Biotic and abiotic chemical weathering of siliciclastic sediments in cold environments

Abstract

Dissertation Summary Chemical weathering of silicate minerals is one of the most important Earth processes, moderating atmospheric carbon dioxide levels by consumption of carbon dioxide during hydrolysis of silicates (Nesbitt and Young, 1982; White and Peterson, 1990; Velbel, 1993; White and Blum, 1995; White et al., 1996; White and Brantley, 2003; White and Buss, 2014). Owing to its significance to the carbon cycle, and sensitivity to climatic conditions, chemical weathering and indices developed to determine the extent of weathering (i.e., Chemical Index of Alteration- CIA) have been the focus of significant studies aimed at investigating implications for paleoclimate in both terrestrial and extraterrestrial settings (i.e., Nesbitt and Young, 1982; Nesbitt and Young, 1989; Soreghan and Soreghan, 2007; Yang et al., 2016; Siebach et al., 2017; Deng et al., 2022). Chemical weathering leaves physical, chemical, and mineralogical signatures on rocks, sediments, and the aquatic environment, both via abiotic and biotic pathways. Therefore, weathering signatures studied on Earth are analogs for extraterrestrial signatures of surface alteration processes (i.e., Cannon et al., 2015; Olsson-Francis et al., 2017). However, abiotic and biotic weathering pathways in cold environments (i.e., within glacial settings) and subsequent weathering signatures remain poorly understood. This dissertation investigates biotic and abiotic weathering signatures and pathways within various glaciated settings, with the focus on Antarctica and Iceland as climatic and mineralogical analogs of Mars. Non-glaciated settings are also investigated to compare weathering signatures generated within cold and hot climates. Chapters within this dissertation are formatted as peer-reviewed journal publications (in prep. or published). The Chemical Index of Alteration (CIA) was developed to quantify the extent of weathering based on major oxides within silicates that are significantly associated with weathering: Al2O3, CaO, Na2O and K2O (Nesbitt and Young, 1982). CIA has been largely used to interpret paleoclimate, and correlated with climate parameters (mean annual temperature, MAT, and mean annual precipitation, MAP), especially with MAT within tropical soil profiles and watersheds on felsic bedrock (i.e., Nesbitt and Young, 1989, Rasmussen et al., 2011; Yang et al., 2016; Joo et al., 2018a). However, these correlations don’t seem to apply to glaciated settings (Deng et al., 2022), and various studies discuss shortcomings of applying CIA when mafic rock types are involved, as mafic major oxide components (such as FeO and MgO) are not incorporated within CIA calculations, and CIA is highly dependent on CaO that can result in artificially underestimated CIA values when the source rock contains high CaO (Nesbitt et al., 1996; Siebach et al., 2017; Mangold et al., 2019; Berger et al., 2020). Despite these documented issues, CIA remains widely utilized for various depositional settings, including potentially glaciated environments on Earth and Mars (i.e., Nesbitt and Young, 1982; Balburg and Dobrzinski 2011; Marra et al., 2017; Hurrowitz et al., 2017; Wang et al., 2020). In Chapter 1, I investigate and compare weathering signatures (mineralogy, chemistry, grain size, and surface area) within mud-sized (<63 µm) sediments from both cold glaciated and hot non-glacial settings on felsic-intermediate bedrock to assess paleoclimatic implications of chemical weathering, attempting to decouple inherited provenance signatures from climatic signals. Use of ternary plots that are commonly used for chemical weathering and paleoclimate studies, such as A-CK-N, A-CKN-FM (Nesbitt and Young, 1982) and MFW (Ohta and Arai, 2007), illustrate that the effects of provenance and mafic mineral sorting (towards finer grain sizes) overshadow weathering trends (Nesbitt et al., 1996; von Eynatten et al., 2012; Mangold et al., 2019), except for tropical soils and fluvial muds from Puerto Rico. In addition, data from very different climatic settings have overlapping CIA values (indicative of weak to intermediate weathering) that also clustered together on A-CK-N diagrams, suggesting that assessing climatic trends using this method may lead to erroneous interpretations. In an attempt to remove the provenance signature from the data, we normalized sediment CIA values to the CIA values determined for their bedrock sources and tested the correlation with MAT and MAP. Though R2 values obtained from this multi-provenance and climate data set were enhanced, and showed better correlation of CIA with MAP, removing the tropical watershed from the data set eliminated any expected correlations. Overall, Chapter 1 shows that: 1) CIA and ternary plots for weathering are most useful when applied to tropical settings with uniform bedrock composition, where elemental weathering trends can be directly traced from the bedrock to first-cycle material (soil profiles/paleosols); 2) CIA values of muds from glacial settings overlap with values observed in hot and humid climates; and 3) no correlations were observed between climatic parameters (mean annual precipitation and temperature) and CIA in non-tropical fluvial sediments, suggesting that CIA is not a useful metric for modeling paleoclimate in glaciated settings.

Microbial organisms catalyze chemical weathering owing to their metabolic biproducts (organic acids, carbon dioxide, extracellular polymeric substances) which locally decrease the pH, and metabolic activity (i.e., photosynthesis) that significantly increases the pH of the overall weathering solution (Welch and Ullman, 1999; Montross et al., 2013; Olsson-Francis et al., 2012; Olson-Francis et al., 2017). Solute fluxes observed in the Antarctic McMurdo Dry Valleys (Gooseff et al., 2002; Marra et al., 2017; Stumpf et al., 2012) exceed expected abiotic weathering fluxes, which previous studies have attributed to microbial weathering (i.e., Lyons et al, 2015), as well as abiotic factors such as production of fresh high surface area silicates via glacial grinding priming them for chemical weathering (Anderson et al., 1997, Anderson, 2005; Stumpf et al., 2012; Marra et al., 2017). However, the effects of psychrophilic microbes in chemical weathering processes in Antarctica (as well as other glaciated settings) are not well known. Permafrost soil surface temperatures can reach 12°C due to radiative austral summer heating (Balks et al., 2002; Dolgikh et al., 2015), providing optimum growth conditions for cold-tolerant cyanobacterial mats (e.g., Kleinteich et al., 2012) that are widespread in meltwater stream banks and cryptoendolithic habitats of the topsoil (Cary et al., 2010; Cowan et al., 2010). In Chapter 2, I investigate the role of the Antarctic benthic mat-forming (non-axenic) cyanobacterium, Leptolyngbya glacialis, on chemical weathering of (felsic-intermediate, Antarctic and basaltic, Iceland) glacial sediments at 12oC, representing permafrost surface temperatures, testing the hypothesis that microbial life increases weathering rates and solute fluxes within glacial settings. Results show silicate weathering rates in felsic sediments are three times faster with microbes than without, whereas biotic and abiotic weathering rates observed in mafic sediments are comparable, likely due to faster chemical weathering rates in basaltic sediments which directly provide nutrients to the microbes, reducing the need for direct microbial-facilitated weathering (scavenging). Results also show that microbes increase the solution pH and lead to up to four times higher bicarbonate concentration, suggesting they may play a key role in carbonate deposition in both felsic and mafic settings. Production of Fe-(hydr)oxide nano minerals and neo-formed clays may be potential inorganic biosignatures as they are closely associated with microbial biofilms, and similar phases were not observed in abiotic reactors. Note that this chapter has been published in Permafrost and Periglacial Processes (Demirel-Floyd et al., 2022), partially fulfilling doctoral degree requirements for the OU School of Geosciences.

Cyanobacteria have also played important roles within Earth history such as atmospheric oxygenation and the evolution of multicellular life (Lyons et al., 2014), while also surviving across multiple climatic extremes such as the Neoproterozoic “Snowball Earth” episodes (Hoffman et al., 1998; Fairchild and Kennedy, 2007; Ye at al., 2015; Brocks et al., 2017; Shizuya et al., 2021). These resilient organisms also are known to endure multiple environmental extremes (UV radiation, desiccation, salt, cold, etc.) within Antarctic glacial habitats (Gilichinsky et al., 2007; Cary et al., 2010; Cowan et al., 2010, Anesio and Laybourn-Parry, 2012), where they lead the primary production and play a fundamental role in Antarctic biogeochemical cycles (McKnight et al., 2004; Smith et al., 2017). Antarctic cyanobacterial mats also increase weathering rates, and therefore impact nutrient fluxes at warmer surface soil temperatures (12oC), as described in Chapter 2 (Demirel-Floyd et al., 2022). Though they are widespread in cold meltwater streams (McKnight et al., 1999, 2004; Van Horn et al., 2016), the role of cyanobacterial mats in cold temperature weathering is not well known. In Chapter 3, I investigate biotic and abiotic silicate weathering rates and nutrient release at different temperatures (4°C and 12°C) and nutrient conditions (10 and 1000 times diluted), using the same felsic-mixed sourced Antarctic glaciofluvial sediments and basaltic-sourced Icelandic glacio-volcanic outwash sediments used in Chapter 2, testing the hypothesis that polyextremophilic cyanobacterial weathering rates increase under colder and nutrient-stressed conditions via enhanced production of extracellular polymeric substance (EPS) resulting in release of organic acids. Results show limited evidence of biological weathering of silicate minerals at cold temperature, yet microbe-mineral interactions still affect nutrient concentrations, particularly for Ca, Mg, Mn, P and N. However, increased nutrient and salt concentrations also increased the rate of solute release from the silicate sediments, even under abiotic conditions. These results indicate that concentration and chemistry of weathering solutes (salts) are important factors controlling weathering rates and nutrient fluxes in cold settings. This chapter will soon be submitted to Geomicrobiology Journal for initial peer review.