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1. Late Pleistocene drivers of climate and vegetation in the Western Sahel from leaf wax biomarker isotopes

   Lupien, Rachel (June 2022, Abstracts American Quaternary Association National Conference)

   The Sahel is highly sensitive to flooding, droughts, and wildfires, risking food and other resources on which nearly 100 million people depend. Understanding how natural variations of precipitation and vegetation fluctuate during high-amplitude glacial- interglacial cycles can help constrain the regional sensitivity to a wide range of external forcings. Further, the interactions between climate and ecosystem changes remain uncertain for sub-Saharan Africa due to the lack of long, highly-resolved, quantitative, terrestrial records. Here we present precipitation and vegetation records from ~215 ka to present, derived from long leaf wax hydrogen (δDwax) and carbon (δ13Cwax) isotopes, respectively. These geochemical records are derived from ODP Site 959 in the Gulf of Guinea, where westerly winds and major river systems transport Western Sahel-sourced terrestrial leaf waxes. We find that, unlike many African records that are precessionally- driven, obliquity plays an important role in West African late Pleistocene hydroclimate, suggesting that a cross-equatorial insolation gradient may be more important in this area and certainly that drivers of orbital-scale precipitation change are regionally-specific. Further, vegetation changes appear to have a complex relationship with hydroclimate over this mid-late Pleistocene interval. A potential shift in this climate-environment coupling at MIS6 ~130 ka, which is a time when there is also a shift in forcing mechanisms in East Africa, suggests that the global boundary condition changes associated with large glacial- interglacial cycling may affect equatorial climate. 

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2. Exploring Radiative Forcing from Pliocene Boundary Conditions and CO2

   https://doi.org/10.5194/egusphere-egu24-11853  

   Kravette, Noah; Feng, Ran; Dvorak, Michelle (April 2024, Copernicus)

   Past climate states hold valuable insights into future climate change. Among those states, mid-Pliocene (3.3 - 3.0 Ma) is often studied as an important analog to near future climate change following an intermediate warming pathway. This time interval featured topography and geography like present-day, yet with retreated polar ice sheets and expanded boreal forests, potentially reflecting equilibrium earth system responses to CO2 forcing at a centennial to millennial time scale. Despite the prolific research on Pliocene climate, little is known about the amount of radiative forcing, especially from changing boundary conditions, that drives the Pliocene climate. Existing constraints mainly focused on well-mixed greenhouse gases and aerosols. Here, we applied the methodology commonly used to quantify radiative forcing of future climate and its sources to constrain radiative forcing of the mid-Pliocene climate using three generations of Community Earth System Models (CCSM4, CESM1.2, and CESM2). To calculate ERF, the difference in net top of the atmosphere radiative fluxes is computed between a pre-industrial control and a mid-Pliocene simulation. Both are carried out with prescribed pre-industrial sea surface temperature. The three mid-Pliocene simulations separately feature a 400 ppm CO2 (the level of mid-Pliocene), mid-Pliocene geography and topography, and mid-Pliocene ice and vegetation. Changing atmospheric temperature, water vapor, surface albedo, and clear vs total sky radiative fluxes are further extracted from these simulations to calculate radiative adjustments with published radiative kernels for CESM. In our preliminary results with CESM1.2, we found that ERF is 1.754 W m-2 for CO2 forcings, 1.143 W m-2 for vegetation and ice sheet forcing, and -0.339 W m-2 for geographic and topographic forcing. Further, ERF from boundary condition changes mostly arises from changing surface albedo with 1.626 W m-2 for vegetation and ice sheet changes and –0.54 W m-2 for geographic and topographic changes respectively. Radiative adjustments from water vapor responses tend to amplify the instantaneous forcing with the most profound effect induced by vegetation and ice sheet changes. These results underscore the importance of constraining radiative forcing from changes in boundary conditions, which is potentially key to understanding drivers of past climate warmth and inter-model spread in simulated past climate states. 

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3. Carbon dioxide fluxes of Arctic coastal ecosystems controlled by seasonal patterns of land‐to‐ocean connectivity

   https://doi.org/10.1002/lno.70101  

   Spera, Alina_C; Lougheed, Vanessa_L (June 2025, Limnology and Oceanography)

   Abstract The strength of coastal Arctic Ocean CO₂uptake is vulnerable to landscape‐scale changes such as hydrological intensification, changing climate, and alterations to terrestrial and aquatic biogeochemistry. Across a period of 4 yr (2019–2023) and three distinct sampling periods, we visited five coastal ecosystems of the Beaufort Sea with varying barrier island coverage to understand drivers of Arctic coastal CO₂flux. The ice cover sampling period was characterized by the highest pCO₂saturation and dissolved O₂undersaturation. We observed a > 100μatm difference in pCO₂over shallow depths (up to 2 m) at 73% of ice‐cover site visits. Notably, the geomorphology of barrier islands and channels controlled the flushing of colder Beaufort Sea waters across the systems and influenced the strength and appearance of both vertical thermohaline and pCO₂stratification. The ice breakup period reflected spring freshet and was a net CO₂sink during sampling, likely related to freshwater riverine dilution, CaCO₃dissolution from sea ice melt, and water column algal activity. During the open water sampling period, the interaction of marine and terrestrial contributions predicted the strength of the CO₂efflux, with freshwater inputs introducing higher temperatures and organic material, which increased remineralization. The capacity of these systems to act as CO₂sources or sinks varies throughout the year and is largely driven by geomorphic conditions. Any spatially integrative studies of CO₂flux or coastal productivity should consider the physical and biogeochemical heterogeneity of Arctic coastal ecosystems. 

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4. Patterns and Mechanisms of Northeast Pacific Temperature Response to Pliocene Boundary Conditions

   https://doi.org/10.1029/2021PA004370  

   Brennan, Peter R.; Bhattacharya, Tripti; Feng, Ran; Tierney, Jessica E.; Jorgensen, Ellen M. (June 2022, Paleoceanography and Paleoclimatology)

   Abstract Ocean‐atmosphere dynamics in the north Pacific play an important role in the global climate system and influence hydroclimate in western North America. However, changes to this region's mean climate under increased atmospheric greenhouse gas concentrations are not well understood. Here we present new alkenone‐based records of sea surface temperature (SST) from the northeast Pacific from the mid‐Piacenzian warm period (approximately 3.3–3.0 Ma), an interval considered to be an analog for near‐future climate under middle‐of‐the‐road anthropogenic emissions. We compare these and other alkenone‐based SST records from the north Pacific to fully‐coupled climate model simulations to examine the impact of mid‐Pliocene CO₂and other boundary conditions on regional climate dynamics and to explore factors governing model disagreement about regional temperature patterns. Model performance varies regionally, with Community Earth System Model 1.2 (CESM 1.2) and CESM2 performing best in regions with greater warming like the California Margin, though these models underestimate the warming evidenced in our new proxy record and others from the region. Single forcing simulations reveal a strong influence for prescribed land surface changes and higher CO₂levels on coastal warming patterns along the California Margin in CESM2. Furthermore, differences in shortwave and longwave radiation and circulation between the models, likely related to changes in the atmospheric component of the model, may play a key role in the ability of models to capture regionally‐varying patterns of Pliocene warmth. Regional patterns of temperature change inferred from geochemical records could therefore help to understand the impacts of different model parameterization schemes on regional climate patterns. 

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5. Orbital-scale climate and environmental responses of the Western Sahel to shifts in Cenozoic boundary conditions

   Lupien, Rachel; Uno, Kevin T; deMenocal, Peter B (December 2024, AGU Abstracts)

   Nearly 100 million people live in and depend on the Sahel for agriculture and natural resources. The region is sensitive to natural climate and environment variations caused by the seasonal movement of the tropical rainbelt. In the paleoclimate record, insolation plays a clear role on West African Monsoon strength, but responses to other forcings like temperature, greenhouse gases, ice volume, and land surface cover are unclear due to the lack of highly resolved, terrestrial records that span major global and regional shifts through time. Here we present leaf wax precipitation and vegetation records from several targeted study windows throughout the last 25 million years, derived from long-chain n-alkane hydrogen (δDwax) and carbon (δ13Cwax) isotopes, respectively, in a sediment core from ODP Site 959 in the Gulf of Guinea, where westerly winds and major river systems transport Western Sahel-sourced material. Analyses of trend and variability document a range of rainfall and vegetation responses to orbital forcings in different boundary conditions in the Oligocene, Miocene, Pliocene, and Pleistocene. We find that both the climate and environment was more variable in times of higher CO2 and global temperatures, suggesting an increase in ecosystem instability moving forward into the future. Because of the high resolution and temporal coverage of these new biomarker isotope records, we can examine relationships between precipitation and vegetation fluctuations, even prior to C4-expansion when there was a strong correlation despite minimal variation in δ13Cwax in a C3 world. Further, we find a wetting trend throughout the record, demonstrating that vegetation on long timescales was decoupled from hydroclimate and that the terrestrial ecosystem may face aridification, contradicting some model projections. 

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