Nearly a decade after Heinrich published his article, the proximal causes of Heinrich events seem clear: different sedimentological environments created by different glaciological conditions.
But it is still unclear what factors and forces ultimately lay behind these abrupt variations. And so scientists continue working to unlock the mysteries of Heinrich events by obtaining more cores, employing new analytical techniques, and seeking to
uncover connections between North Atlantic climatic variations and changes elsewhere in the world.
This slide, for example, shows data from core ODP-609 indicating changes across event H-4. The top graph shows the inverse relationship between the
percentage of the left-coiling planktonic foraminifera N. pachyderma and the percentage of lithic fragments (note the interval in which no foraminifera were present). The bottom graph represents changes in the isotopic composition of oxygen (the ratio of the abundant 16O and rare 18O isotopes of oxygen) contained in N. pachydermas' calcium carbonate shells. This index of the relative heaviness of oxygen isotopes, known as d18O, demonstrates a decrease of nearly 1.5 o/oo during H-4. The more
negative d18O values (often referred to as lighter) during Heinrich events is interpreted to be evidence of plumes of extremely fresh glacial meltwater which flowed into the normally salty North Atlantic. As documented by Bond et al (1992), not only
does the presence of left coiling foraminifera N. pachyderma indicate the deep southward invasion of polar water during the time of an Heinrich event, but the drop in salinities indicated by the d18O measurements was probably significant enough to temporarily shut down thermohaline circulation in the North Atlantic.
Thomas G. Andrews
NOAA Paleoclimatology Program and INSTAAR, University of Colorado, Boulder
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