Typical zircon grains from another deposit
I have been looking again at the Clarke and Kirkland paper which purports to demonstrate that Salisbury Plain was never glaciated -- on the basis of the zircon and apatite record contained in four river samples.
Detrital zircon–apatite fingerprinting challenges glacial transport of Stonehenge’s megaliths.
Anthony J. I. Clarke & Christopher L. Kirkland
Nature Communications Earth & Environment | ( 2026) 7:54
https://doi.org/10.1038/s43247-025-03105-3
Anthony J. I. Clarke & Christopher L. Kirkland
Nature Communications Earth & Environment | ( 2026) 7:54
https://doi.org/10.1038/s43247-025-03105-3
https://brian-mountainman.blogspot.com/2026/01/new-study-on-salisbury-plain-zircon.html
• Hydraulic Sorting: Rivers spatially fractionate minerals by density, grain size, and morphology. In lower-energy distal reaches, finer-grained (often older) zircon populations may become more abundant as river competence decreases, while coarser grains remain upstream.
• Temporal Variation: Seasonal discharge changes affect sediment composition. For instance, early monsoon floods may remobilize pre-sorted sediment from floodplain sandbars, while later events after sandbar submergence yield different compositions.
• Zircon Fertility: Not all source rocks produce zircons at the same rate. Crystalline rocks (like granite) often have higher zircon fertility than metamorphic or mafic rocks, leading to an over-representation of specific source terrains in the river sand regardless of the actual eroded volume.
• Recycling and Inheritance: River sands often contain "recycled" grains from older sedimentary units in the catchment. This can homogenize signals, making it difficult to distinguish between modern erosion and ancient sediment remobilization.
(b) Lab Processing Biases
Biases in the lab are often "anthropogenic" and stem from the physical separation and selection of grains:
• Grain Size Fractionation: Standard heavy mineral separation (e.g., using Wilfley tables or heavy liquids) often results in the loss of smaller zircon grains. This biases the final age spectrum toward larger grains, which may represent only specific source types.
• Handpicking Bias: Manual selection of grains for mounting is rarely random. Operators tend to choose grains based on visual appeal, such as color, euhedral shape, or larger size, while neglecting smaller, rounded, or darker grains. Bulk-mounting is often recommended to mitigate this.
• Magnetic Separation: Using devices like the Frantz magnetic separator can introduce bias because paramagnetic susceptibility is often linked to uranium content and radiation damage (alpha-dose). Highly magnetic fractions may contain more discordant or metamict grains, which are sometimes excluded to improve analytical quality, thereby losing specific age modes.
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Another major issue is that in the study there were no control sites. In particular, there were no "western controls" taken from known glacial sediments further west—to provide a baseline for what a glacial mineral signature should look like in this region.
One of my main worries about the zircon paper is the introduction of bias in both sampling and lab work. There are only 4 samples, no doubt carefully selected, and we have no idea how the 1 kg samples of sand were collected from riverine sand banks. Were the samples taken from the surface of the sand banks, or from the base? Or all mixed up? From what we can see, they were "bulk samples"........
I have been digging about in the literature on zircon and apatite fingerprinting, and this all seems rather relevant:
==================
In zircon dating, biases can significantly alter age spectra, leading to inaccurate interpretations of sediment provenance. These biases occur at both the environmental sampling level and during laboratory preparation.
(a) Sampling of Sandy Beds in Rivers
Sampling river sands introduces "natural" or "environmental" biases that can cause certain age populations to be over- or under-represented:
I have been digging about in the literature on zircon and apatite fingerprinting, and this all seems rather relevant:
==================
In zircon dating, biases can significantly alter age spectra, leading to inaccurate interpretations of sediment provenance. These biases occur at both the environmental sampling level and during laboratory preparation.
(a) Sampling of Sandy Beds in Rivers
Sampling river sands introduces "natural" or "environmental" biases that can cause certain age populations to be over- or under-represented:
• Hydraulic Sorting: Rivers spatially fractionate minerals by density, grain size, and morphology. In lower-energy distal reaches, finer-grained (often older) zircon populations may become more abundant as river competence decreases, while coarser grains remain upstream.
• Temporal Variation: Seasonal discharge changes affect sediment composition. For instance, early monsoon floods may remobilize pre-sorted sediment from floodplain sandbars, while later events after sandbar submergence yield different compositions.
• Zircon Fertility: Not all source rocks produce zircons at the same rate. Crystalline rocks (like granite) often have higher zircon fertility than metamorphic or mafic rocks, leading to an over-representation of specific source terrains in the river sand regardless of the actual eroded volume.
• Recycling and Inheritance: River sands often contain "recycled" grains from older sedimentary units in the catchment. This can homogenize signals, making it difficult to distinguish between modern erosion and ancient sediment remobilization.
(b) Lab Processing Biases
Biases in the lab are often "anthropogenic" and stem from the physical separation and selection of grains:
• Grain Size Fractionation: Standard heavy mineral separation (e.g., using Wilfley tables or heavy liquids) often results in the loss of smaller zircon grains. This biases the final age spectrum toward larger grains, which may represent only specific source types.
• Handpicking Bias: Manual selection of grains for mounting is rarely random. Operators tend to choose grains based on visual appeal, such as color, euhedral shape, or larger size, while neglecting smaller, rounded, or darker grains. Bulk-mounting is often recommended to mitigate this.
• Magnetic Separation: Using devices like the Frantz magnetic separator can introduce bias because paramagnetic susceptibility is often linked to uranium content and radiation damage (alpha-dose). Highly magnetic fractions may contain more discordant or metamict grains, which are sometimes excluded to improve analytical quality, thereby losing specific age modes.
==========================
Another major issue is that in the study there were no control sites. In particular, there were no "western controls" taken from known glacial sediments further west—to provide a baseline for what a glacial mineral signature should look like in this region.
Needless to say, this debate is not over........ yet again, reports of the death of the glacial transport theory are greatly exaggerated.
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