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1. Ge and Si Isotope Behavior During Intense Tropical Weathering and Ecosystem Cycling

   https://doi.org/10.1029/2019GB006522  

   Baronas, J. Jotautas ; West, A. Joshua ; Burton, Kevin W. ; Hammond, Douglas E. ; Opfergelt, Sophie ; Pogge von Strandmann, Philip A. E. ; James, Rachael H. ; Rouxel, Olivier J. ( August 2020 , Global Biogeochemical Cycles)

   Chemical weathering of volcanic rocks in warm and humid climates contributes disproportionately to global solute fluxes. Geochemical signatures of solutes and solids formed during this process can help quantify and reconstruct weathering intensity in the past. Here, we measured silicon (Si) and germanium (Ge) isotope ratios of the soils, clays, and fluids from a tropical lowland rainforest in Costa Rica. The bulk topsoil is intensely weathered and isotopically light (mean ± 1σ:δ³⁰Si = −2.1 ± 0.3‰,δ⁷⁴Ge = −0.13 ± 0.12‰) compared to the parent rock (δ³⁰Si = −0.11 ± 0.05‰,δ⁷⁴Ge = 0.59 ± 0.07‰). Neoforming clays have even lower values (δ³⁰Si = −2.5 ± 0.2‰,δ⁷⁴Ge = −0.16 ± 0.09‰), demonstrating a whole‐system isotopic shift in extremely weathered systems. The lowland streams represent mixing of dilute local fluids (δ³⁰Si = 0.2 − 0.6‰,δ⁷⁴Ge = 2.2 − 2.6‰) with solute‐rich interbasin groundwater (δ³⁰Si = 1.0 ± 0.2‰,δ⁷⁴Ge = 4.0‰). Using a Ge‐Si isotope mass balance model, we calculate that91 ± 9% of Ge released via weathering of lowland soils is sequestered by neoforming clays,9 ± 9% by vegetation, and only0.2 ± 0.2% remains dissolved. Vegetation plays an important role in the Si cycle, directly sequestering39 ± 14% of released Si and enhancing clay neoformation in surface soils via the addition of amorphous phytolith silica. Globally, volcanic soilδ⁷⁴Ge closely tracks the depletion of Ge by chemical weathering (τ_Ge), whereasδ³⁰Si and Ge/Si both reflect the loss of Si (τ_Si). Because of the different chemical mobilities of Ge and Si, aδ⁷⁴Ge‐δ³⁰Si multiproxy system is sensitive to a wider range of weathering intensities than each isotopic system in isolation.

    

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2. The Brachiopod δ 11 B Record Across the Carboniferous‐Permian Climate Transition

   https://doi.org/10.1029/2019PA003838  

   Legett, S. A. ; Rasbury, E. T. ; Grossman, E. L. ; Hemming, N. G. ; Penman, D. E. ( September 2020 , Paleoceanography and Paleoclimatology)

   We present the δ¹¹B of well‐preserved brachiopod fossils coupled with geochemical modeling to examine how seawater boron responded to abrupt and dynamic climate changes in the Late Paleozoic. The Late Carboniferous, a time of major coal formation and glacioeustatic sea level changes, is characterized by relatively stable brachiopod δ¹¹B of 15–17‰, similar to values seen in modern brachiopods. Brachiopod δ¹¹B dropped by ~5‰ in the early Permian and then restabilized at a new value of 10‰ within a few million years. Mass balance models of seawater δ¹¹B reproduced the overall trends in our brachiopod data but failed to capture the large drop in δ¹¹B in the early Permian. Published seawater⁸⁷Sr/⁸⁶Sr and δ^44/40Ca data based on brachiopod shells also shift to lower values in the early Permian, suggesting a common control on all three seawater isotope systems. The Permian terrestrial record of evaporites and eolian deposits suggests a prolonged reduced delivery of dissolved weathering products to the ocean, accounting for the change in seawater⁸⁷Sr/⁸⁶Sr. This reduced weathering, in turn, led to increased atmospheric CO₂and lowered seawater pH, which may have significantly decreased major removal mechanisms for seawater calcium and boron leading to declines in both isotope systems. We propose that boron removal via coprecipitation in carbonates and adsorption onto clay minerals was significantly diminished due to a reduction in the availability of the borate aqueous species caused by lowered seawater pH.

    

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3. Trophic ecology of Caribbean sponges in the mesophotic zone

   https://doi.org/10.1002/lno.11668  

   Macartney, Keir J. ; Slattery, Marc ; Lesser, Michael P. ( December 2020 , Limnology and Oceanography)

   Sponges are a crucial component of Caribbean coral reef ecosystem structure and function. In the Caribbean, many sponges show a predictable increase in percent cover or abundance as depth increases from shallow (< 30 m) to mesophotic (30–150 m) depths. Given that sponge abundances are predicted to increase in the Caribbean as coral cover declines, understanding ecological factors that control their distribution is critical. Here we assess if sponge cover increases as depth increases into the mesophotic zone for three common Caribbean reef sponges,Xestospongia muta,Agelas tubulata, andPlakortis angulospiculatus, and use stable isotope analyses to determine whether shifts in trophic resource utilization along a shallow to mesophotic gradient occurred. Ecological surveys show that all target sponges significantly increase in percent cover as depth increases. Using bulk stable isotope analysis, we show that as depth increases there are increases in the δ¹³C and δ¹⁵N values, reflecting that all sponges consumed more heterotrophic picoplankton, with low C:N ratios in the mesophotic zone. However, compound‐specific isotope analysis of amino acids (CSIA‐AA) shows that there are species‐specific increases in δ¹³C_AAand δ¹⁵N_AAvalues.Xestospongia mutaandP. angulospiculatusshowed a reduced reliance on photoautotrophic resources as depth increased, whileA. tubulataappears to rely on heterotrophy at all depths. The δ¹³C_AAand δ¹⁵N_AAvalues of these sponges also reflect species‐specific patterns of host utilization of both POM and dissolved organic matter (DOM), its subsequent re‐synthesis, and translocation, by their microbiomes.

    

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4. The upper ocean silicon cycle of the subarctic Pacific during the EXPORTS field campaign

   https://doi.org/10.1525/elementa.2021.00087  

   Brzezinski, Mark A. ; Varela, Diana E. ; Jenkins, Bethany D. ; Buck, Kristen N. ; Kafrissen, Sile M. ; Jones, Janice L. ( January 2022 , Elementa: Science of the Anthropocene)

   Diatoms are major contributors to marine primary productivity and carbon export due to their rapid growth in high-nutrient environments and their heavy silica ballast. Their contributions are highly modified in high-nutrient low-chlorophyll regions due to the decoupling of upper-ocean silicon and carbon cycling caused by low iron (Fe). The Si cycle and the role of diatoms in the biological carbon pump was examined at Ocean Station Papa (OSP) in the HNLC region of the northeastern subarctic Pacific during the NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field study. Sampling occurred during the annual minimum in surface silicic acid (Si(OH)4) concentration. Biogenic silica (bSi) concentrations were low, being in the tens of nanomolar range, despite high Si(OH)4 concentrations of about 15 μM. On average, the >5.0-µm particle size fraction dominated Si dynamics, accounting for 65% of bSi stocks and 81% of Si uptake compared to the small fraction (0.6–5.0 μm). Limitation of Si uptake was detected in the small, but not the large, size fraction. Growth rate in small diatoms was limited by Fe, while their Si uptake was restricted by Si(OH)4 concentration, whereas larger diatoms were only growth-limited by Fe. About a third of bSi production was exported out of the upper 100 m. The contribution of diatoms to carbon export (9–13%) was about twice their contribution to primary productivity (3–7%). The combination of low bSi production, low diatom primary productivity and high bSi export efficiency at OSP was more similar to the dynamics in the subtropical gyres than to other high-nutrient low-chlorophyll regions. 

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5. A seawater throttle on H 2 production in Precambrian serpentinizing systems

   https://doi.org/10.1073/pnas.1921042117  

   Tutolo, Benjamin M. ; Seyfried, Jr, William E. ; Tosca, Nicholas J. ( June 2020 , Proceedings of the National Academy of Sciences)

   Since the initial discovery of low-temperature alkaline hydrothermal vents off the Mid-Atlantic Ridge axis nearly 20 y ago, the observation that serpentinizing systems produce abundant H₂has strongly influenced models of atmospheric evolution and geological scenarios for the origin of life. Nevertheless, the principal mechanisms that generate H₂in these systems, and how secular changes in seawater composition may have modified serpentinization-driven H₂fluxes, remain poorly constrained. Here, we demonstrate that the dominant mechanism for H₂production during low-temperature serpentinization is directly related to a Si deficiency in the serpentine structure, which itself is caused by low SiO₂(aq) concentrations in serpentinizing fluids derived from modern seawater. Geochemical calculations explicitly incorporating this mechanism illustrate that H₂production is directly proportional to both the SiO₂(aq) concentration and temperature of serpentinization. These results imply that, before the emergence of silica-secreting organisms, elevated SiO₂(aq) concentrations in Precambrian seawater would have generated serpentinites that produced up to two orders of magnitude less H₂than their modern counterparts, consistent with Fe-oxidation states measured on ancient igneous rocks. A mechanistic link between the marine Si cycle and off-axis H₂production requires a reevaluation of the processes that supplied H₂to prebiotic and early microbial systems, as well as those that balanced ocean–atmosphere redox through time.

    

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