A 50,000 year beryllium-10 record from Gulf of California sediments.

Abstract

The cosmogenic radionuclide ¹⁰Be from a marine sediment core was studied to help understand late Quaternary variations in the geomagnetic field and cosmic rays. The primary objective of this study was to see if the ¹⁰Be anomalies observed in polar ice cores could be observed in mid-latitude marine sediments. A partly-varved sediment core from the upper 50 meters of Leg 64 (DSDP) in the Gulf of California was determined to be ideal. A chronology of the core was determined by radiocarbon analysis, sedimentation rates, and oxygen-isotope stratigraphy. It was found that radiocarbon analysis of total carbon content was reliable only for the sediments younger than 16,000 B.P. Older sediments contain a modern ¹⁴C component added during diagenesis. Radiocarbon analysis of the Holocene sediments showed that the laminations in the sediments formed yearly. Thus, a reasonably reliable chronology for the lower sections of the core was determined from the relatively constant and high sedimentation rate. In addition, data from the authigenic fraction of the sediments were gathered for aluminum, iron, manganese, calcium, magnesium, and beryllium. From these data, the determination of the history of the waters of the Gulf of California was possible. Hydrothermal activity, glacial meltwater events, and the changing water masses in the Gulf of California possibly have left a record in the core studied. Analysis of the ¹⁰Be data along with the elemental data shows that the ¹⁰Be concentrations in the authigenic fraction of the sediments tracked the production rate of this isotope in the atmosphere. The ¹⁰Be concentrations at this site were little affected by diagenesis, hydrothermal activity, and terrigenous input. Most of the ¹⁰Be deposited at site 480 originated from the open sea. The changing geomagnetic dipole moment for the Holocene is seen in the ¹⁰Be data. The production rate for ¹⁰Be during the late Pleistocene tracked the changing dipole moment of the Earth as shown in other studies. Two ¹⁰Be anomalies correspond with the Mono Lake and Laschamp geomagnetic excursions. It was determined that the geomagnetic excursions could not have produced these anomalies. The data for the ¹⁰Be anomalies are consistent with a previous hypothesis for supernovae shock waves and the generation of cosmic rays. It is proposed that a series of such shock waves compress the heliosphere and affect the magnetosphere of the Earth. For such a mechanism to produce the observed geomagnetic excursions, either an enhanced interplanetary magnetic field must be externally imposed on the magnetosphere, or external forcing creates a corresponding non-linear response from the magnetodynamo of the Earth.