Collaborative Research: Provenance of Alkenones & Holocene Temperature Evolution of the NW Atlantic

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This project will produce a set of 12 new sea-surface temperature reconstructions along the western North Atlantic to examine the accuracy of alkenone-based ocean temperature records. Temperature data over the past several thousand years show a mismatch between land-, sea-, and climate model-based estimates. Climate models suggest that temperature remained relatively unchanged, while land-based records shows slight warming. Oddly, sea surface temperatures show 4-6°C cooling over the same period of time. But concerns exist about how well these records reflect temperature conditions in the region. Alkenones are produced by algae which may be biased by the temperature of the season when algae are most abundant. They can also be carried to the North Atlantic from elsewhere by ocean currents and reflect temperature conditions of those regions. This study will attempt to resolve these issues by examination of alkenones collected directly in the water column throughout the year and comparing these to alkenones deposited on the seafloor nearby. Hydrogen isotope ratios and radiocarbon data will provide a test of how much material is transported from different sources. Alkenone temperature data will be compared to other temperature proxies (e.g. Mg/Ca of foraminifera) in the region. The project creates significant learning opportunities for middle and high school teachers who will team with scientists in the classroom to develop climate-based curricular instruction and participate in shipboard research during sediment coring operations. It will also provide significant support for undergraduate students and an early career postdoctoral student. The global composite of Holocene temperature proxy records indicates 0.5-1°C of cooling over the last 8,000 years, driven largely by a 2°C cooling in northern hemisphere marine records. Transient climate simulations, however, show no such cooling, and continental temperature records in eastern North America and Europe indicate slight warming. This “Holocene Conundrum” is one of the most prominent unmet challenges facing the climate science community. The northern hemisphere trend is largely driven by 4-6°C cooling in three alkenone SST records from the NW Atlantic. To determine whether this data-model mismatch indicates a gap in understanding of the climate system, these anomalous proxy records must be examined in the context of the factors influencing alkenone deposition in NW Atlantic slopewaters and compared to other SST proxies in the same locations for which samples are not available and analyses are presently lacking. Both deficiencies will be addressed in this research by measuring hydrogen isotope values and radiocarbon ages of alkenones, along with Mg/Ca ratios in planktonic foraminifera, from sediment cores and archived sediment trap samples collected in 2004-2007 in the NW Atlantic slopewater region. Alkenone SST records can be biased by lateral transport of slow-sinking alkenones via surface and abyssal currents, or by seasonal production, both potential causes for the data-model mismatch. This project will take advantage of the large hydrogen isotope gradient in NW Atlantic surface waters to ascertain alkenone synthetic provenance by measuring 2H/1H ratios in alkenones. Provenance will be further assessed by measuring alkenone 14C ages to determine the extent of pre-aging during advection. Comparison of alkenone temperatures and 14C ages to fast-sinking, foraminiferal Mg/Ca temperatures and calcite 14C ages, across a series of locations and depths, with seasonal resolution, will allow for the full characterization of advective or seasonal biasing in either proxy relative to in situ and satellite SSTs. Having established the impact of these potential biases, a robust re-assessment of NW Atlantic Holocene SSTs will be made via 12 new reconstructions (6 each from the two independent proxies) from 6 planned coring sites on depth transects east of New Jersey and Nova Scotia, allowing a possible solution to the “Holocene conundrum”.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.