Preservation of overmature, ancient, sedimentary organic matter in carbonate concretions during outcrop weathering

Abstract

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.

Microbially mediated fossil concretions and their characterization by the latest methodologies: a review

“…Stable carbon isotope ratios of bulk organic matter ( δ 13 C org ) can be measured after removal of carbonate by reaction with dilute acid. Similarities in δ 13 C org of Upper Cretaceous Holz Shale concretions (California) and host sediment was interpreted as evidence that OM was not strongly degraded during concretion formation ( Loyd, 2017 ). Tripp et al (2022) compared δ 13 C org with compound-specific δ 13 C values of organic compounds from a Mazon Creek concretion (Illinois) preserving a coprolite (see below).…”

A Perspective on Concretions: Deciphering Diagenesis from Earth to Mars

“…Isotopic trends are helpful for understanding the parent water compositions, such as oxygen and carbon isotopes in carbonate concretions (see summary of Mozley and Burns, 1993) as well as understanding the potential role of carbonate concretions in the marine carbon cycle (Gaines and Vorhies, 2015;Loyd, 2017;Loyd et al, 2020). Sulfur isotope studies of pyrite concretions indicate the role of bacterial sulfate reduction (e.g., Coleman and Raiswell, 1981;Strauss and Schieber, 1990;Khan et al, 2022).…”

Formation of large carbonate concretions in black cherts in the Gufeng Formation (Guadalupian) at Enshi, South China

“…Well‐preserved carbonate concretions recorded the original nature of the sediments and can thus be regarded as time‐capsules, preserving the “memory” of the sediments (Dale, John, Mozley, Smalley, & Muggeridge, ; El Albanni, Vachard, Kuhnt, & Thurow, ). As a result, they can provide insights into the geochemical evolution of pore waters in the host rocks during burial and diagenesis (Coleman & Raiswell, ; Curtis & Coleman, ; Dale et al, ; Loyd, ). The growth of carbonate concretions is a passive infill process, where host sediment porosity is replaced by concretionary cement (Raiswell, ).…”