Determining the rates of marine sediment accumulation is critical in reconstructing past changes in productivity and carbon burial, but the ability to reconstruct such flux flux rates over long geologic timescales is been limited by radioactive decay (e.g., 230Thxs) and by variability in the proxy production rate or source (e.g., 4He). This collaborative project, involving researchers from the University of Hawaii at Manoa and the California Institute of Technology, develops the use of extraterrestrial particles in marine sediments as flux proxies, potentially extending scientists? ability to reconstruct changing accumulation rates from tens of thousands of years to hundreds of millions of years. By combining two independent, stable tracers of particulate extraterrestrial matter in marine sediments-- helium (He) and osmium (Os) isotopes-- the researchers will be able to distinguish between changing extraterrestrial flux to Earth and terrestrial focusing of extraterrestrial particles during transport to the seafloor.
The method will be applied over a time window of 10 to 4 million years before present, a time period known to encompass an episode of greatly increased extraterrestrial flux to the Earth resulting from collisions in the asteroid belt. The researchers will compare records from a number of different sediment cores from the Pacific Basin, with accumulation rates varying by a factor of more than twenty, to determine whether or not particulate extraterrestrial He is preferentially focused to high sedimentation rate sites relative to Os. Lastly, the researchers will test whether a reliable age model can be developed for Pacific red clays, which would greatly improve the potential of these sediments as paleoceanographic archives.
In terms of broader impact, funding supports education and training of a postdoctoral researcher and an undergraduate student and fosters international collaborations. The research will also develop a potentially important new tool that would unlock new potential for studying how important environmental parameters, including productivity and terrestrial aridity, change over long time periods.
Extraterrestrial matter is delivered to the Earth both in the form of rare large bodies, like the multi-km impactor that terminated the Cretaceous period 65 million years ago, and in the form of tiny dust grains that are continually produced by collisions between asteroids and in the tails of active comets. The arrival of extraterrestrial debris on the Earth can be detected by only a few very unusual chemical indicators, such as the platinum group metal osmium, and the very rare variety of helium, 3He. Because these species arrive from outer space, their flux to Earth is independent of terrestrial processes, such as climate change. As a result they have been proposed as tracers by which the flux to the seafloor of purely terrestrial matter can be obtained, provided their flux from space is not too variable. For example, if the flux of extraterrestrial 3He is known and constant, and one measures the ratio of, say, calcium carbonate to 3He in a marine sediment, by multiplying the two together, a point measurement of the removal rate of carbonate from the ocean can be obtained. In a similar fashion the concentrations of constant flux proxies can be used as chronometers by which a mapping of sediment depth to time can be made. These two approaches are very broadly useful, but currently limited by uncertainties in the flux and behavior of these so-called constant-flux extraterrestrial tracers. This grant supported work to investigate variations in extraterrestrial flux as well as the relationship between the 3He and Os flux to better understand the applicability of these tracers for use as constant flux proxies. One part of the work focused on an unusual event at about 8 million years ago, when a collision destroyed a 140 km diameter object in the asteroid belt. A very clean record of the 3He flux during this event was obtained from two different sites, one of which is shown in the accompanying figure (from a very well-studied rock outcrop of appropriate age in Italy, called Corvi Beach). Two important results of this work are: a) establishment of a quantitative understanding of how the 3He flux changed over this event, and b) a tentative correlation with osmium data, suggesting that, contrary to previous understanding, the Os flux is sensitive to variations in the flux of extraterrestrial dust to Earth after an asteroid collision. The latter provides a means to establish a quantitative relationship between the dust-derived 3He and Os fluxes, which will be useful in future sediment flux studies while also providing insight to how extraterrestrial dust gets delivered to Earth and how it behaves once it enters the Earth system. In related work, 3He was used as a constant-flux proxy to better understand an important climatic excursion that occurred 55 million years ago. This event is thought to have been driven by a large and rapid influx of carbon dioxide into Earth’s atmosphere, thus making it a potential natural analog to ongoing human-induced climate change. Chronologies developed from 3He concentrations provide indications of sustained climate change after the event occurred (i.e., an irreversible change in climatic static), as well as a better understanding of the fate of the carbon dioxide and the consequences of its neutralization by dissolution of calcium carbonate on the sea bed.
