The amplifying effect of Indonesian Throughflow heat transport on Late Pliocene Southern Hemisphere climate cooling

Earth and Planetary Science Letters 500 (2018) 15–27

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Earth and Planetary Science Letters www.elsevier.com/locate/epsl

The amplifying effect of Indonesian Throughflow heat transport on Late Pliocene Southern Hemisphere climate cooling David De Vleeschouwer a,∗ , Gerald Auer b , Rebecca Smith c , Kara Bogus d , Beth Christensen e , Jeroen Groeneveld a , Benjamin Petrick f , Jorijntje Henderiks g , Isla S. Castañeda c , Evan O’Brien h , Maret Ellinghausen a , Stephen J. Gallagher i , Craig S. Fulthorpe j , Heiko Pälike a a

MARUM-Center for Marine and Environmental Sciences, University of Bremen, Klagenfurterstraße 2-4, Bremen, 28359, Germany Department of Biogeochemistry, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Kanagawa, 237-0061, Japan Department of Geosciences, University of Massachusetts at Amherst, Amherst, MA 01003, USA d International Ocean Discovery Program, Texas A&M University, College Station, 77845-9547, USA e Environmental Studies Program, Adelphi University, Garden City, NY, 11530, USA f Max-Planck-Institut für Chemie, Mainz, 55128, Germany g Department of Earth Sciences, Uppsala University, Uppsala, 75236, Sweden h Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843, USA i School of Earth Sciences, University of Melbourne, Melbourne, 3010, Australia j Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin, 78758-4445, USA b c

a r t i c l e

i n f o

Article history: Received 22 December 2017 Received in revised form 16 July 2018 Accepted 23 July 2018 Available online xxxx Editor: M. Frank Keywords: Indonesian Throughflow Pliocene astronomical forcing Leeuwin Current West Pacific Warm Pool M2 event

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a b s t r a c t An unusually short glaciation interrupted the warm Pliocene around 3.3 Ma (Marine Isotope Stage (MIS) M2). Different hypotheses exist to explain why this glaciation event was so pronounced, and why the global climate system returned to warm Pliocene conditions relatively quickly afterwards. One of these proposed mechanisms is a reduced equator-to-pole heat transfer, in response to a tectonically reduced Indonesian Throughflow (ITF). The ITF is a critical part of the global thermohaline ocean circulation, transporting heat from the Indo-Pacific Warm Pool to the Indian Ocean. When ITF connectivity is reduced, the water and heat supply for the Leeuwin Current, flowing poleward along Australia’s west coast, is also diminished. To assess the possible relationship between mid-Pliocene glaciations and latitudinal heat transport through the Indonesian Throughflow, we constructed a multi-proxy orbitalscale record for the 3.7–2.8 Ma interval from International Ocean Discovery Program (IODP) Site U1463, off northwest Australia. The comparison of the Site U1463 record with paleoclimate records from nearby Site 763 and West Pacific Warm Pool Site 806 allows for a detailed regional reconstruction of Pliocene paleoceanography and thus for testing the proposed hypothesis. An astronomically-paced decrease in potassium content characterizes the late Pliocene interval of U1463. This record documents the increasing aridity of northwest Australia, periodically alleviated by reinforced summer monsoon precipitation under summer insolation maxima. The δ 18 O record of the planktonic foraminifer Globigerinoides sacculifer correlates exceptionally well with the sea surface temperature (SST) record from Site 806 in the West Pacific Warm Pool, even during MIS M2. Hence, Site U1463 preserves an uninterrupted ITF signal even during Pliocene glaciations. However, the U1463 δ 18 OG.sacculifer record exhibits a 0.5h offset with the nearby Site 763A record around MIS M2. This implies that Site 763A, about 500 km west of U1463, more closely tracks Indian Ocean SST records across MIS M2. The U1463 data reveal that heat-transport through the Indonesian Throughflow did not shut down completely during MIS M2, but rather its intensity decreased prior to and during MIS M2, causing Site 763A to temporarily reflect an Indian Ocean, rather than an ITF signal. We conclude that ITF variability significantly influenced latitudinal heat transport by means of the Leeuwin Current and hence contributed to the relative intensity of MIS M2. We propose the ITF valve between the Pacific and Indian Ocean as a positive

Corresponding author. E-mail address: [email protected] (D. De Vleeschouwer).

https://doi.org/10.1016/j.epsl.2018.07.035 0012-821X/© 2018 Published by Elsevier B.V.

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feedback mechanism, in which an initial sea level lowering reduces ITF heat transport, in turn amplifying global cooling by advancing the thermal isolation of Antarctica. © 2018 Published by Elsevier B.V.

1. Introduction The mid-Pliocene Warm Period (3.29–2.97 Ma) is often used as an analogue for on-going climate change and its potential consequences, considering that the current atmospheric CO2 concentration (Tans and Keeling, 2017) has recently reached Pliocene values, estimated around 400 ppm (Haywood and Valdes, 2004; Haywood et al., 2011 and references therein). The mid-Pliocene warm period also is the most recent period in the geological past with warmer than present climates, stable over tens of thousands of years (De Schepper et al., 2014; Haywood et al., 2013). In contrast, worldwide Pliocene climate records suggest significant climate variability, with evidence for brief cool intervals interrupting mid-Pliocene warmth (De Schepper et al., 2013; Dolan et al., 2015). Marine Isotope Stage (MIS) M2 (also known as the ∼3.3 Ma event) is such an interval that is globally recognized as a cooling event, interrupting the relatively warm background climate state. The large positive isotope excursion associated with this Pliocene glaciation event is an unusual and extraordinary feature of the benthic isotope stack (LR04, Lisiecki and Raymo, 2005) and megasplice (DV17, De Vleeschouwer et al., 2017b). The magnitude of the excursion (0.64h in the LR04 stack, 0.74h in the DV17 megasplice) suggests significant paleoclimate variability across MIS M2. Haug and Tiedemann (1998) interpreted this interval as a “failed attempt of the climate system to enter a full glacial state”. Besides the significant positive oxygen isotope excursion in globallydistributed benthic records, global sea level reconstructions indicate an important sea level low-stand around ∼3.3 Ma, after a glacio-eustatic sea-level fall of several tens of meters (Dwyer and Chandler, 2009; Miller et al., 2012; Naish and Wilson, 2009; Rohling et al., 2014). Different hypotheses have been put forward to explain why this glaciation event was so pronounced, and why the global climate system returned to warm Pliocene conditions relatively quickly. One of the proposed mechanisms is a reduced equator-to-pole heat transfer, in response to a tectonically reduced Indonesian Throughflow (De Schepper et al., 2014; Karas et al., 2011a, 2011b; Sarnthein et al., 2017). Indeed, the geometry of the Indonesian Throughflow (ITF) exerts an important control on the heat transport from the Pacific to the Indian Ocean (Fig. 1). Although the tectonic history of the ITF is complex, the general trend on a Myr-timescale indicates that the Indonesian seaway became more restricted with time in response to the northward movement of the Australian plate (Cane and Molnar, 2001; Gallagher et al., 2009; Molnar and Cronin, 2015). In the early Pliocene, the Throughflow was still relatively unbound by the Maritime Continent, whereas a more restricted connection between the Pacific and Indian Ocean is suggested from the Late Pliocene onwards (Christensen et al., 2017; De Schepper et al., 2014; Gallagher et al., 2009, 2014; Karas et al., 2011b, 2017). Changes in ITF connectivity also influence Leeuwin Current intensity, transporting warm, low-salinity, nutrient-deficient water poleward along the west Australian coast. The Leeuwin Current is unique in that it is the only southward flowing eastern boundary current in the Southern Hemisphere, linking the Timor Passage in the ITF region with southeast Australia (Church et al., 1989). The Leeuwin Current is a surface current that is only up to ∼300 m deep (Fig. 4 in Furue et al., 2017; Fig. 3 in Wijeratne et al., 2018; Fig. 9 in Woo and Pattiaratchi, 2008) and exhibits significant seasonal variability (Ridgway and Godfrey, 2015). On the northwest Australian shelf,

Fig. 1. Surface oceanography in the Indo-Pacific Region (adapted from Gallagher et al., 2009). Orange dots indicate the position of studied sites U1463 (northwest Australian Shelf), 763A (eastern Indian Ocean), 214 (equatorial Indian Ocean) and 806 (West Pacific Warm Pool). (For interpretation of the colors in the figure(s), the reader is referred to the web version of this article.)

surface boundary current is strongest from April till June, when transport exceeds 2 Sv at a section across the shelf at 116.7◦ E (Wijeratne et al., 2018), just 100 km west of Site U1463 studied in this paper. South of the North West Cape, the Leeuwin Current strength builds up, with a seasonal maximum around 8 Sv in June (Wijeratne et al., 2018). Seasonal variability in Leeuwin Current System strength is governed by the passage of a sea level pulse, which itself originates in the seasonal movements of wind stresses (Furue et al., 2017). The Leeuwin Current surface water constitutes a warm-water low-density cap, sufficient to restrain upwelling to water depths between 1000 and 500 m (Feng et al., 2007; Holloway, 1995; Pearce, 1991; Thompson et al., 2011; Waite et al., 2007). Here, we present new orbital-scale paleoclimate records from IODP Site U1463 (18◦ 57.9 S, 117◦ 37.4 E, Gallagher et al., 2017a), on the northwest Australian Shelf, downstream of the outflow of the ITF and upstream of the Leeuwin Current. We juxtapose the new U1463 paleoclimate archive with contemporaneous records in the eastern Indian Ocean (Site 763A, Karas et al., 2011b) and the West Pacific Warm Pool (Site 806, O’Brien et al., 2014; Wara et al., 2005) to reveal that the ITF outflow was reduced during the M2 event, leading to a more dimensionally constrained and lower intensity Leeuwin Current. 2. Materials and methods IODP Expedition 356 drilled sediments on the northwest Australian shelf and in the Perth Basin (Gallagher et al., 2017b), yielding long and continuous climate archives that already proved useful in detailing the million-year scale climate evolution of western Australia throughout the Neogene (Christensen et al., 2017; Groeneveld et al., 2017). Site U1463 was drilled at a present-day water depth of 145 m, which implies that the modern-day Leeuwin

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Fig. 2. Site U1463 summary. Core recovery and core images for Holes B, C and D. The age-depth model shows the shipboard age model produced from select calcareous nannofossil (solid circles) and planktonic foraminifers (open circles) datums (Gallagher et al., 2017a). Sedimentation rates assume a linear sedimentation between datums. The red rectangle indicates the stratigraphic interval considered in this study. mbsf = core depth below sea floor.

Current system dictates water column characteristics at Site U1463. During the Pliocene, however, Site U1463 was located at greater water depth (475–1200 m water depth) with benthic foraminifera indicating an outer shelf to upper bathyal marine environment (Gallagher et al., 2017a). In other words, Site U1463 was more susceptible to interactions between the Leeuwin Current and the Western Australian Current during the Pliocene (Fig. 1). A shipboard splice for Site U1463 was produced to a drilled depth of 275.88 mbsf (Fig. 2), which corresponds to a composite depth of 302.08 mcd (Fig. 3) (Gallagher et al., 2017a). In the framework of this Late Pliocene study, we sampled the stratigraphic interval between 268.84 and 312.81 mcd (i.e. 246.64–285.81 mbsf on Fig. 2) at 20 cm spatial resolution (from sections on- and off-splice). Based on the comparison of stable isotope data from on-splice and off-splice sections, we applied one small change to the shipboard splice: The composite depth of Core U1463B-30H (off-splice) was increased by 60 cm compared to the shipboard splice to better fit the equivalent core U1463C-30H (on-splice) (Supplementary Fig. 1, Supplementary Tables 1 and 2). Due to the combination of on- and off-splice isotope measurements, our isotope record has a median resolution of 2 kyr.

2.1. Oxygen and carbon stable isotope analyses A total of 410 stable carbon (δ 13 C) and oxygen (δ 18 O) isotope measurements were made on calcite tests of the shallow dwelling planktonic foraminifer Globigerinoides sacculifer (without sac-like chamber). Specimens were picked from the 315–355 μm size fraction to avoid size effects in δ 18 O values (Elderfield et al., 2002) and to be methodologically consistent with the nearby δ 18 Osacculifer record for ODP Site 763A (Karas et al., 2011b). All samples were measured on a Finnigan MAT 251 gas isotope ratio mass spectrometer connected to a Kiel III automated carbonate preparation device at the Center for Marine Environmental Sciences (MARUM), University of Bremen, Germany. Data are reported in standard deltanotation versus V-PDB. We calibrated all measurements against the in-house standard (ground Solnhofen limestone), which in turn is calibrated against the NBS-19 reference. Over the measurement period the standard deviations of the in-house standard (N = 164) were 0.04h for δ 13 C and 0.05h for δ 18 O. We affirm δ 18 Osacculifer as a proxy for sea surface temperature (SST) at U1463 through the direct comparison of oxygen isotope measurements with twelve TEX86 analyses. Two of these

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Fig. 3. U1463 Proxy records in the depth domain. (A) Spliced NGR-derived K (wt.%) record (in color) compared to the spliced XRF-derived K record (in gray). (B) Spliced NGR-derived U/Th record (in color) compared to NGR-derived U/Th data from off-splice sections (gray) and compared to six ICP-MS measurements (pink diamonds, ICP-MS data from De Vleeschouwer et al., 2017a). (C–D) Spliced planktonic carbon and oxygen isotope records (in color) compared to planktonic carbon and oxygen isotope data from off-splice sections (in gray). Black triangles indicate the stratigraphic position of biostratigraphic datums. mcd = meter composite depth.

twelve TEX86 temperature reconstructions have been previously published in Table S3 in Christensen et al. (2017). The ten additional TEX86 analyses were carried out in the same lab (University of Massachusetts Amherst) using the same method (described in Christensen et al., 2017). The cross-plot of oxygen isotopes and temperature data shows the correlation of the TEX86 -derived SST and the nearest δ 18 Osacculifer measurement from the same Hole (Fig. 4A). The downhole comparison between TEX86 and δ 18 Osacculifer measurements also includes off-splice isotope measurements (Fig. 4B) and thus allows for a comparison of the two proxies using data from different holes, but on the same composite depth scale. Both the cross-plot and downhole comparison corroborate δ 18 Osacculifer as a proxy for SST (Fig. 4) in the studied Pliocene interval of U1463.

2.2. X-ray fluorescence (XRF) core scanning The elemental composition of U1463 sediments was measured on the archive-half core surfaces using the third-generation Avaatech XRF core scanner at the XRF Core Scanning Facility of the Gulf Coast Repository at Texas A&M University. Measurements were taken at a spatial resolution of 2 cm, at source energies of 9 kV (no filter, 0.25 mA) and 30 kV (Pd filter, 1.25 mA) and a 6 s count time for each measurement at each energy. A few core sections that were too disturbed to obtain smooth surfaces, could not be scanned and appear as data gaps in Fig. 3A. Element intensities were obtained by processing raw X-ray spectra using the iterative least-square software (WIN AXIL) package from Canberra Eurisys.

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Fig. 4. Validation of δ 18 OG.sacculifer as a paleotemperature proxy by comparison with TEX86 sea surface temperature (SST) estimates. (A) Cross-plot showing the relationship between TEX86 -derived SST and the nearest oxygen isotope measurement from the same hole. Labels indicate meter composite depth (mcd) of the TEX86 samples. (B) Downhole comparison of TEX86 SST’s and δ 18 OG.sacculifer from on- and off-splice sections (colored and gray records, respectively).

2.3. NGR-derived K, U and Th data We quantified the potassium (K), uranium (U) and thorium (Th) content of U1463 sediments using the freely-available MATLAB algorithm of De Vleeschouwer et al. (2017a). The algorithm deconvolves natural gamma radiation (NGR) spectra, routinely measured on cores during IODP expeditions on DV JOIDES Resolution, into its three components. The spatial resolution of NGR measurements at U1463 is 20 cm in Cores U1463B-1H to 31F, Cores U1463C-1H to 31H and Cores U1463D-1H to 31H, and at 10 cm resolution in deeper cores (Fig. 3A–B). The NGR-derived and XRF-derived potassium data exhibit excellent overlap. It should be noted that the absolute K (wt.%) content of the cored sediment was quantified for the NGR-derived data only. The XRF-derived data provide insight in relative variability only, as this dataset is expressed in counts of X-ray photons. Yet, we aligned the zero-level of both records in Fig. 3A. Pink diamonds on Fig. 3A represent discrete ICP-MS measurements of the U/Th ratio (data from De Vleeschouwer et al., 2017a) and compare very well to the NGR-derived estimates of that ratio. 2.4. Total organic carbon analysis We decalcified ∼0.1 g of freeze-dried and ground sample using step-wise addition of 2N HCl to determine total organic carbon (TOC). After decalcification, samples were washed with distilled water to remove excess HCl and dried. TOC was analyzed using a Leco CS-300 carbon and sulfur analyzer. 2.5. Spectral analyses Wavelet transform of depth- and time-series was carried out using the R-package “biwavelet” (Gouhier et al., 2016), which is based on the original wavelet program written by Torrence and Compo (1998). All other spectral analyses in this study were carried out using the multitaper method (MTM) with three 2π -tapers (Thomson, 1982) and LOWSPEC background estimation (Meyers, 2012), as implemented in the R-package “astrochron” (Meyers, 2014). The confidence levels were calculated applying the LOWESSbased (Cleveland, 1979) procedure of Ruckstuhl et al. (2001). Depth-to-time conversion and bandpass filtering was carried out using functions “tune” and “bandpass” from the same R-package.

preserved calcareous nannofossils and planktonic foraminifers (Gallagher et al., 2017a). In the studied Late Pliocene interval, the C2An–C2Ar reversal was recognized at 310.5 mcd in Hole U1463B as well as in Hole U1463C (Gallagher et al., 2017a). We further constrained the biostratigraphic age control by applying an astrochronologic approach, based on the U1463 δ 13 Csacculifer and K (wt.%) data. The carbon isotope record is suitable for this purpose, as it exhibits the imprint of all three astronomical parameters: eccentricity, obliquity (or tilt) and precession (Fig. 5B and 5E). In addition, we preferred to avoid building an astrochronology based on the U1463 oxygen isotope record, as this dataset is central to the paleoclimate and paleoceanography interpretations in this paper and thus susceptible to circular reasoning. We visually correlated the carbon isotope record to a La2004 (Laskar et al., 2004) eccentricity–tilt–precession (ETP) composite with equal weight of the three astronomical parameters (Fig. 5D, Imbrie, 1985). We set the precession contribution to be positive when perihelion occurs during the austral summer half year, and negative during the austral winter half year. Hence, maxima ETP correspond to maxima in seasonality in the Southern Hemisphere. We use ETP rather than a local insolation signal because ETP exhibits a direct eccentricity imprint. In the older part of the studied interval (305–323 mcd), the K (wt.%) record exhibits a strong periodic character, which we correlated to the La2004 eccentricity–tilt (ET) composite. The astrochronologic age model consists of 10 tie-points between depth and age (Fig. 5C–F). These tie-points are constrained by three biostratigraphic datums in the studied interval (Fig. 5F), which have been determined shipboard during IODP Expedition 356 (Gallagher et al., 2017a). At the upper end of the studied interval, the imprint of a 405-kyr long eccentricity cycle in the δ 13 Csacculifer record is clear from 274 to 287 mcd (Fig. 5D), with high-amplitude precession cycles at the 405-kyr cycle maximum (Fig. 5D–E). The biostratigraphic calcareous nannofossil datums of Discoaster tamalis (2.8 Ma, 272.26 mcd) (Backman et al., 2012; Raffi et al., 2006) and Sphenolithus spp. (3.54 Ma, 298.88 mcd) allow for a clear-cut correlation of this stratigraphic expression of a 405-kyr cycle with its equivalent in the astronomical solution between 3.213 and 2.821 Ma (405-kyr cycle #8 counting back from the present day, Laskar et al., 2004). 4. Results and discussion

3. Astrochronologic age model and time-series analyses

4.1. Paleoceanographic change in the Indo-Pacific region across MIS M2

Site U1463 holds a complete stratigraphic sequence from the late Miocene to the early Pleistocene, with abundant and well-

The Site U1463 planktic G. sacculifer oxygen isotope record is similar to the equivalent record from Site 763A (data from

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Fig. 5. U1463 Astrochronologic age model. (A) Sedimentation rate of the astrochronologic age model. (B–E) The U1463 K (wt.%) and δ 13 CG.sacculifer records and their wavelet-transforms in the time domain. White dashed lines on the wavelet transforms trace the imprint of eccentricity, obliquity and precession. We use an Eccentricity– Tilt–Precession (ETP) composite (in light blue, Laskar et al., 2004) as a tuning target for δ 13 CG.sacculifer and a Eccentricity–Tilt composite (in red) as a tuning target for K (wt.%). (E–F) The U1463 K (wt.%) and δ 13 CG.sacculifer records in the depth domain. Black triangles indicate the stratigraphic position of calcareous nannofossil datums that served as the basis for the astrochronologic age model, as well as a shipboard magnetostratigraphic datum (3.596 Ma). Biostratigraphic datums come from Table T6 in Gallagher et al. (2017a) and consist of Top Discoaster tamalis (2.80 Ma), Top Sphenolithus spp. (3.54 Ma) and Top Reticulofenestra pseudoumbilicus (>7 μm) (3.70 Ma). mcd = meter composite depth.

Karas et al., 2011b), to the southwest (Fig. 1), with variable (ranging between −0.75 and −1.85h) yet stationary isotopic values from 4.0 to 3.0 Ma, and a distinct trend to heavier isotopes after 3.0 Ma (Fig. 6D). Both records have similar δ 18 Osacculifer values in the studied time interval (3.7–2.8 Ma). The similarities between the Site U1463 and Site 763A δ 18 Osacculifer records are not unexpected, as both sites are downstream of the ITF and within the present influence of the Leeuwin Current (Fig. 1). Nevertheless, the high degree of covariance ceases during a ∼130-kyr long interval spanning the M2 glacial event. The ∼130-kyr long interval extends from 3.38 to 3.25 Ma and is characterized by a ∼0.5h offset between both records (Fig. 6D). Interestingly, a similar offset is

present between the δ 18 Osacculifer records of Sites 763A and 806 (in the West Pacific Warm Pool, data from Wara et al., 2005) (Fig. 6E). These patterns suggest that heat-transport via the ITF did not cease completely during MIS M2. However, ITF volume significantly decreased during MIS M2, causing Site 763A to temporarily fall outside the influence of ITF outflow waters and the Leeuwin Current. This also implies that the Leeuwin Current was more constrained and extended over a narrower range at that time. The suggestion that U1463 continues to record an ITF signal, even during MIS M2, is corroborated by the strong agreement between the Site 806 Mg/Ca sea surface temperature (SST) record (O’Brien et al., 2014; Wara et al., 2005) and the evolution of the Site U1463

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Fig. 6. U1463 time-series. (A) The global eustatic sea level curve (Miller et al., 2012) suggests a drop in sea level of several tens of meters at MIS M2. (B) U1463 NGR-derived U/Th time-series (along splice), compared to an austral Tilt–Precession composite (Laskar et al., 2004) (C) U1463 NGR-derived K (wt.%) time-series (along splice) compared to obliquity (Laskar et al., 2004). (D) U1463 δ 18 OG.sacculifer time-series compared to the same proxy from Site 763A (Karas et al., 2011b). (E) Comparison between Site 763A and Site 806 δ 18 OG.sacculifer time-series (Wara et al., 2005). The gray vertical rectangle indicates a ∼130-kyr interval that is characterized by a ∼0.5h offset between Site 763A, and Sites U1463 and 806.

δ 18 Osacculifer record (Fig. 7B). These contemporaneous isotope and Mg/Ca SST records from either side of the IFT unambiguously reveal that heat transport from the Pacific to the Indian Ocean did not completely shut down during MIS M2. Nonetheless, the Site 763A δ 18 Osacculifer time-series is very similar to the equatorial Indian Ocean Mg/Ca SST record from Site 214, especially during the MIS M2 interval (Fig. 7C). This pattern may be interpreted along the lines of the hypothesis proposed by Cane and Molnar (2001) that was further advanced by Karas et al. (2009). These authors suggest that the switch in the source waters of the ITF from warm South Pacific to cooler North Pacific waters was a response to the gradual restriction of the Indonesian Seaway. Changes in the ITF source water are also likely to occur on orbital time scales, with a weaker Leeuwin Current and a reduction in Indonesian Throughflow Waters sourced from the West Pacific Warm Pool during the last five glacials (Spooner et al., 2011), which serve as interesting analogues for the MIS M2 glacial event. However, this interpretation does not explain the offset in oxygen isotope composition between Site U1463 and Site 763 during the M2 interval (Fig. 6D).

In order to explain the U1463–Site 763 offset, we suggest that the sea level drop associated with the M2-event (e.g. Miller et al., 2012) enhanced the effect of a tectonically reduced ITF. In this scenario, the sea level change across the M2 event considerably affected heat transport from the Pacific to the Indian Ocean, by limiting the total throughflow volume during periods of sea level low stand and vice versa. Reduced heat transport through the ITF directly impacted the Leeuwin Current, considering that the ITF represents the major source for this current. Our δ 18 Osacculifer data show that Site U1463 was still influenced by Leeuwin Current activity during MIS M2, whereas Site 763A was not for a ∼130 kyr-long interval straddling the event. The Leeuwin Current thus had a more limited spatial extent during MIS M2. This would imply that the Leeuwin Current was constrained to an even narrower strip along the northwest Australian coast, similar to today’s configuration between November and January, when the Leeuwin Current is at its weakest (Holloway, 1995; Smith et al., 1991; Wijeratne et al., 2018). As a consequence, the ability of the Leeuwin Current to suppress upwelling off northwest Australia diminished, allowing the equatorward southeast trade

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Fig. 7. (A) U1463 δ 18 OG.sacculifer time-series compared to the same proxy from Site 806 (Wara et al., 2005). (B) Comparison between the U1463 δ 18 Osacculifer and Site 806 Mg/Ca sea surface temperature (SST) time-series (O’Brien et al., 2014; Wara et al., 2005). (C) Comparison between the Site 763 δ 18 Osacculifer (Karas et al., 2011b) and Site 214 Mg/Ca sea surface temperature (SST) time-series (Karas et al., 2009). The gray vertical rectangle indicates a ∼130-kyr interval that is characterized by a ∼0.5h offset between Site 763A on the one hand, and Sites U1463 and 806 on the other hand.

winds to induce Ekman transport and upwelling at Site 763A, resulting in surface water cooling and an increase in salinity. Moreover, a temporary decline in Leeuwin Current influence at Site 763A allows for an increased influence of the West Australian Current, bringing cool, nutrient-rich and saline waters from the South Indian Ocean. At Site U1463, however, the Leeuwin Current still continued to be sufficiently strong during MIS M2 to suppress an upwelling-induced surface cooling or increase in salinity. Such temporary reduction in Leeuwin Current influence at Site 763A and a continuous influence at U1463 could explain the observed ∼0.5h offset between both Sites. Interestingly, two similar events with a ∼0.5h increase in δ 18 Osacculifer , have been identified in Early Pleistocene strata at Site 763A by Sinha et al. (2006). These authors denoted these events as PL-1 (2.22 Ma) and PL-2 (1.83 Ma), reporting evidence for upwelling at these times. Similar to our interpretation, Sinha et al. (2006) associate the PL-1 and PL-2 events with weakening of heat transport through the ITF due to lowering sea level. However, Sinha et al. (2006) ascribe the sea level lowering to stronger El Niño conditions, whereas we postulate an eustatic sea level fall as the originator of the observed paleoceanographic change across the MIS M2 glacial event. 4.2. Astronomical pacing of the climate transition from humid Pliocene to arid Pleistocene Christensen et al. (2017) interpreted the K (wt.%) content of U1463 sediments (Fig. 6C) as a measure for continental humidity and we follow that interpretation in this study. Christensen et al. (2017) also proposed uranium as a non-traditional indicator for aridity in northwest Australia with peaks in U content exceeding

7 ppm during their “Arid Interval” (<2.4 Ma). Uranium in marine sediments consist of a detrital and an authigenic component (Mo et al., 1973). In the “Arid interval” described by Christensen et al. (2017), U has a predominant detrital origin driven by windblown material from the Australian desert, where uranium-bearing igneous rocks are exposed (Schofield, 2009). Here, we focus on a slightly older time-interval, in which the sedimentary U content varies between 1 and 2 ppm (Fig. 8A). We interpret these considerably lower U content to be the result of a smaller detrital component. Hence, the uranium in the studied interval is of mixed origin: detrital as well as authigenic. Uranium in seawater tends to be more soluble in oxidizing environments but under reducing conditions authigenic U can be preserved in the sediment, where it is complexed with organic carbon and can be incorporated into authigenic minerals (Auer et al., 2016; McManus et al., 2005). We illustrate the association between uranium and organic carbon in Fig. 8A, exhibiting co-variance between the NGR-derived U content and total organic carbon (TOC). Yet, the co-variance is not perfect, implying that the detrital component of the uranium content cannot be neglected. Contrary to U, thorium is unaffected by redox conditions and remains insoluble (Algeo and Maynard, 2004; Myers and Wignall, 1987; Tribovillard et al., 2006). We use thorium to estimate the detrital fraction and interpret U/Th as a paleoproductivity proxy, where high U/Th values indicate enrichment of authigenic U over the detrital background. The apparent correlation between NGR-derived U/Th and XRF-derived Ni/Ti (Ni is a proxy for the organic C sinking flux; Tribovillard et al., 2006) supports this interpretation (Fig. 8B). The studied interval starts in the “Humid Interval” (sensu Christensen et al., 2017, 3.3–5.5 Ma), a period during which

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Fig. 8. U/Th as a paleoproductivity proxy. (A) U1463 NGR-derived U/Th depth-series (along splice), compared to total organic carbon (TOC) depth-series (along splice). The co-variance between U content and TOC suggests the presence of authigenic U. (B) U1463 NGR-derived U/Th depth-series (along splice), compared to the XRF-derived Ni/Ti depth-series (along splice). mcd = meter composite depth.

high K content and low U/Th indicates widespread wet conditions across northwest Australia and low paleoproductivity on the northwest Australian shelf (Fig. 6B–C). On tectonic time scales, the progressive restriction of the ITF by the emerging Maritime Continent puts an end to this humid interval and leads to a cooler and drier climate in the latest Pliocene and early Pleistocene (Christensen et al., 2017; Godfrey, 1996; Krebs et al., 2011; Song et al., 2007). In our data, this climate transition is characterized by a long-term decrease in K (wt.%) and increase in U/Th (Fig. 6B–C). The long-term decrease in our K (wt.%) time-series is clearly paced by eccentricity as well as by obliquity and precession on 104 -yr timescales (Figs. 6C and 9B). This astronomical signature suggests the importance of astronomically-paced Intertropical Convergence Zone (ITCZ) dynamics. On the one hand, ITCZ dynamics are driven by the local insolation signal: under higher southern hemisphere summer insolation, the low-pressure cell over the Australian continent is strengthened and positioned farther south (Fig. 8b in Wyrwoll et al., 2007). As a result, the northwest trade winds, prevailing during the summer monsoon in northwest Australia, are enhanced and advect more moisture from the tropical Indian Ocean (Figs. 4c and 8a in Wyrwoll et al., 2007). On the other hand, global climate state does have a strong influence on the latitudinal width of the ITCZ, with a broader latitudinal range under warm global climates. The first mechanism is largely driven by eccentricity-modulated precession as this astronomical parameter has the strongest influence on southern hemisphere summer insolation at low- and mid-latitudes. Southern hemisphere summer insolation is greatest when the Earth reaches its perihelion at the southern hemisphere summer solstice (precession maximum), in combination with a high-eccentricity orbit. In contrast, southern hemisphere summer insolation is reduced when Earth reaches its aphelion at the southern hemisphere summer solstice (precession minimum). Model experiments estimate the amplitude of precession-driven precipitation variability in northwest Australia is up to 5 mm/day under present-day boundary conditions (Fig. 8a in Bosmans et al., 2015; Fig. 3b in Mohtadi et al., 2016; Wyrwoll et al., 2007). The obliquity imprint in the U1463 K (wt.%) record most likely represents the influence of global climate state

Fig. 9. LOWSPEC MTM power spectra of the U1463 (A) U/Th, (B) K (wt.%) (C) δ 18 OG.sacculifer and (D) δ 13 CG.sacculifer time-series (colored spectra in the foreground), compared to the MTM power spectrum of an eccentricity–tilt–precession (ETP) composite (gray spectra in the background). The dashed lines represent the 95% confidence level (CL).

on the latitudinal range of the ITCZ, with a deeper penetration of the ITCZ into the Australian continent when obliquity maxima sustained warm climate states in the ‘obliquity world’ of the late Pliocene. Our K (wt.%) time series thus indicates that increasing aridity of northwest Australia (long-term decreasing K (wt.%)) was periodically alleviated by reinforced summer monsoon precipitation when maxima in eccentricity-modulated precession led to

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maxima in summer insolation and maxima in obliquity sustained a wider latitudinal ITCZ range. After 3.0 Ma, when the sediment K (wt.%) content is low and exhibits little variability, U/Th develops a high variability. This U/Th variability exhibits a co-variance with obliquity and precession (Fig. 6B) as well as with the Miller et al. (2012) sea level curve (Fig. 6A). However, the U/Th power spectrum (Fig. 9A) suggests a precession-dominated signal, whereas obliquity prevails in the sea level curve (which is a re-scaling of the LR04 stack). We interpret the increase in U/Th variability as the establishment of a similar-to-modern Leeuwin Current associated with enhanced phytoplankton productivity during austral autumn and winter (Koslow et al., 2008; Rousseaux et al., 2012). Today, a distinct temperature gradient exists between the warm Leeuwin Current water and the cool overlying air during austral autumn and winter. This gradient causes sea surface cooling, resulting in a less effective warmwater cap and a deepening of the mixed-layer. Under these circumstances, the surface waters get replenished by nutrients leading to a bioproductivity peak (Rousseaux et al., 2012). The U/Th power spectrum (Fig. 9A) suggests that the water–air temperature gradient was stimulated on precessional time-scales. This astronomical signature suggests stronger temperature gradients when the Earth was close to the aphelion during austral winter. This astronomical configuration would have caused even cooler winter air temperatures but would have had only a minor influence on the equatorial West Pacific Warm Pool SST, the source area for the Leeuwin Current. The co-variation between U/Th and the sea level curve indicates that the described mechanism not only acted on the beat of precession (as suggested by the U/Th power spectrum in Fig. 7), but also on the pulse of glacial–interglacial cycles, dictating sea level. We hypothesize that this relationship exists because the air over the northwest Australian Shelf cools more intensely than the surface water during interglacial–glacial transitions. Indeed, the northwest Australian Shelf is dominated by vigorous southeast trade winds during austral autumn and winter, bringing relatively cold continental air to the studied region (Godfrey and Ridgway, 1985; Godfrey, 1996). 4.3. The Indonesian Seaway as an amplifier of MIS M2 glaciation and deglaciation Our data indicate a significant decrease, but not a complete shutdown, of ITF heat transport across MIS M2. The associated decrease in inter-ocean heat transport would have major consequences for the ability of the global thermohaline circulation to redistribute heat across ocean basins. A less efficient thermohaline circulation would for example explain the pronounced drop in SST during MIS M2 at Site 1087, in the southern Benguela region (Petrick et al., 2015). The Leeuwin Current off Western Australia would also experience the direct consequences of a decline in ITF heat transport during MIS M2. A weakening of this southward flowing current implies the weakening of pole-ward heat transport in the Indian Ocean. Such a circulation system promotes the thermal isolation of Antarctica and can thus act as an amplifier of the MIS M2 glaciation. McKay et al. (2012) report Southern Ocean cooling and increased seasonal persistence of Antarctic sea ice from 3.3 Ma onwards. These authors state that Antarctic cooling led to the northward migration of the Southern Ocean frontal system and an expansion of the circumpolar westerly winds. Such climatic changes would allow for a stronger Indian Ocean gyre. The West Australian Current is the northward flowing component of the counter-clockwise Indian Ocean moving gyre, and an intensification of this current could have contributed to the spatial constriction of the southward flowing Leeuwin Current. Hence, we suggest that ITF heat transport and paleoenvironmental change in Antarctica and the Southern Ocean are connected through a

positive feedback loop. This hypothesis is supported by similar astronomical signatures in the oxygen isotope record of U1463 and the iceberg-rafted debris record from Site U1361, offshore the East Antarctic Ice Sheet (Patterson et al., 2014). In both records, the imprint of obliquity reaches a local minimum between 3.45 and 3.25 Ma, i.e. just prior to and during the MIS M2 (Fig. 10). Both southern hemisphere records thus reflect an astronomical signature that is significantly different from the LR04 global benthic oxygen isotope stack, characterized by a rather stable obliquity imprint throughout the late Pliocene. The temporary decline of obliquity power occurring simultaneously in the low (U1463) and high (U1361) latitudes of the southern hemisphere can be interpreted as an indication for a short-term dominance of low-latitude mechanisms steering southern hemisphere climate dynamics. Interestingly, the period of low obliquity power largely corresponds to the period of weakened Leeuwin Current, designated by the isotopic offset between U1463 and Site 763. This similarity in timing suggests that the tectonic restriction of the Indonesian Seaway could be the low-latitude mechanism influencing southern hemisphere climate dynamics before and during MIS M2. Hence, we suggest that the Indo-Pacific connection through the Banda sea was tectonically reduced from 3.45 Ma onwards (Fig. 1c–d in Christensen et al., 2017), causing a lessening of the heat transfer between the Pacific and Indian Ocean and thus a decline in the equator-to-pole heat transport. This development therefore contributed to the thermal isolation of Antarctica and ultimately to the severe MIS M2 glaciation event, provoked by the coincidence of an obliquity and eccentricity minimum at 3.3 Ma. Nonetheless, the global climate system returned back to the relatively warm Late Pliocene climate state around 3.25 Ma (Lisiecki and Raymo, 2005) and the Leeuwin Current increased in strength again at that time (Fig. 6). This could be explained by a significant influence of sea level on the efficiency of the ITF. Indeed, the 3.45 to 3.25 Ma interval represents a prolonged period of relatively low sea level in the Miller et al. (2012) sea level reconstruction, amplifying the tectonic restriction of the Indonesian seaway. However, the major transgression around 3.25 Ma seems to have re-established Indo-Pacific heat transfer and thus latitudinal heat transport, therewith instigating a positive feedback mechanism. It lasted until ∼2.7 Ma before the onset of significant Northern Hemisphere glaciation took place, and the Earth experienced the pronounced glacial cycles of the Pleistocene Epoch. Pleistocene interglacial periods are characterized by a stronger Leeuwin Current due to a better connectivity with the West Pacific Warm Pool (Gallagher et al., 2009, 2014; Spooner et al., 2011 and references therein). Paleoceanographic change in the Indo-Pacific region across MIS 2 was thus quite similar to paleoceanographic variability observed during Pleistocene glacial– interglacial cycles. However, an important question remains to be answered in future studies: Why did the ITF positive feedback mechanism amplify the paleoceanographic response to astronomical forcing so vigorously at 3.3 Ma, but failed to intensify astronomical cycles in the time span between MIS M2 (3.3 Ma) and the first major Northern Hemisphere cycle of glaciation and deglaciation (2.7 Ma)? 5. Conclusions The Late Pliocene was a major period of climatic transition for the Australian climate system, as well as for ocean circulation in the Indo-Pacific Region, in response to the tectonic restriction of the Indonesian Seaway. The long-term change in hydroclimate in northwest Australia during the Late Pliocene (i.e. the transition from the Humid to the Arid Interval, sensu Christensen et al., 2017) was firmly paced by astronomical “Milankovitch” insolation forcing. This long-term climatic transition involves a gradual evolution towards today’s monsoonal system in northwest Australia, in which

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Fig. 10. Quantitative assessment of changing astronomical imprints. Cumulative power in the precession (0.04–0.06 cycles kyr−1 ), obliquity (0.02–0.035 cycles kyr−1 ), and eccentricity (0–0.015 cycles kyr−1 ) bands for the U1463 δ 18 OG.sacculifer time-series, the tuned iceberg-rafted debris data from Site U1361 (adapted from Patterson et al., 2014) and the LR04 δ 18 O stack. Both southern hemisphere records display a minimum in obliquity power prior to and during MIS M2, in contrast to the LR04 stack, showing a rather constant obliquity imprint at that time. Calculation method for the U1463 δ 18 OG.sacculifer time-series is identical to the methodology applied by Patterson et al. (2014).

the austral summer monsoon alleviates the otherwise dry conditions in northwest Australia, and in which the austral autumn and winter experience an increase in mixed layer depth, nutrients and productivity. The effects of the ongoing restriction of the Indonesian Seaway were strongly amplified during the MIS M2 glaciation event, however, latitudinal heat transport via the ITF did not shut down completely at that time. This conclusion is supported by the observation that Site U1463 continues to preserve evidence of an ITF signal throughout MIS M2, whereas Site 763A temporarily reflects an Indian Ocean signal. We propose that latitudinal heat transport via the ITF and paleoclimatic changes around Antarctica are connected through a positive feedback loop. Hence, the Indonesian Throughflow plays an influencing role as a southern hemisphere driver for late Pliocene change across MIS M2. 6. Competing interest statement The authors have no competing interest to declare. Acknowledgements This research used samples and data provided by the International Ocean Discovery Program (IODP). The German Science Foundation (DFG) provided funding for this research through project VL96/1-1, project number 319497259. The Vocatio Foundation provided additional funding through a scholarship to DDV, laureate in the 2016 promotion. GA’s contribution was funded by JSPS grant 17H07412. Funding was provided by the Australian IODP office and the ARC Basins Genesis Hub (IH130200012) to S.J.G. DDV is a postdoctoral researcher and HP is the principal investigator in ERC Consolidator grant “EARTHSEQUENCING” (grant agreement 617462). We thank two anonymous reviewers for their constructive feed-

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