Re: Purbeck Beds

["Richard W. Travsky" <rtravsky@uwyo.edu>]

On Fri, 6 Feb 2004, Donna Braginetz wrote:
> The Ananova article reads:
> 
> **  Bone from 20-tonne dinosaur found in quarry
> The fossil has come from rocks known as the Purbeck Beds, in Portland,
> Dorset, formed in an arid lagoon about 130 million years ago.
> 
> What, exactly, is an "arid lagoon"? This is a contradiction in terms,
> no? Can someone more fully describe what the environment that produced
> the Purbeck Beds was like?

Huh. Well, that got me curious, did the google routine. Three hits and two
of them are the article. The other is

 http://www.ucs.louisiana.edu/~rrt4630/colombia.html

Some sort of wetlands project.

 The objectives of the project are to document the changes in the
 hydrological properties of mangrove wetlands as a response to landscape
 freshwater diversions towards a river-dominated, arid lagoon-delta
 ecosystem ( Cienaga Grande de Santa Marta, extension ~ 1280 km2) and
 determine the productivity and structural changes of mangrove wetlands
 during the rehabilatation process.

Looking up "Cienaga Grande de Santa Marta", several hits. This link
seemed to exlpain a bit:

http://data.ecology.su.se/MNODE/South%20America/cienegagrande/cienegagrandebud.html

 The Cienaga Grande de Santa Marta (CGSM) is a lagoon-delta ecosystem
 which forms the exterior delta of the Magdalena River. [...] This system
 can be classified as a type I setting (river-dominated, arid, with low
 tidal amplitude) containing fringe, basin, and riverine mangroves (Thom
 1982). [...] The coastal climate zone is arid tropical, with 6-7
 dry-months a year and an annual deficit of 1,031 mm because
 evapotranspiration (1,431 mm year-1) largely exceeds precipitation (400
 mm year-1). [...] Freshwater diversion from the lagoon-delta complex in 
 this arid coastal region has resulted in hypersalinization of mangrove
 soils   [...]

Hmmm. A lagoon that apparently gets a little dry seasonally.