Long-chain alkenones, organic molecules created by a select number of haptophyte algae, are ideal organic compounds for paleoclimate applications because of their species specificity, preservation in the geologic record as far back as 120 Ma, and prevalence in open marine, marginal marine and even lacustrine environments around the globe. Alkenone-producing haptophytes mainly reside in the shallower depths of the photic zone, and the alkenone unsaturation index, UK’37, has a strong correlation with sea surface temperatures, confirming a relationship with sea surface conditions. Hydrogen isotope ratios measured on long-chain alkenones (d2HC37) reflect growth water hydrogen isotope ratios in culture, and in the natural environment, albeit with different responses due to the biological hydrogen isotope fractionation, expressed as fractionation factor a. Fractionation is the difference in isotope ratios between growth water and isotope ratios of organic compounds, and it occurs intracellularly during various biosynthetic steps in the creation of organic compounds, but it is also affected by salinity, light intensity, and other environmental factors.
Investigations of the effect of different environmental parameters on the relationship between a and salinity in culture increases knowledge about the complex nature of hydrogen isotope ratios of alkenones, and helps clarify which parameters have the most significant effects on d2HC37 ratios – salinity, species, and low light intensity. We investigate this further in the natural environment by examining surface sediments sampled from different locations to cover a combined salinity gradient from 7 – 39. This compilation shows different results from culture experiments, but aligns with other environmental datasets. Using samples from ODP 1234, a down-core record off the coast of Chile covering the last 150 ka, we show that d2HC37 ratios are recording global climate trends over the last 150 ka. We reconstruct relative changes in salinity for the site. Results show that d2HC37 ratios are useful for understanding changes in hydrology of the surface ocean in the geologic record.
