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1. Warm High‐Elevation Mid‐Latitudes During the Miocene Climatic Optimum: Paleosol Clumped Isotope Temperatures From the Northern Rocky Mountains, USA

   https://doi.org/10.1029/2020PA003991  

   Methner, K. ; Mulch, A. ; Fiebig, J. ; Krsnik, E. ; Löffler, N. ; Bajnai, D. ; Chamberlain, C. P. ( June 2021 , Paleoceanography and Paleoclimatology)

   Interrupting a long‐term Cenozoic cooling trend, the Miocene Climatic Optimum (MCO; ca. 17–15 Ma) represents a time interval characterized globally by warmer than present temperatures, lower ice volume, and elevated pCO₂levels. Establishing quantitative Neogene temperature estimates is an important element in the effort to explore the long‐term changes in the carbon cycle and associated climate feedbacks, yet terrestrial temperature records are still sparse. Here, we present a clumped isotope (Δ₄₇) temperature record of the MCO from intermontane basins in the Northern Rocky Mountain (NRM) region. Arikareean (22.7–21.5 Ma) to Barstovian (16.9–14.7 Ma) paleosol carbonates from the Hepburn's Mesa Formation (Montana), supplemented with data from fossil localities in western Idaho. These records yield Δ₄₇‐temperatures ranging from 17°C to 24°C, which are rather warm given the high elevation sites and are further relatively stable (mean of 21 ± 2°C) leading into and during the MCO until ca. 14.7 Ma. At ca. 14.7 Ma, we observe low Δ₄₇‐temperatures (8°C–10°C) concomitantly with elevated Δ₄₇‐temperatures (ca. 22°C). In line with recently suggested climate stability in the NRM region leading into the MCO, our Δ₄₇‐temperature record, combined with carbon isotope (δ¹³C) and reconstructed soil water oxygen isotope (δ¹⁸O_sw) values, indicates rather stable climate and environmental conditions throughout the MCO. Combining available recordsmore »from inland sites in the western United States (NRM, Mojave region) points to prevailing stable continental climates even during the MCO.

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2. The Early to Middle Eocene Transition: An Integrated Calcareous Nannofossil and Stable Isotope Record From the Northwest Atlantic Ocean (Integrated Ocean Drilling Program Site U1410)

   https://doi.org/10.1029/2019PA003686  

   Cappelli, C. ; Bown, P. R. ; Westerhold, T. ; Bohaty, S. M. ; de Riu, M. ; Lobba, V. ; Yamamoto, Y. ; Agnini, C. ( December 2019 , Paleoceanography and Paleoclimatology)

   The early to middle Eocene is marked by prominent changes in calcareous nannofossil assemblages coinciding both with long‐term climate changes and modification of the North Atlantic deep ocean circulation. In order to assess the impact of Eocene climate change on surface water environmental conditions of the Northwest Atlantic, we developed calcareous nannoplankton assemblage data and bulk stable isotope records (δ¹⁸O and δ¹³C) across an early to middle Eocene interval (~52–43 Ma) at Integrated Ocean Drilling Program Site U1410 (Southeast Newfoundland Ridge, ~41°N). At this site, early Eocene sediments are pelagic nannofossil chalk, whereas middle Eocene deposits occur as clay‐rich drift sediments reflecting the progressive influence of northern‐sourced deep currents. Between the end of Early Eocene Climatic Optimum (EECO) and the Ypresian/Lutetian boundary, calcareous nannofossils switched from an assemblage mainly composed of warm‐water and oligotrophic taxa (Zygrhablithus,Discoaster,Sphenolithus,Coccolithus) to one dominated by the more temperate and eutrophic reticulofenestrids. The most prominent period of accelerated assemblage change occurred during a ~2‐Myr phase of relatively high bulk δ¹⁸O values possibly related to the post‐EECO cooling. Although the dominance of reticulofenestrids persisted unvaried throughout the middle Eocene interval, early Lutetian (~47.4 to 47 Ma) stable isotope records indicate a reversal in the paleoenvironmetal trendsmore »suggesting a potential restoration of warmer conditions. Importantly, our data indicate that the ~2‐Myr interval immediately following the EECO was crucial in establishing the modern calcareous nannofossil assemblage structure and also reveal that the establishment ofReticulofenestra‐dominated assemblage occurred prior to the onset of persistent deep current system in the Northwest Atlantic.

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3. Nonlinear response of ecosystem respiration to multiple levels of temperature increases

   https://doi.org/10.1002/ece3.4658  

   Chen, Ning ; Zhu, Juntao ; Zhang, Yangjian ; Liu, Yaojie ; Li, Junxiang ; Zu, Jiaxing ; Huang, Ke ( January 2019 , Ecology and Evolution)

   Global warming exerts profound impacts on terrestrial carbon cycles and feedback to climates. Ecosystem respiration (ER) is one of the main components of biosphere CO₂fluxes. However, knowledge regarding how ER responds to warming is still lacking. In this study, a manipulative experiment with five simulated temperature increases (+0℃ [Control], +2.1℃ [warming 1, W1], +2.7℃ [warming 2, W2], +3.2℃ [warming 3, W3], +3.9℃ [warming 4, W4]) was conducted to investigate ER responses to warming in an alpine meadow on the Tibetan Plateau. The results showed that ER was suppressed by warming both in dry and wet years. The responses of ER to warming all followed a nonlinear pattern. The nonlinear processes can be divided into three stages, the quick‐response stage (W1), stable stage (W1–W3), and transition stage (W4). Compared with the nonlinear model, the linear model maximally overestimated the response ratios of ER to warming 2.2% and 3.2% in 2015 and 2016, respectively, and maximally underestimated the ratio 7.0% and 2.7%. The annual differences in ER responding to warming were mainly attributed to the distinct seasonal distribution of precipitation. Specially, we found that the abrupt shift response of ER to warming under W4 treatment in 2015, which might be regulatedmore »by the excitatory effect of precipitation after long‐term drought in the mid‐growing season. This study highlights the importance of the nonlinearity of warming effects on ER, which should be taken into the global‐C‐cycling models for better predicting future carbon–climate feedbacks.

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4. Reconstructing Pliocene West Pacific Warm Pool Hydroclimate Using In Situ Microanalyses on Fossil Planktic Foraminifer Shells

   https://doi.org/10.1029/2019PA003772  

   Wycech, J. ; Kelly, D. C. ; Fournelle, J. ; Kitajima, K. ; Kozdon, R. ; Orland, I. J. ( June 2020 , Paleoceanography and Paleoclimatology)

   A controversial aspect of Pliocene (5.3–2.6 Ma) climate is whether El Niño‐like (El Padre) conditions, characterized by a reduced trans‐equatorial sea‐surface temperature (SST) gradient, prevailed across the Pacific. Evidence for El Padre is chiefly based on reconstructions of sea‐surface conditions derived from the oxygen isotope (δ¹⁸O) and Mg/Ca compositions of shells belonging to the planktic foraminiferTrilobatus sacculifer. However, fossil shells of this species are a mixture of multiple carbonate phases—pre‐gametogenic, gametogenic (reproductive), and diagenetic calcites—that formed under different physiological and/or environmental conditions and are averaged in conventional whole‐shell analyses. Through in situ measurements of micrometer‐scale domains within Pliocene‐aged shells ofT. sacculiferfrom Ocean Drilling Program Site 806 in the western equatorial Pacific, we show that the δ¹⁸O of gametogenic calcite is 0.6–0.8‰ higher than pre‐gametogenic calcite, while the Mg/Ca ratios of these two phases are the same. Both the whole‐shell and pre‐gametogenic Mg/Ca records indicate that average early Pliocene SSTs were ~1°C warmer than modern, with present‐day SSTs being established during the latest Pliocene and early Pleistocene (~3.0–2.0 Ma). The measurement of multiple calcite phases by whole‐shell δ¹⁸O analyses masks a late Pliocene to earliest Pleistocene (3.6–2.2 Ma) decrease in seawater δ¹⁸O (δ¹⁸O_sw) values reconstructed from in situ pre‐gametogenic δ¹⁸O and Mg/Ca measurements. Ourmore »novel δ¹⁸O_swrecord indicates that sea‐surface salinities in the west Pacific warm pool were higher than modern prior to ~3.5 Ma, which is consistent with more arid conditions under an El Padre state.

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5. Lower soil moisture and deep soil temperatures in thermokarst features increase old soil carbon loss after 10 years of experimental permafrost warming

   https://doi.org/10.1111/gcb.15481  

   Pegoraro, Elaine F. ; Mauritz, Marguerite E. ; Ogle, Kiona ; Ebert, Christopher H. ; Schuur, Edward A. G. ( December 2020 , Global Change Biology)

   Almost half of the global terrestrial soil carbon (C) is stored in the northern circumpolar permafrost region, where air temperatures are increasing two times faster than the global average. As climate warms, permafrost thaws and soil organic matter becomes vulnerable to greater microbial decomposition. Long‐term soil warming of ice‐rich permafrost can result in thermokarst formation that creates variability in environmental conditions. Consequently, plant and microbial proportional contributions to ecosystem respiration may change in response to long‐term soil warming. Natural abundance δ¹³C and Δ¹⁴C of aboveground and belowground plant material, and of young and old soil respiration were used to inform a mixing model to partition the contribution of each source to ecosystem respiration fluxes. We employed a hierarchical Bayesian approach that incorporated gross primary productivity and environmental drivers to constrain source contributions. We found that long‐term experimental permafrost warming introduced a soil hydrology component that interacted with temperature to affect old soil C respiration. Old soil C loss was suppressed in plots with warmer deep soil temperatures because they tended to be wetter. When soil volumetric water content significantly decreased in 2018 relative to 2016 and 2017, the dominant respiration sources shifted from plant aboveground and young soil respirationmore »to old soil respiration. The proportion of ecosystem respiration from old soil C accounted for up to 39% of ecosystem respiration and represented a 30‐fold increase compared to the wet‐year average. Our findings show that thermokarst formation may act to moderate microbial decomposition of old soil C when soil is highly saturated. However, when soil moisture decreases, a higher proportion of old soil C is vulnerable to decomposition and can become a large flux to the atmosphere. As permafrost systems continue to change with climate, we must understand the thresholds that may propel these systems from a C sink to a source.

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