Geoscience Reference
In-Depth Information
presence of diatom species (including
h alassionema nitzschioides
) that live
in waters with enhanced upwelling and diatoms (
Neodenticula seminae
) that
live in cooler, subarctic waters.
h ere is an increased concentration of biogenic silica and organic carbon in
the sediments (from an increased l ux of diatoms remains to the bottom), provid-
ing independent evidence for enhanced biological productivity as a result of the
increased upwelling of cold, nutrient-rich coastal waters. Moreover, a decrease
in the abundance of diatoms (
Pseudoeunotia doliolus
) that live in warm, low-
nutrient waters also supports the idea that conditions were cooler and nutrient-
rich as the result of increased upwelling. h e abundance of nutrient-rich waters
would have allowed all marine organisms along the coast to l ourish, including
those in the intertidal zone that provided food for humans.
Geochemist Timothy Herbert at Brown University, working on the same
cores as Barron, was able to make inferences about sea surface temperatures
using the fossil remains of the cell walls in a particular species of phytoplank-
ton,
Emiliana huxleyii.
h ese microscopic plants alter the rigidity of their
cell walls in response to changing water temperature. h ey do this by chang-
ing the number of double bonds in their lipid molecules, compounds known
as alkenones that behave much like butter, a saturated fat that is a solid at
room temperature. (Its structure is “saturated” with hydrogen atoms, so the
carbon atoms are held to each other with single bonds.) Unsaturated fat, like
oil, is a liquid at room temperature because it has a greater number of double
bonds between its carbon atoms. Phytoplankton have evolved the ability to
change the number of double bonds in their cell walls to maintain somewhat
l uid, or less rigid, cell walls as the water temperature changes. h ese changes
in the ratio of double to single bonds can be measured in the laboratory.
h e alkenone results show that coastal surface water temperatures during
the mid-Holocene were 1-2°C (1.8-3.5°F) cooler than conditions immediately
before or at er. h e upwelling of cooler, nutrient-rich waters would have led
to more prolii c phytoplankton blooms, providing more food for organisms
higher on the food chain: mollusks, i sh, seabirds, and, ultimately, humans.
coastal redwoods: upwelling and fog
Some of the most dramatic evidence for increased coastal upwelling during
the mid-Holocene comes not from the ocean but from the coastal mountains,
where magnii cent coast redwood trees grow between southern Oregon and
