ALBERT

Description: Late Cretaceous (100-73 Ma) pelagic limestones were measured for helium concentration and isotopic composition to characterize the interplanetary dust flux using 3He as a tracer. In the Bottaccione section near Gubbio, Italy, three intervals of elevated 3He concentration were detected: K1 in the Campanian stage at ~79 Ma, K2 in the Santonian stage at ~ 85 Ma, and K3 in the Turonian stage at ~91 Ma. All three of these episodes are associated with high 3He/4He and 3He/non-carbonate ratios, consistent with their derivation from an enhanced extraterrestrial 3He flux rather than decreased carbonate sedimentation or dissolution. While K2 is modest in magnitude and duration and thus is of limited significance, K1 and K3 are each identified by a few myr interval with an ~4-fold enhancement in mean 3He flux compared with pre-event levels. Samples from ODP Hole 762C in the Indian Ocean spanning both K2 and K3 (93-83 Ma) confirm the presence of a peak in the Turonian stage, suggesting that K3 is a global event. The K1 and K3 3He events are similar in most respects to the two peaks previously detected in the Cenozoic, suggesting a similar origin. These have been attributed to a major asteroid collision in the Late Miocene and to a shower of either comets or asteroids in the Late Eocene. Based on the age and temporal evolution of K1, we suggest that it most likely records the collision which produced the Baptistina asteroid family independently dated at ~80 Ma. The K3 event is less easily explained. It is characterized by an unusually spiky and erratic temporal progression, suggesting an unusual abundance of very 3He rich particles not previously seen in the sedimentary 3He record. We suggest this episode arises either from a comet shower or from an asteroid shower possibly associated with dust-producing lunar impacts.

Description: The Paleocene-Eocene Thermal Maximum (PETM; ~56 Ma) is associated with abrupt climate change, carbon cycle perturbation, ocean acidification, as well as biogeographic shifts in marine and terrestrial biota that were largely reversed as the climatic transient waned. We report a clear exception to the behavior of the PETM as a reversing climatic transient in the eastern North Atlantic (Deep-Sea Drilling Project Site 401, Bay of Biscay) where the PETM initiates a greatly prolonged environmental change compared to other places on Earth where records exist. The observed environmental perturbation extended well past the d13C recovery phase and up to 650 kyr after the PETM onset according to our extraterrestrial 3He-based age-model. We observe a strong decoupling of planktic foraminiferal d18O and Mg/Ca values during the PETM d13C recovery phase, which in combination with results from helium isotopes and clay mineralogy, suggests that the PETM triggered a hydrologic change in western Europe that increased freshwater flux and the delivery of weathering products to the eastern North Atlantic. This state change persisted long after the carbon-cycle perturbation had stopped. We hypothesize that either long-lived continental drainage patterns were altered by enhanced hydrological cycling induced by the PETM, or alternatively that the climate system in the hinterland area of Site 401 was forced into a new climate state that was not easily reversed in the aftermath of the PETM.

Description: To better understand the composition, characteristics of helium diffusion, and size distribution of interplanetary dust particles (IDPs) responsible for the long-term retention of extraterrestrial 3He, we carried out leaching, stepped heating, and sieving experiments on pelagic clays that varied in age from 0.5 Ma to ~90 Myr. The leaching experiments suggest that the host phase(s) of 3He in geologically old sediments are neither organic matter nor refractory phases, such as diamond, graphite, Al2O3, and SiC, but are consistent with extraterrestrial silicates, Fe-Ni sulfides, and possibly magnetite. Stepped heating experiments demonstrate that the 3He release profiles from the magnetic and non-magnetic components of the pelagic clays are remarkably similar. Because helium diffusion is likely to be controlled by mineral chemistry and structure, the stepped heating results suggest a single carrier that may be magnetite, or more probably a phase associated with magnetite. Furthermore, the stepped outgassing experiments indicate that about 20% of the 3He will be lost through diffusion at seafloor temperatures after 50 Myrs, while sedimentary rocks exposed on the Earth's surface for the same amount of time would lose up to 60%. The absolute magnitude of the 3He loss is, however, likely to depend upon the 3He concentration profile within the IDPs, which is not well known. Contrary to previous suggestions that micrometeorites in the size range of 50-100 µm in diameter are responsible for the extraterrestrial 3He in geologically old sediments [Stuart, F.M., Harrop, P.J., Knott, S., Turner, G., 1999. Laser extraction of helium isotopes from Antarctic micrometeorites: source of He and implications for the flux of extraterrestrial 3He flux to earth. Geochimica et Cosmochimica Acta, 63, 2653-2665, doi:10.1016/S0016-7037(99)00161-1], our sieving experiment demonstrates that at most 20% of the 3He is carried by particles greater than 50 µm in diameter. The size-distribution of the 3He-bearing particles implies that extraterrestrial 3He in sediments record the IDP flux rather than the micrometeorite flux.