Ana Christina Ravelo

Regional climate shifts caused by gradual global cooling in the pliocene epoch

The Earths climate has undergone a global transition over the past four million years, from warm conditions with global surface temperatures about 3 °C warmer than today, smaller ice sheets and higher sea levels to the current cooler conditions. Tectonic changes and their influence on ocean heat transport have been suggested as forcing factors for that transition, including the onset of significant Northern Hemisphere glaciation ∼2.75 million years ago, but the ultimate causes for the climatic changes are still under debate. Here we compare climate records from high latitudes, subtropical regions and the tropics, indicating that the onset of large glacial/interglacial cycles did not coincide with a specific climate reorganization event at lower latitudes. The regional differences in the timing of cooling imply that global cooling was a gradual process, rather than the response to a single threshold or episodic event as previously suggested. We also find that high-latitude climate sensitivity to variations in solar heating increased gradually, culminating after cool tropical and subtropical upwelling conditions were established two million years ago. Our results suggest that mean low-latitude climate conditions can significantly influence global climate feedbacks.

The Pliocene Paradox (Mechanisms for a Permanent El Niño)

During the early Pliocene, 5 to 3 million years ago, globally averaged temperatures were substantially higher than they are today, even though the external factors that determine climate were essentially the same. In the tropics, El Niño was continual (or “permanent”) rather than intermittent. The appearance of northern continental glaciers, and of cold surface waters in oceanic upwelling zones in low latitudes (both coastal and equatorial), signaled the termination of those warm climate conditions and the end of permanent El Niño. This led to the amplification of obliquity (but not precession) cycles in equatorial sea surface temperatures and in global ice volume, with the former leading the latter by several thousand years. A possible explanation is that the gradual shoaling of the oceanic thermocline reached a threshold around 3 million years ago, when the winds started bringing cold waters to the surface in low latitudes. This introduced feedbacks involving ocean-atmosphere interactions that, along with ice-albedo feedbacks, amplified obliquity cycles. A future melting of glaciers, changes in the hydrological cycle, and a deepening of the thermocline could restore the warm conditions of the early Pliocene.

Patterns and mechanisms of early Pliocene warmth

About five to four million years ago, in the early Pliocene epoch, Earth had a warm, temperate climate. The gradual cooling that followed led to the establishment of modern temperature patterns, possibly in response to a decrease in atmospheric CO2 concentration, of the order of 100 parts per million, towards preindustrial values. Here we synthesize the available geochemical proxy records of sea surface temperature and show that, compared with that of today, the early Pliocene climate had substantially lower meridional and zonal temperature gradients but similar maximum ocean temperatures. Using an Earth system model, we show that none of the mechanisms currently proposed to explain Pliocene warmth can simultaneously reproduce all three crucial features. We suggest that a combination of several dynamical feedbacks underestimated in the models at present, such as those related to ocean mixing and cloud albedo, may have been responsible for these climate conditions.

Enhanced circulation during a warm period

The Early Pliocene is the most recent interval in which equilibrium oceanic conditions can be studied in the context of global warmth relative to today. To characterize thermohaline circulation during this warm period (4.4-3.1 Ma) we combined new benthic foraminiferal isotopic data from the southeast Atlantic with published data, and reconstructed Pacific and Atlantic Ocean nutrient distributions. The data indicate enhanced ventilation of the deep Atlantic and intermediate Pacific during the Early Pliocene. Enhanced ventilation implies that delivery of surface water to high latitudes in western boundary currents of North Pacific and Atlantic subtropical gyres was probably also enhanced. Future modeling of this warm period needs to reconcile reduced meridional surface temperature gradients with enhanced subtropical gyre and thermohaline circulation.

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.

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.

No iron fertilization in the equatorial Pacific Ocean during the last ice age

The equatorial Pacific Ocean is one of the major high-nutrient, low-chlorophyll regions in the global ocean. In such regions, the consumption of the available macro-nutrients such as nitrate and phosphate is thought to be limited in part by the low abundance of the critical micro-nutrient iron. Greater atmospheric dust deposition could have fertilized the equatorial Pacific with iron during the last ice age—the Last Glacial Period (LGP)—but the effect of increased ice-age dust fluxes on primary productivity in the equatorial Pacific remains uncertain. Here we present meridional transects of dust (derived from the 232Th proxy), phytoplankton productivity (using opal, 231Pa/230Th and excess Ba), and the degree of nitrate consumption (using foraminifera-bound δ15N) from six cores in the central equatorial Pacific for the Holocene (0–10,000 years ago) and the LGP (17,000–27,000 years ago). We find that, although dust deposition in the central equatorial Pacific was two to three times greater in the LGP than in the Holocene, productivity was the same or lower, and the degree of nitrate consumption was the same. These biogeochemical findings suggest that the relatively greater ice-age dust fluxes were not large enough to provide substantial iron fertilization to the central equatorial Pacific. This may have been because the absolute rate of dust deposition in the LGP (although greater than the Holocene rate) was very low. The lower productivity coupled with unchanged nitrate consumption suggests that the subsurface major nutrient concentrations were lower in the central equatorial Pacific during the LGP. As these nutrients are today dominantly sourced from the Subantarctic Zone of the Southern Ocean, we propose that the central equatorial Pacific data are consistent with more nutrient consumption in the Subantarctic Zone, possibly owing to iron fertilization as a result of higher absolute dust fluxes in this region. Thus, ice-age iron fertilization in the Subantarctic Zone would have ultimately worked to lower, not raise, equatorial Pacific productivity.

North Pacific Intermediate Water circulation enhanced by the closure of the Bering Strait

The Bering Strait provides a shallow connection that allows freshwater to flow from the North Pacific into the North Atlantic, but this passage was closed during past glacials when sea level was at least 50 m lower than at present. Climate models investigating Bering Strait closure predict that this mechanism increases the salinity in the North Atlantic and reduces the salinity in the North Pacific, inducing a Pacific-Atlantic seesaw in meridional overturning circulation and poleward heat flux. However, the Pacific circulation response to Bering Strait closure, and thus the seesaw theory, has not been tested by long paleoceanographic records. We present long records of foraminiferal δ18O and δ13C from Integrated Ocean Drilling Program Site U1342 in the Bering Sea, which provide the first evidence of enhanced North Pacific Intermediate Water when the Bering Strait was closed during each of the extreme glacials of the last 1.2 Myr. These results suggest that orbital-scale variations in North Pacific Intermediate Water are coherent and in phase with variations in upper North Atlantic Deep Water, but are unrelated to changes in lower North Atlantic Deep Water. Together, these results provide evidence for systematic, orbital-scale variability in North Pacific Ocean circulation and may challenge the idea of an orbital-scale Pacific-Atlantic seesaw.

Enhanced subarctic Pacific stratification and nutrient utilization during glacials over the last 1.2 Myr

The relationship between climate, biological productivity, and nutrient flux is of considerable interest in the subarctic Pacific, which represents an important high-nitrate, low-chlorophyll region. While previous studies suggest that changes in iron supply and/or physical ocean stratification could hypothetically explain orbital-scale fluctuations in subarctic Pacific nutrient utilization and productivity, previous records of nutrient utilization are too short to evaluate these relationships over many glacial-interglacial cycles. We present new, high-resolution records of sedimentary δ15N, which offer the first opportunity to evaluate systematic, orbital-scale variations in subarctic Pacific nitrate utilization from 1.2 Ma. Nitrate utilization was enhanced during all glacials, varied with orbital-scale periodicity since the mid-Pleistocene transition, was strongly correlated with enhanced aeolian dust and low atmospheric CO2, but was not correlated with productivity. These results suggest that glacial stratification, rather than iron fertilization, systematically exerted an important regional control on nutrient utilization and air-sea carbon flux.

Export production fluctuations in the eastern equatorial Pacific during the Pliocene‐Pleistocene: Reconstruction using barite accumulation rates

Export production is an important component of the carbon cycle, modulating the climate system by transferring CO2 from the atmosphere to the deep ocean via the biological pump. Here we use barite accumulation rates to reconstruct export production in the eastern equatorial Pacific over the past 4.3 Ma. We find that export production fluctuated considerably on multiple time scales. Export production was on average higher (51 g C m−2 yr−1) during the Pliocene than the Pleistocene (40 g C m−2 yr−1), decreasing between 3 and 1 Ma (from more than 60 to 20 g C m−2 yr−1) followed by an increase over the last million years. These trends likely reflect basin-scale changes in nutrient inventory and ocean circulation. Our record reveals decoupling between export production and temperatures on these long (million years) time scale. On orbital time scales, export production was generally higher during cold periods (glacial maxima) between 4.3 and 1.1 Ma. This could be due to stronger wind stress and higher upwelling rates during glacial periods. A shift in the timing of maximum export production to deglaciations is seen in the last ~1.1 million years. Results from this study suggest that, in the eastern equatorial Pacific, mechanisms that affect nutrient supply and/or ecosystem structure and in turn carbon export on orbital time scales differ from those operating on longer time scales and that processes linking export production and climate-modulated oceanic conditions changed about 1.1 million years ago. These observations should be accounted for in climate models to ensure better predictions of future climate change.