Expanded Florida reef development during the mid-Pliocene warm period

The opening and closing of ocean gateways has played a major role in shaping global climate by altering oceanic and atmospheric circulation. This study uses planktonic foraminifer assemblage-based sea surface temperature (SST) estimates (Modern Analog Technique) to examine the eastern tropical Pacific (ETP) and subtropical Northwest Atlantic (NWA) during the early stages of shoaling of the Central American Seaway (CAS). In the subtropical NWA (DSDP 103 and ODP 1006), the 5.2 to 5.1 Ma interval is characterized by an increase in SST and sea surface salinity, indicating a strengthening of the Florida Current (FC) and Gulf Stream (GS). Sea surface temperature in the ETP Warm Pool (DSDP Site 84) remained relatively stable between 6.9 and 5.1 Ma, during which El Niño-like conditions persisted. A slight cooling is observed after this interval (with synchronous warming in the NWA), followed by the onset of major cooling at ∼3.2 Ma, both of which are preceded by a shallowing of the thermocline. Stepwise cooling is attributed to enhanced Atlantic meridional overturn cicrulation (AMOC), which caused a shoaling of the main tropical thermocline, thereby strengthening the Walker Circulation and weakening the Pacific North Equatorial Counter Current. Antithetical changes in surface current strength in the NWA and ETP suggest a southward migration of the Intertropical Convergence Zone ∼4.3 Ma. During the mid-Pliocene, SST in the Panama Basin was ∼0.8°C cooler than today, while the subtropical NWA was only ∼1.1°C warmer. This corroborates evidence for reduced meridional SST gradients during the mid-Pliocene as well as the hypothesis that more vigorous ocean circulation-particularly in the NWA-was critical during this period. The timing of SST changes in the ETP and NWA (∼5.1 Ma) suggest that the termination of permanent El Niño and enhanced AMOC did not contribute significantly to the onset of major Northern Hemisphere glaciation (NHG), as both of these events occur well before the beginning of the glacial cycles. However, these processes may have contributed to the development of the small ice sheets of the late Miocene and early Pliocene, but were most likely only preconditioning factors for the onset of major NHG. However, changes in SST and relative thermocline position suggest that high-latitude processes and global cooling may have influenced thermal structure in the ETP. The SST estimates provided indicate that even in its early stages, the shoaling of the CAS had significant implications for low-latitude ocean circulation and thermal structure, as well as for some of the most significant global climate events of the late Neogene.

Ecological interactions are ubiquitous on tropical coral reefs, where sessile organisms coexist in limited space. Within these high-diversity systems, reef-building scleractinian corals form an intricate interaction network. The role of biotic interactions among reef corals is well established on ecological timescales. However, its potential effect on macroevolutionary patterns remains unclear. By analysing the rich fossil record of Scleractinia, we show that reef coral biodiversity experienced marked evolutionary rate shifts in the last 3 million years, possibly driven by biotic interactions. Our models suggest that there was an overwhelming effect of staghorn corals (family Acroporidae) on the fossil diversity trajectories of other coral groups. Staghorn corals showed an unparalleled spike in diversification during the Pleistocene. But surprisingly, their expansion was linked with increases in both extinction and speciation rates in other coral families, driving a nine-fold increase in lineage turnover. These results reveal a double-edged effect of diversity dependency on reef evolution. Given their fast growth, staghorn corals may have increased extinction rates via competitive interactions, while promoting speciation through their role as ecosystem engineers. This suggests that recent widespread human-mediated reductions in staghorn coral cover, may be disrupting the key macroevolutionary processes that established modern coral reef ecosystems.

Late Pliocene continental climate and vegetation variability in the Arctic-Atlantic gateway region prior to the intensification of Northern Hemisphere glaciations

<p>The tropical Pacific thermocline structure is critical to tropical sea surface temperatures (SSTs) and variability. During the mid-Pliocene warm period (~3 Ma), the zonal SST gradient was reduced due to relatively warm SST in the Eastern Equatorial Pacific; we call this mean state “El Padre.” How did the equatorial thermocline contribute to this reduced zonal SST gradient? Here we summarize published Mg/Ca (surface and subsurface dwelling foraminifera) and alkenone records and generate new SST estimates from Mg/Ca and alkenones. The subsurface dwelling <em>Globorotalia tumida</em> Mg/Ca-based temperature records from the eastern and western equatorial Pacific show mid-Pliocene warm period subsurface temperatures warmer than today; El Padre included a basin-wide thermocline that was relatively warm, deep, and weakly tilted. We compare the published and newly generated SST and subsurface temperature records to the Pliocene Modeling Intercomparison Project (PlioMIP1) and show that few models capture the magnitude and spatial pattern suggested by the temperature records. Those models that do corroborate the temperature records have warm subsurface temperatures in the Eastern Equatorial Pacific that dynamically link to warm SSTs in the cold tongue. This highlights the need to accurately model thermocline dynamics and mid-latitude conditions, where equatorial thermocline waters originate, in order to gain an understanding of the underlying processes that explain the mid-Pliocene warm period.</p>

The West Antarctic Ice Sheet (WAIS) is considered one of the tipping points of the Earth System. Its retreat due to climate change progressively results in sea level rise, affecting large numbers of the world's population. We aim to understand the potential consequences of a future WAIS collapse by implementing a mid-Pliocene Warm Period (MP) Antarctic Ice Sheet configuration, based on reconstructions, where the WAIS is severely reduced.We perform simulations with the EC-EARTH3.3 model at low resolution spanning 1400 years under 280, 400, and 560 ppmv of CO2 and derive the mean state of the last 200 years of simulation and the variability of climate patterns for the entire runtime.With the near removal of the WAIS, we find a non-linear response in Antarctic Bottom Water (AABW) formation to increasing CO2 levels. The AABW formation is highly sensitive to increased stratification, which results from sea surface temperature increase driven by current climate change. With the presence of a modern WAIS, Antarctic surface air temperature and Southern Ocean sea surface temperature are positively correlated to atmospheric CO2, and we see a strengthening of the positive phase of the Southern Annular Mode (SAM). This affects mid-latitude westerlies and reinforces the negative feedback between surface warming and AABW formation.However, with the near removal of WAIS, we observe a dampening in the otherwise doubling of atmospheric warming observed with increasing CO2 and the same pattern occurs for the SAM. This results in a non-linear behaviour of AABW formation, where the AABW is suppressed up to 4 Sv during a longer period compared to the control experiments (modern WAIS), followed by a recovery to pre-industrial strength levels that is not sustained under 560 ppmv. This response also induces a further weakening of the Atlantic Meridional Overturning Circulation (AMOC) and a reduced reach of the AABW transport into the Atlantic and Pacific Oceans, with potential cascading effects on the global climate.Our longer simulations reveal that the AABW formation thresholds are highly dependent on atmospheric CO2concentrations and the freshwater input into the surrounding basins of the Antarctic region. These results suggest that WAIS retreat already deeply impacts societal development, but a collapse would induce a new climate regime that needs further investigation to allow for climate adaptation.

Given the threats that tropical cyclones (TC) pose to people and infrastructure, there is significant interest in how the climatology of these storms may change with climate. The global historical record has been extensively examined, but it is short and plagued with recurring questions about its homogeneity, limiting its effectiveness at assessing how TCs vary with climate. Past warm intervals provide an opportunity to quantify TC behavior in a warmer-than-present world. Here, we use a TC-resolving (∼25 km) global atmospheric model to investigate TC activity during the mid-Pliocene warm period (3.264-3.025 Ma) that shares similarities with projections of future climate. Two experiments, one driven by the reconstructed sea surface temperatures (SSTs) and the other by the SSTs from an ensemble of mid-Pliocene simulations, consistently predict enhanced global-average peak TC intensity during the mid-Pliocene coupled with longer duration, increased power dissipation, and a poleward migration of the location of peak intensity. The simulations are similar to global TC changes observed during recent global warming, as well as those of many future projections, providing a window into the potential TC activity that may be expected in a warmer world. Changes to power dissipation and TC frequency, especially in the Pacific, are sensitive to the different SST patterns, which could affect the viability of the role of TCs as a factor for maintaining a reduced zonal SST gradient during the Pliocene, as recently hypothesized.

The depositional setting for northern most Atlantic coral reef development during the Mid-Pliocene Warm Period is a gentle sloping mixed carbonate–siliciclastic ramp. Five core transects document the distribution of the reef complex in an area approximately 65 by 12 km, and nine repetitive depositional facies are identified. Paleoecological and sedimentological evidence documents facies development along a water depth–energy gradient. The mid-Pliocene Tamiami Formation is characterized by coral boundstone developed over level skeletal rudstone depositional units dominated by mollusks. Hyotissa haitensis (Sowerby), one of the last Gryphaeid oysters, is the dominant fossil found in the most continuous skeletal facies and overlies deeper water green clay facies, the only facies with pelagic foraminifera. Ground penetrating radar documents reef depositional topography, onlapping stratigraphy and two episodes of reef growth. Two cycles of deposition are recognized, separated by subaerial exposure. The coral boundstone and the skeletal rudstone exhibit both high primary and secondary porosity and overlie the impermeable clay facies. The upper surface of the coral boundstone lies at ~ 4 m in elevation whereas contemporaneous estuarine deposits are found to the north at elevations of 20–25 m. High porosity bank reef complexes along a shallow dipping ramp provide an alternative to the standard model of reef and porosity development along the outer shelf margin. Understanding the differences in associated facies between these two depositional environments permits better interpretation of observed heterogeneities in subsurface geobodies associated with inner shelf and platform settings.

Global reconstructions of Pliocene climate provide important insights into how the climate system operates under elevated temperatures and atmospheric CO2 levels. These reconstructions have been used extensively in paleoclimate modeling experiments for comparison to simulated conditions, and as boundary conditions. Most previous work focused on the Late Pliocene interval known as the mid Piacenzian Warm Period (mPWP), the interval originally identified by the U.S. Geological Survey Pliocene Research, Interpretation and Synoptic Mapping Project (PRISM) as the PRISM interval or Mid Pliocene Warm Period. The term Mid Pliocene Warm Period is a misnomer due to changes to the geological time scale, and its use should be discontinued. The Pliocene Model Intercomparison Project (PlioMIP), now in its third phase, is expanding to include a focus on the Early Pliocene (Zanclean). PlioMIP3 experiments will allow comparison of environmental and climatic conditions before and after closure of the Central American Seaway (CAS). PlioMIP3 used the annual insolation pattern at the top of the atmosphere to determine time slices in the Zanclean that have orbital configurations that are most similar to modern. Two have been selected by PlioMIP and adopted by PRISM for inclusion in future studies: PRISM5.1 (4.474 Ma) and PRISM5.2 (4.870 Ma). Here we establish the stratigraphic framework for these Early Pliocene time slices and furnish information to help locate these intervals in proxy records of paleoenvironmental data using oxygen isotope stratigraphy, paleomagnetic stratigraphy, biostratigraphy, and biochronology (calibrated planktic foraminifer and calcareous nannofossil events).

<p>The mid-Pliocene Warm Period (mPWP) is the most recent time slice (3.264–3.025 Ma) during which average global surface temperatures were 2–3°C warmer than preindustrial conditions, within the range estimated by the Intergovernmental Panel on Climate Change (IPCC) for the end of the 21<sup>st </sup>Century. Global mPWP sea surface temperature (SST) compilations indicate enhanced warming in the NE Atlantic and Nordic Seas, with anomalies of >6°C based on alkenone methods (Dowsett et al., 2012). However, this warming far exceeds the more conservative SST estimates (a rise of 2−3°C) predicted by the Pliocene Research, Interpretation and Synoptic Mapping (PRISM) reconstructions and leading climate models (including HadCM3). Here, we present new mid-Pliocene alkenone SST records from four regional drilling sites (IODP Site U1308, DSDP Site 552, ODP Site 642 and ODP Site 907) to further examine the magnitude of warming in the NE Atlantic and Nordic Seas, and to evaluate regional discrepancies between proxy and model SST estimates. We demonstrate mid-Pliocene SSTs peaked up to 21.5°C and 19.7°C in the NE Atlantic and Nordic Seas, respectively, consistent with existing studies (Robinson et al., 2008; Robinson, 2009). However, we reveal the majority of these SST estimates are derived from GC injections of relatively low total alkenone concentrations (<50 ng/µl), which are susceptible to warming biases caused by chromatographic irreversible adsorption (Grimalt et al., 2001). We subsequently filtered and applied a mathematical correction to our new data to rectify for these warming biases, which results in a reduction in mPWP SSTs, by up to 3.2°C, across all four sites. The corrected (and cooler) alkenone SST records indicate the magnitude of warming in the NE Atlantic and Nordic Seas may be significantly less than previously thought, helping to reduce and explain regional discrepancies between proxy- and model-based SST reconstructions.</p>

Postglacial reef development in the South-West Pacific: case studies from New Caledonia and Vanuatu

Characterized by similar-to-today CO2 levels (∼400 ppm) and surface temperatures approximately 3°–4°C warmer than the preindustrial, the mid-Pliocene warm period (mPWP) has often been used as an analog for modern CO2-driven climate change and as a constraint on the equilibrium climate sensitivity (ECS). However, model intercomparison studies suggest that non-CO2 boundary conditions—such as changes in ice sheets—contribute substantially to the higher global mean temperatures and strongly shape the pattern of sea surface warming during the mPWP. Here, we employ a set of CESM2 simulations to quantify mPWP effective radiative forcings, study the role of ocean circulation changes in shaping the patterns of sea surface temperatures, and calculate radiative feedbacks during the mPWP. We find that the non-CO2 boundary conditions of the mPWP, enhanced by changes in ocean circulation, contributed to larger high-latitude warming and less-stabilizing feedbacks relative to those induced by CO2 alone. Accounting for differences in feedbacks between the mPWP and the modern (greenhouse gas–driven) climate provides stronger constraints on the high-end of modern-day ECS. However, a quantification of the forcing of non-CO2 boundary condition changes combined with the distinct radiative feedbacks that they induce suggests that Earth system sensitivity may be higher than previously estimated. Significance Statement Climate records from past warm intervals in Earth’s geologic history are frequently used to constrain modern-day equilibrium climate sensitivity. Yet, climate warmth during these periods is not solely driven by the radiative effect of CO2, but also by environmental and geographic features, such as large-scale ice sheet and vegetation changes, that are not expected to occur in the near future. Using the mid-Pliocene warm period as an example, we find that the surface patterns of warming induced by non-CO2 paleoenvironmental boundary conditions lead to a more-sensitive climate state than the modern. This implies that near-term warming under CO2 forcing may be smaller than expected but that on geological time scales, future warming may be significantly larger.

This study evaluates interannual-to-decadal sea surface temperature (SST) variability in the mid-Pliocene Warm Period within the Pliocene Model Intercomparison Project (PlioMIP). Our results show significantly reduced variability at low latitudes and mid-latitudes in the mid-Pliocene in comparison to the pre-industrial climate. At high latitudes of both hemispheres, the SST variability has increased. Latitudinal changes are likely driven by changes in the meridional SST gradient. Results with respect to the main Atlantic SST modes of variability show that the Atlantic Multidecadal Variability shifts southward and expands eastward due to a southward shift in the North Atlantic Drift position. The Atlantic Meridional Mode amplitude weakens due to increased SST gradient between its two poles. The South Atlantic Subtropical Dipole significantly shifts its southwestern pole towards the South American coast. Moreover, all Atlantic modes of variability have shifted their respective frequencies towards lower values. Our analyses on the PlioMIP simulation results provide a useful constraint in future projections associated with a warmer world when assessing Atlantic SST variability.

This paper reviews North Atlantic shelf seas palaeoclimate during the interval 4-3Ma, prior to and incorporating the 'Mid-Pliocene warm period' (ca 3.29-2.97Ma). Fossil assemblages and stable isotope data demonstrate northwards extension of subtropical faunas along the coast of the Carolinas-Virginia (Yorktown and Duplin Formations) relative to the present day, suggesting a more vigorous Florida Current, with reduced seasonality and warm water extending north of Cape Hatteras (reconstructed annual range for Virginia 12-30 degrees C). This interpretation supports conceptual models of increased meridional heat transport for the Pliocene. Sea temperatures for Florida (Lower Pinecrest Beds) were similar to or slightly cooler than (summers 25-27 degrees C) today, and were probably influenced by seasonal upwelling of cold deep water. Reduced seasonality is also apparent in the Coralline Crag Formation of the southern North Sea, with ostracods suggesting winter sea temperatures of 10 degrees C (modern 4 degrees C). However, estimates from Pliocene bivalves (3.6-16.6 degrees C) are similar to or cooler than the present day. This 'mixed' signal is problematic given warmer seas in the Carolinas-Virginia, and climate model and oceanographic data that show warmer seas in the 'Mid-Pliocene' eastern North Atlantic. This may be because the Coralline Crag Formation was deposited prior to peak Mid-Pliocene warmth.

The evolutionary development of tropical coral reefs is presently ascribed to the association of corals with symbiotic algae (zooxanthellae) and to the enhancement of calcification by light. Contrary to this idea, the calcification rate in a non-reef-building tropical coral ( Tubastrea faulkneri ) without symbiotic algae was the same as the light-enhanced rate in a zooxanthellate reef-building coral ( Galaxea fascicularis ). The mechanisms of calcification, however, differed between the two species. Instead of being “light-enhanced,” calcification in corals with algae was “dark-repressed.” The evolutionary development of coral reefs may therefore not be related to light-enhanced calcification resulting from the association of corals with symbiotic algae.

Determination of (palaeo-)ecological preferences of dinoflagellates by applying Detrended and Canonical Correspondence analysis to Late Pliocene dinoflagellate cyst assemblages of the south Italian Singa section