Search USGS⌕ Search

Geology topics

J.C. Lynch

Publications and source records attributed to J.C. Lynch.

7 recordsLinked to original sources
USGS2010

Surface elevation change and susceptibility of different mangrove zones to sea-level rise on Pacific high islands of Micronesia

Mangroves on Pacific high islands offer a number of important ecosystem services to both natural ecological communities and human societies. High islands are subjected to constant erosion over geologic time, which establishes an important source of terrigeneous sediment for nearby marine communities. Many of these sediments are deposited in mangrove forests and offer mangroves a potentially important means for adjusting surface elevation with rising sea level. In this study, we investigated sedimentation and elevation dynamics of mangrove forests in three hydrogeomorphic settings on the islands of Kosrae and Pohnpei, Federated States of Micronesia (FSM). Surface accretion rates ranged from 2.9 to 20.8 mm y −1 , and are high for naturally occurring mangroves. Although mangrove forests in Micronesian high islands appear to have a strong capacity to offset elevation losses by way of sedimentation, elevation change over 6½ years ranged from −3.2 to 4.1 mm y −1 , depending on the location. Mangrove surface elevation change also varied by hydrogeomorphic setting and river, and suggested differential, and not uniformly bleak, susceptibilities among Pacific high island mangroves to sea-level rise. Fringe, riverine, and interior settings registered elevation changes of −1.30, 0.46, and 1.56 mm y −1 , respectively, with the greatest elevation deficit (−3.2 mm y −1 ) from a fringe zone on Pohnpei and the highest rate of elevation gain (4.1 mm y −1 ) from an interior zone on Kosrae. Relative to sea-level rise estimates for FSM (0.8–1.8 mm y −1 ) and assuming a consistent linear trend in these estimates, soil elevations in mangroves on Kosrae and Pohnpei are experiencing between an annual deficit of 4.95 mm and an annual surplus of 3.28 mm. Although natural disturbances are important in mediating elevation gain in some situations, constant allochthonous sediment deposition probably matters most on these Pacific high islands, and is especially helpful in certain hydrogeomorphic zones. Fringe mangrove forests are most susceptible to sea-level rise, such that protection of these outer zones from anthropogenic disturbances (for example, harvesting) may slow the rate at which these zones convert to open water.

Ecosystems↗
USGS2007

Evaluation of marsh development processes at Fire Island National Seashore: Recent and historic perspectives

Purpose and significance of the study: Salt marshes are dynamic environments, increasing in vertical elevation and migrating, often landward, as sea level rises. With sea level rise greater than marsh elevation increase, marshes can be submerged, marsh soils become waterlogged, and plant growth becomes stressed, often resulting in conversion of vegetation-dominated marsh to mudflat or open water habitat. Given that the rate of sea level rise is expected to accelerate over the next century and that some marshes in the northeast are becoming submerged (e.g., Jamaica Bay, NY), it is important to understand the processes that control marsh development. More specifically, the objectives of this project were to quantify vertical marsh elevation change in relation to recent rates of sea-level rise and to investigate factors or processes that are most influential in controlling the development and maintenance of Fire Island salt marshes.

Report↗
USGS2005

Groundwater control of mangrove surface elevation: shrink and swell varies with soil depth

We measured monthly soil surface elevation change and determined its relationship to groundwater changes at a mangrove forest site along Shark River, Everglades National Park, Florida. We combined the use of an original design, surface elevation table with new rod-surface elevation tables to separately track changes in the mid zone (0?4 m), the shallow root zone (0?0.35 m), and the full sediment profile (0?6 m) in response to site hydrology (daily river stage and groundwater piezometric pressure). We calculated expansion and contraction for each of the four constituent soil zones (surface [accretion and erosion; above 0 m], shallow zone [0?0.35 m], middle zone [0.35?4 m], and bottom zone [4?6 m]) that comprise the entire soil column. Changes in groundwater pressure correlated strongly with changes in soil elevation for the entire profile (Adjusted R2 5 0.90); this relationship was not proportional to the depth of the soil profile sampled. The change in thickness of the bottom soil zone accounted for the majority (R2 5 0.63) of the entire soil profile expansion and contraction. The influence of hydrology on specific soil zones and absolute elevation change must be considered when evaluating the effect of disturbances, sea level rise, and water management decisions on coastal wetland systems.

Estuaries↗
USGS2003

Global carbon sequestration in tidal, saline wetland soils

Wetlands represent the largest component of the terrestrial biological carbon pool and thus play an important role in global carbon cycles. Most global carbon budgets, however, have focused on dry land ecosystems that extend over large areas and have not accounted for the many small, scattered carbon -storing ecosystems such as tidal saline wetlands. We compiled data for 154 sites in mangroves and salt marshes from the western and eastern Atlantic and Pacific coasts, as well as the Indian Ocean, Mediterranean Ocean, and Gulf of Mexico. The set of sites spans a latitudinal range from 22.4°S in the Indian Ocean to 55.5°N in the northeastern Atlantic. The average soil carbon density of mangrove swamps (0.055 ± 0.004 g cm -3 ) is significantly higher than the salt marsh average (0.039 ± 0.003 g cm -3 ). Soil carbon density in mangrove swamps and Spartina patens marshes declines with increasing average annual temperature, probably due to increased decay rates at higher temperatures. In contrast, carbon sequestration rates were not significantly different between mangrove swamps and salt marshes. Variability in sediment accumulation rates within marshes is a major control of carbon sequestration rates masking any relationship with climatic parameters. Globally, these combined wetlands store at least 44.6 Tg C yr -1 and probably more, as detailed areal inventories are not available for salt marshes in China and South America. Much attention has been given to the role of freshwater wetlands, particularly northern peatlands, as carbon sinks. In contrast to peatlands, salt marshes and mangroves release negligible amounts of greenhouse gases and store more carbon per unit area.

Global Biogeochemical Cycles↗
USGS1999

Restoring marsh elevation in a rapidly subsiding salt marsh by thin-layer deposition of dredged material

Thin-layer deposition of dredged material on coastal marsh by means of high-pressure spray dredging (Jet-Spray??2) technology has been proposed as a mechanism to minimize wetland impacts associated with traditional bucket dredging technologies and to restore soil elevations in deteriorated marshes of the Mississippi River delta. The impact of spray dredging on vegetated marsh and adjacent shallow-water habitat (formerly vegetated marsh that deteriorated to open water) was evaluated in a 0.5-ha Spartina alterniflora-dominated salt marsh in coastal Louisiana. The thickness of dredged sediment deposits was determined from artificial soil marker horizons and soil elevation change was determined from sedimentation-erosion tables (SET) established prior to spraying in both sprayed and reference marshes. The vertical accretion and elevation change measurements were made simultaneously to allow for calculation of shallow (~5 m depth) subsidence (accretion minus elevation change). Measurements made immediately following spraying in July 1996 revealed that stems of S. alterniflora were knocked down by the force of the spray and covered with 23 mm of dredged material. Stems of S. alterniflora soon recovered, and by July 1997 the percent cover of S. alterniflora had increased three-fold over pre-project conditions. Thus, the layer of dredged material was thin enough to allow for survival of the S. alterniflora plants, with no subsequent colonization by plant species typical of higher marsh zones. By February 1998, 62 mm of vertical accretion accumulated at this site, and little indication of disturbance was noted. Although not statistically significant, soil elevation change was greater than accretion on average at both the spray and reference marshes, suggesting that subsurface expansion caused by increased root biomass production and/or pore water storage influence elevation in this marsh region. In the adjacent shallow water pond, 129 mm of sediment was deposited in July 1996 as a result of spraying, and despite initial shallow subsidence and continual erosion through February 1998, water bottom elevation was raised sufficiently to allow S. alterniflora to invade via rhizome growth from the adjacent marsh. Hence, thin-layer deposition of dredged material at this site was effective at restoring and maintaining marsh elevation after 1.5 years. However, if the open water sediment deposits are not soon completely stabilized via further vegetative colonization, erosion may eventually lower elevations to the level where emergent vegetation cannot persist.

Ecological Engineering↗
USGS1996

Marsh vertical accretion in a Southern California Estuary, U.S.A

Vertical accretion was measured between October 1992 and March 1994 in low and high saltmarsh zones in the north arm of Tijuana estuary from feldspar market horizons and soil corings. Accretion in the Spartina foliosa low marsh (2-8.5 cm) was related almost entirely to episodic storm-induced river flows between January and March 1993, with daily tidal flooding contributing little or no sediment during the subsequent 12 month period of no river flow. Accretion in the Salicornia subterminalis high marsh was low (~1-2 mm) throughout the 17-month measuring period. High water levels in the salt marsh associated with the storm flows were enhanced in early January 1993 by the monthly extreme high sea level, when the low and high marshes were flooded about 0.5 m above normal high tide levels. Storm flows in January-March 1993 mobilized about 5 million tons of sediment, of which the low salt marsh trapped an estimated 31,941 tonnes, including 971 tonnes of carbon and 77 tonnes of nitrogen. Sediment trapping by the salt marsh during episodic winter floods plays an important role in the long-term maintenance of productivity of Tijuana estuary through nutrient retention and maintenance of marsh surface elevation. The potential exists, however, for predicted accelerated rates of sea-level rise to out-pace marsh surface elevation gain during extended periods of drought (i.e. low sediment inputs) which are not uncommon for this arid region.

California↗