Abstract

Four supernovae (SNe), exploding <=300 pc from Earth, were recorded 44, 37, 32, and 22 kyr ago in the radiocarbon (¹⁴C) record during the past 50 kyr. Each SN left a nearly identical signature in the record, beginning with an initial sudden increase in atmospheric radiocarbon, when the SN exploded, followed by a hiatus of 1500 yr, and concluding with a sustained 2000 yr increase in global radiocarbon due to γ-rays produced by diffusive shock in the SN remnant (SNR). For the past 18 kyr excess radiocarbon has decayed with the ¹⁴C half-life. SN22kyrBP, is identified as the Vela SN that exploded 250 ± 30 pc from Earth. These SN are confirmed in the ¹⁰Be, ²⁶Al, ³⁶Cl, and NO_3^- geologic records. The rate of near-Earth SNe is consistent with the observed rate of historical SNe giving a galactic rate of 14 ± 3 kyr^-1 assuming the Chandra Galactic Catalog SNR distribution. The Earth has been used as a calorimeter to determine that ≈2 × 10⁴⁹ erg were released as γ-rays at the time of each SN explosion and ≈10⁵⁰ erg in γ-rays following each SN. The background rate of ¹⁴C production by cosmic rays has been determined as 1.61 atoms cm^-2 s^-1. Approximately 1/3 of the cosmic ray energy produced by diffusive shock in the SNR was observed to be emitted as high-energy γ-rays. Analysis of the ¹⁰Be/⁹Be ratio in marine sediment identified 19 additional near-Earth SNe that exploded 50-300 kyr ago. Comparison of the radiocarbon record with global temperature variations indicated that each SN explosion is correlated with a concurrent global warming of ≈3°C-4°C.