Loyd S J
Department of Geological Sciences, California State University, Fullerton, CA, USA.
Geobiology. 2017 Jan;15(1):146-157. doi: 10.1111/gbi.12194. Epub 2016 Jul 6.
Concretions are preferentially cemented zones within sediments and sedimentary rocks. Cementation can result from relatively early diagenetic processes that include degradation of sedimentary organic compounds or methane as indicated by significantly C-depleted or enriched carbon isotope compositions. As minerals fill pore space, reduced permeability may promote preservation of sediment components from degradation during subsequent diagenesis, burial heating and outcrop weathering. Discrete and macroscopic organic remains, macro and microfossils, magnetic grains, and sedimentary structures can be preferentially preserved within concretions. Here, Cretaceous carbonate concretions of the Holz Shale are shown to contain relatively high carbonate-free total organic carbon (TOC) contents (up to ~18.5 wt%) compared to the surrounding host rock (with <2.1 wt%). TOC increases with total inorganic carbon (TIC) content, a metric of the degree of cementation. Pyrite contents within concretions generally correlate with organic carbon contents. Concretion carbonate carbon isotope compositions (δ C ) range from -22.5 to -3.4‰ (VPDB) and do not correlate strongly with TOC. Organic carbon isotope compositions (δ C ) of concretions and host rock are similar. Thermal maturity data indicate that both host and concretion organic matter are overmature and have evolved beyond the oil window maturity stage. Although the organic matter in general has experienced significant oxidative weathering, concretion interiors exhibit lower oxygen indices relative to the host. These results suggest that carbonate concretions can preferentially preserve overmature, ancient, sedimentary organic matter during outcrop weathering, despite evidence for organic matter degradation genetic mechanisms. As a result, concretions may provide an optimal proxy target for characterization of more primary organic carbon concentrations and chemical compositions. In addition, these findings indicate that concretions can promote delayed oxidative weathering of organic carbon in outcrop and therefore impact local chemical cycling.
结核是沉积物和沉积岩中优先被胶结的区域。胶结作用可能源于相对早期的成岩过程，包括沉积有机化合物或甲烷的降解，这可由显著贫碳或富碳的碳同位素组成表明。随着矿物填充孔隙空间，渗透率降低可能有助于在随后的成岩作用、埋藏加热和露头风化过程中保存沉积物成分不被降解。离散的宏观有机残骸、宏观和微观化石、磁性颗粒以及沉积构造可以优先保存在结核中。在这里，霍尔兹页岩的白垩纪碳酸盐结核与周围的母岩相比（母岩总有机碳含量<2.1 wt%），显示出相对较高的无碳酸盐总有机碳（TOC）含量（高达约18.5 wt%）。TOC随总无机碳（TIC）含量增加，TIC含量是胶结程度的一个指标。结核中的黄铁矿含量通常与有机碳含量相关。结核碳酸盐碳同位素组成（δ¹³C）范围为-22.5至-3.4‰（VPDB），与TOC没有很强的相关性。结核和母岩的有机碳同位素组成（δ¹³C）相似。热成熟度数据表明，母岩和结核中的有机质都处于过成熟状态，并且已经演化到超过油窗成熟阶段。尽管总体上有机质经历了显著的氧化风化，但结核内部相对于母岩表现出较低的氧化指数。这些结果表明，尽管有证据表明存在有机质降解的成因机制，但碳酸盐结核在露头风化过程中可以优先保存过成熟的古老沉积有机质。因此，结核可能为表征更原始的有机碳浓度和化学成分提供一个最佳的替代目标。此外，这些发现表明结核可以促进露头中有机碳的延迟氧化风化，从而影响局部化学循环。
