Early Chemical Evidence of Life

Question 1

If life evolved prior to first microfossil evidence, how can we identify it?

Answer 1

geochemical signatures in the organic fraction of rocks

Question 2

kerogen

Answer 2

heterogenous, high molecular weight organic mixture formed when organic carbon of dead organisms undergoes diagenesis

Question 3

what is a key to the search for chemical evidence of life

Answer 3

kerogen

Question 4

isotope

Answer 4

different form of the same element (same number of protons and electrons) with different number of neutrons - changes the molecular weight of the element-
same electronic structure and behave in nearly the same manner

Question 5

kinetic isotope effect

Answer 5

heavier isotopes (more neutrons) will react more slowly than lighter isotopes, impacting the kinetic behavior of the isotopes, leading to a natural separation in isotopes

Question 6

what scale is difference in kinetic energy between isotopes stated

Answer 6

ratio to a standard in per mil delta notation
(example delta ^13C = -40 0/00 or delta ^13C = 10 0/00)

Question 7

reference for standard for Carbon 13

Answer 7

belemnite from PeeDee Formation

Question 8

How can delta ^13C = -40 0/00 be expressed in words

Answer 8

enriched in carbon 12 or depleted in carbon 13 (because of negative value of the ratio of C13/C12)

Question 9

How do isotopes provide evidence for life?

Answer 9

Life is lazy - autotrophs will use the light carbon first because it takes less energy to use it, leaving a reservoir enriched in the heavier C13 isotope

Question 10

Where in the autotroph processes does carbon isotope fractionation take place?

Answer 10

1 - diffusion of inorganic carbon into the cell
2 - fixing the inorganic carbon into organic carbon

Question 11

signature in rock that helps to identify biogenic processes

Answer 11

Kerogen that in enriched in light carbon isotopes as compared to the associated reservoir of inorganic carbon that is enriched in heavy carbon isotopes

Question 12

rough fractionation that can be produced by autotrophic microbes

Answer 12

20 0/00

Question 13

Hadean chemical signatures

Answer 13

4.4 Ga Jack Hills Zircon
Light carbon in graphite inclusion
not claiming life, just organic
contested

Question 14

earliest Archean chemical signature

Answer 14

3.85 Akilia Greenland
light carbon in graphite inclusions in apatite
back and forth, most recent conclusion is sedimentary BIF with light graphite

Question 15

partial Archean chemical signature

Answer 15

3.8 Isua, Greenland
metamorphosed shale
graphite enriched in C12 but no reservoir to compare to

Question 16

Archean chemical evidence of life

Answer 16

3.5 Coonterunah, Australia
Kerogenous limestone
light carbon Kerogen with carbonate reservoir showing a fractionation of 20 0/00
convincing evidence of autotrophic activity