SOURCE: BBC News (6-1-10)
Founded in 1607, Jamestown in Virginia was the first successful English settlement in North America.
Chemical analysis of shells thrown away from 1611-1612 shows that the James River where the oysters were harvested was much saltier then than it is today.
This was due to decreased flow from surrounding freshwater rivers.
For this to have been the case, rainfall must have been much lower when these oysters were growing.
US researchers have published details of the work in Proceedings of the National Academy of Sciences (PNAS) journal.
After sailing from London, the colonists selected Jamestown island, on the James River (named in honour of their king), as a secure location for their settlement.
The location had the advantage of a deep water channel allowing the English ships to ride close to shore.
But the island was swampy and overrun by mosquitoes.
And the latest evidence, combined with previous data, suggests the colonists could not have chosen a worse time to establish their settlements.
Juliana Harding from the College of William and Mary in Gloucester Point, US, and colleagues analysed oyster shells retrieved in 2006 from a well dug by the colonists.
The well was in use only for a short time before being converted into a rubbish pit, either because it ran dry or was infiltrated by salty water.
The team looked at values for a particular isotope, or form, of oxygen laid down in the shells.
The levels of this isotope - known as oxygen-18 - in oyster shells are controlled by the temperature and salinity of the water they grow in.
The team compared oxygen-18 values in the 17th Century James River oyster shells with those from their modern day counterparts.
They found that the winter salinity of the river was much higher during the early 1600s than it is today.
This suggests winter rainfall was considerably lower than modern levels, confirming historical accounts of drought conditions at the time.
Previous data based on tree rings and historical documents show that the arrival of the English colonists in Virginia coincided with a severe regional drought.
The years 1606-1612 were the driest in nearly eight centuries.
"Shortages of food and fresh drinking water, combined with poor leadership, nearly destroyed the colony during its first decade," the authors of the latest study write in PNAS.
During what became known as the "Starving Time" from 1609-1610, some 80% of the colonists died.
Seasonal cycles in oxygen-18 values, along with archaeological data, allowed the researchers to show that the oysters were collected in different seasons between 1611 and 1612.