Core site

The selected marine sediment core, MD05-2925, is 2,843 cm in length and was recovered in June 2005 during the IMAGES XIII-PECTEN (Past Equatorial Climate: Tracking El Niño) cruise on board the R.V. Marion Dufresne of the French Polar Institute (IPEV). The core site is located at the southern margin of the Western Pacific Warm Pool, 110 km to Fergusson Island, 50 km off southeastern tip of PNG (Fig. 1 and Supplementary Fig. 1).

The core sediment is composed of a mixture of biogenic carbonate and silty clay40. The chlorophyll level of 0.2 mg m−3 (ref. 41) for surrounding surface water in eastern PNG suggests low regional productivity. The dissolved-oxygen concentrations are high (>3 ml l−1) through the whole water column including bottom waters of eastern PNG42. The local benthic oxygen flux, reflecting organic matter remineralization, is only 0.1 mol m−2 per year (ref. 43). It is lower than the values of 0.8 mol m−2 per year for the reducing margins (notably in the eastern boundary upwelling systems and North Indian Ocean)43. These data indicate an oxidative condition at this study site. The upper 1,510 cm was used in this study.

Age model

The age model was established based on accelerator mass spectrometry (AMS) radiocarbon (14C) dates (Supplementary Table 1) and oxygen isotope stratigraphy (Supplementary Fig. 12). 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 software44 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 Sea45. The chronology was based on linear interpolation between calibrated 14C dates (Supplementary Table 1).

For the depths >292 cm, the age model was developed by correlating the composite benthic foraminiferal oxygen isotopic data of core MD05-2925 to the LR04 stack record46 (Supplementary Fig. 12). Composite benthic foraminiferal oxygen isotope data are established with benthic foraminifera (>250 μm, 2–4 individuals each depth), including the Uvigerina spp. (201 samples), Cibicidoides wuellerstorfi (11 samples) and Bulimina spp. (7 samples) at core depths of 157–1,897 cm (Supplementary Fig. 13). Measurement of δ18O data, relative to Vienna Pee Dee Belemnite carbonate standard, was performed on a Micromass IsoPrime isotope ratio mass spectrometer with 1σ reproducibility of ±0.05‰ (ref. 47). δ18O offsets of C. wuellerstorfi (+0.64‰)48 and Bulimina spp. (−0.11‰)49 from Uvigerina spp. were corrected. This age model is supported by the last occurrence of G. ruber (pink) occurred at depths of 830–835 cm, corresponding to 129.8 kyr BP (Supplementary Fig. 12), consistent with the observation in the southern South China Sea50.

Screening for diagenesis

Scanning electron microscopy images of 30 uncleaned individuals of planktonic foraminifera G. ruber (white, s.s. 250–300 μm) at six depths of 477 (50.1 kyr BP) and 617 cm (81.6 kyr BP) with low REE content, 527 (56.8 kyr BP) and 577 cm (73.2 kry BP) with high REE content and 877 (135.0 kyr BP) and 917 cm (146.1 kyr BP) with moderate REE content (Supplementary Fig. 14) were carefully screened. Thirty more uncleaned individuals picked from six depths (87, 267, 787, 1,087, 1,317 and 1,477 cm), respectively, at marine isotope stages 1, 2, 5, 6, 7 and 8 were also checked with scanning electron microscopy. No nodules of Mn-Fe oxides were noticeable and all shell walls were intact and primitive (Supplementary Fig. 14). Additional careful inspection under microscope did not observe Mn-Fe oxides for 1,200 tests from the selected 12 depths. For conservative consideration, we still applied a full cleaning procedure on all samples.

Measurement of foraminiferal trace elements

REE contents of down-core planktonic foraminifera G. ruber (white, s.s. 250–300 μm) were measured (Supplementary Fig. 2). Although no Mn-Fe nodules were noticeable (Supplementary Fig. 14), G. ruber tests were cleaned with a full cleaning procedure for foraminiferal trace metal analysis, modified from refs 51, 52. About 20 foraminiferal individuals were gently crushed, placed in a Teflon vial and washed sequentially with the following reagents (all at pH 8.5–9.0): (i) ethanol+H 2 O, (ii) 1% H 2 O 2 , (iii) 0.56 M NH 4 Cl and (iv) 0.43 M NH 2 OH. Cleaned tests, polished with 10−3 M HNO 3 to dissolve a possible thin post-depositional magnesium-rich surface layer51,52, were rinsed with ultrapure water three times to wash off the residues of chemicals and then dissolved in 5% HNO 3 for instrumental analyses. All chemical procedures were performed on a class-100 laminar-flow bench in a class-10,000 clean room in the High-precision Mass Spectrometry and Environment Change Laboratory (HISPEC), Department of Geosciences, National Taiwan University.

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 ICP-SF-MS, Thermo Fisher ELEMENT II, equipped with a dry introduction Cetac ARIDUS20 system. Two-month 2σ reproducibility is ±1.9–6.5%. Mg/Ca, Mn/Ca and Fe/Ca ratios with respective 2σ errors of ±0.23%, ±0.68% and ±2.7% were determined on the same ICP-SF-MS, equipped with a quartz Scott-type double-pass spray chamber53.

An insignificant correlation between Mg/Ca and Fe/Ca data (Supplementary Fig. 3a) indicates the effectiveness of the cleaning techniques. Moreover, the measured REE/Ca patterns (Fig. 2) are different from shale-like patterns for uncleaned foraminifera with greater light REE (LREE) contents enrichment and unclear Ce anomalies54. We also tested our cleaning procedure/analytical technique by an interlaboratory comparison for analysing REE/Ca ratios of benthic foraminifera C. wuellerstorfi sample from core GGC-15 (ref. 20). The results showed that our REE data replicate measurements using a REE cleaning method at the University of Cambridge (Fig. 5 of ref. 20). Detailed instrumentation and fidelity of our methodology for foraminiferal test REE/Ca determination are described in ref. 20.

Nd isotopic measurement

Planktonic foraminifera G. ruber and sediment (<63 μm) samples were collected from two depth intervals 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 (Supplementary Fig. 2). The picked planktonic foraminifera samples were cleaned with the same protocol for REE/Ca ratio analysis and then dissolved in 2 M HNO 3 . The sediment samples were first cleaned with 10% CH 3 COOH to remove carbonate, and subsequently cleaned with a reductive reagent (1 M NH 2 OH·HCl in 25% CH 3 COOH) to remove possible Fe-Mn phases on the sample surface55. The cleaned sediment samples were decomposed in a mixed solution of HF, HClO 4 and HNO 3 , and then dissolved in 2 M HNO 3 .

Neodymium in the 2 M HNO 3 dissolved samples was extracted by a two-stage column separation56. 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, 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 (or ±0.27 ɛ; 2σ, n=13). All 143Nd/144Nd ratios were calibrated to the reported value relative to the La Jolla standard value of 0.511858 (ref. 57). Sample 143Nd/144Nd ratios [(143Nd/144Nd) sample ] are expressed as ɛ notation defined by an equation of ɛ Nd =[(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 (ref. 58).

Modelling simulation

The simulated precipitation and climatological records used in this study are from an orbital-accelerated transient run using FOAM conducted by Kutzbach et al.21 and re-analysed by Shi et al.22. FOAM, a fully coupled, mixed-resolution, and high-throughput general circulation model, provides a good simulation of mean condition and variability59. With a factor of 100, FOAM was integrated for 2,820 years under orbital forcing only to obtain climate evolution over the past 282 kyr. Changes in global ice volume/sea level and greenhouse gases were not considered. The spatial resolution is set to 4° × 7.5° for atmosphere and 1.4° × 2.8° for ocean. Because of the limitation of orbital acceleration, it is difficult for the deep ocean to reach equilibrium so that the full potential of the deep ocean feedback cannot be achieved. However, in previous studies21,22,60, the responses of monsoon precipitation, mostly considered as a response to the changes in the atmosphere-surface ocean system, to the orbital insolation can be successfully retrieved in the annual variability. A detailed description on the transient experiment is available in ref. 21.