminifera. Pleistocene M. vertebralis from the GAB range from 2.7‰ to 5‰, 2–3‰ higher than the Esperance equilibrium values. Both foraminifera and cements display a positive correlation between δ18O and δ13C (Fig. 3). The correlation between the δ18O and δ13C values for both the Holocene and Pleistocene foraminifera (as a set) has a slope of 0.86 with an r2 value of 0.67. Similarly, the correlation for the cements has a slope of 0.84 with an r2 value of 0.67.

δ18 Oeq = 25:778−3:3334ð43:704 + TÞ0:5 + δ18 Ow

5. Discussion

Interpreted ages are also listed.

ð1Þ

where δ18Oeq is the oxygen isotope composition of calcite (VPDB), δ18Ow is the oxygen isotope composition of seawater (VSMOW) and T is temperature of the water (°C). The annual temperature variation for seawater at Esperance of 16–20 °C (Cann and Clarke, 1993) was used in the calculation. The δ18Ow values for regional ocean water were assumed to be 0‰ VSMOW (Rahimpour-Bonab et al., 1997). The δ18Oeq values of the calcite were adjusted for the MgCO3 content of the

5.1. Age of the tests The depths from which unstained M. vertebralis of the GAB (Fig. 2A) were recovered (50–100 m) indicate that they were probably not taken from living populations. M. vertebralis live today in tropical waters of no greater than 50 m depth, and thrive in the temperate waters no greater than 20 m at Esperance, Western Australia. Three of

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J.M. Rivers et al. / Sedimentary Geology 213 (2009) 113–120 Table 2 Locations, depths, and isotopic compositions of cement from the Great Australian Bight Sample number

Latitude

Longitude

Depth

δ18O

δ13C

ACM-77 ACM-105 ACM-48 ACM-49 ACM-67 ACM-72 GAB-04 GAB-12 GAB-48 GAB-53 PL94-03

32° 36.63′ S 32° 38.06′ S 34° 26.03′ S 34° 05.07′ S 33° 24.09′ S 32° 18.03′ S 32° 14.94′ S 32° 42.04′ S 33° 53.57′ S 33° 26.30′ S 34° 10.00′ S

132° 13.63′ E 129° 29.99′ E 134° 00.16′ E 134° 05.02′ E 132° 51.11′ E 133° 06.99′ E 128° 00.26′ E 129° 59.58′ E 125° 04.98′ E 125° 21.81′ E 134° 30.00′ E

75 65 96 84 94 51 36 70 182 62 80

2.5 2.1 2.2 1.6 2.1 1.3 1.5 1.5 2.1 1.9 2.3

2.9 3.0 2.6 2.3 2.7 1.5 2.3 2.2 2.6 2.4 3.0

shoreward after deposition (Fig. 1). This is unlikely for two reasons. Firstly, the disposition of relict grains on the southern Australian margin reflects the general trend observed in the Holocene environment wherein mollusc- and coralline algae-rich sediments are located inboard and bryozoan-dominated assemblages outboard (Rivers et al., 2007). Therefore, hydraulic sorting does not appear to have caused significant post-depositional migration of the sand and gravel fractions. Secondly, finer grained sediments appear to be transported seaward, not shoreward (James et al., 1994). A more likely explanation for anomalous ages of the M. vertebralis, given their location, is that the dated grains are a mixture of those deposited during the end of MIS 3 and those deposited after inundation during the sea-level rise associated with MIS 2. Highly altered, Fe-stained relict M. vertebralis, whose pores are infilled with micritic Mg-calcite cement (Fig. 2B), are interpreted to be older than the unstained grains. Based on the depth distribution of these relict grains, as well as previous studies of relict foraminifera (Li et al., 1996 and 1999) and 14C dates, James et al. (1997) and Rivers et al. (2007) argued that the relict grains were deposited during marine isotope stages 3 and 4, and exposed to the meteoric environment during the last glacial maximum. Thus, unstained grains of the GAB were mostly formed during inundation of the shelf after the last glacial maximum (MIS 2), whereas stained M. vertebralis tests likely accumulated in interstadial sea-level stands of MIS 3 and 4, when sealevel fluctuated between ~30 and 70 m below present (Fig. 4). 5.2. Explaining the δ18O and δ13C values of Pleistocene M. vertebralis Tests and Carbonate Cement

Fig. 2. Photographs of the three types of Marginopora vertebralis used in this study: A) a modern specimen collected from living populations at Esperance, WA, showing no alteration, B) a white specimen from the Great Australian Bight (GAB) mid-shelf, with dissolution features and empty chambers, and C) a relict specimen from the GAB midshelf with Fe-oxide staining and chambers infilled with micritic cement.

the five 14C dates on groups of ~ 10 tests of white M. vertebralis with minimal discoloration and with empty chambers indicates late MIS 2 or early MIS 1 formation of the tests (Table 3), verifying the antiquity of tests from the GAB. Because of the exponential nature of 14C decay, the dates are most reflective of the youngest tests, and therefore are considered minimum average ages, with most grains in the group likely older. Two samples that have corrected dates of between 17– 22 ka, implying deposition during the last glacial maximum. These older samples, however, were collected from depths of 62 and 55 m respectively, whereas sea-level during the last glacial maximum was ~ 120 below current level (Fig. 4). One possible explanation for this is that these grains were transported in the marine realm 50–100 km

Inasmuch as the characteristics of ocean water throughout the GAB are broadly similar to those of Esperance, Pleistocene M. vertebralis from the GAB are 18O- and 13C-rich with respect to expected equilibrium δ18O and δ13C values of carbonate from modern seawater. Three possible explanations for these differences include vital effects, diagenesis, and differences between late Pleistocene and modern GAB oceanography. Vital effects normally involve incorporation of metabolically 12Crich CO2 into skeletal material (e.g. Dillman and Ford, 1982). Such effects have been invoked for anomalously low δ13C values of M.

Table 3 Radiocarbon ages derived for unstained Marginopora carbonate particles Sample latitude

Sample longitude

Depth (m)

Uncorrected and uncalibrated age (BP)

Age corrected for marine reservoir effect and calibrated

32° 39.60′ S 33° 16.80′ S 32° 49.20′ S 34° 31.60′ S 32° 22.05′ S

128° 29.40 E 125° 18.00 E 125° 57.60 E 122° 58.20 E 131° 31.00 E

55 62 56 95 55

9030 ± 80 16,590 ± 150 12,740 ± 100 10,740 ± 100 20,130 ± 170

7630 ± 140 17,280 ± 160 12,090 ± 200 9960 ± 280 21,420 ± 350

Ages are shown both in conventional years before present and calibrated years corrected for the marine reservoir effect (refer to text).

J.M. Rivers et al. / Sedimentary Geology 213 (2009) 113–120

Fig. 3. Isotopic composition of the three types of Marginopora vertebralis used in this study and Mg-calcite cements from the Great Australian Bight (GAB). MIS refers to marine isotope stage.

vertebralis in the tropical realm (Wefer and Berger, 1980; Wefer et al., 1981). Isotopic data from modern Esperance populations show that in waters of the southern margin, M. vertebralis does have δ18O values in isotopic equilibrium with modern seawater, but the δ13C values are higher by ~ 1‰ with respect to equilibrium values. Incorporation of metabolic 12C-rich CO2 into the skeletal structure would result in low δ13C values in M. vertebralis, the opposite to what is observed. Even if the assumed δ13C value of DIC is too low by 1‰ (Rahimpour-Bonab et al., 1997), thereby explaining the slightly higher δ13C value of Holocene foraminifera relative to equilibrium values, the additional 2‰ higher values of the Pleistocene grains is puzzling. The three main pathways of early diagenesis in this realm are carbonate dissolution, especially of aragonitic components, precipitation of Fe-oxide coatings, and intragranular precipitation of micritic Mg-calcite cements (~12 mol% MgCO3) (Rivers et al., 2008). Early stage dissolution and Fe-oxide staining would have little effect on the carbon and oxygen isotopic composition of M. vertebralis. Infilling of test chambers with micritic cements, however, could affect the measured isotopic compositions of these grains. Of the white Pleistocene M. vertebralis from the GAB, only specimens with no apparent cement infill were chosen for this study. Inasmuch as the relict Pleistocene M. vertebralis from the GAB (which are infilled with micritic cement) have similar values as the empty white Pleistocene M. vertebralis from the GAB (Fig. 3), there is no evidence that such infilling by cement has a primary impact on the isotopic compositions of the foraminifera. Further, if the infilling cement primarily determined the isotopic composition of the foraminfera grains, the carbon isotopic composition of the cement and M. vertebralis would be the same. Because the carbon isotopic composition of the foraminifera grains is distinct from that of the cement, the isotopic composition of the grains apparently reflects that of the skeletal material. There does, however, appear to be a secondary impact on the isotopic compositions of the relict grains by the cements that infill them. The GAB cements have the same range of δ18O values (1 to 3‰) as the Pleistocene M. vertebralis tests from the region, but the δ13C values are distinctly lower by ~ 1‰ (Fig. 3). Thus, the δ13C values of grains containing marine cements should be lowered slightly, but the δ18O values would be unaffected. In support of this, the isotopic compositions of relict M. vertebralis trend toward lower δ13C values by an average of 0.5‰ when compared with unstained equivalents (Fig. 3). This shift is interpreted to reflect the influence of the relatively 13 C-depleted marine cement that infills the relict grains. The δ18O value of seawater and water temperature are the two primary extrinsic controls on the oxygen isotopic composition of carbonate secreted in the marine realm. If it is assumed that GAB M.

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vertebralis were secreted in equilibrium in modern ocean water of normal salinity, then, correcting for Mg content of M. vertebralis (12 mole %), ambient water temperatures during formation would range between 6° and 15 °C. These values are below the measured ocean water temperatures for the region (15°–22 °C) (James et al., 2001), confirming the foraminfera did not form in modern ocean water. The calculated values are also lower than the temperature tolerance of this large foraminifera (18 °C) (Murray, 1973). The depth of recovery and 14C ages suggest that the GAB M. vertebralis were deposited during MIS 2, 3 or 4, at times when sea level fluctuated between 30 and 70 m below current level. If the temperature of ocean water during Pleistocene M. vertebralis growth is assumed to be 18 °C (the lower limit of M. vertebralis tolerance), and the sea water δ18O value is assumed to be between 0 and +1‰ (VSMOW) during these interstadials (e.g. Schrag et al., 2002), then the skeletal δ18O values should be between 0.3 and 1.3‰, still too low to account for the 1–3‰ measured. The most plausible explanation for the anomalously high δ18O values of Pleistocene M. vertebralis are changes in Pleistocene seawater in the GAB that caused enrichment in 18O relative to normal seawater. Preferential removal of 16O during evaporation enriches remaining seawater in 18O (Craig and Gordon, 1965) while increasing salinity. The effect of evaporation on the isotopic composition of invertebrate skeletal material secreted in hypersaline marine waters has been documented for aragonitic otoliths in fish. In a study of fish from Tuamoto Archipelago, Dufour et al. (1998) found an increase in the δ18O values of 0.14‰ in fish bone per 1 ppt increase in seawater salinity. Likewise, in hypersaline Shark Bay on the western Australian coast, Bastow et al. (2002) reported a 0.10‰ increase in fish otoliths per 1 ppt increase in salinity. Corlis et al. (2003) measured the effect of evaporation on δ18O of waters from Spencer Gulf, a saline inverse estuary directly east of the GAB (Fig. 1). Using their data, we calculate an enrichment factor of 0.11‰ δ18O increase per ppt increase in salinity in February (summer) and 0.15‰ per ppt in June (winter) for gulf waters of greater than 38 ppt salinity. All of these factors are in reasonable agreement with the isotopic and salinity enrichment ratio of Craig and Gordon (1965) of 0.11‰ per ppt for low latitude shallow ocean waters. Therefore, modifying the oxygen isotope equilibrium equation of Kim and O'Neil (1997) to account for salinity increases resulting from evaporation gives: δ18 Oeq = 25:778−3:3334ð43:704 + TÞ0:5 + δ18 Ow + 0:72 + 0:114ðS−36Þ ð3Þ where δ18Oeq is the isotopic value of precipitated carbonate (VPDB), δ18Ow is the isotopic value of seawater (VSMOW), T is temperature of precipitation (°C), 0.72 is the correction for the Mg content of M.

Fig. 4. Late Quaternary sea level fluctuations (after Chappell and Shackleton, 1986). The last glacial maximum (LGM) is shown.

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vertebralis (Tarutani et al., 1969) and S is ocean water salinity (ppt). Assuming an ocean temperature of 18 °C, a δ18O value of 0.5‰ (VSMOW) for Pleistocene sea water, δ18O values from 1 to 3‰ for δ18Oeq, a range from 38 to 56 ppt is calculated for the salinity. Open shelf waters along the southern Australian margin currently have salinities of b38 ppt (Ridgway and Condie, 2004). Hypersaline waters of the southern Australian margin are restricted to inboard embayments, the largest being Spencer Gulf (Fig. 1). Spencer Gulf is an inverse estuary with head waters reaching annual mean salinities of 45 ppt (Corlis et al., 2003). M. vertebralis have been identified there in sediments deposited during MS 5e (last interglacial), but are not found living in the estuary today (Cann and Clarke, 1993). On the west coast of Australia, however, in saline waters of outer Shark Bay (but not the more saline stromatolite environments of Hamelin Pool), M. vertebralis currently live in waters with salinities of 35–56 ppt, and temperatures of 17–25 °C (Davies, 1970). Carbonate banks of Shark Bay host seagrass-algal communities. Such communities produce coralline-algal-rich sediments similar to Pleistocene sediments of the GAB (Rivers et al., 2007). Importantly, the encrusting foraminifer Nubecularia Lucifuga, ubiquitous in GAB Pleistocene sediments from which M. Vertebralis were taken, is described as a “main foraminiferal species” in Posidonia seagrass communities at Shark Bay (Davies, 1970). A major constituent of Pleistocene sediments from the GAB (30% of relict grains) are abraded intraclasts composed of sand- to siltsize skeletal grains within a micrite matrix (Rivers et al., 2007). Such grains were interpreted to have formed in muddy, low energy environments, whereas the GAB shelf today is a high-energy stormdominated environment. As a result of baffling, seagrass bank sediments in Shark Bay have as much as 30% fine-grained matrix (Davies, 1970). Such environments are envisioned to be similar to the environments during the Pleistocene in which intraclasts formed in the GAB. Thus, the most parsimonious explanation for the anomalously high δ18O values of Pleistocene M. vertebralis of the GAB is that during MIS 2, 3 and 4, the GAB was covered by vast hypersaline seagrass environments similar to those of Shark Bay today. Unfortunately, M. vertebralis were not available from Shark Bay to analyze for comparison of isotopic composition. Independent support for the formation of waters of elevated salinity on the GAB shelf during lower sea-level stands is provided by Swart et al. (2000), who demonstrated that pore-waters recovered from slope sediments south of the GAB were of elevated salinity. They hypothesized that these waters formed on the shelf during lower sealevel stands, and were forced into the slope sediments under a hydrostatic head. The high δ18O values of some GAB cements (N2‰) suggest that these precipitates also formed in saline environments during the Pleistocene (Fig. 3). The positive correlation between the δ18O values of the cement with δ13C values mirrors that of the Holocene and Pleistocene M. vertebralis. Such a correlation points toward the probability that many of the cements formed during the Pleistocene in environments similar to that in which the M. vertebralis grew. The inferred age of the cement is supported by the observation that skeletal pores of many Pleistocene sediments are infilled with cement while most Holocene grains are empty. Because the cement and the M. vertebralis have similar isotopic trends, it is probable that the δ13C values of both the cement and biogenic carbonate reflect changes in marine dissolved inorganic carbon rather than vital effects. One plausible hypothesis for the increase in 13C with salinity is the sequestration of 12C due to intense photosynthesis in these environments. Lazar and Erez (1992) studied marine-derived brines inhabited by microbial mat communities. During early stages of evaporation (in waters of salinities of 40–60 ppt), δ13C values of brine waters increased with salinity due to photosynthesis. Microbial mat communities are common at Shark Bay (Davies, 1970), and similar effects might be expected. Such a process would influence the carbon isotopic signatures of both skeletal carbonate and cement.

6. Implications for paleoceanography Modern seafloor environments in water depths of b20 m are restricted to an inner strip of the GAB that extends only a few kilometers from the shoreline (Fig. 1). Coastal regions of southern and western Australia with areally extensive marine water bodies of less than 20 m depth include the inverse estuaries Shark Bay (Davies, 1970) and Spencer Gulf (Nunes and Lennon, 1986). These restricted bodies are affected by both solar heating and high evaporation due to the semi-arid Australian climate (Hesse et al., 2004). Study of the sediments in these embayments as well as the open-ocean waters fronting them (James et al., 1997, 1999), indicates these evaporitic estuarian environments have distinct sediments and water chemistries, and hydrologic regimes largely independent of open ocean water dynamics, such as upwelling. Holocene benthic carbonate production is, to some extent, inhibited on the open shelves fronting these two extensive shallow-water bodies, due to the seaward flow of saline bottom waters. During portions of MIS 2, 3 and 4 when sea level fluctuated between 30 and 70 m below modern levels, shallow environments migrated over the vast flat GAB shelf, extending as much as 100 km from shore (Fig. 1). The isotopic compositions of GAB M. vertebralis from these periods indicate that these shallow waters were hypersaline, as much as 56 ppt, in spite of the northward movement of the subtropical convergence zone indicated by studies of planktonic foramifera from the southern Indian Ocean (Prell et al., 1979) and bryozoan mounds on the GAB slope (James et al., 2004). Bathymetry plays a key role in determining the nature of the past depositional environments of the southern margin and possibly the western margin. Bathymetric profiles of the southwestern Australian shelf (James et al., 1999) suggest shallow environments would have also been more extensive than today in that region during these interstadials. The isotopic composition of ooids from the northwest shelf of Australia (James et al., 2004) points to more saline conditions there as well during postglacial-maximum inundation. If modern analogies of ocean systems are indicative of past systematics, saline currents during MIS 2, 3 and 4 probably emanated from these coastal regions, as they do today from Spencer Gulf and Shark Bay (James et al., 1999), and inhibited production on portions of the open shelves, contributing to the low rates of deposition in these cool–water realms. This, in turn, contributes to the occurrence of older, relict sediment on the ocean floor in all of these regions (James et al., 1999, 2004; Rivers et al., 2007). Open unrimmed shelves and slow sedimentation rates in the cool– water carbonate realm lead to mixing of sediments deposited at different times from highly variable sedimentary environments. This can complicate interpretations of cool–water carbonate deposits. As an example, if for this study a benthic foraminifera with broader temperature tolerance than M. vertebralis had been chosen from the shallow GAB MIS 2 sediment, the δ18O values might have been interpreted to reflect lower water temperature during secretion, due to upwelling of cold deep ocean water, which occurs locally on the shelf today (James et al., 2001). The likelihood of such a misinterpretation is magnified in the rock record, where sediments have undergone greater alteration, regional paleoceanography systematics are poorly understood, eustasy patterns are generalized, and the environmental tolerances of the biota might not be ascertainable. Due in part to the Leeuwin Current, GAB water temperatures today (15–22 °C) are similar to those of Esperance, Western Australia (16– 20 °C). During MIS 2, the warm-water Leeuwin Current was not flowing (Wells and Wells, 1994) and sedimentological evidence indicates that the then southern margin mid-shelf was bathed in cooler, more nutrient-rich waters when compared with those of the present day (James et al., 2004; Rivers et al., 2007). Therefore, the presence of a warm-water foraminifer during MIS2 and subsequent disappearance during MIS 1 presents a conundrum. In the cooler waters of the southern Australian coast, M. vertebralis appear to grow

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in water depths of no more than 20 m (Cann and Clarke, 1993). Such habitats, wide-spread across the GAB during low-stands, are now restricted to isolated near-shore embayments (James et al., 2001). During the Holocene, confined pockets of M. vertebralis may have died off during periodic upwelling events and could not be replenished from the Western Australian stock. Thus, the white MIS 2 M. vertebralis were stranded as the ocean continued to rise above them during the Holocene. 7. Conclusions M. vertebralis, a large photosymbiotic foraminifera recovered from the Great Australian Bight, grew in shallow waters (b20 m) during interstadials. White tests, which have some dissolution features and only minor staining were deposited on the vast, flat GAB mid-shelf mostly during the waning stages of MIS 2. These grains were stranded as sea-level rose above them and the euphotic habitat diminished. Relict tests, Fe-oxide stained and cement-filled grains recovered from similar depths, are interpreted to be older and have been deposited during previous interstadials (MIS 3 and 4). The isotopic composition of the M. vertebralis indicates shallow portions of the GAB were bathed in highly saline waters as much as 56 ppt or more during the late Pleistocene. The Pleistocene sedimentary assemblage suggests environments analogous to those of outer Shark Bay expanded across the region during interstadial periods of MIS 2, 3 and 4. Such environments formed when seawater inundated the flat mid-shelf, and shallow-water environments of b20 m depth extended from the shore line to a distance of ~100 km. Shelf bathymetry plays a key role in determining environments of deposition on this extensive carbonate platform. Acknowledgements Research was supported by the Natural Sciences and Engineering Research Council of Canada (NPJ, TKK). Samples were collected using CSIRO R.V. Franklin cruises No. FR03/89, FR02/91, FR06/94, FR07/95, FR03/98, and the captain and crew members of each are thanked for their assistance. Radiocarbon dating was performed at the Isotrace Radiocarbon Laboratory, Toronto, Ontario, Canada. A. Vuletich and K. Klassen (Queen's University) assisted with laboratory analyses. We would like to thank Hank Chafetz, Brian Jones, and one anonymous reviewer for their helpful reviews. References Barker, R.W., 1960. Taxonomic notes on the species figured by H. B. Brady in his report on the foraminifera dredged by H.M.S. Challenger during the years 1873-1876. Special Publication, vol. 9. Society of Economic Palaeontologists and Mineralogists, Tulsa, Oklahoma. Bastow, T.P., Jackson, G., Edmonds, J.S., 2002. Elevated salinity and isotopic composition of fish otolith carbonate: stock delineation of pink snapper, Pagras auratus, in Shark Bay, Western Australia. Marine Biology 141, 801–806. Cann, J.H., Clarke, J.D.A., 1993. The significance of M. vertebralis (Foraminifera) in surficial sediments at Esperance, Western Australia, and in last Interglacial sediments in northern Spencer Gulf, South Australia. Marine Geology 111, 171–187. Cann, J.H., Belperio, A.P., Murray-Wallace, C.V., 2000. Late Quaternary Paleosealevels and Paleoenvironments Inferred from Foraminifera, Northern Spencer Gulf, South Australia. Journal of Foraminiferal Research 30, 29–53. Cann, J.H., Murray-Wallace, C.V., Riggs, N.J., Belperio, A.P., 2006. Successive foraminiferal faunas and inferred palaeoenvironments associated with the postglacial (Holocene) marine transgression, Gulf St Vincent, South Australia. The Holocene 16, 224–234. Chappell, J., Shackleton, N.J., 1986. Oxygen isotopes and sealevel. Nature 324, 137–140. Corlis, N.J., Veeh, H.H., Dighton, J.C., Herczeg, A.L., 2003. Mixing and evaporation processes in an inverse estuary inferred from δH and δO. Continental Shelf Research 23, 835–846. Coulbourn, W.T., Resig, J.M., 1975. On the use of benthic foraminifera as sediment tracers in a Hawaiian bay. Pacific Science 29, 99–115. Craig, H., Gordon, L.I., 1965. Deuterium and oxygen-18 variations in the ocean and the marine atmosphere. Proceedings of a Conference on Stable Isotopes in Oceanographic Studies and Paleo-temperatures. Lischi and Figli, Pisa, Italy, pp. 9–130. Davies, J.L., 1970. Geographical variation in coastal development. Longman, London.

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