Assessing the distribution of time in sediments and sedimentary rocks is critical to constraining rates and dynamics of climate change, mass-extinction events, and the biological recovery following mass extinction events. While sediments have often faithfully recorded climate changes and mass-extinction events, teasing out the mechanism driving these events from the geologic record has proven to be quite challenging, in part because of the inability to accurately determine the duration of such events. Our research utilizes a relatively new technique, involving measurements of extraterrestrial ³He in seafloor sediments, to investigate the process(es) driving cyclical carbonate sedimentation across the Cretaceous-Tertiary (K-T) boundary and the rates of climate change and biological recovery following the K-T impact.

Striking cyclicity in carbonate sedimentation rates in Late Cretaceous through Early Tertiary sediments has been recognized for over two decades (Fig. 1). However, do all bedding couplets really represent ~20-24 kyr? For example, in the Triassic Latemar carbonate sequence (Italy) approximately 600 cycles are recognized. Tuning the carbonate sedimentation rates to the orbital frequencies indicate that the 600 cycles represent ~12 Myrs. U-Pb dating of zircons in volcaniclastic layers within the Latemar platform, however, indicates a maximum time span of 4.7 Myr for the cycles. Thus, a strong case can be made that all sedimentary cycles need not be related to precessional forcing. In addition to determining whether or not the cycles represent ~20 kyr in duration, it is important to understand the physical processes that give rise to the sedimentary cycles. Are these cycles a result of fluctuations in carbonate production, dissolution, or was continental weathering equally important? For example recent work by DeMenocal and co-workers using radiocarbon based sedimentation rates from ODP Site 658C off Cap Blanc, Mauritania, show that gradual changes in insolation forcing resulted in rapid and dramatic increases in the aeolian flux off the west coast of Africa at 5.5 cal. ka BP. At this site, the increase in terrigenous flux was as important as the decrease in carbonate flux in the reduction of the carbonate content of the sediments from ~50% to 30%.

Answering the questions posed above would provide important checks on timescales derived using cyclostratigraphy, such as the pace of events across the Cretaceous-Tertiary boundary, and the response of continents and oceans to Milankovitch forcing. However, there are few available tools that can be utilized to accurately constrain events on timescales of a few thousand years. Accumulation rates of interplanetary dust particles (IDPs) may provide a new tool with which to investigate sedimentation rates at such high resolution. My prior work has shown that the accretion rate of IDPs across the K-T boundary is constant. Therefore, the concentration of these particles in sediments will be governed by sedimentation rates. Implantation of helium by the solar wind tremendously enriches IDPs in ³He compared to terrestrial rocks. By using ³He as a tracer of the IDP concentration in sediments, sedimentation rates at a resolution of 1000 years, and perhaps down to a hundred years, can be determined.

The two primary goals of this proposal are: 1) use ³He-based sedimentation rates to estimate durations of limestone-marl couplets across the K-T over a sufficient number of cycles to verify the claim that they represent precessional cycles and, more importantly, quantify variability in sedimentation rates within a carbonate cycle. Independent knowledge of the distribution of time within a bedding couplet will enable fluxes to be calculated quantitatively on sub-Milankovitch timescales, crucial pieces of information required to investigate causal mechanisms driving sedimentary cycles. 2) use ³He-based sedimentation rates to constrain the pace of climate and productivity changes recorded within the K-T clay, and estimate the duration of important biostratigraphic zones (such as P1A, P1B) to characterize the biological/ecological recovery in the first 100 kyr following the K-T impact. Hence, the proposed research will provide a precise chronological framework for testing models for biotic recovery and evolutionary dynamics.

To assess the distribution of time in sediments deposited following the K-T impact event, the well-studied stratigraphic sections exposed in southeast Spain (Agost and Caravaca) will be sampled for helium measurements. To test the hypothesis that Milankovitch forcing induces carbonate cyclicity in Late Cretaceous-Early Tertiary sediments, ³He-based sedimentation rates will be used to determine duration of well-recognized limestone-marl beds in sediments from DSDP cores (site 516F on the Rio Grande Rise and sites 528 and 529 on the Walvis Ridge) and from Agost (Spain).

The research will provide an important calibration between two techniques: ³He and cyclostratigraphy. Because ³He is preserved in the geological record for at least 480 Myrs, a detailed study will illustrate the potential of applying ³He-based sedimentation rates over a large fraction of the Phanerozoic to probe timescales of climate changes, mass extinctions events, and the biological recovery following mass extinction events.

Obviously, the use of extraterrestrial ³He as a chronometer is not restricted to the K-T, and the technique can be used to address a number of interesting problems regarding climate change in the Quaternary.

To learn more about IDPs or the use of ³He as a chronometer click here