It is becoming increasingly urgent to identify the mechanisms causing such changes, which could enable us to estimate their impact on contemporary ecosystems. That is why it is so important to analyse rapid climate changes from the past. The last serious climate change in the Northern Hemisphere took place near the end of the last glacial period — called the ice age — during what is known as the Younger Dryas ca.
This was a period of violent cooling lasting approximately 1, years, near the end of which there was an equally sudden warming. Determining the exact chronology was of key importance for understanding the course of climate changes in the Younger Dryas. The first attempts at dating the changes in this period were based on studying oceanic sediments and Greenland ice cores.
None of these archives of nature, however, enabled the chronology of the period in question to be re-created accurately. Professor Goslar used an analysis of varved sediments in selected lakes for its determination. We secured samples that allowed radiocarbon dating to the Early Viking Period 9th century and the identification of several plant species in small…. Radiocarbon Dating Ancient Grease. Willard Libby's original scintillation-counting method demanded large sample sizes and a lot of time per sample.
The sample size meant that many interesting things couldn't be…. I got a great letter from Reggae Roger Wikell, which I publish in translation with the permission of Roger and Mattias Pettersson with the awesome metal hair. For context, note that these two scholar friends of mine are the area's foremost authorities on Mesolithic sites that have ended up on…. The nuclei are mostly made of fecal pellets or quartz fragments, even though some ooids have grown over ancient ooids or microbialite fragments.
These sands are locally cemented, forming grainstones. The thickness of ooid sands is highly variable.
Towards deeper areas, the ooid sands overlie a finely laminated green to grey clay sediment, mostly composed of quartz, smectite, illite and ca. In section, clay sediments exhibit a wedge-shape morphology widening toward the surface associated with gypsum crystals Fig. Thin crusts of microbial mats can also be observed on the surface of the clay strips Fig.
The microbial mats show green or red pigments Fig. Polygonal architectures of the emersed White Rock Bay ooid sand flat.
The different vertical shades of green suggest preferential fluid pathways through the clays. The presence of an infra-millimetre thin red-coloured layer 0. Geological field mapping GPS, facies, sampling, etc. Field observations were combined with aerial and satellite images. The dark orange to dark green pigments of the modern living mats in the aerial and satellite images allow us to distinguish between submersed living microbial deposits and light-coloured uncolonized ooid sands or clays.
This mapping approach required suitable visibility through the atmosphere and water column, and images showing water turbidity, cloud cover or waves were discarded. Aerial images have been imported into ArcGIS in order to perform the mapping. The first approach consists in converting the pixel showing microbial deposits into black pixel and integrating them into a shapefile layer. The percentage of points i. For this we defined high- and low-density areas of microbial deposits in aerial images, which match with the perception in the field.
These results have been placed on a metric topographic map for the GSL floor. The distribution of the microbial, non-microbial deposits and relevant sedimentary structures along Antelope Island is mapped in Figs.
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It complements the maps published by Eardley and Baskin et al. A comparison of the temporal series of aerial images since provides insights into the migration and burying dynamics of the microbial deposits Fig. For instance, on the western Antelope Island margin, microbial deposits appeared to be intermittently buried by loose sediments Fig. This process likely results in a ca.
Only considering the Antelope Island area, our mapping results reach a third of the total extent proposed by Eardley Therefore, its initial estimation is likely an underestimation as i a significant part of these structures can be buried under the ooid sand or clay sediments and ii they are partially eroded around the shoreline Fig.
Detailed maps of the western side of Antelope Island. Illustrations of sediment removal.
Most microbial deposits around the margin of Antelope Island were found between 7. In the temporarily exposed zone of the shore facing steep topographic area, cow-pie structures are preserved along the flat. They are absent in the embayment areas like White Rock and Bridger bays; Fig.
In addition, the cow-pie macrofabrics are located along linear belts following isobaths and are locally observed in a relative paleohigh induced by the presence of unusual conglomeratic deposits Fig. Lakeward, cow-pie structures give way to domes and columns, which are organized as isolated circular structures or coalesce, merging into clusters. Locally, the centre of the polygons can be colonized as well. Deeper occurrences of microbialites were reported e. This abrupt slope break marks the transition from a shallow platform shoreward with abundant microbial deposits to a deeper area with rare occurrences of microbialite Fig.
Along the slope break, scattered lens-shaped detachments of microbial structures have been observed Fig.
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Living microbial mats develop on top of microbialites cow pie and domes and columns and cemented ooid sands documented in the submersed area. They can also persist on the shore where emersion is limited to a short time, e. Therefore, the upper occurrence of the living mat, corresponding to the boundary between living and ancient structures, is a good indicator of the annual mean elevation of the shoreline.
Such conditions are suitable for the formation of desiccation polygons Warren, The extended polygonal networks observed in the embayments area are indeed related to desiccation processes. The polygons are locally encrusted by microbial mats probably associated with fluid circulations in the underlying crack system Bouton et al. Both the sedimentary structures and the evaporite can be used to define the emersion conditions and to estimate the shoreline position.
Based on a uniformitarianism approach, we therefore consider that similar structures with the same underlying processes are preserved in the fossil record. The occurrence of ca. Considering a growth rate of 0. Along a shore-to-lake transect, microbialites are frequently observed aligned and following polygons.
Considering their similarity with the present-day shoreline, the metre- to kilometre-long linear arrangements of microbial structures following the isobaths and parallel to the shoreline are interpreted as paleoshorelines. The microbialites track approximate shorelines, as they must form in at least predominantly submerged environments. Each alignment represents a specific water elevation during periods of stable lower water levels. The development of the cow-pie structures along the shoreline may constitute an initial step in the formation of larger structures, resulting in submersed alignments.
In the submersed area between The same polygonal geometry associated with microbial mats interpreted as desiccation structures have been observed in the emersed embayment area. Therefore, the submersed polygons are considered remnants of desiccation polygons related to past stable low water levels of the GSL.
The regular pattern of polygons and their morphology are indicative of desiccation rather than synaeresis formed under subaqueous conditions Nichols, This extensive polygon development requires a protracted and high-amplitude drop in the lake water level. Their presence and stability shown by old USGS aerial images indicate that they formed prior to and that they have been stable since then.
They are similar to the polygonal structures identified in Great Basin playas Neal et al. Giant desiccation polygons have been recognized in the recent shore domain of White Rock Bay this study; Fig. The submersion of polygons following a flooding may trigger the preferential development of microbial structures at the edges of the polygons, i. This process can be enhanced by preferential fluid migration through cracks and underlying fractures Fig.
We hypothesize here that the formation of the desiccation polygons occurs during emersion but that the establishment of mineralizing microbial deposits associated with the polygons allows the preservation of this peculiar sedimentary geometry through time Gerdes, ; Bouton et al.