Time's rising tide may swamp Delta marshes

Mar 16, 2010 by Anne M. Stark
A view of the Delta. Photo by MaryAnn Harral

While marshes in the Sacramento-San Joaquin Delta area are currently keeping pace with rising sea levels, they may not be sustainable under future sea-level increases.

The current rate of vertical soil formation or accretion may not be enough to keep rising marshes from being flooded in the future. These results are part of a new study in the January 2009 issue of the journal, Estuaries and Coast, by Lab scientist Tom Brown in collaboration with Judith Drexler and Christian de Fontaine of the U.S. Geological Survey.

Using the radiocarbon measurement capabilities of the Lab’s Center for Accelerator Mass Spectrometry, Brown analyzed macrofossils in peat soil samples to determine how quickly peat has formed over the past 6,700 years. At four of the marsh sites, peat has accreted from .03 to .49 centimeters per year.

Marshes form where hydrologic, geomorphic and ecological factors are conducive to the initial and continued buildup of mineral sediment and organic matter. In tidal freshwater marshes, this accrual of material results in the formation of organic, peat soils.

To date, most marsh studies looking at peat accretion rates have only focused on short timescales (from 10 years up to 100 years). But Drexler, de Fontaine and Brown suggest that more studies are needed that estimate peat formation processes over timescales that approach or incorporate the entire lifetime of a marsh. In this study, the timescale went back more than 6,000 years.

“Long-term rates at the millennial scale can provide a broader context for understanding the ability of marshes to continue providing crucial ecosystem services under scenarios of rising sea levels associated with global ,” Brown said.

The Delta contains a long continuous peat record, which makes it an ideal area for studying marsh levels for the last several millennia. The Delta, which is the landward limit of the San Francisco Bay Estuary, was once a 1,400-square-kilometer tidal marsh region that began forming about 6,700 years ago.

Between the 1860s and 1930s, the Delta was transformed by drainage and extensive levee building into an agricultural landscape with about 57 farmed islands and tracts. Drainage of marshes and subsequent oxidation of organic peat soils in the agricultural areas resulted in massive losses of soil and subsidence of the land surface up to eight meters below mean sea level.

Brown and his colleagues determined peat accretion rates over the past several thousand years to understand the vulnerability of the marshes to possible flooding from sea level rise.

“There have been studies on the peat distribution and thickness to date, but no one really knows exactly how peat started forming in the Delta and how quickly it has accreted through time,” Brown said.

An aerial view of the Sacramento-San Joaquin Delta. Photo courtesy of U.S. Department of Interior/Bureau of Reclamation

Brown radiocarbon dated macrofossils found in the peat to determine the rate of peat accretion. The mean accretion rate in each marsh was well within the 0.1 to 0.2 centimeter estimated rate of sea-level rise during the past 6,000 years in the Estuary and very close to the estimated 20th century sea level rise of 0.17 centimeters per year.

However, sea level rise estimates for 2090-2099, from the Intergovernmental Panel on Climate Change, span a range of 0.15 to 0.97 centimeters per year and the central IPCC estimate of 0.38 centimeters per year is greater than the mean accretion rates for all the marshes in the study.

Although peat accretion rates in marshes have been known to increase to compensate for sea level rise, it is unclear whether the Sacramento-San Joaquin Delta marshes are capable of such large increases in accretion rates.

In addition, in the Delta region, there has been a decline in the amount of suspended sediment in the Sacramento and San Joaquin rivers due to upstream dam construction.

As sediment is an important component in the peat that forms in the marshes, declining sediment availability may make it even harder for marshes to keep pace with future rise.

Explore further: Warm US West, cold East: A 4,000-year pattern

Provided by Lawrence Livermore National Laboratory

not rated yet
add to favorites email to friend print save as pdf

Related Stories

Newest marsh villain: the periwinkle snail

Dec 19, 2005

Oil companies, levees and the burrowing nutria have been blamed for destroying Louisiana's marshes -- and now a new culprit arrives: the periwinkle snail.

Healthy coastal wetlands would adapt to rising oceans

Mar 28, 2007

Tidal marshes, which nurture marine life and reduce storm damage along many coastlines, should be able to adjust to rising sea levels and avoid being inundated and lost, if their vegetation isn't damaged and their supplies ...

New study fuels Louisiana subsidence controversy

Jul 21, 2006

While erosion and wetland loss are huge problems along Louisiana's coast, the basement 30 to 50 feet beneath much of the Mississippi Delta has been highly stable for the past 8000 years with negligible subsidence ...

Recommended for you

Warm US West, cold East: A 4,000-year pattern

8 minutes ago

Last winter's curvy jet stream pattern brought mild temperatures to western North America and harsh cold to the East. A University of Utah-led study shows that pattern became more pronounced 4,000 years ago, ...

New study outlines 'water world' theory of life's origins

2 hours ago

(Phys.org) —Life took root more than four billion years ago on our nascent Earth, a wetter and harsher place than now, bathed in sizzling ultraviolet rays. What started out as simple cells ultimately transformed ...

Agriculture's growing effects on rain

22 hours ago

(Phys.org) —Increased agricultural activity is a rain taker, not a rain maker, according to researchers at Pacific Northwest National Laboratory and their collaborators at the University of California Los ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

1 / 5 (1) Mar 16, 2010
"While marshes in the Sacramento-San Joaquin Delta area are currently keeping pace with rising sea levels, they may not be sustainable under future sea-level increases."

So keeping with a billion year old tradition, somewhere else, new marshes will form.

More news stories

UN weather agency warns of 'El Nino' this year

The UN weather agency Tuesday warned there was a good chance of an "El Nino" climate phenomenon in the Pacific Ocean this year, bringing droughts and heavy rainfall to the rest of the world.

Tech giants look to skies to spread Internet

The shortest path to the Internet for some remote corners of the world may be through the skies. That is the message from US tech giants seeking to spread the online gospel to hard-to-reach regions.

Patent talk: Google sharpens contact lens vision

(Phys.org) —A report from Patent Bolt brings us one step closer to what Google may have in mind in developing smart contact lenses. According to the discussion Google is interested in the concept of contact ...

Wireless industry makes anti-theft commitment

A trade group for wireless providers said Tuesday that the biggest mobile device manufacturers and carriers will soon put anti-theft tools on the gadgets to try to deter rampant smartphone theft.