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1. Decreased Stratification in the Abyssal Southwest Pacific Basin and Implications for the Energy Budget

   https://doi.org/10.1029/2021GL094322  

   Zhang, Helen J.; Whalen, Caitlin B.; Kumar, Nirnimesh; Purkey, Sarah G. (September 2021, Geophysical Research Letters)

   Abstract As the abyssal oceans warm, stratification is also expected to change in response. This change may impact mixing and vertical transport by altering the buoyancy flux, internal wave generation, and turbulent dissipation. In this study, repeated surveys of three hydrographic sections in the Southwest Pacific Basin between the 1990s and 2010s are used to estimate the change in buoyancy frequency. We find that below the°C isotherm,is on average reduced by a scaling factor of, a 12% reduction, per decade that intensifies with depth. At°C, we observe the biggest change:, or a 29% reduction per decade. Within the same period, the magnitude of vertical diffusive heat flux is also reduced by about, although this estimate is sensitive to the choice of estimated diffusivity. Finally, implications of these results for the heat budget and global ocean circulation are qualitatively discussed. 

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2. Influence of Thermal Stratification on the Structure and Evolution of the Martian Core

   https://doi.org/10.1029/2021GL095198  

   Greenwood, Sam; Davies, Christopher J.; Pommier, Anne (November 2021, Geophysical Research Letters)

   Abstract The apparent end of the internally generated Martian magnetic field at 3.6–4.1 Ga is a key event in Martian history and has been linked to insufficient core cooling. We investigate the thermal and magnetic evolution of the Martian core and mantle using parameterized models and considered three improvements on previous studies. First, our models account for thermal stratification in the core. Second, the models are constrained by estimates for the present‐day areotherm. Third, we consider core thermal conductivity,, values in the range 5–40 Was suggested by recent experiments on iron alloys at Mars core conditions. The majority of our models indicate that the core of Mars is fully conductive at present with core temperatures greater than 1940 K. All of our models are consistent with the range ofW. Models with an activation volume of 6 (0)require a mantle reference viscosity of Pa s. 

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3. Realistic Electron Diffusion Rates and Lifetimes Due to Scattering by Electron Holes

   https://doi.org/10.1029/2021JA029380  

   Shen, Yangyang; Vasko, Ivan Y.; Artemyev, Anton; Malaspina, David M.; Chu, Xiangning; Angelopoulos, Vassilis; Zhang, Xiao‐Jia (August 2021, Journal of Geophysical Research: Space Physics)

   Abstract Plasma sheet electron precipitation into the diffuse aurora is critical for magnetosphere‐ionosphere coupling. Recent studies have shown that electron phase space holes can pitch‐angle scatter electrons and may produce plasma sheet electron precipitation. These studies have assumed identical electron hole parameters to estimate electron scattering rates (Vasko et al., 2018,https://doi.org/10.1063/1.5039687). In this study, we have re‐evaluated the efficiency of this scattering by incorporating realistic electron hole properties from direct spacecraft observations into computing electron diffusion rates and lifetimes. The most important electron hole properties in this evaluation are their distributions in velocity and spatial scale and electric field root‐mean‐square intensity (). Using direct measurements of electron holes during a plasma injection event observed by the Van Allen Probe at, we find that when4 mV/m electron lifetimes can drop below 1 h and are mostly within strong diffusion limits at energies below10 keV. During an injection observed by the THEMIS spacecraft at, electron holes with even typical intensities (1 mV/m) can deplete low‐energy (a few keV) plasma sheet electrons within tens of minutes following injections and convection from the tail. Our results confirm that electron holes are a significant contributor to plasma sheet electron precipitation during injections. 

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4. Synchronous Emplacement of the Anorthosite Xenolith‐Bearing Beaver River Diabase and One of the Largest Lava Flows on Earth

   https://doi.org/10.1029/2021GC009909  

   Zhang, Yiming; Swanson‐Hysell, Nicholas L.; Schmitz, Mark D.; Miller, James D.; Avery, Margaret S. (October 2021, Geochemistry, Geophysics, Geosystems)

   Abstract New geochronologic and paleomagnetic data from the North American Midcontinent Rift (MCR) reveal the synchronous emplacement of the Beaver River diabase, the anorthosite xenoliths within it, and the Greenstone Flow—one of the largest lava flows on Earth. A U‐Pb zircon date of 1091.83  0.21 Ma (2) from one of the anorthosite xenoliths is consistent with the anorthosite cumulate forming as part of the MCR and provides a maximum age constraint for the Beaver River diabase. Paired with the minimum age constraint of a cross‐cutting Silver Bay intrusion (1091.61  0.14 Ma; 2), these data tightly bracket the age of the Beaver River diabase to be 1091.7  0.2 Ma (95% CI), coeval with the eruption of the Greenstone Flow (1091.59  0.27 Ma; 2)—which is further supported by indistinguishable tilt‐corrected paleomagnetic pole positions. Geochronological, paleomagnetic, mineralogical and geochemical data are consistent with a hypothesis that the Beaver River diabase was the feeder system for the Greenstone Flow. The large areal extent of the intrusives and large estimated volume of the volcanics suggest that they represent a rapid and voluminous ca. 1,092 Ma magmatic pulse near the end of the main stage of MCR magmatism. 

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5. Euphotic zone nitrification in the California Current Ecosystem

   https://doi.org/10.1002/lno.11348  

   Stephens, Brandon M.; Wankel, Scott D.; Beman, J. Michael; Rabines, Ariel J.; Allen, Andrew E.; Aluwihare, Lihini I. (October 2019, Limnology and Oceanography)

   Abstract Nitrification, the microbial conversion of ammonium to nitrite then to nitrate, occurs throughout the oceanic water column, yet the environmental factors influencing the production of nitrate in the euphotic zone (EZ) remain unclear. In this study, the natural abundances of N and O isotopes (δ¹⁵N and δ¹⁸O, respectively) in nitrate were used in an existing model framework to quantify nitrate contributed by EZ nitrification in the California Current Ecosystem (CCE) during two anomalously warm years. Model data estimated that between 6% and 36% of the EZ nitrate reservoirs were derived from the combined steps of nitrification within the EZ. The CCE data set found nitrification contributions to EZ nitrate to be positively correlated with nitrite concentrations () at the depth of the primary nitrite maximum (PNM). Building on this correlation, EZ nitrification in the southern California Current was estimated to contribute on average 20% ± 6% to EZ nitrate as inferred using the PNMof the long‐term California Cooperative Oceanic Fisheries Investigation (CalCOFI) survey record. A multiple linear regression analysis of the CalCOFI PNMtime series identified two conditions that led to positive deviations in. Enhanced PNM, and potentially enhanced EZ nitrification, may be linked to (1) reduced phytoplankton competition for ammonium () andas interpreted from particulate organic carbon:chlorophyll ratios, and/or (2) to increased supply of(and thenoxidation to) from the degradation of organic nitrogen as interpreted from particulate organic nitrogen concentrations. 

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