OBSERVATION OF 23 SUPERNOVAE THAT EXPLODED <300 pc FROM EARTH DURING THE PAST 300 kyr

Four supernovae (SNe), exploding <= 300 pc from Earth, were recorded 44, 37, 32, and 22 kyr ago in the radiocarbon (C-14) 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 gamma-rays produced by diffusive shock in the SN remnant (SNR). For the past 18 kyr excess radiocarbon has decayed with the C-14 half-life. SN22kyrBP, is identified as the Vela SN that exploded 250 +/- 30 pc from Earth. These SN are confirmed in the Be-10, Al-26, Cl-36, and NO3- 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 approximate to 2 x 10(49) erg were released as gamma-rays at the time of each SN explosion and approximate to 10(50) erg in gamma-rays following each SN. The background rate of C-14 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 gamma-rays. Analysis of the Be-10/Be-9 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 approximate to 3 degrees C-4 degrees C.