Pliocene reversal of late Neogene aridification

Abstract

The Pliocene epoch (5.3-2.6 Ma) represents the most recent geological interval in which global temperatures were several degrees warmer than today and is therefore considered our best analog for a future anthropogenic greenhouse world. However, our understanding of Pliocene climates is limited by poor age control on existing terrestrial climate archives, especially in the Southern Hemisphere, and by persistent disagreement between paleo-data and models concerning the magnitude of regional warming and/or wetting that occurred in response to increased greenhouse forcing. To address these problems, here we document the evolution of Southern Hemisphere hydroclimate from the latest Miocene to the middle Pliocene using radiometrically-dated fossil pollen records preserved in speleothems from semiarid southern Australia. These data reveal an abrupt onset of warm and wet climates early within the Pliocene, driving complete biome turnover. Pliocene warmth thus clearly represents a discrete interval which reversed a long-term trend of late Neogene cooling and aridification, rather than being simply the most recent period of greater-than-modern warmth within a continuously cooling trajectory. These findings demonstrate the importance of high-resolution chronologies to accompany paleoclimate data and also highlight the question of what initiated the sustained interval of Pliocene warmth.

Keywords: Neogene; aridification; paleoclimate; pollen; speleothems.

Fig. 1.

Conservative estimates of the age ranges of Southern Hemisphere vegetation records accepted by Salzmann et al. (3) as indicative of Late Pliocene (Piacenzian, 2.6–3.6 Ma) terrestrial vegetation. Sample code numbers are those used by Salzmann et al. Colors represent record type (pollen, wood, vertebrate, sediment). Bold colors indicate records clearly falling within the Late Pliocene, and faint colors indicate records either falling outside of the Late Pliocene or with broader age ranges. Of the 32 records, only 6 based on plant fossil data can be confidently assigned a Late Pliocene age. For Makapan, two age estimates are provided, reflecting uncertainty whether the record can be attributed to the Pliocene as a whole or to the Late Pliocene.

Fig. 2.

Locality map showing Nullarbor caves in southern Australia and sites mentioned in the text. The map was produced using Ocean Data View (odv.awi.de).

Fig. 3.

Late Miocene, Pliocene, and Middle Pleistocene vegetation change in semiarid southern Australia. Monte Carlo simulations of the late Miocene–Pliocene (A) and Middle Pleistocene (B) U-Pb–dated Nullarbor speleothem pollen record, accounting for Gaussian uncertainties in speleothem ages and in pollen percentage counts, and of the Nullarbor mean annual precipitation reconstruction derived from the pollen assemblages. The chenopod-dominated Middle Pleistocene assemblage is very similar to the composition of Late Pleistocene and Holocene pollen records (C) from Nullarbor cave/doline infills (43), confirming that the speleothem pollen assemblages register the surrounding vegetation in comparable ways to conventional fossil pollen records. U/Pb ages ±2σ errors are shown for 13 polleniferous samples (color-coded by cave), against the backdrop of other Nullarbor speleothems investigated for pollen (gray).

Fig. 4.

Nullarbor mean annual precipitation reconstruction and proxies for Southern Hemisphere and northern Pacific Ocean sea surface temperatures. (A) Alkenone-derived SST records from north Pacific Ocean ODP sites 1208, 1010, and 1021 (12); warm intervals are indicated by pink shading (for locations, see Fig. 2); note the separate time axis compared with B–H. (B) Benthic foraminifera δ¹⁸O record (44) with 10-ka smoothing (dark purple line), change points at 6.14 and 3.45 Ma. (C) Mg/Ca-derived SSTs, ODP Site 590B, south-western Pacific (34); change points are at 5.49 and 4.35 Ma. (D) Mg/Ca-derived SSTs, ODP Site 763, north-eastern Indian Ocean (33); the warm interval is defined by change points at 5.26 and 3.44 Ma, indicated by yellow shading. (E) Monte Carlo simulation of Nullarbor late Miocene–Pliocene mean annual precipitation. (F) Ice-rafted debris (>125 µm), ODP Site 1165, Prydz Bay (45); change points are at 5.22, 5.04, 3.22, and 2.95 Ma. (G) Opal deposition rate, ODP Site 1095, Bellinghausen Sea, interpreted at this site as a proxy for open water, hence SST (46); change points are at 5.27 and 3.02 Ma. (H) Proxy reconstructions of atmospheric pCO₂, derived from alkenones [turquoise (31) and yellow (32) shading, green circles with error bars (30)], and from δ¹¹B [gray shading (47)]. Red horizontal lines are significant change points in the mean for B–D and F and G. Shaded boxes indicate the duration of intervals of warming.