The graph on the left shows how the lithic fraction would oscillate in a hypothetical core extracted from a continental shelf margin in the North Atlantic. How could such different sediments be
created in the same location? In short, the environmental conditions during sediment deposition on the ocean floor changed rapidly over time as the position and dynamics of the large continental ice sheets shifted. The precise glaciological conditions
during a Heinrich event remain a mystery, but all of the scientific evidence points to Heinrich events requiring a significant drop in sea surface temperatures, a reduction in the flux of planktonic foraminifera, and an extension of the ice sheet onto
the continental shelf, where deposition of ice rafted detritus, (IRD) occurred.
The figure on the right shows the probable differences in ice extent and sediment processes during Heinrich events and the corresponding non-Heinrich intervals. The
key features of sediment delivery during Heinrich events are the presence of icebergs, meltwater plumes and probably massive gravity flows called turbidites. All of these processes, occurring within cold stadial events, produce sediments with high
lithic content and deposit ice rafted detritus. In contrast, non-Heinrich events are distinguished by hemipelagic sediments with high foraminifer content. They are formed at the margin between coastal and pelagic environments and are deposited during
warm interstadial periods. Because Heinrich and non-Heinrich sediments formed in different environmental conditions, these events tell us much about fluctuations in climatic conditions in the North Atlantic over the last 60,000 years.
John T. Andrews, Thomas G. Andrews
NOAA Paleoclimatology Program and INSTAAR, University of Colorado, Boulder
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