Sedimentary rocks from the late Archean (2.7 Ga) contain 2-alpha-methylhopanes, which are biomarkers specific to cyanobacteria. What is the significance of this finding?
ACyanobacteria evolved only 2.7 billion years ago
BThe presence of cyanobacterial biomarkers indicates that oxygenic photosynthesis existed by 2.7 Ga -- several hundred million years before the Great Oxidation Event -- suggesting oxygen production preceded its atmospheric accumulation, with sinks (reduced minerals, volcanic gases) consuming O2 until the balance tipped
CThe biomarkers were produced by non-biological processes
DThe rocks have been contaminated by modern organisms
2-alpha-methylhopanes are diagnostic of cyanobacteria, the only organisms that perform oxygenic photosynthesis. Their presence at 2.7 Ga indicates that O2 production had begun, but atmospheric O2 levels remained low until ~2.4 Ga (GOE) because oxygen sinks exceeded sources. This molecular fossil evidence predates the first unambiguous inorganic evidence for oxygenation, illustrating the power of biomarkers to detect biological innovation before its planetary-scale consequences.
Question 2 True / False
All organic matter in sediments eventually converts to petroleum given sufficient burial depth and time.
TTrue
FFalse
Answer: False
Petroleum generation requires organic matter of appropriate type and adequate thermal maturation. Type I kerogen (algal/lacustrine) generates oil; Type II (marine) generates oil and gas; Type III (terrestrial plant) generates mainly gas. But many organic-rich rocks never reach the thermal window for oil generation (60-160 C), remaining immature. Over-maturation destroys oil, leaving only methane or graphite. And organic matter preserved in oxidizing environments is mostly degraded before burial. Petroleum generation requires the right organic matter type, sufficient burial temperature, and appropriate timing.
Question 3 Short Answer
Explain how the UK37' (alkenone unsaturation index) functions as a sea surface temperature proxy.
Think about your answer, then reveal below.
Model answer: Certain marine haptophyte algae (notably Emiliania huxleyi) synthesize long-chain alkenones (C37 methyl ketones) with varying numbers of double bonds. The ratio of di-unsaturated to tri-unsaturated alkenones (UK37') varies linearly with the growth temperature of the organism: colder waters produce more tri-unsaturated alkenones. Because alkenones are resistant to diagenesis and preserve their unsaturation pattern for millions of years, measuring UK37' in marine sediment cores provides a quantitative paleotemperature proxy calibrated against modern surface ocean temperatures. This is one of the most widely used organic geochemical proxies in paleoceanography.
Alkenones are molecular thermometers: the organism adjusts membrane lipid composition in response to temperature, and this adjustment is preserved in the sedimentary record.