Petra Simonne Dekens

Evidence for El Niño–like conditions during the Pliocene

The modern tropical Pacific Ocean is characterized by strong east-west asymmetry in sea surface temperature and subsurface thermocline depth coupled to easterly trade winds and zonal atmospheric, or Walker, circulation. Walker circulation and the “normal” east-west asymmetry of sea surface temperature and thermocline depth break down temporarily during El Niño events. Since these temporary deviations from the “normal” tropical climate state are known to have global impacts, it is important to consider whether permanent shifts in the mean tropical Pacific climate state are an integral part of global climate change on longer time scales. To understand the link between tropical conditions and global warmth, we focus our study on the early Pliocene, the most recent period in Earth’s history of sustained global warmth relative to today. A data synthesis of tropical paleoceanographic data, including a new alkenone unsaturation index ( U 37 )–based sea surface temperature record from the eastern equatorial Pacific, indicates that, in the early Pliocene, the east-west asymmetry in sea surface temperature and thermocline depth was reduced compared to today and the tropical Pacific was in a permanent El Niño–like state. Thus, the “normal” mean state of the modern tropical Pacific is not a persistent feature of Earth’s climate over long time scales. INTRODUCTION Studies of the el niño Southern Oscillation (enSO) phenomenon indicate that, through atmospheric teleconnections, small changes in the pattern of tropical Pacific sea surface temperature (SSt) have a global impact on interannual time scales (Alexander et al., 2002). Although the mechanisms responsible for the enSO do not directly apply to studies of climate changes on longer time scales, enSO events provide a clear example of how changes in the distribution of SSt across the Pacific Ocean can have far-field climate effects such as higher than average rainfall in the southwestern United States and higher than average temperature in temperate regions of north America. the potential global effect of small, long-term changes in the tropical SSt pattern is substantiated by modeling studies (Yin and Battisti, 2001; Barreiro et al., 2005). However, while the impact of changes in the mean SSt pattern of the tropical Pacific on global climate is recognized, the circumstances under which they could occur are difficult to predict. For example, in simulations of future climate change forced with enhanced greenhouse gases, climate models do not give consistent results in the tropical Pacific: some predict no long-term changes; some predict el niño–like mean conditions; still others predict la niña–like mean conditions (cane, 2005; collins, 2005). Because the instrumental record is too short to examine multidecadal and longer-term climate changes, paleoceanographic studies are needed to establish whether modern mean tropical SSt patterns across tropical basins are stable over long time periods. these data-based studies can then be used to test and improve theoretical and computer models of long-term climate change, including those that are used to predict future climate change. While much can be learned from studying the extreme globally cool climate of the last Glacial Maximum (lGM), it is also important to focus on past periods of global warmth prior to the ice ages of the past few million years. Paleoceanographic studies generally indicate that the mean SSt of the Pacific tropical ocean was stable within a few degrees over millions of years, yet these studies rarely include enough data to characterize the east-west SSt difference across the Pacific. For example, the Pliocene warm period (ca. 4.5–3.0 Ma) (Fig. 1) has been the focus of much interest among paleoclimatologists because of the need to understand climate processes in past times of global warmth. landmark studies, such as those by the Pliocene research, interpretation, and Synoptic Mapping (PriSM) group, including compilations of oceanic and terrestrial data (Dowsett et al., 1996, 2005; thompson and Fleming, 1996) and modeling studies (Haywood et al., 2000; Sloan et al., 1996), indicate that the Pliocene was significantly warmer than today, especially in extratropical regions. However, the PriSM reconstructions include very little data from the tropical Pacific Ocean and therefore do not provide insight into changes in tropical SSt patterns. crowley (1996) pointed out the urgent need for more tropical data in order to further constrain the mechanisms that explain global climate conditions in the Pliocene, and in the last decade, several studies were conducted that focus on the tropical Pacific utilizing Pliocene-age material obtained by the Ocean Drilling Program.

Late Miocene decoupling of oceanic warmth and atmospheric carbon dioxide forcing.

Deep-time palaeoclimate studies are vitally important for developing a complete understanding of climate responses to changes in the atmospheric carbon dioxide concentration (that is, the atmospheric partial pressure of CO2, pCO2). Although past studies have explored these responses during portions of the Cenozoic era (the most recent 65.5 million years (Myr) of Earth history), comparatively little is known about the climate of the late Miocene (∼12–5 Myr ago), an interval with pCO2 values of only 200–350 parts per million by volume but nearly ice-free conditions in the Northern Hemisphere and warmer-than-modern temperatures on the continents. Here we present quantitative geochemical sea surface temperature estimates from the Miocene mid-latitude North Pacific Ocean, and show that oceanic warmth persisted throughout the interval of low pCO2 ∼12–5 Myr ago. We also present new stable isotope measurements from the western equatorial Pacific that, in conjunction with previously published data, reveal a long-term trend of thermocline shoaling in the equatorial Pacific since ∼13 Myr ago. We propose that a relatively deep global thermocline, reductions in low-latitude gradients in sea surface temperature, and cloud and water vapour feedbacks may help to explain the warmth of the late Miocene. Additional shoaling of the thermocline after 5 Myr ago probably explains the stronger coupling between pCO2, sea surface temperatures and climate that is characteristic of the more recent Pliocene and Pleistocene epochs.

A 5 million year comparison of Mg/Ca and alkenone paleothermometers

Geochemical sea surface temperature (SST) proxies such as the magnesium to calcium ratio (Mg/Ca) in foraminifera and the alkenone unsaturation index (UK′37) are becoming widely used in pre-Pleistocene climate records. This study quantitatively compares previously published Mg/Ca and UK′37 data from Ocean Drilling Program (ODP) site 847 in the eastern equatorial Pacific to assess the utility of these proxies to reconstruct tropical SST over the last 5 Ma. Foraminiferal Mg/Ca–SST calibrations that include a dissolution correction are most appropriate at this location because they provide SST estimates for the youngest sample that are close to modern mean annual SST. The long-term trends in the two records are remarkably similar and confirm a ∼3.5°C cooling trend from the early Pliocene warm period to the late Pleistocene noted in previous work. Absolute temperature estimates are similar for both proxies when errors in the dissolution correction used to estimate SST from Mg/Ca are taken into account. Comparing the two SST records at ODP site 847 to other records in the region shows that the eastern equatorial Pacific was 2–4°C warmer during the early Pliocene compared to today.

The evolution of the equatorial thermocline and the early Pliocene El Padre mean state

The tropical Pacific thermocline strength, depth, and tilt are critical to tropical mean state and variability. During the early Pliocene (~3.5 to 4.5 Ma), the Eastern Equatorial Pacific (EEP) thermocline was deeper and the cold tongue was warmer than today, which resulted in a mean state with a reduced zonal sea surface temperature gradient or El Padre. However, it is unclear whether the deep thermocline was a local feature of the EEP or a basin-wide condition with global implications. Our measurements of Mg/Ca of Globorotalia tumida in a western equatorial Pacific site indicate Pliocene subsurface temperatures warmer than today; thus, El Padre included a basin-wide thermocline that was relatively warm, deep, and weakly tilted. At ~4 Ma, thermocline steepening was coupled to cooling of the cold tongue. Since ~4 Ma, the basin-wide thermocline cooled/shoaled gradually, with implications for thermocline feedbacks in tropical dynamics and the interpretation of TEX86-derived temperatures.