Absolute dating radiocarbon dating. The development of accelerator mass spectrometry (AMS) dating, which allows a date to be obtained from a very small sample, has been very useful in this regard. Other radiometric dating techniques are available for earlier periods. One of the most widely used is potassium–argon dating (K–Ar dating). Potassium-40 is a radioactive isotope of potassium that decays into argon-40. The half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14, allowing much older samples to be dated. Potassium is common in rocks and minerals, allowing many samples of geochronological or archeological interest to be dated. Argon,
Stable-isotope probing organisms were consuming existing carbohydrates and which were using carbohydrates more recently produced from photosynthesis. When DNA is the biomarker, SIP theoretically can be performed using isotopically labeled C, H, O or N, though C is used most often. Cupples et al. (2007) demonstrated a weaker DNA buoyant density shift in when N- versus C-labeled substrates were used in pure culture, conversely a very strong buoyant density shift was observed when both labels were used. Stable-isotope probing Stable-isotope probing (SIP) is a technique in microbial ecology for tracing fluxes of nutrients in biogeochemical cycling by microorganisms. A substrate is enriched