Alkenone Paleothermometry in Coastal Settings: Evaluating the Potential for Highly Resolved Time Series of Sea Surface Temperature

Abstract

The unsaturation ratios of alkenones have been applied widely to reconstruct pelagic sea surface temperatures (SSTs). However, applications to costal settings have been hampered by the effects of salinity and nutrient dynamics. Here we present a 4.5-year-long record of alkenones in water column particulate organic matter from sites in Narragansett Bay (RI, USA) spanning wide ranges in salinity and nutrients, and a 300-year alkenone record from two Narragansett Bay sediment cores. Particulate organic matter results suggest that there are two distinct alkenone producing populations. One is a widespread population that blooms in summer (July–September) and displays alkenones typical of Group III producers—the basis for the widely applied marine U k’37 temperature index. The other population is confined to brackish waters in the Providence River during spring salinity minima and displays alkenone profiles consistent with Group I production—the first detection of Group I in a brackish nonlacustrine environment. The sedimentary alkenone profiles consist of only Group III alkenone distributions, allowing for the reconstruction of a 300-year-long record of Narragansett Bay SST. Comparison of the most recent 100 years of this reconstruction with local and regional instrumental SST records shows excellent agreement, confirming a regional pattern of SST rise in the coastal northeastern United States that exceeds global or hemispheric rates. Group III alkenone production in coastal settings may be common and may open the way to high resolution local and regional SST reconstruction.