Ancient
pollen1(花粉) and
charcoal2(木炭) preserved in deeply buried
sediments3 in Egypt's Nile
Delta5 document the region's ancient droughts and fires, including a huge drought 4,200 years ago associated with the
demise6 of Egypt's Old Kingdom, the era known as the pyramid-building time. "Humans have a long history of having to deal with climate change," said Christopher Bernhardt, a researcher with the U.S. Geological Survey. "Along with other research, this study geologically reveals that the evolution of societies is sometimes tied to climate variability at all scales -- whether decadal or
millennial8."
Bernhardt conducted this research as part of his Ph.D. at the University of Pennsylvania, along with Benjamin Horton, an associate professor in Penn's Department of Earth and Environmental Science. Jean-Daniel Stanley at the Smithsonian Institution also participated in the study, published in July's edition of Geology.
"Even the
mighty9 builders of the ancient pyramids more than 4,000 years ago fell victim when they were unable to respond to a changing climate," said USGS Director Marcia McNutt. "This study
illustrates10 that water availability was the climate-change Achilles Heel then for Egypt, as it may well be now, for a planet topping seven billion thirsty people."
The researchers used pollen and charcoal preserved in a Nile Delta
sediment4 core dating from 7,000 years ago to the present to help resolve the physical
mechanisms11 underlying12 critical events in ancient Egyptian history.
They wanted to see if changes in pollen assemblages would reflect ancient Egyptian and Middle East droughts recorded in archaeological and historical records. The researchers also examined the presence and amount of charcoal because fire frequency often increases during times of drought, and fires are recorded as charcoal in the geological record. The scientists suspected that the proportion of wetland pollen would decline during times of drought and the amount of charcoal would increase.
And their suspicions were right.
Large decreases in the proportion of wetland pollen and increases in
microscopic13 charcoal occurred in the core during four different times between 3,000 and 6,000 years ago. One of those events was the
abrupt14 and global mega-drought of around 4,200 years ago, a drought that had serious societal
repercussions15, including famines, and which probably played a role in the end of Egypt's Old Kingdom and
affected16 other
Mediterranean17 cultures as well.
"Our pollen record appears very sensitive to the decrease in precipitation that occurred in the mega-drought of 4,200 years ago," Bernhardt said. "The vegetation response lasted much longer compared with other
geologic7 proxy18 records of this drought, possibly indicating a sustained effect on delta and Nile basin vegetation."