Early Pliocene Warm Period Research Articles

Dampened El Niño in the Early Pliocene Warm Period

AbstractEl Niño‐Southern Oscillation (ENSO) is the strongest mode of interannual climate variability, and its predicted response to anthropogenic climate change remains unclear. Determining ENSO's sensitivity to climatic mean state and the strength of positive and negative feedbacks, notably the thermocline feedback, will help constrain its future behavior. To this end, we collected ENSO proxy data from the early and mid‐Pliocene, a time during which tropical Pacific zonal and vertical temperature gradients were much lower than today. We found that El Niño events had a reduced amplitude throughout the early Pliocene, compared to the late Holocene. By the mid‐Pliocene, El Niño amplitude was variable, sometimes reduced and sometimes similar to the late Holocene. This trend in Pliocene ENSO amplitude mirrors the long‐term strengthening of zonal and vertical temperature gradients and verifies model results showing dampened ENSO under reduced gradients due to a weaker thermocline feedback.

Climate change and tectonic activity during the early Pliocene Warm Period from the ostracod record at Lake Qinghai, northeastern Tibetan Plateau

The Early Pliocene Warm Period (EPWP, 5–3Ma) is sometimes thought to be a useful analogue for a future warmer world, and thus the boundary conditions and drivers of climate in the EPWP may provide valuable lessons for understanding how a future warmer world might unfold. Lake Qinghai is located on the northeastern margin of the Tibetan Plateau (TP) and is affected by both Monsoon climate and Westerlies circulation. It is sensitive to the climate drivers of these systems. Its sediments, accumulated over the Cenozoic period, are a rich source of information for climate, tectonics and environmental changes of the period.We present a high-resolution ostracod record from a Lake Qinghai sediment core with a record of the period 5.10–2.60Ma, thus covering the EPWP. Ostracods appear at 4.63Ma and are most abundant until 3.58Ma, while a body of water was present at the core site. This suggests a phase of humid climate and an intensified Asian Summer Monsoon (ASM), which is consistent with a warmer and wetter climate in the early Pliocene. Within this period the ostracod record shows some variabilities in lake level with deeper periods suggesting more intense ASM compared to those with shallower water. The disappearance of ostracods at 3.58Ma may provide evidence for the uplift of Qinghai Nanshan (south of Qinghai Lake) since this is when the ASM intensified.

Repeated advance and retreat of the East Antarctic Ice Sheet on the continental shelf during the early Pliocene warm period

Diatom analysis of a sediment core recovered at IODP Site U1358 on the continental shelf off the Adélie Coast indicated that the lower section of the core contained an assemblage dating back to the Thalassiosira innura Zone of the lower Pliocene that ranges from 4.2 to 5.12Ma. Based on lithological descriptions at both a macro- and micro-scale of this early Pliocene part of the core, four facies were interpreted from the diamictons representing the progressive advance and retreat of the grounding line over the site. Facies 1a and 1b contain a distinct directional signal from the orientation of the a-axis of clasts with several phases of fabric development along with both brittle and ductile deformation features that are common in sediments that have been subglacially deformed. Facies 1c and 1d are finely laminated and were deposited in open marine conditions. The four facies within the depositional model provide for the first time direct evidence for ice advancing across the shelf adjacent to the Wilkes Subglacial Basin on at least four occasions separated by three periods of open marine conditions indicating retreat of grounded ice inland of the site during a warmer than present early Pliocene. The times of open marine conditions are correlated with previous findings from the neighbouring rise sites that also indicated an oscillating ice margin. This has significant implications because firstly it suggests a dynamic East Antarctic Ice Sheet (EAIS) that is probably far more sensitive to climatic and oceanic forcing even during relatively short time periods than had previously been thought. Secondly it suggests that proxies used to interpret the advance and retreat of the grounding line from the rise can be linked with direct evidence of grounding line migration from the shelf. It also has important implications for the future behaviour and sensitivity of the EAIS under present continuing warming conditions. Together with results from the rise, this paper provides a crucial ice extent target for a new ice sheet model of this region during the Pliocene.

Fourier Grain-Shape Analysis of Antarctic Marine Core: The Relative Influence of Provenance and Glacial Activity On Grain Shape

Abstract Grain shape is useful as a depositional, environmental, diagenetic, and provenance proxy. Here we conduct the first study of down-core grain-shape variation in Antarctica. This study further examines the utility of grain shape as a depositional proxy, namely of glacial influence, in the James Ross Basin and Joinville Plateau of the Antarctic Peninsula from the Eocene to the present. Fourier grain-shape analysis was used to quantify the grain shape of 31 samples spanning the glacial history of the Antarctic Peninsula Ice Sheet. A total of 6,442 quartz grain peripheries were digitized and described through the twenty-first Fourier harmonic. Sediment provenance has a greater influence on grain roughness than glacial activity in the study area. However, provided a similar sedimentary source, secondary changes in roughness correlate with changes in the extent of the Antarctic Peninsula Ice Sheet since the late Oligocene. Grain roughness increased with increased glacial influence during the middle Miocene, after an early Pliocene warm period, and just above a Pliocene regional glacial unconformity. Conversely, grain roughness decreased with less glacial influence during the late Oligocene and during an early Pliocene warm period. This study further illustrates the usefulness of grain shape in distinguishing sediment provenance and environmental conditions in Antarctic settings.

Evolution of marine sedimentation in the Bering Sea since the Pliocene

Sediment of the Bering Sea, derived mainly from biogenic, glaciomarine, and, secondarily, riverine sources, reflects the history of oceanographic changes within the basin and climatic changes on the adjacent continents. Integrated Ocean Drilling Program (IODP) Expedition 323 recovered cores that reveal the evolution of sedimentation in the Bering Sea over the past 5 m.y., a period that includes globally significant events such as the early Pliocene warm period, the onset of extensive Northern Hemisphere glaciation, and the Pleistocene glacial-interglacial and millennial-scale climate cycles. To begin to understand the Bering Sea regional response to and role in these global climate change events, we examined the sedimentary constituents of Expedition 323 sites U1339, U1343, and U1344 on the Bering Slope, and U1340 and U1341 on Bowers Ridge. New particle size and petrographic analyses, combined with shipboard lithostratigraphic and physical property data, are used to characterize sediment types and texture and its distribution through space and time. The sediment comprises mainly two components, opaline diatom valves and siliciclastic grains (mainly clay and fine silt size). Approximately 40% of the variance in particle size can be explained by the abundance and preservation of diatom valves, a rough indicator of biogenic opal productivity. Particle size data indicate that productivity was generally higher during interglacials compared to glacials, and higher during the Pliocene warm period, decreasing as Northern Hemisphere glaciation intensified ∼3 m.y. ago. Although the abundance of diatoms in the sediment varied, diatom ooze and diatom mud are the dominant lithologies at Bowers Ridge, indicating that there was a persistent supply of diatoms to the sediment in the open Bering Sea during the past 5 m.y. This study provides a comprehensive view of sediment types and sedimentation processes; future work should be aimed at validating our interpretations of past changes in productivity and siliciclastic sedimentation mechanisms with multiple additional proxies.

A deep Eastern Equatorial Pacific thermocline during the early Pliocene warm period

During the early Pliocene warm period (∼4.6–4.2Ma) in the Eastern Equatorial Pacific upwelling region, sea surface temperatures were warm in comparison to modern conditions. Warm upwelling regions have global effects on the heat budget and atmospheric circulation, and are argued to have contributed to Pliocene warmth. Though warm upwelling regions could be explained by weak winds and/or a deep thermocline, the temporal and spatial evolution of the equatorial thermocline is poorly understood. Here we reconstruct temporal and spatial changes in subsurface temperature to monitor thermocline depth and show the thermocline was deeper during the early Pliocene warm period than it is today. We measured subsurface temperature records from Eastern Equatorial Pacific ODP transect Sites 848, 849, and 853 using Mg/Ca records from Globorotalia tumida, which has a depth habitat of ∼50–100m. In the early Pliocene, subsurface temperatures were ∼4–5°C warmer than modern temperatures, indicating the thermocline was relatively deep. Subsurface temperatures steeply cooled ∼2–3°C from 4.8 to 4.0Ma and continued to cool an additional 2–3°C from 4.0Ma to present. Compared to records from other regions, the data suggests the pronounced subsurface cooling between 4.8 and 4.0Ma was a regional signal related to restriction of the Isthmus of Panama, while continued cooling from 4.0Ma to present was likely related to global processes that changed global thermocline structure. Additionally, the spatial evolution of the equatorial thermocline along a N–S transect across ODP Sites 853, 849 and 848 suggests an intensification of the southeast trades from the Pliocene to present. Large-scale atmospheric and oceanographic circulation processes link high and low latitude climate through their influence on equatorial thermocline source water regions and consequently the equatorial thermocline. Through these low latitude/high latitude linkages, changes in the equatorial thermocline and thermocline source water played an important role in the transition from the warm Pliocene to the cold Pleistocene.

Teleconnections in a warmer climate: the pliocene perspective

Migrations toward altered sea surface temperature (SST) patterns in the Indo-Pacific region are present in the recent observational record and in future global warming projections. These SSTs are in the form of “permanent” El Nino-like (herein termed “El Padre”) and Indian Ocean Dipole (IOD)-like patterns. The Early Pliocene Warm Period, which bears similarity to future warming projections, may have also exhibited these Indo-Pacific SST patterns, as suggested by regional terrestrial paleo-climatic data and general circulation model studies. The ability to corroborate this assessment with paleo-data reconstructions is an advantage of the warm Pliocene period that is not afforded by future warming scenarios. Thus, the Pliocene period provides us with a warm-climate perspective and test bed for understanding potential changes to future atmospheric interactions given these altered SST states. This study specifically assesses how atmospheric teleconnections from El Padre/IOD SST patterns are generated and propagate to create the regional climate signals of the Pliocene period, as these signals may be representative of future regional climatic changes as well. To do this, we construct a holistic diagnostic rubric that allows us to examine atmospheric teleconnections, both energetically and dynamically, as produced by a general circulation model. We incorporate KE′, a diagnostic adapted from the eddy kinetic energy generation field, to assess the available energy transferred to these teleconnections. Using this methodology, we found that relative to our Modern Control experiments, weaker atmospheric teleconnections prevail under warm Pliocene conditions, although pathways of propagation still appear directed toward the southwestern United States from our tropical Pacific sector forcing. Propagation directly emanating from the Indian Ocean forcing sector appears to be largely blocked, although indirect teleconnective pathways appear traversing the Asian continent toward the North Pacific. The changes in the atmospheric circulation of Indian Ocean region in response to the underlying specified SST forcing (and indicated by Pliocene paleo-data) may have a host of implications for energy transfer out of and into the region, including interactions with the Asian jet stream and changes to the seasonal monsoon cycle. These interactions warrant further study in both past and future warm climate scenarios.

EPRISM: A case study in multiple proxy and mixed temporal resolution integration

As part of the Pliocene Research, Interpretation and Synoptic Mapping (PRISM) Project, we present the ePRISM experiment designed 1) to provide climate modelers with a reconstruction of an early Pliocene warm period that was warmer than the PRISM interval (~3.3 to 3.0 Ma), yet still similar in many ways to modern conditions and 2) to provide an example of how best to ntegratemultiple-proxy sea surface temperature (SST) data from time series with varying degrees of temporal resolution and age control aswe begin to build the next generation of PRISM, the PRISM4 reconstruction, spanning a constricted time interval. While it is possible to tie individual SST estimates to a single light (warm) oxygen isotope event, we find that the warm peak average of SST estimates over a narrowed time interval is preferential for paleoclimate reconstruction as it allows for the inclusion of more records of multiple paleotemperature proxies.

An approach to quantifying Pliocene ice sheet dynamics via slope failure frequencies recorded in Antarctic Peninsula rise sediments

AbstractUnderstanding of glacially driven sedimentary transport systems across the shelf to the slope and subsequently to deep sea sediment bodies along the Pacific continental margin of Antarctic Peninsula is crucial for interpreting ice sheet dynamics. Here we quantify slope-failure frequencies recorded in Pliocene core intervals of ODP Site 1095. We used the relationship between long-term sedimentation rate and marine carbon burial efficiency to calculate glacial or interglacial specific sedimentation rates. Using the decompacted average length of glacial-interglacial cycles it was possible to solve a set of linear equations to derive average half-periods of 61.59 and 59.77 kyr respectively for the time interval 5.8–3.2 Ma. The resulting frequency distribution of slope failures reflects short and rapid but cyclic ice advances every ∼375 years. Short retention times between slope loading and slope failure are supported by biogenic silica dissolution analyses. This study demonstrates the potential of the rise record to improve models of orbitally controlled size variations of the West Antarctic ice sheet and confirms the hypothesis of a highly dynamic ice sheet during the early Pliocene warm period.

High‐amplitude variations in North Atlantic sea surface temperature during the early Pliocene warm period

We provide the first continuous, orbital‐resolution sea surface temperature (SST) record from the high‐latitude North Atlantic, a region critical to understanding the origin of the Plio‐Pleistocene ice ages and proximal to regions that became frequently glaciated after ∼2.7 Ma. We analyzed sediments from Ocean Drilling Program Site 982 over the last 4 Ma for their alkenone unsaturation index and compared this surface water signal to a benthic δ18O record obtained from the same section. We find that while ocean surface temperatures were significantly warmer (∼6°C) than modern temperatures during the early Pliocene, they were also as variable as those during the late Pleistocene, a surprising result in light of the subdued variance of oxygen isotopic time series during the interval of 3–5 Ma. We propose two possible explanations for the high orbital‐scale SST variability observed: either that a strong, high‐latitude feedback mechanism not involving large continental ice sheets alternately cooled and warmed a broad region of the northern high latitudes or that by virtue of its location near the northern margin of the North Atlantic Drift, the site was unusually sensitive to obliquity‐driven climate shifts. On supraorbital time scales, a strong, sustained cooling of North Atlantic SSTs (∼4.5°C) occurred from 3.5 to 2.5 Ma and was followed by an interval of more modest cooling (an additional 1.5°C) from 2.5 Ma to the present. Evolutionary orbital‐scale phase relationships between North Atlantic SST and benthic δ18O show that SST began to lead δ18O significantly coincident with the onset of strong cooling at Site 982 (∼3.5 Ma). We speculate that these changes were related to the growth and subsequent persistence of a Greenland ice sheet of approximately modern size through interglacial states.

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.

Warm upwelling regions in the Pliocene warm period

Given the importance of upwelling processes to coastal productivity and regional climate, it is critical to study the role of upwelling regions within the context of global climate change. We generated sea surface temperature (SST) records for the last 5 million years in three important upwelling regions: the eastern equatorial Pacific, the California margin, and the Peru margin. Prior to ∼3.0 Ma, SSTs at all sites were significantly warmer than today (by 3–9°C), indicating that cold upwelling regions that characterize the modern Pacific Ocean did not exist in the early Pliocene warm period (4.6 to 3.1 Ma), Earth's most recent period of sustained global warmth. Alkenone, phosphorus, and organic carbon mass accumulation rate records indicate that changes in productivity and SST were decoupled and that upwelling of nutrient enriched water occurred even when SSTs were warm during the early Pliocene. Thus the long‐term trends in SST are likely explained by changes in the temperature of upwelled water rather than in the strength of upwelling‐favorable winds alone. The fact that gradual cooling of upwelling regions began before the onset of significant Northern Hemisphere glaciation provides further evidence that the growth of ice sheets and their influence on atmospheric winds alone can not explain the cooling of upwelling regions. Our results suggest that the long‐term average SSTs of upwelling regions are influenced by global changes in the depth and/or temperature of the ventilated thermocline.

Late Neogene interglacial events in the James Ross Island region, northern Antarctic Peninsula, dated by Ar/Ar and Sr-isotope stratigraphy

New outcrops of Late Neogene sedimentary deposits discovered on James Ross and Vega islands, northern Antarctic Peninsula, are fossiliferous and contain mainly fragmented pectinids amongst other as yet unstudied biota. The sedimentary deposits are interbedded with fresh volcanic units, thus providing the unusual opportunity to investigate the ages of the sedimentary rocks using two independent isotopic systems ( 40Ar/ 39Ar and 87Sr/ 86Sr). The fossils record past periods of warmer conditions (interglacials) like today. However, our results demonstrate unequivocally that some of the macrofossils present in the sedimentary rocks are reworked and the host sediments are, in these cases, much younger than the ages of their included fossils would suggest. Nevertheless, the Sr ages date these records of interglacial events in the region independently of the age of the strata in which they occur. Conversely, the volcanic rocks show abundant field evidences that they are coeval with the underlying sedimentary deposits, and hence their Ar ages reliably date the timing of their deposition. Our results indicate that at least three generally warmer periods are represented in the James Ross Island region: at 6.5–5.9, 5.03–4.22 and< 0.88 Ma, although our data do not have the resolution for identifying Milankovitch-scale cyclicity. However, the Early Pliocene warm period is particularly well represented and the James Ross Island data may well be capturing a higher proportion of the longer-lasting interglacials that characterised that period. We also present evidence for an interglacial in the Late Pliocene (at 2.54 + 0.86/− 0.36 Ma). Our data suggest that both Antarctic Peninsula and the East Antarctic ice sheets responded in a similar way to long-term regional shifts in climate, but the Antarctic Peninsula is more sensitive to short-term warming, as occurs today.