The role of orbital forcing in the Early Middle Pleistocene Transition

Abstract

The Early Middle Pleistocene Transition (EMPT) is the term used to describe the prolongation and intensification of glacial–interglacial climate cycles that initiated after 900,000 years ago. During the transition glacial–interglacial cycles shift from lasting 41,000 years to an average of 100,000 years. The structure of these glacial–interglacial cycles shifts from smooth to more abrupt ‘saw-toothed’ like transitions. Despite eccentricity having by far the weakest influence on insolation received at the Earth's surface of any of the orbital parameters; it is often assumed to be the primary driver of the post-EMPT 100,000 years climate cycles because of the similarity in duration. The traditional solution to this is to call for a highly nonlinear response by the global climate system to eccentricity. This ‘eccentricity myth’ is due to an artefact of spectral analysis which means that the last 8 glacial–interglacial average out at about 100,000 years in length despite ranging from 80,000 to 120,000 years. With the realisation that eccentricity is not the major driving force a debate has emerged as to whether precession or obliquity controlled the timing of the most recent glacial–interglacial cycles. Some argue that post-EMPT deglaciations occurred every four or five precessional cycle while others argue it is every second or third obliquity cycle. We review these current theories and suggest that though phase-locking between orbital forcing and global ice volume may occur the chaotic nature of the climate system response means the relationship is not consistent through the last 900,000 years.

Introduction

The Early Middle Pleistocene Transition (EMPT; previously known as the Mid-Pleistocene Transition or Revolution; Berger and Jansen, 1994, Head et al., 2008) is the last major ‘event’ or transition in a secular trend towards more intensive global glaciation that characterizes the late Cenozoic (Zachos et al., 2001). The earliest recorded onset of significant regional glaciation during the Cenozoic was the widespread continental glaciation of Antarctica at about 34 Ma (e.g., Zachos et al., 2001, Huber and Nof, 2006, Sijp et al., 2009). Perennial sea ice cover in the Arctic has occurred throughout the past 14 Ma (Darby, 2008, Schepper et al., 2014). Glaciation in the Northern Hemisphere lagged behind, with the earliest recorded glaciation on Greenland occurring before about 6 Ma (e.g., Larsen et al., 1994, Thiede et al., 2011). Schepper et al. (2014) have identified a number of key Pliocene glacial events which may have been global and occurred at 4.9–4.8 Ma, ∼4.0 Ma, ∼3.6 Ma and ∼3.3 Ma. It is not until the Pliocene–Pleistocene transition that the long-term cooling trend culminates in the glaciation of Northern Europe and North America around 2.6 Ma (Maslin et al., 1998). The extent of glaciation did not evolve smoothly after this, but instead was characterized by periodic advances and retreats of ice sheets on a hemispherical scale – the ‘glacial–interglacial cycles’.

The EMPT is the marked prolongation and intensification of glacial–interglacial climate cycles initiated sometime between 900 and 650 ka (Fig. 1). Before the EMPT, global climate conditions appear to have responded primarily to the obliquity orbital periodicity (Imbrie et al., 1992, Tiedemann et al., 1994, Clark et al., 2006, Elderfield et al., 2012) through glacial–interglacial cycles with a mean periodicity of ∼41 kys. After about 900 ka, starting with Marine Oxygen Isotope Stage (MOIS) 22, glacial–interglacial cycles start to occur with a longer duration and a marked increase in the amplitude of global ice volume variations (Elderfield et al., 2012, Rohling et al., 2014). The increase in the contrast between warm and cold periods may also be in part due to the extreme warmth of many of the post-EMPT interglacial periods as similar interglacial conditions can only be found at ∼1.1 Ma, ∼1.3 Ma and before ∼2.2 Ma. Fig. 2 shows time-series analysis of the ODP 659 (Tropical East Atlantic ocean) benthic foraminifera oxygen isotope record spanning the EMPT (Mudelsee and Stattegger, 1997). The analysis suggests the EMPT was a two-step process with the first transition at about 900 ka, when there is a significant increase in global ice volume but the 41 ky climate response remains. This situation persists until the second step, about 700 ka, when the climate system finds a three-state solution and strong quasi-100 ky climate cycles begin (Mudelsee and Stattegger, 1997). This is consistent with the more recent evidence from ODP Site 1123 in the Southern Pacific ocean, which shows a step like increase in ice volume during glacial periods starting at MOIS 22 at about 900 ka (Elderfield et al., 2012).

During the EMPT there seems to be a shift from a two stable climate state system to a system with three quasi-stable climate states (Fig. 3). These three states roughly correspond to: 1) full interglacial conditions, 2) moderate glacial conditions such as MOIS 3 that are analogous to the glacial periods prior to the EMPT and 3) maximum glacial conditions for example MOIS 2, the Last Glacial Maximum (LGM). This has also added confused to the definition of the EMPT as many of the intermediate climate periods have been overlooked such as the weak interglacial at ∼740 ka, which does not have it own defined MOIS, or the double warm peaks during MOIS 15, 13, and 7.