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1. High-resolution 4D electrical resistivity tomography and below-ground point sensor monitoring of High Arctic deglaciated sediments capture zero-curtain effects, freeze–thaw transitions, and mid-winter thawing

   https://doi.org/10.5194/tc-19-401-2025  

   Cimpoiasu, MO; Kuras, O; Harrison, H; Wilkinson, PB; Meldrum, P; Chambers, JE; Liljestrand, D; Oroza, C; Schmidt, SK; Sommers, P; et al (January 2025, The Cryosphere)

   Abstract. Arctic regions are under immense pressure from a continuously warming climate. During the winter and shoulder seasons, recently deglaciated sediments are particularly sensitive to human-induced warming. Understanding the physical mechanisms and processes that determine soil liquid moisture availability contributes to the way we conceptualize and understand the development and functioning of terrestrial Arctic ecosystems. However, harsh weather and logistical constraints limit opportunities to directly observe subsurface processes year-round; hence automated and uninterrupted strategies of monitoring the coupled heat and water movement in soils are essential. Geoelectrical monitoring using electrical resistivity tomography (ERT) has proven to be an effective method to capture soil moisture distribution in time and space. ERT instrumentation has been adapted for year-round operation in high-latitude weather conditions. We installed two geoelectrical monitoring stations on the forefield of a retreating glacier in Svalbard, consisting of semi-permanent surface ERT arrays and co-located soil sensors, which track seasonal changes in soil electrical resistivity, moisture, and temperature in 3D. One of the stations observes recently exposed sediments (5–10 years since deglaciation), whilst the other covers more established sediments (50–60 years since deglaciation). We obtained a 1-year continuous measurement record (October 2021–September 2022), which produced 4D images of soil freeze–thaw transitions with unprecedented detail, allowing us to calculate the velocity of the thawing front in 3D. At its peak, this was found to be 1 m d−1 for the older sediments and 0.4 m d−1 for the younger sediments. Records of soil moisture and thermal regime obtained by sensors help define the conditions under which snowmelt takes place. Our data reveal that the freeze–thaw shoulder period, during which the surface soils experienced the zero-curtain effect, lasted 23 d at the site closer to the glacier but only 6 d for the older sediments. Furthermore, we used unsupervised clustering to classify areas of the soil volume according to their electrical resistivity coefficient of variance, which enables us to understand spatial variations in susceptibility to water-phase transition. Novel insights into soil moisture dynamics throughout the spring melt will help parameterize models of biological activity to build a more predictive understanding of newly emerging terrestrial landscapes and their impact on carbon and nutrient cycling. 

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2. Geochemical conditions regulating chromium preservation in marine sediments

   https://doi.org/10.1016/j.gca.2023.03.003  

   Bruggmann, S.; Severmann, S.; McManus, J. (May 2023, Geochimica et Cosmochimica Acta)

   In the marine sediment record, concentrations and isotope ratios of chromium (Cr) can be used to reconstruct ocean biogeochemical conditions. These reconstructions rely on a detailed understanding of the chemical pathways that Cr undergoes as it is transferred from the water column to the sediment record. We examined Cr concentrations in marine pore fluids and sediments from six continental margin sites, which can be grouped into two basic environments: (1) sites where sediments are oxygenated and rich in solid phase Mn (herein termed oxic), and (2) sites where sediments are organic C (Corg)-rich and oxygen is depleted (anoxic). We found Cr concentrations to be lower (maximum of 12 nM in pore fluids and 124 ppm sediment solid phase) at oxic sites compared with anoxic sites (maximum of 77 nM and 184 ppm). Our findings confirm previously published interpretations of dissolved Cr in pore fluids (Brumsack and Gieskes, 1983; Shaw et al., 1990). In oxic surface sediments, particulate Cr(III) can be oxidised by Mn oxides, which leads to elevated concentrations of dissolved Cr co-occurring at the same depth as elevated Mn concentrations in the sediment. Under these oxidising conditions, down-core sediments contain relatively low solid-phase Cr concentrations. In oxic sediments, Cr speciation reveals that most of the pore fluid Cr is in the Cr(VI) state. At the site where Mn oxide-rich sediments rest below an oxic water column, oxidative loss of Cr from the sediment to the bottom water leads to the lowest estimated Cr burial efficiency of the sites examined here. Under anoxic Corg-rich conditions, both pore fluids and sediment solid phases contain high Cr concentrations, with 40–80% of dissolved pore fluid Cr present as Cr(III). This enrichment of Cr appears to be tightly linked to the presence of high total organic carbon (TOC) content and scavenging of Cr by (organic) particles in the water column. Combined, these data highlight the strong dependence of Cr on both sedimentary redox conditions as well as biological productivity. Based on the data from modern continental margin sediments, we propose that Cr concentrations and isotope compositions of the authigenic sediment fraction may record a combination of redox conditions and biological productivity in the water column. If confirmed by Cr isotope analyses, these findings will add support for the notion that Cr may serve as a proxy for ocean biological and chemical sedimentological conditions. Thus, careful assessment of the impact of organic matter on Cr is required for reconstructions of redox conditions with sedimentary records. 

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3. High temperatures and low soil moisture synergistically reduce switchgrass yields from marginal field sites and inhibit fermentation

   https://doi.org/10.1111/gcbb.13119  

   Chipkar, Sarvada; Kahmark, Kevin; Bohm, Sven; Hussain, Mir Zaman; Joshi, Leela; Krieg, Karleigh M.; Aguado, Jacob; Cassidy, Jasmine; Lozano, Pablo; Garland, Kevin; et al (February 2024, GCB Bioenergy)

   Abstract ‘Marginal lands’ are low productivity sites abandoned from agriculture for reasons such as low or high soil water content, challenging topography, or nutrient deficiency. To avoid competition with crop production, cellulosic bioenergy crops have been proposed for cultivation on marginal lands, however on these sites they may be more strongly affected by environmental stresses such as low soil water content. In this study we used rainout shelters to induce low soil moisture on marginal lands and determine the effect of soil water stress on switchgrass growth and the subsequent production of bioethanol. Five marginal land sites that span a latitudinal gradient in Michigan and Wisconsin were planted to switchgrass in 2013 and during the 2018–2021 growing seasons were exposed to reduced precipitation under rainout shelters in comparison to ambient precipitation. The effect of reduced precipitation was related to the environmental conditions at each site and biofuel production metrics (switchgrass biomass yields and composition and ethanol production). During the first year (2018), the rainout shelters were designed with 60% rain exclusion, which did not affect biomass yields compared to ambient conditions at any of the field sites, but decreased switchgrass fermentability at the Wisconsin Central–Hancock site. In subsequent years, the shelters were redesigned to fully exclude rainfall, which led to reduced biomass yields and inhibited fermentation for three sites. When switchgrass was grown in soils with large reductions in moisture and increases in temperature, the potential for biofuel production was significantly reduced, exposing some of the challenges associated with producing biofuels from lignocellulosic biomass grown under drought conditions. 

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4. Changing climate response of Northeast Ohio white oaks, USA: Is it tree age or site age?

   Wiles, G; Wiesenberg, N; Pollock, M; Denes, C; Cooper, T; Smith, D; Wiles, M (February 2025, Dendrochronologia)

   White oak, a keystone species of the broadleaf forests of the North American Midwest, has a significant role in providing ecosystems services in a region experiencing warming and increasingly pluvial conditions. A one- hundred-year-old white oak stand in an arboretum, along with two second growth (~200-year-old) stands from Northeast Ohio have consistently responded positively to summer (June-July) precipitation over the past century, whereas four nearby old growth sites (>300 years old) have lost their moisture sensitivity since about the mid 1970s. This “fading drought signal,” which has been previously reported, appears to be more a result of the legacy of land use at the individual sites rather than tree age. The younger oak stands and their relative sustained drought sensitivity is also related to their history of recently attaining the canopy and similar responses associated with intervals of selective logging. All sites are strongly, negatively correlated with summer (June- July) maximum monthly temperatures and in general the maximum temperatures are negatively correlated with precipitation in those months. Future warming in the Midwest is projected to see increases in spring precipitation and likely decreases in late summer precipitation linked to a northward migration of the North American Westerly Jet. This projected decrease in summer precipitation coupled with an increase in maximum and min- imum summer temperatures in the coming decades would increase the moisture stress on these trees. Our ex- amination of these varying climate responses with respect to site characteristics and forest age can help future assessments of tree health and the forest’s ability to sequester carbon, as well as facilitate efforts to reconstruct climate by using a range of tree sites for intervals when sensitivity in old growth sites is lost. 

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5. Primary to post‐depositional microbial controls on the stable and clumped isotope record of shoreline sediments at Fayetteville Green Lake

   https://doi.org/10.1111/gbi.12609  

   Leapaldt, Hanna_C; Frantz, Carie_M; Olsen‐Valdez, Juliana; Snell, Kathryn_E; Trower, Elizabeth_J; Ingalls, Miquela (July 2024, Geobiology)

   Abstract Lacustrine carbonates are a powerful archive of paleoenvironmental information but are susceptible to post‐depositional alteration. Microbial metabolisms can drive such alteration by changing carbonate saturationin situ, thereby driving dissolution or precipitation. The net impact these microbial processes have on the primary δ¹⁸O, δ¹³C, and Δ₄₇values of lacustrine carbonate is not fully known. We studied the evolution of microbial community structure and the porewater and sediment geochemistry in the upper ~30 cm of sediment from two shoreline sites at Green Lake, Fayetteville, NY over 2 years of seasonal sampling. We linked seasonal and depth‐based changes of porewater carbonate chemistry to microbial community composition, in situ carbon cycling (using δ¹³C values of carbonate, dissolved inorganic carbon (DIC), and organic matter), and dominant allochems and facies. We interpret that microbial processes are a dominant control on carbon cycling within the sediment, affecting porewater DIC, aqueous carbon chemistry, and carbonate carbon and clumped isotope geochemistry. Across all seasons and sites, microbial organic matter remineralization lowers the δ¹³C of the porewater DIC. Elevated carbonate saturation states in the sediment porewaters (Ω > 3) were attributed to microbes from groups capable of sulfate reduction, which were abundant in the sediment below 5 cm depth. The nearshore carbonate sediments at Green Lake are mainly composed of microbialite intraclasts/oncoids, charophytes, larger calcite crystals, and authigenic micrite—each with a different origin. Authigenic micrite is interpreted to have precipitated in situ from the supersaturated porewaters from microbial metabolism. The stable carbon isotope values (δ¹³C_carb) and clumped isotope values (Δ₄₇) of bulk carbonate sediments from the same depth horizons and site varied depending on both the sampling season and the specific location within a site, indicating localized (μm to mm) controls on carbon and clumped isotope values. Our results suggest that biological processes are a dominant control on carbon chemistry within the sedimentary subsurface of the shorelines of Green Lake, from actively forming microbialites to pore space organic matter remineralization and micrite authigenesis. A combination of biological activity, hydrologic balance, and allochem composition of the sediments set the stable carbon, oxygen, and clumped isotope signals preserved by the Green Lake carbonate sediments. 

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