Biochemically induced diagenesis of Jurassic micrite: evidence from phase analysis, carbon, oxygen, and strontium isotopes (Franconian Alb, Germany)

IF 1.8 3区地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY International Journal of Earth Sciences Pub Date : 2024-06-25 DOI:10.1007/s00531-024-02431-9

Wolfgang Blendinger

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Abstract

The marine Upper Jurassic rocks of the Franconian Alb consist largely of micritic carbonate of partly dolomitized reef mounds and bedded basinal limestone. All carbonates were lithified in the shallow (centimeters, meters) subsurface and have a wide range of ∂¹³C (≤ + 3‰ to − 10‰VPDB) but always negative ∂¹⁸O (− 1 to − 6‰VPDB). Dolomite and reef limestone show the highest ∂¹⁸O and ∂¹³C values. The most negative ∂¹³C (≥ − 10‰) occurs mainly as cement in dolomite of a basinal, partly dolomitic, biostrome interval. Basinal limestone shows intermediate ∂¹³C values. Because freshwater diagenesis and elevated temperatures cannot explain the observed isotope values, pH is here considered a major factor influencing the isotope signal of micritic limestone. The bulk sediment isotope signal was reset to lower values, from an original lime mud with ∂¹³C ≥ 3‰ and a ∂¹⁸O of ≥ + 1‰, as a result of biochemically induced diagenesis. Carbonate, probably mostly aragonite but occasionally including dolomite, was dissolved in a zone where low pH developed as a result of organic matter degradation. Dissolved carbonate was translocated by diffusion and re-precipitated as cement (ca. 50vol%) in a zone with elevated pH where all in situ lime mud ∂¹⁸O was reset. Imported cement carbonate precipitated in equilibrium with the pore fluid with negative isotope values, whereas ∂¹³C of the in situ lime mud remained unmodified. The negative shift of the bulk ∂¹³C and ∂¹⁸O is variable and depends on pH and the contribution of ¹²C from anaerobic sulfate reduction in the zone of cement precipitation. This produced an ubiquitous covariance of ∂¹⁸O and ∂¹³C. Incorporation of seawater-derived Mg²⁺ during recrystallization of carbonate can account for the local dolomitization. Elevated ⁸⁷Sr/⁸⁶Sr ratios are explained as a result of interaction of clay minerals with the stationary pore fluids. This study shows that the isotopic signal produced by biochemically induced shallow submarine subsurface carbonate diagenesis can be indistinguishable from freshwater diagenesis, that ∂¹⁸O and ∂¹³C of the bulk rock are always reset, and that carbonates can show, in the presence of clay minerals, elevated ⁸⁷Sr/⁸⁶Sr ratios even when the pore fluids were never exchanged.

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侏罗纪微晶岩的生物化学诱导成岩作用：相分析、碳、氧和锶同位素提供的证据（德国弗兰肯阿尔布地区）

摘要弗兰肯阿尔布的海相上侏罗统岩石主要由部分白云石化的礁丘和层状基底石灰岩的微晶碳酸盐组成。所有碳酸盐都是在浅层（厘米、米）次表层形成的，∂13C 的范围很广（≤ + 3‰ 至 - 10‰VPDB），但∂18O 始终为负值（- 1 至 - 6‰VPDB）。白云岩和珊瑚礁石灰岩的 ∂18O 和 ∂13C 值最高。最负∂13C（≥ - 10‰）主要出现在基性部分白云质生物群落的白云岩胶结物中。基性石灰岩显示出中等的 ∂13C 值。由于淡水成岩作用和温度升高无法解释观测到的同位素值，因此 pH 值被认为是影响微晶质石灰岩同位素信号的主要因素。由于生化诱导的成岩作用，大量沉积物的同位素信号被重置为较低的值，原始石灰泥的∂13C ≥ 3‰，∂18O ≥ + 1‰。碳酸盐（可能主要是文石，但偶尔也包括白云石）溶解在因有机物降解而形成低 pH 值的区域。在 pH 值升高的区域，溶解的碳酸盐通过扩散转移，并重新沉淀为水泥（约 50vol%）。进口的水泥碳酸盐与孔隙流体平衡沉淀，同位素值为负，而原地石灰泥浆的∂13C仍未改变。大量 ∂13C 和 ∂18O 的负偏移是可变的，取决于 pH 值和水泥沉淀区厌氧硫酸盐还原产生的 12C 的贡献。这就产生了 ∂18O 和 ∂13C 的普遍共变。碳酸盐再结晶过程中海水衍生的 Mg2+ 的掺入可以解释局部白云石化的原因。87Sr/86Sr 比值升高的原因是粘土矿物与固定孔隙流体相互作用的结果。这项研究表明，生化诱导的浅海次表层碳酸盐成岩作用产生的同位素信号与淡水成岩作用无法区分，大块岩石的∂18O和∂13C总是重置的，而且在粘土矿物存在的情况下，即使孔隙流体从未发生交换，碳酸盐也会显示出87Sr/86Sr比率升高。

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来源期刊

International Journal of Earth Sciences 地学-地球科学综合

CiteScore

4.60

自引率

4.30%

发文量

120

审稿时长

4-8 weeks

期刊介绍： The International Journal of Earth Sciences publishes process-oriented original and review papers on the history of the earth, including - Dynamics of the lithosphere - Tectonics and volcanology - Sedimentology - Evolution of life - Marine and continental ecosystems - Global dynamics of physicochemical cycles - Mineral deposits and hydrocarbons - Surface processes.