# Geochronology/Marker horizons
## Marker beds

The "Allerød Chronozone, 11,800 to 11,000 years ago".
"Lommel (1) is in northern Belgium, near the border with the Netherlands. At 12.94 ka (2), this site was a large late Glacial sand ridge covered by open forest at the northern edge of a marsh. More than 50 archaeological sites in this area indicate frequent visits by the late Magdalenians, hunter-gatherers who were contemporaries of the Clovis culture in North America. Throughout the Bölling-Allerod, eolian sediments known as the Coversands blanketed the Lommel area. Then, just before the Younger Dryas began, a thin layer of bleached sand was deposited and, in turn, was covered by the dark layer marked "YDB" above. That stratum is called the Usselo Horizon and is composed of fine to medium quartz sands rich in charcoal. The dark Usselo Horizon is stratigraphically equivalent to the YDB layer and contains a similar assemblage of impact markers (magnetic grains, magnetic microspherules, iridium, charcoal, and glass-like carbon). The magnetic grains have a high concentration of Ir (117 ppb), which is the highest value measured for all sites yet analyzed. On the other hand, YDB bulk sediment analyses reveal Ir values below the detection limit of 0.5 ppb, suggesting that the Ir carrier is in the magnetic grain fraction. The abundant charcoal in this black layer suggests widespread biomass burning. A similar layer of charcoal, found at many other sites in Europe, including the Netherlands (3), Great Britain, France, Germany, Denmark, and Poland (4), also dates to the onset of the Younger Dryas (12.9 ka) and, hence, correlates with the YDB layer in North America."
Usselo is the type site for the 'Usselo Soil', the 'Usselo horizon' or 'Usselo layer', a distinctive and widespread Weichselian (Lateglacial) buried soil, paleosol, found within Lateglacial eolian sediments known as 'cover sands' in the Netherlands, western Germany, and western Denmark; classified as either a weakly podzolized Arenosol or as a weakly podzolized Regosol, where numerous radiocarbon dates, optically stimulated luminescence dates, pollen analyses, and archaeological evidence from a number of locations have been interpreted to show that the Usselo Soil formed as the result of pedogenesis during a period of landscape stability during the Allerød oscillation that locally continued into the Younger Dryas stadial as a marker bed.
The abundant charcoal, which is found in the Usselo Soil, and contemporaneous Lateglacial paleosols and organic sediments across Europe, may have been created by wildfires caused by a large bolide impact, based upon the reported occurrence of alleged extraterrestrial impact indicators and hypothetical correlations with Clovis-age organic beds in North America. However, the contemporaneous nature of the Usselo Soil with Clovis-age organic beds in North America, the presence of impact indicators within it, and the impact origin of the charcoal may only be apparent.

## Marker events

"What must underlie discussion of the definition of the TJB is the well accepted concept that global correlateability should be the main emphasis in the selection of a GSSP (e.g., Cowie et al., 1986; Remane et al., 1996; Gradstein et al., 2004; Walsh et al., 2004). As Remane et al. (1996: 79) expressed it, “the boundary definition will normally start from the identification of a level which can be characterised by a marker event of optimal correlation potential.” Thus, our goal here is to evaluate the possible marker events that could be used to define the TJB and to argue that an ammonite-based marker event has optimal correlation potential. This marker event is the LO of Psiloceras tilmanni in the New York Canyon section of Nevada."
In the diagram on the right, the Permian-Triassic boundary is at the base of the Induan limestone that occurs within the Yinkeng Formation.
"The Global Stratotype Section and Point (GSSP) of the Permian-Triassic boundary [...] is defined at the base of Hindeodus parvus horizon, i.e. the base of Bed 27c of Meishan section D, Changxing County, Zhejiang Province, South China."
"Hindeodus parvus is now recognized as the index fossil" occurring in the Zone above the P-T boundary.
The Induan was the earliest part of the Triassic Period, and lasted from about 251.2 to about 251.902 Ma.

## Organic horizons

"Volcanic ash soils (Andosols) may offer great opportunities for paleoecological studies, as suggested by their characteristic accumulation of organic matter (OM). However, understanding of the chronostratigraphy of soil organic matter (SOM) is required. Therefore, radiocarbon dating of SOM is necessary, but unfortunately not straightforward. Dating of fractions of SOM obtained by alkali-acid extraction is promising, but which fraction (humic acid or humin) renders the most accurate $^{14}$ C dates is still subject to debate. To determine which fraction should be used for $^{14}$ C dating of Andosols and to evaluate if the chronostratigraphy of SOM is suitable for paleoecological research, we measured $^{14}$ C ages of both fractions and related calibrated ages to soil depth for Andosols in northern Ecuador. We compared the time frames covered by the Andosols with those of peat sequences nearby to provide independent evidence. Humic acid (HA) was significantly older than humin, except for the mineral soil samples just beneath a forest floor (organic horizons), where the opposite was true. In peat sections, $^{14}$ C ages of HA and humin were equally accurate. In the soils, calibrated ages increased significantly with increasing depth. Age inversions and homogenization were not observed at the applied sampling distances. [In] Andosols lacking a thick organic horizon, dating of HA renders the most accurate results, since humin was contaminated by roots. On the other hand, in mineral soil samples just beneath a forest floor, humin ages were more accurate because HA was then contaminated by younger HA illuviated from the organic horizons."

## Soil horizons

"Two fire-history parameters (time-since-fire [TSF] and fire extent) were related to three landscape parameters (landform [hill slope or terrace], aspect, and forest composition) at 83 sites in a 730-ha low-elevation (less than ∼200 m) area of a mountainous watershed. We dated fires using tree rings (18 sites) and 120 soil-charcoal radiocarbon dates (65 sites). Comparisons among multiple radiocarbon dates indicated a high probability that the charcoal dated at each site represented the most recent fire, though we expect greater error in TSF estimates at sites where charcoal was very old (>6000 yr) and was restricted to mineral soil horizons. TSF estimates ranged from 64 to ∼12 220 yr; 45% of the sites have burned in the last 1000 yr, whereas 20% of the sites have not burned for over 6000 yr. Differences in median TSF were more significant between landform types or across aspects than among forest types. Median TSF was significantly greater on terraces (4410 yr) than on hill slopes (740 yr). On hill slopes, all south-facing and southwest-facing sites have burned within the last 1000 yr compared to only 27% of north- and east-facing sites burning over the same period. Comparison of fire dates among neighboring sites indicated that fires rarely extended >250 m. During the late Holocene, landform controls have been strong, resulting in the bias of fires to south-facing hillslopes and thus allowing late-successional forest structure to persist for thousands of years in a large portion of the watershed. In contrast, the early Holocene regional climate and forest composition likely resulted in larger landscape fires that were not strongly controlled by landform factors."

## Surface horizons

"The reliability of radiocarbon ages based on soil organic matter (SOM) from Holocene buried soils in Middle Park, Colorado, is assessed by comparison with ages of charcoal. On average, $^{14}$ C ages of SOM from buried surface horizons are 880 ± 230 $^{14}$ C yr younger than charcoal ages from the same horizon. Humic acid (HA) and low-temperature (400 °C) combustion residue (LT) fractions are 390 ± 230 and 1290 ± 230 $^{14}$ C yr younger than charcoal ages, respectively, and HA ages are on average 860 ± 140 $^{14}$ C yr older than LT fractions."

## Tuffaceous horizons

Heatherdale Shale is in the upper Normanville Group, mid-Botomian Stage, upper Lower Cambrian.
One soft-bodied fossil has been discovered from this site - a poorly-preserved Isoxys valve was cracked out in the lab. Isoxys is a nonmineralizing bivalved arthropod known only from Lower and Middle Cambrian rocks.
"Only two of several tuffaceous horizons from the Stansbury and Arrowie Basins have been dated (i) a date of 522.0 ± 2.1 Ma from the Heatherdale Shale of the Stansbury Basin, about 400 m above latest Atdabanian archaeocyathids and (ii) a date of 522.0 ± 1.8 Ma from the lower part of the Billy Creek Formation in the Arrowie Basin. Neither date is regarded as reliable."
"In the Stansbury Basin, Cooper et al. (1992) produced a mean 206 Pb/238 U Sensitive High Mass Resolution Ion Microprobe (SHRIMP) age of 526 ± 4 Ma with standard SL13 on zircons separated from a tuff bed within the upper part of the Heatherdale Shale at Sellicks Hill, Fleurieu Peninsula. Further analysis, plus new zircon data, enabled Jenkins et al. (2002) to revise this age to 522 ± 2.0 Ma. However, this age is not very well biostratigraphically constrained in the area of outcrop. It is overlain unconformably by the thick (∼8–10 km), predominantly flyschoid sediments of the Kanmantoo Group that contains very few, poorly preserved trilobites and brachiopods (Jago and Haines, 1997). The Kanmantoo Group is intruded by an early syntectonic granitoid known as the Rathjen Gneiss, which has a U–Pb date of 514 ± 4 Ma (Foden et al., 1999)."
"The tuff horizon is quite close to the only known trilobite fauna within the Heatherdale Shale. This comprises a few poorly preserved specimens of the trilobite Atops briandailyi (Jago et al., 1984; Jenkins and Hasenohr, 1989; Jenkins et al., 2002). The tuff horizon is over 400 m stratigraphically higher than the only reasonably well constrained biostratigraphic horizon in the Fleurieu Peninsula Cambrian succession. This horizon contains archaeocyaths in the top of the Sellick Hill Formation and the bottom part of the Fork Tree Limestone that Zhuravlev and Gravestock (1994) considered to be latest Atdabanian. Based on both biostratigraphy and sequence stratigraphy, Gravestock (1995) correlated the Heatherdale Shale with the biostratigraphically controlled successions of Yorke Peninsula and the Flinders Ranges. He suggested that the Heatherdale Shale should be correlated with the Mernmerna Formation and Oraparinna Shale that contain Pararaia janeae Zone (equivalent to the Botoman) trilobites in the Central Flinders Ranges."
"With respect to Yorke Peninsula, Gravestock (1995) correlated the upper Heatherdale Shale to the upper part of the Koolywurtie Member of the Parara Limestone; this contains archaeocyaths of the Syringocnema favus beds, implying a middle to late Botoman age (Zhuravlev and Gravestock, 1994). This is supported by the work of Zhou and Whitford (1994) who reported a U–Pb age of 525 ± 8 Ma with standard SL13 from a felsic tuff within the Cymbric Vale Formation of western New South Wales; Jenkins et al. (2002) recalculated this age to 517.8 ± 2.1 Ma. Both archaeocyath (Zhuravlev and Gravestock, 1994) and trilobite faunas (Jago et al., 1997; Paterson, 2005) from the Cymbric Vale Formation support a mid to late Botoman age."

## Volcanic horizons

"To compare our radiocarbon ages with ages derived from volcanic horizon identification with di-electrical profiling (DEP)/electrical DC conductivity (ECM) measurements, the age difference between trapped air and the ice matrix must be known. The age of the ice matrix at pore closure, at this site, can be calculated from the accumulation rate (62 mm water equivalent/yr), the -10 m temperature (-38.5°C) and the initial density of the snow pack (325 kg/m$^3$) [7,9,11], which leads to 740 yr. According to Schwander and Stauffer [12], the average age difference between the air captured in the ice and the ice matrix is equal to the age of the ice matrix at a density of 815 kg/m$^3$. For this site [Dronning Maud Land, Antarctica], this leads to 670 yr (estimated error ± 100 yr), [...]. (At 815 kg/m$^3$ ca. 50% of the air which will be eventually in the ice has been trapped.)"

## Hypotheses

* Each time frame or span of time in geochronology has at least one dating technique.
* Late Jurassic and Upper Jurassic are different, Late is a time frames, Upper is layer.
* The overall size of—or efficiency of carbon export from—the biosphere decreased at the end of the Great Oxidation Event (GOE) (ca. 2,400 to 2,050 Ma).