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^st 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.

How to cite: Hall, J., Jones, S., Dunkley Jones, T., and Bendle, J.: Mid-Pliocene warming: reducing discrepancies between geological archives and climate models in the NE Atlantic and Nordic Seas, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-220, https://doi.org/10.5194/egusphere-egu21-220, 2020.

The Pliocene epoch (5.3-2.6 million years ago) is the last time Earth experienced atmospheric carbon dioxide levels comparable to present day anthropogenic levels. As such, this time interval is a potential analogue for future, warmer Earth system states. One enigmatic feature of Pliocene climate is a reduced east-west sea surface temperature gradient in the equatorial Pacific (indicative of reduced equatorial upwelling) coinciding with enhanced biological productivity in the eastern equatorial Pacific (indicative of enhanced equatorial upwelling).  Here we use boron isotopes to investigate these dynamics and to reconstruct the zonal surface pH gradient across the Pliocene equatorial Pacific. We find a strengthened pH gradient relative to modern (with more acidic conditions in the east than the west) despite a reduced temperature gradient at this time. These findings are in contrast to modern-day dynamics in which temperature and acidity co-vary, such that the reduction of the zonal temperature gradient during an El Niño event is accompanied by reduced acidity (as well as reduced upwelling and productivity) in the eastern equatorial Pacific. We show that this decoupling between changes in the pH and temperature gradients is consistent with biogeochemically enabled model simulations of Pliocene climate containing an active Pacific meridional overturning circulation and a weakly stratified equatorial thermocline. This reorganization of Pacific circulation and the onset of north Pacific deep water formation allows old, acidic, more nutrient-rich waters to reach the eastern equatorial Pacific despite weak wind-driven upwelling rates, accounting for the low pH values we observe there as well as previous evidence of enhanced productivity.

How to cite: Shankle, M., Burls, N., Fedorov, A., Thomas, M., Penman, D., Ford, H., Jacobs, P., Planavsky, N., and Hull, P.: Decoupled changes in upwelling and acidity in the eastern equatorial Pacific during the Pliocene, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-680, https://doi.org/10.5194/egusphere-egu21-680, 2021.

Modelling results from PlioMIP2 (the Pliocene Model Intercomparison Project Phase 2) focussing on MIS KM5c; ~3.205Ma, suggest that global mean surface air temperature was 1.7 – 5.2 °C higher than the preindustrial.  This warming was amplified at the poles and over land.  The results are in reasonable agreement with paleodata over the ocean.   

Over the land the situation is more complicated.  Model and data are in very good agreement at lower latitudes, however at high latitudes an initial data-model comparison shows much warmer mPWP temperatures from data than from models.   

Here we consider possible reasons for this data-model discord at high latitudes.  These include uncertainties in model boundary conditions (such as CO₂ and orbital forcing), and whether there are local site-specific conditions which need to be accounted for.  We also show that the seasonal cycle in mPWP temperatures at these high latitude sites has no modern analogue.  This could lead to inaccuracies when comparing model derived mean annual temperatures with quantitative climatic estimates from palaeobotanical data using Nearest Living Relative methods.

How to cite: Tindall, J., Haywood, A., Salzmann, U., and Dolan, A.: Data-model Comparison for the mid-Pliocene Warm Period, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4802, https://doi.org/10.5194/egusphere-egu21-4802, 2021.

The Miocene experienced both, ice-house periods with continental ice-sheets covering both poles and warm greenhouse conditions with strongly increased global temperature, glacier retreat and sea-level rise. The Mid-Miocene Climatic Optimum (MMCO) is the most pronounced warming event in the last 24 Ma, standing out in a time of protracted cooling. The MMCO is marked by a period of intensive global warming between ca. 17 and 15 Ma. The subsequent Mid-Miocene Climate Transition (MMCT), in contrast, was affected by global temperature decline, growth of Antarctic ice sheets, sea level fall and marine biota overturn.

Miocene climate conditions were intensely studied on both, global and regional scales, based on i.a. marine isotope records and continental paleobotanical and mammalian fossil data sets. Despite the dense data sets continental Miocene temperature evolution still remains unclear owing to a large range of inferred temperatures and/or poor age constraints of the associated records.

Here, we present a long-term terrestrial climate record that covers the time interval between ~20 and ~13 Ma and is based on stable (δ¹⁸O) and clumped isotope (Δ₄₇) geochemical data. We apply Δ₄₇ thermometry on terrestrial foreland basin sediments to reconstruct the Middle Miocene continental temperature evolution for central Europe. Pedogenic carbonates from well dated fossil soils from several sites in the Northern Alpine Foreland Basin (Switzerland) reveal warm and stable temperatures for the early Miocene (20 – 19 Ma), followed by overall strongly enhanced variability in temperatures with maximum values attained between ca. 17 and 14 Ma. We observe a highly dynamic transition to cooler climates at the end of the MMCO and a subsequent rapid temperature decline of approximately 20°C after 14 Ma during the MMCT. The highly variable temperature patterns during the cooling period coincide with phases of high seasonality in the precipitation pattern as derived from oxygen isotope compositions of soil water.

How to cite: Krsnik, E., Methner, K., Löffler, N., Kempf, O., Fiebig, J., and Mulch, A.: Terrestrial Middle Miocene (Δ47) temperature record reveals highly dynamic climate for the Central Europe, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15056, https://doi.org/10.5194/egusphere-egu21-15056, 2021.

The Miocene climate was dynamic, oscillating between major glaciation events and greenhouse conditions (the so-called Miocene Climatic Optimum or MCO). However, forcing factors responsible for climatic transitions from one state to another are not fully understood, partly because palaeoclimatological records from northern mid to high latitudes are scarce.

To better resolve climatic changes of the Miocene epoch in the northern middle latitudes we studied a unique, nearly complete sedimentary record (Sdr. Vium borehole) spanning the upper Aquitanian to the Tortonian of the North Sea Basin. Newly obtained sea surface temperatures (SSTs) from our Miocene core revealed that the North Sea Basin was up to 20°C warmer than today, reaching the temperature maximum during the worldwide MCO (Herbert et al. 2020). Our high-resolution δ¹³C, TOC and C/N records, as well as elemental detrital ratios (Si/Al, Zr/Rb, Zr/Al) derived from XRF reveal important changes in the source of organic matter and detrital coarse fraction of the sediment. During the Miocene the location of the Sdr. Vium borehole was situated in a proximal setting, with water depths varying between 0 and ~200 m, partly due to advancing and retreating delta lobes and partly due to relative sea level changes. We observe that the depositional environment had a large impact on our record. By far the most important of these changes is a condensed interval associated with phosphatization, pyritization, and glauconite, associated with a major shift from a dark brown, organic-rich, bioturbated silty clay with thin sand lenses (the Hodde Formation) towards a green and brown clay with high concentrations of green glaucony pellets of fine sand grade (the Ørnhøj Formation). This shift is related to the subsidence of the North Sea Basin and marks the onset of a sediment-starvation in the basin.

How to cite: Sliwinska, K. K., Bojesen-Koefoed, J., Dybkjær, K., Herbert, T., Lear, C. H., Rasmussen, E. S., Soltau, E. M., Thibault, N. R., and Vickers, M. L.: The Miocene warmth from the North Sea Basin perspective, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15183, https://doi.org/10.5194/egusphere-egu21-15183, 2021.

The tectonic opening of the Fram Strait (FS) was critical to the water exchange between the Atlantic Ocean and the Arctic Ocean, and caused the transition from a restricted to a ventilated Arctic Ocean during early Miocene. If and how the water exchange between the Arctic Ocean and the North Atlantic influenced the global current system is still disputed. We apply a fully coupled atmosphere-ocean-sea-ice model to investigate stratification and ocean circulation in the Arctic Ocean in response to the opening of the FS during early to middle Miocene. Progressive widening of the FS gateway in our simulation causes a moderate warming, while salinity conditions in the Nordic Seas remain similar. On the contrary, with increasing FS width Arctic temperatures remain unchanged and salinity changes appear to steadily become stronger. For a sill depth of ~1500 m, we achieve ventilation of the Arctic Ocean due to enhanced import of saline Atlantic water through a FS width of ~105 km. Moreover, at this width and depth, we detect a modern-like three-layer stratification in the Arctic Ocean. The exchange flow through FS is characterized by vertical separation of a low salinity cold outflow from the Arctic Ocean confined to a thin upper layer, an intermediate saline inflow from the Atlantic Ocean below and a cold bottom Arctic outflow. Using a significantly shallower and narrower FS during the early Miocene, our study suggests that the ventilation mechanisms and stratification in the Arctic Ocean are comparable to the present-day characteristics.

How to cite: Hossain, A., Knorr, G., Jokat, W., and Lohmann, G.: A modern prototype three-layer stratification in the Arctic Ocean since the Miocene, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9956, https://doi.org/10.5194/egusphere-egu21-9956, 2021.

The Eocene-Oligocene Transition (~34.4–33.7 Ma) marks a major step in the long-term evolution from the greenhouse climate of the Early Palaeogene to the icehouse regime of the Late Neogene and Quaternary. However, it remains uncertain which landmasses were covered by ice sheets during the Early Oligocene Glacial Maximum (~33.7–33.2 Ma), an interval of peak glaciation inferred from deep-sea benthic foraminifera oxygen isotope records that immediately follows the Eocene-Oligocene Transition. The scarcity of Late Eocene and Early Oligocene continental and shallow-marine records in both Arctic and Antarctic regions has prevented the reconstruction of environmental conditions and ice-sheet extent during the Early Oligocene, which is critical for assessing ice–ocean–atmosphere interactions during early stages of the Cenozoic icehouse. Here, we present the first Early Oligocene shallow-marine record from the Pacific margin of West Antarctica, recovered from the central Amundsen Sea Embayment shelf on RV Polarstern expedition PS104 at Site 21. Marine mudstones recovered at this site document the presence of a vegetated archipelago at a palaeo-latitude of 73.5°S. Pollen assemblages and organic biomarker proxies indicate a cool-temperate Nothofagus-dominated forest situated within a productive marine archipelago. No evidence for marine terminating ice was detected in the cores from Site 21, thus indicating that the West Antarctic Ice Sheet was small or entirely absent during the Early Oligocene.

How to cite: Klages, J. P., Hillenbrand, C.-D., Bohaty, S. M., Salzmann, U., Bickert, T., Lohmann, G., Gohl, K., Kuhn, G., Titschack, J., Müller, J., Bauersachs, T., Frederichs, T., Larter, R. D., Hochmuth, K., Ehrmann, W., Rodríguez Tovar, F. J., Schmiedl, G., van de Flierdt, T., Spiegel, C., and Eisenhauer, A. and the Science Team of Expedition PS104: West Antarctic archipelago covered by cool-temperate forests during early Oligocene glaciation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1538, https://doi.org/10.5194/egusphere-egu21-1538, 2021.

At the junction of warmhouse and coolhouse climate phases, the Eocene Oligocene Transition (EOT) is a key moment in the history of the Cenozoic climate. Yet, while it is accompanied by severe extinctions and biodiversity turnovers, terrestrial climate evolution remains poorly resolved. On lands, some fossil and geochemistry records suggest a particularly marked cooling in winter, which would have led to the development of more pronounced seasons (higher Mean Annual Range of Temperatures, MATR) in certain regions of the Northern Hemisphere. This type of climate change should have had consequences on biodiversity and an implication in some of the fauna and flora renewals described at the EOT. However, this season strengthening has been studied only superficially by model studies, and questions remain about the geographical extent of this phenomenon and the associated climatic processes. Although other components of the climate system vary seasonally (e.g., precipitation, wind), we therefore focus on the seasonality of temperatures only.

In order to better understand and describe temperature seasonality change patterns from the middle Eocene to the early Oligocene, we use the Earth System Model IPSL-CM5A2 and a set of simulations reconstructing the EOT through three major climate forcings: pCO2 decrease (1120/840 to 560 ppm), the Antarctic ice-sheet (AIS) formation, and the associated sea-level decrease (-70 m). 

Our results suggest that seasonality changes across the EOT rely on the combined effects of the different tested mechanisms which result in zonal to regional climate responses. Sea-level changes associated with the earliest stage of the AIS formation may have also contributed to middle to late Eocene MATR reinforcement. We reconstruct strong and heterogeneous patterns of seasonality changes across the EOT. Broad continental areas of increased MATR reflect a strengthening of seasonality (from 4°C to > 10°C increase of the MATR) in agreement with MATR and Coldest Month Mean Temperatures (CMMT) changes indicated by a review of existing proxies. pCO2 decrease induces a zonal pattern with alternating increasing and decreasing seasonality bands. In the northern high-latitudes, it results in sea-ice and surface albedo feedback, driving a strong increase in seasonality (up to 8°C MATR increase). Conversely, the onset of the AIS is responsible for a more constant surface albedo, which leads to a strong decrease in seasonality in the southern mid- to high-latitudes (> 40°S). Finally, continental areas emerged due to the sea level lowering cause the largest increase in seasonality and explain most of the global heterogeneity in MATR changes patterns. The seasonality change patterns we reconstruct are consistent with the variability of the EOT biotic crisis intensity across the Northern Hemisphere.

How to cite: Toumoulin, A., Donnadieu, Y., Tardif, D., Ladant, J.-B., Licht, A., Kunzmann, L., and Dupont-Nivet, G.: Continental temperature seasonality from Eocene Warmhouse to Oligocene Coolhouse — A model-data comparison, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8613, https://doi.org/10.5194/egusphere-egu21-8613, 2021.

Ocean currents can transport sinking particles hundreds of kilometers from their origin at the ocean surface to their burial location, resulting in an offset between sea surface temperatures (SSTs) above the burial site and the particle’s origin. Quantifying this offset in particles carrying molecules used in SST proxies can reduce uncertainty in paleoclimate reconstructions. In the Mediterranean Sea, where δ¹⁸O_foraminifera, U^K’₃₇- and TEX₈₆-based SSTs can exhibit large offsets from surface conditions, understanding the possible contribution of lateral transport to proxy bias can provide additional insight when interpreting paleoclimate records.

In this study, Lagrangian particle tracking experiments are performed using the NEMO flow field to simulate transport and allow for a quantitative estimate of transport bias. The model determines the ocean surface origin locations of foraminifera and sedimentary particles that carry alkenones or GDGTs to compare with surface sediment datasets for δ¹⁸O_foraminifera, U^K’₃₇ and TEX₈₆, respectively. A range of sinking speeds appropriate for the export of organic matter (6, 12, 25, 50, 100, 250, and 500 m/d) is used in the model to represent different export modes (i.e., individual coccoliths, coccospheres, aggregates), where the three fastest sinking speeds can also represent sinking foraminifera. Results show that lateral transport bias is generally small within the Mediterranean Sea and cannot explain the large offsets in proxy-based SST reconstructions in this basin.

How to cite: Rice, A., Nooteboom, P., van Sebille, E., Peterse, F., Ziegler, M., and Sluijs, A.: Particle tracking model results suggest little lateral transport bias in inorganic and organic SST proxies in the Mediterranean Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7881, https://doi.org/10.5194/egusphere-egu21-7881, 2021.

Lycophytes (club mosses) represent a distinct lineage of vascular plants with a long history including numerous extant and extinct species. They enriched the soil carbon pool through newly developed root-like structures and promoted soil microbial activity by providing organic matter. They enhanced soil carbon dioxide (CO₂) via root respiration and also modified soil hydrology. These effects had the potential to promote the dissolution of silicate minerals, thus intensifying silicate weathering. The weathering of silicate rocks is considered one of the most significant geo-chemical regulators of atmospheric CO₂ on a long (hundreds of thousands to millions of years) timescale. The motivation for this study is to achieve an increased understanding of the realized impacts of lycophytes on silicate weathering and past climate. To this end, it is necessary to quantify physiological characteristics, spatial distribution, the carbon balance, and hydrological impacts of early lycophytes. These properties, however, cannot be easily derived from proxies. Hence, as a first step, a process-based model is developed here to estimate net carbon uptake by these organisms at the local scale, considering key features such as root distribution, stomatal regulation of water loss, and root respiration.
The model features ranges of key physiological traits of lycophytes to predict the emerging characteristics of the lycophyte community under any given climate by implicitly simulating the process of selection. In this way, also extinct plant communities can be represented.
In addition to physiological properties, the model also simulates weathering rates using a simple limit-based approach and estimates the biotic enhancement of weathering by lycophytes. We run the Lycophyte model, called LYCOm, at seven sites encompassing various climate zones under today's climatic conditions. LYCOm is able to simulate realistic properties of lycophyte communities at the respective locations and estimates an average NPP ranging from 245 g carbon m^-2 year^-1 in Costa Rica to 126 g carbon m^-2 year^-1 in Estonia. Our limit-based weathering model predicts a chemical weathering rate ranging from 0.026 to 0.31 mm rock a^-1 , thereby highlighting the potential importance of lycophytes at the local scale for enhancing chemical weathering. Our modeling study establishes a basis for assessing biotic enhancement of weathering by lycophytes at the global scale and also for the geological past. 

How to cite: Halder, S. and Porada, P.: Impacts of early vegetation on biogeochemical cycles, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12315, https://doi.org/10.5194/egusphere-egu21-12315, 2021.