Abstract
There is substantial interest in stopping and reversing the effects of subsidence in the Sacramento–San Joaquin Delta (Delta) where organic soils predominate. Also, the passage of California Assembly Bill 32 in 2006 created the potential to trade credits for carbon sequestered in wetlands on subsided Delta islands. The primary purpose of the work described here was to estimate future vertical accretion and understand processes that affect vertical accretion and carbon sequestration in impounded marshes on subsided Delta islands. Using a cohort-accounting model, we simulated vertical accretion from 4,700 calibrated years before present (BP) at a wetland area located within Franks Tract State Recreation Area (lat 38.059, long −121.611, hereafter, “Franks Wetland”)—a small, relatively undisturbed marsh island—and at the Twitchell Island subsidence-reversal demonstration project since 1997. We used physical and chemical data collected during the study as well as literature values for model inputs. Model results compared favorably with measured rates of vertical accretion, mass of carbon sequestered, bulk density and organic matter content. From 4,700 to model-estimated 350 years BP, the simulated rate of vertical accretion at Franks Wetland averaged about 0.12 cm yr-1, which is within the range of rates in tidal wetlands worldwide. Our model results indicate that large sediment inputs during the last 150 to 200 years resulted in a higher accretion rate of 0.3 cm yr -1. On Twitchell Island, greater organic inputs resulted in average vertical accretion rates as high as 9.2 cm yr -1. Future simulations indicate that the managed impounded marsh will accrete highly organic material at rates of about 3 cm yr -1. Model results coupled with GIS analysis indicate that large areas of the periphery of the Delta, if impounded and converted to freshwater marsh, could be restored to tidal elevations within 50 to 100 years. Most of the central Delta would require 50 to 250 years to be restored to projected mean sea level. A large portion of the western Delta could be restored to mean sea level within 50 to 150 years (large areas on Sherman, Jersey, and Bethel islands, and small areas on Bradford, Twitchell, and Brannan islands, and Webb Tract). We estimated that long-term carbon sequestration rates for impounded marshes such as the Twitchell Island demonstration ponds will range from 12 to 15 metric tons carbon ha-1 yr-1. Creation of impounded marshes on Delta islands can substantially benefit levee stability as demonstrated by cumulative force and hydraulic gradient calculations.
Highlights
The 300,000-ha Sacramento–San Joaquin Delta is a critical natural resource, important agricultural region and the hub for California’s water supply
Peat vertically accretes concomitantly with sea level rise (Jelgersma et al 1993; Mitsch and Gosselink 2000) and when organic matter accumulates at a faster rate than it decomposes (Boelter and Verry 1977; Mitsch and Gosselink 2000)
Using physical and chemical data for cores collected throughout the Delta, Drexler et al (2009a, 2009b) provided insight into vertical accretion of peat and demonstrated fundamental differences among islands where mineral input was more dominant (e.g., Browns Island) because of sediment input from the Sacramento River and those where organic accretion predominated near the San Joaquin River (Figure 1)
Summary
The 300,000-ha Sacramento–San Joaquin Delta is a critical natural resource, important agricultural region and the hub for California’s water supply. Within an area of about 81,000 ha in the central Delta, an estimated 4.5 billion m3 of soils with a high organic matter content (peat) accreted during the last 6,700 years as sea level rose (Shlemon and Begg 1975; Drexler et al 2009a, 2009b; Mount and Twiss 2005). Peat vertically accretes concomitantly with sea level rise (Jelgersma et al 1993; Mitsch and Gosselink 2000) and when organic matter accumulates at a faster rate than it decomposes (Boelter and Verry 1977; Mitsch and Gosselink 2000). Farming is becoming more difficult and some subsided islands contain areas that are no longer arable because of excessive wetness
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