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Abstract

The retrieval of useful environmental information from the annual layers of natural high-resolution records such as tree rings, coral banding, or marine varves first requires reconstruction and validation of the chronological sequence in which this information has been preserved. This chapter describes the detailed reconstruction of a varve chronology from the central Gulf of California which encompasses most of the 20th century. The chronology was developed from a set of five box cores taken from the anaerobic slope off Guaymas, Sonora, and consists of a unique sequence of lamina couplets identifiable throughout the study area stretching more than 20 km along the slope. The reconstruction was initially hampered by the fact that each core site has suffered a history of small-scale mass movement that has disrupted the stratigraphic continuity of its lamina sequence. Because these breaks generally do not occur at the same level in all of the cores, a continuous varve stratigraphy could be developed by piecing together short lamina sequences from four of the five box cores available. The chronostratigraphy commences with the winter/spring lamina of the year 1908. This lower boundary is imposed by a stratigraphic discontinuity that cuts across all five core sites and is inferred to be a sediment gravity flow triggered by a nearby large-magnitude earthquake in the autumn of 1907. The surface lamina is dated from the time of collection in 1978. Independent 210Pb dating of specific lamina intervals indexed to the lamina stratigraphy from all five cores strongly supports our interpretation that the reconstructed varved interval represents 70.5 years of continuous sedimentation.

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