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Welcome to PlioWiki, a wiki maintained by the community involved in the PLIOMAX project that investigates relative sea level (RSL) during the Pliocene. Please keep in mind this is a work in progress and your input is welcome.


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Goals of PLIOMAX project

The Pliomax project aims to increase the accuracy of global sea level estimates for the mid-Pliocene warm period, between 3.3 and 2.9 million years ago. Numerous proxy methods suggest that atmospheric CO2 levels at that time ranged between 350 and 400 ppm, a maximum value that will soon be exceeded. Likewise, mean global surface temperature is estimated to have been 2-3°C greater than today. Thus, the mid-Pliocene warm period provides both a natural analogue for a higher CO2 world as well as a testing ground for climate and ice sheet models that are being used to predict the future response of Earth’s climate to increasing levels of greenhouse gases (GHG). However, our ability to calibrate and verify model performance under different CO2 and climate conditions is limited by the accuracy of available paleoclimate data and, in particular, our knowledge of past sea level (SL), a reflection of polar ice volume. Currently, few estimates of mid-Pliocene SL exist and they range from +5m to >+40m (“+” represents the elevation of sea level relative to present). These SL estimates reflect a large range of uncertainty in the sensitivity of polar ice sheets, including the East Antarctic Ice Sheet (EAIS), to a modest, ~2-3°C, global warming.

The goal of the Pliomax project is to facilitate the study of nearshore Pliocene stratigraphic sections around the world. It has become increasingly clear that all evidence for past SL, whether from the last glacial maximum 20,000 years ago or from three million years ago, needs to be evaluated within the context of glacial isostatic adjustment models of crustal loading[1]. Such models make predictions as to where diagnostic responses of eustatic SL to a given ice volume change are likely to be observed. In other words, no one site can perfectly record global SL; dozens of mid-Pliocene SL estimates from globally distributed locations, a "fingerprint", are needed in order to constrain the Pliocene SL[2].

The Pliocene ice sheets

Definition of mid-Pliocene warm period (MPWP)

The MPWP has classically been understood to be the warm interval just prior to the onset of the northern hemisphere (NH) cooling trend that culminated in the establishment of widespread NH glaciation by 2.5 Ma. Here we define the MPWP as extending from the M2-M1 marine isotope stage (MIS) transition to the G17-G16 MIS transition as shown on the LR04 benthic isotope stack[3]. Using the LR04 time scale[3], this interval extends from ~3.28 Ma to ~2.94 Ma. Within this 340,000 year interval, four intervals of maximum warmth and/or minimum ice volume are observed in the benthic isotope record; MIS KM5, KM3, K1, and G17. The alterations between warmer and cooler climate are paced by Milankovitch orbital cycles[4]. While we are most interested in determining the maximum sea level rise (minimum ice volume) during these warm events, it would also be useful to determine the range, or amplitude, of ice volume change associated with these cycles. Around the world, the warm conditions that prevailed during the mid-Pliocene began to decline by 2.7 Ma, with both marine and terrestrial records indicating global cooling. By ~2.5 Ma (MIS 100), evidence for significant continental ice growth in the circum-North Atlantic and circum-North Pacific regions is found both on land and in the sea[5][6]. This transitional period is typically referred to as the intensification of northern hemisphere glaciation due to the well-documented expansion of ice sheets on North America and Scandanavia at this time. What is less clear is whether a significant cooling occurred in the south polar regions as well.

Ice-volume record for the Plio-Pleistocene using the LR04 benthic δ18O stack and timescale[3]. Geomagnetic reversal stratigraphy is shown above the x-axis. PLIOMAX will target four super-interglacial events, G17, K1 and KM3 (orange bars) that are well constrained by magneto- and biostratigraphy. Oxygen isotope inferred sea level changes, assuming no temperature, salinity, diagenetic, or vital effect overprints are shown on the scale on the right.

Direct evidence for the size and location of ice sheets during MPWP

In the northern hemisphere, it is certain that glaciers reaching sea level were at least periodically present in Greenland and Scandinavia during the MPWP. This is evident from small, but continuous, amounts of ice-rafted detritus (IRD) in cores from the Vøring Plateau and off the coast of East Greenland [7]. It seems possible that ice sheets reaching the sea may have been present in the circum-Arctic region as well. However, the absence of IRD in open North Atlantic sediment prior to 2.8 Ma suggests that no widespread Northern Hemisphere glacial advances took place during the mid-Pliocene [8][9][10]. Likewise, the appearance of widespread IRD in the North Pacific Ocean does not occur until after 2.7 Ma[11]. It would be interesting to determine if IRD is present in the Norwegian-Greenland Sea specifically during the warmest marine isotope stages in the PLIOMAX interval. This may be difficult to determine due to the scarcity of carbonate in this region which makes chronology challenging.

In the southern hemisphere, studies conducted on and around Antarctica are suggestive, if not directly indicative, of a warmer, partially deglaciated EAIS during the MPWP; such evidence includes extensive paleosol development[12], increased smectite in near-shore sediment[13] and less regional ice-rafted material and sea ice[14]. However, it is impossible to determine from these studies the magnitude of deglaciation that may have occurred at this time, including the locations of terrestrial ablation margins and grounded marine-based ice margins around the East Antarctic continent.

Dynamic behavior of the EAIS margin throughout the Plio-Pleistocene is suggested by evidence of less continental ice, reduced sea-ice cover, and inland penetration of warmth in the Prydz Bay region[15][16]as well as a significant melting of the Ross Ice Shelf associated with warming near 1.0 Ma[17]. It is possible that the almost completely ice-covered and poorly studied EAIS coastline (which generally lies between 65°S and 70°S over more than 7000 km) could have been periodically deglaciated (with ice margins retreating inland) at various times during the late Pliocene and/or early Pleistocene. This possibility is the subject of a long-standing debate in the literature about the meaning of paleontological evidence in the Sirius Formation that some interpret as indicative of open marine conditions in the interior basins of Antarctica during the middle Pliocene[18]. Such an interpretation would be consistent with an EAIS retreat at that time although the mode of emplacement of the Sirius formation microfossils remains controversial[19].

In contrast to EAIS, strong evidence for periodic collapse of the WAIS during the MPWP has been documented from sediment recovered in the Ross Sea by the Andrill drilling project[20]

Indirect evidence for ice volume during the MPWP

Indirect or proxy methods can also be used to infer the volume of polar ice sheets at times in the past. Two approaches are commonly used by the paleoclimate community: a) geochemical measurements of the oxygen isotopic change of seawater due to the growth and decay of O-16 enriched ice sheets[21]; and b) estimation of past sea level change, based on shoreline indicators, as a direct response to ice mass change. Each method has its own advantages and disadvantages -- this site focuses on the second.

Field-based determination of paleo-sea level utilizes fundamental principles of geomorphology and morphostratigraphic succession combined with relative and absolute geochronological dating of paleo-shoreline/sea level deposits[22]. For the MPWP, we believe that the best estimates of SL highstands will come from diagnostic coastal sedimentary deposits where local and regional tectonics are minimal or can be constrained by reference to younger, known highstand deposits (for instance, from MIS5e or MIS11). These paleoshoreline estimates can then be evaluated within the context of predicted (modeled) glacio-isostatic and epeirogenic effects. Finally, it is important to recognize that one is typically observing evidence for maximum warmth highstands (in an orbital cycle sense) and thus are documenting the maximum amount of deglaciation that occurred during the low-amplitude, high-frequency climate cycles of the early and middle Pliocene.

It has been said that no perfectly stable margin exists[23] and thus no “silver bullet” site exists that will reflect eustatic SL change during the MPWP. Every margin around the globe will have a unique tectonic, isostatic, and epeirogenic (dynamic topography) history. While tectonic histories are fundamentally local, glaciohydro-isostatic effects will have broad regional patterns that will vary systematically around the globe[24][25][26][27]. Thus, even if SL rose 20 m and there were no local tectonic offsets, that 20 m would still be expressed over a range of elevations depending on the global distribution of glacio-isostatic and dynamic topography effects. With a broad geographic distribution of mid-Pliocene SL estimates (which is being compiled in RSLmap), one may eventually be able to “fit”, through inverse modeling, the most likely SL history (maximum eustatic rise) consistent with the global distribution of shorelines and predicted (modeled) GIA and dynamic topography responses.

Volume of modern ice reservoirs

While there is plenty of evidence to suggest that less ice existed on the planet during the MPWP, determining exactly where and how much ice melted during this slightly warmer period has proven far more difficult to determine. Today, most of the water tied up in ice on the Earth’s surface resides in Antarctica (~61 m sea level equivalent, SLE[28]). Only 7 m SLE is locked in the Greenland ice sheet (GIS) and far less (~0.5 m SLE) is locked in mountain glaciers and ice caps[28]. The well-documented and rapid retreat of mountain glaciers around the world over the last 40 years is believed to have contributed 0.50 ± 0.18 mm/yr (or about 2 cm total) to the observed rise in sea level[29]. Far less certain is the long-term contribution of the GIS and Antarctic ice sheets to sea level changes over the last few decades. However, summarizing a decade of recent satellite data, Shepherd and Wingham[28] estimate that ice sheet mass imbalances contribute 0.35 mm/yr (or ~10%) to the present rate of sea level rise. They further conclude that most of this ice loss is due to acceleration of ice streams and glaciers into the ocean, congruent with a recent and growing consensus among glaciologists that ice sheets are far more dynamic, and capable of movement on far shorter timescales, than previously accepted wisdom would have allowed[30]. The growing appreciation of the dynamic behavior of even large continental ice sheets has led many scientists to conclude that current IPCC estimates for future sea level rise due to melting are probably underestimates and that improvements in ice sheet models are urgently needed.

Ultimately, ice sheets are at the mercy of the competing forces of ablation and accumulation. On the East Antarctica ice sheet (EAIS) today, virtually no melting occurs and precipitation is limited by low air temperature. Most ablation is due to calving of icebergs from ice margins at sea level. By contrast with the EAIS, the West Antarctic and Greenland ice sheets experience widespread summer melting (ablation) in low altitude coastal regions that is offset by accumulation inland.

Review of Pliocene climate

Paleo CO2 estimates

Proxy evidence for Pliocene atmospheric carbon dioxide suggest levels may have varied between 350 and 400 ppmv[31][32], ~35% higher than pre-industrial levels and comparable to current values (394 ppmv in spring of 2012).

Published studies of Pliocene greenhouse gas concentrations

Sea surface temperature (SST) reconstructions

Evidence is abundant for widespread and persistent surface ocean warmth during the mid Pliocene. At 3.0 Ma, high-latitude SSTs may have been elevated by as much as 7°C with respect to modern values, and mid-latitude SSTs elevated by 3–4°C with respect to modern values[33]; estimates of low-latitude SSTs at this time are not significantly higher than at present[34]. Note however that some SST reconstructions may be biased by assumptions about the temperature tolerance of extinct species. One notable difference in low-latitude SST patterns is observed in the equatorial Pacific region where the characteristic east-west SST gradient was considerably reduced relative to today[35][36][37]. From these data, numerous investigators have concluded that the Pacific Ocean was in a permanent “El Nino”-like state during the preglacial Pliocene[38][39][40][41][42].

Published studies of Pliocene SSTs

Land temperature reconstructions

Published Studies of land surface temperature during Pliocene

Thermohaline circulation and latitudinal temperature gradients

In addition to higher CO2 levels which would have promoted global, and especially high-latitude, warmth, stronger thermohaline circulation and thus enhanced meridional transport may also have contributed to increased warmth at high latitudes[43][44] [45]. Both reconstructed and modeled equator-pole SST gradients are less than the observed modern gradient[46].

Published studies of Pliocene ocean and atmosphere circulation

Synoptic reconstructions of Pliocene surface temperatures

The PRISM (Pliocene Research, Interpretation and Synoptic Mapping) Project is the most comprehensive and detailed reconstruction of global climate and environmental conditions older than the last glacial maximum. The project focused on the mid Pliocene warm interval from 3.3-3.0 Ma and began with the PRISM1 reconstruction that was based upon 64 marine and 74 terrestrial sites including data sets representing vegetation and land ice, monthly SST and sea ice, sea level and topography[47][48]. An expanded PRISM2 reconstruction[49] consisted of 28 global scale data sets on a 2° latitude by 2° longitude grid. These data sets included additional marine and terrestrial proxy reconstructions[50] as well as additional marine sites, including from the Mediterranean and Indian Ocean, that improved spatial coverage. In PRISM2, SST estimates also were recalculated using an improved core-top calibration data set[51]. While PRISM1 assumed sea level was at +35m, sea level was assumed to be at only +25m in PRISM2 in keeping with isotopic data thought to be more accurate[52] and not available when PRISM1 was released. PRISM2 also used numerical climate model results to guide the areal and topographic distribution of Antarctic ice although such an exercise is clearly highly dependent on the value assumed for sea level (and hence global ice volume).

Most recently, PRISM3 incorporates improvements to the SST and sea-ice proxy data sets, enhanced coverage of terrestrial vegetation data, as well as deep-water temperatures in the context of a three-dimensional ocean (PRISM3D). These changes will allow the PRISM data set to be used in more sophisticated climate modeling experiments. In particular, the SST reconstruction has been enhanced by inclusion of maximum and minimum probable warming limits.

Numerical modeling studies of Pliocene climate

Pliocene ice sheet models

In a recent simulation of Antarctic ice sheet history spanning the last 5 Ma, and which used a new ice sheet model with realistic ice shelves and migrating grounding lines[53], the West Antartic Ice Sheet (WAIS) exhibited highly dynamic behavior with dramatic retreats during a number of late Pliocene and Pleistocene Antarctic “super-interglacials”. The present ice configuration was compared with the smallest Antarctic ice volume obtained in the 5 Ma simulation, that was estimated to be equivalent to ~7m of sea level rise. If coeval with full Greenland deglaciation, this would imply a maximum sea level rise of ~14 m. In these simulations, most of the ice loss in Antarctica was from the marine-based WAIS and was mainly caused by increased sub-ice ocean melt rather than sea level rise (destabilizing the ice sheet) or surface melt. Additional ice loss from Antarctica would require significant surface melt over the flanks of the terrestrial East Antartic Ice Sheet (EAIS), melting that may be underestimated given the simple parameterized climate used to drive the ice model.

Published modeling studies of Pliocene ice sheets

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External Links

PALSEA A key question for future climate change prediction is the sensitivity of ongoing sea-level rise to increases in temperature. Peak Quaternary temperatures and sea levels were similar to modern (or meters higher) with some interglacials slightly cooler and others significantly warmer so that the paleo data set covers the range of temperature increases and sea level increases suggested for the next century. PALSEA is a working group aiming to define how we may place empirical constraints on sea-level rise over the next century using paleodata.

MEDFLOOD MEDFLOOD proposes the creation of a comprehensive, coherent, spatially explicit and updatable database containing Holocene and MIS 5.5 relative sea level data available in literature for the Mediterranean basin. The database will be freely available online and will be updatable by the scientific community. The database, coupled with considerations on vertical land movements due to tectonics, volcanic and isostatic effects, will sustain the projections of future sea level and the building of coastal flooding maps for the countries involved in the project.

Sealevel.climatecentral Threats from sea level rise and storm surges to all 3000+ coastal towns, cities, counties and states in the lower forty-eight states www.climatecentral.org

PRISM The PRISM (Pliocene Research, Interpretation and Synoptic Mapping) Project was initially devised to reconstruct surface conditions from a focused stratigraphic interval (3.264 - 3.025 Ma) that was similar to what we may expect in the near future. A major component of the PRISM reconstruction has been the systematic documentation of the magnitude and variability of Pliocene sea surface temperature (SST) on a global scale. The main purpose of efforts to reconstruct SST and other paleoenvironmental parameters is to provide a conceptual model and synoptic view of the earth during an interval considerably warmer than modern. The acceptance of future global warming has increased interest in documenting and modeling Earth response to past episodes of warming.

PlioMIP General Circulation Models (GCMs) are routinely used to simulate and predict Earth’s past, present and future climate. Although models broadly agree, significant differences exist in the detail of their predictions. Unfortunately, paleoclimate modeling studies often utilize only a single model, meaning the results obtained may be highly model-dependant. To combat this bias, the international Palaeoclimate Modelling Intercomparison Project (PlioMIP) was initiated to coordinate and encourage the systematic study of several GCMs and to assess their ability to simulate large changes in paleoclimate. In 2007 PlioMIP coverage was extended beyond the Last Glacial Maximum and the mid-Holocene climatic optimum to the mid-Pliocene, a time period identified by the IPCC as a potential analogue for future climate change.

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