Murky Waters

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Scientists are digging deep into lake sediments along the Sierra Nevada mountain range – the major source of California’s water supply — to understand how the hydrology of this mountain range has varied over the last millennium. The research study will offer valuable insight to better prepare for a warmer and drier California future.

A research team, led by Scripps Institution of Oceanography at UC San Diego graduate student Lydia Roach and research meteorologist Dan Cayan, collected sediment cores from Swamp Lake, an isolated lake near the Hetch Hetchy reservoir in Yosemite National Park’s northwest corner. Protected within the confines of one of the country’s oldest national parks, Swamp Lake sediments have remained largely undisturbed by humans throughout the Holocene epoch, which began 11,700 years ago and continues to the present, thus offering researchers a rare glimpse at past climate conditions.

The lake’s remote location posed several challenges for the research team trying to collect important sediment that lay 20 meters (66 feet) below the lake’s bottom.

“Transporting fragile frozen sediment cores from a lake in Yosemite to a freezer at Scripps was probably the greatest logistical challenge of this project,” said Roach, a student of Scripps paleoclimate expert Chris Charles.

During the most recent field expedition in September 2007, the researchers used a combination of mules and human porters to transport their gear, including several coolers of dry ice and the necessary supplies to assemble a raft that researchers needed to collect the samples, seven miles from Hetch Hetchy to Swamp Lake along an overgrown trail.

The Scripps team is interested in collecting the sediment cores to analyze the hydrogen isotope ratios of specific organic compounds, such as fatty acids and other lipids during episodes of severe “megadroughts” believed to have occurred in California’s climate history over the past 1,100 years.

Once at the field site, the Scripps research team assembled the raft to support four scientists and their equipment to begin coring through the lake bottom without disturbing the finely layered sediment structure.

To collect the sediments in the soupy Swamp Lake bottom, Roach used a special coring technique called freeze coring. By lowering a hollow, weighted, aluminum wedge filled with a dry ice-ethanol slurry into the upper sediment and letting it sit for 10 to 15 minutes to freeze the sediment to the aluminum, the researchers were able to retrieve a crust of frozen sediment around the aluminum wedge with fine layers perfectly intact.

By pursuing this new method to investigate historic hydrologic variability in the Sierra Nevada Mountains, Roach hopes to offer a better understanding of how future climate change will affect California’s vital freshwater resources over the coming decades and centuries.

Thus far, the study is showing promising preliminary results indicating that hydrogen isotope ratios of these compounds may in fact be sensitive to the amount of snow that falls in and around the watershed of swamp lake. This snowfall variability is linked to a variety of local and remote climate processes including the El Niño Southern Oscillation (ENSO) and trends of increasing mean annual temperature.

California is heavily dependant on Sierra Nevada snowpack as a freshwater resource. The researchers hope that by understanding how this resource has varied over not just the 20th century but the past several centuries, they can better predict how it will be impacted by future climate change.

Roach has begun analyzing older sediments, roughly 500 to 1,000 years old, to investigate hydroclimate variability near Swamp Lake and the Sierra Nevada in general over the last millennium.

-- Annie Reisewitz

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