Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Clim. Past Discuss., 11, 5683–5725, 2015 www.clim-past-discuss.net/11/5683/2015/ doi:10.5194/cpd-11-5683-2015 CPD © Author(s) 2015. CC Attribution 3.0 License. 11, 5683–5725, 2015 This discussion paper is/has been under review for the journal Climate of the Past (CP). Palaeogeographic Please refer to the corresponding final paper in CP if available. controls on the climate system Palaeogeographic controls on climate and D. J. Lunt et al. proxy interpretation Title Page D. J. Lunt1, A. Farnsworth1, C. Loptson1, G. L. Foster2, P. Markwick3, C. L. O’Brien4,5, R. D. Pancost6, S. A. Robinson4, and N. Wrobel3 Abstract Introduction 1School of Geographical Sciences, and Cabot Institute, University of Bristol, Bristol, BS8 1SS, Conclusions References UK Tables Figures 2Ocean and Earth Science, University of Southampton, and National Oceanography Centre, Southampton, SO14 3ZH, UK 3Getech Plc, Leeds, LS8 2LJ, UK J I 4Department of Earth Sciences, University of Oxford, Oxford, OX1 3AN, UK J I 5Department of Geology and Geophysics, Yale University, New Haven, CT 06511, USA 6School of Chemistry, and Cabot Institute, University of Bristol, Bristol, BS8 1TS, UK Back Close Received: 16 November 2015 – Accepted: 1 December 2015 – Published: 14 December 2015 Full Screen / Esc Correspondence to: D. J. Lunt ([email protected]) Printer-friendly Version Published by Copernicus Publications on behalf of the European Geosciences Union. Interactive Discussion 5683 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract CPD During the period from approximately 150 to 35 million years ago, the Cretaceous– Paleocene–Eocene (CPE), the Earth was in a “greenhouse” state with little or no ice at 11, 5683–5725, 2015 either pole. It was also a period of considerable global change, from the warmest pe- 5 riods of the mid Cretaceous, to the threshold of icehouse conditions at the end of the Palaeogeographic Eocene. However, the relative contribution of palaeogeographic change, solar change, controls on the and carbon cycle change to these climatic variations is unknown. Here, making use climate system of recent advances in computing power, and a set of unique palaeogeographic maps, we carry out an ensemble of 19 General Circulation Model simulations covering this D. J. Lunt et al. 10 period, one simulation per stratigraphic stage. By maintaining atmospheric CO2 con- centration constant across the simulations, we are able to identify the contribution from palaeogeographic and solar forcing to global change across the CPE, and explore the Title Page underlying mechanisms. We find that global mean surface temperature is remarkably Abstract Introduction constant across the simulations, resulting from a cancellation of opposing trends from Conclusions References 15 solar and paleogeographic change. However, there are significant modelled variations on a regional scale. The stratigraphic stage–stage transitions which exhibit greatest Tables Figures climatic change are associated with transitions in the mode of ocean circulation, them- selves often associated with changes in ocean gateways, and amplified by feedbacks J I related to emissivity and albedo. Our results also have implications for the interpreta- J I 20 tion of single-site palaeo proxy records. In particular, our results allow the non-CO2 (i.e. palaeogeographic and solar constant) components of proxy records to be removed, Back Close leaving a more global component associated with carbon cycle change. This “adjust- ment factor” is illustrated for 7 key sites in the CPE, and applied to proxy data from Full Screen / Esc Falkland Plateau, and we provide data so that similar adjustments can be made to any Printer-friendly Version 25 site and for any time period within the CPE. Interactive Discussion 5684 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1 Introduction CPD Over the last 150 million years, the climate of the Earth has experienced change across a broad range of timescales, from geological (10’s of millions of years), to orbital 10’s– 11, 5683–5725, 2015 100’s of thousands of years), to millenial, to decadal. 5 Variability on timescales from geological to orbital has been characterised by mea- Palaeogeographic surements of the isotopic composition of oxygen in the calcium carbonate of benthic controls on the 18 foraminifera (δ Obenthic, e.g. Friedrich et al.(2012); Cramer et al.(2009), Fig.1); this climate system indicates, in the broadest sense, a long-term cooling (and increasing ice volume) trend from the mid Cretaceous (∼ 100 million years ago, 100 Ma) to the modern. Imprinted D. J. Lunt et al. 10 on this general cooling are several shorter geological-timescale variations such as cooling through the Paleocene (65 to 55 Ma), and sustained warmings in the early 18 Title Page Eocene (55 to 50 Ma) and middle Miocene (17 to 15 Ma). The δ Obenthic record also shows evidence for orbital scale variability in icehouse periods (for example Quaternary Abstract Introduction glacial–interglacial cycles, from around ∼ 2 Ma) and greenhouse periods (for exam- Conclusions References 15 ple Paleogene hyperthermals, ∼ 55 Ma), and other events occurring on sub-geological timescales, such as the Eocene–Oligocene boundary (34 Ma) and Cretaceous Oceanic Tables Figures Anoxic Events (OAEs, ∼ 100 Ma). A methodology for reconstructing the global-mean surface temperature of the past J I ∼ 65 million years has been developed by Hansen et al.(2013), making assumptions 18 J I 20 about the relationship between δ Obenthic and deep ocean temperature, and between deep ocean temperature and surface temperature (Fig.1). The assumptions mean that Back Close the absolute values of temperature need to be treated with considerable caution, but the record indicates global mean surface temperatures of ∼ 25 ◦C in the Paleocene, Full Screen / Esc peaking at over 28 ◦C in the early Eocene. From the early Eocene to the present day, ◦ Printer-friendly Version 25 there is a general cooling trend, with temperatures decreasing to ∼ 24 C degrees by ◦ the late Eocene. Today, global mean surface temperature is close to 15 C. Interactive Discussion Although it has long been thought that greenhouse gas concentrations are the pri- mary cause of Cretaceous and Eocene warmth (Barron and Washington, 1984; Barron 5685 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | et al., 1995), the reasons for variability within this period are currently largely unknown. Possible candidates for the forcing on the Earth system on these timescales include CPD changes in solar forcing, direct tectonic forcing, and greenhouse gas forcing (most 11, 5683–5725, 2015 likely CO2) through changes in the carbon cycle, or some combination of these. 5 The solar forcing is related to a increase in solar constant, resulting from an increas- ing luminosity of the sun over 10’s of millions of years. This itself is due to continued Palaeogeographic nuclear fusion in the core of the Sun, converting hydrogen into helium, reducing the controls on the core’s density (Fig.2, Gough, 1981). As a result, the core contracts and temperature climate system increases. The increase in luminosity is a consequence of this core temperature in- D. J. Lunt et al. 10 crease. According to the solar model of Gough(1981), the solar constant increases nearly linearly from ∼ 1348 Wm−2 at 150 Ma to 1365 Wm−2 for the modern. Also over the timescales of tens of millions of years, plate tectonics has led to major Title Page changes in the position and configuration of the continents, and bathymetric and to- pographic depths and heights. These changes can have a direct effect on climate, for Abstract Introduction 15 example through changing the global distribution of low-albedo (ocean) vs. high-albedo Conclusions References (land) surfaces (e.g. Barron et al., 1980), and/or changing ocean gateways leading to modified ocean circulation (e.g. Zhang et al., 2011), and/or topography modifying the Tables Figures area of land above the snowline (e.g. Foster et al., 2010) or atmospheric circulation (e.g. Ruddiman and Kutzbach, 1989). Palaeogeographical reconstructions for certain J I 20 periods in Earth’s history exist (e.g. Scotese, 2001), but not always at high tempo- ral resolution, nor in a form which can readily be implemented into a climate model. J I In Sect. 2.1 we present a new set of palaeogeographical reconstructions which span Back Close the Cretaceous–Paleocene–Eocene, from approximately 150 to 35 Ma (Fig. S1 in the Full Screen / Esc Supplement). 25 The greenhouse gas forcing could itself be indirectly related to tectonic changes, Printer-friendly Version through changes in the balance of sources (e.g. volcanism) and/or sinks (e.g. weath- ering of silicate rocks, Raymo et al., 1988) of CO2, or other greenhouse gases, and/or Interactive Discussion changes to the sizes of the relevant reservoirs (e.g. due to changes in the residence time of carbon in the ocean due to changes in ocean circulation, Lauderdale et al., 5686 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2013, themselves driven ultimately by tectonic changes. In addition, greenhouse gases will also likely be modified in response to climate changes caused by the solar or di- CPD rect tectonic forcing. Efforts have been made to reconstruct the history of CO over 2 11, 5683–5725, 2015 geological timescales (e.g. see compilation in Fig.3, Honisch et al., 2012). However, 5 CO2 proxies are still associated with relatively large uncertainties, despite currently un- dergoing a period of rapid development (e.g. Zhang et al., 2013; Franks et al., 2014; Palaeogeographic Martinez-Boti et al., 2015). controls on the Disentangling these various forcings on long-term climate evolution is a key chal- climate system lenge. Previous work has often been in a modelling framework, and has focused on D.