A researcher reconstructs sea surface temperatures using both foram δ¹⁸O and alkenone UK'₃₇ from the same sediment core and finds the two proxies disagree by 3°C in one interval. What is the most scientifically appropriate interpretation?
AOne proxy must be wrong; discard whichever disagrees with modern analogues for that region
BThe disagreement is within combined uncertainties of ±2°C and should be ignored
CThe proxies measure temperature through different chemical systems and can be differentially affected by diagenesis, ice-volume effects (δ¹⁸O), or sediment reworking (alkenones) — the disagreement is informative and warrants multi-proxy investigation
DAlkenones are always more reliable than δ¹⁸O in tropical settings, so the foram data should be disregarded
Proxy disagreements are scientifically valuable, not merely problematic. Foram δ¹⁸O records both temperature and seawater isotopic composition (which changes with ice volume), so ice-volume corrections introduce uncertainty. Alkenones bypass carbonate chemistry entirely but can be physically reworked from older sediments. A 3°C discrepancy may reflect a real phenomenon (e.g., a freshwater event that shifted seawater δ¹⁸O without changing temperature), diagenetic alteration of one proxy, or reworking. Multi-proxy disagreement demands investigation, not arbitrary dismissal of one record.
Question 2 Multiple Choice
Why does a higher UK'₃₇ value correspond to warmer sea surface temperatures?
AWarmer water contains more dissolved carbon, which increases the total alkenone production rate
BThe C₃₇:₂ alkenone is more thermally stable and preferentially survives preservation in warmer sediments
CCoccolithophores increase membrane unsaturation at cold temperatures and decrease it at warm temperatures; higher UK'₃₇ means more di-unsaturated (fewer double-bond) alkenones, reflecting warmer growth conditions
DWarmer water dissolves the C₃₇:₃ form preferentially, leaving a higher ratio of C₃₇:₂ in the sediment
The biological mechanism is membrane fluidity regulation. At cold temperatures, coccolithophores synthesize more unsaturated alkenones (more double bonds) to maintain membrane flexibility — a form of homeoviscous adaptation. At warm temperatures, more saturated forms (fewer double bonds) maintain appropriate membrane rigidity. UK'₃₇ = [C₃₇:₂] / ([C₃₇:₂] + [C₃₇:₃]): more di-unsaturated (two double bonds, C₃₇:₂) relative to tri-unsaturated (three double bonds, C₃₇:₃) means higher UK'₃₇, reflecting warmer growth temperature.
Question 3 True / False
Alkenone paleothermometry is particularly valuable because it records temperature through organic molecular structure, making it chemically independent of the carbonate system and a powerful cross-check for oxygen isotope proxies.
TTrue
FFalse
Answer: True
This independence is alkenone paleothermometry's greatest strength. Foram δ¹⁸O records are affected by both temperature and the isotopic composition of seawater (δ¹⁸Osw), which itself varies with ice volume. Disentangling these two signals requires additional corrections. Alkenones record temperature purely through the degree of organic molecular unsaturation — no mineral dissolution, no isotopic fractionation corrections, no ice-volume dependence. In intervals where foram records are complicated by diagenesis or ice-volume uncertainty, alkenones can provide an independent temperature constraint.
Question 4 True / False
Because alkenones are produced by surface-dwelling coccolithophores, they reliably record sea-floor bottom water temperatures at the sediment site.
TTrue
FFalse
Answer: False
Alkenones are produced in the surface ocean by photic-zone coccolithophores — they record sea surface temperatures (SST), not bottom water temperatures. After the organisms die, the alkenone-containing particles sink and are preserved in the underlying sediment. The preserved UK'₃₇ signal reflects the temperature of the surface waters where the coccolithophores lived and grew, not the deep-water conditions at the sea floor where the sediment accumulates.
Question 5 Short Answer
Why is physical reworking of alkenones a concern when reconstructing paleotemperatures, and what independent evidence is needed to rule it out?
Think about your answer, then reveal below.
Model answer: Alkenones can be transported by bottom currents or bioturbation from older sediment layers into younger ones (or vice versa). If reworked alkenones from a different time period contaminate a sample, the measured UK'₃₇ would reflect a mixture of temperatures from multiple time intervals, biasing the paleotemperature estimate. Independent chronological controls — radiocarbon dating, biostratigraphy, or correlation with well-dated reference cores — are needed to verify that the alkenones in a given sediment horizon actually formed at the time the sediment was deposited.
This is why alkenone paleothermometry is most reliable in sediment cores with good age control and minimal evidence of bioturbation or current winnowing. High-sedimentation-rate sites are generally preferred because reworking mixes a smaller relative fraction of the signal. The concern about reworking is not unique to alkenones — all sedimentary proxies face it — but the fact that alkenone molecules can survive for millions of years makes them particularly susceptible to long-distance reworking from ancient outcrop material in some settings.