Geoscience Reference
In-Depth Information
During warmer climate conditions, bristlecone pines are able to survive at
higher elevations, and the tree line moves upslope. h e radiocarbon ages of
ancient tree stumps along the upper elevations of the White Mountains indi-
cate ages of between 5,700 to 4,100 years ago, suggesting a warmer climate.
More evidence of warmth was obser ved in glaciers in the mountain ranges of
the western United States that retreated during the mid-Holocene.
coastal conditions
Sediment cores retrieved from along the continental shelf of the Pacii c
Northwest coast contain the remains of marine organisms that provide clues
to past oceanic conditions during the mid-Holocene. h ese clues include the
minuscule fossilized remains of microscopic plants (phytoplankton) living
in the sunlit surface waters of the oceans, which convert carbon dioxide and
sunlight into energy (in a process known as photosynthesis), providing the pri-
mary fuel for the entire marine food web. Some phytoplankton secrete a silica
cell wall that cannot be digested by the tiny marine animals (zooplankton)
that feed on and then excrete them. As the excrement (or “fecal pellets,” i lled
with these hard, tiny cells) sinks slowly to the bottom, it eventually becomes
invaluable data for the oceans' sedimentary archive.
John Barron, a micropaleontologist at the U.S. Geological Survey in
Menlo Park, California, has devoted his career to studying the fossil remains
of phytoplankton. He is the world 's expert on diatoms, a type of phytoplank-
ton that secretes silica cell walls shaped like minuscule hat boxes, footballs, or
other fanciful forms—elegant ornamentation that helps them stay al oat in
the sunlit surface waters of the ocean by increasing their surface area. Barron
has analyzed the dif erent diatom species found in this region and noted that
each is adapted to specii c ocean conditions, including water temperature,
oxygen content, nutrient levels, and water depth.
Barron and his colleagues determined the relative abundances of a dozen
or so species found in sediment cores recovered from the continental shelf in
the eastern Pacii c, from Baja California to Washington state. h e oceano-
graphic conditions of this region, just of the coast of western North America,
play a major role in inl uencing climate along the coast, thereby determining
its biological resources. Barron's research i ndings reveal that cold, nutrient-
rich waters were upwelling along the coast more ot en and more extensively
between 6,300 and 5,800 years ago. His evidence consists of the increased
