Introduction

The Sacramento–San Joaquin Delta of California was 

once a 1,400 square kilometer (km2) tidal marsh, which contained a vast layer of peat ranging up to 15 meters (m) thick 

(Atwater and Belknap, 1980). Because of its favorable climate 

and highly fertile peat soils, the majority of the Delta was 

drained and reclaimed for agriculture during the late 1800s 

and early 1900s. Drainage of the peat soils changed the conditions in the surface layers of peat from anaerobic (having no 

free oxygen present) to aerobic (exposed to the atmosphere). 

This change in conditions greatly increased the decomposition 

rate of the peat, which consists largely of organic (plant) matter. Thus began the process of land-surface subsidence, which 

initially was a result of peat shrinkage and compaction, and 

later largely was a result of oxidation by which organic carbon 

in the peat essentially vaporized to carbon dioxide (Deverel 

and others, 1998; Ingebritsen and Ikehara, 1999). Because of 

subsidence, the land-surface elevation on farmed islands in 

the Delta has decreased from a few meters to as much as 8 m 

below local mean sea level (California Department of Water 

Resources, 1995; Steve Deverel, Hydrofocus, Inc., written 

commun., 2007).

The USGS, in collaboration with the University of 

California at Davis, and Hydrofocus Inc. of Davis, California, 

has been studying the formation of the Delta and the impact 

of wetland reclamation on the peat column as part of a project 

called Rates and Evolution of Peat Accretion through Time 

(REPEAT). The purpose of this report is to provide results on 

the age of the remaining peat soils on four farmed islands in 

the Delta.

Age Determination of the Peat

In the summer of 2005, peat cores were collected using 

a Livingstone corer from Sherman Island, Webb Tract, Venice 

Island, and Bacon Island in the Sacramento–San Joaquin 

Delta, as well as four relatively undisturbed Delta marsh 

islands. The farmed islands currently are being used to produce corn, safflower, wheat, alfalfa, and pasture. Bulrush seeds 

and charcoal samples found throughout the peat cores were 

sent for radiocarbon age determination (for example Byrne 

and others, 2001; Watson, 2004) to the Center for Accelerated 

Mass Spectrometry, Lawrence Livermore National Laboratories, Livermore, California.

Age Determination of the Remaining Peat in the 

Sacramento–San Joaquin Delta, California, USA

By Judith Z. Drexler, Christian S. de Fontaine, and Donna L. Knifong

Results from radiocarbon age determination showed 

that the bottom of the peat column dates back to nearly 7,000 

years ago, indicating that the Delta started forming at about 

this time. In addition, the uppermost layer of intact peat that 

has not been physically disturbed by plowing or disking and 

has not been exposed to the atmosphere is mostly between 

4,100 and 4,800 years old on Sherman Island, Webb Tract, and 

Venice Island, and 4,900 to 5,300 years old on Bacon Island 

(fig. 1). These dates refer to peat situated in the center of the 

islands, which are much more subsided than the perimeter of 

the islands (Mount and Twiss, 2005). Ages of peat along the 

perimeter of the islands are younger, except on Webb Tract 

where the coring location was near the island drain. This location most likely was subject to greater oxidation of peat than 

usually occurs near a levee. Therefore, the age of peat along 

the perimeter of Webb Tract may be somewhat younger in 

areas distant from the drain.

At the farmed island sites, there was no peat younger 

than approximately 3,000 years old, because the younger peat 

layers have already been lost through subsidence processes. 

In fact, most of the peat from these sites was over 4,000 years 

old (fig. 1). In contrast, peat cores collected from the unfarmed 

marsh islands, which are also being analyzed as part of the 

REPEAT project, contained peat that dated to within the past 

100 years (not shown). These results indicate that, since reclamation, agricultural practices on the four farmed islands have 

largely consumed peat formed between approximately 150 

to 4,000 years ago. Because the four farmed islands studied 

for this project are similar in land use and water management to other islands in the central and western Delta region, 

it is highly likely that a similar pattern of peat consumption 

pertains to this entire part of the Delta. As farming continues 

in the Delta, land-surface subsidence will consume older and 

older layers of peat, mainly through the process of oxidation, 

unless the practice of keeping an aerobic root zone is discontinued.

References Cited

Atwater, B.F. and Belknap, D.F., 1980, Tidal-wetland deposits of the Sacramento–San Joaquin Delta, California: Pacific Coast Paleogeography 

Symposium v. 4, p. 89–103.

Byrne, R., Ingram, B.L., Starratt, S., Malamud–Roam, F., Collins, J.N., and 

Conrad, M.E., 2001, Carbon-isotope, diatom, and pollen evidence for late 

Holocene salinity change in a brackish marsh in the San Francisco Estuary: 

Quaternary Research v. 55, p. 66–76.

Open-File Report 2007–1303
California Department of Water Resources, 1995, Sacramento–San Joaquin 

Delta Atlas: The Resources Agency, Sacramento, California, 71 p.

Deverel, S.J., and Rojstaczer, S.A., 1996, Subsidence of agricultural lands in 

the Sacramento–San Joaquin Delta, California: Role of aqueous and gaseous carbon fluxes: Water Resources Research v. 32, p. 2359–2367.

Deverel, S.J., Wang, B., and Rojstaczer, S., 1998, Subsidence of organic soils, 

Sacramento–San Joaquin Delta, in Borchers, J.W. (ed.), Land subsidence; 

 case studies and current research; proceedings of the Dr. Joseph F. Poland 

symposium on Land subsidence: Special Publication—Association of 

Engineering Geologists, Sudbury, Mass., v. 8, p. 489–502.

Ingebritsen, S.E. and Ikehara, M.E., 1999, Sacramento–San Joaquin Delta: 

the sinking heart of the state, in Galloway, D., Jones, D.R. and Ingebritsen, 

S.E. (eds.), Land Subsidence in the United States: U.S. Geological Survey 

Circular 1182, p. 83–94.

Mount, J. and Twiss, R., 2005, Subsidence, sea level rise, and seismicity in 

the Sacramento–San Joaquin Delta: San Francisco Estuary and Watershed 

Science v. 3, issue 1, 18 p., http://repositories.cdlib.org/jmie/sfews/vol3/

iss1/art5

Rojstaczer, S.A. and Deverel, S.J., 1995, Land subsidence in drained histosols 

and highly organic mineral soils of California: Soil Science Society of 

America Journal v. 59, p. 1162–1167.

Watson, E.B., 2004, Changing elevation, accretion, and tidal marsh plant 

assemblages in a south San Francisco Bay tidal marsh: Estuaries, v. 27, 

no. 4, p. 684–698.

Oakley

Rio Vista

Antioch

1215230 12145 1213730 12130

38

07

30

38

37

52

30

Sacramento River

San Joaquin River

0 2 4 MILES

0 2 4 KILOMETERS

Sherman Island

Webb Tract

Venice 

Island

Bacon

 Island

(6,600) (6,700)

(6,500)

(6,500)

Age, in years, of intact peat surface:

EXPLANATION

3,200 – 3,800

4,100 – 4,800

4,900 – 5,300

Coring locations

(6,500) Maximum age, in years, of peat on island

DELTA

AREA

San Joaquin 

River

Sacramento

River

Sacramento

San Stockton Francisco

Modified from U.S. Geological Survey National Hydrography Data Set (NHD), 1:100,000 scale, 2001,

Albers Equal-Area Conic Projection

Figure 1. Estimated age of the surface of the intact peat that underlies the physically disturbed and oxidized soil layer currently 

being farmed in the Sacramento–San Joaquin Delta, California. Marsh island sites are not shown.

2 Age Determination of the Remaining Peat in the Sacramento–San Joaquin Delta, California, USA

For more information contact:

Judith Drexler, 

U.S. Geological Survey, 

6000 J Street, Placer Hall, 

Sacramento, CA 95819-6129

 jdrexler@usgs.gov