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1. Impacts of Paleoecology on the TEX ₈₆ Sea Surface Temperature Proxy in the Pliocene‐Pleistocene Mediterranean Sea

   https://doi.org/10.1029/2018PA003494  

   Polik, Catherine A.; Elling, Felix J.; Pearson, Ann (December 2018, Paleoceanography and Paleoclimatology)

   Abstract The TEX₈₆proxy, based on the distribution of isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs) from planktonic Thaumarchaeota, is widely used to reconstruct sea surface temperature (SST). Recent observations of species‐specific and regionally dependent TEX₈₆‐SST relationships in cultures and the modern ocean raise the question of whether nonthermal factors may have impacted TEX₈₆paleorecords. Here we evaluate the effects of ecological changes on TEX₈₆using one Pliocene and two Pleistocene sapropels from the Mediterranean Sea. We find that TEX₈₆‐derived SSTs deviate from‐derived SSTs before, during, and after each sapropel event.‐derived SSTs vary by less than 6 °C, while TEX₈₆‐derived SSTs vary by up to 15 °C within a single record. Compound‐specific carbon isotope compositions indicate minimal confounding influence on TEX₈₆from exogenous sources. Some of the variation can be accounted for by changes in nitrogen cycling intensity affecting thaumarchaeal iGDGT biosynthesis, as demonstrated by an inverse relationship between TEX₈₆and δ¹⁵N_TN. TEX₈₆‐derived SSTs also consistently show warm anomalies in the Pleistocene, while the Pliocene samples exhibit both warmer and cooler relative offsets. These anomalies result from systematic differences between Plio‐Pleistocene iGDGT distributions and both modern Mediterranean and modern, globally distributed core top samples. Through characteristic GDGT distributions, we suggest the existence of three distinct endemic populations of Thaumarchaeota in the Pliocene, Pleistocene, and modern Mediterranean Sea, respectively. Importantly, these communities prevailed during both sapropel and oligotrophic conditions. Our results demonstrate that ecological and community‐specific effects must be considered when applying the TEX₈₆proxy to paleorecords. 

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2. Bayesian Calibration for the Arctic Sea Ice Biomarker IP ₂₅

   https://doi.org/10.1029/2024PA005048  

   Fu, C_Y; Osman, M_B; Aquino‐López, M_A (March 2025, Paleoceanography and Paleoclimatology)

   Abstract Sea ice plays multiple important roles in regulating the global climate. Rapid sea ice loss in the Arctic has been documented over recent decades, yet our understanding of long‐term sea ice variability and its feedbacks remains limited by a lack of quantitative sea ice reconstructions. The sea ice diatom‐derived biomarker has been combined with sterols produced by open‐water phytoplankton in the index as a sea ice proxy to achieve semi‐quantitative reconstructions. Here, we analyze a compilation of over 600 published core‐top measurements of paired with brassicasterol and/or dinosterol across (sub‐)Arctic oceans to calculate a newln() index that correlates nonlinearly with sea ice concentration. Leveraging sediment trap and sea ice observational studies, we develop a spatially varying Bayesian calibration (BaySIC) for ln() to account for its non‐stationary relationship with sea ice concentration and other environmental drivers (e.g., sea surface salinity). The model is fully invertible, allowing probabilistic forward modeling of the ln() index as well as inverse modeling of past sea ice concentration with bi‐directional uncertainty quantification.BaySICfacilitates direct proxy‐model comparisons and palaeoclimate data assimilation, providing the polar proxy constraints currently missing in climate model simulations and enabling, for the first time, fully quantitative Arctic sea ice reconstructions. 

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3. Scale‐Dependent Drivers of Air‐Sea CO ₂ Flux Variability

   https://doi.org/10.1029/2024GL111911  

   Fay, Amanda R; Carroll, Dustin; McKinley, Galen A; Menemenlis, Dimitris; Zhang, Hong (October 2024, Geophysical Research Letters)

   Abstract In climate studies, it is crucial to distinguish between changes caused by natural variability and those resulting from external forcing. Here we use a suite of numerical experiments based on the ECCO‐Darwin ocean biogeochemistry model to separate the impact of the atmospheric carbon dioxide (CO₂) growth rate and climate on the ocean carbon sink — with a goal of disentangling the space‐time variability of the dominant drivers. When globally integrated, the variable atmospheric growth rate and climate exhibit similar magnitude impacts on ocean carbon uptake. At local scales, interannual variability in air‐sea CO₂flux is dominated by climate. The implications of our study for real‐world ocean observing systems are clear: in order to detect future changes in the ocean sink due to slowing atmospheric CO₂growth rates, better observing systems and constraints on climate‐driven ocean variability are required. 

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4. Enhanced ClNO ₂ Formation at the Interface of Sea-Salt Aerosol

   https://doi.org/10.1021/acs.jpclett.4c02289  

   Moon, Seokjin; Limmer, David T (September 2024, The Journal of Physical Chemistry Letters)
5. Alternate Histories: Synthetic Large Ensembles of Sea‐Air CO ₂ Flux

   https://doi.org/10.1029/2021GB007174  

   Olivarez, Holly C.; Lovenduski, Nicole S.; Brady, Riley X.; Fay, Amanda R.; Gehlen, Marion; Gregor, Luke; Landschützer, Peter; McKinley, Galen A.; McKinnon, Karen A.; Munro, David R. (May 2022, Global Biogeochemical Cycles)

   Abstract We use a statistical emulation technique to construct synthetic ensembles of global and regional sea‐air carbon dioxide (CO₂) flux from four observation‐based products over 1985–2014. Much like ensembles of Earth system models that are constructed by perturbing their initial conditions, our synthetic ensemble members exhibit different phasing of internal variability and a common externally forced signal. Our synthetic ensembles illustrate an important role for internal variability in the temporal evolution of global and regional CO₂flux and produce a wide range of possible trends over 1990–1999 and 2000–2009. We assume a specific externally forced signal and calculate the rank of the observed trends within the distribution of statistically modeled synthetic trends during these periods. Over the decade 1990–1999, three of four observation‐based products exhibit small negative trends in globally integrated sea‐air CO₂flux (i.e., enhanced ocean CO₂absorption with time) that are within one standard deviation of the mean in their respective synthetic ensembles. Over the decade 2000–2009, however, three products show large negative trends in globally integrated sea‐air CO₂flux that have a low rate of occurrence in their synthetic ensembles. The largest positive trends in global and Southern Ocean flux over 1990–1999 and the largest negative trends over 2000–2009 fall nearly two standard deviations away from the mean in their ensembles. Our approach provides a new perspective on the important role of internal variability in sea‐air CO₂flux trends, and furthers understanding of the role of internal and external processes in driving observed sea‐air CO₂flux variability. 

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