The micromorphology of Younger Dryas-aged black mats from Nevada, Arizona, Texas and New Mexico

Abstract

Black mats are organic-rich sediments and soils that form in wet environments associated with spring discharge. Micromorphological and geochemical analyses of 25 black mats dating to the Younger Dryas Chronozone (12.9–11.7 ka) and early Holocene were conducted to determine their composition and depositional environment. Samples were collected from Arizona, New Mexico, Texas and Nevada. Micromorphological analyses were conducted on thin sections using polarized and blue fluorescent light. These analyses determined that black mats contain humic acids, fine (5–20 μm) plant fragments, diatoms, phytoliths, and gastropods. The dominant type of organic matter in black mats is derived from herbaceous plants, contradicting previous studies that supported algal or charcoal sources. Differences in the micromorphological characteristics of the samples revealed that black mats formed as three different types, organic horizons, moist soils and ponded sediments, depending on their topographic position in relation to the water table. The microscopic evidence found in black mats supports the presence of widespread wet environments in Nevada and Arizona during the Younger Dryas Chronozone, clearly indicating a sustained period of greater effective moisture, optimal for spring discharge and black mat formation.

Comprehensive analysis of nanodiamond evidence relating to the Younger Dryas Impact Hypothesis

“…These and other similar deposits have been described as organic-rich, silty sediments (e.g. Brakenridge, 1981;Quade et al, 1998;Baker et al, 2008), although their total organic carbon content varies (most cases <5 wt%) and is not correlated with sediment texture and color (see Pinter et al, 2011;Harris-Parks, 2014. In fact, such deposits vary in color and white-colored diatomites have been described as 'black mats' (Haynes, 2008).…”

Sediments from around the lower younger dryas boundary (se arizona, usa): implications from la‐icp‐ms multi‐element analysis

“…Therefore, it is appropriate to suggest that if the microscopic charcoal fragments are responsible for the described trace element features, such fragments are present only in the LYDB sediments, but not in the black mat (cf. Harris‐Parks ). Such microscopic fragments, according to geochemical characteristics, are not present in the analyzed samples of the Clovis channel sand either.…”

“…This black mat is a very reliable stratigraphic marker dating the Allerød‐Younger Dryas transition. The major hypotheses on the origin of the black mat suggest that it was formed by either water‐transported organic material, or as the result of a heavy deposition of algae in a shallow fresh‐water reservoir (see Haynes , and Harris‐Parks for more references), or in response to periods of spring‐fed stream activation when groundwater oxidized the organic material (Quade et al . ; Harris‐Parks ).…”

“…the black mat (e.g. Haynes 2007Haynes , 2008Pigati et al 2009Pigati et al , 2012Ballenger et al 2011;Harris-Parks 2016). Overall, the black mat is a general term that includes, in addition to the dark organic-rich deposits, some Younger Dryas marls and diatomites that are light grey in color (e.g.…”

“…This black mat is a very reliable stratigraphic marker dating the Allerød-Younger Dryas transition. The major hypotheses on the origin of the black mat suggest that it was formed by either water-transported organic material, or as the result of a heavy deposition of algae in a shallow fresh-water reservoir (see Haynes 2007Haynes , 2008 and Harris-Parks 2016 for more references), or in response to periods of spring-fed stream activation when groundwater oxidized the organic material (Quade et al 1998;Harris-Parks 2016). However, the reported presence of charcoal (Haynes 2007), soot, polycyclic aromatic hydrocarbons and glass-like carbon in or around the lower Younger Dryas boundary (LYDB) allowed some authors to suggest extensive wildfires and decomposition of charred wood as a response to a short and probably catastrophic event Marlon et al 2009;Mahaney et al 2010).…”