Spatial Distribution of Features: Spheroids

Diagenetic concretions are masses of minerals formed by diagenesis (the process of sediment turning into sedimentary rock), and can be formed in situ or weathered out. They vary in morphology: they may have visible sedimentary laminations (fine discrete layers), and they can be spherical, discoidal, or elongate in shape. They do not displace sedimentary layers. Their interiors may be solid, rinded, or layered.

Spherical concretions can form in self-organized (non-random) or random spacing within the three dimensions of the host rock, and their spatial distributions are linked to how the spheroids formed.

Clusters of concretions (elongate along sedimentary laminae) can be biotic when nucleated on organics concentrated on bedding planes. Also, clusters of concretions are observed on organic nuclei such as preserved bones. Irregular shapes can also preserve biotic nuclei, such as entombing shells or other fossils. 

Created at: 
2021-09-22
Updated at: 
2022-08-23
Curator: 
Svetlana Shkolyar
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Definition
PRO
CON

In mudrocks, where fluid flow (and thus reactant transfer) is restricted, spheroidal concretions can have diverse mineralogies between concretions within an outcrop and may be more likely to involve biomediated reactions.
[Contributor: Sally Potter-McIntyre] [Congruence, General]

Environments
Mars: Surface
e

Spheroidal concretions often  result in biologically-involved precipitation [Sally Potter-McIntyre]


eEvidence for mudrock‐hosted carbonate concretions being biologically mediated [Contributor: Sally Potter-McIntyre]

eBioturbation influenced the morphology of concretions in the Morrison Formation.
[Contributor: Sally Potter-McIntyre]

Clusters of concretions can form on organic precursors such as bones, shells, or small masses of organic material. 
[Contributor: Sally Potter-McIntyre]
 
[Congruence, General]

Environments
Exoplanets: Surface, Mars: Surface, Mars: Subsurface
Reviewer Andy Czaja: " This argument seems to state that the features have a biological origin because they form on/around biological precursors. But the abiotic signal strength argument pointed out concretions that had self-organized spatial arrangement were abiological, even though they nucleated on organic material in mudrocks. This seems contradictory, unless I am missing something."
eConcentric growth occurs when a organic matter nucleates the precipitation reaction. 

eCarbonate concretions typically form around some nucleus (a shell fragment, organic matter or a grain).

eMudrock‐hosted carbonate concretions may form in the presence of organic material like soft body remains [Sally Potter-McIntyre]

 
Definition
PRO
CON

Concretions are easily preserved because of the preferential cementation that renders them harder than the surrounding host rock. If the host rock is sufficiently denuded, the concretions are typically still preserved and collected in topographic low spots. 
[Contributor: Sally Potter-McIntyre]
 
[Survivability, General]

Environments
Exoplanets: Surface, Mars: Surface, Mars: Subsurface
eIn lab studies of iron oxide concretion formation, the precipitation of hydrated iron sulfate, driven by the increasing pH, caused the greenish rind to become red and harder. [Sally Potter-McIntyre]

 
 

When the host rock is denuded, biotic concretions accumulate in topographic lows and loose their original, source-diagnostic spatial distribution.

[General]

Environments
Mars: Surface
eCopper Harbor Formation structures are examined and interpreted as abiotic concretions because biogenic origin hypothesis is rejected. Conglomerates examined accumulated in a relative topographic low area.

Definition
PRO
CON

Self-organized spacing, measured as the distribution on two-dimensional surfaces of neighboring spheroids, is a common feature of abiotic spheroidal concretions. This is because they form in near stagnant water tables where precipitation is triggered by supersaturation within a pore space, where spacing is controlled by chemical self-organization, abundance of reactants, rate of reactant transfer, and time.
[Contributor: Sally Potter-McIntyre] [Congruence, General]

Environments
Exoplanets: Surface, Mars: Surface, Mars: Subsurface
eNavajo sandstone concretions display self-organization, attributed to abiotic reactions 

eAbiotic models of end-members of macro- and micro-concretions are described [Sally Potter-McIntyre]

eConcretions studied showed evidence of periodic self-organization through diffusion of Fe and O. [Sally Potter-McIntyre]

 
Definition
PRO
CON

With certain, more soluble, minerals, concretions may be dissolved while still in the subsurface, for example, carbonate concretions in an acidic fluid, complicating detection. 
[Contributor: Sally Potter-McIntyre]
 
[Survivability, General]

Environments
Exoplanets: Surface, Mars: Surface, Mars: Subsurface
eWith certain, more soluble, minerals, concretions may be dissolved while still in the subsurface, for example, abiotic carbonate concretions in an acidic fluid.  [Sally Potter-McIntyre]

Concretions are easily preserved in some cases even if the host rock is denuded.  

[Survivability]

Environments
Mars: Surface
ePaleogeomorphology of the Ordovician Ordos Basin is described and concretions are noted.

 
 

In self-organized spheroidal concretions, which are formed abiotically, spacing is random and no obvious biosignatures are preserved.  [Contributor: Sally Potter-McIntyre]
 
[Congruence, General]

Environments
Exoplanets: Surface, Mars: Surface, Mars: Subsurface
From reviewer Hank Rainwater: "I assume that self-organization leads to random spacing, but does this imply that spacing other than random is controlled by some other process that is relevant to the argument?"
eLack of obvious self-organized spacing suggests a random nucleation on organics which provide kinetically favorable nucleation sites. Self-organized nucleation occurs via diffusion.

With certain, more soluble, minerals, concretions may be dissolved while still in the subsurface [Survivability]

Environments
Mars: Surface
eCertain soluble, minerals, concretions may be dissolved while still in the subsurface. Carbonate concretions in an acidic fluid are exemplified here.