Time scales across the Cretaceous-Tertiary boundary: A new approach

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ABSTRACTCyclical variations in the carbonate content, represented by limestone-marl bedding couplets, in Late Cretaceous-Early Tertiary sediments may be related to the Earth's precessional period of ~20-24 thousand years (kyr). However, do all bedding couplets really represent ~20-24 kyr? If so, did orbital forcing produce these variations by driving fluctuations in carbonate production in the ocean or was continental weathering equally important? Answering these questions 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. 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.Intellectual Merit: 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).Broader Impact: The proposed work will provide an important calibration between two techniques: 3 He 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. The proposed research also incorporates graduate student education and involves them in research with goals that are of societal significance. Involvement in exciting research is important for recruiting and retaining talented individuals in the earth sciences. Such individuals are needed to address the growing environmental challenges facing humankind.