Aragonite is calcite’s best friend at the seafloor

Aragonite is about 50% more soluble than calcite in seawater and its pelagic production is dominated by pteropods. Moreover, it could account for a large fraction of marine CaCO3 export. The aragonite compensation depth (ACD, the depth at which accumulation is balanced by dissolution) is generally very close to the aragonite saturation depth, i.e. within a few hundred metres. Conversely, the calcite compensation depth (CCD) can be 1-2 kilometres deeper than the calcite saturation depth. That aragonite disappears shallower than calcite in marine sediments is coherent with aragonite&#8217;s greater solubility, but why is the calcite lysocline, i.e. the distance between its compensation and saturation depths, much thicker than its aragonite equivalent?Here, we suggest that at the seafloor, the addition of a soluble CaCO3 phase (aragonite) results in the preservation of a predeposited stable CaCO3 phase (calcite), and term this a negative priming action. In soil science, priming action refers to the increase in soil organic matter decomposition rate that follows the addition of fresh organic matter, supposedly resulting from a globally increased microbial activity (Bingeman et al., 1953). Using a new 3D model of CaCO3 dissolution at the grain scale, we show that a conceptually similar phenomenon could occur at the seafloor, in which the dissolution of an aragonite pteropod at the sediment-water interface buffers the porewaters and causes the preservation of surrounding calcite. Since aragonite-producing organisms are particularly vulnerable to ocean acidification, we expect an increasing calcite to aragonite ratio in the CaCO3 flux reaching the seafloor as we go further in the Anthropocene. This could, in turn, hinder the proposed aragonite negative priming action, and favour chemical erosion of calcite sediments.&#160;Reference:&#160;Bingeman, C.W., Varner, J.E., Martin, W.P., 1953. The Effect of the Addition of Organic Materials on the Decomposition of an Organic Soil. Soil Science Society of America Journal 17, 34-38.

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Recommendations for isolation of humin fraction from soil material

10.5194/egusphere-egu21-8315 ◽

2021 ◽

Author(s):

Jerzy Weber ◽

Elżbieta Jamroz ◽

Andrzej Kocowicz ◽

Magdalena Debicka ◽

Jakub Bekier ◽

...

Keyword(s):

Organic Matter ◽

Soil Analysis ◽

Spectroscopic Properties ◽

Soil Science ◽

Spectroscopic Techniques ◽

Science Center ◽

Soil Science Society ◽

Mineral Fraction ◽

Freeze Dried ◽

Humin Fraction

Methods of isolation of the humin fraction can be divided into two main groups: (1) extraction of humic (HA) and fulvic (FA) acids followed by extraction of humin with different organic solvents, and (2) extraction of HA and FA followed by removal of soil mineral fraction. To isolate the large amounts of humin necessary to study the interactions of this fraction with pesticides, we examined some modifications of the latter method.The first step was to separate HA and FA according to a modified IHSS method (Swift 1996). HA and FA were extracted with 0.1 M NaOH with a 5:1 ratio of extractant to soil. 20 hours shaking was found to be more effective, but 4 hours shaking provided the advantage of being able to extract twice a day, &#160;which ultimately shortened the procedure time.The HA and FA free residue was then digested to remove mineral components. We used several (up to 8 weeks) digestions with 10% HF/HCl as higher concentrations of HF can result in structural alteration of the organic compounds (Hayes et al. 2017). While HF/HCl treatment can lead to hydrolysis and loss of polysaccharide and protein materials (Stevenson 1994), the advantage of using HF is the removal of paramagnetic compounds (such as Fe), which facilitates the use of spectroscopic techniques to characterize humin. In contrast to the procedures for only increasing the concentration of organic matter (Schmidt et al. 1997), the sample was digested until the mineral fraction not complexed with humin was completely digested. We tested different modes of mineral fraction digestion in 10% HF/HCl using polyethylene centrifuge bottles. Occasional shaking once a day had the same effect as continuous shaking. It takes 6 weeks to digest 200 g of pure sand in a 1000 cm3 bottle, when the HF/HCL was weekly replaced. After replacing HF/HCl every 2 weeks, the digestion time of the same material increased to 8 weeks.After treatment with HF/HCl, the residue was rinsed with 10% HCl to remove secondary minerals. The residue was washed with distilled water until the neutral pH and then dialyzed to a negative Cl&#8722; test with AgNO3. Then the humin fraction was freeze dried.&#160;&#160;LiteratureHayes M.H.B., Mylotte R., Swift R.S. 2017. Humin: Its Composition and Importance in Soil Organic Matter. In: Sparks D.L. (ed) Advances in Agronomy, Vol. 143, Academic Press, Burlington, 47&#8211;138.Schmidt, M.W.I., Knicker, H., Hatcher, P.G., K&#246;gel-Knabner, I. 1997. Improvement of 13C and 15N CPMAS NMR spectra of bulk soils, particle size fractions and organic material by treatment with 10% hydrofluoric acid. European Journal of Soil Science, 48, 319-328.Stevenson F.J. 1994. Humus Chemistry; Genesis, Composition, Reaction. 2nd ed. John Wiley & Sons., New York.Swift R.S. 1996. Organic matter characterization. In: Sparks, D.L., et al. (Ed.), Methods of Soil Analysis. Part 3. Chemical Methods - Soil Science Society of America, Book Series no 5,&#160; 1011-1069.&#160;AcknowledgementsThis work was supported by the National Science Center (NCN) Poland (project No 2018/31/B/ST10/00677 &#8220;Chemical and spectroscopic properties of soil humin fraction in relation to their mutual interaction with pesticides").

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