Abstract
Over the last 5 million years, the global climate system has evolved toward a colder mean state, marked by large‐amplitude oscillations in continental ice volume. Equatorward expansion of polar waters and strengthening temperature gradients have been detected. However, the response of the mid latitudes and high latitudes of the Southern Hemisphere is not well documented, despite the potential importance for climate feedbacks including sea ice distribution and low‐high latitude heat transport. Here we reconstruct the Pliocene‐Pleistocene history of both sea surface and Antarctic Intermediate Water (AAIW) temperatures on orbital time scales from Deep Sea Drilling Project Site 593 in the Tasman Sea, southwest Pacific. We confirm overall Pliocene‐Pleistocene cooling trends in both the surface ocean and AAIW, although the patterns are complex. The Pliocene is warmer than modern, but our data suggest an equatorward displacement of the subtropical front relative to present and a poleward displacement of the subantarctic front of the Antarctic Circumpolar Current (ACC). Two main intervals of cooling, from ~3 Ma and ~1.5 Ma, are coeval with cooling and ice sheet expansion noted elsewhere and suggest that equatorward expansion of polar water masses also characterized the southwest Pacific through the Pliocene‐Pleistocene. However, the observed trends in sea surface temperature and AAIW temperature are not identical despite an underlying link to the ACC, and intervals of unusual surface ocean warmth (~2 Ma) and large‐amplitude variability in AAIW temperatures (from ~1 Ma) highlight complex interactions between equatorward displacements of fronts associated with the ACC and/or varying poleward heat transport from the subtropics.
Highlights
The last 5 Ma of Earth history are marked by two significant transitions that represent both a change in mean global climate state and an evolving response to external forcing by solar radiation
These conditions are coeval with high abundances of nannofossil species characteristic of modern surface waters to the south of the Subtropical Front (STF) (e.g., Coccolithus pelagicus and Calcidiscus leptoporus) being recorded at Ocean Drilling Program (ODP) Site 1172 in the southwest Tasman Sea (44°57′S; Figures 1 and 6) [Ballegeer et al, 2012]. These results suggest that the late Pliocene STF was positioned between Deep Sea Drilling Project (DSDP) Site 593 and ODP 1172, representing a relatively minor but equatorward displacement compared to modern
We address the relative paucity of temperature data from surface and intermediate-depth waters of the midlatitude and high latitude of the Southern Hemisphere through analysis of DSDP Site 593 in the Tasman Sea, southwest Pacific
Summary
The last 5 Ma of Earth history are marked by two significant transitions that represent both a change in mean global climate state and an evolving response to external forcing by solar radiation. The onset or intensification of northern hemisphere glaciation (INHG) is usually defined at ~2.7 Ma, but occurs within a broader window of cooling and increasing continental ice [e.g., De Schepper et al, 2013; Lisiecki and Raymo, 2005; Rohling et al, 2014]. Explanations for the INHG and MPT have tended to focus on the evolution of the Northern Hemisphere ice sheets [Clark et al, 2006; Haug et al, 2005], but changes to Antarctic ice sheet extent and circulation in the surrounding Southern Ocean have been detected By decoupling the temperature and ice volume contributions to benthic foraminifera oxygen isotope composition (δ18O) in the deep northwest Pacific, Woodard et al [2014] proposed that Antarctic ice volume increased from ~3.15 to 2.75 Ma, prior to INHG, and Elderfield et al [2012] argued that a stepped increase in ice volume during marine isotope stages (MIS) 22–24 (~0.9 Ma) might be linked to ice sheet growth in the Ross Sea sector
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