Also, the oxygen-isotope ratio in fossil shellfish (as their life processes prefer the lighter oxygen isotope) has been used to help determine ancient temperatures. During an ice age, because proportionally more oxygen-16 is retained in ice sheets and does not flow back to the oceans, the ocean’s surface becomes enriched in oxygen-18 and that difference can be discerned in fossil shells. Sediments are usually laid down in annual layers, and in some places, such as the off of Venezuela's coast, undisturbed sediments have been retrieved and analyzed, which has helped determine when ice sheets advanced and retreated during the present ice age.Mass spectrometers have been invaluable for assigning dates to various rocks and sedimentary layers, as radioactive isotopes and their daughter isotopes are tested, including , , , and . Also, the ratios of elements in a sample can be determined, which can tell where it originated. Many hypotheses and theories have arisen, fallen, and been called into question or modified by the data derived from those increasingly sophisticated methods, and a few examples should suffice to give an idea of what is being discovered.The moon rocks retrieved by astronauts are still being tested, as new experiments and hypotheses are devised. In 2012, which resulted from testing moon rocks for the ratios (both are stable isotopes), and it has brought into question the hypothesis that the Moon was formed by a planetary collision more than four billion years ago. The titanium ratio was so much like Earth’s that a collision with Earth forming the Moon has been questioned (as very little of the hypothesized colliding body became part of the Moon). The collision hypothesis will probably survive, but it may be significantly different from today’s hypothesis. , as well as , and their ages confirm that geologists have derived, and meteorite dates provide more evidence that our .In the Western Hemisphere, the and civilization collapses of around a thousand years ago, or the Mississippian civilization collapse of 500 years ago, have elicited a great deal of investigation. From New Age ideas that the Anasazi and Mayan peoples “ascended” to the Eurocentric conceit that the was European in origin, many speculations arose that have been disproven by the evidence. It is now known that the Anasazi and Mayan culture collapses were influenced by epic droughts, but that was only the proximate cause. The ultimate cause was that those civilizations were not energetically sustainable, and the unsustainable long before Europeans invaded North America. The Anasazi used logs to build their dwellings that today . Scientists have used strontium ratios in the wood to determine where the logs came from, as well as dating the wood with and analyzing , and a sobering picture emerged. The region was already arid, but agriculture and deforestation desertified the region around , which was the heart of Anasazi civilization. When Anasazi civilization collapsed, at Chaco Canyon they were hauling in timber from mountains more than 70 kilometers away (the strontium ratios could trace each log from the particular mountain that it came from). When the epic droughts delivered their final blows, Anasazi civilization collapsed into a morass of starvation, warfare, and cannibalism, and the forest has yet to begin to recover, nearly a millennium later.Another major advance happened in the late 20th century: the ability to analyze DNA. was discovered in 1953. In 1973, . In 2003, . was accomplished in 2005, for orangutans in 2011, and for in 2012. The comparisons of human and great ape DNA have yielded many insights, but the science of DNA analysis is still young. What has yielded far more immediately relevant information has been studying human DNA. The have been identified. Hundreds of falsely convicted Americans have been released from prison, and nearly 20 from , due to Human DNA testing has provided startling insights into humanity's past.
This simple version of a traditional science experiment makes a great class project to demonstrate that oxygen is a by-product of photosynthesis. This project is best started in the morning of a sunny day. Elodea is the chosen plant because it gives off oxygen in the form of bubbles, which you should be able to see.
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I have koi in outside fish ponds. At times during the long summer days there is an occurrence called an algae bloom where the water in the pond becomes filled with very small suspended algae. During the day there is no problem with the respiration of the koi that I have in the pond… but because the algae use up so much available oxygen during the night and do not add any O2 to the water…my koi in the very early morning hours before the sunlight starts photosynthesis of the algae run out of the amount of oxygen they need for respiration and are forced to breathe atmospheric O2 at the surface of the pond! They gasp for O2 out of the water from the atmosphere where there is enough available for them to survive. My point is in water ponds there is a semi closed environment where plants can use up so much oxygen at night that they force the fish to get their oxygen elsewhere. When days become shorter the algae bloom will naturally diminish if I wait it out and do not do massive water changes or resort to killing the floating algae with a chemical plant killer algaecide that will not kill my fish if used in the proper doses. Plants do use O2 at night and do not give off any O2 in darkness!