6af). Composition and texture of displacive layers in carbonate concretions of Xiamaling Formation (Jizhentun section). This was possible only with the bacterial colonization and the right sedimentation rate. For instance, calcite with high Mn/Fe ratio usually dominates in a Mn reduction zone (e.g. The growth of carbonate concretions in dark shale is most commonly estimated to take place over a time-range of 103 to >106years (Sells-Martnez 1996). 7g). To better elucidate recent advance and divergence in the study of the Xiamaling Formation, different subdivision schemes from the outcrop and drillcore successions for this formation are compared in Fig. Eiler, and F.A. Trefry et al. Concretion morphology, classification and genesis. Whiticar, M.J. 1999. Sells-Martnez 1996; Loyd et al. Dong-Jie Tang. 1.351.40Ga) of North China Platform, abundant carbonate concretions occur in a ~6-m-thick interval of organic-poor green silty shale (Figs. Minerals as well as synthetic phases (MINM2553) were used as reference standards. a Sparsely distributed carbonate concretions in green-yellow silty shale; b Densely packed carbonate concretions in green-yellow silty shale; c Concretions wrapped by green silty shale laminae (arrows); d A concretion with sharp contact with the surrounding and lateral shale laminations; e and f Two concretions with lateral shale laminae (dash lines and arrows), and the laminae thickness in concretions is expanded by an order of magnitude relative to the laminae in adjacent shales. Marine Geology 113 (12): 2740. Brennan, L.A. Hardie, and R.V. 2013). Springer Nature. 2004). 2018), while pyrite commonly concentrates in a sulfate reduction zone (e.g. Guo et al. 2004; Meyer et al. These concretions range in size from a few millimeters to as much as 0.7m (2.3ft) in length and 12cm (0.39ft) in thickness. In recent years, multiple approaches have been used to decipher the potential of microbial processes for their formation, of which analysis of mineralogy, CS isotopes and REE are principal aspects (e.g. Journal of the Geological Society 157: 187200. One possibility is that dehydration hardens the outer shell of the concretion while causing the interior matrix to shrink until it cracks. In the middle part of the Member IV of the Xiamaling Formation (ca. 2016. Geochimica et Cosmochimica Acta 52 (5): 12031219. Zhang, S.C., X.M. Chemical Geology 291: 152165. [60] Somewhat weathered and eroded giant cannonball concretions, as large as 6 meters (20 feet) in diameter, occur in abundance at "Rock City" in Ottawa County, Kansas. 6c and d). Iams, T.E. Bau, M., and P. Dulski. Zhang et al. The laminae in R-horizons could be laterally extended into adjacent shales, while the D-horizons, vertically alternated with R-horizons, could not be extended into the adjacent shales, which is the simple marks for identification of R-horizons and D-horizons. [44] Whether the initial cementation was concentric or pervasive, there is considerable evidence that it occurred quickly and at shallow depth of burial. Zhao, and H. Weissert. carbonate calcium concretion strange modified disc scienceblogs feldt advisor lotta stergtland setting track thanks museum county right Planavsky, N.J., C.T. 8; Haley et al. 2012. 2012), ankerite and siderite more often occur in an iron reduction and methanogenesis zone (e.g. They are commonly several centimeters in thickness, but in the R-horizon dominated intervals some laterally continuous D-horizons are quite thin (<1mm) (Fig. Zhang, S.C., X.M. Journal of the Geological Society 157: 239251.

Evans, H.C. Wu, H.Y. 5c and d) has been interpreted as a primary depositional feature, and their preservation could be ascribed to passive porefilling carbonate growth prior to the significant compaction of clays (e.g. Although reasons for this have not been fully addressed, low concentrations of nitrate, nitrite, and Mn-hydroxides in sediments of most environments were often quoted as the potential causes. 5c and d), and calcite geodes in D-horizons (Fig. 2017, 2018). In the Connecticut River Valley, these concretions are often called "claystones" because the concretions are harder than the clay enclosing them. In most of the samples, calcite in D-horizons exhibits dull orange or black color under CL (Fig. 2004; Gutjahr et al. Kansas pop rocks are concretions of either iron sulfide, i.e. Almost all the calcite in the concretions has low MnFe in nuclei but high MnFe in rims with average Mn/Fe ratio close to 3.3. The oxic degradation of sedimentary organic matter 1400 Ma constrains atmospheric oxygen levels. Earth and Planetary Science Letters 394: 3037. Thamdrup, B., H. Fossing, and B.B. Shi, X.Q. Roberts, A.P., and R. Weaver. However, among these processes, denitrification and Mn reduction processes have been commonly identified in marine sediments (Haley et al. The interval studied in this paper is in the Member IV or Unit 1. Dong, J., S.H. Also, when they are either cut or hammered, they produce sparks and a burning sulfur smell. 1989; German and Elderfield 1990; Johnson et al. 2006). As the MnFe contents in the nuclei are lower than those in the rims and are black in color under CL, we could further deduce that the calcite precipitation was initiated in the nitrate reduction zone above the Mn reduction zones (Fig.

As shown in Table1 and illustrated in Fig. Tang et al. Orbital forcing of climate 1.4 billion years ago. Connelly, B.M. -110 to 30; Whiticar 1999), has been recognized as the signature for AOM (e.g. Contrary to what has been published on the Internet, none of the iron sulfide concretions, which are found in the Smoky Hill Chalk Member were created by either the replacement of fossils or by metamorphic processes. 1.401.35Ga), North China, however, carbonate concretions occur in organic-poor green silty shales (avg. Various microbial processes in suboxic to anoxic zones of marine sediments have the ability to induce the formation of carbonate concretions by increasing the environmental alkalinity and pH (Curtis et al. Canfield, D.E., and B. Thamdrup. 4c). 2017, 2018). [61] The Moeraki Boulders, Ward Beach boulders and Koutu Boulders of New Zealand are examples of septarian concretions, which are also cannonball concretions. [43][44][38] For example, if the porosity after early cementation varies across the concretion, then later cementation filling this porosity would produce compositional zoning even with uniform pore water composition. 2014; Gaines and Vorhies 2016). Subsequent growth likely occurred in the Mn reduction zone with slightly increased burial depth, therefore the high MnFe rims were formed. Zhou, and Q. Shi.

Mn/Fe ratios are enriched in our samples compared with those of average global carbonate (0.29; Turekian and Wedepohl 1961) and of manganous samples (Tostevin et al. Curtis, C.D., M.L. 2012; Dong et al. Bau, M., P. Mller, and P. Dulski. 2017a). Bjerrum, E.U. For example, in modern oceans the average sulfate concentration is as high as 28mM (Lowenstein et al. Lithology and major sedimentary structures in the middle part of the Member IV of the Xiamaling Formation (Jizhentun section). The 1.35 Ga diabase sills from the northern North China craton: Implications for breakup of the Columbia (Nuna) supercontinent. Their reduction and dissolution would significantly influence the REE composition of pore fluids and could subsequently be recorded by the carbonate concretions formed in these zones (Haley et al. [45][46][47][38] In many cases, there is clear evidence that the initial concretion formed around some kind of organic nucleus. When the iron-bearing groundwater came into contact with more oxygen-rich groundwater, the reduced iron was converted back to insoluble iron oxide, which formed the concretions. Geo-Marine Letters 4 (2): 125130. 8). 1994; Haley et al. Rare earth elements in the water column of Lake Vanda, McMurdo Dry Valleys, Antarctica. 2004; Luo et al. Concretions as agents of soft-tissue preservation: A review. Bottrell, S.P. Evidence for surface reaction-controlled growth of carbonate concretions in shales. Isotopic studies of mudrock-hosted concretions from rock record show that they are commonly characterized by conspicuously depleted and variable 13C signatures that can reach low values of 10 (e.g. 3e and f). 2012; Dong et al. Application of the Ce anomaly as a paleoredox indicator: The ground rules. 2016. German, C.R., and H. Elderfield.

2006; Loyd et al. 7h; Table 2) also support the idea that calcite precipitation was facilitated by Mn reduction in the sediments, while the slight MREE enrichment indicates a partial contribution from iron reduction (Fig. Growth mechanisms and environmental implications of carbonate concretions from the ~1.4Ga Xiamaling Formation, North China.

They are found throughout the fossil record but are most common during periods in which calcite sea conditions prevailed, such as the Ordovician, Jurassic and Cretaceous. Additional fractures formed during subsequent episodes of shallow burial (during the Cretaceous) or uplift (during the Paleogene). Loucks, K.L. Curtis et al. [42], It is commonly thought that concretions grew incrementally from the inside outwards. Canfield. They are found where submarine erosion has concentrated early diagenetic concretions as lag surfaces by washing away surrounding fine-grained sediments. 2001. They are sparse in some layers (Fig. Reinhard, and N.J. Planavsky. The word 'concretion' is derived from the Latin concretio "(act of) compacting, condensing, congealing, uniting", itself from con meaning 'together' and crescere meaning "to grow". Decho, D.H. Buckley, J.R. Field emission scanning electron microscope, Inductively Coupled Plasma Mass Spectrometry. Raiswell, R., S.H. 2016; Tang et al. Bjerrum, E.U. In the Member IV of the Xiamaling Formation (ca. Chemical Geology 161 (13): 291314. [18] They appear in nodular patches, concentrated along bedding planes,[18] or protruding from weathered cliffsides. Carbonate concretions provide unique records of ancient biogeochemical processes in marine sediments, and have the potential to reflect seawater chemistry indirectly. Raiswell, R. 1988. If this deduction is the case, many similar concretions formed in nitrate and Mn reduction zones with seawater-like carbon isotope composition should exist in the Mesoproterozoic strata and further studies are required to test this explanation. [46][38], One model for the formation of septarian concretions in the Staffin Shales suggests that the concretions started as remirigid masses of flocculated clay. Precambrian Research 225: 7785. 2016. [34][35] Septarian nodules are characteristically found in carbonate-rich mudrock. The study was supported by the National Natural Science Foundation of China (No. 2002. Biogeosciences 14 (8): 21332149.