Origin, growth, and characteristics of calcareous concretions in the varved sediments of a Glacial Lake
Graphical abstract
Introduction
Concretions are discrete structures typically formed through the localized deposition of cement around the nucleus (McCoy, 2014) and have been universally found in different types of depositional environments of widely varying geological ages throughout the world, including sedimentary soils (e.g., deltaic deposits, marine sediments, glaciolacustrine sediments) (Dionne and Cailleux, 1972; Hight et al., 2003; Jasper, 1998; Theakstone, 1981) and sedimentary rocks (e.g., sandstones, shales, mudrocks) (Clifton, 1957; Loope et al., 2011; Mcbride and Milliken, 2006; Morad and Eshete, 1990). Usually, concretions from different kinds of sediments vary significantly in origin and have different types of cement, such as carbonates (Mavotchy et al., 2016; Sellés-Martínez, 1996), and iron oxides (Chan et al., 2005, Chan et al., 2004), as well as less common phosphate, sulfur, sulfate, and siliceous minerals (Loope et al., 2011; Pardi, 1984; Sellés-Martínez, 1996). On the other hand, concretions can record or even preserve historical changes in geochemical environments through progressive growth, providing valuable indicators of the timing of diagenetic processes upon burial (Hudson et al., 2001; McBride et al., 2003; Mozley and Davis, 2005). Therefore, understanding their formation and growth mechanisms plays an essential role in developing key insights on the geological processes and events in history, such as diagenesis and deposition, among others (Dale et al., 2014). Moreover, analysis of the concretion's geochemistry has greatly advanced the understanding of the natural environments and the degree of changes in the chemistry of early porewater.
As the most common ones, calcareous concretions are widely distributed in marine sediments, are highly enriched in carbonates compared to the surrounding host matrices, and typically stand out from their host by sharp boundaries (Berner, 1968a; Coleman, 1993; Dix and Mullins, 1987). A widely accepted speculation is that the carbonate concretions are formed through solute precipitation in the pores of host sediments that gradually bonds the host particles into hard, rock-like solid (Raiswell, 1971). Their mineralogical composition also varies considerably in different depositional environments. For example, the carbonates cover a large range, including calcite (CaCO3), dolomite (CaMg(CO3)2), siderite (FeCO3), rhodochrosite (MnCO3), and all of their mixtures. Moreover, many of the carbonate concretions are spherical or oval and contain various kinds of well-preserved fossils in their centers. In recent decades, stable carbon isotopes have been utilized to identify the source of the carbon that formed the carbonates in concretions (Curtis et al., 1972; Gautier, 1982; Hudson, 1978), and the results unequivocally showed that most concretionary carbonates are derived from microbiological oxidation of organic matter accompanied by processes such as sulfate reduction or methanogenesis (Raiswell and Fisher, 2000). The formation process of such concretions has been explained by diffusion and rapid syn-depositional reactions involving organic solutes and other porewater constituents (Yoshida et al., 2018b).
The calcareous concretions formed in the Connecticut Valley varved clay (CVVC), a glaciolacustrine deposit in the glacial Lake Hitchcock (New England, USA), have attracted the attention of geologists for centuries (Hitchcock, 1841; Sheldon, 1900; Tarr, 1935). According to the literature, when compared with other stereotypical carbonate concretions from marine sediments, most of ovaline or spherical shape, the CVVC concretions have some peculiar characteristics: (1) no traces of nucleus or fossil have ever been found in any of these concretions; (2) they have various novel morphologies; (3) they are very young and were formed after the deposition of the CVVC sediments (i.e., <15,000 years). Their distribution in the varved sediments, mode of occurrence, appearance, composition, geochemistry, and origin have long been studied (Levy, 1998; Pardi, 1984; Sheldon, 1900; Tarr, 1935). However, due to the limitations of prior experimental methods and techniques, most of the previous studies on CVVC concretions are relatively descriptive and lack data-supported evidences. Moreover, most prior works were based on a limited number of concretions and hence their results remain fragmental or incomplete. The origin and formation of these concretions are not entirely clear, with considerable confusions and contradictions, probably due to the unique varved stratification of the host sediments.
Focusing on these questions and misunderstandings, this paper presents an in-depth, comprehensive study of a large number of CVVC concretions found from undisturbed tube samples recovered from nearly the entire CVVC stratum, via multiple techniques, including X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), nanoindentation mapping, and isotope analysis, to gain a complete understanding of their composition, microstructure, mechanical properties, and isotope geochemistry and thus reveal the chemical and physical characteristics of the concretions, with a particular purpose to elucidate the formation and role of calcite cement. An integration of these results and findings provides solutions and insights to some long-standing questions about the origin, growth, and mechanisms of formations of these concretions. Moreover, an in-depth understanding of the concretion characteristics is inspiring to many interdisciplinary problems in geology and geotechnical engineering. For example, a common phenonemon in geotechnical engineering is that the properties of undisturbed soils are usually stronger than that of the remolded counterparts due to the existence of cementation among soil particles. In addition, this study can also serve as a latest reference to the analysis and characterization of the engineering properties of rocks and other cementitous maerials such as concretes.
Section snippets
Geological settings
The Connecticut Valley varved clay (CVVC), as with other varved sediments typically found in the glaciated regions of North America and Northern Europe (Zolitschka et al., 2015), was formed during the retreat of the Late Pleistocene ice sheet approximately 18,200 to 12,500 years ago (Ashley, 1975; Flint, 1956; Ridge et al., 2012; Rittenour et al., 2000). It is a lacustrine deposit in the glacial Lake Hitchcock, New England, USA. Fig. 1 shows the geographical location of Lake Hitchcock and
Concretion samples
The studied concretion samples were collected from the newly extracted CVVC samples at the US National Geotechnical Experimentation Site (NGES) (DeGroot and Lutenegger, 2003) situated on the University of Massachusetts Amherst (UMass Amherst) campus in western Massachusetts, USA (Fig. 1). In fact, these concretions were found to be abundant in the unexposed varved clay below the groundwater table but not in the oxidized crust layer near the ground surface. Their shapes and morphologies vary
Visual inspection and identification
Fig. 2 shows typical concretions found from the CVVC sediments below the groundwater table at the NGES. Visually, all concretions are hard and consolidated stones with a light grey color. Unlike their strongly weathered counterparts with brown-colored outer friable rinds that were found in other exposed CVVC sites (Levy, 1998), the concretions buried beneath the groundwater are unweathered. Moreover, they have relatively rounded surfaces as sharp boundaries from the surrounding soil matrix. The
Experimental research findings
Compositional analyses and microscopic observations show that the CVVC concretions are highly heterogeneous composites that consist of the original CVVC sediments (~40 wt%) formed via gravitational settling in the glacial lake and the post-depositionally precipitated calcite (~60 wt%) acting as the pore infills and cementation agent. In other words, these concretions directly grow in the previously-deposited CVVC by cementing the sediment particles with neoformed calcite. Due to the layered
Conclusions
This paper describes a systematic study of the characteristics of the CVVC concretions probed by a wide array of investigations, including XRD and EDS-based compositional analyses, mechanical property mapping by nanoindentation, selection dissolution, microstructure examination by optical and electron microscopy, and stable isotope analysis, with an overall goal to resolve some long-standing questions regarding their origin, growth, and other formation mechanisms. An integration of the above
Author statement
Yongkang Wu: Investigation, Methodology, Conceptualization, Writing - Original draft.
Shengmin Luo: Investigation, Writing - Review & Editing.
Dongfang Wang: Investigation, Writing - Review & Editing.
Stephen J. Burns: Investigation.
Emily Li: Writing - Review & Editing.
Don J. DeGroot: Resources, Supervision.
Yuzhen Yu: Resources, Funding acquisition.
Guoping Zhang: Supervision, Conceptualization, Methodology, Validation, Writing - Review & Editing, Funding acquisition, Resources.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was partially supported by the internal startup fund awarded to G. Zhang from the University of Massachusetts Amherst. Some of the experimental work made use of the Shared Experimental Facilities (i.e., X-ray diffractometer) at the Massachusetts Institute of Technology, supported in part by the MRSEC Program of the National Science Foundation under award number DMR-1419807.
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