Pliocene research at Exeter

The broad consensus among climate model predictions for the future suggests an increase in the expanse of arid to semi-arid zones in a warmer world and an increased occurrence of ‘mega-droughts’ in such regions. Yet extensive study of the record of terrestrial climate and climate model simulations for the warm Pliocene (~3 to 5 millions of years ago, the last time atmospheric carbon dioxide levels were comparable to today) indicates noticeably wetter than modern conditions in modern arid and semi-arid regions, including those in southwest North America, South America, Europe, and Australia (e.g. Jimenez-Moreno et al., 2010; Goldner et al., 2011; Salzmann et al., 2013).

Terrestrial records of precipitation balance provide powerful insights into Pliocene climate but are, by their nature, rare, discontinuous in coverage and often suffer from age control limitations. By contrast, marine records immediately downwind of continents, which for the Pliocene have been largely underutilized for addressing this problem, provide well-dated, high resolution and continuous archives of terrestrial climate.

We are utilizing such records (e.g. Lang et al., in review for QSR) to reconstruct detailed histories of terrestrial climate (e.g. aeolian dust deposition, a proxy for continental humidity/aridity) from key locations benefiting from excellent age control from the (I)ODP archive to shed new light on the regional evolution of terrestrial climate during the warm Pliocene. Key outcomes will be to provide an independent mean to test the competing hypotheses available to explain early Pliocene warmth and wetness and improve our understanding of the fate of mid-latitude climate in response to anthropogenic global warming.

Key papers

• Goldner, A., Huber, M., Diffenbaugh, N., Caballero, R. 2011. Implications of the permanent El Niño teleconnection “blueprint” for past global and North American hydroclimatology. Clim. Past, 7, 723–743, 2011.
• Jimenez-Moreno, G., Fauquette, S., and Suc, J.P., 2010. Miocene to Pliocene vegetation reconstruction and climate estimates in the Iberian Peninsula from pollen data. Review of Palaeobotany and Palynology 162, 403–415.
• Lang, D.C, Bailey, I., Wilson, P.A., Beer, C., Bolton, C.T., Newsam, C., Spencer, M.R., Friedrich, O., Gutjhar, M., Foster, G.L., Cooper, C.J., Milton, A., Lithogenic dust flux to the mid-latitude North Atlantic Ocean: Insight into the transition on North America from the warm humid Pliocene to the glaciated Quaternary, in review for QSR.
• Salzmann, U., Dolan, A.M., Haywood, A M., Chan W.-L., Hill, D.J., Abe-Ouchi, A., Otto-Bliesner, B., Bragg, F., Chandler, M. A., Contoux, C., Dowsett, H.J., Jost, A., Kamae, Y., Lohmann, Lunt, D. J., Pickering, S.J., Pound M.J., Ramstein, G., Rosenbloom, N.A., Sohl, L., Stepanek, C., Ueda, H, Zhang, Z.  (2013): Challenges in reconstructing terrestrial warming of the Pliocene revealed by data-model discord. Nature Climate Change. 

Maturation of northern hemisphere ice-sheets during the Pliocene is reflected in the culmination of a 1 Myr-long increase in benthic foraminifer δ¹⁸O over ~3.5–2.5 Ma with evidence for ice-rafted debris (IRD) deposition in the northern high latitude oceans from 2.75 Ma. The magnitude of IRD deposition in the subpolar North Atlantic, however, is not simply linearly related to the volume of continental ice-sheets. Moreover, without knowledge of the provenance of this IRD potentially misleading inferences can be made regarding the spatial evolution of continental ice-sheet growth during Pliocene iNHG. If we are to understand the drivers of Pliocene NHG we therefore must better understand the sources of IRD deposited in the subpolar North Atlantic Ocean. To do this we measure the geochemistry (e.g. Pb-isotope composition) and age (e.g. via Ar-Ar dating) of large numbers of individual sand-sized ice-rafted feldspars and amphiboles, respectively.

Key findings

One way in which our provenance research has contributed to the iNHG debate has been to address the meaning of the major onset of IRD deposition in the Nordic Seas and subpolar North Atlantic Ocean during marine isotope stage G6, 2.72 Ma, in terms of ice-sheet extent. Previously, for example, it is hypothesized that this event records the Pliocene onset of major NHG due to a synchronous advance of North American Laurentide, Scandinavian and Greenland ice-sheets to their marine calving margins during MIS G6 (Kleiven et al., 2002; Palaeogeog. Palaeoclim. Palaeoecol).

Recently, by measuring the Pb-isotope composition of many individual (>150 µm) sand-sized ice-rafted feldspars deposited at DSDP Site 611 during full glacial MIS 100 (~2.5 Ma) we demonstrated that the provenance of this IRD is comparable to that deposited during the last glacial maximum in the North Atlantic Ocean. By late Quaternary standards, the onset of ‘major NHG’ is perhaps best defined by the first time during iNHG when northern hemisphere ice-sheets expanded into the mid latitudes (Head et al., 2008). Based on the terrestrial record of NHG, we can be confident that this condition was met in North America during MIS 100 (Balco and Rovey II, 2010). We therefore used the Pb-isotope composition of ice-rafted feldspars deposited in the North Atlantic Ocean during MIS 100 as a reference template against which to test for the attainment of major, widespread NHG during other glacials. Our data show that a persistent pattern in terms of IRD provenance and iceberg calving sources consistent at our study site with major NHG during MIS 100, is not established at Site 611 during the onset of major IRD deposition in the subpolar North Atlantic Ocean at 2.72 Ma during MIS G6, as previously hypothesised by Kleiven et al. (2002), but ~80 kyr later during MIS G2 (2.64 Ma).

Key papers

• Bailey, I., Hole, G.M., Foster, G.L., Wilson, P.A., Storey, C.D., Trueman, C.N., Raymo, M.E., 2013. An alternative suggestion for the Pliocene onset of major northern hemisphere glaciation based the geochemical provenance of North Atlantic Ocean ice-rafted debris, Quaternary Science Reviews, 75C, 181–194.
• Bailey, I., Foster, G.L., Wilson, P.A., Jovane, L., Storey, C.D., Trueman, C.N., Becker, J., 2012. Flux and provenance of ice-rafted debris in the earliest Pleistocene sub-polar North Atlantic Ocean comparable to that of the last glacial maximum, Earth and Planetary Science Letters 341-344, 222-233.
• Bailey, I., Liu, Q., Swann, G.E.A., Jiang, Z., Sun, Y., Zhao, X., Roberts, A.P., 2011. Iron fertilisation and biogeochemical cycles in the sub-Arctic northwest Pacific during the late Pliocene intensification of Northern Hemisphere Glaciation, Earth and Planetary Science Letters 307(3–4), 253-265.
• Bailey, I., Bolton, C.T., DeConto, R.M., Pollard, D., Schiebel, R., Wilson, P.A., 2010. A low threshold for North Atlantic ice rafting from ‘low-slung and slippery’ late Pliocene ice sheets, Paleoceanography 25, PA1212.

The end of early Pliocene warmth (~3 Ma) occurred during a time of increasing climate extremes at both the orbital and suborbital-scale. Changes in global sea-level, which were previously dominated by fluctuations in the volume of the west Antarctic ice-sheet increased due to long-term intensification of northern hemisphere glaciation. The growth of large ice-sheets on circum-North Atlantic landmasses is thought to have led to the amplification of climate variability on suborbital time scales (McManus et al., 1999). To date, however, only a handful of adequately resolved and temporally limited records are available (e.g. Bartoli et al., 2006; Bolton et al., 2010) to test these hypotheses and existing studies have produced contradictory conclusions.

To address this gap in our knowledge, we are examining expanded Pliocene records recovered from Northeast North Atlantic Ocean sediment drifts by the Ocean Drilling Program to better understand the relationship between northern hemisphere ice-sheet expansion and both orbital and suborbital variations in climate during the Pliocene iNHG.

Key papers

• Bartoli, G., Sarnthein, M., Weinelt, M., 2006. Late Pliocene millennial-scale climate variability in the northern North Atlantic prior to and after the onset of Northern Hemisphere glaciation, Paleoceanography 21, PA4205.
• Bolton, C.T. et al, 2010. Millennial-scale climate variability in the subpolar North Atlantic Ocean during the late Pliocene, Paleoceanography 25, PA4218.
• McManus, J. et al, 1999. A 0.5‐million‐year record of millennial‐ scale climate variability in the North Atlantic. Science 283, 971–975.

The ocean, through its ability to globally redistribute heat and partition carbon dioxide, is believed to play a key role in driving and amplifying climate change during Quaternary glaciations on orbital to millennial timescales. Little is known, however, about the changes in Atlantic Meridional Overturning Circulation (AMOC) associated with the culmination of Earth’s most recent major climate transition, the intensification of northern hemisphere glaciation (iNHG) by ~2.5 Ma.

Evidence for a long-term weakening of AMOC (Hodell & Venz-Curtis, 2006) and suggestions of sub-orbital deep ocean circulation changes during iNHG (Becker et al., 2006) are primarily based on benthic carbon isotopes records. Carbon isotopes may be biased, however, by changes in carbon cycling and associated changes in the isotopic composition of northern and southern deepwater sources during this interval of major global change. To address this problem we are generating new proxy records of ocean circulation, independent of the carbon cycle, using Nd-isotopes measured on fish debris, to better constrain the evolution of AMOC during iNHG.

Key papers

• Becker, J.F. et al. Lourens, L.J., Raymo, M.E., 2006. High-frequency climate linkages between the North Atlantic and the Mediterranean during marine oxygen isotope stage 100 (MIS100). Paleoceanography 21, PA3002.
• Hodell, D. A., and K. A. Venz-Curtis (2006), Late Neogene history of deepwater ventilation in the Southern Ocean, Geochem. Geophys. Geosyst., 7, Q09001.
• Lang, D.C, Bailey, I. et al, Lithogenic dust flux to the mid-latitude North Atlantic Ocean: Insight into the transition on North America from the warm humid Pliocene to the glaciated Quaternary, in review for QSR.

The Indian monsoon is one of the most powerful climatic phenomena in the modern world, supplying freshwater to over a billion people annually. One of the key issues of our time is understanding how the monsoon system will respond to rising global temperatures over the coming decades and centuries, in the face of anthropogenic climate change.

In combination with sophisticated modelling studies, one of the best ways of understanding how the Indian monsoon may behave in the high CO2 future is to reconstruct how it behaved in the high CO2 world of the geological past. The Pliocene (5.33­ to 2.58 million years ago) is thought to be the last time that atmospheric CO2 was at the levels we are currently experiencing (~400 ppmv), but conditions were quite different with higher average surface temperatures and higher global sea levels during the Pliocene relative to today. We have recently made great strides in our understanding of Pliocene climate dynamics, but the behaviour of the powerful Indian monsoon at this time remains mysterious.

Expedition 353 

From November 2014 to January 2015, International Ocean Drilling Program (IODP) Expedition 353, the “iMonsoon” cruise, collected deep-sea sediments from the Bay of Bengal and the Andaman Sea, encompassing the core regions of summer monsoon precipitation for the first time. These cores, which contain sediments spanning the Miocene to Holocene in age, will be the basis of many exciting research projects in the coming years, with the aim of reconstructing the behaviour of the Indian monsoon over the last 8 million years, including the warm Pliocene.

Dr Kate Littler sailed as a shipboard sedimentologist on IODP Exp 353 and so has a large number of Pliocene-age core samples collected from the Bay of Bengal and the Andaman Sea. She is working with national and international collaborators, students and postdocs, to generate very high-resolution geochemical, sedimentological and paleontological records from these cores, to reconstruct past precipitation and sea surface temperatures and see how these varied on orbital time scales. Dr Littler is collaborating with scientists from the University of Exeter, University of Bristol, Open University, University of Bremen (Germany), United States Geological Survey (USA), Pusan National University (South Korea) and others.

In November 2015, Dr Littler was awarded a UKIODP Moratorium Award (£31k) to generate the oxygen isotope record for the Pliocene at Site U1445, in collaboration with colleagues at the Open University and NIGL. This award will allow a robust age model to be generated that will form the back-bone of future work using sophisticated organic and inorganic geochemical proxies.

Key papers

• Clemens SC, Kuhnt W, LeVay LJ, and the Expedition 353 Scientists, (Littler K). (2015) International Ocean Discovery Program Expedition 353 Preliminary Report.
• Clemens et al., 2008. Southern Hemisphere forcing of Pliocene δ18O and the evolution of Indo-Asian monsoons. Paleoceanography 23 (4)
• Zhang et al., 2009. Mid-Pliocene Asian monsoon intensification and the onset of Northern Hemisphere glaciation. Geology 37 (7), 599-602
• Zhisheng et al., 2001. Evolution of Asian monsoons and phased uplift of the Himalaya-Tibetan plateau since Late Miocene times. Nature 411, 62-66