Has the Earth been exposed to supernovae within the last 300 kyr?

Has the Earth been exposed to supernovae within the last 300 kyr?

Melott, A. L., I. G. Usoskin, G. A. Kovaltsov,
and C. M. Laird, 2015, Has the Earth been exposed
to numerous supernovae within the last 300 kyr?
International Journal of Astrobiology. 14(3):375–378
doi:10.1017/S1473550414000512

"ABSTRACT: We show that Firestone (2014) contains
numerous errors in the application of past research.
F14 overestimates likely nitrate and 14C production
from moderately nearby supernovae by about four
orders of magnitude. Moreover, the results are based on
wrongly selected (obsolete) nitrate and 14C datasets.
The use of correct and up-to- date datasets does not
confirm the claimed results. The claims in the paper
are invalidated."

PDF file at http://cc.oulu.fi/~usoskin/personal/Melott_2015.pdf
and http://arxiv.org/abs/1406.5141 .

Also, there is "Comment on: “Observation of 23
Supernovae that exploded <300 pc from Earth
during the past 300 kyr” by R.B. Firestone"
arXiv.org > astro-ph > arXiv:1406.5141v1
Astrophysics > Earth and Planetary Astrophysics at:
http://arxiv.org/abs/1406.5141v1 ,
http://arxiv.org/find/astro-ph/1/au:+Melott_A/0/1/0/all/0/1 ,
and
http://fermilab.voxcharta.org/2014/06/19/comment-on-observation-of-23-supernovae-that-exploded-300-pc-from-earth-during-the-past-300-kyr-by-r-b-firestone/

Yours,

Paul H.