time layers
tree rings
tree-time / dendrochronology. Look at a cut log, look at its rings. growth in trees occurs near the bark, while the deeper-in core layers are preserved rings of past growth.
The seasons / environment affect the growth rate of the tree , creating generally a visual difference between spring / summer+autumn ring growth in temperate regions, hence 1 ring = 1 seasonal cycle. Generally, spring is associated with less-dense, rapid growth, while summer (sometimes into autumn) growth is denser.
width of rings can also be used more finely, wider rings = more growth that year, so probably better conditions... overlapping ring-dates between trees can lead to a long, multi-tree chronology for the area.
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ice cores
layers are built by annual snow, compacted into ice underneath the layers forming above. Ice traps atmosphere , how we can know the rough % oxygen/carbon dioxide/etc of past atmosphere circumstances. Heat doesn't flow well through ice, so even past temperatures can be trapped in large sheets.
dust and marine salt-ions can be trapped within, giving further depth to past ecologies.
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lake cores
mud at the bottom of the lake builds up in undisturbed layers, trapping things that land in the lake and sink.
Trapped pollen can be observed under a microscope & can identify what plant it is from, helping the understanding of how plants spread ; such techniques have been used to find out when some crops were domesticated & cultivated1.
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sedimentary & metamorphic rocks
an iconic time-layering. sediment builds up in layers, compresses under the weight of the layers above and becomes stone. Traps things in there, and they may become coal, fossils, etc. Other things are trapped: the ripples in sediment formed by waves, so that we may know past sand-beaches.
with enough time, these layers become distorted by geological processes, becoming Wiggly (should they move downwards), getting turned non-horizontal, getting weathered (should they be exposed to air/water/etc), so on.
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scutes
scutes: plates of a tortoise/turtle's carapace. They grow more quickly in warm seasons than cold ones, forming a ring much like a tree's, even mirroring that same seasonality. elderly animals may have worn-down scutes so particularly-old animals are difficult to age exactly2.
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otoliths
inner-ear-bones. in fish, the large otoliths also form rings with age, so fish-ages can be counted. Again, they are seasonal, growing faster in the summer and slower in the winter.
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stromatolites
bacterial fossils / fortresses. on the sea-floor, bacteria can grow into a mat, which is sticky... sand and such gets stuck, forming a layer, and the bacteria then migrates to the top of that layer. repeat. sometimes, more than sediment is stuck : microfossils, for example3.
stromatolites used to be commonplace, but they do not form without enough of a bacterial mat: in modern times, the mat struggles to form under grazing pressure from things such as snails, shrimp, and so on. Modern stromatolites -- their mats formed of different bacteria than the ancient ones -- can be found in places too harsh for their predators to inhabit.
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baleen
whale baleen, the filter-teeth they have, grows in a sturdy sheet. The filter-bristles are formed by the rubbing of baleen-on-baleen, breaking the sheet's ends apart. To compensate for continual wear, the baleen grows.
hormones, which can indicate stress, pregnancy, etc , and isotopes, which indicate different water conditions and therefore different locations / food, are stored in the baleen4. while the growth/wearing means it is not a complete record of a whale's life, it is still an invaluable and rich slice.
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hyrax middens
the rock hyrax, Procavia capensis, are herbivores common in Southern Africa. they have a habit of having a designated "bathroom" that they do their business in, generation after generation ... this builds up layers, much like layers of sediment, also trapping markers like pollen types and quantities5.
to the hyraxes, to the rocks they clamber, the trees that break those rocks, all the others: delightful archival work, everyone!
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thanks for reading!🪱🦀
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1 ) Ã…kesson, C. M., McMichael, C. N. H., & Bush, M. B. Late-Holocene maize cultivation, fire, and forest change at Lake Ayauch, Amazonian Ecuador. Sage Journal, 2023 33(5).
2 ) Attenborough, D. Life in Cold Blood.
3 ) Seckbach, J., & Oren, A. Microbial Mats
4 ) Holland, J. S. A whale's baleen bristles reveal the story of its life. Smithsonian Magazine, 2017.
5 ) Chase, B. M., et al. Rock hyrax middens: A palaeoenvironmental archive. Quaternary Science Reviews, 2012 56:107-125.
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images:
Olea europaea subsp. cuspidata by Rotational ; Mediterranean rocky coast of Libya by Koperczak ; Stromatolite, Greysonia sp., Vendian, Bolivia by Daderot ; Hyrax midden stratification by DrBMChase.
