Time series analysis of magnetic susceptibility variations in deep marine sedimentary rocks: A test using the upper Danian-Lower Selandian proposed GSSP, Spain

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It has been clearly demonstrated that past climate fluctuations are recorded in marine sedimentary rocks. However, for many reasons, extracting the climate signature is difficult. Initial low-field mass-specific magnetic susceptibility (MS) data can potentially provide a measure of climate variability and thus become a proxy characterizing climate cyclicity in a wide range of marine sediments. This is due to the fact that climate change (warm, wet versus cold, dry) drives cyclic weathering and erosional variations that are recorded as the detrital components of marine sediment that dominates the MS. To test the utility of MS to yield climate proxies in marine sediments showing major changes in lithology, we have sampled the well-studied Danian/Selandian boundary interval (Lower Paleocene) at Zumaia (Zumaya), Spain. This interval represents a dramatic, rapid lithologic change from Danian carbonate-dominated limestone-marl couplets to a detrital-dominated marl-shale sequence in the Selandian, indicating onset of a major regression-erosional event beginning in the lowest Selandian. Sampling included a continuous sequence from the uppermost Danian Stage (3.71 m) into the lowermost Selandian Stage (5.2 m), a suite of 175 samples collected at 5 cm intervals. Our results indicate that MS measurements reflect changes in detrital sediment at the site, first by closely tracking high-frequency limestone-marl couplets, second with a large, rapid shift toward higher MS values beginning at the Danian/Selandian boundary resulting from a major regression, and third by tracking low-frequency climate-controlled variations known to have occurred during deposition of these sediments. MS zones developed from the cyclicity observed throughout the sequence, supported by time series analysis using Fourier Transform (FT) methods applied to the MS results, exhibit Milankovitch cyclicities in the precessional (19-24 kyr), obliquity (41-54 kyr) and eccentricity bands (100 kyr). This is in excellent agreement with previous FT work on the section using measured variations in cyclic bed thicknesses. With the new MS data set and FT results, we then developed a Floating Point Time Scale (FPTS) for the sequence sampled (covering ∼ 550 kyr through the Danian/Selandian boundary interval), yielding a time-scale resolution for the uppermost Danian to ∼ 10,000 years. However, only the ∼ 100,000 year eccentricity band for the Selandian is sufficiently well developed for an FPTS estimate, and yields a time-scale resolution of ∼ 50,000 years. Our test of the utility of MS data sets in this varying depositional setting demonstrated that these data can provide a climate proxy that is not disrupted by large lithologic changes. © 2008 Elsevier B.V. All rights reserved.

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Palaeogeography, Palaeoclimatology, Palaeoecology

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