Long chain alkenones preserved in Miocene lake sediments

Over the last few decades, the occurrence of long chain alkenones (LCAs) in Quaternary lacustrine settings has been reported, the distributions being remarkably different from those of open ocean settings. We report here the occurrence of LCAs with modern lake-type composition from Middle Miocene sedimentary strata in the Dahonggou Section, Qaidam Basin, northern Tibetan Plateau. This is the first report of LCAs with preserved lake compositions in pre-Quaternary lacustrine sediments. The discovery implies that the producers of modern lacustrine LCAs could originate from similar lake species living at least as far back as the Miocene epoch. It also sheds new light on the potential of &ldquo;alkenones&rdquo; as palaeotemperature indicators in lacustrine systems back to at least 10.5 Ma.</p

Mysterious abrupt carbon-14 increase in coral contributed by a comet

A large and sudden increase in radiocarbon (C-14) around AD773 are documented in coral skeletons from the South China Sea. The C-14 increased by similar to 15 parts per thousand during winter, and remain elevated for more than 4 months, then increased and dropped down within two months, forming a spike of 45 parts per thousand high in late spring, followed by two smaller spikes. The C-14 anomalies coincide with an historic comet collision with the Earth&#39;s atmosphere on 17 January AD 773. Comas are known to have percent-levels of nitrogen by weight, and are exposed to cosmic radiation in space. Hence they may be expected to contain highly elevated C-14/C-12 ratios, as compared to the Earth&#39;s atmosphere. The significant input of C-14 by comets may have contributed to the fluctuation of C-14 in the atmosphere throughout the Earth&#39;s history, which should be considered carefully to better constrain the cosmic ray fluctuation.</p