Material and Methods Summary
Chapter 2. Material and Methods Summary
MD05-2925
Site MD05-2925 (9.3oS, 151.5oE, water depth 1661 m) was collected during the 2005 PECTEN (Past Equatorial Climate: Tracking El Niño) cruise which supported by International Marine and Climate Changes (IMAGES) Project on the research vessel Marion Dufresen. MD05-2925 is located at the northern slope of Woodlark Basin in the Solomon Sea, which is the passage of surface and subsurface water masses between low- and middle-latitude South Pacific Ocean gyre and cross equatorial currents. The New Guinea Counter Current and South Equatorial Current, as low-level western boundary currents, flow northwest through the Solomon Sea, transporting southern subtropical waters to the equatorial region via Vitiaz Strait [Melet et al., 2010a; Melet et al., 2010b; Cravatte et al., 2011; Grenier et al., 2011;
Melet et al., 2011]. The average sea surface temperature (SST) is 28.5 oC [Locarnini et al., 2010], with seasonally apart from the core region of the warm surface water mass in the Indo-Pacific warm pool (IPWP).
The precipitation sources are mainly comes from the south Indian Ocean and the Coral Sea [Gimeno et al., 2012], and are focused during the July-August-September (JAS). Intertropical convergence zone (ITCZ) and the South Pacific Convergence (SPCZ) are integrated here, which resulting as a bridging from the above the regional Carbonate Compensation Depth (CCD). The chlorophyll level of
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0.2 mg/m3 [Radenac et al., 2012] for surrounding surface water in eastern Papua New Guinea (PNG) suggests low regional productivity. The dissolved-oxygen concentrations are high (>3 mL/L) through the whole water column including bottom waters of the eastern PNG [Garcia et al., 2010]. The local benthic oxygen flux, reflecting organic matter remineralization, is only 0.1 mol/m2/yr [Jahnke, 2003]. It is lower than the values of 0.8 mol/m2/yr for the reducing margins [notably in the eastern boundary upwelling systems (EUBS) and North Indian Ocean, Jahnke, 2003].
These data indicates an oxidative sea floor condition at this study site.
Oxygen isotope
Planktonic foraminifera, Globigerinoides ruber (white, s.s., 250-300 μm), were hand picked under microscope for the oxygen isotope measurement. Each samples contained 7-10 individuals and were immersed with methanol and ultrasonicated for 10 seconds, and rinsed with deionized water 5 times. Samples were immersed afterward in the hyperchloride sodium (NaOCl) for 24 hours, and then measured by an isotopic ratio mass spectrometer (IRMS), Micromass IsoPrime, housed in the National Taiwan Normal University. Long-term precision is 0.10‰
(2RSD, N = 701) with respect to Vienna Pee Dee Belemnite (VPDB) [Lo et al., 2013].
Trace elements/Ca (TE/Ca) measurements
For TE/Ca (mainly Mg and REEs) ratio measurements, 20-30 individuals were gently crushed and transported into 1.5 mL Teflon vial. The clean procedure is as follows: (1) Foraminiferal fragments were immersed with ethanol, ultrasonicated for 20 minutes, and then rinsed by Milli-Q ultra-pure water 3 times. (2) An aliquot of 0.45 mL 3% H2O2 was added for 2 hours to decompose organic material. (3) NH4Cl
22
(0.45 mL, 1.0 N) was added for 20 minutes to adsorb cations on chamber surface. (4) NH2OH (0.45 mL, 0.01 N) was added for 3 hours to dissolve metallic oxides. (5) Diluted nitric acid (1 mL, 0.005 N) was added to polish the high-Mg content surface.
A sector field inductive coupled plasma mass spectrometer (SF-ICP-MS), Thermo Electron Element II, housed at the High-Precision Spectrometry and Environment Change Laboratory (HISPEC), Department of Geosciences, National Taiwan University, was used to determine trace element/Ca ratios following the methodology developed by Lo et al., [2014]. Two-year reproducibility for Mg/Ca analysis is
±0.21% (1 RSD).
We measured REE/Ca records of the planktonic foraminifer G. ruber (white, s.s. 250-300 μm). REE/Ca ratios were calculated using the ion beams of 46Ca, 139La,
140Ce, 141Pr, 146Nd, 147Sm, 153Eu, 160Gd, 159Tb, 163Dy, 165Ho, 166Er, 172Yb and 175Lu, detected on an inductively coupled plasma sector field mass spectrometer (ICP-SF-MS), Thermo Fisher ELEMENT II, equipped with a dry introduction Cetac ARIDUS [Shen et al., 2011] system. Two-month 2-sigma reproducibility is ±1.9-6.5%. The detailed instrumental settings and analytical methodology are described in Shen et al.
[2001].
Nd isotope:
Planktonic foraminifer G. ruber and sediment (<63 m) samples were collected from two depths of 472-477 cm (49.5-50.1 kyr BP, 580 individuals, >250 μm) and 537-542 cm (58.8-60.6 kyr BP, 250 individuals, >250 μm) of core MD05-2925. The picked planktonic foraminifer samples were cleaned with the same protocol for REE/Ca ratio analysis and then dissolved in 2 M HNO3. The sediment samples were first cleaned with 10% CH3COOH to remove carbonate, and
23
subsequently cleaned with a reductive reagent (1 M NH2OH∙HCl in 25% CH3COOH) to remove Fe-Mn phases on the sample surface [Bayon et al., 2004]. The cleaned sediment samples were decomposed in a mixed solution of HF, HClO4, and HNO3, and then dissolved in 2 M HNO3.
Neodymium in the 2 M HNO3 dissolved samples was extracted by a two-stage column separation [Pin and Zalduegui, 1997]. The REE fraction in the solution was purified from the remaining major and trace elements using Eichrom RE resin.
Neodymium was subsequently separated from the other REE with Eichrom Ln resin.
Neodymium isotopic compositions were measured by a multi-collector ICP-MS (MC-ICP-ICP-MS), Thermo Fisher Neptune, in the HISPEC. The measured
143Nd/144Nd ratios were normalized to 146Nd/144Nd = 0.7219 using an exponential law.
La Jolla standard was measured at 0.511811±0.000014 (2, n = 13). All 143Nd/144Nd ratios were calibrated to the reported value relative to the La Jolla standard value of 0.511858 [Lugmair et al., 1983]. Sample 143Nd/144Nd ratios [(143Nd/144Nd)sample] are expressed as ε notation defined by an equation of εNd = [(143Nd/144Nd)sample/(143Nd/144Nd)CHUR-1]×104, where the 143Nd/144Nd ratio of CHUR standard for Chondritic Uniform Reservoir [(143Nd/144Nd)CHUR] is 0.512638 [Jacobsen and Wassergurg, 1980].
δ18OSW-IVC calculation
To extract seawater δ18O (δ18OSW) values, we used a cultural based equation, SST = 16.5 − 4.8 × (δ18OC − δ18OSW) [Bemis et al., 1998] and a constant offset of 0.27‰ between carbonate VPDB and Vienna Standard Ocean Water (VSMOW) scales. Ice volume corrected δ18OSW (δ18OSW-IVC) was calculated using the method proposed by Waelbroeck et al. [2002]. We only calculated δ18OSW-IVC for the last
24
termination, because the complication of long-term sea level reconstruction.
Age model
A series of planktonic foraminiferal AMS 14C dates at 19 different depths, including 200 individuals of Globigerinoides sacculifer (>500 μm) each, from the upper 292 cm of the core were measured. Dates were calibrated to calendar ages (before 1950 AD) using CALIB 6.0.1 software [Stuiver et al., 2010] with a reservoir age difference (ΔR) estimated from the Marine Reservoir Correction Database (http://calib.qub.ac.uk/marine/). The calculated weighted mean ΔR value is 64 ± 23 years for the selected four sites around the Solomon Sea [Petchey et al., 2004].
The chronology was based on linear interpolation between calibrated 14C dates [Figure 2-1, Table 2-1].
Composite benthic foraminiferal oxygen isotope data are established with benthic foraminifera (>250 μm, 2-4 individuals each depth), including the Uvigerina spp. (171 samples), Cibicidoides wuellerstorfi (9 samples), and Bulimina spp. (6 samples) [Figure 2-2]. The δ18O offset calibration between Uvigerina spp. and C.
wuellerstorfi is +0.64‰ [Shackleton and Opdyke, 1973], and between Uvigerina spp.
and Bulimina spp. is -0.11‰ [Oba et al., 2006].
For core samples below 292 cm, the age model was constructed by correlating the composite benthic foraminiferal oxygen isotopic data of core MD05-2925 to the LR04 stack record [Lisiecki and Raymo, 2005]. All age control points are summarized in Table 2-2 In addition, the age model is supported by two planktonic/nannofossil biostratigraphic events. The last occurrence (LO) of G. ruber (pink) occurred between the depth of 830 and 835 cm, average dated 126.8 kyr, which is consistent with the observation in the southern South China Sea [Lee et al., 1999].
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The first occurrence (FO) of Emiliania huxleyi was observed between the depth of 1550 and 1580 cm, with an age estimate of about 293-299 kyr and within Marine Isotope Stage (MIS) 8. The overall sedimentation rate is ~10 cm/kyr. The relatively high sedimentation rates ranged from 10-40 cm/kyr occurred at upper section of MD05-2925 from the depth of 0-300 cm which represents ~170 years per sample. For lower section, each sample represents ~906 years.
Radiative forcing calculation
Details of the ΔRFGHG calculation should consider all the major greenhouse gases, and calculate the differences between certain past time and the pre-industrial greenhouse gases level ([CO2]0 = 280 ppm, [CH4]0 = 700 ppb, and [N2O]0 = 720 ppb, Ramaswamy et al., 2001). The full equations to determine ΔRFGHG are listed below:
ΔRFCO2 = 4.841 ln ([CO2]/[CO2]0) + 0.0906 (√[CO2] – √[CO2]0)
EQ1 ΔRFCH4 = 0.036 (√[CH4] – √[CH4]0) –
[0.47 ln{1 + 2.01x10-5 ([CH4] [N2O]0)0.75 + 5.31x10-15 [CH4] ([CH4] [N2O]0)1.52}] – [0.47 ln{1 + 2.01x10-5 ([CH4]0 [N2O]0)0.75 + 5.31x10-15 [CH4]0 ([CH4]0 [N2O]0)1.52}]
EQ2
The contribution of N2O to CH4-induced radio forcing, however, is too small, and the EQ2 could be simplified as:
ΔRFCH4 = 0.036 (√[CH4] – √[CH4]0)
EQ3
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The sum of EQ1 and EQ3 is the total ΔRFGHG during the past 360 kyrs, however, the CH4 only contribute <5% of the RF. Thus in this study we only consider the RF contributed by CO2 (EQ1).
Non-overlapping binned method
To build stacked N- and S-IPWP records, we followed the suggestions by Leduc et al. [2010] and considered three criteria for this dataset: (1) sites location within 12oN to 15oS, which is the main IPWP range [Yan et al., 1992; Gagan et al., 2004], (2) little or no influence by coastal upwelling, and (3) usage of specific proxies, Mg/Ca-derived SST and δ18OC records, of planktonic foraminifer, G. ruber (white, s.
s.). We adopted the age model for sites, ODP 806, MD97-2140, MD97-2141, MD98-2162, MD98-2170, MD98-2176, and MD98-2181. For records with available original radiocarbon ages from sites, including MD01-2378, MD01-2390, MD98-2165, and MD06-3067, we recalculated the age models using new CALIB 6.0.1 program [Stuvier et al., 2010]. The sea level change effect on δ18OSW was also corrected. The MATLAB code of a non-overlapping binned method was provided by Dr. D. W.
Oppo and Dr. B. K. Linsley [Oppo et al., 2009; Linsley et al., 2010]. We divided the total data into every 400-yr window and calculated the mean and standard error of mean for each time window.
ODP 806 (0.3oN, 159.4oE, water depth 2520 m, Lea et al., 2000), MD97-2140 (2.0oN, 141.7oE, water depth 2547 m, de Garidel-Thoron et al., 2005), ODP 871 (5.6oN, 172.3oE, water depth 1255 m, Dyez and Ravelo, 2012), TR163-19 (2.3oN, 91oW, water depth 2348 m, Lea et al., 2000), and ODP 1240 (0.0oN, 86.5oE, water depth 2921 m, Pena et al., 2008) have also been collected to calculate the average climatic sensitivity [Table 2-4]. Due to the effect of time resolution, we resampled
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ODP 806, ODP 871, TR163-19, and MD97-2140 into 4-kyr, and ODP 1240 into 1-kyr time resolution and then compared to the same time resampled Antarctica ΔT and pCO2 records to calculated ΔRFGHG.
EOF analysis
We applied an empirical orthogonal function (EOF) analysis of modern SST dataset [1950-2004 AD, Reynolds et al., 2002] for a sector from 20oS – 20oN, and 100oE- 180oE to determine the boundary between North- and South-IPWP [Chapter 3].
EOF1 factor identified clearly different SST variation groups between equator. EOF2 shows minor (9.7%) but significant inter-annual zonal (ENSO) control on the SST patterns.
FOAM
The simulated precipitation and other climatological records [Chapter 4]
were calculated from an orbital-accelerated transient run using the coupled fast ocean-atmosphere model (FOAM, Kutzbach et al., 2008; Shi et al., 2011). With a factor of 100, the experiment was integrated for 2840 years under the orbital forcing only to obtain the climate evolution during the past 284 kyr. Changes in global ice volume/sea level and greenhouse gases were neglected. The spatial resolution was set to 4°×7.5° for atmosphere and 1.4°×2.8° for the ocean. A detailed description is available in Kutzbach et al. [2008], and Shi et al. [2011].
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1 NSF-Arizona AMS Laboratory in University of Arizona (U. Arizona), Tucson, USA.
2 Rafter Radiocarbon Laboratory, Institute of Geological and Nuclear Science (GNS), New Zealand.
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Table 2-2. Depth-age pairs of control points by δ18O graphic fitting to LR04 stack [Lisiecki and Raymo, 2005].
Depth (cm) Age (kyr)
389 35.7
489 54.4
568 72.2
651 84.9
739 100.6
803 118.3
844 129.1
883 141.6
997 165.2
1112 192.0
1320 244.6
1432 260.6
1525 287.8
1702 324.4
1767 334.4
1866 358.6
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Table 2-3. Selected sites for stacked N- and S-IPWP records.
Core Location
(Latitude, and Longitude) References North-IPWP group
(orange circles in Figure 3-1 and 3-S2) ODP 806 0.3oN, 159.4oE Lea et al., 2000
MD97-2140 2.0oN, 141.7oE de Garidel-Thoron et al., 2005 MD98-2181 6.3oN, 125.83oE Stott et al., 2002; 2004
MD06-3067 6.5oN, 126.5oE Bolliet et al., 2011 MD97-2141 8.8oN, 121.3oE Rosenthal et al., 2003 MD01-2390 12.1oN, 113.24oE Stenike et al., 2006
South-IPWP group
(green circles and star in Figure 3-1 and 3-S2) MD98-2162 4.4oS, 117.5oE Visser et al., 2003 MD98-2176 5.0oS, 133.4oE Stott et al., 2004 MD05-2925 9.3oS, 151.5oE This Study MD98-2165 9.7oS, 118.3oE Levi et al., 2007 MD98-2170 10.6oS, 125.4oE Stott et al., 2004 MD01-2378 13.1oS, 121.7oE Xu et al., 2008
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Table 2-4. Selected sites for greenhouse gases radiative forcing calculation.
Core Location
(Latitude, and Longitude) References North-IPWP group
ODP 806 0.3oN, 159.4oE Lea et al., 2000
MD97-2140 2.0oN, 141.7oE de Garidel-Thoron et al., 2005 ODP 871 5.6oN, 172.3oE Dyez and Ravelo., 2012
South-IPWP and EEP group TR163-19 2.3oN, 91oW Lea et al., 2000 ODP 1240 0.0oN, 86.5oE Pena et al., 2008 MD05-2925 9.3oS, 151.5oE This Study
36
Figure 2-1. Age model of core MD05-2925. Blue cross symbols denote calibrated AMS 14C dates used for upper 292 cm. (A) a composite MD05-2925 benthic foraminiferal δ18O record, (B) MD05-2925 G. ruber δ18O record. Dashed lines are the age control points by comparing with (C) global composite LR04 [Lisiecki and Raymo, 2005].
37
Figure 2-2. Composite benthic foraminiferal oxygen isotope record of core MD05-2925. Cross symbols with dark gray dashed line denote the oxygen isotope data for Uvigerina spp. Red dots and green triangles are the corrected 18O data of C.
wuellerstorfi [Shackleton and Opdyke, 1973] and Bulimina spp. [Oba et al., 2006], respectively.
38