The amplifying effect of Indonesian Throughflow heat transport on Late Pliocene Southern Hemisphere climate cooling
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 (0.64‰ in the LR04 stack, 0.74‰ 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 globally-distributed 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, Karas et al., 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, Gallagher et al., 2014; Karas et al., 2011b, Karas et al., 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, 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.
Section snippets
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 Current system dictates water column
Astrochronologic age model and time-series analyses
Site U1463 holds a complete stratigraphic sequence from the late Miocene to the early Pleistocene, with abundant and well-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 sacculifer
Paleoceanographic change in the Indo-Pacific region across MIS M2
The Site U1463 planktic G. sacculifer oxygen isotope record is similar to the equivalent record from Site 763A (data from Karas et al., 2011b), to the southwest (Fig. 1), with variable (ranging between −0.75 and −1.85‰) 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 sacculifer values in the studied time interval (3.7–2.8 Ma). The similarities between the Site U1463 and Site 763A sacculifer
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
Competing interest statement
The authors have no competing interest to declare.
Acknowledgments
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
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