Pliocene reversal of late Neogene aridification J. M. Kale Snidermana,1, Jon D. Woodheada, John Hellstroma, Gregory J. Jordanb, Russell N. Drysdalec,d, Jonathan J. Tylere, and Nicholas Porchf aSchool of Earth Sciences, University of Melbourne, Parkville, VIC 3010, Australia; bSchool of Biological Sciences, University of Tasmania, Private Bag 55, Hobart, TAS 7001, Australia; cSchool of Geography, University of Melbourne, Parkville, VIC 3010, Australia; dEnvironnements, Dynamiques et Territoires de la Montagne, UMR CNRS, Université de Savoie-Mont Blanc, 73376 Le Bourget du Lac, France; eDepartment of Earth Sciences, University of Adelaide, Adelaide, SA 5001, Australia; and fSchool of Life and Environmental Sciences, Deakin University, Burwood, VIC, 3125, Australia Edited by Edouard Bard, Centre Européen de Recherche et d’Enseignement des Géosciences de l’Environnement, Aix-en-Provence, France, and accepted by the Editorial Board December 30, 2015 (received for review October 13, 2015) The Pliocene epoch (5.3–2.6 Ma) represents the most recent geological term cooling/aridification trend and Pliocene warmth remains interval in which global temperatures were several degrees warmer unclear. Thus, was Pliocene warmth a discrete interruption of late than today and is therefore considered our best analog for a future Cenozoic cooling/aridification trends (6), or was the Pliocene anthropogenic greenhouse world. However, our understanding of merely an interval immediately preceding the abrupt steepening of Pliocene climates is limited by poor age control on existing terrestrial these trends associated with the onset of extensive Northern climate archives, especially in the Southern Hemisphere, and by per- Hemisphere glaciation (12, 13)? sistent disagreement between paleo-data and models concerning the To place the temporal evolution of Southern Hemisphere Plio- magnitude of regional warming and/or wetting that occurred in re- cene vegetation and climate on a firmer chronological footing, we sponse to increased greenhouse forcing. To address these problems, generated fossil pollen records from radiometrically dated spe- here we document the evolution of Southern Hemisphere hydrocli- leothems (secondary cave carbonate deposits such as stalagmites mate from the latest Miocene to the middle Pliocene using radiomet- and flowstones) from southern Australia. Pollen assemblages were rically-dated fossil pollen records preserved in speleothems from extracted from samples collected in five caves on the Nullarbor semiarid southern Australia. These data reveal an abrupt onset of Plain (Fig. 2 and SI Appendix,Fig.S1), a large (200,000 km2) karst warm and wet climates early within the Pliocene, driving complete province uplifted above sea level during the late Miocene (14). biome turnover. Pliocene warmth thus clearly represents a discrete Consistent with its position in Southern Hemisphere subtropical interval which reversed a long-term trend of late Neogene cooling latitudes, the Nullarbor Plain is currently semiarid, receiving mean and aridification, rather than being simply the most recent period of annual precipitation (MAP) of ca. 180–270 mm (SI Appendix,Fig. greater-than-modern warmth within a continuously cooling trajec- S1), with a weak winter-maximum. Mean annual temperature is tory. These findings demonstrate the importance of high-resolution ∼18 °C. The vegetation is largely treeless chenopod shrubland, chronologies to accompany paleoclimate data and also highlight the question of what initiated the sustained interval of Pliocene warmth. grading coastward into sparse, low open woodlands dominated by Acacia (Mimosaceae) or Eucalyptus (Myrtaceae). Nullarbor spe- paleoclimate | pollen | speleothems | aridification | Neogene leothems grew during the late Neogene (15) but negligible calcite speleothem growth occurs today. Recent development and refine- ment of the uranium-lead (U-Pb) chronometer has allowed high- ur knowledge of Pliocene climates is based predominantly on precision geochronology of such ancient speleothem samples the rich marine sediment record (1), but understanding of O (15–18). Our fossil pollen record is composed of 13 polleniferous Pliocene climates on land remains limited because the few existing samples (out of 81 explored for pollen), the oldest dated at 5.59 ± terrestrial archives tend to have poor age control and are of short 0.15 Ma, in the latest Miocene, and the youngest 0.41 ± 0.07 Ma, in duration. This deficiency is no more evident than in paleovegetation records, which are integral to modeling Pliocene climates because vegetation is both a critical indicator of regional precipitation and Significance EARTH, ATMOSPHERIC, also makes a large contribution to planetary albedo (2, 3). Several AND PLANETARY SCIENCES syntheses of Pliocene vegetation have been compiled (3, 4), but The warm climates of the Pliocene epoch are considered our their value is hampered by substantial uncertainty surrounding the best analog for a future anthropogenic greenhouse world. synchroneity of the records. For example, vegetation syntheses have However, understanding of the nature of Pliocene climate focused on the Late Pliocene (the Piacenzian Age, 3.6–2.6 Ma) variability and change on land is currently limited by the poor because this period is considered likely to be a closer biological and age control of most existing terrestrial climate archives. We geological analog for future warming than the Early Pliocene (1, 5). present a radiometrically dated history of the evolution of However, only 6 of 32 Southern Hemisphere records interpreted by Southern Hemisphere vegetation and hydroclimate from the Salzmann et al. (4) as documenting the nature of Late Pliocene latest Miocene to the middle Pliocene. These data reveal a sharp vegetation are both based on plant fossils and can confidently be increase in precipitation in the Early Pliocene, which drove com- assigned to the Late Pliocene; the remaining records have such poor plete vegetation turnover. The development of warm, wet early chronologies that their possible ages range from Late Miocene to Pliocene climates clearly reversed a long-term Southern Hemi- Early Pleistocene, or the records only infer the nature of vegetation sphere trend of late Neogene cooling and aridification, high- ∼ indirectly from geomorphology or vertebrate fossils (SI Appendix, lighting the question of what initiated this sustained, 1.5-My- Table S1). As a result, current understanding of the response of long interval of warmth. Southern Hemisphere vegetation to Late Pliocene climates (2–4) ≥ Author contributions: J.M.K.S, J.D.W., J.H., and G.J.J. designed research; J.M.K.S., J.D.W., J.H., may conflate the climate and vegetation history of 5Myofthelate G.J.J., and N.P. performed research; J.M.K.S., J.D.W., J.H., G.J.J., J.J.T., and N.P. analyzed data; Cenozoic (Fig. 1), an interval that is characterized by global cooling and J.M.K.S., J.D.W., J.H., G.J.J., R.N.D., and J.J.T. wrote the paper. – (6) and, in subtropical- to midlatitudes, increasing aridity (7 10). The authors declare no conflict of interest. This uncertainty is problematic for two reasons. First, it has become This article is a PNAS Direct Submission. E.B. is a guest editor invited by the Editorial clear that the peak of Pliocene warmth globally was not generally Board. within the Late Pliocene but occurred earlier, within the Early 1To whom correspondence should be addressed. Email: [email protected]. Pliocene (11). Second, largely because of a lack of continuous This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. and well-dated records, the relationship between the longer- 1073/pnas.1520188113/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1520188113 PNAS | February 23, 2016 | vol. 113 | no. 8 | 1999–2004 Downloaded by guest on September 30, 2021 Early Late Early Late to estimate the prevalence of the living relatives as a function of Pleistocene Pliocene Pliocene Miocene modern mean annual precipitation (MAP) (SI Appendix,Figs.S3 Foz de Amazonas (44) and S4). We used a Monte Carlo approach to account for propa- Sterkfontein (131) Late Other Mahakam (175) Pliocene ages gation of uncertainties from the Gaussian 2σ error associated with DSDP 262 (177) Pollen, terrestrial sediments Laetoli (126) each speleothem age and from the GAM-derived probability dis- Lauca (45) Pollen, marine sediments Yallalie (185) Vertebrates tributions of the MAP estimates (SI Appendix, Supplementary DSDP 532 (127) Vertebrates, possible age Materials and Methods ODP 1123 (199) Wood and Fig. S5). This analysis indicates that in ODP 1082 (128) Sedimentological evidence ODP 815/823 (182-183) the latest Miocene and earliest Pliocene before ca. 5Ma,MAPin ODP 765 (179) the Nullarbor region was ca. 480 mm (320–875 mm, at the 95% Linda Valley (193) Kowai Fm (196) confidence level; Figs. 3 and 4E) (i.e., slightly wetter than today). Five Fingers (197) Makapan (130) The absence or scarcity of pollen of the forest taxa that dominate Buenos Aires (48) Hapuku (180) samples later in the Pliocene (e.g., Eucalyptus or Banksia)implies Malawi (125) S. South America (46-47) that MAP was unlikely to be close to the upper end of the Ivirin (178) reconstructed interval. We therefore conclude that by the end of Tadmor Gp (198) E Highlands, NSW (189) the Miocene, a semiarid climate had developed in the Nullarbor Lk George (188) Lachlan Fm (189) region that persisted for at least several hundred ka of the Early Lake Tay (186) Butchers Ck (184) Pliocene. However, soon after 5 Ma, MAP rose rapidly (within N. Eyre Peninsula. (180) ≤ Wipajiri/Tirari Fms (180-181) ca. 100 ka, or within 350 ka, at the 95% confidence level) to ca. Namba Fm (180) – Tempe Downs (180) 1,220 mm (ca. 635 2,100 mm, at the 95% confidence level) and Langebaanweg(129) was maintained for ca. 1.5 My at levels two to four times higher than today. 2 3 4 5 6 7 Age (Ma) The timing of biome turnover in the Early Pliocene Nullarbor pollen record corresponds closely to abrupt increases in sea-surface Fig.