Abstract:Isolated spherical carbonate concretions observed in marine sediments are fascinating natural objet trouve because of their rounded shapes and distinct sharp boundaries. They occur in varied matrices and often contain well preserved fossils. The formation process of such concretions has been explained by diffusion and rapid syn-depositional reactions with organic solutes and other pore water constituents. However, the rates, conditions and formation process of syngenetic spherical concretions are still not ful… Show more
“…After the precipitation of goethite, the pore space of the sandstone was filled, which would have decreased the diffusion coefficient of the sandstone and caused the migration rate of the reaction front to decrease ( 27 ). We performed simple numerical simulations of CaCO 3 dissolution and Fe 3+ precipitation to estimate formation time scales of Fe-oxide concretions (see the Supplementary Materials).…”
“…After the precipitation of goethite, the pore space of the sandstone was filled, which would have decreased the diffusion coefficient of the sandstone and caused the migration rate of the reaction front to decrease ( 27 ). We performed simple numerical simulations of CaCO 3 dissolution and Fe 3+ precipitation to estimate formation time scales of Fe-oxide concretions (see the Supplementary Materials).…”
“…Measurements were calibrated with rock reference samples issued by the Geological Survey of Japan (GSJ: Geochemical Reference Sample Data Base, GSJ: https://gbank.gsj.jp/geostandards/welcome.html ). The estimated analytical uncertainties were 1% to 2% for SiO 2 and CaO and 5% for other major elements 18 , 20 .…”
Section: Methodsmentioning
confidence: 95%
“…The width of the reaction front ‘L’, diffusion coefficient of SiO 2 (aq) ‘D’ and concretion growth rate ‘V’ have a relationship: ‘L = D/V’ that can be shown on a simple diagram (Fig. 4 ) 18 , 20 . Figure 4 Diffusion–growth rate cross-plot.…”
Section: Discussionmentioning
confidence: 99%
“…In a general sense, commonly observed enrichment of Si to form silica concretions has been explained by diffusion accompanied by silica super-saturation in the pore-water 3 , 15 . A related process is diffusion dominated, three dimensional migration of aqueous solutes when cementation conditions are isotropic as recognized also in many carbonate concretions 16 – 20 . Carbonate concretions that are morphologically similar to silica concretions have been found to contain organic material at their centres and micro-organisms, especially chemosynthetic anaerobes, have been shown to play an important role in concretion formation 21 – 25 .…”
Isolated silica concretions in calcareous sediments have unique shapes and distinct sharp boundaries and are considered to form by diagenesis of biogenic siliceous grains. However, the details and rates of syngenetic formation of these spherical concretions are still not fully clear. Here we present a model for concretion growth by diffusion, with chemical buffering involving decomposition of organic matter leading to a pH change in the pore-water and preservation of residual bitumen cores in the concretions. The model is compatible with some pervasive silica precipitation. Based on the observed elemental distributions, C, N, S, bulk carbon isotope and carbon preference index (CPI) measurements of the silica-enriched concretions, bitumen cores and surrounding calcareous rocks, the rate of diffusive concretion growth during early diagenesis is shown using a diffusion-growth diagram. This approach reveals that ellipsoidal SiO2 concretions with a diameter of a few cm formed rapidly and the precipitated silica preserved the bitumen cores. Our work provides a generalized chemical buffering model involving organic matter that can explain the rapid syngenetic growth of other types of silica accumulation in calcareous sediments.
“…However, some bones are partially or entirely encased in carbonate-cemented mudstone that is very hard and generally leaves the bones affected by the removal process. The formation of a carbonate case is thought to result from the release of organic constituents, particularly fatty acids, to the surrounding matrix [35,36]. This and the production of ammonia from protein degradation probably caused the precipitation of carbonate in the matrix.…”
Over twenty years of work on the Hanson Ranch (HR) Bonebed in the Lance Formation of eastern Wyoming has yielded over 13,000 individual elements primarily of the hadrosaurid dinosaur Edmontosaurus annectens. The fossil bones are found normally-graded within a fine-grained (claystone to siltstone) bed that varies from one to two meters in thickness. Almost all specimens exhibit exquisite preservation (i.e., little to no abrasion, weathering, and breakage), but they are disarticulated which, combined with our sedimentological observations, suggests that the bones were remobilized and buried after a period of initial decay and disarticulation of Edmontosaurus carcasses. Because of the large number of recovered fossil elements, we have been able to gain deeper insight into Edmontosaurus biostratigraphy including disarticulation and transport of skeletal elements. The most common postcranial elements in the bonebed are pubes, ischia, scapulae, ribs, and limb bones. By contrast, vertebrae, ilia, and chevrons are rare. The most common craniomandibular bones include dentaries, nasals, quadrates, and jugals, whereas the premaxillae, predentaries, and braincase bones are underrepresented. Thus, overall, chondrocranial and axial elements, as well as distal elements of the limbs, are distinctly underrepresented. We hypothesize that following decay and disarticulation, hydraulic winnowing removed the articulated sections (e.g., vertebral columns) and the small, distal-most elements before, or at the same time, the remaining bones were swept up in a subaqueous debris flow that generated the deposit. Comparison of the HR Bonebed with other widely dispersed Upper Cretaceous hadrosaurid-dominated bonebeds reveals many shared attributes, which suggests similar processes at work in the formation of these bonebeds across space and time. This in turn reflects similar behavior by populations of these species around the world resulting in similar modes of death, becoming interred in similar depositional settings.
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