Radiocarbon and Deepwater Mass Changes in the Western North Atlantic During the Past 2000 Years: A Late Holocene Deepwater Study


OCCI Funded Project: 2005

Proposed Research

This project aims to characterize Holocene* climate variability in the western North Atlantic through radiocarbon dating of sediments taken from critical areas in the North Atlantic.  Strong ocean-atmosphere interactions make the western North Atlantic a climatic “center of action” - Subtropical surface waves feed the Gulf stream, southwestward flowing surface waters reflect the local subpolar climate as well as conditions in the Labrador and Greenland Seas. Deeper waters from these regions sculpt the continental margin and focus sediments in drift deposits containing high-resolution records of deep circulation and other paleoclimate signals. In these sediment drift deposits paleoceanographers have identified large changes in the Atlantic Ocean’s “conveyor” circulation during glacial times. Prior to the Holocene, suppression or reduction of the conveyor has resulted in rapid and sustained episodes of cold climate. 

Evidence of Holocene climate change is known from many locations around the globe, but the cause is unknown. One hypothesis is that cold events of recent millennia, such as the Little Ice Age, reflect the same processes as during glaciation.  The problem in testing this idea is that the Holocene climate signals are much smaller, and the data are noisier and sometimes ambiguous. Recently, we have used radiocarbon (14C) in benthic forams as a tracer for the strength of the conveyor in sediments from the Bermuda Rise in the Sargasso Sea. This region is known to be an area of high sediment accumulation rates, which preserve the history of millennial- and centennial-scale variability in climate extending through today. 

The preliminary 14C results from the Bermuda Rise show that, during the Little Ice Age of a few hundred years ago, there was increased production of North Atlantic Deep Water (NADW). This 14C method has never before been used for events such as the Little Ice Age, and the first results suggest that, in the recent past, there was actually more NADW than today. Importantly, this result is the opposite of what one would expect for a cool event, based on the conveyor’s behavior during glacial times.  If supported by further data, this result suggests that the climate system operates differently in the Holocene than during glaciation.  To confirm this important result we propose three tasks: (1) to complete the data series for the last 1000 years on the Bermuda Rise, (2) to extend the series to earlier cold events in the Holocene, and (3) to develop similar data series from two other locations in the western North Atlantic. Altogether, we plan on obtaining 30 Accelerated Mass Spectrometer (AMS) 14C dates from the Bermuda Rise samples and 50 AMS 14C dates from other locations.

*Holocene: The geologic period since the end of the retreat of the glaciers about 11,000 years ago to the present time; also known as the present interglacial epoch.