dioxide into sugars. As they do this, plants incorporate a
quantity of a naturally occurring radioisotope, carbon-14.
After plants die, the carbon-14 deteriorates due to
radioactive decay. When archaeologists find a shipwreck,
they can date the ship’s wood by comparing the remaining
carbon-14 fraction in it to that expected from atmospheric
carbon-14. This gives an estimate of when the tree was cut
down. Only about 20 shipwrecks have been dated to as
long ago as 500 BC . These were probably not Roman
ships, and could well have been Carthaginian, for example.
But then the number of Roman shipwrecks increases
rapidly. Around the time of the birth of Christ, they reached
a peak of 180.
Shipwrecks are a powerful way of tracing the economic
contours of the Roman Republic, and they do show
evidence of some economic growth, but they have to be
kept in perspective. Probably two-thirds of the contents of
the ships were the property of the Roman state, taxes and
tribute being brought back from the provinces to Rome, or
grain and olive oil from North Africa to be handed out free
to the citizens of the city. It is these fruits of extraction that
mostly constructed Monte Testaccio.
Another fascinating way to find evidence of economic
growth is from the Greenland Ice Core Project. As
snowflakes fall, they pick up small quantities of pollution in
the atmosphere, particularly the metals lead, silver, and
copper. The snow freezes and piles up on top of the snow
that fell in previous years. This process has been going on
for millennia, and provides an unrivaled opportunity for
scientists to understand the extent of atmospheric pollution
thousands of years ago. In 1990–1992 the Greenland Ice
Core Project drilled down through 3,030 meters of ice
covering about 250,000 years of human history. One of the
major findings of this project, and others preceding it, was
that there was a distinct increase in atmospheric pollutants
starting around 500 BC . Atmospheric quantities of lead,
silver, and copper then increased steadily, reaching a peak
in the first century AD . Remarkably, this atmospheric
quantity of lead is reached again only in the thirteenth
century. These findings show how intense, compared with
what came before and after, Roman mining was. This
upsurge in mining clearly indicates economic expansion.